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Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine Ming-Hui Sun,†a Shao-Zhuan Huang,†a Li-Hua Chen,*a Yu Li,*a Xiao-Yu Yang,*a Zhong-Yong Yuan*d and Bao-Lian Su*abc Over the last decade, significant effort has been devoted to the applications of hierarchically structured porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion. The hierarchy of porosity, structural, morphological and component levels in these materials is key for their high performance in all kinds of applications. The introduction of hierarchical porosity into materials has led to a significant improvement in the performance of materials. Herein, recent progress in the applications of hierarchically structured porous materials from energy conversion and storage, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine is reviewed. Their potential future applications are also highlighted. We particularly dwell on the relationship between hierarchically porous structures and properties, with examples of each type of hierarchically structured porous material according to its chemical composition and physical characteristics. The present review aims to open up a new avenue to guide the readers to quickly obtain

Received 22nd February 2016

in-depth knowledge of applications of hierarchically porous materials and to have a good idea about

DOI: 10.1039/c6cs00135a

selecting and designing suitable hierarchically porous materials for a specific application. In addition to

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focusing on the applications of hierarchically porous materials, this comprehensive review could stimulate researchers to synthesize new advanced hierarchically porous solids.

1. Introduction Hierarchically structured porous materials integrate multiple levels of porosity and structure and exhibit porous structure on more than one length scale from micro (o2 nm), meso(2–50 nm) to macropores (450 nm).1 The multiple levels of pore sizes usually comprise bimodalities such as micro–meso, meso–macro, and micro–macro, or even trimodalities such as micro–meso–macro and meso–meso–macro.2 Owing to their diversity and performance, hierarchically porous structures have attracted considerable attention as an

a

State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China. E-mail: [email protected], [email protected] b Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium. E-mail: [email protected]; Fax: +32 81 72 54 14; Tel: +32 81 72 45 31 c Clare Hall, University of Cambridge, Cambridge, CB2 1EW, UK. E-mail: [email protected] d Collaborat Innovat. Ctr. Chem. Sci. & Engn. Tianjin, Key Lab. Adv. Energy Mat. Chem., Minist. Educ., Coll. Chem., Nankai Univ., Tianjin 300071, China † These authors contributed equally to this work.

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important family of functional materials in recent years.3,4 Generally, hierarchically porous materials are highly porous, with high surface area, large accessible space, low density, variable chemical compositions and interconnected hierarchical porosity at different length scales, which are very favorable for light harvesting, electron and ion transport, and mass loading and diffusion, endowing them with technological importance in energy storage and conversion, catalysis, photocatalysis, adsorption, separation, gas sensing, and biomedicine.5,6 Over the last decade, numerous approaches have been developed to fabricate hierarchically porous materials.6 For example, a soft templating method, mainly using surfactants as soft templates, is usually employed for the synthesis of materials with dual mesoporous structures.6–9 Colloidal crystal templating is a common strategy to fabricate hierarchically porous materials, in which colloidal crystals are utilized as hard templates to create macropores, and smaller pores (small macropores, mesopores, or micropores) can exist within the solid wall skeleton.9 Some other important strategies, such as supercritical fluid (SCF) technology, emulsion templating, freeze-drying, and breath figure (BF) templating, are also used to synthesize the hierarchically structured porous materials.9,10

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The very innovative and versatile self-formation approach to well-defined and advanced hierarchically porous materials based on the chemistry of metal alkoxides and other precursors has been largely applied in the synthesis of porous materials.1,6,9 Additionally, some methods altering the chemical and physical parameters, such as phase separation, templating replication and post-treatment, have also been successfully employed.11–13 In addition to the extensive research on the design and synthesis of hierarchical porous materials, the investigation of the applicability of hierarchically porous materials is currently under rapid development.14 For example, hierarchically porous materials are widely used in the energy conversion area. Firstly, the hierarchically porous structures can increase the optical

path length and enhance the adsorption of dye molecules, thereby improving the light-harvesting efficiency, which can be effectively used as photoanodes for dye-sensitized solar cells (DSSCs). Secondly, as mentioned above, hierarchical porous structures favor efficient light harvesting, so they are applicable to photocatalytic H2 production, in which the separation and transfer of photogenerated electrons and holes can be effectively promoted. Thirdly, hierarchically porous architectures have been used in fuel cells (FCs), in which reactants flow into the cell and reaction products flow out. This characteristic needs the anodes and cathodes to be highly porous to facilitate the diffusion of the fuels and chemicals produced, thus improving the current density and conversion efficiency. Electrical energy storage

Li-Hua Chen received his BS from Jilin University in 2004 and PhD degrees in inorganic chemistry from Jilin University, China (2009), and in inorganic materials chemistry from the University of Namur, Belgium (2011). In 2011–2012, he held a researcher position at the University of Namur with Prof. Bao-Lian Su working on hierarchically porous zeolites. He is currently a full professor working in the State Key Laboratory of Li-Hua Chen Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, China. His research is aimed at new porous materials with hierarchical porosity for catalysis.

Yu Li received his BS from Xi’an Jiaotong University in 1999 and obtained his PhD from Zhejiang University in 2005. He worked in EMAT at the University of Antwerp with Prof. G. Van Tendeloo in 2005 and then in CMI at the University of Namur with Prof. Bao-Lian Su in 2006. Currently, he is a ‘‘Chutian’’ Professor at the Wuhan University of Technology. His research interests include the design and synthesis of nanomaterials, synthesis of hierarchically porous materials, and their applications in the fundamental aspects of energy and environment.

Xiao-Yu Yang earned his BS degree from Jilin University in 2000 and his joint PhD degree from Jilin University, China and University of Namur, Belgium (co-education) in 2007. After a postdoctoral fellowship at the UNamur, he worked as a ‘‘Charge´ de Recherches’’ at the F.N.R.S. (National Foundation of Scientific Research, Belgium). He is currently working as a professor at the State Key Laboratory of Xiao-Yu Yang Advanced Technology for Material Synthesis and Processing. He was a recipient of the Program for New Century Excellent Talents in University by the Chinese Ministry of Education in 2011, a ‘‘Chutian Scholar Professor’’ at the Wuhan University of Technology in 2010 and a ‘‘Qiu-Shi Outstanding Postgraduate’’ of Hong Kong in 2005. His research is aimed at self-assembly of materials, porous materials, catalysis, and cell-surface-engineering.

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Yu Li

Zhong-Yong Yuan received his BSc degree in Chemistry at Zhejiang Normal University in 1990, and obtained his PhD degree in Physical Chemistry at Nankai University in 1999. After his postdoctoral research at the Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, he joined the Laboratory of Inorganic Materials Chemistry at the University of Namur, Belgium in Zhong-Yong Yuan 2001. In 2005, he was appointed as a Professor in Nankai University. In 2006, he was awarded the ‘‘Program for New Century Excellent Talents in University’’ by the Ministry of Education. His research interests are mainly focused on the self-assembly of hierarchical nanoporous and nanostructured materials for energy and environmental applications. He is the coauthor of 170 ISI publications, 3 book chapters and 9 patents.

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applications, including supercapacitors and batteries (lithium ion batteries, lithium–sulfur batteries, lithium–air batteries, sodium-ion batteries and magnesium-ion batteries), are also among the applications that can profit from hierarchically porous materials, because a hierarchically porous structure can facilitate charge transfer through the electrode/electrolyte interface, reduce the ion diffusion pathway, and accommodate volume changes during cycling. High performance heterogeneous catalysts and photocatalysts are extremely desirable due to their green chemistry character, low energy routes and less waste generation during product isolation. Hierarchically porous structures, which can stabilize high active site dispersion and enhance reagent diffusion during catalytic reactions, have been widely used in catalysis and photocatalysis.15,16 Firstly, the incorporation of macropores into mesoporous architectures can increase light scattering and potentially light harvesting, thus improving photocatalytic efficiency. On the other hand, the introduction of macropores or mesopores into conventional microporous zeolites can minimise diffusion barriers, and potentially enhance the dispersion of active sites during catalyst preparation.17–19 Hierarchically porous materials, especially hierarchically monolithic silica materials, have also been considered as potential adsorbents for gas and liquid separation owing to their high permeability.20 In this regard, the high permeability and homogeneous flow-through the porous structure of macroporous silica are now used for various purposes related to

Bao-Lian Su is a member of the Royal Academy of Belgium, a fellow of the Royal Society of Chemistry and a life member of Clare Hall, University of Cambridge and holds ‘‘Chaire Francqui au titre Belge’’. He joined the faculty at the University of Namur and created the Laboratory of Inorganic Materials Chemistry (CMI) in 1995. He is currently the Director of the Laboratory of Inorganic Materials Chemistry (CMI), Bao-Lian Su University of Namur, Belgium. His is an ‘‘Expert of the State’’ in the framework of the Chinese Central Government program of ‘‘Thousands Talents’’ and ‘‘Changjiang Professor’’ appointed by Chinese Ministry of Education. He is a ‘‘Strategical Scientist’’ at the Wuhan University of Technology. He has received a series of honours and awards such as the First Prize Invention Award of Sinopec (1992, China), the A. Wetrems Prize (2007, Royal Academy of Belgium) and the IUPAC Distinguished Award for Novel Materials and their synthesis (2011). His current research fields include the synthesis, the properties and the molecular engineering of organized, hierarchically porous and bio-inspired materials, living materials, leaf-like materials and the encapsulation of living organisms for artificial photosynthesis, nanotechnology, biotechnology, cell therapy and biomedical applications.

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separation science. Also, as a class of efficient adsorbents with abundant adsorption sites, hierarchically porous materials are widely used for removing harmful pollutants such as dyes and heavy metal ions from the environment. Furthermore, hierarchically structured porous semiconductors possess large areas for surface adsorption and reaction processes and can facilely aid gas diffusion and mass transport, which greatly improve the sensitivity and response time of gas sensors. Importantly, as the hierarchically porous materials can improve bioactive behavior, enhance drug diffusion, loading and release, and ensure high enzyme loading and quick enzyme immobilization rates, they have also earned significant attention for biomedical applications, such as bone tissue engineering, drug delivery and enzyme immobilization. All of the above applications are summarized in Table 1 and their detailed description will be presented in our review. Since there is widespread interest in the development of hierarchically porous materials, their synthesis and applications have been reported by some excellent reviews.1,6,9,12–21 However, most of the reviews just focus on the synthesis and applications of one kind of material, or on materials in one application. There is no report describing all the families of functional hierarchically porous materials and their corresponding wide range of applications. In addition, the hierarchically porous materials are now highly researched and their applications are extended to more and more fields. The importance of such materials and their applications have become increasingly prominent. Therefore, it is of great importance to review the wide range of applications of hierarchically structured porous materials. In this critical and comprehensive review, we will describe the recent progress in the applications of hierarchically structured porous materials from energy conversion and storage, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine, and some proposed potential applications. This review is organised as follows: the application of a variety of hierarchically porous materials with different chemical compositions, physical properties and hierarchies in energy conversion and storage, a key field of current interest, is firstly commented in Sections 2 and 3, respectively. The high performances of hierarchically porous structures in energy conversion technologies such as dye sensitized solar cells (DSSCs), fuel cells (FCs) and photocatalytic H2 production and in energy storage technologies such as Li ion batteries, Na ion batteries, Mg ion batteries, Li–S batteries, Li–air batteries and supercapacitors as electrode materials are commented. It is well known that hierarchically porous materials are beneficial for heterogeneous catalysis and photocatalysis due to enhanced mass diffusion, excellent active site accessibility and efficient light harvesting. Therefore, the effects of hierarchically porous structures of these materials on their catalytic and photocatalytic activity in a variety of reactions are illustrated in Section 4. For heterogeneous catalysis and photocatalysis, hierarchical porosity is essential for industrial adsorption and separation processes. The improvements achieved by the introduction of hierarchical porosity in adsorbents for adsorption and separation are discussed in Section 5. The hierarchically porous materials have been largely used in the sensing field as well. The high sensing performances owing to

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Table 1 Applications of hierarchically structured porous materials in energy conversion and storage, photocatalysis, catalysis, adsorption, separation, sensing and biomedicine

Applications

Tpyes

Structural properties

Positive effects in applications

Energy converEnergy sion and storage conversion

DSSCs Fuel cells Photocatalytic hydrogen production Supercapacitors Lithium ion batteries Lithium–sulfur batteries Lithium–air batteries Sodium-ion batteries Magnesium-ion batteries

(i) Porous hierarchy (ii) High surface area; (iii) Short diffusion length

(i) Efficient light-harvesting, especially in biomaterials replica or biocomposites; (ii) Fast charge separation and high current density; (iii) High gas permeability; (iv) High storage density; (v) Fast electron and ion transport; (vi) Small resistance

(i) Porous hierarchy (ii) High surface area; (iii) Tunable pore size (iv) Large pore volume

(i) High accessibility of bulky molecules; (ii) High diffusion rate of reactant and product; (iii) Usually heteroatoms of zeolites or supported nanometal particles as active sites

Energy storage

Catalysis and photocatalysis

Photocatalysis Traditional Metal catalysts catalysis Metal oxide catalysts Zeolites Metals loaded on hierarchically porous inert supports

Adsorption and separation

Adsorption Separation

(i) Homogeneous flow-through (i) High permeability; pore structure; (ii) Usually monolithic column used (ii) High surface area; (iii) Controlled pore structures and surface properties;

Sensing

ZnO-based sensors Other metal/bimetal oxide based sensors Graphene-based sensors

(i) Porous hierarchy (ii) High surface area; (iii) Short diffusion length

(i) Large surface adsorption positions and reacting areas; (ii) Facile gas diffusion and mass transport

Biomedicine

Bone tissue engineering

(i) Porous hierarchy (ii) High surface area; (iii) Large pore volume

(i) Improved bioactive behavior and easy for cell penetration, tissue ingrowth; (ii) Enhanced drug diffusion, loading and release; (iii) High enzyme loadings and quick enzyme immobilization rates

their hierarchical porosity in sensor devices are reviewed in Section 6. The hierarchy is present everywhere in nature and in our body. The wide range of applications of hierarchically structured porous materials in biomedicine are demonstrated in Section 7. A comprehensive summary and outlook pointing out the future research directions of hierarchically porous materials in new fields are given in Sections 8 and 9, respectively. We sincerely hope that the readers will enjoy this complete and comprehensive review. This review also illustrates the relationship between the hierarchical structure and properties of these materials, which highlights the importance of developing methods for designing and controlling a specific pore structure. Therefore, the present paper aims to open up a new avenue for guiding the readers to quickly obtain in-depth knowledge of hierarchically porous materials, thereby promoting the development and applications of hierarchically porous materials.

2. Hierarchically porous structures for energy conversion Hierarchically porous nanostructures have been widely used for solar energy conversion and chemical energy conversion.21–25

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As one of the clean energy sources with the best potential, solar energy conversion devices, which include commercial silicon solar cells, organic solar cells, quantum dot solar cells, DSSCs, photosynthetic solar cells, and photocatalytic water splitting systems, have been well studied.21 Fuel cells (FCs), another potential clean energy sources, have also been widely investigated as chemical energy conversion systems. Considering that there have been many interesting reviews on solar energy conversion, this section only pays attention to the recent progress in typical hierarchically porous structures for DSSCs and photocatalytic H2 production. On the contrary, there are few reviews on FCs dealing with hierarchically porous structures. Therefore, we also focus on hierarchically porous structures for FC utilization in this section. 2.1 Hierarchically porous structures for dye sensitized solar cells Generally, a regular DSSC device consists of a working electrode, dye molecules, electrolytes, a spacer, and a counter electrode. The working electrode contains the as-prepared hierarchically porous structured materials, where a great amount of dyes can be loaded inside the hierarchically porous structures. On the other hand, the hierarchically porous structures are beneficial for light harvesting via multiple light scattering and reflections.

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It is well known that the macrochannels in hierarchically porous materials play a role as light harvesters, allowing the deep penetration of light in materials due to the scattering effect of the longer pathway length in macrochannels.21 The utilization of hierarchically structured porous materials enhances light absorption, resulting in high power conversion efficiency. So far, the most widely studied hierarchically porous nanostructures are TiO2, ZnO and the composites based on them due to their easily controlled porous structures,26–28 despite many other materials also being used for DSSCs. 2.1.1 Titanium oxide-based hierarchical porous structures for dye sensitized solar cells. Cho et al.29 synthesized a hierarchical porous TiO2 structure by beam assembly and/or lithography in which the nanoscale pore size and macroporous structure thickness are controlled. The hierarchically porous TiO2 structures are sensitized with N719 dye molecules and assembled with a Pt coated counter electrode filled with an iodine/iodide-based electrolyte. The working electrodes with two kinds of different macroporous film thicknesses (4 mm and 6 mm) and two different mesopore diameters (50 nm and 90 nm) are examined. The efficiency of the 50 nm porous electrodes is 5.00%, which is 4.5 times higher than that of the 90 nm porous electrodes (3.44%). For the 50 nm mesoscale porous electrodes, the short-circuit photocurrent density ( Jsc) values for the thicknesses of 6 mm and 4 mm are 10.3 mA cm 2 and 7.29 mA cm 2, respectively, resulting in an efficiency for the 6 mm thickness that is 1.41 times higher than that for the 4 mm thickness. The maximum efficiency for this hierarchically porous TiO2 structure is 5.00% with 50 nm pores and a 6 mm thickness, where the open-circuit voltage (Voc) and the fill factor (FF) are 0.77 V and 0.63, respectively. This result is comparable to the efficiency of conventional nanocrystalline TiO2 electrodes with the same thickness due to the strong suppression of charge recombination in the hierarchically porous TiO2 electrodes. Zhao et al.30 reported hierarchically structured three-dimensionally ordered macroporous (HS-3DOM) TiO2 with controlled macropore sizes (ca. 85–155 nm) for DSSCs. The hierarchically structured porous TiO2 layers fabricated from HS-3DOM TiO2 exhibit two kinds of morphologies, including one with nanoparticles filled in macropores and another with nanoparticles blocked on the surface of macropores. The results show that the TiO2 photoanode with hierarchically structured three-dimensionally ordered macropores (a macropore size of 105 nm) gives a current density of 20.6 mA cm 2 and a very high photo-toelectrical energy conversion efficiency of 9.7%. Such high power conversion efficiency (PCE) is ascribed to the special morphology of the TiO2 photoanode: the ordered and open structure resulted in high dye adsorption and the hierarchically porous structure resulted in enhanced light scattering and improved charge collection efficiency. Hwang et al.31 prepared hierarchically structured porous TiO2 spheres (HSP-TiO2) using an electrostatic spray technique and utilized them as photoelectrodes for highly efficient DSSCs. The DSSCs with HSP-TiO2 photoelectrodes showed a high energy conversion efficiency of over 10% under illumination of light at 100 mW cm 2. Frequency dependent measurements

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demonstrated that HSP-TiO2 shows a higher electron diffusion coefficient and a longer lifetime than nanostructured TiO2 due to the large surface area, strong scattering effects and high porosity present in the hierarchically porous architecture. The presence of high hierarchical porosity once again shows the advantage for light absorption by the light scattering effect. Huo et al.32 have synthesized hierarchically porous TiO2 hollow spheres (HPHSs). These as-prepared HPHSs have central cavities, macropores on shells and mesopores accumulated by TiO2 nanocrystallites. This unique hierarchically porous structure endows the TiO2 spheres with high specific surface area and excellent light scattering properties. The presence of TiO2 HPHSs as a light harvesting layer in DSSCs improves the photoelectric conversion efficiency (Z) by 38.2% (from 5.00% to 6.91%), compared with that of single layer P25 films (Fig. 1). Yu et al.33 have synthesized hollow anatase TiO2 (HA-TiO2) spheres composed of anatase nanocrystals, which not only showed low density, high specific surface area, and hierarchically porous structures, but also exhibited high light-collection efficiency and fast motion of charge carriers. The optimal efficiency of HA-TiO2 obtained at 600 1C is 4.82%, which is higher than that of P25 (4.35%). The enhanced performance of HA-TiO2 is due to its high surface area and hierarchical nanoporous structure. Liu et al.34 have synthesized TiO2 hollow spheres with a diameter of approximately 250 nm for DSSCs. TiO2 hollow spheres showed enhanced performance (Jsc = 8.0 mA cm 2, Voc = 0.732 V, FF = 65%, and Z = 3.79%) compared with P25 TiO2 nanocrystals ( Jsc = 7.25 mA cm 2, Voc = 0.702 V, FF = 59%, Z = 2.96%) measured at 100 mW cm 2, AM1.5G. The improved performance is primarily due to the enhanced light scattering by the TiO2 hollow spheres and the highly porous structure of the hollow sphere electrode, allowing the highly viscous non-volatile electrolytes to thoroughly penetrate the electrode. 2.1.2 Zinc oxide-based hierarchical porous structures for dye sensitized solar cells. Li et al.35 reported hierarchically

Fig. 1 (a) TEM image, (b) the corresponding SAED pattern, (c) TEM image of TiO2 HPHSs, (d) SEM image of a macropore, (e) IPCE spectra and (f) normalized IPCE spectra of DSSCs based on P25 films, dense TiO2, and TiO2 HPHSs. (Reproduced from ref. 32 with permission, Copyright American Chemical Society, 2013).

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porous nanosheet-assembled ZnO microspheres (PNMSs) for DSSCs. An overall light conversion efficiency of up to 5.16% has been achieved using this unique structure, which is superior to the reference porous nanosheet-assembled ZnO microflowers (PNMFs), porous dispersed ZnO nanosheets (PDNs), and previously reported nanostructures. The high performance of PNMSs can be attributed to (1) their special morphology and, in particular, large external pores for light absorption and propagation and quick electrolyte diffusion, (2) the nanopores embedded in the nanosheet for more efficient electrolyte diffusion, (3) the large surface area for dye adsorption, (4) the single crystal nature of the nanosheet facilitating a direct electron pathway and reducing the hole–electron recombination and the intersectional contact with neighboring nanosheets, thus avoiding high resistance in nanoparticle-based spheres. The outstanding property of hierarchical macro-nanoporosity was again evidenced by strong light absorption and propagation, high dye adsorption and quick electrolyte diffusion. Qiu et al.36 have synthesized a superstructure of meso/microporous single-crystalline ZnO nanoplates for DSSCs. This special structure showed a decent energy conversion efficiency of 5.05% owing to the finely separated bicontinuous transport pathways of electrons and holes and the enhanced light reflection and scattering as a result of again the hierarchical porosity. Very recently, Zheng et al.37 have designed a hierarchically porous nanocomposite electrode structure consisting of hierarchical iodine-doped zinc oxide (I-ZnO) aggregates for DSSCs. A ZnO compact layer (CL) is developed to suppress electron recombination by reducing back electron transfer from FTO to the electrolyte. A TiO2 protective layer (PL) is developed as a barrier to retard the formation of Zn2+/dye complexes and hence to enhance the stability of I-ZnO based cells under full sunlight outdoors. Hence, such hierarchical structure gave a distinguished PCE of 6.79% for the I-ZnO + CL + PL DSC, with 36% efficiency enhancement in comparison with the unmodified I-ZnO cell (5.01%). Moreover, the presence of a hierarchical structure induced a significant improvement in the stability of the I-ZnO + CL + PL cell as compared with I-ZnO. Most recently, perovskite solar cells have attracted much attention.38 The conversion efficiency of this new class of solar cells can reach up to 20%. This can largely accelerate the replacement of commercial silicon solar cells for practical utilization. Since the hierarchically porous nanostructures can act as excellent light harvesters and extend electron–hole separation time for solar cells, developing hierarchically porous perovskite nanostructures is urgent from this point of view. 2.2 Hierarchically porous structures for photocatalytic hydrogen production Hydrogen production by solar driven photocatalysis from renewable resources, such as water, has attracted tremendous interest and has been regarded as a promising approach for clean energy generation and environmental remediation in recent years. Generally, for H2 production on a semiconductor, the potential of the conductive band (CB) edge should be more negative than E(H+/H2) (0 V at pH 0), while the potential of the valence band (VB) edge should be more positive than E(O2/H2O)

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(1.23 V at pH 0). So far, various photocatalysts (such as TiO2, CdS, ZnS, BiWO3, and g-C3N4) have been developed for hydrogen production. There are many excellent reviews on various structures for photocatalytic H2 production.39–45 Therefore, in this section, we will only focus on the most recently developed hierarchically porous structures. Zhou et al.46 have prepared hierarchically porous zeolite Beta anchored and highly dispersed with Pt/CdS NPs into the mesopore channels by a facile successive two-step pore modification technology. A highly efficient and stable H2 evolution rate of 3.09 mmol h 1 g 1 has been successfully achieved by using the prepared novel catalyst under mimic sunlight irradiation. The synergetic catalytic effect between highly dispersed CdS and Pt nanoparticles (NPs), as well as the interaction between CdS species and the zeolite matrix, contributed to the enhanced separation and transfer of photogenerated electrons and holes, thus improving its photo-electrochemical properties and H2 evolution rate. Furthermore, this novel hierarchical porous structure can effectively protect CdS NPs from photocorrosion, thus exhibiting high stability in the photocatalytic system. Wang et al.47 designed a flexible hierarchically porous bionanocomposite foam of ZnxCd1 xS/BC via organizing ZnxCd1 xS nanoparticles into the hierarchical architecture of bacterial cellulose (BC) through a templating mineralization and ion exchange/seeded growth method. The optimized Zn0.09Cd0.91S/BC exhibits a high H2 evolution rate of 1450 mmol h 1 g 1 under visible light irradiation and an excellent apparent quantum efficiency of 12% at 420 nm. This is due to the synergy of high specific surface area, narrow band gap, multilength scale structural hierarchy and the intrinsic properties of Zn0.09Cd0.91S. Although various nanostructures have been used for photocatalytic H2 production, the utilization of hierarchically porous structures is still limited. Considering the merits of the special structures for light harvesting and photogenerated electron and hole separation and transport, great efforts should be made to design photocatalysts showing catalytic activity in the visible light region. In particular, developing noble metal-free hierarchically structured porous photocatalysts with nitrogen doping, sulfur doping, graphene and g-C3N4 modification should be a very promising route to enhance the H2 evolution rate. 2.3

Hierarchically porous structures for fuel cells

Generally, fuel cells, involving hydrogen or small organic molecules, can convert chemical energy from a fuel into electrical energy in a constant temperature process. The increasing interest in clean and sustainable energy sources has stimulated the development of fuel cells as one of the most promising power sources with high energy conversion efficiency and low emissions. Fuel cells are made up of three segments: the anode, the electrolyte, and the cathode, where the catalyst on the anode can oxidize the fuel into a positively charged ion and a negatively charged electron and the catalyst on the cathode turns the ions into waste chemicals. Therefore, the catalysts on both electrodes are very important for a fuel cell. From the viewpoint of the catalysts used in fuel cells, the high surface areas, large pore

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volumes and, in particular, interconnected hierarchical macronanoporosity of the supports on both electrodes are essential owing to the fact that higher surface areas, larger pore volumes and interconnected hierarchical macro-nanoporosity can allow a better dispersion for the active catalysts and offer an open network around the active catalysts for facile diffusion of fuels and products. So far, porous carbon-supported Pt and Pt-based catalysts have been generally considered to be the most common electrocatalysts for fuel cells. However, the high cost, quick CO poisoning and slow kinetics of fuel oxidation severely restrict their commercial applications. Pursuing low-cost electrocatalysts with highly improved kinetics is urgent. Great efforts have been made to search for efficient, durable and inexpensive alternatives to Pt-based catalysts, such as Pd-based catalysts and metal-free catalysts. In this review, we focus on the typical metal-free, transition metals–nonmetal and metal oxide catalysts supported on hierarchically porous carbon/graphene (HPC/HPG) structures for highly efficient fuel cells. 2.3.1 Noble metals loaded on hierarchically porous carbon/ graphene for fuel cells. Li et al.48 have reported the synthesis of hierarchically porous carbons with both micro- and mesopores through direct carbonization of assembled nanoparticles of the zeolitic imidazolate framework (ZIF-8) at different temperatures. The porous carbon has been used as a support for the Pd electrocatalyst for methanol electrooxidation in alkaline media for the first time. The results show that the Pd catalyst on the support of ZIF-8-derived carbon (ZC) prepared at 1000 1C (Pd/ZC-1000) displayed extremely high catalytic activity and electrochemical stability for methanol electrooxidation. The catalytic activity of Pd/ZC-1000 is 5 times higher than that of Pd/XC-72R (commercial carbon support) at the same Pd loading. The electrochemical measurements of Pd/ZC-1000 show a slower current decay over time and the highest current, proving a higher tolerance to the carbonaceous species generated during methanol oxidation. Zhang et al.49 have presented Pd nanoparticles loaded on hierarchically porous graphene wrapped nickel foam (Pd/GF/Ni) architectures (Fig. 2a–d). Cyclic voltammogram (CV) curves show that the electrochemically active surface area of Pd/GF/Ni is significantly larger than that of Pd/GF and Pd/C catalysts (Fig. 2e). The onset potential for ethanol oxidation on the Pd/GF/Ni electrode is about 0.5 V, which is much more negative than that of the Pd/GF electrode ( 0.3 V) and comparable to that of the commercial Pd/C electrocatalysts (Fig. 2f). Moreover, in the positive-going potential scan, the peak current density is about 6.2- and 2-fold larger than that of our Pd/GF and commercial Pd/C catalyst, respectively, even when the Pd content in Pd/GF/Ni is as low as 1.9 wt%. Fig. 2g shows the stability of the Pd/GF/Ni electrode. Its peak current density exhibits a negligible change after the 100th cycle, revealing the excellent electro-chemical stability of the Pd/GF/Ni catalyst. The chronoamperometry curves of Pd/GF/Ni, Pd/GF, and Pd/C catalysts shown in Fig. 2h reveal that Pd/GF/Ni shows a slower current decay over time and much higher current densities in comparison with Pd/GF and Pd/C catalysts, indicating a higher tolerance of Pd/GF/Ni to carbonaceous species generated

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Fig. 2 Low- and high-magnification SEM images of (a and b) Pd/GF/Ni and (c and d) Pd/GF. (e) CVs of the Pd/GF/Ni, Pd/GF and Pd/C catalysts measured in 1 M KOH. (f) CVs of the Pd/GF/Ni, Pd/GF and Pd/C catalysts measured in 1 M KOH with 1 M ethanol at 50 mV s 1. (g) The corresponding 1st and 100th CVs of Pd/GF/Ni. (h) The chronoamperometry curves of Pd/GF/Ni, Pd/GF and Pd/C catalysts measured in 1 M KOH with 1 M ethanol. (Reproduced from ref. 49 with permission. Copyright Royal Society Chemistry, 2015).

during ethanol oxidation and much higher electroactivity for ethanol oxidation than Pd/GF and Pd/C catalysts. The authors attribute the superior electrocatalytic performance of Pd/GF/Ni to its special 3D hierarchically porous graphene network with ultrafine monodispersed Pd-nanoparticle loading. 2.3.2 Non-metal doped hierarchically porous carbon/graphene for fuel cells. Nitrogen-doped carbon/graphene materials are the most widely studied candidates for oxygen reduction reaction (ORR). Nitrogen-doped or sulfur-doped HPC/HPG materials are considered as one of the most active catalysts for ORR. He et al.50 have reported the fabrication of a 3D hierarchically porous nitrogendoped carbon catalyst under KOH-assisted annealing (denoted as NC-A) with micro-, meso-, and macro-porosity in one structure with a super-high surface area (2191 cm2 g 1) and an interconnected pore system. The NC-A sample displays a nearly equal ORR activity to that of the Pt/C catalyst in terms of their close E1/2 ( 0.133 and 0.128 V for NC-A and Pt/C, respectively) and JL (4.54 and 4.66 mA cm 2 for NC-A and Pt/C, respectively). Furthermore, they noted that pyrrolic and pyridinic N atoms are less effective ORR catalytic sites. Tao et al.51 have synthesized a novel class of N-doped 3D hierarchically porous carbon materials (HPC-Ns). The metal-free HPC-Ns show excellent

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electrocatalytic performance for ORR. The current density of the HPC-Ns obtained at 850 1C ( 4.14 mA cm 2) is extremely close to that of commercial Pt/C ( 4.24 mA cm 2), which are much higher than those of N-doped mesoporous carbon CMK-3-N ( 3.46 mA cm 2) and microporous carbon AC-N ( 2.34 mA cm 2) in alkaline solution. Liu et al.52 have reported nitrogen-doped hierarchically porous carbon spheres (N-HCS) via a nanocasting method. N-HCS obtained after carbonization at 1000 1C (N-HCS-1000) exhibits a hierarchical micro–meso– macroporous structure with a relatively high surface area of 1413 m2 g 1 and a notably large pore volume of 2.96 cm3 g 1. The N-HCS-1000 material demonstrates excellent activity with high current density and onset potential very close to that of the commercial Pt/C catalyst in ORR in alkaline media. It also shows better methanol crossover resistance and higher stability than the commercial Pt/C catalyst. Liang et al.53 have demonstrated a hierarchically porous structure of meso/microporous nitrogen-doped carbon (denoted as meso/micro-PoPD), with a high specific surface area of 1280 m2 g 1 (Fig. 3a). Cyclic voltammetry measurements in an O2-saturated electrolyte show that the peak potential at 0.83 V versus the reversible hydrogen electrode (RHE) afforded by the meso/micro-PoPD catalyst is comparable to that of the commercial 20 wt% Pt/C catalyst (hereafter the Pt/C catalyst) at 0.84 V. Particularly, the hierarchically porous meso/micro-PoPD catalyst exhibits substantial improvement in ORR activity, as reflected in the shift of the half-wave (E1/2) potential to 0.85 V, well comparable to Pt/C (E1/2 = 0.85 V) with the same loading of 0.1 mg cm 2. After the catalyst loading of meso/micro-PoPD is increased to 0.5 mg cm 2, an improved E1/2 value of 0.87 V is obtained, being 20 mV more positive than that of the Pt/C catalyst and higher than all previously reported heteroatom-doped carbon metal-free catalysts. They also pointed out that the catalytic activity of PoPD-derived catalysts is dependent on the proportion of doped quaternary nitrogen, instead of the total amount of nitrogen species. The rotating ring-disk electrode (RRDE) study reveals a high selectivity of the meso/ micro-PoPD catalyst towards the four-electron reduction of oxygen compared with other samples (Fig. 3b). The electrochemical durability of meso/micro-PoPD and Pt/C catalysts is investigated by cycling between 0.6 V and 1.0 V at 50 mV s 1 in 0.1 M KOH under an O2 atmosphere. The results show a very little change in half-wave potential (B10 mV) for meso/micro-PoPD even after 10 000 cycles, while there is B40 mV loss of half-wave potential for the Pt/C catalyst under the same conditions, suggesting the superior durability of the meso/micro-PoPD catalyst. Furthermore, the full-cell (Zn–air battery) performance of the prepared catalyst outperforms the state-of the-art Pt/C catalyst. Li et al.54 have prepared hierarchically micro–mesoporous N-doped activated carbon (NMAC) demonstrating excellent electrocatalytic performance for ORR both in alkaline and acidic media with high electrocatalytic activity and long-term operation stability. In particular, NMAC exhibits prominent stability in acidic media, remaining stable for about 7 hours and retaining 88.5% of the initial current density after 12 hours in 0.5 M H2SO4 solution. Wang et al.55 have prepared a series of novel nitrogen-doped hierarchical carbon monoliths (NCMs) with macroporous scaffolds composed

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Fig. 3 (a) TEM image of meso/micro-PoPD and the electrocatalytic performance of the catalysts: (b) H2O2 yields plot of meso/micro-PoPD, reference materials, and Pt/C catalyst. RDE and RRDE are recorded in 0.1 M KOH. Electrode rotation speed, 1600 rpm; scan rate, 10 mV s 1. Scale bar: 30 nm. (Reproduced from ref. 53 with permission, Copyright Macmillan Publishers Limited, 2014).

of interconnected mesoporous rods. The pyridinic and graphitic N dominate in the NCMs. The NCMs obtained at 750 1C exhibited comparable catalytic activity but superior long-term durability and methanol tolerance to commercial Pt/C for ORR with a four-electron transfer pathway in alkaline media. Xu et al.56 have synthesized hierarchically nanoporous N-doped carbon nanowires (N-CWs) for ORR in 0.1 M NaOH. Specifically, in addition to the improved kinetic current density and overpotential, the N-CWs prepared at 700 1C show optimized ORR performance with a 4-electron-transfer number, very close to the commercial Pt/C catalyst. Ning et al.57 have fabricated hierarchically porous N-doped carbon nanoflakes (HPNCNFs). The HPNCNF-900 catalyst, obtained at 900 1C, exhibits excellent ORR activity and durability in alkaline media outperforming the state-of-the-art Pt/C catalyst at a moderate loading. Gokhale et al.58 have developed a N-doped hierarchically porous carbonbased electrocatalyst, showing remarkable electrocatalytic activity towards ORR in 0.1 M KOH solution. Oxygen reduction follows the desired 4-electron transfer mechanism involving the direct reduction pathway. Furthermore, the carbon catalyst also exhibits good electrochemical stability and fuel tolerance. Cui et al.59 have demonstrated heteroatom (N and S) co-doped hierarchically

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porous carbon (NeS-HC). The resultant dual-doped NeS-HC catalyst exhibits significantly enhanced electrocatalytic activity, long-term operation stability, and tolerance to the crossover effect compared with commercial Pt/C for ORR in 0.1 M KOH. The DFT calculations reveal that the dual doping of S and N leads to the redistribution of spin and charge densities, which might be responsible for the formation of a large number of carbon atom active sites. Graphene has recently received great attention due to its intriguing physical, chemical, and mechanical properties. Porous graphene nanostructures are of great interest for applications in energy conversion. Shin et al.60 have reported an ice-templated self-assembly approach for the integration of two-dimensional graphene nanosheets into 3D hierarchically porous graphene nanoscroll networks, demonstrating promising electrocatalytic activity for ORR in 0.1 M KOH. Qiu et al.61 have produced an intercalated graphene sheet (GSs) and graphitic carbon nitride (GCN) composite (GS/GCN) for ORR. The obtained material possesses 100% catalytic selectivity towards the four-electron pathway, and its ORR activities outperform any other existing GCN-based catalysts. Importantly, the GS/ GCN composite exhibits improved tolerance against methanol and enhanced long-term stability, compared with the commercial Pt/C catalyst. He et al.62 have reported a novel hierarchically porous architecture as an efficient bioanode, consisting of biocompatible chitosan and vacuum-stripped graphene (CHI/ VSG) for microbial fuel cells (MFC). With the hierarchical pores and unique VSG, an optimized bioanode delivers a remarkable maximum power density of 1530 mW m 2 in a mediator-less MFC, 78 times higher than that of the carbon cloth anode. Wang et al.63 have prepared crumpled, sheet-like, sulfur-doped graphene (SG) with an appropriate sulfur content, hierarchically robust porous structure, large surface area/pore volume, and highly graphitized texture (Fig. 4a). The CVs of S-doped graphenes exhibit a nearly rectangular shape, showing high conductivity with superior capacitive current. A well-defined oxygen reduction peak at 20.32 V (vs. SCE) is observed for S-doped graphenes in O2-saturated 0.1 M KOH solution. SG-800 (obtained at 800 1C) demonstrates a very high current density of 3.68 mA cm 2. The electrocatalytic selectivity of SG-800 and 40% Pt/C towards the electro-oxidation of methanol shows that S-doped graphenes have higher selectivity for ORR with a remarkable improved ability to avoid the cross effects than commercial Pt/C. The corresponding onset potential of SG-800 for the ORR is about 20.15 V, close to that obtained from CV measurements (20.17 V). The Koutecky–Levich (K–L) plots of SG-800 exhibit good linearity, with the highest electron transfer numbers (3.7–3.8) (Fig. 4b). Furthermore, SG-800 retained 89% of the initial catalytic current after 3.5 h, indicating the stability of the S-doped graphene for ORR, whereas the commercial Pt/C maintained a relative current of only 70%. All the above examples show clearly the advantage of hierarchical porosity in fuel cells in terms of stability and power density improvement. In addition, Yang et al.64 have reported hierarchically porous nitrogen-doped carbon nanofibers (ANCNFs) for ORR in a neutral environment. Compared with inactivated nitrogen-doped carbon

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Fig. 4 (a) SEM images of SG-800. (b) The electron transfer numbers at SG-800 and CG-800 calculated based on the K–L equations in the potential range of 20.5, 20.6 V (vs. SCE). (Reproduced from ref. 63 with permission. Copyright Macmillan Publishers Limited, 2015).

nanofibers (NCNFs), ANCNFs exhibit a more positive onset potential, higher current density as well as higher electron transfer number, highlighting the importance of the chemical activation process. MFCs equipped with an ANCNF catalyst exhibit a high power output of 1377  46 mW m 2, which is B1.5 times the output of the NCNF cathode (921  29 mW m 2), and nearly 4 times that of a plain cathode (341  9 mW m 2) owing to its hierarchical structure. 2.3.3 Transition metal–nonmetal doped hierarchically porous carbon/graphene for fuel cells. Reducing Pt loading or resorting to precious-metal-free catalysts is highly desirable from the point of view of sustainability, cost and stability. Developing preciousmetal-free electrocatalysts with high activity and good ORR durability has therefore become a hot topic. So far, only Fe/C or Co/C catalysts have demonstrated reasonable initial activity and power performance for the ORR in the acidic medium. For example, Liang et al.65 have developed a porous Fe–N–C hybrid material (denoted as Fe–N–CNT–OPC), composed of hierarchically ordered porous carbon (OPC) microblocks interlinked via in situ grown carbon nanotubes (CNTs) (Fig. 5a and b). Cyclic voltammetry indicates a distinct ORR peak centered at ca. 0.23 V for Fe–N–CNT–OPC, over 100 mV more positive than that of Fe–N-CNF–OPC (CNF is carbon nanofiber), showing a better ORR performance. Such a hierarchically porous Fe–N–CNT–OPC material demonstrates a very positive ORR onset

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Fig. 5 Typical structural characterization of Fe–N–CNT–OPC and analogues: (a) SEM, and, (b) TEM image of Fe–N–CNT–OPC. ORR evaluations of Fe–N–CNT–OPC and analogues: (c) peroxide yield (solid) and electron transfer numbers (dash); (d) ORR current (pattern column) and the corresponding cost-efficiency (color column) of the samples at different potentials. The catalyst loading on the electrode for all tests is 80 mg. (Reproduced from ref. 65 with permission. Copyright Wiley-VCH, 2014).

potential within 18 mV close to that of Pt/C at low overpotential. Due to the hierarchical porosity interlinked via CNTs, Fe–N–CNT–OPC also shows a very sharp current increase and thus ensures its ORR current exceeds that of Pt/C, and the ORR current remains stable when the potential is more negative, up to 0.8 V. The rotating ring-disk electrode (RRDE) study shows that Fe–N–CNT–OPC has lower ring current compared with Fe–N–CNF–OPC, owing to a larger portion of oxygen being directly reduced to OH without intermediate peroxides on Fe–N–CNT–OPC. The amount of peroxide and the electron transfer numbers of the catalysts during ORR have been calculated (Fig. 5c), showing an electron transfer number up to 3.99 at an over-potential as low as 0.1 V. Fe–N–CNT–OPC heated at 900 1C shows the best ORR onset potential. This Pt-free material’s cost economy is also evaluated as shown in Fig. 5d, being far more economical than the commercial Pt/C. In addition, Deng et al.66 have prepared an ordered hierarchically porous carbon codoped with N and Fe (Fe-NOHPC) with a large surface area (1172.5 m2 g 1) and a pore volume of 1.03 cm3 g 1. Compared with commercial Pt/C, it demonstrates much better ORR catalytic activity and higher stability as well as higher methanol tolerance in an alkaline electrolyte. Wang et al.67 synthesized cobalt nanoparticles embedded in N-doped CNFs with a hierarchical pore structure (HP-Co-NCNFs). When used as an electrocatalyst for the ORR, HP-Co-NCNFs exhibits an outstanding electrochemical performance in terms of activity, methanol tolerance, and durability due to its special hierarchically

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porous structure. Chao et al.68 have prepared hierarchically structured yolk–shell Co and N co-doped porous carbon microspheres (YS-Co/N-PCMs). The YS-Co/N-PCM catalyst exhibits high catalytic activity for ORR in alkaline media for fuel cells, compared with the commercial Pt/C catalyst and many non-precious-metal doped carbon-based catalysts reported previously in terms of resistance to methanol crossover and longterm stability. Furthermore, the YS-Co/N-PCM catalyst also shows higher catalytic activity toward oxygen evolution reaction (OER) than that of the commercial Pt/C catalyst. Elumeeva et al.69 have fabricated high-nitrogen-doped carbon aerogels modified with iron and cobalt for the ORR using the rotating disk/ring-disk electrode at the relevant half-cell reaction. Iron composites show an amorphous, quasi-atomic distribution of the metal, while iron/cobalt composites present very small, well-supported nanoparticles, demonstrating favorable performances compared with the commercial Pt/C catalyst. 2.3.4 Metal oxides loaded on hierarchically porous carbon/ graphene for fuel cells. As another typical electrocatalysts, transition metal oxides loaded in a carbon matrix have also been developed for solid oxide fuel cell (SOFC) systems. For example, Jiang et al.70 have synthesized a biological low-cost, ecofriendly method for the synthesis of Mn2O3 micro/nanocubes by calcination of MnCO3 precursors in an oxygen atmosphere, which are fabricated by dissimilatory metal-reducing Shewanella loihica PV-4 in the presence of MnO4 as the sole electron acceptor under anaerobic conditions. After calcining the MnCO3 precursors at 500 1C and 700 1C, porous Mn2O3-500 and Mn2O3-700 still show microcubic morphology, while their edge lengths are decreased to 1.8 mm by thermal decomposition. Electrochemical measurements demonstrate that the porous Mn2O3-500 micro-/nanocubes exhibit promising catalytic activity towards the ORR in an alkaline medium, owing to a synergistic effect of the overlapping molecular orbitals of oxygen/manganese with the hierarchically porous structures, which are favorable for oxygen adsorption. In addition, these Mn2O3 micro-/nanocubes possess better stability than commercial Pt/C catalysts and methanol-tolerance in alkaline solution. Kumar et al.71 have prepared a homogeneous, hierarchically porous nanotubular TiO2 structure with uniform dimensions, which behaves as an efficient bifunctional electrocatalyst for oxygen and hydrogen evolution reactions in neutral medium. Hu et al.72 have fabricated IrO2 electrocatalyst powders with 3D hierarchically ordered macroporous structure (3DOM) toward the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer. Compared with IrO2 prepared by a simple pyrolysis method at 450 1C, the 3DOM IrO2 (450 1C) exhibits an over 2 times enhancement in the BET surface area, voltammetric charges, and OER activity. Hierarchically porous metal oxide composites have also been developed for solid oxide fuel cell (SOFC) systems. For example, Zhao et al.73 synthesized a hierarchically porous metal oxide composite, LiNiCuZn-oxide (LNCZO), which is used as a symmetrical, anode and cathode, for the SDC-LiNaCO3 (LNSDC) electrolyte based LTSOFCs, achieving a maximum power density of 132 mW cm 2 at 550 1C. A single-component fuel cell device based on LNCZO and ionic conductor LNSDC

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demonstrates a peak power output of 155 mW cm 2. Chen’s group74 have fabricated a hierarchically porous nanocathode network (Sm0.5Sr0.5CoO3–Gd0.1Ce0.9O2 d, SSC–GDC) for high performance low temperature SOFCs. The hierarchically porous straight open GDC cathode skeleton and NiO–GDC anode substrate prepared by freeze-drying tape-casting facilitate mass transport. The cell with straight open electrodes and hierarchically porous cathodic network demonstrates a maximum power density of 0.65 W cm 2 at 500 1C and impressive stability for more than 500 h at 400 1C using H2 as a fuel and ambient air as an oxidant (Fig. 6). They have also fabricated high performance low temperature solid oxide fuel cells (LTSOFCs) with both acicular anodes and cathodes with a thin Gd-doped ceria (GDC) electrolyte film. The novel electrode architecture with an acicular Ni–Gd0.1Ce0.9O2 d (Ni–GDC) anode and a hierarchically porous cathode with nano-size Sm0.5Sr0.5CoO3 (SSC) particles exhibits a significantly high power output of 1.44 W cm 2 and an extremely low cell polarization resistance of 0.0379 V cm 2 at 600 1C.75 Furthermore, they fabricated a hierarchically porous Sr2Fe1.5Mo0.5O6 d–Gd0.1Ce0.9O1.95 (SFM–GDC) ceramic anodesupported SOFC with a GDC electrolyte film having a high hierarchical porosity and low tortuosity factor, facilitating gas diffusion in the anode during fuel cell operation. Peak power density of cells with a La00.6Sr0.4Co0.2Fe0.8O3–GDC (LSCF–GDC) cathode reaches 0.22 W cm 2 at 700 1C when using H2 as the fuel and ambient air as the oxidant. The SFM–GDC anode showed excellent sulfur tolerance when using H2 with 50 ppm H2S.76 Moreover, a thin nano samarium doped ceria (SDC) layer catalyst on the wall surface of the Ni–yttrium-stabilized zirconia (Ni–YSZ) has also been prepared. Single cells with a nano SDC layer show very stable cell performance and a peak power density of 0.65 W cm 2 at 800 1C using methane as the fuel owing to the SDC layer effectively preventing the formation or growth of nickel carbide (onset of coking).77

Fig. 6 SEM images of the cross-section of the cell (a) and the cathode (b). Typical electrochemical impedance spectra (c) of the cell with a hierarchically porous rough SSC–GDC cathode nano-network prepared by the FC-G method at 400, 450 and 500 1C, respectively. (Reproduced from ref. 74 with permission. Copyright Elsevier Ltd, 2014).

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3. Hierarchically porous structures for energy storage Hierarchically porous nanostructures have also been widely used for energy storage.78,79 In this section, we mainly focus on hierarchically porous structures for supercapacitors and lithium-ion batteries. The utilization of hierarchically porous structures in the most recently developed lithium–sulfur batteries, lithium–air batteries, sodium-ion batteries and magnesium-ion batteries is also discussed. 3.1

Hierarchically porous structures for supercapacitors

Supercapacitors (SCs), also called electrochemical capacitors (ECs) or ultracapacitors (UCs), have attracted much attention due to the depletion of fossil fuels and the impacts of environmental pollution that the international community faces today. Generally, SCs can be divided into two types: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors according to the energy storage mechanisms. For EDLCs, the capacitance is produced by charge separation at the electrode/electrolyte interface, and active carbons with high surface area are commonly used in commercial products. For pseudo-capacitors, the capacitance is mainly based on transition metal oxides via reversible faradaic reactions occurring at or near a solid electrode surface. Conventionally, supercapacitors deliver high power density (1–10 kW kg 1) but low energy density (1–10 W h kg 1) as they involve non-solid state ion diffusion and only store energy by surface adsorption reactions of charged species on an electrode material.79 A great deal of research effort has been made to develop electrodes for both EDLCs and pseudocapacitors. This requires developing novel materials to construct electrodes with high capacities and long cycling abilities. In this part, we focus on the hierarchically porous carbon- and/or graphene-based hierarchically porous structures for EDLCs and transition metal oxide based hierarchically porous structures for pseudocapacitors. The enhancement of pseudocapacitive properties is due to the hierarchically porous architecture offering fast ion/electron transfer and alleviating structural degradation caused by volume expansion during the cycling process. 3.1.1 Hierarchically porous carbon- and/or graphene-based hierarchically porous structures for electrochemical double layer capacitors. Electrochemical double-layer capacitors (EDLCs) based on carbon materials with high power density and long cycling life have attracted much attention.78,79 A variety of carbon materials, such as activated carbon (AC), mesoporous carbon (MC), ordered mesoporous carbon (OMC), graphene, carbon nanotubes (CNTs), carbon nanoflakes (CNFs), hierarchically porous carbon (HPC) and carbide-derived carbon, have been synthesized and studied for electrochemical energy storage. Therefore, we will discuss the typical hierarchically porous carbon- and/or graphenebased hierarchically porous structures for EDLCs. Many efforts have been made to develop hierarchically porous structure based materials for EDLCs. Wang et al.80 have synthesized uniform hierarchical yolk–shell carbon spheres (YS-CSs) with a microporous yolk, a mesoporous shell and a macroporous hollow cavity by using a cationic surfactant

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cetyltrimethylammonium bromide (CTAB) as a template, resorcinolformaldehyde (RF) as a carbon source and tetraethoxysilane (TEOS) as an assistant pore-forming agent. When used as an electrode material for a supercapacitor, YS-CSs exhibits high performance with a high specific capacitance and a good rate capability (75% retention at a high current density of 20 A g 1). Kim et al.81 have fabricated a 3D hierarchically porous carbon electrode containing both nanoscale and microscale porosity via the colloidal self-assembly of monodisperse starburst carbon spheres (MSCSs). This MSCS structure facilitates ion and electron transport (electrode electrical conductivity B0.35 S m 1). The capacitance of the 3D-ordered porous MSCS electrode is B58 F g 1 at 0.58 A g 1, 48% larger than that of a disordered MSCS electrode. At 1 A g 1 the capacitance of the ordered electrode is 57 F g 1 (95% of the 0.24 A g 1 value), which is 64% higher than the capacitance of the disordered electrode. The ordered electrode preserves 95% initial capacitance after 4000 cycles. Very recently, Long et al.82 have synthesized hierarchically porous graphene-like carbon (PGC) materials through the hydrothermal treatment of fungus and the subsequent carbonization process. Layer-stacking PGC materials derived from the cell walls of fungus have a high surface area (1103 m2 g 1), high bulk density (B0.96 g cm 3) and a hierarchically interconnected porous framework. The PGC electrode shows an ultra-high volumetric capacitance of 360 F cm 3 and excellent cycling stability with 99% capacitance retention after 10 000 cycles (Fig. 7). More importantly, the as-assembled symmetric supercapacitor delivers a superior volumetric energy density of 21 W h L 1. In addition, various hierarchically porous carbons with different porosities and morphologies have been synthesized and used for EDLCs. Amali et al.83 have synthesized a 3D hierarchically porous carbon framework with micro-, mesoand macropores, which exhibited excellent performance as a supercapacitor electrode. The best sample demonstrates a specific capacitance of 211 F g 1 at 10 mV s 1, which is only slightly lower than 206 F g 1 at 100 mV s 1, corresponding to a decay of only 2%. Huang et al.84 have designed a bowl-like carbon sheet (BCS) with interconnected channels and hierarchical porosity. These BCSs maintain 105 F g 1 at 100 A g 1 and a rectangular degree at a high scan voltage of 1 V s 1. Chen et al.85 synthesized hierarchically porous tubular carbon (HPTC) with a large surface area of 1094 m2 g 1 by selectively removing lignin from natural wood. Furthermore, by KOH activation, HPTC has a surface area of up to 2925 m2 g 1. The activated HPTC demonstrates an electrochemical capacitance of 246 F g 1 at 2 mV s 1. After 5000 cycles, over 90% capacitance retention is still maintained. Ding et al.86 designed a peanut shell with hierarchically porous carbon nanosheets (PSNC) for a sodium ion capacitor. The optimized PSNC obtained at 800 1C (PSNC-3-800) delivers a capacitance of 213 F g 1 at a current density of 0.1 A g 1, giving a surface normalized capacitance of 8.9 mF cm 2 (based on BET surface area analysis). Particularly, PSNC-3-800 delivers a capacity of 119 F g 1 at 25.6 A g 1, compared with 36 F g 1 from commercial active carbon.

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Fig. 7 (a) SEM image of the obtained densely porous layer-stacking carbon (PGC) material. The arrow shows the peeled carbon nanosheet and the dotted curve shows the place where the sheets are peeled from. (b) HRTEM image of PGC, showing an interconnected porous system and the electrochemical characteristics of PGC in 6 M KOH aqueous electrolyte in a three-electrode system. (c) The gravimetric capacitance of HTC, PGC and A-HTC electrodes at the current densities from 0.5 to 20 A g 1. (d) Comparison of the volumetric and gravimetric capacitances of the PGC electrode with other carbon electrodes in aqueous electrolytes. (e) Cycling stability of the PGC electrode after 10 000 cycles at 200 mV s 1. The inset shows the capacitance retention change of initial 3000 cycles (left) and the CV curves before cycling and after 10 000 cycles (right). (Reproduced from ref. 82 with permission. Copyright Elsevier Ltd, 2015).

Nitrogen modification of carbon skeletons can modulate the electronic properties of carbon and produce additional functional groups on the carbon surface, resulting in an enhancement in the electrochemical performance in addition to hierarchical structures. Wan et al.87 have demonstrated the synthesis of N-enriched hierarchically porous carbons (HPCs) and used them as electrode materials for supercapacitors. The HPC-800 sample activated at 800 1C demonstrates a high pseudocapacitance and the largest specific capacitance of 641.6 F g 1 at a current density of 1 A g 1 in a 6 mol L 1 KOH aqueous electrolyte when measured in a three-electrode system. Furthermore, the HPC-800 electrode exhibits an excellent rate capability (443.0 F g 1 remained at 40 A g 1) and good cycling stability (94.3% capacitance retention over 5000 cycles). Guo et al.88 have fabricated a hierarchically porous carbon monolith with variable sizes in multi-length-scale porosity, a nitrogen and boron co-doped graphitized framework, and high

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mechanical strength. With such heteroatom doped skeleton structures and fully interconnected macropores, mesopores and micropores, the hierarchically porous carbon (CNB-3) shows outstanding electrochemical performance: a superior high gravimetric capacitance of 247 F g 1, an interfacial capacitance of 66 mF cm 2 (based on the discharge current density of 0.5 A g 1) and a high volumetric capacitance of 101 F cm 3. A capacitance retention of 96.2% is maintained after 4000 cycles, due to its unique skeleton structure and high conductivity (Fig. 8). Our group has recently reported a new class of hierarchical mesoporous carbons (HMC) with nitrogen modification on the basis of the well-adjusted self-assembly of polyhedral oligosilsesquioxanes (POSS) and amphiphilic triblock copolymers (PEO–PPO–PEO) (Fig. 9a and b).89 The obtained carbon materials with a high specific surface area of over 2000 m2 g 1 and a large pore volume of over 1.19 cm3 g 1 possess both quite uniform micropores with the size of B1 nm and highly ordered mesopores with the size of B4 nm, owing to the molecular-scale templating effect of POSS siloxane cages as well as the good assembly compatibility between the block copolymers and the aminophenyl-functionalized POSS used. Nitrogen functionalities with a relatively high content (B4 wt%) are spontaneously incorporated into those carbon materials. Benefiting from the uniform microporosity and the nitrogen doping, the specific capacitance of the POSS-derived hierarchically porous carbons reaches B160 F g 1 in an ionic liquid electrolyte and B210 F g 1 in 1 M H2SO4 aqueous electrolyte, when measured at a current

Fig. 8 SEM (a) and TEM images (b) of CNB-3 and the long cycle life of CNB-3 over 4000 cycle numbers (c), the first 10 cycles of GC curves (d), and the last 10 cycles of GC curves (e) tested at a current density of 1 A g 1 with a two electrode system. (Reproduced from ref. 88 with permission. Copyright Royal Society Chemistry, 2013).

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Fig. 9 (a) TEM and (b) high-resolution TEM (HRTEM) images of HMC viewed along the [001] direction. The inset in panel (a) shows the corresponding fast Fourier transform (FFT) diffractograms. (c) Specific capacitance of HMC and RC at different current densities. (Reproduced from ref. 89 with permission. Copyright Elsevier Ltd, 2016).

density of 0.25 A g 1 in a symmetrical two-electrode cell. More importantly, the highly ordered mesopores facilitate fast transportation of ions to the fine micropores to achieve excellent power performance. The hierarchical carbon sample with a hexagonal mesostructure and a high mesoporosity displays the best rate capability with 94% and 97% of capacitance retention in an ionic liquid and 1 M H2SO4, respectively, with the current density ranging from 0.25 to 10 A g 1 (Fig. 9). By combining the self-assembly strategy with rich POSS chemistry, many other hierarchical hybrid materials or carbon materials with unique electrochemical properties can be synthesized. Moreover, the surface modification of porous carbon by conducting polymers is also an efficient method to improve the capacitance of low capacities owing to their high conductivity and stability, simple synthesis and good environmental compatibility. Lin et al.90 have designed a unique CNF conjugated polyaniline (CNF–PANI) to generate a binder-free CNF-based composite paper electrode. Morphological investigations reveal 3D hierarchically porous interconnected network structures with loosely-stacked hyperbranched PANI nanobundles onto the skeleton of CNFs. The flexible CNF–PANI nanohybrid paper exhibits excellent electrical conductivity (0.31 S cm 1) and specific capacitance (167 F g 1), which is much larger than that of original CNFs (2.5 F g 1). Graphitization of hierarchically porous carbon via high temperature treatment, combining with graphene and incorporation of CNTs has also been widely used to modify and increase the

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conductivity of porous carbon. Bian et al.91 have designed hierarchical yolk–shell graphitized carbon, which exhibits higher electrical capacitance than that of AC due to the increased electrical conductivity in addition to the advantage of hierarchical porosity. Liu et al.92 have reported the hierarchically ordered mesoporous carbon/graphene (OMC/GA) aerogel composites possessing interconnected macroporous graphene networks for supercapacitors. The resulting composites demonstrated outstanding electrochemical performances: the capacitance of OMC/GA-2 is B197 F g 1 at 0.5 A g 1 and 141 F g 1 at 10 A g 1. An all-solid-state supercapacitor (ASSS) based on OMC/GA with vertical mesopores exhibits an outstanding specific capacitance (44.3 F g 1) at 5 mV s 1, excellent cycling stability (7.4% loss after 1000 cycles), and high power density during the fast charge/ discharge process (E3545 W h kg 1 at less 3.6 s). Achour et al.93 have reported the design of hierarchical composite electrodes consisting of porous and nanostructured TiN grown on vertically aligned CNTs as high-performance electrodes for microsupercapacitors. The electrodes deposited on silicon substrates exhibited a capacitance as high as 18.3 mF cm 2 at 1 V s 1. Particularly, this capacitance is maintained over 20 000 cycles. Chen et al.94 have synthesized a class of graphitized porous carbon particles with a hierarchically porous structure and graphitized shells for supercapacitors. At a current density of 1 A g 1, the graphitized-carbon electrode shows a capacitance of 102 F g 1, higher than those of CNTs and AC based electrodes. The graphitized-carbon electrode has a high specific capacity of 115 F g 1 at 0.1 A g 1 and 83% of the capacitance (95 F g 1) at 10 A g 1 (Fig. 10a). The graphitized-carbon supercapacitor shows a gravimetric energy density of 30 W h kg 1 at a power density of 270 W kg 1, or 24 W h kg 1 at 25 kW kg 1. When considering material density, graphitized-carbon presents a maximum volumetric energy density of 22.5 W h L 1, about twice that of AC (13.5 W h L 1). The device retains an energy density of 18 W h L 1 at 18.8 kW L 1, significantly higher than that of AC (6 W h L 1 at 10 kW L 1) as shown by the Ragone plot in Fig. 10b. After 5000 cycles, an B98% capacitance retention is obtained at 2 A g 1. Significantly, Huang et al.95 synthesized hierarchically ordered porous carbon (HOPC) without/with partially graphitic nanostructures for supercapacitors. The partially graphitic 3D HOPC-g-1000 (g = graphitic, treatment temperature at 1000 1C) nanostructure demonstrates a gravimetric specific capacitance of 73.4 F g 1 at 3 mV s 1. No obvious specific capacitance fading is observed for HOPC-g1000 over 5400 cycles. In addition, the specific capacitance per surface area of partially graphitic HOPC is higher than that of conventional porous carbons, CNTs, and modified graphene (10–19 mF cm 2) due to the good electrical conductivity and high speed charge diffusion in hierarchically porous structure. 3.1.2 Metal oxide-based hierarchically porous structures for pseudocapacitors. Compared with the EDLCs, pseudocapacitors store charges via a Faradaic process, i.e. redox reactions on an electrode, resulting in higher energy density. This makes the pseudocapacitors based on transition metal oxides receive much attention. The structures of the electrodes are then very important for such a redox reaction between the active materials

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Fig. 10 (a) Specific capacitance as a function of current density, (b) Ragone plots of various carbon electrodes (aerosol-carbon particles, AC, CNT, LN-porous carbon, SK2200, Ness4600). (Reproduced from ref. 94 with permission. Copyright Wiley-VCH, 2011).

and electrolyte ions during the charge–discharge process. The requirement for the full contact, fast ion diffusion, massive active sites and porous structures is very crucial for developing high performance supercapacitors and all these requirements need hierarchical structure. Therefore, the design of materials with a hierarchical porous structure is a promising approach to meet these requirements. 3.1.2.1 Nickel oxide-based hierarchically porous structures for supercapacitors. Among the transition metal oxides, nickel oxide (NiO) is the most widely studied material for supercapacitors due to its high abundance, low cost, high specific capacitance and high chemical/thermal stability. Tu’s group96 has prepared a hierarchically porous NiO film with a substructure of a NiO monolayer hollow-sphere array and a superstructure of porous net-like NiO nanoflakes. Such a NiO film exhibited weaker polarization, better cycling performance and higher specific capacitance due to its hierarchically porous structure in comparison with the dense NiO film. The specific capacitance of the hierarchically porous NiO film is 309 F g 1 at 1 A g 1 and 221 F g 1 at 40 A g 1, respectively, much higher than that of the dense NiO film (121 F g 1 at 1 A g 1 and 99 F g 1 at 40 A g 1)

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in 1 M KOH. Furthermore, they also synthesized a hierarchically porous NiO film using NiO triangular prisms and porous NiO nanoflakes.97 The hierarchically porous NiO film demonstrates a high discharge capacitance and excellent rate capability with 232 F g 1, 229 F g 1, 213 F g 1 and 200 F g 1 at 2, 4, 10, and 20 A g 1, respectively. The specific capacitance of 87% is maintained from 2 A g 1 to 20 A g 1. This hierarchically porous NiO film also exhibits good cycling stability and a specific capacitance of 348 F g 1 after 4000 cycles. Recently, a series of NiO materials with hierarchically porous structure and various morphologies have been investigated and used for supercapacitors. For example, Xu et al.98 have reported unique hierarchically porous NiO nanosheet based nanotubes, which demonstrate a specific capacitance of 588 F g 1 after 1000 cycles at 3 A g 1, leading to only 5.2% capacity loss. In particular, the specific capacitance reaches 960 F g 1 after 1000 cycles at 10 A g 1, leading to only 1.2% capacity loss. Han et al.99 have designed the porous nanosheets constructed hierarchical NiO nanospheres for supercapacitors. Electrochemical results show that the hierarchically porous NiO obtained via the trisodium citrate assisted route demonstrates a high rate charge–discharge performance of 463 F g 1 at 4.5 A g 1. It also exhibits a much longer cycling stability (95% capacitance retention after 1000 cycles at 0.5 A g 1) as compared with the NiO prepared without sodium citrate (182 F g 1 at 4.5 A g 1; 70% capacitance retention after 1000 cycles at 0.5 A g 1). Yan et al.100 have prepared hierarchically structured porous NiO hollow spheres with a nanosheet shell. A supercapacitor electrode based on the NiO hollow spheres exhibits a specific capacitance of 346 F g 1 at 1 A g 1 after 2000 cycles, owing to the unique hollow-sphere architecture providing fast ion/electron transfer. Zhang et al.101 have synthesized hierarchically porous NiO-modified diatomite structures for supercapacitors. This unique NiO-modified hierarchically porous diatomite structure exhibits a specific capacitance of 218.7 F g 1 and excellent cycling stability (90.61% retention after 1000 cycles) in 1 M KOH electrolyte. Park et al.102 have prepared hierarchical porous a-Ni(OH)2 SCs, exhibiting a high specific capacitance of 1582 F g 1 in 1 M potassium hydroxide (KOH) at a scan rate of 2 mV s 1 and a capacitance retention of almost 74% after 1000 cycles at a current density of 5 mA cm 2. Pan et al.103 have reported porous Co/NiO core/shell nanowire arrays via growing the NiO nanoflake shell on Co nanowires, which exhibit good cycling stability and a specific capacitance of 956 F g 1 at 2 A g 1 and 737 F g 1 at 40 A g 1. Han et al.104 have synthesized hierarchically porous NiO nanotube arrays showing enhanced pseudocapacitive performance of NiO according to in-depth electrochemical analyses. It is evident that a hierarchically porous structure is essential for good cycling stability, high capacitance retention and high current density. 3.1.2.2 Other metal oxide/hydroxide-based hierarchically porous structures for supercapacitors. Yuan et al.105 have prepared a hierarchical net-like interconnected porous Co3O4 nanoflake film for supercapacitors, which demonstrates excellent cycle life and high specific capacitances (443 F g 1 at 2 A g 1 and

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334 F g 1 at 40 A g 1). Duan and Cao106 have synthesized a hierarchically porous Co3O4 film composed of a Co3O4 monolayer hollow-sphere array and porous net-like Co3O4 nanoflakes. As a cathode material for the supercapacitor, the hierarchically porous Co3O4 film exhibits a noticeable capacitance of 454 F g 1 at 2 A g 1 as well as quite good cycling stability and high capacitance retention. Dhawale et al.107 have fabricated a hierarchically porous CoOOH thin film with uniform morphology and highly ordered macropores. The prepared electrode shows a maximum specific capacitance of 387 F g 1 at 1 mA cm 2, with a high energy of 19 W h kg 1 and a power density of 1713 W kg 1. In addition, the porous CoOOH electrode demonstrates good stability with a cycling efficiency 490% after 5000 cycles. Zhu et al.108 have synthesized hierarchically porous CuFe2O4 nanospheres, demonstrating high specific capacitance and good retention for SCs. The hierarchical CuFe2O4 nanospheres showed the highest capacitance of 334 F g 1 and 88% capacitance retention after 600 cycles at 0.6 A g 1. 3.1.2.3 Metal oxide/metal oxide composite-based hierarchically porous structures for supercapacitors. Although HPMOs have been widely synthesized for pseudo-capacitors, the low electrical conductivity results in much lower capacitance than the theoretical values estimated from their redox reactions. Accordingly, great efforts have been made to improve their capacitive performances. Many better conductive porous substrates, including nickel foam (NF), HPC, HPG and CNTs, have been adopted to incorporate with the transition metal oxides. Wei et al.109 have designed 3D ZnO–NiO mesoporous architectures as electrochemical capacitors. An electrochemical study shows that the 3D ZnO–NiO composite obtained at 400 1C demonstrates a high specific capacitance of 2498 F g 1 at 2.6 A g 1. In addition, good rate capability at high current densities and excellent long-term cycling stability (about 3.0% loss of the maximum specific capacitance after 2000 cycles) are obtained owing to its morphological characteristics of mesoporous and nanosheet self-assembled architecture, as well as a rational composition of the two constituents. Tang et al.110 have reported the hierarchically porous Ni–Co oxide microflowers with porous flakes. The pseudocapacitive results showed that the Ni–Co oxide demonstrates a good reversibility with a high specific capacitance (834.93 F g 1 at 1 mV s 1 scan rate). When prepared as an electrode, the Ni–Co oxide/activated carbon asymmetric supercapacitor demonstrates a high specific capacitance (60 F g 1 with a 1 mV s 1 scan rate) and a high reversibility of specific capacitance at 37 F g 1 at a high current density of 20 mA cm 2. Zhang et al.111 have prepared hierarchically porous MnO2 nanosheets on 1D H2Ti3O7 and anatase/TiO2 (B) nanowires for high performance supercapacitors. After incorporation of activated graphene, MnO2/TiO2 exhibits a more optimized electrochemical performance with a specific capacitance of 120 F g 1 at 0.1 A g 1 (based on MnO2 + TiO2) than MnO2/H2Ti3O7. MnO2/TiO2/activated graphene (AG) demonstrates a better energy density of 29.8 W h kg 1 than MnO2/H2Ti3O7/AG, while maintaining desirable cycling stability due to its special structure.

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Leontyeva et al.112 have prepared a hierarchically porous NiO/C nanocomposite. The synthesized NiO/C composite exhibits pseudo-capacitive behavior with the mass capacitance value as high as 1012 F g 1 and 750 F g 1 according to CV and discharge curves, respectively. Particularly, a specific capacitance of NiO of 2573 F g 1 is obtained with 97% utilization of NiO in the NiO/C composite. Zhang et al.113 have fabricated a hierarchically porous NiO nanoflake array on carbon cloth, which exhibits a high specific capacitance of 660 F g 1 at 2 A g 1. The integrated flexible electrode shows excellent mechanical behavior and long-term cycling stability with 82% retention after 4000 cycles at 1 A g 1. Zhao et al.114 have fabricated 3D hierarchically porous microstructure of manganese oxide (MnOx) electrodeposited on Ni foam supported CNTs (MnOx/CNT/NF) for the supercapacitor electrode. The MnOx/CNT/NF electrode with a MnOx loading mass of 100 mg demonstrates a high specific capacitance of 462 F g 1 with good long cycle stability at 5 A g 1. Zhang et al.115 have synthesized a hierarchically porous nanoflake-like nickel hydroxide and mesoporous carbon composite, showing a specific capacitance of 2570 F g 1. In addition, Deng et al.116 have reported a nano-architectured CuO electrode with a 3D hierarchically porous structure on NF. Because of its unique hierarchical nanoarchitecture, the CuO nanoribbon-on-Ni-nanoporous/Ni foam (CNRNP) electrode shows a more excellent supercapacitance than a conventional electrode. The CNRNP electrode also exhibits a more superior kinetic performance compared with CuO nanoflake-on-Ni foam (CNFNF) and flake-like CuO (FLC) electrodes. In addition, an exceptionally large specific capacitance of 800 F g 1 (deducting the substrate capacitance from the total) for the CNRNP electrode is obtained at 200 mV s 1. Recently, Lang et al.117 have designed hierarchically porous hybrid gold/MnO2 composites (Fig. 11a and b). The gold/MnO2 composites are used as active charge storage electrodes in a simple supercapacitor device. The nanoporous gold/MnO2 electrodes exhibit improved electrochemical performance compared to that of the bare nanoporous gold framework (Fig. 11c and d). This is probably due to the porous metal/oxide structure, in which the nanocrystalline MnO2 grows epitaxially into the internal surface of the highly conductive nanoporous gold, allowing easy and efficient access of both electrons and ions so as to afford a fast redox reaction at high scan rates. Sun et al.118 have reported a hierarchical Ni@MnO2 structure consisting of MnO2 nanowires supported on hollow Ni dendrites for highperformance supercapacitors. At a MnO2 mass loading of 0.35 mg cm 2, the Ni@MnO2 electrode demonstrates a specific capacitance of 1125 F g 1. In addition, a remarkable high rate performance (766 F g 1 at 100 A g 1) is achieved. Electrochemical tests in a two-electrode configuration for the Ni@MnO2 structure with a high MnO2 loading of 3.6 mg cm 2 show a high specific power of 72 kW kg 1. Chou et al.119 developed hierarchically porous MnO2/HPC nanocomposites for asymmetric supercapacitor applications. The MnO2/HPC electrodes achieve higher specific capacitance values (196 F g 1) than those of pure carbon electrodes (60.8 F g 1), and maintain superior rate capability in neutral electrolyte solutions. The asymmetric supercapacitor consisting of a MnO2/HPC cathode and a HPC anode shows

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Fig. 11 (a) SEM image of a nanoporous gold/MnO2 film with a MnO2 plating time of 10 min. (b) Bright-field TEM image of the nanoporous gold/MnO2 hybrid with a MnO2 plating time of 20 min. (c) Cyclic voltammograms for bare nanoporous gold electrodes and nanoporous gold/MnO2 electrodes for three different plating times at a scan rate of 50 mV s 1. (d) Volumetric capacitances of both nanoporous gold/MnO2 and Ag65Au35/MnO2 electrodes as a function of plating time. Inset: Volumetric capacitance versus mass ratio for nanoporous gold/MnO2 electrodes. Capacitance is estimated from the cyclic voltammograms at a scan rate of 50 mV s 1. (e) Charge–discharge (voltage versus time) curves at a current density of 0.5 A g 1 and (f) specific capacitance (Cs) versus discharge current density for bare nanoporous gold electrodes and for nanoporous gold/MnO2 electrodes for three different plating times. All data are taken in a 2 M Li2SO4 solution at room temperature. (Reproduced from ref. 117 with permission. Copyright Macmillan Publishers Limited, 2011).

excellent performance with energy and power densities of 15.3 W h kg 1 and 19.8 kW kg 1 at a cell voltage of 2 V, respectively. Li and Wang120 have successfully synthesized a coated core structure consisting of an internal graphitized carbon sphere and an external MnO2 layer, which not only possesses a high surface area and hierarchical porosity, but also has improved electrical conductivity. Such structural characteristics enable the obtained composite to show a specific capacitance of 583 F g 1 at 1 A g 1. Peng et al.121 have prepared MnO2/C nanocomposites with hierarchical pore structure. The optimized MnO2/C nanocomposite exhibits high specific capacitance and excellent rate capability. Li et al.122 have studied amorphous manganese oxide (a-MnOx) on hierarchically porous 3D graphene sheet-carbon nanotube (GS-CNT) structure, producing a specific capacitance of MnOx of 1200 F g 1. The specific energy and specific power of a-MnOx/GS-CNT are as high as 46.2 W h kg 1 and 33.2 kW kg 1, respectively. Zhang et al.123 have designed hierarchically structured MnO2/graphene/carbon fibre (CF) and 3D porous

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graphene hydrogel (GH) wrapped copper wire (CW). The assembled asymmetric supercapacitor devices using MnO2/graphene/CF as the positive electrode and GH/CW as the negative electrode can be cycled reversibly in a high-voltage region of 1.6 V, delivering a high areal energy density of 18.1 mW h cm 2 and a volumetric energy density of 0.9 mW h cm 3. He et al.124 have synthesized SnO2/C microspheres with hierarchical porosity. The SnO2/C porous microspheres possess the characteristics of pseudocapacitors and ECDLs. Experimental results show that the SnO2/C microspheres exhibit good conductivity as well as electro-cycling stability, and deliver a super specific capacitance of up to 420 F g 1 in galvanostatic charge/discharge measurements and a high energy density of 34.2 W h kg 1. Chen et al.125 have synthesized hierarchically porous networks consisting of V2O5/CNT nanowire composites, which delivered a capacitance of up to 440 and 200 F g 1 at 0.25 and 10 A g 1, respectively. Asymmetric devices based on these nanocomposites and an aqueous electrolyte exhibit an excellent charge/ discharge capability, and high energy densities of 16 W h kg 1 at a power density of 75 W kg 1 and 5.5 W h kg 1 at a high power density of 3750 W kg 1, competitive with Ni–MH batteries (Fig. 12). Furthermore, they noted that the hybrid asymmetric capacitor using the hierarchically structured V2O5/CNT nanowire composite

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as the anode operated at a maximum voltage of 2.8 V delivers a maximum energy of B40 W h kg 1. Huang et al.126 have designed a 3D hierarchically porous NiO–graphene hybrid film, which demonstrates a specific capacitance of 540 F g 1 at 2 A g 1 with 80% capacitance retention after 2000 cycles, much higher than that of the bare NiO nanoflake film (370 F g 1, 66% capacitance retention). In addition, they pointed out that the NiO–graphene hybrid demonstrates enhanced reaction kinetics with 50% response time for NiO/NiOOH reaction and 79% charge-transfer resistance compared with those of the bare NiO film. Zhang et al.127 have prepared a 3D graphene wrapped nickel foam (Ni/GF) architecture. The optimized hierarchically asymmetric supercapacitor exhibits excellent stability in a high-voltage region of 1.8 V and remarkable cycling stability with 90.2% capacitance retention after 10 000 cycles. A high energy density of 1.23 mW h cm 3 is obtained. Tang et al.128 have studied graphene-decorated hierarchically porous nickelian heterogenite ((Ni,Co)OOH), showing great enhancement of electrochemical properties. The contributed capacitance value of (Ni,Co)OOH in (Ni,Co)OOH–rGO-10 h (decorated with 10 h reduced graphene oxide) is 1045 F g 1, 3.5 times higher than that in the pure sample (296 F g 1). Mitchell et al.129 have developed highly porous hierarchical flexible nanosheets of NiCo2O4–graphene oxide (NiCo2O4–GO), demonstrating a specific capacitance of 1078 F g 1 at 1 mA with great cycling stability. Table 2 summarizes the electrochemical performance of the various hierarchically porous carbon and/or graphene based materials for supercapacitors. 3.2

Fig. 12 (a) SEM image of a V2O5/CNT nanocomposite demonstrating nanowire V2O5 networks penetrated with CNTs, (b) TEM image of a nanocomposite showing embedded CNTs within the V2O5 nanowires and (c) Ragone plots of Na-ion and Li-ion asymmetric supercapacitors made from a V2O5/CNT nanocomposite anode and an AC cathode, a symmetric supercapacitor made from the same AC, and various supercapacitor devices recently developed. All of the data are based on the mass of electrode materials. (Reproduced from ref. 125 with permission. Copyright American Chemical Society, 2012).

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Hierarchically porous structures for lithium ion batteries

Considerable attention has been paid to electrochemical energy storage devices, especially rechargeable lithium ion batteries (LIBs), with both high power and high energy densities, which result in their applications in electric vehicles and portable electronic devices. However, many potential electrode materials of LIBs are limited by slow Li+ diffusion, poor electron transport in electrodes and increased resistance at the electrode/electrolyte interface at high discharge–charge rates. Various hierarchically porous structures (e.g. nanoscale size, nanoporous or hierarchically nano/macro-structure) have been investigated as anode or cathode materials to improve the electrochemical performances by providing good access of the electrolyte to the electrode surface, shortening the Li+ insertion/extraction pathway and facilitating charge across the electrode/electrolyte interface, resulting in excellent capacity, long cycle life and good rate performance. Three types of mechanisms of electrode materials for LIBs have been widely accepted: the intercalation–de-intercalation reaction, the conversion (redox) reaction and the alloying– dealloying reaction.130 The typical intercalation–de-intercalation reaction is mainly for the layered structures, such as the commercial graphite, carbon-based structures, and some of the metal oxides (TiO2, V2O5, Li4Ti5O12, etc.).131–133 The typical conversion (redox) reaction is for the metals and metal oxides, while the typical alloying–dealloying reaction is mainly for some elements such as Si, Sn, Ge and their oxides. In this section, we discuss

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BET surface areas (m2 g 1) 182 122.88 205.6 165 196.8 290 — 114 — — 405.3 — 1046 1260 655 357 1103 1094 1555.4 277 376 18.93 296 254 1208 — 48.4 135 — — — 57.7 57.8 196.1 — 322 137 107 132.2 437 1032.2 — 1543 114

HPS NiO nanospheres Hierarchical NiO moss decorated diatomites Hierarchically porous a-Ni(OH)2 HPS Co/NiO core/shell nanowires HPS NiO film HPS NiO film HPS NiO@MnO2 Sodium-ion pseudo-capacitors NiO nanoflakes on porous graphene frameworks Manganese oxide on HPG-CNT structure Graphene/MnO3 and graphene/polypyrrole hybrid architecture MnO2/graphere/carbon fibers and all-solid-state asymmetric Yolk–shelled carbon spheres MSCSs Hierarchically porous carbon Carbon sheets Porous layer-stacking carbon

Tubular carbon HPCs Polyaniline HPC Carbon nanofiber-silsesquioxane–polyaniline HOPC Ordered mesoporous carbon/graphene HPGP Titanium nitride and carbon nanotube composites

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3D ZnO–NiO architectures Porous Ni–Co oxide Hierarchically porous Co3O4 film Hierarchically porous Co3O4 film Porous CoOOH thin-film Porous CuFe2O4 nanospheres HPS MnO2 nanosheets 3D nanoarchitectrued CuO electrodes NF-supported CNTs HPS Ni(OOH)2/mesoporous carbon HPS NiO based nanocomposites CBD CFP MnO2/HPC Porous MnO2/C hybrids MnO2/C nanocomposites SnO2@C composites V2O5 nanowire/CNT composites

1

1

2

2.6 1 mV s 1 2 2 40 0.6 0.1 10 mV s 1 5 5 mA cm 2 1 2 — 1000 mV s 1 1 2 1 10

2 mV s 1 1 0.67 0.5 0.1–1.0 200 mV s 5 mV s 1 0.1 1Vs 1

4.5 0.25 2 mV s 1 2 2Ag 1 1Ag 1 12.5 60C 2 — — 5 mA cm 20 0.58 1 100 200 mV s

Current density (A g 1)

2498 834.93 454 443 334 334 120 880 462 2570 840 660 — 196 583 130 420 200

463 218.7 1580 956 232 309 1097 75 540 1200 1.23 mW h cm — 90 58 204 105 360 F cm 3 374 246.4 641.6 2200 247 167–102 47.1 44.3 115 18.3 mF cm 2

Capacitance (F g 1)

3

1000 4000

75 94.3 88 93 96.32 86 102 82 — 60 — 79 91 —





97 85

— 497 99 96 90 94.3 93 96.2 61 100 92.6 98 —

89 81 80 80 98.58 90.2 90 75 95

95 90.61 74 98

Capacity retention (%)

6 M KOH 6 M KOH 1 M NEt4BF4 0.5 M K2SO4; 1 M Et4NBF4 3 M KOH 1 M KOH 2 M KOH 2 M KOH 2 M KOH 1 M KOH 1 M Na2SO4 3 M KOH 0.5 M Na2SO4 2 M KOH 1 M NaOH 2 M KOH 1 M KOH 1 M Na2SO4 0.1 M Na2SO4 1 M Na2SO4 1 M KOH 1 M NaClO4

2 M KOH 1 M KOH 1 M KOH 1 M Na2SO4 1 M NaClO4 2 M KOH IR drop in DI water 0.5 M Na2SO4 1 M Na2SO4 6 M KOH 1 M Na2SO4 1 M H2SO4 6 M KOH 6 M KOH 1 M Na2SO4 6 M KOH 6 M KOH 1 M H2SO4 6 M KOH

5 M KOH 1 M KOH

Measurement condition 3-electrode, 3-electrode, 3-electrode 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 2-electrode, 2-electrode, 3-electrode, 3-electrode, 2-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode 3-electrode, 3-electrode, 2-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 2-electrode, 3-electrode, 2-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode, 2-electrode, 3-electrode, 3-electrode, 3-electrode, 3-electrode,

109 110 106 105 107 108 111 116 114 115 120 113 104 119 120 121 124 125

85 87 89 88 90 95 92 94 93

99 101 102 103 97 96 118 125 126 122 127 123 80 81 83 84 82

Ref.

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— — — 4100 2000 —



2000 2000 2500 3000 5000 600 3000 5000 500

5000 5000 500 4000 500 45400 1000 5000 20 000

1000 1000 1000 6000 4000 4000 1500 900 2000 1500 10 000 10 000 — 4000 200 3000 10 000

Cycle numbers

The electrochemical performance of various hierarchical porous carbon and/or graphene based materials for supercapacitors

Structures

Table 2

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the recently developed hierarchically porous structures of current interest for LIBs. We try to include the typical hierarchically porous structures such as hierarchically porous carbons, hierarchically porous graphenes, hierarchically porous metal oxides, hierarchically porous polyanion compounds, and hierarchically porous Si and Ge as electrode materials for LIBs. 3.2.1 Carbon- and/or graphene-based hierarchically porous structures for lithium ion batteries. Nowadays, carbon materials, such as graphite, are the most commonly used anode materials for LIBs owing to their low cost and abundance. However, the low theoretical capacity of 372 mA h g 1 cannot meet the everincreasing requirements of various intelligent mobile electronic devices and popularization of electric vehicles or hybrid-electric vehicles. On the other hand, the graphite carbon materials used in LIBs usually suffer from large irreversible capacity due to the formation of a solid electrolyte interphase (SEI) layer on their large surfaces. It is well known that the electrochemical performance and stability of a carbonaceous electrode highly depend on its crystallinity, texture, and morphology on the micro- and nanoscale. Hierarchically porous carbon (HPC)/graphene (HPG) have attracted enormous attention due to their high specific surface area, excellent conductivity, strong adsorption capacity as well as chemical stability. In addition, these characteristics are also beneficial for the loading of active substances, the diffusion of the electrolyte, and the improvement of the charge transport. Therefore, HPC/HPG-based hierarchically porous structures are considered as ideal carriers for electrode materials. Moreover, doping of heteroatoms, such as N, B or S, can also effectively improve the electronic properties and the electrochemical activity of carbon materials, resulting in their excellent ability to store Li ions. Wang et al.134 have synthesized hierarchically porous graphitic carbon spheres via a biomimetic approach based on template (pluronic F127 micelle cluster)-induced self-assembly of a-cyclodextrin. When used as an anode for LIBs, the hierarchically porous graphite carbon spheres demonstrate high reversible capacity (ca. 700 mA h g 1 at 50 mA g 1), good cycling stability, and remarkably outstanding high-rate performance (ca. 600, 450, and 290 mA h g 1 obtained at 1, 10, and 30 A g 1, respectively), which is among the best pure carbon materials available for LIB applications. The high performance is attributed to a synergism of the high degree of graphitization, large surface area with hierarchically distributed pore sizes as well as heteroatom doping. Yu et al.135 have reported a novel hierarchically architectured porous material composed of a micro-sized porous carbon sphere matrix embedded with hollow nanocapsules (HNs-HPCS). The hollow nanocapsules embedded within the carbon interior store large amounts of Li ions, while hierarchical pores are favorable for the fast transportation of Li ions in electrolytes to a great degree, and thus made the micro-sized material a promising anode material for high-performance LIBs. As a result, the hierarchically porous structure delivers a very high capacity of 805 mA h g 1 at 0.1 A g 1, and the capacity of B210 mA h g 1 is obtained at 20 A g 1. Ning et al.136 have synthesized S-doped porous carbons (S-PCs) as anodes for LIBs, which resulted in a much thinner SEI-layer and hence significantly enhanced coulombic efficiency

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in the first cycle to 81.0% compared with the undoped porous carbon of 39.6%. The Li storage capacity of the S-PCs reaches 1781 mA h g 1 at the current density of 50 mA g 1. Cao et al.137 have designed a 3D hierarchically macro–mesoporous graphene with very high conductivity (61 500 S m 1). This graphene based material exhibits enhanced lithium storage performance of 580 mA h g 1 after 300 cycles at 1 A g 1. Chi et al.138 have synthesized a novel composite of HPC supported polydimercaptothiadiazole (PDMcT)–(polyaniline) PANI (HPC/PDMcT– PANI) for LIBs. The initial discharge capacity of the HPC/ PDMcT–PANI composite reaches 310 mA h g 1. Moreover, the HPC/PDMcT–PANI composite also shows higher specific capacity and better cycling stability than that of its two binary composites PDMcT–PANI (291 mA h g 1) or HPC/PDMcT (253 mA h g 1). Hao et al.139 have reported highly interconnected 3D crystalline carbon frameworks with hierarchically porous channels, exhibiting a good cycle performance and rate capability for LIBs. Sun et al.140 have synthesized 3D hierarchically porous nitrogen–sulfur codoped graphene-like microspheres (3D NS-GSs), presenting a superior capacity with excellent cycling stability. Zhao’s group141 has designed a hierarchically ordered core/shell graphitic carbon material for LIBs. Xiao et al.142 have reported a hierarchically porous Mo2C–C (HP-Mo2C–C) hybrid for LIBs, displaying excellent lithium storage performance in terms of specific capacity, cycling stability and rate capability. Wei et al.143 have fabricated hierarchically structured nanocarbon electrodes comprising aligned multiwalled CNTs and carbon nanohorns (CNHs), providing a significant performance enhancement to solid Li-battery performance. 3.2.2 Metal oxide-based hierarchically porous structures for lithium ion batteries. The electrochemical performance of metal oxides depends strongly on the morphology, crystalline structure, crystallinity, crystallite size, compositions and surface area. Decreasing the particle size to the nanoscale allows higher reversible capacity and faster rates due to the increased surface contact area and the shortened path length for Li+ diffusion. This however leads to an enhanced irreversible capacity loss due to the undesired side reaction. Therefore, hierarchically porous metal oxides have attracted special interest for LIBs due to their unique properties such as a high specific surface area, narrow pore size distribution and good permeation. 3.2.2.1 Binary oxide-based hierarchically porous structures for lithium ion batteries. Various metal oxides have been synthesized and used for LIBs. There are also interesting reviews on their synthesis and related properties. In this section, we mainly focus on the recently developed typical binary oxides with hierarchically porous structures, such as TiO2, cobalt oxides, NiO, manganese oxides, iron oxides, SnO2 and V2O5. The recently developed hierarchically porous TiO2 structures will be discussed with particular attention. 3.2.2.1.1 Iron oxide-based hierarchically porous structures for lithium ion batteries. Iron oxides (Fe2O3 and Fe3O4) have attracted considerable attention owing to their high theoretical capacities, low toxicity, low price and natural abundance.131 The low electrical conductivity and poor cycle stability however

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limit their application for LIBs due to the mechanism based on the conversion (redox) reaction. Synthesizing nanostructures and introducing a carbonaceous matrix and hierarchically porous structures are promising strategies to overcome the drawbacks and to promote their electrochemical performance. Huang et al.144 have reported the design of ordered hierarchically porous 3D electrodes with entrapped Fe2O3 nanoparticles. In contrast to previous reports on hierarchically porous electrodes from irregular self-assembly or post-incorporation of active nanoparticles, their strategy relies on in situ formation and entrapment of active nanoparticles inside the simultaneously formed ordered hierarchically 3D porous carbon, in which the periodic macroporous–mesoporous carbon is directly integrated with the open-porous current collector (Ni foam, NF) without an organic binder, and the electrode active nanoparticles (Fe2O3) are spatially entrapped inside the periodic porous carbon. As a result, the designed Fe2O3/C 3D electrode demonstrated ultrahigh rate capabilities and long-term cycling stability without capacity decay (Fig. 13). Particularly, the Fe2O3/C 3D electrode delivers the capacity of 804.59 mA h g 1 after 500 cycles at 4.0 A g 1, corresponding to 79.90% of the theoretical capacity of Fe2O3 (1007 mA h g 1). The reversible capacity of the Fe2O3/C 3D electrode reaches 733.35 mA h g 1 when cycled at 8.0 A g 1 for 500 cycles (Fig. 13c). The Fe2O3/C 3D electrode shows long-term cycling stability with more than 92% of the theoretical capacity (928.22 mA h g 1) retained after 1000 cycles at 2.0 A g 1. Zhang et al.145 have produced hierarchically porous Fe2O3 microspheres assembled by well-crystalline nanoparticles.

Fig. 13 (a and b) TEM images with low magnifications, (c) rate capabilities of the electrodes at the current densities of 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 8.0 and 1.0 A g 1. (Reproduced from ref. 144 with permission. Copyright Wiley-VCH, 2014).

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As an anode material for rechargeable LIBs, the Fe2O3 microsphere exhibits a stable and reversible capacity of 705 mA h g 1 after 430 cycles at 100 mA g 1. Yue et al.146 synthesized porous Fe2O3 with hierarchical structure for LIBs, showing an initial capacity of 634 mA h g 1 and an efficiency of over 90% after 20 cycles at 100 mA g 1. Mao et al.147 have prepared aligned nanorods constructed hierarchically porous Fe2O3 nanostructures. The hierarchically porous Fe2O3 nanostructure obtained at 400 1C exhibits high reversible discharge capacity (901.3 mA h g 1 at 0.2C), superior rate performance (221.3 mA h g 1 at 1000 mA h g 1) and excellent cycling stability (416.4 mA h g 1 after 1000 cycles at 5000 mA g 1). Lei et al.148 have synthesized the nanoparticles constructed hierarchically porous Fe3O4 spheres, delivering a reversible capacity of 511 mA h g 1 after 50 cycles at 100 mA g 1. Jing et al.149 have prepared a nano/micro hierarchically porous flower-like Fe3O4/carbon nanocomposite for LIBs, exhibiting high capacity and good cycle stability (1030 mA h g 1 up to 150 cycles at B150 mA g 1), as well as enhanced rate capability. Han et al.150 have designed carbon-coated Fe3O4 coaxial nanotubes with hierarchical porosity. When used as an anode material for a lithium-ion half-cell, the carbon-coated hierarchically porous Fe3O4 nanotubes show excellent cycling performance with a specific capacity of 1020 mA h g 1 after 150 cycles at 200 mA g 1. Even at a higher current density of 1000 mA g 1, a capacity of 840 mA h g 1 is retained after 300 cycles with no capacity loss. In addition, a superior rate capability is obtained with a stable capacity of 355 mA h g 1 at 8000 mA g 1. Chen et al.151 have synthesized hierarchically porous Fe3O4/carbon nanocomposites. Due to the favorable nanostructure, the synthesized Fe3O4/C composites with 18.8 wt% carbon exhibit a capacity of 580 mA h g 1 at a current density of 1000 mA g 1. 3.2.2.1.2 Titanium oxide-based hierarchically porous structures for lithium ion batteries. As typical intercalation metal oxides, Ti-based nanostructures (anatase, rutile, TiO2-B, Li4Ti5O12, etc.) have recently received increasing attention as promising Li-ion battery anode materials, owing to their low cost, non-toxicity, low volume change, excellent recharge ability, improved safety over graphite and relatively high lithium insertion potential (1.5–1.8 V vs. Li/Li+).26,27,152,153 Hwang et al.154 have reported a hierarchical meso–macroporous TiO2 with three-dimensionally interconnected pore networks. The hierarchical TiO2 coin cells (h-TiO2/Li) are assembled for galvanostatic charge–discharge tests over the potential range from 1.0 to 3.0 V vs. Li/Li+. The h-TiO2/Li cell exhibits a reversible capacity of 215 mA h g 1, which is larger than that of mesoporous TiO2 (194 mA h g 1) at 0.2C. In addition, the h-TiO2 electrode shows better rate capability: the reversible capacity of 113 mA h g 1 at 20C (1C = 167 mA g 1, hereafter for all the TiO2 structures without mention). Our group has recently synthesized two kinds of three dimensionally ordered macroporous TiO2 samples, one with disordered worm-like mesopores formed by aggregation of nanoparticles (3DOM) and another with straight mesopores generated by the surfactant templating strategy (3DOMM) (Fig. 14a).155a 3DOM TiO2 possesses a first discharge capacity of 248 mA h g 1 corresponding to 0.74 lithium inserted per

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formula unit of TiO2, while the first discharge capacity of 3DOMM TiO2 is 235 mA h g 1, corresponding to 0.70 lithium inserted per formula unit of TiO2. Both samples showed higher initial capacities than the practical composition of Li0.6TiO2, due to the presence of hierarchical meso–macroporosity ensuring that the electrolyte ions are transported much more smoothly, and their thin walls provide continuous and shorter path lengths for Li ion diffusion in the solid phase. In addition, the high surface area also supplies more active sites for the storage of Li ions. 3DOM TiO2 has a higher initial capacity and a lower irreversible capacity than those of 3DOMM TiO2 at the beginning of the reaction, indicating that the electrolyte could easily penetrate into the larger worm-like mesopores (7 nm). As a result, an initial capacity of 208 mA h g 1 is obtained and a reversible capacity of 126 mA h g 1 is retained after 200 charge–discharge cycles for the 3DOM TiO2 electrode, with a capacity retention of 61% (Fig. 14b). Remarkably, an initial discharge capacity of 203 mA h g 1 is obtained and a much higher reversible capacity of 144 mA h g 1 is retained after 200 cycles with a capacity retention of 71% for the 3DOMM TiO2 electrode. Furthermore, an excellent reversible capacity of 123 mA h g 1 is obtained after 200 cycles at 2C. These results verified that 3DOMM TiO2 is suitable for high-rate applications due to its higher specific surface area and bicontinuous porous structure, leading to more active sites, a shorter path length and a more continuous transport path for Li ion diffusion and insertion. Nanostructured hierarchically porous TiO2 hollow microspheres, with the merits of nanostructures and microstructures,

have received great attention, owing to the fact that such spheres offer more active sites for Li+ insertion, accommodate volume changes during charge/discharge cycles and reduce the diffusion pathway for Li+ and electron transport to enhance the LIB performance. Shen et al.156 have synthesized hierarchically porous hollow TiO2 microspheres. The electrochemical measurement results show that the initial Li insertion/extraction capacity of the electrodes obtained at 180 1C for 15 h is 158 mA h g 1 and 151 mA h g 1 at 5C, respectively. In addition, the reversible capacity is 131 mA h g 1 after 100 cycles at 5C, indicating excellent cycling stability and good high rate performance. Following the very promising results above using TiO2 with hierarchical porosity, our group has designed hierarchically porous nanorod chain-constructed TiO2 hollow microspheres (HNC-TiO2-HMSs) via a facile one-pot fluorine-free solvothermal alcoholysis route using a TiCl4 and isopropanol reaction system (Fig. 15a and b).157 Such highly porous hollow microspheres with hollow cavity, straight nanorod chains and straight nanochannels can highly facilitate the charge diffusion and Li+ insertion and buffer the volume change during the Li+ insertion/extraction process. When the HNC-TiO2-HMS electrode is discharged at 1, 2, 5, 10, 20 and 30C for more than 100 cycles, reversible capacities of 216, 184, 140, 112, 88 and 79 mA h g 1 are obtained, respectively (Fig. 15c). The very long cycle experiment shows the extraordinarily high stability of HNC-TiO2-HMSs owing to its particular hierarchically porous structure enabling fast and efficient electron/Li+ transport. The post-mortem studies on the HNC-TiO2-HMS based anode demonstrate that the hierarchical nanorod constructed hollow sphere structure is retained after the electrochemical reaction, indicating the structural and electrochemical stability of the HNC-TiO2-HMSs material (Fig. 16a and b). Interestingly, after lithium insertion, numerous small nanoparticles with a diameter of B5 nm are randomly distributed on the surface of the nanorods (Fig. 16c). Fig. 16d further reveals that the nanoparticles are homogeneously distributed on the nanorods. The lattice spacings of the nanoparticles and the nanorod

Fig. 14 (a) SEM image of the prepared 3DOMM TiO2, (b) charge– discharge curves of 3DOM TiO2 and 3DOMM TiO2 at a current of 0.2C for the first cycle. (Reproduced from ref. 155a with permission. Copyright Royal Society Chemistry, 2014).

Fig. 15 (a) SEM image and (b) TEM image of the HNC-TiO2-HMSs. (c) Rate capability and cycle performances at various discharge–charge rates of HNC-TiO2-HMSs. (Reproduced from ref. 157 with permission. Copyright Elsevier Ltd, 2015).

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Fig. 16 The post-mortem characterization of the HNC-TiO2-HMS anode material after 100 discharge–charge cycles at 1C: (a and b) SEM images, (c) TEM and (d) HRTEM images. The inset in (b) is the SEM image of the corresponding sample. The TEM images clearly show the Li2Ti2O4 nanoparticles formed on the surface of TiO2 nanorods. The inset in (c) is the schematic illustration of the Li2Ti2O4 nanoparticles. (Reproduced from ref. 157 with permission. Copyright Elsevier Ltd, 2015).

correspond to the (102) crystal plane of anatase and the (400) crystal plane of Li2Ti2O4 (space group: F3m3, lattice constants: a = b = c = 8.375 Å). Very recently, hierarchical nanosheet-constructed yolk– shell TiO2 (NYTiO2) porous microspheres have been prepared through a well designed one-pot template-free solvothermal alcoholysis process using tetraethylenepentamine (TEPA) as the structure directing reagent. Such a yolk–shell structure possesses a highly porous shell and a dense mesoporous core.158 The outer, 2D nanosheet-based porous (15 nm) shell and the nanocrystal-based inner mesoporous (3 nm) core provide a stable, porous framework, effective grain boundaries and a short diffusion pathway for Li+ and electron transport, facilitating lithium insertion/extraction. The voids between the core and the shell could not only store the electrolyte by the capillary effect and facilitate charge transfer across the electrode/electrolyte interface but also buffer the volume change during Li+ insertion/extraction. When the NYTiO2-400-based electrode is discharged for more than 100 cycles at 1C, a reversible capacity of 225 mA h g 1 is retained, leading to a lithium insertion coefficient of 0.67. A reversible capacity of 185 mA h g 1 is obtained after 100 cycles at 2C. Furthermore, when the current density is increased to 5C, 10C, 20C and 30C, the reversible capacity is decreased to 137, 113, 76 and 67 mA h g 1 after 100 cycles, respectively. Even at 50C for 470 cycles, the reversible capacity remained at 58 mA h g 1 (Fig. 17). Such high electrochemical performance can be attributed to the hierarchically porous yolk–shell structure with an outer porous nanosheet shell and an inner mesoporous core. The homogeneously distributed 5–10 nm Li2Ti2O4 nanocrystallites are also observed on the surface of the nanosheets upon cycling (Fig. 18). EELS elemental mapping indicates that the distribution of O and Ti elements is uniform. However, the Li element is non-uniformly distributed in the selected area (Fig. 18b). Superimposed elemental maps (Fig. 18e) showed

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Fig. 17 The rate and cycling performance of the yolk–shell TiO2 (NYTiO2) porous microsphere. The insets are the TEM image and schemed structure of NYTiO2 porous microsphere. (Reproduced from ref. 158 with permission. Copyright Royal Society Chemistry, 2015).

Fig. 18 (a) HAADF-STEM and (b–e) STEM EELS mapping images: (b) Li; (c) O; (d) Ti; (e) color map (red: Li; blue: Ti; green: O). (Reproduced from ref. 158 with permission. Copyright Royal Society Chemistry, 2015).

that the Li element is mainly focused in B7 nm islands, indicating the existence of Li2Ti2O4 nanocrystallites. The synergy of the yolk–shell structure with dual mesopores in the shell and core and Li2Ti2O4 nanocrystallites endow the hierarchical NYTiO2 with high reversible capacity, excellent rate capability and outstanding cycle performance. In addition, Liu et al.159 have reported tiny octahedra constructed hierarchically porous anatase TiO2 microspheres for LIBs, which deliver a large capacity of 157.3 mA h g 1 after 200 cycles at 1C, high rate performance and excellent cycling stability. They have also prepared nanorod constructed hierarchically porous rutile TiO2 microspheres, demonstrating a large reversible charge–discharge capacity of 160.4 mA h g 1 after 100 cycles at 1C.160 Shen et al.161 have reported a new anatase/ rutile TiO2 nanocomposite microsphere (ART) electrode for LIBs, which exhibits a reversible capacity of 103 mA h g 1 after 100 cycles at 30C. Our group has further designed a hierarchical mesoporous anatase/rutile TiO2 nanowire bundle (HM-TiO2-NB) superstructure with an amorphous surface and straight nanochannels through a templating method at a low temperature under acidic and wet conditions (Fig. 19a).162 The HM-TiO2-NB superstructure demonstrates high reversible capacity, excellent cycling performance and superior rate capability. Most importantly, a self-improving phenomenon of Li+ insertion capability based

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Fig. 19 (a) The schematic illustration of the Li+ insertion mechanism and (b) rate and cycling performance (bottom) of HM-TiO2-NB. (Reproduced from ref. 162 with permission. Copyright Wiley-VCH, 2015).

on two simultaneous effects: the crystallization of amorphous TiO2 and the formation of Li2Ti2O4 crystalline dots on the surface of TiO2 nanowires, has been clearly observed during the Li+ insertion process. When discharged for 100 cycles at 1C, HM-TiO2-NB exhibits a reversible capacity of 174 mA h g 1. Even when the current density is increased to 50C, a very stable and extraordinarily high reversible capacity of 96 mA h g 1 is delivered after 50 cycles (Fig. 19b). To increase the electron and ion conductivity of TiO2, surface modification has been adopted. Zhu et al.163 have synthesized hierarchically porous TiO2 based composites (pure TiO2 and TiO2/carbon (TiO2/C) composite). The hierarchically porous TiO2/C composite calcined at 500 1C delivers a remarkable discharge capacity of 132 mA h g 1 at 1C after 100 cycles, and excellent rate capability (over 96 mA h g 1 at 30C). Kim’s group164 has synthesized hierarchically porous anatase TiO2 microspheres. The prepared TiO2 microspheres exhibited a high reversible discharge capacity of 212.3 mA h g 1 at 0.1C, a high-rate performance of 77.9 mA h g 1 at 8C, and an excellent capacity retention of 497% after 100 cycles at 1C due to the nanosized TiO2, porous structure, carbon coating and Ti3+ incorporation. Hasegawa et al.165 have prepared hierarchically porous N-doped TiO2 materials, which deliver high discharge capacity and good cycle performance as negative electrodes for Li-ion batteries. Zhang et al.166 have synthesized hierarchically porous titanium dioxide/ graphitic carbon microspheres (TiO2/GCM) for LIBs. The obtained TiO2/GCM composite microspheres demonstrated a more stable cycling performance, larger reversible capacity, and better rate capability, compared with that of the graphitic carbon microspheres. Huang et al.167 have synthesized hierarchically porous TiO2-B nanoflowers for LIBs. TiO2-B demonstrates high reversible capacity (285 mA h g 1 at 1C), excellent cycling performance and superior rate capability (181 mA h g 1 at 10C after 100 cycles). Chen et al.168 have synthesized unique TiO2-B nanosheets/anatase nanocrystals co-anchored on nanoporous graphene sheets. The electrodes composed of the nanohybrid exhibited superior rate capability (160 mA h g 1 at ca. 36C; 154 mA h g 1 at ca. 72C) and excellent cyclability, owing to the synergistic effects of conductive graphene with numerous nanopores and the pseudocapacitive effect of ultrafine TiO2-B nanosheets and anatase nanocrystals.

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Fig. 20 SEM image (a), TEM (b) and HRTEM (c) micrographs of the as-prepared porous TiO2-B spheres constructed by nanotubes. Electrochemical properties of the TiO2-B material. (d) Galvanostatic discharge– charge curves at different current rates; (e) the first two CV curves at a sweep rate of 0.2 mV s 1; (f) rate performance; (g) comparison of rate capability between this work and other previously reported works at various current densities; (h) cycling performance at rate of 10C over 1000 cycles. (Reproduced from ref. 169 with permission. Copyright Macmillan Publishers Limited, 2015).

Most recently, our group has synthesized hierarchically structured porous TiO2-B spheres via a hydrothermal process using the amorphous titania/oleylamine composite as a self-sacrificing template (Fig. 20a and b).169 Such a unique structure exhibits superior lithium storage performance. The porous TiO2-B delivers a high capacity of 221 mA h g 1 even at 10C, which is much better than those of TiO2-B nanoribbons (93 mA h g 1 at 5C) and commercial P25 nanopowders (62 mA h g 1 at 5C), respectively owing to its characteristic pseudocapacitive behavior and hierarchically porous structure for fast lithium storage. Compared with TiO2-B nanoribbons, the small diameter and tube wall thickness of the nanotubes shorten the diffusion lengths of lithium ions. Moreover, the high specific surface area of the porous material provides a large electrode–electrolyte contact area for rapid electrochemical reactions. In addition, the electrochemical performance of hierarchically porous TiO2-B spheres is superior to TiO2-B with other nanostructures (Fig. 20d), such as TiO2-B nanorods on reduced graphene oxide (RGO), graphene/TiO2-B nanowires, elongated TiO2-B nanotubes, mesoporous TiO2-B microflowers, and porous TiO2-B nanosheets (see references therein). Fig. 20e shows that the capacity starts at 221 mA h g 1 and maintains at 211 mA h g 1 over 200 cycles, with a capacity loss of 4.5%. Even after 1000 cycles, the reversible capacity of 154 mA h g 1 is retained with a capacity loss of 30%. The superior cycling performance certainly demonstrated the high stability of the hierarchically

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Fig. 21 (a) XRD pattern, (b) SEM image, (c) TEM image and SAED pattern (inset) and (d) TEM and HRTEM (inset) of the TiO2-B material after discharge– charge measurements at 10C for 1000 cycles. (Reproduced from ref. 169 with permission. Copyright Macmillan Publishers Limited, 2015).

porous structure and the good accommodation of volume/ strain changes during lithium insertion–extraction. Interestingly, the ex situ characterization of the TiO2-B material after cycling (Fig. 21) reveals that the crystalline and porous spherical structures have been well preserved and the nanotubes are still highly interconnected. In addition, some newly formed nanodots are found to be attached on the nanotubes, corresponding to cubic LiTiO2 (space group: Fm3m). The observation of these isolated cubic LiTiO2 nanodots could explain the full lithiation capability of the TiO2-B material and possible reasons for capacity fading after longterm electrochemical cycling. 3.2.2.1.3 Cobalt oxide-based hierarchically porous structures for lithium ion batteries. Cobalt oxides have received increasing attention as anode materials in LIBs due to their excellent chemical and physical properties, such as high theoretical capacity (715 mA h g 1 for CoO and 890 mA h g 1 for Co3O4). Unfortunately, cobalt oxide anode electrodes suffer from rapid capacity fading due to large volume changes, aggregation, and intrinsically low electronic conductivity. Various methods have been developed to improve the cycling stability of cobalt oxide, including reducing size, using special morphologies and modifying the surface with carbon/graphene layers. Our group has developed an effective method to design a hierarchically porous CoO film via introducing micropores in CoO nanoparticles and self-assembly on copper foil for LIBs (Fig. 22a–c).170 By adjusting the CoO precursor/organic template ratio, CoO nanoparticles with a different particle size of 50 nm, 25 nm and 13 nm and a microporosity of 1 nm for samples 1, 2 and 3, respectively, have been obtained. All the as-synthesized CoO electrodes show a very highly reversible capacity and cycling stability for lithium storage. Fig. 21e shows

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Fig. 22 HRTEM images of as-synthesized CoO nanoparticles with different molar ratios of Co to oleylamine: (a) 2 : 100 for sample 1, (b) 1 : 100 for sample 2, and (c) 0.5 : 100 for sample 3 (d) voltage profiles for the CoO during the 1st cycle under a current density of 0.2C, (e) cycling performance of the CoO samples under a current density of 0.2C. (Reproduced from ref. 170 with permission. Copyright Royal Society Chemistry, 2013).

that the first discharge/charge capacities are about 673.7/ 539 mA h g 1 for commercial powder, 1099/901 mA h g 1 for sample 1, 1209.6/1000.3 mA h g 1 for sample 2 and 1432.8/ 1200 mA h g 1 for sample 3, respectively, corresponding to the coulombic efficiency of B80%, 82%, 82.7% and 83.8%, respectively. After 50 cycles, the capacities of CoO samples 1–3 reduced to, respectively, 17%, 37%, and 65% of the initial capacities. The capacity differences between the CoO samples 3 and 2 or 1 after 50 cycles become more evident, compared with that after the first cycle. These results indicated that the hierarchically porous structure is helpful to promote the electrochemical performance of CoO. Wu et al.171 have prepared nickel foam supported carboncoated CoO wall arrays (CoO@C–Ni) assembled with ultrathin nanosheets. Such a unique structure leads to fast electron transport and excellent structural stability. Due to these advantageous structural features, the hierarchical CoO@C wall arrays supported on nickel foam with strong adhesion exhibit improved rate capabilities and good cycling stability without capacity decay. Guan’s group172 has fabricated vertically aligned hierarchically porous polycrystalline Co3O4 nanostructures for LIBs. Co3O4 nanotubes obtained at 300 1C show an initial discharge capacity of up to 1293 mA h g 1 and the retention of a charge capacity of 895.4 mA h g 1 after 10 cycles at 35 mA g 1. The initial discharge capacities of porous Co3O4 nanorods and Co3O4 microspheres are 1250.77 and 1008.62 mA h g 1 at 35 mA g 1, respectively.173 However, after 10 cycles, the porous Co3O4 nanorods and Co3O4 microspheres show charge capacities of only 709.8 mA h g 1 and 538.2 mA h g 1 at 35 mA g 1, respectively. Guo et al.174 have synthesized unique hierarchically porous spherical Co3O4 superstructures. These superstructures present a high specific capacity of 1750 mA h g 1 after the first cycle, and the capacity retention remains constant at 1600 mA h g 1 after 50 cycles at 180 mA g 1. Sun et al.175 have obtained a hierarchically porous structured Co3O4/C composite, which

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exhibits a remarkable reversible capacity of 1079 mA h g 1 after 50 cycles at 0.1 A g 1 and a reversible capacity of 540 mA h g 1 at 7 A g 1. Park and Lee176 have synthesized hierarchically mesoporous flower-like Co3O4/carbon nanofiber (Co3O4/CNF) composites for LIBs. After the initial capacity of 1446 mA h g 1, the specific capacity levels off at 911 mA h g 1 without capacity fading even after 50 cycles at 200 mA g 1. 3.2.2.1.4 Nickel oxide-based hierarchically porous structures for lithium ion batteries. Nickel oxide (NiO), other than its excellent properties for supercapacitors, is a promising electrode material for LIBs due to its higher theoretical capacity of 718 mA h g 1 and higher volumetric energy density (approximately 2 times and 5.8 times of graphite, respectively). Nevertheless, similar to other transition metal oxides, it has the disadvantage of poor cycling stability owing to pulverization and a large specific volume change during cycling. Porous NiO nanostructures are considered to have much better electrochemical performances than their dense counterparts owing to their large surface area and open edge geometry to accommodate the volume change. It is believed that introducing a carbon matrix in hierarchically porous NiO nanostructures could further improve the lithium ion battery performance. Bai et al.177 have synthesized hierarchically porous structured NiO microspheres. The 3D hierarchical mesoporous NiO microspheres delivered a reversible capacity of 800.2 mA h g 1 after 100 cycles at a high current density of 500 mA g 1. Hu et al.178 have obtained porous tremella-like NiO hierarchical nanostructures with ultrathin nanoflakes for LIBs. The initial discharge and charge capacities are 1374 and 1091 mA h g 1 at 0.1C (1C = 718 mA g 1), respectively. The capacity is maintained at 830 mA h g 1 after 30 cycles at 0.1C. Yuan et al.179 have prepared a hierarchically ordered porous NiO array film for LIBs. The discharge capacity of porous array NiO film is 518 mA h g 1 after 50 cycles at 1C, higher than that of the dense NiO film (287 mA h g 1). Xia et al.180 have reported hierarchically porous NiO/C microspheres for LIBs. The specific capacities of the porous NiO/C microspheres after every 10 cycles at 0.1, 0.5, 1, and 3 A g 1 are about 698, 608, 454 and 352 mA h g 1 respectively. Very recently, our group has reported a binder-free (3D) macro– mesoporous electrode architecture via self-assembly of 3 nm NiO nanodots on macroporous nickel foam for a high performance supercapacitor-like lithium battery (Fig. 23).181 This electrode architecture provides a hierarchically 3D macro–mesoporous electrolyte-filled network that simultaneously enables rapid ion transfer and ultra-short solid-phase ion diffusion. Benefitting from the structural superiority owing to the interconnected porous hierarchy, the electrode exhibits supercapacitor-like high rate capabilities with high lithium battery capacities during the discharge–charge process: a very high capacity of 518 mA h g 1 at an ultrahigh current density of 50 A g 1. It exceeds at least B10 times than that of the state-of-the-art graphite anode, which shows only B50 mA h g 1 at B2 to 3 A g 1 as an anode for Li-ion batteries. The method of preparation of the 3D interconnected hierarchically macro–mesoporous electrode provides an efficient new binder-free electrode technique towards the development of high-performance supercapacitor-like Li-ion batteries.

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Fig. 23 (a) Schematic illustration of the monodisperse 3 nm NiO nanodot preparation, (b) the self-assembly of 3 nm NiO nanodots on macroporous Ni foam, (c) SEM image, (d) HRTEM image of self-assembled 3 nm NiO on NF, (e) the specific capacity of the 3 nm NiO/NF for Li-ion battery at 1 A g 1 for 1000 cycles. (Reproduced from ref. 181 with permission. Copyright Elsevier Ltd, 2016).

3.2.2.1.5 Manganese oxide-based hierarchically porous structures for lithium ion batteries. Manganese oxides (MnO, MnO2, Mn2O3, and Mn3O4) have attracted much attention owing to their high theoretical capacities, low toxicity, low price, thermal stability and natural abundance. However, the low electrical conductivity and poor cycle stability limit their application for LIBs. One effective way to improve the electroactivity and rate capability is the preparation of hierarchical nanostructures with special morphologies via decreasing the electron and lithium ion diffusion lengths. Constructing nanocomposites with a carbonaceous matrix is an effective approach to enhance the electrical conductivity, as well as buffer volume expansion/ contraction during the cycling process. In addition, designing hierarchically hollow or porous micro/nanostructures is also a very important strategy to enhance the electrochemical performance. Jiang et al.182 have synthesized a hierarchically porous MnOx microsphere morphology composed of nanoparticles of MnO and Mn3O4 for LIBs. The MnOx microspheres exhibit a high specific capacity of 1018, 901 and 757 mA h g 1 with nearly 100% capacity retention after 100 cycles at 100, 200 and 500 mA g 1, respectively, owing to the porous structures buffering the volume change during the discharge/charge processes, and the nano-hybrids enhancing electron and ion transport in the electrode materials. Our group has designed two types of hierarchical mesoporous urchin-like Mn3O4/carbon microspheres (HM-MO/C-MS) via the in situ carbonization of the newly synthesized lamellar manganese alkoxide (Mn-DEG) along with the crystallization of Mn3O4 in air (MO-A) and nitrogen (MO-N) for LIBs, respectively (Fig. 24a–c).183 Such unique HM-MO/C-MS with high surface area provides obvious advantages including a large contact area with an

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Fig. 24 (a) SEM images of MO-A, SEM image (b) and TEM image (c) of MO-N. (d) Charge/discharge capacities at various current densities and cycling performance at a high current density of 1500 mA g 1. (Reproduced from ref. 183 with permission. Copyright Elsevier Ltd, 2015).

electrolyte, a short transport path for Li+ ions, a low resistance for charge transfer, and superior structural stability. Compared with MO-A, MO-N shows a better cycling stability and a higher reversible specific capacity of around 915 mA h g 1 after 50 cycles of discharge and recharge, close to the theoretical capacity of Mn3O4 (937 mA h g 1). The good performance of MO-N is attributed to the more stable hierarchical framework of MO-N that can bear repeated lithiation and delithiation reaction and the single crystalline nanorods that can facilitate the charge transfer on the electrode/electrolyte interface. Also, the hierarchically porous MO-N demonstrates better rate capability than that of MO-A (Fig. 24d). Such superior rate performance of the MO-N sample is ascribed to the attractive structure of this material: hierarchical three dimensional frameworks, high porosity distribution and composited with conductive carbon, which is very important for high-power applications. Mn2O3 is another promising anode material, which has high theoretical capacity (1018 mA h g 1), low cost, significant thermal stability and lower operating voltage. For example, our group has recently fabricated bicontinuous hierarchically porous Mn2O3 single crystals (BHP-Mn2O3-SCs) with uniform parallelepiped geometry (Fig. 25a–c).184 The monodispersed BHP-Mn2O3-SCs exhibits high specific surface area and three dimensional interconnected bimodal mesoporosity throughout the entire crystal. Such a hierarchical interpenetrating porous framework not only provides a large number of active sites for Li ion insertion, but also good conductivity and a short diffusion length for Li ions, leading to high lithium storage capacity and enhanced rate capability. Furthermore, owing to their specific porosity, these BHP-Mn2O3-SCs as anode materials can accommodate volume expansion/contraction that occurs with lithium insertion/extraction during discharge/charge processes, resulting in their good cycling performance. It is shown that the size of a Mn2O3 single crystal can affect the electrochemical properties of materials due to the ion diffusion path length. BHP-Mn2O3-SCs

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Fig. 25 (a) SEM image, TEM images (b and c) of BHP-Mn2O3-SCs. The inset in (b) is the SAED pattern. (d) Cycling performance at 100 mA g 1, (e) rate capability of BHP-Mn2O3-SCs. (Reproduced from ref. 184 with permission. Copyright 2015, Nature Publishing Group).

with a size of B700 nm display the best electrochemical performances with large reversible capacity (845 mA h g 1 at 100 mA g 1 after 50 cycles, Fig. 25d), high coulombic efficiency (495%), excellent cycling stability and superior rate capability (410 mA h g 1 at 1 A g 1, Fig. 25e). These values are among the highest reported for Mn2O3-based bulk solids and nanostructures. Besides Mn3O4 and Mn2O3, MnO2 is also extensively studied as an anodic material for LIBs due to its high storage capacity, low cost, low toxicity and natural abundance. Recently, our group has designed the unique walnut-shaped porous MnO2/ carbon nanospheres (P-MO/C-NSs) with high monodispersity via in situ carbonization of amorphous MnO2 nanospheres.185 Polyvinylpyrrolidone (PVP) is utilized as both the surfactant for morphology control and as a carbon source for carbon scaffold formation accompanied by MnO2 crystallization. Such a unique walnut-shaped porous nanostructure with an intimate carbon layer provides a large contact area with an electrolyte, a short transport path length for Li+, low resistance for charge transfer and superior structural stability. The P-MO/C-NS electrode demonstrates high lithium storage capacity (1176 mA h g 1 at 100 mA g 1, Fig. 26a), very good cycling stability (100% capacity retention versus the second cycle) and excellent rate capability (540 mA h g 1 at 1000 mA g 1, Fig. 26b). The deep oxidation of Mn2+ to Mn3+ in P-MO/C-NSs results in an extraordinarily high capacity of 1192 mA h g 1 at a current density of 1000 mA g 1 after a long period of cycling, very close to the maximum theoretical reversible capacity of MnO2 (1230 mA h g 1). The high lithium storage capacity and rate capability can be attributed to the enhanced reaction kinetics owing to the hierarchically organized walnut-shaped porous nanostructure with an intimate carbon layer. 3.2.2.1.6 Tin oxide-based hierarchically porous structures for lithium ion batteries. Tin oxide (SnO2), featuring low cost and toxicity and a high theoretical capacity of 790 mA h g 1, is an ideal anode candidate for LIBs. However, rapid capacity fading due to pulverization caused by a rather large volume change

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step-increase and sudden-recovery current, showing a far better recovery capability of 94.3% (Fig. 27). This high electrochemical performance is the result of the synergistic effect of the porous, hierarchically photonic microstructure and the presence of carbon, which ensures an effective flow of the electrolyte and a short diffusion length of lithium ions, considerable buffering room, and aggregation of SnO2 particles in the special structure during the discharge/charge processes.

Fig. 26 (a) Cycling performance at 100 mA g 1, (b) rate capability of P-MO/C-NSs. (Reproduced from ref. 185 with permission. Copyright Royal Society Chemistry, 2016).

hinders its further application. Effective strategies to enhance its electrochemical performance include reducing its dimensions to shorten the ionic and electrical path length and reduce pulverization impact. The coating carbon or other conducting layers can improve the electrical conductivity and introducing hierarchically porous nanostructures can increase surface areas for reaction, interfacial transport, or dispersion of active sites. Gurunathan et al.186 have prepared hierarchical nanoporous SnO2 hollow microspheres (HMS). They have investigated the electrochemical properties of SnO2-HMS as a negative electrode material for LIBs by employing three different binders: polyvinylidene fluoride (PVDF), the Na salt of carboxy methyl cellulose (Na-CMC) and Na-alginate. The SnO2-HMS electrode with the Na-alginate binder delivers a discharge capacity of 800 mA h g 1, higher than Na-CMC (605 mA h g 1) and PVDF (571 mA h g 1) used as binders at 1C. After 50 cycles, the discharge capacities are 725 mA h g 1, 495 mA h g 1, and 47 mA h g 1, respectively. EIS results confirm that the SnO2HMS electrode with Na-alginate as a binder has much lower charge transfer resistance than the electrode with Na-CMC and PVDF binders. Zhang’s group designed a hierarchically porous structured carbon/SnO2 composite from a photonic hierarchical structure,187 demonstrating high reversible capacities, good cycling stability, and excellent high-rate discharge performance. A capacitance of B572 mA h g 1 after 100 cycles, 4.18 times that of commercial SnO2 powder (137 mA h g 1), is observed after a

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3.2.2.1.7 Vanadic oxide-based hierarchically porous structures for lithium ion batteries. Vanadic oxide (V2O5) is another typical intercalation metal oxide for LIBs. Its dimensions, morphology, porosity, and texture are important for the electrochemical performance.188 Hierarchically porous V2O5 nanomaterials, benefiting from the merits of nanostructure, porous structure and hierarchical structure, have been considered to be a useful way to improve their electrochemical performance. However, the synthesis of hierarchically porous V2O5 nanostructures is still a challenge. An et al. have fabricated 3D porous V2O5 hierarchical microplates for LIBs, which exhibited an excellent rate capability and a stable capacity of 110 mA h g 1 after 100 cycles at 2 A g 1.189 Remarkably, as the temperature increased to 40 and 60 1C, the increased capacities of 141 and 143 mA h g 1 are obtained after 100 cycles at a current density of 1 A g 1, respectively, which are higher than those measured at room temperature due to the gradually promoted electrochemical kinetics during the temperature-rise process. Our group has synthesized hierarchically porous V2O5 microspheres using intertwined laminar nanocrystals or crosslinked nanobricks. These microspheres exhibit high and stable Li+ storage and maintain reversible Li+ storage capacities of up to 10 and 85 mA h g 1 over 100 cycles at 0.5 and 1C, respectively. Their superior Li+ storage performance could mainly be ascribed to the improved electrode/electrolyte interface, the reduced Li diffusion path and relieved volume during lithiation and delithiation in hierarchically porous microspheres.189 3.2.2.2 Ternary oxide-based hierarchically porous structures for lithium ion batteries. Ternary oxides can be used as either anodes or cathodes in LIBs. For the ternary oxides as anode materials for LIBs, Li4Ti5O12 is one of the widely studied materials due to its unique structures, such as ‘‘zero-strain’’ in the lattice on charge/discharge and the higher Li+ insertion/ extraction potential (B1.55 V vs. Li+/Li) with the theoretical capacity of 175 mA h g 1, resulting in an excellent cycle life and enough safety. For the ternary oxides as cathode materials for LIBs, spinel LiMn2O4 is one of the most widely synthesized materials owing to its intrinsic advantages of low cost, high abundance, better safety and environmental compatibility. Thus, it is important to design and prepare hierarchically porous structures with improved capacity, cycle life and rate performance. Shen et al.190 have synthesized hierarchically porous Li4Ti5O12 microspheres as anode materials for LIBs. The hierarchically porous Li4Ti5O12 microspheres annealed at 700 1C possess the perfect crystallization and optimal particle size, which deliver an initial discharge capacity of 162.4 mA h g 1 at 1C (1C = 175 mA g 1), a specific charge capacity of 92.3 mA h g 1 at

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discharge capacity of 105 mA h g 1 is obtained at 10C (1C = 148 mA g 1), and a capacity retention of about 90% is achieved after 500 cycles at 10C. Wu et al.196 have synthesized crystalline hierarchical hollow porous LiMn2O4 microcubes as cathode materials for LIBs, which exhibit excellent rate capability with initial discharge capacities of 110 (0.5C), 100 (1C), 90 (2C), and 68 (5C) mA h g 1, respectively. After 200 cycles with current density constantly changing from 0.5C to 5C and back to 1C, the reversible capacity of HP-LiMn2O4 microcubes is still maintained above 100 mA h g 1. Sun et al.197 have fabricated hierarchically porous donut-shaped LiMn2O4 comprising aggregated single-crystalline nanoparticles. The as-obtained donutshaped LiMn2O4 delivers excellent rate capability and high-rate cycling stability. Surprisingly, even at a high charge/discharge rate of 10C, above 95% capacity retention is achieved after 500 cycles.

Fig. 27 (a) SEM image and (b) cycling performance of the hierarchically porous structured carbon/SnO2 composite. The insets in (a) are the photograph of butterfly (upper left) and enlarged SEM image (upper right), respectively. (Reproduced from ref. 188 with permission. Copyright American Chemical Society, 2015).

the high rate of 20C, and an excellent capacity retention of 147.4 mA h g 1 after over 200 cycles at a rate of 2C. Hasegawa et al.191 have reported hierarchically porous structured Li4Ti5O12 with various morphologies. The flowerlike Li4Ti5O12 structure delivers the best electrochemical performance. It demonstrates an initial discharge capacity of 165 mA h g 1 at 0.1C (1C = 175 mA g 1) and delivers the reversible high rate capacities of 155 mA h g 1 and 121 mA h g 1 at 10C and 30C, respectively. After 500 cycles at 5C, more than 98% of the initial capacity is retained. Sun et al.192 have fabricated a hierarchically porous NiCo2O4/NiO hollow dodecahedron as an anode material for LIBs. Such a hollow dodecahedron NiCo2O4/ NiO demonstrates a high reversible capacity of 1535 mA h g 1 at 0.2 A g 1 and good cycling stability (97.2% retention after 100 cycles). Giri et al.193 have reported the synthesis of 3D hierarchically porous ZnCo2O4 ‘‘wrinkled-paper-like’’ structures. The results demonstrate that the incorporation of B23% carbon in the matrix largely promotes their performance as anode materials for LIBs, delivering an excellent initial specific capacity of 527 mA h g 1 at 100 mA g 1. After 50 cycles, no obvious fading is observed. Pikul et al. have interdigitated 3D bicontinuous nanoporous LiMnO2 electrodes for LIBs.194 The assembled lithium ion microbatteries exhibit power densities of up to 7.4 mW cm 2 mm 1 and energy densities of up to 15 mW h cm 2 mm 1, which equal or exceed that of the best supercapacitors, and are 2000 times and 2 times higher than that of other reported microbatteries. Cheng et al.195 have prepared hierarchically porous LiMn2O4 nanorods as cathode materials for rechargeable LIBs. An initial

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3.2.2.3 Complex oxide-based hierarchically porous structures for lithium ion batteries. The development of cathode materials with high energy and power densities is crucial to meet the requirements of practical applications, such as electric vehicles (EV), hybrid electric vehicles (HEV), and stationary energy storage. Synthesis of complex oxides is considered as a promising solution. Great efforts have been made to find high-capacity cathode materials based on LiNi1 xMxO2, because of their very high practical capacities (220–230 mA h g 1) at high voltages (4.4–4.6 V). The problem is that at such high operating voltages, these materials can largely react with the electrolyte owing to the instability of tetravalent nickel in the charged state, leading to quick capacity fading. This usually enables these materials to operate reversibly at a potential range below 4 V, leading to low capacities of 150 mA h g 1. To improve the stability of these materials, the most accepted strategy is the introduction of Mn and/or Co into the transition-metal layer stabilizing the transition-metal oxide framework, because part of Mn and/or Co cannot change the valence state of the compounds during charge and discharge processes. Zhou et al. have fabricated hierarchically porous LiNi0.5Mn1.5O4 hollow microstructures.198 The resultant LiNi0.5Mn1.5O4 hollow structures show that the discharge capacity decreased only very slightly from 118 to 117, 115, 111.5, and 104 mA h g 1 as the current density increased from 1 to 2, 5, 10, and 20C (1C = 147 mA g 1), respectively. When the current density returns back to 5C, a discharge capacity of about 116 mA h g 1 is recovered. The hollow structure also demonstrates stable cycling performance. For example, after 200 cycles at 2C, 96.6% of the initial capacity can be retained. Sun et al. have synthesized hierarchically porous nickel-rich layered LiNi0.5Co0.10Mn0.15O2 microspheres (FCG) for LIBs.199 Fig. 28a shows the rate capability of the FCG material along with the inner composition (IC, LiNi0.86Co0.10Mn0.04O2) and outer composition (OC, LiNi0.70Co0.10Mn0.20O2) materials, both of which are synthesized by the conventional constant concentration approach. When discharged at the C/5 rate (B44 mA g 1), the IC, FCG and OC materials deliver a reversible capacity of 210.5, 197.4 and 188.7 mA h g 1, respectively, because the IC material has the highest nickel content and the OC material

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Fig. 28 (a) Comparison of rate capabilities of the FCG with the IC and OC materials (upper cutoff voltage of 4.3 V versus Li+/Li), (b) cycling performance of half-cells using the FCG, IC and OC materials cycled between 2.7 and 4.5 V versus Li+/Li using a constant current of C/5 (44 mA g 1). The electrolyte used is 1.2 LiPF6 in EC/EMC (3 : 7 by volume) with 1 wt% vinylene carbonate as an electrolyte additive. The cells are characterized between 3.0 and 4.2 V with a constant current of 1C. (Reproduced from ref. 199 with permission. Copyright Macmillan Publishers Limited, 2012).

has the lowest nickel content. However, when discharged at the 5C rate, the FCG material delivers the highest reversible capacity because of its highest lithium-ion diffusion coefficient. The electronic conductivity results showed that the IC material has the highest value of 1.67  10 4 S cm 1, followed by the FCG material, 3.10  10 5 S cm 1, and the OC material, 7.30  10 6 S cm 1. Therefore, they noticed that the high rate capability of the FCG material has no strong correlation with the electronic conductivity, but mostly originates from the special percolated aligned hierarchical nanorod network that shortens the diffusion pathway of lithium ions in the particle. Both the IC material (highest nickel content) and the FCG material deliver a higher capacity of 220.7 and 215.4 mA h g 1, respectively, whereas the OC material (lowest nickel content) shows a lower capacity of 202 mA h g 1. The coloumbic efficiency of FCG is higher (94.8%) compared with both the IC and OC electrodes (91%) owing to a well-developed aligned nanorod network in the FCG material that facilitates LiC diffusion, and thus high lithium utilization. The reversible capacity of the IC material decreases markedly with cycling (Fig. 28a). This rapid capacity fading is mainly caused by the direct exposure of a high content of a Ni(IV)-based compound to a non-aqueous electrolyte at a high potential; this exposure leads to the chemical decomposition of both the surface of the electrode material and the electrolyte. In contrast, the OC material has a higher manganese content and a lower oxidizing capability

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towards non-aqueous electrolytes. Therefore, this material has a lower reversible capacity, but a much better capacity retention. Fig. 28b also shows that the FCG material has combined advantages of a high capacity from the high nickel content in the bulk and a high electrochemical stability from the high manganese content on the surface. The authors assembled a pouch cell using the FCG material as the cathode and mesocarbon microbeads (MCMB, graphite) as the anode for LIBs. This cell is cycled between 3.0 and 4.2 V with a constant current of 1C (33 mA). The full-cell shows an outstanding capacity retention after 1000 cycles both at room and high temperatures. They have also fabricated pouch-type full-cells; the cells are cycled to 4.3, 4.4 and 4.5 V at 1C rate. In all cases, the cells exhibit excellent cycling performance. The capacity of the cells increases with cutoff voltage owing to the higher lithium utilization at high voltage. The cell cycles to 4.5 V showing very minor capacity fading at 55 1C possibly caused by a limited reactivity between the charged cathode and the electrolyte. In addition, Remith and Kalaiselvi have synthesized novel lithium rich layered Li1.2Mn0.6Ni0.1Co0.1O2 microspheres containing hierarchically arranged and interconnected nanostructures.200 This Li1.2Mn0.6Ni0.1Co0.1O2 cathode demonstrates an appreciable discharge capacity of 242 mA h g 1 at 50 mA g 1. Zhang et al. have prepared a lithium-rich cathode material Li1.2Ni0.13Mn0.54Co0.13O2 with hierarchically porous microrod structure.201 Electrochemical measurements show high capacities, good cyclability and outstanding rate capability. It delivers discharge capacities of 280.7, 254.8, 232.3, 225.6, 201.7 and 172.7 mA h g 1 at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C (1C = 250 mA g 1), respectively. Chen et al. have synthesized a hierarchically porous layered lithiumrich oxide, 0.5Li2MnO3–0.5LiMn1/3Ni1/3Co1/3O2 as a cathode for LIBs, delivering an initial discharge capacity of 262 mA h g 1 at 0.1C and 135 mA h g 1 at 4C, and possessing a capacity retention of 83% after 200 cycles at 4C.202 3.2.3 Polyanion compound-based hierarchically porous structures for lithium ion batteries. Polyanion compounds have been synthesized and used as cathode materials for LIBs. Among these polyanion materials, olivine LiFePO4 has attracted much attention due to its low cost, environmental compatibility, high theoretical specific capacity of 170 mA h g 1 and especially superior safety performance. Introducing hierarchically porous nanostructures, benefiting from large surface areas for reaction, interfacial transport, or dispersion of active sites at different length scales of pores and shortened diffusion paths or reduced diffusion effect, can largely improve their electrochemical properties. Doherty et al. have synthesized a hierarchically porous monolithic LiFePO4/C composite as a cathode material for LIBs.203 The results show the discharge capacities for LiFePO4 of 140 mA h g 1 at 0.1C and 100 mA h g 1 at fast discharge rates of 5C. Furthermore, they have produced a hierarchically porous LiFePO4 electrode material. The templated LiFePO4 samples give the best discharge capacities of 160 mA h g 1 at 0.1C and 115 mA h g 1 at 5C.204 Qin et al. have prepared a hierarchically porous and conductive LiFePO4 bulk electrode as a cathode material for LIBs.205 It exhibits an excellent specific discharge capacity of 156 mA h g 1 at a constant current of 0.448 mA.

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The discharge capacity approaches 8.22 mA h (volumetric capacity: 164 A h L 1), equalling that of conventional film electrodes of 1.35C. As the constant current increases to 0.895 mA, the cell still presents a specific capacity of 151 mA h g 1 and a capacity of 7.97 mA h (volumetric capacity: 159 A h L 1). Yang et al.206 have prepared a hierarchically porous LiFePO4/ N-doped carbon nanotube (N-CNT) composite. The LiFePO4/ N-CNT composite demonstrates a reversible discharge capacity of 138 mA h g 1, higher than 113 mA h g 1 for the LiFePO4/CNT composite at a current density of 17 mA g 1. Asfaw et al. have fabricated a novel 3D hierarchically porous LiFePO4/C composite.207 The composite electrode exhibits impressive cyclability and rate performance at different current densities. Footprint area capacities of 1.72 mA h cm 2 at 0.1 mA cm 2 (lowest rate B0.05C) and 1.1 mA h cm 2 at 6 mA cm 2 (highest rate B0.3C) are obtained in the range 2.8 to 4.0 V. Hasegawa et al.208 have synthesized hierarchically porous Li2FeSiO4/C monoliths with controlled macropores. The electrodes prepared from the Li2FeSiO4/C composites show that the smaller macropore size (equal to the thinner macropore skeletons) and the presence of micro- and mesopores in the macroporous skeletons (hierarchically porous structure) are beneficial for a better electrode in the case of Li2FeSiO4, with extremely low ionic and electrical conductivities. Fei et al.209 have incorporated B50 nm Li3V2(PO4)3/C core–shell nanospheres into a porous carbon framework. The Li3V2(PO4)3/C nanocomposite demonstrated an initial discharge capacity of 130 mA h g 1 at 0.1C, approaching its theoretical capacity (133 mA h g 1). At a high current rate of 10C, the nanocomposite delivers an impressive long cycle life and remarkable capacity retention (90% after 1200 cycles) with above 99% coulombic efficiency during the charge–discharge process. Tan et al. have synthesized a hierarchically porous Li2FeP2O7/C composite.210 The hierarchical pores in Li2FeP2O7/C result in high cycling stability and rate-capability. After 100 cycles at 1C, 93.8% of the initial capacity is retained. The discharge capacity is 62.1 mA h g 1 at the current density of 4C, being a promising cathode material for advanced rechargeable LIBs. 3.2.4 Silica or germanium-based hierarchically porous structures for lithium ion batteries. Among the anode materials for LIBs, silica (Si) can theoretically accept 4.4Li+ (Li4.4Si) per atom, resulting in a high capacity of 4200 mA h g 1, and therefore has recently received much attention. In addition, Si has also low working potential and abundant resources. However, due to its low electrical conductivity, severe volume expansion and fast capacity fading, its practical application has been significantly hindered. Generally, porous nanostructures can accommodate better the volume change during the lithium insertion/extraction process, shorten the lithium ion transfer path, and offer sufficient contact and enhance charge transfer at the interface between the electrode and the electrolyte. It is essential to design 3D hierarchically porous Si/C nano-hybrid architectures to improve the electrochemical performance of Si anode materials. Magasinski et al. have assembled Si particles into carbon black (C–Si) rigid spherical granules for LIBs.211 The specific reversible capacity of the sample with B50 wt% of Si reaches B1950 mA h g 1 at C/20. The specific capacity of the Si

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nanoparticles alone is estimated as B3670 mA h g 1, which is over 5 times higher than that of the theoretical capacity of graphite, 6 times that of high-performance graphitic anodes and nearly 18 times that of the annealed carbon black. The overall carbon contribution is estimated as B230 mA h g 1 (115 mA h/0.5 g). The volumetric capacity is determined to be 1270 mA h cm 3 at C/20, which is higher than B620 mA h cm 3 for graphitic anodes. The specific capacity of C–Si granules at 1C and 8C is 1590 and 870 mA h g 1, respectively, being B82 and 45% of that at C/20, which is much higher than graphites. The differential capacity curves displayed broad lithiation (Li insertion) peaks at 0.21 and 0.06 V, and a narrower delithiation (Li extraction) peak at 0.5 V. The C delithiation peaks commonly observed at 0.2 V are too small to be visible, because of the tiny contribution of carbon to the overall anode capacity. The SEM studies of the anode particles after cycling demonstrate exceptional robustness of the C–Si granules. Shen et al. have synthesized carbon-coated hierarchically porous silicon as an anode material for LIBs.212 It demonstrates reversible capacities of 1700, 1560, 1300, 1000 and 850 mA h g 1 at a current density of 0.1, 0.4, 0.8, 1.6 and 3.2C, respectively, indicating good rate capability. Jung et al. have developed unique carbon-coated 3D porous Si (c-SiRH) layers from the rice husk.213 Taking advantage of the interconnected hierarchically porous structure naturally existing in rice husks, the converted c-SiRH exhibits excellent electrochemical performance as a lithium battery anode (Fig. 29). After 50 cycles, c-SiRH, carbon-coated Si NPs (c-Si NPs), and Si nanoparticles (Si NPs) retain 100%, 36%, and 20% of their initial capacities, respectively (Fig. 29a). After 200 cycles, c-SiRH still shows outstanding 100% capacity retention of the original capacity (1554 mA h g 1) at 1C (1C = 2000 mA g 1). In addition, c-SiRH exhibits good rate capability at the rate from 0.2 A g 1 to 10 A g 1 (Fig. 29b). Even when the discharge rate is increased 50 times from 0.1C to 5C, the c-SiRH retains 95.3% of the original discharge capacity (1686 mA h g 1). The SEM image of the c-SiRH electrode taken after 50 cycles shows the well maintained interconnected porous structure. Recently, Li et al. have fabricated 3D interconnected porous silicon/carbon (Si/C) hybrid architectures for LIBs.214 The 3D porous Si/C hybrid exhibits an excellent reversible capacity of 1552 mA h g 1 and a coulombic efficiency of nearly 100% after 200 cycles at a current density of 200 mA g 1, and demonstrates a greatly enhanced rate capability with a reversible capacity of 1057 mA h g 1 even after 50 cycles at 2.0 A g 1. Ren et al. have prepared hierarchically porous silicon/carbon microspheres (GPSCMs) which when used as the anode materials for LIBs, the average charge capacity is found to be 589 mA h g 1 at a current density of 50 mA g 1, much higher than that of the commercial graphite microspheres (GMs).215 Germanium (Ge)-based materials have also been studied as promising anodes in LIBs to replace the graphite anode because of their high theoretical capacity (B1600 mA h g 1), excellent lithium conductivity, and high electrical conductivity. Xiao and Cao have synthesized a Ge nanoparticles/N-doped carbon (Ge/NC) monolith with a hierarchically porous structure for LIBs.216 The as-prepared Ge/NC hybrid exhibits high capacities

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Fig. 29 (a) Delithiation capacities of c-SiRH, c-Si NPs, and bare Si NPs over cycling. (b) Delithiation capacities at various discharge rates from 0.1C to 5C (1C = 2 A g 1). (Reproduced from ref. 213 with permission. Copyright National Academy of Sciences, 2013).

of up to 1240.3 mA h g 1 at 0.1 A g 1 and 813.4 mA h g 1 at 0.5 A g 1 after 90 cycles. It also demonstrates good cycling stability and excellent rate capability. Yu et al. have designed a 3D bicontinuous Au/a-Ge thin film electrode.217 It delivers a very high reversible capacity of 1066 mA h g 1 after 100 cycles at 0.2C (1C = 1600 mA g 1). In addition, this material preserves 47.5% of its 1C capacity when cycled at 60C with a capacity of 360 mA h g 1. Table 3 lists the detailed performance of the reviewed materials. From the results of various hierarchically porous structures for LIBs, it can be seen that the materials with hierarchically porous structures can enhance their electrochemical performance. Developing various materials with HPSs is thus very important for their practical LIB application. Particularly, as the cathode materials determine the capacity of LIBs, developing the cathode materials with HPSs is more urgent for LIBs. Though high temperature calcination is often required for the preparation of cathode materials, development of novel strategies is a bidding need. It is believed that through designing and synthesizing various materials with HPSs for LIBs and understanding the reactions between the electrode and electrolyte, the capacity, cycle life and rate performance of these materials can be largely improved. 3.3 Hierarchically porous structures for lithium sulfur batteries At present, Li-ion batteries have been widely used in portable electronic devices and will play a significant role in large-scale

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energy storage. However, the theoretical gravimetric capacity (B370 mA h g 1) is too low to overcome the problems of limited range in electric vehicles or hybrid electric vehicles, and their cost is too high to sustain the commercial viability of electrified transportation. Being one of the most abundant elements on earth, sulfur has received much attention due to its capacity to accept up to two electrons per atom at B2.1 V vs. Li/Li+ with a high theoretical capacity of 1675 mA h g 1 and a theoretical energy density of B2600 W h kg 1. Unfortunately, sulfur undergoes a series of compositional and structural changes during cycling. This involves low electrical conductivity, soluble polysulfides, insoluble sulfides and large volume changes, leading to its poor utilization, low coulombic efficiency of the sulfur cathode, and fast capacity fading. Therefore, trapping the polysulfides, enhancing the electrical conductivity and accommodating the volume change of the sulfur electrode in a conducting matrix are essential to improve the electrochemical performance of the lithium–sulfur (Li–S) batteries. It has been accepted that hierarchically structured porous matrices are appropriate for improving both rate and capacity retention performance of the sulfur electrode. In the fabrication of Li–S batteries, the hierarchically porous carbon materials are the most widely used matrices because of their large surface area and hierarchically porous structure. The soluble polysulfide intermediates can thus be trapped within the hierarchically porous cathode and the volume expansion can effectively be alleviated. Moreover, the electron transport properties of the carbon materials can provide an electron conducting network and promote the utilization efficiency of sulfur in the cathode. Wang et al. have incorporated sulfur in hierarchically porestructured carbon (HPC) pillars for Li–S batteries.218 The composites demonstrate enhanced electrochemical performance due to the unique porous structure of the HPC matrix and its strong interaction with sulfur. Here, we would like to give a detailed explanation of their sample, HPC-50S (50 wt% sulfur), for Li–S battery analysis. Generally, two discharge plateaus at B2.25 V and 1.98 V are observed in the initial cycle corresponding to the two-step reaction of sulfur with lithium. Only one plateau is observed in the charging process at B2.34 V. In the second cycle, the discharge plateaus rises a little bit while the charge plateau slightly becomes lower, well consistent with the CV results, implying that the dynamic performance of the HPC/S composite is improved after the initial cycle. In the CV plots, the reduction peaks at 2.28 V and 2.00 V are usually ascribed to the formation of lithium polysulfides (Li2Sn, 4 o n o 8) and the reduction of the higher order (n 4 4) lithium polysulfides to lithium sulfides (Li2S2 and Li2S), respectively. In this process, some polysulfides dissolve into the electrolyte, diffused to the lithium electrode and lead to a shuttle effect, responsible for the low coulombic efficiency and capacity degradation. The oxidation peak at 2.41 V is associated with the lithium extraction and conversion to lithium polysulfides (Li2Sn, 2 o n o 8). It is noteworthy that a weak broad peak is observed at 2.82 V, due to the formation of S8. This means that the generated lithium polysulfides can be well confined in the micropores of HPC and reversibly converted to the higher polysulfides and even S8.

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Review Article Table 3

The electrochemical performance of the various hierarchically porous carbon and/or graphene based materials for LIBs

Samples

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BET surface areas (m2 g 1)

Graphitic HPC spheres

328

Hollow HPC microspheres

192

Sulfur-doped HPC 3D HPS graphene HPC/PDMCT–PANI 3D crystalline carbon frameworks 3D nitrogen–sulfur codoped graphene-like microspheres

10 917 611.1 687 861

Core/shell graphitic carbon materials HP–Mo2C–C CNT-carbon nanohorns (CNHs) 3D meso–macroporous TiO2

731 200.6 1025 158 139

3DOMM TiO2 Hollow TiO2 microspheres HNC-TiO2-HMSs

97.8 43.8

Yolk–shell TiO2 microspheres

168

HPS anatase TiO2 microspheres HPS rutile TiO2 microspheres Anatase/rutile TiO2 microspheres Anatase/rutile TiO2 nanowire bundles

244 131.7 54 117.8

Anatase TiO2/C composites

170

Anatase TiO2 microspheres Nitrogen-doped anatase TiO2 HPS TiO2/graphitic carbon microspheres HPS TiO2-B nanoflowers TiO2-B/anatase TiO2 on graphene sheets TiO2-B nanowires microspheres

43.9 41 176.94 214.6 277.1 295

Fe2O3 nanoparticles in 3D HPC

82.7

HPS Fe2O3 microspheres HPS Fe2O3 microspheres HPS Fe2O3 microspheres

15 46.85 56.1

HPS Fe3O4 spheres HPS flower-like Fe3O4/C composite HPS Fe3O4/C composite HPS CoO film 3D CoO/C composite

— 61 238.2 56 24.4

3D Co3O4/C composite HPS HPS HPS HPS HPS HPS HPS HPS HPS HPS

19.5

polycrystalline Co3O4 Co3O4 nanorods Co3O4 microspheres Co3O4 microspheres Co3O4/C composite flower-like Co3O4/CNF composite NiO microspheres tremella-like NiO nanoflakes NiO array film NiO/C microspheres

HPS CNF/NiO core–shell nanocables HPS MnOx (MnO–Mn3O4) microspheres Fe-doped HPS MnxOy materials HPS urchin-like Mn3O4/C microspheres (N2)

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41.07 63.01 53.84 98 332 30.27 61.8 171.58

— 68.2 — 70

Current rate (mA g 1)

Reversible capacity (mA h g 1)

30 000 50 100 20 000 50 1000 150 100 100 500 300 100 10 0.2C 20C 1C 4C 5C 1C 10C 1C 10C 1C 1C 30C 1C 50C 1C 30C 1C 200 0.5C 10C 6000 1C 10C 2000 4000 8000 100 100 200 5000 100 150 100 0.2C 500 1000 500 1000 35 35 35 180 100 200 500 0.1C 1C 100 500 1000 3000 200 500 1500 100 1500 100

290 700 805 210 1781 580 310 352 1117 725 420 1196.8 352 215 113 144 106 131 216 112 225 113 157.3 160.4 103 174 96 132 96 212.3 171 189 181 155 270 226 928.22 804.59 733.35 705 634 901.3 416.4 511 1030 789 780 804 420 697 320 895.4 709.8 538.2 1600 1079 911 800.2 830 518 698 608 454 352 825 757 400 915 480 376

Cycling numbers 50 50 100 10 300 20 50 80 80 10 100 5 10 10 200 200 100 100 100 100 100 200 100 100 100 50 100 10 100 10 50 100 100 10 1000 1000 500 500 430 20 10 1000 50 150 100 50 60 10 60 10 10 10 10 50 50 50 100 30 50 10 10 10 10 50 100 900 50 200 50

Voltage range

Ref.

0–3

134

0.01–3

135

0.01–3 0.005–3 1.8–3.8 0.005–3 0.01–3

136 137 138 139 140

0.001–3 0.01–3 1.0–3.0

141 142 143 146

1.0–3.0

147

1.0–3.0 1.0–3.0

148 149

1.0–3.0

150

1.0–3.0 1.0–3.0 1.0–3.0 1.0–3.0

151 152 153 154

1.0–3.0

155

1.0–3.0 1.0–3.0 1.0–3.0 1.0–3.0 1.0–3.0 1.0–3.0

156 157 158 159 160 161

0.005–3

162

0.01–3 0.02–3

163 164 165

0.005–3 0.01–3 0.01–3 0.05–3 0.01–3

166 167 168 170 171

0.01–3

171

0–3 0.01–3 0.01–3 0.01–3 0.01–3 0–3 0.01–3 0.01–3 0.01–3 0.05–3

172 173 173 174 175 176 177 178 179 180

0–3 0.05–3 0.01–3 0.01–3.0

181 182 183 184

0.01–1.5

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(continued)

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3D CNF/Mn3O4 coaxial nanocables Nanoporous SnO2 hollow microspheres HPS carbon coated SnO2/C composite Bioinspired HPS SnO2/C composite 3D porous V2O5 hierarchical microplates HPS Li4Ti5O12 microspheres HPS Li4Ti5O12 materials HPS NiCo2O4/NiO hollow dodecahedron 3D HPS ZnCoO2 materials Nanorods constructed HPS V2O5 spheres HPS LiV3O8 spheres HPS LiMn2O4 nanorods HPS LiMn2O4 microcubes HPS donut-shaped LiMn2O4 HPS LiNi0.5Mn1.5O4 hollow microstructures HPS nickel-riched LiNi0.5Co0.1Mn0.15O2 microspheres HPS lithium-rich Li1.2Mn0.6Ni0.1Co0.1O2 microspheres HPS lithium-rich Li1.2Ni0.13Mn0.54Co0.13O2 microrods HPS lithium-rich larered 0.5Li2MnO3–0.5LiMn1/3Ni1/3Co1/3O2 HPS monolithic LiFePO4/C composite HPS LiFePO4 materials HPS LiFePO4 bulk electrode HPS LiFePO4/N-doped carbon nanotubes composite 3D HPS LiFePO4/C composite HPS Li2SiO4/C monoliths Li3V2(PO4)3/C core–shell nanospheres HPS Li2FeP2O7/C composite Porous Si–C composite particles Carbon coated HPS silicon Carbon-coated 3D porous Si 3D porous silicon/carbon hybrid architectures HPS silicon/carbon microspheres HPS Ge nanoparticles/N-doped carbon monolith 3D bicontinuous Au/Ge thin film

BET surface areas (m2 g 1) 280 — 214 36 32 57.5 30 126.3 356 — — 8.6 78.4 — — — 13.8 3.4 8.74 150 90–100 16 11.6 — 38 69 49.3 24 76 47.3 126.7 80–90 268.1 —

The initial discharge capacities of HPC-39S (39 wt% sulfur), HPC-50S (50 wt% sulfur) and HPC-58S (58 wt% sulfur) are 1774, 1466 and 1224 mA h g 1 at 0.1C (1C = 1675 mA h g 1) respectively (Fig. 30a). The results also indicate that HPC/S with the lowest sulfur content displays the highest capacity retention and sulfur utilization, consistent with the literature. The capacity of HPC-58S decreases to 456 mA h g 1 after 40 cycles while those of HPC-39S and HPC-50S remain at 690 mA h g 1 and 524 mA h g 1 after 50 cycles, respectively. The rate performance of HPC-39S and HPC-50S composites decreases with increasing current density. A discharge capacity of 267 mA h g 1 is obtained at 5C. In addition, the discharge capacity is recovered to 497 mA h g 1 at 0.1C (Fig. 30b). In addition to this, Yu et al. have synthesized hierarchically porous carbon monoliths via a hydrothermal nanocasting method using hierarchically meso–macroporous silica monolith templates.219 A large amount of sulfur has been inserted within the carbonaceous scaffolds reaching an initial discharge capacity of 1305 mA h g 1 at a current density of 0.1C. After 25 cycles, a discharge capacity of 469 mA h g 1 still remained. Xu et al. have reported the encapsulation of sulfur into a 3D hierarchically porous carbon nanoplate (HPCN) derived from one-step pyrolysis of metal–organic frameworks (MOF-5).220

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Current rate (mA g 1)

Reversible capacity (mA h g 1)

100 1C 1000 50 2000 2C 5C 200 100 70 100 10C 1C 10C 2C 0.2C 50 0.1C 4C 0.1C 0.1C

760 725 400 572 110 147.4 160 1535 527 201 262 94.5 100 82 121 215 180 245.7 135 140 160

0.1C 0.2 mA cm 5 0.1C 10C 1C C/20 400 2000 200 50 100 500 0.2C

2

138 1.65 mA h cm — 124 95 1950 920 1554 1552 560 1240.3 813.4 1066

Cycling numbers 50 50 100 100 100 200 500 100 50 50 50 500 200 500 200 100 35 50 200 10 10

2

100 60 — 100 1200 100 100 110 200 200 50 90 90 100

Voltage range

Ref.

0.01–3 0.01–2 0–2 0.05–2 2.4–4 1–2.5 1–2 0.01–3 0.01–3 1.8–4 2–4 3–4.4 3–4.3 3.2–4.3 3.5–5 2.7–4.5 2–5 2–4.8 2–4.8 2.2–4 2.2–4 2–4.2 2.5–4.2 2.8–4 1.5–4.5 3–4.3

185 186 187 188 189 190 191 192 193 166 166 195 196 197 198 199 200 201 202 203 204 204 206 207 208 209

2.4–4.3 0–1.1 0.005–1.5 0.01–1 0.01–2 0.005–2 0.01–1.5

210 211 212 213 214 215 216

0.005–1.2

217

When evaluated as a cathode for Li–S batteries, the HPCN–S composite shows high specific capacity and excellent cycling performance: an initial discharge capacity of 1177 mA h g 1 at 0.1C and a discharge capacity of 730 mA h g 1 after 50 cycles at 0.5C with the coulombic efficiency of 97%. They have also encapsulated sulfur into hierarchically porous carbon (HPC) derived from the soluble starch with a template of needle-like nanosized Mg(OH)2, resulting in a weight percent of sulfur of up to 84 wt% in S/HPC.221 When evaluated as the cathode for Li–S batteries, the S/HPC composite exhibits a high initial discharge capacity of 1249 mA h g 1 and retained a discharge capacity of 562 mA h g 1 with a coulombic efficiency as high as 94% after 100 cycles at 1C. Even at 2C, S/HPC still delivers a discharge capacity of 822 mA h g 1 and retains 419 mA h g 1 after 100 cycles. Chen’s group has prepared a carbon–sulfur composite via impregnating sulfur in porous hollow carbon spheres (PHCSs).222 The carbon–sulfur composite with 50.2 wt% sulfur exhibits an initial discharge capacity of 1450 mA h g 1 and a reversible discharge capacity of 1357 mA h g 1 after 50 cycles at a 0.05C rate. At a higher rate of 0.5C, the capacity is B800 mA h g 1 after 30 cycles. Zhang et al. have fabricated novel hierarchically porous carbon materials by carbonization of silk cocoon with KOH activation.223

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Fig. 30 (a) The cycling performance, and (b) the rate performance of the HPC/S composites. The inset of (a) is the potential profile of the as-prepared HPC in the first two cycles. (Reproduced from ref. 218 with permission. Copyright Elsevier Ltd, 2013).

The prepared carbon–sulfur composite exhibits a high initial discharge capacity of 1443 mA h g 1, and 804 mA h g 1 is retained at a rate of 0.5C with a coulombic efficiency at B92% after 80 cycles. Yang et al. have fabricated nitrogendoped hierarchically porous coralloid carbon/sulfur composites (N-HPCC/S) for Li–S batteries.224 With a sulfur content (58 wt%) in the total electrode weight, the N-HPCC/S cathode demonstrates a high initial discharge capacity of 1626.8 mA h g 1 and remains high up to 1086.3 mA h g 1 after 50 cycles at 100 mA g 1, about 1.86 times that of activated carbon. Particularly, the reversible discharge capacity is still 607.2 mA h g 1 after 200 cycles at 800 mA g 1. You et al. have synthesized a hierarchical micro–mesoporous activated graphene (a-MEGO) with a high surface area for Li–S batteries.225 Due to its extremely high surface area and pore volume, a-MEGO achieves a sulfur loading as high as 75 wt%, leading to a high overall energy density. Meanwhile, the hierarchically micro–mesoporous structure provides an effective platform for confining polysulfides. As a result, the composite exhibits a high specific capacity of 789 mA h g 1 (based on composite) at 0.15C with a coulombic efficiency of B100%. The capacity retention of a-MEGO/S can reach 76% after 200 cycles at 1C. Bao et al. have synthesized a hierarchical architecture of multiwalled carbon nanotubes@mesoporous carbon (MWCNT@ Meso-C) for Li–S batteries.226 The initial discharge capacity of the MWCNT@Meso-C/S sulfur cathode reaches as high as 1343 mA h g 1 at 0.5C. After 50 cycles, the discharge capacity still remained at 540 mA h g 1. CV and EIS tests indicate the low resistance and good kinetic characteristics of the

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MWCNT@Meso-C/S sulfur cathode. They have also fabricated a hierarchical architecture of graphene oxide@mesoporous carbon (GO@Meso-C) using graphene oxide@metal–organic framework hybrid material (GO@MOF-5) as both the template and precursor.227 The initial and 100th cycle discharge capacity of the hierarchical GO@Meso-C/S sulfur cathode is as high as 1122 mA h g 1 and 820 mA h g 1 at 0.2C respectively. Guo’s group has designed a 3D hierarchically ordered mesoporous carbon/sulfur composite slice coated with a thin TiO2 layer as a cathode for Li–S batteries.228 The hierarchical architecture provides a 3D conductive network for electron transfer, open channels for ion diffusion and strong confinement of soluble polysulfides. In addition, the TiO2 coating layer further effectively prevents the dissolution of polysulfides and improves the strength of the entire electrode, thereby enhancing the electrochemical performance. As a result, after TiO2 coating, the electrode exhibits excellent cycling performance, with a discharge capacity of 608 mA h g 1 at 0.2C and 500 mA h g 1 at 1C after 120 cycles, respectively. They have also prepared an activated hierarchically micro–macroporous carbon from cotton.229 This hierarchically micro–macroporous carbon structure could load 68 wt% sulfur. The prepared hierarchically porous carbon– sulfur composite electrode shows excellent cycling stability and good performance: a reversible capacity of 760 mA h g 1 after 200 cycles at 0.2C. Klose et al. have synthesized hierarchically porous hollow carbon nano-onions for a Li–S battery, which acts as an effective host for the active material, yielding capacities of up to 550 mA h g 1 after 40 cycles at 0.2C.230 Meng and Gao have prepared porous carbon with micropores and mesopores by direct carbonization of MOFs and they are loaded with sulfur for Li–S batteries.231 The sample with 46.3 wt% sulfur exhibits an initial discharge capacity of 1272 mA h g 1 at 0.01C and 934 mA h g 1 at 0.1C with good cycle performance. Wang et al. have prepared a hierarchically porous carbon material as the conductive matrix in the sulfur cathode for rechargeable lithium batteries by an in situ two-step activation method using sucrose as the carbon source, CaCO3 as the template, and (CH3COO)2CuH2O(Cu(Ac)2) as an additive.232 The result shows that Cu(Ac)2 addition is very helpful for sulfur dispersion in the active porous carbon, which exhibits an initial discharge capacity of 1397 mA h g 1 (refer to the weight of S) at 0.1C and a capacity retention of about 60% over 100 cycles. Xia et al. have also prepared S-doped 3D porous carbon architecture from a metal–organic gel (MOG) template, which demonstrates excellent electrochemical performance as a cathode material for a Li–S battery.233 Very recently, Wu et al. have developed a 3D free-standing hierarchical graphene-based porous carbon (GPC) film capsulated sulfur for flexible Li–S batteries (Fig. 31a and b).234 The hierarchically porous carbon with an encapsulation of 41 wt% sulfur demonstrates a reversible capacity of 1017 mA h g 1, 865 mA h g 1, and 726 mA h g 1 at 0.2C, 0.5C and 1C respectively and B10.5% capacity loss after 300 cycles at 1C. In addition, a flexible Li–S battery is assembled by using a freestanding GPC film with sulfur (2 cm  3.5 cm) as a cathode,

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Fig. 31 (a) The top-view and (b) cross-sectional view of the graphenebased porous (GPC)-sulfur films. (c) The second charge–discharge profiles for the GPC-sulfur cathode films at the bent and flat states at 0.5C. (d) The cycle performance for the GPC-sulfur cathode films at 0.5C and 1C and inset shows that a bent cell is encapsulated in the glass bottle filled with argon. (Reproduced from ref. 234 with permission. Copyright Royal Society Chemistry, 2015).

lithium as an anode, glass fiber as the separator, and hollow Al foil and steel foil as current collectors. The film cathode in the bent state displays a similar charge–discharge profile as that in the flat state at 0.5C, and the discharge capacity reaches 815 mA h g 1. After 20 cycles at 0.5C and 1C, the film cathode still demonstrates a high discharge capacity of 679 mA h g 1, indicating good electrochemical performance in the bent state (Fig. 31c and d). After connecting with an LED, the battery lights the LED under the state of bending and even folding (Fig. 31d), suggesting its potential application in flexible electronics. In this part, we have surveyed recent studies on HPSs for Li–S batteries. Significant improvements have been accessed but challenges still remain. Designing novel nanocomposite materials with hierarchy have already demonstrated a significant impact in the field. More attention should be paid to overcoming the high solubility issue of the intermediate lithium polysulfides in organic electrolytes. Developing a selective coating layer on the lithium metal anode or a solid-state electrolyte allowing only Li+ transport should be an effective strategy to solve this problem. In addition, the widely used characterization tools and methodologies for LIBs, such as in situ XRD, in situ Raman and in situ IR, should help us to understand the fundamental mechanism of the chemical and physical processes. 3.4

Hierarchically porous structures for lithium air batteries

The increasing demand for ultra-high energy density storage systems makes Li–O2 batteries a topic of much interest recently due to their highest specific capacity (B11 000 mA h g 1). This allows the Li–O2 battery to deliver substantially high energy density (B3500 W h kg 1), very close to the currently used gasoline fuel. In Li–O2 batteries, the desired reaction on the air electrode can be simplified as 2Li+ + O2 - Li2O2. However, the unavoidable side reactions in Li–O2 batteries can produce many unwanted

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by-products e.g. Li2O, ROCO2Li, Li2CO3, Li2CO3/LiCOO–R/ LiOH, which cover the electrodes resulting in very low capacity, rate performance and cyclability.235 In fact, even the desired reaction also creates many related problems mentioned above. Insoluble Li2O2 and Li2O are often formed at the cathode upon discharge via Li+ formation and the reduction of molecular O2, which gradually block the electrolyte and oxygen pathways and eventually lead to low electrochemical performance. Therefore, design and synthesis of highly effective cathodes for high performance Li–O2 batteries is an urgent need. Hierarchically porous structures become the first choice for developing desired cathodes with high performance due to their special structures that facilitate electrolyte permeation and oxygen diffusion and provide sites to accommodate Li2O2 deposition, etc. Li et al. have developed a hierarchical nitrogendoped micron-sized honeycomb-like porous carbon by using nano-CaCO3 particles as a hard template and sucrose as the carbon source, followed by thermal annealing at 800 1C in ammonia.236 When used as a cathode for Li–O2 batteries, the material demonstrated enhanced activity for oxygen reduction reaction and oxygen transfer ability. As a result, a superior discharge capacity of up to 12 600 mA h g 1 is achieved at 0.1 mA cm 2 (140 mA g 1), about 4 times higher than that of commercial Ketjenblack carbon. Remarkably, improved cycling stability is also obtained due to the special hierarchically porous structure and enhanced conductivity by nitrogen doping. Song et al. have prepared the hierarchically porous carbons (HPCs) by sol–gel self-assembly technology at different surfactant concentrations.237 Using the HPCs as the cathode electrode, the discharge capacity is increased with the specific surface area of HPCs. Particularly, the HPC obtained at a CTAB concentration of 0.27 mol L 1 exhibits good capacity retention through controlling discharge depth to 800 mA h g 1 and the highest discharge capacity of 2050 mA h g 1 at 0.1 mA cm 2. Lim et al. have constructed hierarchically porous electrodes composed of well-aligned CNTs fibrils (woven CNT) for Li–O2 batteries, demonstrating significantly enhanced cycle stability and rate capability.238 Even at 4000 and 5000 mA g 1, the woven CNT air electrode can deliver capacities of B2100 and 1700 mA h g 1 after 20 cycles, respectively. The controlled porous framework of these woven CNT electrodes enables effective formation/ decomposition of Li2O2 by providing facile accessibility of oxygen to the inner side of the air electrode and preventing the clogging of pores by discharge products, even during the deep discharge according to the observation of the discharge products. Xiao et al. have designed and fabricated 3D hierarchically interconnected macro–mesoporous air electrodes constructed using functionalized graphene sheets (FGS) by a colloidal micro-emulsion approach (Fig. 32a and b).239 The interconnected macro–mesopores ensured oxygen diffusion into the interior carbon and provided tri-phase regions for oxygen reduction reaction and Li–O2 discharge product storage. In addition, the FGS contains electroactive groups such as carboxyl, epoxy, hydroxyl, and defects, which can accelerate oxygen reduction under Li+-containing conditions. At 0.1 mA cm 2, the discharge capacity of FGS with a C/O ratio of 14 (C/O = 14)

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of about 600 mA h g 1 with high recharge efficiency (B90%) and good capacity retention is obtained owing to the robust a-MnO2 structure with good activity. Wang et al. have synthesized a freestanding, hierarchically porous carbon (FHPC) derived from graphene oxide (GO) gel in nickel foam without any additional binder.241 The GO with appropriate acidity via its intrinsic COOH groups is very important to form the framework of a 3D gel. When used as a cathode for a Li–O2 battery, the capacity reaches 11 060 mA h g 1 at 0.2 mA cm 2 (280 mA g 1). In particular, a high capacity of 2020 mA h g 1 is obtained even at a current density of 2 mA cm 2. Very recently, Han et al. have prepared a MnO2/hierarchically porous carbon (HPC) nanocomposite via in situ redox deposition and growth of MnO2 on HPC.242 Owing to the mild reaction conditions, MnO2 is uniformly distributed on the surface of HPC, without destroying the hierarchically porous nanostructure. When the MnO2/HPC nanocomposite was employed as the cathode for non-aqueous Li–O2 batteries, the MnO2/HPC nanocomposite exhibits excellent performance compared with HPC and Super-P (Fig. 33): low charge overpotential, good rate

Fig. 32 (a and b) SEM images of the FGS sample. Electrochemical performances of Liair batteries using FGS as the air electrode. (c) The discharge curve of a Li–O2 cell using FGS (C/O = 14) as the air electrode (PO2 = 2 atm). (d) The same Li–O2 cell as in (a) but tested in the pure argon atmosphere. (e) Discharge curve of a pouch-type Li–air cell made from FGS (C/O = 14) operated in ambient environment (PO2 = 0.21 atm, relative humidity = 20%). The inset shows the prototype of the pouch cell. (f) The discharge curve of a Li–O2 cell using FGS (C/O = 100) as the air electrode (PO2 = 2 atm). (Reproduced from ref. 239 with permission. Copyright American Chemical Society, 2011).

reached 15 000 mA h g 1 carbon with a plateau at around 2.7 V (Fig. 32c). This corresponds to a specific energy of 39 714 W h kg 1 carbon, giving an average voltage of 2.65 V. To exclude possible electrochemical contributions from the decomposition of organic functional groups on the graphene sheets, the electrode produced with the FGS paper is also discharged to 2.0 V in pure argon. A negligible discharge capacity less than 40 mA h g 1 is obtained (Fig. 32d). They have used a pouch-type cell to evaluate its potential for practical application operated in an ambient environment with an oxygen partial pressure of 0.21 atm and 20% relative humidity. The specific capacity based on the carbon weight exceeds 5000 mA h g 1 at 0.1 mA cm 2 (Fig. 32e). The discharge curve of FGS with a C/O ratio of 100 (C/O = 100) is also measured for Li–O2 batteries. The capacity delivered from this FGS with reduced defect/functional group concentrations is about one-half of that of FGS (C/O = 14) (Fig. 32f). This finding is consistent with the observed Li2O2 particle densities after discharge, indicating that the defects in the cathode material is helpful for Li2O2 deposition on the surface. In addition, Zeng et al. have synthesized highly crystalline a-MnO2 with a hierarchical bimodal porous structure by facile reduction of KMnO4 in acidic solution.240 The a-MnO2 electrodes display an initial discharge capacity of 900 mA h g 1 at a current density of 0.1 mA cm 2. After prolonged cycling, a constant value

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Fig. 33 (a) SEM of the synthesized MnO2/HPC composite materials. (b) First discharge–charge curves of Li–O2 batteries with Super P, HPC and MnO2/HPC at a current density of 100 mA g 1. (c) Cyclability and terminal discharge voltages of three electrodes at a current density of 350 mA g 1. The current density and specific capacity are calculated on the basis of the total catalyst weight. (Reproduced from ref. 242 with permission. Copyright Tsinghua University Press and Springer, 2015).

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capacity and long cycle stability up to 300 cycles with a controlled capacity of 1000 mA h g 1 at 100 mA g 1. The excellent electrode performance is attributed to the combination of the multi-scale porous network of a shell-connected carbon support and highly dispersed MnO2 nanostructure facilitating the transportation of ions, oxygen and electrons. Although impressive improvements have been obtained recently, there are plenty of challenges that need to be resolved in a Li–O2 battery in comparison to already widely used LIBs: the air electrode catalyst, the instability of electrolytes, lithium dendrite growth on the anode, by-products, high overpotential, etc. The hierarchically porous structures have many more advantages compared to other solid materials and can further contribute to the improvement of Li–O2 battery performance by the optimization of pore size, pore shape and proportion of macropores, mesopores and micropores, and by controlling the morphologies of the structures. With efforts devoted to the fundamental studies on electrolytes, electrode design and the over cell design, high performance Li–O2 batteries are expected in practical application soon. 3.5

Hierarchically porous structures for sodium ion batteries

Lithium-ion batteries have been widely accepted for portable electronic devices due to their high energy density. This makes them the prime candidates to power next generation of electric vehicles and plug-in hybrid electric vehicles. The demand for Li will be significantly increased in the future. However, the resources of Li in the earth are limited. Pursuing other sustainable energy storage systems in advance becomes urgent. Sodium-ion batteries (NIBs), naturally, are one of the most important potential candidates for such a replacement. Sodium (Na) and Li belong to the same main cluster element in the periodic table and they share similar chemical properties in many aspects. The fundamental principles of NIBs and LIBs are identical. Although NIBs have lower energy densities and operating voltages than those of LIBs, there are several reasons to investigate NIBs. On one hand, as battery applications extend to large-scale storage such as electric buses or stationary electrical grid storage, high energy density is less critical. On the other hand, the low cost and abundance of Na in earth will become an advantage when a large amount of alkali is required for large-scale applications, though at this point, this situation does not exist yet. However, we have to gain the fundamental information on Na-based systems in advance. Similar to LIBs, the electrochemical performance of NIBs, such as specific capacity and operation voltage, are mainly determined by the electrode materials. Therefore, seeking good electrode materials is still a major challenge in advancing NIB technology. Several good reviews on the electrode materials for NIBs have already been published.243–245 In this review, we will only briefly highlight the most recent developments in the electrode materials with hierarchically porous structures for NIBs to show the significant benefit of hierarchically porous structures for NIBs. Ding et al. have synthesized hierarchically porous carbon (HPC) from a peat moss biomass precursor, resulting in sheet-like

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walls in the postpyrolyzed carbons that are as thin as 60 nm.246 The as-prepared HPC has a highly ordered pseudographic structure with a highly dilated graphene interlayer spacing that tremendously improves the high rate performance through facile electrolyte access and further reduces the Na ion diffusion distance. In order to improve the high rate performance, mild air activation at 300 1C is employed to create sufficient porosity to further reduce the Na ion diffusion distance. The optimized structure (carbonization at 1100 1C) exhibits a stable cycling capacity of 298 mA h g 1 after 10 cycles at 50 mA g 1, with B150 mA h g 1 of charge accumulating between 0.1 and 0.001 V with negligible voltage hysteresis in that region and nearly 100% cycling coulombic efficiency. In addition, a superb cycling capacity of 255 mA h g 1 after 210 cycles at 100 mA g 1 is achieved and a high rate capacity of 203 mA h g 1 at 500 mA g 1 is reached. Yan et al. have proposed a strategy to prepare a sandwich-like hierarchically porous carbon/graphene (G@HPC) composite by a facile ionothermal process, which can overcome the disadvantages of non-graphitized carbon materials.247 Such a hierarchical structure on both sides of graphene facilitates Na+ diffusion within the bulk electrode material, while the graphene sandwiched by porous carbon guarantees fast electron transport. The nanocomposite demonstrates outstanding electrochemical performances in terms of high specific capacity, long cycle life and high rate capability. G@HPC shows a specific capacity of 670 mA h g 1 with a sloping region at 1.50 V to 0.01 V. This indicates Na+ insertion into parallel layers. Moreover, G@HPC exhibits high reversible capacity and stable cycling performance. A high capacity of 400 mA h g 1 is achieved after 100 cycles, with a high coulombic efficiency approaching 100%. A galvanostatic discharge–charge test involving 1000 cycles is further carried out at a high current density of 1 A g 1 (Fig. 34c). After activation at 100 mA g 1 in the initial cycle, the specific capacity stabilizes at 250 mA h g 1, and coulombic efficiency approaches 100% after several cycles. Remarkably, a specific capacity of 250 mA h g 1 remains without

Fig. 34 (a) SEM images, (b) TEM images of the G@HPC composite, (c) cycling performance under 1 A g 1. (Reproduced from ref. 247 with permission. Copyright Wiley-VCH, 2014).

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any decay after 1000 cycles. Wenzel et al. have synthesized a hierarchically porous carbon with defined interconnected macropores (2–4 mm) and mesopores (5 nm).248 This hierarchical porosity enables fast ion transport and short diffusion lengths. The as-prepared HPC shows a stable capacity upon cycling and reached a coulombic efficiency of 95.2% after 20 cycles and 97.9% after 40 cycles at 74.4 mA g 1. From the 40th cycle onwards the capacity fades, however, it still remains at 80 mA h g 1 up to 125 cycles. At higher currents (2C and 5C) the capacity still exceeds 100 mA h g 1, while reaching high coulombic efficiencies (99.8% after 25 cycles). Su et al. have prepared hierarchical hollow V2O5 nanospheres with predominantly exposed {110} facets.249 When applied as the cathode material for Na-ion batteries, the V2O5 hollow nanospheres exhibit a high initial capacity of 229.68 mA h g 1 at 20 mA g 1. After 100 cycles, a discharge capacity as high as 141 mA h g 1 is obtained. It also demonstrates superior high rate capacity and cycling performance. Fig. 35c displays the high reversibility of the V2O5 hollow nanospheres over 100 cycles at high current densities. A reversible capacity of 109.5 mA h g 1 is retained after 100 cycles at 160 mA g 1. As the current rate increases to 320 mA g 1, the discharge capacity of 93.1 mA h g 1 is retained after 100 cycles. Fig. 35d shows the rate capability of the V2O5 hollow nanosphere electrode at different current rates, indicating the excellent capacity retention at various current densities. A capacity of 149 mA h g 1 is recovered when current density reversed back to 40 mA g 1. The outstanding electrochemical performance can be ascribed to the unique hierarchically porous hollow nanospherical architecture, which can accommodate large strain without pulverization, and the exposed {110} facets provide ample space for Na+ ion insertion and extraction. Liang et al. have synthesized hierarchically porous Li4Ti5O12 microspheres composed of several hundred nanometer-sized primary particles.250 The Na-storage behavior of the as-prepared

Fig. 35 (a) SEM images of V2O5 nanospheres. (b) Typical TEM image of a single V2O5 nanosphere. (c) Discharge capacities versus cycle number at current densities of 40, 80, 160, 320, and 640 mA g 1. (d) Rate performance of V2O5 hollow nanospheres at various current densities. (Reproduced from ref. 249 with permission. Copyright Royal Society Chemistry, 2014).

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Li4Ti5O12 is reported for the first time. The average Na-storage voltage is about 1.0 V and the reversible capacity is around 145 mA h g 1 at 0.1C. Most importantly, it is found that the Na-storage mechanism in Li4Ti5O12 is relevant but different from that of Li-storage in Li4Ti5O12: instead of forming an expected single phase of Li4Na3Ti5O12, the final product after three Na insertions might be a mixture of LiNa6Ti5O12 and Li7Ti5O12. Hasegawa et al. have also reported hierarchically structured porous Li4Ti5O12 for a sodium-ion battery.191 It shows poor rate capability due to the slow diffusion kinetics of the Na+ ion. The discharge capacity at 0.1C is 158 mA h g 1. The discharge capacities at 1C and 2C are quickly decreased to 104 mA h g 1 and 82 mA h g 1, respectively. However, increasing the operating temperature could drastically improve the rate capability. When the operating temperature is increased from room temperature to 40 1C, the discharge capacities at 1C and 2C increase to 127 mA h g 1 and 113 mA h g 1, respectively. At 60 1C, the rate performance is further enhanced to 105 mA h g 1 and 63 mA h g 1 at 10C and 30C, respectively. Significant degradation of the electrode upon cycling is observed together with impaired coulombic efficiency. These new findings indicate that the development of an electrolyte as well as a binder for use at 40–60 1C is required. Liu et al. have fabricated a 3D hierarchically porous Na3V2(PO4)3/C cathode electrode with 20–30 nm Na3V2(PO4)3 nanoparticles uniformly encapsulated in interconnecting onedimensional carbon nanofibers using a simple and scalable electro spinning method.251 The Na3V2(PO4)3/C cathode shows an initial charge capacity of 103 mA h g 1 and a discharge capacity of 101 mA h g 1 (calculated for the total mass of Na3V2(PO4)3/C) at 0.1C, and retains stable discharge capacities of 77, 58, 39 and 20 mA h g 1 at 2C, 5C, 10C and 20C, respectively. Moreover, because of the efficient 1D sodium-ion transport pathway and the highly conductive network of Na3V2(PO4)3/C, the electrode also exhibits high overall capacities even when cycled at high currents. Wang et al. have designed a new hierarchically porous Na3V2(PO4)3/C architecture by a facile one-pot synthesis.252 The prepared Na3V2(PO4)3/C composite consists of spherical particles filled with hierarchical pores and interconnected nanochannels, resulting in the honeycomb-type architecture. It not only enables easier electrolyte penetration, but also provides a high-efficiency electron/ion transport pathway for fast sodium intercalation. The Na3V2(PO4)3/C composite demonstrates improved sodium diffusion capability and decreases electrochemical resistance for the honeycomb-structured microball in comparison with the microsized nonporous reference samples. Moreover, it also delivers superior high rate capability and cycling stability. A capacity of B101.2 mA h g 1 is obtained after 200 cycles at a 1C rate, which retains 93.6% of the initial capacity. Even at 20C, it still delivers a high capacity of 80.2 mA h g 1 corresponding to 71% capacity retention. Zhao et al. have constructed a 3D hierarchically porous NaTi2(PO4)3/C structure by a facile self-assembly strategy.253 The NaTi2(PO4)3/C composite constitutes a 3D porous framework with a bicontinuous electronic conductive skeleton, showing a

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wafer-like structure. When used as an anode in an aqueous system, the wafer-like composite exhibits better sodium intercalation kinetics and enhanced high-rate capability than nonporous samples. Moreover, a full aqueous rechargeable sodium battery is fabricated using the wafer-like NaTi2(PO4)3/C composite as the anode and Na0.44MnO2 as the cathode. The cell exhibits superior high rate property and ultralong-life performance, delivering 64% capacity at 30C and retaining 67% capacity after 400 cycles at alternate 50 and 5C. On the basis of recent studies on NIBs, some fundamental knowledge on electrode materials, electrolytes and full cell design has been obtained. The most interesting results from previous reports suggest that electrode materials that do not function well in Li-intercalation compounds may work well with Na.254 However, the problems of NIBs at present, such as low capacity, low cycle life and low rate performance, still need to be resolved. From the results on electrode materials, hierarchically structured porous nanostructures are still most promising materials for advanced sodium-ion batteries. 3.6 Hierarchically porous structures for magnesium ion batteries Magnesium (Mg) is a benign and abundant metal being the 5th most abundant element in the earth’s crust. Its specific volumetric capacity reaches 3833 mA h cc 1, higher than that for Li metal (2046 mA h cc 1), which make it one of the most promising metal anodes for high energy-density batteries. However, Mg-ion batteries have still not gained appreciable interest from scientists and companies. A very good review on rechargeable Mg batteries about the 1st and 2nd breakthroughs has been commented.255 Li et al. have synthesized a unique MWNT/C/Mg1.03Mn0.97SiO4 nanocomposite by the direct scattered growth of MWNTs on Mg1.03Mn0.97SiO4 nanoparticles via interfacial amorphous carbon phase bonding through a simple one step CVD process.256 This nanocomposite presents a high reversible capacity of about 300 mA h g 1 at 0.2C (1C = 314.6 mA g 1) and good cycling stability at 0.5C. The rate capability and cyclability can be attributed to a desired kinetic transport response of magnesium ions and electrons as well as the mechanical integrity of the MWNTs anchoring the active Mg1.03Mn0.97SiO4 during electrochemical cycling. Furthermore, they have prepared 3D hierarchically porous MgCoSiO4 for rechargeable magnesium batteries.257 The porous MgCoSiO4 sample at 700 1C shows high discharge capacities and better long term cycling stability (about 120 mA h g 1 after 200 cycles at 0.25C, 1C = 305.7 mA g 1). When increasing the rate to 0.5 and 1C, the discharge capacities at the 20th cycle were still maintained at B110 and 80 mA h g 1, respectively. It is suggested that the interconnected wall structure, dual porosity and high surface area reduce the solid-state diffusion lengths for Mg diffusion, facilitate electrolyte penetration into particles and provides a large number of active sites for charge-transfer reactions. The rechargeable Mg batteries are less studied although they have high volumetric capacity. The fundamental studies on the structure of active Mg ions in relevant electrolyte solutions, the adsorption–deposition–dissolution mechanism on Mg anodes,

This journal is © The Royal Society of Chemistry 2016

Review Article

and the mechanisms of Mg insertion into electrode materials are still highly expected towards the development of practical rechargeable Mg batteries. 3.7 Hierarchically porous structures for other energy storage technologies The hierarchically porous structures have also been studied for other types of energy storage technologies, such as hydrogen storage and thermal storage. A very good review on these types of energy storage technologies has already been published. In this review, we only mention the most recent developments in hydrogen storage and thermal storage. For example, Grundy and Ye have prepared a hierarchically nanoporous activated carbon for hydrogen storage, which shows 2.66 wt% H2 adsorption at 77 K and 1 bar.258 Yang et al. have synthesized MOFderived hierarchically porous carbon (MDC) with exceptional porosity for hydrogen storage.259 The as-prepared MDC samples demonstrate excellent H2 storage performance. The MDC-1 sample exhibits 3.25 wt% H2 storage capacity at 77 K and 1 bar, which surpasses the performance of all other materials characterized to date. Particularly, at 100 bar and 298 K, MDC-1 exhibits 0.94 wt% H2 storage capacity due to high ultramicroporosity leading to the efficient storage of H2 at low pressure and room temperature. Wang et al. have prepared a phase change material (PCM) composite by loading pure PCM in the hierarchically porous silica (HPS) matrix.260 The surface properties of the hybrid silica material are effectively controlled via organic silane modification. An extra energy increase (about 10% of the total enthalpy value) is revealed during the phase change process due to the strong van der Waals interaction between the modified surface and PCM. The latent heat value of the PCM (including HPS) composite is 163 J g 1. The PCM in modified HPS shows high thermal storage capability (about 99.34% compared to pure PCM) owing to the functional surface groups being not only helpful to reach high loading capacity for PCM (up to 3.5 times that of HPS’s weight), but also beneficial to enhance the energy storage capacity in confinement.

4. Hierarchically porous structures for photocatalysis and catalysis Recently, synthesis and applications of hierarchically porous materials, which consist of multiple porosities on different levels from micro-, meso- to macro-, have received increasing attentions. The introduction of micro–mesopores to macroporous solids can provide accurate size and shape selectivity and catalytically active sites for guest molecules while the macropore system can minimize diffusion barriers and thus enhance the mass transport. Hierarchically porous materials have also been recognized as good light harvesters due to the presence of macrochannels allowing deep penetration of light and provoking the light scattering effect leading to the longer pathway of light for its higher utilization efficiency.21 Thus, hierarchically porous materials are applicable to photocatalytic and catalytic reactions involving large molecules or to viscous systems.

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Generally, hierarchically porous materials can be used as (photo) catalysts or (photo) catalyst supports in photocatalytic and organic catalytic fields due to their enhanced catalytic performances. In this section, we thoroughly summarized the catalytic and photocatalytic applications of hierarchically porous materials. These hierarchically porous (photo) catalysts have been introduced mainly with a focus on their applications in photocatalytic or organic reactions, with examples from the literature. Table 4 summaries the structural properties of various hierarchically porous materials for photocatalysis and catalytic applications. 4.1

Hierarchically porous structures for photocatalysis

Environmental pollution problems generated from hazardous waste, contaminated groundwater and toxic air contaminants become more and more serious with continuous social development, which greatly affects human health. In order to solve this serious dilemma, extensive research is underway to develop efficient strategies for the elimination of these hazardous chemical compounds. Heterogeneous photocatalysis is proven to be an efficient environmental control method. Semiconductors are of high importance and most commonly used heterogeneous catalysts or catalyst supports in photocatalysis because of their interesting acid–base and redox properties. Their appropriate situation and width of band gap allows light absorption, generating electrons (e ) and holes (h+), which can subsequently induce redox reactions. In this part, for the sake of clarity, we only focus on the very recent progress in the improvement of the photocatalytic performance of semiconductors by the introduction of hierarchically porous structures. For a more comprehensive review on the applications of hierarchically porous semiconducting metal oxides and composites in photocatalysis, readers can read our very recent review paper which is ref. 21. In this recent review paper, we have described the advantage of hierarchically porous materials for photocatalysis, in particular, how the light absorption efficiency can be largely improved by the light harvesting and light scattering effect in macropores in hierarchically porous materials in addition to the large surface area, easy mass transport and high hydroxyl group concentration in detail and in depth. 4.1.1 Titanium oxide-based hierarchically porous structures for the photocatalyst. Among various semiconductor oxides, titania appears to be the most promising photocatalyst for environmental remediation and energy conversion purpose due to its many desirable properties, such as high chemical and thermal stability, low cost and environmental benignity. However, its high intrinsic band gap (3.2 eV) allows only adsorption of the ultraviolet part of solar irradiation. Recently, introducing porosity into TiO2 has proven to be an effective strategy to improve light absorption efficiency and so also its photocatalytic efficiency. These porous TiO2 photocatalysts have superior photocatalytic properties than those of non-porous materials. Much effort has been made to fabricate TiO2 photocatalysts with hierarchically porous structures, which exhibit improved mass diffusion properties. A dual template technique is

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generally used to synthesize hierarchically porous TiO2 materials. In this procedure, colloidal crystals or polystyrene latexes and long-chain surfactants or copolymer micelles are generally used to create macropores and mesopores.261–263 For example, Liu et al.262 have synthesized hierarchically ordered macro/ mesoporous titania inverse opal (TiO2-IO) films using a polyethylene glycol (PEG)-associated sol–gel method. Macroporous templates are obtained using the self-assembly of monodispersed polystyrene (PS) microspheres into opal structures, which are subsequently infiltrated with titanium alkoxide precursors containing PEG 2000 as the mesopore directing agent. After the removal of PS and PEG 2000 by calcination, TiO2-IO hierarchical structures with macro/mesopores are formed (Fig. 36a and b). Fig. 36c shows the comparison of catalytic activity for the TiO2-IO films prepared from TiO2 sols containing different quantities of PEG 2000. The photocatalytic activity of the macro/ mesoporous TiO2-IO films containing PEG 2000 is significantly improved compared with the pure TiO2-IO films and this is attributed to their highly organized hierarchical macro/ mesoporous structures. Larger and more mesopores embedded in the macroporous wall provided more active sites for the photocatalytic reactions, improved the mass transfer and maximized the use of photogenerated electron–hole pairs. The value of the rate constant (k) increases as the PEG 2000 content in the sol–gel solution increases. It is notable that the k value of the sample IO-PEG-2.1 wt% is almost six times higher than that is observed for the pure TiO2-IO film and is attributed to the significantly increased accessible surface area of the macro/mesoporous TiO2-IO films. In addition, larger and more mesopores embedded in the macroporous wall can improve the interconnection of macropores. This results in an increase in the atmospheric oxygen mass transport rate, which is beneficial to the photocatalytic reaction. Furthermore, bio-templates can also be used to synthesize hierarchically porous TiO2.264,265 For example, Li et al.264 have synthesized highly ordered macro–mesoporous anatase titanium dioxide (TiO2) flakes by a sol–gel method in an acidic medium, using fresh natural rose petals and triblock copolymer P123 as dual templates. The photocatalytic activity of the porous TiO2 flakes is evaluated by photodegrading rhodamine B (RhB) under ultraviolet (UV) light. Furthermore, the obtained TiO2 flakes exhibit a higher photocatalytic activity than the commercial photocatalyst Degussa P25, resulting from their hierarchically porous structure. Besides, template-free methods can also produce hierarchically porous TiO2. For example, efficient TiO2 photocatalysts with hierarchically meso–macroporous structure have been prepared by Su and co-workers via a spontaneous self-formation procedure based on the reaction of titanium alkoxides with water.266 Various hierarchically meso–macroporous titanias, obtained via a spontaneous self-formation process using titanium isopropoxide, have been synthesized by three synthesis routes: in water (A) without and (B) with hydrothermal treatment and (C) in a surfactant Brij 56 aqueous solution with hydrothermal treatment, respectively. The product TiO2-A obtained by a spontaneous self-formation process in water media presents well-defined

This journal is © The Royal Society of Chemistry 2016

This journal is © The Royal Society of Chemistry 2016

Traditional catalysis

SnO2 Pd/Al2O3

Metal oxides as catalyst support

Hollow spheres with two-level hierarchical pores Macro–mesoporous Porous nanostructured assemblies Assembles of two dimensional nano-sheets Microspheres composed of nanoribbons Meso–microoporous Macro–mesoporous Macro–mesoporous Mesoporous nanowire core & branch nanoflakes shell Macro–mesoporous Macro–mesoporous Macro–mesoporous Macro–mesoporous Macro–meso–microporous

Macro–mesoporous Hierarchical mesoporous

Hierarchical mesoporous

Hierarchically porous SiO2 frameworks and TiO2 nanoparticles Dual-mesoporous Macro–mesoporous Dual-mesoporous Macro–mesoporous Macro–mesoporous Meso–microporous Macro–mesoporous

Dual-mesoporous Macro–mesoporous Macro–mesoporous

Suzuki cross coupling reactions Selective hydrogenation of pyrolysis gasoline Selective hydrogenation of pyrolysis gasoline CO oxidation Catalytic transformation of 2-propanol and n-butanol Total oxidation of butan-1-ol and toluene N-Butanol catalytic oxidation

Conversions of o-xylene into CO2 CO oxidation Catalytic oxidation of CO Methanol electro-oxidation

Powders Powders Powders Powders Powders Powders Powders Powders Powders

Dimethyldichlorosilane synthesis

Powders

Synthesis of 5-benzyl-1H-tetrazole

CO oxidation Degradation of acetaldehyde

Powders Powders Powders

Combustion of CH4

Catalytic oxidation of CO CO conversion Esterification reaction of acetic acid with n-butanol Electrochemical detection of H2O2

Azo dye acid orange II (AO7) Rhodamine B (RhB) Phenol Rhodamine B (RhB) Rose bengal Methyl orange (MO) Phenol

Rhodamine B (RhB)

Powders Foams Frameworks Nanowire arrays Powders

Powders Films Powders Powders Powders Powders Powders

Fibres

Frameworks

Methylene blue (MB) Rhodamine B (RhB) Rhodamine B (RhB) Methylene blue (MB) methyl orange (MO) Rhodamine B (RhB) Rhodamine B (RhB) Acetone Methyl orange (MO) Methylene blue (MB) Rhodamine B (RhB) Methyl orange (MO) Rhodamine B (RhB) Rhodamine B (RhB)

Type of reactions/organics

308 303 304 305 298 299 300

294 295 296 297

293

292

290 291

289

276 286 287 288

276 277 278 279 280 281 282

275

261 262 264 265 266 267 268 269 263 271 272 273 274

Ref.

Chem Soc Rev

Pt/Al2O3 Ni–TiO2

Co3O4–CeO2 CuO/TiO2 Co3O4/NiO

CuO

ZnO

CeO2 Gd doped CeO2 ZrO2 Co3O4

Silicon

Iron oxides Graphitic carbon nitride

Mixed metal oxides

Single metal oxides

Non-TiO2-based

Doped TiO2 Composites CdS/TiO2 ZrO2/TiO2 Polypyrrole– TiO2 TiO2 nanocrystals on SiO2 fibres CeO2 ZnO

Film Film Flake Powders Powders Powders Foams Powders Film Monoliths Powders

Macro–mesoporous

TiO2-based

Photo-catalysis

TiO2

Hierarchically porous structure Morphology

Chemical composites

Application

Table 4 The structural parameters and performance of various hierarchically porous materials for catalysis and photocatalysis applications

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Application

Zeolite-based

Chemical composites

Table 4 (continued)

3520 | Chem. Soc. Rev., 2016, 45, 3479--3563

Ru/b eta Meso–microporous

Meso–microporous

Micro–meso–macroporous TiMeso–microporous containing b zeolites Meso–microporous Macro–meso–microporous Meso–microporous

TS-1

Micro–meso–macroporous

Powders

Powders

Powders

Powders

Powders

Powders Fibres Powders

Macro–meso–microporous Meso–microporous

Powders Skeletons Monoliths Powders

Micro–macroporous

Aqueous-phase hydrogenation of furfural Fischer–Tropsch (FT) synthesis Phenol hydroxylation Epoxidation of oct-1-ene and epoxidation of cyclohexene Catalytic oxidative desulfurization of fuel oil Oxidative desulphurization Oxidative desulfurization of bulky and small reactants Epoxidation of styrene and 2,4,6-trimethylstyrene Epoxidation of unsaturated ketones, e.g. 2-cyclohexen-1-one, with hydrogen peroxide as an oxidant Epoxidation of cyclohexene and 1-dodecene Methanol conversion to light olefins (MTO) reaction

339 340 341 342

329 330 331 332 333 334 335 337 338

328

324 325 326 327

321 322 323

315 316 317 318 319 320

314

312 313

302 309 306 307 310 311

301

Ref.

Review Article

SAPO

Ti–Si

Y

Gas-phase hydrogenation of 1,3-butadiene Oxidation of toluene Ethanol steam reforming reaction Hydrodesulfurization reaction Selective oxidation of benzyl alcohol Protection of benzaldehyde with pentaerythritol Condensation of benzaldehyde with hydroxyacetophenone Condensation of benzaldehyde with 1-pentanol Cracking of branched polyethylene Alkylation of toluene with benzyl chloride, condensation of benzaldehyde with n-butyl alcohol and acetalization of cyclohexanone with methanol Aldol condensation of benzaldehyde with glycol, aldol condensation of benzaldehyde with n-butyl alcohol Synthesis of dimethyl ether (DME) from methanol Fischer–Tropsch synthesis Phenol tert-butylation reaction Cracking of 1,3,5-triisopropylbenzene Cracking of 1,3,5-triisopropylbenzene Acid-catalyzed reactions: cumene cracking and a-pinene isomerization Cracking of 1,3,5-triisopropylbenzene Cracking reaction of iso-butane Condensation of benzaldehyde with glycerol, alkylation of benzene with benzyl alcohol, condensation of benzaldehyde with hydroxyacetophenone Esterification of benzyl alcohol with hexanoic acid Esterification reaction of high free fatty acid (FFA) oils Deep hydrogenation of bulky aromatic pyrene Hydrodesulfurization (HDS) of 4,6dimethyldibenzothiophene Fischer–Tropsch (FT) synthesis

Film Powders Monolith Powders Powders

Meso–microporous

Reduction of p-nitrophenol by NaBH4

Powders

Porous and tubular nanostructures Macro–mesoporous Macro–mesoporous

Pd/beta Meso–microporous

Beta Pure

Pd–TiO2 Pd–Nb2O5 and Ta2O5 Ni/Mg–Al–O Co–Mo–Ni–TiO2 Al2O3 Al–Si ZSM-5 zeolites

Pt–TiO2

Type of reactions/organics

Hierarchically porous structure Morphology

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357 Foams

353 354 355 356 Foams Powders Powders

Hierarchically porous Macro–mesoporous hydrogen silsesquioxane (HPHSQ) Hierarchically porous Macro–mesoporous organosilica (HPOS) Other hierarchically por- Macro–mesoporous ous inert supports

Macro–mesoporous Hierarchically porous silica (HPS)

Monoliths

343 344 345 346 347 348 349 350 351 352

Reduction of 4-nitrophenol Methanol oxidation Degradation of methyl violet Degradation of acid fuchsin Chemoselective hydrogenation of chalcone Oxygen reduction reaction Degradation of orange II Oxidation of cyclooctene Catalytic oxidation of CO and o-DCB Mizoroki–Heck cross-coupling reaction of 4-iodotoluene and n-butyl acrylate Reduction of 4-nitrophenol CO oxidation Reduction of 4-nitrophenol Oxidative esterification of p-nitrobenzyl alcohol with methanol CO oxidation Hierarchically porous car- Dual mesoporous bons (HPC) Macro–meso–microporous Macro–mesoporous Supported catalysts

Chemical composites

Powders Powders Fibers Powders Powders Foams Powders

Ref. Type of reactions/organics

Review Article

Application

Table 4 (continued)

Hierarchically porous structure Morphology

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Fig. 36 SEM (a) and TEM (b) images of the TiO2-IOs prepared from the TiO2 sols without PEG 2000 content (IO-PEG-0 wt%); (c) Photocatalytic degradation of RhB on TiO2 membranes prepared with different PEG 2000 content (0, 1.4, 2.1 and 2.8 wt%). (Reproduced from ref. 262 with permission. Copyright Elsevier, 2015).

macropores with a pore size range of 2–8 mm. It can be clearly visualized from SEM images that the TiO2 particles of around 100 nm in size located within the macropore walls are not closepacked in texture. The morphologies of the products obtained by method B in water media with hydrothermal treatment are similar to those obtained using method A. Hierarchically meso– macroporous titanias constructed from closely aggregated particles are also synthesized by using method C in a surfactant Brij 56 aqueous solution by the hydrothermal treatment process. TiO2-C presents well-defined macropores with a pore size range of 1–5 mm (Fig. 37a). Macroporous structures of the products are fully retained after calcination at 500 1C for 3 h (Fig. 37b). Comparison of the photocatalytic activity of titania products obtained using different synthesis methods can be found in Fig. 37c. TiO2-C exhibits the best photocatalytic activity, and can fully photodegrade the dye molecules in 35 min due to its higher crystallinity and more mesopores generated by the surfactant compared to TiO2-A and TiO2-B. This part of the work again highlights and confirms the importance of macropores as light harvesters in the enhancement of light absorption by the light scattering effect and deep light penetration into the materials.21 Our group has synthesized a series of hierarchically meso– macroporous TiO2 with inverse opal structure.267 These materials show a significant light enhancement leading to a highly exalted photocatalytic activity. A slow photon effect has been revealed with this kind of ordered meso–macroporous inverse opal structure.267 These studies present significant progress in improving the light absorption efficiency and can open a new exciting avenue for the design of new high performance photocatalytic materials with applications in all the fields related to light

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Fig. 37 SEM images of as-synthesized and calcined titania products obtained by methods C: TiO2-C (a) and TiO2-C-500 (b); photodegradation of rhodamine B (c) of the as-synthesized TiO2 products:TiO2-A, TiO2-B, and TiO2-C. (Reproduced from ref. 266 with permission, Copyright Elsevier, 2015).

absorption including solar cells, optical telecommunications and optical computing. Yu and co-workers have also successfully prepared trimodal sponge-like macro-/mesoporous titania with biphase (anatase and brookite) structures by hydrothermal treatment of the precipitates of tetrabutyl titanate in pure water without using any templates and additives.268 The resultant hierarchically porous titania powders display excellent photocatalytic activity due to their special pore-wall structure and their photocatalytic activity is three times higher than that of P25. Xia et al.269 have also synthesized hierarchically porous TiO2 materials by a controllable template-free aqueous method. Their porous textures and crystalline structures are tailored by varying the pH of the starting aqueous solution and the crystallization method. The hierarchical titania prepared by the hydrothermal method at a pH of 2 has much higher efficiency with an apparent reaction rate of 9.10  10 2 min 1, surpassing that of Degussa P25. Furthermore, Sanchez and his co-workers have synthesized hierarchically porous like macroporous films with mesoporous walls induced by coupling nanobuilding block and breath approaches without polymeric templates.270 The calcined honeycomb-like porous TiO2 shows 1.6 times higher activity in the decomposition of methylene blue than the nonporous one. The superior photocatalytic activity of the calcined macro–mesoporous TiO2 film can be explained on the basis of its hierarchical porosity. The porous morphology makes it possible to irradiate UV light onto the film inside effectively, and to achieve the generation of a large amount of active oxygen and radical species, resulting in the high photocatalytic activity.

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As mentioned above, titania has been widely used as a photocatalyst because of its strong redox power, high photocorrosion resistance, cost effectiveness and long-term stability. However, its high intrinsic band gap allows only absorption of the ultraviolet part of the solar irradiation. In order to overcome this shortcoming, impurity doping in TiO2 has been considered as a promising strategy. In particular, transition metal and nonmetal doping has been demonstrated to be more effective for the extension of the photocatalytic activity of TiO2 into the visible region. For example, hierarchically porous crystalline nitrogendoped titania monolithic material is fabricated by annealing a TiO2 porous monolith under a modest flow of ammonia gas.271 Nearly 50% of Rhodamine B in aqueous solution is efficiently degraded by a N-doped TiO2 porous monolith with the mixedphase of anatase and rutile under visible light within 120 min. Besides, some novel and efficient photocatalysts based on TiO2 coupled with narrow band gap semiconductors, such as CdS272 and ZrO2,273 have been successfully synthesized. For example, hierarchical macro–mesoporous ZrO2–TiO2 composite materials are prepared based on a facile surfactant self-assembly method with titanium isopropoxide and zirconium n-butoxide as precursors.273 The ZrO2 content in the composites (2.0–9.8%) is easily controlled by adjusting the molar ratio of the two precursors, while maintaining the porous structures of the materials. SEM results demonstrated that the ZrO2–TiO2 composites are characterized by parallel channels (50–500 nm) and interparticle pores (B8 nm) in the walls (Fig. 38a–d). The photocatalytic activities of the ZrO2–TiO2 photocatalysts are evaluated by degradation of RhB under UV light irradiation (254 nm, 500 mW cm 2). It can be found that the absence of a catalyst (blank), pure ZrO2 and pure TiO2 has no apparent effect on the degradation of RhB, while ZrO2–TiO2 photocatalysts present obviously enhanced UV light photocatalytic activity compared with the undoped TiO2 sample. The photoactivity is improved gradually with the increase of zirconium molar content. With 6.9% of zirconium

Fig. 38 SEM images of ZrO2–TiO2 composites (a) ZrO2–TiO2 (2.0%), (b) ZrO2–TiO2 (4.1%), (c) ZrO2–TiO2 (6.9%), and (d) ZrO2–TiO2. (Reproduced from ref. 273 with permission, Copyright Elsevier, 2015).

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doped in TiO2, the as-prepared ZrO2–TiO2 (6.9%) sample exhibits a superior photocatalytic activity that could degrade RhB by 86.9% in 3 h. According to the kinetics analysis of a photocatalytic process, the apparent rate constant K of the ZrO2–TiO2 (6.9%) composite is about 3.0 times that of the undoped TiO2 sample. The higher photocatalytic activities of the ZrO2–TiO2 photocatalysts compared to the TiO2 sample indicate that incorporation of zirconia is crucial in achieving high rates of degradation. In addition, doped zirconium ions can induce surface defects or crystallinity changes in semiconductors, which act as an electron trap slowing down its recombination rate and prolonging the lifetime of the hydroxyl radicals. In addition, the integration of organic phase on TiO2 at the nanoscale level can produce much higher photocatalytic properties than either of the constituent phases. For example, novel hierarchically macro–mesoporous polypyrrole–TiO2 composites are synthesized by polymerization of pyrrole with macro/ mesoporous TiO2 as a photosensitizer and template.274 The sample of polypyrrole–TiO2 shows higher photoactivity and photocurrent than TiO2 under visible light irradiation due to its ordered macropores, uniform polypyrrole sensitizer layer, and relatively low hole–electron recombination rate. In addition, nanosized TiO2 catalysts are always assembled on/in a solid carrier to avoid their easy aggregation and difficult separation problems. For example, hierarchically porous TiO2/ SiO2 fibers, in which the hierarchically porous SiO2 fibers confer a platform to assemble TiO2 nanoparticles with a uniform spatial distribution, are prepared via an electrospinning technique combined with subsequent high-temperature calcination treatment.275 The resultant hierarchically porous TiO2/SiO2 fibers have shown an enhanced photocatalytic activity with B95% degradation of rhodamine B as compared with B75% using common TiO2 nanoparticles and 77% using commercially available Degussa P25 after 120 min illumination. 4.1.2 Other metal oxide-based hierarchically porous structures as photocatalysts. Despite the importance of TiO2, other metal oxide semiconductors such as cerium oxide,276 zinc oxide277,278 and iron oxides279 are also widely used as photocatalysts to efficiently control the environmental pollution. For example, hierarchically porous CeO2 nanopalm leaves are synthesized by using a facile solvothermal method at 180 1C utilizing cerium nitrate and ethylene glycol (Fig. 39a).276 The CeO2 palm leaves exhibited a significant photocatalytic performance in degrading azo dye acid orange II in an aqueous solution under UV radiation (Fig. 39b). Hierarchically porous zinc oxide (ZnO) spherical nanoparticles are synthesized through a self-assembly pathway.277,278 Fig. 39c is a low-magnification TEM image of a larger ZnO particle, confirming that this larger particle contains many smaller ZnO nanoparticles with uniform size. The aggregation of these nanoparticles leads to the formation of mesovoids. Fig. 39d compares the photodegradation activity of phenol under UV irradiation of ZnO nanoporous materials, TiO2 nanoparticles (PC-500), commercial ZnO powder, and ZnO nanopowder. The porous ZnO nanoparticles show superior photoactivity compared to TiO2 nanoparticles, commercial ZnO powder, as well as ZnO nanopowder in the decomposition of phenol in waste water.

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Fig. 39 (a) Low bright-field TEM images of CeO2 nanopalm leaves, (b) photocatalytic degradation of AO7 dye and AO7 dye with CeO2 nanopalm leaves under variation UV-Vis light irradiation time. (d) Low-magnification cross-sectional TEM image of the as-obtained porous ZnO nanoparticles. (f) Curves of the residual fraction of the phenol as a function of UV irradiation time when using porous ZnO nanoparticles, TiO2 nanoparticles (PC-500), commercial ZnO powder and ZnO nanopowder. (Adapted with permission from ref. 276 (a and b). Copyright Springer, 2013, from ref. 278 (c and d). Copyright American Chemical Society, 2007).

Hierarchically porous a-FeOOH nanoparticles are controlled and prepared via a facile polystyrene (PS) microsphere-templated method.279 The results show that the walls are formed by the agglomeration of the nanoparticles, leading to significant textural mesoporosity within the walls of the structure. The resultant sample consists of a uniform structure with ordered macropores. UV adsorption measurements suggest that the a-FeOOH nanoparticles exhibit high adsorption performance and certain photocatalytic efficiency. The high surface area of the a-FeOOH sample can provide more unsaturated surface sites exposed to the reactants and the mesopores in the photocatalyst enabling storage of more reactant molecules. Thus, the high specific surface area, hierarchically porous structure, and good adsorptive ability are responsible for achieving better photodegradation performance. 4.1.3 Polymer semiconductor-based hierarchically porous structures as photocatalysts. In addition, polymer semiconductors such as graphitic carbon nitride also showed intrinsic semiconductor-like adsorption in the blue region of the visible spectrum.280,281 For example, hierarchically porous graphitic carbon nitride (hp-g-CN) is successfully derived from melamine polymerized microstructures via pyrolysis under Ar without the involvement of hard templates or surfactants as porogens.280,281 The SEM image (Fig. 40a) confirms that it is porous in nature and consists of many curved nanosheets with lamellar morphology. The TEM image (Fig. 40b) also reveals layered platelet-like morphology with obvious porous structures. The photocatalytic activity of the as-prepared hp-g-CN is evaluated by the degradation of methyl orange (MO) under visible light irradiation. Fig. 40c shows that only 35.4% MO can be photodegraded by bulk g-C3N4 in 3 h.

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4.2

Fig. 40 (a) SEM image and (b) TEM image of hp-g-CN and (c) photocatalytic degradation of MO dye over hp-g-CN, bulk g-C3N4 under visible light irradiation. (Adapted with permission from ref. 281, Copyright The Royal Society of Chemistry, 2014).

In contrast, the use of hp-g-CN results in 90% photodegradation of MO in the same time frame, which is much better than the commercial P25 benchmark photocatalyst. The superior catalytic activity of hp-g-CN over bulk g-C3N4 can be attributed to the following three reasons: (1) hp-g-CN has a much higher surface area, leading to a higher adsorption capacity for MO and the exposure of more active sites. (2) The hierarchically porous feature of hp-g-CN enhances the diffusion of MO and the degraded products for fast degradation reactions. (3) The nanostructured nature favors faster photogenerated carrier separation, thereby resulting in improved photoactivity. 4.1.4 Silicon-based hierarchically porous structures as photocatalysts. Compared to the above mentioned visible-light responsive photocatalysts, silicon (Si) has been proposed as an alternative high performance photocatalyst with a wide spectral response. For example, hierarchical macro–mesoporous silicon has been fabricated through electro-assisted chemical etching using a silicon wafer as a substrate.282 The designed hierarchically porous silicon material is composed of a macroporous infrastructure and nanoporous surface (denoted NP-MPSi). The photocatalytic activity of NP-MPSi is evaluated by its ability to photocatalytically degrade phenol. Nearly 10% of the phenol is removed in the dark in the presence of NPMPSi, owing to the high adsorption of phenol on NP-MPSi. For direct photolysis under Xe lamp irradiation without NP-MPSi, about 53% of the phenol is removed. In contrast, more than 95% of the phenol is removed after 5 h of photocatalytic reaction in the presence of NP-MPSi, indicating a significantly improved photocatalytic oxidation capability compared with MPSi.

3524 | Chem. Soc. Rev., 2016, 45, 3479--3563

Hierarchically porous structures for catalysis

Over 90 percent of chemical products obtained by petroleum refinery and fine chemical industries are synthesized by conventional catalytic processes. The catalyst plays a key role in all the chemical processes, and its performance directly affects the whole catalytic reaction. Besides, the removal of organic wastes such as benzene, toluene, and xylene which are harmful to the environment is an important research topic for environmental treatment systems. Complete catalytic oxidation techniques that convert an organic compound contaminant into CO2 and H2O are effective pathways to remove volatile organic compounds (VOCs). Hierarchically porous catalysts are of recent scientific and technological interest due to their improved diffusion performances and high surface area. These hierarchically porous catalysts are generally classified into three categories according to their compositions: hierarchically porous metal oxides or metals, hierarchically porous zeolites and hierarchically porous material-based catalysts. In this section, we thoroughly discuss hierarchically porous materials mainly focusing on their catalytic applications in organic reactions, with examples from the literature. 4.2.1 Metal-based hierarchically porous structures as catalysts. Gao et al.283 have successfully demonstrated a simple and fast approach for the direct growth of hierarchically micro/ nanostructured metallic Cu microspheres at room temperature in the absence of any hard template. In the reduction of 4-nitrophenol, the as-prepared porous Cu microspheres present much better catalytic performances than those of commercial Cu powder and commercial Cu sheets. Hollow coral-like Pt mesostructures have been prepared by means of the replacement reaction between Ag and H2PtCl4 simply at room temperature with Ag corals as sacrificial templates.284 These hollow corallike Pt mesocrystals exhibit an enhanced catalytic performance for methanol oxidation because of their special structure and morphology. Fig. 41a and b show that the synthesized materials

Fig. 41 (a) Low-magnification and (b) high-magnification SEM images of Pt corals, (c) TEM image of Pt corals and (d) a single branch of a Pt coral. (Reproduced from ref. 284 with permission. Copyright Elsevier, 2013).

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exhibit well-defined hollow coral-like mesostructures. Fig. 41c shows that the synthesized Pt mesostructures have a coral-like morphology and a strong contrast difference between their edges (dark) and center (bright), which indicates that the Pt mesostructures possess a hollow interior. As seen in Fig. 41d, the wall thickness of the hollow Pt mesostructures is 5–7 nm and the hollow Pt mesocorals consist of a large number of Pt nanoparticles and pores. Cyclic voltammogram (CV) curves of the three catalysts recorded at room temperature in an argon gas-purged 0.1 M HClO4 solution are studied. The electrochemical surface areas (ECSAs) of the hollow Pt corals are the largest compared to those of Pt spherical mesocages and commercial Pt blacks. The largest ECSA of Pt mesocorals may be due to their intrinsic structure, consisting of a hollow interior, small nanoparticles, and abundant pores. The electrocatalytic measurement of three catalysts is performed in an argon gas-purged 0.1 M HClO4/0.5 M CH3OH solution using a glassy carbon rotating disk electrode at room temperature with a scan rate of 50 mV s 1. Comparing peak specific activity for methanol oxidation reaction, the hollow Pt corals exhibit the highest activity, 2.1 times greater than that of Pt black. The Pt corals show greatly improved catalytic activity for methanol oxidation because of their special structure and morphology. 4.2.2 Metal oxide-based hierarchically porous structures as catalysts. Synthesis of metal oxides with hierarchical structures is of great interest due to their high catalytic activity, good chemical stability and excellent thermal stability. Hierarchically porous metal oxides can be used alone or in combination with other metal oxides or as catalyst supports to immobilize noble metal nanoparticles. 4.2.2.1 Single metal oxide-based hierarchically porous structures as catalysts. As one of the most important rare-earth metal oxides, ceria is a superior catalyst and catalyst support and has been extensively investigated for various catalytic reactions due to its outstanding oxygen storage capability associated with its excellent redox properties. The catalytic performance of ceria is mainly dependent on its morphology and structure. Therefore, porous ceria has shown great potential as a versatile catalyst and catalyst support.285,286 Qian et al.285 have reported a novel diatom-based scaffold for the synthesis of hierarchically biomorphic CeO2 with special porous structure via incorporating Ce ions into the frustule. The obtained CeO2 exhibits high catalytic activity in CO oxidation owing to its unique hierarchical structure and periodic meso–macro scale features. Li et al.286 have reported a facile electrochemical deposition route for the direct growth of porous Gd-doped CeO2. The SEM images of porous Gd-doped CeO2 foam structures are shown in Fig. 42a. It can be easily found that the foam structures have a higher density of pores, and the sizes of pores are 200–500 nm. Catalytic activities of porous Gd-doped CeO2, porous CeO2, and CeO2 nanoparticles as a function of reaction temperature are shown in Fig. 42b. The CO conversion increases with increasing reaction temperature for all samples. Porous Gd-doped CeO2 shows much higher activity than porous CeO2 and CeO2 nanoparticles. For porous Gd-doped CeO2, a 47% CO conversion is achieved at

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Fig. 42 (a) SEM images of porous Gd-doped CeO2, (b) CO conversion as a function of temperature for the as deposited porous Gd-doped CeO2 (10 atom% Gd) (1), porous CeO2 (2), and CeO2 nanoparticles (3). (c) Typical SEM image of the prepared sample MoSZ-5% and the inset is TEM image of the prepared MoSZ-5% sample, and its corresponding electron diffraction pattern. (d) Conversion rates of esterifications in 4 h by using catalysts HZr, MoZr, MoSZ-2.5%, MoSZ-5%, MoSZ-10% and reference sample R for different cycling numbers. (e) SEM and (f) TEM images of synthesized 3D macro/mesoporous Co3O4 and (g) CO conversion and O2 selectively to CO2 over 3D macro-/nanoporous Co3O4 and Co3O4 nanoparticles catalyst, respectively. (Adapted with permission from ref. 286 (a and b). Copyright The American Chemical Society, 2009, from ref. 287 (c and d). Copyright Elsevier, 2009, from ref. 290 (e–g). Copyright Elsevier, 2013).

180 1C, while only 23% and 8% CO conversions are obtained at the same temperature for porous CeO2 and CeO2 nanoparticles, respectively. The large difference in catalytic activity can be partly due to the variation in surface area. It is well-known that the first and the last step of the catalytic process is mainly related to the adsorption and desorption, respectively, of gas molecules on the surface of the catalyst. Therefore, the porous structures with high surface area allow better contact with gas molecules, and accordingly show better catalytic performance compared with CeO2 nanoparticles. A novel hierarchically porous molybdenum-promoted SO42 / ZrO2 (MoSZ) solid acid with a nanocrystallized framework of the tetragonal phase and a high surface area has been synthesized via a composite surfactant-assisted route combined with a hydrothermal treatment process.287 Fig. 42c shows the representative SEM and TEM images of the synthesized hierarchical ZrO2 material. It exhibits a macroscopic network structure with

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a well-defined regular array of macropores, indicating a regular and well-distributed macroporous structure. Such a hierarchically porous structure provides perfect pore tunnels for molecular diffusion in the catalytic reactions. Fig. 42d shows the change in the conversion rates of acetic acid reacted with n-butanol for 4 h over prepared MoSZ, HZr, MoZr (sample without sulfate adsorption) and reference samples. The conversion of acetic acid reaches a maximum of 5% sulfate loading (MoSZ-5%). The novel mesoporous solid acid exhibits better catalytic performance than normal ZrO2 samples without hierarchically porous structure in the esterification reaction of acetic acid with n-butanol and can preserve the acid better than normal solid acids after several recycle experiments. The hierarchically porous structure contributes to the greatly enhanced acidic catalytic performance for the esterification reaction. The small amount of Mo incorporation into the ¨nsted zirconia framework can generate a certain amount of Bro acid sites, which are helpful in maintaining the acidity after esterification reactions for several cycles. Spinel Co3O4 is a technologically important multifunctional material with great applications as a heterogeneous catalyst. Nanostructured Co3O4 with high specific surface area and enhanced catalytic activity is particularly attractive. In the past decade, various nanostructures of cobalt oxides such as nanowires,288 hollow spheres289 and 3D macro/mesoporous structure290,291 have been synthesized, and their enhanced catalytic performances have been demonstrated. Xu et al.288 have synthesized freestanding Co3O4 nanowire arrays on a nickel substrate via a facile template-free procedure. The as-synthesized Co3O4 nanowires exhibit a hierarchically porous structure inside their architectures. The Co3O4 nanowire array shows a much high sensitivity and a relatively wide linear range for the electrochemical catalysis of H2O2. Liang et al.289 have synthesized Co3O4 hollow spheres with tunable hierarchical pores. Due to their high specific surface area and abundance of well interconnected mesopores, the resultant Co3O4 hollow spheres exhibit excellent catalytic performance and durability in methane combustion. Deng et al.290 have synthesized three-dimensional macro/ mesoporous tricobalt tetraoxide Co3O4 via a facile self-sustained decomposition using the complexes of metallic salts and organics (metal–organic complexes). The morphology of as-prepared material characterized by SEM is shown in Fig. 42e. The as-synthesized powder shows a coral-like shaped particle. TEM images (Fig. 42f) further confirm that the coral-like shaped particles with a size of tens of micrometers consisted of interconnected spherical particles forming a 3D hierarchically porous structure with typical bimodal porous morphology, in which the walls of the macropores contain smaller mesopores. The irregular Co3O4 nanoparticles are uniform in size. The catalytic performances of 3D hierarchically porous Co3O4 are investigated by means of the CO oxidation reaction. The results obtained for the porous Co3O4 and Co3O4 nanoparticles catalysts are shown in Fig. 42g. The porous Co3O4 catalyst exhibits excellent CO oxidation catalytic activity which is significantly enhanced compared to that of Co3O4 nanoparticles due to the lower conversion temperature and higher conversion efficiency. In addition, 3D macro-/nanoporous Co3O4 has a high selectivity for CO.

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Zinc oxide, a functional n-type semiconductor with a wide band gap of 3.37 eV, has been investigated extensively for its special catalytic properties. Besides, 3D assembled hierarchically porous ZnO structures are of key importance for their catalytic applications. Sinhamahapatra et al.292 have reported the controlled synthesis of 3D hierarchically porous ZnO architectures made of two dimensional nano-sheets through the calcination of a hydrozincite intermediate. In the synthesis of 5-benzyl-1H-tetrazole, the synthesized porous 3D assembled ZnO catalyst results in 85% yield while nanocrystalline and bulk ZnO result in 68% and 51% yield. Although the surface area of the ZnO nanoparticles is more than that of the synthesized porous 3D assembled ZnO, the poor catalytic activity is due to its less effective surface area for agglomeration. Besides, the synthesized catalysts show excellent recyclability. Cupric oxide, an important p-type semiconductor metal oxide, has attracted great interest in recent years owing to its widespread application in catalysis due to its excellent catalytic and chemical properties. Su et al.293 have prepared hierarchical dandelion-like CuO (HD-CuO) microspheres composed of nanoribbons via a facile hydrothermal method. The investigation of the Rochow reaction shows that the HD-CuO catalyst has a better catalytic performance than the commercial CuO microparticles and the commercial CuO–Cu2O–Cu catalyst in dimethyldichlorosilane synthesis, due to its well-developed hierarchically porous structure and higher specific surface area.293 Wu et al.294 have prepared open porous hierarchically structured microcrystalline a-MnO2 containing almost 100% Mn4+ ions on its surface by a novel redox-precipitation method, where Mn(NO3)2 and KOH are titrated into excess KMnO4 solution at pH 8. For comparison, MnOx is also prepared using a conventional precipitation method. The synthesized catalyst exhibits good low-temperature reducibility, and converts 100% o-xylene into CO2 at 220 1C, 50 1C lower than the MnOx prepared by the conventional method. The surface concentration of Mn4+ ions plays a key role in its high catalytic activity for the complete oxidation of o-xylene. In addition, the open porous structure and presence of a small amount of potassium ion in the microcrystalline a-MnO2 channel may also be responsible for its excellent catalytic performance. 4.2.2.2 Mixed metal oxide-based hierarchically porous structures as catalysts. As mentioned above, single metal oxide has been found to be an excellent catalytic material in many catalytic oxidation reactions. Their activity in total catalytic reactions can be further enhanced by mixing with other metal oxides. For example, CeO2 is widely used in the promotion of catalytic reactions for CO oxidation. However, CeO2 has some problems for its practical industrial application mainly due to its relatively low catalytic activity. The activity of ceria in total oxidation reactions can be greatly enhanced by combination with other transitional metals like Co, Cu, Ti, Ni, and so on. Three dimensional macro/mesoporous Co3O4–CeO2 is prepared via a facile self-sustained decomposition of metal–organic complexes.295 Porous Co3O4–CeO2 exhibits 3D hierarchically porous structure with typical bimodal porous morphology, in

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which the walls of the macropores contain smaller mesopores. TEM investigations give further insight into the morphology and the structural features of the as-synthesized material. The porous 20% Co–CeO2 material has 3D hierarchical architecture which consists of nanoparticles. The mesopores are formed by the aggregation of nanoparticles and the macropores are self-assembled by nanoparticles. The wall thickness of the macropores is ca. 300–400 nm and the walls consist of many fine nanoparticles. To evaluate their catalytic activity for CO oxidation, the catalytic performances of 3D hierarchically porous Co3O4–CeO2 catalysts are evaluated under a reaction stream with a gas composition of 1.0 vol% CO balanced by fresh air with 20 vol% O2. The activity of porous Co–CeO2 catalysts is greatly enhanced after doping Co, suggesting that integrating cobalt oxides with ceria can further enhance the redox behavior for CO. The catalytically active Co and Ce sites in Co3O4–CeO2 composite oxides can cooperatively enhance the catalytic activity. Owing to their significant activity and lower cost, supportedcopper oxide catalysts, such as CuO/TiO2,296 have been found to be effective for the catalytic oxidation of CO. Hierarchically mesoporous–macroporous titanium dioxide (MMTD) is synthesized by the hydrolysis of tetrabutyl titanate in the absence of surfactants, which exhibits a porous hierarchy of wormhole-like mesostructure in the framework of macrochannels.296 CuO nanoparticles are then supported on the MMTD by a deposition–precipitation method, retaining high surface areas and hierarchical porosity. Fig. 43a shows the SEM images of the 400 1C-calcined 8%-CuO/MMTD catalyst. The channel-like macroporous structure is preserved after CuO loading and high temperature calcination. TEM images (Fig. 43b) further confirm that the wormhole-like mesopores in the macroporous frameworks result from the nanoparticle assembly. The nanoparticles are still uniform but little bigger than the MMTD sample. The catalytic performance of the hierarchical MMTD supported CuO catalysts is tested by low-temperature catalytic CO oxidation (Fig. 43c). The observed activity of the pure MMTD support in CO oxidation is quite low, while the addition of CuO significantly increases the catalytic activity of all the samples. The synergistic effect between CuO and the MMTD support, highly dispersed CuO nanoparticles, mesoporous– macroporous framework, high surface area and uniform distribution of the nanoscale particle size are responsible for the high catalytic activity of CuO/MMTD nanocatalysts for low-temperature CO oxidation. A self-supported Co3O4/NiO core/shell nanowire array is prepared by the combination of hydrothermal synthesis and electro-deposition methods.297 The Co3O4/NiO core/shell nanowire array possesses a hierarchically porous architecture consisting of a mesoporous core and a branched nanoflake shell. It is clearly shown that the Co3O4 nanowire is tightly wrapped by the electrodeposited NiO nanoflake shell. Besides, the selected area electron diffraction (SAED) pattern of the nanoflakes shell reveals the existence of cubic polycrystalline NiO, indicating the formation of a Co3O4/NiO core/shell nanowire array. The electro-oxidation properties of methanol of the Co3O4/NiO

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Fig. 43 (a and inset) SEM and (b) TEM images of a 400 1C-calcined 8%CuO/MMTD catalyst, (c) catalytic activity for CO oxidation of CuO/MMTD catalysts with different CuO loading amounts calcined at 400 1C. (Adapted with permission from ref. 296, Copyright Springer, 2009).

core/shell nanowire array are elucidated by cyclic voltammetry (CV) in the 0.5 M methanol + 1 M KOH solution. The Co3O4/NiO core/shell nanowire array shows high anodic current density with almost two times larger than that of the single Co3O4 nanowire array. The enhanced electrocatalytic reactivity is mainly due to the unique porous core/shell architecture providing several major advantages: (1) both Co3O4 and NiO are catalysts for direct electro-oxidation of methanol. They work together and provide potential synergistic effects contributing to the higher electrocatalytic reactivity. (2) The directly grown array ensures good mechanical adhesion and electrical connection to the current collector and avoids the use of polymer binders and conducting additives. (3) The porous core/shell array architecture enables the full exposure of both active materials to the electrolyte and provides a short diffusion path for both electrons and ions, thus leading to faster kinetics, lower overpotential and higher electro-catalytic reactivity. 4.2.3 Noble metals loaded/doped on metal oxide-based hierarchically porous structures for catalysis. Although noble metal nanoparticles (Pt and Pd) exhibited high performance at lower temperatures, the high cost, limited availability and easy aggregation have severely restricted their wide application. Thus, the immobilization of noble metal nanoparticles on supported materials has attracted significant interest. Owing to its superior physical and chemical properties, TiO2 with a well-defined hierarchically porous structure has also been widely used as a catalytic support for a variety of

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Fig. 44 (a) SEM micrographs of calcined 3% Nb–TiO2, (b) intrinsic reaction rate of isopropanol decomposition in the presence of x% Nb–TiO2 catalysts, (c and d) TEM images of SnO2 IOs after Pd NP immobilization. (e) Linear sweep voltammogram of formic acid oxidation using Pd SnO2 IOs. The inset shows the CV curve for blank SnO2 IO before Pd NP immobilization. (Adapted with permission from ref. 298 (a and b), Copyright The Royal Society of Chemistry and the Centre National de la Recherche Scientifique, 2014, from ref. 307 (c–e). Copyright Chemical Industry and Engineering Society of China, 2014).

heterogeneous catalysts (such as niobium,298–300 platinum300,301 and palladium302). Adding a second active component into these supports can combine the advantages of a promoter with improved diffusion through hierarchically porous structure. Finol et al.298 have shown that in the catalytic decomposition of 2-propanol and catalytic oxidation of n-butanol, the incorporation of niobium into hierarchical macro–mesoporous TiO2 catalysts results in a marked increase of the catalytic activity in comparison with the pure TiO2 catalysts. Fig. 44a shows that the meso and macro porosities in 3% Nb–TiO2 are retained after calcination. The macropores are arranged parallel to each other with a funnel-like shape. The macroscopic framework is composed of titania nanoparticles, which agglomerate to form the edges and the walls of the macro-channels. Concurrently this aggregation of nanoparticles creates a mesoporous network with a wormhole-like array. Fig. 44b shows the evolution of the intrinsic reaction rate of isopropanol (IPA) decomposition as a function of temperature for the x% Nb–TiO2 hierarchical catalysts. In the absence of Nb, isopropanol starts to be converted at 200 1C while for samples containing 3 and 5% Nb, the conversion of IPA started at 175 1C. In the absence of Nb, acetone and propene are produced in similar proportions, indicating that the basic/redox and acid sites are both used. However, in the presence of Nb (within the catalyst), propene is almost the only product observed. This result can be explained by the increase in the strength of the acid sites by Nb addition, which favours the isopropanol decomposition reaction into propene. They have further analyzed the influence of platinum and niobium introduction on hierarchically porous TiO2

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supports and catalytic performance in the total oxidation of n-butanol. Platinum catalysts supported on hierarchically porous TiO2 modified by niobium have been synthesized and tested for the complete oxidation of n-butanol. The incorporation of niobium into Pt/TiO2 catalysts leads to a significant increase in catalytic activity in comparison with the catalyst without niobium addition at a relatively low temperature.300 Ortel et al.302 have synthesized mesoporous Pd nanoparticle/ TiO2 catalysts (pluronic F127 template) and hierarchical macro– mesoporous Pd nanoparticle/TiO2 catalysts (templates F127 and PMMA spheres). Both of the synthesized catalysts show similar high catalytic activity in the selective hydrogenation of 1,3-butadiene although their pore structures differ significantly. This is the expected behavior when mass transport in the catalyst by pore diffusion is rapid compared with the reactions kinetics. Furthermore, Su et al.299 have synthesized hierarchically micro– meso–macroporous Nb doped TiO2 via a self-formation procedure and employed it as a catalytic support. By depositing noble metals such as Pd and Pt on Nb/TiO2, different catalytic performances are obtained. A marked increase in activity and CO2 selectivity is achieved when depositing Pt on Nb–TiO2 whereas an improvement in low temperature activities is achieved when depositing Pd on Nb–TiO2. Al2O3 is a particularly important material because of its application in catalysis. Recently, porous alumina has attracted much attention due to its special properties. The hierarchically ordered pore system as well as the crystalline nature of the sample make it a very promising material in various applications. Using hierarchically ordered porous Al2O3 as a supporter of active materials (e.g. Pd,303,304 Pt305) can enhance the diffusion of the reaction species, as well as the dispersion state and accessibility of metal nanoparticles. Novel palladium catalysts on alumina supports with the hierarchically macro-/mesoporous structure are prepared by a spontaneous self-assembly procedure and applied to selective hydrogenation of pyrolysis gasoline.304 In comparison with a commercial catalyst without the hierarchically porous structure, these novel catalysts exhibited much better catalytic performance, that is, higher activity and selectivity, mainly due to their unique structures of hierarchical mesopores and macropores. Bian et al.305 have firstly reported a procedure using soft/ hard dual templates to produce g-alumina with hierarchically ordered mesopores and macropores (HOMA). After loading with Pt nanoparticles for CO oxidation, the Pt/HOMA catalyst shows much better catalytic properties than Pt/commercial Al2O3, suggesting that the hierarchically porous structure is a better support for Pt nanoparticles because it facilitates the dispersion and formation of smaller Pt nanoparticles. A hierarchical macro– mesoporous monolithic Ni/Mg–Al–O catalyst is prepared via an impregnation route using polystyrene foam as the template. Compared with the pelletized sample that is prepared without a template, the macro–meso-porous Ni/Mg–Al monolith exhibits superior reactivity in terms of H2 production and also has lower CH4 yields at 700 1C and 800 1C. Furthermore, the monolithic catalyst maintains excellent activity and H2 selectivity after 100 h on-stream at 700 1C, as well as good resistance to coking and

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metal sintering.306 TiO2 presents a low metal–support interaction and excellent activities. However, the relatively low surface area and the poor thermal stability of the active anatase phase make TiO2 unsuitable for industrial applications. To overcome this problem, TiO2–Al2O3 mixed oxides have been used as supports taking advantage of the beneficial features of both TiO2 and Al2O3. Liu et al.307 have synthesized hierarchically macro-/mesoporous structured Al2O3 and TiO2–Al2O3 materials as supports to prepare novel Co–Mo–Ni hydrodesulfurization (HDS) catalysts. The effect of the hierarchically macro-/mesoporous structure on the HDS activity of the catalyst is studied. As the size of catalyst particles is 0.9–2.0 mm and internal diffusion limitations cannot be neglected, the apparent activities are quite different for the two catalysts. The hierarchically porous catalyst shows much higher HDS activities than the commercial catalyst, suggesting that the hierarchically porous structure can enhance the diffusion of substances to the active sites of the catalyst. A high surface area 3D ordered SnO2 inverted opal (IO) with walls composed of interconnected nanocrystals is reported using a facile approach with tin acetate precursors.308 Fig. 44c shows highly dispersed Pd nanoparticles immobilized in SnO2 IO. A high resolution TEM image in Fig. 44d displays nanocrystals with an interplanar d-spacing of 0.33 and 0.26 nm, corresponding to SnO2(110) and SnO2(101), respectively. The lattice fringes with a d-spacing of 0.23 nm belong to Pd(111). Pd NPs are found to be dispersed primarily around the SnO2 nanocrystals; no dominant facet relationship between the crystalline Pd and SnO2 is found and the arrangement is randomly distributed. The electrocatalytic activity of SnO2–Pd IOs is also investigated for formic acid oxidation (FAO). The catalytic performance of SnO2–Pd IOs is assessed in Suzuki cross coupling reactions displaying excellent catalytic activity at room temperature without activity loss after 3 cycles. SnO2–Pd IOs also show potential for fuel cell applications and in electrocatalytic oxidation reactions, with improved formic acid oxidation compared with commercial Pd/C (Fig. 44e). The enhanced catalytic performance is attributed to the uniform dispersion of small diameter Pd NPs and the hierarchical porosity throughout the IO network, facilitating access to the catalytically active sites. The hierarchical porosity and order at multiple length scales are well suitable for a range of catalytic processes involving liquids and gases. Hierarchically porous niobium and tantalum oxides have been synthesised using a self-formation process based on the hydrolysis and condensation kinetics of the metal ethoxide precursors, or a self-assembly process using a non-ionic polyethylene glycol template.309 The deposition of Pd nanoparticles is achieved using the wet impregnation method on calcined supports. The catalytic systems are pre-reduced in hydrogen to investigate their efficiency in the total oxidation of toluene and have increased activity in comparison with a series of reference catalysts based on hierarchically porous TiO2 and ZrO2, with minimal benzene formation. Our group has been very active in the application of hierarchically porous metal oxides for catalysis in water-gas shift reactions and in the oxidation of VOCs.1,6,9,18,298,299 A series of

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reviews have summarized our previous studies.1,6,9,18 Very recently, our group has achieved excellent results using hierarchically porous mixed metal oxides or hierarchically porous transitional metal doped metal oxides as catalyst supports for the abatement of VOCs.298,299 Pd and/or Au have been loaded on Nb-doped and V-doped macroporous–mesoporous TiO2 as catalysts (PdVTi, AuVTi and PdAuVTi) for the total oxidation of toluene and propene. Owing to hierarchically porous systems, these catalysts offer better performance compared with other porous systems.298 A very recent work of our group on a hierarchically nanostructured porous group of Vb metal oxides (by the self-formation mechanism with alkoxide precursors1,6,9) used as catalytic supports for Pd in the catalytic remediation of VOCs (toluene) has shown a prudent effect of hierarchically porous textures on the catalytic performances of obtained catalysts.298 Pd has been also loaded on V-doped hierarchically porous TiO2 formed by the self-formation phenomenon1,6,9 for the oxidation of butanone and a mixture of butanone and toluene. This efficient catalytic performance has been attributed at least partly to the hierarchically porous systems, offering easier mass transfer, larger active surface area, higher coke resistance and higher dispersion of metal active counterparts.298 Gold catalysts loaded on hierarchically porous systems have been used for the oxidation of CO and VOCs. A very recent review detailed the catalytic performances of these kinds of catalysts.299 For further discussion, readers can follow recent reviews on noble metal based catalysts supported on hierarchically macro–mesoporous metal oxide supports for the total oxidation of VOCs and the application (in general) of hierarchically porous catalysts cited in ref. 299. 4.2.4 Zeolite-based hierarchically porous structures for catalysis. Recently, the concept of introducing supplementary pores (meso-/macropores) into zeolite materials to fabricate hierarchically structured zeolites has gained surging interest due to the combined advantages of both multimodal porous systems and zeolite crystals. Hierarchically structured zeolites possess a superior catalytic activity compared with conventional zeolites, since they couple the catalytic features of both porous systems and zeolite crystals with improved accessibility and matter transport as a result of the additional larger porous networks all within a single system.1,18,299 4.2.4.1 Aluminosilicate zeolite-based hierarchically porous structures for catalysis. Aluminosilicate zeolites with intricate micropores and strong acidity have been widely used as heterogeneous catalysts in the petrochemical and fine chemical industries. ZSM-5 is an MFI-type zeolite with a special 10-ring channel structure. It has been widely adopted in catalysis especially in the shape selective catalysis in various industrial processes. Surfactants have been widely used as mesoporous templates to synthesize micro–mesoporous zeolites. For example, Narayanan et al.310 have synthesized nanosized ZSM-5 zeolite hexagonal cubic micro-blocks with different Si/Al ratios by using a nonionic surfactant as a mesoporous template. The as-synthesized ZSM-5 zeolite catalysts show excellent catalytic activities in the selective oxidation of benzyl alcohol. Liu et al.311 have synthesized ZSM-5 single crystals with b-axis-aligned mesopores using

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a designed cationic amphiphilic copolymer as a mesoscale template. This sample shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5. For example, in the condensation of benzaldehyde with hydroxyacetophenone, conventional ZSM-5 and mesoporous ZSM-5 (ZSM-5-M) give low (18.7%) and medium (45.7%) activities, respectively. In contrast, ZSM-5 single crystals with b-axisaligned mesopores (ZSM-5-OM) are very active, giving a conversion of 90.8%. Considering the similarities of ZSM-5, ZSM-5-M and ZSM-5-OM in terms of Si/Al ratios, aluminum distribution and acidic strength, as well as a larger particle size of ZSM-5OM than those of ZSM-5 and ZSM-5-M, the superior catalytic activities in the conversion of bulky substrates over ZSM-5-OM compared to that over ZSM-5-M should be directly assigned to the contribution of b-axis-aligned mesopores in ZSM-5-OM. It is possible that almost all b-axis-aligned mesopores in zeolite crystals are opened to the surface of ZSM-5 crystals and are accessible by bulky molecules. In contrast, a majority of the disordered mesopores in the ZSM-5 crystals may be located in the interiors of the crystals and hardly accessible by the bulky molecules. Ryoo et al.312 have successfully designed appropriate bifunctional surfactants that direct the formation of mesopores and micropores in zeolite structures simultaneously and yields 2 nm thickness of MFI zeolite nanosheets corresponding to the b-axis dimension of a single MFI unit cell (Fig. 45a and b). The catalytic performance of the MFI nanosheets is investigated using large organic molecules so that diffusion of the reactant molecules constrains the reaction. As expected, the catalytic activities of the MFI nanosheets are much higher than those of the conventional MFI zeolite. Another remarkable feature of the

Fig. 45 (a) SEM and (b) TEM images of MFI nanosheets with a multilamellar structure and coke deposition in MFI zeolite catalysts during methanol-to-gasoline conversion: (c) conventional MFI zeolite. (d) unilamellar MFI zeolite. (Reproduced from ref. 312 with permission, Copyright Macmillan Publishers Limited, 2009).

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MFI nanosheets is their increased catalytic lifetime in methanolto-gasoline conversion. Owing to methanol’s small size, there is no significant difference in the initial catalytic activity between the ultra-thin and the conventional MFI zeolite. With time on-stream, the MFI nanosheets are deactivated far more slowly than the conventional MFI (Fig. 45c and d). These enhanced catalytic activities can be attributed to a large number of acid sites located at the mesopore surface of MFI nanosheets, with the unilamellar MFI generally exhibiting higher activities owing to its larger external surface area after calcination. In addition, their reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. Besides, the synthesized MFI nanosheets also possess a long catalytic lifetime due to slow deposition of coke exclusively at external zeolite surfaces arising from facile mass transfer of coke precursors out of the zeolite micropores. Hard templates such as carbon are also used to synthesize hierarchically porous ZSM-5. For example, Tao et al.313 have prepared hierarchical ZSM-5 microspheres with nanorod orientedstructures by hydrothermal treatment of a carbon–silica monolith. The products exhibit large external surface and secondary mesoporous structures, resulting from the assembly of zeolite nanorods, which can lower the diffusion limitation for guest molecules. Several catalytic reactions including the alkylation of toluene with benzyl chloride, condensation of benzaldehyde with n-butyl alcohol and acetalization of cyclohexanone with methanol are conducted to verify the catalytic activity of hierarchical ZSM-5 and conventional ZSM-5. For the alkylation of toluene and condensation of benzaldehyde, where large molecules are involved, Hier-ZSM-5 shows a higher activity than Con-ZSM-5, while for the acetalization of cyclohexanone involving small molecules, the catalytic performance is very similar for both the ZSM-5 catalysts. For the alkylation of toluene, the conversion of toluene over Hier-ZSM-5 is 73.5%, which is nearly 2.5 times higher than Con-ZSM-5 (30.0%). Moreover, the selectivity percentage of the mono-alkylated/di-alkylated product over Hier-ZSM-5 is 90 : 10, while only a 1% di-alkylated product is obtained over Con-ZSM-5, indicating that the external acid sites facilitate the formation of the di-alkylated product. Dealumination is a conventional method and can be achieved by steam treatment at relatively high temperatures between 500 and 600 1C, or acid leaching using nitric or hydrochloric acid. The zeolitic framework can be partially removed due to the continuous hydrolysis of Al–O–Si linkages in the zeolites by steam at high temperatures, which leaves mesopores in the crystals.314 However, this mesoporosity is formed at the expense of zeolite crystallinity. Desilication, the selective extraction of Si from a zeolite framework by alkali treatment, has proved to be a more efficient method for creating mesoporosity in the zeolite crystals, since the amorphous silica fragments can be completely removed by an alkaline solution and the mesopores within the zeolites are open. As a result, alkaline-treated zeolites with mesoporosity exhibit good catalytic properties. Rutkowska et al.315 have synthesized a micro–mesoporous ZSM-5 zeolite by desilication as a catalyst for the synthesis of dimethyl ether

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(DME) from methanol. The catalytic performance of ZSM-5 is improved by generation of mesoporosity using the desilication method (alkaline leaching of framework Si). Furthermore, hierarchically mesoporous ZSM-5 can also be used as a novel catalytic support. For example, mesoporous ZSM-5 catalysts can serve as supports for a cobalt catalyst in Fischer–Tropsch synthesis.316 Application of such a catalyst reduces wax production compared with that of the conventional Co/SiO2 catalyst. The hierarchical structure combined with the acidity of meso-ZSM-5 improves the selectivity to the C5–18 fraction, due to large mesopores suppressing hydrocarbon cracking and preventing CH4 formation. The relatively large cobalt crystallites over the Co/SiO2 catalyst and the Co/meso-ZSM-5 catalyst result in high reducibility, which is possibly attributed to lower CO2 selectivity. Micro–macroporous structured zeolites are another popular and important members of the hierarchically porous zeolite family. The introduction of a macroporous system into a zeolite matrix results in a shorter diffusion path length hence faster mass transfer can be achieved in catalytic reactions. Meanwhile, a macroporous structure is believed to greatly increase resistance to coke deposition, which is a problem in conventional microporous zeolite catalysts. This thus increases the recyclability of the catalyst. Many efforts have been devoted to the development of various zeolite based materials with a macroporous structure. Using polystyrene (PS) microspheres as a hard template is a very effective way to create a macroporous system in zeolitic materials. In this case, hollow micro– macroporous structures can be formed by using PS microspheres as sacrificial templates. Xu et al.317 have synthesized micro– macroporous structured ZSM-5 zeolite using PS spheres as templates. The synthesized samples showed a high phenol conversion and selectivity of 2,4-di-tert-butyl phenol during the reaction owing to the presence of a hierarchical porosity and strong acidity of the pore walls. In addition, macroporous monoliths that not only serve as a macroporous scaffold but also as initial nutrients for the growth of a zeolitic microporous framework can also be used to synthesize hierarchical micro– macroporous zeolitic materials. For example, a hierarchically porous ZSM-5 monolith shows by transforming the skeleton of a macroporous silica gel high catalytic activity and a long catalytic lifetime for the catalytic cracking of large molecules.318,319 Recently, the conventional alkaline desilication procedure has been found to be an efficient method for the construction of hollow micro–macroporous zeolite architectures. For example, Mei et al.320 have synthesized HZSM-5 zeolite microboxes with a regular hollow core prepared by an alkaline desilication of ZSM-5 single-crystals (Fig. 46a and b). Model acid-catalyzed reactions, such as cumene cracking and a-pinene isomerization, are used to evaluate the catalytic performance of the HZSM-5 zeolite microbox. The former reaction is carried out in a pulse microreactor at 250, 300 and 350 1C, respectively. The latter reaction is performed at 150 1C in a three necked round bottom flask. As presented in Fig. 46c, the activity of the HZSM-5 microbox catalyst for gas phase cumeme cracking reaction is obviously higher than that of the parent single-crystalline HZSM-5.

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Fig. 46 TEM image and ED pattern of (a) the parent HZSM-5 and (b) HZSM-5 microboxes, (c) conversion of cumene cracking at 250, 300 and 350 1C over a HZSM-5 microbox catalyst and parent single-crystalline zeolite. The conversion data have been normalized to the same number of acid sites. Dashed-blocks: HZSM-5 microbox; solid-blocks: parent HZSM-5. (d) Conversion of a-pinene isomerization as a function of reaction time at 150 1C over a HZSM-5 microbox catalyst and parent single crystalline zeolite. The conversion data have been normalized to the same number of acid sites. (Reproduced from ref. 320 with permission, Copyright Macmillan Publishers Limited, 2009).

The difference in activity is reduced as the reaction temperature is increased, which may arise from the reduction of difference in molecular diffusion limitation with increasing temperature. The enhanced activity can be attributed to the improved transport properties of the zeolite microbox, since the hollow voids in the zeolite crystal shorten the diffusion path of the reactant and product molecules to and from the acid sites on the zeolite. A similar phenomenon is observed for the acid-catalyzed liquidphase a-pinene isomerization reaction, as illustrated in Fig. 46d. These results show that the zeolite microbox offers perspective as an efficient catalyst in zeolite catalysis. Potentially the desilication method can be used for the design and fabrication of industrial zeolite catalysts providing high activity and shape selectivity. Hierarchical catalysts having both micro–meso–macroporous structure and strong catalytic activity are much more desirable. Today, there is still a big challenge in introducing both mesopores and macropores into zeolitic materials. A novel quasi-solid-state crystallization method is employed to fabricate hierarchically porous ZSM-5 that has micro–meso–macroporous structures with high catalytic activities.321 The synthesized samples present a macroscopic network with relatively homogenous and straight channel-shaped macropores, which are arranged parallel to each other. The macropore walls are constructed entirely from uniform zeolite nanocrystals with high crystallinity, which result in relatively uniform mesopores or mesovoids in the products (Fig. 47). Such a micro–meso–macroporous structure with strong acidity has great advantages in catalytic reactions involving large organic molecules, as diffusion constraints and/or adsorption of

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Fig. 47 (a–c) SEM and (d and e) TEM investigations of micro–meso– macroporous aluminosilicate (MMM(2)). (Reproduced from ref. 321 with permission, Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 2011).

reactant molecules onto the strong acid sites are the main concern. Catalytic activities for the cracking of the large molecule hydrocarbon 1,3,5-triisopropylbenzene (TIPB) over various catalysts are compared. The kinetic molecular dimension of TIPB is much larger than the entrance dimensions of MFI zeolites, and such molecules cannot penetrate into the internal channels of zeolite ZSM-5. Cracking reactions can thus be realized only at the external surface of ZSM-5 crystals. With a contact time of 24 ms, HZSM-5, a commercial product with a Si/Al ratio around 75, is much less active (23.1%) owing to its relatively small pore size with respect to the large diameter of the reactant molecules. Al-MCM-41 and MCM-41 show no activity. In contrast, the synthesized samples with the same contact time yield the highest activity (88.6%), suggesting that the presence of large meso– macroporosity favors the access of TIPB molecules to the active sites. Recently, Liu et al.322 have synthesized hierarchical ZSM-5 zeolite fibers with macro–meso–microporosity by coaxial electrospinning. Uniform and tunable macropores are realized for the first time on such a kind of fibrous zeolite by regulating the rates of inner fluid in coaxial electrospinning. The as-prepared hierarchical ZSM-5 hollow fibers exhibit higher yield of propylene and good anti-coking stability than conventional ZSM-5, ZSM-5 nanocrystals and ZSM-5 solid fibers in the cracking reaction of iso-butane. The excellent catalytic performances are attributed to the combined effect of suitable acidity and hierarchical porosity on the promotion of the diffusion and the accessibility to the catalytic active sites. Zeolite Beta has been widely used as a catalyst for petrochemical processes, such as cracking, isomerization, alkylation and acylation owing to its unique three-dimensional microporous system with 12-ring openings, together with its suitable physicochemical properties. However, the major disadvantage in the catalytic properties of this microporous material is the slow diffusion of bulky molecules, which may limit the reaction rate and accelerate catalyst deactivation. Moreover, due to two different crystallographic phases which co-exist in the Beta zeolitic framework and can generate many defects in zeolite

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systems, the pore size is further reduced. In order to solve this problem, Xiao et al.323 have successfully synthesized mesoporous aluminosilicate zeolite Beta single crystal (Beta-MS) from a commercial cationic polymer that acts as a dual functional template to generate zeolitic micropores and mesopores simultaneously. Beta-MS exhibited a remarkably higher catalytic activity than conventional Beta in acid-catalyzed reactions involving larger molecules. Such superior catalytic activities of Beta-MS can be attributed to their highly mesoporous structures, which provide more accessible active sites and facilitate molecular diffusion of large substrates. Hierarchically structured porous material constructed from zeolite Beta nanocrystals with a micro-dual meso–macroporous structure can also be synthesized from a quasi-solid state crystallization transformation process in a glycerin medium (Fig. 48a–l).324,325 These hierarchically structured porous systems are homogeneously distributed throughout the final product, which greatly improves the accessibility of reactant molecules to the active sites of the materials used as catalysts. Different MMM-Beta materials have been obtained as a function of crystallization time. The MMM-Beta-8 (with a crystallization time of 8 hours) catalyst shows a superior catalytic esterification activity (45.5% in 48 h) compared with the normal Beta nanocrystals (26.3% in 48 h) (Fig. 48m). These results might be directly attributed to their stronger acidity, higher surface areas and the improved accessibility of the catalysts due to the introduction of a well-defined hierarchically meso–macroporous structure. Moreover, these novel hierarchical multiple pore

Fig. 48 SEM images of products MMM-Beta series: (a and b) MMM-Beta-3; (c and d) MMM-Beta-5; (e and f) MMM-Beta-8, TEM images of products MMM-Beta series: (g and h) MMM-Beta-3; (i and j) MMM-Beta-5; (k and l) MMM-Beta-8. (m) Conversion profiles of the esterification of palmatic/ oleic acids mixture into esterified alkyl esters as a function of the reaction time over different catalysts at 130 1C. (Reproduced from ref. 325 with permission, Copyright Elsevier, 2013).

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structures with strong acidic properties lead to superior advantages in catalytic reactions. Furthermore, hierarchically porous Beta can also be used as a novel catalytic support.18,299 For example, palladium particles supported on mesoporous Beta zeolite (Pd/Beta-H) have been used as catalysts for the hydrogenation of bulky aromatic pyrene. The Pd/BetaH catalyst shows a higher activity and selectivity for the products of deep hydrogenation than a conventional Beta zeolite-supported Pd catalyst (Pd/Beta) and Pd catalyst supported on mesoporous materials (Pd/Al-MCM-41) and alumina (Pd/g-Al2O3).326 In addition, in the presence of 200 ppm sulfur, the catalytic hydrogenation of naphthalene and pyrene over Pd/Beta-H exhibits good sulfur tolerance, compared with Pd/Al-MCM-41.327 Cheng et al.328 have reported that the use of mesoporous Beta as a support significantly improves the product selectivity of a supported ruthenium catalyst in Fischer–Tropsch (FT) synthesis. The mesoporosity and the unique acidity of meso-Beta probably contribute to the selective hydrocracking of the primary heavier hydrocarbons formed on Ru nanoparticles into gasoline-range liquid fuels. Y zeolite, a kind of solid acid material with uniform microporous structure, has been widely used as a component of fluid catalytic cracking (FCC) catalysts for refining oil and as a material for adsorbing and removing gaseous emissions because of its good hydrothermal stability and strong acidity. However, the innate small microporous channel will severely limit the diffusion performances. The hierarchical zeolites, which combine both the mesoporous structure advantage and the strong acidity, viewed as promising materials for catalytic applications, have attracted great attention over the past decades. Awala et al.329 have reported the rational design of template-free nanosized FAU zeolites with exceptional properties, including extremely small crystallites (10–15 nm) with a narrow particle size distribution, high crystalline yields (above 80%), micropore volumes (0.30 cm3 g 1) comparable to their conventional counterparts (micrometresized crystals), Si/Al ratios adjustable between 1.1 and 2.1 and excellent thermal stability leading to superior catalytic performance in the dealkylation of a bulky molecule, 1,3,5-triisopropylbenzene, probing sites mostly located on the external surface of the nanosized crystals (Fig. 49). Xing et al.330 have developed a two-step method consisting of acid leaching and base leaching to create hierarchical pores on a general microporous Y zeolite. The synthesized hierarchical zeolites Y (Y-AB, A: acid leaching, B: base leaching) are then used to support Co as catalysts to catalyze the hydrogenation of carbon monoxide to form hydrocarbons through Fischer–Tropsch synthesis (FTS) reaction. The CO conversion and C5-11 selectivity on Co/Y-AB catalysts are significantly increased compared with those on the pristine Y supported Co catalyst due to their optimized hydrocracking and isomerization function afforded by the hierarchical zeolite Y ¨nsted acid/Lewis acid (B/L) ratio and textural with the strong Bro properties. 4.2.4.2 Titanosilicate zeolite-based hierarchically porous structures for catalysis. Titanium silicalite-1 (TS-1) zeolite is considered to be a green chemistry catalyst, which is widely used in a series of

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Fig. 49 Highly crystalline FAU-type zeolites with nanosized particles and octahedral morphology. TEM pictures of ultra-small zeolite Y crystals with size of 10 nm (sample Y-10) (a) and zeolite Y crystals with size of 70 nm (Y-70) (b) nanosized zeolites; the corresponding high-magnification images of single nanocrystals are presented as insets. (c) Superior catalytic activity of the nanosized FAU compared with a commercial sample. Conversion of 1,3,5-triisopropylbenzene over Y-10, Y-70, zeolite Y crystals with size of 400 nm (Y-400) and a commercial zeolite Y sample provided by UOP (LZY-62) with crystal size of 1–2 mm and Si/Al of 2.5. (Reproduced from ref. 329 with permission, Copyright Macmillan Publishers Limited, 2015).

oxidation reactions using H2O2 as an oxidant under mild conditions. However, TS-1 suffers from severe diffusion limitations owing to its intrinsic microporosity. Therefore, synthesis of hierarchically porous zeolites has drawn intensive interest because of their improved catalytic performance. Hierarchically porous TS-1 zeolites have been obtained by acid and alkaline post-synthesis treatments331 or using hard templates such as carbon black or carbons obtained from sucrose carbonization.332–334 Likewise, the use of organosilane compounds as soft-templates has been found to be a successful strategy to obtain hierarchical TS-1 zeolitic materials.335 Jacobsen and co-workers have developed a hard-template method to prepare hierarchically porous TS-1 exhibiting superior catalytic activity towards epoxidation of cyclohexene due to the improved accessibility to active sites.332 Fang et al.333 have synthesized mesoporous TS-1 with an additional mesopore system formed by packing the TS-1 nanocrystals via the nanocasting route using nanoporous carbon (CMK-3) as a template. The catalytic performance of mesoporous TS-1 in the H2O2 oxidation of aromatic thiophene, the model sulfur

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containing molecule in transportation fuels, is tested. Compared with common TS-1, mesoporous TS-1 shows improved catalytic performance for thiophene oxidation and is able to catalyze the oxidation of a bulky sulfur containing molecule such as dimethyldibenzothiophene.336 Chen et al.334 have synthesized a series of hierarchically porous TS-1 zeolites using cheap tetrapropylammonium bromide as a microporous template and the carbon material from sucrose carbonization as a mesoporous template. The catalysis results show that hierarchically porous TS-1 zeolites not only show high removal rates of small molecular thiophene, but also exhibit high activities in the oxidation of bulky benzothiophene, which are superior to conventional TS-1. Interestingly, when a soft template, hexadecyltrimethoxysilane, is used to prepare hierarchical TS-1, the obtained material possesses the characteristics of both micro- and mesoporous materials and is applied as a catalyst in the oxidation of thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Compared with conventional TS-1 (CTS-1) and mesoporous material Ti-MCM-41, HTS-1 exhibits excellent catalytic performance in the oxidation of bulky and small reactants in the presence of H2O2.335 The novel quasi-solid state crystallization approach has been successfully extended to the synthesis of micro–meso– macroporous TS-1 zeolite.337 All the pores, at three length scales, within the synthesized zeolites are interconnected, which improves accessibility during catalytic reactions. Moreover, a unique structure in which many large aggregates constructed from zeolite nanocrystals and bonded together by the interconnecting amorphous region is observed in the final product MMM-TS-1 (Fig. 50). This novel structure greatly improved the stability of the hierarchically porous structure. More importantly, the tetrahedrally coordinated Ti species in this catalyst are stable during catalytic processes and thermal treatment. Catalytic activities of MMMTS-1(3) (this number indicates that the sample is obtained by 3 hour crystallization) and nanosized TS-1 in the epoxidation

of styrene and 2,4,6-trimethylstyrene are compared. Styrene conversion with the MMM-TS-1(3) catalyst can reach 85%, which is higher than that obtained over normal TS-1 nanocrystal catalysts (72%). This is directly attributed to the higher surface area of MMM-TS-1(3) of 580 m2 g 1 compared to TS-1 nanocrystals (260 m2 g 1), and also the well-defined meso–macroporous structure. Furthermore, the selectivity of MMM-TS-1(3) catalysts is far superior to that of TS-1 nanocrystals. Noteworthily, the conversion in the epoxidation of larger molecules (2,4,6-trimethylstyrene) with MMM-TS-1(3) as a catalyst reaches 45%, while the conversion is just 5% for TS-1 nanocrystals, as 2,4,6-trimethylstyrene molecules are too large to enter the micropore channels of the nanocrystals. The MMM-TS-1(3) catalyst has a much larger external surface area due to the presence of mesopores, which means many more active sites exist in this catalyst than in TS-1 nanocrystals. Meanwhile, macropores facilitate the penetration of reactants. Ti-containing b zeolites, with large 12-ring pore channels, are more efficient than TS-1 in the oxidation of bulky molecules. To further improve the diffusion performances, hierarchical Ti-b zeolites receive more and more attention. Tang et al. synthesized a hierarchically porous Ti-b zeolite via a simple and tunable solid-state reaction in which dealuminated microporous b zeolite precursors are mixed with organometallic precursor titanocene dichloride and then calcined in air to produce microporous Ti-b zeolites with various Ti contents.338 Their utilization in cyclohexene oxidation shows that the resultant materials are highly active. Ren et al.339 have prepared Ti-containing hierarchical b zeolites via a simple and Ti-contenttunable two-step post-synthesis procedure. In the catalytic process of alkene epoxidation, the synthesized Ti-mb zeolites showed high catalytic activities. In particular, when a bulky molecule, i.e., 1-dodecene, is used, the benefit of the extra mesoporous structures in Ti-mb is significantly observed and the substrate conversion (11.3%) is found to be nearly twice that with microporous Ti-mb (6.8%).

Fig. 50 (a and b) SEM and (c and d) TEM investigations of micro–meso– macroporous zeolite TS-1 (MMM-TS-1(3), the number in parenthesis indicates the synthesis times (hours)). (Reproduced from ref. 337 with permission, Copyright Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2011).

4.2.4.3 Aluminophosphate zeolite-based hierarchically porous structures for catalysis. In the family of aluminophosphate (ALPO)-based molecular sieves, silicoaluminophosphates (SAPOs) have considerable potential as acidic catalysts for the conversion of hydrocarbons. For example, microporous crystalline silicoaluminophosphate SAPO-34 is currently widely used as an efficient industrial catalyst for methanol-to-olefin (MTO) conversion. However, their relatively small micropores have significantly lowered their catalytic performances. With the introduction of mesoor/and macropores into microporous SAPO-34 zeolite crystals, the resulting hierarchical structure can markedly enhance the MTO performance.340–342 For example, Sun et al. have successfully synthesized hierarchically porous SAPO-34 catalysts using an organosilane surfactant as the mesopore director by direct hydrothermal crystallization.341 The hierarchically porous SAPO-34 crystals are obtained as cubic aggregates of small cubic-like nanocrystals (Fig. 51a–d). Catalytic tests of methanol conversion are carried out at 673 K in a fixed-bed reactor over the catalysts. The results are shown in Fig. 51e. Significantly, the hierarchically porous SAPO-34 catalysts exhibit a remarkably

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Fig. 51 SEM images of hierarchically porous SAPO-34 (SH1) (a and b) and SH2 (c and d) (SH1 stands for the hierarchically porous SAPO-34 catalysts from the starting gels with the optimized molar compositions of 1.0Al2O3 : 1.0P2O5 : 3.0morpholine (Mor, the template for micropores): 0.6SiO2 : 80 H2O : 0.1[3-(trimethoxysilyl)propyl]-octadecyldimethyl-ammonium chloride (TPOAC, the template for mesopores) under hydrothermal conditions at 180 1C and SH2 stands for the hierarchically porous SAPO-34 catalysts from the starting gels with the optimized molar compositions of 1.0Al2O3 : 1.0 P2O5 : 3.0Mor : 0.6 SiO2 : 80H2O : 0.15TPOAC under hydrothermal conditions at 180 1C and (e) methanol conversion variation with time-onstream over conventional microporous SAPO-34 (SM) and hierarchically porous SAPO-34 (SH1 and SH2) catalysts. (Reproduced from ref. 341 with permission, Copyright The Royal Society of Chemistry, 2014).

prolonged catalytic lifetime and enhanced light olefin selectivity compared with the conventional microporous SAPO-34 catalyst. Particularly, the lifetime of sample SH2 (466 min) with hierarchically porous structure is over four-times higher than that of SM (106 min) with only microporous structure. The prominent difference in the catalytic performance of hierarchically porous SAPO-34 and conventional microporous SAPO-34 catalysts can be explained by the difference in the level of porosity, acidity, as well as crystallite size. The generated mesopore in the hierarchically porous SAPO-34 can greatly enhance the transfer of the reaction products from the narrow pore to outside space and reduce coke formation and thus prolong the lifetime. Furthermore, the decreased acidic strength and acidic concentration of hierarchically porous SAPO-34 catalysts can also retard coke formation and thus prolong the catalyst lifetime. Yang et al. have prepared a novel hierarchical SAPO-34 monolith by the dry gel conversion of the amorphous silicoaluminophosphate monolith.340 The obtained material possesses macropores and mesopores besides micropores. The macroporous framework of the hierarchical SAPO-34 is constructed by interconnected spherical aggregates of cubic SAPO-34 crystals and mesopores formed by close stacking of cubic SAPO-34 crystals. Catalytic tests show that the hierarchical SAPO-34 possesses high catalytic activity compared to the conventional microporous SAPO-34 for methanol

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to light olefin (MTO) reaction. The better activity is mainly assigned to the existence of cubic SAPO-34 crystals and the hierarchical porosity. 4.2.5 Metals loaded on hierarchically structured porous supports as catalysts. Metal nanoparticles as heterogeneous catalysts have been attracting a great deal of attention due to their excellent catalytic performances. However, such metal nanoparticles generally have a severe aggregation problem. In order to solve this problem, they are usually immobilized on a solid support. The pore architecture of catalyst supports is an important factor facilitating the accessibility of reactants to catalytic sites, which is the key to improving catalytic activities. Numerous research studies have been devoted to design appropriate supported catalysts to achieve high catalytic activity, facile separation, reusability and recyclability. Among all the supports available, hierarchically porous materials containing well defined threedimensional open macro–mesoporous architecture, which facilitates the transfer of reactant molecules to catalytic sites and easy separation of the catalysts from reaction media, are the most widely used supports. In addition to using the above-mentioned materials having catalytic activities as catalyst supports, other inert materials such as carbon, silica and organoslilica can also be used as catalyst supports due to their high surface area, surface functionality and high chemical stability. 4.2.5.1 Metals loaded on hierarchically structured porous carbons for catalysis. Jiang et al. have utilized a functionalized, porous hard templating route together with hydrothermal and further carbonization treatment to fabricate micrometer-sized hierarchically porous carbon spheres with high monodispersity and hierarchical pores.343 The as-synthesized porous carbon spheres are further employed as a matrix for loading noblemetal nanoparticles to prepare Au/C composite microspheres by an in situ approach. The deposited Au nanoparticles with the size of 15–30 nm are well dispersed in the pores of porous carbon spheres. The reduction of 4-nitrophenol (4-NP) by sodium borohydride is chosen as a model reaction to evaluate the catalytic activity of Au/C composite microspheres. The Au/C composite microspheres possess higher performance toward the reduction of 4-nitrophenol than many other currently available AuNP-supported materials which may be caused by the hierarchically porous structure of the carbon matrix and the AuNPs highly dispersed within pore channels. Zhao et al.344 have synthesized honeycomb-like hierarchically porous carbons by a facile self-assembly strategy, in which the template/carbonprecursor composite is prepared in a one-pot approach by directly evaporating the reactant mixture solution of tetraethyl orthosilicate and phenolic resin. The as prepared hierarchically porous carbon is used as the support for a Pt catalyst for methanol oxidation. Pt nanoparticles are deposited on the as-prepared hierarchically porous carbon through a chemical reduction method (Fig. 52a–d). The catalytic activity and stability of the as-prepared porous carbon supported Pt catalyst (Pt/PC) for methanol oxidation have been investigated by cyclic voltammetry (CV) using a nitrogen-saturated 1.0 M CH3OH + 0.5 M H2SO4 solution. For comparison, the catalytic properties of a

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Fig. 52 TEM images of the as-prepared carbon-supported Pt catalysts: (a and b) porous carbon supported Pt catalyst (Pt/PC), (c and d) carbon black (Vulcan XC-72)-supported Pt catalyst (Pt/VC) and (a) cyclic voltammograms of carbon-supported catalysts in a nitrogen-saturated 1.0 M CH3OH + 0.5 M H2SO4 solution at a scan rate of 20 mV s 1, (b) the longterm stability of the ratio of forward anodic current peak (If) to reverse anodic current peak (Ir) for the catalysts at a scan rate of 100 mV s 1, (Reproduced from ref. 344 with permission, Copyright The Royal Society of Chemistry, 2009).

carbon black (Vulcan XC-72)-supported Pt catalyst (Pt/VC) have also been evaluated under the same conditions. Fig. 52e shows the CV curves of the as-prepared Pt/PC (solid line) and Pt/VC (dash line) catalysts obtained. The peak value of If for the Pt/PC catalyst is larger than that for the Pt/VC catalyst, suggesting a relatively high catalytic activity and promoted utilization of the noble-metal catalyst. The long-term stabilities of If/Ir for the two catalysts have also been investigated from the 20th to 180th CV cycles at a higher scan rate of 100 mV s 1. As can be seen from the curves displayed in Fig. 52f, the value of If/Ir for the Pt/PC catalyst decreases from 1.71 to 1.17 during the initial 20–80th cycles and remains constant in the following cycles, which is higher than that for the Pt/VC catalyst throughout the test. On one hand, compared with carbon black that consists of aggregated nano-grains, the as-prepared porous carbons show plenty of macropores and the thin walls with mesopores, which will facilitate the mass transport and improve the reacting efficiency. On the other hand, the high surface area of the support is always beneficial to obtain a favorable dispersion of metal particles and increase the utilization rate of the catalyst, while the microporous surface with small curvature will help to anchor the Pt particles to alleviate the agglomeration and promote the stability of the catalyst during continuous cycling. Hierarchically porous CaFe2O4/carbon fiber hybrids with enhanced microwave induced catalytic activity for the degradation of methyl violet (MV) from water are synthesized from kapok by a novel two-step process coupling pore-fabrication and nanoparticle assembly.345 The as-prepared samples exhibit characteristic hollow fiber morphology, and CaFe2O4 nanoparticles are

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uniformly dispersed on the surface of hollow carbon fibers (HCF). It is found that the microwave induced degradation of MV over CaFe2O4/HCF had a high reaction rate and a short processing time. Under the conditions of 50 mg L 1 and 50 mL MV solution, 0.3 g L 1 catalyst dose and a microwave power of 800 W, completely degradation of MV is obtained after 6 min under microwave irradiation. Hierarchically porous Cu–Ni/C composite catalysts are successfully fabricated by a bio-inspired route involving impregnation and calcination processes. Macroporous networks composed of interwoven carbon fibers with nano Cu–Ni particles, which had a high surface area (538 m2 g 1), are prepared by template-directed synthesis employing tissue paper as a bio-template.346 The obtained hierarchically porous structures exhibited high catalytic activity for acid fuchsin degradation in the MICO process due to the synergetic effects between the microwave (MW), porous carbon and Cu–Ni nano particles. The degradation of acid fuchsin is a fast process under MW irradiation. Not only the chromophore group but also the naphthalene ring and benzene ring in the acid fuchsin molecule can be destroyed mostly. Ni addition has a significant effect on acid fuchsin degradation because it enables the formation of more hydroxyl radicals under MW irradiation. Palladium nanoparticles supported on N-doped hierarchically porous carbon have been developed as a highly efficient, reusable and environmentally benign heterogeneous catalyst for the selective hydrogenation of various a,b-unsaturated carbonyls to their corresponding saturated carbonyls under mild conditions (Fig. 53a and b).347 Complete conversion of a series of

Fig. 53 (a) TEM image and (b) HRTEM image of N-doped carbon material, (c) the time-activity profile under different temperatures. (d) Temperature dependence of the rate of reaction in the Arrhenius coordinates for Pd/ CN. (Reproduced from ref. 347 with permission, Copyright The Royal Society of Chemistry, 2014).

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a,b-unsaturated carbonyls is achieved with excellent selectivity (499%) within 4 h. Moreover, the catalyst can be easily recovered by centrifugation and recycled up to 8 times without apparent loss of activity and selectivity. The considerable catalytic performance is attributed to the hierarchically porous network and incorporation of nitrogen atoms. The hierarchically porous network and mesoporosity inhibit the limitation of mass transfer of the reactant to the active sites, promote the free diffusion of molecules and shorten the diffusion distances in the channels. The introduction of nitrogen atoms leads to not only a very stable and uniform dispersion of Pd but also additional electronic activation of the metal nanoparticles (Fig. 53c). Sulfur, trace nitrogen and iron codoped hierarchically porous carbon foams (HPCFs) are fabricated by directly pyrolyzing a sulfurenriched conductive polymer, poly(3,4-ethylene-dioxythiphene)– polystyrenesulfonic acid aerogel, under an argon atmosphere.348 This simple pyrolysis treatment results in the molecular rearrangement of heteroatom sulfur, adjacent carbons and trace nitrogen/ iron from oxidants to form active catalytic sites of HPCFs. At the same time, the high porosity of HPCFs provides a large surface area for the uniform distribution of active sites, and allows rapid oxygen transport and diffusion. As a result, these HPCFs exhibit enhanced catalytic performances for oxygen reduction reaction via a direct four-electron reduction pathway in an alkaline electrolyte due to their high specific surface areas and hierarchically porous structures, as well as the homogeneously distributed active sites. Besides, they also display a higher stability and better methanol/ CO tolerance than the commercial Pt/C catalyst. 4.2.5.2 Metals loaded on hierarchically structured porous silica for catalysis. Miao et al.349 have prepared a novel Fe2O3/hierarchically porous silica (HPS) hybrid catalyst via the wetness impregnation method. Fe2O3 particles with the size of hundreds of nanometres formed in the porous skeleton of HPS. The obtained Fe2O3/HPS shows satisfactory catalytic ability for the photo-Fenton degradation of azo-dye Orange II. More than 90% of the dye is degraded under vis-light irradiation. The catalyst worked well in the pH range of 2.5–8.5, which is much wider than many normal traditional homogeneous Fenton systems. In addition, Fe2O3/HPS is stable, environmentally -friendly and could be reused more than 10 times without obvious loss of activity. Sen et al.350 have synthesized a novel vanado-silicate catalyst by using a polystyrene latex and a tri-block copolymer as a macroporous and mesoporous template (Fig. 54). Silica gel and hydrated vanadyl sulfate are used as the silica and vanadium source. The resulting porous vanado–silicate nanocomposite exhibits interconnected macroporous windows and meso–microporous walls containing well dispersed vanadyl species and it is used for the oxidation of a bulky organic molecule (cyclooctene). The vanadium species in the hierarchically ordered porous vanado-silicate showed an exceptionally high turn over number (TON) for the oxidation of cyclooctene. The hierarchically porous silica (HPS) supported CuO/CeO2 nanocrystal (CuO/CeO2/HPS) is prepared by sol–gel and multihydrothermal processes.351 CuO/CeO2/HPS still retains the co-continuous skeleton and through-macroporous structure.

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Fig. 54 (a and b) SEM images and (c and d) TEM images of vanado-silicate catalyst (MH 0.01) and a comparative study of oxidation of cyclooctene to various products using vanado-silicate catalyst (MH 0.01) calcined at 500 1C (MH 0.01/calcined) and V-impregnated hierarchically ordered porous silica (V-impregnated HOPS). (Reproduced from ref. 350 with permission, Copyright The Royal Society of Chemistry, 2012).

CuO/CeO2/HPS presents much better catalytic activity than the HPS supported CuO nanocrystal (CuO/HPS), the HPS supported CeO2 nanocrystal (CeO2/HPS) and CuO/CeO2 for CO and 1,2-dichlorobenzene (o-DCB) oxidation. This indicates that the effect of the HPS support on catalytic activity is as important as the synergetic effect between CuO and CeO2. Among all four CuO/CeO2/HPS catalysts (HPS-1, HPS-2, HPS-3, HPS-4, which are prepared by impregnating HPS in 10 ml of 1.5, 2, 2.5 and 3 mol l 1 Ce(NO3)3 aqueous solutions, respectively and 10 ml of 0.5 mol l 1 Cu(NO3)2 aqueous solution), HPS-4 is more satisfactory, with the 100% conversion of CO at 110 1C and 98.8% conversion of o-DCB at 500 1C, indicating that the loadings of CeO2 and CuO have a great influence on CO and o-DCB oxidation, and the effective dispersion of the active component CuO may have a positive effect on the catalytic activity using an on-site reduction methodology via a single-step three-hour process.352 The obtained Pd@HSQ catalyst has been employed in the Mizoroki–Heck cross-coupling reaction. High accessibility of reactant molecules, undetectable leaching of Pd nanoparticles and easy separation of the monolith from liquid media provide high catalytic activity, reusability and easy handling.

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Fig. 55 (a) (From left to right) digital camera images of the original dried HSQ, progress of reduction of metal ions in E3 (E3 refered to the reaction conditions of HSQ : H2O : acetone = 1 : 56 : 259 in molar ratio) at different reaction times, and the obtained crack-free monolith embedded with Au1Pd1 nanoparticles (hereafter E3 will be referred to as Au1Pd1). (b and c) SEM images of the original HSQ and E3, respectively. (d) HAADF-STEM image of E3. (e)–(g) EDS element mapping results of the same. (h) Plot of Ct/C0 against the reaction time for ten successive catalytic cycles. (Reproduced from ref. 353 with permission, Copyright The Royal Society of Chemistry, 2012).

4.2.5.3 Metals loaded on hierarchically structured porous hydrogen silsesquioxane for catalysis. Shimada et al. have synthesized hierarchically porous hydrogen silsesquioxane (HSQ) monolith supported mono-, bi-, tri- and tetrametallic metal alloy nanoparticles with controlled compositions and loadings via a low-temperature, single-step, liquid-phase methodology (Fig. 55a–g).353 The composition of Au and Pd in the immobilized nanoparticles is 1 : 1, and loading is 4 mol% with respect to the HSQ monolith in E3 (E3 refers to Au1Pd1 and the ratio of HSQ : H2O : acetone = 1 : 56 : 259). The catalytic activities of these nanocatalyst-embedded monoliths have been investigated using a model reduction reaction of 4-nitrophenol (4-NP). The bimetallic nanoparticles synthesized with different compositions show a unique dependence of the catalytic activity on the alloy composition. In most cases, metal alloy nanoparticles exhibit higher catalytic activity compared with their monometallic counterparts. This originates from the combined effects of the nanoparticle size and changes in the electronic properties of the nanoparticles by alloying. The highest catalytic activity is achieved for the Pd1Rh4-embedded monolith with 4 mol% loading. The PdRh-embedded monolith is then tested for application in a continuous flow reactor. The high conversion and feasibility at high flow rates demonstrate its potential as a heterogeneous catalyst that can be used in a continuous flow reactor. In addition, the Pd1Rh4 nanoparticle-embedded monoliths show acceptable reusability (Fig. 55h). 4.2.5.4 Metals loaded on hierarchically structured porous organosilica for catalysis. Grathwohl et al. have prepared platinum

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containing and platinum free hybrid ceramic foams by using polysiloxanes and platinic acid as precursors and expanded polystyrene beads as templates to generate macropores.354 For the analysis of their catalytic properties, CO oxidation is used as a test reaction. The thin struts of the materials allow a decrease of mass transport limitation caused by the microporosity. Furthermore, by coating the templates with the Pt containing precursor solution and infiltrating with Pt free solution, monoliths are generated with cell walls being coated with a thin Pt containing layer. The Pt loading of these materials is only 32% of the materials. Thus, mass transport limitation can be avoided in conjunction with good catalytic performances while minimizing the expensive Pt content. Gao et al.355 have developed hierarchically porous organosilica microspheres having mesoporous and macroporous structures by a template-free method using a designed flexible-bridged organosilica precursor. Then, the monodispersed Ag nanoparticles with mean sizes of 20–40 nm supported on organosilica microspheres are in situ prepared via a classical silver mirror reaction. The resultant Ag/hierarchically porous organosilica microspheres exhibit remarkable performance for the reduction of 4-nitrophenol to yield 4-nitroaniline. After seven successive cycles of the reaction, the conversion still remained 100%. The catalytic activity highly depends on the large surface areas of Ag NPs and direct contact between the reactant molecules and Ag NPs. The hierarchically porous organosilica microspheres offer a large number of active sites for the uniform growth and distribution of Ag NPs, leading to a large available surface area for electron transfer. In addition, the hierarchically porous structure is beneficial for the diffusion of reactant molecules. The above synergistic effects endow the Ag/hierarchically porous organosilica microspheres with excellent catalytic activity. 4.2.5.5 Metals loaded on other hierarchically porous supports for catalysis. Zhong et al.356 have developed a general and environmentally friendly synthesis route for embedding Co nanoparticles in hierarchically porous nitrogen doped graphite to catalyze the aerobic oxidation of alcohols to esters at room temperature under base-free and atmospheric conditions. The catalytic system features a broad substrate scope for aromatic and aliphatic alcohols as well as diols, giving their corresponding esters in good to excellent yields at room temperature and under atmospheric conditions without the assistance of any base additives. Adam et al.357 have synthesized platinum- and zinc/platinum-containing hybrid ceramics by the pyrolytic conversion of a mixture of precursors. The accessibility of the platinum particles inside the material and their efficiency for catalytic purposes are analyzed by using CO oxidation as a model reaction. The synthesized hierarchical ordered monolithic materials are macroporous foams with microporous struts providing a large specific surface area in the struts that is easily accessible by gaseous reactants (convection). For a more improved mass transport, the generation of macroporous foam with meso- and microporous struts is favorable. In addition, the catalytic activity in CO oxidation experiments is strongly dependent on the pyrolysis temperature and the presence of zinc.

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5.1

Hierarchically porous structures for adsorption

Nowadays, large amounts of waste chemicals are being generated, which cause serious environmental pollution. Water pollution is extremely harmful to human health. Watersoluble pollutants, including dyes and heavy metal ions, and air pollutants such as volatile organic compounds (VOCs) can be easily adsorbed by the human body through drinking and breathing and lead to numerous physical and mental diseases. Among various methods used for eliminating the chemical compounds in water, adsorption is considered to be an efficient method to separate a wide range of water pollutants. Hierarchically porous materials are widely used for removing harmful pollutants from the environment. In this section, we mainly focus on the adsorption of water-soluble pollutants and air pollutants by various hierarchically porous materials. 5.1.1 Hierarchically porous structures for adsorption of water-soluble pollutants. Dyes have become one of the most serious water pollutants with the development of industry and technology, which is one of the most serious problems facing mankind today.358,359 Hierarchically porous materials containing multi-level pores present enhanced adsorption performance compared with single-sized porous materials due to their improved accessibility and high surface area.360,361 Recent progress has focused on the design and synthesis of highly efficient adsorbents with novel porous hierarchy. Carbon materials with hierarchically porous structures are considered as promising candidates for the application of adsorption of dye molecules due to their very high surface area, porous hierarchy and low cost. For example, diatomite-templated carbons with macroporosity derived from replication of a diatom shell and microporosity derived from structure-reconfiguration of the carbon film have been developed by Liu et al., exhibiting higher adsorption capacity for methylene blue than the commercial activated carbon.362 In this work, diatomite is used as both a template and an adsorbent. Subsequently, they have demonstrated the enhanced porosity of diatomite-template carbons by two-step physical activation methods using CO2 and H2O as activation agents (Fig. 56).363 The pore parameters, such as the specific surface area and pore volume, as well as the micropore volume, show a great increase after two-step activation and are 2–3 times larger than those of the original carbon. The carbon products after two-step activation possess greatly increased pore parameters and a larger adsorption capacity for methylene blue than the original carbon; the maximum Langmuir adsorption capacity of the CO2-activated carbon for MB is 505.1 mg g 1.363 Metal oxides or bimetal oxides are intriguing adsorbent materials due to their special acid–base bifunctional activities.364 These metal oxides allow the fabrication of high surface area materials owing to the amorphous nature of the materials, which is considered as the key factor for their high adsorption capacity.365,366 However, though correlation between surface area and adsorption capacity is often documented, pore

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Fig. 56 SEM images of (a) the diatom shell of Dt (inset: edge macropores of diatom shell), (b) carbon pillars C/Dt-CO2, (c and d) carbon tubes of C/Dt-CO2; TEM images of (e) carbon pillars of C/Dt-CO2, (f) carbon tubes of C/Dt-CO2, (g) micropores of C/Dt-CO2; (h) the MB adsorption isotherms of C/Dt, C/Dt-CO2, and C/Dt-H2O; (i) linear fitting plots based on Langmuir isotherm model for MB adsorption. (Reproduced from ref. 363 with permission, Copyright Elsevier Ltd, 2013).

properties are equally important factors to consider when designing sorbents with high adsorption capacities.367 It is believed that the accessibility of the adsorbate to the internal surfaces is an essential factor for improving the adsorption performance apart from a high surface area.368–370 For example, Caruso’s group has demonstrated that the larger mesopores of micro–meso–macroporous amorphous TiO2/ZrO2 sorbents revealed higher polymer loading capacities irrespective of the surface area.369 Later, they found that hierarchically porous amorphous TiO2/ZrO2 beads with micropores and small mesopores exhibited lower uranyl and bisphosphonate sorption capacities than corresponding crystalline beads with no micropores and larger mesopores.368–370 Recently, they reported temperatureinduced modulation of alginate or hybrid alginate/TiO2/ZrO2 beads produced using a sol–gel templating technique to prepare hierarchically porous millimeter-sized amorphous TiO2/ZrO2 beads with adjustable mesopore sizes while maintaining high surface areas to fully harness the potential that high surface area amorphous matrices can offer.367 This work highlights the importance of enlarged mesopores within an amorphous matrix to attaining higher adsorption capacities.367 Fast adsorption of organic pollutants in water and very high adsorption capacity for dye molecules have been demonstrated by using novel hierarchically-packed tin dioxide sheets371 and hierarchical macro–mesoporous zirconia respectively.372 Moreover, a series of hybrid materials with hierarchically porous structures have been successfully synthesized and used as adsorbents for removing dye molecules, such as SiC ultrathin

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fibers,373 silicon–carbon–nitrogen hybrid materials,374 MIL-101 metal–organic frameworks,375 and layered double hydroxide (LDH),376 and so on. These resultant materials show remarkable adsorption performance due to their unique hierarchically porous structures created in the adsorbents. For example, the porous Si–C–N hybrid material with a high specific surface area and interconnected macropores displays excellent removal capability for dyes with triphenyl structures and a novel selective adsorption nature towards dyes with triphenyl structure compared with those dyes with azo benzene structures.374 Hierarchical mesostructured MIL-101 exhibits remarkably accelerated adsorption kinetics for dye removal in comparison with the bulk MIL-101 crystals.375 Although many hierarchically porous materials with remarkable adsorption performance have been synthesized, the use of powder counterparts has poor operation flexibility due to the complex and expensive filtration or centrifugation procedures for collecting the powder. Compared with the powder counterparts, the use of monoliths as industrial adsorbents presents wide operation flexibility. Moreover, given the separation problem of the powder adsorbents, magnetic separation as a promising strategy to fix this problem has been paid more and more attention. In this context, recently effort has been made to fabricate magnetic materials or magnetically-separable monoliths with hierarchically porous structures as industrial adsorbents.377–381 For example, a series of hierarchically porous iron oxides including a-Fe2O3, Fe3O4 and g-Fe2O3 with different hollow379 and bowknot-like structures have been fabricated by a precursor thermal conversion method.378 These porous iron oxide superstructures exhibit ferromagnetic properties at room temperature and selective adsorption ability for different dye molecules. The g-Fe2O3 hierarchically nanostructured hollow microspheres show a better adsorption ability over salicylic acid (SA) than methylene blue (MB) and basic fuchsin (BF), and Fe3O4 hierarchically nanostructured hollow microspheres have the best performance for adsorbing MB.379 However, the hierarchically a-Fe2O3 bowknots show better adsorption ability for Congo red (CR) than Fe3O4 and g-Fe2O3 superstructure.378 Moreover, very recently, Liu et al. have synthesized magneticallyseparable hierarchically porous graphitic carbon monoliths containing magnetic nanoparticles as magnetically separable adsorbents for dyes.380,381 These monoliths are fabricated via multi-component co-assembly in a polyurethane (PU) foam scaffold and possess a hierarchically macro–mesoporous structure with a high specific surface area (725 m2 g 1), large pore size (7.2 nm) and pore volume (0.74 cm3 g 1), and high saturation magnetization (2.3 emu g 1), which make them ideal candidates for the adsorption of dyes in aqueous solution (Fig. 57).381 Welldispersed and homogeneous metallic Ni nanoparticles can be incorporated into a carbon matrix (Fig. 57b and c). The resultant monoliths exhibit good adsorption characteristics desirable for application in the adsorption of methyl orange (MO) (440 mg g 1) (Fig. 57e) and significant easy separation under an external magnetic field.381 5.1.2 Hierarchically porous structures for adsorption of heavy metal and metal ions. Toxic heavy metal and metal ion

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Fig. 57 (a) Photographs and SEM of HPGCM-Ni-900; typical TEM (b) and HRTEM images (c) of HPGCM-Ni-900; magnetization curve (d) of HPGCM-Ni-900 recorded at room temperature; Insets are the demonstration of magnetic separation of HPGCM-Ni-900 by applying an external magnet (inset in d). (Reproduced from ref. 381 with permission, Copyright Wiley-VCH, 2015).

contaminated natural water has become a serious environmental problem. To solve this problem, exploration of efficient heavy metal and metal ion removal methods is highly desired. Adsorption is the most attractive removal method because of its simplicity, convenience and high removal efficiency. Among various materials available for adsorption, metal oxides with hierarchically porous structures are appealing adsorbents due to their good chemical and thermal stability, customizable morphology and high surface area.368,382–385 Besides, the interconnected hierarchical pore system can facilitate rapid diffusion and transportation of guest molecules as well as promote access to the active sites.386,387 For example, Sun et al. have synthesized hierarchically nanostructured TiO2 spheres with controllable morphologies based on a novel amphiphilic polymeric TiO2 precursor.385 The as-obtained spheres have hierarchical structures with specific surface areas larger than 200 m2 g 1 and mean pore sizes of several nanometers. The as-obtained specific hierarchically nanostructured TiO2 spheres exhibit a considerably higher adsorption capability for Cr(VI) anions in aqueous solution compared with the previously reported TiO2 nanomaterials, showing a high potential for heavy metal ion sequestration applications (Fig. 58a–d). Nie et al.388 have synthesized hierarchical meso–macroporous g-Al2O3 hollow microspheres by a microwave-assisted hydrothermal route, using KAl(SO4)2 and urea as raw materials. The results show that the obtained g-Al2O3 hollow microspheres are about 0.8–1.2 mm in diameter with a shell thickness of approximately 200 nm and the high surface area of 242 m2 g 1. The prepared g-Al2O3 hollow microspheres also exhibit enhanced adsorption performance for Cu2+ in solution than those prepared by the hydrothermal (HT) method and commercial g-Al2O3. Xu et al.384 have synthesized hierarchically porous CeO2–ZrO2 nanospheres and examined their suitability as arsenic sorbents. CeO2–ZrO2 hollow nanospheres show an adsorption capacity of 27.1 and 9.2 mg g 1 for As(V) and As(III), respectively, at an equilibrium arsenic concentration of 0.01 mg L 1 (the standard for drinking water) under

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Fig. 58 SEM images of (a) hierarchically porous TiO2 spheres, (b) hollow TiO2 spheres, and (c) raspberry-like TiO2; (d) typical adsorption isotherms of the as-prepared mesoporous TiO2 spheres and P25 with varying compositions; (Adapted with permission from ref. 385 (a–d), Copyright Royal Society Chemistry, 2014).

neutral conditions, indicating a high arsenic removal performance of the adsorbent at low arsenic concentrations. Such a great arsenic adsorption capacity is attributed to the high surface hydroxyl density and the presence of a hierarchically porous network in the hollow nanospheres. The adsorption of arsenic on the hollow nanospheres is found to be completed through the formation of a surface complex by substituting hydroxyl with arsenic species. A field application of the adsorbents for removal of arsenic in groundwater demonstrates that they are promising for practical drinking water treatment. Drisko et al.389 have also prepared zirconium titanium mixed oxide beads with porosity on multiple length scales. In this facile synthesis, the bead diameter and the macroporosity can be conveniently controlled through minor alterations in the synthesis conditions. The precursor solution consisted of poly(acrylonitrile) dissolved in dimethyl sulfoxide to which was added the block copolymer Pluronic F127 and metal alkoxides. The millimeter sized spheres are fabricated with different macropore dimensions and morphologies through dropwise addition of the precursor solution into a gelation bath consisting of water (H2O beads) or liquid nitrogen (LN2 beads). H2O beads and liquid nitrogen beads obtained after calcination (550 1C in air) have surface areas of 140 and 128 m2 g 1, respectively, and various pore architectures. The H2O-derived beads have much larger macropores (5.7 mm) and smaller mesopores (6.3 nm) compared with the LN2-derived beads (0.8 mm and 24 nm, respectively). The macropore diameter and morphology greatly affected surface accessibility. Beads with larger macropores reached adsorption equilibrium much faster than the beads with a more tortuous macropore network. Metal phosphonates, an important class of inorganic– organic hybrid materials, have been extensively used as adsorbents. A large number of hybrid open-framework microporous

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or mesoporous metal phosphonate materials with various chemical compositions and organic groups have been developed and exhibit excellent performances. For example, Ma et al.390 have synthesized a family of hybrid surface-phosphonated titania, titania-phosphonate, and titanium phosphonate porous materials with different organic groups in the network by utilizing a series of organophosphonic acids as the coupling molecules. It was revealed that the integrity of organic groups was retained inside the framework of the synthesized hybrids. All the synthesized adsorbents have the capacity to adsorb heavy metal ions with definite selectivity, which depend on the nature and positions of organic functional groups.390 Later, they synthesized inorganic–organic hybrid titanium phosphonate materials with a hierarchically porous structure by a mild solvent evaporation strategy using 1-hydroxyethylidene1,1-diphosphonic acid as an organophosphorus coupling molecule.391 The hybrid materials are used as adsorbents for the liquid phase adsorption of Cu2+ ions in water and the gas phase adsorption of CO2, showing high adsorption capacity and good reusability, which make them promising adsorbents for practical application in environmental remediation.391 Recently, they have developed hollow manganese phosphonate microspheres of an inorganic–organic hybrid with hierarchically porous shells through a template-free hydrothermal method using ethylene diamine tetra(methylene phosphonic acid) as the coupling molecule.392 The adsorption process follows pseudo-second order reaction kinetics, as well as the Langmuir isotherm, indicating that Cu2+ is a monolayer adsorbed on the hybrid by chemical complexation. Moreover, Zhu et al.393 have synthesized hierarchically porous zirconium phosphate (ZrP) monoliths with tunable compositions (from Zr(HPO4)2 (Zr : P = 1 : 2) to NaSICON (Na super ionic conductor)-type ZrP (Zr : P = 1 : 1.5)) as well as macropore sizes (from 0.5 to 5 mm). The as-synthesized ZrP monoliths with a high reactive surface area (600 m2 g 1) and relatively high mechanical strength (a Young’s modulus of 320 MPa) are applied to ion adsorption. A simple syringe device inserted tightly with the ZrP monolith as a continuous flow setup is demonstrated to remove various toxic metal ions in aqueous solutions, which shows promising results for water purification.393 Porous silicas are also used as adsorbing materials to anchor binding groups for pollutants due to their large surface areas, well-defined pore structures and tunable surface properties. Various functional groups have been grafted onto the surface of mesoporous silicas to selectively adsorb different compounds. For example, Shi et al.394 have fabricated hybrid hierarchically porous silica adsorbents by the sol–gel method and modified them with thiol or sulfonic groups. The adsorption capacity of sulfonic acid functionalized hierarchically porous silica microfoam for some basic dyes, such as fuchsin basic, is approximately 1145.2 mg g 1. A mixture of hierarchically porous silica micro-foam and sulfonic acid functionalized hierarchically porous silica micro-foam shows good purification capacity to simulate multicomponent wastewater, containing different inorganic and organic chemicals. The removal rate for any of the pollutants is more than 90%, and can even reach 100% for

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metal ions and dyes.394 Furthermore, carbon spheres have also been used for metal ion adsorption. For example, Zhao et al.395 have prepared carbon spheres with tunable morphologies and porous structures using successive steps: liquefaction, resinification, self-assembly, carbonization, and KOH activation. The diameter of the carbon spheres is in the range of 10–25 mm, and the vermicular pore size of the carbon spheres has a range of 2.0–5.0 nm (mesopores) and 1.5–2.0 nm (micropores). The carbon spheres have a high surface area of 1064 m2 g 1 and a large pore volume of 0.503 cm3 g 1. The adsorption capacities of hierarchical carbon spheres for removal of Cr(III) and Pb(II) are 70% and 90%, respectively. Layered double hydroxides (LDHs), anionic clays, are well documented as effective ion exchangers/adsorbents for removal of a variety of anionic pollutants. Zhou et al.396 have synthesized lithium/aluminum layered double hydroxides (Li/Al-LDHs) via a facile hydrothermal route, using lithium and aluminum chloride mixed solutions with various molar ratios (Li+/Al3+ = 2, 3, 4, 5) as precursors and urea as a precipitating agent. The Li/Al-CLDHs contain three types of hierarchically porous organizations such as small mesopores (ca. 4.5–10 nm), large mesopores (ca. 40–50 nm) and macropores (ca. 200–500 nm). The as-prepared Li/Al-CLDH samples exhibit an excellent adsorption capacity of 158.7 mg g 1 towards fluoride species in water. The superior sorption capacity of Li/Al-CLDHs is attributed to their unique hierarchically porous structures and high specific surface areas.396 5.1.3 Hierarchically porous structures for adsorption of air pollutants: VOCs and special organic molecules. Another important application of hierarchically porous materials is the removal of volatile organic compounds (VOCs) by adsorption. VOCs are toxic or even carcinogenic (such as benzene) and the most common air pollutants emitted from the chemical, petrochemical, pharmaceutical, building materials, and printing industries.397 Most VOCs are the main sources of photochemical reactions in the atmosphere, which lead to various environmental hazards.398 Many studies have been conducted to design and synthesize hydride composites with desired hierarchically porous structures for the adsorption of benzene399–403 and benzene derivatives.404–406 Yuan’s group has reported the preparation of hierarchically porous nanocomposites of diatomite-based ceramic monoliths coated with silicalite-1 nanoparticles for benzene adsorption.400 The hierarchical porosity of the resultant nanocomposites is due to the inherent micropores of Sil-1, the mesopores resulting from the stacking of Sil-1, and the hierarchical macropores of ceramic supports. Given their porous hierarchy, the nanocomposites exhibit a much higher benzene adsorption capacity (133.3 mg g 1 (Sil-1)) compared with that of a commercial micron-sized ZSM-5 product (66.5 mg g 1) and a synthesized Sil-1 (SilSYN, 94.7 mg g 1). Moreover, adsorption–desorption rate constants of the nanocomposites are three and five times higher than those of the ZSM-5 and SilSYN, respectively.400 Subsequently, they investigated the effects of desilication and NaOH etching pretreatment on optimizing the porosity parameters of the resultant nanocomposites, resulting in the improvement of their benezene adsorption performance.399,401 Xiao’s group has successfully

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synthesized hierarchically porous polymers in acetone medium through a simple solvothermal polymerization.403 In their work, a controllable transition from macro/mesoporosity to meso/ microporosity was achieved by simply changing the amount of acetone. The resultant nanoporous materials have a large BET surface area (up to 420 m2 g 1), large pore volumes (up to 1.73 cm3 g 1), and superhydrophobicity due to their highly porous structures, which endow these nanoporous materials with an outstanding adsorptive capacity and very high selectivity to toxic volatile organic compounds (VOCs).403 Moreover, a series of adsorbents with significant porous hierarchy and various chemical compositions have been developed for the removal of benzene derivatives, such as a carbon-based hybrid adsorbent for four benzene derivatives including 4-chlorophenol, phenol, benzoic acid and 4-hydroxylbenzoic acid in aqueous solution,404 and porous aerogels with hierarchically porous structure based on imine chemistry for the adsorption of aromatic toluene.405 Recently, some attempts have also been made to fabricate hierarchically porous materials for removal of organic compounds, including phosphates,407,408 lysozyme,409 neonicotinoid insecticides,410 phenylurea herbicides,406 ammonium,408 microcystin-LR411 and mercaptan.412 In addition, hierarchically porous materials with strong hydrophobicity have also been used as superadsorbents for oil spillage cleanup. Compared with the traditional adsorbents with single porosity for the treatment of oil spillage, such as zeolites,413 wool fibers,414 kapok fibers,415 and polystyrene fibers,416 hierarchically porous materials are considered as the more valuable candidates owing to their higher oil loading capacity, lower density, higher selectivity of oils against water and higher adsorption capacity.377,417–420 For example, hierarchically porous sponges from microfibrillated cellulose fibers exhibit rapid and high selectivity adsorption performance for various oils and an ultra-high adsorption capacity of 88–228 g g 1 (Fig. 59) because of their ultralow density (0.0024 g cm 3) and high porosity (up to 99.84%).418 The adsorbed oil can be readily and rapidly recovered by means of simple mechanical squeezing, while the superabsorbent could be reused at once without any other treatment (Fig. 59). The superabsorbent shows excellent recyclability and can be reused for at least 30 cycles while still maintaining its high oil adsorption capacity (137 g g 1 for pump oil) (Fig. 59). Moreover, hierarchical macro–mesoporous carbon monoliths,377 hierarchically porous PVDF/nano-SiC foams419 and hydrophobic monolithic hierarchically porous silica420 have also been prepared as the candidates for the application of oil removal. 5.1.4 Hierarchically porous structures for CO2 capture. Anthropogenic CO2 emission from the burning of fossil fuels has become a crucial matter in view of its great environmental and societal implications.421–423 Therefore, there is a great need to develop practical and efficient CO2 capture methods at anthropogenic point sources424,425 and effective sorbent materials.426 Recently, the physical adsorptive removal of CO2 by porous solids has attracted much attention.427–436 The key pore parameters that determine if the adsorbents have potential for use in CO2 capture are the surface area and pore volume together with the pore size.

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Fig. 59 SEM images of 0.5 MCF (microfibrillated cellulose fibers (MCF) aqueous suspensions with concentrations of 0.5%) are prepared MCF sponges: (A) unmodified 0.5 MCF0, (D) modified 0.5 MCF0 (prepared without the treatment revolution of MCF aqueous suspensions with concentrations of 0.5%); optical images of water and oil droplets on the unmodified 0.2 MCF6000 (prepared after the treatment revolution (6000r) of MCF aqueous suspensions with concentrations of 0.2%) (B) methyltrimethoxysilane (MTMS) modified 0.2 MCF6000 (E); the hydrophobicity comparison of pristine and hydrophobic 0.2 MCF6000 (C). A mirrorreflection phenomenon occurred for the MTMS modified 0.2 MCF6000 sponge when it is immersed into water (F). Droplets of water and pump oil are colored with CuSO4 and oil red O, respectively. High efficiency in oil recovery and adsorbent reusability of the hydrophobic 0.2 MCF6000 sponge (a–h). (Reproduced from ref. 418 with permission, Copyright Royal Society Chemistry, 2015).

Hierarchically porous materials are considered effective adsorbents for CO2 capture due to their significantly improved mass transport facilitated by the macropores and high surface area and pore volume arising from micro-/mesopores.1 Among these promising adsorbents, hierarchically porous carbons, owing to their low cost, high resistance to both alkaline and basic media, high hydrophobicity, high thermal stability and easy-to-design pore structures, are considered to be the most promising adsorbents for CO2 capture.437–446 For example, Estevez et al.437 have reported an ice templating coupled with hard templating and physical activation approach for the synthesis of hierarchically porous carbon monoliths (HPCs) with tunable porosities across all three length scales (macro- meso- and micro) (Fig. 60a and b). These HPCs present an ultrahigh surface area of 2096 m2 g 1 and a specific pore volume of B11.4 cm3 g 1, leading to an exceptional ability when used as scaffolds for amine based CO2 capture, achieving a maximum CO2 capacity of 4.2 mmol g 1 (Fig. 60c–f). Srinivas et al. have developed a method for obtaining highly hierarchical micro–meso–macroporous carbons with a simultaneously high surface area (up to 2730 m2 g 1) and

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ultra-high total pore volume (up to 5.5 cm3 g 1) by carefully controlled carbonization of in-house optimised metal–organic frameworks (MOFs).438 Given their rich pore properties, the resultant materials show significantly enhanced CO2 adsorption (e.g., over 27 mmol g 1 at 30 bar and 27 1C) at high-pressures and temperatures of up to 75 1C, compared with their MOF counterparts and other porous carbons reported in the literature.438 Graphene-based carbons with a similar hierarchical micro–meso–macroporous structure prepared by carbon dioxide activation are also synthesized for efficient CO2 adsorption.444 Besides hierarchically porous carbons, zeolites with porous hierarchy are also considered to be the most promising adsorbents for CO2 capture due to their high surface area and tunable porosities.447,448 A successful example demonstrated by Akhtar et al. shows the utilization of binderless zeolite 13X monoliths with a hierarchical porosity as adsorbents produced by slip casting followed by pressureless thermal treatment. These hierarchically porous zeolite 13X monoliths display a CO2 uptake of more than 29 wt%.448 In addition, porous organic polymers (POPs) with hierarchical porosity have also been used for CO2 capture.449,450 The broad range of pores allows for faster transport of molecules through the hierarchically porous POPs, resulting in increased diffusion rates and faster CO2 uptake compared with POPs with micropores only.449 To improve the CO2 adsorption capability of the hierarchically porous carbons, another efficient strategy is to introduce basic nitrogen sites onto their surface, which can further promote the interaction between CO2 molecules and the carbon surface.451–453 Li et al.439 have reported that hierarchically porous N-doped carbon microflowers with a high-surface area and, importantly, a large micropore volume present remarkably high CO2 adsorption capacities of 6.52 and 19.32 mmol g 1 at 0 1C (273 K) and at two pressures, 1 bar and 20 bar, respectively. N-doped hierarchically porous carbon materials with a tunable surface area, pore volume, narrow average mesoporous size and importantly high N content (4.54%) synthesized in a large-scale fabrication system reported by Liu et al.454 show good performance as adsorbents for selective CO2 capture. Zhu et al.441 have described a facile and efficient ‘‘spheridization’’ method to produce nitrogen-enriched hierarchically porous carbon spheres millimeters in diameter, with intricate micro-, mesoand macro-structural features. Given the highly developed microporous structures and the relatively high pyridinic nitrogen content inherited from the co-polymerized acrylonitrile and acrylamide precursor, incorporated within the hierarchically porous structures, the resulting carbon spheres show a relatively high CO2 uptake of 16.7 wt% under 1 bar of CO2, and particularly, an exceptional uptake of 9.3 wt% under a CO2 partial pressure of 0.15 bar at 25 1C. 5.2

Hierarchically porous structures for separation

Materials with a high specific surface area have long been utilized as adsorbents and separation media for both gas and liquid chromatography. Since the invention of liquid chromatography in packed columns by Tswett, many efforts have been

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made to develop packing materials starting from finely crushed particles, through spherical particles, to spherical unimodal-sized particles.1 After the widespread development of gas chromatography for simple and quick separation, the versatility of liquid chromatography is expanded to a broader range of analytes owing largely to the development of column technology.455 To improve the performance of liquid chromatography, smaller particles have been used as packing materials for increasing the number of theoretical plates. However, the columns packed with smaller particles can increase the plate numbers only in the case when they are used in an instrument that can generate and endure high enough pressure to drive the liquid at an acceptable linear velocity.1 Compared with the conventional particle-packed columns, monolithic columns with co-continuous and hierarchically porous structures can provide better total performance owing to their much faster separation at much lower backpressure.456–460 In this section, several representative examples of the hierarchically porous materials for the application of separation are described. Hierarchically distributed macro–mesoporous monolithic silica materials have found prominent application as novel separation media for high performance liquid chromatography (HPLC) owing to their high permeability.1,455 The thin skeletons lead to high efficiency based on fast equilibration of solutes between a mobile liquid phase and a stationary solid phase on the mesopore walls, while the large macropores contribute to high permeability.455 For example, Minakuchi et al. have developed a series of SiO2-based monolithic columns with hierarchically porous structure, demonstrating outstanding achievements for the applications of high-throughput screening and complex mixture separation.461,462 The macropores in the resultant monoliths allow facile transport of fluids and the mesopores enlarged the surface area to establish contact between the fluid and the solid surface.463 Shi’s group has demonstrated the synthesis of silica spheres possessing both mesopores and penetrable macropores for the application for HPLC.464,465 The resultant materials exhibited fast separation for benzene, benzaldehyde and benzyl alcohol (in two minutes at a flow rate of 7 mL min 1), at much lower (o1/2) pressures under the same chromatography conditions in comparison with a commercial Kromasil ODS column.464 These hierarchical macro–mesoporous silica materials have also been used for the separation of seven aromatic hydrocarbons and six sulfonamides, respectively, and show fast and low pressure separation.466 Besides SiO2-based columns, metal oxide based columns have also attracted much attention recently owing to their excellent pH and thermal stability, great mechanical strength and ability to selectively adsorb organophosphate compounds.467–469 An initial strategy is to modify the skeleton surfaces of SiO2 monoliths to fabricate hybrid compounds for monolithic non-SiO2 columns, such as ZrO2-coated SiO2 in fused SiO2 capillaries470,471 and TiO2coated SiO2.472,473 Recently, the synthesis of pure TiO2 and ZrO2 monoliths have been achieved successfully for the application of liquid separation.463,474–476 For example, Konishi et al.463 have reported that anatase TiO2 monolithic gels with well-defined bicontinuous macropores and microstructured skeletons possess

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Fig. 60 SEM images of HPC scaffolds: (a) a similar interconnected macroporous structure after etching; representative TEM images of (b) KCU-C 12-1 (KCU-C 12-1 refered to the sample with average colloidal silica size of 12 nm and mass ratio of silica to glucose of 1). In addition, scale bars for TEM images are 200 nm for the larger images and 20 nm for the inset images); (c) CO2 sorption capacity of polyethyleneimine (PEI)KCU-C 4-3 (73 wt% PEI) composite (KCU-C 4-3 refered to the sample with average colloidal silica size of 4 nm and mass ratio of silica to glucose of 3); (d) capacitive frequency response of KCU-C 4-1-4 (KCU-C 4-1-4 refered to the sample with average colloidal silica size of 4 nm and mass ratio of silica to glucose of 1 and the time for CO2 activation is 4 h) measured in symmetrical two electrode configuration; (e) variation of specific capacitance with sweep rate, measured from cyclic voltammetry data for KCU-C 4-1-4 in 1 M H2SO4 within the potential range 0–1 V (vs. Ag/AgCl); (f) Ragone plot for KCU-C 4-1-4 and GMCS-NH3 (NH3 treated, hierarchical, graphene mesoporous carbon spheres) measured in symmetrical two electrode configuration using different current densities in 1 M H2SO4 within the potential range 0–1 V. (Reproduced from ref. 437 with permission, Copyright Royal Society Chemistry, 2013).

phospho-sensitivity and exhibit excellent chromatographic separation of phosphorus-containing compounds. In contrast to the SiO2 rod column that has no ability to separate these phosphates, the resultant TiO2 rod columns exhibit separation efficiency due to their ability to bind to the phosphate group.463 Hasegawa et al.477 developed a separation medium based on hierarchically porous TiO2 monoliths for HPLC. The obtained TiO2 monolithic columns calcined at 200 and 400 1C showed good separation of polar benzene derivatives as well as organophosphates, and the theoretical plate number of the TiO2 column calcined at 400 1C was more than 48 000 m 1 in the normalphase separation of polar benzene derivatives.477 Moreover, Ma et al.478 have demonstrated the application of a series of

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hybrid materials based on hierarchical micro-/mesoporous metal phosphonate materials for the separation of basic compounds of o-toluidine, propranolol, and pyridine.

6. Hierarchically porous structures for sensing Gas sensors play an important role in environmental monitoring, controlling chemical processes, and occupational safety and health. The ideal sensors should not only possess physical and chemical stability because they are exposed to oxidizing or reducing atmospheres at high temperature for a long time, but also have high sensitivity and selectivity to a gas at low concentration, and detect the target gas quickly and accurately. Many efforts have been made to develop new strategies to fabricate novel gas sensors with high performances. Most of the studies on gas sensors focused on metal oxide semiconductors owing to the fact that the chemical interaction of gas molecules with the semiconductor surface leads to remarkable changes in electrical conductivity.479 In fact, in the 1960s the gas sensing capability of semiconducting oxides was investigated and the adsorption of reducing gas on metal oxides was found to result in a change in the electrical conductivity of the oxides.480,481 This phenomenon has attracted extensive attention for the development of gas sensors based on various semiconducting oxides.482 A series of metal oxide semiconductors (MOS), such as SnO2, ZnO, TiO2, Fe2O3, In2O3, WO3, and so on, have been developed as gas-sensing materials for detection of many gaseous pollutant.483–488 Recently, metal oxide semiconductors with hierarchically porous structures have attracted much attention as sensors. Compared with other nanostructures, hierarchically porous structures can provide a large surface-tovolume ratio which greatly facilitates gas diffusion and mass transport in sensor materials, thus improving the sensitivity and response time of the gas sensor.489 6.1 Zinc oxide-based hierarchically porous structures for sensors Among transition-metal oxides, one of the key wide-bandgap II–VI compound semiconductors, zinc oxide, an n-type semiconductor with a wide band gap of 3.3 eV, is a primary material to develop metal oxide gas sensors because of its tremendous potential applications in piezoelectric nanogenerators, nanolasers, solar cells, gas sensors and so on.490–496 As one of the most prominent materials for gas sensors, ZnO has already shown good response to gaseous pollutants such as H2S, NOx, and benzene,497,498 and explosive gases such as H2, CO, ethanol and acetone.499–501 Recently, a series of hierarchically porous ZnO architectures with specific morphologies which are composed of interconnected ZnO nanosheets/nanoparticles with high porosity have been reported.484,489,502–507 These materials are usually prepared by a hydrothermal process using zinc carbonate or zinc carbonate hydroxide precursors combined with subsequent calcination. For example, Zeng et al.502 have developed a facile and template-free solution method for

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Fig. 61 FESEM images of cross sections of carbonized wood pyrolyzed at 600 1C for 2 h in vacuum condition and wood-templated bulk ZnO samples calcined at 600 1C in air: (a) Fir-templated ZnO; (b) Lauantemplated ZnO. Gas sensing response of ZnO calcined at 600 1C with different templates (c) at the working temperature of 332 1C. Gas sensing response of Fir-templated ZnO calcined at different temperatures (d) at the working temperature of 332 1C. (Reproduced from ref. 507 with permission, Copyright Elsevier Ltd, 2009).

the preparation of a hierarchically porous ZnO nanosheet thin film used as a microstructure CO sensor. The resultant sensor exhibits high response to CO with the response time of 25 s and low cross response to common interfering gases at 300 1C (Fig. 61). Hierarchically assembled porous ZnO sensors with flower-like nanostructures exhibit good response and reversibility to some organic gases, such as ethanol,508 acetone,508,509 and formaldehyde.509 Liu et al.507 have reported the gas sensing ability of bioinspired hierarchically porous ZnO ceramics prepared by a convenient and low-cost bioinspired method using Lauan and Fir woods as templates. The resultant woodtemplated ZnO has inherited the well-aligned porous frameworks and microstructural characteristics from Lauan or Fir templates (Fig. 61). The material displays high sensitivity and selectivity to H2S and much higher (5.1 times) gas response than non-templated ZnO because of its hierarchically porous structure (Fig. 61). The sensing response strongly depends on the calcination temperature and wood template because of different grain sizes and porous microstructure. Fir-templated ZnO calcined at 600 1C has the best sensing properties including the highest gas sensing response, the highest selectivity coefficients of H2S and the shortest response and recovery time (Fig. 61). Higher porosity and surface area can provide more surface adsorption positions and reacting areas for oxygen and test gases and help gases transfer more quickly, leading to the

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increase of gas response.507 These unique hierarchically porous structures with specific morphologies provide a large contact surface area with more reactive sites for electrons, oxygen ions and target gases and abundant channels for gas diffusion and mass transport, resulting in the excellent gas sensing performances including high sensitivity and short response and recovery times.504a Doping of ZnO with various elements, such as noble metals, rare metals, transition metals, or metal oxides, has been reported to be a useful method to improve electrical conductivity when they are used in gas sensing devices.510–514 For example, Fe-doped ZnO microspheres with lotus-like morphology constructed using hierarchically porous nanosheets are prepared through a hydrothermal route.505 The results showed that the structural disturbance of ZnO crystals induced by Fe doping improves their ethanol and acetone response signal values to several reductive gases. In this work, Fe doped micro–nanostructured ZnO shows a significantly higher gas response signal value than the undoped ZnO, and the optimum amount of Fe doping is 1 at%, while excessive Fe doping reduces the gas sensing response.505 The 3D hierarchically porous ZnO superstructures employed as a support to load Au nanoparticles (AuNPs) to construct hybrid nanomaterials combining the high surface accessibility of porous materials and catalytic activity of small AuNPs demonstrate excellent ethanol and methanol sensing properties in terms of higher sensitivity and very fast response.515 6.2 Other metal/bimetal oxide-based hierarchically porous structures for sensors Besides ZnO, other metal oxide and bimetal oxide based semiconductors with hierarchically porous structure have also been used as gas sensors, such as WO3,516,517 In2O3,518,519 SnO2,520,521 CuO,48 TiO2,522 NiO,523 CuO/ZnO,524 and ZnO– CdO525 tungsten oxide (WO3), an n-type wide band gap semiconductor with a bandgap of 2.8 eV made up of WO6 octahedra units which are capable of forming unique layer/tunnel structures by corner/edge/face-sharing, is a very promising sensing material owing to its excellent sensitivity to NO/NO2.526,527 For example, Bai et al.516 have developed nanosheet-assembled tungsten oxide microspheres with intrinsic non-stoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization as sensing materials and for fast diffusion of gas molecules.516,528 The maximum response of tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO2 gas. The firstprinciples calculations reveal that the NO2 molecule is most likely adsorbed at the terminal O1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO2-sensing properties.516 Wang et al.517 have reported bio-templated fabrication of hierarchically porous WO3 hollow microspheres from lotus pollens for NO gas sensing at low temperatures. The WO3 microspherebased sensor exhibits a high sensitivity (S = 46.2) to 100 ppm NO gas with a pretty fast response and recovery speed (62 s/ 223 s) at 200 1C. Fang et al.518 have prepared hierarchically porous In2O3 nanolamellae with two levels of nanopores

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including nanogaps of 5–50 nm present between the nanoparticles and concave nanopits of 3 nm on the surface of each nanoparticle. These nanopits form negative curvatures on the surfaces of the nanoparticles, leading to a high density of atomic steps and thus enhancing surface activity. The high density of atomic steps on the surface of the nanoparticles enhances the surface activity, leading to a formaldehydedetection limitation of 80 ppm and response/recover times as short as 5 s/1.3 s respectively.518 Jia et al.519 have achieved the design and fabrication of the hierarchically micro/nanostructured sensors with desired sensing performance. They have presented a successful example to fabricate hierarchical micro/nanostructured porous In2O3 film-based gas sensors on the ceramic tube based on a simple and flexible layer-by-layer strategy using the monolayer colloidal crystal with different sizes of colloidal spheres as templates (Fig. 62). Such films have biperiodic ordered structures and can be fully lifted off from the substrate and present a freestanding property, as well as tunable structures and morphologies (Fig. 62a–c).519 The resultant film-based sensors exhibit both higher sensitivity and much faster response to an NH3 atmosphere than the corresponding conventional nanostructured ones. Significantly, the gas-sensing parameters (i.e., response time and the sensitivity) are well-controlled separately in a large range simply by changing the pore sizes in different layers of the porous film in this work (Fig. 62d and e).519 Viet et al.520 have simultaneously investigated the gas sensing performance of SnO2 nanowires and their hierarchical nanostructures by testing with liquefied petroleum gas and NH3 gas at different concentrations and operating temperatures, showing their highly promising performance in gas sensing. Mesoporous SnO2 consists of uniform spheres with two distinct hierarchical pore sizes of 5.0 nm and 52 nm, showing promising response to methane gas with low cross-sensitivity to water.521 This work demonstrates that spherical mesoporous

Fig. 62 FESEM images of hierarchical micro/nanostructures with different pore size combinations: (a) 1000/100 nm; (b) 1000/200 nm; (c) 1000/ 350 nm. Sensitivity as a function of NH3 concentration (d) and diagram of tR versus S in 2000 and 100 ppm (inset) NH3 (e) at 60 1C for the different double-layer hierarchically structured porous In2O3 films (1000/100, 1000/200, 1000/350 nm). (Reproduced from ref. 519 with permission, Copyright American Chemical Society, 2009).

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SnO2 materials exhibit stronger response to CH4 than the nonspherical materials. P-type hierarchically porous CuO microspheres comprising numerous platelet-like building blocks radiating from the center of the microstructure exhibit higher responses to ethanol, propanol, and acetone in comparison with that of commercial CuO powder.48 The response and recovery time are below 20 s for 200 ppm of the tested gases. Although gas sensors based on semiconducting metal oxides provide safe detection of reducing gases, such as toxic or flammable gases, they still have some limitations and challenges in terms of sensitivity, selectivity, and long-term stability. To overcome the disadvantages of semiconducting metal oxide gas sensors, great efforts have been made to develop metal oxides focusing on the preparation procedure, doping, and composition of metal oxides.525 For example, the sensing properties can be improved greatly by loading of Ag nanoparticles48 or Pd nanoparticles522,529 on the hierarchically porous architectures. Cai et al. have demonstrated the synthesis and application of coral-like ZnO–CdO composites with 3D hierarchically porous structures for isopropanol (IPA) gas-sensing.525 The results show that the sensor based on the ZnCd10 composite exhibits good response, high selectivity, and quick responses to IPA gas at an operating temperature of 248 1C, which are 2–17 times that of the sensors fabricated using porous ZnO and ZnCdx (x = 2.5, 5.0 and 20) composites.525 A 2D-graphene/2D-NiO nanosheet-based hybrid nanostructure displays responsivity/sensitivity two orders higher than that of a NiO nanosheet toward NO2 at the 1 ppm level.530 Moreover, other metal oxide based materials with hierarchically porous structures have been used as promising sensors, such as Fe2O3531 and Al2O3.529 6.3 Graphene-based hierarchically porous structures for sensors Graphene-based materials, exhibiting large specific surface areas, high conductivity, and unique electrochemical properties, have also been used as high performance biosensors.532–535 Shi et al.534 have reported hierarchically porous 3D graphene films on gold substrates with a large surface area, excellent binding strength, high conductivity, and distinct interfacial microenvironments as electrochemical adenosine triphosphate (ATP) and thrombin (Tob) sensors. As selected examples, the resultant aptasensors in the assay of ATP and Tob exhibit high sensitivity, excellent selectivity, stability and reproducibility, as well as promising potential in real serum sample analysis. The results demonstrate that the surface area, as well as interfacial microenvironments, plays a critical role in molecular recognition.534 3D porous and redox-active Prussian blue-in-graphene (PB@G) aerogel modified electrodes show very low limit of detection (5  10 9 M) and a wide linear range (0.005–4 mM) in H2O2 electrochemical detection, owing to their extremely porous nature, ample surface area and high electrical conductivity.535 6.4 Other chemical compound-based hierarchically porous structures for sensors Some hierarchically porous materials with specific chemical composition have also been developed and used as sensors

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for detection of explosives, such as hierarchical TNT imprinted porous SiO2 films for rapid and selective detection of explosives,536 hierarchically porous CoOOH derived from Morphol-butterfly wings for carbon monoxide detection at low temperatures,537 a honeycomb-like hierarchical ionic liquid([BSmim]HSO4)–AuNP–porous carbon composite modified boron-doped diamond (BDD) electrode as an efficient novel acetylcholinesterase (AChE) biosensor for the detection of organophosphate pesticides,538 and hierarchically structured nanoporous poly(ionic liquid) membranes for fiber-optic pH sensing.539 Moreover, hierarchically porous zeolitic based materials have also been developed as sensors,540,541 such as hydrogen peroxide biosensors based on the direct electrochemistry of haemoglobin immobilized on gold nanoparticles in a hierarchically porous MFI zeolite.541

7. Hierarchically porous structures for biomedicine Hierarchically porous materials have attracted much attention due to their multilevel porous structure and high surface areas, which can provide many novel properties and have important prospects in practical biological processes, such as bone tissue engineering, drug delivery and enzyme immobilization. Thus, tremendous efforts have been made to synthesize hierarchically porous biomaterials. It is important to carefully examine the hierarchical porosity found in nature. Only by imitating the porous structure in nature we can obtain synthetic materials that perform a role similar to that of hierarchically structured porous natural materials. 7.1 Hierarchically porous structures for bone tissue engineering Defects and functional disorders of bone have become a global health care problem. Bone repair has become a major clinical and societal need with the increasing aging population and social development. Bone tissue engineering, mainly focused on applying the principles of biology and engineering to the development of viable substitutes that restore and maintain the function of human bone tissues, has emerged as a valid approach for the current therapy for bone generation. Among various materials available, hierarchically porous materials are most widely used materials due to their high surface area and large pore volume which improve their bioactive behavior and allow them to be loaded with osteogenic agents to promote new bone formation. However, design and synthesis of appropriate hierarchically structured biomaterials for tissue replacement or tissue regeneration still remains as a great challenge of chemistry, physics, biology and materials science. The ideal porous materials used for bone tissue engineering applications should be primarily nontoxic to the human body and have hierarchical and interconnected porosity close to 90% analogous to bone to allow cell penetration, tissue ingrowth, and eventually vascularization upon implantation. Among various hierarchical biomaterials, bioactive ceramics, porous calcium phosphates,

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Ti-based, hierarchically porous scaffolds and biodegradable polymers are considered as most promising materials for bone tissue engineering. 7.1.1 Bioactive ceramics-based hierarchically porous structures for bone tissue engineering. Bioactive ceramics such as bioactive glasses and calcium silicate have received great attention due to their ability to form direct bonds with living bone after implanation.542 Great efforts have been made to develop bioactive ceramics with hierarchically porous structure for bone tissue regeneration.543–547 For example, by using block copolymers and polyurethane sponges as co-templates, Mg-, Zn- and Sr-doped hierarchical macroporous and mesoporous bioactive glass scaffolds have been synthesized by Wang et al. and show no cytotoxicity.548 In their work, the gradual release of Ca, P, Si, Mg, Zn and Sr into the culture medium from these scaffolds contributed to the enhancement of the proliferation and activity of mesenchymal stem cells, making them good candidates for bone substitute materials. Jagadeesan et al.546 have prepared a hierarchically porous bioactive glass of composition 80 mol% SiO2 and 15 mol% CaO (MBGH) using block copolymer and glucose-derived amorphous carbon spheres as templates. The preliminary biocompatibility test using human fibroblast cells shows that MBGH is not toxic to the cells.546 Han et al.547 have prepared highly ordered hierarchical meso– macroporous bioglasses with a well-interconnected 2D/3D pore structure using mushroom stalk and block copolymers as dual templates. The drug loading and release test indicates that the loading amount of the sample can reach 33.59 wt% and the release amount can reach close to 75 wt% after 48 h. It takes 4 h to induce the formation of hydroxyapatite, presenting good biocompatibility and enhanced adherence of HeLa cells.547 Other methods such as modified polymer templating549 and biotemplating,550 can also be used to synthesize hierarchically porous bioactive glass with various porous architectures. The resultant materials all exhibited favorable biocompatibility in vitro and promising excellent potential applications in the field of biomaterials. Moreover, bioceramics can also be used in combination with silk. For example, Xu et al.551 have prepared hierarchically porous structured bioceramic-silk (BC-silk) scaffolds by combining 3D-plotting with the freeze-drying method. The obtained hierarchically porous BC-silk scaffolds possess distinct apatite mineralization and mechanical strength. Compared with BC scaffolds, BC-silk scaffolds present significantly enhanced cell attachment, proliferation and bone-related gene expression.551 7.1.2 Calcium phosphate-based hierarchically porous structures for bone tissue engineering. A calcium phosphate cement scaffold (CPC) has been widely used as a bone graft substitute, but undesirable osteoinductivity and slow degradability greatly hamper their clinic application. To solve these problems, calcium phosphates are generally used in combination with other materials, such as calcium silicate552 and magnesium.553 For example, Zhang et al.552 have prepared a recombinant human bone morphogenetic protein-2 (rhBMP-2)-loaded calcium silicate/ calcium phosphate cement scaffold (CSPC) with hierarchical pores (Fig. 63a–c). The obtained CSPC scaffold presents interconnected

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Fig. 63 3D topology of the porous CSPC scaffold observed by micro-CT (a) and the surface morphology of the porous CSPC scaffolds by SEM (b 40, c 1000). Bone regeneration in a cavity defect after various CPCbased scaffolds implantation in the rabbit femur in 8 weeks. (d) Micro-CT images (left) of the bone regeneration in the defects (red circle) and tridimensional reconstruction images (right) of the defect sites (red: new bone formed, blue: scaffold residue). (e) Quantitative analysis of the new bone volume after 8 weeks of surgery by micro-CT (*p o 0.05, compared with CPC; *p o 0.05, CSPC/rhBMP-2 vs. CPC/rhBMP-2). (f) Quantitative analysis of the new bone area after 8 weeks of surgery by histological observation (*p o 0.05, compared with all the other groups). (Reproduced from ref. 552 with permission, Copyright Elsevier Ltd, 2013).

macropores on the order of 200–500 mm and micropores of 2–5 mm. Compared with the calcium phosphate cement scaffold (CPC), CPC/rhBMP-2 and CSPC scaffolds, the rhBMP2-loaded CSPC scaffold induces more enhanced osteogenic differentiation in vitro and significantly promotes ectopic bone formation and bone regeneration in a rabbit femur cavity defect model (Fig. 63d–f).552 Wei et al.553 have prepared hierarchical 3D microporous/macroporous magnesium–calcium phosphate (micro/ma-MCP) scaffolds containing magnesium ammonium phosphate hexahydrate and hydroxyapatite. The micro/macroporous MCP scaffolds with porosities varying from 52% to 78% show open macropores with the sizes of 400–500 mm, and the degradability of micro/ma-MCP scaffolds is significantly enhanced as compared with ma-MCP and CPC scaffolds. The cell culture experiments indicated that micro/ma-MCP scaffolds promote MG63 cell attachment, proliferation and differentiation, showing excellent biocompatibility. Histological evaluation results confirmed that micro/ma-MCP scaffolds exhibit high efficiency of bone regeneration.553 7.1.3 Titanium scaffold-based hierarchically porous structures for bone tissue engineering. Titanium (Ti) and its alloys have become the most widely used load-bearing orthopedic and dental implant materials in clinical practice due to them possessing excellent mechanical properties, biocompatibility,

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corrosion resistance and high strength-to-weight ratios. Nevertheless, these metals are bioinert and cannot well establish a tight and chemical bond with host bones, which easily cause subsequent wear and tear, thus ultimately resulting in implant failure. The mismatch of the mechanical properties between bone and the Ti implant is another major problem in orthopedic surgery. Because the Ti implant has higher mechanical strength, bone can be loaded insufficiently, thus resulting in bone resorption due to stress shielding. These disadvantages have urged researchers to seek strategies to overcome them, and now many methods have been attempted. For example, Ti-based, hierarchically porous scaffolds anchored to Ti substrates are prepared by synthesizing hydroxyapatite–calcium carbonate–Ti three-layer spheres and combining a modified plasma spraying process and an anodic oxidation treatment.554 These hierarchically porous scaffolds are composed of 100–350 mm interconnecting macropores, 0.2–90 mm pores and B100 nm nanopores with 470% porosity and demonstrated good cellular and mechanical compatibility, high osteoconductivity and osteoinductivity, and sturdy osteointegration.554 A bioinspired nano–microstructured octacalcium phosphate (OCP)/silk fibroin (SF) composite coated on titanium is obtained through a mild electrochemically induced deposition method.555 In vitro cell culture tests demonstrate that the presence of OCP/SF composite coatings, with highly ordered and hierarchically porous structure, greatly enhances cellular responses.555 Moreover, a novel methodology for producing hierarchically structured Ti alloy foams is reported combining gel casting of a TiO2 precursor with electrolytic reduction via the FFC Cambridge process.556 These foams are hierarchically porous, with both the traditional large (4300 mm) highly interconnected pores and, uniquely, wall struts also containing micron scale (0.5–5 mm) interconnected porosities. The typical properties of an 80% porous Ti foam are in the range required for biomedical implant applications.556 7.1.4 Biodegradable polymer-based hierarchically porous structures for bone tissue engineering. Biodegradable polymers, such as poly(lactic acid) (PLA), poly(glycolic acid) PGA and poly(3-capro-lactone) (PCL), belonging to the family of poly(a-hydroxyesters), have emerged as a class of biomaterials of growing interest for application in tissue engineering. Among them, PCL is widely used because of its good biocompatibility, reproducible mechanical and physical properties and processability, but its high hydrophobicity and low degradability in vivo make it less suitable for long-term applications. In order to solve this problem, Guarino et al.557 have prepared a fibre-reinforced composite scaffold for bone tissue engineering applications. These composites are composed of poly-L-lactide acid (PLLA) fibres embedded in a porous PCL matrix, and are obtained by synergistic use of phase inversion/particulate leaching techniques and filament winding technology. For these resultant materials, a porosity degree as high as 79.7% and the bimodal pore size distribution showing peaks at ca. 10 and 200 mm diameter, respectively, accounting for 53.7% and 46.3% of the total porosity are achieved.557 The degrees of micro- and macroporosity determine the structural and biological properties of the composite. Concerning its degradation properties, over 5 weeks,

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the treatment with NaOH solution (pH 12) induces the most significant changes, including the significant structural rearrangements of the more hydrophilic PLLA component, increase in crystallinity and decrease in the polymeric chain length. Furthermore, encouraging results are obtained in cellular studies in the case of human osteoblasts (HOB) and marrow stromal cells (MSC): the latter are found to be more active on the composite, due either to a slightly higher number of cells compared with HOB, or to a higher activity of the cells.557 7.2

Hierarchically porous structures for drug delivery systems

Drug delivery systems (DDS) can significantly improve the pharmacological properties of conventional drugs. In general, drug delivery carriers should have high surface area and good biocompatibility since the structure of carriers can greatly affect the performance of DDS. Hence, tremendous efforts have been made to fabricate hierarchically porous materials. Such hierarchical structures facilitate drug molecules to diffuse and transport through the hierarchical pores. The pore structure is another important factor to meet the requirements for drug loading and release. Drug delivering systems often require a controllable diffusion rate of molecules through pores to control the whole process, which is decided by the pore size and pore length. Recently, many materials with different chemical compositions and morphologies, including plate-like,558 films,559 nanocubes,560 microspheres561–564 and hybrid gels,565 have been synthesized to obtain excellent DDS performance. For example, a highly uniform, hierarchically nanoporous silica film with two distinct pore sizes of 200 nm and 7 nm is prepared and used as a drug delivery agent (Fig. 64a and b).559 When loaded with ampicillin, this hierarchically porous film shows over 8 times longer inhibition of E. coli growth than both the inverse opal film and the mesoporous film (Fig. 64c). Hierarchically porous silica nanotubes obtained by a simple hydrothermal method using a surfactant-polyelectrolyte template exhibit high loading capacities for ibuprofen (IBU), which are 362 mg g 1 and 509 mg g 1 for solid and hollow nanocubes, respectively.560 A high drug-loading capacity (DLC) has been achieved for hollow carbonated hydroxyapatite (HCHA) microspheres with mesoporous and well-defined 3D network structures constructed using nanoplates as building blocks with mesoporous and hollow structures due to their high specific surface area and hierarchically porous structure.561 Seisenbaeva et al.564 have prepared hierarchically porous microparticles of metal oxides by a simple one-step biomimetic procedure. The produced materials have high open porosity, high thermal stability and a strong selective trend toward adsorption of ligands containing a phosphate or phosphonate moiety, which makes the obtained microparticles interesting as a potential matrix for smart drug release. Application of chiral ligands offers chiral precursors, which provide an approach for the synthesis of oxide materials possessing chiral surfaces.564 Moreover, molecularscale self-assembly of a 3D aluminosiloxane (Al–O–Si) hybrid gel network synthesized via the cocondensation of hydrolyzed alumina (AlOOH) and (3-aminopropyl)trimethoxysilane (APS) presents a much higher loading capacity than MCM-41, with maximum

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Fig. 65 Specific activity of CAT immobilized in PAH–PDA (black), purePDA (red), and PSS–PDA (green), as a function of recycling number. (Reproduced from ref. 566 with permission, Copyright American Chemical Society, 2013).

Fig. 64 (a and b) TEM images of hierarchically porous silica film at different magnifications. (c) Comparison the total adsorbed amount of the rhodamine by summing up all released rhodamine from each film. Red, purple and blue lines represent the result of hierarchically porous silica film, inverse opal silica film, and mesoporous silica film, respectively. (Reproduced from ref. 559 with permission, Copyright Royal Society Chemistry, 2011).

loading amounts of 567 mg g 1 of IBU, 273 mg g 1 of aspirin (ASP) and 640 mg g 1 of IBU, 301 mg g 1 of ASP, respectively, whereas MCM-41 has maximum loading amounts of 359 mg g 1 of IBU and 213 mg g 1 of ASP.565 7.3 Hierarchically porous structures for enzyme immobilization Enzyme immobilization has received increasing attention due to the benefits of immobilized enzymes with respect to enhanced stability, repeated use, facile separation from reaction mixtures, and the prevention of enzyme contamination in products. Hierarchically porous materials are commonly used as supports for enzyme immobilization. Owing to their high surface areas and hierarchically porous structure, relatively high enzyme loadings and quick enzyme immobilization rates can be realized.566– 570 In addition, the pore sizes of hierarchically porous materials can be tuned to be comparable to the diameter of enzymes. Substantial amounts of the immobilized enzyme can still be adsorbed on the surface of the support when the sample is dispersed/immersed in solution by surface modification of the porous support. For example, Shi et al.566 have prepared three kinds of mussel-inspired polydopamine microcapsules with different wall structures by a template-mediated method. When serving as the carrier for catalase (CAT) immobilization, these enzyme-encapsulated polydopamine (PDA) microcapsules show

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distinct structure-related activity and stability (Fig. 65). In particular, poly(allylamine hydrochloride) (PAH)–PDA microcapsules with a wall of highly interconnected networks displayed several significant advantages, including increases in enzyme encapsulation efficiency and enzyme activity/stability and a decrease in enzyme leaching in comparison with other two types of PDA microcapsules (Fig. 65).566 He et al.567 have designed ZnO–CaO–SiO2–P2O5 bioactive nanostructures with versatile hierarchical nanoporous structures, unique surface chemistry and potential for optimizing the catalytic reaction of enzymes. This new type of solid supporting material, namely, bioactive nanostructured phosphosilicate glass, has both high affinity for enzyme molecules and a favored microenvironment that results in high immobilization efficiency with enhanced enzyme loading and stability compared with conventional approaches.567 Yun et al.568 have demonstrated the synthesis and application of hierarchical meso–macroporous-giant-porous bioactive glass/ poly-e-caprolactone (PCL) composite scaffolds, which exhibit good molding capabilities, mechanical properties, three dimensionally well-interconnected pore structures, bioactivities, and biocompatibilities in vitro. Depending on the amount of NaCl, the scaffolds also show unique sponge-like properties, but still retain better mechanical properties than general salt leaching derived PCL scaffolds. Moreover, Davis et al. have reported the attractive properties of diatom-derived carbon and gold-bearing replica microparticles as support materials for enzyme immobilization.571 Upon shape-preserving inorganic conversion and appropriate surface functionalization, the flow-through catalytic activities of the samples are noticeably enhanced relative to the starting silica frustules and to conventional enzyme support materials. Furthermore, after normalization with respect to the amount of enzyme loading, the specific enzyme activities of all of the samples are similar and reproducibly higher than for conventional enzyme supports that lack frustule morphology.571 In addition, hierarchically porous materials have also been used in the fields of bovine serum albumin (BSA) separation,572–574 cell adsorption/response.575–577 bioelectronics,578,579 glucose detection,580,581 osteoblast carriers,582 biocatalysis,583 and nerve guidance channels.584 For example, hierarchically porous layered double hydroxide (LDH)/Al2O3 composites synthesized on a large scale with uniform mesochannels, high surface area and large pore volume exhibit a much higher adsorption

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capacity for bovine serum albumin (BSA) (90.27%) than calcined LDH particles.572 Moreover, compared with bare LDH particles, the paper-based Ni–Al LDH/Al2O3 composite presents a high binding capacity for bioseparation, and the bioseparation of adsorbed composites shows near complete (over 95%) desorption in a high negative charge density of salt solution.573 Recently, Fu et al.575 have reported a very facile and straight forward method for preparing a hierarchically porous chitosan–PEG–SiO2 (CPS) biohybrid through an in situ silica sol–gel process within a chitosan– PEG hydrogel. Rat blood cell adsorption experiments demonstrate that the hierarchically porous CPS biohybrid has excellent capacity for quick cell adsorption/concentration due to its unique hierarchical structure and the embedded positively charged biopolymer (chitosan).575 In addition, Sen et al.583 have demonstrated a biocatalytic application of novel hierarchically ordered porous magnetic nanocomposites with interconnected macroporous windows and meso–microporous walls containing well dispersed magnetic nanoparticles. The catalytic conversion values are found to be 10 2 order in magnitude compared with the value obtained using pure lipase (Candida rugosa) in a homogeneous medium.583 Tsujimura et al.579 have reported enzymemodified MgO templated porous carbon forms as promising electrode materials for the elaboration of efficient bioelectrochemical devices. This work demonstrates a carbon electrode designed to achieve efficient enzymatic electrolysis by exploiting a hierarchically porous structure based on macropores for efficient mass transfer and mesopores for high enzyme loading.579 Very recently, Wei et al. have prepared hierarchically porous NaCoPO4–Co3O4 hollow microspheres via a simple hydrothermal method. It is found that the as-prepared hierarchically porous NaCoPO4–Co3O4 hollow microspheres exhibit good catalytic activity for the oxidation of glucose, showing a fast amperometric response time of less than 5 s, and the detection limit is estimated to be 0.125 mM.581

8. Summary Hierarchically porous materials have attracted continuously increasing interest. Various applications of hierarchically porous materials are introduced in detail in this review. Hierarchically structured porous materials are widely used in the fields of energy storage and conversion, catalysis, photocatalysis, adsorption, separation, sensors, biomedicine and CO2 capture since they can provide large surface areas for reaction, high dispersion of active sites at different length scales of pores, shortened mass diffusion paths or minimized diffusion barriers, easier accessibility for guest objects and improved mass transport. In addition, they can also act as host materials to stabilize or incorporate other active components. Hierarchically structured porous materials can be effectively used in energy conversion and storage. Their involvement in energy conversion, including photosynthesis, photocatalytic H2 production, DSSCs and FCs, is systematically described in this review. Energy storage applications such as LBs, NBs, Mg ion, Li–air, and Li–S batteries and supercapacitors based

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on hierarchically porous materials are also introduced in detail. By finely tuning the hierarchy, morphology and chemical composition of the porous materials at different length scales, their energy storage performance can be significantly enhanced. For photocatalysis, the incorporation of macropores into mesoporous materials can significantly enhance light harvesting and light scattering as well as increase multiple internal reflections, thus improving their photocatalytic efficiency. In addition, the presence of macropores can minimize diffusion limitations, improve the internal mass transport and enhance the in-pore active sites accessibility by guest molecules especially in liquid phase reactions. As for traditional catalysis, due to the easy mass transport and large pore volume of macroporous systems, hierarchically porous structures will improve the mass transport ability (especially for the diffusion of large molecules or in viscous systems), reduce coke deposition and improve the tolerance of coke deposition, thus increasing the catalyst’s lifetime. In addition, materials with hierarchically porous structures have long been utilized as adsorbents and separation media. Their multiple pore sizes can be expected to combine reduced diffusion resistance and high surface areas for yielding improved overall reaction and adsorption/separation performances. Additionally, pore size, volume, structure, and interconnectivity are also essential for improving mass transport. As for gas sensors, hierarchically porous structures can provide a large surface-to-volume ratio that can greatly facilitate diffusion and mass transport in sensor materials, thus improving their sensing performance. Hierarchically porous materials are also widely used as biomaterials due to their high surface area and large pore volume which would improve their bioactive behavior and allow them to be loaded with various functional groups to obtain excellent new properties and biocompatibility. In conclusion, great progress has been made in the field of hierarchically porous materials. The integration of multi-scale porosities into one material can offer a large surface area, attendant dispersion of metal or acid/base functions, and superior mass transport of reactants and products to/from active sites located inside the framework, thus improving the overall performances, adsorption/separation ability and/or structural properties of the catalyst. The hierarchically porous materials have become an evolving field of important current interest.

9. Outlook During the past few decades, hierarchically porous materials with diverse porous structures and various compositions have been synthesized by many available synthetic strategies. However, their applications in the fields of energy storage and conversion, photocatalysis, catalysis, adsorption, separation, sensing and biomedicine are still under exploration since for each application, materials with specific chemical compositions, morphology and certain hierarchically porous structures are needed. Hierarchically porous materials have generated and

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will continue to generate new material systems for a largely extended field of applications. The vast majority of recently synthesized hierarchically porous materials are in powder form, which cannot always be applied to practical production. Thus the future design of tailored hierarchically porous materials should develop in the form of a device, monolith or film. Currently, the most efficient strategy to synthesize porous materials with hierarchical pore dimensions and pore structures is to use various templates. Thus hierarchically porous materials cannot be synthesized on a large scale owing to such methodological limitations, which seriously restrict their further use and development. Therefore, effort to develop new synthetic strategies to realize large scale production of hierarchically porous materials is ongoing.

Acknowledgements This work is supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R52) of the Chinese Ministry of Education. B. L. Su acknowledges the Chinese Central Government for an ‘‘Expert of the State’’ position in the Program of the ‘‘Thousand Talents’’, the Chinese Ministry of Education for a ‘‘Changjiang Chaire Professor’’ position and a Clare Hall Life Membership at the Clare Hall College and the financial support of the Department of Chemistry, University of Cambridge. L. H. Chen, Y. Li and X. Y. Yang acknowledge Hubei Provincial Department of Education for the ‘‘Chutian Scholar’’ program. This work is also financially supported by NFSC-21301133, NFSC-51472190, ISTC-2015DFE52870, SRF for ROCS SEM ([2015]311), Hubei Provincial Natural Science Foundation (2015CFB428, 2014CFB160).

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