Facile preparation of N-doped mesocellular graphene

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Facile preparation of N-doped mesocellular graphene foam from sludge flocs for highly efficient oxygen reduction reaction† Daixin Ye,‡a Li Wang,‡b Ren Zhang,ab Baohong Liu,a Yi Wang*ab and Jilie Kong*a The use of environmental waste products as materials for the production of energy is an extremely attractive prospect for both economic and social development. Sludge flocs (SFs) are environmental waste products that are difficult to handle. We used these SFs as a source of carbon and nitrogen for the preparation of N-doped mesocellular graphene foam (SF-NMGF) via a simple one-step pyrolysis method. The particular composition and structure of the SFs meant that the resultant SF-NMGF had a large Brunauer–Emmett–Teller surface area and consisted of a graphitic framework surrounded by ultrathin nanosheets. The material contained foam-like mesopores with a size centred at about 15 nm and the N was incorporated homogeneously with a high percentage (40.5 at%) of graphitic-N. As a result of these unique properties, the SF-NMGF had an excellent electrocatalytic activity with 4e when used as a metalfree catalyst for the oxygen reduction reaction (ORR). Specifically, the prepared SF-NMGF catalyst

Received 27th April 2015 Accepted 11th June 2015

exhibited a high diffusion-limited current, superior durability and better immunity towards methanol

DOI: 10.1039/c5ta03060a

crossover for the ORR in alkaline solution than a commercial 20 wt% Pt/C catalyst. The synthesis of the SF-NMGF can be scaled up at low cost, which will be beneficial for both sludge handling and the

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development of materials for the ORR.

Introduction The recycling of environmental wastes to produce energy materials is an extremely attractive prospect for both economic and social development because it encompasses both environmental and energy concerns. Our increasing energy demands can be met while, at the same time, the quality of our environment is improved without further depletion of our natural resources.1,2 However, little work has so far been reported that relates such environmental requirements with the development of sources of energy.3,4 Sludge is a major by-product of wastewater treatment plants.5 Its disposal causes a number of environmental problems worldwide because it contains both harmful heavy metals and pathogens.6,7 Although some traditional routes (e.g. discharge to the oceans, incineration, composting and land application) have been developed for the disposal of sewage sludge, these have either a high capital cost or adverse environmental impacts.8,9 There is therefore a need to develop more

a

Department of Chemistry, Innovative Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China. E-mail: [email protected]; jlkong@fudan. edu.cn

b

Center of Analysis and Measurement, Fudan University, Shanghai 200433, China

† Electronic supplementary 10.1039/c5ta03060a

information

(ESI)

available.

‡ Daixin Ye and Li Wang contributed equally to this work.

This journal is © The Royal Society of Chemistry 2015

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DOI:

environmentally benign sustainable techniques for sludge handling and management. Fuel cells are the most promising devices for the generation of clean energy and electrocatalysts are crucial in the oxygen reduction reaction (ORR).10,11 Unfortunately, traditional Pt-based materials, which have long been regarded as superior ORR electrocatalysts, are oen too expensive for practical applications and have limited availability and insufficient durability.12,13 As an alternative to Pt-based catalysts, metal-free nitrogen-doped carbon materials (e.g. graphene,14 carbon nanotubes15 and mesoporous carbon16) have become an active eld of research for the ORR as a result of their low cost, excellent electrocatalytic activity, fuel crossover effects and good durability. However, nitrogen-doped carbon electrocatalysts are usually fabricated using expensive carbon sources and labourintensive post-doping techniques,17,18 which consequently increase costs and unavoidably cause non-uniform doping or damage to the synthesized carbon structures.19 In addition, the pyrolysis of common carbon sources usually results in nonporous materials with a low degree of graphitization (low surface area/pore volume and low conductivity) resulting from the absence of a template effect and metal catalysis, respectively.20,21 Considered from these two aspects, the recycling of sludge or its derivatives to produce ORR electrocatalysts is a highly practical and attractive prospect. First, the microorganisms in sludge are potential C and N sources for N-doped carbon

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materials. Second, sludge contains heavy metals, which could act as catalysts for the graphitization of N-doped carbon materials. Third, sludge is rich in inorganic nanoparticles, which could act as the template for porous materials with large surface areas. More importantly, sludge and its derivatives are environmental waste products that are very difficult to dispose safely and, as a consequence, are both cheap and plentiful. The utilization of sludge to produce ORR electrocatalysts would be an environmentally benign and sustainable technique to manage this waste product. We used sludge ocs (SFs), a spongy sludge derivative containing microorganisms (Fig. S1A–C†) to synthesize an SFderived N-doped mesocellular graphene foam (SF-NMGF) via a simple one-step pyrolysis at 900 C under an atmosphere of N2. The SF-NMGF had foam-like mesopores with a large surface area and a graphitic framework with uniform doping. As an electrocatalyst for the ORR, the SF-NMGF showed excellent catalytic activity, stability and tolerance to methanol poisoning effects in alkaline media. This work provides a robust strategy to bridge environmental requirements and the development of sustainable energy sources.

Experimental Reagents and apparatus Reagents such as FeSO4$7H2O, NaHCO3 and CaCl2 were purchased from Sinopharm Chemical Reagent Co. Ltd (China). Aqueous Naon was purchased from Sigma-Aldrich (St Louis, MO, USA). All the chemicals were used without further purication. KOH solution (0.1 M) was used as the electrolyte. Double-distilled water was used in all the experiments. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectrometry/mapping experiments were performed on a JEM-2010 instrument with an acceleration voltage of 200 kV. For the TEM observations, the samples were dispersed in ethanol and supported on a holey carbon lm on a Cu grid. Scanning electron microscopy was performed on a Hitachi S-4800 scanning electron microscope operated at 20 kV. X-ray diffraction (XRD) measurements were obtained on a PANalytical X'pert PRO diffractometer with Cu Ka radiation at a scan rate of 4 min1 in the 2q range 10–80 . Raman spectra were obtained on a Labram-1B (Dilor, France) confocal microscope Raman spectrometer with 632.8 nm incident laser light. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were performed on a Pyris Diamond TG/DTA thermogravimetric analyser (PerkinElmer). N2 adsorption– desorption isotherms at 77 K and pore size distribution curves were measured using a Tristar 3000 system. The samples were outgassed for more than 5 h at 250 C before measurements. X-ray photoelectron spectrometry (XPS) was carried out on an RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Al Ka radiation (hn ¼ 1486.6 eV) as the X-ray source for excitation. Optical photographs were taken using a readily available Canon camera. Inductively coupled plasma atomic emission spectrometry (ICP-AES) for the determination of inorganic elements

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was carried on a PerkinElmer OPTIMa 8000 ICP-AES instrument, while some organic elements were determined using a Vario EL cube elemental analyser. Electrochemical experiments Electrochemical experiments, including linear sweep voltammetry (LSV) and amperometry, were carried out on a CHI 660E electrochemical workstation (Chenhua, Shanghai, China). All the electrochemical experiments used a conventional threeelectrode electrochemical system consisting of a working electrode, a Pt foil auxiliary electrode and a saturated calomel electrode. A glassy carbon rotating disc electrode (GCRDE) was used as the basal working electrode. All experiments were performed in 0.1 M KOH. Synthesis of the SF-NMGF The SFs were rst prepared from activated sludge waste in the wastewater treatment plants. Typically, the SFs were obtained from a sequencing batch reactor fed with synthetic wastewater and used for the cultivation of aerobic granules (Table S2†). The cultivation procedure has been reported previously.22 These bioocs were an intermediate product between the activated sludge and the aerobic granules. They were mainly produced during the initial phase of cultivation and were frequently washed out from the reactor. This abandoned biomass was recycled to prepare the graphene materials studied in this work. The Raman spectra showed that the SFs did not contain any graphitic carbon before pyrolysis (Fig. S1†). The XRD results showed that there were some crystallized inorganic species, evidenced by a few small diffraction peaks (Fig. S2†). XPS, ICP-AES and elemental analysis revealed that the SFs were composed of the major elements C, O, H, N, Ca and Na and minor elements P, S, Mg, K and Fe (Fig. S3 and Table S3†). To synthesize the SF-NMGF, dry SFs were placed in a pipe furnace and carbonized for 5 h at 900 C under an N2 atmosphere. The black powder recovered was dispersed with stirring in an HCl solution to remove the inorganic nanoparticles and some metal nanoparticles, such as Fe. Aer centrifugation, washing and drying, the SF-NMGF were obtained. Preparation of modied electrodes and electrochemical tests Prior to use, the GCRDE was polished with 0.3 and 0.05 mm a-Al2O3 powder until a shiny mirror surface was obtained. It was then ultrasonicated for 10 min each in ethanol and doubly distilled water. Finally, it was dried in a stream of high-purity nitrogen before use. SF-NMGF samples (2 mg) were dispersed in a solution containing 768 mL of double-distilled water, 32 mL of 5 wt% aqueous solution of Naon and 200 mL of ethanol. A homogenous catalyst ink was obtained by ultrasonicating this mixture. A 10 mL volume of the resulting catalyst ink was dropped onto the cleaned GCRDE surface to prepare the SF-NMGF/GCRDE and the modied electrode was allowed to dry under an infrared lamp for 10 min. The obtained modied electrodes were preserved in a refrigerator at 4 C aer washing with doubly distilled water. The same method was used to prepare the Pt/C/GCRDE.


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Results and discussion


Characterization of the SF-NMGF hybrid material Fig. 1A shows a representative TEM image of the SF-NMGF. The SF-NMGF was shaped like a vesicle, similar to the morphology of the microorganisms in the SFs (Fig. S4D†). The magnied TEM images show that the SF-NMGF was made up of many ultrathin nanosheets (Fig. 1B and S5A and S5B†), which coincided with graphene. The STEM images showed that the nanosheets were interleaved with each other to form foam-like mesopores (Fig. 1C and S5C†), which were similar to the spongy structures formed by the numerous microorganisms in the SFs (Fig. S4C and D†). The HRTEM images further revealed mesocellular pores in the SF-NMGF (Fig. 1D). The carbon framework of the SF-NMGF was graphitized and well-resolved lattice fringes, attributed to the (002) planes of graphitic carbon (graphene),23 were clearly observed (Fig. 1A, inset, and Fig. S5D†). This was further validated by the XRD results (Fig. S6A†), which showed two typical (002) and (100) reections of graphitic carbon at 2q values of 24.7 and 43.6 , respectively, similar to reduced graphene.24 The Raman spectrum (Fig. S6B†) showed clear signals similar to the symmetrical A1g and E2g modes of graphitic carbon atoms at 1346 cm1 (D-band), 1580 cm1 (G-band) and 2810 cm1 (2D-band).25 The TG/DTG curves (Fig. S6C†) indicated that the SF-NMGF sample did not rapidly decompose by combustion when heated in air until the temperature exceeded 450 C.26


It could be inferred from elemental mapping (Fig. 1E and S7A–D†) and the EDX pattern (Fig. S7E†) that the SF-NMGF contained carbon, nitrogen and oxygen and that N was homogeneously doped in the carbon network. All these results suggested that the prepared SF-NMGF was an N-doped mesocellular graphene foam with graphitization. The SF-NMGF had a BET surface area of 370 m2 g1, a pore volume of 0.69 cm3 g1 and the pore diameters were centred around 15 nm, as demonstrated by the typical type IV N2 adsorption–desorption isotherm and the pore size distribution curve (Fig. 2A). Based on these observations, the formation of N-doped mesocellular graphene using SF was deduced (Scheme 1). As potential sources of carbon and nitrogen, numerous microorganisms (bacteria) in the SF were assembled around the inorganic nanoparticles (the inorganic elements and their contents are given in Table S1†) to form spongy aggregates. Notably, the ionic Fe2+ added as nutrition for the microorganisms in activated sludge (Table S2†)27 could be absorbed by the microorganisms and subsequently transformed into embedded Fe nanoparticles in the microorganisms in the SFs. Aer calcination at 900 C under an N2 atmosphere, the microorganisms containing C and N, and, in particular, their membranes, were transformed into N-doped carbon. The inorganic nanoparticles in the SFs may act as solid supports to prevent the fusing of carbon, resulting in the formation of nanosheets and mesocellular pores aer removal by a solution of HCl. The Fe nanoparticles could also act as a catalyst for the graphitization of the carbon nanosheets (graphene).28 Therefore SFs are appropriate precursors for the N-doped mesocellular graphene foam and the SF-NMGF can be prepared simply on a large scale. Further characterizations of the SF-NMGF by XPS are shown in Fig. 2B and C. The survey spectrum (Fig. 2B) shows three typical peaks of C1s, N1s and O1s and corresponding atomic

(A) N2 adsorption–desorption isotherms and (inset) corresponding pore size distribution curve of the SF-NMGF. A sharp increase in nitrogen absorption at a high relative pressure confirmed the existence of mesocellular pores in the SF-NMGF. (B) XPS survey of the SF-NMGF; inset chart shows the percentages of carbon, nitrogen and oxygen based on XPS data. (C) High-resolution C1s XPS spectra of the SF-NMGF. (D) High-resolution N1s XPS spectra of the SF-NMGF; inset shows the content of different types of nitrogen in the SF-NMGF.

Fig. 2

Fig. 1 (A and B) TEM images of the SF-NMGF at different magnifications (inset, HRTEM image of SF-NMGF). (C) STEM image of the SF-NMGF. (D) HRTEM image of the SF-NMGF. (E) EDX mapping of the SF-NMGF.


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Scheme 1 Preparation of the SF-NMGF using sludge flocs as precursors. (A) Sludge flocs, (B) SF-NMGF containing inorganic nanoparticles and (C) pure SF-NMGF.

percentages of C (78.7 at%), N (4.5 at%) and O (16.8 at%) were obtained (inset, Fig. 2B). The C1s high-resolution spectrum was deconvoluted into a dominant peak at 284.4 eV and a small broad peak at 285–291 eV (Fig. 2C), which were indexed as sp2hybridized graphitic carbon and carbon-bonded oxygen/ nitrogen congurations, respectively.29,30 The N1s high-resolution spectrum revealed that the SF-NMGF had been doped with four related N species of pyridinic (398.4 eV), pyrrolic (400.0 eV), graphitic (401.8 eV) and other types of N (406.5 eV).31 Their corresponding atomic percentages were 23.0, 24.5, 40.5 and 12.0 at%, respectively (Fig. 2D). The large amount of graphitic-N in the SF-NMGF could be attributed to the effect of Fe-coordinated pyrolysis, which would also improve the catalytic activity in the ORR.28,32

Electrocatalytic properties towards the ORR As a result of its unique graphitic and mesocellular porous structure and highly active N-doping, the SF-NMGF was expected to be a highly efficient ORR electrocatalyst. Measurements were therefore made using an SF-NMGF hybrid modied GCRDE (SF-NMGF/GCRDE) to investigate its activity in the ORR. For comparison, measurements were also made on a commercial 20 wt% platinum on carbon black (Pt/C) electrode. Fig. 3 shows that the ORR onset potential for the SF-NMGF electrode only deviated by 50 mV from that of the Pt/C electrode and that the SF-NMGF electrode had a higher diffusion-limited current density than the Pt/C electrode. This indicates that the SF-NMGF electrode has a comparable electrocatalytic activity to the commercial Pt/C electrode in alkaline media. Some metal-free catalysts for the ORR have been reported previously.33,34 However, most of the catalysts still exhibit a weaker activity than commercial 20% Pt/C in alkaline media. The improved catalytic performance observed for the metal-free SF-NMGF electrode could be a result of (1) the high percentage of graphitic-N in the SF-NMGF, which has been demonstrated to be benecial to the ORR,35 and (2) the foamlike mesopores with a large surface area and the graphitic framework with uniform N-doping, which both offer a high probability of exposure of the active sites. These favourable features may contribute to the excellent catalytic performance for oxygen reduction.

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Kinetics, stability and selectivity study To explore the ORR in a more quantitative manner, the voltammetric proles in an O2-saturated 0.1 M KOH electrolyte showed that the current density was enhanced by an increase in the rotation rate (from 225 to 2025 rpm; Fig. 4A). The number of electrons transferred per oxygen molecule involved in the ORR process was determined by the Koutecky–Levich equation, which relates the current density J to the rotation rate of the electrode u: J1 ¼ JL1 + JK1 ¼ B1u1/2 + JK1

(1)

B ¼ 0.62nFC0D2/3v1/6

(2)

JK ¼ nFkC0

(3)

where J is the current density, JK and JL are the kinetic- and diffusion-limited current densities, u is the angular velocity of the disc (u ¼ 2pN, where N ¼ the rotation frequency), n is the overall number of electrons transferred during oxygen reduction, F is the Faraday constant (F ¼ 96 485 C mol1), D is the diffusion coefficient of O2 in the 0.1 M KOH electrolyte (1.9 105 cm2 s1), C0 is the bulk concentration of O2 (1.2 103 mol L1), v is the kinetic viscosity of the electrolyte (0.01 cm2 s1) and k is the electron transfer rate constant. Fig. 4B shows the Koutecky–Levich curves at different potentials. The straight, parallel lines of J1 vs. u1/2 imply a rst-

Fig. 3 LSV plots of different materials in O2-saturated 0.1 M KOH at a sweep rate of 5 mV s1 and a rotation rate of 1600 rpm.


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commercial Pt/C catalyst showed a signicant 25% decrease in current density over 4000 s of continuous operation in 0.1 M KOH, exhibiting an obvious decay in activity over time. This result suggested that the stability of the SF-NMGF catalysts was superior to that of the Pt/C catalyst.


Conclusions Fig. 4 (A) LSV plots of SF-NMGF/GCRDE at different rotation rates in a

0.1 M KOH solution saturated with O2. (B) K–L plots of J1 vs. u1/2 obtained from the LSV data at different potentials.

order reaction towards dissolved oxygen. The n value for the SFNMGF hybrid was derived to be 4.6, 4.3, 4.1, 4 and 4.2 at potentials of 0.3, 0.4, 0.5, 0.6 and 0.7 V, respectively. These results suggested a four-electron process for the ORR on the SF-NMGF hybrid electrode and are similar to those obtained for the ORR catalysed by a high-quality commercial Pt/C catalyst in the same 0.1 M KOH electrolyte. This excellent performance may be inuenced by the high content of quaternary-N, which enhanced the catalytic performance via the intermediate HOc radicals.36,37 The ideal catalyst for the ORR should show satisfactory tolerance to the fuel molecule (e.g. methanol). Therefore the catalyst was exposed to fuel molecules to test for possible crossover effects (Fig. 5A). A sharp decrease in the current was observed for the Pt/C electrocatalyst with the addition of 2.0 M methanol. However, the SF-NMGF electrocatalyst did not show an obvious change aer the addition of methanol. This indicated that the SF-NMGF electrocatalyst had a higher selectivity towards the ORR than the commercial Pt/C. The stability of the SF-NMGF/GCRDE was tested and compared with a reference Pt/C catalyst at a constant voltage of 0.75 V for 4000 s in an O2-saturated 0.1 M KOH solution (Fig. 5B). The activity normalized to the initial activity as a function of time served as a criterion for catalyst stability. Over 4000 s, the ORR activity of the SF-NMGF catalysts retained almost all their initial value, which is a promising result for further applications in alkaline fuel cells. However, the

Fig. 5 (A) Chronoamperometric responses of the SF-NMGF and Pt/C electrodes in O2-saturated 0.1 M KOH with the rapid addition of 2 M methanol. (B) Chronoamperometric responses (percentage of current retained versus operation time) of the SF-NMGF and Pt/C electrodes kept at 0.70 V versus RHE in O2-saturated 0.1 M KOH.


A facile and low cost pyrolysis strategy was used to synthesize a high yield of high-quality SF-NMGF. This method uses waste SFs as the sole starting material without any synthetic chemicals. The SF-NMGF obtained had a high BET surface area, abundant mesocellular pores, a graphitic framework, a homogeneous incorporation of N with a high percentage (40.5 at%) of graphitic-N, and exhibited high electrocatalytic activity, operational stability and methanol-tolerance in the ORR. This work has conrmed the feasibility of using environmental waste products as the starting material to produce high-quality and high-performance carbon-based energy materials on a large scale and at low cost. This method could be a link between an environmental need and the requirement to develop more sustainable sources of energy.

Acknowledgements This work was supported by grants from the National Natural Science Foundation of China (21303022, 21175029, and 21335002) and the Natural Science Foundation of Shanghai City of China (13ZR1451400).

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