Constructing highly-efficient electron transport

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Keywords: Nickel cobaltate; Nickel molybdate; Transport channel; Uninterrupted ...... [32] H. Wang, Q. Gao, L. Jiang, Facile Approach to Prepare Nickel Cobaltite.

Accepted Manuscript Constructing highly-efficient electron transport channels in the 3D electrode materials for high-rate supercapacitors: the case of [email protected] hierarchical nanostructures Peng Zhang, Jinyuan Zhou, Wanjun Chen, Yuanyuan Zhao, Xuemei Mu, Zhenxing Zhang, Xiaojun Pan, Erqing Xie PII: DOI: Reference:

S1385-8947(16)31223-2 http://dx.doi.org/10.1016/j.cej.2016.08.131 CEJ 15697

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 June 2016 19 August 2016 29 August 2016

Please cite this article as: P. Zhang, J. Zhou, W. Chen, Y. Zhao, X. Mu, Z. Zhang, X. Pan, E. Xie, Constructing highly-efficient electron transport channels in the 3D electrode materials for high-rate supercapacitors: the case of [email protected] hierarchical nanostructures, Chemical Engineering Journal (2016), doi: http://dx.doi.org/ 10.1016/j.cej.2016.08.131

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Constructing highly-efficient electron transport channels in the 3D electrode materials for high-rate supercapacitors: the case of [email protected] hierarchical nanostructures Peng Zhanga, Jinyuan Zhoua, Wanjun Chenb, Yuanyuan Zhao a, Xuemei Mua, Zhenxing Zhanga, Xiaojun Pana,*, Erqing Xiea,* a

School of Physical Science and Technology, Lanzhou University, Lanzhou 730000,

China b

Key laboratory for Electronic Materials of the State Ethnic Affairs Commission,

College of Electrical Engineering, Northwest University for Nationalities, Lanzhou 730030, China *Corresponding

authors.

Tel.:

+86-0931-8912616,

E-mail

addresses:

[email protected] (X. Pan), [email protected] (E. Xie). ABSTRACT It’s demonstrated that transport channels of electrons are very crucial to the performances

of

supercapacitor

electrodes

and

different

morphologies

of

nanomaterials usually imply different properties on electron transport in them. Hence, we

constructed

two

types

of

NiCo [email protected]

hierarchical

core-shell

nanostructures, in which NiCo 2O4 scaffolds are in form of uninterrupted nanosheet arrays (UNSAs) or nanoneedle arrays (NNAs) and NiMoO4 hierarchies in form of nanosheets, and investigated electron transport properties of their resultant electrodes. Results showed that NiCo [email protected] and [email protected] electrodes respectively exhibit high areal capacitances of 7.29 F cm-2 and 5.96 F cm-2

(current density of 2 mA cm-2), both of which are much improved compared with the previous

work.

And

more

interestingly,

the

capacitances

from

[email protected] electrodes are enhanced by 22% ~ 39% compared to those from NiCo [email protected] ones at various current densities. And theoretical simulations and electrochemical impedance spectroscopy results confirmed that compared to the NNA ones, the UNSA scaffolds can provide more accessible and efficient electron transport channels (especially at high-rate charge-discharge processes), which leads to a much lower charge-transfer resistance and superior rate capability.

Furthermore,

the

assembled

asymmetric

supercapacitors

of

[email protected]//active carbon show a high energy density (52.6 Wh kg-1 at 332.4 W kg-1) and a high power density (2632.8 W kg-1 at 36.9 Wh kg-1). Keywords: Nickel cobaltate; Nickel molybdate; Transport channel; Uninterrupted nanosheet arrays; Supercapacitors 1. Introduction As one of the most important energy storage devices, supercapacitors (SCs), also known as electrochemical capacitors, often hit a fatal bottleneck in real application compared to other chemical batteries including li-ion batteries, i.e., energy density [1-6]. To achieve high energy density for SCs, pseudocapacitive materials such as MnO2, Fe2O3, NiO, Co3O4, etc., have been introduced into their electrodes [7-12]. However, these common pseudocapacitive materials often show a low electrical conductivity, which greatly limits their utility efficiency in capacitance. Recently, a type of bimetallic oxides, such as NiCo 2O4, CoMoO4, ZnCo2O4, NiMoO4, etc., have

attracted high attention in field of supercapacitors due to their much higher electrical conductivity

and

high

specific

capacitance

compared

to

the

traditional

pseudocapacitive oxides [13-19]. Thus, much effort has been endeavoured to fabricate various bimetallic oxide nanomaterials and applied them into SCs for high performances [20-24]. Among bimetallic oxides, NiCo2O4 have attracted much interest and been considered as one of the most promising and scalable electrode materials for SCs due to its excellent electrical conductivity and high electrochemical activity [25-29]. Several different types of NiCo 2O4 nanostructures, including nanoparticle/spheres [30, 31], nanowire/needles [26, 27, 32, 33], nanotubes [34, 35] and nanosheets [36-38] have been synthesized for applications in SCs. For instance, Lu and co-workers synthesized NiCo 2O4 nanoparticles through an epoxide-driven sol-gel method and a high-specific capacitance of 1400 F g-1 under a mass loading of 0.4 mg cm-2 was achieved [30]. However, NiCo 2O4 nanoparticle/spheres or electrospun nanotubes need to be mixed with polymeric binder and carbon black, which will inevitably decrease the electrical conductivity of the electrode materials and sacrifice overall energy storage capacity. In contrast, self-supported NiCo2O4 nanoarrays show apparent advantages including fast electron and ion transport and large electroactive surface area. For instance, Zhang and co-workers synthesized NiCo 2O4 nanowire arrays via a surfactant-assisted hydrothermal method and obtained high specific capacitances (1283 F g-1 at 1 A g-1 and 1010 F g-1 at 20 A g-1 under a mass loading of 1.2 mg cm-2 ) [26]. Lou and co-workers synthesized interconnected mesoporous NiCo 2O4

nanosheets on various conductive substrates via solution methods and obtained high areal capacitances (3.51 F cm-2 at 1.8 mA cm-2 and 1.37 F cm-2 at 48.6 mA cm-2 under a mass loading of 1.2 mg cm-2) [36]. And Yu and co-workers synthesized ultrathin mesoporous NiCo2O4 nanosheets on carbon fiber paper and obtained high specific capacitances (1422 F g-1 at 1 A g-1 and 999 F g-1 at 20 A g-1 under a mass loading of 0.8 mg cm-2) [37]. However, their rate capabilities are still not very satisfactory, which will restrict the performances of SCs especially at high-rate charge-discharge processes. And the mass loading of active materials still need to be improved to increase the energy-storage capacity of SCs, which is very important for practical applications. On the other hand, to further increase the energy density and rate capability of SCs, the utilization of active materials should be improved. Thus, series of three-dimensional (3D) hierarchical structures based on NiCo2O4 nanoscale scaffolds have been prepared for the electrodes of SCs [39-44]. For example, Zhang and co-workers synthesized [email protected] core-shell nanowire arrays on Ni foam with a large areal capacitance of 6.88 F cm-2 at 30 mA cm-2 and a good rate capability of about 80% from 10 to 80 mA cm-2 [45]. In another study, Wang and co-workers reported NiCo [email protected] nanocolumn arrays on Ni foam and a high specific capacitance of 1705.3 F g-1 at 5 mA cm-2 (1.62 F cm-2) is achieved [46]. In addition, NiCo2O4 nanoflake/sheet scaffolds supported core-shell arrays have been synthesized via solution methods, and demonstrate excellent electrochemical performance with high specific/areal capacitances, good rate capabilities and cycling stabilities [40, 43,

47]. These studies suggest that the 3D hierarchical architecture can provide more accessible electroactive sites and easy charge transport, which accordingly result into their improved electrochemical performance. However, how the nanoscale morphologies of the NiCo2O4 scaffolds affect the electron transport in the 3D electrode materials was often ignored in the previous work. Therefore, it is important and instructive to investigate the effect of morphologies of NiCo2O4 nanoscale scaffolds on their electron transport behaviors in the constructed 3D electrode materials. Herein, take the case of [email protected] hierarchical core-shell nanostructures, we have designed two types of NiCo2O4 scaffolds (uninterrupted nanosheet arrays (UNSAs) and nanoneedle arrays (NNAs)) to investigate how the nanoscale morphology affects the electron transport and capacitances of the counterpart electrodes.

And

the

resultant

two

electrodes

are

labelled

as

[email protected] and [email protected], respectively. As is expected, both NiCo [email protected] 3D electrodes are capable of delivering high areal and specific capacitances, e.g., at current density of 2 mA cm-2, 7.29 F cm-2 and 1941 F g-1 for NiCo [email protected] electrodes, which are respectively 22.3% and 24.4% higher than those from [email protected] ones (5.96 F cm-2 and 1560 F g-1). And, the [email protected] electrodes exhibit excellent rate capability (84.1%)

with

30-fold

increase

in

current

density

compared

with

[email protected] ones (73.5%), which might be due to that the hierarchical UNSAs electrode can deliver much better reversibility and lower charge-transfer

resistance (0.758 Ω) than the NNAs one (1.438 Ω) during redox reactions. Further theoretical simulation of electron transports in 3D electrode materials suggests that the large lateral space and tight interconnection between UNSAs are beneficial to the electrochemical performances of electrodes. In addition, the assembled asymmetric supercapacitors of [email protected]//active carbon yielded the maximum energy density of 52.6 Wh kg-1 at power density of 332.4 W kg-1, implying its prospect in SCs. 2. Experimental 2.1 Material Synthesis All reagents used in this work are of analytical grade and were used without further purification. Prior to the synthesis, the Ni foam (2×1 cm2) was carefully cleaned using 3 M HCl solution in an ultrasound bath for 10 min in order to remove the possible NiO layer, and then successively cleaned using acetone and deionized (DI) water for 10 min each. Synthesis of NiCo2O4 UNSAs and NNAs: The NiCo 2O4 UNSAs were synthesized through a hydrothermal reaction combing with a subsequent annealing process. In a typical synthesis, 0.728 g of Co(NO3)2·6H2O, 0.364 g of Ni(NO3)2·6H2O, and 1.05 g of hexamethylene tetramine (HMT) were dissolved in 100 mL of DI water, followed by magnetic stirring until a uniform mixture solution formed. Then the cleaned Ni foams were fixed on a glass slide and immersed into this mixture solution to grow bimetallic carbonate hydroxide precursor at 95 °C for 10 h. After cooling down to room temperature (RT), the samples were rinsed for several minutes using DI water

under assistance of ultrasonication, and dried in air at 60 °C. To obtain NiCo 2O4 UNSAs, the samples were further annealed under Ar atmosphere at 300 °C for 2 h with heating rate of 2 °C min-1. As for the synthesis of NiCo2O4 NNAs, their process is similar to that of NiCo2O4 UNSAs, except replacing HMT using urea. The final mass loadings of NiCo 2O4 UNSAs and NNAs on Ni foam are about 1.6 and 1.81 mg cm-2, respectively. Synthesis of NiMoO4 hierarchies on NiCo2O4 scaffolds: In a typical process, 0.249 g of Ni(CH3COO)2·4H2O, 0.2 g of (NH4)6Mo 7O24·4H2O, and 0.24 g of urea were dissolved in 40 mL of DI water, followed by an intense ultrasonic treatment for 10 min. The obtained solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave. At the same time, NiCo2O4 UNSA or NNA scaffolds fixed on glass slides were immersed into the solution, followed by autoclaving at 140 °C for 1 h. After cooling to RT, the resultant samples were rinsed using DI water for several times, and further annealed under Ar atmosphere at 400 °C for 2 h with heating rate of 2 °C min-1. The final mass loadings of NiCo [email protected] and [email protected] on Ni foam are about 3.76 and 3.83 mg cm-2, respectively. 2.2 Characterization The X-ray diffraction (XRD) patterns were recorded on a Philips X’pert pro diffractometer (Cu Kα, λ=1.5406 Å). The morphologies and crystal structures were examined using field emission scanning electron microscopy (FESEM; Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM; FEI Tecnai F30, operated at 200 kV).

2.3 Electrochemical Measurements The electrochemical measurements of the samples were carried out on an electrochemical workstation (CS310, Wuhan Corrtest Instruments Co. Ltd., China) at RT. As for the test of single electrode, a three-electrode configuration was used to measure the cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) behavior, and electrochemical impedance spectroscopy (EIS). During the test, a piece of [email protected] on Ni foam was used as the working electrodes, Ag/AgCl as reference electrode, a Pt foil as counter electrode, and 3 M KOH aqueous solution as the electrolyte. EIS measurements were conducted in the frequency range from 0.01 to 100 kHz with an alternating-current perturbation of 5 mV. Prior the the assembly of asymmetric supercapacitor devices, a type of Ni foam/active carbon (Ni/AC) composites were prepared by dipping Ni foams into AC ink (including 85 wt % AC, 10 wt % acetylene black, and 5 wt % polyvinylidene fluoride binder dispersed in N-methyl-2-pyrrolidone solvent) and subjected to “dip and dry” coating cycles to insure the uniform coating of AC. During the electrochemical tests, the [email protected] on Ni foam and Ni/AC composites were respectively used as positive electrode and negative electrode, and these two slices of electrodes were assembled into a sandwich configuration using a ~20 µm thick membrane as the separator and 3 M KOH solution as the electrolyte. And the specific capacitance, energy and power densities of the asymmetric supercapacitors were calculated using the total mass of both positive and negative electrodes, excluding the current collectors.

3. Results and discussion 3.1 Structures and morphologies of [email protected] electrodes The synthetic strategy used to fabricate the [email protected] electrode materials is schematically illustrated in Fig. 1a, in which two steps are mainly involved: i) Hydrothermal process combining with a post-calcination to grow NiCo2O4 UNSA scaffolds on Ni foam; ii) Uniform anchoring ultrathin NiMoO4 nanosheets onto the surfaces of NiCo2O4 UNSA scaffolds, forming a type of intriguing hierarchical architecture of NiCo [email protected] After growth of NiCo2O4-UNSAs or [email protected], the color of the Ni foam changed from metallic white to black (Fig. 1b). And from SEM images shown in Figs. 1c, a type of NiCo 2O4 nanosheet arrays were uniformly grown on the Ni foam’s surface and interconnected with each other to form a UNSA structure. And the grown nanosheets are ultrathin with a lateral size of above 2 µm. This type of UNSA structure might facilitate the ions’ diffusion and electrons’ transport during charge/discharge processes. Fig. 1d shows the typical SEM image of NiCo [email protected] electrodes. The NiCo2O4-UNSAs were uniformly coated with a layer of dense tiny NiMoO4 nanosheets, inspiringly remaining their uninterrupted nanosheet structure. From the magnified SEM images, the tiny NiMoO4 nanosheets are ultrathin with a lateral size around 100 nm and also interconnected and well-distributed on the surface of NiCo2O4 nanosheets, forming a highly porous surface morphology. It suggests that this type of porous hierarchical architecture would provide abundant open space, effective transport accesses, and electroactive surface sites, which might benefit their

capacitive behaviors. Further TEM measurements were carried out to investigate the microstructures of NiCo2O4 and [email protected] electrodes, as shown in Figure 2. As can be seen from Fig. 2a, the grown NiCo 2O4 nanosheet possesses a smooth surface without folded edges or wrinkles, and its planar size can be estimated to be about 2 µm in length and 1 µm in height. And the corresponding HRTEM image and SAED patterns (Fig. 2b) indicate the polycrystalline characteristic of cubic spinel NiCo2O4, in which the d-spacings of 0.285 and 0.204 nm correspond to the distance of the (220) and (400) planes, respectively, which are consistent with the XRD pattern shown in Fig. S1a. The diffraction peaks located at 2θ=18.9°, 31.1°, 36.7°, 44.6°, 59.1°, and 65° (marked with clubs) correspond to the (111), (220), (311), (400), (511), and (440) crystalline planes of cubic spinel NiCo2O4 phase (JCPDS 20-0781), respectively. After coated with a layer of dense tiny NiMoO4 nanosheets, as shown in Fig. 2c, the NiCo 2O4 scaffold’s surface exhibits a crumpling silk-like morphology. And the dark strips on the scaffold are the formed curling edges or wrinkles of NiMoO4 nanosheets during the sample preparation process for TEM test. And HRTEM image shown in Fig. 2d reveals a d-spacing of ca. 0.21 nm, corresponding to the (330) crystal plane of monoclinic NiMoO4, which is also consistent with the XRD results (Fig. S1a). And the main diffraction peaks located at 2θ= 28.8° correspond to the (330) crystal plane of monoclinic NiMoO4 phase (JCPDS 33-0948). The corresponding selected area electron diffraction (SAED) pattern of the shell shows well-defined diffraction rings, indicating the polycrystalline characteristic of NiMoO4 (inset of Fig. 2d). In addition,

the compositions of NiCo [email protected] and their core-shell configurations can be further confirmed by energy dispersive X-ray spectrometer (EDX) spectra which were recorded in the shell region and center region, respectively, as shown in Fig. S1b. The center region consist of Co, Mo, Ni, and O except Cu element from the used TEM grids, while the shell only of Mo, Ni, and O elements, implying their core-shell structure. 3.2 Structures and morphologies of [email protected] electrodes Fig. 3a shows the synthetic strategy to fabricate the NiCo [email protected] electrodes, in which the former UNSAs change into NNAs. From the digital photographs of the obtained samples, the change in color of Ni foam are almost the same as that the case of UNSA samples. Fig. 3c and d present the SEM images of the NiCo2O4 NNA and NiCo [email protected] samples, respectively. It can be seen that the needle-like NiCo2O4 nanoarrays are uniformly grown on the Ni foam. The NiCo2O4 NNAs possess a sharp tip with length about 1 µm. After coated with NiMoO4 nanosheets (Fig. 3d), the NiCo2O4 NNAs change to a tree-like hierarchical structure, keeping their arrayed morphology. From TEM image of the NiCo 2O4 (Fig. 3e), the NiCo2O4 nanoneedles are composed of numerous nanocrystallites with size of several

nanometers.

Fig.

3f

shows

the

hierarchical

structure

of

[email protected] electrodes, in which the NiCo2O4 cores are tightly wrapped with ultrathin NiMoO4 nanosheets with width of about 150~200 nm. And the folded feature of NiMoO4 nanosheets is highly similar to that of [email protected] aforementioned. Furthermore, from the magnified TEM image (Fig. 3g), a large

quantity of mesopores with a size range of 2~6 nm can be found from NiMoO4 nanosheets, which might be mainly caused by the gas release during the pyrolysis of the Ni-Mo precursor [21]. 3.3 Electrochemical properties of single electrode To evaluate the electrochemical performances of the as-synthesized products, the NiCo2O4 and [email protected] hierarchical nanostructures on Ni foam were fabricated as binder-free electrodes for electrochemical tests. Fig. 4a presents the representative cyclic voltammetry (CV) curves of [email protected] electrode measured with scanning rates ranging from 2 to 50 mV s-1. All CV curves exhibit a pair of well-defined redox peaks within a potential window of -0.2 to 0.6 V, suggesting the presence of the reversible Faradaic reactions and pseudocapacitive characteristics. With the increase in the scanning rates, the anodic and cathodic peaks shift slightly towards positive and negative potentials, respectively. Even at scanning rate of 50 mV s-1, the CV curve still retains a pair of symmetric redox peaks, implying that the [email protected] hierarchical structures are beneficial to fast reversible redox reactions. And with decreasing the scanning rates, the anodic peaks gradually change into two components, which might be attributed to the redox reactions related to M-O/M-O-OH, where M refers to Ni and Co ions [32, 40, 48]. This observation also demonstrates that the covering of NiMoO4 nanosheets does not block the inner NiCo2O4 nanosheets from participating in redox reactions, which might be due to the porous structure of NiMoO4 nanosheets. Fig. 4b compares CV curves from four types of electrodes at same scanning rate of

10 mV s-1, i.e., NiCo 2O4 UNSAs, NiCo2O4 NNAs, NiCo [email protected] and [email protected] As can be seen, the integrated areas of CV curves from the [email protected] composite electrodes are much larger than those of the NiCo 2O4 nanoscale scaffolds, indicating much larger pseudocapacitive performances from the hierarchical

electrode

materials.

Interestingly,

the

CV

curves

of

[email protected] electrodes exhibit a much smaller interval (0.298 V) between anodic peak and cathodic peak than that of NiCo [email protected] electrodes (0.517 V). These results suggest that the NiCo [email protected] could deliver a more easy redox reversibility and a lower charge-transfer resistance at the interface between electrode material and electrolyte, especially at high rate charge-discharge process [36]. Fig. 4c shows the typical GCD curves of NiCo [email protected] electrodes recorded at different current densities within a potential window of 0~0.45 V. Their GCD curves possess a well-defined potential plateaus, illustrating a high pseudocapacitive behavior. And the GCD curves are nearly symmetrical without an obvious iR drop at low current densities, indicating good reversibility of the redox reactions and low internal resistance. Fig. 4d further compares GCD curves from four types of electrodes stated above at current density of 5 mA cm-2. Similar to the CV data, the GCD from [email protected] hierarchical electrodes deliver much longer discharge time than the NiCo 2O4 electrodes, indicating higher areal capacitances. More GCD curves of NiCo2O4 UNSA/NNA and NiCo [email protected] electrodes at different current densities are presented in Fig. S2. Using these GCD curves, the

areal capacitances and specific capacitances of four electrodes at various current densities are calculated and depicted in Fig. 4e. The areal capacitances of [email protected] electrodes are calculated to be 7.29, 6.94, 6.78, 6.58, 6.31 and 6.13 F cm-2 at the current densities of 2, 5, 10, 20, 40 and 60 mA cm-2, respectively, indicating excellent rate capability (84.1%) during the charge-discharge process. This small decrease in areal capacitances at high current densities is mainly caused by the decrease in the utilization of the active material due to the NiMoO4 coating layers, in which the electrolyte diffusion into the inner NiCo 2O4 is always limited by the time constraint during fast charge-discharge process, thus only the NiMoO4 shell can be utilized for charge storage [49]. For [email protected] electrodes, the areal capacitances are calculated to be 5.96, 5.44, 5.05, 4.80, 4.51 and 4.38 F cm-2 at the current densities of 2, 5, 10, 20, 40 and 60 A cm-2, respectively. In contrast, the NiCo2O4 UNSA (NNA) electrodes show similar capacitances, which are calculated to be 1.47 (1.62), 1.41 (1.47), 1.35 (1.38), 1.28 (1.27), 1.23 (1.21) and 1.20 (1.15) F cm-2 at the current densities of 2, 5, 10, 20, 40 and 60 mA cm-2, respectively. It can be seen that the areal capacitances of [email protected] hybrid electrodes are much

higher

than

those

from

NiCo2O4

nanostructured

electrodes.

This

electrochemical superiority of [email protected] hybrid electrodes is mainly attributed to the incorporation of ultrathin NiMoO4 nanosheets and the hierarchical architecture. The ultrathin and mesoporous features of NiMoO4 nanosheets would greatly extend the electroactive surface area, and also facilitate ion transport within the electrodes. On the other hand, the arrayed NiCo 2O4 scaffolds can serve as the

conductive link between current collector and NiMoO4 shell. Thus, a type of bi-functional channels for electron transfering to the collector and electrolyte ions accessing to the surface of active material are constructed by the inner NiCo2O4 core and the open voids among the interconnected NiMoO4 nanosheets. And it is important to

note

that

the

capacitance

retentions

of

NiCo2O4

NNA

and

[email protected] electrodes are about 71 % and 73.5% at 60 mA cm-2, respectively, while those of NiCo 2O4 UNSA and [email protected] electrodes are ~81.6% and ~84.1%, respectively. In addition, the cycling stabilities of NiCo 2O4 and [email protected] hierarchical electrodes are also investigated by GCD tests at high current density (50 mA cm-2), as shown in Fig. 4f. For the pristine NiCo 2O4 UNSA and NNA electrodes, their capacitances remain about 88.5% (1.08 F cm-2) and 83.4% (1.0 F cm-2) of their initial values after 5000 cycles, respectively. Encouragingly, after coating with NiMoO4 nanosheets, the [email protected] electrodes still keep a good long-term electrochemical stability (82.2% retention after 5000 cycles at 50 mA cm-2), which is much higher than that of [email protected] ones (70.6% retention). The good cycle stability from NiCo2O4 UNSA scaffolds can be further proved by SEM images of the electrodes after 5000-cycle tests. As shown in Fig. S3, the morphologies of [email protected] electrodes are well maintained without noticeable collapse and agglomeration, while for [email protected] electrodes, the 3D hierarchical structure can hardly be observed, and the active material becomes much denser than the initial ones. So, the uninterrupted NiCo 2O4 nanosheets can support

each other to relieve deformation during high-rate charge-discharge processes, and accordingly benefit the cycling stability of the whole electrodes. From the above results and analysis, it is found that both NiCo 2O4 UNSA and [email protected] electrodes exhibit higher rate capability and cycling stability than NiCo2O4 NNA and [email protected] electrodes. This might be mainly attributed to their structural uninterruptability and stability from NiCo 2O4 UNSA scaffolds. To further investigate the kinetics within this type of UNSA structures, we compare the EIS spectra from the above four types of electrodes, as shown in Fig. 5a. In principle, the slope of curves in low frequency area reflects the electrolyte diffusion in electrode materials, the intercept at the real part (Z′) reflects the bulk resistance (Rb) of the electrochemical system, and the diameter of the semicircle reflects the interfacial charge-transfer resistance (Rct). It can be seen that [email protected] electrodes show the best ideal straight line in low frequency area, indicating the lowest ion diffusion resistance. While the other electrodes can also exhibit slightly worse ion diffusion behaviors. Fig. 5b shows the enlarged EIS spectra in high frequency area. There are significant differences in the diameters of semicircles. By fitting the EIS data, the Rcts are calculated to be 0.822, 0.758, 1.553 and 1.438 Ω for NiCo 2O4 UNSAs, [email protected], NiCo 2O4 NNAs, and [email protected] electrodes, respectively. As is expected, [email protected] electrodes deliver the lowest Rct. This result suggests that the excellent rate capability and cycling stability are mainly due to low charge-transfer resistances in UNSA structure of NiCo 2O4. Besides, compared the EIS

spectra before and after 5000 cycles, the curve shapes keep almost unchanged with the exception of a slight change in Rct (~0.232 Ω after 5000 cycles), as shown in Fig. S4. Additionally, to make better understanding of the effect of nanoscale morphology on the charge-transfer performances, a typical electron-transfer mechanism in 3D electrode materials was also proposed, as shown in Fig. 5c. From the above results, it is seen that NiCo 2O4 nanosheets have much larger lateral size (~ 2 to 4 µm), while NiCo2O4 nanoneedles are ~10 nm at the tip, and ~100 nm at the bottom. As schematically illustrated, the NiCo 2O4 nanosheets can provide more transport channels for charges, resulting a lower resistance. While for NiCo 2O4 nanoneedle scaffold, the electrical conductivity can be greatly limited by its cusp feature. On the other hand, in comparison to the mutually independent NiCo 2O4 nanoneedles, the criss-cross feature of NiCo 2O4 nanosheets can further improve the conductive structural interconnectivity. Based on such an advantageous NiCo 2O4 UNSA scaffold, the obtained NiCo [email protected] hierarchical structures can provide more favourable transport channels for electrons and thus more active redox reactions especially at high rate charge-discharge processes. Base

on

the

above

electrochemical

performance,

the

obtained

[email protected] hybrid electrodes in this work are apparently superior or comparable to many of the previously reported 3D hierarchical nanostructures, as summarized in Table S1. These results suggest the great promise of this unique hierarchical nanostructure for high performance pseudocapacitive electrodes

characterized in terms of high areal capacitance, excellent rate capability and long cycling stability. 3.4

Electrochemical

test

of

[email protected]//AC

asymmetric

supercapacitors In order to illustrate the potential application of [email protected] electrode materials for electrochemical energy storage, a type of asymmetric supercapacitors of [email protected]//AC were assembled. Prior to the assembly, the masses of [email protected] and AC should be well balanced. Fig. 6a shows the CV curves of [email protected] and AC/Ni foam electrodes with a scan rate of 10 mV s-1 in 3 M KOH. After calculation of their corresponding capacitances, the mass ratio between the [email protected] and AC is designed to be 1:5.4 based on the following equation: m+/m-=C-∆V-/C+∆V+, where C represents the specific capacitance values and ∆V is the potential range. Fig. 6b shows the CV curves of the assembled asymmetric supercapacitor with different window potentials at scan rate of 10 mV s-1. It is can be seen that the suitable electrochemical window potential of the cell can be extended to 1.8 V. Fig. 6c shows the typical GCD curves of the cells at various current densities with a potential window of 0-1.6 V. The specific capacitances were calculated from the corresponding GCD curves and the results are plotted in Fig. 6d. Based on the total weight of electrodes (i.e. mass of NiCo [email protected] + AC), we obtain the maximum specific capacitance of 148 F g-1 (3.6 F cm-2) at 10 mA cm-2. Moreover, the asymmetric supercapacitor also delivers an impressive high energy density of 52.6 Wh kg-1 at a

power density of 332.4 W kg-1, and still remains 36.9 Wh kg-1 at a power density of 2632.8 W kg-1, which are much higher than those of the asymmetric supercapacitors based on various reported Ni-Co compounds and other ternary compounds, such as NiCo2O4//AC (17.72 Wh kg-1) [50], NiCo2O4-RGO//AC (23.3 Wh kg-1 at 324.9 W kg-1) [51], β-NiMoO4-CoMoO4·xH2O//AC (28 Wh kg-1 at 100 W kg-1 and 18 Wh kg-1 at 1000 W kg-1) [52], NiCo [email protected]//AC (35 Wh kg-1 at 163 W kg-1) [42], and [email protected] NWSAs//AC (21.7 Wh kg-1 at 157 W kg-1) [44]. To further demonstrate the practical application of our asymmetric supercapacitor, two prototype devices can be connected in series and charged at 80 mA cm-2 for 20 s to power a 5 mm green light-emitting diode (LED) and a 5 mm red LED, respectively, as shown in Fig. 8f. And the red LED can be lighted for 70 min, as shown in Fig. S5. These results demonstrate that the [email protected] electrodes would have outstanding potentials for use in practical energy storage devices. 4. Conclusions In summary, two types of NiCo [email protected] hierarchical nanostructures have been fabricated on Ni foam via a facile two-step hydrothermal approach. The resultant [email protected] hierarchical electrodes exhibit highly enhanced electrochemical performances

compared

to

the

NiCo 2O4

electrodes.

Moreover,

the

[email protected] electrodes deliver a high areal capacitance of 7.29 F cm-2 at 2 mA cm-2, remarkable rate capability of 84.1% at 60 mA cm-2, and good cycling stability (82.2% after 5000 cycles), which are much higher than those of [email protected] ones. The origin of these enhancements in capacitive

behaviours should be due to that the NiCo [email protected] electrodes can provide more favourable transport channels for electrons, better reversibility, and lower charge-transfer resistance than the NiCo [email protected] one, and the uninterrupted porous network formed by tight interconnection between NiCo 2O4 nanosheet scaffolds and the large lateral size are crucial to the optimization of electrochemical performances. Finally, a type of asymmetric supercapacitors based on [email protected] electrodes shows the maximum energy density of 52.6 Wh kg-1 at a power density of 332.4 W kg-1, suggesting its great potential for energy storage applications. Besides, this research would give a general reference for the rational design of other active materials for high-performance supercapacitors. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 11474135 and 51572118) and the Fundamental Research Funds for the Central Universities (No. lzujbky-2015-110). References [1] Y. Gogotsi, P. Simon, True Performance Metrics in Electrochemical Energy Storage, Science Magazine vol. 334 (2011) 917-918. [2] P. Simon, Y. Gogotsi, B. Dunn, Where Do Batteries End and Supercapacitors Begin?, Science 343 (2014) 1210-1211. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat Mater 7 (2008) 845-854. [4] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366-377. [5] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359-367. [6] I.S. Ike, I. Sigalas, S. Iyuke, Understanding performance limitation and suppression of leakage current or self-discharge in electrochemical capacitors: a review, Phys. Chem. Chem. Phys 18 (2016) 661-680. [7] P. Yang, Y. Ding, Z. Lin, Z. Chen, Y. Li, P. Qiang, M. Ebrahimi, W. Mai, C.P.

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(2016) 120-125. [36] G. Zhang, X.W. Lou, General Solution Growth of Mesoporous NiCo 2O4 Nanosheets on Various Conductive Substrates as High-Performance Electrodes for Supercapacitors, Adv. Mater. 25 (2013) 976-979. [37] F. Deng, L. Yu, G. Cheng, T. Lin, M. Sun, F. Ye, Y. Li, Synthesis of ultrathin mesoporous NiCo2O4 nanosheets on carbon fiber paper as integrated high-performance electrodes for supercapacitors, J. Power Sources 251 (2014) 202-207. [38] J. Du, G. Zhou, H. Zhang, C. Cheng, J. Ma, W. Wei, L. Chen, T. Wang, Ultrathin Porous NiCo 2O4 Nanosheet Arrays on Flexible Carbon Fabric for High-Performance Supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 7405-7409. [39] D. Kong, W. Ren, C. Cheng, Y. Wang, Z. Huang, H.Y. Yang, Three-Dimensional [email protected] Coaxial Nanowire Arrays on Carbon Textiles for High-Performance Flexible Asymmetric Solid-State Supercapacitor, ACS Appl. Mater. Interfaces 7 (2015) 21334-21346. [40] W. Zhou, D. Kong, X. Jia, C. Ding, C. Cheng, G. Wen, NiCo 2O4 Nanosheets Supported Hierarchical Core-shell Arrays for High-Performance Supercapacitors, J. Mater. Chem. A 2 (2014) 6310-6315. [41] L. Yu, G. Zhang, C. Yuan, X.W. Lou, Hierarchical NiCo [email protected] core-shell heterostructured nanowire arrays on Ni foam as high-performance supercapacitor electrodes, Chem. Commun. 49 (2013) 137-139. [42] K. Xu, W. Li, Q. Liu, B. Li, X. Liu, L. An, Z. Chen, R. Zou, J. Hu, Hierarchical mesoporous NiCo [email protected] core-shell nanowire arrays on nickel foam for aqueous asymmetric supercapacitors, J. Mater. Chem. A 2 (2014) 4795-4802. [43] X. Liu, S. Shi, Q. Xiong, L. Li, Y. Zhang, H. Tang, C. Gu, X. Wang, J. Tu, Hierarchical [email protected] 2O4 Core/Shell Nanoflake Arrays as High-Performance Supercapacitor Materials, ACS Appl. Mater. Interfaces 5 (2013) 8790-8795. [44] D. Cheng, Y. Yang, J. Xie, C. Fang, G. Zhang, J. Xiong, Hierarchical [email protected] core-shell hybrid nanowire/nanosheet arrays for high-performance pseudocapacitors, J. Mater. Chem. A 3 (2015) 14348-14357. [45] Z. Gu, H. Nan, B. Geng, X. Zhang, Three-dimensional [email protected] core/shell nanowires for electrochemical energy storage, J. Mater. Chem. A 3 (2015) 12069-12075. [46] C. Cui, J. Xu, L. Wang, D. Guo, M. Mao, J. Ma, T. Wang, Growth of [email protected] Nanocolumn Arrays with Superior Pseudocapacitor Properties, ACS Appl. Mater. Interfaces 8 (2016) 8568-8575. [47] J. Hu, F. Qian, G. Song, L. Wang, Hierarchical Heterostructures of [email protected] (X = Ni, Co) as an Electrode Material for High-Performance Supercapacitors, Nanoscale Research Letters 11 (2016) 1-7. [48] J. Xiao, L. Wan, S. Yang, F. Xiao, S. Wang, Design Hierarchical Electrodes with Highly Conductive NiCo 2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors, Nano Lett. 14 (2014) 831-838. [49] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X.-b. Zhao, H.J. Fan, High-Quality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for

Electrochemical Energy Storage, ACS Nano 6 (2012) 5531-5538. [50] C.-T. Hsu, C.-C. Hu, Synthesis and characterization of mesoporous spinel NiCo2O4 using surfactant-assembled dispersion for asymmetric supercapacitors, J. Power Sources 242 (2013) 662-671. [51] X. Wang, W.S. Liu, X. Lu, P.S. Lee, Dodecyl sulfate-induced fast faradic process in nickel cobalt oxide-reduced graphite oxide composite material and its application for asymmetric supercapacitor device, J. Mater. Chem. 22 (2012) 23114-23119. [52] B. Senthilkumar, D. Meyrick, Y.-S. Lee, R.K. Selvan, Synthesis and improved electrochemical performances of nano β-NiMoO4–CoMoO4· xH2O composites for asymmetric supercapacitors, RSC Adv. 3 (2013) 16542-16548.

Figure captions: Fig.

1

(a)

Schematic

illustration

of

the

two-step

synthesis

of

[email protected] directly on Ni foam. (b) Digital photographs of Ni foam, NiCo2O4 UNSAs and NiCo [email protected] on Ni foam. (c) SEM images of NiCo2O4 UNSAs. (d) SEM images of [email protected] Fig. 2 (a, b) Low-magnification TEM and HRTEM images of a single NiCo 2O4 nanosheet (inset: SAED pattern of NiCo 2O4 nanosheet). (c, d) Low-magnification TEM and HRTEM images of [email protected] hierarchical nanosheet (inset: SAED pattern of NiMoO4 taken from the shell region). Fig.

3

(a)

Schematic

illustration

of

the

two-step

synthesis

of

[email protected] directly on Ni foam. (b) Digital photographs of Ni foam, NiCo2O4 NNAs and [email protected] on Ni foam. (c) SEM images of NiCo2O4 NNAs. (d) SEM images of [email protected] at different magnifications. (e, f) TEM images of NiCo 2O4 NNAs and [email protected], respectively. (g) Observation of the mesoporous feature of NiMoO4 shell (the boxed region in (f)). Fig. 4 (a) CV curves of the NiCo [email protected] electrode at varied scanning rates. (b) Comparison of CV curves for the two types of NiCo 2O4 and [email protected] hierarchical electrodes at the same scanning rate of 10 mV s-1. (c) GCD curves of the NiCo [email protected] electrode at varied current densities. (d) Comparison of GCD curves for the two types of NiCo2O4 and [email protected] hierarchical electrodes at a current density of 5 mA cm-2. (e) Areal capacitances of the

four electrodes as a function of current density. (f) Cycling stabilities of the four electrodes at current densities of 50 mA cm-2. Fig. 5 (a) EIS analysis of the two types of NiCo 2O4 and NiCo [email protected] hierarchical electrodes. (b) Magnified semicircles in high frequency area. (c) Schematic illustration of electron transport channels in UNSA and NNA 3D electrodes. Fig. 6 (a) CV curves of the AC and [email protected] at a scanning rate of 10 mV s-1 measured by a three-electrode configuration. (b) CV curves of the [email protected]//AC asymmetric supercapacitor at different potential windows

at

a

scan

rate

of

10

mV

s-1.

(c)

GCD

curves

of

the

[email protected]//AC asymmetric supercapacitor at different current densities. (d) Specific and areal capacitances of the asymmetric supercapacitor as a function of current density. (e) Ragone plots of the asymmetric supercapacitor. The values reported for other devices are added for comparison. (f) Digital photograph of a 5 mm green-LED and a 5 mm red-LED lighted by prototype device.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Highlights:  Two types of hierarchical nanostructures were constructed using NiCo 2O4 scaffolds.  Areal capacitance of 7.29 F cm-2 is obtained for [email protected] electrodes.  The UNSA scaffold can provide more efficient electron transport channels.  The [email protected] electrodes show high rate capability (84.1% at 60 mA cm-2).