Facile Growth of Caterpillar-like NiCo2

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Facile Growth of Caterpillar-like NiCoS Nanocrystal Arrays on Nickle Foam for High-Performance Supercapacitors Xiaojuan Chen, Di Chen, Xuyun Guo, Rongming Wang, and Hongzhou Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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ACS Applied Materials & Interfaces

Facile Growth of Caterpillar-like NiCo2S4 Nanocrystal Arrays on Nickle Foam for High-Performance Supercapacitors Xiaojuan Chen,1,2,3 Di Chen,2 Xuyun Guo,4 Rongming Wang,2,* and Hongzhou Zhang3 1 Department of Physics, Beihang University, Beijing 100191, P. R. China. 2 Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, P. R.

China. E-mail: [email protected]

3 School of Physics & CRANN, Trinity College Dublin, Dublin, Ireland. 4 Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China KEYWORDS: NiCo2S4, conductive, nanosheets@nanowires, ammonium fluorid， supercapacitor ABSTRACT: Ternary cobalt nickel sulfides as a novel and efficient electrode material in supercapacitors has recently gained extensive interests. Herein, we firstly report a highly conductive caterpillar-like NiCo2S4, composed of NiCo2S4 nanosheet core and nanowires shell grown on Ni foam via a facile and cost-effective chemical liquid process. Growth mechanism of the NiCo2S4 nanosheets@nanowires (NSNWs) structure was also investigated in detail by analyzing time-dependent experimental as well as the amount of additive ammonium fluoride in solution. Furthermore, the 1

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electrochemical measurements were performed among three different morphologies of NiCo2S4 including nanosheets, nanosheets@nanoparticles and NSNWs structure, which were obtained from different reaction stages. Because the NSNWs structure has relatively high electroactive surface area, conductivity and effective electron transport pathways, the as-prepared NiCo2S4 NSNWs structure comparing with two other morphologies exhibits the maximum specific capacity of 1777 F/g at 1 A/g and the highest capacitance retention (83% after 3000 cycles) at a high scan rate of 10 A/g with a mass loading density of 4.0 mg/cm2. These results indicate that the NiCo2S4 NSNWs structure has great potential in supercapacitors. INTRODUCTION Supercapacitors,1-2 as an important member in the alternative energy family, has attracted great attention since their irreplaceable merits of the high power density, long cycling life, and fast charging/discharging rate.3-4 These unique characteristics would make supercapacitors widely used in hybrid electric vehicles,5 portable electronic devices,6 and backup energy systems.7 Generally, supercapacitors can be divided into two types on different charge storage mechanisms. Pseudocapacitors (PCs) based on redox reactions, possess larger specific capacitance than electrical double layer capacitors (EDLS, based on electrostatic accumulation). However, the faradaic process of PCs is normally slower, which limits the power density of PCs than EDLS.8-9 Especially in a high current density condition, the high specific capacitance for PCs cannot be maintained without a rapid charge transfer rate. In general, the charge transfer rate of pseudocapacitors can be improved by exploring the 2


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high conductivity of electrode materials or designing the electrode with effective charge transport pathways. Among various pseudocapacitance electrode materials, transition metal oxides (such as MnO2,10-11 CoOx,12-13 NiCo2O4, 14-16 etc.) have been investigated intensively due to their high theoretical capacitance, low toxicities and costs. Recently, NiCo2S417-21 with spinel structure has drawn more attention. In AB2S4 spinel structure, nickel (Ni3+) occupied the octahedral sites and cobalt (Co3+ and Co2+) is distributed over both octahedral and tetrahedral sites.22 Besides, NiCo2S4 possesses rich redox reactions with mixed valence state,23-24 which also exhibits ultrahigh electrical conductivity about 103 S/cm.18, 22 In addition, NiCo2S4 is cost-effective, and available in various favouring ion diffusion morphologies,25 such as nanowires,26 urchin like structures,27-28 making it acting as an attractive candidate for high performance supercapacitors. Indeed, it has been evidenced that NiCo2S4 materials possess high-rate capacitance performance in supercapacitors.17, 29 As for designing electrodes, binder-free supercapacitor is an effective strategy to reduce internal resistance for high rate capacitance. For example, directly grow active materials on conductive substrate (e.g., carbon fiber paper, carbon cloth, Ni foam, etc.) can dramatically improve the capacitive performance of supercapacitors, because 3D substrate can promote the active surface area, shorten charge transport pathways and without using any polymer binder. To date, NiCo2S4 nanostructures with various shapes grown on Ni foam have been reported.30 Specifically, Liu et al found 3D porous NiCo2S4 nanonetworks on Ni foam(NF) shows an excellent property with 3



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specific capacitance of 1501.2 F/g at 1 A/g.31 In addition, 3D hybrid nanostructures such

as

Co3O4@NiCo2O4,32

NiCo2O4@NiO,33

NiCo2S4@MnO234

and

NiCo2S4@Ni(1-x)Cox(OH)235 arrays on NF have been reported for high-performance supercapacitors, which are assigned to large specific surface area, and synergistic effect between different components. However, the core material in these structures usually contributes little capacitance, since the core is almost completely blocked from the access of electrolyte by shell, and this complex core-shell producing process is costly. To the best of our knowledge, few literatures on the synthesis of hierarchical NiCo2S4 nanosheet@nanowires on Ni foam with high capacitive performance were reported from the simply and effectively liquid solution process till now. In this paper, a 3D hierarchical NiCo2S4 NSNWs arrays structure grown directly on NF is successfully synthesized by a facile hydrothermal and subsequent sulfurization treatment. The possible formation mechanism of the NiCo2S4 NSNWs structure is also proposed by analyzing the evolution process of Ni-Co precursor and tuning the amount of NH4F in the reaction, respectively. Electrochemical measurements were performed among NiCo2S4 nanosheet@ nanowires (NSNW, S1),

NiCo2S4

nanosheet@nanoparticles (NSNP, S2) and NiCo2S4 porous nanosheets (NS, S3), the specific capacitance for S1, S2 and S3 electrode is estimated to be 1777, 1238 and 1010 F/g, respectively, at the same current density of 1 A/g. Moreover, the S1 electrode also shows the best cycling stability at high current density of 10 A/g than other morphologies. This is all due to the structure supervisor for S1 electrode: (1) the multi-direction grown nanowires on the surface of nanosheets can dramatically 4


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increase the interface between the electrode and the electrolyte, enabling more reaction sites. (2) The established rapid charge transport pathways from NF to high conductive nanosheets to nanowires，which favors to high-rate capacitance. These results suggest that the NiCo2S4 NSNWs structure can act as a high performance electrode material for supercapacitors. EXPERIMENT SECTION All chemicals were used as received without further purification. Briefly, Nickel foam (approximately 1×5 cm) was carefully cleaned by HCl solution to remove the surface oxide layer and then washed with ethanol and deionized water. The top side of the NF was then covered with a polytetrafluoroethylene tape to avoid contacting reactants. In a typical procedure, the Ni-Co precursors with various nanostructures on NF were first prepared from the hydrothermal process. Namely, the reagents (1 mmol NiCl2·6H2O, 2 mmol CoCl2·6 H2O, 10 mmol CO(NH2)2, and 10 mmol NH4F) were dissolved in 50 mL distilled water and stirred to form a clear pink solution. Then the aqueous solution and the Ni foam were placed in a 100 mL Teflon lined stainless-steel autoclave. The autoclave was sealed and then maintained at 95°C for 12 h, 10 h and 8 h for the synthesis of Ni-Co precursor NSNWs, Ni-Co precursor NSNP and Ni-Co precursor NS, respectively. Afterwards, the samples were collected by centrifugation and washed with acetone for several times, then dried at 80°C for 6 h. Finally, the NiCo2S4/NF was obtained by placing Ni-Co precursor/NF in a 50 mL autoclave with a solution containing Na2S·9H2O (1.5 mmol) in an electric oven at 120 ˚C for 8 h. The NiCo2S4/NF products were washed successively with deionized water and ethanol to 5



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remove any residual ionic species, and finally dried in vacuum for 24 h. The morphology of samples was characterized by using field-emission scanning electron

microscopy

(FESEM,

HITACHI,

S4800)

equipped

with

X-ray

energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM, JEOL, JEM-2200F, 200 kV). Their phase and chemical state were characterized by using X-ray diffraction (XRD, X ’Pert Pro MPD system, Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS, ESCALAB MK II, Al Kα photon source, C1s 284.8 eV). Electrochemical measurements The electrochemical measurements were carried out in a three-electrode glass cell connected to an electrochemical workstation. Ni foam loaded with the S1 (4.0 mg/cm2), S2 (4.5 mg/cm2) and S3 (4.7 mg/cm2) was used as the working electrode, respectively. And a Pt wire and a Hg/HgO electrode were used as the counter and reference electrodes. The electrochemical performance of the samples was evaluated on a CHI 660D (CH Instruments) workstation for cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) tests in 5 M KOH aqueous solution. EIS measurements were carried out using this apparatus over a frequency range of 100 kHz to 0.01 Hz at 0 V with an AC amplitude of 5 mV. RESULTS AND DISCUSSION The fabrication process of hierarchical NiCo2S4 NSNWs grown on NF is schematically illustrated in Figure 1. In the first step, pure sheet-like Ni-Co precursors were vertically grown on the surface of 3D NF. Extending the reaction time, a large 6


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scale of nanoparticles was formed and then grew to nanowires on the surface of nanosheets backbone, forming Ni-Co precursors NSNWs. In the second step, Ni-Co precursors NSNWs were converted to NiCo2S4 NSNWs by an anion exchange reaction in sodium sulfide solution. To illustrate morphology and microstructure of the NiCo2S4 NSNWs, SEM and TEM measurements are performed. In Figure 2a and its inset, typically caterpillar-like NiCo2S4 arrays are homogeneous deposited on the surface of NF, suggesting that our hydrothermal method is effective to grow this structure on NF. Moreover, the NiCo2S4 structure is well preserved during sulfurization process (Figure S1). Close observation reveals that the NiCo2S4 structure is composed by nearly vertical nanosheets aligned on NF and many multi-directional nanowires grown on the nanosheets, and there is considerable inter-nanowire space, it will help electrolyte easy access to nanosheets core and nanowires shell in electrochemical reaction, improving the utilization rate of the electrode materials. As shown in Figure S1a, the length of a NSNWs structure is in the range of 2-5 µm. On the surface of NiCo2S4 NSNWs (Figure 2b), several rough nanowires from bottom core about 50 nm to tip around 30 nm are clearly seen, it is noted that the porous nanowires are composed by the adjacent nanoparticles with diameter of ∼10 nm, which can be manifested in Figure 2c. In the set of Figure 2b, the selected-area electron diffraction (SAED) pattern of a single nanowire shows the ring patterns, which are well indexed to the NiCo2S4 planes of (111), (311), (400), (422), (511), and (440), unveils its polycrystalline structure. In Figure 2c, a HRTEM image taken from a NiCo2S4 7



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nanowire reveals lattice fringes with interplanar spacing of 0.542 nm corresponding to the (111) plane of NiCo2S4, the corresponding fast-Fourier-transform (FFT) pattern (inset in Figure 2c) also confirms its polycrystalline structure. Moreover, the TEM and elemental EELS map images (Figure 2d) results indicate the elements Ni, Co and S are uniformly distributed in the NiCo2S4 nanowire. Figure S2a shows the XRD pattern of the NiCo2S4 NSNWs sample on NF, where the diffraction peaks at 26.8°, 31.6°, 38.3°, 50.5°, and 55.3° consistent well with the spinel type cubic phase of NiCo2S4 (JCPDS no. 20-0782), respectively.36 There are also have two peaks at 44.4° and 52.0° can be assigned to the Ni substrate, the last two clear peaks at 29.8° and 52.1° may correspond to Co9S8 (JCPDS no.86-2273). The existence of Co9S8 phase in NiCo2S4 product is ascribed to the incomplete sulfurization of the samples using Na2S as a sulfur source, which was verified by previous reports.37 The chemical bonding state of each component element on the surface of the NiCo2S4 sample is characterized by XPS. Figure 3a shows a wide XPS spectrum of the NiCo2S4 NSNWs sample. The peaks at 164.1, 781.0 and 856.3 eV correspond to S 2p, Co 2p and Ni 2p, respectively, indicating the existence of S, Co and Ni elements in the as-obtained sample, further confirmed in Figure S2b. Figure 3b-d show high-resolution individual XPS spectra of Ni 2p, Co 2p and S 2p, respectively. Using a Gaussian fitting method, Ni 2p spectrum (Figure 3b) and Co 2p spectrum(Figure 3c) both can be well fitted with two spin-orbit doublets, and two shake-up satellites (marked by ‘‘Sat.’’). In Figure 3b, the peaks at 855.6 eV for Ni 2p3/2 and 873.1 eV for Ni 2p1/2 suggest the existence of both Ni2+ and Ni3+. The 8


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intense satellite peaks indicate that Ni3+ is the majority species in the crystal lattice. For the Co 2p spectrum in Figure 3c, a doublet containing peaks at 778.4 eV for Co 2p3/2 and 793.6 eV for Co 2p1/2 are demonstrated, suggesting the coexistence Co3+ and Co2+ in the NiCo2S4. The weak satellite peaks indicate that the majority of cobalt is Co2+. According to the above mentioned analysis, the near surface of NiCo2S4 sample has compositions of Co2+, Co3+, Ni2+, and Ni3+, which agree with the results from the literatures

26, 38

. As for S 2p spectrum (Figure 3d), it can be divided into two main

peaks located at around 161.4 and 162.5 eV and one shake-up satellite at about 168.4 eV.39 The peak at 162.5 eV is close to binding energy of S 2p3/2 due to of metal-sulfur bonds, while the peak at 161.4 eV near binding energy of S 2p1/2 may be attributed to the sulfur ions at low coordination, which is generally related to sulfur vacancies. The relative intensity of 2p1/2 in S2p spectrum is 37.1 %, indicating a higher sulfur vacancy concentration in NiCo2S4. Some literatures29, 40 reported that high proportion sulfur vacancies in sample can improve the conductivity of NiCo2S4, further increase specific capacitance, but disorder crystal structure from the sulfur vacancies may weaken cyclic performance. Moreover, Figure S4 displays the N2 adsorption-desorption isotherm curves of three samples, the Brunauer-Emmett-Teller (BET) surface areas of S1 sample, S2 sample, S3 sample are calculated to be 57.28, 50.16 and 49.73 m2/g, respectively. Apparently, S1 sample shows the relatively highest surface area comparing with other samples.

Growth mechanism Based on above results, the morphology of NiCo2S4 with NSNWs structure was well 9



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inherited from Ni-Co precursor after a sulfurization process. Hence, the NSNWs structure of NiCo2S4 is decided by Ni-Co precursor formation process. In that experiment, reaction time and the concentration of NH4F in solution play very important role. So we carefully studied the evolution process of the Ni-Co precursor/NF and the parameter of NH4F, respectively. Figure 4a-d show the SEM images of Ni-Co precursors obtained at different reaction stages. When reaction time arrived at 480 min (Figure 4a), many nanosheets were observed and aligned on the Ni foam. After 540 min (Figure 4b), numerous nanoparticles were formed and covered on the surface of the nanosheets. Until to 600 min (Figure 4c), the smooth nanosheets arrays were completely decorated by nanoparticles to become volume-expanded nanosheets. With the reaction proceeding, many nanoparticles were elongated to nanowires on the surface of the nanosheets, the nanowires became bigger and longer out from the nanosheets core. Up to 600 min (Figure 4d), uniform Ni-Co precursor NSNWs were collected. Figure 4e-h show the morphology of the obtained Ni-Co precursors at various concentrations of NH4F. Without NH4F, only 1D Ni-Co precursor nanowires arrays can be achieved (Figure 4e). When the concentration of NH4F is at 0.2 M, NSNWs arrays can be clearly obtained (Figure 4f). However, at 0.24 M, there are only nanosheets arrays (Figure 4g). Increasing to 0.3 M, a mass of hexahedron arrays appeared (Figure 4h). From the four group experimental results, we can summarize that NH4F benefits the growth of 2D and 3D Ni-Co precursors on NF in aqueous solution, such as the nanosheets and hexahedron structure. But in the same condition without NH4F, it is favorable to the growth of 1D nanowire. These 10


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different morphologies of Ni-Co precursors are attributed to NH4F. According to relevant work41 and Ni-Co precursor chemical reactions equations (see Supporting Information), NH4F is easy decomposed in hot aqueous solution, reducing the solution pH value. However, the Ni-Co precursor is not stable in acidic environment. Especially for lower dimension nanomaterial with relatively high surface area, it hardly appears in high concentration of NH4F condition. In contrast, the 2D or 3D nanostructure can form in high amount of NH4F. According to above discussions, the possible growth mechanism of the NSNWs arrays on NF can be explained as followings: at the beginning of this reaction, the higher concentration of NH4F in solution reduced the pH valve in solution benefits the growth of nanosheets on NF. However, with the reaction going on, NH4F was gradually used out, the rising up pH value will help the growth of nanowires and load on the surface of nanosheets, resulting in the formation of NSNWs structure. Electrochemical properties To demonstrate the superiority of this unique NSNWs structure in supercapacitors, we next investigate the electrochemical performance of three different morphologies NiCo2S4, including NSNW (S1), NSNP (S2) and porous NS (S3) obtained in three different reaction stages, as shown in Figure S3. Figure 5a displays the cyclic voltammogram (CV) curves of the S1, S2, and S3 electrodes at the scan rate of 1 mV/s. Clearly, three pairs of well-defined redox peaks are observed in CV curve of S1, which are mainly originated from the redox processes of Co2+/Co3+/Co4+ and Ni2+/Ni3+

based

on

the

three

reversible

reactions: 11



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NiCo2S4+OH-+H2O ⇔ NiSOH+2CoSOH+e-;

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CoSOH +OH- ⇔ CoSO+H2O+e-; Co9S8

+OH- ⇔Co9S8OH+e-. Compared with the CV curves of S2 and S3, the S1 electrode

exhibits the highest current densities and broader voltage range. In addition, the galvanostatic charge-discharge measurements of the three electrodes were conducted at the same current density of 1 A/g, as shown in Figure 5b, the discharge time for S1, S2 and S3 electrode is estimated to be 855, 619 and 505 s, respectively. These measurement results indicate S1 electrode shows the highest specific capacitance compared to S2 and S3, which can be attributed to the unique 3D NSNWs structure with the largest surface region providing the larger ion accessible area. Meanwhile, the diameter of nanowires under the maximum diffusion distance range of electrolyte,42 making sure the active materials sufficiently participating in the electrochemical reaction. The electrochemical performance of NiCo2S4 NSNWs was further investigated systematically. Figure 6a shows typical CV curves of the S1 electrode with the potential window of 0-0.6 V at various scan rates. With increasing scan rate, the redox current increase continuously, and the position of the oxidation and reduction peaks shift toward higher and lower potentials, respectively, implying the fast redox reactions at the interface between the electrode and electrolyte. Figure 6b shows the charge/discharge voltage vs time profiles at current densities from 1 to 20 A/g in the voltage window of 0-0.5 V. The pseudocapacitive behavior is further confirmed by the asymmetrical curves with well-defined plateaus. According to the specific capacitance formula C = I ∆t / m ∆U, where I is the discharge current, ∆t refers to the 12


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discharge time, ∆U denotes the voltage range, and m is the mass of the active material, the specific capacitance of S1 electrode can be calculated to 1777, 1461, 1200, 982, 834, 720 F/g at current densities of 1, 2, 5, 10, 15, and 20 A/g, respectively, as shown in Figure 6c (black curve). Similarly, the specific capacitance of S2 and S3 electrodes were also evaluated, as depicted by the red and blue curves in Figure 6c. Evidently, the S1 electrode possesses the highest specific capacitance among three electrodes at any current densities. Even at a current density as high as 20 A/g, the specific capacitance of the S1 electrode can still reach an impressive value of 720.0 F/g, this demonstrates the excellent rate capability of the S1 electrode. The cycling property, a critical factor for practical application in supercapacitors, is also evaluated by repeated charge-discharge measurements at a current density of 10 A/g, as shown in Figure 6d. As usual, the specific capacitance decreases with the increasing of cycle number. After 3000 cycles, 83 % of the specific capacitance of S1 electrode can be maintained, which is obviously superior to the S2 (66%) and S3 (73%). Interestingly, at the beginning of the profiles, the S2 electrode demonstrates a higher specific capacitance than that of S3 (744 vs 706 F/g), yet its specific capacitance declines sharply with the increasing of cycle number. Up to 3000 cycle, the specific capacitance of S2 is even lower than that of S3, which probably due to the particles on the surface of nanosheets fell off during long-term cycling. The SEM images of the S1, S2 and S3 electrodes after the cycling test are all shown in Figure S5. Obviously, the structure of S2 and S3 electrode materials were more seriously damaged than S1 electrode materials after long-term cycles. 13



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Figure 7 presents the Nyquist plots of the S1 electrode carried out in the frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. EIS can be fitted by an equivalent circuit, which is composed of an internal resistance Rs, an interfacial charge transfer resistance Rct, a Warburg resistance W, and the double layer capacitance Cdl. The internal resistance (Rs) and the charge transfer resistance (Rct) were measured to be 0.37 and 0.80 Ω, respectively. It manifests that NiCo2S4 NSNWs has lower resistance, and good ion/electron transfer, which were mainly attributed to the high conductivity of NiCo2S4 material and without any binder. EIS Nyquist plots of S2 and S3 electrodes are also displayed in Figure S6. Remarkably, S1 has the smallest interfacial charge transfer resistance comparing with the other electrodes. The above electrochemical tests indicate NiCo2S4 NSNWs structure has superior capacitive performance in energy storage. Here we packed two NiCo2S4 symmetry supercapacitors device with aluminum-plastic foils in Figure S7a. Then the two NiCo2S4 symmetry supercapacitors were assembled in series and charged by using a CR2032 Lithium Battery (2.5 V) as a constant voltage sources. After charging for only 10 s, this device could power a red LED (0.03 W) and keep it lighting for 60 mins, as shown in Figure S7b-d.

CONCLUSION In summary, a 3D caterpillar-like NiCo2S4 was successfully synthesized on NF via a two-step solution reaction. The possible growth mechanism of the NSNWs structure has also been proposed. When applied as electrode materials in supercapacitors, the 14


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NiCo2S4 NSNWs structure exhibits the highest specific capacitance and cycling stability, comparing to two other morphologies of NiCo2S4 electrode materials. These results suggest that the NiCo2S4 NSNWs structure electrode material may hold great potential in supercapacitors.

Figure 1. Schematic illustration of the fabrication processes of NiCo2S4 NSNWs arrays on the NF.

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Figure 2. SEM images of NiCo2S4/NF (a and inset) obtained at different magnifications. TEM image (b) of nanowires from NiCo2S4 NSNWs, the inset in (b) shows SAED pattern of one individual NiCo2S4 nanowire. HRTEM image (c) of a NiCo2S4 nanowire and the inset is its corresponding FFT image. TEM image (d) and the corresponding compositional EELS elemental map images of a single NiCo2S4 nanowire.

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Figure 3. XPS spectra of NiCo2S4 NSNWs arrays on NF: (a) survey, (b) Ni 2p, (c) Co 2p, and (d) S 2p.

Figure 4. SEM images of the intermediates obtained at different time for Ni-Co precursor: (a) 480, (b) 540, (c) 600, and (d) 720 min；SEM images of Ni-Co precursor samples synthesized with different concentrations of NH4F: (e) 0 mmol, (f) 10 mmol, (g) 12 mmol, and (h) 15 mmol.

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Figure 5. (a) CV curves of the S1, S2, and S3 electrodes measured at a scan rate of 1 mV/s. (b) Galvanostatic discharge curves of the S1, S2, and S3 electrodes tested at a current density of 1 A/g.

Figure 6. (a) Cyclic voltammograms of the NiCo2S4 at various scan rates ranging from 1 to 10 mV/s; (b) Galvanostatic charge-discharge curves of S1 electrode ranging from 1 to 20 A /mg; (c) Specific capacitance as a function of current density for the S1, S2, and S3 electrodes; (d) Specific capacitance vs. cycle number at a current density of 10.0 A/g for the S1, S2 and S3.

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Figure 7. EIS Nyquist plots of the NiCo2S4 NSNWs electrode. The inset shows the equivalent circuit diagram used to analyze the EIS data.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 51371015, 51331002，11674023), and the Fundamental Research Funds for the Central Universities (FRF-BR-15-009B). ACCSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

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Graphical Abstract 161x91mm (150 x 150 DPI)


Facile Growth of Caterpillar-like NiCo2

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