A flexible and high-performance all-solid-state supercapacitor device based on Ni₃S₂ nanosheets coated ITO nanowire arrays on carbon fabrics

Abstract

A Ni₃S₂ coated indium tin oxide (ITO) core–shell structure on flexible carbon fabrics (CF@ITO@Ni₃S₂) was prepared by electrodepositing Ni₃S₂ nanosheets on ITO nanowire arrays grown on flexible carbon fabrics by chemical vapor deposition. The ITO nanowires on carbon fabrics formed a conductive support which offered a large contact surface with the electrolyte and hence was accessible for ion diffusion. Ni₃S₂ nanosheets were uniformly deposited on the ITO nanowire. The maximum mass loading of Ni₃S₂ on ITO nanowire array coated carbon fabrics could reach about a quadruple higher amount than that on the bare carbon fabrics. The prepared CF@ITO@Ni₃S₂ electrodes exhibited excellent capacitive performance compared with Ni₃S₂ coated bare carbon fabric electrodes (CF@Ni₃S₂). High areal capacitance of 3.85 F cm⁻² and gravimetric capacitance of 1865 F g⁻¹ were achieved when the mass loadings of Ni₃S₂ on the ITO nanowire arrays were around 4.12 mg cm⁻² and 0.96 mg cm⁻², respectively. The sample with 0.96 mg cm⁻² of Ni₃S₂ could also deliver 1372 F g⁻¹ when charge–discharge current density reached 50 mA cm⁻², indicating the excellent rate capability of the structure. The assembled all-solid-state full cell based on symmetric electrodes obtained a relatively high areal capacitance of 736 mF cm⁻² at 8 mA cm⁻², which delivered a maximum energy density of 1.02 mW h cm⁻³ at a power density of 39.9 W cm⁻³. The outstanding capacitive performance suggests that the CF@ITO@Ni₃S₂ device was promising for application in an inexpensive energy storage system.

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Introduction

Nowadays, the development of portable electronics, hybrid vehicles, and grid-scale energy storage systems has led to urgent demands for energy storage devices with high energy density, short charging time and long cycle life.^1–3 Compared to lithium ion batteries, supercapacitors possess higher power density and a faster charge–discharge rate, which make them potential candidates for the above applications.^4–7 However, the energy density of supercapacitors still exhibits at a low level. Therefore, exploring supercapacitors with high energy density remains a crucial question.

On the basis of different charge storage mechanism, supercapacitors were categorized into electric double layer capacitors (EDLCs)^8–10 and pseudocapacitors. Pseudocapacitors (generally with transition metal oxides/hydroxides,^11–16 sulfides,^17–20 nitrides,^21,22 carbides²³ and conducting polymers²⁴ as active materials), in which charge was stored by the reversible faradaic reaction on the surface of the active materials, showed higher specific capacitance than EDLCs. Since good electrical conductivity of the active materials is an important factor deciding the electrochemical properties, transition metal sulfides obtained much attention in the field of supercapacitors for its better electrical conductivity than transition metal oxides.²⁵ So far, transition metal sulfides such as MoS₂, Ni₃S₂, CoS₂, and CuS has been found versatile materials in various electrochemical applications such as supercapacitors, lithium ion batteries,²⁶ counter electrodes of dye-sensitized solar cells^27,28 and so on, exhibiting their excellent electrochemical activity. For example, the theoretical gravimetric capacitance for Ni₃S₂ in a three-electrode system can be achieved about 2400 F g⁻¹. However, the reported gravimetric capacitance for Ni₃S₂ in the literature was just about 1500 F g⁻¹.^29,30 The reason is probably that although transition metal sulfides exhibit higher electrical conductivity than transition metal oxides, the demand of good electric transfer ability for the fast faradic redox actions is still not be satisfied, which leads to the weak rate performance of sulfide-based supercapacitors. Several strategies have been reported to improve the performance for sulfide-based supercapacitor electrodes. One of them is developing various nanostructures with high surface area such as nanowires,^31,32 nanosheets,^33,34 nanotubes,³⁵ spheres³⁶ and so on, providing more active sites for the redox reaction and shortening the charge transfer length, which is beneficial for fast charge–discharge rate. Another approach, which is considered as a general strategy, is to combine active materials with conductive and high-specific-area supports such as carbonaceous materials (graphene,³⁷ carbon nanotube,³⁸ carbon fiber³⁹), conductive oxides (Sb doped SnO₂,⁴⁰ Sn doped In₂O₃,^41,42 etc.). Binder-free active materials would also be preferred, which eliminate the contact resistance between the nanostructures and improve the cycle life.^43–46

Based on the above consideration, we designed a “core–branch–shell” nanostructure and combined high-specific-area supports and nanostructured active materials to improve the electron transfer in the active materials as much as possible. Conductive single-crystallin ITOe nanowires with high specific area were chosen as conductive supports for active materials loading. Nickel sulfides nanosheets were acted as active materials for supercapacitor electrodeposited on the supports. To obtain portable and wearable devices, flexible carbon fabrics were chosen as substrates for current collection, so the device can be flexibly bent. Solid-state electrolyte was chosen as the electrolyte to assemble all-solid-state supercapacitor devices, which can solve the electrolyte leakage problem in traditional liquid-electrolyte-based devices and be more feasible for the device flexibility.^47–49

Experimental

Preparation of ITO nanowire arrays on carbon fabrics (CF@ITO)

As shown in Fig. 1, ITO nanowire arrays were grown on carbon fabrics by chemical vapor deposition method. Before deposition, the carbon fabrics were ultrasonic cleaned by water, ethanol and acetone in sequence and dried under N₂ flow. Carbon fabrics were then sputtered with about 10 nm thick gold nanoparticles using ion sputtering equipment. The ITO nanowires were grown on the Au sputtered carbon fabrics in tube furnace. Indium and tin powder were purchased from Aladdin and the mass ratio for deposition was 10 : 1. The source was kept at 800 °C under a pressure of 40 Pa and an air flow of 0.5 sccm. The carbon fabrics were placed on the right and left side of the source with the distance about 0.5 cm. The growth time for ITO nanowires was 30 min. Different lengths of the ITO nanowires could be obtained by placing the substrates at different distance from the air inlet.

Fig. 1 Scheme for preparation of Ni₃S₂ nanosheets coated ITO nanowire on carbon fabrics.

Preparation of Ni₃S₂ on ITO nanowires based carbon fabrics (CF@ITO@Ni₃S₂)

Ni₃S₂ nanosheets were electrodeposited on ITO nanowires based carbon fabrics in a three-electrode electrochemical system as shown in Fig. 1. The ITO nanowires based carbon fabrics, a Pt sheet (1 × 1 cm²) and a saturated Ag/AgCl acted as the working electrode, counter electrode and reference electrode, respectively. The precursor solution for electrodeposition of Ni₃S₂ was consisted of 50 mM NiCl₂·6H₂O and 1 M thiourea (TU). Ni₃S₂ nanosheets were electrodeposited by the cyclic voltammetry (CV) method with the potential range from −1.2 to 0.2 V and scan rate 5 mV s⁻¹. Then the obtained CF@ITO@Ni₃S₂ were washed thoroughly with DI water and dried in a vacuum oven for 12 h at 60 °C. The probable growth mechanism⁵⁰ was demonstrated as follows:

Ni²⁺ + 2e⁻ → Ni

TU + Ni²⁺ → (NiTU)²⁺

TU + 2e⁻ → S²⁻ + CN⁻ + NH₄⁺

2S²⁻ + O₂ + 2H₂O + 3Ni → Ni₃S₂ + 4OH⁻

Preparation of the all-solid state symmetric cells

6 g of the powder of polyvinyl alcohol (PVA) was dissolved in 10 mL of deionized water at 90 °C with constant stirring for 1 h. When the solution became clear, 3 g of KOH in 20 mL deionized water was mixed in the solutions. The solution was then stirred at 90 °C for 30 min. Then two pieces of the CF@ITO@Ni₃S₂ electrodes were immersed into the PVA–KOH solution for 10 min. The all-solid-state symmetric supercapacitor device was prepared by assembling the electrodes with a filter paper in-between as separator until the water evaporated.

Electrochemical measurements

The electrochemical measurements were conducted in a three-electrode electrochemical system in a 2 M KOH aqueous solution. The prepared samples were used as the working electrode, a Pt sheet (1 × 1 cm) as the counter electrode and Ag/AgCl as the reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS) tests were carried out on a CHI660E electrochemical workstation (Chenhua, Shanghai). The electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternate current voltage with 5 mV amplitude in a frequency from 0.1 Hz to 100 kHz at open circuit potential.

The capacitance values were calculated from galvanostatic charge–discharge curves according to the following equations:

half-cell:

full cell:

where the areal capacitance (C) applies to a single electrode and the cell capacitance (C′) applies to the full cell only, I is the applied current in the measurement, ΔV is the operation voltage, S is the area of active material on the single electrode, S′ is the area of active material on the device.

Results and discussion

The morphologies and crystal structure of nanowires were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in low magnification SEM images in Fig. 2(a and b), ITO nanowires were uniformly and vertically aligned on the carbon fibers. The high-magnification SEM image (Fig. 2(c)) showed that the diameter of the nanowire was around 100 nm. The nanowire possessed cubic crystalline phase with smooth side surface and faceted shape, indicating the good crystallinity of the nanowires, which could also be seen in the TEM image in Fig. 2(d). The high-resolution TEM images in Fig. 2(e) showed clear and continuous lattice fringes of the nanowire, revealing the single-crystalline nature and further verifying the good crystallinity of the nanowire. The lattice spacing of 0.29 nm matches well with the interplanar spacing (222) planes of cubic In₂O₃, indicating the nanowire grows along (222) crystallographic direction. The inset of Fig. 2(e) shows the SAED pattern of the ITO nanowire, which reveals the single crystalline nature of the nanowire. The nanowires on carbon fabrics in Fig. S1(a1–a3), (b1–b3) and (c1–c3)† were corresponding to different lengths of ITO nanowires on carbon fabrics named as L1, L2, L3. As can be seen in the figure, with the length of the ITO nanowire increasing, the surface contact area of the electrode increased, which can load more active materials.

Fig. 2 (a–c) SEM images of ITO nanowires on carbon fabrics (d) low-resolution TEM image of ITO nanowires. (e) lattice fringes of the ITO nanowires with its selected area electron diffraction pattern in the inset.

Fig. 3(a and b) show the SEM images of CF@ITO@Ni₃S₂ in different magnifications using the ITO nanowire L2. The low-magnification SEM image in Fig. 3(a) showed that the electrodeposited active materials were uniformly coated on the ITO nanowire and no other product stacked between the ITO nanowires. From the higher-magnification SEM image in Fig. 3(b), we can see that the Ni₃S₂ coating on ITO nanowires showed nanosheet morphology with sizes of about 30 nm. Fig. 3(c) presents a typical HR-TEM image of ITO@Ni₃S₂ structure. As seen in Fig. 3(c), the ITO nanowire was coaxially deposited with Ni₃S₂ shell, forming a core–shell structure. A higher resolution image of the Ni₃S₂ coated ITO nanowires has been present in Fig. S2.† As seen in the figure, the Ni₃S₂ was porous with small nanosheets. The elemental analysis of the ITO@Ni₃S₂ coated carbon fabrics via energy dispersive X-ray spectroscopy (EDS) showed that elements In, Sn, Ni and S existed in the sample.

Fig. 3 (a) SEM image of Ni₃S₂ nanosheets coated ITO nanowires on carbon fabrics in low-resolution. (b) High-resolution SEM image of Ni₃S₂ nanosheets coated ITO nanowires. (c) Low-resolution TEM image of Ni₃S₂ nanosheets coated ITO nanowire. (d) EDX mapping of the Ni₃S₂ nanosheets coated ITO nanowires with In, Sn, S and Ni elements existing in it.

The crystal structures of CF@ITO@Ni₃S₂ were identified by X-ray powder diffraction (XRD). As can be seen from Fig. 4(a), the distinct diffraction peaks located at 21.5°, 30.6°, 33.1°, 35.5°, 41.8°, 45.7°, 51.0° and 59.1° degrees can be indexed to the (211), (222), (321), (400), (332), (431), (440) and (541) planes of In₂O₃ (JCPDS card no. 06-0416), respectively. The diffraction peaks at 21.7°, 31.1°, 37.7° and 55.2° are corresponding to (101), (110), (003), (122) planes of heazlewoodite phase Ni₃S₂ (JCPDS no. 44–1418). XRD patterns of different lengths of ITO nanowires on carbon fabrics were shown in Fig. S3.† As shown in the figure, when we increased the length of ITO nanowires, the intensity of the peak at 30.6° corresponding to (222) plane of In₂O₃ increased and the density of the peak at 26.4° corresponding to the H-graphite phased carbon fabrics decreased. The compositions of the CF@ITO@Ni₃S₂ electrode were further investigated by X-ray photoelectron spectra (XPS). The full XPS spectrum was displayed in Fig. 4(b). As seen in the figure, Ni 2p and S 2p peaks were present in the sample, revealing that nickel sulfide were successfully electrodeposited on the ITO nanowires. Two peaks at 855.2 and 872.8 eV in the Ni 2p spectrum in Fig. 4(c) were assigned to Ni 2p_1/2 and Ni 2p_3/2, respectively. The additional peaks around 875 eV and 882 eV could be ascribed to other nickel compounds such as nickel hydroxides. The peak at around 162.6 eV in Fig. 4(d) belonged to S 2p, indicating the deposited film was composed of S²⁻. These XPS results are quite confirmed the presence of Ni₃S₂. The peak at around 167.7 eV and 169 eV could be ascribed to the inconsequential amount of Ni_xS_y.³³

Fig. 4 XRD pattern (a), full XPS spectra (b), Ni 2p XPS spectrum (c) and S 2p XPS spectrum (d) of the CF@ITO@Ni₃S₂ electrode.

To evaluate the electrochemical performance of CF@ITO@Ni₃S₂ electrode, various electrochemical measurements were performed using a three-electrode system with a 2 M KOH aqueous solution as electrolyte, platinum foil (1 × 1 cm²) as counter electrode and an Ag/AgCl electrode as reference electrode. The CV curves of the CF@ITO@Ni₃S₂ electrodes with different lengths of ITO nanowires were present in Fig. S4(a).† And the calculated areal capacitances of the above samples were shown in Fig. S4(b).† The average mass loadings of Ni₃S₂ on different lengths of ITO nanowires were 0.58 mg cm⁻², 0.96 mg cm⁻² and 1.08 mg cm⁻², respectively, when electrodeposited with 2 cycles of Ni₃S₂. As seen in the figure, the areal capacitance increased with the increased length of the ITO nanowires. The reason was that with the increase of the length of nanowires, the surface contact area of the CF@ITO enlarged, which led to more mass loading of Ni₃S₂ nanosheets. Electrochemical characterizations of the feasible nanosheets coated on bare carbon fabrics (CF@Ni₃S₂) and CF@ITO@Ni₃S₂ using L2 electrodeposited with different cycles of Ni₃S₂ nanosheets were further carried out to investigate the influence of mass loading of Ni₃S₂ nanosheets on the electrochemical performances of the samples. The average mass of the ITO nanowires for L2 was 2.90 mg cm⁻². Fig. 5(a) shows the cyclic voltammetry (CV) curves of CF@Ni₃S₂ with different electrodeposition cycles of Ni₃S₂ nanosheets at the scan rate of 10 mV s⁻¹ in the potential range from −0.2 to 0.8 V. Three typical loading masses around 0.18 mg cm⁻², 0.42 mg cm⁻² and 1.07 mg cm⁻² were obtained, corresponding to 1, 3 and 5 electrodeposition cycles, respectively. As shown in Fig. 5(a), all CV curves of the samples showed a pair of redox peaks with an analogous shape, representing typical pseudocapacitive behaviors of the sulfide electrodes in the KOH electrolyte. The redox peaks is probably ascribed to the redox reactions of Ni₃S₂ with the alkaline electrolyte: Ni₃S₂ + 3OH⁻ ↔ Ni₃S₂(OH)₃ + 3e⁻. With the increasing loading of Ni₃S₂ nanosheets on the bare carbon fabrics, the anodic peaks shifted to higher potential while the cathodic peaks shifted to lower potential, which is probably a result of the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reactions. Fig. 5(b) showed the CV curves of ITO nanowire array on carbon fabric without Ni₃S₂ loading (0 mg cm⁻²) and CF@ITO@Ni₃S₂ electrodeposited with different cycles. Five typical mass of around 0.77, 0.96, 1.6, 2.22, 4.12 mg cm⁻² were obtained, corresponding to 1, 2, 3, 4, 5 cycles. From the CV curves we can see that the current through ITO nanowire array on carbon fabric without Ni₃S₂ loading is small and no distinct redox peaks were shown, indicating low capacitive performance of the conductive supports. When loaded with Ni₃S₂, the CV curves of CF@ITO@Ni₃S₂ electrodes show similar shapes to those of CF@Ni₃S₂, but with enhanced electrical current response, indicating the advantages of the ITO nanowires on carbon fabrics for the capacitive performance in supercapacitors. Fig. 5(c) and (d) show the galvanostatic charge–discharge curves of samples at 5 mA cm⁻² with different electrodeposition cycles on bare carbon fabrics and ITO nanowires coated carbon fabrics, respectively. A well-defined plateau during the discharge processes can be observed, suggesting excellent pseudocapacitive behavior of as-prepared samples. As summarized in the figures, the discharge time increased with the increasing loading mass of Ni₃S₂ nanosheets. When the Ni₃S₂ nanosheets masses on bare carbon fabrics and ITO nanowires coated carbon fabrics increased to about 1.07 and 2.22 mg cm⁻², a little voltage decay appeared, probably for that the thickness of Ni₃S₂ nanosheets was too large to transfer charge efficiently.

Fig. 5 CV curves of Ni₃S₂ nanosheets on bare carbon fabrics (a) and ITO nanowires coated carbon fabrics (b) electrodeposited with different cycles at 10 mV s⁻¹. Charge–discharge curves of Ni₃S₂ nanosheets on bare carbon fabrics (c) and ITO nanowires coated carbon fabrics (d) electrodeposited with different cycles at 5 mA cm⁻². (e) Areal capacitances (blue color) and gravimetric capacitances (black color) of CF@ITO@Ni₃S₂ electrodes (marked as points) and the CF@Ni₃S₂ electrodes (marked as stars) as a function of different loading masses of Ni₃S₂ nanosheets at 5 mA cm⁻² when electrodeposited with different cycles. (f) EIS plots of Ni₃S₂ nanosheets on bare carbon fabrics and ITO nanowires coated carbon fabrics electrodeposited with different cycles.

The areal capacitances and gravimetric capacitances of the CF@Ni₃S₂ and CF@ITO@Ni₃S₂ electrodes were summarized in Fig. 5(e) as a function of the Ni₃S₂ mass loading. The detailed performance parameters of the CF@ITO@Ni₃S₂ and CF@Ni₃S₂ electrodes were summarized in Tables S1 and S2.† When directly deposited on bare carbon fabric, the mass loading of Ni₃S₂ for CF@Ni₃S₂ was relative low, which was limited by the low surface area of the carbon fabric. When electrodeposited for 5 cycles, the mass loading of Ni₃S₂ on carbon fabric reached a saturation value of around 1 mg cm⁻². The increase of mass loading was terminated even though the deposition cycles increase further, because the deposited Ni₃S₂ suppressed the electrodeposition process. The mass loading of Ni₃S₂ on ITO nanowire arrays is much larger than that on bare carbon fabric due to the enhanced surface area of the nanowire array. When electrodeposited for 2 cycles on the ITO nanowire arrays, the mass loading of Ni₃S₂ was larger than the saturation mass loading on bare carbon fabric. The mass loading on ITO nanowire arrays increased further when the deposition cycle increased and reached a high mass loading of 4.12 mg cm⁻² when deposited for 5 cycles. One of the results benefited from the enhanced mass loading is the enhancement of the areal capacitance. The largest areal capacitance of CF@Ni₃S₂ electrode is around 0.8 F cm⁻², while the areal capacitance of CF@ITO@Ni₃S₂ electrode can reach 3.85 F cm⁻² when the deposited Ni₃S₂ for 5 cycles. Gravimetric capacitance is another importance parameter of capacitors. For both CF@Ni₃S₂ and CF@ITO@Ni₃S₂ electrodes, the value of gravimetric capacitance were relatively small when deposited Ni₃S₂ for the first cycle, which is because the Ni₃S₂ phase was difficult to form in the first deposition cycle. When electrodeposited for 2 or 3 cycles, the gravimetric capacitance of both electrodes increased significantly. When the electrodeposition cycles increased further, the gravimetric capacitance of both electrodes decreased because a thicker Ni₃S₂ film can bring a higher internal resistance to the electrode. The highest gravimetric capacitance of CF@ITO@Ni₃S₂ electrode based on Ni₃S₂ mass was 1865 F g⁻¹ when the electrodeposition cycle was 2, which was much larger than that of CF@Ni₃S₂ electrode (845 F g⁻¹). The value was also higher than those of other Ni₃S₂ based supercapacitors in the literature such as Ni₃S₂/ZnO electrodes (1529 F g⁻¹),⁵¹ NiCo₂O₄@Ni₃S₂ core–shell electrode (1716 F g⁻¹),⁵² Ni₃S₂/graphene (1022.8 F g⁻¹)⁵³ and so on.

Electrochemical impedance spectroscopy (EIS) was utilized to investigate the charge transfer resistance in the above samples. The measurements were carried out in a three-electrode system with a 2 M KOH aqueous solution as electrolyte, platinum foil (1 × 1 cm²) as counter electrode and an Ag/AgCl electrode as reference electrode. The Nyquist plots were shown in Fig. 5(f) with a fitted equivalent circuit (inset). The semicircles in the high frequency represent R_ct, which are corresponding to the faradic charge transfer resistance. Constant phase elements CPE1 and CPE2 in the equivalent circuit are used to express the double-layer capacitance and pseudocapacitance. The fitting parameters of the equivalent circuit elements are listed in Table S3.† From the fitting parameters we can see that the charge transfer resistance R_ct of CF@ITO@Ni₃S₂ (1.43–2.03 Ω) electrodes were larger than that of CF@Ni₃S₂ electrodes (0.27–0.32 Ω) due to the extra resistance of the ITO nanowires. The capacitance can be reflected in the Q value of CPE in the fitting result. Q₁ of CPE1 and Q₂ of CPE2 represent the double-layer capacitance and pseudocapacitance, respectively. The fitting result showed that the double-layer capacitance of the both CF@ITO@Ni₃S₂ and CF@Ni₃S₂ were two order of magnitude smaller than the pseudocapacitance. The pseudocapacitance of both CF@ITO@Ni₃S₂ and CF@Ni₃S₂ electrodes were increased with the electrodeposition cycles. The pseudocapacitance of CF@ITO@Ni₃S₂ electrode (1.00 F, 5 cycles) is much larger than that of CF@Ni₃S₂ electrode (0.38 F, 5 cycles), which agreed well with the galvanostatic charge–discharge results.

Fig. 6(a) showed the CV curves for the CF@ITO@Ni₃S₂ electrode with 2 cycles of Ni₃S₂ electrodeposition at different scan rates. As seen in the figure, with the scan rates increasing, the shape of the CV curves didn't change significantly, indicating the good electrical conductivity of the prepared ITO nanowires. The anodic peaks shift to higher potential while the cathodic peaks shift to lower potential, which can be attributed to the pretty low ion diffusion rate compared to fast electronic neutralization during the redox reaction. The CV curves for other samples were present in Fig. S5 and S6.† They showed the same trend as the CF@ITO@Ni₃S₂ electrode with 2 cycles of Ni₃S₂. Fig. 6(b) shows the charge–discharge curves at different current densities. The almost symmetric charge and discharge curves indicate good capacitive performance of the prepared sample. The gravimetric capacitances of CF@ITO@Ni₃S₂ and CF@Ni₃S₂ electrodes with different electrodeposition cycles were summarized in Fig. 6(c) as a function of different charge–discharge current densities. The gravimetric capacitance values for all samples were listed in Table S4.† For all CF@Ni₃S₂ electrodes, the capacitance retentions were below 65.5% when the current density was at 50 mA cm⁻², showing relative weak rate performance. However, when we introduced ITO nanowire arrays in the system, the performance distinctly improved. As seen in the figure, all CF@ITO@Ni₃S₂ electrodes showed the maximum gravimetric capacitances at 5 mA cm⁻² when the current density ranged from 2 mA cm⁻² to 50 mA cm⁻². The result was probably for the electrodes activation or better electrolyte diffusion after the first measurements at 2 mA cm⁻².⁵⁴ For the CF@ITO@Ni₃S₂ electrode with 2 cycles of Ni₃S₂ electrodepositions, the maximum gravimetric capacitance could reach 1865 F g⁻¹ at 5 mA cm⁻². When the current density increased to 50 mA cm⁻², the gravimetric capacitance could still remain 1372 F g⁻¹, with the capacitance retention about 79.4%. These values were higher than some reported works, such as Ni–Co sulfide nanowire arrays,³¹ Ni₃S₂@MoS₂ core/shell nanorod arrays⁵⁵ and so on. The improvement in rate performance when introduced ITO nanowires in the systems could be attributed to the excellent electrical conductivity of the one-dimensional ITO nanowires for better electrons transfer and electrolyte diffusion.

Fig. 6 (a) The CV curves of the CF@ITO@Ni₃S₂ electrode electrodeposited with 2 cycles at different scan rates. (b) The galvanostatic charge–discharge curves for the CF@ITO@Ni₃S₂ electrode electrodeposited with 2 cycles at different current densities. (c) The areal capacitances for the CF@ITO@Ni₃S₂ electrodes (marked as points) and the CF@Ni₃S₂ electrodes (marked as stars) at different current densities from 2 mA cm⁻² to 50 mA cm⁻². (d) The cyclic stability test for the CF@ITO@Ni₃S₂ electrode electrodeposited with 2 cycles at 40 mA cm⁻².

Cyclic stability tests of the sample with ITO nanowires were examined using galvanostatic charge discharge cycles with a constant current density of 40 mA cm⁻² over 2000 cycles and the curve was shown in Fig. 6(d). After 2000 charge–discharge cycles under large current density of 40 mA cm⁻², the areal capacitance of the sample remained about 81.44% of its initial value, indicating not bad stability. Furthermore, the sample exhibited nearly 95% coulombic efficiency over the entire stability test. The result was comparable³⁸ to or higher^29,56 than the reported literature about Ni₃S₂ stability. And it can be seen that the introduction of ITO into the electrodes played no negative influence on the stability at work in alkali electrolyte. The EIS curves of the CF@ITO@Ni₃S₂ electrodes before and after cyclic stability were present in Fig. S7.† According to the fit parameters listed in Table S5,† the Q₂ values, which represent the pseudocapacitance performance, decreased after 2000 cycles.

To evaluate the capacitive performance of CF@ITO@Ni₃S₂ electrodes in full cell, flexible all-solid state symmetric supercapacitor device was fabricated composing of two CF@ITO@Ni₃S₂ electrodes with PVA/KOH polymer as electrolyte. The schematic diagram for the full cell was shown in Fig. 7(a). Fig. 7(b) showed the CV curves of the symmetric supercapacitor device under different scan rates. The CV curves showed quasi-rectangle shape without distinct redox peaks. The nearly symmetric charge and discharge curves at different current densities in Fig. 7(c) indicate the full cell good capacitive performance. The mass loading of active material for each electrode was about 4.03 mg cm⁻². The thickness for the whole device was about 0.1 cm. The areal capacitance was corresponding to about 0.74, 0.48, 0.44 and 0.4 F cm⁻² when the current density was 8, 24, 40 and 80 mA cm⁻². The results were substantially larger than those previously reported such as Ni₃S₂@CoS//AC ASC (0.69 F cm⁻²),⁵⁷ Co₃O₄@C@Ni₃S₂//AC ASCs (0.327 F cm⁻² at 20 mA cm⁻²)⁵⁸ and so on. The energy density and power density of the full cell device were calculated according to the following equations:

where C is the areal capacitance of the symmetric cell device, ΔV is the operating voltage of the cell, and Δt is the discharge time. The energy density of the full cell based on the symmetric electrodes can achieve 1.02 mW h cm⁻³ at a power density of 39.9 mW cm⁻³ and still maintain 0.56 mW h cm⁻³ at a high power density of 399.9 mW cm⁻³. This means that in a few seconds, the device can be charged to a relative high volumetric energy density of 0.56 mW h cm⁻³. Ragone plots were shown in Fig. 7(d). The obtained maximum volumetric energy density is considerably higher than some reported supercapacitor devices such as H-TiO₂@MnO₂//H-TiO₂@C-ASCs (0.30 mW h cm⁻³, 0.18 W cm⁻³),⁵⁹ H-MnO₂//RGO-ASC (0.25 mW h cm⁻³, 1.01 W cm⁻³),⁶⁰ MnO₂//Fe₂O₃-ASC (0.55 mW h cm⁻³),⁶¹ ZnO@MnO//graphene-ASCs (0.234 mW h cm⁻³, 0.134 W cm⁻³),⁶² Co₃O₄//graphene-ASC (0.62 mW h cm⁻³, 1.47 W cm⁻³)⁶³ and was a little lower than some devices such as Co₃O₄/RuO₂ SCs (1.44 mW h cm⁻³).⁶⁴ Moreover, the prepared full cell exhibited an excellent rate performance, which was better than some reported devices.^61,63 The results above confirm that the CF@ITO@Ni₃S₂ electrode is very promising as a high-performance electrode for supercapacitor devices.

Fig. 7 (a) The schematic diagram of the full cell. (b) The CV curves of the full cell at different scan rates showing quasi-rectangle shape without distinct redox peaks. (c) The galvanostatic charge–discharge curves for the full cell at different current densities. (d) Ragone plots of the all-solid-state symmetric supercapacitor device with other supercapacitor devices as comparison (e) the cyclic stability test for the full cell at 24 mA cm⁻². (f) Capacitance retention when the device was bent in different curvature radiuses.

As a critical parameter to determine the energy storage performance for practical applications, the long-term cycling stability of our symmetric supercapacitor device was also tested under current density of 24 mA cm⁻² (Fig. 7(e)). The full cell can maintain 86.72% after 2000 charge–discharge cycles, which was better than the half cell stability test in a three-electrode system. The EIS curves of the CF@ITO@Ni₃S₂ electrodes before and after cyclic stability were present in Fig. S8.† According to the fit parameters listed in Table S5,† the Q₂ values for the devices, which represent the pseudocapacitance performance, decreased after 2000 cycles.

The flexible symmetric supercapacitor device was bent under different curvature radius to characterize the retention. Fig. 7(f) shows the capacitance retention as a function of the curvature radius ranging from 7.5 cm to 1.5 cm. As seen in the figure, even though the curvature radius was kept as 1.5 cm, the retention value could keep 90.5%. This performance can be attributed to the good flexibility of the carbon fabrics and good contact between the active materials and conductive substrate.

Conclusions

In summary, CF@ITO@Ni₃S₂ of “core–branch–shell” structure was successfully prepared by electrodepositing Ni₃S₂ nanosheets on the chemical vapor deposited ITO nanowires. The ITO nanowires were uniformly coated on the carbon fabrics to form conductive supports with high surface area. Ni₃S₂ nanosheets were uniformly deposited on the ITO nanowires, which also enhanced the surface area of the active area and was accessible for ion diffusion. The prepared structure exhibited excellent capacitive performance compared with CF@Ni₃S₂. High areal capacitance of 3.85 F cm⁻² of the prepared CF@ITO@Ni₃S₂ electrode can be achieved with the mass of Ni₃S₂ around 4.12 mg cm⁻². When the mass loading of Ni₃S₂ was about 0.96 mg cm⁻², the maximum gravimetric capacitance of 1865 F g⁻¹ was obtained at 5 mA cm⁻². When charge–discharge current density reached 50 mA cm⁻², the sample with 0.96 mg cm⁻² of Ni₃S₂ can also deliver 1372 F g⁻¹, indicating the excellent rate capability of the structure. The assembled full cell based on symmetric electrodes obtained a relative high areal capacitance of 736 mF cm⁻² at 8 mA cm⁻², which delivers a maximum energy density of 1.02 W h cm⁻³ at a power density of 39.9 mW cm⁻³. More importantly, the full cell device maintained about 86.72% of its initial capacitance after 2000 cycles. Such excellent capacitive performance suggested that the CF@ITO@Ni₃S₂ electrode showed a great potential application as an inexpensive energy storage system.

Acknowledgements

This work was supported primarily by the National Natural Science Foundation of China (61377051, 21476104), the National Basic Research Program of China (2013CB632404), the Natural Science Foundation and Key Research Project of Jiangsu Province (BK20130053, BK20141233, BE2015090). Jianguo Liu also thanks to the support of of Jiangsu Province Natural Science Foundation for Distinguished Young Scholars (BK20150009) and Qing Lan Project of Jiangsu Province, China. We would like to thank Mr Yangrunqian Wang for his help in language checking to improve the level of English throughout our manuscript.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14174a

‡ They contribute this work equally.

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