Enhanced performance of mesoporous NiCo
 2
 S
 4
 nanosheets fibre‐shaped electrode for supercapacitor

Among various power storage devices, supercapacitors have attracted much attention in the past few years due to their high power density, long-life cycle and short charge-discharge times [1, 2]. Supercapacitors can be classified into Faradaic redox reaction pseudocapacitors (FPCs), electrochemical double-layer capacitors (EDLCs) and hybrid supercapacitors according to their energy store mechanisms [3, 4]. So far, pseudocapacitors have been developed deeply since, FPCs have higher energy density than EDLCs. Electrode materials have vital influence on the electrochemical performance of supercapacitors. NiCo2S4 with many shapes, such as sphere [5], nanoparticle [6], nanowhisker [7] and nanoflake [8], has been widely reported owing to its high electrical conductivity, thermal stability and super electrochemical performance [9]. Sulphur is abundant and less electronegative than oxygen, so transition metal sulphide has more flexible structure and higher electrochemical activity than transition metal oxide or hydroxide. Much work indicated that NiCo2S4 was prepared with two steps: in the first step, Ni– Co precursor was fabricated with solvothermal route and then the precursor was sulphured to obtain NiCo2S4 [10]. NiCo2S4 can inherit the structure of Ni-Co precursor to a large extent. As we know, electrodeposition method can prepare Ni–Co LDH with 3D nanostructure easily. However, Ni–Co LDH prepared using this technique has not been reported to fabricate NiCo2S4 electrode material for supercapacitor. In this paper, hierarchical Ni–Co LDH precursor on nickel wire was fabricated by a simple electrodeposition process, and an ordinary sulphuration method was employed to prepare NiCo2S4 fibre-shaped electrode. NiCo2S4 electrode showed far higher electrochemical performance than Ni–Co LDH.

and 20 mmol NH4Cl were mixed with 100 mL deionized water and stirred for 15 min with a magnetic stirrer. The Ni-Co LDH was deposited on the surface of nickel wire in an electrochemical work-station for 15 min at the steady current of 0.02 A. Finally, the obtained products were washed for several times with deionized water and ethanol, and dried at 70 • C for 12 h.
2.4 mmol Na 2 S⋅9H 2 O was dissolved in 50 mL deionized water and stirred for 30 min. Next, 30 mL of the solution and the nickel wire coated with Ni-Co LDH were transferred into a Teflon-lined autoclave of 50 mL for sulphuration reaction at 100 • C for 10 h. After that, the nickel wire was taken out and washed with deionized water and ethanol, finally dried at 60 • C for 12 h. During sulphuration process, Ni-Co LDH was transformed to NiCo 2 S 4 . The whole fabrication of NiCo 2 S 4 fibreshaped electrode was shown in Figure 1.
The X-ray diffraction (XRD) analysis with Cu Kα-radiation, high-Resolution transmission electron microscope (HREM, Tecnai G 2 F20), scanning electron microscope (SEM, Hitachi SU-8200) were used to characterize the structure, phase and morphology of the products. X-ray photoelectron spectroscopy (XPS) was carried out to investigate the elemental compositions.
The electrochemical studies of Ni-Co LDH and NiCo 2 S 4 electrodes were tested in a three electrode system in 1 M KOH aqueous solution, in which Ni-Co LDH and NiCo 2 S 4 electrodes were used as working electrode, platinum plate and standard calomel electrode (SCE) as counter electrode and reference electrode, respectively. Cyclic voltammogram (CV) and galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were done in CHI660E electrochemical station at room temperature. The cycle life of the electrodes was measured with repeated GCD test.

RESULTS AND DISCUSSION
The crystalline structure of Ni-Co LDH and NiCo 2 S 4 was examined by XRD analysis, as shown in Figure A1.  Figure A1(a) [11]. The three strong peaks at 44.  440) and (553) crystal planes of the cubic NiCo 2 S 4 phase as shown in Figure A1(b). It is worth noting that after sulphuration, the peaks of nickel wire became weak obviously and the two peaks at 51.6 • and 76.1 • cannot be detected almost, meaning that nickel wire was involved in this sulfurization reaction. To further investigate the composition and chemical element of NiCo 2 S 4 , XPS spectra are shown in Figure 2. Figure 2(a-d) shows the XPS spectra of NiCo 2 S 4 . The elements C, S, Ni and Co were detected in the full-scan spectrum of the sample (Figure 2(a)). As shown in Figure 2(b,c), the Ni 2p and Co 2p spectra can be fitted with two spin-orbit doublets and two shake-up satellites. In Ni 2p spectrum (Figure 2(b)), the binding energy at 863 eV in Ni 2p3/2 and 883.6 eV in Ni 2p1/2 agreed with the spin-orbit of Ni 2+ , and the binding energy at 956.12 eV in Ni 2p3/2 and 873.97 eV in Ni 2p1/2 were the characteristic of Ni 3+ . Figure 2(c) indicates that Co 2p3/2 and Co 2p1/2 peaks located at the binding energies of 781.13 and 796.86 eV were accompanied by shake-up satellites, implying that the cobalt in NiCo 2 S 4 are Co 2+ and Co 3+ states. As for  Figure 2(d), the peaks at 161.79 and 168.46 eV corresponded to the S 2p3/2 and S 2p1/2, respectively, representing the common metal-sulphur bond.
The morphology Ni-Co LDH nanosheets on Ni wire was investigated by SEM (Figure 3(a-c)). Ni-Co LDH was deposited on Ni wire densely and uniformly (Figure 3(a)). From the inset of Figure 3(a), the thickness of Ni-Co LDH can be estimated to be about 11 μm. Ni-Co LDH exhibited nanosheetlike shape with thickness of about 5 nm (Figure 3(b,c)). These nanosheets were interconnected with each other, forming a 3D network structure with many micropores. The structure of Ni-Co LDH was further investigated by TEM ( Figure A2). The sample shows plate-like morphology ( Figure A2(a)), consistent with the SEM result. The lattice fringes with interplanar spacing of 0.27 nm can be identified ( Figure A2(b)), corresponding to the (100) plane of Ni-Co LDH. In the selected area electron diffraction (SAED) pattern of Ni-Co LDH, two standard concentric circles corresponding to (006) and (108) planes can be clearly seen.
The structural information of NiCo 2 S 4 was shown in Figure 4. NiCo 2 S 4 displays nanosheet-like shape with 3D network structure, which is extremely similar to that of Ni-Co LDH. This suggests NiCo 2 S 4 inherits the structure of Ni-Co LDH.
Numerous fine nanoparticles attached on the nanosheets can be seen clearly in Figure 4(b), which should be the product of sulphuration reaction. Plate-like shape of NiCo 2 S 4 can be also observed in TEM image (Figure 4(c)). In addition, it's found there are many nanopores in the nanosheets, which is different from that of Ni-Co LDH with a relatively smooth surface ( Figure A2(a)). NiCo 2 S 4 exhibited clear lattice fringes with interplanar spacing of 0.32 nm in Figure 4d, assigned to the (220) crystal plane of NiCo 2 S 4 . SAED pattern of NiCo 2 S 4 also confirms its polycrystalline structure. Figure 5a shows the typical CV curves of Ni-Co LDH fibreshaped electrode at the scam rate of 10-80 mV/s in the potential window of 0-0.8 V. Ni-Co LDH shows two distinct pairs redox peaks, revealing that the electrochemical performance characteristics mainly result from Faradaic pseudocapacitance, related to Co 2+ /Co 3+ /Co 4+ and Ni 2+ /Ni 3+ redox process [12]. Clear discharge platforms can be observed in GCD curves, in accordance with the reduction peaks ( Figure A3(a)).
For NiCo 2 S 4 fibre-shaped electrode, its redox peaks are unconspicuous in CV curves ( Figure 5(b)), which exhibit near rectangular appearance. This indicates double-layer capacitance makes larger contribution on its performance. GCD curves of NiCo 2 S 4 electrode with no discharge platform also confirm the dominant charge-discharge mechanism is different from that of Ni-Co LDH precursor ( Figure 5(c)). The specific capacitances of Ni-Co LDH and NiCo 2 S 4 fibre-shaped electrodes were calculated based on their GCD curves, as shown in Figure A3(b). NiCo 2 S 4 electrode exhibits specific capacitance of 1974.55, 1800 and 1463.64 mF/cm 2 at current densities of 2, 3, 5 mA/cm 2 , respectively, which is nearly three times than that (630.77, 567.69 and 538.46 mF/cm 2 ) of Ni-Co LDH ( Figure A3(b)). At a high current density of 10 mA/cm 2 , the specific capacitance of NiCo 2 S 4 can reach 1127.27 mF/cm 2 , while for LDH, the value is only 523.08 mF/cm 2 . After 5000 cycles at 10 mA/cm 2 , it's observed that the specific capacitance of Ni-Co LDH electrode decreases rapidly and only 65.3% FIGURE 5 (a) CV curves of Ni-Co LDH at various scan rates from 10 to 80 mV/s, (b) CV curves of NiCo 2 S 4 with the scan rates of 5-100 mV/s, (c) GCD curves of NiCo 2 S 4 , (d) cycling stability of the two electrodes at 10 mA/cm 2 capacitance can be retained, while for NiCo 2 S 4 electrode, capacitance retention can reach 86.2% ( Figure 5(d)). Evidently, sulphuration treatment generates a positive effect on Ni-Co LDH precursor prepared by electrodeposition technique. This is due to that sulphur-containing functional group in NiCo 2 S 4 has higher pseudocapacitance performance and better structural stability compared with hydroxyl group in Ni-Co LDH. Our work may provide new idea for the performance improvement of LDH used as supercapacitor electrode material.

CONCLUSION
In summary, mesoporous NiCo 2 S 4 nanosheet arrays were successfully prepared through sulfurizing 3D hierarchical Ni-Co LDH, which was fabricated on nickel wire by electrodeposition technique. NiCo 2 S 4 fibre-shaped electrode showed a superior electrochemical performance compared to Ni-Co LDH precursor. The specific capacitance can reach 1974.55 mF/cm 2 at 2 mA/cm 2 for NiCo 2 S 4 electrode (630.77 mF/cm 2 for Ni-Co LDH) and after 5000 cycles, 86.2% capacitance retention can be obtained (only 65.3% for Ni-Co LDH).