## 3 Results
Furthermore, the magnified TEM images in Figure 4c-e show that the Ni-Co sulfide nanowires are hollow internally with a sheath thickness of \(\sim\)15 nm (Figure 4d). The nanotube and rough surface are very both effective in increasing the electroactive sites. The lattice fringes of HTEM image in Figure 4f and g can be assigned to the (220) and (220) crystal plane of Co\({}_{3}\)S\({}_{4}\), (201), and (202) plane of Ni\({}_{3}\)S\({}_{2}\), respectively. The sample contains the elements of Co, Ni, and S evidenced by EDS pattern in Figure 4h, demonstrating that it is completely in sulfuration. To demonstrate the electrochemical performance and properties of Ni-Co sulfide nanosheet-nanotube arrays, the comparative experiment was carried out between the NiCo\({}_{2}\)O\({}_{4}\) electrode and the Ni-Co sulfide electrode are shown and plotted in Figure 5. Firstly, a comparison of the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves at 1 mV s\({}^{-1}\) and 1 mA cm\({}^{-2}\) of NiCo\({}_{2}\)O\({}_{4}\) and the corresponding Ni-Co sulfide nickel foam are shown in Figure 5a,b. Conspicuously, the integral area of the CV curve and the discharge time of Ni-Co sulfide is larger than Ni-Co oxides, implying excellent electrochemical energy storage capacity of the synthesized Ni-Co sulfide nanotube. Figure 5c shows the CV curves of Ni-Co sulfide at different scan rates from 1 to 10 mV s\({}^{-1}\) ranging in a potential window between \"0 to 0.6 V\". The pair redox peaks can clearly be observed and visible in all the CV curves leading to different pseudo-capacitive characteristics, during the electrochemical performance test and are directly related to the Faradaic redox reactions to M-O/M-O-OH [29], where M = Ni or Co. In Figure 5c, as the scan rate increases, the redox current increases; in the result, the corresponding anodic/cathodic peaks shifted towards higher and lower potential specifically at 8 mV s\({}^{-1}\), and at 10 mV s\({}^{-1}\), respectively with the large potential separation. This phenomenon is caused by the kinetic irreversibility in the redox process, due to Ohmic resistance and polarization behavior of the electrode [30]. The galvanostatic charge discharge curves of Ni-Co sulfide electrode with. Hg/HgO reference electrode in the potential window ranging between 0 to 0.5 V at different selected current densities changing from 1 to 8 mA cm\({}^{-2}\), as shown in Figure 5d. It can clearly observe that distinct plateau regions of the Ni-Co sulfide galvanostatic charge discharge are matching with the redox peaks in CV curves, indicates high pseudocapacitive characteristics of the Ni-Co sulfide. The cycle stability of supercapacitors for long-term is another critical issue to further evaluate the application of different structure electrodes. The cycling tests for the four synthesized electrodes were carried out by the repeated charging-discharging measurement at current density of 1 mA cm\({}^{-2}\) over 3000 cycles. The long cycle performance and stability at a current density of 1 mA cm\({}^{-2}\) are shown in Figure 5e, the Ni-Co sulfide nanotube achieved their peak (2.31 F cm\({}^{-2}\)) after 30 cycles and then became quite stable with 95.2% of the retained specific capacitance after 3000 cycles, while, the counter-part NiCo\({}_{2}\)O\({}_{4}\) nanowire array exhibited activation induced peak (2.05 F cm\({}^{-2}\)), which is attributed the enhanced conductivity and surface sites [31], then, followed by an obvious capacity decline to 1.91 F cm\({}^{-2}\) after 3000 cycles. We can see that the cycling performance of Ni-Co oxide supercapacitor increases at the beginning and then decreases during the charging/discharging measurements. This is because the electrode cannot be completely wetted at the pristine cycles and has to go through a slow activation process for the electrode [32]. Furthermore, compared with previous work, as shown in Table S1, we clearly see that the Ni-Co sulfide nanosheet-nanotubes is a kind of excellent electrode material for supercapacitors.