## 3 Results and Discussion
### Electrochemical Properties and Characterization of NiCo\({}_{2}\)S\({}_{4}\)/rGO Composites
At different current densities of 1, 2, 5, 7, and 10 A g\({}^{-1}\), the initial specific capacitances of the HSC device were obtained as 121.2, 107.8, 92.0, 86.6, and 77.5 F g\({}^{-1}\), respectively. Moreover, the HSC device demonstrated a fine cycling performance, with a capacitance retention rate of 83.8% after 1000 cycles at a current density of 1 A g\({}^{-1}\) (Figure 10e). The coulombic efficiency of this hybrid device is also shown in Figure 10e, which has remained above 98.6% during the entire cycle test process. Figure 10f shows the corresponding IR drop of the device at different current densities. It had the smallest IR drop of 0.092 V at a current density of 1 A g\({}^{-1}\), and the IR drops at 2, 5, and 7A g\({}^{-1}\) were 0.119 V, 0.126 V, and 0.180 V, respectively, demonstrating a small increase of the IR drop within such a current density range. When the current density reached 10 A g\({}^{-1}\), the IR drop increased to 0.658 V sharply. Therefore, the HSC device had a good electrochemical performance at low current densities. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **Methods** & **Capacity** & **Capacity Retention Rate** \\ \hline NiCo\({}_{2}\)S\({}_{4}\)/rGO & & 198 mAh g\({}^{-1}\) & 83.6\% \\ (This work) & MISR & (1 A g\({}^{-1}\)) & (1 A g\({}^{-1}\), 1000 cycles) \\ Tube-like NiCo\({}_{2}\)S\({}_{4}\)[49] & Solvothermal method & 116.4 mAh g\({}^{-1}\) & 75.9\% \\  & & (3 A g\({}^{-1}\)) & (10 A g\({}^{-1}\), 5000 cycles) \\ Ni\({}_{1.5}\)Co\({}_{1.5}\)S\({}_{4}\)[12] & Polyol method & 167.0 mAh g\({}^{-1}\) & 108\% \\  & & (1 A g\({}^{-1}\)) & (10 A g\({}^{-1}\), 2000 cycles) \\ NiCo\({}_{2}\)S\({}_{4}\) arrays on nickel foam [50] & Hydrothermal method & 102.2 mAh g\({}^{-1}\) & 85\% \\  & & (0.5 A g\({}^{-1}\)) & (5 A g\({}^{-1}\), 5000 cycles) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrochemical comparison of NiCo-based sulfides synthesized by different methods. Figure 9: _Cont._  
There are two other important electrochemical properties of electrode materials that have often been ignored by researchers, i.e., the self-discharge and leakage current of hybrid devices. In this work, the device was firstly charged to 1.5 V under a very small current density, and then kept at this voltage for 1 h. The current change over such time was recorded to obtain the leakage current curve, as shown in Figure 11a. It can be observed that the leakage current dropped rapidly from the initial 1.9 mA to 0.33 mA within the first 10 min. After that, the leakage current decreased slowly and was finally stabilized at 0.25 mA. For water-based hybrid supercapacitors studied in the past, a leakage current of 0.0255 mA per mg was regarded as an extremely low value, indicating that our device had excellent cycle stability. In addition, in order to further evaluate the charge retention performance of the hybrid device, a self-discharge test was performed by charging the device to 1.5 V with the same method used in the leakage current test, and then removing the power supply and letting it stand for 30 min, as shown in Figure 11b. Unlike the leakage current, self-discharge maintained a mild trend throughout the test. The initial voltage was 1.460 V and remained at 1.216 V after 1 h with a potential difference of 0.244 V, which indicates that this device has an excellent self-discharge performance and hence will have an appealing prospect in the field of energy storage. Figure 10: _Cont._  
Figure 9: (a) CV curves at various scan rates, (**b**) galvanostatic charge and discharge diagram at different current densities, (**c**) EIS spectra, and (**d**) cycling performance of the NiCo\({}_{2}\)S\({}_{4}\)/rGO composites; SEM images of NiCo\({}_{2}\)S\({}_{4}\)/rGO composites (**e**) before and (**f**) after 1000 charge-discharge cycles.