## 3 Results and Discussion
### Electrochemical Properties and Characterization of NiCo\({}_{2}\)S\({}_{4}\)/rGO Composites
This meant that some oxygen-containing functional groups still existed on the graphene surface [42,46], which might help anchor the NiCo\({}_{2}\)S\({}_{4}\) species on graphene more tightly and strengthen the structure of the hybrid electrodes. Figure 8 presents TEM and SEM images of the NiCo\({}_{2}\)S\({}_{4}\)/rGO composites. It shows that the composites had a much rougher lamellar structure as compared with the smooth wrinkles of rGO (Figure 5d) due to the anchoring of the bimetallic sulfides on the rGO surface. The NiCo\({}_{2}\)S\({}_{4}\) anchoring on rGO appeared uniform, which would take full advantage of the excellent conductivity and large specific surface area of rGO. In addition, the tight anchoring of NiCo\({}_{2}\)S\({}_{4}\) composites on rGO built a barrier-free high-speed transmission channel, which would allow the charge to diffuse quickly and effectively and improve the overall conductivity. From the low-magnification SEM image (Figure 8b), the overall morphology and size distribution of the particles are clearly demonstrated. The particles in the high-magnification SEM image (Figure 8c) exhibited characteristic graphene wrinkles with a grainy surface as a consequence of uniform attachment of NiCo\({}_{2}\)S\({}_{4}\) nanoparticles. Such attachment could act as a spacer between graphene layers [41], which significantly reduces the agglomeration behavior of rGO. Therefore, NiCo\({}_{2}\)S\({}_{4}\) and rGO complemented each other and significantly enhanced the superelectric properties of the electrode materials. Similar three-electrode systems of the NiCo\({}_{2}\)S\({}_{4}\) composites were built to test the electrochemical properties of NiCo\({}_{2}\)S\({}_{4}\)/rGO, including cyclic voltammetry (CV, Figure 9a), galvanostatic charge and discharge (GCD, Figure 9b), electrochemical impedance (EIS, Figure 9c), and cycling performance (Figure 9d). Using 6 M KOH aqueous solution as the electrolyte, CV curves were obtained at the scan rate range 2 to 40 mV\({}^{-1}\). The CV curves were similar to that of the NiCo\({}_{2}\)S\({}_{4}\) composites, illustrating remarkable Faraday redox peaks caused by the interactions between Co\({}^{2+}\)/Co\({}^{3+}\) or Ni\({}^{2+}\)/Ni\({}^{3+}\) cations and the OH\({}^{-}\) anions from the KOH electrolyte. The CV curve of the material at a scan rate of 5 mV s\({}^{-1}\) was remarkably symmetrical and the potential difference of the redox peaks was only 209 mV, which was much smaller than that of NiCo\({}_{2}\)S\({}_{4}\) (259 mV), indicating that the electrochemical reversibility was greatly improved. In addition, as the scan rate increased, a shift in the redox peak could be observed, which was due to the increase in the internal diffusion resistance of the electrode [47]. In order to further explore the potential of NiCo\({}_{2}\)S\({}_{4}\)/rGO composites as electrode materials, the constant current charge-discharge performance at different current densities (1 to 20 A g\({}^{-1}\)) was tested in 6 M KOH electrolyte with the voltage window being set to 0 to 0.5 V (vs. Hg/HgO). Figure 9b illustrates that the initial specific capacity values of the NiCo\({}_{2}\)S\({}_{4}\)/rGO composite were 198.0, 194.1, 188.3, 176.0, 171.4, and 161.1 mAh g\({}^{-1}\), respectively, under different current densities of 1, 2, 5, 7, 10, and 20 A g\({}^{-1}\). Based on the initial specific capacity at the fixed current density of 1 A g\({}^{-1}\), which corresponded to the best performance, the retention rates at another five current densities of 2, 5, 7, 10, and 20 A g\({}^{-1}\) were 98.0%, 95.1%, 88.8%, 86.5%, and 81.4%, respectively. This shows that an excellent rate performance was achieved by the NiCo\({}_{2}\)S\({}_{4}\)/rGO composite material to  
Figure 7: (a) XPS spectrum; (b) Ni 2p XPS; (c) Co 2p XPS; (d) S 2p XPS; (e) C 1s XPS of NiCo\({}_{2}\)S\({}_{4}\)/rGO composites.