## 3 Results and Discussion
The testing results illustrate that the NiCo\({}_{2}\)S\({}_{4}\) electrode had a relatively lower bulk resistance and smaller charge transfer resistance as compared with the Co\({}_{3}\)S\({}_{4}\) and NiS electrodes, demonstrating that the doping of Co into NiS can reduce the intrinsic resistance of the NiCo\({}_{2}\)S\({}_{4}\) electrode and accelerate the electron transport capability. In the low-frequency region, the slope of the curve was related to the Warburg impedance, which was caused by ion diffusion [39]. The curve slope of the NiCo\({}_{2}\)S\({}_{4}\) electrode was close  
Figure 2: (**a**) XRD analyses; (**b**) cycling stability under a current density of 1 A g\({}^{-1}\); (**c**) CV measurements at a scan rate of 5 mV s\({}^{-1}\); (**d**) Nyquist spectra (EIS) of the NiS, Co\({}_{3}\)S\({}_{4}\), and NiCo\({}_{2}\)S\({}_{4}\) composites prepared in MISR. to 90\({}^{\circ}\), which indicates that the ion diffusion in the electrolyte was much faster than that of NiS and Co\({}_{3}\)S\({}_{4}\). To explore the effects of micromixing efficiency of the reactor on the co-precipitation process, NiCo\({}_{2}\)S\({}_{4}\) composites were prepared using MISR and STR. By SEM observation of the materials in Figure 3a,b, it was found that both composites had an irregular shape. However, the NiCo\({}_{2}\)S\({}_{4}\) composite prepared by STR was more agglomerated than the sample prepared by MISR, with a larger particle size and uneven distribution. The reason for this phenomenon was that the micromixing performance of MISR was better than that of traditional STR, which was similar to other composite materials prepared by MISR and STR as previously reported [40]. Compared with STR, the porous structure of the composite material obtained by MISR can lead to a larger surface area, which is a great advantage for electrode materials to achieve excellent electrochemical performance. In order to investigate the distribution of the Ni, Co, and S elements in the NiCo\({}_{2}\)S\({}_{4}\) particles, EDS mapping analysis was performed as illustrated in Figure 3c, showing uniform distribution of all three elements and no other impurity elements observed, which suggests that the products were of high purity. Porosity and specific surface area are important textural properties that affect the electrochemical performance of materials, and can be obtained by N\({}_{2}\) adsorption and desorption isotherms. The N\({}_{2}\) adsorption and desorption curves of the MISR-prepared NiCo\({}_{2}\)S\({}_{4}\) shown in Figure 3d displayed typical IV adsorption behavior. The obvious hysteresis loop with relative pressure (\(P/P_{0}\)) in the range 0.6-1.0 indicates that the material had an ordered mesoporous structure. The pore size distribution (PSD) deduced by the separate desorption curve shows that the NiCo\({}_{2}\)S\({}_{4}\) composite had a wide size distribution between 10 and 100 nm. The sharp peak around 20 nm might be caused by the interspace between NiCo\({}_{2}\)S\({}_{4}\) particles. The ordered mesoporous structure of NiCo\({}_{2}\)S\({}_{4}\) resulted in a BET surface area (\(S_{\rm BET}\)) of 55.3 m\({}^{2}\) g\({}^{-1}\), which was much larger than the \(S_{\rm BET}\) values of NiS and Co\({}_{3}\)S\({}_{4}\) of 41.5 and 32.7 m\({}^{2}\) g\({}^{-1}\), respectively. Previous studies reported that a Faraday redox reaction only occurred on a thin surface of the active electrode materials [40]. Increasing  
Figure 3: SEM images of **(a)** the MISR-prepared and **(b)** the STR-prepared NiCo\({}_{2}\)S\({}_{4}\). (c) Element distribution (EDS) of the MISR-prepared NiCo\({}_{2}\)S\({}_{4}\); (d) N\({}_{2}\) adsorption/desorption isotherms and PSDs (the insertion) of the MISR-prepared NiCo\({}_{2}\)S\({}_{4}\). BET surface area could enhance the effective contact between electrolyte ions and active materials, thereby promoting the electrochemical performance of the prepared NiCo\({}_{2}\)S\({}_{4}\) composite materials. The Nyquist plots of the NiCo\({}_{2}\)S\({}_{4}\) prepared by MISR and STR are presented in Figure 4a.