## 3 Results and Discussion
### Electrocatalytic Properties  
The electrocatalytic HER activities of Ni\({}_{3}\)Se\({}_{4}\), MoSe\({}_{2}\), and Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) with different ratios were investigated in the 0.5 M H\({}_{2}\)SO\({}_{4}\) solution in a three-electrode cell. For reference, commercial Pt/C (10 wt%) was also tested for comparison. As shown in Figure 5a, all Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites showed low onset overpotentials at a cathode current density of 1 mA/cm\({}^{2}\) (\(\eta_{1}\)). Specifically, Sample 1 showed the lowest onset overpotential of \(\sim\)145 mV, while the overpotential for Samples 2 and 3 were \(\sim\)160 and \(\sim\)214 mV, respectively. Further, increasing the negative potential gave rise to a rapid increase in cathode current density. The overpotentials at the cathode current density of 10 mA/cm\({}^{2}\) (\(\eta_{10}\)), which are usually regarded as indicators of HER performance [49], were 206, 242, and 310 mV for Samples 1, 2, and 3, respectively. All of them showed a decrease in \(\eta_{1}\) and \(\eta_{10}\) compared with pure MoSe\({}_{2}\) nanosheets. The Tafel plots of catalyst samples and Pt/C are displayed in Figure 5b. The linear regions of Tafel plots derived from the polarization curve can be analyzed using the Tafel equation:  
Figure 4: (a) FE-SEM image of Ni\({}_{3}\)Se\({}_{4}\), (b) HRTEM image of Ni\({}_{3}\)Se\({}_{4}\), (c) FE-SEM image of Ni\({}_{3}\)Se\({}_{4}\)@MoSe\({}_{2}\), and (d) HRTEM image of Ni\({}_{3}\)Se\({}_{4}\)@MoSe\({}_{2}\). \(\eta\) = b \(\log\) j + a, where \(\eta\) is overpotential, b is Tafel slope, and j is current density [50; 51]. Compared to pristine MoSe\({}_{2}\) nanosheets (Tafel slope of 80 mV/dec), Sample 1 showed the lowest Tafel slope (65 mV/dec), while a slightly higher Tafel slope value was observed for Sample 2 (76 mV/dec) and Sample 3 (96 mV/dec).The HER performance of pure Ni\({}_{3}\)Se\({}_{4}\) nanoparticles was also investigated for comparison. They exhibited inferior performance towards HER. These results indicate that HER catalytic activity originates from MoSe\({}_{2}\) rather than from the inactive Ni\({}_{3}\)Se\({}_{4}\). This observation is also confirmed by EIS, which was performed to investigate the impedance properties and the electron transfer kinetics during the HER [51]. The charge transfer resistance (R\({}_{\text{ct}}\)) obtained from the impedance spectra (Figure 5c) showed that Ni\({}_{3}\)Se\({}_{4}\) nanoparticles were conductive and had lower R\({}_{\text{ct}}\) than pure MoSe\({}_{2}\) nanosheets, although they were not good for HER. We attribute the enhanced HER activity of MoSe\({}_{2}\) to the incorporation of Ni\({}_{3}\)Se\({}_{4}\) nanoparticles, which have large surface areas and can improve the conductivity of MoSe\({}_{2}\) nanosheets and hence promote electron transfer between MoSe\({}_{2}\) and the electrolyte. The \(\eta_{1}\), \(\eta_{10}\), Tafel slope, and R\({}_{\text{ct}}\) values of the three samples, along with those of Ni\({}_{3}\)Se\({}_{4}\), MoSe\({}_{2}\) are summarized in Table 1.  
The stability of the Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites was evaluated by cyclic voltammetry tests from \(-0.4\) to \(0.2\) V vs. RHE at 50 mV/s for 1000 cycles. The polarization curves of the Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites showed negligible activity change after 1000 cycles, indicating their durability towards HER. Notably, \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\eta_{1}\) (mV) & \(\eta_{10}\) (mV) & Tafel Slope (mV/dec) & \(R_{\text{ct}}\) (\(\Omega\) cm\({}^{2}\)) \\ \hline Ni\({}_{3}\)Se\({}_{4}\) & 240 & 330 & 109 & 108 \\ Ni\({}_{3}\)Se\({}_{4}\)–MoSe\({}_{2}\) (1:2) & 145 & 206 & 65 & 68 \\ Ni\({}_{3}\)Se\({}_{4}\)–MoSe\({}_{2}\) (1:1) & 160 & 242 & 76 & 98 \\ Ni\({}_{3}\)Se\({}_{4}\)–MoSe\({}_{2}\) (2:1) & 214 & 310 & 96 & 156 \\ MoSe\({}_{2}\) & 210 & 295 & 80 & 134 \\ \hline \hline \end{tabular}
\end{table}