## 4 Results and Discussion
In an alkaline medium, because H\({}^{+}\) is not available from the electrolyte, the HER initiates from the dissociation of water (\(H_{2}O\)) molecules to give protons (i.e., \(H^{+}\)), which follows both the Volmer step (4) and the Heyrovsky step (5) of alkaline HER. Meanwhile, the Tafel step is similar to that in the acidic medium. The overall HER process in an alkaline medium is depicted in Equation (6). \[H_{2}O+e^{-}\to H^{ads}+OH^{-} \tag{4}\]  
\[H^{ads}+H_{2}O+e^{-}\to OH^{-}+H_{2} \tag{5}\]  
Figure 3.0  
Moreover, Tafel slope analysis was employed to assess the HER performance of nanocatalysts, which provides key information about the reaction pathways of HER. To further get insight into the HER mechanism, the Tafel plots (linear portion only) were simulated by using the following Tafel equation:  
Figure 3.1  
where \(b\): the Tafel slope, \(j\): the current density, \(j_{0}\): the exchange current density. A smaller Tafel slope indicates a faster hydrogen generation rate (i.e., increased HER kinetics). The Tafel slopes of experimental NCs are plotted in Figure 4b,d for alkaline and acidic mediums, respectively, and summarized in Figure 3.2 Accordingly, Ni@Pd-Pt NC exhibits the lowest Tafel slope of 42 mV dec\({}^{-1}\) in an alkaline medium, which is exactly the same as that of standard Pt/C. Whereas in an acidic medium Ni@Pd-Pt NC achieves a competitive low Tafel slope of 47 mV dec\({}^{-1}\), which is very close to standard Pt/C catalyst (32 mV dec\({}^{-1}\)). Generally, if the Tafel slope is between 40 and 120 mV dec\({}^{-1}\), then the Heyrovsky reaction is the rate-determinative step (RDS). In this case, the electrochemical desorption step severely hampers the overall reaction process [32]. Accordingly, in the present study, HER followed the Heyrovsky reaction. Furthermore, the stability test results of Ni@Pd-Pt NC in both alkaline and acidic mediums have been added in Figure S3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**0.1 M KOH**} & \multicolumn{2}{c}{**0.5 M H\({}_{2}\)SO\({}_{4}\)**} \\ \hline \multirow{2}{*}{**Samples**} & **Overpotential at** & **Tafel Slope** & **Overpotential at** & **Tafel Slope** \\  & **10 mA cm\({}^{-2}\)(mV)** & **(mV decade\({}^{-1}\))** & **10 mA cm\({}^{-2}\)(mV)** & **(mV decade\({}^{-1}\))** \\ \hline Pd/C & 227 & 196 & 251 & 199 \\ Pt/C & 26 & 42 & 37 & 32 \\ Ni@Pd & 173 & 156 & 158 & 215 \\ Ni@Pd-Pt & 11.5 & 42 & 37 & 47 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The hydrogen evolution reaction (HER) parameters of experimental NCs under investigation in alkaline and acidic medium. Figure 4: **(a)** The linear sweep voltammetry (LSV) and related **(b)** Tafel plots of experimental NCs under investigation in alkaline medium (0.1 M KOH). **(c)** LSV curves and related **(d)** Tafel plots of experimental NCs under investigation in acidic medium (0.5 M H\({}_{2}\)SO\({}_{4}\)). All the LSV curves were measured in 0.1 M KOH electrolyte solution at a scan rate of 2 mV s\({}^{-1}\). Cyclic voltammetry (CV) technique (depicted in Figure S4) provides further structure-performance-correlation. The CV curves were obtained in an alkaline (0.1 M KOH) electrolyte (N\({}_{2}\)-saturated) with 2 mV/s sweeping rate at room temperature. As depicted in Figure S3, Pd/C and Ni@Pd NCs exhibit a smeared peak profile at lower potentials (\(<\)0.4 V vs RHE. ), which is attributed to the diffusion of H\({}^{+}\) ions in the Pd crystal, resulting in a higher overpotential compared to Ni@Pd-Pt NC, hence, comparatively suppressed HER activity. In the case of Ni@Pd-Pt NC, sharp and narrow current responses for H\({}^{+}\) desorption and adsorption (i.e., peaks H/H\({}^{+}\)) are observed, which confirms the presence of sub-nanometer scaled Pt-clusters on the Ni@Pd surface, resulting in low resistance and enhanced Faradaic process. This leads to improved HER kinetics compared to Pd/C and Ni@Pd NCs.