## 4 Results and Discussion
and deconvoluted to analyze the emission signals from various oxidation states of \"Pd\"; the obtained parameters are summarized in Table 1. Accordingly, the higher ratio of Pd (0) comparative to Ni@Pd confirms that Pt-clusters protect the Pd-atoms from surface oxidation in Ni@Pd-Pt NC. Moreover, the intensity of signals (i.e., emission peaks) in XPS spectrum is directly proportional to the target atom's electron density. Hence, the higher intensity for Ni@Pd-Pt NC indicates its higher index of 3d electrons in Pd atoms, which proves electron relocation from Ni-to-Pd atoms and further to Pt-atoms. Besides, the reduced binding energy (BE) of Ni@Pd-Pt NC also confirms the electronegativity gap and lattice strain drove electron relocation from Ni atoms to Pd atoms. XPS spectra of Ni@Pd-Pt NC at the Pt-4f orbital and Ni@Pd NC at the Ni-2p orbital have been depicted in Figures S1 and S2, respectively. It is a well-known fact that the electronic structure of the catalysts has a major effect on their electrochemical HER performance. Accordingly, superior HER performance is expected from Ni@Pd-Pt NC as compared to Ni@Pd. The HER activities of Ni@Pd-Pt and control samples were investigated by using a linear sweep voltammetry (LSV) technique (Figure 4a,c). The LSV curves were obtained at a rotation rate of 1600 rpm, 2 mV s\({}^{-1}\) scan rate and an applied potential window of \(-0.20\) and 0.20 V at room temperature. The N\({}_{2}\) saturated 0.1 M KOH (for alkaline medium) and 0.5 M H\({}_{2}\)SO\({}_{4}\) (for acidic medium) electrolytes were used for LSV measurement. As-obtained HER parameters in both alkaline and acidic mediums have been summarized in Table 2. The overpotential is supposed to be the minimum potential needed to start the HER at the surface of catalyst at any cathodic current density. It is evident from Figure 4a that Ni@Pd-Pt NC outperforms all other NCs under investigation in an alkaline medium. The Ni@Pd-Pt NC exhibits significantly reduced overpotential of 11.5 mV, which is 161.5 mV lesser than that compared to Ni@Pd at the cathodic current density of 10 mA cm\({}^{-2}\) in an alkaline medium. Of most importance, Ni@Pd-Pt NC surpasses the HER performance of standard Pt/C catalyst (30 wt.% Pt) and exhibits relatively less overpotential in an alkaline medium (14.5 mV). On the other hand, Ni@Pd-Pt NC also demonstrates competitive HER performance as that of standard Pt/C catalyst in acidic medium. The Ni@Pd-Pt NC achieves a low overpotential of 37 mV in an acidic medium, which is similar to Pt/C and 121 mV lower as compared to Ni@Pd NC (Figure 4c). Such an enhanced HER activity of Ni@Pd-Pt NC compared to Ni@Pd and Pt/C catalysts is due to the fact of unique charge relocation on the NC surface and high lattice strain. Moreover, an even closer inspection of LSV curves gives a clear insight into the surface chemical identities of Ni@Pd and Ni@Pd-Pt nanocatalysts. Accordingly, the intense peaks observed before 0 mV at the cathodic current density of 4.5 mA cm\({}^{-2}\) and 8 mA cm\({}^{-2}\) in alkaline and acidic medium, respectively, reflects the presence of PdPt and NiPd alloys on the Ni@Pd-Pt surface. The HER mechanism proceeds through a series of reactions in both the alkaline and acidic mediums. As shown in Scheme 2, in the acidic medium, adsorbed hydrogen ions (i.e., \(H^{ads}\)) were formed on reactive sites of the catalyst surface in the first (discharge) step (commonly known as the Volmer reaction (1)), which was followed by an electrochemical desorption step (known as the Heyrovsky reaction (2)) and a recombination step (i.e., the Tafel reaction) (3). \[H_{3}O^{+}+e^{-}\to H^{ads}+H_{2}O \tag{1}\]  
\[H^{ads}+H_{3}O^{+}\to H_{2}+H_{2}O \tag{2}\]  
\[2H^{ads}\to H_{2} \tag{3}\]  
Besides the alkaline medium, HER proceeds via a different pathway based in an acidic medium.