## 1 Introduction  
Hydrogen has been investigated as a potential candidate for the implementation of green-energy technologies, however, sustainable hydrogen production is the key prerequisite to this end [1, 2]. It is a well-known fact that hydrogen does not exist freely in nature but is largely available in compounds such as water and hydrocarbons, therefore, efficient generation of hydrogen is imperative for use in various applications. Recently, great efforts have been devoted to the sustainable generation of hydrogen from electrochemical water splitting. Hydrogen evolution reaction (HER) plays a major role in electrochemical water splitting [3]. However, the HER incurs a very high overpotential and requires an omnipotent nanocatalyst (NC) to lower down the overpotential and to trigger the HER kinetics. Thus far, carbon-supported platinum (Pt/C) is the state-of the-art electrocatalyst that boosts the HER kinetics by virtue of its excellent activity, high turnover frequency, relatively low overpotential and high current density [4; 5]. Since the widespread application of Pt is limited by its high material cost and scarcity, minimization of Pt-loading or even the discovery of alternatives to Pt-based catalysts are key steps for industry efficient hydrogen production [6]. By keeping the aforementioned issues in view, enormous efforts are devoted towards designing alternative catalysts with earth abundance, cost-effective and superior activity over the past several decades including transition metal oxides [7; 8], sulfides [9; 10], carbides [11; 12], nitrides [13; 14], borides [15; 16], phosphides [17; 18] and metal-free catalyst [19; 20], which can overcome the economical limitations and stability issues. Additionally, some noble metals (e.g., Pd, Ru) are also used to trigger the HER kinetics. According to a volcano plot, Pd is supposed to be a possible alternative of Pt towards the HER because the adsorption energy of Pd is nearly similar to that of Pt, therefore, it can be a potential HER catalyst [21; 22]. Moreover, Pd has high hydrogen (H\({}_{2}\)) adsorption energy immediately next to Pt and thus can absorb a considerably large amount of H\({}_{2}\) from both the gas phase and electrolytes. In addition, Pd is nearly five times more economical as compared to that of Pt. While Pd-based catalysts have the potential to replace Pt in HER, a narrow gap in HER catalytic activity still exists as compared to that of Pt-based catalysts. Numerous alternative strategies such as alloying Pd with late-transition metals [23], morphology and composition control [24], coupling with optimized support materials [25], tuning the particle size and so on have been directed to fill this gap. However, none of them has outperformed Pt-based catalysts in HER. Based on the aforementioned arguments and in line with recent developments, in the present study, we synthesized a ternary NC comprising hierarchical Ni@Pd NC with Pt-clusters decoration on the surface via a \"self-aligned\" heterogeneous nucleation-crystal growth mechanism on carbon black support (namely Ni@Pd-Pt). Our previous studies demonstrated that Pt-clusters decoration effectively shelters the catalyst surface from oxidation and creates intense active sites for HER facilitation [26; 27; 28; 29; 30; 31]. In such a unique structure, Pt-clusters are decorated on the Ni@Pd surface and at the heteroatomic interfaces of Ni-to-Pd; where the ligand effect (due to difference in electronegativity of adjacent atoms) and lattice strain (due to lattice mismatch) plays a key role at the heterogenous interfaces and relocate electrons to the NC surface, hence, the formation of catalytic active sites takes place, which boosts the HER kinetics. Besides, neighboring Pd reaction sites share the intermediate steps during HER, therefore, boost the activity of Ni@Pd-Pt in HER. Hence, the Ni@Pd-Pt NC delivers a lower overpotential of 11.5 mV at the cathodic current density of 10 mA cm\({}^{-2}\).