## 4 Noble Metal-Based Electrocatalysts for ORR and HER
### Pt-Based Catalysts in ORR and HER
In addition to the conventional synthesis methods, Cu@Pt core-shell NCs are also prepared via de-alloying and galvanic replacement processes. To this end, [PERSON] et al. prepared surface de-alloyed Pt-Cu NCs by using the electrochemical method and achieved 4-times improved mass activity as compared to that of the baseline benchmark in ORR [41]. Moreover, [PERSON] et al. used the galvanic replacement reaction to prepare carbon-supported Pt@Cu core-shell nanoparticles and as-prepared Pt@Cu NC outperformed the commercial Pt-catalyst [42]. Meanwhile, similar kinds of structures have also been employed to boost the HER kinetics. [PERSON] et al. used the atomic layer deposition (ALD) technique to deposit Pt on tungsten carbide and produced core-shell structures with Pt-loading reduced by nearly 10-times as compared to that of bulk-Pt catalyst while maintaining similar HER performance [43]. Meanwhile, [PERSON] et al. reported on reduced graphene oxide (rGO) supported AuPt@Pt core-shell nanocrystals to catalyze HER in both acidic and alkaline mediums. As-prepared AuPt@Pt core-shell nanocrystals exhibited improved HER performance with an onset potential (\(-25\) mV) and a small Tafel slope (\(33\) mV decade\({}^{-1}\)) in \(0.5\) M H\({}_{2}\)SO\({}_{4}\), and an onset potential (\(-43\) mV) and small Tafel slope (\(73\) mV decade\({}^{-1}\)) in \(0.5\) M KOH [44]. Although, core-shell structure NCs seems to be a perfect design, the minimal Pt-loading constraint (i.e., the formation of the Pt-monolayer) and the delicate controllability of perfect core-shell structure with complete Pt-shell coverage is still a challenging task, and can result in structure failure (corrosion of core crystal or segregation of Pt) in the harsh redox conditions. Pt atoms are the only, unified active sites on the core-shell structured NCs, therefore, the selectivity of intermediate species in ORR and HER between surface active sites is suppressed. Consequently, irrespective of the improvement, the unified surface chemical identities affect the traffic of intermediate species and thus suppress the ORR and HER kinetics. Following this line of research, recent pioneer studies demonstrated that surface decoration of atomic-to-nanoscale clusters on core-shell or cluster-in-cluster structured heterogeneous catalysts further facilitates the kinetics of electrochemical reactions. Taking the advantages of local synergetic collaborations between active sites, [PERSON] et al. decorated highly active atomic Pt-clusters on carbon nanotube-supported Ni@Pd core-shell structure catalysts (NPP) for ORR application. Moreover, they also controlled the depth and distribution of Pt-clusters on the Ni@Pd surface via changing the adsorption time of Pt\({}^{4+}\) ions. The ORR performance of NPP NCs changed while changing the adsorption time of Pt\({}^{4+}\) ions due to variation in charge density on the surface. In the optimum case, NPP NCs had the highest specific activity (\(0.732\) mAcm\({}^{-2}\)) when the adsorption time was \(2\) h. (Figure 5) [45]. The same group has also synthesized tri-metallic NCs consisting of a sub-nanometer Pt-cluster's mask decorated with Pd nano-islands on a NiOx support. They suppressed the Ni-oxide  
Figure 4: (**a**) The cyclic voltammetry and (**b**) linear sweep voltammetry curves of CoPt nanocatalysts (NCs) reduced in different environments. (Reproduced with permission from [40], American Chemical Society, 2019)  
structure via optimized Pt-loading. In the optimum case, 9 wt.% Pt-decorated Ni@Pd NC achieved the mass activities of 1523.7 mA mg\({}^{-1}\) and 671.5 mA mg\({}^{-1}\) at 0.85 V and 0.9 V vs. reversible hydrogen electrode (RHE), respectively [46]. The surface decoration with Pt-atomic clusters not only triggers the ORR activity but also triggers HER activity. [PERSON] et al. also synthesized atomic Pt-cluster decorated Ni@Pd hierarchical structured bimetallic NCs with 2 wt.% Pt-content by using a wet chemical reduction method.