## 4 Noble Metal-Based Electrocatalysts for ORR and HER
### Pt-Based Catalysts in ORR and HER
Among various compositions of the Pt\({}_{\text{x}}\)Ni alloys, Pt\({}_{1.5}\)Ni exhibited the highest mass activity of 1.96A/mgPt\({}^{-1}\) at 0.9 V in ORR [33]. In addition, [PERSON] et al. demonstrated that Pt-Ni alloy nanoparticles fabricated on a reduced graphene oxide (rGO) support exhibited excellent electrocatalytic catalytic activity towards HER in alkaline mediums [34]. Apart from controlling the bulk composition, various potential strategies such as thermal annealing have been demonstrated to manipulate the near-surface region of catalysts. For instance, [PERSON] et al. successfully formed a Pt enriched layer over Pt-M alloy surfaces by using thermal annealing [35]. The Pt-based alloys exhibited enhanced performance towards ORR and HER as compared to that of a commercial benchmark catalyst. The improved activity of Pt-based alloys is due to the change in the electronic structure, i.e., increase of the d-band vacancy, geometric effect, steric effect, and ligand effect [36]. Mainly, whenever an alloy forms, it creates the lattice contraction and eventually leads to changes in the Pt-Pt inter-atomic distance, which favors the dissociative adsorption of oxygen. As a result, it better tunes the ORR activity of alloys compared to pure Pt alone. Unfortunately, despite their good activity, dissolution of the transition metal alloyed in the Pt-M catalysts is a major drawback because these transition metals (M) are electrochemically soluble in low pH conditions. Hence, extensive research is needed to solve the stability issues and their intimate contact within the system. To surpass the severe issues of Pt-based alloys, such as leaching of non-noble metal and less structural stability in acidic conditions, researchers have come up with a new approach, where, the less stable transition metals are protected with a monolayer (shell) of active Pt. In this way, the problems associated with the dissolution of transition metals are addressed and the interaction of the two metals at the heterogenous binary interface is expected to change the properties of Pt. In such heterogeneous archetypes, a combination of ligand effect (due to electronegativity difference between two atoms) and lattice strain (attributed to the lattice mismatch between intra-particle domains of heterogeneous NCs) tunes the electronic and geometric properties of the material for enhanced ORR activity facilitation [37]. Co-existence of these two effects leads to a stronger electron relocation between the active sites (Pt-shell) and neighboring atoms, which results in a substantially suppressed binding energy of intermediate species, hence, the dramatically enhanced catalytic activity of core-shell structured NCs is obvious. Researchers optimized various compositions and configurations to achieve high performances in the last decades. For example, [PERSON] et al. developed Cu@Pt core-shell electrocatalysts and tested for ORR. As-prepared Cu@Pt NC achieved a peak power density of 0.9 W cm\({}^{-2}\) at Pt loadings of as low as 0.24 mg cm\({}^{-2}\) on each cathode and anode [38]. To further decrease the Pt-content, [PERSON] et al. prepared a Cu@Pt core-shell NC on a Vulcan XC-72R carbon support by using a two-step reduction method, where ethylene glycol (EG) was used as the solvent and reducing agent. As-synthesized NC was employed towards ORR and Cu@Pt NC with an atomic ratio of Cu/Pt of 2.73:1 exhibited tremendous performance [39]. The core-shell structured NCs, in which the active metal such as Pt is decorated only on the surface is also a possibility for increasing Pt-utilization and reducing Pt loading. In this context, [PERSON] et al. also synthesized a similar kind of catalyst with ultra-low loading (5 wt.%) and high utilization of Pt (Co\({}_{95}\)Pt\({}_{5}\)) via the deposition-precipitation method in different reduction environmentsand found that Co@Pt NC reduced in a CO-environment with only 5 wt.% of Pt surpassed a commercial catalyst and performed in an outstanding manner towards ORR (Figure 4) [40].