**3. Recent progress of PGMs electrocatalysts**

Over the past decades, PGMs-based electrocatalysts have been mostly studied using various approaches for wide-range applications such as in fuel cell technology (HOR, AOR and ORR), hydrogen evolution reactions, and oxygen evolution reactions. These applications' behaviour relies on the best choice of catalysts, thus far PGMsbased electrocatalysts are taking the lead. The most commonly used PGM electrocatalysts over the years for fuel cells, HER and OER are platinum-based in acidic medium, Pd and Ru-based electrocatalysts in alkaline medium, while for OER they are iridium, Pt-based as well as PGMs free [18]. Traditionally, these electrocatalysts have significantly improved the catalytical performance with lower

overpotential and faster reaction kinetics. In addition, one of the most significant roles played by PGM-based and PGM-free electrocatalysts was to enhance the mass activity by turning the physicochemical properties, such as structural or electronic effects and change the adsorption/desorption of the intermediates on the surface of the catalysts. In this book chapter, we summarize the recent progress of PGMs and PGMs free electrocatalysts that are classic and believed to have potential applications in HER, OER, ORR and HOR for sustainable energy conversion.

#### **3.1 Pt and Pd alloys-based electrocatalysts**

Since Pt-based electrocatalysts were first coined for the development of fuel cells, HER and OER are seen as cost reductions and possible commercialization over pure Pt catalysts. Therefore, adding two or more non-noble metals onto to Pt surface can be of great importance to enhance its desorption as well as complete oxidation of CO intermediates and accelerate the reaction kinetics [19]. Since electrocatalysts are surface effect dependent, they have the propensity to agglomerate on the surface leading to a decrease in surface area and affecting the catalytical performance [20]. Therefore, the use of advanced supporting materials such as carbon black, carbon nanotubes, graphene, carbon nanofibers, carbon nano-horns and heteroatom-doped carbon material will allow the increase in surface area, high porosity, high electrical conductivity and high dispersion of Pt-based electrocatalysts [19].

Although Pt-based electrocatalysts have been chosen as the best candidate thus far, their ability to operate only in an acidic medium hinders the development of efficient and practical AOR, HOR, HER and ORR [21] in the future. In acidic environments, most electrocatalysts are unstable, easily decomposed and corrosive. Therefore, an urgent needs to develop new electrocatalysts that can easily operate in alkaline medium to mitigate these obstacles. To date, Pd-based electrocatalysts have previously been indicated as an alternative to replace Pt due to their excellent properties [22, 23]. These different approaches have resulted in high-efficiency AOR, HOR, OER and ORR. Various types of Pt-based electrocatalysts were investigated for the development of HER, HOR, HOR and ORR in an alkaline environment. The investigation has revealed the higher exchange current density or mass activity at a low overpotential of 0.05 or 0.1 V. Developing electrocatalysts with high activity and durability is of the most significance. Recently, the development of PGMs-free electrocatalysts for the application of fuel cells, OER and HER has become the current topic of interest. In the recent 5 years, extensive work has been published regarding the utilization of PGMs free in alkaline conditions. Designing highly efficient and long-term stable non-noble metals with advanced support material to increase the surface area will result in high electrochemical activity and excellent performance of HER, HOR, ORR and OER.

#### **3.2 PGM-based electrocatalysts for application in ORR and OER**

Oxygen reduction reaction (ORR) as a cathodic reaction for fuel cells has gained a lot of attraction for the development of next-generation energy conversion [24]. The ORR plays a huge role in fuel cells as it controls the whole device's performance. ORR reaction mechanism has two reaction pathways that occur during the reduction of oxygen with the aid of electrocatalysts. The proposed mechanism involves the direct four electrons (4e) step where the reduction of O2 to water/OH (acidic or basic electrolyte) occurs without H2O2 formation and the two electrons (2e) transfer is
