**2. Platinum group metal catalysts**

Platinum group metals consist of six (6) principal elements and they are also known as precious metal, and these metal includes Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Osmium (Os), Iridium (Ir) and Platinum (Pt) [3]. They are often classified into two categories namely, light (Ru, Rh, Pd) and heavy PGMs (Os, Ir and Pt) based on whether they are based on 4d electron shells or 5d electron shells based on their atomic number [4]. The PGMs are used in various applications across different applications; however, in this section, we will focus on their fabrication methods and use in electrocatalysis [5]. PGMs are applied in numerous energy conversion and storage as electrocatalysts and as energy carriers. PGMs are key electrocatalyst materials that are employed in numerous electrochemical energy conversions, including fuel cell and hydrogen generation to name a few. In electrocatalysis, the PGMs are applied in the proton exchange membrane, and they are mainly responsible for oxygen reduction and hydrogen oxidation reactions at the anode and cathode of the fuel cell [6]. However, recently there has been a drive to substitute PGM-based catalysts with non-PGM catalysts, and this was due to the relatively high costs associated with the former [7]. The use of these relatively non-expensive, high abundance and good activity has further progressed the improvement of energy materials [8].

#### **2.1 Fabrication methods of PGMs electrocatalysts**

Fabrication of PGMs nano-electrocatalysts has been the search of interest for many years to improve the properties for a wide range of applications. In nanotechnology, nanostructure PGMs electrocatalysts are fabricated using two (2) synthetic approaches, i.e. "Top-down" and "Bottom-up" techniques [9]. The top-down mainly focuses on forming the nano-scale materials from bulk *via* physical crushing milling, plasma etching and lithographic methods (i.e., photo-lithology, soft lithology, colloidal lithology) to name a few. Whereas the "Bottom-up" counterpart implies the fabrication of PGMs electrocatalysts, starting from molecular/atomic scale *via* selfassembly of atoms and molecules. Bottom-up techniques include chemical vapour deposition, molecular beam epitaxy, sol-gel method, green synthesis, hydrothermal and chemical reduction to name a few. Bottom-up has shown many advantages

*Recent Progress on Metal Hydride and High Entropy Materials as Emerging Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.113105*

compared to its counterpart that as the control over shape size, cost-effective, short methods with low consumption. The later approach usually involves the use of a metal salt which is reduced in the liquid medium in the presence of a stabilizing agent to form relatively stable nanoparticles [10]. Examples include the use of platinum chloride salt as a Pt source, sodium borohydride as the reducing agent and citrate moiety as the stabilizing agent [11]. An example would include the use of electron beam lithography to produce metal nanoparticles with the desired dimensions [12]. In addition, excellent electrochemical activity demands a robust and unified synthetic approach to design phases of catalyst materials.

#### **2.2 Intrinsic, geometric properties of PGMs-M electrocatalyst**

Generally, the use of metallic nanocatalysts that are not anchored onto a support structure is relatively unstable. Furthermore, we will highlight the effects each method has on the geometric properties and the intrinsic performance of the catalysts in application. They tend to undergo Ostwald ripening and coalesce producing larger particles with reduced surface area and eventually loss of performance as electrocatalysts hence the use of various mesoporous structures as support materials [13]. The synthetic approach is found to control the selectivity, mass activity, agglomeration, surface morphology, atomic scale and size of nanomaterials-based catalysts. Zhang et al. have reported the electrodeposition (self-supporting electrode) of Pt/C for the oxidation of liquid fuels, such as ethanol and methanol. The method showed uniformly dispersed nanoparticles due to weak Vander Waal force between Pt and substrate and compared with the traditional method [14]. Notably, the influence of geometric properties such as shape, size and morphology influences the electrochemically active surface area due to increasing active sites which further improves the catalytic activity. Mkhohlakali et al. argued that the best descriptor for catalytic activity of PdTe is best described by geometric features. Furthermore, AFM and DFT showed rugged surface morphology and High O\* binding energy respectively, which enhanced the oxidation of ethanol reaction intermediates [15]. Mesoporous structures have been used successfully to fabricate electrocatalysts with unique structures and speciality morphology. However, great care is required in controlling the pore sizes and pore distribution and their specific surface area as these factors affect the catalyst properties and their performance during application [16]. In the past decade, various studies have been conducted in template development for electrocatalysts [17]. The high adsorption capacity often directly relates to better metal loading and enhanced electrocatalysis performance.
