*4.3.1 Emerging HEMs towards ORR*

A major issue that prevents the practical use of fuel cells and metal-air batteries is the oxygen reduction reaction's (ORR) sluggish kinetics. Pt-based catalysts have exhibited high activity for ORR; however, the high cost of Pt limits their application []. Therefore, it is still critically necessary to improve the ORR activity of Pt-based nanostructures and reduce the Pt loading quantity. Thus, developing noble metal with Pt activity by alloying with other metals provides \*OH to enhance ORR performance. Alloying with 5 or more metals in one solid phase solution makes HEAs rather than binary, ternary. HEAs are increasingly being investigated concerning ORR; this is because of their many unique mechanical and physical properties, which make them particularly suitable as structural materials [74]. The higher stability and activity of PdCuMoNiCo HEA [83] compared to 20% Pt/C was due to the synergistic effect of its hollow structure. Recently, Chen et al. reported the remarkable 15.8 folds of higher mass activity (1.738 A mg<sup>1</sup> ), 0.90 V and higher stability of PtFeCoNiCu than the commercial Pt/C towards ORR in polymer electrolyte membrane fuel cell (PEMFCs)- [84], as indicated in **Figure 5**.

Al-Cu-Ni-Pt-Mn np-HEA exhibited (with low Pt loading 20%Pt) exhibited a higher electrochemical activity towards ORR as compared to the commercial Pt/C [74].

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

**Figure 5.**

*Schematic representation of ORR polarization curve of PtFeNiCu and commercial Pt/C electrocatalysts scanned in oxygen saturated 0.1 M HClO4 at 25 C at a scan rate of 10 mVs<sup>1</sup> and rotation rate of 1600 rpm. Reproduced with permission from the literature [84].*

#### *4.3.2 Progress of HEAs on OER*

The oxygen evolution reaction (OER), one of the most crucial steps in water splitting and other renewable energy storage and conversion techniques like metal-air batteries and fuel cells, has a four-electron transfer process, which exhibits slow reaction kinetics and a high reaction energy barrier, resulting in a high overpotential. Noble metal oxides such as RuO2/IrO2 have been ideal/active candidates for OER. In the search for highly active electrocatalysts, transition metal-based electrocatalysts emerged as potential candidates for OER electrocatalysts. With the synergistic effect possessed by HEAs and their oxide, there is an extensive search for HEAs for OER application. Qiu used the dealloying method to prepare the quaternary Ir-based HEAs such as AlNiCoIrX (i.e. X = M0, Cu, Cr, V, Nb) for OER and Li et al. reported the FeNiMoCrAl thin film electrocatalysts deposited using sputter deposition, and HEAs electrocatalyst showed superior electrochemical performance with low potential and higher current density as compared to binary and ternary counterparts [**10.1002/ smll.202106127**].

#### *4.3.3 Advances of HEAs towards green (HER)*

The challenge associated with hydrogen production is a sufficiently effective catalyst towards hydrogen production [4]. In contrast to traditional binary and ternary alloy, the HEAs possess the advantage of lattice distortion, which often provide the diffusion pathways and interstitial sites for hydrogen atoms, leading to intriguing hydrogen properties. Feng et al. reported the HEA NiCoFePtRh denoted at us-HEA is the smallest size HEA ever reported [82]. The high activity was characterized by the mass activity is about 28.3 Amg-1 at 0.05 V for HER in acidic solution (0.5 M H2SO4) which is 74.5 folds higher than the commercial Pt/C. These later findings are attributed to the lattice distortion of single-solid solution HEAs which promote the synergistic effect.

#### *4.3.4 PGMs-HEAs towards EOR*

In 2022, Fan et al. reported on the HEAs composed of PdPtCuAgAu nanowires for methanol oxidation reaction (MOR) for fuel cell application. Combining 5–6 PGMs to

**Figure 6.**

*Illustrate the comparison of current density and mass activity using cyclic voltammetry (CV) plot of the HEAs and commercial Pt/C in 1.0 M MeOH in 0.5 M H2SO4 solution at 5mVs-1 (a-c) and of PGM-HEAs with commercial catalyst (Pd/Pt) (d-f) towards EOR. Reproduced with permission from literature [85, 86].*

make large PGMs-HEAs, creates a variety of active sites on their surface to catalyze the multistep reactions. Recently, Wu et al. have reported the 6 principal PGM elements to catalyze ethanol oxidation reaction, and PGMs-HEAs exhibited higher activity by 12.8-folds than the commercial Pt/C in terms of current density, mass (intrinsic) activity as shown in **Figure 6**(**c**-**d**) [85]. This increase is attributed to the scissoring of the ethanol C-C bond which is the key step for the 12e- pathway for EOR by adding the second and third, fourth elements.

#### **4.4 Challenges and advantages of PGMs and high entropy for energy conversion**

As much PGMs-based electrocatalysts have been a leading material for energy conversion, PGMs occurs in trace amount in the earth crust and this scarcity inhibits the commercialization of PGMs. In addition, the high possibility of poisoning by reaction by-products on the surface of electrocatatalyst has become the biggest challenge. For instance, the (i) H-H (heron sky) forms Pt-H and decreases the electrochemical activity on the catalyst surface during the hydrogen evolution reaction. (ii) The acetate by-which is the EOR reaction product (COOH\*) on the catalysis surface poison the catalysts surface and subsequently decreases the stability, selectivity and catalysts overall performances. (iii) Furthermore, the O-O (metal atom (M-OO) to form superoxo-metal complex which results to yielding of two electrons triggering the Pauling's model in oxygen reduction reaction instead of 4 electron pathway why which is an undesirable electrochemical reaction. During the review of the literature, the researcher's approach's trend been found that the are numerous efforts to find a simple and facile methodthat can selectively form the low-dimension catalyst materials (1D to 2D) that possess highly active sites for electrocatalytic activity. The bottom-up fabrication methods has been promising to fine-tune the development of active electrocatalysts with high selectivity. Nonetheless, this is low commercialization of energy conversion technologies that are assembled with

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

PGMs-based electrocatalysts. The new emerging type of alloy enables the mixing of PMGs elements into solid solution, which enhances the properties of and the performance of PGMs HEAs; however, shortcomings associated with HEAs formation is the fabrication method which requires the specialized synthesis method. It was observed during the review of this book chapter that, the new facile method, low temperature and pressure method is required for the development of HEAs.
