*3.2.1 PGM-based electrocatalysts for application in OER in acidic and alkaline environments*

Oxygen evolution reaction is based on the 4-electrons process and their reaction mechanism is found to be difficult whether conducted in the alkaline or acidic medium leading to high overpotential. The high overpotential causes the efficiency and the performance of OER to be sluggish for the formation of hydrogen from the water-splitting process. On that note, suitable electrocatalysts are needed to break down the O-H bond to form the O – O bond, accelerate the kinetics of OER and enhance the overall efficiency. Ideally, a catalyst should be able to follow the Sabatier principle whereby it is stated that a catalyst should not be too weak nor too strong to bind oxygen. The computational studies were performed on the potential catalysts and the volcano curve gives the relationship between the catalytical activity of the catalysts and bond strength. Therefore, Trassati et al. have reported that RuO2 has shown a high OER activity on the volcano shape between the oxidizing enthalpy and OER activity [28]. An extensive search has been conducted for RuO2 base electrocatalysts, ranging from the monometallic (Ru, Ru, and Ir, Ru, and the mixed IrO2 and RuO2) in both acidic and basic environments. The development of these materials from fundamental to commercialization is hindered by the slow kinetic and high cost of Ru and Ir [29–35]. In the past decade, researchers have reported the use of second or third metal oxides (IrO2-SnO2, RuO2-TiO2, IrO2-MnO2, RuO2-ZrO2, IrO2- Ta2O5 and IrO2-IrO2-Ta2O5) to design binary and ternary electrocatalysts not only to minimize the noble metal content but also to reduce the overpotential towards OER. In earlier studies, it was reported that the binary catalysts/multi-metal oxides outperform monometallic catalysts because of the ability to promote bifunctional (bonding interaction (M-OH)) activity and lower the overpotential kinetics [36, 37]. Lee et al. have reported the use of highly active metals and stable three-dimensional mesoporous Ir-Ru binary based on mixed metal oxides and metal/metal support as OER catalysts. Their structural and electrochemical behaviour was investigated for the OER activity and mesoporous IrO2/RuO2 with a molar ratio of (1:10 Ir/Ru) gave a lower overpotential of 300 mV at 10 mA cm<sup>2</sup> as compared to IrO2/RuO2 with a high overpotential of 340 mV. After the stability test was performed on both catalysts ran for over 2 h, the overpotential was as low as 22 mV for mesoporous IrO2/RuO2 and 44 mV for IrO2/RuO2 [33]. In addition, Huang et al. have also affirmed that coupling RuO2 with oxides such as TiO2 with a high valence state can enhance the intrinsic stability under an acidic medium without compromising the performance. The electrochemical behaviour revealed that the RuO2/HTI/TI composite has low

overpotential of 220 and 265 mV with the current densities of 10 and 50 mAcm<sup>2</sup> and gave the mass activity of 1760 60 mA gRu <sup>1</sup> at 290 mV overpotential, which is 7.5 times higher than pure RuO2 nanoparticles. The study also revealed the high stability of composite for 11 hr. with a current density of 500 mAcm<sup>2</sup> and mass loading of 0.1 mg cm<sup>2</sup> [38]. These findings highlight that the incorporation of metal oxides into noble metals is promising electrocatalyst for OER [39]. These results were better as compared to amorphous Ir atomic clusters, and IrO2 nanoneedles [40]. More work has been conducted on the OER electrocatalysts and interestingly zirconium oxides were also investigated because they are less likely to be corroded and possess a homogenous dispersion property among other reported oxides when dispersed on noble metals. Liu et al. studied the ternary metal oxides Ti/IrO2-RuO2-ZrO2 for OER activity and the electrochemical investigation shows the high electrocatalytic activity and good stability when the Ru content is 21 wt%, which emphasizes that Ru content plays a critical role in designing the efficient OER catalyst [41]. Furthermore, it has been reported earlier that the catalysts for OER applications in acidic environments should have high Ru content for better performance [42]. However, a pioneering study by Niu et al. demonstrates that the addition of transition metals onto Ru can minimize the noble metal content in an acidic medium and also contributes to regulating the electronic structural interaction. They aimed to investigate the low loading RuO2 (2.51 wt%) unto (Mn, Co)3O4 as highly efficient catalysts succeeded for OER application. The RuO2/(Co, Mn)3O4 catalysts show a high performance with an overpotential reported to be 270 mV at a current density of 10 mA cm<sup>2</sup> , superior to commercial RuO2, and RuO2/Co2O3/CC catalysts [42]. Though PGM electrocatalysts have exhibited remarkable catalytical performance for OER, high cost and poor chemical stability in alkaline environments impede the development of tenable applications [43]. Therefore, the electro-design of non-noble electrocatalysts such as Co-based, Ni-based metals, and multi-metal oxides (spinel, layered hydroxides, perovskites, etc.) as active OER showed the best alternative is an alkaline environment [37, 44]. In general, the spinel metal oxides are expressed as follows AB2O4, whereby cation A has a charge of 2+, occurred at the tetrahedral sites (A M+2) and cation B with M+3 occurs at the octahedral sites of the close-packed structure [37]. Among them all, spinel ferrites, such as NiFe2O4 and CoFe2O4 have been widely studied as efficient and promising catalysts for OER activity and other applications [36]. In addition, layered double hydroxides (LDHs) were considered a class of synthetic layered terrestrial 3d transition metals with high electrocatalytical activity for OER application. **Table 1** summarizes the electrocatalytic activity of mono-metallic, bimetallic and trimetallic towards different electrocatalytic reactions.

#### *3.2.2 PGM-based electrocatalyst for hydrogen evolution reaction*

Hydrogen evolution is the production of hydrogen gas through a water-splitting process and the reverse is utilizing that Hydrogen gas as fuel in fuel cell applications [50]. Both the HER and HOR processes occur with the aid of electrocatalysts at the cathodic and anodic sides during electrochemical reactions, respectively. Thus far, PGM electrocatalysts such as Pt, Ru, Pd, Ir and Rh are the most efficient, and more active and have advanced tremendously in the past years to lower the overpotential while enhancing the reaction rate [51, 52]. The ultimate goal in the development of electrocatalysts under acidic conditions for both HER and HOR reactions is to understand the concept of adsorption-free energy of hydrogen (ΔGH) because the theory can describe intensively the binding strength of intermediate H\*on the catalyst *Recent Progress on Metal Hydride and High Entropy Materials as Emerging Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.113105*


**Table 1.**

*Summary of electrocatalysts and their electrocatalytic activity.*

surface [53]. The surface of the electrocatalysts should neither be too strong nor too feeble to bind hydrogen, so it enables the surface to form hydrogen gas easily by improving the kinetics of HER as expressed in equation [50, 54]. This is according to the empirical rule obtained from the Sabatier principle where it is expected that ΔGH of the electrocatalysts should be close to zero [53]. Nørskov and his coworkers were inspired by these experimental findings and later decided to conduct the computational work utilizing density functional theory (DFT) on various metallic surfaces and the outcomes were the same as the one attained from the volcano shape. The volcano curve investigates a linear relationship between the hydrogen evolution reactions's (HER) exchange current density and the hydrogen binding energy (HBE) on the metallic surface [54]. This is basically to show that the variation of the measured exchange current densities is well comprehended by using a facile kinetic model. The volcano shape in Figure displayed the role PGMs electrocatalysts play in HER reactions and those electrocatalysts gave the best performance in enhancing the kinetics in an acidic medium. Thus far, Pt and Pd (which are closer to the highest peak at the volcano) are leading efficient HER catalysts, with low overpotential and showing rapid kinetics [55]. It is desirable to introduce the support materials as well as the nonnoble metals to eliminate the problem while maintaining the high catalytic activity [55]. Alloying PGMs with non-noble metals (Fe, Au, Co, Ni, Cu, etc.) and with their corresponding metal oxides/hydroxides by turning intrinsic and extrinsic properties towards HER [51]. The table summarizes the electrocatalysts used for HER. The development of non-noble metals as an alternative to Pt and Pd has gained attractive attention to date as bifunctional electrocatalysts owing to their low cost and possessing good corrosive resistance in alkaline medium [56]. So far electrocatalysts, such as cobalt, Ni, Mn, Fe based, were widely explored and the performance in HER gives high electrocatalytical behaviour. Even though sluggish kinetics and lower ECSA in most of the electrocatalysts were observed, however, alloying and the use of support

materials combat the problems. The support materials also play a crucial role in PGMs-free alloys as they can increase the surface area.

#### *3.2.3 Reaction mechanism for HER in acidic and alkaline media*

To comprehend the application of electrocatalysts towards HER reaction, it is advisable to first understand the catalytic reaction mechanism evolved throughout. The reaction can take place in two conditions: acidic and alkaline conditions and are described as follows. The hydrogen evolution reaction can be expressed in the following equation, where the hydrogen intermediates are formed throughout the process to produce hydrogen gas: the process includes three major steps in an acidic environment. This reaction step involves the adsorption and desorption of H\* intermediates on the surface. The initial step involves the adsorption of a proton into the electrocatalyst surface – M-Hads where M represents the active material with one electron migrating (R1), followed by the second step which can be derived from either Heyrovsky or Tafel reaction because this particular step-dependent on the coverage of hydrogen intermediates on the surface of the active site, subsequently, the final step involves the coverage of two hydrogens adsorbed on the active site to form hydrogen atom [57, 58]. In an alkaline environment, the HER reactions occur in two steps mainly, the Volmer and Heyrovsky reactions, whereas the Tafel reaction remains the same with the acidic step [59]. Briefly, the initial step involves the formation of M-Hads and OH� on the active site followed by the interaction of water with the adsorbed hydrogen and electron to form a hydrogen atom (**Table 2**) [60].

### **3.3 Binary and ternary electrocatalysts towards electro catalytic reaction**

Researchers have approached the drawback associated with sluggish kinetic and 2 electron pathways during ORR by introducing the second and third elements. The enhancement of the catalyst activity is ascribed to the synergistic effect and functional activity of two elements and the O� O-species formed by exophilic species. Lankiang et al. reported the binary and ternary electrocatalysts such as Pt70Pd15Au15 using the micro-emulsion method, and the electrocatalysts were tested for ORR in in O2-saturated 0.1MHClO4 by rotating disk electrode. Recent research on PtPd-based catalysts has shown an enhancement in the electroactivity towards ORR in acidic medium, which is explained by a synergetic effect between Pd and Pt.

### **3.4 PGM-based electrocatalysts for application in HOR**

The PGMs electrocatalysts for application in hydrogen-oxygen reaction (HOR) have gained much attention in the past decades. Thus far, the widely studied PGM


**Table 2.**

*Summary of reaction mechanism in both acidic and alkalinity.*

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

electrocatalysts are Pt, Pd, Ru, Ir, Re and Rh on HOR in fuel cells for electrochemical conversion purposes. Inspired by Sabatier principles it states that the best electrocatalysts for HOR should possess a moderate adsorption strength. The HOR reaction pathway in the alkaline medium can be obtained following the elementary steps: The R1 and R2 which are Tafel and Heyrovsky are reactions where the Hads intermediates are chemically absorbed on the active site and where M represents the active site for hydrogen adsorption. Later, the absorbed hydrogen together with hydroxyl ions forms water and therefore electrons are released from the surface. In addition, the Tafel reaction step as differs from Heyrovsky ought two vicinal active sites to allow the adsorption of two hydrogen intermediates (bond length between H-H is approximately 0.74 Å. Therefore, it is generally accepted that in a basic electrolyte, the kinetics of HOR is derived from hydrogen binding energy (HBE). HBE is a very important factor as it affects the efficiency of HOR, sluggish kinetics results in weak or strong adsorption interaction between binding PGMs adsorbate and intermediates. As it was observed from the volcano curve when the pH (from 0 to 14) is increased the use of PGMs aids in binding the hydrogen stronger hence, the PGMs are found on top of the curve [56].
