*4.2.3 Solvothermal co-reduction technique*

So far, several methods for the preparation of HEA have been investigated to address the drawbacks associated with high-temperature preparation methods for HEAs. Solvothermal co-reduction which is analogous to hydrothermal has attracted considerable interest in the preparing HEAs and HEAs oxides. The word "solvothermal" was first introduced in the early 1980s to distinguish this specific

#### **Figure 3.**

*Comparative electrocatalytic performance showing HER polarization curve of commercial electrocatalysts Pt/C and Rh/C and HEAs towards HER in 0.5 M H2SO4 solution. Reprinted with permission from literature [75].*

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

#### **Figure 4.**

*Schematic illustration of carbothermal strategy using carbon substrate.*


#### **Table 3.**

*Summary of various HEAs prepared by different synthetic methods, size and their catalytic application.*

reaction from others in other solvents [77]. The solvothermal method is known to be the most effective synthetic route for HEAs with controlled sizes and morphology. Wang et al. prepared small-size (5 nm) HEAs for application in water oxidation reactions, particularly OER and HER [78]. In this method, an auto-clave is a closed system where the mixture is reacted with solvent (i.e. organic or non-aqueous) under a controlled temperature (i.e. between 29 and 98°C) and pressure. The process uses straightforward autoclaves that have been moderately heated. There are various HEAs reported that are prepared by the solvothermal co-reduction method. A summary of different HEAs, with different properties and fabricated using methods, is listed in **Table 3**.

#### **4.3 Application of high entropy materials: Recent progress**

The cock-tail effect of high entropy alloys promotes the synergistic effect which is beneficial in energy conversion and electrocatalysts and increases the investigation of HEAs for ORR, OER, HEA and ethanol oxidation reaction [61]. The discovery of emerging efficient electrocatalyst materials such as high-entropy alloys for use in renewable energy sources such as fuel cells, batteries and water electrolysis is imperative to the development of energy conversion because it is the carbon-neutral approach. Having oxygen reduction reaction (ORR) and oxygen evolution reaction as a key reaction in fuel cell and zin-air batteries respectively are a good example of an electrochemical energy conversion reaction, which requires better electrocatalysts. In addition, hydrogen production through hydrogen evolution on the cathode and oxygen evolution on the anode during electrochemical splitting into chemical energy is an intriguing pathway to convert chemical energy stored in water into electricity [81]. Electrocatalysis is a key asset in the transition towards sustainability because it enables the net-zero-carbon synthesis of value-added chemicals and chemical fuels, including power-to-X approaches. Examples include energy conversion through fuel cells, hydrogen evolution, hydrogen oxidation of fuel cells, ORR, OER reaction in zin-air batteries and water electrolysis. The energy conversion system that occurs in fuel cells, water electrolysis (H2O splitting) and metal-air batteries is driven/formed by ORR and OER has a cell reaction. The efficiencies of this electrochemical energy conversion system are hampered by sluggish/slow reaction kinetics, which requires the development of active electrocatalysts to overcome these bottlenecks [1]. The complexity associated with these reaction intermediates requires promising bifunctional (i.e. with multiple active sites) electrocatalyst materials such as HEAs [64, 82]. Many attempts have been made to develop HEAs, specifically, FCC structured type of HEAs has become a current subject of intense investigation as emerging electrocatalysts for OER and ORR. The active catalyst sites of HEAs and the unique physico-chemical properties of HEAs have attracted great interest in OER and ORR. In the section below, the continuous work on the development of HEA as electrocatalysts towards electrochemical conversion, particularly OER and ORR will be reviewed and discussed.
