**4. High entropy materials**

High entropy materials (HEMs) are emerging routes for the production of highperforming electrocatalysts due to their intrinsic tenability and the coexistence of several possibilities and these materials may result in earth-abundant catalysts for efficient electrochemical energy storage and conversion. High-entropy materials were first proposed in 2004 and have been applied in a range of systems and applications. They are a promising class of disordered multicomponent materials with tailorable properties/functionalities (and maybe unparalleled performances). HEMs are a new class of materials that have just been invented and exhibit extraordinary properties that outperform those of conventional alloy (i.e., binary, ternary alloys) material. This concept was first introduced recently by Cantor et al. and Yeh et al. in 2004 [61, 62] based on the composition, and HEAs/HEMs materials are defined as a single phase (alloy) multi-component system with 5 or more major metal element of which each having a near equiatomic (equimolar ratio) and each element has at least concentration between 5 and 35% content [1, 63]. These materials are also known as multi-principal component alloys, in contrast to traditional alloys which have 1 principal metal element dominant. Another definition is based on the thermodynamic configurational entropy of mixing (Δ*Smix*, as shown in Eq. (2)), the following formula can be used to express the aforementioned definition

$$N\_{major} \ge \dots \ge \dots \ge \emptyset, c\_i \le \mathfrak{Z}\mathfrak{z}, \mathfrak{Z}\mathfrak{z} \in \mathfrak{W} \land n\_{minor} \ge 0c\_j \le \mathfrak{Z}\mathfrak{Y} \tag{1}$$

Where *cimajor and inminor* are the number of primary and secondary elements, respectively, and *ci*∧*cj* are the atomic percentages of primary and secondary elements, respectively.

$$
\Delta S\_{\text{mix}} = -R\sum\_{i=1}^{n} c\_i lnc\_i = -R\sum\_{n} \frac{1}{n} \ln \frac{1}{n} = Rlnn \tag{2}
$$

Where R is the gas constant, n is several major elements, and *Ci* is the concentration of the components. Compared to conventional compounds, HEAs with more than 5 elements trigger the formation of a single-phase solution (structure) with lattice distortion (which is beneficial for gas sorption and hydrogen storage). There are various factors affecting the formation of HEAs (1) Enthalpy of mixing (Δ*Smix*), (2) Atomic size difference, (3) Electronegativity difference, (4) Valence of the electron concentration, (5) Effect of kinetics on phase formation, (6) Geometric parameter and (7) excess of entropy. Based on the high entropy effect phenomenon, the high entropy of mix Δ*Smix* must be higher than 1.5R.

$$
\Omega = T\_m \frac{\Delta \mathcal{S}\_{\text{mix}}}{\Delta H\_{\text{mix}}} \tag{3}
$$

The enthalpy entropy relationship during solidification is symbolized as (magma). This parameter is calculated based on Eq. (3). Where Tm and H denote the melting temperature and the enthalpy of mixing, respectively. When *Ω*> 1, the solid single solution phase is expected in the alloy structure, while in the case of *Ω* <1 is attributed to the formation of an intermetallic compound or phase separation.

According to Yeh et al., another attractive property of multiple elements (large HEAs) with different characteristics that enhance the performance in diverse applications are (i) sluggish diffusion, (ii) high entropy effect and (iii) cocktail effect (which describes the synergistic response, beneficial for energy conversion) [64]. As illustrated in **Figure 1(a)** and **(b)** HEAs exist in various structures namely (i) facecantered cubic, (ii) body-cantered cubic (bcc) and (iii) hexagonal closely packed (hcp). The characteristic features, generation and classification of HEMs will be discussed in the following section.

#### **4.1 Generations and category of HEAs**

Refractory element-based HEAs are frequently categorized into the first, second and third generations based only on the time of their formation [65]. In contrast to traditional alloys (i.e. binary and ternary, with 1–2 principal elements), the first generation of high entropy alloys which comprised of a single phase with at least 5 principal elements at equal concentration with BCC single-solid phase. These typical HEAs were established in 2004 as Canter alloys such are AlCoCrNiMn, FeCrMnNiCo, and Mo25Nb25Ta25W25. Due to the thermal stability (i.e. resilience in high temperatures), these HEAs were being applied in aerospace that is wind turbine blades. The second generation consists of at least four principal elements with dual phases at

**Figure 1.**

*Schematic representation of high entropy alloy. Reproduced with permission from reference [1].*

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

non-equiatomic started to emerge because of the high density and low corrosion resistance of the first-generation HEAs. The metals with a high density such as Ta, Nb and Mo in the first generation were substituted with light and corrosion-resistant elements such as Al, Fe and Cr to name a few and transitioned from single to dual phases such as Fe50Mn30Co10Cr10 [66]. The HEAs formation strategy is reviewed and discussed in the following section.

#### **4.2 Preparation methods**

Ever since the discovery of HEAs in 2004, there has been an extensive search for suitable methods to form a single-solid solution of such materials [67]. The common methods to prepare 3D and low-dimensional structures of HEAs are abundant and mainly include (i) Vacuum induction melting, (ii) Vacuum arc furnace melting, (iii) Top-down chemical dealloying, (iv) bottom carbothermic shock, (v) Solvothermal co-reduction, (vi) Ball milling and (vii) Sputtering. In the following section, the various preparation methods will be discussed.
