Recent Progress on Metal Hydride and High Entropy Materials as Emerging Electrocatalysts for Energy Storage and Conversion

Abstract

The global energy demand and energy crisis such as the use of fossil fuel for energy conversion and storage have created a need for the development of clean and sustainable renewable energy sources such as fuel cells, batteries, supercapacitors, solar. However, commercialization of renewable energy devices relies heavily on exploring and devising highly functional and stable materials. High entropy materials are emerging, high-performing electrocatalysts due to their intrinsic tenability; hence, these materials may result in earth-abundant catalysts for efficient electrochemical energy storage and conversion. In this chapter, advancements in the energy storage and conversion efficiencies of emerging materials, i.e. high entropy and metal hydrides, as well as their counterparts, i.e. PGMs and MOFs, respectively are discussed. Their applications in fuel cells, hydrogen and oxygen evolution reactions, hydrogen storage, and batteries are deliberated. Furthermore, computer modeling (density functional theory) and machine learning are factored in to supplement the catalytic processes in energy generation and storage reactions.

1. Introduction

The new and advanced materials such as high entropy materials, metal hydrides and MOFs enables the realization of renewable energy technology (i.e. Fuel cell, zinc-air batteries, green hydrogen technology, oxygen reduction) [1]. The high entropy alloy’s (HEA’s) unique structural and morphological characteristics, tuneable chemical composition and functional properties have attracted considerable interest in the field of renewable energy technology [1]. In order to develop environmentally sustainable energy sources, innovative methods for designing catalytic nanoparticles are required [2]. Fuel cells and hydrogen generation have gained substantial attention as an alternative to conventional energy conversion technologies to overcome the consequences that arise with the utilization of fossil fuels. These issues are in connection with the depletion of fossil fuel reserves, demand for carbon-neutral energy sources, climatic changes concerns and economic consideration. Furthermore, energy storage has become the research of interest for renewable energy sources such as metal-organic frameworks, and metal hydride to store hydrogen and batteries materials.

This chapter presents the emerging materials (i.e. high entropy, metal hydride), their counterparts (i.e. binary and ternary PGMs and PMGs-MOFs) and the application of these materials for energy storage and conversion. The synthetic/fabrication approach of high entropy alloys (HEAs) and the computational (i.e. machine learning and density functional theory) approach for energy storage and conversion catalyst screening process are discussed. In addition, the intrinsic properties, geometric properties, and challenges associated with electrocatalysts in energy conversion, current status, conclusions, and future perspective are also fully elucidated.

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 self-assembly 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 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.

3. Recent progress of PGMs electrocatalysts

Over the past decades, PGMs-based electrocatalysts have been mostly studied using various approaches for wide-range applications such as in fuel cell technology (HOR, AOR and ORR), hydrogen evolution reactions, and oxygen evolution reactions. These applications’ behaviour relies on the best choice of catalysts, thus far PGMs-based electrocatalysts are taking the lead. The most commonly used PGM electrocatalysts over the years for fuel cells, HER and OER are platinum-based in acidic medium, Pd and Ru-based electrocatalysts in alkaline medium, while for OER they are iridium, Pt-based as well as PGMs free [18]. Traditionally, these electrocatalysts have significantly improved the catalytical performance with lower overpotential and faster reaction kinetics. In addition, one of the most significant roles played by PGM-based and PGM-free electrocatalysts was to enhance the mass activity by turning the physicochemical properties, such as structural or electronic effects and change the adsorption/desorption of the intermediates on the surface of the catalysts. In this book chapter, we summarize the recent progress of PGMs and PGMs free electrocatalysts that are classic and believed to have potential applications in HER, OER, ORR and HOR for sustainable energy conversion.

3.1 Pt and Pd alloys-based electrocatalysts

Since Pt-based electrocatalysts were first coined for the development of fuel cells, HER and OER are seen as cost reductions and possible commercialization over pure Pt catalysts. Therefore, adding two or more non-noble metals onto to Pt surface can be of great importance to enhance its desorption as well as complete oxidation of CO intermediates and accelerate the reaction kinetics [19]. Since electrocatalysts are surface effect dependent, they have the propensity to agglomerate on the surface leading to a decrease in surface area and affecting the catalytical performance [20]. Therefore, the use of advanced supporting materials such as carbon black, carbon nanotubes, graphene, carbon nanofibers, carbon nano-horns and heteroatom-doped carbon material will allow the increase in surface area, high porosity, high electrical conductivity and high dispersion of Pt-based electrocatalysts [19].

Although Pt-based electrocatalysts have been chosen as the best candidate thus far, their ability to operate only in an acidic medium hinders the development of efficient and practical AOR, HOR, HER and ORR [21] in the future. In acidic environments, most electrocatalysts are unstable, easily decomposed and corrosive. Therefore, an urgent needs to develop new electrocatalysts that can easily operate in alkaline medium to mitigate these obstacles. To date, Pd-based electrocatalysts have previously been indicated as an alternative to replace Pt due to their excellent properties [22, 23]. These different approaches have resulted in high-efficiency AOR, HOR, OER and ORR. Various types of Pt-based electrocatalysts were investigated for the development of HER, HOR, HOR and ORR in an alkaline environment. The investigation has revealed the higher exchange current density or mass activity at a low overpotential of 0.05 or 0.1 V. Developing electrocatalysts with high activity and durability is of the most significance. Recently, the development of PGMs-free electrocatalysts for the application of fuel cells, OER and HER has become the current topic of interest. In the recent 5 years, extensive work has been published regarding the utilization of PGMs free in alkaline conditions. Designing highly efficient and long-term stable non-noble metals with advanced support material to increase the surface area will result in high electrochemical activity and excellent performance of HER, HOR, ORR and OER.

3.2 PGM-based electrocatalysts for application in ORR and OER

Oxygen reduction reaction (ORR) as a cathodic reaction for fuel cells has gained a lot of attraction for the development of next-generation energy conversion [24]. The ORR plays a huge role in fuel cells as it controls the whole device’s performance. ORR reaction mechanism has two reaction pathways that occur during the reduction of oxygen with the aid of electrocatalysts. The proposed mechanism involves the direct four electrons (4e⁻) step where the reduction of O₂ to water/OH⁻ (acidic or basic electrolyte) occurs without H₂O₂ formation and the two electrons (2e⁻) transfer is where O₂ is firstly reduced directly to H₂O₂ as intermediates and later to H₂O [24]. The former reaction is ideally applicable in fuel cells and the latter reaction produces H₂O₂ which is good for the green synthesis process, for the treatment of wastewater and disinfection [25]. The good ORR performance should obey the Sabier principle where the adsorption strength of these intermediates (OOH*, OH* and O*) should neither be too weak nor too strong to bind the active site. In theory, according to the volcano plot the different electrocatalysts are demonstrated based on their catalytical activity performance. The use of the model assists in knowing why Pt and Pd with their alloys are better cathode electrocatalysts over other metals before performing the experimental analysis [26]. Then from now on, Pt and Pd-based electrocatalysts have greatly been investigated as ORR catalysts due to their electronic structure for O⁻ (from dissociation of O₂) and OH⁻ binding energies [27].

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, 30, 31, 32, 33, 34, 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⁻² 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⁻² and gave the mass activity of 1760 ± 60 mA g_Ru ⁻¹ 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⁻² and mass loading of 0.1 mg cm⁻² [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⁻², 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.

Catalysts	Electrolytes	Mass loading	Mass activity	Stability	Over potential	Refs
IrO2-SnO2	0.1 M H2SO4	1.2 mg cm^−2				[45]
Ru-RuO2-NC	0.5 M H2SO4		1.35 A g^−1	40 h at 10 mAcm^−2	176 mV	[46]
Ru-NiO/Co3O4					269 mV at 100 mAcm^−2
Ir-Co3O4	0.5 M H2SO4	2.88 wt% Ir		130 h at 10 mAcm^−2	225 mV at 10 mAcm^−2	[47]
Ti/IrO2-RuO2-ZrO2
RuO2(Mn,Co)3O4	0.5 M H2SO4	2.51 wt% Ru	366.5 A/g Ru	1000 cycles between 0.867 to 1.667 V	270 mV at 10 mA cm^−2	[42]
Ru3MoCeOx	0.5 M H2SO4		1469.5 A/g		164 mV at 10 mA cm^−2	[48]
NiFe LDH/CB	1 M KOH	0.16 mg cm^−2		12 h with	300 mV at 10 mA cm^−2	[49]

Table 1.

Summary of electrocatalysts and their electrocatalytic activity.

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 (ΔG_H) because the theory can describe intensively the binding strength of intermediate H*on the catalyst 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 ΔG_H 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 non-noble 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-H_ads 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-H_ads 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].

Reaction mechanism	Acidic	Alkaline
Volmer reaction	$H^{++e^{-\to M-H_{\mathit{ads}}}}$(R1)	$H_{2}O$+$e^{-\to M-H_{\mathit{ads}}}$+ ${\mathit{OH}}^{-}$
Heyrovsky reaction	$H^{++e^{-+M-H_{\mathit{ads}}\to H_2}}$(R2)	$H_{2}O$+$e^{-+M-H_{\mathit{ads}}\to ={\mathit{OH}}^{-+H_2}}$
Tafel reaction	$M-H_{\mathit{ads}}+M-H_{\mathit{ads}}\to H_2$ (R3)

Table 2.

Summary of reaction mechanism in both acidic and alkalinity.

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 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 H_ads 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].

4. High entropy materials

High entropy materials (HEMs) are emerging routes for the production of high-performing 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 ($∆S_{\mathit{mix}}$, as shown in Eq. (2)), the following formula can be used to express the aforementioned definition

$$N_{\mathit{\text{major}}}\ge 5,5\in \%,c_i\le 35,35\in \%\wedge n_{\mathit{\text{minor}}\ge }0c_j\le 5\%$$E1

Where ${\mathit{ci}}_{\mathit{\text{major}}}\mathit{and}{\mathit{in}}_{\mathit{\text{minor}}}$ are the number of primary and secondary elements, respectively, and $c_i\wedge c_j$ are the atomic percentages of primary and secondary elements, respectively.

$$∆S_{\mathit{mix}}=-R\sum _{i=1}^n{c}_i{\mathit{lnc}}_i=-R\sum _n\frac{1}{n}\mathit{ln}\frac{1}{n}=\mathit{\text{Rlnn}}$$E2

Where R is the gas constant, n is several major elements, and $C_i$ 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 ($∆S_{\mathit{mix}}$), (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 $∆S_{\mathit{mix}}$ must be higher than 1.5R.

$$\Omega =T_m\frac{{\mathit{\Delta S}}_{\mathit{mix}}}{{\mathit{\Delta H}}_{\mathit{mix}}}$$E3

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 $\Omega$> 1, the solid single solution phase is expected in the alloy structure, while in the case of $\Omega$ <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) face-cantered 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.

Figure 1.

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

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 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.

4.2.1 Vacuum arc melting method

Guler et al. have reported another second generation of HEMs using vacuum arc melting followed by a heat treatment process. The materials were composed of AlCoCrFeNiTix with (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) stoichiometry [68]. Ti was incorporated in the most studied HEAs (AlCoCrNi). The materials were heated at various temperatures that is 500, 700 and 1000°C. Zhang et al. reported the equimolar NbZrTiCrAl HEAs prepared by the vacuum arc melted. The equimolar HEAs were prepared with oxidation response (treated) vacuum arc melted NbZrTiCrAl HEA at 800–1200°C (Figure 2).

Figure 2.

Schematic illustration of vacuum arc method. Reprinted with permission from literature [69].

The HEAs prepared using this type of method show fine and conformal (uniform) grains, uniform chemical composition and high density [70]. However, it is difficult and tedious to evenly combine five or more metal species into a single nanostructured phase and precisely shape the HEAs into low dimensional (₁D, ₀D) structures to obtain diverse applications (owing to their intrinsic and thermodynamic instability) using the tradition wet chemical method. To overcome this shortcoming, there have been many approaches developed for the fabrication of HEAs to low dimensional structure, including boot-up and top-down to fabricate HEAs. Some of these methods include the sputtering technique, solvothermal deposition, carbothermal shock method and chemical dealloying method [71]. Recently, HEAs have been prepared successfully by employing the carbothermal shock and chemical dealloying method. The section below will discuss the methods for the fabrication of HEAs.

4.2.2 The bottom-up carbothermal shock method

The carbothermal shock (CTS) method relies on thermally shocking the metal salt covered with carbon nanofibers at around 1730°C, followed by rabid quenching [72, 73, 74]. As illustrated in Figure 3, this method is a facile throughput to fabricate HEAs on carbon support by rapid heating and ultrafast cooling at about 105–2000 K.S⁻¹. The rapid increase of temperature from ambient temperature to high temperature (≥1700°C) and cooling fast to 25°C is the promising solution to dictate the alloying of multicomponent at ultrafast kinetics [76]. This method was first introduced in 2018 by Yao et al. by developing the HEAs nanoparticles with PtPdRhRuCe composition for application in ammonium oxidation [72]. Through optimizing the method parameters namely, the concentration of the metal precursor, the shock duration, substrate and cooling rate a uniform dispersion and narrow particle size distribution can be achieved (Figures 3 and 4) [67].

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].

Figure 4.

Schematic illustration of carbothermal strategy using carbon substrate.

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 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.

High entropy alloy composition	Fabrication method	Size (nm)	Catalytic application	Ref
PtPdRhRuCe	Carbothermal		Ammonium oxidation	[73]
Al-Cu-Ni-Pt-Mn	Top-down de-alloying	15	ORR	[74]
np- p-PtRuCuOsIr	Chemical dealloying	60	ORR	[79]
FeNiMoCrAl	Sputter-deposition	10	OER	[80]
(Co,Cu,Fe,Mn,Ni)3O4	Solvothermal	5	OER	[78]
[78]; NiCoFePtRh	Vacuum arc; Facile chemical coreduction	25; 1.62	-; HER	-; [75]

Table 3.

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

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.

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⁻¹), 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.

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⁻¹ and rotation rate of 1600 rpm. Reproduced with permission from the literature [84].

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].

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 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.

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].

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 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.

5. Energy storage: MOFs

MOFs are known as coordination polymers developed from solid-state/zeolite chemistry and coordination chemistry [87, 88]. The use of MOFs in gas is extremely important because they are porous materials [89]. Ever since the establishment of MOFs by Hoskins and Robson, MOFs have grabbed great attention in porous materials because of their ability to store gases. Yaghi et al. went on to popularize research on MOFs after that, particularly after MOF-5 was reported [90]. It is theoretically possible to construct a MOF that is well-suited for a desired application (such as sensors, catalysis or separation) by carefully choosing nodes and linkers (including flaws, such as missing nodes and/or linkers). It’s noteworthy that MOFs have an ad hoc naming scheme that often uses numbers (e.g. MOF-5) or names derived from the universities from which they originated (e.g. NU-1000) [91].

5.1 Properties of MOFs

Metal-organic frameworks (MOFs) are crystalline substances with a high surface area, high porosity, and the ability to efficiently adsorb hydrogen. At 77 K, MOFs exhibit good gravimetric hydrogen capabilities, and some of them have exceeded the US Department of Energy (DOE) objective. Numerous MOFs have been reported to display permanent porosity with pore windows between 5 and 25 Å. Through ligand extension, MOFs with large interior surface areas, extending 10,000 m²/g, and extremely high porosity (up to 90% free volume) have been created. The measurement of gas isotherms has proven crucial in determining if permanent porosity has been achieved after guests have left. MOFs can be created with the right tailoring to function as extremely selective molecular sieves, sensors or catalysts. Among other capabilities of MOFs, gas storage is one of the most promising uses for metal-organic frameworks. As shown in Figure 1, the isotherm forms, which are typically Type I with little to no hysteresis, show that durable microporous structures exist under reversible gas physisorption of tiny molecules (Figure 7).

Figure 7.

Schematic representation of type I isotherms of MOFs (MOF 177, MOF-5, MOF-2) N2 sorption measured at −196,15°C showing the conformal microspores. Reprinted with permission from literature.

5.2 Metal hydrides for hydrogen storage

Due to its high gravimetric energy density and environmental benefits, hydrogen has been proposed as a promising alternative to the widely used fossil fuels as an energy source [92]. Hydrogen can be generated and separated from a variety of sources including water, fossil fuels and biomass [93]. The use of hydrogen as an energy carrier can however be impeded by the lack of safe, energy-efficient and cost-effective storage systems [94]. The most common storage modalities of hydrogen include (i) pressurized gas, (ii) cryogenic liquid and (iii) solid fuel via adsorption onto porous materials [95]. Since the storage of hydrogen in MOFs was innovatively proposed by Rosi et al. [96], a plethora of research has been conducted on the modification and application of MOFs as hydrogen storage materials [97, 98, 99]. The tunability, topological structure and nanoconfined environments of MOFs provide ideal conditions for hydrogen capture, storage and release with considerable safety, convenience and efficiency [100]. Other properties of MOFs that make them attractive for hydrogen storage include their porous structures, high specific surface areas, exposed metal nodes, facile fabrication procedures, controllable chemical functionality and amenability to scale-up [101]. Although MOFs are favourable for hydrogen storage, high storage capacities (up to 4.5–7.5 wt%) are normally achieved at cryogenic temperatures (77 K) and high pressures [87, 94, 102]. The strong pressure and temperature dependence as well as storage capacity requirements of hydrogen physisorption on MOFs therefore limit their practical application [103]. Several approaches have thus been considered to improve the hydrogen storage capacities of MOFs, including the formation of composites by adding dopants and substituting the metal nodes within the MOFs [104]. Some researchers have even proposed nanoconfinement of other materials inside MOFs as an alternative approach to enhancing hydrogen storage [105]. Alternatively, several other materials have emerged as feasible candidates for efficient hydrogen storage such as in metal hydrides. Hydrogen forms metal hydrides with some metals and alloys leading to solid-state storage under moderate temperature and pressure [106]. The reaction of hydrogen with a metal to form a metal hydride results in the generation of heat, i.e. an exothermic reaction. When hydrogen is then required, the stored heat (ΔH) is utilized to release hydrogen from the hydride in an endothermic reaction [107]. Since hydrogen becomes part of the chemical structure of the metal, cryogenic temperature or high pressure are not required to break these chemical bonds; hence, most metal hydrides absorb and desorb hydrogen at ambient temperature and close to atmospheric pressure [108]. Metal hydrides have a higher hydrogen storage density than gaseous or liquid hydrogen; hence, they are volume-efficient storage materials [108]. The use of metal hydrides for hydrogen storage is also favourable over-pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures [21]. Nanostructured metal hydrides have particularly gained attention due to their improved reversibility, altered heats of hydrogen absorption/desorption and nano interfacial reaction pathways with fast rates [109].

Examples of commonly used metal hydrides as solid-state hydrogen storage materials are binary metal hydrides which include MgH₂, TiH₂ and AlH₃ [110, 111]. The low cost and good reversibility of MgH₂ have made it particularly popular, as well as the fact that it holds the highest energy density (9 MJ/kg) among all the reversible hydrides that apply to hydrogen storage [112]. The sorption kinetics involved in hydrogen generation with MgH₂ are however quite sluggish and it has a high thermodynamic stability requiring temperatures that exceed 300°C for the desorption of hydrogen [113]. Upon alloying, metal hydrides exist either as intermetallic or complex hydrides (alanates, borohydrides and nitrides) which have also been more extensively studied due to their high hydrogen storage capacities [109].

Alternative metals have also been proposed as additives to aid the shortcomings of these metal hydrides. Palladium, for instance, acts not only as a catalyst to facilitate the uptake and dissociation of hydrogen in other metal hydrides but it can also protect the surface from corrosion [28]. Although palladium can absorb large volumetric quantities of hydrogen at room temperature and atmospheric pressure to form palladium hydride, it (as well as other PGMS) is not often considered as a sole hydrogen storage material since it is somewhat expensive and has a low gravimetric hydrogen density [113]. Extensive research has been conducted on the use and optimization of various metal hydrides to optimize the conditions and increase the hydrogen storage capacity of these materials as shown in Table 4.

Hydride type	Hydride	Tabs (°C)	Pabs (bar)	Hydrogen storage capacity (wt%)	Ref
Binary hydrides	MgH2	30	30	6.7	[114]
Complex hydrides	LiBH4	300	100	9.20	[115]
	Mg(BH4)2	290–350	1	14.82	[116]
	Ca(BH4)2	350–400	30–50	9.60	[117]
	NaBH4	350	40	10.80	[118]
Intermetallic hydrides	LaNi4.5Sn0.5	25	7.50	0.95	[119]
	TiFe  +  X wt.% Zr	25	—	1.20	[120]
	Ti1.1CrMn	23	330	1.80	[121]

Table 4.

Summary of hydrogen storage parameters and efficiencies of different metal hydrides.

5.3 Application of MOFs and metal hydrides in batteries

The power output of these renewable energy resources such as solar, hydro and wind power is highly fluctuating and intermittent which invites the parallel implementation of electrochemical energy conversion and storage technologies, such as rechargeable batteries [122]. Such storage technologies make sustainable energy utilization easy and efficient [123]. Rechargeable lithium-ion batteries (LIBs) with zero emissions, now particularly dominate the energy storage and conversion devices market, which reduces our reliance on conventional energy resources [124]. The development of high-capacity electrode materials for LIBs, however, is still necessary to meet the sustained growing demand for energy. Thus, research centred on the optimization of efficient conducting materials in energy storage devices such as batteries has soared.

The high porosity, versatile functionalities, diverse structures and controllable chemical compositions of MOFs offer various possibilities for generating adequate electrode materials for rechargeable batteries [125]. The porous structure of MOFs enables a facile electrolyte penetration and ion transportation, while the designable components promise the incorporation of electroactive sites, offering infinite possibilities for the search for candidate electrode materials for different battery systems [126]. Despite these attractive features, MOFs (and their derivatives) as electrode materials face various challenging issues, which impede their practical applications. They suffer from poor electrical conductivity, low tap density, and irreversible structural degradation upon the charge/discharge processes [125].

Metal hydrides on the other hand have been widely investigated not only in LIBs but also in nickel-metal hydrides (Ni-MHy) batteries [127, 128]. Due to their attractive properties, Ni-MHy batteries have often been used for both electric and hybrid vehicles because they provide several advantages compared to lead-acid batteries [129]. In the conventional Ni-MH battery configuration, the charge-discharge processes occur as depicted in Figure 8.

Figure 8.

Ni-MHy batteries. Reprinted with permission from [129].

Metal hydrides have received increasing interest as materials in these kinds of batteries, both as electrodes and ion conductors [130]. Metal hydride-based materials have the potential to be negative materials for LIBs, owing to their high theoretical Li storage capacity, relatively low volume expansion, and suitable working potential with very small polarization [131]. They also owe their efficiency to their large specific capacity and low voltage hysteresis compared to other conversion materials used for LIBs [124]. Among various metal hydrides, MgH₂ has particularly gained popularity as an anode material for LIBs [132]. Various modification techniques including the addition of TiH₂, as well as other materials (catalyst/carbon or suitable binders) and nanocrystallization, have been implemented to remedy the drawbacks associated with the electrochemical performances of metal hydrides [133]. These include their kinetics limitations, structural reorganization, capacity fading and volumetric change during the discharge/charge process [124].

For instance, Yang et al. fabricated MgH₂-based composites with expanded graphite (EG) and TiO₂ using a plasma-assisted milling process to improve the electrochemical performance of MgH₂. The resulting MgH₂–TiO₂–EG composites showed an increase in the initial discharge capacity and cycling capacity compared with a pure MgH₂ electrode. A stable discharge capacity of 305.5 mAh·g⁻¹ could be achieved after 100 cycles for the 20 h-milled MgH₂–TiO₂–EG-20 h composite electrode and the reversibility of the conversion reaction of MgH₂ could be greatly enhanced [134].

Additionally, Mo et al. designed a three-dimensional hierarchical metal hydride/graphene composite (LiNa₂AlH₆/3DG) that showed outstanding cycling stability with LiBH₄ as a solid electrolyte. An ultra-high capacity of 861 mAh·g⁻¹ at the current density of 5 A g⁻¹ and a long cycle life of 500 cycles with capacity retention of 97% was achieved [135]. Moreover, Weeks et al. analysed and compared the physical and electrochemical properties of an all-solid-state cell utilizing LiBH₄ as the electrolyte and aluminium as the active anode material. An initial capacity of 895 mAh·g⁻¹ was observed and is close to the theoretical capacity of aluminium due to the formation of a LiAl (1:1) alloy. This demonstrated the possibility of utilizing other high-capacity anode materials with a LiBH₄-based solid electrolyte in all-solid-state batteries [136].

5.4 Challenges and advantages of MOFs and metal hydride

Many beneficial features have evolved into a wide variety of MOFs that is high degree of porosity, high surface area, flexible architecture, multifunctional chemical properties and tuneable structure. MOFs are widely used in many applications, including catalysts, supercapacitor, adsorbents, sensors, environmental protection and drug delivery, due to their simplicity of design and homogeneous and fine-tuneable pore architectures. However, a number of disadvantages have also limited the use of these innovative materials in practical applications, including high production costs, poor selectivity, low capacity and challenges with recycling and regeneration. Furthermore, poor electrical conductivity and stability of conventional MOFs inhibit development and application. With many short coming associated with MOFs, synthetic challenges in the discipline emerge from understanding and regulating both structural and compositional complexity because of the enormous array of conceivable topologies and compositions. In addition, despite the enormous variety of known structural types, there have been no definitive findings of shape-selective catalysis in MOFs although zeolite community has a highly established understanding of shape-selective catalysis. Zhang et al. reported the attempts to improve MOFs shapes and widen their application through the introduction of graphene as a template to grow the MOFs which avoids agglomeration, and diminish poor electrical conductivity and stability [137].

6. A computational approach for conversion: Density functional theory and artificial neural network

Machine learning is one of the most powerful too of artificial intelligence and has been utilized for numerical prediction, classification, efficiency and pattern recognition. Among ML tools, artificial neural network (ANN) has become the popular nonlinear algorithm, adaptive structure, tunable and easy to train for various applications such as catalyst, biology and energy [138]. In general, ANN architecture is comprised of at least three (3) layers, i.e. input layer, hidden layer and output layer as depicted in Figure 8. Each layer contains numerous neurons that connect to the next layer, where they connect it represented by weights. As depicted in Figure 9, ANN is composed of single layers (Hidden layers) that represent the parameters such as current and cell potential. The output is represented by the efficiency of the system. ML utilizing the artificial neural network has attracted considerable interest in energy conversion and electrocatalytic reaction to predict the throughput using the algorithms. ML has attracted many researchers in the field of catalysis. Lu et al. leverage the use of neural networks and density functional theory (DFT) for predicting the surface defects for oxygen reduction reaction. Mehiritz et al. report the first model of electrooxidation of ethanol using an artificial neural network (ANN) utilizing the differential evolution (DE) algorithm. The best results (Model) were obtained with a single hidden layer [140].

Figure 9.

Typical example of feed-forward approach using artificial neural network architecture for input/output variables, output representing system efficiency, (b) schematic cartoon illustrating the possible adsorption site (charge depletion, and charge accumulation), (c) partial PDOS showing ORR. Reprinted from the literature [139].

In addition, theoretical techniques, such as quantum-chemical modeling employing density functional theory (DFT, Vienna ab initio simulation package (VASP) methodologies, are used to expedite the process of finding the ideal material. This route enables us to evaluate different catalytic materials without using experimental analysis, as well as determine the catalyst’s adsorption qualities and the impact of the material’s composition and structure on the kinetics of the catalytic process. Nrskov and co-authors first use DFT-based techniques to compute the adsorption energy of reactants and intermediates. In this section, the efforts to identify the highly active catalysts and relevant electrocatalysis reaction mechanisms have been studied, particularly focusing on the d-band centre location, the electronegativity of the central atom to the neighboring, along with density of state (DOS). These theoretical properties are utilized in electrochemical systems to determine the appropriate descriptor for catalytic activity, specifically towards the ORR and OER. As illustrated in Figure 9, this is the typical schematic diagram depicting the DOS pattern and d-band centre level. There has been extensive research on the theoretical modeling of energy conversion and storage to screen the catalyst process. For example, Sunday et al. reported reactive molecular dynamics on surface PtNiFe heteroatoms to model the catalyst mechanism. Based on the energy barrier, it was found that NiO efficient than monometallic Ni and Pt, the HOads and Hads are likely easier on NiO during water oxidation (dissociation) [141]. Mkhohlakali et al. used the DFT and found the appropriate descriptor for the EOR catalyst process and the geometric and electronic effect has been found as the best descriptor for PdTe towards ethanol oxidation in alkaline media [15]. Furthermore, DFT has also been used for energy storage, particularly hydrogen storage. Kabelo and co-workers have described the hydrogen hybridization between boron and yttrium in yttrium-doped borophene adsorption on borophene materials using DFT indicating the potential use of borophene for hydrogen storage [142]. In addition, adsorption energies, diffusion barriers and reaction activation barriers for reactants, intermediates and products on the catalyst surface are very important microscopic characteristics that can be obtained from first principles calculations based on the density functional theory (DFT) in heterogeneous catalysis. In many energy conversion processes, DFT is used to estimate the rates of the fundamental processes and identify the rate-limiting phases. Shen et al. employed DFT to predict the mechanism for CO₂ reduction on Cobalt porphyrin and it was found that the key intermediates formed when Co is in CO1 and the results were agreed with the experimental results with more details [143]. There is an increasing search for prediction of CO₂ reduction using DFT, and Gao et al. reported the theoretical calculation for CO₂ to CO on Co-quaterypyridine complex surface [144]. In the case of metal hydrides, there is yet no material that can fully satisfy the requirements for the practical onboard application, despite the fact that solid-interstitial and non-interstitial (state) hydrides have made notable advancements in terms of materials synthesis, mechanistic understanding, and performance enhancement.

7. Conclusion and future perspective

Based on the reported research trends, new emerging materials such as HEAs and metal hydrides show the potential and future technology in energy conversion and storage respectively. The second generation of HEAs illustrate the higher electrocatalytic efficiency and stability for both anode and cathode materials because of increased active sites, enhanced reaction kinetics and CO- poison tolerance dues to single-solid solution as compared to conversional PGMs alloys. Both the electrochemical and physicochemical priorities affect the efficiency of energy conversion and storage. The future work following this trend is the consideration of “giant” HEAs compounds. Furthermore, the overall findings therefore not only depict the use of metal hydrides as anode materials but also as solid-state electrolytes. Beyond the conventional Ni-MHy battery, the potential of these hydrides has been demonstrated at a lab-scale upon optimization of a range of innovative energy storage concepts, including the MgH₂-Li chemistry. Extensive research however still needs to be conducted to promote the development of new batteries of higher energy density and lower cost. Computer modeling demonstrates a better approach to understanding the interfacial electrochemical properties and physico-chemical properties. In addition, the efficiency of the electrochemical systems was well predicted machine learning through artificial neural networks, which shows the efficient potential approach to predict the efficiency of fuel cell, hydrogen storage and batteries. Among the algorithm approach, the differential evolution utilizing the single hidden layer shows the efficient and electrocatalytic reaction kinetics prediction. The modeling of structural formation and electrochemical properties of metal hydrides, high entropy and MOFs should be mandatory to strengthen the synthesis and better understanding of electrocatalysts. Although advances centred on solving energy problems have soared with the discovery and implementation of various emerging catalysts, further work still needs to be conducted to ensure sustainable energy systems and technologies. In addition to high efficiency, catalysts for energy generation and storage should result in technologies that are affordable, abundant and generate clean energy.