High-Entropy Alloys: Innovative Materials with Unique Properties for Hydrogen Storage and Technologies for Their Production

Abstract

This paper presents a review of a number of works devoted to the studies of high-entropy alloys (HEAs). As is known, HEAs represent a new class of materials that have attracted the attention of scientists due to their unique properties and prospects of application in hydrogen power engineering. The peculiarity of HEAs is their high entropy of mixing, which provides phase stability and flexibility in developing materials with given characteristics. The main focus of this paper is on the application of HEAs for solid-state hydrogen storage, their physicochemical and mechanical properties, and synthesis technologies. Recent advances in the hydrogen absorption properties of HEAs are analyzed, including their ability to efficiently absorb and desorb hydrogen at moderate temperatures and pressures. Prospects for their use in the development of environmentally safe and efficient hydrogen storage systems are considered. The work also includes a review of synthesis methods aimed at optimizing the properties of HEAs for hydrogen energy applications.

1. Introduction

In the context of the global energy crisis and increasing environmental pollution, hydrogen energy is known to be one of the most promising alternatives to fossil fuels [1,2]. Hydrogen energy has attracted particular attention as a potential replacement for fossil fuels due to its cleanliness, non-toxicity, renewability, wide distribution, and high energy density compared to other fuels [3,4]. In addition to hydrogen production using renewable sources, hydrogen storage plays a key role in the development of hydrogen energy systems and, moreover, material-based storage can be an efficient and stable solution for hydrogen transportation and storage [5,6]. However, the development of alloys nowadays is still limited to the classical design paradigm. At the same time, there is a general trend that the chemical complexity in alloys is steadily increasing with time, as clearly shown in Figure 1 [7]. Figure 1 shows the evolution of metallic materials in terms of configurational entropy (ΔS_conf) over time, from the Bronze Age to the present day. Materials are classified into three zones: low-entropy (ΔS_conf < 1R), medium-entropy (1R $\le$ ΔS_conf $<$ 1.5R), and high-entropy alloys (ΔS_conf $\ge$ 1.5R). It is shown how an increase in the chemical complexity of alloys (from bronze and iron to modern HEAs) is accompanied by an increase in configurational entropy. Modern HEAs such as CoCrFeMnNi and AlCrFeNiCo are presented in the zone of high-entropy alloys, emphasizing their role as next-generation materials.

Since the end of the 20th century, as is well known, traditional metallic materials have been giving way in practice to new classes—high-entropy materials. HEAs usually involve several elements, which are usually present in equiatomic or nearly equiatomic proportions. Cantor et al. [8] and Yeh et al. [9] simultaneously put forward such an alloying concept using multibasic elements. Cantor et al. obtained a 20-component alloy containing 5 at.% Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Mo, Nb, Al, Cd, Sn, Pb, Bi, Zn, Ge, Si, Sb, and Mg. The Gibbs phase rule allows the formation of up to 21 phases (at constant pressure) at equilibrium, but it was found that the predominant phase was a face-centered cubic (FCC) solid solution phase containing mainly Fe, Ni, Cr, Co, and Mn. Based on this, FeCrMnNiCo alloy was developed, which forms only the solid solution phase. The HEAs developed by Yeh et al. also consisted mainly or entirely of solid solution phases, contrary to what was expected. Yeh et al. attributed this result to the high configurational or mixing entropy of a solid solution containing multiple elements. In equilibrium, the phase with the lowest free energy ($∆$G) is formed. Increasing $∆$S (entropy) will increase the probability that the phase will be stable. Other names have been proposed for HEAs such as multicomponent alloys, multi-principal element alloys (MPEAs), complex concentrated alloys (CCAs), and concentrated solid solution alloys. In the present study, in order to recognize the original work of the authors who first introduced the concept of high-entropy alloys, the term HEAs is used to describe all the systems considered.

Nowadays, HEAs are increasingly becoming the subject of intensive research [10,11,12,13] due to their unique properties, including high hardness, corrosion resistance, and thermal stability. These properties make them promising candidates for diffusion barriers, high-performance catalytic systems, and innovative tools for solid-state hydrogen storage and other advanced technological applications. Research has focused on their microstructure, mechanical performance, and potential applications at extreme temperatures and conditions. The large degree of lattice distortion caused by the mismatch in atomic radii of the different constituent elements is of great interest for hydrogen storage [14,15,16,17,18,19].

Despite significant progress in the study of HEAs, many aspects of their behavior under real-world operating conditions remain unexplored. One such challenge is the need to better understand the mechanism of hydrogen’s interaction with HEAs at the atomic level. Studies show that high desorption energy is a possible mechanism, and in this regard, current research has focused on developing HEAs with improved reversibility of hydrogen chemisorption–desorption at room temperature and reducing their fabrication cost. In addition, the effects of microstructural factors such as grain size, local lattice distortions, and phase distribution are still poorly understood. These factors can have a significant impact on the hydrogen absorption properties and stability of HEAs.

In connection with the above, the aim of this paper is to review the research on HEAs as innovative materials for solid-state hydrogen storage. The main attention is given to the analysis of their physicochemical properties, synthesis methods, and critical evaluation of existing approaches to their application. Special attention is also paid to the hydrogen absorption characteristics of HEAs, their kinetics, and their efficiency under realistic operating conditions.

2. Relevance of Research on the Properties of HEAs for Hydrogen Storage

The conceptual approach to modifying the properties of materials by doping is a long-established approach in materials science. In the context of the development of materials for solid-state hydrogen storage, it has become obvious that the introduction of additional chemical elements makes it possible to significantly adjust their physicochemical and functional properties [20]. A considerable amount of research has focused on binary and ternary systems based on magnesium, alanates, amides, and intermetallides, including such well-known compounds as LaNi₅, TiFe, and materials based on titanium and zirconium. These systems are characterized by the presence of a basic element (or several elements), with the other components present in relatively low concentrations.

Within such systems, there is a tendency to form solid solutions or intermetallic compounds with improved hydrogen absorption characteristics. Significant advances in tuning the properties of metal hydrides in recent decades have been achieved precisely through studies of such systems. Nevertheless, historically, the multicomponent approach was predominantly limited to the study of the effect of minimal alloying additives on the hydrogen absorption properties of already well-known systems. This process was traditionally associated with high financial and time costs due to the need for labor-intensive and systematized research.

The HEA paradigm has significantly broadened the horizons of materials science, opening access to unexplored regions of the central parts of multicomponent phase diagrams, where the formation of alloys with simple crystal structures is possible. This approach has stimulated the development of new theoretical tools and methods to evaluate a wide range of unknown compositions [21]. In the context of hydrogen storage materials, such capabilities provide fundamentally new approaches to fine-tune properties. HEAs exhibit improved characteristics such as enhanced gravimetric capacity under moderate conditions, complete reversibility of hydrogen-absorbing cycles at room temperature, and no need for activation processes. These properties have already been experimentally confirmed in a number of works [22,23].

In addition, current research on HEAs is shedding light on the fundamental mechanisms of interactions between multibasic elements and hydrogen at both the local and mid-atomic levels, clarifying how these interactions determine the hydrogen absorption characteristics of alloys [24,25,26]. Based on these findings, it becomes possible to hypothesize that HEA compositions can be purposefully designed to meet the specific requirements of hydrogen storage materials. Moreover, the integration of advanced computational methods and machine learning tools, already actively applied in the field of hydrogen research [27], provides powerful tools for accelerated exploration of the vast space of potential HEA compositions.

The remaining unresolved scientific and technological challenges concerning the realization of efficient hydrogen storage systems in the context of future mobile applications necessitate the development of innovative materials and fundamentally new storage concepts, which is discussed in detail in a number of fundamental works [28,29]. Within this framework, special attention should be paid to the research of HEAs capable of providing enhanced performance characteristics, including increased gravimetric capacity, as well as achieving high equilibrium pressures compatible with system operation at pressures up to 35 MPa [30]. Such research aimed at unlocking the potential of HEAs as hydrogen storage materials could significantly contribute to the transformation of this field, opening new perspectives for its scientific and technological development.

3. Structural Features and Properties of HEAs

It is generally accepted that HEAs are defined as systems containing at least five major elements, with the concentration of each element varying from 5 to 35 atomic percent [9]. These elements, acting as equal components, form complex thermodynamic conditions that fundamentally distinguish HEAs from traditional binary or ternary systems. In addition to major elements, HEAs often contain minor components with concentrations below 5 at.% [31]. The term “high-entropy” is due to the fact that the entropy of mixing occurring in the liquid or solid phases of these alloys is much higher than in standard systems with fewer components [32]. This leads to the fact that the entropy effect in HEAs is much more pronounced, having a critical impact on the thermodynamic stability of the alloys. When developing the concept of HEAs, the thermodynamic characteristics of the alloy as a closed system and the influence of these characteristics on the structure and performance properties of alloys are guided.

The most important parameter in solid solution formation is the Gibbs free energy:

$$∆\mathrm{G}=∆H_{mix}-T·∆S_{mix}$$

where $∆$H_mix is the enthalpy of mixing, T is the absolute temperature, and $∆$S_mix is the entropy of mixing.

The entropy S of a system has an additive property. For pure elements, the entropy is determined only by the thermal motion of atoms. In the case of alloys, in addition to the vibrational contribution, there is a configurational contribution to the entropy due to the different arrangement of atoms in the lattice and the appearance of vacancies. There are also small contributions to entropy from the electronic and magnetic components of disorder. As a result, the total mixing entropy includes four components: configurational entropy (S_conf), vibrational entropy (S_v), magnetic entropy (S_m), and electronic entropy (S_e) [33].

Thus, the entropy of mixing is as follows:

∆S_mix = ∆S_conf + ∆S_v + ∆S_m + ∆S_e

According to Boltzmann’s hypothesis on the relationship between entropy and system complexity [34], the change in entropy per mole (∆S_mix) during the formation of a solid solution of n elements with equal molar concentration can be calculated using the following equation:

∆S_mix = klnw

where k is the Boltzmann constant (1.38·10⁻²³ J·K⁻¹), and w is the number of equally probable microstates. The number of microstates w, describing all possible configurations of the distribution of atoms of different elements in the crystal lattice, is determined through the factorials of the number of atoms of each element:

$$w={\displaystyle \frac{N!}{N_A!\cdot N_B!\cdot \cdot \cdot \cdot N_n!}}$$

where N is the total number of atoms in the system, and N_A!, N_B!, and N_n! are the factorials of the number of atoms of each of the components.

Thus, with equal molar fractions of each element, the entropy of mixing will have the following form:

$$∆S_{\mathit{mix}} =-R{\displaystyle \sum _{i=1}^n{c}_i\ln }{c}_i=-R\ln 1/n=R\ln n$$

where R is the gas constant (R = 8.314 (J/K·mol)), c_i the molar fraction of the ith element, and n the total number of the constituent elements [33]. At the equiatomic ratio c_i = 1/N, the change in entropy of the system when forming a solid solution of N elements with a fraction c_i of each element will be S_mix = RInN. An increase $∆$S_mix leads to a decrease in the Gibbs free energy of the alloy and hence an increase in the stability of the solid solution. For a five-element alloy with equal elemental fractions, $∆$S_mix = 1.61R. Figure 2 shows an example demonstrating the formation of a five-component equiatomic alloy [33]. In Figure 2A, the colored circles represent one of the five different elements (A, B, C, D, E). Here, the atoms are arranged in an ordered manner to form a regular crystal structure. This is a model for an ordered phase where each atom occupies a strictly defined position in the crystal lattice. In Figure 2B, the colored circles are arranged chaotically, reflecting a high-entropy structure (or random distribution of atoms), where different atoms occupy positions in the crystal lattice randomly.

In addition to high entropy, there are other key parameters that determine the formation of amorphous phases and solid solutions in multicomponent alloys. These parameters include atomic size difference (δ), enthalpy of mixing ∆H_mix and entropy of mixing ∆S_mix [35]:

-

Enthalpy of mixing [36]:

$$\Delta H_{mix}={\displaystyle \sum 4{\Omega }_{ij}{c}_i{c}_j}$$

where c_i and c_j are the content (at. %) of the i-th and j-th elements in the alloy, respectively, and ${\Omega }_{ij}=4\Delta H_{AB}^{mix}$ is a concentration-dependent parameter characterizing the interaction between the elements in solid solution.

-

The average difference in atomic radii:

$$\delta r=100\%\sqrt{{\displaystyle \sum c_i{\left(1-r_i/\overline{r}\right)}^2}}$$

where c_i is the content (at.%) of the i-th element in the alloy, r_i is the atomic radius of the i-th element in the alloy, and $\overline{r}=\Sigma c_i{r}_i$ is the average atomic radius of the alloy.

Thermodynamic parameter (Ω) determining the average melting point of elements:

$$\Omega =T_m∆S_{mix}/|∆H_{mix}$$

where $T_m=\Sigma c_i{T}_{mi}$ is the melting point of the elements.

Calculations taking into account the parameters Ω and δr have shown that the formation of simple solid solutions in HEAs is more likely when the conditions Ω $\ge$ 1.1 and δr $\le$ 6.6 are met, while the formation of a multiphase structure consisting of solid solutions and intermetallic phases occurs at 1.1 $<$ Ω $<$ 10 and δr $>$ 3.8. Despite possible errors, the parameter Ω can serve as a sufficiently reliable “tool” for distinguishing between the formation of solid solutions and intermetallic phases in multicomponent systems. The high value of the Ω parameter (more than 1.1) and the low value of δ (less than 6.6%) indicate the probability of formation of solid solutions in HEAs. Thus, the simplicity of phases in such alloys, contrary to the expectations of a complex microstructure, is related to their unique crystal structure and entropic effects, which significantly affect their mechanical and physicochemical properties.

Thus, HEAs tend to form single-phase solid solutions with simple structures such as volume-centered cubic (BCC) or face-centered cubic (FCC) [15]. One of the key characteristics of HEAs that ensure their application in hydrogen energy is their ability to form thermodynamically stable solid solutions, due to which a uniform distribution of atoms in the crystal lattice is achieved. What structure will form can often be predicted a priori from the valence electron concentration of the VEC:

$$VEC={\displaystyle \sum _{i=1}^n{c}_i}{\left(VEC\right)}_i$$

where c_i is the atomic fraction of element i and the valence electron concentration VEC_i is present in HEAs with N different elements. At VEC $\ge$ 8, a single FCC phase is formed in the alloy, at 6.87 ≤ VEC ≤ 8, a mixture of FCC and BCC phases are present in the alloy, and at VEC $<$ 6.87, only one BCC lattice phase is present in the alloy.

Alloys with BCC structures are characterized by high strength and resistance to wear, which makes them suitable for creating durable materials for hydrogen storage under pressure. In turn, FCC structures provide high corrosion resistance and stability at low temperatures, which is especially important for hydrogen storage under cryogenic conditions. HEAs exhibit unique hydrogen absorption properties due to the high defect density caused by lattice distortion. The introduction of elements with different atomic radii, such as aluminum or nickel, contributes to the modification of the local environment and the formation of multiphase structures that optimize hydrogen absorption and desorption processes. For example, the addition of aluminum to CoCrFeNi or CoCrFeMnNi alloys can induce a transition from FCC to BCC structures, which improves the kinetics of interaction with hydrogen [37,38,39]. Recent studies have demonstrated that FCC structures can be transferred to a metastable state using transformational plasticity effects [40,41]. This has allowed HEAs to overcome the traditional trade-off between strength and ductility, which is particularly important for materials designed for hydrogen storage at high pressures or cryogenic temperatures. The introduction of elements with strong interatomic interactions or minimal modification of the composition can stimulate the formation of secondary phases, improving the sorption properties of HEAs. For example, the addition of aluminum and nickel to HEAs often leads to the formation of ordered phases such as B2 (as in NiAl) or L1₂ (as in Ni₃Al) [42,43]. Moreover, systems containing aluminum, nickel, and titanium can form L2₁-type phases similar to Ni₂AlTi, which enhances the hydrogen absorption properties of the alloys. These ordered phases are variations of simple FCC or BCC structures and can be considered solid solutions with increased configurational entropy [44]. Since such phases are often present as nanoscale coherent particles, they significantly improve the mechanical properties of alloys through dispersion-strengthening effects. For hydrogen power generation, this is critical because the high mechanical stability of the material ensures long and safe hydrogen storage. In [45], HEA systems were divided for the first time into four families, shown in Figure 3, based on their mechanical properties. The first one is soft solid solution HEASs including only 3d transition metals. The second group is combinations of transition metals with large-atomic-radius elements such as Al/Ti/V/Mo. The third group is based on refractory metals. Several other alloy systems have also been studied.

Another important class of high-entropy materials is represented by high-entropy ceramic systems (HECs), which are characterized by a multicomponent composition involving at least five different cationic species. These materials demonstrate superior performance compared to traditional ceramic composites [46]. Among the most studied representatives of high-entropy ceramics are oxides (HEOs), nitrides (HENs), carbides (HECs), borides (HEBs), hydrides (HEHs), silicides (HESis), and sulfides (HESs) [47]. Currently, high-entropy oxides (HEOs) represent one of the most deeply studied subcategories of high-entropy ceramics, demonstrating significant scientific and technological potential in such advanced areas as hydrogen generation [48], the creation of magnetic and dielectric functional elements [49,50], the development of battery systems [51], and in thermal barrier coatings [52]. However, in the context of studying the photocatalytic properties of HEOs, no fundamentally significant attempts have been made to systematically analyze their performance in comparison with their unoxidized HEA counterparts. Edalati et al. [53] successfully synthesized the high-entropy oxide TiZrNbHfTaO₁₁ via high-pressure and oxidation methods, demonstrating their superiority in photocatalytic CO₂ conversion compared to traditional photocatalysts such as BiVO₄ and the anatase form of TiO₂. This superiority is due to their defective, strained two-phase crystal structure and optimized electronic band configuration. Similar findings were made for TiZrNbHfTaO₆N₃ oxynitride compounds [54], which demonstrated significantly higher photocatalytic activity than commercial samples such as the reference P25 TiO₂. Edalati et al. [53] used the high-pressure torsion (HPT) technique to prepare HEAs from a mixture of elemental metal powders. Subsequent oxidation of the formed pellets in air at 1100 °C for 24 h resulted in the preferential oxidation of the surface regions of the particles, which is attributed to the high temperature stability and refractory nature of the constituent metals. In the early stages of this process, a dense oxide layer is formed, which effectively blocks further diffusion of oxygen into the particles. Considering the key role of configurational entropy in determining the properties of materials, it becomes important to compare its value for traditional engineering alloys such as microalloyed steel and aluminum systems with HEAs. Microalloyed steel, an example of a high-performance material that is widely used in various industrial applications, is characterized by a comparatively low configurational entropy. This is due to the limited number of alloying elements such as carbon, manganese, niobium, and vanadium, which ensures the optimization of mechanical properties with minimal compositional complexity. Similarly, aluminum alloys used in the aerospace and automotive industries exhibit moderate entropy due to the limited number of additive elements (e.g., magnesium, copper, silicon). However, HEAs, due to their high configurational entropy, offer an alternative for applications in extreme conditions such as high temperatures or corrosive environments.

4. Methods of HEA Synthesis

HEAs can be synthesized by all known synthesis methods for common alloys and vary depending on the target characteristics and applications, including both classical metallurgical processes and modern methods of powder technology, as well as mechanical activation [33]. In Figure 4, we have shown the methods and types by which HEAs can be synthesized. The key aspect is the control of the microstructure and phase composition, as they directly determine the mechanical, thermal, and hydrogen storage properties.

The HEAs investigated for hydrogen storage are most often synthesized by arc melting and high-energy ball milling methods. Also, HEAs for hydrogen storage (HS materials) can be obtained by laser-assisted deposition (LENS), injection molding, and melt spinning (rapid cooling). The principle of arc melting (AM) is based on the creation of a plasma between the electrode and crucible in a vacuum or controlled atmosphere [55,56]. The use of rapid cooling techniques improves the microstructure and the formation of solid solutions. A schematic representation of the arc melting method using an electric arc is shown in Figure 5. A tungsten electrode creates an arc with a temperature of 3000–4000 °C, under which the material is placed on a graphite substrate. A water-cooled copper plate is used to cool the structure. Electrical power is supplied from an external power source.

This approach is effective for the production of significant volumes of material while ensuring its chemical homogeneity, which has been confirmed in a number of studies [58,59]. However, during the melting process, especially in arc furnaces, there remains a high risk of oxidation of active components (e.g., titanium or aluminum), which requires strict control of the protective environment and increases equipment and process costs. In addition, this method is limited in its ability to produce non-equilibrium and fine-grained structures or nanomaterials, as melting and slow cooling processes tend to produce a coarse-grained structure that negatively affects the mechanical properties of the alloy. The non-equilibrium structure limits the use of the material for HS with respect to storage capacity, thermodynamics, kinetics, and hydride phase. Figure 6 shows surface images of HEAs ingots synthesized by vacuum arc melting, where significant porosity and irregular topography are noted.

In mechanical activation (MA), ball milling (BM) is also a promising synthesis technique and is often used to synthesize refined microstructures with equilibrium and/or non-equilibrium phases. Figure 7 shows the mechanism of high-energy ball milling. Material particles are trapped between the grinding balls and undergo compression, flattening, and fracture formation. The alternating fracture and cold welding process pulverizes the particles and mixes them uniformly. It has been successfully applied to prepare alloys or hydrides for Mg-based hydrogen storage [62,63], and it has been effective in producing nanostructures for improved thermodynamics and kinetics [64,65]. To the best of our knowledge, Zepon et al. [66] were the first to report the synthesis of hydrogen storage HEAs using this method. In particular, the alloy MgZrTiFe_0.5Co_0.5Ni_0.5 and its hydride form were obtained by high-energy ball milling carried out for 24 h at a rotation speed of 600 rpm in an argon atmosphere under a pressure of 0.7 MPa Ar or 3.0 MPa H₂. De Marco and co-authors [67] successfully synthesized homogeneous alloys MgVCr and MgVTiCrFe by high-energy ball milling conducted for 24–72 h under 3 MPa of H₂ pressure, despite the known thermodynamic immiscibility of Mg with V and Cr. However, MgVTiCrFe exhibited poor performance in the context of hydrogen storage, including a capacity of only 0.37 wt% and poor cycling stability. Cardoso et al. [68] synthesized MgAlTiFeNi using ball milling in a hydrogen atmosphere (3 MPa) for 24 h or in an argon atmosphere for 30 h. The main alloy phase, which has a BCC structure, did not show significant hydrogen sorption. Instead, TiH₂ and Mg₂FeH₄ hydride compounds were identified in the samples. Although the hydrogen desorption temperature was lower than that of commercial MgH₂ (which is an advantage for hydrogen storage under moderate conditions), the capacity of the alloy was relatively small (~1 wt%).

Laser additive techniques such as selective laser melting (SLM), laser-assisted mesh formation (LENS), and directed energy deposition (DED) are also used to synthesize HEAs [70]. High-energy lasers melt metal powders layer by layer, creating complex geometries and specialized microstructures. SLM enables rapid solidification, forming a fine microstructure and improving mechanical properties. Studies on AlCrFeCoCu alloy (Figure 8) have shown excellent anti-corrosion properties due to phases in simple lattices (BCC and HCP). However, the size of the components to be machined limits the application of SLM and DED in the production of large parts.

The LENS method was applied by Kunze et al. to a series of experiments on the synthesis of HEAs such as TiZrNbMoV [72], ZrTiVCrFeNi [73], and LaNiFeVMn [74] in the context of the development of hydrogen storage materials. High cooling rates (in the range of 10³–10⁶ K/s) during solidification are expected to minimize phase segregation and ensure the formation of a homogeneous microstructure. The schematic in Figure 9 demonstrates the laser cladding process with powder feed. The laser beam heats the substrate, creating a melt zone where the powder is fed with argon gas. A shielding gas protects against oxidation and a control computer manages the process parameters to ensure high fusion accuracy.

As studies have shown, obtaining alloys close to the calculated chemical composition is possible with strict adherence to synthesis conditions, such as the use of a high-power laser, powders with appropriate characteristics, and optimal processing parameters [73,75]. Montero et al. [76] observed that HEA samples of identical nominal composition (Ti_0.325V_0.275Zr_0.125Nb_0.275) fabricated by different methods including reactive ball milling (RBM), BM, and AM show significant differences in hydrogen storage properties. For example, samples prepared by the AM method are characterized by higher hydrogen uptake kinetics and reduced desorption temperature compared to their counterparts fabricated by the BM method. At the same time, hydrides synthesized by the RBM method showed the lowest desorption temperatures (220 °C and 260 °C for the two peaks).

One of the promising modern technologies is powder metallurgy, which is a highly efficient and versatile method widely used for the synthesis of HEAs, especially in the production of parts with strict geometrical characteristics. In this context, spark plasma sintering (SPS) as a powder metallurgy technique is an efficient method to create complex multicomponent materials. This is due to the possibility of careful control of the microstructure, the use of relatively low sintering temperatures, and the achievement of high material densities. Such characteristics make this method particularly attractive for creating new materials with improved mechanical, thermal, and physical properties. Based on the definition of SPS, it can be noted that the simultaneous application of high temperature and pressure results in the rapid densification and consolidation of powders. This process is able to create a material with a more homogeneous microstructure and increased grain boundary density, which contributes to the improvement of mechanical properties such as tensile and compressive strength, impact toughness, stiffness, and ductility [77,78]. Figure 10 shows the setup for SPS. Electrodes connected to a current source create an electrical pulse through the powder material in the mold while applying pressure. The temperature is monitored by a thermocouple and cooling is provided by a water jacket. The process is controlled through an SPS controller. Due to the rapid heating and low holding time at high temperatures, SPS prevents undesirable grain boundary diffusion and grain growth processes, which helps to maintain a highly active material surface. Thus, Moravcik et al. [79] consolidated Ni_1.5Co_1.5CrFeTi_0.5 alloy using the SPS technique. The results showed a flexural strength of 2593 MPa, tensile strength of 1384 MPa, elongation to failure of 4.01%, and elastic modulus of 216 GPa. These results were significantly higher than the same HEAs produced by the casting method.

Park and coworkers [80] synthesized HEA TaNbHfZrTi powders using hydrogenation–dehydrogenation reaction and investigated their sintering behavior using SPS. The initial ingot consisted of a single solid–solid BCC phase, but after annealing in a hydrogen atmosphere, it absorbed hydrogen and converted to a hydride phase. This caused brittleness, allowing it to be easily pulverized into powder. The hydrogen was then removed from the powders by vacuum heat treatment, resulting in the formation of a two-phase microstructure with nanoscale BCC and HCP phases. After sintering the powders at 1000 °C, densification was complete and the HCP phase was retained, but uneven grain growth was observed. Upon sintering at 1100 °C, the structure transformed into a single BCC phase with an equiaxed microstructure, and grain growth became uniform. The grain size decreased from 227.8 μm in the original ingot to 22.5 μm in the compact sintered at 1100 °C.

In our works [81,82,83], the main regularities of the formation of the phase composition and structure of the powder composition of the Ti-Al-Nb system in the process of preliminary MA and subsequent SPS were revealed. The high microstructural stability of the two-phase alloy (O + B2) based on the Ti-Al-Nb system in the process of thermocyclic hydrogen sorption has been established and shown. The problems of creating new two-phase high-temperature IMCs on the basis of the Ti-Al-Nb system and developing a method for obtaining hydrogen-accumulating materials with increased sorption capacity have been set and solved. The method proposed by the authors promotes the creation of active states of metal particles in the powder composition of the Ti-Al-Nb system. The use of the MA method at the stage of pre-treatment of powder compositions leads to a non-equilibrium structure with an increased number of defects in the crystal structure of metal particles. It is shown that in the process of MA, most of the aluminum component dissolves in the lattices of Ti and Nb, by interpenetration, forming solid solutions of (Ti, Al) and (Nb, Al) and various intermediate compounds. It was found that during the MA process, as a result of multiple effects of “cold welding” of metal particles and their fracture, layered composite particles of multifaceted shapes are formed, the size of which depends on the duration of the MA process.

The features of structure and phase formation of mechanically activated powder compositions of the Ti-Al-Nb system depending on the temperature of the subsequent IPS have been revealed. It has been established that changes in the phase composition of samples at the sintering temperature of 1000 °C are characterized by the processes of dissolution of titanium and niobium particles by the liquid phase on the basis of aluminum. The increase in sintering temperature up to 1200 °C leads to the expansion of the areas of homogeneous elemental composition and to the formation of phases with signs of lamellar and/or Vidmanstett distribution. The structure of titanium particles undergoes the first-priority dissolution at SPS with the formation of a homogeneous phase of AlTi₃ composition. The kinetics of dissolution of niobium grain material are characterized by the joint penetration of titanium and aluminum into the niobium structure with the formation of new phases. SPS at 1300 °C and a pressure of 1500·10⁵ Torr provides the formation of a continuous macrohomogeneous, mainly two-phase (B2 + O) structure with a high content of orthorhombic NbAlTi₂ and cubic B2 phases. Evaluation of the ratio of the main crystalline phases in the structure, which are identified as orthorhombic NbAlTi₂ and B2 phases, by the Rietveld method in the software “HighScore” (https://www.malvernpanalytical.com/en/products/category/software/x-ray-diffraction-software/highscore). gives values at the level of 60% and 40%.

Thus, SPS represents the most promising HEA synthesis method for hydrogen storage applications, combining key characteristics such as high density, microstructure stability, and improved sorption properties.

5. Hydrogen Sorption Properties of HEAs and Prospects of Their Application in Hydrogen Power Engineering

Hydrogen is known to be a useful energy carrier, and its production and storage have been the subject of intense research for many decades. Hydrogen remains the most valuable source of renewable and sustainable solutions to reduce the global consumption of fossil fuels. However, the main challenge of this environmentally friendly fuel is its storage, and therefore, it remains a subject of ongoing research [84,85,86]. In this context, HEAs have attracted attention not only due to their unique mechanical properties but also due to their prospects as hydrogen storage materials. One of the current approaches to control the thermodynamic stability of hydrides for hydrogen technologies is the use of multicomponent high-entropy hydrides, which are synthesized on the basis of HEAs. In the process of hydrogen absorption, the formation of hydride phases in the alloy matrix takes place. The process of the interaction of HEAs with hydrogen and the formation of high-entropic hydride is illustrated in Figure 11 [87]. The process is reversible and is determined by the hydrogen gas pressure (P) relative to the equilibrium pressure (P_eq). Here, the colored circles represent the atoms of the different elements that make up the high-entropy alloy. The structure shows that the atoms are distributed chaotically, which is characteristic of HEAs. After interaction with hydrogen, hydride is formed. Hydrogen atoms occupy inter-nodal positions in the lattice. At a pressure of P $>$ P_eq, hydrogen is absorbed by the alloy, forming hydride. At pressure P $<$ P_eq, hydrogen is released from the hydride, returning the structure to its original state. Thus, the key parameter of the system is the equilibrium pressure, which determines the conditions under which hydrogen absorption and desorption occurs at given temperatures. Optimization of this pressure in hydride HEAs allows efficient hydrogen storage and release at operating temperatures and pressures to be achieved, which makes them promising materials for hydrogen storage systems.

HEAs have been observed to possess high configurational entropy (ΔS_conf ≥ 1.5R), which leads to unique crystal transformations that create favorable conditions for phases suitable for hydrogen storage [88]. Studies have shown that single-phase HEAs with a BCC structure have significant potential for hydrogen storage [89,90,91]. Alloys with BCC and Laves phases show high reactivity with hydrogen at room temperature, which makes them promising materials for hydrogen technology [92,93]. The influence of HEAs favors the formation of C14-type single-phase structures, and the maximum hydrogen storage capacity is closely related to the enthalpy of hydrogen formation. Thus, multicomponent high-entropy alloys with BCC and Laves phases offer many possibilities for efficient hydrogen storage in practical applications. Importantly, each element in the alloy plays a different role, depending on its covalent binding energy to hydrogen atoms, making it critical to optimize the production steps of HEAs to improve their storage capacity. High-entropy alloys containing about 95 wt.% of the stable C14 Laves phase are considered particularly promising for use in hydrogen storage systems [94]. For example, Figure 12 shows the absorption–desorption curves for LaNiFeVMn alloy before and after activation. It is evident that after activation, the hydrogen absorption rate increases significantly compared to the original, inactivated material. Studies have also shown that multiphase alloys consisting of a mixture of solid solution with a BCC structure and intermetallides of the Laves type are characterized by high hydrogen capacity and fast kinetics of hydrogen absorption and release [95].

For practical applications, hydrogen desorption must occur in conditions close to ambient, i.e., temperatures in the range of 1 to 100 °C and pressures between 1 and 10 atm [96,97]. Kao et al. [98] reported the hydrogen storage properties of CoFeMnTi_xV_yZr_z alloy with a C14 Laves-type single-phase structure. They found that the CoFeMnTi₂VZr alloy exhibited a maximum hydrogen storage capacity of up to 1.8 wt.% at room temperature. In [99], they investigated the hydrogen storage properties in ZrTiVCrFeNi alloy synthesized by the laser cladding method (LMD). The hydrogen absorption and desorption curves for this alloy are shown in Figure 13A [99]. After synthesis, the material was annealed at 1000 °C for 24 h to improve its chemical composition. The main phase of the alloy is a C14 Laves structure with a small amount of α-Ti solid solution formed by laser synthesis and subsequent annealing. Hydrogen capacity measurements were carried out at pressures up to 100 bar and temperatures of 50 °C, with activation of the sample at 500 °C for 2 h. Figure 13A shows that the maximum hydrogen capacity was 1.81 wt.% after synthesis and 1.56 wt.% after annealing. However, the equilibrium pressure for desorption was too low for complete hydrogen release.

In comparison, the TiZrVCrFeNi alloy synthesized by vacuum arc melting shows reversible hydrogen storage at room temperature without prior activation (see Figure 13B) [99]. In this material, the hydrogen storage capacity was 1.6 wt.% in the first cycle and increased to 1.7 wt.% in the third cycle. In addition, the alloy showed complete hydrogen desorption with minimal hysteresis on the pressure–composition–temperature isotherms, which is also presented in Figure 13B.

The kinetics of hydrogen storage in HEAs are determined by many factors such as temperature, pressure, chemical composition, and microstructure of the alloys. Temperature and pressure play a key role in these processes. The thermodynamic characteristics of HEAs directly affect their efficiency and hydrogen storage capacity, such as thermodynamic stability, hydrogen absorption heat, entropy, free energy, and heat of reaction. The hydrogen storage capacities of the high-entropy Zr-Ti-Ni-Cr-Mn alloy were studied by Fukagawa et al. [100], and its composition pressure isotherms were measured at 323 K. While calculating the enthalpy of hydride formation (∆H) and entropy (∆S), the authors found that ∆H decreases with decreasing lattice parameter and concluded that ∆S corresponds to the degree of freedom arising from hydrogen desorption and absorption and has a constant value of 130 J/mol·K. All samples showed good reversibility. In addition, as the nickel content increased, the hydrogen absorption decreased and the hysteresis improved. Moore et al. [101] studied a high-entropy Ti-Zr-Nb-Hf-Ta alloy and, by means of calculations, found that vibrational entropy combined with conformational entropy accurately predicted the decomposition of hydride HEAs.

Table 1. Summary of registered BCC HEAs, their characteristics, and hydrogen storage capacity. ^a: the crystal lattice parameter.

Normalized Chemical Com- Position Ordered by Atomic Number	Nominal Composition	Synthesis and Processing	VEC	Alloy Phase	Hydride Phase	H2 Absorp. Capacity (wt%)	H/M	Rev. H2 Capacity (wt%/H/M)	H2 Absorp. Kinetics	Hydride Decompos. Onset/Peak Temperatures (K)	Ref.
Ti0.2V0.2Zr0.2Nb0.2Mo0.2	TiZrNbMoV	LENS (300 W)	4.8 a	BCC (major) NbNi4 (minor)	FCCTiHx	2.3 (323 K)–1.78 (after activation 673 K)	-	-	2.3 wt% in 1380 s (303 K, 8.5 MPa H2)	-	[72]
Ti0.2V0.2Zr0.2Nb0.2Mo0.2	TiZrNbMoV	LENS (1000 W three times)	4.8 a	BCC (major) Zr-rich (Ppt)	BCC (major) Zr-rich (Ppt)	0.59 (323 K) 0.61 (after activation 673 K)	-	-	0.59 wt% in 1380 s (303 K, 8.5 MPa H2)		[72]
Ti0.22V0.22Zr0.22Nb0.11Hf0.22	TiVZrHfNb0.5	Arc melting	4.33	BCC	BCT	-	1.82 (293 K)	-	-	~573/-	[102]
Ti0.22V0.22Zr0.11Nb0.22Hf0.22	TiVZr0.5HfNb	Arc melting	4.44	BCC	FCC	-	1.99 (293 K)	-	-	~593/-	[102]
Ti0.22V0.22Zr0.22Nb0.22Hf0.11	TiVZrHf0.5Nb	Arc melting	4.44	BCC	FCC	-	2.00 (293 K)	-	-	~593/-	[102]
Ti0.22V0.11Zr0.22Nb0.22Hf0.22	TiV0.5ZrHfNb	Arc melting	4.33	BCC	FCC	-	1.96 (293 K)	-	-	~573/-	[102]
Ti0.11V0.22Zr0.22Nb0.22Hf0.22	Ti0.5VZrHfNb	Arc melting	4.44	BCC	FCC	-	1.97 (273 K)	-	-	~573/-	[102]
Ti0.25V0.25Zr0.04Nb0.25Ta0.21	TiVZr0.15NbTa0.85	Arc melting	4.71 a	BCC	FCC (major) BCT (minor)	-	$~1.9$	-	-	-	[103]
Ti0.25V0.25Zr0.125Nb0.25Ta0.125	TiVZr0.50NbTa0.50	Arc melting	4.63 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.25V0.25Zr0.19Nb0.25Ta0.06	TiVZr0.74NbTa0.26	Arc melting	4.57 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.2Zr0.2Nb0.2Mo0.2Hf0.2	TiZrHfMoNb	Arc melting	4.6	BCC	FCC	1.18	-	-	-	~540/575	[104]
Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2	TiZrNbHfTa	Arc melting (homogenized by induction heating)	4.4	BCC	FCC	-	~2.0 (573 K)	-	-	$~593/~$648	[105]
Ti0.25V0.25Zr0.04Nb0.25Ta0.21	TiVZr0.15NbTa0.85	Arc melting	4.71 a	BCC	FCC (major) BCT (minor)	-	$~1.9$	-	-	-	[103]
Ti0.25V0.25Zr0.125Nb0.25Ta0.125	TiVZr0.50NbTa0.50	Arc melting	4.63 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.25V0.25Zr0.19Nb0.25Ta0.06	TiVZr0.74NbTa0.26	Arc melting	4.57 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.2Zr0.2Nb0.2Mo0.2Hf0.2	TiZrHfMoNb	Arc melting	4.6	BCC	FCC	1.18	-	-	-	$~$540/575	[104]
Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2	TiZrNbHfTa	Arc melting (homogenized by induction heating)	4.4	BCC	FCC	-	~2.0 (573 K)	-	-	$~$$593/~$648	[105]
Ti0.25V0.25Zr0.04Nb0.25Ta0.21	TiVZr0.15NbTa0.85	Arc melting	4.71 a	BCC	FCC (major) BCT (minor)	-	$~1.9$	-	-	-	[105]
Ti0.25V0.25Zr0.125Nb0.25Ta0.125	TiVZr0.50NbTa0.50	Arc melting	4.63 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.25V0.25Zr0.19Nb0.25Ta0.06	TiVZr0.74NbTa0.26	Arc melting	4.57 a	BCC	FCC (major) BCC (minor)	-	~1.9	-	-	-	[103]
Ti0.25V0.25Nb0.25Hf0.25	TiVHfNb	Arc melting	4.50	BCC	FCC	-	1.99 (293 K)	-	-	$~593/-$	[103]
MgVAlCrNi	MgVAlCrNi	High-energy ball milling	-	BCC	BCC	-	~0.15	-	-		[105]
Mg28V28Al19Cr19Ni6	Mg28V28Al19Cr19Ni6	High-energy ball milling	-	BCC	BCC	-	~0.15	-	-		[105]
Mg26V31Al31Cr6Ni6	Mg26V31Al31Cr6Ni6	High-energy ball milling	-	BCC	BCC	-	-	-	-	-	[105]

summarizes the reported BCC HEAs, their characterization, and hydrogen storage performance.

The cycling properties of HEAs during hydrogen absorption and desorption cycles are an important indicator of their performance as hydrogen storage materials. They mainly include stability and cycle life. According to Montero et al. [106], the hydrogen storage capacity of Ti_0.30V_0.25Zr_0.10Nb_0.25Ta_0.10 alloy gradually decreased after the first cycle and reached a stable state at the eighth cycle, when it was approximately 86% of the initial capacity (1.71 H/M; 2.19 wt.%). The maximum hydrogen uptake at room temperature was 2.0 H/M (2.5 wt.%). The structure of the alloy and its hydrides did not undergo significant phase segregation and oxidation despite a slight decrease in hydrogen storage capacity.

Along with this, in [107], we experimentally proved the high thermal stability of the sorption properties and structural-phase state of the two-phase alloy of the Ti-Al-Nb system at repeated (10 cycles) high-temperature (500–600 °C) processes of sorption/desorption by hydrogen. According to the results of the calculation of sorption parameters of the two-phase alloy of the Ti-Al-Nb system (Dh = 9.1·10⁻⁵ cm²/s), it is established that hydrogen diffuses through grain boundaries of the B2 phase and interlamellar boundaries of the O phase. The maximum content of hydrogen absorbed by the two-phase alloy (B2 + O) of the Ti-Al-Nb system is 1.91 wt.%. At the same time, it should be noted that the increase in the amount of thermocyclic loading in the hydrogen environment practically does not lead to visible changes in the morphology of the main phases. After repeated thermocyclic impacts in hydrogen medium, the predominantly two-phase structure of alloys undergoes transformation with the formation of a new structure. In the phase composition of samples, the formation of hydrides as independent phases was not revealed. Hydrogen absorption can be associated with the formation of an ordered or disordered solid solution on the basis of already existing crystalline O and B2 phases.

All the above studies to date demonstrate that HEAs with a BCC structure are very promising materials for hydrogen storage. Especially attractive is the hydrogen-to-metal ratio (H/M) of 2.5 reported by Salberg et al., which is significantly higher than that of conventional BCC alloys and transition metals. However, the lack of similar data in other HEAs underscores the need for further in-depth study of this phenomenon to assess its versatility and potential. Existing studies demonstrate that the hydrogen storage performance of BCC HEAs can be significantly optimized by varying their chemical composition (see Table 1). This allows the development of a broad class of materials capable of reversible hydrogen absorption at both room and elevated temperatures. The influence of synthesis and processing methods on microstructure and compositional homogeneity remains an important area for further research, as the elimination of dendrites and homogenization can significantly improve hydrogen storage efficiency.

The issues of desorption kinetics and the behavior of alloys under repeated absorption–desorption cycles, which still remain insufficiently studied, deserve special attention. A systematic study of these aspects, including hydrogenation mechanisms (one-step or two-step) and the influence of the valence electron parameter (VEC) on segregation and thermodynamic stability, may open new perspectives for the design of BCC HEAs with improved performance.

The extensive composition of BCC HEAs and the possibility of their adaptation to specific requirements make them extremely attractive candidates for the development of new hydrogen storage materials. However, further improvement of their properties requires an integrated approach including experimental studies, modeling, and development of new methods for the synthesis and destabilization of hydride phases.

6. Conclusions

One of the main challenges in the implementation of hydrogen technologies remains to ensure the efficient and safe storage of hydrogen. Metal hydrides have been explored for decades as promising materials for such applications, and in this context, HEAs open new horizons. This class of materials shows unique opportunities due to their chemical flexibility, ability to form different crystal structures (FCC, BCC, Laves, HCP), and customizability of hydrogen absorption properties. Studies have already demonstrated the high hydrogen absorption capacity of HEAs, including for BCC-structured systems that exhibit hydrogen-to-metal (H/M) ratios as high as 2.5. This far exceeds the capabilities of traditional metals and alloys. However, more experimental data and a better understanding of the mechanisms underlying the interaction of HEAs with hydrogen are required to confirm these capabilities.

Existing synthesis methods such as AM, BM, and SPS allow us to vary the microstructural characteristics of alloys, including the elimination of dendritic structures and increasing composition homogeneity. This, in turn, has a positive effect on the hydrogen absorption properties. Nevertheless, the influence of microstructural changes caused by different processing methods on hydrogen absorption remains poorly understood. Further optimization of synthesis and processing methods may be a key factor to maximize material performance. Thermodynamic modeling of HEA-H systems, integrated with methods for predicting phase formation and stability, can significantly accelerate the process of finding and developing new compositions. This will allow the use of high-performance computational methods to explore the vast space of possible alloys. A systematic study of the relationship between internal structures, compositions, and their effects on hydride properties could provide key answers to questions concerning the optimization of hydrogen storage materials. BCC HEAs stand out as the most promising materials for hydrogen storage due to their high configurational entropy, flexibility in composition, and excellent hydrogen absorption–desorption reversibility. Despite the confirmed high absorption rate, a systematic study of desorption processes and material behavior under repeated cycling remains underdeveloped.

HEAs thus represent a promising class of materials with the potential to transform hydrogen storage technologies. Their extensive chemical composition, ability to form different phases, and prospects for fine-tuning properties open up a wide range of applications. Integrating experimental studies with thermodynamic modeling and machine learning will accelerate the development of new materials, providing a solid foundation for the next phase of hydrogen energy development.