Phase Formation, Microstructural Evolution, and Surface Performance of High-Entropy Alloys for Electrocatalysis and Corrosion Resistance: A Review

Abstract

High-entropy alloys (HEAs) are a unique metallic alloy that was initially recognized for the possibility of stabilizing simple solid-solution phases through high configurational entropy. Research over the past two decades, however, has shown that their behavior is far more complex. Phase formation, microstructural evolution, and surface performance arise from the combined influence of composition, atomic interactions, processing history, and the surrounding environment. This paper reviews the connections between these aspects moving from the bulk alloy to the surface. The thermodynamic and empirical criteria utilized for prediction of phase formation and reasons behind ignoring the factors such as ordering, segregation, metastability, and processing defects are described. Further, the influence of casting, rapid solidification, coating deposition, and thin-film processing on the microstructure that will interact with catalytic or corrosive environment is reviewed. Electrocatalysis and corrosion resistance are considered as two strongly coupled surface phenomena rather than separate fields of application. Quantitative comparison of exemplary high-entropy alloy systems shows the influence of the alloying approach and surface development on the catalytic properties, surface reconstruction, selective dissolution, passive film formation, and localized corrosion. The potential of CALPHAD modeling, density functional theory, machine learning, and multi-objective optimization for a better alloy selection in the field of high-entropy alloys is reviewed as well. We identified that the success of HEA design is not only in choosing the right composition but rather in controlling the phases, defects, interfaces, and surface of the HEA.

1. Introduction

High-entropy alloys (HEAs) are metallic materials designed using multiple principal elements rather than one dominant base element [1,2]. This concept questions the conventional alloy-design approach, where a major element such as Fe, Ni, Al, Ti, or Cu forms the matrix and small additions are used for strengthening, corrosion resistance, or phase control [1,2,3,4]. In HEAs, multiple elements are introduced in relatively high concentrations, creating a large composition space and many possible local atomic environments [2,5,6]. Because of this compositional flexibility, HEAs are being studied for structural applications, high-temperature service, wear resistance, corrosion protection, catalysis, electrocatalysis, hydrogen production, and energy conversion [6,7,8,9,10,11,12,13,14,15,16,17]. This design contrast is illustrated in Figure 1.

The original HEA concept was strongly associated with the idea that high configurational entropy could stabilize simple solid-solution phases and suppress the formation of brittle intermetallic compounds [1,2]. However, later reviews and experimental studies showed that entropy alone cannot explain phase stability in multi-component alloys [3,4,18,19,20]. Instead, the phase formation is controlled by the interaction of the configurational entropy, the mixing enthalpy, the atomic size difference, the valence electron density, the electronegativity difference, the cooling rate, the diffusion kinetics, and the processing route [18,19,20,21,22].

This review explains HEA concepts as being more than the statement that “high entropy stabilizes solid solutions”. Here, the impact of the alloy chemistry, the processing, and the environment on the phase structure and performance are highlighted. The interrelation between phase formation, microstructure development, and surface performance of high-entropy alloys (HEAs) is provided. The criticality of this approach is that the essential functional HEA applications rely on the surfaces and interfaces rather than the bulk phases. In electrocatalysis, the actual catalytic action occurs at the catalyst–electrolyte interface, where the phenomena such as adsorption, charge transfer, oxidation, reduction, surface reconstruction, and elemental dissolution take place [9,10,11,12,13,14,15,16,17]. The resistance to corrosion in materials is determined by the passive film formation, localized dissolution, selective phase corrosion, grain boundary chemistry, porosity, and electrolyte interaction.

Thus, bulk HEAs, coating HEAs, and electrocatalytic HEAs are treated as related topics. Bulk systems are accounted for by phase-selection and microstructural principles. Coatings show how fast processing and interfaces change those states. Electrocatalytic systems show how local chemistry and reconstruction control activity and durability. All three are susceptible to corrosion, as coatings and catalysts need to be able to resist aggressive chemical or electrochemical environments [9,13,16,23,24,25,26,27,28]. This review is structured around the structure–processing–microstructure–surface–performance relationship that emerges and is summarized in Figure 2.

Thus, the questions addressed in this review are the following: (i) how is selection of bulk phases and defects at the surface achieved through composition and processing; (ii) how does the inherited surface structure affect electrocatalytic activity and reconstruction; (iii) how does the electrolyte, as well as the temperature, affect passive film chemistry, failure, and dissolution; and (iv) how can these two characteristics be optimized together rather than separately?

2. Fundamental Concepts in High-Entropy Alloy Design

2.1. Definition and Design Philosophy

The HEAs are generally considered to be alloys containing five or more principal elements in roughly equiatomic or high concentrations [1,2]. However, modern HEA research is also welcoming medium-entropy alloys, complex concentrated alloys, refractory HEAs, non-equiatomic HEAs, high-entropy oxides, high-entropy nitrides, and high-entropy nanomaterials [3,4,5,6,14,17,29]. Many important alloys do not follow the original equiatomic definition, hence the broader term “complex concentrated alloys” is often used to refer to alloys with multiple principal elements and chemically complex solid solutions [3,4].

The design concept of HEAs is different from conventional alloy design. Minor elements are added to conventional alloys to modify the dominant matrix. The large number of elements in HEAs strongly influences phase stability, lattice distortion, electronic structure, diffusion, and surface behavior [1,2,3,4,5]. This makes HEAs attractive for electrocatalysis, as the surface can contain many different atomic neighborhoods that may offer a wide distribution of adsorption energies of reaction intermediates [9,10,11,12,13,14,15,16,17]. It also makes HEAs attractive for corrosion protection because selected combinations of elements such as Cr, Ni, Co, Fe, Al, Ti, and Mo may promote passivation, strengthen the matrix, or improve resistance to localized attack [8,23,24,25,26,27,28].

2.2. Thermodynamic Basis of Phase Formation

The basic thermodynamics behind HEA engineering is typically described using Gibbs free energy of mixing:

$$\Delta G_{mix}=\Delta H_{mix}-T\Delta S_{mix}$$

(1)

where $\Delta G_{mix}$ is the Gibbs free energy of mixing,

$\Delta H_{mix}$ is the enthalpy of mixing,

T is temperature, and

$\Delta S_{mix}$ is configurational entropy.

• For an ideal random solution, the configurational entropy is expressed as:

  $$\Delta S_{mix}=-R{\displaystyle \sum _{i=1}^N}{C}_i\ln C_i$$

  (2)

  where R is the universal gas constant and $C_i$ is the atomic fraction of component i.

For an equiatomic alloy, configurational entropy increases as the number of elements increases since $\Delta S_{mix}$ decreases with increasing N in Equation (2). This reduction in $\Delta G_{mix}$ may help stabilize disordered solid-solution phases at high temperatures. Nonetheless, configurational entropy alone cannot ensure formation of one particular solid solution. Negative mixing enthalpies may lead to ordered intermetallic, whereas strongly positive enthalpies may induce segregation or even complete immiscibility [4,18,19]. Lattice distortion due to atomic size difference could be introduced, and valence electron concentration (VEC) can be employed to predict FCC, BCC, or a combination of both FCC and BCC structures for transition-metal HEAs [18,19,20,21,22]. Phase prediction thus requires the use of several descriptors instead of one thermodynamic descriptor.

Figure 3 introduces the main thermodynamic and empirical descriptors used to screen HEA phase formation and shows how these parameters collectively guide FCC, BCC, mixed-phase, or non-solid-solution tendencies.

2.3. Phase-Selection Parameters

Empirical and semi-empirical descriptors are useful for a first-pass screening of the very large composition space of HEAs. $\delta$ is the atomic size difference, an estimate of size misfit. $\Delta H_{mix}$ is the net enthalpic tendency towards mixing, segregation, or ordering. $\mathsf{\Omega }=T_m\Delta S_{mix}/|\Delta H_{mix}|$ compares the entropy term to the enthalpy term. VEC reflects an electronic preference for FCC or BCC-type structures. The most commonly cited numerical thresholds for bulk transition-metal HEAs are summarized in

Table 1. Empirical phase-selection thresholds for bulk HEA screening.

Descriptor	Criterion	Interpretations
$\delta$ and $\Delta H_{mix}$	$\delta \le 6.6\%$ and $-15\le \Delta H_{mix}\le 5$ kJ mol−1	Broad solid-solution formation window.
$\mathsf{\Omega }$ with $\delta$	$\mathsf{\Omega }\ge 1.1$ and $\delta \le 6.6\%$	Entropy-assisted solid-solution screening criterion.
VEC: FCC	$\mathrm{VEC}\ge 8.0$	FCC tendency in transition-metal HEAs.
VEC: BCC or mixed	$\mathrm{VEC}<6.87$ for BCC; $6.87\le \mathrm{VEC}<8.0$ for mixed FCC + BCC	Electronic-structure tendency rather than a proof of equilibrium phase constitution.

[19,20,21].

The screening parameters are based on the data and represent screening criteria and not any fundamental phase laws. They cannot distinguish disordered BCC from chemically ordered B2, cannot resolve short-range order, and cannot predict temperature-dependent decomposition. For example, the Cantor alloy may be a single FCC phase after conventional processing but can develop additional phases including sigma phase precipitation after long-term exposure at certain temperatures [30,31]. More generally, HEAs that have been shown to satisfy nominal solid-solution windows can still order or separate, as specific pair interactions, competing intermetallics, and thermal history are not captured by one scalar threshold [32,33,34]. Additional deviations from bulk screening assumptions are imposed by rapid solidification, thin-film growth, electrodeposition, substrate interaction, nanoscale stabilization, oxidation, reconstruction, and selective dissolution [7,9,11,23,35,36].

Therefore, phase selection parameters need to be correlated with CALPHAD, diffraction, microscopy, surface analysis, and electrochemical testing. This chain of evidence that distinguishes from the phase and surface state that actually occurs during processing and service is a useful compositional screen. Recent experiments performed on rapidly processed, mechanically alloyed, and multi-phase HEAs also indicate that nominal descriptor windows are not enough to uniquely identify the phases retained after processing [37,38,39,40,41,42]. Broader assessments show competing inter-metallics, ordering, and temperature-dependent transformations instead of an entropy-only rule [43,44].

2.4. The Four Core Effects and Their Limitations

Early studies by Yeh et al. and Cantor et al. showed that multi-component alloy systems could form relatively simple solid-solution phases despite their chemical complexity [1,2]. As illustrated in Figure 4, this observation led to the development of the so-called four core effects of high-entropy alloys: the high-entropy effect, severe lattice distortion effect, sluggish diffusion effect, and cocktail effect. These effects are useful for explaining why HEAs can exhibit unusual phase stability, mechanical properties, catalytic behavior, and corrosion resistance. However, modern studies have also shown that these effects should be treated as guiding concepts rather than universal rules [3,4,5].

More recent assessments also emphasize that the four core effects should not be interpreted as fixed laws, because local chemical ordering, precipitation, phase decomposition, and processing history can significantly modify the expected behavior of HEAs [31,32,33,34].

2.4.1. High-Entropy Effect

The most basic idea of HEAs is the high-entropy effect. Thus, the configurational entropy arising from mixing of several major elements can lower the Gibbs free energy of the alloy and favor simple FCC, BCC, or HCP solid solutions [1,3,5]. At sufficiently high temperature, this entropy contribution can aid in stabilizing a disordered solid solution [3,5,18,19].

The high-entropy effect is often invoked to explain the ability of chemically complex alloys to inhibit large-scale formation of brittle intermetallics. Zhang et al. proposed solid solution rules for multi-component alloys, and Yang et al. proposed criteria based on size difference, mixing enthalpy, and thermodynamic stability [19,20]. Further studies by Guo and Liu have shown that the competition between solid-solution and amorphous formation is related to the more general thermodynamic background [18]. Therefore, entropy is not a complete explanation of phase selection, merely one factor.

Limitation: The stability of phases is not controlled by entropy alone. The resulting configuration depends on the mixing enthalpy, the difference in atomic size, the valence electron density, electronegativity, the processing conditions, and the temperature [3,18,19,20,21]. In particular, VEC has been demonstrated to be very important for FCC and BCC tendencies in transition-metal HEAs [21]. Similar CALPHAD studies have shown that phase stability is a function of both temperature and composition, and an as-processed alloy which appears to be stable might decompose upon annealing [45,46,47].

2.4.2. Severe Lattice Distortion Effect

The strong lattice distortion effect is caused by the presence of dissimilar atoms of HEA alloys that have distinct atomic radius, bonding behavior, and electronic configurations. Upon occupation of a common crystal lattice by these dissimilar atoms, lattice distortions and strain field effects take place [3,5,48]. Such a lattice distortion effect may influence the movement of dislocations, diffusion, electronic configuration, adsorption effects, and chemical reactivity on the surface. For structural HEAs, this lattice distortion effect causes strengthening, while in electrocatalytic HEAs, the lattice distortion effect modifies the electronic environment of surface atoms [11,13,49].

In electrocatalysis, lattice distortion plays a particularly crucial role since it influences the strength of interaction between surface atoms and intermediates like H*, OH*, O*, and OOH*. In the case of traditional single-metal catalysts, adsorption centers are relatively homogeneous. When it comes to HEAs, every individual surface atom may have a unique atomic environment. As a result, there will be a wide range of values for the energy of adsorption that could raise the possibility of locating a nearly optimal active center for HER, OER, ORR, CO₂ reduction, and water-splitting reactions [9,10,13,15,17].

Limitation: Lattice distortion is not always severe or necessarily beneficial. Ideal random mixing is not always achieved, as local environments can show short-range order, clustering, segregation, and compositional variations [50,51,52]. First-principles calculations show that composition variation impacts the fault energies and local stability, while the short-range order in CrCoNi-type alloys impacts physical properties [50,52]. The real structures are therefore more complicated than a mere ”distorted random lattice”. However, the distortion can improve the adsorption at some positions, but it can also destabilize the catalyst surface or change the active site distribution [8,23,24,25]. Instead, it should be seen as a structural variable that can be tuned.

2.4.3. Sluggish Diffusion Effect

The sluggish diffusion effect predicts that atomic diffusion in HEAs is slower than in conventional alloys due to each atom experiencing a complicated and fluctuating local environment [3,5]. An atom moving in an HEA lattice encounters different neighbors and different local energy barriers, and diffusion may become more difficult. This effect has been used to explain the good thermal stability, slow grain growth, delayed phase transformation, and improved high-temperature performance of some HEAs [3,5,6].

For microstructural evolution, sluggish diffusion is often connected to the stability of fine microstructures, delayed precipitation, and resistance to coarsening. In coatings and thin films, slower diffusion may help retain protective microstructures during thermal or electrochemical exposure [7,35,36,53]. In corrosion-resistant HEAs, diffusion behavior can also influence passive-film formation and elemental redistribution at the surface. Elements such as Cr, Al, Ni, Co, Fe, and Mo can enrich or deplete at the surface depending on the electrolyte and applied potential, which then affects passivation and corrosion resistance [8,23,24,25].

Limitation: Diffusion in high-entropy alloys (HEAs) is not always slow and is affected by factors such as composition, crystal structure, temperature, defect density, grain boundaries, phase constitution, etc. [3,4,5]. Diffusion through grain boundaries, surfaces, and defects can be much faster than lattice diffusion [8,23,24]. Thus, surface atoms might be involved in reactions and/or restructuring during electrocatalysis and corrosion, even when bulk diffusion is slow. Not every alloy, transport pathway, or operational condition is characterized by sluggish diffusion.

2.4.4. Cocktail Effect

The cocktail effect is a term used to describe properties arising from the interaction of multiple primary constituents rather than simply averaging their individual behaviors [1,3,4]. Various constituents can impart passivation, catalytic activity, electrical conductivity, or structural integrity, leading to a composite property profile that is difficult to attain using traditional binary or ternary alloys.

This effect is highly relevant to this review because electrocatalysis and corrosion resistance require more than one property at the same time. A good HEA electrocatalyst should have high activity, good electrical conductivity, abundant active sites, surface stability, and resistance to dissolution. Recent HEA catalysis studies show that multi-element compositions can create diverse adsorption sites and tune the electronic structure of the surface [9,10,11,12,13,14,15,16,17]. For example, high-entropy alloy electrocatalysts have been explored for water splitting, oxygen evolution reaction, CO₂ reduction, and other electrochemical reactions because the combination of transition metals can produce multi-site catalytic behavior [9,10,11,12,13,14,15,16,17]. In corrosion studies, the cocktail effect can also support passivation when elements such as Cr, Al, Mo, or Ni contribute to protective oxide or hydroxide film formation [8,23,24,25].

Limitation: The cocktail effect is challenging to measure, as a perceived synergy may be influenced by one or two elements. Brittle intermetallics, phase segregation, galvanic microcells, surface degradation, or the deterioration of a stable passive layer may also be influenced by additional factors [3,8,18,23,24,25]. An initially potent catalyst may deteriorate due to the leaching of the active component or transformation into a less efficient oxide/hydroxide layer. Similarly, passivating agents are ineffective in preventing corrosion when microsegregation results in localized anodic and cathodic areas. The cocktail effect is most accurately interpreted as an interaction between composition, structure, and properties rather than a mere advantage of chemical intricacy.

3. Phase Formation and Microstructural Evolution in Bulk HEAs

3.1. Bulk Synthesis Routes

Bulk high-entropy alloys (HEAs) can be produced through arc melting, vacuum induction melting, directional solidification, mechanical alloying, spark plasma sintering, hot pressing, and powder metallurgy methods [1,2,3,5,6,54,55]. These processes are vital in the formation of the primary phase composition, grain microstructure, dendritic segregation, precipitation, and chemical homogeneity of the alloy. Arc melting and casting are widely used approaches due to their ability to produce bulk specimens via repetitive melting and flipping. Still, even when using repetitive melting, perfect chemical homogeneity is not achieved in some cases since segregation can happen due to the disparity in the melting point, atomic size, and chemical interaction [2,6,22].

Mechanical alloying and powder metallurgy yield HEAs with non-equilibrium or enhanced microstructures. Supersaturated solid solutions and nanocrystalline powders are obtained by repeated fracturing, cold welding, and plastic deformation. These can be built up by spark-plasma sintering or hot pressing with minimal grain growth [54,55]. These routes are particularly relevant to refractory HEAs and systems with different melting points, but oxygen contamination, residual porosity, and oxide inclusions may lead to poor surface stability and corrosion resistance.

Synthesis becomes a parameter of microstructural design, not simply a process of fabrication. The bulk alloy becomes a homogeneous solid solution, develops a dendritic segregation pattern, forms a multi-phase structure, or preserves a metastable phase depending on the selected trajectory. These features lead to segregation, secondary phases, interfaces, and porosity that then control passivation and electrochemical reconstruction.

3.2. Solidification, Dendritic Segregation, and Phase Separation

The non-uniform solidification of the constituting elements of high-entropy alloys can cause the formation of dendritic and interdendritic zones in the course of the bulk solidification process. The nominal equiatomic or near-equiatomic compositions of the HEAs are often characterized, but the actual local composition in the solidified microstructure may not be the desired bulk composition. This is due to the different melting points, atomic radii, chemical affinities, diffusivities, and partitioning behavior during cooling of the alloying elements [2,6,22]. During solidification of the liquid alloy, some elements may tend to concentrate preferentially in the dendritic areas, and some elements may tend to concentrate in the interdendritic areas. Thus, the alloy has an equiatomic overall composition, but the local compositional variations could be large within the microstructure.

Dendritic segregation is important in view of its effect on the phase stability, hardness, ductility, corrosion resistance, and catalytic efficiency of the material. Segregation could lead to the formation of localized regions having varying degrees of hardness, ductility, or brittleness. For corrosion application, chemically different dendrites and interdendritic regions could have different reaction rates in the electrolyte solution and would result in galvanic interaction or selective dissolution. For electrocatalysis, local compositional difference could play a role in the spatial location of the active site and influence the reaction rate and adsorption characteristics. Thus, in addition to the solidification behavior, the dendritic segregation should be considered as part of the microstructure of HEAs affecting the surface efficiency of the alloys. Based on studies on the Cantor-type and other multi-component alloy systems, it has been found that even FCC HEAs could change their microstructure due to precipitation and local ordering upon thermal treatment [30,31,56].

Al is present in Al-containing HEAs, which has been proved to promote the formation of BCC or ordered B2 phase due to its large effect on lattice structure, atom size difference, and chemical ordering [21,22]. This is striking, as it shows how the addition of a single component can change the phase equilibrium of a multi-component alloy. For example, the addition of aluminium disturbs the FCC structure and facilitates the formation of BCC/B2-type structure, which may increase the strength and hardness but at the expense of ductility if the ordered phase is present in excess or irregularly dispersed. Therefore, Al-containing HEAs represent ideal examples to understand the role of composition on phase selection and the role of phase selection on final properties.

CoCrFeMnNi Cantor-type high-entropy alloys are generally referred to as FCC solid solutions due to their relatively stable FCC configurations and excellent ductility [2]. But even these apparently simple systems can show short-range ordering and chemical variations that affect deformation and phase stability [50,51,52,57]. The single phase label is insufficient to describe the architecture at the atomic scale. XRD indicates a single predominant phase, but nanoscale organization or localized variability can influence mechanical properties, corrosion behavior, and surface reactivity.

Refractory HEAs commonly show BCC phases because refractory elements such as Nb, Mo, Ta, W, V, and Ti tend to favor BCC crystal structures [6,48,58,59]. These alloys are important because they demonstrate how elemental selection can guide phase formation toward high-strength and high-temperature-stable microstructures. However, refractory HEAs may also experience segregation, density-related limitations, processing difficulty, and brittle phase formation depending on composition and thermal history. Thus, the solidification behavior of bulk HEAs provides a foundation for understanding why phase formation cannot be predicted from configurational entropy alone. Instead, the observed microstructure results from the combined effects of thermodynamics, kinetics, elemental partitioning, cooling rate, and processing route.

3.3. FCC, BCC, B2, Sigma, and Multi-Phase Structures

Many HEAs form FCC, BCC, B2, sigma, or mixed FCC/BCC structures depending on the alloy chemistry, valence electron concentration, atomic-size mismatch, mixing enthalpy, and processing route [2,6,21,22]. These phases are usually identified using X-ray diffraction because each crystal structure produces characteristic diffraction peaks. For example, the XRD patterns shown in Figure 5 illustrate the phase evolution in Al_xCrCuFeNi₂ alloys, where FCC and BCC reflections appear as the alloy composition changes. This type of result is important because it shows that changing the concentration of one element can shift the balance between FCC and BCC phase formation, even when the alloy still belongs to the same general HEA family.

The observation of both FCC and BCC peaks in these systems suggests that the formation of HEA phases is not simply dictated by configurational entropy. Although the increased entropy may stabilize the disordered solid solutions, the electronic and thermodynamic properties of the alloying elements also affect the final phase composition. Especially, the introduction of aluminum is associated with the improvement of BCC or ordered B2 structures because of the effects of aluminum on lattice distortion, atomic size difference, and chemical ordering [21,22]. On this basis, XRD evidence such as Figure 5 provides a practical way to show how phase selection varies with composition.

While FCC HEAs tend to have relatively higher ductility and plastic deformability, BCC HEAs can exhibit extremely high strength and hardening, although often with lower ductility at room temperature [6,51,52]. This difference is important because phase structure influences not only mechanical behavior but also corrosion resistance and surface performance. FCC structures may provide better toughness and damage tolerance, whereas BCC or ordered B2 phases may improve hardness and wear resistance. However, ordered B2 phases can also lead to embrittlement if they become excessive or poorly distributed [21,22]. Similarly, sigma and other topologically close-packed phases may degrade mechanical properties and corrosion resistance if they form in large amounts, because they may consume beneficial elements from the matrix and introduce brittle or chemically unstable regions [3,18,46].

The broader meaning of this phase behavior is illustrated in Figure 6. George et al. showed that when multiple elements are mixed, the possible outcomes are not limited to one random solid solution; the alloy may undergo spinodal decomposition, form multiple solid solutions, form a single solid solution, or develop an intermetallic phase within a solid-solution matrix [4]. This is directly relevant to HEAs because the term “high entropy” can sometimes give the impression that a single-phase solid solution should always form. In reality, different elements may prefer to mix, segregate, order, or decompose depending on the free-energy landscape, mixing enthalpy, atomic-size difference, electronic structure, temperature, and cooling rate.

Consequently, the assumption that single-phase materials are always better than multi-phase materials, or vice versa, does not hold universally. The performance of an HEA depends not only on how many phases form but also on how those phases are distributed, whether they are chemically stable, and how they interact with the operating environment. A deliberately designed multi-phase microstructure may improve strength, wear resistance, or catalytic performance by introducing hard phases, heterointerfaces, and chemically diverse surface sites. In contrast, an uncontrolled multi-phase structure may increase brittleness, reduce corrosion resistance, or generate micro-galvanic coupling between phases with different electrochemical potentials [8,23,24,25,26,28].

This clarification is of great importance for electrocatalysis and corrosion resistance. While a bulk FCC, BCC, or hybrid FCC/BCC phase may be stable as synthesized, the surface is prone to oxidation, hydroxylation, segregation, dissolution, or reconstruction in electrochemical conditions [10,11,13,23,24]. Thus, phase formation cannot be treated as a crystallographic problem only. It is to be considered as part of a larger context of microstructure-surface-performance relationship where phase composition affects the mechanical integrity, corrosion behavior, catalytic efficiency, and durability.

3.4. Bulk HEAs as a Foundation for Surface Studies

Bulk HEAs provide the thermodynamic and microstructural foundation for understanding surface behavior in coatings, thin films, and electrocatalysts. They help clarify how alloy composition, solidification history, phase constitution, segregation, and thermal stability influence the final material structure. However, bulk phase stability should not be treated as direct evidence of surface stability. In corrosion and electrocatalysis, the exposed surface may oxidize, dissolve, segregate, reconstruct, or form oxide/hydroxide-rich layers during operation [8,10,11,13,23,24].

Therefore, bulk HEAs are more appropriate for use as a reference material than as a full indicator of surface behavior. Although the bulk HEA of FCC, BCC, or multi-phase alloy might be stable after preparation, the surface chemistry of the material can be highly different in corrosive or electrochemical conditions. The difference here is very critical, as it is the surface area on which the electrolyte reaction takes place that controls the corrosion resistance and electrocatalytic behavior of the material. Hence, it is very important to study the bulk microstructure of HEAs with surface analysis and electrochemistry.

4. HEA Coatings, Thin Films, and Surface Engineering

4.1. Importance of HEA Coatings

HEA coatings are highly applicable in cases where surface protection is required but not the whole component formed from expensive or chemically complex alloys. With HEAs coated on the surface only, they provide an effective means to increase the properties of the alloy such as wear resistance, corrosion resistance, oxidation resistance, and catalytic activity [7,35,36,45,53,60,61,62]. This approach is particularly relevant in harsh environments, where degradation often begins at the surface rather than in the bulk.

HEA surface coatings play a very crucial role in electrocatalysis, wherein the active catalyst layer can act as a surface coating. Film coatings such as electrodeposited film, sputtered film, thermal spray coatings, and chemical deposits can alter surface chemistry and create more active sites for electrochemical reactions [10,11,12,13,14,15,16,17,36,45,49,53,61,62]. Thus, HEA coatings should be considered not only as protective layers but also as surface-engineering platforms for controlling corrosion resistance, catalytic performance, and long-term durability. The principal surface-engineering techniques considered here are shown in Figure 7.

The simplified diagram shows only the basic deposition geometry. The relationships between process and microstructure are included here to avoid cluttering the picture. Laser cladding is a process consisting of localized melting, rapid solidification, metallurgical bonding, and possible dilution with the substrate. In thermal spraying, the lamellar coating is formed by the impact of splats, and porosity, oxide formation, and inter-splat adhesion are of great importance. Magnetron sputtering/PVD allows for a careful thickness control and usually results in dense, smooth, nanocrystalline, or metastable coatings. Electrodeposition employs an external current, which controls the thickness of the coating, grain size, and composition through bath chemistry and electrical parameters. Electroless deposition is an auto-catalytic process where there is no use of any external electrical current, and it is useful due to its property to give a conformal coating to complex shapes. The structure of the deposit and its adhesion depend a lot on the nature of the solution and the surface activation [7,10,36,53,61,63].

4.2. Laser Cladding and Rapid Solidification

Laser cladding is studied extensively for its localized melting, rapid solidification, and metallurgical bonding to the substrate [35,36,60,61]. A laser then melts the alloy powder (injected or pre-placed) on a thin layer of substrate. Hence, the development of the coatings depends on the composition and on the process parameters such as laser power, scanning velocity, powder feed rate, overlap, shielding gas, and dilution [35,36,61]. They affect the lifetime of the melt pool, the cooling speed, the shape of the heat-affected zone, the residual stress, and the probabilities of porosity, cracks, unmelted particles, and oxidation. According to the recent studies of laser cladding, heat treatment, reinforcement size, dilution, and processing conditions can affect the phase composition, defect density, wear resistance, and corrosion property of laser clad [64,65,66,67,68].

Fast solidification during laser cladding is a crucial factor that influences grain refinement, inhibition of diffusion, increased solubility of alloys, stabilization of metastable phases like FCC, BCC, B2, or a combination of different crystal lattices [45,53,61]. At the same time, the strong thermal gradients in laser cladding can cause significant residual stresses and crack generation in materials. Additionally, substrate dilution can shift the HEA coating from the equiatomic or near-equimolar composition affecting the stability of alloy phases, dendritic segregation, hardness, and corrosion [35,61,62]. Thus, laser cladding should be considered a coupled composition–processing problem rather than only a deposition method.

Laser-cladded HEA coatings provide high wear resistance and oxidation and corrosion resistance for surface functionalities when the coating is dense, well adhered, and promoted by passivating elements such as Cr, Al, Ni, or Mo [26,28,53,69] However, high configurational entropy alone does not necessarily lead to corrosion resistance. Porosity, fissures, oxide contaminants, dendritic/interdendritic galvanic interactions, and substrate dilution can still lead to corrosion [8,23,24,62]. The objective of laser-cladded HEA coatings is to possess a high coating density and uniform surface chemistry.

4.3. Thermal Spray Coatings

The methods of thermal spraying, including high-velocity oxy-fuel (HVOF), high-velocity air-fuel (HVAF), plasma spraying, and cold spraying, are extensively employed for deposition of HEA-based coatings. Using such techniques enables protective and functional layers to be deposited on engineering materials without producing an expensive multi-element alloy throughout their volume [7,35,36,62]. Such a methodology is highly desirable in cases of corrosion resistance, wear resistance, and other forms of surface engineering, as degradation usually starts from the surface first. Typically, the process of thermal spraying includes heating, accelerating, and depositing the feedstock material onto a substrate. However, the degree of particles’ heating, melting, oxidation, deformation, and cohesion depends greatly on the type of thermal spraying technique utilized and the process parameters. Thus, while HVOF uses high temperature and high particle velocity, producing rather dense coatings, partial oxidation during deposition is possible. On the contrary, the use of low temperatures and high particle velocity in HVAF could minimize the formation of oxides while increasing particle deformation and density [7,35,62]. As a result, some specific characteristics of HEA thermal-sprayed coatings, such as splat layers, interlamellar boundaries, oxide inclusions, unmelted or partially melted particles, residual porosity, and rapid solidification phases, should be expected [7,35,53].

Thermal-sprayed microstructure is very important because it influences surface integrity, mechanical protection, and corrosion properties of coatings. Different process parameters including feedstock powder preparation, particle size distribution, spraying temperature, particle velocity, the atmosphere used for spraying, spray stand-off distance, and substrate state can affect coating phase formation, porosity, oxide content, residual stresses, adhesion strength, and surface roughness [7,35,36,60]. The importance of HVOF and HVAF spraying in cases where corrosion and wear resistance are required lies in the fact that relatively dense HEA coatings can be produced, which will have solid interparticle bonding, oxide-controlled content, and other beneficial properties [7,35,62,69]. Nevertheless, in the case of HEA thermal spraying, the corrosion resistance of the coating does not depend on the nominal HEA composition alone. Although the selected HEA includes corrosion-resistant materials, such as chromium, aluminum, nickel, cobalt, iron, molybdenum, etc., its corrosion resistance will be influenced by defects in the microstructure of the coating, which can promote electrolyte penetration. These defects include pores, cracks, oxide-rich splat interfaces, lamellae, and phase separation sites. As soon as electrolyte reaches such areas, localized dissolution, failure of the protective film, or micro-galvanic effects can occur because there will be at least two different chemical phases [8,23,24,62].

The AlCrFeCoNi coatings studied by Löbel et al. clearly show the correlation between processing, microstructure and corrosion. As seen in Figure 8, the HVOF and HVAF coatings have the same nominal HEA composition but different cross-sectional microstructures. The analysis shows that the spraying method can influence the density of the coating, lamellae structure, arrangement of oxides, and the degree of porosity. These distinctions are important because a more uniform and dense coating may prevent the movement of corrosive agents through the coating and reduce access of the electrolyte to the interior of the coating and the substrate. Conversely, oxide dense surfaces, interconnected voids, and layered imperfections may interfere with the continuity of the passive film and promote localized corrosion. Thus, thermal spray processing should be regarded as a microstructural design strategy for controlling the corrosion resistance, surface life, and long-term performance of HEA coatings rather than simply as a coating application method.

4.4. Magnetron Sputtering, PVD (Physical Vapor Deposition), and Thin Films

The composition, thickness, deposition rate, substrate temperature, and environment of HEA films can be controlled by magnetron sputtering and the PVD methods [36,53,70]. The techniques can provide metastable solid solutions, nanocrystalline or amorphous structures, and textured films depending on the power, chamber pressure, substrate temperature, gas composition, and post-deposition annealing [36,45,53]. Thin-film repositories are therefore ideal for systematic investigations of compositional and dimensional effects. The latest films of high-entropy alloys prepared by co-sputtering and magnetron sputtering deposition demonstrated that deposition parameters can affect the columnar growth, metastability, mechanical properties, biocompatibility, and electrochemical properties [71,72,73,74,75]. The characteristics of films can deviate from bulk behavior due to surface diffusion, nucleation, interaction with the substrate, and the build up of internal stresses [36,45,53]. Thus, equilibrium thermodynamics is not enough: a composition that yields a stable bulk phase can produce a metastable or amorphous film if deposited rapidly. In addition, the substrate affects texture, grain size, adhesion, and stress.

This feature proves to be particularly advantageous for electrocatalysis, where sputtered composition gradients and even controlled surfaces enable systematic studies of composition-activity associations [9,13,15,70]. Sputtering and PVD are coating techniques and experimental tools to study the correlation between phase composition and adsorption, surface chemistry, catalytic activity, and corrosion.

4.5. Electrodeposition and Electroless Deposition

Electrodeposition and electroless deposition are low-cost scalable techniques to deposit the catalyst and protective coatings on complex geometries [10,11,36,63,76]. Electrodeposition requires a conductive substrate, whereas electroless deposition can occur on activated insulators. Both approaches are capable of producing thin, adherent coatings without requiring the high temperatures of casting, thermal spraying, or laser cladding and are compatible with nickel foam, carbon cloth, carbon paper, and metal-foil electrodes [10,11,63].

The production of multi-component HEA coatings is considerably more complex than the application of binary or ternary alloys. The reduction potential, complexation and hydrolytic behavior, and deposition rate constant differ for each metal ion. Therefore, the final composition of the deposited coating may differ from the original composition depending on the exact control of the deposition parameters. Such optimization is necessary for obtaining the target coating composition, microstructure, phase composition, and surface roughness [13,63,77]. Electroless deposition also depends on reducing agents, as the metal ions are reduced chemically and not forced by an external electrical current.

Electrodeposition provides several advantages in the production of porous or nanostructured HEA films with a large electrochemically active surface area for electrocatalytic applications. The pores increase the electrolyte’s accessibility, expose more active sites, speed up charge transfer reactions, and allow the bubbles to escape during electrolysis [10,11,63]. However, it is important to distinguish between the catalytic role of the support and the role of the applied layer. Nickel foams could enhance the oxygen evolution activity in alkaline solutions, leading to misleading activity values when substrate controls, blanks, mass loading normalization, and electrochemical active surface area adjustments are not taken into account [11,63,78].

Thus, active HEA layers with controlled thickness, architecture, and available surface area can be produced by electrochemical and electroless deposition. The main limitations are the inconsistent incorporation of different elements and the difficulty of discriminating the intrinsic activity of the coating from the response of the substrate. It is critical to study in detail the composition, substrate controls, durability assessments, and post-reaction surface characterization.

5. High-Entropy Alloys for Electrocatalysis

5.1. General Advantages of HEAs in Electrocatalysis

HEAs are attractive electrocatalysts because they provide many chemically distinct surface sites [10,11,12,13,14,15,16,17,49]. In monometallic catalysts, most surface atoms have similar adsorption properties. In HEAs, different neighboring atoms create a distribution of adsorption energies, which may allow some sites to approach optimal binding strength for intermediates [12,14,49]. This is important for reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction, nitrogen reduction, and alcohol oxidation [12,13,14,15,16,17,49].

The high-dimensional composition space of HEAs also allows the combination of elements that perform different catalytic functions. One element may promote water dissociation, another may tune hydrogen adsorption, another may improve conductivity, and another may improve corrosion resistance [10,11,12,13,15,16,17,49]. This is why transition-metal HEAs containing Fe, Co, Ni, Cu, Mo, Mn, Cr, V, Zn, and related elements are being explored as noble-metal-free or noble-metal-lean catalysts for water splitting [10,11,13,16,17].

More recent electrocatalysis studies further show that the catalytic advantage of HEAs comes from the formation of many different local adsorption environments on one surface. Instead of having one uniform active site, HEA surfaces may contain many possible arrangements of neighboring atoms, allowing adsorption energies and reaction pathways to be tuned across a broad distribution of surface sites [9,10,12,13,15,17,63,79,80,81,82].

5.2. HER and OER Mechanisms

In hydrogen evolution reactions (HERs), the main elementary steps are the Volmer, Heyrovsky, and Tafel reactions. In acidic media, HER mainly involves proton adsorption and reduction:

Volmer Step

$$\mathrm{M}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\iff \mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}$$

(3)

Heyrovsky Step

$$\mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\iff \mathrm{M}+{\mathrm{H}}_2$$

(4)

Tafel Step

$$2\mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}\iff {\mathrm{H}}_2+2\mathrm{M}$$

(5)

whereas in alkaline media, the reaction also requires water dissociation before hydrogen adsorption can occur [10,11,63].

Volmer Step

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}+\mathrm{M}\iff \mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}+\mathrm{O}{\mathrm{H}}^{-}$$

(6)

Heyrovsky Step

$$\mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\iff {\mathrm{H}}_2+\mathrm{O}{\mathrm{H}}^{-}+\mathrm{M}$$

(7)

Tafel Step

$$2\mathrm{M}-{\mathrm{H}}_{\mathrm{ads}}\iff {\mathrm{H}}_2+2\mathrm{M}$$

(8)

where M is an active site.

This additional water-dissociation step makes alkaline HER more kinetically demanding. An ideal HER catalyst should bind hydrogen neither too strongly nor too weakly; very strong H adsorption can block active sites, while very weak adsorption limits hydrogen formation. HEAs are attractive for HER because their multi-component surfaces create different local atomic environments, allowing hydrogen adsorption energy to be tuned through elemental interaction, lattice distortion, and electronic structure modification [12,14,49].

The oxygen evolution reaction (OER) is more complex due to the involvement of multiple proton-coupled electron transfer steps and oxygen-containing intermediates such as OH*, O*, and OOH*. The OER is generally slower than HER and needs a higher overpotential because of the four-electron transfer and O–O bond formation [10,11,13,16]. OER is generally realized via the formation of surface species resembling transition metal oxyhydroxides in alkaline media, especially on Fe, Co, and Ni-based catalysts. Thus, redox-active transition-metal-containing HEAs could offer abundant active sites and tunable surface chemistry for OER. Elements like Fe, Co, Ni, Mn, and Mo can change roles by modifying the electronic configuration, the intensity of the adsorption, the conductivity, and the surface reconfiguration dynamics [10,11,13,17].

One should not assume that the catalytic surface of an HEA will be the same as the as-prepared alloy. In operation, oxidation, hydroxylation, segregation, dissolution, and reconstruction may occur, especially at OER potentials [11,13,16,63]. Thus, the active phase could be a reconstituted layer rather than the true metal surface. In interpreting HER and OER results, one should consider durability, post-reaction composition, and corrosion resistance, with sustained activity rather than initial overpotential being the relevant metric.

The analysis is consistent with the basic principles of electrocatalysis. The performance of water-splitting catalysts is intimately connected to the adsorption-energy descriptors, reaction overpotentials, and kinetic parameters such as Tafel slopes [83,84,85,86,87,88,89]. Thus, the evaluation of HEA electrocatalysts should not only consider the multi-element composition but also the way elements influence the adsorption energies, charge transfer, reaction kinetics, and surface stability during HER and OER processes.

5.3. HEAs for Water Splitting

Water-splitting processes require efficient catalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Conventional noble-metal catalysts such as Pt for HER and IrO₂/RuO₂ for OER show high catalytic activity, but their large-scale use is limited by high cost, scarcity, and possible instability under some industrially relevant operating conditions [10,11,78]. High-entropy alloys are being considered as promising alternatives because they provide broad compositional flexibility, multiple active sites, and tunable electronic structures while potentially reducing dependence on expensive noble metals [10,11,16].

Recent studies on noble-metal-free and noble-metal-lean HEA electrocatalysts suggest that the manipulation of composition, morphology, nanostructuring, and active-site design plays a crucial role in water-splitting efficiency [10,11,16,63]. Transition metals such as Fe, Co, Ni, Mn, Cu, and Mo are commonly used in different functions for water dissociation, hydrogen adsorption, oxygen-intermediate adsorption, electronic conductivity, and surface reconstruction. Therefore, HEAs can provide several adsorption environments rather than a single kind of catalytic site, which is helpful to the balance of different requirements for HER and OER [12,13,14]. Recent OER research has explored porous alloys, HEA/MoC heterostructures, vacancy-engineered FeCoNiCrV, dual-phase PtNiCuMnMo, and ultrathin two-dimensional HEAs, showcasing a range of strategies to tailor the binding of oxygen intermediates and the active surface area [90,91,92,93,94,95]. Recent HER studies also have used carbon supports, Ru single atoms, ultrathin nanowires, and patterned electrodes to separate composition effects from morphology and conductivity [96,97,98,99]. These results are complementary to kinetically controlled nanoparticle synthesis, liquid-metal routes, Mo-coordinated alloys, and sputtered-film electrodes [75,100,101,102,103,104].

Bifunctional HEA materials are particularly appealing, as the same catalytic system can catalyze both HER and OER in a full water-splitting device. However, HER and OER demand different surface chemistries, and a good catalyst for one reaction may not be stable in the potential window required for the other. OER is particularly challenging, as high anodic potentials can lead to oxidation, oxyhydroxide formation, leaching of element(s), or surface reconstruction [11,13,63]. Thus, the overpotential and Tafel slope alone should not be the only parameters for the assessment of bifunctional HEAs. Other considerations include long-term resilience, current density, Faradaic efficiency, catalyst loading, electrolyte condition, substrate effect, and post-reaction surface analysis [10,11,16,78].

Recent assessments and experimental studies on water splitting suggest that HEA-based catalysts can be prepared as alloys, oxides, oxyhydroxides, nanoporous structures, or reformed surface layers depending on the synthesis method and operating conditions [10,11,16,77,78,86,105,106,107,108]. The present investigations demonstrate that the efficiency of water splitting depends on the initial composition of the HEA and the active surface formed during the electrochemical evaluation.

One example of bifunctional catalysts is the H-FeCoNiCuMo catalyst reported by Huang et al., but this alone cannot establish a general HEA performance trend [109]. It is therefore interpreted here together with Fe–Co–Ni, Mo/Cr-containing, and noble-metal-containing systems in

Table 2. Comparison of representative HEA electrocatalysts for HER and OER. n.r. denotes not reported in the particular cited study.

Catalyst	Rxn.	Electrolyte	$\mathit{\eta }$ (mV)	Tafel (mV dec−1)	Area Metric	Stability	Reference
MnFeCoNi	OER	1 M KOH	302 at 10	83.7	n.r.	>20 h	[110]
np-AlNiCoFeMo	OER	1 M KOH	240 at 10	46	nanoporous; ECSA n.r.	cycling reported	[107]
np-AlNiCoIrMo	OER	0.5 M H2SO4	233 at 10; 255 at 20	55.2	nanoporous; ECSA n.r.	7000 cycles at 60 mA cm−2	[108]
H-FeCoNiCuMo	HER	0.5 M H2SO4/1 M KOH	9 at 10	n.r.	hierarchical porosity; ECSA n.r.	long-term electrolysis reported	[109]
np-NiCoFeMoMn	HER/OER	1 M KOH	150/350 at 1000	29 (HER)	self-supported nanoporous electrode	>375 h	[111]
CrMnFeCoNi	OER	1 M KOH	265 at 10	37.9	ECSA-normalized data reported	1200 Zn–air cycles at 5 mA cm−2	[112]
FeCoNiCuTi	HER/OER	1 M KOH	64.9/175 at 10	36.81/44.31	$C_{dl}=34.1$ mF cm−2	cycling and chronopotentiometry	[113]
B-NiCoCuMoMn	OER	1 M KOH	200 at 10	110	ECSA n.r.	48 h	[114]
PtPdFeCoNi	OER	0.1 M KOH	306 at 10	n.r.	nanoparticle ECSA reported	3000 cycles	[115]

and Figure 9. The comparison shows that the low overpotential can be achieved from various combinations of intrinsic kinetics, porosity, reconstruction, and catalyst loading, so a defensible comparison requires the reaction, electrolyte, current density, Tafel slope, durability, and surface-area information to be read together.

Table 2 also highlights a reporting inconsistency: Many important studies are missing ECSA or double-layer capacitance. Stability protocols range from a few hours to cycling or device testing. Therefore, the values only assess performance in similar situations. They do not provide an overall evaluation of compositions. The self-supported np-NiCoFeMoMn electrode is highly relevant at industrially relevant current densities, since it was able to sustain both half-reactions at 1000 mA cm⁻², while many other studies only report at 10 mA cm⁻² [111]. Future reports should include currents normalized to geometry, mass, and ECSA, corrections for uncompensated resistance, catalyst loading, Faradaic efficiency, and the flux of dissolved metals.

5.4. Surface Reconstruction During Electrocatalysis

A major challenge of HEA electrocatalysis is that the catalytic surface can deviate from the initially prepared alloy surface during operation. Prior to testing, an HEA catalyst can be prepared as a metallic solid solution, nanostructured alloy, thin film, or electrodeposited layer. However, in HER or OER processes, the surface is exposed to applied voltage, electrolyte ions, adsorbed intermediates, gas evolution, and local pH gradients. Such conditions may cause oxidation, hydroxylation, elemental segregation, selective dissolution, surface roughening, amorphization, or surface reconstruction [9,10,11,13,16]. Recent studies also show evidence for Mo-dependent reconstruction, targeted phase dissolution, surface oxide formation induced by dealloying, and reconstruction of bulk or electrodeposited high-entropy alloy electrodes [116,117,118,119,120].

This distinction matters for OER because high oxidizing potentials can transform transition-metal HEA surfaces into FeOOH-, CoOOH-, NiOOH-, or mixed oxyhydroxide-rich layers. These reconstructed layers may be the active OER phase rather than the pristine metal surface [11,13,63]. Although the surfaces of HER may remain metallic in nature, processes such as adsorption, local coordination, elemental leaching, and surface relaxation can still take place, owing to the contribution of multiple elements to water dissociation, hydrogen binding, and charge transfer [10,12,14,109].

5.4.1. Kinetic Stages and Dissolution Sequence

Reconstruction is better understood as a chronological development, not as a binary change of “before/after”. Initial activation or induction process: Wetting, double-layer charging, adsorption, and the initial oxidation occurrences. In a transitory state: selective dissolution, outward cation movement, incorporation of oxygen/hydroxide, surface roughening, and nucleation of an amorphous oxyhydroxide layer. Finally, a dynamic equilibrium can be achieved between oxide formation, dissolution, redeposition, and catalytic turnover. The temporal evolution of each regime depends on the historical potential, scanning velocity, electrolyte composition, particle sizes, and phase or defect arrangement of the precursor; ex situ observations do not properly represent the rates [115,121,122,123].

The makeup and potential also affect elemental order. Aluminum, manganese, and copper are readily dissolved. Chromium forms a protective oxide at potentials below transpassivity but dissolves at high anodic potentials. Nickel, cobalt, and iron are incorporated and oxidized into the active oxyhydroxide framework. Molybdenum may alter the electronic configuration or dissolve as a soluble molybdate. These are trends rather than a universal sequence of steps, as local coordination, short-range order, passivation, complexation, and pH affect the dissolution sequence. Therefore, the combination of time-resolved ICP-MS with operando XAS/Raman is essential to elucidate the true sequence of events for each HEA.

The importance of order is illustrated by quantitative examples. The FeCoNiCrMn thin film exhibits an OER overpotential of 263 mV at 10 mA cm⁻² and a Tafel slope of 50.9 mV dec⁻¹ after 48 h of chloride conditioning and 400 cyclic-voltammetry scans. The reconstructed surface composition was Ni > Mn > Fe > Co > Cr [121]. Selective dissolution of copper in nanoporous NiCuFeCoMn resulted in the enrichment of active species of nickel, cobalt, and iron and delivered a performance of 292 mV at 10 mA cm⁻², a slope of 53.85 mV dec⁻¹, and stability for 200 h [122]. For example, PtPdFeCoNi nanoparticles developed an amorphous Fe/Co/Ni oxyhydroxide-dominant layer during the oxygen evolution reaction, while the same surface showed relative stability under oxygen reduction reaction conditions, suggesting that the active phase is determined by the ongoing reaction [115].

5.4.2. Separating Intrinsic Activation from Surface-Area Gain

An increase in post-reconstruction activity alone is not proof of improved intrinsic kinetics. Selective dissolution can improve roughness and (ECSA, Electro-Chemical Surface Area), with concomitant change in site chemistry. Therefore, a rigorous reconstruction–activity correlation should report film thickness or reconstructed fraction, element-specific dissolution vs. time, ECSA before and after activation, and both geometric and ECSA-normalized currents. If the geometric activity increases and the specific activity remains unchanged, the increase in surface area is the parsimonious explanation. If both increase and spectroscopy shows a new active coordination, an intrinsic electronic or chemical contribution is better supported. This distinction relates precursor processing to catalytic benchmarking here and to corrosion loss.

Neither only preliminary XRD, SEM, or TEM is sufficient to determine the catalytic state. Operando and post-test XAS, XPS, Raman spectroscopy, high-resolution TEM, atom-probe tomography, and ICP analyses are needed to study the stability of the surface, the formation of the active layer, or selective dissolution [11,13,16,63]. Reconstruction is not necessarily deterioration: regulated transformation may lead to active species, while unregulated transformation may corrode metals, destabilize films, or remove active spots. So, an important HEA benchmark should be the capability to generate and maintain an active surface for prolonged periods of operation in HER, OER, or overall water splitting [10,11,16,109].

This dynamic-surface interpretation is consistent with the recent HEA electrocatalysis literature, which shows that surface activity can be improved by nanoporous structures, high-entropy atomic environments, and reconstructed oxide or oxyhydroxide layers [11,12,77,106,107,108,124,125,126]. However, the same processes that create active sites can also accelerate elemental leaching or structural degradation if the reconstructed surface is not stable.

6. Corrosion Behavior and Durability of HEAs

6.1. Why Corrosion Matters in HEA Systems

Corrosion resistance is one of the most important requirements for HEAs used as bulk alloys, coatings, and electrocatalysts. For HEA coatings, corrosion resistance determines whether the coating can protect the underlying substrate during long-term exposure. For HEA electrocatalysts, durability depends not only on catalytic activity but also on resistance to dissolution, oxidation, and surface degradation during operation. Bulk HEAs must also resist degradation in aggressive environments such as marine, acidic, alkaline, and high-temperature media [8,23,24,25]. HEAs containing Cr often show improved passivation due to the formation of oxide-rich surface films. Corrosion resistance depends not only on the Cr content but also Cr distribution, phase constitution, surface defects, and local galvanic interactions [8,23,24].

Recent corrosion studies have shown the effect of passive film chemistry, elemental distribution, defect concentration, and localized galvanic interaction on the resistance of multi-principal-element alloys. This is especially true for coatings and electrocatalysts, where surfaces are accessible and afford protective and functional capabilities.

From

Table 3. Comparison of representative HEA corrosion results across chloride, acidic, and high-temperature environments.

Alloy/Condition	Environment	${\mathit{E}}_{\mathit{corr}}$ (V)	${\mathit{i}}_{\mathit{corr}}$ (μA cm−2)	Breakdown/Passive Metric	Interpretation	Reference
AlCrFe2Ni2	3.5 wt.% NaCl	$-0.25$	0.049	$E_{pit}=0.07$ V	Multi-phase alloy; localized attack limits the passive range.	[127]
AlCrFe2Ni2 FCC phase	3.5 wt.% NaCl	$-0.20$	0.022	$E_{pit}=1.00$ V	FCC constituent is substantially more resistant than the BCC constituent.	[127]
AlCrFe2Ni2 BCC phase	3.5 wt.% NaCl	$-0.34$	0.141	$E_{pit}=-0.07$ V	Phase partitioning creates a localized-corrosion pathway.	[127]
SLM CoCrFeMnNi, XY	3.5 wt.% NaCl	$-0.346$ (SCE)	0.123	passive region reported	Orientation/solidification structure changes the local response.	[128]
Cast CoCrFeMnNi	3.5 wt.% NaCl	$-0.264$ (SCE)	0.120	passive region reported	Similar $i_{corr}$ does not imply identical pit morphology.	[128]
SLM CoCrFeNiN0.07	0.5 M H2SO4	n.r.	n.r.	$i_{pass}=6.1$; film 9.37 nm	Cr-rich p–n film; lower defect density than HIP/cold-rolled states.	[129]
Laser-clad AlCrFeCoNi, C2	3.5 wt.% NaCl	$-0.327$	0.236	$R_{ct}=224.2$ kΩ cm2	Processing parameters control pores, phases, and barrier resistance.	[130]
Al0.5Ti3Zr0.5NbxMo0.2	3.5 wt.% NaCl	n.r.	n.r.	breakdown up to 8.68 V (SCE)	Nb-assisted BCC design produces an unusually broad passive range.	[131]
AlCoCrNiNb0.2	air, 100 h, 1000/1100 °C	—	—	mass gain 0.65/1.31 mg cm−2	High-temperature oxidation follows scale-growth rather than aqueous $i_{corr}$ metrics.	[132]

, it can be seen that “HEAs show good corrosion resistance” is not enough. Chloride evaluations require $i_{corr}$, passive-current density, and pit/breakdown potential. Acid or alkaline tests also require film stability and dissolution flux. High-temperature tests are studied by mass-gain kinetics and scale spallation. The localized corrosion path can be modified by the phase composition and processing orientation of a single nominal alloy. Cross-study rankings are meaningful only if surface polish, reference electrode, scan rate, immersion time, and temperature are similar.

6.2. Passive Films and Localized Corrosion

The most corrosion-resistant high-entropy alloys (HEAs) are made of passive layers of oxides and hydroxides of elements such as Cr, Al, Ti, Ni, Fe, Co, and Mo. The effective passive layer can reduce the corrosion current density and delay the metal disintegration. However, the passivity is susceptible to degradation due to the presence of pores, chemical instability, and depletion of protective elements in the film, leading to pitting and localized corrosion [24,25,26,27,28]. The multi-component and multi-phase high-entropy alloys (HEAs) are susceptible to phase-specific corrosion due to the different electrochemical activities of the phases and can induce micro-galvanic couples that promote localized degradation [8,23,24,25,26,27,28]. The pores, splat boundaries, oxide films, and microcracks in HEA coatings act as pathways for corrosive species to infiltrate the coating interior or substrate [7,35,62]. Thus, the corrosion behavior should be evaluated considering the chemical composition and microstructural properties of the material. Localized corrosion behavior has been recently connected with chemical ordering, surface grinding, shot peening, spraying, antibacterial additives, and tribocorrosion [133,134,135,136,137,138]. In the case of refractory and oxidation-resistant systems, the presence of high-valence oxides, the deposition conditions, and the exposure to elevated temperatures can change the degradation process governing the system [139,140,141]. These results agree with earlier corrosion analysis and standards [142,143,144,145,146].

In HEA coatings, processing-induced features such as oxide-rich boundaries, cracks, pores, unmelted particles, and dilution zones may disturb the continuity of the passive film and reduce corrosion resistance, even if the alloy contains passivating elements [7,28,35,36,61,62,69]. So corrosion resistance should be understood from the viewpoint of the surface chemistry and microstructure of coatings.

6.2.1. Composition Gradients and Film Structure

HEA passive films are chemical grades, not homogeneous mixtures of all alloying elements. The inner barrier is compact and enriched in the most stable oxides, e.g., Cr₂O₃, Al₂O₃, TiO₂, or Nb₂O₅, while the outer exchange layer is more hydrated with Fe-, Co-, and Ni-bearing oxides/hydroxides. During the first seconds or minutes of polarization, less stable species dissolve and the barrier is enriched in passivating cations. In a 38Ni–21Cr–20Fe–13Ru–6Mo–2W alloy, the 3–4 nm film was a non-stoichiometric oxide solid solution enriched in Cr and Ru and depleted in Fe and Ni, which indicates that the nanoscale structure need not consist of discrete equilibrium crystals [147]. However, long-term exposure can lead to an initially favorable picture: CrCoNi retained a compact Cr-rich film after 28 days in 1 M H₂SO₄, while CrMnFeCoNi formed a porous Fe/Mn-rich corrosion layer with preferential Ni dissolution [148]. Potential, temperature, stress, and heat treatment have also been shown to alter the passive-film chemistry, semiconductor response, and breakdown susceptibility in CoCrFeNi-, CrFeCoNi-, TRIP-, and eutectic HEA systems [149,150,151,152,153,154].

The film could be amorphous or nanocrystalline and could have a graded bilayer rather than a single stoichiometric phase. At higher temperatures, the microstructure is different, with diffusion processes leading to larger scale sizes and more crystalline structures, often with an inner Al₂O₃ barrier layer and an outer Cr₂O₃-rich layer and near-parabolic mass-gain kinetics if diffusion through the scale is rate-controlling [132,155,156]. This is the reason why the aqueous polarization and high-temperature oxidation data are combined in Table 3 but not combined into a single numerical ranking.

6.2.2. Semiconductor Properties and Growth Kinetics

In the passive potential range, the Mott–Schottky evaluation often differentiates between p-type and n-type reactions. The p-type contribution mainly comes from cation vacancies in the outer oxide, and the n-type contribution from oxygen vacancies or metal interstitials in the inner barrier. Carrier concentrations for the CoCrFeNiMo_x films have been determined to be in the range of ${10}^{21}$ to ${10}^{22}$ cm⁻³. The increase in carriers is typically associated with more electrically active point defects and enhanced transport properties. However, the presence of Mo may enhance the carrier density but also reduce the passivation due to its competition with the Cr enrichment [157]. Processing may be just as important. The SLM CoCrFeNiN_0.07 produced a 9.37 nm chromium-enriched p–n film with a passive current of 6.1 μA cm⁻² compared to the thinner and more flawed films with passive currents of 13.8 and 19.6. The values are μA cm⁻² after HIP and cold rolling, respectively [129]. A n–p heterojunction is also observed in CoCrFeMoNi, in which iron-containing species are thermodynamically more stable at high pH [158].

We must find out what is going on here. The film growth is driven by the field-assisted ion migration and generation of point defects and impeded by the increasing thickness of the barrier. Unlimited linear growth is much less plausible than logarithmic or inverse-logarithmic behavior for thin aqueous films. Pores, phase interfaces, inclusions, and coating splat borders act as sites for pit initiation, where the defect flux is locally enhanced. But high temperature on a continuous basis results in lattice and grain-boundary diffusion on a much thicker scale with nearly parabolic kinetics. Protection cannot, therefore, be deduced from composition alone, and a complete study of passive films requires a combination of XPS depth profiling or atom-probe tomography (composition gradient), TEM/diffraction (amorphous versus crystalline structure), Mott–Schottky/EIS (defect and transport properties) and time-resolved thickness or mass gain (growth law).

6.3. Corrosion–Electrocatalysis Coupling

Corrosion and electrocatalysis are closely related phenomena. In water splitting, the dissolution, oxidation, surface restructuring, and passive-film degradation can be induced by the applied potential, electrolyte environment, gas evolution, local pH changes, and repeated redox cycles [10,11,12,13,14,15,16,63,78]. A catalyst may exhibit good initial activity due to surface reconstruction, but the same processes may result in material degradation in long-term operation. Therefore, the corrosion-relevant durability tests such as dissolution experiments, stability at high current densities, chronoamperometry, chronopotentiometry, accelerated degradation testing, and post-test composition analysis should be performed in the investigations of electrocatalytic HEAs [11,13,16,78]. Meanwhile, the influence of surface films on the electrocatalytic performance should also be considered in corrosion research of HEAs. Such aspects need to be considered simultaneously in the development of HEAs with high catalytic activity and strong corrosion resistance.

This activity–durability coupling defines the inputs needed for predictive design in Section 7: phase stability alone is insufficient, and useful models must include surface segregation, potential-dependent oxidation, adsorption, defect-assisted transport, and element-resolved dissolution. The comparative tables in Section 5 and Section 6 provide the experimental observables against which such models should be validated.

7. Predictive and Computational Tools for HEA Design

7.1. CALPHAD

CALPHAD (CALculation of PHAse Diagrams) is a common approach to assess phase stability of multi-component alloys. It predicts equilibrium phase diagrams, phase fractions, transformation temperatures, and phase fields as a function of the composition before synthesis, which is helpful in the large HEA compositional space [46,47,55,59].

The reliability of CALPHAD is only as good as the database it uses. Many high-dimensional systems are not sufficiently constrained by existing binary and ternary evaluations [46,47,57]. Thus, the forecasts may be reliable for well-evaluated subsystems but uncertain in other areas.

CALPHAD is mainly concerned with equilibrium or near-equilibrium conditions, whereas coatings, thin films, nanoparticles, and electrodeposits are often prepared under far-from-equilibrium conditions. Phases that are not present in equilibrium phase diagrams can be kinetically stabilized by quenching at high rates, limited diffusion, substrate effects, nanoscale dimensions, and deposition kinetics [36,45,53,63,105]. Direct characterization of HEA coatings and electrocatalysts will enhance CALPHAD prediction.

In this assessment, CALPHAD is most useful in conjunction with experimental and surface characterization. It can be used to direct compositions to optimal bulk stability, and the formation and stability of the expected phases are characterized by XRD, microscopy, electrochemical evaluation, and corrosion examination. CALPHAD is a phase design tool and not a total predictor of surface behavior; a seemingly stable bulk may still undergo oxidation, dissolution, segregation, or reconstruction. Recent CALPHAD-related studies combine thermodynamic calculations with phase-field modeling, high-throughput screening, interpretable learning, and experimental validation; but they still suffer from database and non-equilibrium limitations [159,160,161,162].

7.2. Density Functional Theory

Density functional theory (DFT) provides atomistic insight into the energetic and electronic factors that control phase stability, bonding, segregation, and surface reactivity in high-entropy alloys. In HEA research, DFT is especially useful because it can estimate formation energies, mixing enthalpies, adsorption energies, electronic structure, charge redistribution, and the relative stability of different crystal structures. These quantities are important because phase formation in HEAs is not governed by configurational entropy alone; it also depends on the balance between enthalpic interactions, competing intermetallic phases, lattice preference, and temperature-dependent stabilization effects [51,52,163].

For electrocatalytic HEAs, DFT is commonly used to calculate adsorption energies of key reaction intermediates such as H*, OH*, O*, and OOH*. These adsorption energies can act as descriptors for HER and OER activity because they help explain whether a surface binds reaction intermediates too weakly, too strongly, or near optimally. DFT can also reveal how different neighboring atoms in a multi-component surface modify the electronic structure and create a distribution of active sites rather than one uniform catalytic site [12,15,17,82]. This is particularly important for HEAs because the local atomic environment around an active site can vary strongly from one surface position to another.

However, the main challenge is that HEAs contain a very large number of possible atomic arrangements. A single nominal composition can have many different local configurations, short-range ordering states, segregation patterns, and surface terminations. Direct DFT calculation of every possible arrangement is therefore computationally expensive. To address this, DFT is often combined with simplified thermodynamic models, special quasi-random structures, cluster expansion, Monte Carlo simulations, machine-learning potentials, or descriptor-based screening. These approaches allow DFT information to be used more efficiently across large composition spaces [15,51,52,82,163].

A useful example is the high-throughput DFT-based thermodynamic screening reported by Chen et al. [163]. In that study, more than 658,000 equimolar quinary alloy combinations were evaluated using a regular-solution Gibbs free-energy model with binary interaction terms obtained from DFT calculations. The stability of each candidate alloy was assessed by comparing the Gibbs free energy of the solid solution against competing phases, including intermetallic compounds. This approach identified more than 30,000 possible single-phase equimolar HEAs and showed that phase stability depends on the combined effects of mixing enthalpy, melting point, configurational entropy, and competition from intermetallic phases.

Importantly, Chen et al. did not stop at computational prediction. They selected two predicted compositions, BCC AlCoMnNiV and FCC CoFeMnNiZn, synthesized them experimentally, and confirmed their structures using backscattered-electron scanning electron microscopy and X-ray diffraction. This provides a clear example of how DFT-based thermodynamic screening can move from prediction to experimental validation, as shown in Figure 10.

This example illustrates that DFT is most useful when combined with synthesis and characterization. CALPHAD is useful for determining the stability of equilibrium phases if reliable datasets are available and DFT can provide first-principles energetic data for less studied chemical domains. Later on, machine learning can accelerate the search by learning from the computed and experimental data. Hence, DFT should be regarded as a component of an integrated HEA design approach that connects thermodynamic prediction, surface features, synthesis, characterization, and performance assessment rather than as a standalone tool.

7.3. Machine Learning

Models trained on compositional, thermodynamic, atomic, processing, and experimental descriptors can predict phase stability, mechanical properties, corrosion resistance, catalytic performance, and processing behavior [81,164,165,166,167]. This is especially important when small changes in composition result in large changes in phase constitution, microstructure, surface chemistry, or performance. Recent developments in inverse design and coating optimization have highlighted the need for Bayesian, generative, and interpretable models within the framework of predictions combined with experimental or physics-based validations [168,169,170,171].

Machine learning can be useful in predicting the phase formation of alloys, such as whether the alloy will form a solid solution, multiple phases, intermetallic compounds, or an amorphous structure [167,172]. Machine learning can aid in corrosion research by revealing the links between alloy composition, manufacturing techniques, crystalline structure, and surface features that determine corrosion resistance. The phenomena under investigation are affected by more than one variable [165,166]. Machine learning can be used in electrocatalysis to explore large composition spaces of high-entropy alloys and to predict catalytic parameters such as adsorption energies, overpotential trends, and activity–stability relations [15,81,82].

Instead of using random selection, methodologies such as active learning, Bayesian optimization, and multi-objective optimization can suggest candidates to improve a target attribute or reduce model uncertainty. Ways for anticipated improvement are focused on enlightening tests, and multi-faceted ways examine activity, corrosion resistance, stability, cost, and durability [15]. Therefore, machine learning acts as a supportive tool for decision-making instead of replacing synthesis and experimental validation.

Recent ML works have taken a step further by integrating high-throughput data, descriptor selection, interpretable models, and optimization algorithms to speed up HEA design [164,166,172,173,174,175,176]. These techniques are especially useful when multiple objectives are to be considered simultaneously, such as phase stability, hardness, corrosion resistance, catalytic activity, cost and processability.

7.4. Multi-Objective Optimization

A major limitation of many HEA studies is that optimization is customarily performed for one property at a time. Researchers in electrocatalysis may be more focused on low overpotential or high current density and less on metal dissolution, long-term stability, corrosion resistance, and surface reconstruction. For coatings, hardness or wear resistance is often emphasized, but corrosion behavior, adhesion, porosity, residual stress, and coating–substrate compatibility are often overlooked. Similarly, in bulk HEAs, single-phase formation can be regarded as the primary design goal even when the final application is strongly dependent on surface properties, passive-film stability, or catalytic activity [11,16,77,78,177].

A multi-objective optimization framework provides a more realistic approach because HEA performance is often governed by several competing requirements. An ideal HEA electrocatalyst should exhibit low overpotential and Tafel slope, high current density, long-term stability, minimal metal dissolution, strong corrosion resistance, low cost, and reproducible synthesis. For HEA coatings, optimization should jointly consider corrosion and wear resistance, porosity, adhesion, residual stress, phase stability, coating thickness, and processing feasibility [7,35,36,53,60,69,178].

One attribute may be increased at the expense of another. A robust passivation could kill the catalytic efficacy, a highly reactive surface might dissolve or restructure, and an overrigid coating might become brittle due to the buildup of intermetallics or poorly connected phases. Therefore, the design of HEA should consider the trade-off between the phase stability, activity, durability, corrosion resistance, cost, and processing reliability. Figure 11 shows that this trade-off can be achieved by coupling machine learning with multi-objective optimization and experimental validation.

8. Integrated Cross-Section Analysis of Phase Formation, Electrocatalysis, and Corrosion

The earlier parts offer separate evidentiary foundations on phase selection, processing, electrocatalysis, corrosion, and predictive design. Together, they provide a more accurate notion that the relevant design variable is not only the nominal composition but the resultant state generated by a particular sequence of composition, processing, and environmental factors. Elemental reservoirs and segregation pathways are dictated by bulk phases. Defects, interfaces, and available surface area are introduced through processing. Electrochemical exposure creates an operational surface, whose activity and stability may differ from the material as prepared [7,9,11,24,25].

You can see the tradeoff from the comparative analysis conducted at the beginning. From Table 2, low overpotential may be related to nanoscale area, noble-metal content, or an activated oxyhydroxide surface. From Table 3, durability is strongly dependent on electrolyte, processing defects, and passive-film stability. The values measured in different media or normalizations cannot be placed on one universal ranking. Instead, a defensible HEA design must link each reported metric to the phase, microstructure, surface state, and test condition that produced it.

8.1. Cross-Property Trade-Offs

Opposites are catalytic surfaces and protective surfaces. Porosity, surface roughness, small grain size, and structural rebuilding can increase the electrochemically active surface area, but the same characteristics can also promote selective dissolution or act as pathways for localized corrosion. Thick layers and dense Cr-, Al-, or Ti-rich barriers prevent movement of the electrolyte. However, a very passive surface may hide sites for hydrogen evolution reaction or oxygen evolution reaction. The aim is to control the surface development, providing sufficient activation to produce active sites but not so much dissolution or film degradation that irreversible losses occur [10,23,147].

In this equilibrium, phase composition is a combination of elemental availability and spatial configuration. Structuring or separating a phase can enhance a coating and induce a localized galvanic pair. A supposed single-phase matrix can decrease the phase–scale coupling but still retain the chemical heterogeneity at the nanoscale. Likewise, the metal precursor of an OER catalyst can provide Fe, Co, and Ni to an active oxyhydroxide layer, but selective depletion of Mn, Cr, or other less stable elements could either expose new sites or compromise surface stability. These links are summarized in

Table 4. Integrated HEA design trade-offs across phase formation, electrocatalysis, and corrosion.

Design	Phase/Microstructure Consequence	Electrocatalysis Consequence	Corrosion/Durability Consequence	References
Rapid or non-equilibrium processing	Retains metastable phases, fine grains, residual stress, and defects.	Can raise accessible area and diversify active sites.	Defects and composition gradients can interrupt passivity.	[7,35,36]
Porosity and roughness	Increase surface-to-volume ratio and local transport paths.	Often improve geometric current density.	Can accelerate ingress, dissolution, and coating/substrate attack.	[10,26,27]
Elemental segregation or ordering	Creates chemically distinct regions and phase boundaries.	Changes adsorption and reconstruction locally.	May promote selective attack or micro-galvanic coupling.	[9,24,25]
Passivating additions	Favor Cr-, Al-, Ti-, Mo-, or Nb-bearing surface species.	May stabilize the surface or block catalytic sites, depending on potential.	Can strengthen the barrier film when the elements remain uniformly available.	[23,28,147]
Non-equiatomic optimization	Assigns different functional weight to each element.	Tunes adsorption, conductivity, and active-phase formation.	Tunes film chemistry, phase stability, critical-material use, and cost.	[15,57,179]

.

Therefore, equiatomic composition is a convenient experimental benchmark but not a universal ideal. Application-oriented compositions must be assessed against criteria like phase and processing viability, inherent and area-normalized activity, element-specific dissolution, passive-film durability, operational lifespan, and variability in underlying data [57,81,179,180].

8.2. Sustainability, Criticality, and Cost

Resource optimization is not caused by chemical complexity. Noble-metal-free Fe–Co–Ni–Mn–Mo systems eliminate the direct use of platinum-group metals but can still depend on critical or supply-sensitive elements and still require energy-intensive nanoporous processing. In contrast, PtPdFeCoNi and AlNiCoIrMo-type HEAs can decrease the amount of a single precious element compared to a pure noble metal catalyst, but the advantage should be presented as activity per mass of Pt-group metal, not only overpotential [10,107,108,115]. Dilution only matters if the entire electrode can be achieved with the same current, efficiency, and lifetime at a lower critical-material loading. A new oxide-derived powder-metallurgy method demonstrates that processing yield and densification can be addressed directly as sustainability factors, rather than only inferred from composition [181].

Processing tips the ecological and financial balance even further. A thin layer can save alloy stock and extend the life of a large substrate. However, sputtering losses, thermal-spray overspray, laser energy, electrodeposition bath losses, and discarded porous coatings contribute to material and energy requirements [7,35,36,63]. Selective dissolution is not only a durability loss but also a material liberation mechanism with electrocatalysts. A longer service life and recoverability are preferred to a small gain in the initial overpotential.

Additional factors such as catalyst or coating yield, energy intensive processing stages, distribution of critical elements among functional domains, Pt-group-metal mass activity where applicable, operational durability at specific current or conditions, leaching of elements, and recovery alternatives at end of life should also be considered in future assessments. These metrics can be directly incorporated into the multi-objective process with activity, corrosion resistance, and phase stability [166,175,179]. This analysis supports the sustainability and cost argument in the Abstract but does not mean that all HEAs are inherently cost efficient or better for the environment.

8.3. Data and Model Integration

The best prediction framework is hierarchical. Empirical thresholds (Table 1) provide fast screening, CALPHAD evaluates competing bulk phases, DFT provides local energetic and adsorption properties, and ML is used to rank the candidates while taking into account uncertainty. Then, experiments taking into account the processing should confirm the expected phase and microstructure. This should be followed by electrochemical evaluations and surface analyses after operation or in operando conditions. In the process, instead of focusing only on the composition objectives [15,163,164,165,166,167], we should re-integrate the results back into the model as data labeled by conditions.

9. Research Gaps and Priorities

The remaining gaps are methodological rather than compositional. The integrated analysis leads to the summary of the priorities in

Table 5. Priority gaps and the evidence needed to close them.

Gap	Why Current Evidence Is Insufficient	Priority Action	References
Transfer of bulk phase rules	Equilibrium descriptors omit deposition kinetics, size, substrate, and surface exposure.	Build processing-labeled phase datasets and validate predictions after synthesis and service.	[7,32,36]
Activity–durability coupling	Initial overpotential can improve while dissolution or reconstruction shortens life.	Pair activity with long-duration operation, ICP dissolution, and post/operando surface analysis.	[11,121,122]
Testing comparability	Loading, iR correction, area normalization, electrolyte, surface finish, and scan protocol vary widely.	Adopt benchmark electrocatalysis and corrosion metadata and report uncertainty.	[24,25,84,87]
Sustainability and cost data	Critical-material loading, process yield, energy use, release, and recovery are rarely reported together.	Add mass and service-based economic and environmental metrics to screening.	[10,179,181]
Model–experiment closure	CALPHAD, DFT, ML, synthesis, and electrochemistry are often performed as separate studies.	Use active-learning loops in which experiments update phase, surface, and durability models.	[15,163,164,166]

.

In the near term, standardized reporting and studies are crucial to differentiate intrinsic activity from surface-area enhancement. These studies must report catalyst loading, substrate controls, reference electrode calibration, iR compensation, ECSA or other rationalized normalization, Faradaic efficiency, lifetime at a given current density, and post-mortem composition. Corrosion studies require surface preparation, electrolyte composition, pH, temperature, exposed surface area, scan rate, immersion time, EIS model used, and morphology after corrosion [24,25,84,87].

Moving forward, composition selection with processing control must correlate with post-test evidence. CALPHAD and DFT narrow the initial search space, while machine learning methods rank candidates, balancing competing targets. Experimental validation of material changes during operation, such as elemental dissolution, passive-film formation, surface reconstruction, and activity loss, is essential. Instead of a single favorable measurement for HEA selection, the balance among catalytic activity, corrosion resistance, durability, material demand, and manufacturing cost must be considered.

10. Conclusions

It is evident from this review that the behavior of HEAs cannot be estimated based on the number of alloying elements or nominal compositions only. The key point lies in the formation of specific material state through the process of composition, preparation, and exposure to the operating environment. Phase bulk and processing defects are the factors determining which elements and interfaces will appear at the surface, whereas further electrochemical/corrosion exposure may result in passivation, segregation, selective dissolution, and reconstruction. Thermodynamic approaches, CALPHAD modeling, DFT, and machine learning can help to limit the search space and reveal significant patterns; however, they cannot replace experiment and direct surface analysis. Hence, the electrocatalytic and corrosion properties should always be considered along with the electrolyte, applied loading, the method of normalization, preparation procedure, and the time of testing. Future advances in this field rely on the correlation between the prepared phase, microstructure, and the surface formed in the process of operation. There should also be a consideration between activity, corrosion resistance, durability, resource requirements, and production costs for a viable HEA. An HEA of interest cannot be defined as the alloy with the lowest overpotential or the widest passive range.