High-Entropy Alloys Trade Hydrogen Capacity for Practical Storage Performance

A review of high- and medium-entropy alloys for solid-state hydrogen storage finds that the most promising materials are not those with the largest hydrogen uptake alone, but those that can balance capacity, activation, kinetics, reversibility and long-term cycling. Refractory body-centered cubic (BCC) alloys can reach approximately 2–3 wt% hydrogen in favorable compositions, while C14 Laves-phase alloys more often operate near room temperature with faster sorption and lower capacities of roughly 1–2 wt%. Multiphase BCC–C14 structures may offer a way to combine the two performance profiles, but the review finds that standardized testing and application-focused evaluation remain essential.

The attraction of these alloys lies in their unusually broad compositional design space. Unlike conventional hydride families, high-entropy alloys and medium-entropy alloys allow phase constitution, hydrogen thermodynamics and microstructure to be adjusted through the choice and proportion of several elements. The source article notes that many of these materials are better understood as complex concentrated alloys, because their behavior is governed not only by configurational entropy but also by enthalpic interactions, lattice distortion and local chemical effects. Refractory elements such as titanium, vanadium, zirconium, niobium and hafnium can provide hydrogen-absorbing structures, while chemical disorder creates a distribution of local hydrogen environments that may affect site occupancy, diffusion and sorption thermodynamics.

Semiempirical descriptors provide the first stage of alloy selection. Atomic-size mismatch estimates lattice distortion; the entropy–enthalpy competition parameter, Ω, weighs configurational entropy against the magnitude of mixing enthalpy; valence electron concentration, or VEC, provides a proxy for phase tendencies; and the radius ratio between larger hydride-forming elements and smaller transition metals offers a geometric guide for Laves-phase packing. Earlier criteria associate VEC values below 6.87 with a greater tendency toward BCC structures, while a value near 6.4 has repeatedly been linked to favorable hydrogen-storage behavior in Laves-related systems. Ratios close to 1.225 are associated with stable C14 or C15 structures, although the review cautions that these are screening tools rather than universal rules. Local ordering, segregation, intermetallic stoichiometry and processing history can overturn predictions based only on average composition.

Those first-pass predictions are increasingly refined with CALPHAD, density functional theory and machine learning. CALPHAD can estimate phase equilibria, hydrogen solubility limits and hydride stability across temperature and pressure, and a model built for single-phase BCC multicomponent alloys reproduced two equilibrium plateaus in the TiVNb system across temperatures from 200 to 235 °C. A separate thermodynamic framework calculated room-temperature pressure–composition–temperature behavior for 440 multicomponent C14 alloys and reproduced the main plateau features and capacities of six experimental alloys. High-throughput workflows can now screen many candidate compositions before expensive atomistic calculations are undertaken, while machine-learning models can learn composition–property relationships from experimental databases. These tools are complementary, however: CALPHAD constrains phase stability, DFT examines local atomic environments and binding energies, and data-driven models depend heavily on the quality and scope of the available datasets.

Density functional theory can estimate formation energies, phase stability, hydrogen solution enthalpies and site-specific hydrogen binding, yet chemical disorder makes accurate modeling expensive. The coherent potential approximation is computationally efficient but averages over local relaxation and short-range order. Special quasi-random structures represent disorder more realistically but require large supercells, while approaches using small sets of ordered structures strike a practical compromise for broad screening. The review stresses that such calculations are most useful for relative comparisons rather than direct forecasts of measured isotherms, activation requirements, kinetic rates or usable capacity. Finite periodic cells commonly represent conditions at 0 K and may omit finite-temperature entropy, surfaces, oxides, defects, segregation and multiphase evolution, so experimental verification remains indispensable.

Performance differences between the major phase families are pronounced. C14 Laves-phase HEAs generally activate more readily, absorb and release hydrogen faster and operate under milder near-room-temperature conditions, but their gravimetric capacity is usually moderate. The equiatomic TiZrCrMnFeNi alloy demonstrated reversible storage of about 1.7 wt% hydrogen at room temperature under moderate pressure, with little or no severe activation requirement. CALPHAD-guided TiZrNbFeNi compositions retained a dominant C14 phase and achieved roughly 1.3–1.8 wt% reversible capacity, depending on phase fraction and homogeneity. By contrast, refractory BCC alloys form hydrides with reported formation enthalpies commonly around −40 to −60 kJ mol⁻¹ H₂, which contributes to low plateau pressures and desorption temperatures often above 300 °C. Their high uptake is therefore not fully accessible under mild operating conditions.

Alloying can moderate that trade-off, but usually cannot eliminate it. Controlled chromium additions to Ti–V–Nb alloys raised plateau pressures and partially destabilized the hydride, while excessive aluminum caused capacity loss and secondary phases. In a combined machine-learning, DFT and experimental study, replacing part of chromium with iron in a TiVNbCr system reduced the absorption enthalpy from −51.6 to −41.3 kJ mol⁻¹ H₂ while preserving reversible BCC-to-FCC hydride transformation, but usable hydrogen capacity also declined. Processing is another major variable. Melting and casting provide reference microstructures, although rapid solidification can leave dendritic segregation that broadens pressure–composition plateaus. Homogenization can improve chemical uniformity and phase distribution, but the right temperature matters: in TiZrCrMnFeNi, annealing at 1,000–1,200 °C increased the C14 fraction and enhanced storage, whereas treatment at 900 or 1,300 °C reduced C14 content and sharply lowered capacity.

Mechanical alloying and severe plastic deformation offer another route to faster activation. Milling creates nanocrystalline or partly amorphous structures, high defect densities and chemically homogeneous powders, enabling chemistries such as magnesium-containing alloys that are difficult to produce by melting. High-pressure torsion, commonly applied at about 5–6 GPa, can refine grains into the sub-100-nanometre range and add non-equilibrium interfaces that provide possible pathways for hydrogen transport. These features can lower activation barriers and accelerate sorption, but grain boundaries and dislocations also trap hydrogen. The same defect population that improves entry may retain hydrogen after desorption, increase hysteresis and reduce reversibility, so deformation is not an unambiguously beneficial treatment. Powder oxidation, milling contamination, pyrophoricity and the lack of a scalable severe-deformation route remain additional barriers.

Multiphase architectures offer perhaps the most promising way to move beyond the capacity-versus-reversibility divide. In BCC–C14 systems, the BCC phase generally acts as the main hydrogen reservoir, while Laves-phase domains and their interfaces are associated with improved activation, nucleation and transport. The optimum phase ratio is not simply the one giving maximum total uptake: in one Ti–V–Mn–Fe–Ni series, the alloy with the greatest BCC fraction achieved the highest uptake, about 1.7 wt%, whereas the C14-rich alloy had the best reversible capacity, about 1.2 wt%. Another dual-phase series showed that an intermediate balance best combined accessible diffusion paths with enough storage sites. This phase-synergy concept is promising, but the review notes that quantitative rules for selecting phase balance, elemental partitioning and interface morphology for a particular operating window do not yet exist.

Long-term testing exposes the limits of short-term capacity claims. Repeated hydrogenation can generate dislocations, vacancies and clusters that act as persistent trapping sites, gradually reducing the mobile hydrogen fraction. In BCC materials, lattice expansion and contraction and symmetry-changing transformations also contribute to local cracking, incomplete reversal and decrepitation. Some high-resolution studies suggest that defect accumulation may be more important than wholesale phase decomposition. By comparison, C14-rich alloys often undergo smaller volume changes and can be more mechanically durable. The TiZrFeMnCrV alloy, for example, retained about 97% of its initial 1.80 wt% capacity after 50 cycles, with later capacities remaining between 1.73 and 1.76 wt%. The review concludes that the field now needs common reporting for activation, equilibration, usable reversible capacity, plateau pressure, hysteresis and cycling conditions, alongside inverse-design workflows that combine CALPHAD, DFT and data-driven prediction. Without application-anchored standards, high-entropy hydrides may continue to look promising in isolated measurements without demonstrating a decisive advantage in storage systems.

Subject of Research: Comparative design, processing and hydrogen-storage performance of high- and medium-entropy alloys

Article Title: Recent advances in hydrogen storage properties of high-entropy alloys: from alloy design to performance

Article References: Edalati, P., & Floriano, R. (2026). Recent advances in hydrogen storage properties of high-entropy alloys: from alloy design to performance. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13672-1

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13672-1

Keywords: hydrogen storage, high-entropy alloys, medium-entropy alloys, C14 Laves phase, BCC alloys, CALPHAD, density functional theory, machine learning, hydrogen kinetics, multiphase materials, Recent, advances