High-entropy alloys and the broader class of compositionally complex alloys have recently attracted significant attention as promising materials for solid-state hydrogen storage. Their potential arises not only from high configurational entropy but also from the possibility of tailoring phase composition, crystal structure, local chemical environment, and defect states that govern hydrogen sorption thermodynamics and kinetics. This review summarizes current understanding of hydrogen interaction mechanisms in HEAs and discusses the role of body-centered cubic (BCC), face-centered cubic (FCC), and Laves phases in determining hydrogen capacity, reversibility, and cyclic stability. The limitations of commonly used descriptors, including valence electron concentration (VEC), atomic size mismatch δ, enthalpy of mixing ΔH_mix, and Ω parameter, in predicting hydrogen storage behavior are critically analyzed. Particular attention is given to the effects of processing methods, phase transformations during hydrogenation/dehydrogenation, and the energetic heterogeneity of interstitial sites in multicomponent systems. The review highlights that future progress will depend on the transition from empirical alloy discovery toward physically informed multiparametric design integrating CALPHAD, DFT modeling, machine learning, and in situ/operando characterization techniques for the development of efficient and durable hydrogen storage materials.

The effects of post-processing heat treatment on the structural and phase state and hydrogen sorption characteristics of an equiatomic TiZrNbVTa high-entropy alloy produced by mechanical activation and spark plasma sintering were investigated. The powder mixture was mechanically activated under argon and consolidated at 1450 °C. Two processing conditions were compared: SPS with a 7 min hold without subsequent treatment, and SPS with a 5 min hold followed by annealing at 1050 °C for 5 h. Phase composition was analyzed by X-ray diffraction. Hydrogenation was performed at 400–600 °C under a hydrogen pressure of 37–40 bar. Hydrogen content was evaluated using the SOAK kinetic method and inert gas fusion analysis with a RHEN-602 analyzer. Both materials were predominantly composed of a Nb-enriched body-centered cubic phase, whereas their secondary-phase compositions differed. After annealing, weak reflections tentatively attributed to ZrO₂ and TaO₂ appeared, whereas reflections previously assigned to Zr, V, and TiO₂ could no longer be resolved. According to the RHEN-602 measurements, the hydrogen content ranged from 1.12 to 1.37 wt.% for the SPS sample and from 0.64 to 2.08 wt.% for the SPS-HT sample between 400 and 600 °C. These results indicate that processing involving high-temperature annealing modifies the phase state and may enhance hydrogen uptake at 500–600 °C.