High-Entropy Alloys as Materials for Solid-State Hydrogen Storage: From Fundamental Principles to Directed Design Strategies

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 ΔHmix, 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.