Mechanism-informed design of high-entropy alloy hydrides for reversible solid-state hydrogen storage

Solid-state hydrogen storage remains a major challenge because practical materials must combine high reversible capacity, moderate operating conditions, fast kinetics, cycling stability, low cost, and scalable processing. High-entropy alloys and related compositionally complex alloys are promising metal hydride-forming materials because their multicomponent chemistry enables tuning of crystal structure, lattice strain, hydrogen-site energetics, phase stability, and absorption–desorption behaviour. This review examines high-entropy alloy hydrides for reversible solid-state hydrogen storage, focusing on alloy composition, BCC/FCC phase constitution, hydride thermodynamics, diffusion kinetics, activation behaviour, and cycling durability. Reported studies show that BCC-dominated Ti-, V-, Zr-, Nb-, and Hf-containing alloys generally offer the best performance, typically reaching 1–3 wt.% hydrogen, with selected systems exceeding 3 wt% under favourable conditions. However, practical use is limited by high alloy density, costly or critical elements, incomplete reversibility, high desorption temperatures, limited cycling validation, and non-standardized testing protocols. Unlike previous capacity-based summaries, this review transforms HEA hydrogen-storage literature into a mechanism-informed, sustainability-aware, and deployment-oriented design framework for identifying practically viable HEA hydrides. Future progress requires standardized, data-driven, and experimentally validated design of lower-density, reversible, and economically viable HEA hydrides.