Structural and thermodynamic insights into Mg-based high-entropy alloys for solid-state hydrogen storage: A review

Magnesium-based materials are among the promising candidates for solid-state hydrogen storage due to their high gravimetric capacity, natural abundance, and low cost; however, their practical application is severely hindered by high desorption temperatures, sluggish hydrogen sorption kinetics, and limited cycling stability. Recently, high-entropy alloys (HEAs) have emerged as an effective strategy to overcome these limitations by introducing multi-element synergistic effects, severe lattice distortion, and tuneable thermodynamic properties. This review provides a comprehensive and critical overview of structural, thermodynamic, and kinetic roles of HEAs in enhancing the hydrogen storage performance of Mg/MgH2 systems. Fundamental HEA design principles, including mixing enthalpy and entropy, atomic size mismatch, valence electron concentration, and phase stability criteria, are systematically discussed in relation to phase formation and hydrogen to metal interactions. The core effects of HEAs, including lattice distortion, sluggish diffusion, and cocktail effects, are discussed in the context of their entropy-driven contributions compared with conventional catalytic effects. Various synthesis routes, including mechanical alloying, arc melting, severe plastic deformation, and chemical methods, are compared in terms of microstructural control, scalability, and industrial feasibility. Furthermore, structure-property relationships governing hydrogen sorption kinetics, thermodynamic destabilization, cycling durability, and degradation mechanisms are summarized based on recent experimental findings. Finally, current challenges, unresolved scientific questions, and future research directions are outlined, with particular emphasis on rational HEA design guidelines, long-term stability, and practical deployment for sustainable hydrogen energy applications.