Catalyst-State Evolution in Reversible Mg/MgH2 Solid-State Hydrogen Storage

Magnesium hydride is a high-capacity solid-state chemical hydrogen-storage material, but its storage performance is constrained by activation requirements, elevated-temperature hydrogen release, slow reversible uptake and release, and degradation during repeated cycling. Catalysts are commonly identified by the precursor added before ball milling, although many oxides, fluorides, carbides, nitrides, metals, and multicomponent additives reconstruct before stable storage cycling is established. This review focuses on catalyst-state evolution in solid-state hydrogen-storage systems in which the reversible Mg/MgH 2 couple remains the principal storage reaction, while secondary catalyst-derived reversible phases are considered where they directly influence Mg/MgH 2 storage behavior. Evidence from Nb 2 O 5 -, TiF3-, TiO 2 -, Ni-, Co-, Cu-, and rare-earth-containing MgH 2 systems is organized across mechanochemical processing, first-cycle reconstruction, Mg-rich and MgH 2 -rich half-cycles, and prolonged cycling. A working catalyst state is considered only when a reaction-state-specific phase, coordination environment, or interface is chemically identified, remains accessible to the Mg/MgH 2 conversion region, and is retained or reproducibly regenerated during cycling. Phase detection is therefore separated from catalytic-function assignment. Catalyst persistence is assessed independently through reversible capacity, uptake and release kinetics measured under matched conditions, chemical-state persistence, and interfacial accessibility. The capabilities and limitations of diffraction, spectroscopy, microscopy, isotope measurements, kinetic analysis, and atomistic calculations are evaluated according to whether they connect catalyst-state evolution with hydrogen-storage performance. The combined evidence indicates that precursor design should control reproducible state trajectories that form within accessible activation conditions, limit irreversible Mg consumption, remain connected to the conversion region, and persist or regenerate during cycling. This storage-focused approach provides practical criteria for comparing catalyst systems and qualifying reproducible MgH 2 materials for reversible solid-state hydrogen storage.