Unveiling Strain‐Driven Catalysis in Magnesium Hydride via Ultrafine High‐Entropy Hydride

The unprecedented, direct and tailored synthesis of ultrafine high‐entropy hydride (HEH) nanoparticles was achieved under ambient conditions via one‐step mechanochemical strategy, and demonstrated their exceptional catalytic function in magnesium hydride (MgH 2 ) hydrogen storage. The partially dehydrogenated HEH phase introduces abundant hydrogen vacancies and coherent lattice strain, effectively destabilizing MgH 2 and facilitating hydrogen release via interface‐mediated lattice distortions. Specifically, the HEH catalyst triggers MgH 2 dehydrogenation at 180 °C and achieves reversible hydrogen absorption at room temperature, surpassing most transition metal alloy catalysts. Kinetic analysis confirms altered reaction models and a substantial reduction in activation energies (from 131.52 to 72.76 kJ mol −1 ). Owing to the interfacial structural stability and reversible catalytic behavior during cycling, the composite maintains exceptional cyclic durability exceeding 6 wt% hydrogen capacity over 40 cycles. The proposed synthesis pathway and the elucidated strain‐induced catalytic mechanism offer a new paradigm for the rational design of HEH catalysts for advanced Mg‐based hydrogen storage materials.