An In Situ-Formed Mg3AlNi2/Mg3AlNi2H x Ternary Hydrogen Pump for Rapid and Reversible Hydrogen Storage in Mg–Al–Ni Alloys

Mg90–xAl10Nix (x = 5, 10, 15, 16, 17, 18, 19, and 20) ternary alloys were synthesized by hydriding combustion synthesis (HCS) to improve the de-/hydrogenation performance of MgH2. Among these alloys, Mg71Al10Ni19 shows rapid hydrogen uptake, reaching approximately 97% of its practical capacity within 500 s at 225 °C, and nearly complete dehydrogenation within 600 s at 300 °C. The hydrogenation and dehydrogenation enthalpies of the Mg/MgH2 are reduced from –76.24 and 80.86 kJ/mol to –74.40 and 76.91 kJ/mol, respectively. The pronounced kinetic enhancement is mainly attributed to in situ-formed Mg3AlNi2, which reversibly accommodates and releases hydrogen while retaining structural stability during cycling. Density functional theory (DFT) calculations and deep potential molecular dynamics (DPMD) simulations reveal the favorable formation of Mg3AlNi2Hx, faster H diffusion in Mg3AlNi2 than in Mg2Ni, and weaker H-related electron localization in Mg3AlNi2Hx than in Mg2NiH4. These results support the Mg3AlNi2/Mg3AlNi2Hx couple as a ternary hydrogen pump. Al3Ni2 may further serve as an electron-transfer mediator between Mg3AlNi2 and MgH2, whereas Mg2Ni/Mg2NiH4 provides an additional hydrogen-transfer pathway. These findings provide a rational strategy for designing high-performance Mg-based hydrogen storage materials through the in situ generation of synergistic intermetallic phases.