Tailoring hydrogen storage performance of Mg-Mg ₂ Ni alloys: synergistic effects of composition and phase formation with first-principles insights

This work investigates the impact of Mg/Ni atomic ratios (75 : 25 and 66.7 : 33.3) on the formation of Mg₂Ni phases and their hydrogen storage performance. Mg-Ni alloys were synthesized by vacuum casting at 1073 K followed by high-energy ball milling and were evaluated through both experimental methods and density functional theory (DFT) simulations. DFT calculations revealed that hydrogen absorption in Mg₂Ni is thermodynamically more favorable than in pure Mg, with enthalpy values consistent with experimental results. Hydrogenation tests at 588 K under 20 MPa demonstrated superior performance for the Mg-25Ni alloy, which achieved a higher storage capacity (3.76 wt%) and faster kinetics than Mg-33Ni (3.53 wt%). The improved performance is attributed to the enhanced formation of MgH₂ and the synergistic interaction between Mg and Mg₂Ni. Kinetic analysis using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model indicated a lower activation energy for Mg-25Ni (56.74 kJ mol^-1), confirming faster desorption kinetics. Pressure-composition-temperature (PCT) isotherms and van't Hoff analysis further supported the favorable thermodynamics of Mg-25Ni. Notably, this alloy exhibited excellent cyclic stability with minimal capacity loss over 10 cycles. These findings establish Mg-25Ni as a promising candidate for high-efficiency, reversible hydrogen storage, bridging fundamental insights with practical material design.