Tailoring hydrogen storage performance of Mg-Mg2Ni alloys: synergistic effects of composition and phase formation with first-principles insights

Abstract

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.

Fig. 1. FE-SEM micrographs of mechanically milled Mg-25Ni (a–c) and Mg-33Ni (d–f) alloys at low magnification.

Fig. 2. FE-SEM micrographs of as-synthesized Mg-25Ni (a–c) and Mg-33Ni (d–f) alloys at low magnification, showing surface morphology after hydrogen absorption.

Fig. 3. TEM and HRTEM images of Mg-25Ni (a–c) and Mg-33Ni (d–f) alloys after hydrogen absorption.

Fig. 4. X-ray diffraction patterns and Rietveld refinement results of Mg-Ni alloys at different stages: (a–c) Mg-25Ni alloy and (d–f) Mg-33Ni alloy in three states: as-synthesized, after initial hydrogenation, and after three hydrogen absorption/desorption (H-cycling) cycles.

Fig. 5. Hydrogenation behavior of Mg-25Ni (a) and Mg-33Ni (b) alloys synthesized at 573 K and 2 MPa.

Fig. 6. Hydrogen absorption and desorption performance of Mg-25Ni (a) and Mg-33Ni (b) alloys at 588 K.

Fig. 7. Hydrogen absorption and desorption kinetics of Mg-Ni alloys: (a) absorption kinetics at 2 MPa under various temperatures (523 K, 573 K, 588 K) for the Mg-25Ni alloy; (b) desorption kinetics at 0.01 MPa under various temperatures (523 K, 573 K, 588 K) for the Mg-25Ni alloy; (c) absorption kinetics at 2 MPa under various temperatures (523 K, 573 K, 588 K) for the Mg-33Ni alloy; (d) desorption kinetics at 0.01 MPa under various temperatures (523 K, 573 K, 588 K) for the Mg-33Ni alloy.

Fig. 8. JMA plots for the hydrogen desorption process of Mg-Ni alloys: (a) Mg-25Ni and (b) Mg-33Ni; (c) plot of ln k versus 1000/RT for the hydrogen desorption process.

Fig. 9. Hydrogen absorption/desorption cycles of the Mg-25Ni alloy after activation at 588 K.

Fig. 10. (a–c) PCT isotherms of Mg-25Ni and Mg-33Ni alloys at 523, 573, and 588 K; (b–d) corresponding van't Hoff plots (ln P vs. 1000/T).

Fig. 11. Schematic illustration of the hydrogen absorption and desorption mechanisms in (a) Mg-25Ni and (b) Mg-33Ni alloys.