Dual tuning of the de-/hydrogenation thermodynamics and kinetics of the Mg–Ni alloy by introducing the Ag–H bond: enhanced hydrogen storage properties at moderate temperatures

Abstract

Mg-based hydride is considered a promising candidate for hydrogen storage. However, the de-/hydrogenation properties at moderate temperatures have limited its application. In this study, Ag micro-alloying is utilized to improve the hydrogen storage properties of the Mg₈₅Ni₁₅ material. Due to the dissolved Ag atoms in the partial Mg₂Ni lattice, the lamellar Mg–Mg₂Ni eutectic transforms into a fishbone Mg–Mg₂Ni eutectic. The results indicate that the thermodynamics and kinetics of de-/hydrogenation in the Mg₈₅Ni_14.8Ag_0.2 alloy have significantly improved. The hydrogenation capacities at 50, 100, and 175 °C are 1.8, 3.0, and 4.1 wt%, respectively. The Mg₈₅Ni_14.8Ag_0.2 hydride starts to decompose only at 178 °C. Owing to the doping of 0.2 at% Ag, the activation energy for dehydrogenation is reduced from 110.6 kJ mol⁻¹ of the Mg₈₅Ni₁₅ hydride to 89.4 kJ mol⁻¹. The dehydrogenation enthalpy value decreases to 65.6 kJ mol⁻¹, and the corresponding platform pressure increases slightly. The optimized thermodynamic properties of dehydrogenation are attributed to the substitution of partial Ni atoms with Ag atoms in Mg₂Ni. This leads to the formation of unstable Ag–H bonds, and the breaking of these bonds results in the instability of the Mg₂Ni(Ag)H₄. The desorption of Mg₂Ni(Ag)H₄ further promotes the decomposition of MgH₂. In addition, the increased lattice spacing provides a wider channel for the transport of H atoms, which explains the improvement in hydrogen de-/absorption kinetics.