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

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 Mg85Ni14.8Ag0.2 alloy have significantly improved. The hydrogenation capacities at 50, 100, and 175 & DEG;C are 1.8, 3.0, and 4.1 wt%, respectively. The Mg85Ni14.8Ag0.2 hydride starts to decompose only at 178 & DEG;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. The onset dehydrogenation temperature of Mg85Ni14.8Ag0.2 reduces to 178 & DEG;C due to the enhanced hydrogen pumping effect of Mg2NiH4 by adjusting the Ni-H bond.