Hybrid activation mechanism of thermal annealing for hydrogen storage of magnesium based on experimental evidence and theoretical validation

Magnesium is a promising candidate for solid hydrogen storage system, but it requires initial activation to absorb hydrogen due to the MgO passivation layer. Thermal annealing is one of the most effective approaches to activate Mg due to its operational simplicity. This study investigates the effect of thermal annealing on initial hydrogen uptake performance of Mg and complements the existing activation mechanism based on experimental evidence and theoretical validation. The thickness of MgO passivation layer is determined to be 15 nm, suppressing the initial hydrogen absorption of Mg powder to be negligible. After thermal annealing treatment, the initial hydrogen absorption is significantly improved to 6.1 wt%. Apart from the passivation film cracks, Mg nanoprecipitates are also observed on the annealed Mg particle surface, indicating that the bare Mg may behave as a defect reservoir with Mg vacancies retained on the surface. Passivation film cracking induced by thermal expansion mismatch paves way for hydrogen getting access to the bare Mg while surface Mg vacancy further reduces the energy barrier of H-2 dissociation. Both molecular dynamics simulations and density functional theory calculations offer supports for the hybrid activation mechanism.