Comparative analysis of hydrogen uptake and release kinetics in MgH₂/C composites synthesized using varied surface areas graphite powders

The synthesis process vastly conditions the temperature behaviour decomposition reactions of magnesium hydride. In our experiments, the MgH₂/C composites were prepared by the high-energy reactive co-milling (HERBM) technique of the elemental magnesium with graphite powders in a hydrogen gas atmosphere. The graphite powders with different initial specific surface areas were admixed before HERBM to magnesium as active dopants. The forming/decomposition kinetics of magnesium hydride, activation energy, and sorption/desorption mechanisms in MgH₂/C composites were explored by the Johnson–Mehl–Avrami–Kolmogorov method. A correlation has been found between the activation energy of sorption/desorption processes and the specific area of graphite powder. The lowest hydrogen desorption activation energy (78 kJ/mol) is for the composite MgH₂/C fabricated using graphite powder with 8.1 m²/g of surface area. In contrast, the lowest hydrogen sorption activation energy (65 kJ/mol) is for the MgH₂/C composite, fabricated by co-milling magnesium and graphite powder with 329 m²/g surface area. It was shown the several concurrent processes rather than a single well-defined one is responsible for the hydrogen uptake/release reactions for a graphite-less sample and graphite-added ones.