Non-stoichiometric Mg-Ni composites for hydrogen storage: role of in-situ Mg$_2$Ni formation in enhancing hydrogen sorption performance

Magnesium-based alloys are promising hydrogen storage materials due to their high capacity but suffer from slow hydrogenation kinetics and high operating temperatures. While high-pressure torsion (HPT) enhances kinetics through microstructural refinement, and nickel addition provides catalytic effects, their combined influence on hydrogen storage performance remains insufficiently characterized. This work investigates Mg-Ni composites with variable Ni content (2–16 at%). After consolidating the powders by HPT and activating them in a Sievert apparatus, in-situ differential scanning calorimetry (DSC) was used to track phase evolution and extract thermodynamic parameters. DSC thermal cycling under hydrogen promoted the in-situ solid-state synthesis of Mg₂Ni from the elemental mixtures. Activation energy analysis revealed systematic reduction with increasing Ni content. While HPT enhanced initial hydrogenation kinetics, thermodynamic parameters (enthalpy, entropy) showed no systematic variation across HPT conditions, as high thermal cycling temperatures (up to 450 °C) led to the annealing of HPT-induced structural defects. The findings demonstrate that compositional tuning through in-situ Mg₂Ni formation dominates hydrogen storage performance, with HPT serving primarily as an initial activation aid.