Hydrogen storage properties of AZ31–carbon black composite: the role of Ni and Ti catalysts at different reaction temperatures

High-energy ball milling was used to create AZ31 composites modified with carbon black (CB), nickel (Ni), and titanium (Ti) in order to address the slow sorption kinetics of magnesium-based hydrogen storage materials. The effect of additive type and concentration on microstructural evolution and kinetic mechanisms was examined in a systematic investigation of the hydrogen storage performance over a temperature range of 375, 350, and 325 °C. Kinetic modeling using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) and Arrhenius equations revealed a quantitative mechanistic dichotomy between the additives. Nickel acts as a potent "surface kinetic accelerator," driving ultra-fast hydrogen dissociation via surface spillover. This mechanism significantly lowered the apparent activation energy (Ea ≈ 141.3 kJ/mol), enabling the AZ31 + 3CB + 1Ni composite to achieve 95% hydrogen saturation in just 702 s at 375 °C and maintain rapid kinetics (1040 s) at 325 °C. Conversely, titanium functions as a "bulk structural modifier." Although Ti agglomeration shifted the rate-limiting step toward bulk lattice diffusion (elevating Ea to 182.0 kJ/mol), it provided robust Zener pinning and deep nucleation pathways. Consequently, while Ni excels in reaction velocity, the AZ31 + 3CB + 1Ti composite successfully prevents high-temperature grain coarsening, achieving an exceptional maximum reversible capacity of 4.00 wt% even at the lowest operating temperature of 325 °C (outperforming the 3.65 wt% of the Ni-doped composite). This study highlights a targeted design strategy that balances ultra-fast kinetics with thermodynamic microstructural stability.