MgH2 with a Pre-Synthesized Ti–Ni Composite Catalyst: Superior Sorption Kinetics and High Saturated Capacity over Physically Mixed Ni/Ti

Abstract Magnesium hydride (MgH2) is a promising solid-state hydrogen storage material owing to its high theoretical capacity, earth abundance, and low cost. However, its practical application is severely hindered by sluggish sorption kinetics and high thermodynamic stability. Herein, we systematically compare the catalytic effects of elemental Ni, elemental Ti, physically mixed Ni/Ti, and a pre-synthesized Ti–Ni composite catalyst on the hydrogen storage performance of MgH2, aiming to understand the advantages and disadvantages of the catalytic performance of Ni and Ti and to exert their synergistic catalytic effect. Our results suggest that Ni is associated with faster initial hydrogenation kinetics and lower onset dehydrogenation temperatures, while Ti correlates with higher equilibrium capacities and more sustained hydrogenation/dehydrogenation behavior at elevated temperatures. These distinct catalytic tendencies are inferred from systematic comparisons of sorption kinetics, phase evolution, elemental distribution, and kinetic modeling. Simple co-doping of Ni and Ti partially combines these benefits but fails to eliminate the intrinsic drawbacks of each element. It is noteworthy that the pre-synthesized Ti–Ni composite catalyst further overcomes the above-mentioned drawbacks and exhibits a more uniform distribution of elements, which further improves both the hydrogen absorption rate and the extent of the reaction in MgH2. As a result, the MgH2–5wt%pre-Ti-Ni-c-c composite absorbs 6.10 wt % H2 within 27.3 s at 275 °C and releases 5.54 wt % H2 within 548 s at 325 °C, with a markedly reduced dehydrogenation enthalpy from 93.28 to 72.25 kJ mol–1. After 10 cycles, its hydrogen absorption and desorption capacity retentions reach 96.3 and 93.5%, respectively, demonstrating improved initial cycling stability compared with BM-MgH2 (82.6%). Mechanistic studies combining phase evolution, kinetic modeling, and DFT calculations attribute the superior performance to the combined action of three components: the NiTi bimetallic phase for a collaborative catalytic effect, reversible Mg2Ni/Mg2NiHx phases that contribute to rapid initial kinetics, and TiO2 that induces stronger lattice distortion. This work demonstrates that the pre-synthesized Ti–Ni composite catalyst, with its superior dispersion, effectively overcomes the limitations of physically mixed Ni/Ti systems and achieves a better balance between rapid initial reaction rates and deeper reactions in actual test results.