Interfacial Topology Engineering of Self-Derived TiO₂ Shells for Nucleation-Controlled Fast Kinetics in MgH₂

Magnesium hydride (MgH₂) is hampered by sluggish kinetics despite its high capacity. Here, we report a solvent-free, single-step mechanochemical strategy that drives the in situ reconstruction of titanium oxide acetylacetonate into a self-derived, high-coverage TiO₂ nanolayer on MgH₂ surfaces. Unlike conventional discrete particle catalysts, this surface-anchored TiO₂ phase maximizes the interfacial contact area. Kinetic analysis establishes a direct link between this high-coverage interfacial topology and nucleation-governed kinetics: the ubiquitous distribution of interfacial active sites shifts the dehydrogenation mechanism to be primarily controlled by 2D nucleation and growth, while rehydrogenation is initially driven by rapid surface-chemisorption. Consequently, the composite unlocks exceptional kinetic activity at low temperatures, releasing 4 wt.% H₂ at 186°C and absorbing 5.8 wt.% H₂ at 26°C within 10 h, retaining 99% capacity over 100 cycles. Density functional theory elucidates the atomistic origin of this enhancement: the unique TiO₂/MgH₂ band alignment channels electrons from hydrogen vacancies into Ti 3d states. This interfacial electronic polarization significantly stabilizes hydrogen vacancies and lowers the desorption activation barrier to 81 kJ mol^-1, validating the interface-dominated kinetic model. This work demonstrates that interfacial topological engineering via in situ reconstruction is a critical pathway to overcoming the kinetic bottlenecks of solid-state hydrogen storage.