The dual role of vacancies and dopants in hydrogen dissociation and diffusion On Mg(0001)

Magnesium hydrides offer superior theoretical hydrogen storage capacities; however, their practical implementation is severely constrained by sluggish sorption kinetics. This study employs first-principles density functional theory (DFT) to elucidate the atomistic mechanisms governing hydrogen interaction with Mg(0001) surfaces, incorporating clean, vacancy-containing, Ti-doped, and Ti–M (M = Nb, V, Zr) co-doped configurations. Two-dimensional potential energy surface mapping reveals that H2 dissociation on pristine Mg(0001) is kinetically hindered by a 0.95 eV activation barrier along the bridge site (90,150). Substitutional Ti-doping significantly enhances surface reactivity, reducing this barrier to 0.25 eV, thereby functioning as a potent catalyst for hydrogen activation. Despite this catalytic benefit, nudged elastic band calculations identify a critical “dual role” for transition metal dopants: while Ti facilitates surface dissociation, it introduces a substantial subsurface kinetic trap characterized by a secondary migration barrier of 0.897 eV. Conversely, surface vacancies act primarily as diffusion promoters; although they increase the surface dissociation barrier to 1.10 eV, they facilitate hydrogen entry by lowering the initial penetration barrier to 0.328 eV. Projected density-of-states analysis confirms that Ti d-states provide the requisite orbital flexibility for facile dissociation but simultaneously induce localized stabilization that arrests long-range mobility. We demonstrate that Ti–V co-doping optimizes the energetic landscape, effectively mitigating Ti-induced trapping by restoring the subsurface barrier to 0.220 eV. This investigation identifies a fundamental Sabatier-type trade-off between surface activation and bulk transport, providing a robust theoretical framework for the strategic design of Mg-based storage materials.