Revealing the Role of Grain Boundaries in Magnesium Hydrogen Storage: Insights into Adsorption and Dissociation

Mg is an attractive hydrogen storage material, yet its practical application is hindered by sluggish hydrogen uptake due to high H₂ dissociation barriers. Although experiments suggest that grain boundaries (GBs) serve as preferential sites for hydride nucleation, the atomic-scale mechanisms remain unclear. In this paper, we employed density functional theory (DFT) calculations to elucidate hydrogen adsorption and dissociation at representative Mg twin boundaries with different misorientation angles. We found that hydrogen adsorption consistently favors Hollow sites at GBs owing to strong Mg–H orbital hybridization. Among the studied configurations, the {10"1" @#x0305;1} twin boundary exhibits the lowest dissociation barrier (0.74 eV), reduced by 34.5% compared with Mg (0001). Strikingly, the dissociation barriers follow a non-monotonic “reversed volcano” trend with GB rotation angle, where intermediate-angle GBs maximize charge transfer into the H₂ σ* orbital and thereby facilitate bond cleavage. This synergy between local free volume, coordination number, and electronic redistribution provides a unified descriptor (χgem) that rationalizes the angular dependence of reactivity. Our findings establish a clear mechanistic link between GB geometry and hydrogen activation, offering design principles for tailoring microstructures to accelerate hydrogen storage kinetics in Mg-based materials.