Magnesium hydride (MgH2) is a promising material for solid-state hydrogen storage due to its high gravimetric hydrogen capacity as well as the abundance and low cost of magnesium. The material's limiting factor is the high dehydrogenation temperature (over 300 °C) and sluggish (de)hydrogenation kinetics when no catalyst is present, making it impractical for onboard applications. Catalysts and physical restructuring (e.g., through ball milling) have both shown kinetic improvements, without full theoretical understanding as to why. In this work, we developed a machine learning interatomic potential (MLP) for the Mg-H system, which was used to run long time scale molecular dynamics (MD) simulations of a thick magnesium hydride surface slab for up to 1 ns. Our MLP-based MD simulations reveal previously unreported behavior of subsurface molecular H2 formation and subsequent trapping in the subsurface layer of MgH2. This hindered diffusion of subsurface H2 offers a partial explanation on the slow dehydrogenation kinetics of MgH2. The kinetics will be improved if a catalyst obstructs subsurface formation and trapping of H2 or if the diffusion of subsurface H2 is improved through defects created by physical restructuring.
© 2024 The Authors. Published by American Chemical Society.