Adding
transition metals (TMs) in Mg-based hydrogen storage materials
has been proposed as a promising approach to improve their storage
performance. It was experimentally shown that adding Ni and V catalysts
in Mg dramatically decreased the formation enthalpies and activation
energies of hydrogenation and dehydrogenation. Herein, we aim to unravel
the roles of Ni and V catalysts in improving the hydrogen absorption
process in Mg-based storage materials using first-principles methods.
Mg2Ni and V clusters deposited on Mg2Ni structures
were modeled, as evidenced by experimental observations. The results
indicate that both V and Ni facilitate spontaneous H2 dissociation
and stabilize hydrogen adsorption. Such strong interactions stem from
the strong hybridization between the molecular orbital of adsorbed
hydrogen and the Ni and V 3d states. The addition of the V cluster
on the Mg2Ni surface also induces surface reconstruction,
and consequently, more strong adsorption sites are available and the
sites with connected Ni are formed, which could promote more facile
diffusion paths of hydrogen spillover from the cluster to the surface
and surface diffusion. Although hydrogen diffusion to a subsurface
is the most kinetically limited step at low hydrogen contents, increasing
hydrogen coverages reduces such barriers by a half. The high hydrogen
coverage also drives surface, subsurface, and under-subsurface diffusion
to be highly thermodynamically favorable. The computational results
suggest that hydrogen absorption into the V/Mg2Ni material
is kinetically and thermodynamically appreciable at operating conditions
of high H2 pressure. The catalytic roles of Ni and V for
the hydrogen absorption process also agree with the phenomenon seen
in ab initio molecular dynamics simulations where the hydrogen absorption
process occurs at a significantly faster rate on the Mg2Ni structure and even faster on the V/Mg2Ni structure
compared to the pure Mg structure. Through systematic computational
investigations, our findings provide in-depth theoretical insights
and guidance on using a combination of TM catalysts to improve the
performance of Mg-based hydrogen storage materials.