Synergistic Effects of V and Ni Catalysts on Hydrogen Sorption Kinetics of Mg-Based Hydrogen Storage Materials: A Computational Study

Abstract

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.