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

Adding transition metals (TMs) in Mg-based hydrogenstorage materials has been proposed as a promising approach toimprove their storage performance. It was experimentally shown thatadding Ni and V catalysts in Mg dramatically decreased the formationenthalpies and activation energies of hydrogenation and dehydrogen-ation. Herein, we aim to unravel the roles of Ni and V catalysts inimproving the hydrogen absorption process in Mg-based storagematerials usingfirst-principles methods. Mg2Ni and V clusters depositedon Mg2Ni structures were modeled, as evidenced by experimentalobservations. The results indicate that both V and Ni facilitatespontaneous H2dissociation and stabilize hydrogen adsorption. Suchstrong interactions stem from the strong hybridization between themolecular orbital of adsorbed hydrogen and the Ni and V 3d states. Theaddition of the V cluster on the Mg2Ni surface also induces surface reconstruction, and consequently, more strong adsorption sitesare available and the sites with connected Ni are formed, which could promote more facile diffusion paths of hydrogen spillover fromthe cluster to the surface and surface diffusion. Although hydrogen diffusion to a subsurface is the most kinetically limited step at lowhydrogen 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 thathydrogen absorption into the V/Mg2Ni material is kinetically and thermodynamically appreciable at operating conditions of high H2pressure. The catalytic roles of Ni and V for the hydrogen absorption process also agree with the phenomenon seen in ab initiomolecular dynamics simulations where the hydrogen absorption process occurs at a significantly faster rate on the Mg2Ni structureand even faster on the V/Mg2Ni structure compared to the pure Mg structure. Through systematic computational investigations, ourfindings provide in-depth theoretical insights and guidance on using a combination of TM catalysts to improve the performance ofMg-based hydrogen storage materials.

Automated summary

This study investigates the effects of adding Ni and V catalysts on the hydrogen absorption process in Mg-based storage materials using first-principles methods. The results show that both Ni and V facilitate hydrogen dissociation and stabilize hydrogen adsorption. The addition of V clusters induces surface reconstruction and provides more strong adsorption sites. The computational investigations suggest that hydrogen absorption into V/Mg2Ni material is both kinetically and thermodynamically feasible at high H2 pressure.