Improvement of Hydrogen Vacancy Diffusion Kinetics in MgH2 by Niobium- and Zirconium-Doping for Hydrogen Storage Applications

Abstract

Transition metal (TM) catalytic dopants are broadly used in hydrogen storage materials to increase H2 desorption and absorption kinetics. We have studied H vacancy formation energy in pure, Nb- or Zr- doped bulk magnesium hydride using density functional theory based calculations. The preferential dopant location was determined by means of occupation energy analysis. Both TM species prefer substitutional locations, Zr being more stable than Nb. Five different sites for H vacancy formation have been considered, all of them near the dopant. The vacancy formation energy decreases, especially in the interstitial Zr system in comparison with a pure one (from 1.35 to 0.51 eV). Concerning diffusion, we consider four paths in the pure and doped systems: the doping with TMs diminishes the activation energy barriers improving the diffusion kinetics, being more considerable for Zr. Effects of a possible spin polarization induced in the system by TM atoms and H vacancies generation have been considered as well.