A computational study of electronic structure, thermodynamics and kinetics of hydrogen desorption from Al- and Si-doped α-, γ-, and β-MgH2

Abstract

First principles calculations of pure and Al- and Si-doped α-, γ-, and β-MgH₂ were performed to investigate the influence of Al and Si as impurities and the presence of high pressure phases on the properties of hydrogen sorption of magnesium hydride. The ab initio plane wave pseudopotential method based on density functional theory within the generalized gradient approximation was used in the present study. The total energies of the considered systems were calculated as a function of cell volume to obtain material properties such as bulk modulus K₀, equilibrium cell volume V₀ and minimum energy E₀(V₀). From the density of states (DOS) analysis, it was shown that doping MgH₂ with Al and Si caused a reduction in the band gaps of each of the three phases. The diminished band gaps made the Mg–H bond more susceptible to dissociation. The destabilization of the hydrides was reflected in the decreased heats of formation of the doped hydrides, with the following ΔH_f order: Si < Al and β < γ < α. A 30.5% reduction in the activation energy barrier E_act for H₂ desorption was calculated for the Al-doped α-MgH₂(001) surface and a 15.5% decrease in E_act of the Si-doped γ-MgH₂(001) surface was deduced, while doping with Al and Si increased the activation energy barrier for the β-MgH₂ (001) surface drastically.