Structures and Electronic and Hydrogen Storage Properties of Magnesium Scandium Hydrides

MgH₂ is well known as a potential hydrogen storage material. However, its high thermodynamic stability, high dissociation temperature, slow absorption, and desorption kinetics severely limit its application. Aiming at these shortcomings, we try to improve the hydrogen storage property of MgH₂ by doping with transition metal Sc atoms. The structures and electronic and hydrogen storage properties of Mg-Sc-H systems have been systematically studied by combining the crystal structure analysis by particle swarm optimization and density functional theory method. The results show that the structure of MgScH₈ with the R3 space group is the most stable one, which is proved to be a wide-band gap (2.96 eV) semiconductor. The possible decomposition pathways, which are crucial for the applicability of R3-MgScH₈ as a hydrogen storage material, are studied, and the pathway of MgScH₈ → ScH₆ + Mg + H₂ is found to be the most favorable one under 107.8 GPa pressure, while above 107.8 GPa, MgScH₈ → Mg + Sc + 4H₂ becomes the most thermodynamically stable pathway and releases the maximum amount of hydrogen. Based on the root mean square deviation calculation, it is found that R3-MgScH₈ begins to melt at 400 K. The result of ab initio molecular dynamics simulations shows that the hydrogen release capacity (4.04 wt %) can be easily achieved at 500 K, thus making MgScH₈ a potential hydrogen storage material.