Tunable hydrogen storage in magnesium–transition metal compounds: First-principles calculations

Magnesium dihydride (MgH₂) stores 7.7 wt % hydrogen but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM) (=Sc,Ti,V,Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of MgₓTM_1-xH₂ compounds, $x=[0--1]$, by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for x0.8 correlates with a phase transition of MgₓTM_1-xH₂ from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, and Cr. Varying the TM and the composition x, the formation enthalpy of MgₓTM_1-xH₂ can be tuned over the substantial range of 0--2 eV/f.u. Assuming however that the alloy MgₓTM_1-x does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content $(x=0.75)$ is close to that of pure Mg.