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

Magnesium dihydride $({\text{MgH}}_{2})$ stores $7.7\text{ }\text{wt}\text{ }%$ hydrogen but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM) $(=\text{Sc},\text{Ti},\text{V},\text{Cr})$ aims at improving the thermodynamic and kinetic properties. We study the structure and stability of ${\text{Mg}}_{x}{\text{TM}}_{1\ensuremath{-}x}{\text{H}}_{2}$ 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 $x\ensuremath{\gtrsim}0.8$ correlates with a phase transition of ${\text{Mg}}_{x}{\text{TM}}_{1\ensuremath{-}x}{\text{H}}_{2}$ 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 ${\text{Mg}}_{x}{\text{TM}}_{1\ensuremath{-}x}{\text{H}}_{2}$ can be tuned over the substantial range of 0--2 eV/f.u. Assuming however that the alloy ${\text{Mg}}_{x}{\text{TM}}_{1\ensuremath{-}x}$ 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.