Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds

Magnesium dihydride ($\mgh$) stores 7.7 weight % 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 Mg$_x$TM$_{1-x}$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 $x\gtrsim0.8$ correlates with a phase transition of Mg$_x$TM$_{1-x}$H$_2$ from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, Cr. Varying the transition metal (TM) and the composition $x$, the formation enthalpy of Mg$_x$TM$_{1-x}$H$_2$ can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy Mg$_x$TM$_{1-x}$ 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.