Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds

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Magnesium dihydride () 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 MgₓTM₁₋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 MgₓTM₁₋ₓH₂ 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 MgₓTM₁₋ₓH₂ can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy MgₓTM₁₋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.