Solid-state reactions and hydrogen storage in magnesium mixed with various elements by high-pressure torsion: Experiments and first-principles calculations

Magnesium hydride is widely known as an interesting candidate for solid-state hydrogen storage. However it is too stable and does not desorb hydrogen at ambient conditions. Although MgH₂ suffers from slow kinetics, its hydrogenation kinetics can be significantly improved by addition of catalysts and/or decreasing the grain size. Reducing the thermodynamic stability of MgH₂ is now the main challenging task. In this study, 21 different elements were added to magnesium in atomic scale by using the High-Pressure Torsion (HPT) technique and different kinds of nanostructured intermetallics and new metastable or amorphous phases were synthesized after HPT (Mg₁₇Al₁₂, MgZn, MgAg, Mg₂In, Mg₂Sn, etc.) or after post-HPT heat treatment (MgB₂, Mg₂Si, Mg₂Ni, Mg₂Cu, MgCo, Mg₂Ge, Mg₂Pd, etc.). In most of the compounds, the desorption temperature decreases by addition of elements, even though that the ternary hydrides are formed only in limited systems such as Mg-Ni and Mg-Co. Appreciable correlations were achieved between the theoretical binding energies obtained by first-principles calculations and the experimental dehydrogenation temperatures. These correlations can explain the effect of different elements on the hydrogenation properties of the Mg-based binary systems and the formation of ternary hydrides.