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

Abstract

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.