Superior MgH2 Kinetics with MgO Addition: A Tribological Effect

Abstract

The kinetics of hydrogen absorption/desorption in magnesium can be improved without any catalysis assistance and MgO was found to be more effective than the best catalyst reported so far, i.e., Nb₂O₅. Herein, a quantitative analysis of the hydrogen kinetics in magnesium modified with MgO was performed in order to identify possible rate controlling mechanisms. While hydrogen absorption was found to be diffusion controlled as commonly reported, hydrogen desorption evolved from nucleation and growth to an interface controlled process depending on the desorption temperature. Comparison with the effect of Nb₂O₅ indicates that similar rate limiting steps occur regardless of the oxide additive. These findings are reconciled by considering the tribological effect of solid oxide additives, as a correlation between oxides electronegativity and improvement in hydrogen kinetics was found. Such a correlation clearly highlights the mechanical effect of solid oxides in facilitating the grinding and stabilisation of small magnesium particles for efficient and fast hydrogen kinetics.