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.
Highlights
Magnesium has been the centre of extensive investigations for its potential use as a medium for storing hydrogen on board vehicles because it is an abundant and low cost metal that can store up to7.6 mass % of hydrogen [1,2]
Several groups have advanced the formation of a nanostructure upon mechanical milling as a possible ground for the kinetics improvements [7,8,9]; some investigations have shown that the nanostructure and associated strain and grain boundary defects may not play a crucial role in the overall kinetic enhancement observed [10,11,12]
Other investigations have found similar results with hydrogen kinetics deteriorating as an oxide layer is allowed to grow at the surface of MgH2 particles [27,34]
Summary
Magnesium has been the centre of extensive investigations for its potential use as a medium for storing hydrogen on board vehicles because it is an abundant and low cost metal that can store up to7.6 mass % of hydrogen [1,2]. The practical use of magnesium as a hydrogen store has always been restrained by the high stability of its hydride ( H300 °Cdes = −74 kJ.mol−1 H2) and poor kinetics when it comes to release or absorb hydrogen [3,4]. When considering hydrogen kinetics in magnesium, two main hypotheses are generally accepted with respect to the mechanisms limiting hydrogen absorption/desorption rates. Several groups have advanced the formation of a nanostructure upon mechanical milling as a possible ground for the kinetics improvements [7,8,9]; some investigations have shown that the nanostructure and associated strain and grain boundary defects may not play a crucial role in the overall kinetic enhancement observed [10,11,12]. The high temperature of 300 C required for hydrogen absorption/desorption leads to strain relaxation and defects annihilation
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