Effect of Microstructural Refinement and Na Addition on Hydrogenation Kinetics of Cast Mg–Al–La Alloy During the First Hydrogen Absorption Process

Abstract

A fine eutectic structure in Mg-based alloys for hydrogen storage systems is advantageous for hydrogen sorption kinetics, since hydrogen diffuses preferentially along interphase boundaries. In our previous study, we have discovered a composition (Mg-4.6%Al-6.3La) in the Mg-rich corner of the MgAl–La alloy system that is characterised by a very fine eutectic structure. Potentially, this alloy could exhibit superior hydrogen absorption kinetics even without the addition of catalytic elements such as Na due to a high density of interphase boundaries. In this work, we have studied the hydrogen absorption kinetics of Mg-4.6%Al-6.3La alloy with and without the addition of Na (0.2%) during the initial absorption process. The addition of Na to Mg-4.6%Al-6.3La alloy coarsened the (Al, Mg)3La phase in the eutectic structure compared to the Na-free Mg-4.6%Al-6.3%La alloy reducing the density of interphase boundaries between Mg and (Al, Mg)3La that can function as hydrogen diffusion pathways. Additionally, in Na-containing alloys, no indication of twinning in the (Al, Mg)3La phase was found. Despite the absence of twinning in the (Al, Mg)3La phase and the low density of interphase boundaries, the Na-containing MgAl–La sample demonstrated faster hydrogen absorption kinetics than the Na-free counterpart. Contrary to previous predictions, our work established that a high density of interphase boundaries and twinning in the intermetallic phase are not always correlated with hydrogen absorption kinetics and it is suggested that the absorption kinetics during the initial hydriding process may be significantly affected by the independent effect of Na.KeywordsMagnesiumHydrogen storageMicrostructureKinetics