Influence of the substitution of La for Mg on the microstructure and hydrogen storage characteristics of Mg20-xLaxNi10 (x=0-6) alloys

In order to enhance the glass forming ability of the Mg2Ni-type hydrogen storage alloy, the Mg in the alloy was partially substituted by La. The alloys Mg20-xLaxNi10 (x = 0, 2, 4, 6) were prepared by casting and rapid quenching. The structures and morphologies of the as-cast and the quenched alloys were studied by XRD, SEM and HRTEM. It was found that no amorphous phase was formed in the as-quenched La-free alloy. But the as-quenched alloys containing La held a major amorphous phase, confirming that the substitution of La for Mg significantly enhances the glass forming ability of the alloys. When La content x <= 2, the major phase in the as-cast alloys is Mg2Ni phase, but with the further increase of La content, the major phase of the as-cast alloys changes into (La,Mg)Ni-3 + LaMg3 phase. Thermal stability of the asquenched alloys was studied by DSC, showing that La content engenders a negligible influence on the crystallization temperature of the amorphous phase. The hydrogen absorption and desorption kinetics of the as-cast and the quenched alloys were measured by an automatically controlled Sieverts apparatus. The results showed that the hydrogen absorption and desorption capacities and kinetics of the as-cast alloys clearly rise with increasing La content. For La content x = 2, the as-quenched alloy displays an optimal hydrogen desorption kinetics at 200 degrees C. The electrochemical measurement showed that the discharge capacities of the as-cast alloys rose with the increase of La content, but those of the as-quenched alloys obtained the maximum values with the variation of La content. The cycle stability of the as-cast and the quenched alloys significantly improved with increasing La content. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.