Effect and mechanism of Mg on crystal structures and electrochemical cyclic stability of Ce_2Ni_7-type La-Mg-Ni-based hydrogen storage alloys

摘要

LaNi3.50 and La0.80Mg0.20Ni3.50 hydrogen storage alloys with single-phase Ce2Ni7-type structure are prepared, respectively. It is found that the expansion of [A(2)B(4)] subunits volumes of Mg-containing alloy is bigger than that of Mg-free alloy, while the expansion of [AB(5)] subunits volumes of the two alloys are almost the same, hence the deviation value of [A(2)B(4)] and [AB(5)] subunits volumes and strain value of super stacking structure for Mg-containing alloy are obviously higher than those of Mg-free alloy, and Mg-containing alloy is easier to be pulverized during charge/discharge cycles. Mg-containing alloy has more apparent cracks on the surface of alloy at the same cycle numbers, which demonstrate that Mg-containing alloy has poorer anti-pulverization. Besides, the presence of Mg hydroxides with a regular hexagon shape makes the surface of Mg-containing alloy become loose enough to be easily oxidized after charge/discharge cycles. The corrosion current density of Mg-containing alloy is larger than that of Mg-free alloy, so Mg-containing alloy is easier to be oxidized. Therefore, reducing the deviation value of [A(2)B(4)] and [AB(5)] subunits of super stacking structure and the corrosion of alloying element Mg is the basic solution to enhance the electrochemical cycling stability of La-Mg-Ni alloy. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.