Microstructural evolution and electrochemical properties of the ultra-high pressure treated La0.70Mg0.30Ni3.3 hydrogen storage alloy

Abstract

This research highlighted the effects of ultra-high pressure (UHP) technique (1–5 GPa) on microstructural evolution and electrochemical performances of La0.70Mg0.30Ni3.3 alloy. Structure analysis showed that the as-cast alloy consisted of (La,Mg)Ni3, (La,Mg)2Ni7, (La,Mg)5Ni19, LaNi5 and LaMgNi4 phases. For all the UHP treated alloys, (La,Mg)5Ni19 phase disappeared via converting into (La,Mg)2Ni7 and LaNi5 phases. At 1 GPa, LaNi5 phase and LaMgNi4 phase fully transformed into (La,Mg)Ni3 and (La,Mg)2Ni7 phases by the peritectic reactions. However, when the pressure was 3 GPa and 5 GPa, the peritectic reactions were uncompleted for the atom diffusion was hindered, which not only caused the minor residual of LaNi5 and LaMgNi4 phases in the alloys, but also resulted in the increase of (La,Mg)Ni3 phase and the decrease of (La,Mg)2Ni7 phase. Electrochemical measurements displayed that the cyclic stability of alloy electrode treated at 1 GPa increased by 8.7% comparing with that of the as-cast alloy electrode, while the cyclic stability reduced when the pressure increased to 3 GPa and 5 GPa. This indicated that the appropriate UHP treatment was favorable for the improvement of the cycle life of alloy electrode. The high rate dischargeability (HRD) of alloy electrodes gradually decreased with the pressure increasing, which mainly resulted from the decrease of the hydrogen diffusion coefficient.