Phase Structure And Electrochemical Properties Of AB₃-type La-Mg-Ni-based Hydrogen Storage Electrode Alloys

Based on the review of the state of the art of the rare earth-magnesium-based hydrogen storage electrode alloys, the AB3-type La-Mg-Ni-based hydrogen storage electrode alloys with higher discharge capacity are selected as the studied object in this work. By means of XRD and SEM microstructure analysis, and galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), linear polarization and anodic polarization electrochemical test methods, the effects of the La/Mg ratio, heat treatment and electrolyte modification on the phase structure and electrode performances of the over-stoichiometric ratio (La, Mg)Ni2.95Coo.25Al0.3 alloys are investigated systematically in order to improve the cycling stability of the alloy electrodes.Firstly, the structure characteristic and electrochemical properties of the La1-xMgxNi2.95Coo.25Al0.3 (x=0.0-0.4) electrode alloys are studied. It is found that all the as-cast alloys present multi-phase structure. With increasing x, the primary phases vary from the binary phase of LaNi5 and La2Ni7 for x=0-0.1, and LaNi5 and La2MgNi9 for x=0.15-0.3, to the ternary phase of LaNi5, La2MgNi9 and LaMg2Ni9 for x=0.35-0.4. The electrochemical examinations show that the activation property of the alloy electrodes maintains unchanged with the increasing Mg content, however, the maximum discharge capacity (Cmax), the cycle stability (S100)and the high rate dischargeability (HRD500) are significantly improved owing to the changes in the phase structure. The optimum composition is found to be x=0.3, at which Cmax, S100 and HRD500 are 341.5 mAh/g,65.6% and 87.3%, respectively.Secondly, the effect of heat treatments on the structure and electrochemical properties of the La0.7Mg0.3Ni2.95Co0.25Alo.3 alloys are revealed. The results indicate that the crystallization, the composition homogenization, the phase constitution and the phase abundance are closely correlated with the heat treatment approaches. It is found that the discharge capacity and the cycling stability of the La0.7Mg0.3Ni2.95Co0.25Al0.3 alloy electrode are improved by the water-cooling treatment while the high dischargeability is slightly decreased. The Cmax and S100 of the alloy annealed at 1173 K for 8 h followed by water cooling are 352.8 mAh/g and 73.8%, respectively.Finally, for further improving the cycling duration of the La-Mg-Ni-based alloy electrodes, MgO and Al2O3 are separately introduced to the KOH electrolyte, and the electrochemical behaviors of the La0.7Mgo.3Ni2.95Coo.2sAlo.3 alloy electodes are investigated. The results show that the activation (Na), the maximum discharge capacity (Cmax) and the initial cycling stability (S10) maintain almost unchanged, but the capacity retention after 100 cycles is improved in some degree. It is believed that the particle pulverization is the primary factor for the capacity degradation during the intial cycling stage, while the degradation of the discharge capacity is attributed to the corrosion of the active elements with the cycling proceeding. The addition of MgO and Al2O3 inhibits in part the segregation and oxidation/corrosion of the the active elements owing to the formation of the Mg(OH)2 and Al2O3 protective films, and consequently improving the cycling endurance for the long-time cycling.