Structure And Electrochemical Properties Of Low-Co La-Mg-Ni-based Hydrogen Storage Electrode Alloys

In this work, the effects of partial substitution of Mg with Ca and Co with Si on the structure and electrochemical properties of the La0.7Mg0.3Ni2.65Co0.75Mn0.1hydrogen storage electrode alloy were first investigated systematically by a series of structural analyses and electrochemical examinations including XRD, SEM, galvanostatic charge-discharge, EIS, linear polarization and anodic polarization. And then, the heat-treatment temperature was further optimized to improve the overall electrochemical properties of the low-Co La-Mg-Ni-based hydrogen storage electrode alloys.Firstly, the structure and electrochemical properties of the La0.7Mg0.3-xCaxNi2.65Co0.75Mn0.1(x=0.05-0.15) alloys was investigated. It was found that all the alloys consist of the (La,Mg)Ni3phase and the LaNi5phase. The presence of Ca changes the relative abundance of the (La,Mg)Ni3phase and the LaNi5phase. With increasing the content of Ca, the abundance of the (La,Mg)Ni3phase was decreased while it was increased for the LaNi5phase. At the same time, a small amount of the (La,Mg)2Ni7phase appeared in the alloys with Ca. Electrochemical investigations revealed that with increasing the content of Ca, the maximum discharge capacity of the La0.7Mg0.3-xCaxNi2.65Co0.75Mn0.1alloys was decreased while the cycling stability and the high dischargeability were improved. The La0.7Mg0.2Ca0.1Ni2.65Co0.75Mn0.1alloy exhibits the optimal overall electrochemical properties as its maximum discharge capacity is about371.7mAh/g, and the HRD value at1250mA/g is52.7%.And then, the effects of the partial substitution of Co with Si on the structure and electrochemical properties of La0.7Mg0.3Ni2.65Co0.75Mn0.1hydrogen storage electrode alloys were systematically investigated. XRD results showed that the abundance of (La,Mg)Ni3phase was decreased while it was increased for the LaNi5phase with increasing the Si content. Moreover, the La2Ni7phase could also be detected in the alloys with Si. Electrochemical examinations indicated that the partial substitution of Si for Co increased significantly the cycling stability of the La0.7Mg0.3Ni2.65Co0.75Mn0.1alloy, and improved its high rate dischargeability although the maximum discharge capacity was decreased. The x=0.15alloy exhibits the optimized overall electrochemical properties as its maximum discharge capacity is323.5mAh/g, the capacity retention is about71.9%after75cycles, and the value of HRD1220isabout55%.Finally, the effects of the heat-treatment temperatures on the structure and electrochemical properties of the optimized La0.7Mg0.3Ni2.65Co0.6Si0.15Mn0.1alloy were further investigated. The results show that the crystallinity and compositional homogeneity of the La0.7Mg0.3Ni2.65Co0.6Si0.15Mn0.1alloy were increased after annealed. With elevating the heat-treatment temperature, the maximum discharge capacity and the cycling stability were first improved and then decreased while the high dischargeability was gradually decreased. The optimal heat-treatment was determined to be annealed at950℃for8h as the maximum discharge capacity is about333.1mAh/g, and the capacity retention is76.6%after100cycles.