Effects Of Substitution Of Zr Or Ti For La On The Structure And Electrochemical Properties Of La-Mg-Ni-Co-based Hydrogen Storage Electrode Alloys

In this thesis, based on the review of the research and development of the non-AB5 type rare earth-based hydrogen storage alloys, the La-Mg-Ni-Co-based hydrogen storage electrode alloys were selected as the study object of this work. By means of XRD (Rietveld) analysis and the electrochemical test methods including the galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potentialstatic discharge etc., the effects of substitution of Zr or Ti for La on the structure and electrochemical properties of La0.7-xZrxMg0.3Ni2.45Mn0.1Co0.75Al0.2 (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) hydrogen storage electrode alloys or La0.7-xTixMg0.3Ni2.45Mn0.1Co0.75Al0.2 (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) hydrogen storage electrode alloys were studied so that we could know about the influence regularity that the substitution of Zr or Ti affect the structure and electrochemical properties of the studied hydrogen storage electrode alloys and the relationship among compositions, the structure and electrochemical properties of the studied hydrogen storage electrode alloys, and improve the overall properties of the hydrogen storage alloys in order to develop a new type rare earth-based hydrogen storage electrode alloy in practical application.The effects of substitution of Zr for La on the structure and electrochemical properties of the La0.7-xZrxMg0.3Ni2.45Mn0.1Co0.75Al0.2 (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) hydrogen storage alloys have been first studied systematically. X-ray powder diffraction (XRD) analysis shows that all the studied hydrogen storage alloys mainly consist of a (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and a LaNi5 phase with the hexagonal CaCu5-type structure. The cell volumes of these two phases decrease with increasing x and the abundances of (La,Mg)Ni3 phase decrease, the abundances of LaNi5 phase increase when La is gradually substituted of Zr. Electrochemical studies indicate that the maximum discharge capacity decreases from 364.5 mAh/g (x=0) to 306.9 mAh/g (x=0.1) and the cyclic stability of the studied alloys become better at an optimum substitution, the high rate dischargeability (HRD) first increases and then decreases when x increases, the high rate dischargeability increases from 55.5% (x=0) to 63.7% (x=0.02) and then decreases to 18.2% (x=0.1) at the discharge current density Id=1000 mA/g. Electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potential-step measurements showed that the electrochemical resistance increases and the exchange current densityIo always decreases, both the limiting current density IL and the hydrogen diffusion coefficient D first increase and then decrease with increasing x from 0 to 0.1. The electrochemical kinetics properties of the alloy with x=0.02 is better. And the overall properties of the alloys is better than others when x=0.02.In the second part of this work, the structure and electrochemical properties of the La0.7-xTixMg0.3Ni2.45Mn0.1Co0.75Al0.2 (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) hydrogen storage alloys have been studied systematically. The studies shows that all the studied hydrogen storage alloys mainly consist of a (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and a LaNi5 phase with the hexagonal CaCu5-type structure by X-ray powder diffraction (XRD) analysis. When x increases, the cell volumes of these two phases decrease, the abundances of (La,Mg)Ni3 phase decrease and the abundances of LaNi5 phase increase. Electrochemical studies indicate that the maximum discharge capacity decreases from 366.6 mAh/g (x=0) to 329.7 mAh/g (x=0.1) and the cyclic stability of the studied alloys become better at an optimum substitution of Ti for La and the alloy with x=0.04 exhibits the best cyclic stability. The high rate dischargeability (HRD) first decreases and then increases, and decreases at last when La is substituted of Ti. Electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potential-step measurements showed t...