| In this thesis, based on the review of the research and development of the AB3 type rare earth-based hydrogen storage alloys, the La-Mg-Ni-Co-Mn-Al-based hydrogen storage electrode alloy La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 was selected as the study object of this work. By means of XRD/Rietveld analyses and the electrochemical tests including the galvanostatic charge-discharge, electrochemical impedance spectra (EIS), linear polarization, anodic polarization, potentialstatic discharge and etc., the relationships among compositions, phase structure and electrochemical properties were systemically studied for developing the new type rare earth-based hydrogen storage electrode alloys with high discharge capacity and long cycling life. The study includes the effect of substitution of Nd for La on the structure and electrochemical properties of La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 hydrogen storage alloy electrode, the effect of substitution of Pr for La on the structure and electrochemical properties of La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 hydrogen storage alloy electrode and the effect of substitution of Cr for Ni on the structure and electrochemical properties of La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 hydrogen storage alloy electrodes.The effects of substitution of Nd for La on the structural and electrochemical properties of the La0.7-xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) hydrogen storage alloys had been studied systematically. The results of X-ray powder diffraction showed that the alloys were all consisted of the (La, Mg)Ni3 phase and the LaNi5 phase, and the lattice parameters and cell volumes of the component phases all decreased with increasing x. With the increase of Nd content, the abundance of the (La, Mg)Ni3 phase decreased from 69.39 wt% (x=0.00) to 49.58 wt% (x=0.30), whereas the abundance of the LaNi5 phase increased from 30.61 wt% (x=0.00) to 50.42wt% (x=0.30). The electrochemical studies showed that the maximum discharge capacity decreased when x increased from 0.00 to 0.30. However, the cycling stability of the alloy electrodes was improved with the increase of Nd content. After 100 charge/discharge cycles, the capacity retentions (S100) increased from 73.2% (x=0.00) to 80.9% (x=0.30). The high rate dischargeability of the alloy electrodes were all rather high increasing from 61.8% (x=0.00) to 70.1% (x=0.10) and then decreasing to 53.7% (x=0.30) at a high discharge current densityof 1000 mA/g. Meanwhile, the results of the electrochemical impedance spectra, linear polarization, Tafel polarization, and hydrogen diffusion coefficient measurements all indicated that the exchange current density Io, the limiting current density IL and the hydrogen diffusion coefficient D of the alloy electrodes also all increased first and then decreased with increasing Nd content in these alloys, which signified that the electrochemical kinetics property of La0.7-xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2(x=0.00-0.30) hydrogen storage alloys had a variety alike with different value of Nd content. Considering the general effect of substitution of Nd for La on the overall performance of the La0.7-xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2(x=0.00-0.30) alloy electrodes, the optimum composition was found to be x=0.10. The alloy electrode can be completely activated within 2 cycles and its maximum discharge capacity is 372.7 mAh/g, the HRD1000 value reaches 70.1%, S100 is 78.4%.The effects of substitution of Pr for La on the structural and electrochemical properties of La0.7-xPrxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x=0.00-0.30) hydrogen storage alloys had been studied systematically. The results of X-ray powder diffraction showed that the alloys were all consisted of the (La, Mg)Ni3 phase and the LaNi5 phase, and the lattice parameters and cell volumes of the component phases all decreased with increasing x. With the increase of Pr content, the abundance of the (La, Mg)Ni3 phase decreased from 69.48 wt% (x=0.00) to 40.52 wt% (x=0.30), whereas the abundance of the LaNi5 phase increased from 30.52 wt% (x=0.00) to 59.48wt% (x=0.30). The electrochemical studies showed that the maximum discharge capacity decreased when x increases from 0.00 to 0.30. However, the cycling stability of the alloy electrodes was improved with the increase of Pr content. After 100 charge/discharge cycles, the capacity retentions (S100) increased from 75.7% (x=0.00) to 80.6% (x=0.30). The results of the high rate dischargeability, the electrochemical impedance spectroscopy, linear polarization, Tafel polarization, and hydrogen diffusion coefficient measurements all indicated that the exchange current density Io, the limiting current density Il and the hydrogen diffusion coefficient D of the alloy electrodes all increased first and then decreased with increasing Pr content in these alloys, which signified that the electrochemical kinetics property of the La0.7-xPrxMg0.3Ni2.45Co0.75Mn0.1Al0.2(x=0.00-0.30) hydrogen storage alloys had a variety alike with different value of Pr content. Considering the general effect of substitution of Pr for La on the overall performance of the La0.7-xPrxMg0.3Ni2.45Co0.75Mn0.1Al0.2(x=0.00-0.30) alloy electrodes, the optimumcomposition was found to be x=0.15. The alloy electrode can be completely activated within 2 cycles and its maximum discharge capacity is 356.8 mAh/g, the HRD1000 value reaches 78.9%, S100 is 79.4%.The effects of substitution of Cr for Ni on the structural and electrochemical properties of La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Crx (x=0.00, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60) hydrogen storage alloys were studied systematically. The results of X-ray powder diffraction and Rietveld analyses showed that the alloys were all consisted of the (La, Mg)Ni3 phase and the LaNi5 phase. Meanwhile, the Cr2Ni3 phase and the LaNi5 phase were found with the increase of Cr content. When x increases from 0.00 to 0.60, the abundance of the (La, Mg)Ni3 phase decreased, whereas the abundance of the LaNi5 phase, the Cr2Ni3 phase and the LaNi5 phase increased, which signified that substitution of Cr for Ni in La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Crx (x=0.0-0.60) hydrogen storage alloys promoted the forming of the LaNi5 phase, the Cr2Ni3 phase and the LaNi5 phase. The electrochemical studies showed that the maximum discharge capacity decreased from 355.2mAh/g (x=0.00) to 234.5mAh/g (x=0.60). The cycling stability of the alloy electrodes was decreased with the increase of Cr content. After 100 charge/discharge cycles, the capacity retentions (S100) were 68.6%(x=0.0), 67.0%(x=0.10), 62.1%(x=0.20), 58.8%(x=0.30), 57.1%(x=0.40), 61.2%(x=0.50), 58.7%(x=0.60), respectively. The results of the high rate dischargeability, the electrochemical impedance spectra, linear polarization, Tafel polarization, and hydrogen diffusion coefficient measurements all indicated that the exchange current density I0, the limiting current density IL and the hydrogen diffusion coefficient D of the alloy electrodes all decreased with increasing Cr content in these alloys. It signified that the electrochemical impedances of La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Crx (x=0.00-0.60) hydrogen storage alloys were increased, but the speed rate of the hydrogen diffusion were decreased which arise the decreas of high rate dischargeability. The decrease of high rate dischargeability can be attributed to the variation of the surface electrocatalytic activity as a result of more Ni content substituted by Cr in these alloys.
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