A Study On The AB₃ Type Of Rare-Earth Hydrogen Storage Electrode Alloys

In this thesis, based on an overall review of the research and development of the non-AB5 type La-Ni based hydrogen storage alloys, the AB3-type (A=rare earth, Mg, Ca or Y, M=Ni, Co, Al, Cu, Zn) hydrogen storage alloys were selected and studied. By means of XRD analysis and electrochemical measurements, the effect of elemental substitution on the structural and electrochemical properties of the AB3-type hydrogen storage alloys has been systematically studied in order to optimize the overall electrochemical properties.The study of Mg substitution for La on the the structural and electrochemicalproperties of La1-xMgxNi2.875Mn0.1Co0.525 (x=0-0.7) hydrogen storage alloys showsthat all alloys are mainly composed of a the La(La,Mg)2Ni9 phase with therhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonalCaCu5-type structure. With increasing Mg content in the alloys, the abundance of theLa(La,Mg)2Ni9 (x=0,LaNi3) phase increases. However the LaNi5 phase abundancefirst increases and then decreases with increasing x. The P-C isotherms curves showthat with increasing Mg content in the alloys the plateau pressure for hydrogenabsorption and desorption increases, while the maximum hydrogen storage capacity[H/M]max is decreased. Electrochemical measurements show that the maximumdischarge capacity of the alloy electrode increases first and then decreases. Withincreasing Mg content in the alloys, the high rate dischargeability and the exchangecurrent density I0, and the limiting current density IL of the alloy increase first andthen decrease, while the electrochemical reaction resistance decreases first and thenincreases. The optimum composition is discovered to lie in x=0.3, at which themaxium discharge capacity, the activation cycles, the capacity decay rate D60,c at thedischarge rate 60mA/g after 80 cycles, the HRD at the discharge current density1000mA/g, the exchange current density I0, and the limiting current density IL of thealloy are 371.2mAh/g, 2, 2.85mAh/g cycle,, 65%, 301.7mA/g, 2313.4 mA/g,respectively.The effect of the cerium content on the structural and electrochemical properties of the La0.7-xCexMgo.3Ni2.875Mn0.1Co0.525 (x=0-0.5) hydrogen storage alloys has been studied systematically. The results show that all alloys are mainly composed of a the La(La,Mg)2Ni9 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonal CaCu5-type structure. With increasing Ce content, the cell volume of La(La,Mg)2Ni9 phase and LaNi5 phase decrease. With increasing Ce content in the alloys, the abundance of the La(La,Mg)2Ni9 phase increases but theLaNi5 phase abundance decreases. The P-C isotherms curves show that with increasing Ce content in the alloys, the plateau pressure for hydrogen absorption and desorption increases, while the maximum hydrogen storage capacity [H/M]max is decreased. With increasing Ce content the discharge capacity decreases and the cycling life can be gradually improved. The kinetics of the electrochemical hydrogen reaction in alloy electrodes increases first and then decreases with increasing Ce content in alloys. The optimum composition is found to lie in x=0.2, at which the maxium discharge capacity, the activation cycles, the capacity decay rate D60,c at the discharge rate 60mA/g after 80 cycles, the HRD at the discharge current density 1000mA/g, the exchange current density I0, and the limiting current density IL of the alloy are 295.6mAh/g, 3, 1.49 mAh/g'cycle, 61.4%, 358.61mA/g, 2702.69 mA/g, respectively.The study of Ce substitution for La in A side and together with Mn substitutionfor Ni in B side on the the structural and electrochemical properties ofLao.7-xCexMgo.3Ni2.975-yMnyCoo.525 (x, y=0-0.4) hydrogen storage alloys shows that allalloys are mainly composed of a the La(La,Mg)2Ni9 phase with the rhombohedralPuNis-type structure and the LaNi5 phase with the hexagonal CaCus-type structure.With increasing Mn content in the alloys, the abundance of...