An Investigation On The Phase Structure And Electrochemical Properties Of The AB₃ And AB₅ Type Ca-contained Rare Earth-based Hydrogen Storage Electrode Alloys

Based on the review of the research and the development of the non-AB₅ type rare earth-based hydrogen storage alloys, the AB₃ type La-Ca-Mg-Ni(Co) -based and AB₅ type Ml-Ca-Ni-based hydrogen storage electrode alloys were designed as the study object of this work. By means of XRD, and the electrochemical test methods including galvanostatic charge-discharge, EIS, linear polarization, and anodic polarization etc., the relationship among compositions, phase structure, and electrochemical properties was studied to develop the new type rare earth-based hydrogen storage alloy with high discharge capacity and long cycling life.The study on the phase structure and the electrochemical properties of the La-Ca-Mg-Ni(x=0.0¹.0) alloy revealed that the alloys mainly consist of the LaNi₃ phase with the rhombohedral PuNi₃-type and the LaNi₅ phase with the hexagonal CaCu₅-type structure. With the increase of the x, the dimension of the a axis in cell lattice and the cell volume decrease monotonously. In the range of the x=0.0⁰.8, the electrochemical properties of the alloy improved, and the optimum composition was found to be x=0.8, at which the maximum discharge capacity ,the activation cycles and the HRD at the discharge current density 1200mA/g(HRD₁₂₀₀) are 411.7mAh/g, 2 and 55.6% respectively. However, the capacity retention after 50 charge/discharge cycles S₅₀ reduces to only 42.0%. As the content of Ca is more than 0.8%, the discharge capacity, the cycling life and HRD decrease.In order to improve the cycling life and HRD, the La_1.2Ca_0.8MgNi₉ alloy was selected to be studied. With the increase of the Co content, the influence upon the phase structure and electrochemical properties caused by the substitution of Co for Ni reveals that the cell volume expand and the maximum discharge capacity of the alloy decreases from 397.9mAh/g(x=0.1) to 276.4mAh/g(x=0.4); the capacity retention after 30 cycles (S₃₀) decrease dramatically; the HRD₁₂₀₀ reaches to 71.6%(x=0.2). The kinetic research indicates that the increase of the charge transfer rate and the diffusion velocity of hydrogen in bulk are the main reason.The phase structure and electrochemical properties of Ml_0.8Ca_0.2 Ni₅ have been studied in different temperature(25,0,-10,-20℃). The alloy has the structure of the hexagonal CaCu₅-type. The alloy has the maximum discharge capacity and the best cycling life while testing in -10℃; however, the HRD of the alloy decrease with the temperature falling. The research reveals that the charge transfer rate is the limited step while the discharge current density is less than 400mA/g and the temperature is higher than -10℃; the diffusion velocity of hydrogen should be considered while the testing temperature is lower than -10℃.