Effects Of W, Nb, Mo Substitution For Nickel On The Phase Structure And Electrochemical Properties Of The La-Mg-Ni-Co Type Hydrogen Storage Electrode Alloys

In this thesis, based on the overall review of the research and development of the non-AB5 type hydrogen storage alloys, the La-Mg-Ni-Co-based hydrogen storage alloy La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 was selected as starting alloy, and the effects of partial substitution of W or Nb for Ni on the structure and electrochemical properties of the La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Mx(M=W, Nb or Mo) hydrogen storage alloys were studied systematically by means of XRD analyses and electrochemical investigations. The relationship among compositions, phase structure and electrochemical properties is evaluated, which can be a basic work for developing the new type rare earth-based hydrogen storage electrode alloys with good overall electrochemical performances.The study of W substitution for Ni on the structure and electrochemical properties of La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Wx (x=0-0.15) hydrogen storage alloys shows that all the alloys are mainly composed of a the (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonal CaCu5-type structure. Moreover, the Co7W6 second phase is formed and increases gradually with increasing x from 0.02 to 0.15; as x reaches 0.15, the separate W phase is formed. But the lattice parameters, cell volume and phase abundance of those phases change rather complicatedly. Electrochemical measurements show that the cycling stability is increased considerably, while the maximum discharge capacity of the alloy electrode increases monotonically from 355.5 mAh/g (x=0) to 319.5 mAh/g (x=0.15) mainly due to the formation of new phases which cannot absorb hydrogen. The kinetics evaluated by the high rate dischargeability, the charge-transfer resistance, the exchange current density I0, the limiting current density Il and the hydrogen diffusion coefficient of the alloy electrode increase first and then decrease with increasing x from 0 to 0.1, and slightly increase again at x=0.15. The catalysis of Co7W6 second phase is not obvious. The optimum composition is found at x=0.05, at which the maximum discharge capacity, the activation cycles, the HRD at discharge current density of 600 mA/g (HRD600) and the capacity retention(S150) after 150 cycles are 339.3 mAh/g, 2, 79.8% and 59.3%, respectively.The study of Nb substitution for Ni on the structure and electrochemical properties of La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Nbx (x=0-0.1) hydrogen storage alloys shows that all alloys consist of the (La,Mg)Ni3 phase and the LaNi5 phase.Furthermore, the AlNb2 second phase is formed and increases gradually with increasing x from 0.02 to 0.1. The lattice parameters, cell volume decrease abnormally with increasing x from 0 to 0.02 and then increase gradually when x increases further. The phases abundance of the (La,Mg)Ni3 phase decreases and that of LaNi5 phase increase with increasing Nb content. Electrochemical studies show that the cycling stability of those alloy electrodes decreases markedly due to the consumption of Al by the formation of AlNb2 which is greatly beneficial to the cycling stability, and the maximum discharge capacity of the alloy electrode decreases from 350.0 mAh/g to 309.0 mAh/g with increasing x, which can be ascribed to the decrease of phase abundance of (La,Mg)Ni3 phase and the high stable hydride of AlNb2 second phase at room temperature [90]. With increasing Nb content, the kinetics of the alloy electrode increases first and then decreases. The optimum composition is found at x=0.02, at which the maximum discharge capacity, the activation cycles, the HRD at discharge current density of 800 mA/g (HRD800) and the capacity retention(S100) after 100 cycles are 351.1 mAh/g, 5, 89.2% and 59.1%, respectively.The study of Mo substitution for Ni on the electrochemical properties of La0.7Mg0.3Ni2.45-xCo0.75Mn0.1Al0.2Mox (x=0-0.1) hydrogen storage alloys shows that the cycling stability is decreased, while the maximum discharge capacity of the alloy electrode increases first from 354.3 mAh/g (x=0) to 361.4 mAh/g (x=0.05) and then decreases to 339.8 mAh/g (x=0.1). The...