Abstract:In order to investigate the influences of the stoichiometric ratio of La/Mg (increasing La and decreasing Mg on the same mole ratio) on the structure and electrochemical performances of the La-Mg-Ni-based A₂B₇-type electrode alloy, the as-cast and the annealed ternary La_0.8+xMg_0.2–xNi_3.5 (x=0-0.05) electrode alloys were prepared. The characterization of electrode alloys by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that all the as-cast and the annealed alloys hold two major phases of (La,Mg)₂Ni₇ and LaNi₅ as well as a residual phase of LaNi₃. Moreover, the increase of La/Mg ratio brings on a decline of (La,Mg)₂Ni₇ phase and a rise of LaNi₅ and LaNi₃ phases. The variation of La/Mg ratio gives rise to an evident change of the electrochemical performances of the alloys. The discharge capacities of the as-cast and the annealed alloys evidently decrease with growing the La/Mg ratio, while the cycle stabilities of the alloys visibly augment under the same condition. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS), the Tafel polarization curves, and the potential step measurements all indicate that the electrochemical kinetic properties of the alloy electrodes increase with the La/Mg ratio rising.

Key words: hydrogen storage alloy; annealing treatment; structure; electrochemical performance; kinetics