Abstract:In order to ameliorate the electrochemical hydrogen storage performance of La-Mg-Ni system A₂B₇-type electrode alloys, a small amount of Si was added. The La_0.8Mg_0.2Ni_3.3Co_0.2Si_x (x=0-0.2) electrode alloys were prepared by casting and annealing. The effects of adding Si on the structure and electrochemical hydrogen storage characteristics of the alloys were investigated systematically. The results indicate that the as-cast and annealed alloys hold multiple structures, involving two major phases of (La, Mg)₂Ni₇ with a Ce₂Ni₇-type hexagonal structure and LaNi₅ with a CaCu₅-type hexagonal structure as well as one residual phase LaNi₃. The addition of Si results in a decrease in (La, Mg)₂Ni₇ phase and an increase in LaNi₅ phase without changing the phase structure of the alloys. What is more, it brings on an obvious effect on electrochemical hydrogen storage characteristics of the alloys. The discharge capacities of the as-cast and annealed alloys decline with the increase of Si content, but their cycle stabilities clearly grow under the same condition. Furthermore, the measurements of the high rate discharge ability, the limiting current density, hydrogen diffusion coefficient as well as electrochemical impedance spectra all indicate that the electrochemical kinetic properties of the electrode alloys first increase and then decrease with the rising of Si content.

Key words: A₂B₇-type electrode alloy; Si additive; structure; electrochemical characteristics