Abstract:The effects of stoichiometric ratioxon the structure, thermodynamic properties and electrode performances of the hydrogen storage alloys Mm(Ni_0.72Co_0.14Mn_0.08Al_0.06)x(Mm=Ce-rich mischmetal,x=4.4~5.0) were studied. The major phases of all alloys are of hexagonal CaCu5type structure. The unit cell volume increases with decreasing stoichiometric ratiox.A small quantity of the second phase Ce₂Ni₇is segregated in the alloy withx <5.0 and the amount of Ce₂Ni₇phase increases with decreasingx.The enthalpy change -ΔHvalue of hydride formation increases with decreasingxbecause of increasing unit cell volume and the Ce₂Ni₇phase forming more stable metal hydride. The discharge capacities of MH electrodes increase with increasingxbut for alloy with x=5.0, and have a maximum 306 mAh/g atx=4.8. The exchange current density of MH electrode reaction increases but the high rate dischargeability decreases with decreasingx.This fact indicated that the hydrogen diffusion in the alloy is a rate-determining step in high rate discharging.

Key words: hydrogen storage alloy　stoichiometric ratio　structure　thermodynamic　property