The effects of
stoichiometric ratio x on the structure, thermodynamic properties and electrode performances of the hydrogen
storage alloys Mm(Ni 0.72 Co 0.14 Mn 0.08 Al 0.06 ) x (Mm=Ce rich mischmetal, x =4.4～5.0) were studied. The major phases of all alloys are of hexagonal CaCu 5 type structure. The unit cell volume increases with
decreasing stoichiometric ratio x. A small quantity of the second phase Ce 2Ni 7 is segregated in the alloy with x<5.0 and the amount of Ce 2Ni 7 phase increases with decreasing x. The enthalpy change -Δ H value of hydride formation increases with decreasing x because of increasing unit cell volume and the Ce 2Ni 7 phase forming more stable metal hydride. The discharge capacities of MH electrodes increase with increasing x but for alloy with x =5.0, and have a maximum 306 mAh/g at x =4.8. The exchange current density of MH electrode reaction increases but the high rate dischargeability decreases with decreasing x. This fact indicated that the hydrogen diffusion in the alloy is a rate determining step in high rate discharging.