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The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging and discharging for a number of cycles. The plate of the negative electrode of an experimental cell in this study was made from a mixture of a multicomponent AB 5 -based alloy powder, nickel powder, and polytetrafluoroethylene (PTFE). The characteristics of the negative electrode, including discharge capacity, exchange current density, and hydrogen diffusivity, were studied by means of the electrochemical experiments and analysis in an experimental cell. The exchange current density of a Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 alloy electrode increases with increasing number of charge/discharge cycles and then remains almost constant after 20 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, increases the hydrogen exchange current densities. Measurement of hydrogen diffusivities for Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 alloy powder shows that the ratio of D/a 2 (D ϭ hydrogen diffusivity; a ϭ sphere radius) increases with increasing number of cycles but remains constant after 20 cycles.

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