Abstract
The degradation mechanism of metal hydride (MH) electrodes employing MmNi 5-basedhydrogen storage alloys which contain cobalt, manganese and aluminium as the components substituted for nickel was examined. At first, experimental cells of an Ni-MH system were assembled to clarify its characteristics as a battery and to investigate its failure mode in cyclic use. An internal pressure increase was observed for the cell as a typical failure mode. It was shown that the pressure increase was brought about by loss of charge reserve in the MH electrode with spontaneous hydride formation due to electrochemical reactions caused by the segregation of alloys. Secondly, the surface layers of the hydrogen storage alloy particles in the used and unused metal hydride electrodes were eluted with ethylenediaminetetraacetic acid solutions and were analysed. Inductively coupled plasma atomic emission spectroscopy showed that the selective segregation of manganese and its deposition as the hydroxide from the alloy particle occurred in the early stage of cyclic use. Deposition rates for lanthanum and nickel hydroxide were rather slower than that for manganese. Addition of cobalt to the alloy was found to suppress the segregation rate of manganese.
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