ELECTROCHEMICAL IMPEDANCE AND DETERIORATION BEHAVIOR OF METAL HYDRIDEELECTRODES

Electrochemical impedance spectroscopy (EIS) was applied to metal hydr ide electrodes. Cole-Cole plots for the electrodes consisted of two ob vious semicircles and a slope related to Warburg impedance. The semici rcle in the high-frequency region was mainly related to the resistance and capacitance between the current collector and the pellet of alloy powder. The semicircle in the low-frequency region, which exhibited a ppreciable dependence on hydrogen content, was attributed to electrode reactions on the alloy particles and double-layer capacitance on the alloy particles. Resistance and capacitance between alloy particles in the electrodes also need to be taken into account. Deterioration proc esses of metal hydride electrodes using a mischmetal-based alloy, MmNi 3.5Co0.7Al0.8, were also studied employing EIS. Deterioration of a met al hydride electrode using copper-coated alloy powder was dominated by a decrease in reactivity of the alloy surface. In contrast, an increa se in the contact resistances and a decrease in the amount of electroc hemically utilizable alloy particles were significant in the deteriora tion of electrodes using uncoated alloy powder. Deterioration of the e lectrodes was avoided to some extent by elevating the hot-press temper ature during electrode preparation.