ELECTROCHEMICAL IMPEDANCE AND DETERIORATION BEHAVIOR OF METAL HYDRIDE ELECTRODES

Electrochemical impedance spectroscopy (EIS) was applied to metal hydride electrodes. Cole-Cole plots for the electrodes consisted of two obvious semicircles and a slope related to Warburg impedance. The semicircle 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 appreciable 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 processes of metal hydride electrodes using a mischmetal-based alloy, MmNi3.5Co0.7Al0.8, were also studied employing EIS. Deterioration of a metal hydride electrode using copper-coated alloy powder was dominated by a decrease in reactivity of the alloy surface. In contrast, an increase in the contact resistances and a decrease in the amount of electrochemically utilizable alloy particles were significant in the deterioration of electrodes using uncoated alloy powder. Deterioration of the electrodes was avoided to some extent by elevating the hot-press temperature during electrode preparation.