The electrochemical impedance of metal hydride electrodes

Abstract

Electrochemical impedance spectroscopy (EIS) and other characterization methods combined with modelling are very useful tools to gain an understanding of the processes governing the charge and discharge reactions in metal hydride electrodes. Impedance measurements were performed in the range from 10 kHz to 0.1 mHz. The proposed model simulated the experimental data better than previously presented models by a smooth fit for all frequencies. Especially the fit at the lower frequencies (diffusion) was notable. The diffusion was modelled using spherical diffusion geometry. To fit the experimental data, equations describing the current distribution in porous electrodes were needed. The different sub-processes taking place during the charge/discharge reaction were identified. The following parameters were found to be of significant importance in order to describe the overall process: charge transfer resistance, double layer capacitance and impedance resulting from diffusion. A continuous improvement of the catalytic properties during electrochemical cycling of the metal hydride electrode was found to take place. This improvement in the catalytic properties was found to be a result of a decrease in the charge transfer resistance. The results confirmed the usefulness of the impedance method for in situ characterization of hydride electrodes.