An impedance model for metal hydride electrodes

A model for the impedance of a granular porous hydride electrode is proposed and developed mathematically. The charging/discharging process consists of charge transfer, adsorption, absorption and hydride forming steps coupled with spherical diffusion in the particles of the material. The developed model is successfully compared with experimental results from a frequency analysis of an AB(5) type hydride forming alloy with misch metal on A sites and B sites containing mainly Ni and minor amounts of Co, Mn and Al. The rates of the different steps in the process are determined by the fitting procedure to the appropriate equivalent circuit and found to vary considerably with the state of charge. The optimum energy efficiency for battery electrodes of the material used is obtained in the range of 40 to 80% depth of discharge. The impedance analysis is found to be an efficient method for characterizing hydride electrodes in situ.