THEORETICAL-ANALYSIS OF METAL HYDRIDE ELECTRODES - STUDIES ON EQUILIBRIUM POTENTIAL AND EXCHANGE CURRENT-DENSITY

A theoretical model for the metal hydride electrode has been developed assuming that hydrogen diffusion in the alloy and charge-transfer at the surface control the discharge process. Theoretical equations for t he dependence of equilibrium potential and exchange current density on the surface hydrogen concentration have been derived. These parameter s have been used to correlate experimental data with the theoretical e lectrode discharge model. Analysis of both the experimental and theore tical discharge curves reveals a potential plateau determined by the m agnitude of the interactions between the hydrogen in the alloy and the unhydrided metal. Neglecting these hydrogen-metal site interactions r esults in simulations predicting the electrode potential varying over the entire duration of discharge. The results also indicate that utili zation of the electrode is controlled by the rate of hydrogen diffusio n in the electrode and by the alloy particle size. Kinetic resistance at the surface is a determining factor of the polarization losses of t he electrode. The variation of equilibrium potential and exchange curr ent density with the state of charge has been characterized experiment ally. These results are compared with the model predictions, and goad agreement is seen.