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 the dependence of equilibrium potential and exchange current density on the surface hydrogen concentration have been derived. These parameters have been used to correlate experimental data with the theoretical electrode discharge model. Analysis of both the experimental and theoretical discharge curves reveals a potential plateau determined by the magnitude of the interactions between the hydrogen in the alloy and the unhydrided metal. Neglecting these hydrogen-metal site interactions results in simulations predicting the electrode potential varying over the entire duration of discharge. The results also indicate that utilization of the electrode is controlled by the rate of hydrogen diffusion in the electrode and by the alloy particle size. Kinetic resistance at the surface is a determining factor of the polarization losses of the electrode. The variation of equilibrium potential and exchange current density with the state of charge has been characterized experimentally. These results are compared with the model predictions, and good agreement is seen.