Discharge Kinetics of the Nickel Electrode

The kinetics of nickel electrode discharge are found to be controlled by solid‐state proton diffusion under normal high rate discharge conditions. As the nickel electrode is discharged, the conductivity of the active material decreases until eventually mixed kinetics are observed where the electrode impedance has significant contributions from both proton diffusion and charge transfer resistance. Further discharge results in the formation of a semiconductor layer at the metal‐active material interface that is depleted in charge carriers and has a relatively high electronic resistance. The depletion layer is responsible for the secondary discharge plateau of the nickel electrode at 0.0 to −0.5 V vs. . Changes in electrode capacitance during depletion layer formation appear to provide a sensitive measure of the uniformity of electrode discharge. The effects of cobalt additives on the kinetics have been experimentally measured, and while cobalt does not change the discharge mechanism, it does increase the ionic and electronic conductivity of the active material allowing a greater depth of discharge before depletion layer formation.