Nucleation of Lead Sulfate in Porous Lead-Dioxide Electrodes

A one-dimensional mathematical model of a porous lead-dioxide lectrode is described and used to investigate l ad-sulfate nucleation and growth during discharge. Derivation of a nucleation rate expression that is based on classical, het-erogeneous nucleation theory is outlined. An electrochemical kinetic expression is derived based on a reaction mecha-nism involving elementary steps, and concentrated ternary electrolyte theory is used in formulating material-transport equations. Nucleation and electrochemical kinetic parameters are estimated by comparison of model results with experi-mental results available in the literature. The interplay of nucleation and growth kinetics of lead sulfate is responsible for the initial minimum in the voltage-time curve that is commonly observed during constant-current discharge. The model simulates the voltage minimum, which is referred to as the coup defouet, and calculates the degree of lead-ion supersatur-ation, the number density of lead-sulfate particles, and the free energy of formation as well as the size of critical nuclei. The model also predicts a disappearance of the voltage minimum with the addition of seed particles for lead-sulfate nucle