IMPEDANCE OF POROUS-ELECTRODES

Equations describing potential and current distribution in semi-infinite- and finite-length cylindrical porous electrodes, assuming Butler-Volmer equation for the pore walls, are presented. in the case of finite-length pores, for arbitrary values of transfer coefficients, only numerical solutions exist. Ac impedances of these electrodes were also simulated for different kinetic and geometric parameters. Complex nonlinear least-squares fit of the simulated impedances to the de Levie's equation, which neglects the potential dependence of the charge transfer resistance, was studied and the approximation errors discussed. This procedure overestimates charge transfer resistances predicted by de Levie's equation (for the same pore geometry), at most by a factor of 2. The double-layer capacitances obtained are also larger than those assumed. Errors of approximations are discussed in relation to earlier experimental work.