Convective-diffusion impedance for a partially blocked rotating-disc electrode

For a partially blocked electrode, the attenuation factor ρ of the diffusion admittance is defined. Its zero-frequency limit coincides with the corresponding steady-state factor for the diffusion limiting current. Calculation of ρ as a function of the frequency and the surface-coverage parameters were performed by the finite-element method using semi-analytical expressions derived from a transient mass-transport equation inspired by the Levich model of a RDE. Comparison between values of ρ so calculated and those drawn from impedance measurements on model electrodes is satisfactory; divergences observed at low and intermediate frequencies are explained by a contribution of the liquid velocity components parallel to the surface, which are neglected in this theory.