Primary and secondary distributions after a small-amplitude potential step at disk electrode coated with conducting film

The set of equations and boundary conditions for the “primary potential/current distribution” after a small-amplitude potential step has been analyzed for a film-coated disk electrode in contact with an electrolyte. The solution of these equations provides the overall short-time resistance of this system, Rtot, which is determined by the short-time resistance of the electrolyte solution in contact with the bare disk electrode, Rs, and the short-time film resistance to the current passage in the normal direction, Rf=Lf/κfπro2 (ro, disk radius; Lf, film thickness; κf, its specific conductivity). The deviation of Rtot from the sum of these resistances, Rs + Rf, originates from a three-dimensional potential/current distribution in solution. Procedures to calculate the film resistance and its specific conductivity on the basis of the measured values of Rtot and Rs have been proposed. Similar analysis has also been carried out for the “secondary potential/current distribution” in the same system. The overall resistance for this regime is related to the short-time solution resistance, Rs, and to the total resistance of the electrode, equal to the sum of the resistance, Rf, and two interfacial resistances, Rm/f and Rf/s. A method to determine the bulk-film parameters, Rf and κf, from data for the secondary distribution is discussed. Advantages and restrictions of the proposed route to transport parameters of a film at the electrode surface are analyzed, in comparison with existing methods of their determination.