Models for mediated reactions at film modified electrodes: controlled electrode potential

Abstract

Models have been developed previously for redox reactions mediated in films on electrode surfaces. As developed by Savéant and coworkers for steady state rotating disk voltammetry, a redox species, P, confined in the film is electrolyzed at the mass transport limited rate to form Q. Q is able to undergo reaction with a species, A, present in solution and able to permeate the film, to regenerate P and form product B; A + Q →k 1 B + P. The reaction may occur either in the bulk of the film or at the film-solution interface. Here, the models are modified to include control of the electrode potential such that the concentration of P and Q at the electrode surface are parameterized by the Nernst equation. As long as no P and Q are lost from the film, the Nernstian condition will govern most electrode systems at steady state. Control of the electrode potential allows (1) simplified determination of the kinetic characteristics for some reaction schemes, and (2) evaluations where a second electrolysis occurs at a potential sufficiently close to the redox potential of P/Q that there are advantages in not applying potential sufficient to electrolyze P at the mass transport limited rate. This includes reactions where the formal potentials for P/Q and A/B are within 200 mV of each other as well as cases where a larger applied potential exceeds the solvent limit or leads to film decomposition. Equations are also provided for the electrolysis of A to B at the electrode surface as governed by the Nernst equation. Methods for evaluating experimental data are outlined.