Kinetics of electrochemical reactions mediated by redox polymer films: Pre-activation (CE) mechanisms

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Abstract

Mechanisms involving catalytic reaction with several successive step may prove important in the mediation of electrochemical reaction by redox polymer films or other redox immobilized coatings. The case of a catalytic reaction involving an activation step prior to electron transfer with the mediator is analysed and discussed in the context of stationary techniques such as rotating disc electrode voltammetry. The overall kinetics are dependent upon three dimensionless parameters which are conveniently chosen as ratios between characteristic current densities and a ratio expressing the competition between the electron-transfer reaction and the reverse step of the activation reaction. Numerical resolution of a single master differential equation leads to the values of the plateau current densities at the first and second waves for any set of values of the three dimensionless parameters. (The first wave is obtained in a potential region where electron transfer between the electrode and the mediator occurs but does not involve the substrate; at the second wave both the mediator and the substrate exchange electrons with the electrode). However, a much simpler approach to the analysis of the overall kinetics involves the description of limiting cases of kinetic behaviour obtained for extreme values of the dimensionless parameters. The expressions for the plateau current densities are given for the various limiting cases and sub-cases as well as the range of validity. This leads to the representation of the overall kinetics by a three-dimensional zone diagram illustrating how the kinetic control is shifted with variations of the parameters. The relationships between the dimensionless parameters and the actual operational parameters (rotation speed, film thickness, concentration) and intrinsic parameters (rate constants, diffusion coefficient, partition coefficient) lead to diagnostic criteria for analysing the kinetics on which procedures for optimizing the catalytic efficiency of the redox coatings should be based.

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