Charge transfer at partially blocked surfaces: A model for the case of microscopic active and inactive sites

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Abstract

Non-linear diffusion effects induced by partial blocking of an electrode surface toward electron transfer are analyzed in the case where the blocking film is sprinkled with a large number of microscopic, active sites. Under such conditions, the average size of the active sites and the average distance between them are small compared to the total diffusion layer. As a result, non-linear diffusion is confined to a layer adjacent to the electrode surface which is thin compared to the total diffusion layer. It follows that, in the framework of relaxation electrochemical techniques, non-linear diffusion can be regarded as occurring under stationary conditions while linear diffusion is time-dependent. The resolution of the diffusion problem is thus greatly simplified, leading to the description of the polarization as a function of only two dimensionless parameters which are simply related to the experimental parameters: fractional coverage, distance between active sites, standard rate constant of electron transfer and time-range. The formulations thus obtained give evidence for a formal similarity with the polarization problem corresponding to the coupling of electron transfer with preceding and following chemical reactions. According to the values of these parameters, two main types of behavior are obtained: in one of them, the polarization curves show a quasi-reversible behavior, while in the other, kinetic-type waves are obtained. The effect of the various parameters on the characteristics of these waves and on the passage from one type of behavior to the other is described. Procedures for deriving estimates of the size of the active sites and the distance between them from the polarization curves are proposed.

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