Electrocatalysis at Redox Polymers

It is often necessary to apply potentials in excess of that required by thermodynamics to drive a particular electrochemical reaction when the kinetics of a redox reaction are slow at a bare electrode. This overpotential can be lowered by accelerating the rate of electrochemical reaction by modifying the electrode surface. For surface-immobilized redox sites, electrocatalysis is accomplished by shuttling electrons by repeated cycling between the catalytic and precatalytic state between the electrode and the electroactive substrate this process is known as mediated electrocataly sis.  

The theory of mediated electrocatalysis has been studied extensively by many authors, and established theories allow mechanistic elucidation and kinetic parameters to be evaluated. An extensive discussion of these appears in Ref. 188. The mediation process can be described by the following reactions  

In this example, the mediating process involves the oxidation of the surface-bound redox species A, but for a mediation process based on the reduction of the surface-bound redox species, the same theory applies. Depending on such factors as film morphology, film thickness, the partition coefficient, diffusion rate of the electroactive species through the film, and electron transfer rates, four types of limiting cases can be identified 

Electron and substrate diffusion (De and Dy, respectively) are so rapid that the rate-controlling step is the rate k of the catalytic reaction. 

The catalytic reaction is so rapid that the rate is controlled by the two diffusion processes, i.e., diffusion of electrons and substrate. 

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Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

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