Electron transport through polymer films

Regarding the question of the rate of electron transport through polymer films, it is not yet clear what ultimate rate can be achieved. In solar energy applications the important issue is whether the rate can be high enough so that the net electron transfer rate is light intensity limited. 

The theory of electron transport through polymer films at the surfaces of electrodes has blossomed under the guidance and development of Saveant and his group, and many others (51-60). Saveant s major contribution was to provide a general, mathematical description of charge transfer during electrocatalytic oxidation or reduction of a substrate in solution. 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Transport through surface films and coatings, including membranes, ionic and electronic conducting polymers, fast ionic conducting solids, and passive corrosion films... 

However, it is important to pay attention to more than just the electronic charging of the polymer film (i.e., to electron exchange at the metal polymer interface and electron transport through the smface layer), since ions will cross the film solution interface in order to preserve electroneutrality within the film. The movement of counterions (or less frequently that of co-ions) may also be the ratedetermining step. 

Much more effectual and very often applied are polymer-coated electrodes. Especially electrochemical polymerization is an attractive method for the immobilization of redox enzymes at electrode surfaces, and/or accumulation of electroactive reactants. An approximative analytical treatment of the response of an amperometric enzymatic electrode leading to plots of fluxes and concentration profiles has been made in [14]. The electron transport through poly-4-vinylpyridine and polystyrene-sulfonate films (widely used for immobilization of redox centers on electrodes) has been studied in [15]. 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

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