Electron Hopping in Assemblies of Redox Centers

Electron transport in electrode coatings containing redox centers is a necessary ingredient of their functioning as a catalytic device. They indeed serve as an electron shuttle between the electrode and the catalyst present inside the film. As discussed in the next section, the same molecule may play the role of catalyst and of electron carrier, since as shown earlier, redox catalysis is possible in these multilayered coatings. They may also be different, as exemplified is Section 4.3.6. 

FIGURE 4.18. Cyclic voltammetric response of an assembly of redox centers as a function of the parameter tif j JDeFv/ 1ZT from bottom to top 1, 3, oo. 

Why does electron hopping between adjacent redox sites behave like a diffusional transport is the question we address now. A first answer could be found in investigations of the diffusion of redox couples in solution in which it was found that the diffusion rate is augmented by electron exchange in solution between the members of the couple.13 The resulting overall diffusion coefficient, Dap, is thus the sum of two terms  

In fact, in most systems of practical interest there is no physical displacement of molecules throughout the film. The redox centers are attached to the macromolecular structure. Even if some physical displacement of the redox molecules around the anchoring points is possible, such motion is obviously too restricted to contribute directly to charge transport. 

Another treatment of the problem, more adapted to this situation, thus consisted of dividing the film in successive monolayers and describing electron transport as self-exchange electron transfer between the reduced and oxidized forms of the redox couple.14 The variation of the concentration with time, t, and distance from the electrode, x, are thus depicted by equations (4.22) and (4.33), as established in Section 6.4.3.15 

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Blauch DN, Saveant JM. Dynamics of electron hopping in assemblies of redox centers. Percolation and diffusion. J Am Chem Soc 1992 114 3323-3332. 

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