Redox Transformations and Transport Processes

The elucidation of the nature of charge transfer and charge transport processes in electrochetnically active polymer films may be the most interesting theoretical problem of this field. It is also a question of great practical importance, because in most of their applications fast charge propagation through the film is needed. It has become clear that the elucidation of their electrochemical behavior is a very difficult task, due to the complex nature of these systems [1-8]. 

In the case of traditional electrodes, the electrode reaction involves mass transport of the electroactive species from the bulk solution to the electrode surface and an electron transfer step at the electrode smface. A polymer film electrode can be defined as an electrochemical system in which at least three phases are contacted successively in such a way that between a first-order conductor (usually a metal) and a second-order conductor (usually an electrolyte solution) is an electrochemi-cally active polymer layer. The polymer layer is more or less stably attached to the metal, mainly by adsorption (adhesion). 

The fundamental observation that should be explained is that even rather thick polymer films, in which most of the redox sites are as far from the metal surface as 100-10,000 tun (this corresponds to surface concentrations of the redox sites r = 10 —10 molcm ), may be electrochetnically oxidized or reduced. 

According to the classical theory of simple electron-transfer reactions, the reactants get very close to the electrode surface, and then electrons can tuimel over the short distance (tenths of a nanometer) between the metal and the activated species in the solution phase. 

Ru(bpy in Nafion], some of these ions can reach the metal surface. However, when the redox sites are covalently bound to the polymer chain (i.e., no free diffusion of the sites occurs), and especially when the polymer chains are connected by chemical cross-linkages (i.e., only segmental motions are possible), an explanation of how the electrons traverse the film should be provided. 

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