Redox Transformations and Transport Processes

Abstract

According to the classical theory of simple electron-transfer reactions, the reactants get very close to the electrode surface, and then electrons can tunnel over the shod distance (some tenths of nanometer) between the metal and the activated species in the solution phase. In the case of polymer-modified electrodes the active parts of the polymer cannot approach the metal surface, because polymer chains are trapped in a tangled network, and chain diffusion is usually much slower than the time scale of the transient electrochemical experiment. Therefore, the transport of electrons can be assumed to occur either via an electron exchange reaction (electron hopping) between neighboring redox sites if the segmental motions make it possible or delocalized electrons can move through the conjugated systems (electronic conduction). The former mechanism is characteristic of redox polymers. In the case of electronically conducting polymers, electrochemical transformation—usually oxidation—of the nonconducting form of these polymers usually leads to a reorganization of the bonds of the macromolecule and the development of an extensively conjugated system. Electron hopping mechanism is likely to be operative between the chains interchain conduction) and defects even in the case of conducting polymers. However, attention has to be paid not only to the “electronic charging” of the polymer film (i.e., to the electron exchange at the metal? polymer interface and the electron transport through the surface layer) since in order to preserve electroneutrality within the film ions will cross the film? solution interface. The movement of counterions (or less frequently that of the co-ions) may also be the rate-determining step. A small disbalance of the charge related to the electrochemical double layers may exist only at the interfacial regions.