Conducting polymers redox processes

As mentioned above in this section, PTh is oxidized with two oxidation peaks at around 0.28 and 0.58 V vs. Ag/Ag" in acetonitrile [1641. When oxidized to the second oxidation peak, however, the oxidation product is not stable and loses its electroactivity with protons released during the reverse scan [175]. When the potential is scanned to the first oxidation peak, the electrochemical conversion is chemically reversible with its oxidation product stable. This process is accompanied by the ion transport to maintain the electroneutrality as for oxidation of other conducting polymers. This process w.is found to be significantly dependent on the electrolyte for its shape as well as kinetics [21c]. Cations were also found to affect the redox processes of PTh [175]. These results indicate that both anions and cations can affect the redox chemistry of PThs as for PPy, depending on relative sizes/diffusion coefficients of anions or cations [176,177]. 

The technology proposed consists in the conducting of redox processes, which proceed in nanoreactors of polymeric matrixes and are accompanied by the reduction of metal ions included into the cavities of organic polymer gels. At the same time, hydrocarbon shells are simultaneously oxidized to carbon. 

Even when they have a partial crystallinity, conducting polymers swell and shrink, changing their volume in a reverse way during redox processes a relaxation of the polymeric structure has to occur, decreasing the crystallinity to zero percent after a new cycle. In the literature, different relaxation theories (Table 7) have been developed that include structural aspects at the molecular level magnetic or mechanical properties of the constituent materials at the macroscopic level or the depolarization currents of the materials. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

The conducting polymers show a significant non-faradaic component of the electrochemical mechanism inside the main range of potentials AEn. Nevertheless, the possibilities of redox processes at the limits and beyond this range of potentials should be taken into account. At the same time, these processes can lead to rapid formation of thin insulating... 

Despite the lack of theoretical models for interfacial recombination processes in excitonic solar cells, it is obvious empirically that those cells which function efficiently must have a very slow rate of recombination. In DSSCs, this can be explained simply by the slow electron self-exchange rate of the I /I2 redox couple and the absence of field-driven recombination. However, in the case of solid-state, high-surface-area OPV cells, such as the conducting polymer/C60-derivative cells [36,39], the slow rate of interfacial recombination is an important problem that is not yet understood. 

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