Redox reactions polymer/electrolyte systems

A second type of electroactive polymer film is the redox polymer which contains localized sites that may be oxidized and reduced. Charge is not distributed along the polymer chain but is localized at specific, pendant redox sites. An example of this type of polymer is poly(vinylferrocene) (PVF) which is shown in Figure 2.1 in both the reduced and oxidized forms. Poly(vinylferrocene) undergoes a reversible redox reaction when used with an appropriate electrolyte (such as LiC104 in acetonitrile) and has been used as a model redox polymer system [17-19]. The oxidation process occurs by removal of electrons and the simultaneous insertion of anions from the electrolyte. The Fe centre in the pendant ferrocene group undergoes oxidation. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Maksymiuk, K. 1996. Charge transfer reactions between conducting polymers and redox systems from electrolyte solutions. Relation between electron transfer kinetics and formal potentials of reacting species.PoZ / Chem 70 (1) 126. 

The number of studies which utilize ionic liquid electrol54e in redox capacitor system is still small, probably due to the difficulty to reproduce the pseudo-capacitive reaction in ionic liquid media. While the principle of pseudo-capacitance of conductive polymer electrodes permits to utilize ionic liquid electrolytes, high viscosity and rather inactive ions of ionic liquid may make their pseudo-capacitive reaction slow. The combination of nanostmctured conductive polymer electrode and ionic liquid electrolyte is expected to be effective [27]. It is far difficult that ionic liquids are utilized in transition metal-based redox capacitors where proton frequently participates in the reaction mechanisms. Some anions such as thiocyanate have been reported to provide pseudo-capacitance of manganese oxide [28]. The pseudo-capacitance of hydrous ruthenium oxide is based on the adsorption of proton on the electrode surface and thus requires proton in electrolyte. Therefore ionic liquids having proton have been attempted to be utilized with ruthenium oxide electrode [29]. Recent report that 1,3-substituted imidazolium cations such as EMI promote pseudo-capacitive reaction of mthenium oxide is interesting on the viewpoint of the establishment of the pseudo-capacitive system based on chemical nature of ionic liquids [30]. 

In this section we present a more detailed treatment of the oxidation/reduction (charging) reaction of an electroactive polymer coating. In particular the effects of a changing Donnan potential and site-site interactions are considered. The polymer is assumed to be in quasi-equilibrium with the electrode, as in Section 3. The redox sites react similarly to a redox species in an electrolyte that is confined to a thin layer cell. The concentrations (activities) of the electroactive sites are described by the Nemst equation, cf. Eqn. 29. The current peaks associated with linear sweeps of the electrode potential are well described in the literature see for instance Ref. 62. We can also characterize such systems by charging curves... 

In the following, the polymeric redox sites are termed P/P to distinguish them from the solution redox species, O/R, which may, of course, also enter into the polymer phase. The mediated (catalytic) reaction is the chemical electron transfer reaction between the redox system from the electrolyte and the polymeric redox sites ... 

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