Polymer-coated electrodes applications

This section describes typical applications of the in-situ electrochemical ESR methodology. Specific examples of radical identification, determination of radical decay mechanisms, polymer-coated electrodes, and spin trapping will be included. 

Small LL interfaces have been used by Girault and co-workers (33-38) and by Senda et al. (39, 40). We have used a small hole formed in a thin glass wall (41-43). Figure 16 shows the voltammetric response of lauryl sulfate anion transport between water and nitrobenzene. Recent analytical applications of these microinterfaces have resulted in construction of gel-solidified probes. The advantage of such a modification is ease of handling (44-47). The immobilization can be extended further to studies of frozen interfaces, or even to solid electrolytes. Significantly, ITIES theory also applies to interfaces that are encountered in ion-doped, conductive, polymer-coated electrodes. 

The application of polymer-coated electrodes was already discussed for high energy batteries (doped polyacetylenes, Sect. X) and liquid junction solar cells (conducting polypyrrole films. Sect. XII). Further information on the application of polymer-modified electrodes can be found in recent reviews... 

Table 1. Electron processes at polymer-coated electrodes and their Applications...

In addition to the reactions described in Sect. 4, polymer-coated electrodes exhilut various electric properties which can be used for electronic devices. The most important of them are charge storage, electrochromism, transport of ions or molecules, and control of electron processes. Their applications are described in this section. 

Polymer-coated electrodes belong to a new scientific field that combines modern aspects of electrochemistry, polymer chemistry, organic chemistry and physics. Research on this topic will be extended in the future with particular interest focused on the industrial applications of these promising new materials. 

Although electrochemistry is an established field of science with many applications, it is still expected that new areas of important applications can be found. In order to reach this objective, the development of new functional materials is required. Polymer-coated electrodes are the most promising candidates, as can be seen from this review article. 

The preparation, mechanisms and possible applications of polymer-coated electrodes have been described in this review. Processes based on the transportation, storage, activation, pumping, and utilization of electrons (Tables 1, 2, 6) which form important electron cycles in nature (Fig. 1) have been explained. 

A considerabel numbers of papers on fundamental studies of polymer-coated electrodes were published in various journals (e.g., Synth. Met., J. Chem. Soc., J. Electro-anal. Chem., Electrochim. Acta, Makromol. Chem., Macromol. Symp. 8 (1987)) in the year after this manuscript was completed. In several companies applied research works are on the way to realize application of these unconventional materials. Recently secondary batteries (polyaniline/Li, polypyrrole/Li) were commercialized. Developments for practical uses on electrochromic display and sensors are also underway. 

Among polymer coated electrodes which have been the object of active investigations during this last decade, particular attention has been paid to the conductive polypyrrole films, obtained by electrooxidation of pyrrole in acetonitrile. Such electrodes have been used to study the electrochemical behaviours of the quinone-hydroquinone redox couple " and tetrathiafulvalene , The controlled release of ferrocyanide from polypyrrole by reduction of the polymer has been demonstrated . As an application, electroinactive anions can be determined using a polypyrrole modified electrochemical detector in flow-injection analysis. Pyrrole can be polymerized from aqueous solutions. Enlarging the modification field, the polymerization step may be preceded by a chemical reaction between pyrrole and another substrate . 

As a final example of typical areas of application of the technique, we consider the information that can be found from the lineshapes of ESR lines and their intensities. In particular, we examine the contributions of electrochemical ESR to the field of polymer-coated electrodes and conducting polymers. 

The effect of the polymer-coating of the electrode appears to be not only the immobilisation of the ruthenium complex but also adsorption of the oxalate ions on the polymer layer. Possible practical applications of such polymer-coated electrodes are in the electrochemical analysis of trace compounds by concentration in the film [24, 25] and electroanalysis of slow reactions, e.g. mediation of enzyme redox processes [22]. 

Empirical kinetics are useful if they allow us to develop chemical models of interfacial reactions from which we can design experimental conditions of synthesis to obtain thick films of conducting polymers having properties tailored for specific applications. Even when those properties are electrochemical, the coated electrode has to be extracted from the solution of synthesis, rinsed, and then immersed in a new solution in which the electrochemical properties are studied. So only the polymer attached to the electrode after it is rinsed is useful for applications. Only this polymer has to be considered as the final product of the electrochemical reaction of synthesis from the point of view of polymeric applications. 

Wire is coated by being passed through a plastic extruder, but most materials are coated with solutions, emulsions, or hot powders. The classic brushing process has been replaced by roll coating, spraying, and hot powder coating. The application of polymers from water dispersions to large objects, such as automobile frames, has been improved by electrode-positon of the polymer onto the metal surface. 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit-... 

A recent example of the application of in situ FTIR reflectance spectroscopy to modified electrodes can be found in the work of Korzeniewski and her group [54] on Pt electrodes coated with films of the polymer polyaniline. Figure 13.6 shows the reflectance spectrum (in AR/R units) of a polyaniline film-coated electrode as a function of applied potential. As the film becomes more oxidized, the band at 1320 cm-1 is shifted to higher energies, a result consistent with an increasing C-N bond strength upon oxidation [54]. The interac-... 

Having debated the mechanism of charge transport within the polymer film, it is now useful to consider a few examples of chemical applications of polymer modified electrodes. Electrodes coated with [Ru(bipy)2Cl(PVP)]Cl or [Ru(bipy)2(py)(PVP)]Cl2 show strong catalytic effects for the reduction of cerium(IV) and the oxidation of iron(II).52... 

In this section, we describe the fabrication of metal complex oligomer and polymer wires composed of bis(terpyridine)metal complexes using the bottom-up method.11 13 This method has an advantage in fabricating organized structures of rigid redox polymer wires with the desired numbers of redox metal complexes. We also present a new electron-transport mechanism applicable to the organized redox polymer wires-coated electrode. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

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