Electroactive polymers redox process

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

Two nitrogen-containing electroactive polymers, polypyrrole (PPY) [21] and polyaniline (PAN) [22], have been of particular interest because of their environmental stability, high electrical conductivity and interesting redox properties associated with the chain heteroatoms. More importantly, PAN has been found to exhibit solution processability [23, 24] and partial crystallinity [25,26]. 

First we review processes that ordinarily accompany the redox switching of an electroactive polymer. The key step is coupled electron/ion transfer, which converts one or more reduced forms of the polymer to one or more oxidized forms of the polymer. Solvent and neutral species transfers and polymer structural changes (reconfigurations or coordination state changes) accompany the switching process under permselective conditions. Under nonpermselective conditions, salt transfer also occurs [12]. 

Under current investigation are the synthesis and properties of multimetallic thin films containing Ru, Os1, and Re1 in a wide variety of coordination environments. Judicious choice of such materials may lead to creation of an electroactive polymer film which would exhibit a bandlike spectrum of reversible, metal-centered redox processes extending from ca. -0.6 to +1.5 V. 

SECM can also be used to study the flux of species produced at a modified electrode surface, such as one with a film of polymer (Section 14.2.3). In one type of experiment, the tip is held at a potential where it can detect an electroactive ion released from the polymer film during a redox process (30-32). For example the SECM was used to detect the release of Br during the reduction of oxidized polypyrrole (PP) in the form, PP" Br . During a reductive cyclic voltammetric scan, Br was found to be released only in a later part of the scan, after an appreciable amount of cathodic charge had passed. This result suggested that during the early phase of the reduction the uptake of cations, rather than the release of anions, maintained charge balance in the film. 

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]. 

First of all, we must realize that the most important property of the electroactive polymer, as far as mediation is concerned, is its redox potential. To mediate a reduction of a solution species, the redox potential of the electroactive layer must be less positive than that of the analyte for the mediated oxidation process, the reverse is the case. This means that the osmium polymers under consideration here which have a redox potential of about 250 mV, are thermodynamically able to mediate the reduction of Fe(III) to FeCII), but not the reverse process (see Fig. 8.24), since the formal potential of the Fe(III/II) couple is 450 mV. The difference in the two redox potentials can be considered the driving force for the mediating process. On the basis of these considerations, it is clear that the mediated reduction of Fe(III) [as in Eq. (38)] is irreversible. 

Electroactive polymers have a number of attractive features that account for this continuing interest. First they present a distributed array of catalytic sites. Thus in contrast to monolayer chemically modified electrodes, there are potentially a much greater number of reactive sites that can contribute to the catalytic current. Since these sites are distributed throughout the film, it is essential to consider the mass transport of reagents into the film and the mass transport of products out of these films when studying the overall kinetics of these processes. The coupled mass transport and kinetics in redox polymer films have been investigated in some detail, and good models exist for these processes. ... 

The oxidation and reduction (redox) processes in electroactive polymers (EAPs) make it possible to use these polymer materials as charge storage devices, either as battery electrodes or as supercapacitors. The potential for reduced cost, weight, and enviromnental impact of EAP electrodes relative to the metals and metal oxides that are traditionally used in such devices makes these polymers attractive alternatives. While inorganic options are limited, EAPs can be tailored to provide specific properties, such as conductivity, voltage window, storage capacity, porosity, reversibility, and chemical and environmental stability. 

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. 

The oxidation or doping process of electroactive polymers involves the simultaneous insertion of charge and a counterion to balance the charge. For redox polymers such as polyvinylferrocene the process may be written... 

As the frequency of voltage perturbation is increased, one may limit the penetration depth (D/jco) of a concentration wave generated by the redox reaction so that it is much smaller than the thickness of the electroactive polymer film. In this region, one measures the kinetics of the charge injection process at the surface (Region III). The impedance characteristic is a semicircle in the Zreal vs —Zjmag impedance plane plot. For the impedance measurement, one may obtain Rct, the charge transfer resistance, and the double layer capacitance Cdl- This procedure was used to calculate the... 

K. Jackowska, A. Kudelski, J. Bukowska, Spectroelectrochemical and EPR determination of the number of electrons transferred in redox processes in electroactive polymers - polyindole films, Electrochimica Acta 1994, 39, 1365. 

Intrinsically conducting polymers (ICPs) are electroactive long-range conjugated polymers. They generally possess reversible redox performance, while metal corrosion is also a redox process therefore, it is possible that ICPs may find application for metal anticorrosion. It is true since the early report for corrosion inhibition performance of ICPs such as polyaniline (PANI) by DeBerry [5]. After more than 20 years of development, now ICPs have received much attention, since th may be a kind of alternative anticorrosion agents instead of the toxic heavy metal in anticorrosion coating, no matter they are used alone or as composite with substrate resin. 

Numerous detection strategies have also been developed for biosensing applications based on combining electrochemistry with SPR detection. Although most of the combined electrochemical and SPR studies utiUzed uniform electrode surfaces with traditional SPR detection, there have been several examples of combined electrochemical systems with SPR imaging, where the optical response of various locations on the electrode surface are investigated simultaneously. Simultaneous electrochemical and SPR analysis has been extensively used in the characterization of various conducting and electroactive polymer films to provide information about polymer assembly, redox transformations, electrochemicaUy catalyzed processes and others applications [48, 49]. 

---