Analyses of Spectroelectrochemical Behavior of Polypyrrole Films Using the Nernst Equation: “Monomer Unit Model” and Polaron/Bipolaron Model

Abstract

q ABSTRACT A series of the absorption spectrum changes in polypyrrole films were analyzed using the Nernst equation by two models. One is the " m o n o m e r unit model" and the other is the "polaron/bipolaron model." In the first model, formal electrode potentials and the n-values were obtained and usec~ to fit calculated hAbs vs. E curves to the experimental plots at three wavelengths. In the second model, the "apparent" excess chemical potentials were introduced to correct for the large deviations between the Nernst equation and the Nernst plots obtained from the absorption spectra. The calculated hAbs vs. E curves were then fitted to the experimental plots. The "apparent" excess chemical potentials were assigned as corrections for species concentrations. Advantages of the monomer unit model over the polaron/bipolaron model are pointed out in the precise analyses of the spectroelectrochemical behavior of the films.Electroconducting polymers, such as polypyrroles, polythiophenes, and polyanilines, have been recently receiving m u c h attention because they can be easily prepared by electrochemical polymerization, are stable in air, and also have unique electronic, electrochemical, and optical properties (1-4). One of the most interesting aspects of these polymers is their spectroelectrochemical behavior (5-7). For these electroconducting polymers, spectroelectrochemical studies have been intensively carried out (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). The absorption spectra are quantitatively analyzed using the Nernst equation, and the formal electrode potentials and "n-values" (the number of electrons transferred during the redox reactions) are then obtained (8-11). Furthermore, Marque and Roneali (11) carried out comparative spectroelectrochemical studies of three-substituted polythiophenes and analyzed the deviations of the Nernst plots) from linearity in terms of (i) the coulombic interactions between charged sites, and (ii) the mechanical work required to expand the polymer lattice by the insertion of counterions and solvent molecules. The c o m m o n and important points and/or assumptions in these studies (8)(9)(10)(11) are that the redox reactions of these polymers can be described by one " m o n o m e r unit," not by several m o n o m e r units of the polymers. From now on, we call the treatment of the redox reactions of electroconducting polymers in terms of m o n o m e r units as "monomer unit model."In addition, redox reactions (including spectroelectrochemical behavior) of the electroconducting polymer (polypyrrole) can be analyzed (12, 13) using the Nernst equation in terms of a polaron/bipolaron model (3). This model treats the polymers in several monomer units having +1 or +2 charges, and describes the redox reactions of the polymers by these units. Therefore, the number of electrons transferred during the redox reactions are + 1 or +2 by this model. According to such analyses (13), excess chemical potentials must be introduced to explain nonNernstian behavior of the polymer. They we...

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