Free-Standing Membranes for the Study of Electrochemical Reactions Occurring at Conducting Polymer/Electrolyte Interfaces Claude Deslouis, ² Marco M. Musiani,* ,‡ and Bernard Tribollet ² UPR 15 du CNRS, 4 pl. Jussieu, 75252 Paris, France, and IPELP CNR, 4 corso Stati Uniti, 35127 PadoVa, Italy ReceiVed: October 26, 1995; In Final Form: January 30, 1996 X A double submerged impinging jet (DSIJ) cell has been built allowing an identical hydrodynamic regime to be established on both faces of a bipolar electrode consisting of either a Pt sheet or a free-standing polypyrrole (PPy) membrane. Both electrode materials have been studied in this cell, in the presence of electrolytes containing redox couples. The experimental current-potential characteristics and impedance diagrams relative to PPy may be analyzed as the result of two parallel paths of charge transport involving (i) convective diffusion of the redox species in solution, electron transfer at the polymer/electrolyte interface, and electron transport within the film and (ii) migration of nonelectroactive ions in solution, ion transfer at the polymer/electrolyte interface, and ion transport within the film. In the most favorable circumstance of a fast redox couple and a low ionic conductivity of the membrane, comparison between Pt and PPy explicitly shows an electrocatalytic effect of the latter. Introduction Among many other possible applications, the use of conduct- ing polymer films as catalytic electrode materials able to mediate redox reactions has been proposed and investigated by several groups. 1-3 Most investigations and theoretical treatments have been devoted to cyclic voltammetry and rotating disk electrode techniques (refs 1 and 4 and references therein); ac impedance has also been explored. 5-8 The current experimental arrange- ment in these investigations is one in which the polymer film is supported on an electrode (often a rotating disk electrode) and dipped in an electrolyte containing the redox couple. In this arrangement, henceforth called modified electrode and schematically represented in Figure 1A, two different interfaces exist: a metal/film interface where only electrons may be exchanged and a film/electrolyte interface where both electrons and ions may be exchanged. Actually, the situation represented in Figure 1A is only a limiting case for fast electron exchange between film and redox couple. More generally this electron transfer may occur within the polymer, after penetration of the redox substrate. Even if one may assume that no substrate penetration occurs, the data analysis must take into account the existence of two interfaces of different nature, each of which may be the site of an impedance. Figure 1B schematically shows a symmetrical arrangement for the study of redox processes occurring at a polymer film/ electrolyte interface (again as a limiting case). In this case the polymer film is a free-standing membrane, symmetrically bathed at both faces by the same electrolyte. Each of the two identical faces allows the transfer of both electrons and ions (only anions are shown since it is assumed that they are responsible for the neutralization of the polymer charge; i.e., the polymer behaves as an anion exchanger). Carriers of both types are also transported across the film. Thus, in the steady state, the overall current may be assumed as due to two additive components, the one resulting from convective diffusion of the redox species in solution, electron transfer at the polymer/electrolyte interface, and electron transport within the film, and the other due to migration of nonelectroactive ions in solution, ion transfer at the polymer/electrolyte interface, and ion transport within the film. As a first approximation, in the non steady state, the respective contributions will be inversely proportional to the impedance of each path. Actually the overall process is more complex since (a) electroactive species (if charged) may contribute to the ionic transport in both the solution and the film and (b) ionic and electronic transport are coupled in the film. ² UPR 15 du CNRS. ‡ IPELP CNR. X Abstract published in AdVance ACS Abstracts, May 1, 1996. Figure 1. Schematic representation of the main charge transfer and charge transport processes occurring on (A, top) a modified electrode and (B, bottom) a free-standing membrane in contact with electrolytes containing a redox couple. Only the limiting case of surface reaction is considered. 8994 J. Phys. Chem. 1996, 100, 8994-8999 S0022-3654(95)03154-6 CCC: $12.00 © 1996 American Chemical Society