ELSEVIER Synthetic Metals 87 (1997) 179-185 Impedance spectroscopy of undoped, doped and overoxidized polypyrrole films J. Mostany *, B.R. Scharifker Departamento de Qufmiea, Universidad Simdn Botfvar, Apartado 89000, Caracas 1080-A, Venezuela Received 2 December 1996; accepted 4 December t996 Abstract The frequency response of polypyrrole thin films was studied in the presence of NO3-, F-, CI- and Br- in the potential region of interest for the study of the insulator-conductor transition. The impedance spectra have been interpreted using the Randles equivalent circuit modified with a constant phase element (CPE), using nonlinear least-squares fitting of the parameters of the circuit to experimental data. The results show that the distribution of relaxation times widens with oxidation of the polymer, indicating the presence of domains with different electrical properties, consistent with hopping of charge carriers in the oxidized state. It is also shown that in F - solution and at moderate potentials, overoxidation leads to irreversible degradation of the electroactive properties, affecting the electronic conductivity and diffusion coefficients of ionic species within the film. Keywords: Polypyrrole; Films; Doping; Impedance spectroscopy; Overoxidation 1. Introduction The electrochemical behaviour of conducting polymers has been extensively studied with the aim of elucidating the nature of ionic doping, conduction and charge accumulation in these materials. Diverse electrochemical and non-electro- chemical techniques have been used for this purpose, among them electrochemical impedance spectroscopy (EIS) [ la]. Whereas transient techniques require large perturbations for the determination of transport phenomena which, in the case of thin films, may be accompanied by important structural changes [2], impedance techniques offer advantages over transient studies mainly due to (i) coverage of a wide range of frequencies, which allows the determination of both kinetic and mass transport parameters, and (ii) small perturbation of the system from its steady state. The frequency response of an electrochemical system to a low-amplitude perturbation might be interpreted either formally, with a detailed physical model based upon a plausible theory [3], or through an empirical equivalent circuit, with faradaic processes, space charges, and electronic and ionic conduction processes rep- resented as ideal components, such as resistors and capacitors, or as distributed elements to account for dispersion in the microscopic properties of non-homogeneous materials [ 1a]. * Corresponding author. Tel.: +58 2 906 3980; fax: +58 2 906 396t; e-mail: jmosta@usb.ve 0379-6779/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved P//S0379-6779 (96)04905-3 The behaviour of polypyrrole depends strongly on its oxi- dation state. Reduced polypyrrole films behave as electronic insulators. Oxidation of the polymer backbone generates charge carriers, which confer electronic conductivity to the polymer, but may also lead to side reactions that alter the molecular structure of the polymer and, hence, change its properties. In the presence of nucleophiles, oxidation ofpoly- pyrrole leads to the formation of substituted products [4]. The presence of a strong nucleophile such as OH -, for exam- ple, leads to the formation of quinone moieties that disrupt the conjugated double-bond system and, hence, the electronic and electrochemical properties of oxidized polypyrrole. Such irreversible modifications of the polymer upon oxidation have been termed 'overoxidation' [5-7] and, although in general lead to degradation of optical properties [ 8 ] and electronic conductivity, may give rise to selective ionic exchange prop- erties [9] and shifts in the doping potential [ 10]. We have recently found [ 11 ] that in F- aqueous solution the solvent participates in the doping process and, as a result, overoxi- dation readily occurs at mild potentials. It is thus important to characterize further the overoxidation processes and to assess their effects on the electrochemical behaviour of con- ducting polymer films. In this work we have studied the frequency response of polypyrrote thin films in the presence of several anions. Data are interpreted by means of the Randles equivalent circuit