ELECTROCHEMICAL CHARACTERISTICS OF ANODICALLY PREPARED CONDUCTING POLYANILINE FILMS ON CARBON-FIBER SUPPORTS

Conducting polyaniline (PANi) films can be electrodeposited on carbon fibre supports, without the additional peak at 0.5 V, by monitoring the anodic potential limit of the cyclic potential sweep deposition. Cyclic voltammetric measurements on samples with different thicknesses (130 and 34 nm) are used to investigate the electroactivity of the PANi films in aqueous 1 M H2SO4 solution and in propylene carbonate (PC). Sufficient stability of PANi upon storage in the dry state and in aqueous 1 M H2SO4 solution is verified. Scanning electron micrographs show that the deposited films are uniform and continuous without fibril development. Electrochemical impedance spectroscopic measurements are carried out in aqueous 1 M H2SO4 solution and in PC at various bias potentials over the frequency range 75 kHz-0.6 Hz. The usefulness of computer simulation and transfer function transformation techniques in the analysis of the a.c. response on the basis of an electrical circuit analog is demonstrated and the simulated spectra are given. A model of the electrochemical interface is discussed and values of kinetic and diffusion parameters are estimated. It is demonstrated that two types of doping are involved in the redox process. A fast doping process results in the formation of an 'internal surface' double-layer polarization capacitance with fractional power behaviour, while the slow one is a bulk 'diffusion-controlled' phenomenon resulting in large capacitance values in the range 500-600 F cm(-3). Diffusion coefficient values for the doping species of the order of 10(-9)-10(-11) cm(2) s(-1) are estimated. The conductivity associated with the PANi film, shows a marked increase when the film is converted from the insulating reduced state (sigma similar to 1.5 mu S cm(-1)) to the highly conducting oxidized state. A 'two-step' undoping process upon reduction of PANi is evidenced.