Anion and Solvent Exchange as a Function of the Redox States in Polyaniline Films

Abstract

Intensive improvements in electroactive polymer research has provided a wide range of new materials with notable technological uses. Considerable interest has been displayed in the development of modified electrodes with electrochemically reactive polymeric materials due to their potential application in energy storage, electrochromic devices, chemical sensors, and a series of other electrochemical devices. 1,2 Novel techniques, such as the electrochemical quartz crystal microbalance (EQCM), 3-9 radio tracer method, 10 probe beam deflection, 11 and scanning electrochemical microscopy, 12 have supplied reliable information about the kinetics and mechanism of synthesis and charging-discharging reaction of those polymers. The insertion and expulsion of ionic species were determined, and their influence on the charge-compensation process was investigated in order to clarify the redox mechanism.Nowadays, high electronic conductivity, resulting from a protonation process in some intermediate oxidation states, in combination with its easy synthesis, processability, and stability makes polyaniline (PANI) films an intensively investigated polymer. 6,13-20 A great deal of work has been devoted to the electrochemical characterizations but now fundamental interest resides in the PANI redox-reaction mechanism. Proton exchange and redox properties have been reported since 1968 and become the subject of deep investigations in the 1980s with the work of MacDiarmid et al. 21 and Genies et al. 22 In their studies, they concluded that irrespective of the method employed for synthesis, polyanilines undergo reversible redox reactions without degradation in a specific potential range. Now, ionic transport in the film, necessary for the maintenance of electroneutrality, is investigated by modern analytical techniques trying to identify the chemical species taking part in the redox process. Authors 3,23,24 have suggested that, in aqueous acid electrolytes, the PANI initial electrochemical oxidation is accompanied by proton expulsion with subsequent anion incorporation when a certain amount of polymer has been oxidized. Incorporation of anions, 25-27 protons expulsion, 28 or both processes, 3,29 are currently reported, because the improvement of such knowledge seems to be a way for obtaining an ideal electrochemical response of PANI, leading to new materials of relevant technological importance. For a better understanding of the structure and properties of electrochemically reactive films, appropriate modifications in the polymeric surface can be made which are of great interest for their technological application.In this present work, it is intended to discuss polyaniline-film formation, charge transfer, and the swelling process during oxidation and reduction of the films in acidic solution. EQCM and cyclic voltammetry experiments are used simultaneously to study the electropolymerized polyaniline film. ExperimentalThe electrochemical polymerization of aniline was carried out by cycling the potential between Ϫ0.2 and 0.75 V [vs. a sat...

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