Electrochemical and Thermal Behavior of Polyaniline in Aqueous Solution Containing  SO 4 2 −  Ions

Abstract

Electrochemical and thermal measurements were carried out simultaneously on polyaniline in 1M ZnSO4 aqueous solution. An exothermic change at lower potential and an endothermic change at higher potential were observed on the surface of the polyaniline electrode during the anodic oxidation. The proportional relation between I hTdt and electric quantity was recognized in the thermal response to anodic constant current. The thermal response obtained here may be mainly attributed to the entropy change for two electrochemical processes of doping of SO~-ions and proton elimination. Temperature-dependence of electrode potential of the po]yaniline oxidized anodically in ZnSO4 aqueous solution was examined to evaluate the entropy change for electrochemical dedoping process of SO~ ions.Conducting polymers have attracted workers' attention as functional materials. In order to use conducting polymers effectively, it is important to make clear doping process which may cause their function. The electrochemical method is useful in synthesis of polymers and analysis of the doping process and also for their application. There have been various papers concerning electrochemical behavior of polyaniline. 1-16 It has been proposed that not only anion doping but also proton elimination may proceed in the anodic oxidation of polyaniline in aqueous solutions. In situ measurements of absorption spectra 7-9'1t-I~ and weight change 5' 14-16 of polyaniline during anodic oxidation have been carried out to get knowledge of the electrochemical doping process. We have reported preliminary results of the thermal behavior of polyaniline in the anodic process. ~7' 18 Thermal methods can provide information on the entropy change for an electrochemical reaction. In this paper, we investigated the electrochemical process of polyaniline in aqueous solution containing SO~ ions mainly from the point of view of entropy change through the analysis of electrochemical and thermal response of polyaniline. ExperimentalPolyaniline was anodically polymerized on a graphite electrode (Toyo Tanso Company, size: 10 × 20 × 0.6 mm 3) by a potential sweep method [potential range: 0.0 -0.8 V vs. saturated calomel electrode (SCE); sweep rate: 10 mV s 1; sweep repetition times: 57] in 1M (M = tool dm -3) I-I2SO4 aqueous solution containing 0.1M aniline. To measure the weight of polyaniline polymerized, the potential of the polyaniline-graphite electrode was kept at -0.20 V for 30 min after polymerization, and then it was rinsed with pure water and dried in vacuum. The weight of polyaniline was 6.5 mg by subtracting the weight of graphite. The polyaniline thus prepared was used as a working electrode. As counter and reference electrodes a platinum wire and an SCE, respectively, were used. Thermal measurement was done as follows. One of two thermistors (Takara Thermistor, Model PXK-76; resistance: 7.326 and 7.328 k12 at 20°C; thermistor constants: 3348 and 3337 K, respectively) was attached to the surface of the polyaniline electrode and the other positioned in the bulk of...

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