Performance of electrochromic cells of polyaniline in polymeric electrolytes

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Polyaniline is currently considered to have good device potential [1-5]. One of the devices for which polyaniline is seriously considered is the electrochromic display. A number of studies on the electrochromism (EC) of polyaniline covering various aspects, such as the mechanism of the colour change [6-8] and the effect of cell parameters on the switching time [9], have been done, but these and similar studies have generally been conducted in liquid media in electrochemical cells. In practical devices it is preferable to employ solid materials in order to minimize the problems of sealing in hazardous liquids. Furthermore, EC devices are usually required to have a thin layer configuration. With these considerations in mind, we have performed experiments with thin layer cells of polyaniline containing a supporting electrolyte such as LiC10 4 dissolved in polyethylene oxide (PEO) or polyvinyl alcohol (PVA). We have also studied cells containing urea dissolved in glycerol as the medium. We report the performance of such cells in this letter. A polymeric electrolyte such as PEO was chosen because the amorphous nature of PEO leads to good ionic conductivity and redox stability up to +3 V [10]. In addition, the combination of PEO/ LiC104 is known to be a very fast ionic conductor, with the Li ÷ being the mobile species [11]. Recently, reports have also appeared which describe solid state electrochromic cells of methylene blue using polyacrylamide [12] and gels of polymethyl methacrylate in electrochromic cells of WO3 [13]. The composition of the electrochromic cells used in this study is given in Table I. We have studied the effects of the medium on the various parameters such as switching time, cycle lifetime and applied voltage on the electrochromic display. The current transients for the switching reaction of polyaniline were analysed to understand the influence of mass transport on the switching reaction in polyaniline. Cells for electrochromic studies were constructed as follows. Polyaniline (nominal thickness 1 #m) was coated with indium tin oxide (ITO) plate by application of alternating voltage ( -0 .1 V to 1 V) in HC1 medium containing 0.1 M aniline. The film was dried and dip coated with the appropriate electrolyte then covered with another plate of ITO. The whole assembly was then sealed with wax to exclude air. The schematic of such an electrochromic cell is shown in Fig. 1. Electrical contacts were made using alligator clips after ensuring that there was minimal contact resistance. The cell was mounted on a stand in an optical bench and illuminated at 632 nm with a tungsten-halogen lamp (Oriel Corp., USA) through a monochromator (Oriel). The transmitted light was monitored by an Si photodiode (Oriel). The diode