Electrochemistry of Conjugated Polymers and Electrochemical Applications
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Electrochemistry of Conjugated Polymers and Electrochemical Applications Alan G. MacDiarmid and Weigong Zheng Introduction The discovery in 1977-781 that transpoly acetylene— (CH)V, the prototype conducting polymer (Figure 1)—could be chemically p-doped (partly oxidized) or n-doped (partly reduced) with a concomitant increase of its conductivity through the semiconducting to the metallic regime introduced new concepts of considerable theoretical and possible technological importance to condensed matter science. In 19792 it was discovered that p- or H-doping of trans-(CH)x could be accomplished electrochemically and that these processes were electrochemically reversible. Polyacetylene is the simplest example of a conjugated polymer, a polymer in which the "backbone" atoms are joined alternately by single and double bonds. All conducting polymers, "synthetic metals," are conjugated polymers, at least in their doped forms. Other conducting polymers, including for example, poly(paraphenylene),3 polypyrrole,4 polythiophene,5 and polyaniline,6 have since been examined as electrochemically active materials. These findings have stimulated much industrial and academic interest in the electrochemistry of conducting polymers and their possible technological applications in for example, energy storage, electrochromic displays, electrochemical drug-delivery systems, electromechanical devices, and light-emitting devices. This article will show the relationship between the doping of a conjugated polymer, the reduction potential of the polymer, and the role of "dopant" ions. These interrelationships have frequently caused 24
considerable confusion in understanding electrochemical doping. Electrochemical synthesis of conjugated polymers and the role of cyclic voltammetry in elucidating the mechanism of electrochemical redox processes involving conjugated organic polymers will also be discussed. This article will also summarize a few selected applications involving electrochemical properties of conjugated polymers. The coverage is intended to be
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=c H
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C/s-(CH)x Figure 1. Cis- andtrans-isomers of the conjugated polymer, polyacetylene, (CH)X.
Standard Reduction Potentials The standard reduction potential E^ of a redox couple is measured with respect to the potential of the standard hydrogen electrode, which is defined as zero at 25°C. All substances must be at unit activity. A hydrogen electrode consists simply of a platinum electrode covered with a thin layer of very finely divided platinum (to increase the rate of H2 adsorption/desorption) immersed in 1.0-molar* aqueous acid in a glass vessel. Hydrogen gas at 1-atm pressure is constantly bubbled across the Pt/solution interface. When the vessel is connected to an adjacent vessel containing for example a 1.0-molar* solution of a silver salt such as AgNO3 into which a silver metal electrode is immersed, a voltage of 0.80 V is observed by a voltmeter connecting the platinum and silver electrodes. The solutions
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