Electrochemical Characteristics of Alkali‐Metal Doped Polyacetylene Electrodes

Abstract

The doping and undoping characteristics of alkali-metal doped polyacetylene are investigated using cyclic voltammetry and complex impedance measurement techniques. Reversibility, charge transfer resistance (Rct), double layer capacitance (Col), and general impedance behavior are discussed:Polyacetylene (PA) can be either partially oxidized or reduced electrochemically (1). When oxidized or reduced, PA becomes conductive with the appropriate counterion being inserted from the electrolyte. Its potential application to energy storage devices has spurred extensive studies on its electrochemical properties. Most studies have been concentrated on oxidized polyacetylene (2-5), which can be used as a cathode in secondary batteries. However, studies on reduced polyacetylene have also attracted attention (5-7). A variety of cations are suitable as counterions for the partially reduced polymer. These include alkali-metal ions and a number of organocations such as alkylammonium, immidazolium, peridinium, sulfonium, and the like. The doping (reduction) and undoping (oxidation) reaction can be represented as followswhere M § is most typically Li § Na § or K § y is the fractional charge per repeat unit, and x is the degree of polymerization. The reduced polymer has been suggested as an anode in secondary batteries (7). In the present work, the basic electrochemical doping and undoping characteristics of PA by alkali metals are studied using cyclic voltammetry and complex impedance measurements. ExperimentalPolyacetylene in the form of freestanding film was prepared using the standard Shirakawa technique (8). After preparation, the film was stored in a refrigerator at a temperature of -40~ inside a dry box before use. Polyacetylene electrodes were constructed from films having a thickness of 0.003 cm and an area of 0.5 • 0.5 cm 2 and were wrapped with expanded nickel metal. Thicknesses were measured with a micrometer exerting gentle pressure in order to minimize compression of the porous structure. Solvents used for the electrolyte solution were tetrahydrofuran (THF) or 2-methyltetrahydrofuran (2-MTHF). These solvents were distilled from a solution with sodium benzophenone. The salts, lithium tetrabuty]borate, LiB(C4Hg)4, and sodium tetraphenylborate, NaB(C~H~)4, purchased from Alfa, were used without further treatment. Potassium tributylpyrrolylborate, KB(C4Hg)3(C4H4N), was prepared in this laboratory according to the procedure by Klemann et al. (9). Each solution was stirred with its alkalimetal amalgam and filtered before use.A three-electrode cell having the configuration sketched in Fig. 1 was used in all of the electrochemical studies. The working electrode v~as a polyacetylene film with typical *Electrochemical Society Active Member. area 0.5 • 0.5 cm 2 wrapped with expanded nickel metal. Alkali metal held within a polyethylene tube was used as a reference electrode. Two alkali-metal foils were positioned on both sides of the working electrode as counterelectrodes. The cell was contained in a glass cell with a Kalrez O-ri...

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