Chemical Synthesis and Electrochemical Properties of Poly(cyano-substituted-diheteroareneethylene) as Conducting Polymers for Electrochemical Supercapacitors

Abstract

Polymers derived from diheteroaryl-(cyanovinylene), synthesized by chemical polymerization of the monomers in the presence of 6 equivalents of FeCl3FeCl3 in chloroform, were characterized by elemental analysis, energy-dispersive analysis by X-ray, and X-ray photoelectron spectroscopy. The electrochemical performances of composite electrodes prepared from chlorinated poly-(E)-α-[(2-thienyl)methylene]-2-thiopheneacetonitrile (poly-1), poly-(E)-α-[(3-methyl-2-thienyl)methylene]-2-thiopheneacetonitrile (poly-2), and poly-(E)-α-[(2-furanyl)methylene]-2-thiopheneacetonitrile (poly-3), with acetylene black (A.B., 45 wt %) and polytetrafluoroethylene (5 wt %) have been investigated in 1 M Et4NBF4Et4NBF4 /acetonitrile using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge cycling. The effect of the structure of the polymers on their electrochemical properties is in good agreement with anticipated effects for methyl and furan groups. The results indicated p- and n-doping levels up to 0.3 electrons per heterocycle for the three polymers. These values make those polymers interesting candidates for use in electrochemical supercapacitors in which specific capacity and energy of about 30 Ah/kg and 55 Wh/kg of polymer may be achieved. The best cyclability was demonstrated with poly-1 and poly-2, in particular during cycling in their n-doping state with a doping level of 0.16 and 0.17 electrons per thiophene unit, respectively, maintained after 1000 cycles. Preliminary charge/discharge galvanostatic cycling with a poly-1-based supercapacitor yielded specific energy and power of 42 Wh/kg and 11 kW/kg of polymer, respectively, during the first 60 cycles and 30 Wh/kg and 9 kW/kg after 1800 cycles, for a discharge time of about 10 s. Moreover, cycling experiments performed separately on negative and positive electrodes have shown that the capacity loss is associated essentially with the n-doping process at the negative electrode. © 2001 The Electrochemical Society. All rights reserved.