Physicochemical and Electrochemical Characterization of Polycyclopenta[2,1-b;3,4-b‘]dithiophen-4-one as an Active Electrode for Electrochemical Supercapacitors

Poly(cyclopenta[2,1- b;3,4- b ‘]dithiophen-4-one) (PCDT) has been characterized by several electrochemical and spectroscopic techniques so that it can be used as an active electrode material in electrochemical supercapacitors. This polythiophene derivative was prepared by the electrochemical polymerization of cyclopenta[2,1- b;3,4- b ‘]dithiophene-4-one (CDT) from a nonaqueous solution (acetonitrile) containing tetraethylammonium tetrafluoroborate. The range of electroactivity of PCDT in nonaqueous media spans at least 2 V. The doping levels measured by cyclic voltammetry and EDAX were found to be 0.19 and 0.14, respectively, for the oxidized-PCDT. Low-frequency capacitance measurements were made by electrochemical impedance spectroscopy to evaluate the film's ability to store charge. Low-frequency capacitances of about ∼70 F/g were found for both the p- and n-doped states. Ionic and electronic resistances were established using both a Randles equivalent circuit and the linear transmission line model to analyze the impedance data of electronically conducting polymers. The film morphology was studied by scanning electron microscopy, and photomicrographs revealed an open and porous structure. X-ray photoelectron spectroscopy was employed to evaluate the electronic properties of the polymer. Negatively charged sulfur atoms were only observed in low content probably due to low negative polaron stability and the sensitivity of the polymer to trace amounts of oxygen and water. Nonetheless, the S 2p σ - species for the n-doped state has clearly been identified in our study as we present here. A conservative estimate of the doping level was found to be ∼0.06. Preliminary galvanostatic charge/discharge cycling experiments indicated an energy density, E, of about 6 (W h)/kg for the active material with a power density, P, of 1 kW/kg for an 18 s discharge time. These values are above the midterm requirements fixed by the U. S. Department of Energy for electrochemical supercapacitors ( E > 5 (W h)/kg and P > 500 W/kg). A discharge capacity decrease was observed during the first 20 cycles but thereafter the discharge capacity remained constant for the next 80 cycles. Additional work is currently under way to improve the stability of the PCDT-based capacitor since this conducting polymer should be very interesting as an active electrode in electrochemical supercapacitors.