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 characterize d by several electrochemical and spectroscopic techniques so that it can be used as an active electrode material in electrochemical supercapacitors. T his polythiophene derivative was prepared by the electrochemical polymeriza tion of cyclopenta[2,1-b;3,4-b'] dithiophene-4-one (CDT) from a nonaqueous solution (acetonitrile) containing tetraethylammonium tetrafluoroborate. Th e 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 capacitan ce measurements were made by electrochemical impedance spectroscopy to eval uate the film's ability to store charge. Low-frequency capacitances of abou t similar to 70 F/g were found for both the p- and n-doped states. Ionic an d electronic resistances were established using both a Randles equivalent c ircuit 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 por ous structure. X-ray photoelectron spectroscopy was employed to evaluate th e electronic properties of the polymer. Negatively charged sulfur atoms wer e only observed in low content probably due to low negative polaron stabili ty and the sensitivity of the polymer to trace amounts of oxygen and water. Nonetheless, the S 2p(sigma-) species for the n-doped state has clearly be en identified in our study as we present here. A conservative estimate of t he doping level was found to be similar to 0.06. Preliminary galvanostatic charge/discharge cycling experiments indicated an energy density, E, of abo ut 6 (W h)/kg for the active material with a power density, P, of 1 kW/kg f or 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 obser ved during the first 20 cycles but thereafter the discharge capacity remain ed 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 electrochemica l supercapacitors.