Pergamon Ekrtrochimica km. Vol. 40, No. 13 14. pp. 2223 -2228. 1995 Copyright 0 1995 Elsevier Science Ltd. Printed in Great Britain. All ri@s reserved 00%4686/95 59.50 + 0.00 zyxwvutsr CHARACTERIZATION BY IMPEDANCE SPECTROSCOPY OF A POLYMER-BASED SUPERCAPACITOR C. ARBIZZANI,* M. MAWRAGOSTINOand L. MENECHELLO Department of Chemistry “G. Ciamician”, University of Bologna, via F. Selmi 2,40126 Bologna, Italy zyxwvutsrqponm (Received 6 April 1995) Abstract-The performance of a polymer redox supercapacitor based on poly(dithieno[3,4-b : 3’,4’-d] thiophene) in liquid and in polymer gel electrolyte was tested for capacity, energy density, power density and self-discharge.The results are very promising and demonstrate the viability of a symmetric super- capacitor based on an n- and p-doped electronically conducting polymer. Key words: polymer electrolyte, supercapacitor, electrochemical capacitor, impedance spectroscopy, n- and p-doped polymer. INTRODUCTION Supercapacitors based either on the double-layer capacitance with high-area electrode materials or on a redox pseudocapacitance with Faradaic-active materials are now attracting much attention as complement-battery devices for electric vehicle power source and for computer power back-up or other applications requiring fast charge-discharge[l, 21. Supercapacitors based on double-layer capac- itance are under going rapid developments and high- performance devices are already on the market. Pseudocapacitance-based supercapacitors, in which Faradaic charge transfer takes place, particularly those with electronically conducting polymers (ECPs) as active materials, are a different version of these devices. ECPs can be positively or negatively charged at suitable potentials with ion insertion in the polymer matrix to balance the injected charge, processes that are known as p- and n-doping, respec- tively. Recently Rudge et aI.[3-61 have described several schemes by which ECPs can be applied as active materials, and we have focused our attention on the one with a p- and n-dopable ECP as the most promising scheme in terms of energy and power characteristics. Unlike that for p-doped ECPs, there are very few reports on n-doped polymers because of the intrinsic difficulties in the n-doping process, whose electro- chemical characteristics are strongly dependent both on counterion size[7] and the solvent’s properties (a greater solvent reactivity in n-doping potential domain). The present study reports experiments evaluating the performance of a symmetric supercapacitor based on the poly(dithieno[3,4-b : 3’,4’-dlthiophene) (pDTT) which, because of the small energy gap between valence and conduction bands, can be suc- * Author to whom correspondence should be addressed. cessfully p- and n-doped in the presence of tetra- lkylammonium salts[8]. Data are reported both for liquid- and solid-state configurations of the device with propylene carbononate (PC)-tetraethyl- ammonium tetrafluoborate (TEABF,) and a gel elec- trolyte based on polyethylene oxide (PEO) and PC- TEABF,; conductivity data of an hybrid electrolyte feasible for this supercapacitor and based on a cross linked polyethylene glycol (PEG) incorporating PC- TEABF, are also reported. EXPERIMENTAL The pDTT films were electrosynthesized from the monomer (DTT) after ref. [9] on carbon paper elec- trodes (TCP-090, Toray Industries, Inc., Japan, 0.25cm2 geometric area and 0.75cm’ effective area. All the data are referred to geometric area). The elec- trosyntheses are performed in acetonitrile (ACN)-TEABF, 0.1 M-DTT 0.015 M at I = 1 mAcm_’ under argon atmosphere at room temperature; film deposition charge was 2.5 C cm- 2. The electrosynthesized polymers, before being tested in supercapacitors, were activated in PC-TEABF, 0.2M by cyclic voltammetry (CV) in the anodic range, until cycling stability was reached (ca. 30 cycles), and then in the cathodic one for the n- dopable electrodes. The gel electrolyte (GE) PE0,gTEABF,-86% w/w PC was prepared in dry box by adding PEO (Union Carbide 4 000 000 MW) to a solution of PC-TEABF, 0.2 M in a ratio PEO : PC = 1 : 6 and kept 10h at 70°C in a sealed vial. The hybrid electrolyte (HE) PEG2,TEABF,-50% w/w PC, a cross-linked polymer incapsulating PC-TEABF, , was prepared from polyethylene glycol (PEG) (Fluka 2000MW) dissolved in PC-TEABF, by an aliphatic tri- isocyanate (Desmodur N3390 from Bayer) cross- linking agent 25% in excess with respect to stechiometric quantity; the reaction at 70°C took 5h. 2223