Supercapacitors are battery-complementary devices for applications demanding high operating power levels. [1][2][3] The increasing interest in such devices is stimulated by the prospect of their use as secondary power sources in electric vehicles (EVs) to provide peak power for acceleration and hill climbing. For EV supercapacitors, the U.S. Department of Energy (DOE) has set the following goals for specific power and energy of a fully packaged device: 500 W kg Ϫ1 and 5 Wh kg Ϫ1 (near-term goals, 1998-2003) and 1500 W kg Ϫ1 and 15 Wh kg Ϫ1 (advanced goals) 4 with a device cycle-life of 10 5 cycles. 1 Two types of supercapacitors are under development: the doublelayer and the redox supercapacitors. In the former, energy storage arises mainly from the separation of electronic and ionic charges at the interface between high-specific-area electrode materials and electrolyte solution, i.e., it is electrostatic in origin. 5 In the latter, fast faradaic reactions take place at the electrode materials at characteristic potentials, as in batteries, and give rise to what is called pseudocapacitance. 6 The targets are the same for both device types: the development of electrode materials with high specific capacitance, for maximizing specific energy, and with low electric resistance, for maximizing specific power, and of high stability to repeated chargedischarge cycles for a long cycle life. Additional requisites include electrolytes with high breakdown potential for greater energy storage and low resistivity for greater power, and for market success, materials with a high performance-to-cost ratio.Electrode materials for double-layer supercapacitors are essentially activated carbon of high specific area up to 2500 m 2 g Ϫ1 ; high performance C/C supercapacitors are already on the market. 1,7 For redox supercapacitors, two classes of electrode materials are being developed: the noble metal oxides for use in aqueous electrolytes, with ruthenium oxide as the best performing, 8 and electronically conducting polymers (ECPs) for use in both aqueous 9 and organic electrolytes. 10 The latter class is the most promising for EV supercapacitors, particularly because it enables devices to work at potentials as high as from 3.0 to 3.2 V, and also because of the lower cost of ECPs with respect to RuO 2 .The faradaic reactions occurring in ECPs are the p-doping and ndoping processes (Scheme 1 with polythiophene as an example).
Supercapacitors are now attracting much attention as battery‐complementary devices for applications requiring high operating power levels, such as in electric vehicle technology. Two types of supercapacitors with different modes of energy storage are currently under study: the double‐layer, and the redox. Electronically conducting polymers represent an interesting class of electro‐active materials for redox supercapacitors and can be applied in different configurations. Performance data for new and conventional devices are presented and compared to those of a double‐layer capacitor with activated carbon electrodes.
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