Requirements for performance characterization of C double-layer supercapacitors: Applications to a high specific-area C-cloth material

Electrochemical capacitors, based on the double-layer capacitance of high specific-area C materials, are attracting major fundamental and technological interest as highly reversible, electrical charge-storage and delivery devices, capable of being operated at high power-densities. A variety of applications have been described in the literature, e.g. for cold-start vehicle assist, in hybrid load-leveling configurations with batteries, fuel-cells, as well as directly with internal combustion engines. Additionally, high capacitance C electrodes have been usefully employed as anodes coupled with battery-type cathodes, e.g. Pb/PbO2, in so-called asymmetric capacitor cells. On account of these perceived various applications, requirements for performance evaluation must be developed in systematic and complementary ways. In the present paper, we examine experimentally the following test procedures as exemplified by application to an high specific-area (ca. 2500m(2) g(-1)) woven C-cloth capacitor electrode material: (i) evaluation of the specific capacitances as a function of charge/discharge rates employing cyclic-voltammetry and dc charging curves; (ii) as in (i), examination of reversibility, and energy-efficiency as a function of electrolyte (H2SO4) concentration, i.e. conductivity; (iii) interpretation of effects in (i) and (ii) in terms of distributed resistance and capacitance in the porous C matrix according to the de Levie model; (iv) interpretation of data obtained in (i) in terms of Ragone plots which, for capacitor devices, require special treatment owing to the fundamental dependence of electrode- (or device) potential on state of discharge; (v) interpretation of self-discharge (SD) kinetics in terms of porous-electrode structure. Performance data for the C-electrode are given for capacitative charging up to high C-rates, extension of operational voltage windows and for SD behaviour. (c) 2005 Elsevier B.V. All rights reserved.