In this study, the performance of a polyaniline (PANI) based supercapacitor where electroactive PANI films were prepared on carbon paper electrodes from a nonaqueous solution with an organic acid (CF3COOH)as the proton source was investigated. The use of nonaqueous media as electrolyte led to an increase of the electroactivity window from 0.75 V in aqueous media up to 1.0 V. Low frequency capacitance, evaluated by electrochemical impedance spectroscopy, of about 150 F/g is reported. Scanning electron microscopy indicated a highly porous material for deposited charges greater than 1 C/cm(2). Constant current charge/discharge cycling of a symmetric supercapacitor based on PANI in nonaqueous medium was performed in a two-electrode cell configuration and a loss of about 60% of the discharge capacity was demonstrated after 1000 cycles. Tetramethylammonium methanesulfonate (Me4NCF3SO3) was also used instead of tetraethylammonium tetrafluoroborate (Et4NBF4) as supporting electrolyte in acetonitrile for the charge/discharge testing of the PANI-PANI capacitor Energy and power densities of approximately 3.5 Wh/kg and 1300 W/kg, respectively, were developed by this supercapacitor for a cell voltage of 1 V and a discharge rime of 20 a. On the other hand, an asymmetrical supercapacitor with polypyrrole and polyaniline used as positive and negative electrodes, respectively, displayed slightly improved performance. Indeed, an energy density of 5 Wh/kg and a power density of 1200 W/kg were reported for discharge time of about 20 s with 1 M Me4NCF3SO3/acetonitrile as electrolyte. (C) 2000 The Electrochemical Society. S0013-4651(99)07-124-4. All rights reserved.