An asymmetric electrochemical capacitor based on polyfluorene/carbon nanocomposite as positive electrode and activated carbon as negative electrode with nonaqueous electrolyte of 1 M tetraethylammonium tetrafluoroborate/propylene carbonate was investigated. From a galvanostatic charge-discharge test, the asymmetric capacitor test cell exhibited cell voltage as high as 3.2 V because of the high redox potential of the positive electrode (similar to 1.1 V vs Ag/Ag+). Specific capacitance obtained for the test cell was 34 F g(-1) (per electrode mass). The maximum energy density of the test cell was 47 Wh kg(-1) (per electrode mass), which was two times higher than that of typical double-layer capacitors based on activated carbon/activated carbon electrodes. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2994627] All rights reserved.