Utilizing the dual functions of activated carbon (AC) both as a conductive agent and an active substance of a positive electrode, a hybrid supercapacitor (AC-MnO2&AC) with a composite of manganese dioxide (MnO2) and activated carbon as the positive electrode (MnO2&AC) and AC as the negative electrode is fabricated, which integrates approximate symmetric and asymmetric behaviors in the distinct parts of 2 V operating windows. MnO2 in the positive electrode and AC in the negative electrode together form a pure asymmetric structure, which extends the operating voltage to 2 V due to the compensatory effect of opposite over-potentials. In the range of 0-1.1 V. both AC in the positive and negative electrode assemble as a symmetric structure via a parallel connection which offers more capacitance and less internal resistance. The optimal mass proportions of electrodes are calculated though a mathematical process. In a stable operating window of 2 V. the capacitance of AC-MnO2&AC can reach 33.2 F g(-1). After 2500 cycles, maximum energy density is 18.2 Wh kg(-1) with a 4% loss compared to the initial cycle. The power density is 10.1 kW kg(-1) with an 8% loss. (C) 2010 Elsevier B.V. All rights reserved.