Asymmetric supercapacitor cells based on MnO2 as positive electrode and graphene as negative electrode have been constructed with 1 M Na2SO4 aqueous electrolyte. Amorphous MnO2 nanoparticles were prepared by reducing KMnO4 in N,N-dimethylformamid (DMF). Graphene was prepared by hydrothermal reduction of exfoliated graphite oxide sheets. The MnO2/graphene asymmetric supercapacitor could operate reversibly at a high cell voltage of 2.0 V. showing an energy density of 25.2 Wh kg(-1) at a power density of 100W kg(-1), which is much higher than those of symmetric supercapacitors based on MnO2/MnO2 (4.9 Wh kg(-1)) and graphene/graphene (3.6 Wh kg(-1)). Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles. (C) 2012 Elsevier B.V. All rights reserved.