Graphene-MnO2 (GR-MnO2) nanocomposites with different MnO2 amounts have been synthesized by a simple redox reaction between GR and KMnO4 at 70 degrees C, in which GR as a supporting substrate to grow MnO2. Ultrathin MnO2 nanoflakes with a layered structure are dispersed on the surface of GR nanosheets, and the incorporation of MnO2 nanoflakes onto GR prevents the agglomeration of GR nanosheets. GR nanosheet suspension reacted with a KMnO4 solution of 1.45 mmol L-1, the obtained nanocomposite (GR-MnO2 (1.45)) electrode shows a specific capacitance of 274 F g(-1) at a scan rate of 10 mV s(-1). An asymmetrical electrochemical capacitor is assembled by using GR-MnO2 (1.45) nanocomposite as positive electrode and GR as negative electrode. It can cycle reversibly in a cell potential of 0-1.8 V and gives a high energy density of 23.9 Wh kg(-1) and a good cycling performance of about 96% of the initial capacitance after 1000 cycles. (C) 2013 Elsevier Ltd. All rights reserved.