Graphene–MnO2 nanocomposite for high-performance asymmetrical electrochemical capacitor

Graphene–MnO2 (GR–MnO2) nanocomposites with different MnO2 amounts have been synthesized by a simple redox reaction between GR and KMnO4 at 70 °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.