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Supercritical CO 2 (SCCO 2 ) is used to synthesize MnO 2 . This process is found to be promising for producing oxide nanorods ($5 nm in diameter) with little agglomeration. With suitable SCCO 2 pressure and temperature, nanocrystalline a-MnO 2 with an extremely high surface area of 245 m 2 g À1 (versus 80 m 2 g À1 for the oxide prepared in ambient air) was obtained. Introduction of graphene nanosheets and carbon nanotubes into MnO 2 with and without the aid of SCCO 2 is compared. SCCO 2 can help debundle the graphene nanosheets and uniformly disperse the MnO 2 nanorods in between, thus preventing graphene restacking. Since the oxide particles are sandwiched between the highly conductive sheets, a high electrochemical utilization is obtained. Accordingly, SCCO 2 -MnO 2 /graphene shows superior supercapacitor properties. The feasibility of using an ionic liquid (IL) as a conductive wrapping agent to improve the performance of the MnO 2 /graphene electrode is demonstrated.Because the intimate contact between the oxide and graphene can be ensured and the interfacial double-layer capacitance can be optimized, the electrode wrapped with an IL shows a higher capacitance, improved high-rate capability, and higher cyclic stability compared to those obtained without IL wrapping.

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