Improved supercapacitor performance of MnO2–graphene composites constructed using a supercritical fluid and wrapped with an ionic liquid

Abstract

Supercritical CO2 (SCCO2) is used to synthesize MnO2. This process is found to be promising for producing oxide nanorods (∼5 nm in diameter) with little agglomeration. With suitable SCCO2 pressure and temperature, nanocrystalline α-MnO2 with an extremely high surface area of 245 m2 g−1 (versus 80 m2 g−1 for the oxide prepared in ambient air) was obtained. Introduction of graphene nanosheets and carbon nanotubes into MnO2 with and without the aid of SCCO2 is compared. SCCO2 can help debundle the graphene nanosheets and uniformly disperse the MnO2 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, SCCO2-MnO2/graphene shows superior supercapacitor properties. The feasibility of using an ionic liquid (IL) as a conductive wrapping agent to improve the performance of the MnO2/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.