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.