Hierarchical porous carbon spheres/graphene composite for supercapacitor with both aqueous solution and ionic liquid

Hierarchical porous carbon spheres/graphene (CSG) composite was prepared by a simple self-assembled method for supercapacitor electrode materials. Carbon spheres (CS) was chosen as the spacer for graphene due to its submicron size. The morphology and structure of as-prepared samples were characterized by SEM, XRD and N2 adsorption/desorption techniques. As a result, the aggregation of graphene was reduced by CS effectively, which leads to an enhanced specific surface area and more ion-accessible pores. This unique structure is conducive to the penetration/mobility of ions, especially for ionic liquid with large ionic sizes. The electrochemical performance of synthesized CSG was investigated and the composite offered an outstanding capacitive behavior. In the two-electrode system, CSG electrode displayed a specific capacitance of 303.8 F g−1 in KOH and 280.0 F g−1 in [BMIM]BF4. Remarkably, the CSG//CSG supercapacitor exhibited a high energy density of 87.5 Wh kg−1 in [BMIM]BF4 electrolyte. After 5000 cycles, the specific capacitance of CSG could still remain 95.3% in KOH and 90.1% in [BMIM]BF4 of the initial value, implying good long-term cycle stability of the composite. The outstanding electrochemical performance can be attributed to the novel hierarchical porous structure.