Coupling of graphene quantum dots with MnO2 nanosheets for boosting capacitive storage in ionic liquid electrolyte

• A new design of graphene quantum dots (GQDs) integrated with MnO 2 is proposed. • GQDs play an important role in optimizing structure and performance of MnO 2 . • Such composite electrode shows superior capacitive storage behaviors in IL electrolyte. • A high-performance flexible IL-based supercapacitor is successfully assembled. • High energy/power densities and strong durability are acquired in flexible device. Utilizing ionic liquid (IL) as electrolyte to fabricate supercapacitor is an effective strategy for increasing its operating voltage and energy density. Although manganese dioxide (MnO 2 ) can exhibit pseudocapacitive storage behavior in some specific IL electrolytes, it still suffers from low specific capacitance as well as unsatisfactory kinetics. Herein, a new design of graphene quantum dots (GQDs) integrated with MnO 2 nanosheets is proposed to construct a GQDs@MnO 2 composite electrode for IL-based supercapacitor. Synergistically coupling of GQDs with MnO 2 nanosheets provides a 3D nanoflower architecture with increased surface area, enhanced electrochemical kinetics and excellent structural integrity. The GQDs play an important role in modifying the density of state and energy bandgap as well as enhance the electronic conductivity of GQDs@MnO 2 electrode. These properties endow the superior capacitive storage, rapid charge–discharge response and high electrochemical reversibility for GQDs@MnO 2 electrode in IL electrolyte. For real-life application, a high-performance flexible IL-based supercapacitor is assembled, which can deliver a high energy density (82.2 Wh kg −1 ), a high power density (11.6 kW kg −1 ), and long-term cycle performance upon the straight and bent states, suggesting its great potential in energy-related technologies and portable electronics.