One-step hydrothermal synthesis of manganese oxide nanosheets with graphene quantum dots for high-performance supercapacitors

MnO₂ is considered to be one of the promising electrode materials for supercapacitors thanks to its ultra-high theoretical capacitance value, but its actual electrochemical performance is not ideal due to its low electrical conductivity and poor stability. Herein, we find that the supercapacitor performance of graphene quantum dots (GQDs)@MnO₂ composite is superior to that of pure MnO₂ electrode. The GQDs@MnO₂ composite is obtained by a highly efficient one-step hydrothermal method, in which KMnO₄ reacts with graphene oxide to produce MnO₂ nanosheets anchored with GQDs in a short time. The GQD@MnO₂ electrode presents high specific capacitance of 246 F g⁻¹ at a scan rate of 1 mV s⁻¹ in Na₂SO₄ electrolyte, and the as-assembled asymmetric supercapacitor (GQDs@MnO₂//activated carbon) exhibits superior energy density of 29.9 Wh kg⁻¹ at power density of 538.0 W kg⁻¹, and good cycling performance (81.3 % retention after 8000 cycles) that was far better than that of pure MnO₂-based supercapacitor. The excellent supercapacitor performance of GQDs@MnO₂ composite results from its enhanced electrical conductivity, good wettability and abundant available contact sites for aqueous electrolyte, which are ascribed to the intrinsic high electrical conductivity as well as the quantum confinement and edge effects of GQDs.