Carbon Dot-Mediated Synthesis of Manganese Oxide Decorated Graphene Nanosheets for Supercapacitor Application

In this work, we demonstrate that carbon dots (CDs) can be used as a dispersing agent for graphene as well as a reducing agent for KMnO₄ for the synthesis of manganese oxide (MnO_x)-graphene hybrid nanocomposites for supercapacitor applications. CDs obtained from the pyrolysis of ammonium citrate under dry heating possess excellent solubility in water due to their oxygen- and nitrogen-containing functional groups. In addition, the sp²-carbon-rich CDs exhibited strong interaction with graphene through π-π stacking for self-immobilizing on graphene in the preparation of water-soluble CD/graphene nanocomposites (CDGs). Interestingly, MnO_x could be grown in situ on CDGs after reaction with KMnO₄ in aqueous solution under a mild reaction temperature (75 °C). Under the mild reaction conditions, CDs undergo sacrificial oxidation for the formation of MnO_x nanoparticles on graphene, whereas the graphene's graphitic carbons are protected. The as-formed nanostructured MnO_x on CDGs (MnO_x-CDGs) was employed to fabricate flexible solid-state supercapacitor which exhibited good capacitance properties (specific capacitance ∼280 F g^-1) with very high charge-discharge cyclic stability (>10-000 cycles) and good capacitance retention at 90° bending angle. Compared to other graphene-based nanocomposites, our one-pot synthesis route for MnO_x-CDGs is relatively green, simple, rapid, and cost-effective and has a great potential for the synthesis of different metal oxide-decorated graphene nanocomposites for energy conversion and storage applications.