Functional Graphene Quantum Dot-Mediated Synthesis of Heterojunction Nb2o5-Graphene Composite for Flexible Symmetrical Supercapacitors with High Energy Density and Wide Voltage Window of 2 V

Nb2O5 has great potential in high-performance supercapacitors because of its multielectron redox, high chemical stability and wide voltage window, but poor electronic conductivity limits its practical applications. The study reported one strategy for functional graphene quantum dot-mediated synthesis of heterojunction Nb2O5 -graphene composite. Firstly, p-type semiconductor, EDTA-functionalized and boron-doped graphene quantum dot (p-GQD), was made via two-step pyrolysis. Then, it was orderly fixed on graphene oxide via π-π stacking, coordinated with Nb(V) ion, reduced into hydrogel with ascorbic acid and thermally annealed in N2. The ressulting Nb2O5-p-GQD-G offers three-dimensional structure, ultrasmall size of Nb2O5 nanorods, PN junction and Schottky heterojunction. Unique structure significantly improves the electronic conductivity, electrochemical active sites and voltage window (2V). The specific capacitance of n-Nb2O5-p-GQD-G electrode reaches to 267.4F g-1 at 1A g-1 . The value is more than 1.5 times that of n-Nb2O5-n-GQD-G electrode, verifying that the formation of PN junction brings an increased capacitance. The symmetrical flexible supercapacitor exhibits excellent supercapacitor performance, including high capacitance (794F g-1 at 0.2A g-1 ), rate-capacity (167.2F g-1 at 2A g-1 ), cycling stability (98.8% capacity retention after 10000 cycles at 1.5 A g-1 ) and energy density (110.3 W h Kg-1 at 200 W kg-1 ).