Synthesis of highly dispersed Nb2O5-graphene heterojunction composites using ethylene diamine tetraacetic acid and boron-functionalized graphene quantum dots for symmetrical flexible supercapacitors with ultrahigh energy density

Nb2O5 is a promising metal oxide electrode material for supercapacitors because of outstanding structural stability and wide electrochemical window, but low inherent conductivity hinders its many applications. This article reports one way for the synthesis of Nb2O5-graphene heterojunction composites using ethylene diamine tetraacetic acid and boron-functionalized graphene quantum dots (p-GQDs). p-GQDs were immobilized on graphene sheets by pi-pi stacking, followed by coordination with Nb(v), reduction into a hydrogel with ascorbic acid and annealing in nitrogen. The resulting Nb2O5-p-GQD-G offers a three-dimensional structure, ultrasmall size of Nb2O5 nanorods, PN junctions and Schottky heterojunctions. These dramatically improve the inherent conductivity, number of electrochemically active sites and a voltage window range. The n-Nb2O5-p-GQD-G electrode indicates a capacitance of 267.4 F g(-1) at the current density of 1.0 A g(-1), which is more than 1.5-fold that of the n-Nb2O5-n-GQD-G electrode, verifying that the formation of PN junctions can enhance the capacitance. The symmetrical supercapacitor with n-Nb2O5-p-GQD-G electrodes provides high specific capacitance (794 F g(-1) at the current density of 0.2 A g(-1) and 167.2 F g(-1) at the current density of 2.0 A g(-1)), cycling stability (a capacity retention of 98.8% after 10 000 cycles at the current density of 1.5 A g(-1)) and energy density (110.3 W h Kg(-1) at the power density of 200 W kg(-1)).

Automated summary

This article presents the synthesis of Nb2O5-graphene heterojunction composites, consisting of a three-dimensional structure, ultra-small Nb2O5 nanorods, PN junctions, and Schottky heterojunctions. These composites significantly enhance the conductivity, electrochemically active sites, and voltage window range. The resulting n-Nb2O5-p-GQD-G electrode exhibits a capacitance of 267.4 F g(-1) at a current density of 1.0 A g(-1), which is 1.5 times higher than the n-Nb2O5-n-GQD-G electrode, demonstrating the enhanced capacitance through the formation of PN junctions. The symmetrical supercapacitor with n-Nb2O5-p-GQD-G electrodes shows high specific capacitance, cycling stability, and energy density.