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

Abstract

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 π–π 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).