Niobium pentoxide nanoparticles decorated graphene as electrode material in aqueous-based supercapacitors: Accurate determination of the working voltage window and the analysis of the distributed capacitance in the time domain

This study report on the physicochemical behavior exhibited by the composite material based on niobium pentoxide nanoparticles (Nb2O5) decorated graphene (Gr) and their promising electrochemical properties in neutral aqueous supercapacitors (SCs). The Nb2O5@Gr composite showed outstanding properties where Gr contributes to electrochemical stability and ultra-fast electronic transport, while Nb2O5 offers fast and reversible solid-state surface redox reactions. Nb2O5 also works as a protective surface blocker for the carbon scaffold. A new methodology is proposed to verify the electrocatalytic activity for electrolyte decomposition in SCs. It was verified the electrocatalytic activity decreased by 7% for water splitting in the presence of Nb2O5, which boosted the overall device characteristics for the energy-storage properties. The Nb2O5@Gr composite electrode increased the energy and power densities from 4 to 18 µWh cm−3 and 0.5 to 2 µW cm−3, respectively. In addition, the cell voltage increased from 0.4 to 0.6 V, while the areal capacitance changed from 27.5 to 150 µF cm−2, and the equivalent series resistance (ESR) underwent a three-fold reduction. These promising findings are associated with synergism between the Nb2O5 nanoparticles, which conferred a pronounced pseudocapacitive contribution, while the Gr worked as an efficient nanosized and localized current collector at the active redox surface sites. These results encourage future works to explore the exciting properties of the Nb2O5@Gr composite electrode in aqueous-based SCs. The distributed capacitance in the time domain was calculated to discriminate the electrostatic and faradaic charge-storage mechanisms occurring in electrical double-layer capacitors and pseudocapacitor devices, respectively.