Self-supported VO(PO3)2 electrode for 2.8 V symmetric aqueous supercapacitors

Abstract

Improving the voltage window of aqueous supercapacitors beyond 2.5 V without diminishing their capacitance performance is a big challenge to achieve excellent energy density because of the undesirable water decomposition. Herein, this study developed a novel vanadium phosphate (VPO) of the VO(PO3)2 aerogel as the electrode to enlarge the device voltage, which could effectively raise HER/OER overpotentials and thus achieving a stable wide electrochemical window of −1 ∼ 1 V (vs. SCE) with remarkably improved capacitance. The VPO microstructure is organized by nanowire-stacked lamellar bricks via engineering a controllable phase-transformation using the V-P-citric acid-aerogel as the precursor. Examined as the supercapacitor electrode, in-situ X-ray powder diffraction analysis uncovers that the VO(PO3)2 undergoes a reversible phase-transformation reaction associated with Na+ intercalation/de-intercalation in Na2SO4 and a typical redox pseudocapacitance process in KOH. For the first time, a 2.8 V symmetric aqueous supercapacitor was further assembled with an energy density of 123.1 Wh kg−1 at the power density of 580 W kg−1, which is much superior to those of all reported V-base supercapacitors. The underlying mechanisms of the VO(PO3)2 on ultrahigh voltage window and charge storage are further elucidated, which provide the fundamental guidance for structuring VPO-based electrodes used for commercial energy storage devices.