Wet oxidative functionalization of carbon nanotube cloth to boost its performance as a flexible supercapacitor electrode

Abstract

Electrochemical recharging behavior of a binder-free flexible carbon nanotube cloth (CNTC) material in respect to aqueous supercapacitor applications is reported. To provide high enough pseudocapacitance, the surface of CNTC was exposed to wet oxidative functionalization by a KMnO4 treatment in an acidic medium. We report the effect of a KMnO4 to CNT molar ratio on capacitance, electrical resistivity and specific surface area of the CNTC material. The change in the composition of oxygen-containing interfacial functional groups attached to the carbon surface is also investigated by X-ray photoelectron spectroscopy and temperature-programmed desorption methods. For the most oxidized CNTC sample the capacitance of ca. 71 F g−1 (29 µF cm−2BET) in 0.5 M H2SO4 (slightly decreasing with a scan rate up to 100 mV s−1) with the retention of ca. 99% during 30,000 recharging cycles as well as specific resistivity of ca. 31 µΩ m are achieved. Finally, two flexible symmetric supercapacitor prototypes based on a 38% H2SO4 solution and operating at 1.3 or 1.5 V were assembled from the freestanding and graphite foil supported functionalized CNTC electrodes, respectively. For the former device, the volumetric capacitance of ca. 22 F cm−3, power density of ca. 10 kW l−1 and energy density of ca. 3 Wh l−1 (per total volume of both electrodes) are demonstrated. The issues of the long-term stability and self-discharge behavior of assembled prototypes are also considered. Reported results allow us to consider the functionalized CNTC material as a promising electrode material for flexible aqueous supercapacitors.