Highly Capacitive Activated Carbon Fiber Clothes for Wide-Voltage (2 V) High-Energy-Density Aqueous Symmetric Supercapacitors

Abstract

Carbon aqueous symmetric supercapacitors are attractive supercapacitor devices owing to the low-cost and high conductivity of porous carbons. However, the limited capacitive charge storage process in carbon symmetric capacitors in comparison to pseudocapacitors or asymmetric supercapacitors severely limit the energy density. The only effective strategy to enhance the energy density of carbon aqueous symmetric supercapacitor is to extend the cell voltage beyond the thermodynamic stability of aqueous electrolytes (1.23 V). Also, in recent studies, researchers have focused on preparing high performance carbon electrodes from commercial carbon fiber clothes by activating the surface usually through chemical and electrochemical routes. However, it is still a major challenge to realize a comprehensive electrochemical performance of high capacitances, excellent rate capability, stability and wide operational potential windows in the activated carbon fiber cloth electrodes. Herein, we successfully activated the surface of commercial carbon fiber clothes using a facile-one-step air calcination at 400 oC for 6 hours strategy to realize a remarkable improvement in surface area, porosity and electrochemical performance. The successful activation of the carbon fiber clothes is fully confirmed through BET tests, SEM, TEM, Raman and contact angle measurement tests. Specifically, the activated carbon exhibits a 450-fold increase in specific surface area (615 m2 g-1 for the activated carbon fiber clothes) as well as rich microporosity, excellent conductivity and superhydrophilicity, which would be beneficial in realizing easy electrolyte penetration and excellent capacitive performance. As expected, the activated carbon fiber cloth achieves ultrahigh areal capacitances of 1553 and 1600 mF cm-2 at 1 mA cm-2 in positive and negative potential windows, good rate capability and excellent cycling stability up to 20000 cycles in 1 M Na2SO4 electrolyte compared to a lowly capacitance of 1.53 mF cm-2 for the commercial carbon fiber clothes. Owing to the large overpotential for hydrogen and oxygen evolution, the activated carbon fiber cloth based symmetric supercapacitor is stable in a wide cell voltage of 2.0 V, displaying high volumetric and gravimetric energy densities of 7.62 mWh cm-3 and 18.2 Wh kg-1, respectively. The stability in this wide potential window is again confirmed using float voltage test method. These results not only report one of the best electrochemical performance of activated carbon fiber clothes, it also provides an effective and simple calcination activation method for realizing high capacitances and takes advantage of the wide operating voltage window to achieve high energy density in carbon textile supercapacitors. Importantly, our studies also showed that the electrochemical performances of metal oxides grown on carbon fiber clothes and annealed in air at 400 oC might be over-estimated as that temperature is sufficient to activate the surface of the carbon fibers.