Expanding the potential window and enhancing the capacitance of carbon materials through moderate electrochemical oxidation

Energy densities of widely used carbon-based supercapacitors can be enhanced by expanding carbon's electrochemical stability potential windows (ESPWs) or by introducing pseudocapacitance to the energy storage/release processes. Nevertheless, maintaining materials' rate performance and cycling stability after these treatments remains challenging. Here, we apply a cyclic voltammetry treatment with various maximum oxidation potentials (MOPs) to introduce oxygenated functionalities onto the electrochemically active surfaces of carbon materials (e.g., activated carbon, graphene, and carbon nanotubes). This treatment expands the carbons' ESPWs by improving oxidation resistance and introduces highly stable proton-involved pseudocapacitance on carbons. By applying moderate MOPs of +0.6 V and +0.7 V (vs. Hg/Hg 2 SO 4 ) in 3 M H 2 SO 4 , the ESPWs of the activated carbon electrodes (5.0 mg cm −2 ) are extended from 1.0 V to 1.2 V and 1.3 V, respectively. Meanwhile, their capacitances increase from 271.5 F g −1 to 369.2 F g −1 and 395.0 F g −1  at a low rate of 10 mV s −1 (from 304.7 F g −1 to 421.9 F g −1 and 472.4 F g −1  at 1 A g −1 ), and from 186.8 F g −1 to 243.7 F g −1 and 217.8 F g −1  at an ultrahigh rate of 1000 mV s −1 , respectively. Thus, the energy densities of corresponding symmetric devices increase by 76.0 % and 101.3 % at 1 A g −1 , and by 110.7 % and 27.2 % at 100 A g −1 , respectively. This work provides a simple and facile way to significantly optimize the active materials for energy storage, which is expected to be extended to other electrode systems for supercapacitors in the near future.