Extremely high-rate aqueous supercapacitor fabricated using doped carbon nanoflakes with large surface area and mesopores at near-commercial mass loading

Achieving a satisfactory energy-power combination in a supercapacitor that is based on all-carbon electrodes and operates in benign aqueous media instead of conventional organic electrolytes is a major challenge. For this purpose, we fabricated carbon nanoflakes （20-100 nm in thickness, 5-μm in width） containing an unparalleled combination of a large surface area （3,000 m2-g-1 range） and mesoporosity （up to 72%）. These huge-surface area functionalized carbons （HSAFCs） also had a substantial oxygen and nitrogen content （N10 wt.% combined）, with a significant fraction of redox-active carboxyl/phenol groups in an optimized specimen. Their unique structure and chemistry resulted from a tailored single-step carbonization-activation approach employing （2-benzimidazolyl） acetonitrile combined with potassium hydroxide （KOH）. The HSAFCs exhibited specific capacitances of 474 F-g-1 at 0.5 A.g-1 and 285 F-g-1 at 100 A.g-1 （charging time 〈 3 s） in an aqueous 2 M KOH solution. These values are among the highest reported, especially at high currents. When tested with a stable 1.8-V window in a 1 M Na2SO4 electrolyte, a symmetric supercapacitor device using the fabricated nanoflakes as electrodes yielded a normalized active mass of 24.4 Wh-kg-1 at 223 W·kg-1 and 7.3 Wh·kg-1 at 9,360 W·kg-1. The latter value corresponds to a charge time of 〈3 s. The cyclability of the devices was excellent, with 93% capacitance retention after 10,000 cycles. All the electrochemical results were achieved by employing electrodes with near-commercial mass loadings of 8 mg-cm-2.