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 (～10 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.