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.