Extremely high-rate aqueous supercapacitor fabricated using doped carbon nanoflakes with large surface area and mesopores at near-commercial mass loading
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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.
Keywordsenergy storage high power activated carbon graphene doped carbon heteroatoms
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The authors are thankful to financial supports from the National Natural Science Foundation of China (Nos. 51402272 and 21471139), Shandong Province Outstanding Youth Scientist Foundation Plan (No. BS2014CL024), Seed Fund from Ocean University of China, and Fundamental Research Funds for the Central Universities.
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