A quantitative evaluation of the large pore-size effect on the electric double-layer capacitance for high voltage by 19F-NMR

Abstract

Electric double-layer capacitors (EDLCs) afford long cycle life and high-power density by dielectric ion adsorption and desorption on the surface of porous materials. However, high potential charges/discharges lower the capacitance of EDLCs. In this study, we quantitatively evaluated the large pore-size effect of activated carbons (ACs) on the electric double layer capacitances at various voltages based on a hierarchical domain model from a structural point of view. ACs with highly developed microporous and mesoporous (2–4 nm) structures were prepared using KOH activation. The AC with small amount of mesopores showed a specific capacitance of 30.35 F/g at a voltage of 2.7 V, where it significantly lowered to 20.93 F/g at a high voltage of 3.3 V. Meanwhile, the highly microporous AC with abundant mesopores had a higher specific capacitance (40.77 F/g at 2.7 V to 38.22 F/g at 3.3 V). 19F-NMR results indicate that the amounts of solvated ions in the mesopores significantly increased in all the ACs after charging at a higher voltage. The mitigation of the high-voltage capacitance drop might occur because of the mesopore structure that functions as a buffer space for the electric double layer.