How carboxylic groups improve the performance of single-walled carbon nanotube electrochemical capacitors?

Abstract

Single-walled carbon nanotube (SWCNTs) is a promising material candidate for fabricating high-performance electrodes in electrochemical capacitors. An intriguing question is what are the key material characteristics of SWCNTs that influence the performance of SWCNT-based capacitors? We grafted SWCNTs with different amounts of carboxylic groups by a surfactant free method. Their density was quantified using a fluorescence labeling method, ranging from 7.3 to 353.2 nmol m−2. SWCNTs were also characterized by scanning electron microscope, N2 physisorption, ultraviolet-visible-near-infrared absorption, Fourier transform infrared, Raman, and X-ray photoelectron spectroscopy. Functionalized SWCNTs show a minor increase in their microspores and mesopores volume, and the total surface area stays ∼322.8 m2 g−1. We correlated SWCNT physiochemical properties with the performance of assembled two-electrode SWCNT capacitors. The specific capacitance, power density and energy density increase with increasing carboxylic group density, reaching the maximum at 146.1 F g−1, 308.8 kW kg−1 and 13.0 Wh kg−1 at the density of ∼250–350 nmol m−2. Potentiostatic electrochemical impedance spectroscopy analysis reveals that introducing an appropriate concentration of carboxylic groups plays two key roles: (1) it decreases the surface resistivity of SWCNT films, thus significantly reducing the equivalent series resistances of capacitors and (2) it enhances the surface wettability of SWCNTs, which not only offers more accessible sites for the physisorption of free electrolyte ions on SWCNT surfaces, but also increases ionic conductivity at electrode–electrolyte interfaces. These results and analysis provide a fundamental understanding of the effect of functionalization on the performance of SWCNT-based electrochemical capacitors, and shed light on a pathway by which electrochemical capacitors can be further improved for practical applications.