Electric Double-Layer Capacitors Using “Bucky Gels” Consisting of an Ionic Liquid and Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) formed gels after being ground with a room-temperature ionic liquid (RTIL). The gels were used as electrodes of electric double-layer capacitors (EDLCs), and the RTIL was used as the electrolyte. The performance of the EDLCs was examined by charge-discharge experiments and was compared with that of the EDLCs using activated carbon electrodes and the RTIL electrolytes. The gels functioned as the electrodes over a wide composition range from 0.02 to 0.12 of SWNT/RTIL (wt/wt), and the retained capacitance increased with increasing the SWNT compositions. The EDLCs with the SWNTs showed higher capacitance than that with the activated carbons in terms of the capacitance per unit surface area, though the gravimetric capacitance was lower. The gel electrodes can be fabricated as thick as without a severe ohmic-drop problem, which may contribute to a simple cell structure. The changes in the performance of the SWNT-EDLCs, with or without the gelation, were apparent, and the gelation greatly contributed to the high performance. This is due to the formation of continuous SWNT and RTIL paths at the molecular level by the gelation.