We studied imidazolium-based ionic liquids mixed with multiwall carbon nanotubes using impedance spectroscopy and nuclear magnetic resonance. The results show a percolation threshold below 1 wt% of carbon nanotubes, which, due to their high dispersibility in imidazolium-based ionic liquids, is 3-to 8-fold lower than the percolation threshold for carbon nanotubes in typical organic solvents. The addition of carbon nanotubes appears to polarize their interface with imidazolium-based ionic liquids and increases the diffusion coefficient of the anions up to 35%, which is likely due to weak van der Waals interactions between the carbon nanotube walls and the cations. These effects produce a 3-to 5-fold increase in electrical conductivity of the electrolyte mixture, at carbon nanotube concentrations that are under the threshold for percolation. [7][8][9] In particular, mixing single-or multiwall CNTs in iodine-free DSCs can increase energy conversion efficiency by 100 to 300%.2,4 For example, Lee et al 4 mixed single-wall CNTs (SWCNTs) in 1-ethyl-3-methylimidazolium iodide to reach an efficiency of 1.9%, compared to 0.4% without SWCNTs. The room-temperature stability of their DSCs, with SWCNTs in the IL, is also reported to be superior to that of conventional DSCs, which use I − /I 2 in an organic solvent. While CNT-ILs show promise to enhance the efficiency of DSCs, the molecular mechanisms that enable the improved performance are not completely clear. 10 Moreover, the solubility of CNTs in ILs, and the compatibility of the mixture, is a processing challenge for several applications. 7,8,11 Because CNT-ILs are mixed ionic-electronic conductors, it is necessary to study the electronic contribution of the ions and CNTs systemically before many applications could be realized. These contributions must be examined above and below the threshold for CNT percolation to fully understand the system, but we found no studies of this type in the literature.CNTs [I] (purity >99%) from IOLITEC; DMF and DMSO from Sigma Aldrich; and MWCNTs and N-MWCNTs from US-nano (US4315 and US4882, respectively). According to product specifications, the MWCNTs had a length of 10-20 μm, outside diameter of 50-80 nm, and density of 2.1 g/cm 3 . The N-MWCNTs had an approximate length of 35 μm and outside diameter of 20-50 nm and about 3 wt% nitrogen content. The solvents, ILs, and CNTs were used as received.We stirred each mixture of MWCNT-IL on a hot plate for 30 minutes, then ultrasonicated the mixture for 30 minutes, followed by another stirring step for 15 minutes, with all steps occurring at a temperature of 65• C. The samples were allowed to cool at room temperature for at least 1 hour, followed by 5 minutes of stirring before taking measurements. To test if the MWCNTs re-agglomerate significantly after time, we made measurements within 5 minutes after the final stirring step, and after 6 hours without additional stirring, and the results only varied by about 5%. Typical sample volumes were approximately 6 mL. To increase the wt% of MWCNTs in ...
