Enhanced Electrical Conductivity of Imidazolium-Based Ionic Liquids Mixed with Carbon Nanotubes: A Spectroscopic Study

Abstract

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 ...

Enhanced thermo-electrochemical power using carbon nanotube additives in ionic liquid redox electrolytes

“…Thermocells are promising alternative devices to harvest waste heat from close-to-room temperature sources (less than 230 degrees celsius). Following our recent demonstration of enhanced electrical conductivity in mixtures of multiwall carbon nanotubes (MWCNTs) and imidazolium-based ionic liquids (IL), 6 here we show that the addition of MWCNTs to redox ILs also enhances the electrical conductivity, and, at low concentration of MWCNTs (less than 0.5 weight percentage), increases the electric power generation of thermocells by as much as 1.3 times the original power. 5 The energy conversion efficiency of thermocells, however, is limited by the slower transport of ions, instead of electrons as in semiconductor thermoelectrics.…”

“…1a). 6,18 Here we show that the combination of these effects can be optimized at low concentrations of MWCNTs to enhance the power of thermocells, despite the reduced open circuit voltage that results from the partially percolated MWCNT networks. 16,17 In addition, the imidazolium cations are attracted to the MWCNT surface by weak van der Waals forces, leading to ion-pair dissociation and faster diffusion of the anions.…”

“…6 In the case of electrolytes with 0.025 M of Co II (bpy) 3 (NTf 2 ) 2 /Co III (bpy) 3 (NTf 2 ) 3 , the redox couple was added before mixing with MWCNTs. 6 In the case of electrolytes with 0.025 M of Co II (bpy) 3 (NTf 2 ) 2 /Co III (bpy) 3 (NTf 2 ) 3 , the redox couple was added before mixing with MWCNTs.…”

“…6 The electrochemical setup consisted of a 2-electrode conguration with platinum electrodes. 6 The electrochemical setup consisted of a 2-electrode conguration with platinum electrodes.…”

Rheological behavior of multiwalled carbon nanotube-imidazolium tosylate ionic liquid dispersions

“…hampered resulting in a decrease of viscosity [41]. Besides, imidazolium-based cations have a great attraction to carbon nanophases [14], and then the interaction between the cation and the anion is suppressed and the anion has a higher mobility [42]. In our case, the presence of low quantities of MWCNTs may disrupt the interaction between the ions and the resulting dispersion shows a lower resistance to flow, as it will be further explained in the following sections.…”