Supercapacitors composed of carbon nanotube (CNT) micropores in the room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4(-)) are studied via molecular dynamics (MD)computer simulations. It is found that the distribution of RTIL ions inside the micropore varies significantly with the pore size. Internal solvation of small (6,6) and (7,7) CNTs with an electrified interior wall is effected almost exclusively via counterions. Surprisingly, these counterions, even though they all have the same charge, lead to a charge density characterized by multiple layers with alternating signs. This intriguing feature is attributed to the extended nature of RTIL ion charge distributions, which result in charge separation through preferential orientation inside the electrified nanotubes. In the case of larger (10,10) and (15,15) CNTs, counterions and coions develop multilayer solvation structures. The specific capacitance normalized to the pore surface area is found to increase as the CNT diameter decreases from (15,15) to (7,7). As the pore size further reduces from (6,6) to(5,5), however, the specific capacitance diminishes rapidly. These findings are in excellent agreement with recent experiments with carbon-based materials. A theoretical model based on multiple charge layers is proposed to understand both the MD and experimental results.

A molecular dynamics computer simulation study of room-temperature ionic liquids. I. Equilibrium solvation structure and free energetics Solvation in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate is studied via molecular dynamics simulations by employing a diatomic solute as a probe. It is found that solvent fluctuations are chacterized by at least two distinct dynamics occurring on vastly different time scales-rapid subpicosecond dynamics arising mainly from anion translations and slow relaxation ascribed to anion and cation diffusions. Fast subpicosecond dynamics are responsible for more than 50% of the entire relaxation of solvent fluctuations in the temperature range 350 KрTр500 K. It is also found that solvent spectral shifts and reorganization free energies in these liquids are comparable to those in ambient water.

Single- and double-walled carbon nanotubes in the armchair configuration solvated in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4 −) are studied via molecular dynamics (MD) computer simulations. Cations and anions show smeared-out, cylindrical shell-like distributions outside of the nanotubes irrespective of the nanotube diameter. The ion distributions inside the nanotubes vary markedly with their diameter. For example, in the case of (n,n) single-walled nanotubes, EMI+ and BF4 − ions separately form single-shell zigzag and chiral distributions for (8,8) and (10,10), respectively, while (12,12) develops a second internal solvation structure. The first internal solvation shell of (15,15) nanotubes consists of alternating layers of cations and anions along the nanotube axis. In the azimuthal direction, these cations and anions, respectively, form a pentagonal structure, whereas the corresponding ions for (20,20) show disordered octagonal structures. The smallest nanotube that allows solvent ions inside the tunnel is (7,7) with a diameter of 0.95 nm, which shows a single file distribution of internal ions. Imidazole rings of cations in the first internal and external solvation shells are mainly parallel to the nanotube surface, indicating π-stacking between the nanotubes and EMI+ ions there.