Molecular modeling of ionic liquids confined in nanoporous carbons

1. Molecular modeling of ionic liquids confined in nanoporous carbons Francisco R. Hung, Cain Department of Chemical Engineering, Louisiana State University A fundamental understanding of the behavior of ILs inside nm-sized pores is crucial for the rational design of electrochemical double-layer capacitors (EDLCs) and dye-sensitized solar cells for energy storage. We have studied the structural and dynamical properties of the IL [bmim+][PF6-] confined inside multi-walled carbon nanotubes using molecular dynamics simulations. The structural properties of the confined ILs affect the electrical capacitance, and the dynamical properties of ILs inside nanopores influences the electrical resistance in an EDLC. The confined ions have different mobilities according to their radial position. The dynamics are measured by the ions’ mean squared displacements (MSDs) in the axial direction as a function of time. The ions in the center of the pore (black lines) move faster than those in the middle layer (red lines), which in turn have faster dynamics than those ions near the walls (blue lines). However, the dynamics are not completely regular. The MSD of the anions in the inner and middle layers are similar in magnitude; in contrast, the cations in the middle layer move faster than those in the inner layer. Furthermore, the cations in the inner and outer layers have larger MSDs than the anions in those layers, but the MSDs of the cations and anions in the middle layers are comparable in magnitude. These results suggest that the dynamics of ILs confined inside MWCNTs are very complex and heterogeneous. Pore size and pore loading (i.e., the amount of IL inside the MWCNT) have a profound effect on the structure and dynamics of the confined IL. The ions tend to form concentric layers in the radial direction. The layering behavior depends directly on the pore size and the pore loading.