A comparative study of alkylimidazolium room temperature ionic liquids with FSI and TFSI anions near charged electrodes

Electric double layer (EDL) structure and capacitance generated by the two series of room temperature ionic liquids containing alkylimidazolium C_nmim (n = 2,4,6,8) cations and bis(fluorosulfonyl) imide (FSO₂)₂N⁻, (FSI) or bis(trifluoromethylsulfonyl) imide (CF₃SO₂)₂N⁻ (TFSI) anions were studied on flat (basal plane graphite) and atomically corrugated (prismatic plane graphite) charged electrode surfaces using atomistic molecular dynamics simulations. On atomically flat surface, generated EDLs in all systems produced a weakly changing differential capacitance (DC) as a function of electrode potential. However, on atomically rough surfaces, ionic liquids with FSI and TFSI anions show substantially different EDL structures and DC dependence. Unlike [C_nmim][TFSI], which generated a camel-shape DC regardless of the cation alkyl tail length, the [C_nmim][FSI] showed a transition from a bell-shape to a camel-shape DC upon increase of the cation alkyl tail length. Analysis of contributions from rearrangement and reorientation of cations and anions indicated that the ability of the FSI anion to respond to changes in electrode potential is the primary driving force for such behavior