Electric double layer structure and capacitance of imidazolium-based ionic liquids with FSI− and Tf− anions at graphite electrode by molecular dynamic simulations

Abstract

Molecular dynamic simulations were performed to investigate the interfacial electric double layer (EDL) structure and differential capacitance (Cd) of ionic liquids (ILs), 1-ethyl-3-methyl-imidazolium bis(fluorosulfonyl)imide (EMIM+/FSI−) and 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate (EMIM+/Tf−) on planar graphite electrode. The differential capacitance curve (Cd − U) of EMIM+/Tf− is asymmetric camel-shaped. The higher Cd of EMIM+/Tf− than the EMIM+/FSI− at 0–0.5 V and 1.1–2.4 V, is attributed to the formation of a thinner EDL of EMIM+/Tf− induced by the smaller size of Tf− compared to FSI−. In addition, because of the fewer interaction sites between cations and Tf− with the more localized negative charge distribution, Tf− anions are expelled away from the charged electrode more easily, which makes more effective screening of EMIM+/Tf− to the charged electrode than that of EMIM+/FSI−. Accordingly, the Cd of EMIM+/Tf− within the potential of −1.1 V to 0 V is higher than that of the EMIM+/FSI−. At highly negative potentials of −2.5 V to −1.1 V, FSI− with more interaction sites with cations are gradually squeezed out from the EDL, and more cations of EMIM+/FSI− accumulate to replace FSI−, leading to a higher Cd of EMIM+/FSI− than EMIM+/Tf−. Meanwhile, the Cd of EMIM+/Tf− decreases sharply due to the thicker EDL resulted from the overcrowded counterions.