Effects of anion on the electric double layer of imidazolium-based ionic liquids on graphite electrode by molecular dynamics simulation

Effects of anion on the electric double layer (EDL) structure and differential capacitance (Cd) of ionic liquid (IL), 1-butyl-3-methyl-imidazolium bis(fluorosulfonyl)imide (BMIM+/FSI−) and 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate (BMIM+/Tf−) on graphite electrode were studied by molecular dynamic simulations. It is found that ILs with FSI− and Tf− anions show substantially different EDL structures and Cd features. The potential-dependent capacitance curve of the BMIM+/FSI− is asymmetric camel-shaped, with the potential of zero charge at the local minimum, while the BMIM+/Tf− shows much weaker Cd dependence on potential between 0 to 1.1 V. Such feature of BMIM+/Tf− indicates formation of Helmholtz-like EDL structure, which consists of a parallel alignment of imidazolium-ring in BMIM+ and the planar graphite electrode. A higher Cd of the both ILs at positive polarization can be attributed to the thinner effective EDL caused by the smaller size of anions compared to the cations. Apart from that, the slightly higher capacitance of BMIM+/Tf− compared to BMIM+/FSI− at positive polarization and the reverse trend at negative polarization is associated with the effectively shortened EDL thickness that resulted from the smaller size of Tf− in contrast to FSI−.