Effects of repulsive interaction on the electric double layer of an imidazolium-based ionic liquid by molecular dynamics simulation

The effects of repulsive interaction on the electric double layer (EDL) and differential capacitance (C-d) of an ionic liquid (IL) 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF6-) on the graphite electrode were studied by molecular dynamics (MD) simulations. The strength of repulsive interaction was studied by manually tuning the parameter lambda (lambda) with lambda = 1.00 for normal Lennard-Jones interaction and smaller lambda for stronger repulsion between IL and the electrode. When lambda changes from 1.00 to 0.25, the dependence of C-d on potential (C-d-U) curves at different repulsions is asymmetrically camel-shaped with higher C-d at the negative polarization than that at the positive due to the thinner effective thickness of EDL from the specific adsorption of BMIM+. Such a trend is opposite in the case of lambda = 0.05. Apart from that, the maximum of C-d at the negative polarization monotonically decreases with increasing repulsion. On the other hand, the maximum of Cd at the positive polarization first increases with increasing repulsion, due to the more effective screening of PF6- by weakening the specific adsorption of BMIM+ as lambda changes from 1.00 to 0.75, and then it decreases with increasing repulsion.