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 (Cd) 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 (λ) with λ = 1.00 for normal Lennard-Jones interaction and smaller λ for stronger repulsion between IL and the electrode. When λ changes from 1.00 to 0.25, the dependence of Cd on potential (Cd-U) curves at different repulsions is asymmetrically camel-shaped with higher Cd 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 λ = 0.05. Apart from that, the maximum of Cd 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 λ changes from 1.00 to 0.75, and then it decreases with increasing repulsion.