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

Abstract

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 (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 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 λ = 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 C_d 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.