Ion-ion correlations across and between electrified graphene layers.

When an ionic liquid adsorbs onto a porous electrode, its ionic arrangement is deeply modified due to a screening of the Coulombic interactions by the metallic surface and by the confinement imposed upon it by the electrode's morphology. In particular, ions of the same charge can approach at close contact, leading to the formation of a superionic state. The impact of an electrified surface placed between two liquid phases is much less understood. Here we simulate a full supercapacitor made of the 1-butyl-3-methylimidazolium hexafluorophosphate and nanoporous graphene electrodes, with varying distances between the graphene sheets. The electrodes are held at constant potential by allowing the carbon charges to fluctuate. Under strong confinement conditions, we show that ions of the same charge tend to adsorb in front of each other across the graphene plane. These correlations are allowed by the formation of a highly localized image charge on the carbon atoms between the ions. They are suppressed in larger pores, when the liquid adopts a bilayer structure between the graphene sheets. These effects are qualitatively similar to the recent templating effects which have been reported during the growth of nanocrystals on a graphene substrate.