  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.
