As observed for Li + , the increase in electrolyte concentration only increases the amount of Cl - near the positively charged electrode, while the position of the peaks is not affected by the concentration changes. However, at 4 M concentration, this is accompanied by broadening of the second peak (Figure ). The density profiles highlight an important feature of the double layer structure formed at the interface. Both anions and cations lie within the outer Helmholtz plane (OHP), extending no further than the first ionic adsorption layer along the z-axis from the graphene surface and, as the electrolyte concentration increases from 1 to 2 M, the density peaks clearly overlap (also see inside snapshots in Figure ). The coexistence of ions and counterions inside the Helmholtz plane, also evident in the non-normalized density profiles (Figure ), suggests an over-screening effect, where the first adsorbed layer of ions provides higher counter charge than that on the electrode surface. The latter is compensated by the adsorption of counterions that reside in between/vicinity of the first layer of ions. The overall phenomenon is evident on the positive electrode already at 1M and is further amplified as concentration increases (also see inside snapshots in Figure ). At the negative surface this over-screening is only visible for the 4M concentration (also see inside snapshot in Figure ) and is significantly weaker when compared to the positive electrode. Overscreening effects and ion crowding phenomena in the EDL have been also predicted by Finney et al. in aqueous NaCl electrolytes on graphite at concentrations greater than ca. 0.6 M 62 . Furthermore, similar effects have been reported in the literature for pure ionic liquids at relatively low applied voltages . All details of placement and thickness of IHP and OHP are reported in the Figure and Table , respectively.