Ion Interactions across Graphene in Electrolyte Aqueous Solutions

The interfacial behavior of graphene is involved in a number of technological processes and applications, ranging from energy storage to sensing and nanofluidics. The organization of ions and structuring of water molecules close to a graphene interface, which represents an atomically thin surface, substantially affect the interfacial physicochemical properties in electrolytes as well as the specific capacitance of supercapacitors. Moreover, adsorption of ions on one side of the ultimately thin material may largely impact the adsorption of additional charge carriers on the opposite side and thus influence the overall supercapacitor performance. However, these phenomena are so far not fully understood. In this study, all-atomic classical molecular dynamics (MD) simulations were conducted with explicitly included polarization, which is essential for accurate description of electrolytes at interfaces in systems containing carbon allotropes. We employed an isotropic polarization model using classical Drude oscillators and adjusted Thole parameters for graphene. This approach improved the classical description of graphene-electrolyte interaction although did not fully cover the inherent anisotropy of graphene polarization because the field components parallel to the graphene sheet were largely reduced but not completely screened as in semimetals. The MD simulations were applied to examine the interface between graphene and potassium halide solutions. The results showed that water molecules formed a well-organized single layer on both sides of graphene, which primarily acted as a hydrophobic structuring agent. This arrangement significantly contributed to effective shielding of ion-ion interactions acting through the graphene sheet. Thus, the ion-specific structuring of adjacent electrolytes on opposing sides of graphene was generally independent. The findings help to understand structuring of electrolyte on graphene-based electrode materials of supercapacitors.