Molecular response of 1-butyl-3-methylimidazolium dicyanamide ionic liquid at the graphene electrode interface investigated by sum frequency generation spectroscopy and molecular dynamics simulations

The ion distribution at the charged three layer graphene-ionic liquid (1-butyl-3-methylimidazolium dicyanamide, [BMIM][DCA]) interface is probed by sum frequency generation spectroscopy (SFG), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG results show the presence of both [BMIM]⁺ cations and [DCA]^- anions at the charged graphene surface, where the anion signal increased with the increasing external potential and the cation signal was little influenced by the external potential. The CV and EIS results suggest that ions form an electrically stable double layer at the charged graphene surface. Molecular dynamics simulations of [BMIM][DCA] near the charged graphene surface suggest that the interface consists of alternating layers of cations and anions that restructure with external potential variation. The experimental data can be interpreted by a double-layer model where the anionic and cationic layers interchange position with each other at the surface depending on the applied potential.