Structure–Capacitance Relationships of Graphene/Ionic Liquid Electrolyte Double Layers

The differential capacitance profile of electrochemical interfaces reflects the physical properties of the double layer. For carbon electrodes and ionic-liquid-based electrolytes, these capacitance profiles are not fully understood. In this work, we utilize constant voltage molecular dynamics simulations to compute differential capacitance profiles of ionic liquids [BMIm+]­[BF4 –] and [BMIm+]­[TFSI–] mixed with acetonitrile and 1,2-dichloroethane, at model graphene electrodes. We find that both pure and 10% mole fraction ionic liquid electrolytes exhibit camel-shaped capacitance profiles with two peaks on either side of a minimum centered at the potential of zero charge. This profile shape results from the electric-field-induced rearrangement of ion structure within the inner layer closest to the electrode interface. At a low potential, the ionic liquid inner layer is concentrated with nonpolar trifluoromethyl and butyl functional groups of the anions and cations, corresponding to the minimum of the capacitance profiles. With increasing voltage, electrostatic interactions of polar/charged functional groups with the electrode surface compete with these nonpolar interactions, leading to ion rearrangement that increases the inner-layer charge density and results in higher capacitance. After the ion restructuring is complete, the response saturates and capacitance diminishes. The presence of organic solvent significantly changes the composition of the inner layer. For example, strong nonpolar interactions between dichloroethane molecules and the graphene surface substantially block ion/electrode contact at moderate potentials. Overall, our simulations highlight the dynamic nature of the inner region of organic electrolyte double layers and the sensitive dependence on electrolyte composition and applied voltage.