Inner Layer Capacitance of Organic Electrolytes from Constant Voltage Molecular Dynamics

The response of electrochemical interfaces to applied voltages dictates both the chemical and physical properties of electrochemical systems. The capacitance of an interface intrinsically depends on its voltage-dependent, microscopic structural and compositional changes, yet detailed characterization of this structure/property relationship is often difficult. In this work, we employ constant potential molecular dynamics simulations to investigate the relationship between capacitance and interfacial structure of supercapacitor systems composed of pristine graphite electrodes combined with several organic solvents as well as [BMIm⁺]­[BF₄^–]/acetonitrile electrolytes at various concentrations. Specifically, we quantify how the total capacitance of acetonitrile, acetone, dichloroethane, and chloroform solvents depends on both inner layer and diffusion layer contributions and evaluate perfect screening of the Helmholtz layer when [BMIm⁺]­[BF₄^–] ions are added to the solvent. Surprisingly, we find that the inner layer capacitances for the organic solvents and [BMIm⁺]­[BF₄^–] solutions are very similar, regardless of the solvent type and largely independent of the ion concentration. This is because of the strong hydrophobic attraction of nonpolar alkyl groups of solvent molecules and ions with the graphene surface, which is similar for the different systems. When high voltage is applied, the electrostatic and hydrophobic interactions at the interface are modulated, leading to reorientation of interfacial ions and solvent molecules. The most significant structural rearrangements occur for acetone and pure [BMIm⁺]­[BF₄^–] ionic liquid, and these rearrangements are correlated to dielectric saturation of the inner layer capacitance at higher voltages. Our results imply that strong hydrophobic forces are an important influence on the double-layer capacitance of carbon-based supercapacitors, no matter the electrolyte composition.