Density Functional Study of Charge Transfer at the Graphene/Ionic Liquid Interface

We use density functional theory to analyze the charge transfer between lithium or magnesium cations and a graphene wall beyond the predictions of classical Marcus theory. To that end, metal atoms are placed in three different kinds of environments: (i) in a vacuum, (ii) among fluorine atoms to simulate a molten salt, and (iii) in an ionic liquid. We prove that a complete charge transfer to the electrodes takes place in all of the studied environments and that the charge transfer process starts at longer distances from the electrode in the ionic liquid. Vertical ionization potentials and vertical electron affinities are studied, and they confirm that the nanoconfined region close to the electrode is a favorable environment for electronic exchange. No significant difference between monovalent and divalent cations was found. Our results suggest a certain catalyzing effect of ionic liquids regarding metal-electrode charge transfer in these densely ionic environments. Moreover, they show that ionic liquids can actually enhance charge transfer to electrodes in electrochemical devices without significantly altering the nature of the process.