Abstract
Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials in supercapacitors. Since energy storage in supercapacitors relies on the electrolyte-electrode interface, here we elucidate the interfacial subnanometer structure of a single component liquid composed solely of cations and anions – an ionic liquid- on electrified graphene. We study the effect of applied potential on the interaction between graphene and a silicon tip in an ionic liquid and describe it within the framework of the Derjaguin-Landau-Verwey-Overbeck (DLVO) theory. The energy is stored in an electrical double layer composed of an extended Stern layer, which consists of multiple ion layers over ~2 nanometers, beyond which a diffuse layer forms to compensate the applied potential on graphene. The electrical double layer significantly responds to the applied potential, and it shows the transition from overscreening to crowding of counterions at the interface at the highest applied potentials. It is proposed that surface charging occurs through the adsorption of the imidazolium cation to unbiased graphene (likely due to π-π interactions) and that the surface potential is better compensated when counterion crowding happens. This study scrutinizes the electrified graphene-ionic liquid interface, with implications not only in the field of energy storage, but also in lubrication.
Highlights
A single component electrolyte composed of ions –an ionic liquid– with a moderate polarizability has been chosen for this study
We propose that the electrical double layer (EDL) force is attractive for the dissimilar graphene-tip system and that the variation of the force with surface potential reflects the change of the EDL with applied potential
While EDL forces have been previously measured between charged or biased surfaces in ILs26, ion pairs are expected to populate the solid-liquid interface in the absence of surface charge and specific interactions, thereby eliminating overscreening and EDL forces, as it occurs on silica surfaces (Fig. 1C)
Summary
A single component electrolyte composed of ions –an ionic liquid– with a moderate polarizability has been chosen for this study. Ionic liquids (ILs) are organic molten salts with low melting point[12]; due to its ionic composition, they are subjected to significant electrostatic interactions Because of their high charge density and wide electrochemical window, and the possibility to tune intermolecular forces and physicochemical properties, ILs are considered to be ideal electrolytes for energy storage[13]. A long-range electrical double layer force with a decay length as large as 13 nm has been measured for several ILs on various substrates[26,27,28,29,30,31] This finding is important since the interfacial capacitance is inversely proportional to the effective thickness of the electrical double layer where the charge is stored, and it affects the stored energy in the supercapacitor. Statistical analysis of the steps resolved in the force-separation curves and modeling of the surface forces provide molecular insight into the electrical double layer of [EMIM][TFSI]
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