Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [C_nmim][TFSI] Ionic Liquids at Graphite Electrodes

Abstract

The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide ([C n mim][TFSI], n = 2, 4, 6, and 8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within ±2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.

The interface between HOPG and 1-butyl-3-methyl-imidazolium hexafluorophosphate

“…51 Theoretical studies confirmed the existence of spatial heterogeneities at the interface due to the formation of polar and non-polar domains of the IL-molecules, as well as the existence of highly compressible alternating multilayers. 52,53,54,55,56 At very high negative potentials we could not find ordered superstructures such as those exist on Au(100) in BMIPF6, where ordered double-row structures of the cations could be observed (Fig. 9.…”

Classical Density Functional Study on Interfacial Structure and Differential Capacitance of Ionic Liquids near Charged Surfaces

“…MD simulations have addressed the unique potential influence on DC curve through analysis of cumulative density of specific groups and atoms in RTILs. 35 …”

Evaluation of the constant potential method in simulating electric double-layer capacitors

“…Merlet et al [24][25][26][27] studied a nanoporous carbon electrode in contact with electrolyte consisting of an ionic liquid or an ionic liquid/acetonitrile mixture. Vatamanu and co-workers [28][29][30][31] investigated ionic liquid electrolytes with carbon or gold electrodes using the smooth particle mesh Ewald (SMPE) 32,33 method to simplify the calculation. The hydration of metal-electrode surfaces was examined by Limmer et al 34,35 In all of these studies, detailed comparisons of the CPM with the fixed charge method have been lacking.…”