Effects of Alkyl Chain Length on Interfacial Structure and Differential Capacitance in Graphene Supercapacitors: A Molecular Dynamics Simulation Study

Abstract

Supercapacitors with graphene electrodes are studied via molecular dynamics simulation. As an electrolyte, we consider three different room-temperature ionic liquids (RTILs), each of which is made up of the same anion BF4−, and different cations, 1-Cn(n=2,4,6)-3-methylimidazolium, respectively. We investigate how the alkyl chain length of the cation affects their interfacial structure and electrical properties for electric double layer capacitors. As a whole, cations and anions make layering structures between two parallel electrodes. Cations in the nearest layer orient predominantly in parallel to the electrode. Imidazolium rings of cations form π-stacking with graphene, then the alkyl chains of cations align parallel to the electrode. Differential capacitances in three RTILs are found to decrease with an increase of the magnitude of electrode potentials. The ion size and orientation affect both structure and capacitance behavior. The parallel orientations of cations become stronger with an increase of the alkyl chain length for the considered RTILs. The differential capacitance tends to decrease with raising the alkyl chain length over a wide range of the electrode potential. This is ascribed to a steric effect caused by larger cation size. It is also found that anodic capacitance is higher than cathodic one due to a higher screening efficiency by small anions, and an asymmetry in the peak of capacitance biased to the cathodic side becomes weaker as the alkyl chain length increases. Comparing electrode charge with ion numbers near the electrodes, with respect to their changes in response to the electrode potential, we find that the interfacial layer of the electrolyte mainly governs capacitive behavior of the systems.