Symmetric effect on electrical double-layer characteristics and molecular assembly interplay in imidazolium-based Ionic liquid electrolytes in supercapacitor models

Abstract

Studies on the ion-layer formation of imidazolium-based ionic liquids have extensively explored how to improve in-depth knowledge of electrical double-layer (EDL) properties. In this computational study, 1-alkyl-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C_nmim][NTf₂]), namely, [C₁mim][NTf₂] and [C₂mim][NTf₂], inside a simulated supercapacitor were investigated to expose an symmetric alkyl chain effect. Molecular dynamic simulations of a supercapacitor model with graphite electrodes were conducted. Changes in charging dynamics and EDL structures at different voltages were studied. Although [C₁mim][NTf₂] equilibrated much quicker than [C₂mim][NTf₂], surface charge development on the symmetrical imidazolium ionic liquid was slower than that on the asymmetrical counterpart. Physical EDL structural analysis showed that [C₁mim][NTf₂] could not rearrange in a rigid co-ion layer, whereby the [C₁mim]⁺ cation stayed adsorbed on the positive electrode throughout all the tested voltages. The strongly attached [C₁mim]⁺ on the electrode surface contributed to low responsiveness in symmetrical [C₁mim][NTf₂], which was supported by lower overall differential capacitance (C_D) magnitude and less sharp C_D wings at high voltage when compared to [C₂mim][NTf₂].