Simulations of Double Layer Structure and Differential Capacitance of Quaternary Ammonium-Based Ionic Liquids: Effect of Switching the Length of Alkyl Chains

Abstract

Quaternary ammonium-based ionic liquids (QaILs) have non-flammability, non-volatility, and thermal/chemical/electrochemical stabilities and are expected to be electrochemically applied. QaILs are superior to well-known imidazolium-based ILs in the wide potential window and high cationic designability. While the number of studies focusing on QaILs is limited,[1] further knowledge is needed to utilize the characteristics of QaILs. In this study, we performed molecular dynamics simulations at the interface between QaILs and the graphene electrode. The QaILs are composed of a common anion, bis(trifluoromethanesulfonyl)amide (TFSA−) and different cations: butyltrimethylammonium (N1114 +) dibutyldimethylammonium (N1144 +), tributylmethylammonium (N1444 +), tetrabutylammonium (N4444 +). By substituting methyl and butyl groups in Qa cations, we investigated how the number of long alkyl chains affects the potential dependence of differential capacitance and the electric double layer structure.[2] The Qa cations have major two orientations on the electrode: lying orientation where all the butyl chains are parallel to the electrode (Figure a) and standing orientation where one or more butyl chains are pointing to the ILs bulk (Figure b). N4444+ only shows standing orientation because of its geometric restriction. Compositions of the first interfacial layer reflect both the cationic orientation and surface charge density. As the electrode is negatively charged up, the compositions indicate the increase in the number of cations and different behavior of butyl chains depending on the Qa cations. Orientation analysis of the butyl chain indicates that the increase in lying orientation occurs at moderately negative (< ~−2 V) potentials, and the increase in standing orientation occurs at further negative potentials.As the potential deceases, the differential capacitance of all the QaILs drastically decreases in the potential range from 0 to ~−2V and gradually decreases at more negative potentials (Figure c). In the latter potential range, the differential capacitances show an upward discrepancy from predicted curves from Kornyshev model.[3][4] The discrepancy begins at the potentials where the standing orientation starts to increase, which implies a correlation between the differential capacitance and the cationic orientation. The discrepancy is greater for QaILs composed of cations with fewer butyl chains. The orientation effect is significant for such cations because those in lying and standing orientation have a great difference in occupation space in the first ionic layer.