Imidazolium Triflate Ionic Liquids’ Capacitance–Potential Relationships and Transport Properties Affected by Cation Chain Lengths

Abstract

In this paper we report the effects of five imidazolium cations with varying alkyl chain lengths to study the effects of cation size on capacitance versus voltage behavior. The cations include ethyl-, butyl-, hexyl-, octyl-, and decyl-3methylimidazolium, all paired with a triflate anion. We analyze the capacitance with respect to the cation alkyl chain length qualitatively and quantitatively by analyzing changes in the capacitance−potential curvature shape and magnitude across several standard scanning protocols and electrochemical techniques. Further, three transport properties (viscosity, diffusion coefficient, and electrical conductivity) are experimentally determined and integrated into the outcomes. Ultimately, we find higher viscosities, lower diffusion coefficients, and lower electrical conductivities when the alkyl chain length is increased. Also, capacitance values increase with cation size, except 1-octyl-3methylimidazolium, which does not follow an otherwise linear trend. This capacitive increase is most pronounced when sweeping the potential in the cathodic direction. These findings challenge the conventional hypothesis that increasing the length of the alkyl chain of imidazolium cations diminishes the capacitance and ionic liquid performance in charge storage.

Understanding the Electric Double-Layer Structure, Capacitance, and Charging Dynamics

“…The opposite trends on the side-chain effects reflect a competition between the depletion of co-ions by the neutral segments (positive effect on capacitance) and the increased overall size of the cations (negative effects). The theoretical results explain why contradictory conclusions were reached regarding the effects of the alkyl chain of imidazolium cations on capacitance …”

“…The theoretical results explain why contradictory conclusions were reached regarding the effects of the alkyl chain of imidazolium cations on capacitance. 210 Figure 11D shows a similar coarse-grained model that was used to investigate the capacitive performance of oligomeric ionic liquids (OILs) in nanoporous electrodes. 211 In contrast to their application to field-effect transistors, the capacitive performance of OILs in nanoporous electrodes is sensitive to the charging potential (Figure 11E).…”

Importance of Anion–Anion Pairing for Capacitance of Carbon/Ionic Liquid Interfaces

“…In addition to tuning the compatibility of electrode nanostructures with ions, experimental characterizations have shown that diverse ion structure and capacitance of ILs at electrode interfaces can be achieved by selecting different chemical structures and compositions of cations ,,− and anions. ,,,− Of particular interest are ILs that possess both polar and nonpolar groups (e.g., alkyl chains on imidazolium cations and −CF 3 groups on certain anions). These ILs form distinctive ion ordering at electrode interfaces because they interact with the electrode surface, together with varying electrostatic and nonpolar interactions between molecular ions.…”

Effect of cation alkyl chain length on 3-sulfopropylmethacrylate-based draw solutes having lower critical solution temperature

“…Because the alkyl chain elongation leads to the enhancement of their van der Waals interaction, resulting in ion aggregation. 66–68 Therefore, [P 4444 ][C 3 S] containing [P 4444 ] + ions with short alkyl chain length show a relatively high conductivity compared to [P 4446 ][C 3 S].…”