Renata Costa

Double layer in room temperature ionic liquids: influence of temperature and ionic size on the differential capacitance and electrocapillary curves

Differential capacity-potential curves, C(E), were obtained from electrochemical impedance spectra (12 kHz-2 Hz) for the interfaces between Hg and a series of alkyl imidazolium-based room temperature ionic liquids having the same anion, bis(trifluoromethanesulfonyl) imide: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [EMIM][Tf(2)N], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [BMIM][Tf(2)N], 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [HMIM][Tf(2)N]. The electrocapillary curves were obtained from drop time measurements and the values of the pzc were calculated. The pzc apparently becomes more negative as the imidazolium alkyl chain length increases. A small effect of the cation is seen on the C(E) curves at negative potentials. The effect of the aromatic nature of the cation is assessed by comparing 1-butyl-1-methylimidazolium bis(trifluoromethanesulfonyl) imide, with 1-butyl-3-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide [BMPyr][Tf(2)N]. The effects of temperature on the capacitance, drop time electrocapillary curve and on the pzc were also obtained. The capacity was found to increase with increasing temperature in the whole range of accessible potentials.

The electrical double layer at the ionic liquid/Au and Pt electrode interface

The role of the electrode material on the interfacial double layer structure of a series of ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([C4MIM][PF6]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4MIM][Tf2N]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM][BF4]) was investigated on gold (Au) and platinum (Pt) electrodes. The aim of this work is to contribute with experimental data to complement theoretical models that are currently under discussion in order to describe the structure of the double layer formed at the interface electrode/ionic liquid. There is still a controversy about the general shape that differential capacitance curves should present and researchers still continue making efforts to correlate the nature of the electrode/ionic liquid with the shape of the curves. Differential capacitance curves at the Au and Pt interface shows that the values of capacity follow the same order C [C4MIM][PF6] < C [C4MIM][Tf2N] < C [C4MIM][BF4]. However, the values of C are considerably lower for Pt when compared with Au for all three liquids studied. The alkyl chain length effect on the differential capacity curves was also studied at the Pt/[CnMIM][Tf2N] interface (where n = 2, 4 and 6). The results follow an expected trend except for the liquid [C6MIM][Tf2N] that shows a value of capacity near to that obtained for [C2MIM][Tf2N]. Since ionic liquids have great applicability in energy storage devices and capacitors it is of great importance to evaluate the dependence of differential capacitance as function of temperature. The results indicate that the capacity increases with increasing temperature for all systems studied in this work.

Electric double layer studies at the interface of mercury–binary ionic liquid mixtures with a common anion

To gain deeper insight into the structure of the electrode–ionic liquid interface differential capacitance–potential curves have been acquired through electrochemical impedance measurements at the mercury electrode. Understanding the role of the nature and size of the ionic species in the ionic liquid, namely the influence of different lengths of alkyl side chains of the imidazolium cation on the structure of the double layer, is the aim of the work being carried out. Differential capacitance–potential curves were obtained at the mercury electrode in contact with various mixtures of room-temperature ionic liquids of varying cation chain length (1-alkyl-3-methylimidazolium, [CnMIM]+ where n = 2, 6 and 12) with the same anion (bis(trifluoromethylsulfonyl)imide, [Tf2N]−). For comparison purposes we have also carried out electrochemical measurements of the pure ionic liquids under the same conditions. Systematic variations in the electrocapillary drop-time curves and capacitance were found with increasing amounts of the cation with longer alkyl chains in the mixture. The electrocapillary curve results may be interpreted as indicating the preferential adsorption of the longer alkyl group on the mercury surface.