New insights into the interface between a single-crystalline metal electrode and an extremely pure ionic liquid: slow interfacial processes and the influence of temperature on interfacial dynamics

Abstract

Ionic liquids are of high interest for the development of safe electrolytes in modern electrochemical cells, such as batteries, supercapacitors and dye-sensitised solar cells. However, electrochemical applications of ionic liquids are still hindered by the limited understanding of the interface between electrode materials and ionic liquids. In this article, we first review the state of the art in both experiment and theory. Then we illustrate some general trends by taking the interface between the extremely pure ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and an Au(111) electrode as an example. For the study of this interface, electrochemical impedance spectroscopy was combined with in situ STM and in situ AFM techniques. In addition, we present new results for the temperature dependence of the interfacial capacitance and dynamics. Since the interfacial dynamics are characterised by different processes taking place on different time scales, the temperature dependence of the dynamics can only be reliably studied by recording and carefully analysing broadband capacitance spectra. Single-frequency experiments may lead to artefacts in the temperature dependence of the interfacial capacitance. We demonstrate that the fast capacitive process exhibits a Vogel-Fulcher-Tamman temperature dependence, since its time scale is governed by the ionic conductivity of the ionic liquid. In contrast, the slower capacitive process appears to be Arrhenius activated. This suggests that the time scale of this process is determined by a temperature-independent barrier, which may be related to structural reorganisations of the Au surface and/or to charge redistributions in the strongly bound innermost ion layer.

The interface between Au(100) and 1-butyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide

“…Still, we found that the interfacial admittance increases with temperature -just as the bulk conductivity does -similarly to what is shown in Figure 7 of reference [18] and Figure 3.b of reference [20] for other Au(100) | IL systems. This finding is in accordance with the conclusions of reference [28] based on temperature-dependent EIS measurements on the Au(111) | [Py 1,4 ][FAP] 3 system.…”

The interface between HOPG and 1-butyl-3-methyl-imidazolium hexafluorophosphate

“…49 They reported fast, slow and an ultra-slow sub-Hz capacitive processes for Au(111) in 1-butyl-1-methylpyrrolidiniumtris(pentafluoroethyl)trifluorophosphate (Py1,4FAP). The ultra-slow capacitive contribution was assumed to be related to a vacancy formation in the surface.…”

Static Capacitance at the Electrochemical Liquid-liquid Interface Between Ionic Liquids and Eutectic Ga-In Alloy Measured Using the Pendant Drop Method

“…In contrast to the progress of the understanding of the EDL of ILs from the theoretical side, experimental confirmation of the predicted IL-peculiar behavior is not straightforward. It has been found, from studies using electrochemical impedance spectroscopy (EIS), that C for the EDL of ILs has strong frequency dependence [11][12][13][14][15][16][17][18] and shows potential hysteresis, 11,[19][20][21] both of which seem to result from the structural ordering of IL interface [22][23][24][25][26][27][28][29] and the ultraslow dynamics [29][30][31][32][33][34][35][36][37][38] of such ordered structure. These tendencies have hampered rigorous evaluation of C experimentally.…”