The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × √3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.

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.

In this paper we present a combined in situ STM, AFM and EIS study on the structure and dynamics of the interfacial layers between Au(111) and two extremely pure ionic liquids, namely [Py 1,4 ]FAP and [EMIM]FAP. The combination of these methods provides valuable information for both neutral and electrified interfaces. In situ STM and AFM results reveal that multilayered morphology is present at the IL/Au(111) interface, with stronger near surface layering detected at higher electrode potentials. The in situ STM measurements show that the structure of the interfacial layers is dependent on the applied electrode potential, the number of subsequent STM scans and the scan rate. Furthermore, in the case of [Py 1,4 ]FAP, the Au(111) surface undergoes herringbone reconstruction, Au(111)(22 x √3), in the cathodic potential regime, and the ultra-slow formation of vacancies in the herringbone structure is probed with in situ STM. EIS measurements reveal the presence of two distinct capacitive processes at the interface taking place on different time scales. The time scale of the fast process is typically in the millisecond range and is governed by the bulk ion transport in the IL, which exhibits a Vogel-Fulcher-Tammann-type temperature dependence. The slow process takes place on a time scale of seconds and is Arrhenius activated. The contribution of this process to the overall interfacial capacitance is

We have measured the frequency-and potential-dependent differential capacitance of the room temperature ionic liquids [EMIm][N(Tf) 2 ] and [BMP][N(Tf) 2 ] at a polycrystalline platinum interface by means of broadband electrochemical impedance spectroscopy. In a frequency range from 1 MHz to 10 Hz, we observe a transition from the bulk capacitance to a nonideal double layer capacitance. Below 10 Hz, the differential capacitance increases strongly with decreasing frequency, and the capacitance exceeds 0.5 mF/cm 2 . This low-frequency behavior points to the existence of slow pseudocapacitive processes which are most likely related to ion adsorption. We have fitted the capacitance spectra by means of an equivalent circuit containing constant-phase elements for the nonideal double layer capacitance and for the pseudocapacitance, respectively. When we plot the double layer capacitance estimated from the CPE parameters versus the dc potential of the working electrode, we find hysteresis effects, i.e., the double layer capacitance depends on the scan direction of the dc potential. The hysteresis is caused by slow processes at the ionic liquid/Pt interface taking place on the time scales of minutes to hours. We suggest that these are the same processes causing the pseudocapacitive behavior at frequencies below 10 Hz.

Electrochemical impedance spectroscopy was used for characterizing the interface between the ultrapure room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, [EMIm]FAP, and a Au(111) working electrode (WE). Plots of the potentialdependent spectroscopic data in the complex capacitance plane (CCP) reveal the existence of three distinct processes taking place on different time scales. At all WE potentials ranging from -0.5 to þ1.0 V versus a Pt pseudo-reference electrode, a highfrequency semicircle was detected in the CCP, which was attributed to the formation of an electrochemical double layer (EDL). At intermediate frequencies, a second capacitive process was observed, which is most likely related to electrode de-/reconstruction in the cathodic regime and to a strong interaction between the Au(111) surface and FAPanions in the anodic regime. When the WE potential becomes either more negative than -0.4 V or more positive than þ0.8 V, a third ultraslow process was detected, which seems to become Faradaic in the low-frequency limit. To extract differential capacitance values for EDL formation and for the second capacitive process, the complex capacitance data were fitted to an empirical Cole-Cole type equation. We find a significant hysteresis in the potential dependence of the differential double-layer capacitance (C EDL ). The capacitance relaxation strength of the second process is particular high at electrode potentials around þ0.4 V and at potentials more negative than -0.4 V.

Electrochemical impedance spectroscopy was used to characterise the interface between the ultrapure room temperature ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and a Au(111) working electrode at electrode potentials more positive than the open circuit potential (-0.14 V vs. Pt pseudo-reference). Plots of the potential-dependent data in the complex capacitance plane reveal the existence of a fast and a slow capacitive process. In order to derive the contribution of both processes to the overall capacitance, the complex capacitance data were fitted using an empirical Cole-Cole equation. The differential capacitance of the fast process is almost constant between -0.14 V and +0.2 V (vs. Pt pseudo-reference) and decreases at more positive potentials, while the differential capacitance of the slower process exhibits a maximum at +0.2 V. This maximum leads to a maximum in the overall differential capacitance. We attribute the slow process to charge redistributions in the innermost ion layer, which require an activation energy in excess of that for ion transport in the room temperature ionic liquid. The differential capacitance maximum of the slow process at +0.2 V is most likely caused by reorientations of the 1-butyl-1l-methylpyrrolidinium cations in the innermost layer with the positively charged ring moving away from the Au(111) surface and leaving behind voids which are then occupied by anions. In a recent Monte Carlo simulation by Federov, Georgi and Kornyshev (Electrochem. Commun. 2010, 12, 296), such a process was identified as the origin of a differential capacitance maximum in the anodic regime. Our results suggest that the time scales of capacitive processes at the ionic liquid/metal interface are an important piece of information and should be considered in more detail in future experimental and theoretical studies.

Results of potential-dependent differential capacitance measurements on the interface between six different ionic liquids and the (111) surface of single-crystalline gold are presented. The measurements were done by means of broadband impedance spectroscopy in a frequency range from 10 mHz to 1 MHz. We discuss the influence of the IL cation, the IL anion and the cations' alkyl chain length on the interfacial capacitance. Our results suggest that (i) there is no simple relationship between the cation size and the value of the differential capacitance, (ii) the general shape of the potentialdependent differential capacitance curve is more strongly influenced by the IL anion, and (iii) experimental differential capacitance curves do not exhibit a simple "camel-" or "bell-shaped" curvature as predicted by mean-field theories. Furthermore, the broadband measurements show that two capacitive processes can be distinguished, which take place on millisecond and second time scales, respectively. While a millisecond time scale is expected for double-layer charging governed by the bulk conductivity of the IL, the existence of a slow process points to additional barriers for charge transport at the interface. The capacitance contribution of the slow process is most pronounced for ILs based on the N-butyl-N-methyl-pyrrolidinium ([Pyr 1,4 ]) cation. A comparison of capacitance data with insitu STM data from previous studies suggests that the slow process is connected to herringbone-type structures at the interface. While the herringbone superstructure of the Au(111) surface is well known in aqueous electrochemistry, a herringbone-type structure of adsorbed ions was described in a recent MD simulation paper by Federov and coworkers (K. Kirchner, T. Kirchner, V. Ivaništšev, M. V. Fedorov, Electrochim. Acta 2013, in press:

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