ARTICLE pubs.acs.org/JPCC
On Capacitive Processes at the Interface between 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate and Au(111) Marcel Dr€uschler,* Benedikt Huber, and Bernhard Roling Department of Chemistry, Philipps University of Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany
bS Supporting Information ABSTRACT: 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 FAP- anions 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 (CEDL). 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.
I. INTRODUCTION Room-temperature ionic liquids (RTILs) have been the object of many recent studies, because most of them show outstanding properties1-3 that are relevant for a broad range of possible (electrochemical) applications. For instance, they can be used as electrolytes in electrochemical supercapacitors,4-7 as gate dielectrics in field-effect double-layer transistors,8 as reaction media for metal deposition,9-12 and as solvents for various chemical and electrochemical reactions.3,13 In supercapacitor applications, the broad electrochemical window of many RTILs offers perspectives for increasing the maximum voltage. However, the usage of RTILs in this field is still hindered by the limited knowledge about the structure and dynamics of the interface between RTILs and typical electrode materials, although some progress has been made in the past years.14,15 RTILs are dense ionic systems, and therefore, the classical mean-field EDL models of Helmholtz, Gouy, Chapman, and Stern cannot be used for describing the interfacial structure. Steric effects due to the finite size of the ions, the individual ionion and ion-electrode interactions, and the variation of the permittivity with distance from the electrode surface have to be taken into account. It is straightforward to consider the finite volume of the ions first, because this can be done at the meanfield level, resulting in analytical expressions for the differential capacitance of the EDL. Recently, Kornyshev presented a r 2011 American Chemical Society
mean-field lattice gas model of the EDL and showed that, at high ion densities, the potential-dependent differential capacitance of the EDL differs strongly from the predictions of the classical Gouy-Chapman model.16 The differential capacitance CEDL is defined as Dq ð1Þ CEDL ¼ DðΔjÞ μ, T , p Here, q denotes the electrode charge, whereas Δj is the potential difference between the electrode and the bulk of the RTIL. CEDL is measured while keeping the bulk chemical potential of the RTIL, μ, the temperature, T, and the pressure, p, constant. In the classical Gouy-Chapman model, ions are treated as point charges leading to an exponentially increase of CEDL with increasing electrode potential. The origin is a very strong accumulation of counterions close to the electrode at high potentials. In contrast, when the finite ion volume is taken into account, a global maximum of CEDL at the point of zero charge (bell-shaped curve) or two distinct maxima very close to it (camel-shaped curve) are predicted, depending on the relative Received: January 14, 2011 Revised: February 17, 2011 Published: March 10, 2011 6802
dx.doi.org/10.1021/jp200395j | J. Phys. Chem. C 2011, 115, 6802–6808
The Journal of Physical Chemistry C ion density, γ, in Kornyshev’s model.16 At high electrode potentials (positive or negative), CEDL drops with increasing potential. The thermodynamic origin of this effect is the large entropy penalty when more and more counterions are accumulated at the electrode. Therefore, the thickness of the EDL grows with increasing electrode potential (lattice saturation effect).16 Shortly after Kornyshev’s publication, Oldham used a modified Gouy-Chapman approach to derive similar expressions for CEDL as in Kornyshev’s theory.16,17 We note that finite ion volume approaches have already been published in the early 40s and 50s,18-21 but later, they have hardly attracted the electrochemists’ attention, maybe except electrochemists working in the field of molten salts.22 The influences on CEDL of the polarizability of ions and of specific ion adsorption were both studied by Lauw et al. using self-consistent mean-field theory (SCMFT) calculations.23,24 They showed that the polarizability of ions leads to a potentialdependent spatial distribution of the dielectric permittivity close to the charged surface and identified the polarizability as a key factor that supports camel-shaped CEDL(Δj) curves. Furthermore, they found that the presence of specifically adsorbed ions changes the potential-dependence of CEDL within a small potential range close to the point of zero charge. The point of zero charge itself is shifted along the potential axis depending on the strength of specific adsorption. At the same time, Tazi et al. used molecular dynamics simulations (MDS) to study the influence of electrode-ion interactions on the potential dependence of CEDL.25 They observed a potential-induced ordering of the ions in a way that they mimic the electrode surface structure. This epitaxial process is only visible if the polarization of the ions and the polarization of the electrode (interaction of the ions with induced image charges inside the metal) are taken into account. This process creates a sharp maximum of CEDL at the potential at which the epitaxial process takes place. Theoretical predictions for CEDL can be tested by means of electrochemical impedance spectroscopy (EIS). The complex ^ (ν) can be calculated from the complex impedance capacitance C ^ ^ ^ (ν))-1, where j denotes the imaginary Z(ν) via C(ν) = (j 3 2πνZ unit and ν the measurement frequency. For an ideal capacitive ^ (ν) levels into a CEDL plateau at low interface, the real part of C frequencies. However, real RTIL/electrode interfaces show deviations from this ideal capacitive behavior. To account for this, the interface has often been described with the help of a so-called constant phase element (CPE) instead of an ideal capacitor. Empirical relations were then used to estimate CEDL from the CPE parameters. Numerous EIS studies on the EDL between different RTILs and solid electrodes have been carried out so far.14,26-37 The obtained CEDL(Δj) curves do not exhibit a clear trend, and the results are, in part, contradictory. In a recent mini-review, Lockett et al. commented on the diversity of CEDL data.14 They discussed the influence of various parameters on CEDL(Δj), in particular, the influence of the RTIL purity, of the electrode material, of the electrode surface preparation, and of the exact measurement method. An important parameter is the structure of the electrode surface. Almost all of these measurements were carried out using either polycrystalline metal surfaces or glassy carbon (GC) electrodes. These electrodes exhibit a heterogeneous surface structure, and it is difficult to assess the influence of the heterogeneities on the CEDL(Δj) data. Consequently, it was the aim of the present study to characterize the interface between a well-prepared single-crystalline
ARTICLE
metal electrode and a RTIL of the highest purity that is available at present. We chose a Au(111) electrode and the RTIL, [EMIm]FAP. In this paper, we show that, by means of EIS, three distinct processes are detectable at the interface. These processes are most easily resolved when plotting the data in the CCP. Two of these processes are clearly capacitive, one of them being EDL formation. We discuss critically the usage of CPE fits for estimating the EDL capacitance CEDL, and we suggest an alternative way based on the empirical Cole-Cole expression. Furthermore, we show that, due to the potential-dependent overlap of different interfacial processes in the CCP, a plot of constant-frequency capacitance data versus the electrode potential may lead to misleading results for capacitance maxima and minima.
II. EXPERIMENTAL SECTION [EMIm]FAP was purchased from Merck KGaA in ultrapure quality (water, alkali metals, and halides contents were below 10 ppm). During the measurements, the home-made electrochemical cell was placed in a glovebox with an ultra-high-purity N2 atmosphere (LABstar, MBRAUN GmbH; H2O and O2 contents of the box
