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Hysteresis Effects in the Potential-Dependent Double Layer Capacitance of Room Temperature Ionic Liquids at a Polycrystalline Platinum Interface Marcel Dru¨schler,*,† Benedikt Huber,† Stefano Passerini,‡ and Bernhard Roling† Department of Chemistry, UniVersity of Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany, and Institute of Physical Chemistry, UniVersity of Mu¨nster, Correnstrasse 30, 48149 Mu¨nster, Germany ReceiVed: December 4, 2009; ReVised Manuscript ReceiVed: January 19, 2010
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/cm2. 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. Introduction The structure and dynamics of the electrochemical double layer in room temperature ionic liquids (RTILs) at electrode interfaces are important for many applications. For instance, the usage of RTILs as electrolytes in electrochemical double layer capacitorssa subgroup of so-called supercapacitorss offers perspectives for improved energy storage.1-6 RTILs exhibit much broader electrochemical windows than the commonly used electrolytes based on aqueous and organic solvents.7-9 Another important field of application is metal deposition from ionic liquids. It was found that the size and morphology of deposited metal particles can be influenced by the choice of the ionic liquids.10 Thus, the kinetics of the metal deposition seems to depend on the structure of the ionic liquid at the substrate interface, i.e., on the structure of the double layer. For classical diluted electrolyte solutions, the structure of the double layer at metal interfaces is commonly described by meanfield models, in particular by the Helmholtz model, the Gouy-Chapman model, and the Stern model. However, these mean-field models are not applicable to double layers in dense ionic systems, such as ionic liquids. In dense systems, steric effects and individual ion-ion interactions can lead to strong charge density correlations in the double layer,11-13 which are ignored in mean-field approaches. Only the finite volume occupied by the ions can be taken into account at the meanfield level.14-16 The finite volume sets an upper limit for the ion concentration at the electrode interface. Thus, when the charge on the electrode increases, the double layer has to become * To whom correspondence should be addressed. E-mail: drueschm@ staff.uni-marburg.de. Phone: +49-6421-28-22305. † University of Marburg. ‡ University of Mu¨nster.
thicker in order to screen this charge. This leads to a drop of the differential double layer capacitance at high electrode potentials. After publication of Kornyshev’s feature article16 in 2007, a number of experimental studies on the potential-dependent double layer capacitance of ionic liquids were carried out.17-21 Unfortunately, the authors of most studies do not give information about the purity of their ionic liquids and do not show cyclic voltammograms. Therefore, it is not possible to assess the influence of impurities on the measured capacitance values. An exception is ref 19 in which Lockett et al. describe in detail the purification of their ILs and the preparation of the working electrode for their electrochemical measurements. The studied ILs were based on imidazolium cations and chloride anions and were quite viscous or even solid at room temperature. Therefore, the capacitance measurements were done in a temperature range from 80 to 140 °C. Remarkably, the authors find hysteresis effects in the potential dependence of the double layer capacitance; i.e., the measured differential capacitance depends on the direction in which the dc potential of the working electrode is changed. The motivation of the present study is to find out whether this type of hysteresis is a universal phenomenon at metal/ionic liquid interfaces and can also be observed in RTILs with relatively low viscosity. Therefore, we have applied broad-band electrochemical impedance spectroscopy for studying the capacitive properties of the interface between a polycrystalline Pt working electrode and two well-known RTILs, namely, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][N(Tf)2]) and N, N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP][N(Tf)2]). Experimental Section [EMIm][N(Tf)2] was obtained from Iolitec in high purity. Residual solvents, traces of water, and adsorbed gases were
10.1021/jp911513k 2010 American Chemical Society Published on Web 02/08/2010
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Figure 2. Cyclic voltammograms of [EMIm][N(Tf)2] (black curve) and [BMP][N(Tf)2] (red curve) at a scan rate of 10 mV/s.
Figure 1. Schematic illustration of the electrochemical cell.
removed by heating the RTIL to 70-80 °C under high vacuum (10-6 mbar) for 72 h. A Karl Fischer titrator (E 547 KF, Metrohm) was used for analyzing the amount of residual water. The water content was found to be below the detection limit of 100 wt ppm. We note that other groups found water contents in the range of a few wt ppm or even lower after drying typical RTILs at elevated temperatures in vacuum.22 [BMP][N(Tf)2] was synthesized as described elsewhere,23 using [BMP][Br] as the precursor. The elemental analysis of both [BMP][N(Tf)2] and [BMP][Br] indicated purity higher than 99.5 wt %.23 However, the IL was subjected to a further purification step. It was dissolved in ethyl acetate (Aldrich, reagent grade) and stirred in a mixture of active carbon (DarcoG60, Sigma-Aldrich) and alumina (acidic, Brockman I, SigmaAldrich). The IL/ethyl acetate/alumina/active carbon weight ratio was 1/0.6/0.5/0.2. The mixture was stirred at 60 °C for 10 h. The solution was then filtered over a 0.2 µm Teflon filter, and most of the ethyl acetate was removed with a rotary evaporator (Resona Labo-Rota C-311) connected to an oil-free pump (PANAR ZA.60S). The final drying step was performed in the dry room using a glass oven (Büchi B-585) connected to a more powerful oil-free vacuum pump (Varian SH-100) at 50 °C for 10 h followed by an additional 10 h at 110 °C. Finally, the IL was sealed in a glass vial and vacuum-sealed in a coffee-baglike envelope. Thus, both RTILs used for this study are highly pure, which is essential for evaluating their electrochemical properties.24,25 In Figure 1 we show a schematic illustration of a homemade electrochemical cell used for both cyclic voltammetry and impedance spectroscopy. The measurements were carried out in a three-electrode configuration with a polycrystalline platinum wire acting as working electrode, a platinum crucible acting as counter electrode, and a silver wire acting as quasi-reference electrode. The preparation of the surface of the working electrode was essential for electrochemical investigations as the surface roughness and adsorbed species may strongly influence the results. Hence, the electrode was polished using 3- and 1-µm diamond paste. Then the surface was rinsed with ethanol and stored under vacuum at ambient temperatures for 24 h. The surface area of the working electrode was kept much smaller than that of the counter electrode. Thus, the contribution of the latter to the overall capacitance is small. Furthermore, the measurement cell exhibits the following features: (i) It can be operated with very small amounts of liquid in the range from 20 to 40 µL. (ii) Measurements can be done under vacuum and under inert gas atmosphere. (iii) The electrodes can be ex-
Figure 3. Idealized equivalent circuit for an electrolyte in contact with an ion-blocking electrode.
changed easily by using a pin connector system. In this study, all measurements were carried out under ultrahigh-purity nitrogen atmosphere (99.999%) in a glovebox. The water and the oxygen contents of the glovebox were below 0.1 ppm. The electrochemical cell was connected to a Novocontrol Modular Measurement System consisting of an Alpha-AK highresolution impedance analyzer and a POT/GAL 30 V/2 A electrochemical interface. First cyclic voltammograms with a scan rate of 10 mV/s were recorded in order to evaluate the electrochemical window. This window was defined as the potential range where the current density does not exceed 200 µA/cm2. Impedance measurements were carried out at different working electrode dc potentials superimposed by an ac voltage of 10 mV rms. The frequency range of the ac voltage extends from 0.01 Hz to 1 MHz. The impedance spectra were fitted to equivalent circuits using the ZView software. Results and Discussion In Figure 2, we show the cyclic voltammograms of [EMIm][N(Tf)2] and [BMP][N(Tf)2]. In the case of [EMIm][N(Tf)2], the width of the electrochemical window is 4.0 V, ranging from -1.6 to 2.4 V versus the Ag quasi-reference electrode. The electrochemical window of [BMP][N(Tf)2] is broader (5.0 V) and ranges from -2.5 to +2.5 V. The cathodic limit of this latter ionic liquid ([BMP][N(Tf)2]) appears to be narrower than previously reported.26 This is most likely caused by dissolved nitrogen, which was reported to be reduced at such cathodic potentials.26 Nevertheless, within the investigated electrochemical window, only a small current peak with a maximum height of 85 µA/cm2 was detected. This confirms the high purity of the RTILs. For understanding the capacitive properties of the RTILs as obtained from impedance spectroscopy, let us first consider an idealized equivalent circuit for an electrolyte in contact with a blocking electrode. This circuit consists of a bulk resistance Rbulk and a bulk capacitance Cbulk acting in parallel. The electrode interface is represented by a double layer capacitance connected in series to the (Rbulk Cbulk) element; see Figure 3. Since Cdl . Cbulk, the real part of the complex capacitance spectrum of this circuit, C′(ω), is given by
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C'(ω) ) Cdl ·
1 + ω2 · τbulk · τdl 1 + (ωτdl)2
Dru¨schler et al.
(1)
with τbulk ) Rbulk · Cbulk and τdl ) Rbulk · Cdl. In the limit of high frequencies, C′(ω) is identical to Cbulk. When ω becomes smaller than 1/(τbulk · τDL)1/2, the real part of the capacitance increases with decreasing frequency, and for ω < 1/τdl, the spectrum levels off into a plateau regime with C′(ω) ) Cdl. Our experimental capacitance spectra exhibit significant deviations from the behavior of the idealized circuit. As an example, we show in Figure 4 a capacitance spectrum of [EMIm][N(Tf)2] at a working electrode potential of -0.4 V. Between 10 Hz and 1 kHz, a double layer capacitance regime is detectable. However in this regime, C′(ω) still increases slightly with decreasing frequency. This nonideal capacitive behavior is well-known from aqueous electrolytes, and its origin has been discussed controversially. It has, for instance, been attributed to the roughness of the electrode surface27,28 and to specific adsorption processes,29 respectively. We note that the frequency range where Lockett et al. detect the nonideal double layer capacitance of the ILs [EMIm][Cl], [BMIm][Cl], and [HMIm][Cl] is similar to ours, namely, below 1 kHz.19 The ionic conductivities of these ILs at elevated temperatures (80-140 °C) are in a similar range as the ionic conductivities of our ILs at room temperature. For instance, the ionic conductivity of [EMIm][Cl] is 18.68 mS cm-1 at 81.5 °C and 30.0 mS cm-1 at 100 °C, respectively,30 whereas the room temperature ionic conductivities of [EMIm][N(Tf)2] and [BMP][N(Tf)2] are 9.2 and 2.2 mS cm-1, respectively.8 Consequently, the time scales of the double layer formation are also similar. A common way to describe this nonideal behavior in equivalent circuits is to replace the capacitance by a constant phase element (CPE); see Figure 5. The complex impedance of a CPE is given by
ZCPE )
1 T(iω)R
Figure 4. Differential capacitance spectrum of [EMIm][N(Tf)2] at a working electrode potential of -0.4 V vs Ag quasireference (red symbols). A weak dispersion is observed at intermediate frequencies followed by an additional strong increase of the differential capacitance at frequencies below 10 Hz. The solid lines denote fits based on the equivalent circuits shown in Figure 3 (black curve; experimental data between 1 kHz and 1 MHz was used for the fit), Figure 5 (green curve), and Figure 8 (blue curve).
Figure 5. Equivalent circuit accounting for the nonideal capacitive behavior of the electrical double layer.
(2)
with the parameters R and T. An exponent R ) 1 corresponds to an ideal capacitance. The green line in Figure 4 shows that the resulting circuit fits the capacitance spectrum at frequencies above 10 Hz. The values of R are in the range from 0.90 to 0.95 for [EMIm][N(Tf)2] and between 0.85 and 0.92 for [BMP][N(Tf)2], respectively. From the circuit parameters R, T, and Rbulk, a double layer capacitance Cdl can be estimated by using the equation31
Cdl )
R
T (R-1) Rbulk
(3)
In Figure 6, the estimated values for Cdl are plotted versus working electrode potential for both RTILs. Our results confirm the existence of hysteresis effects. The values and the potentialdependent features of Cdl are clearly influenced by the direction in which the dc potential of the working electrode is changed. These hysteresis effects must be caused by slow processes at the Pt/RTIL interface taking place on a time scale of minutes to hours. At a given dc potential, the recording of a single differential capacitance spectrum from 1 MHz down to 10 mHz takes about 20 min. Then the dc potential is changed, and a new differential capacitance spectrum is recorded. Thus, a
Figure 6. (a) Differential double layer capacitance (per unit area) of [EMIm][N(Tf)2], derived from eq 3, versus working electrode potential and (b) differential double layer capacitance (per unit area) of [BMP][N(Tf)2], derived from eq 3, versus working electrode potential.
complete measurement for obtaining the potential-dependent data shown in Figure 6 takes several hours. Remarkably, processes much slower than the double layer formation are directly observable in our capacitance spectra; see Figure 4. Below 10 Hz, there is a rise of the capacitance with decreasing frequency, which leads to differential capacitance values above 0.5 mF/cm2. These are most likely pseudocapacitive processes, which have also been found in the case
Double Layer Capacitance of RTILs
Figure 7. Equivalent circuit describing the capacitive behavior over all the entire frequency range by including pseudocapacitive contributions at low frequencies.
of aqueous electrolyte solutions and have been attributed to specific adsorption of ions at the electrode interface.32-36 In an equivalent circuit, these processes can be described by adding a CPE in parallel to the double-layer CPE; see Figure 7. As seen from the blue line in Figure 4, the resulting equivalent circuit leads to a good fit of the capacitance spectrum over the entire frequency range. The hysteresis in the double layer capacitance can now be easily rationalized by assuming that slow pseudocapacitive processes lead to a change in the structure of the electrostatic double layer. An important consequence is that a comparison between experimental and theoretical results for the potentialdependent double layer capacitance is highly problematic, since state-of-the-art theoretical approaches do not take into account processes much slower than the double layer formation. Thus, our results challenge both theory and experiment. In theory, approaches for taking into account pseudocapacitive processes should be developed. To this end, electronic degrees of freedom at the interface have to be considered in addition. In experiment, measuring techniques should be developed for obtaining hysteresis-free double layer capacitance values. This seems realistic, since the time scales of double layer formation and of pseudocapacitive processes differ by orders of magnitude. Conclusions We have shown that the potential-dependent double layer capacitance of the RTILs, [EMIm][N(Tf)2] and [BMP][N(Tf)2], at a polycrystalline Pt interface is characterized by hysteresis effects, which are most likely related to slow pseudocapacitive processes. These pseudocapacitive processes govern the capacitance spectra in a frequency range below 10 Hz. In order to clarify the nature of these processes, a combination of capacitance measurements with other experimental techniques for probing the RTIL/metal interface will be needed, e.g., STM, AFM,37,38 and sum-frequency generation techniques.39-41 Acknowledgment. S.P. is thankful for the financial support of the European Commission within the FP6 STREP Project ILLIBATT (Contract no. NMP3-CT-2006-033181). References and Notes (1) Balducci, A.; Henderson, W. A.; Mastragostino, M.; Passerini, S.; Simon, P.; Soavi, F. Electrochim. Acta 2005, 50, 2233.
J. Phys. Chem. C, Vol. 114, No. 8, 2010 3617 (2) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (3) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Science 2006, 313, 1760. (4) Largeot, C.; Portet, C.; Chmiola, J.; Taberna, P. L.; Gogotsi, Y.; Simon, P. J. Am. Chem. Soc. 2008, 130, 2730. (5) Huang, J.; Sumpter, B. G.; Meunier, V. Angew. Chem. 2008, 120, 530. (6) Chmiola, J.; Largeot, C.; Taberna, P. L.; Simon, P.; Gogotsi, Y. Angew. Chem. 2008, 120, 3440. (7) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. Chem. Phys. Chem. 2004, 5, 1106. (8) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567. (9) Hapiot, P.; Lagrost, C. Chem. ReV. 2008, 108, 2238. (10) El Abedin, S. Z.; Po¨lleth, M.; Janek, J.; Endres, F. Green Chem. 2007, 9, 549. (11) Fedorov, M. V.; Kornyshev, A. A. J. Phys. Chem. B 2008, 112, 11868. (12) Fedorov, M. V.; Kornyshev, A. A. Electrochim. Acta 2008, 53, 6835. (13) Kislenko, S. A.; Samoylov, I. S.; Amirov, R. H. Phys. Chem. Chem. Phys. 2009, 11, 5584. (14) Eigen, M.; Wicke, E. J. Phys. Chem. 1954, 58, 702. (15) Kilic, M. S.; Bazant, M. Z.; Ajdari, A. Phys. ReV. E 2007, 75, 021502. (16) Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545. (17) Alam, M. T.; Islam, Md. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 111, 18326. (18) Alam, M. T.; Islam, Md. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 2601. (19) Lockett, V.; Sedev, R.; Ralston, J.; Horne, M.; Rodopoulos, T. J. Phys. Chem. C 2008, 112, 7486. (20) Islam Md., M.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16568. (21) Alam, M. T.; Islam, Md. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16600. (22) Borisenko, N.; El Abedin, S. Z.; Endres, F. J. Phys. Chem. B 2006, 110, 6250. (23) Appetecchi, G. B.; Scaccio, S.; Tizzani, C.; Alessandrini, F.; Passerini, S. J. Electrochem. Soc. 2006, 153, A1685. (24) Rollet, A. L.; Porion, P.; Vaultier, M.; Billard, I.; Deschamps, M.; Bessada, C.; Jouvensal, L. J. Phys. Chem. B 2007, 111, 11888. (25) Schro¨der, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J. New J. Chem. 2000, 24, 1009. (26) Randstrom, S.; Appetecchi, G. B.; Lagergren, C.; Moreno, A.; Passerini, S. Electrochim. Acta 2007, 53, 1837. (27) De Levie, R. Electrochim. Acta 1965, 10, 113. (28) Pajkossy, T. J. Electroanal. Chem. 1991, 300, 1. (29) Pajkossy, T. J. Electroanal. Chem. 1994, 364, 111. (30) Vila, J.; Varela, L. M.; Cabeza, O. Electrochim. Acta 2007, 52, 7413. (31) Brug, G. J.; Van den Enden, A. L. G.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1984, 176, 275. (32) Grahame, D. C. Chem. ReV. 1947, 41, 441. (33) Damaskin, B. B. Electrochim. Acta 1964, 9, 231. (34) Wendt, H.; Riemenschneider, P. Chem.-Ing.-Tech. 1978, 50, 250. (35) Pajkossy, T.; Kolb, D. M. Electrochim. Acta 2001, 46, 3063. (36) Pajkossy, T. Solid State Ionics 2004, 176, 1997. (37) Atkin, R.; Warr, G. G. J. Phys. Chem. C 2007, 111, 5162. (38) Hayes, R.; El Abedin, S. Z.; Atkin, J. Phys. Chem. B 2009, 113, 7049. (39) Rivera-Rubero, S.; Baldelli, S. J. Phys. Chem. B 2004, 108, 15133. (40) Baldelli, S. J. Phys. Chem. B 2005, 109, 13049. (41) Aliaga, C.; Baldelli, S. J Phys. Chem. B 2006, 110, 18481.
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