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J. Phys. Chem. C 2008, 112, 16600–16608
Capacitance Measurements in a Series of Room-Temperature Ionic Liquids at Glassy Carbon and Gold Electrode Interfaces Muhammad Tanzirul Alam, Md. Mominul Islam, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ReceiVed: May 26, 2008; ReVised Manuscript ReceiVed: July 17, 2008
An extensive study has been done for the first time on the structure of the electrical double layer (EDL) at polarized glassy carbon (GC) and gold (Au) electrode interfaces in a series of room-temperature ionic liquids (RTILs) via the measurement of capacitance-potential curves. The parabolic capacitance-potential curves similar to those observed in high-temperature inorganic molten salts were obtained at GC electrode in all of the RTILs studied. Potential of zero charge (PZC) at GC electrode in imidazolium-based RTILs depends significantly on the electrochemical pretreatment of the electrode surface: Electrochemical oxidation pretreatment generates the oxide surface on GC electrode, which results in a favorable adsorption of positively charged imidazolium cations on the electrode surface and in turn shifts the PZC to the positive direction of potential, whereas at the electrochemically reduced GC electrode, on which the adsorption of the imidazolium cations is less favorable, PZC shifts to the negative direction of potential. Such an effect of electrochemical pretreatment was not observed at the highly oriented pyrolytic graphite electrode. The hump on the anodic side of the capacitance-potential curves at Au electrode in imidazolium-based RTILs results from the π-electronic interaction of the imidazolium ring with the metal electrode, which was substantiated by using nonmetallic electrode and varying the ions of the RTILs. Such an enhanced interaction of the imidazolium ring with a gold electrode, as in the case of anion adsorption, shifts the PZC to the negative direction of potential. Such a hump as that observed at the gold electrode was not observed at the GC electrode. Similarly to the case in high-temperature inorganic molten salts, capacitances at PZC increase with increasing temperature. Different aspects of the obtained capacitance-potential curves are interpreted satisfactorily based on the hitherto proposed concept of the EDL structures. Introduction Room-temperature ionic liquids (RTILs) are considered as versatile electrolytes and proposed as new promising solvents for diverse electrochemical and nonelectrochemical applications and devices1-4 as their physicochemical properties5 such as hydrophobicity/hydrophilicity, melting point, viscosity, density, and polarity can be favorably changed by altering the chemical structures of both the cation and the anion. Moreover, RTILs are unique in electrochemistry because of their dual role as solvents as well as electrolytes, and thus their fundamental data on conductivity, potential window, interfacial properties at various electrodes, and so forth, are of great importance in their practical applications. Methods commonly used for studying the interfacial properties include electrocapillary curve measurement,6 capacitance measurement,7 and surface-related spectroscopic measurements.8-10 Among these methods, capacitance measurement by the impedance technique is extensively used because of its reproducibility and convenience. On the other hand, the measurements of surface tension or surface stress of solid electrodes are not easy, although some attempts have been made to measure the surface energy or at least to determine the potential of zero charge (PZC).7 Few works have been done on the electrochemical determination of the electrical double layer (EDL) structure at the electrode/ionic liquid (IL) interfaces. Most recently, Lockett et * To whom correspondence should be addressed. E-mail: ohsaka@ echem.titech.ac.jp. Tel: +81-45-9245404. Fax: +81-45-9245489.
al.11 reported the potential dependency of the EDL capacitance at glassy carbon (GC) electrode in three 1-alkyl-3-methylimidazolium chloride-based RTILs at several temperatures. Their capacitance-potential curves have a minimum with two side branches. The minimum was designated as the PZC. The differential capacitance and the adsorption of the imidazolium ring were found to increase with decreasing the chain length of the alkyl residue of the imidazolium cation. The temperature dependence of the EDL capacitance was broadly similar to that obtained in high-temperature inorganic molten salts. Nanjundiah et al.12 measured the differential capacitances at carbon (carbon cloth, carbon yarn, and GC) and mercury electrodes in RTILs with a view to understanding the usefulness of using RTILs along with carbon cloth in a practical capacitor. Their impedance measurement at GC electrode did not show the PZC clearly, whereas we reported, as in the case of high-temperature inorganic molten salts, the parabolic capacitance-potential curve with a clear PZC at the electrochemically pretreated (oxidized) GC electrode.13 Capacitance-potential curves were found to vary significantly with the electrode substrates14 and heterogeneity of the RTILs.15 Baldelli et al.16-18 studied the orientation of the ions of RTILs at the electrode interfaces by sum frequency generation spectroscopy. According to their report, both the cation and the anion change their orientation to cope with the applied potential. Orientation of the cation was reported to be influenced by the size of the anion and the substituent of the cation (e.g., alkyl group). Planer cation (e.g., imidazolium) prefers to lie flat at the electrode surface at negative potential
10.1021/jp804620m CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
Glassy Carbon and Gold Electrode Interfaces with its possible orientational change. Kornyshev et al.19,20 have predicted that the capacitance-potential curve should be belland camel-shaped for the ILs with similar anion and cation sizes depending on the so-called lattice-saturation parameter. PZC can be obtained from the maximum of the capacitance if the curve is bell-shaped or from the local minimum between the humps if the curve is camel-shaped. In addition, it has also been predicted19 that for ILs with asymmetric ion size, the maximum of the capacitance will not coincide with the PZC. Recently, Oldham21 has also derived a similar outcome based on the Gouy-Chapman-Stern approach. Previous models of the EDL structures at electrode/solution interfaces have been developed with strong emphasis at the atomically smooth mercury electrode surface.6,7 Data of the EDL at solid electrode/solution interfaces is scarce11 though from a practical point of view (e.g., fuel cell) it is very important to reveal the related information. A common difficulty in revealing the EDL structure at the solid electrode is the presence of microscopic cracks and kinks at the electrode surface. Carbon electrodes differ significantly from those made of metal because of carbon’s anisotropic structure and major difference in behavior after surface treatment.22 The existence of surface oxides is well established, and the type and quantity of functional groups are found to vary greatly with pretreatment history.22-25 Electrochemical pretreatment (ECP) of carbon has been a major research subject as it can greatly increase electron transfer rates for a variety of redox systems.23 Oxygen content of the GC surface increases with its electrochemical oxidation. XPS and elemental analysis indicate an increase of the content of alcoholic, phenolic, as well as carboxylic group on the surface.24,25 Carbonyl groups are detected in the oxidized surface but disappear upon reduction.22-25 Most laboratories have taken the approach of adopting a uniform procedure to produce a surface with unknown structure but reproducible properties. Thus, the effects of the pretreatment procedure on the capacitance-potential curves at GC electrode in RTILs are examined in this study. One of the general aspects of the capacitance-potential curves is the appearance of a hump. Several causes, such as orientation of the solvent molecule at the interface,26-29 change in dipole moment of the solvent, adsorption of ions or ion aggregates,30 structural change of the electrolytes,31 and ion association32 have already been put forward for explaining the appearance of a hump. Mott and Watts-Tobin,27 in an attempt to explain the hump observed in aqueous media, have proposed that at the electrode surface there are two types of water molecules; one with the oxygen end toward the electrode and the other with the hydrogen end, both at an angle of cos-l (1/3) to the normal of the surface. The change of the angle and flipping of the oxygen and hydrogen atoms at the surface with potential is responsible for its occurrence. A hump has been also found to appear in the capacitance-potential curves measured in RTILs.11,13-15 In our previous report, we illustrated that the hump observed at the mercury electrode in RTILs containing a larger alkyl group is the consequence of the specific adsorption of the alkyl group at the electrode surface.14 However, the cause of the appearance of a hump in other cases11,13 is not known yet. Clarification of possible causes for its appearance is of utmost importance for revealing the structure of EDL in RTILs. In this study, we examined the effects of the ECP on the EDL structure at GC/RTILs interfaces. Capacitance-potential curves were measured at both electrochemically oxidized and reduced GC and HOPG electrodes. We used a series of RTILs with a fixed cationic backbone and anionic structure only by
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16601 varying the chain length of alkyl residue, (i.e., 1-alkyl-3methylimidazolium tetrafluoroborate, [RMIBF4], R ) CnH2n+1, where n ) 2, 4, 6, and 8) for clarifying some probable interactions of the imidazolium ring with the gold electrode surface as well as the relation of PZC with the hump and structure of the RTILs. RTILs with totally different cations and anions [N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (N,N-DEMMEABF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFs)] were also used to compare and understand certain aspects of capacitancepotential curves. Capacitances were also measured at three different temperatures at the gold electrode for obtaining a preliminary insight on how the EDL structure varies with temperature. An attempt was made to explain the capacitancepotential curves based on the hitherto proposed concept of EDL structure. Experimental Section Reagents. All of the RTILs of EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), BMIBF4 (1-butyl-3-methylimidazolium tetrafluoroborate), HMIBF4 (1-hexyl-3-methylimidazolium tetrafluoroborate), OMIBF4 (1-octyl-3-methylimidazolium tetrafluoroborate), EMITFs [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide], and N,N-DEMMEABF4 [N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate] with a purity of more than 99% were obtained from either Wako Pure Chemicals Industries (Japan) or Kanto Chemical Co., Inc. (Japan). RTILs were dried under vacuum for 12 h at a temperature of 100 °C. The average water and halide contents were less than 10 and 100 ppm, respectively (Kanto Chemical Co., Inc. (Japan)). N2 (99.99%) gas was of ultrahigh purity and supplied from Nippon Sanso Co., Inc. (Japan). Electrochemical Measurements. Electrochemical experiments were performed in a three-electrode cell containing a gold (Au, diameter ) 1.6 mm), glassy carbon (GC, diameter ) 1 mm), platinum (Pt, diameter ) 1.6 mm) or highly oriented pyrolytic graphite (HOPG) obtained from NT-MDT Co. (Russia) as a working electrode, a spiral platinum wire as a counter electrode, and a homemade silver/silver chloride (Ag/AgCl) as a reference electrode. Preparation of this reference electrode has been described elsewhere.13 Each experiment was done with a freshly polished working electrode. The working electrodes were polished first with emery paper and then with aqueous slurries of fine alumina powders (1 and 0.06 µm) on a polishing cloth followed by rinsing with doubly distilled water and acetone in an ultrasonic bath, each for 10 min, and were finally rinsed with Milli-Q water. Gold and platinum electrodes were further treated electrochemically by successive potential cycling in N2-saturated 0.05 M H2SO4 until the cyclic voltammograms, characteristic of clean gold and platinum electrodes, were obtained. Electrochemical oxidation pretreatment of the GC and HOPG surfaces was performed by repeating the potential scan at a scan rate of 0.5 V/s in the potential range of 0.7 to 1.6 V vs Ag/AgCl/KCl (saturated) for 200 times in N2-saturated 0.5 M H2SO4, whereas the reduction pretreatment was likewise performed in a 1 M NaOH solution in the potential range of -0.8 to -1.4 V. Solarton SI 1260 and SI 1287 were used as impedance/gain phase analyzer and electrochemical interface, respectively, for the measurement of impedance. The time constants of the cell (RC: R, electrolyte resistance; C, interfacial capacitance) containing EMIBF4, EMITFs, BMIBF4, and N,N-DEMMEABF4 were calculated to be 0.059, 0.052, 0.23, and 0.28 ms, respectively, using the values of R and C obtained from the
16602 J. Phys. Chem. C, Vol. 112, No. 42, 2008 Nyquist plots (not shown here) measured at -0.2 V. Thus, impedance measurements were done at a constant frequency (200 Hz) by scanning the electrode potential (i.e., dc potential) from negative to positive direction at a scan rate of 5 mV/s and ac potential with 5 mV peak to peak amplitude was superimposed on the dc potential. From the impedance measurement data, the value of capacitance (C) was derived using the equation -Z// ) 1/(2πfC), where Z// is the imaginary component of impedance.33,34 Capacitance was also derived11,17,35 by fitting the impedance curve (Nyquist plot), measured at a potential in the frequency range of 5 kHz to 20 Hz, with a simple circuit combination (i.e., electrolyte resistance and constant phase element (CPE) in series). To treat the nonideal capacitive behavior of solid electrodes instead of double layer capacitance (Cd) at ideally polarizable electrode, CPE was used. In this case, capacitances were measured at a potential interval of 0.025 V in the potential range studied. Data were fitted using Z-View software supplied by Scribner Associates, Inc. The latter procedure was found to give more intense capacitive hump at the gold and platinum electrodes in imidazolium-based RTILs,13 whereas at the GC electrode both procedures gave almost the same results. Thus, in the cases of gold and platinum electrodes, the capacitance-potential curves obtained from the fitting of the Nyquist plots are used in this study. In accordance with the previous reports by Lockett et al.11 and us13-15 and the in situ spectroelectrochemical data obtained by Baldelli et al.16-18 and Nanbu et al.,9 a minimum of capacitance-potential curve is ascribed to the PZC throughout this study. Sadkowski,36,37 Pajkossy,38 and Neves et al.39,40 have reported the unreliability of the measurement of Cd from impedance spectra of solid electrodes. Thus, throughout this study instead of Cd, the term C is used. RTILs were deoxygenated by purging pure N2 gas for about 30 min, and the gas was kept flowing over the liquid during the electrochemical measurements. All of the experiments were carried out at room temperature (25 ( 2 °C) unless otherwise noted. A continuous water flowing thermostatic bath was used for measuring the capacitances at elevated temperatures. Results Capacitance-Potential Curves at GC/RTILs Interfaces. Figure 1 represents the capacitance-potential curves measured at GC electrode (electrochemically untreated) in N2-saturated (a) BMIBF4 and (b) OMIBF4. The curves are parabolic like those in high-temperature inorganic molten salts.41,42 The potential of minimum capacitance (i.e., 0.08 V in BMIBF4 and 0.11 V in OMIBF4) is designated as the PZC.11-18 These capacitance curves do not show any hump being distinct from those at different electrode substrates in aqueous31 and conventional organic solvents.43 Though the curves are, on the whole, parabolic, the elevation of capacitance on both sides of PZC differs in different RTILs. Compared to BMIBF4, the capacitance rise in OMIBF4 is much lower. Capacitance measurements at GC electrode in some other imidazolium-based RTILs ([RMIBF4], R ) CnH2n+1, where n ) 2, 3, 6) also gave a similar type of parabolic curves. Lockett et al.11 also reported similar parabolic capacitance-potential curve with two side branches (hump) at the GC electrode in three 1-alkyl-3-methylimidazolium chloride-based RTILs. However, the cause of the appearance of humps is unknown. Also shown in Figure 2 are the capacitance-potential curves measured at electrochemically (a) reduced and (b) oxidized GC electrodes in N2-saturated BMIBF4. Similarly to the electrochemically untreated (ECU) GC electrode (Figure 1), the
Alam et al.
Figure 1. Capacitance versus potential curves measured at electrochemically untreated GC electrode in N2-saturated (a) BMIBF4 and (b) OMIBF4.
capacitance-potential curve at the electrochemically reduced (ECR) GC electrode is parabolic with a minimum at -0.01 V, which is more negative than that at the ECU GC electrode, whereas at the electrochemically oxidized (ECO) GC electrode, the minimum was found at a more positive potential (i.e., 0.32 V) than at the ECU GC electrode with a hump in the potential range of 0 to -0.6 V. Figure 3 shows the capacitance-potential curve measured at HOPG electrode in N2-saturated BMIBF4. The curve of a parabolic shape resembles well those at the ECU and ECR GC electrodes (Figure 1 and part a of Figure 2). The capacitance of the HOPG electrode is unusually low, with a value of 3.0 µF cm-2 at the PZC, (-0.075 V). This low capacitance has been attributed to a space charge layer caused by the semimetal character of HOPG.44-46 The basal-plane HOPG is nearly atomically smooth and behaves as a nearly ideal capacitor with no observed frequency dispersion. The capacitance-potential curve was also measured at the HOPG electrode after the ECP in acidic solution (data not shown here). The curve was found to be almost identical in shape to Figure 3 with a little higher value of capacitance. Unlike the GC electrode, the values of PZC of the ECU and ECO HOPG electrodes were found to be the same. Oxidation of the graphite begins at defects (probably some exposed edge plane), and then progresses along the edge plane forming kink and large regions of oxidized edge plane. It has been reported that the HOPG initially delaminates, and then is fractured, probably because of lattice strain induced by the
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Figure 2. Capacitance versus potential curves measured at electrochemically (a) reduced and (b) oxidized GC electrodes in N2-saturated BMIBF4.
Figure 4. Capacitance versus potential curves measured at the gold electrode in N2-saturated (a) EMIBF4, (b) BMIBF4, and (c) OMIBF4.
Figure 3. Capacitance vs potential curve measured at HOPG electrode in N2-saturated BMIBF4.
oxidation process.22,47,48 The increased roughness of the surface is responsible for the higher value capacitance at the ECO HOPG electrode. Capacitance-Potential Curves at Au/RTILs Interfaces. Figure 4 represents the capacitance-potential curves measured at the gold electrode in N2-saturated (a) EMIBF4, (b) BMIBF4, and (c) OMIBF4. These curves are quite different from those at
carbon electrodes in Figures 1 to 3. The differences are prominent especially on the anodic side of the PZC where a hump is observed. The important features of the curves in Figure 4 are noted below. 1. PZC shifts toward the positive direction of potential with increasing the alkyl chain length of the imidazolium cation: the values of PZC in EMIBF4, BMIBF4, and OMIBF4 are -0.51, -0.47, and -0.34 V, respectively (Table 1). 2. Compared to the cathodic side, the capacitance rise on the anodic side of the PZC is higher especially for the imidazolium cations with shorter alkyl chains (parts a and b of Figure 4), whereas in OMIBF4 (part c of Figure 4) it is almost the same on both sides of the PZC as in the case of GC electrode (Figure 1).
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TABLE 1: Values of PZC at GC, Au, and Pt Electrodes in Different RTILs at 25 °C RTILs
dielectric constant49
EMIBF4
12.8 ( 0.6
BMIBF4
11.7 ( 0.6
HMIBF4
8.9 ( 0.9f
OMIBF4 EMITFs N,N-DEMMEABF4
15.2 ( 0.3
PZCg vs Ag/AgCl (V) 0.085a
-0.51 -0.67 0.08a -0.01b 0.32c -0.47
-0.65 0.09a -0.39 0.11a -0.34 0.075a -0.53 -0.15a 0.08
capacitance at PZC, (µF cm-2)
electrodes
12.6 12.8 12.9 11.3 10.1 12.9 11.6 13.9d 16.7e 11.5 9.7 10.1 9.0 9.5 13.1 13.1 11.9 11.1
GC Au Pt GC GC GC Au Au Au Pt GC Au GC Au GC Au GC Au
a Electrochemically untreated GC electrode. b Electrochemically reduced GC electrode. c Electrochemically oxidized GC electrode. d At 50 °C. e At 80 °C. f Dielectric constant of 1-hexyl-3-methylimidazolium hexafluorophosphate. g The accuracy in the PZC measurements is ( 0.01 V.
Figure 5. Capacitance vs potential curve measured at the gold electrode in N2-saturated EMITFs.
Figure 6. Capacitance vs potential curve measured at the gold electrode in N2-saturated N,N-DEMMEABF4.
3. In all the cases, humps are observed only on the anodic side of the PZC, and their height decreases with increasing the alkyl chain length; the smallest hump is observed in OMIBF4 (part c of Figure 4). 4. Unlike the PZC, the hump moves toward the negative direction of potential with increasing the alkyl chain length, and the shape of the capacitance-potential curve becomes parabolic as in the case of high-temperature molten salts.41,42 In fact, in the case of EMIBF4 and BMIBF4, it is parabolic in the potential range of -1 to 0 V (parts a and b of Figure 4). The positions (i.e., peak potentials) of the hump in EMIBF4, BMIBF4, and OMIBF4 are 0.47, 0.39, and 0.18 V, respectively. The explanation for these results will be given in the Discussion section. To investigate the cause of the appearance of a hump on the anodic side of the capacitance-potential curves (Figure 4) at the gold electrode in imidazolium-based RTILs, capacitancepotential curves were measured by changing the ions of the RTIL as well as the electrode. Figure 5 illustrates the capacitance-potential curve measured at the gold electrode in N2-saturated EMITFs. Similarly to the other imidazolium-based RTILs (RMIBF4 (R ) CnH2n+1, n ) 2, 4, and 8), Figure 4), it
exhibits a hump on the anodic side of the PZC. Our findings are correlated well with the previous experiments31 done with the aim of delving into the structure of EDL in conventional organic and aqueous media. That is, we observed a sharp hump in RTILs containing planner aromatic ring (parts a a and b of Figure 4), whereas its sharpness is lost when BF4- is replaced by a bigger anion, TFs- (Figure 5), as proposed previously.31 The peak potential of the hump in EMIBF4 (part a of Figure 4) is 0.47 V, whereas in EMITFs it is ca. 0.2 V, being consistent with the previous result31 that the hump shifts to the negative direction of potential with increasing the size of electrolyte anion. The appearance of a hump in the EMITFs indicates that the hump observed in EMIBF4 (part a of Figure 4) is not related to the anion, BF4-. Figure 6 shows the capacitance-potential curve measured at the gold electrode in N2-saturated N,N-DEMMEABF4. This curve is almost parabolic like those at the GC electrode in BMIBF4 and OMIBF4 (Figure 1) with a hump and PZC at ca. -0.75 and 0.09 V, respectively. The cause of the appearance of hump is unknown. In this RTIL, a hump is not observed on the anodic side of the curve whereas at the gold electrode in all the studied imidazolium-based RTILs there appears a hump on
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Figure 7. Capacitance vs potential curve measured at the platinum electrode in N2-saturated EMIBF4.
the anodic side of the capacitance-potential curves (Figures 4 and 5). Thus, the presence of imidazolium ring in imidazoliumbased RTILs is responsible for the appearance of a hump at the gold electrode. The capacitance-potential curve measured at the platinum electrode in N2-saturated EMIBF4 is shown in Figure 7. The PZC for the Pt/EMIBF4 interface is estimated as -0.67 V. As in the case of the gold electrode, a capacitive hump was observed only on the anodic side (0.11 V) of the PZC. Therefore, it can be considered that the hump in Figure 4 is related to some sort of interaction of imidazolium ring with the metal electrode (discussed later). The sharp increase of the capacitance at potentials more positive than 1.1 V is not clarified. It could be related to a faradaic process. Temperature Effect on Capacitance-Potential Curve. Figure 8 illustrates the temperature dependency of capacitancepotential curves measured at the gold electrode in N2-saturated BMIBF4 at (a) 50 and (b) 80 °C. Similarly to the case in hightemperature inorganic molten salts and aqueous media,26,31,41,42 the capacitance in BMIBF4 increases with increasing temperature. The values of capacitance at the PZC in BMIBF4 at room temperature (25 ( 2 °C, part b of Figure 4), 50, and 80 °C are 11.6, 13.9, and 16.7 µF cm-2, respectively. Recently, Lockett et al.11 have also observed the similar enhancement of the capacitance with increasing temperature at the GC electrode in imidazolium-based RTILs. Grahame26 studied the temperature dependence of the capacitance-potential curves at the mercury electrode in aqueous media. He reported that the hump decreases with increasing temperature. The hump at the Hg/EMIBF4 interface was also found to decrease with increasing temperature by our group.14 In both cases, the hump is considered to result from the orientational change of the solvent/ions at the interface. Discussion Carbon Electrodes. Inspection of the curves (Figures 1 to 3) measured at the GC electrode in imidazolium-based RTILs reveals that the degree of capacitance rise on both sides of PZC decreases with increasing the alkyl chain length and can be attributed to the comparatively smaller value of the dielectric constant49 () (directly proportional to capacitance) and the lesser extent of interaction of the charged moieties of the ions with the electrode due to the increased crystallinity of RTILs containing longer alkyl groups.50-53
Figure 8. Capacitance versus potential curves measured at the gold electrode in N2-saturated BMIBF4 at (a) 50 and (b) 80 °C.
ECP in acidic solution yields an oxide surface in the form of phenolic and carboxylate functional groups on which positively charged imidazolium cations are adsorbed and thus is responsible for the positive shift of the PZC7,22 (Figures 2). On the other hand, ECR of the GC electrode in 1 M NaOH diminishes some oxide functional groups (e.g., carbonyl) on the electrode surface.23-25 Therefore, the surface becomes less prone to adsorb cations compared to the oxidized surface, which is the cause of the relatively negative value of the PZC (Figure 2). The content of the oxide functional groups at the ECU GC electrode surface is moderate, and the value of PZC at the ECU GC electrode (0.08 V, Table 1) is between those at the ECO (0.32 V) and ECR (-0.01V) GC electrodes, supporting the above rationale. The cause of the appearance of the hump in part b of Figure 2 is not clear, but it could be the aftereffect of the intense oxidation of the electrode surface, which results in an oxide layer on the electrode surface with the introduction of the new functional groups containing oxygen (e.g., carbonyl).23,24 Slow faradaic processes of the different oxides at the GC electrode could also be the cause of the hump.22 The O/C ratio at the electrode surface was reported to increase significantly with ECP in acidic solution with a larger background current,22-25 which is the cause of the relatively larger capacitance value obtained at the ECO GC electrode. To justify the cause of the shift of the PZC upon ECP, the capacitance-potential curve was measured at HOPG electrode. In accordance with the reported literature,22 it is considered that the moderate ECP used in this study (Experimental Section)
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do not actually yield an oxide surface on the HOPG electrode, whereas the formation of an oxide surface on the GC electrode with the ECP under the same condition is well documented. The unchanged PZC at both the ECU (Figure 3) and ECO (data not shown) HOPG electrodes is also an indication of the absence of surface oxide on these HOPG electrodes. Oxide surface formation would have resulted in a shift of the PZC toward the positive direction of potential. Therefore, the different degree of the oxide surface formation is considered to be the cause of the relative variation of the PZC and capacitance at the ECU, ECR, and ECO GC electrodes in a given RTIL (Table 1, Figures 1 and 2). According to the Helmholtz model,7 the simplest equation for double layer capacitance is
Cd )
εε0 d
(1)
where, and 0 are the dielectric constant of the medium and permittivity of free space, respectively, and d is the distance between the charging plates of the double layer. Variation of Cd with potential and electrolyte concentration suggests that either ε or d depends on these variables. In the case of RTILs, the influence of electrolyte concentration is always constant as the cation and anion exist at their optimum concentrations all the time. The observed capacitance-potential curves can not be explained based on the change in because the dielectric constant should be largest at the PZC and should decrease on both sides of the PZC. The alternative is the change in the thickness of the compact layer due to electrostriction,54,55 which had been proposed earlier to explain the changes in capacitance. The enhancement of capacitance on both sides of the PZC (Figures 1 to 3), like those in high-temperature inorganic molten salts, suggests the presence of pseudocapacitance due to the adsorption of ions. The specific adsorption only forces the ions to be packed into the compact layer and thereby increases the capacitance. RTILs are composed of molecular ions, and thus the change of orientation of the molecular cation with potential is quite apprehensible and should be more prominent than that of simple cations or anions (e.g., Na+, I-). From the crystallographic data of 1-alkyl-3-methyl imidazolium salts,50-53 it is known that the anion resides in the center of the cation ring, and the cation changes its angle with respect to the normal of the electrode surface to cope with the potential change.16-18 At a highly negatively charged surface, the imidazolium ring becomes parallel to the electrode surface, and consequently the distance (d) is decreased, which in turn may increase the capacitance.16-18 So, orientational changes of the ions, pseudocapacitance due to their adsorption, and the variation of the distance of the charged interfaces can be reckoned as responsible factors for the capacitance changes. Metal Electrodes. The large change of capacitance on the anodic side of the PZC (parts a and b of Figure 4) at the gold electrode could be due to the deviation of the system from ideal polarization due to specific adsorption, for example, adsorption pseudocapacitance. According to Graves and Inman,41 it could be due to the reversible charge transfer of the excess surface ions. Devanthan and Tilak31 suggested that the high values in capacitance are due to adsorption pseudocapacitance where no charge crosses over to the electrode. In the present case, the pseudocapacitance may arise from the π-electronic interaction of imidazolium ring with the gold electrode. Aromatic and unsaturated organic compounds exhibit an attraction toward metals (gold, mercury) due to their π-bonding ability. The effect of π electrons on adsorption has been demonstrated from a
comparative study on the conjugated unsaturated hydrocarbons and their hydrogenated derivatives.31 Those studies showed that the unsaturation enhances their adsorption on the anodic side when compared with the saturated compounds. Aromatic cations like anilinium, dimethylanilinium, pyridinium, and so forth, adsorb on the cathodic as well as anodic sides of electrocapillary maxima. The adsorption on the anodic side has been attributed to the π-electronic interaction with mercury.56-59 These effects can be qualitatively formulated by considering the polarizability of the π electrons of imidazolium ring. Because aromatic imidazolium compounds trend to adsorb flat on the electrode surface as proved by Baldelli et al.8,16-18 and Nanbu et al.,9 the π electrons of the imidazolium ring are considered to interact with the vacant orbital of metal electrode on the anodic side of the PZC. At negative potential beyond the PZC, the imidazolium cation lies flat at the gold surface, but with increasing the potential toward the positive direction it changes its orientation from parallel to perpendicular by altering its tilt and torsional angle, which in turn lessens the overlapping of the π orbital of the imidazolium cation with the vacant orbital of the gold electrode and thus assists the desorption of the imidazolium cation. This desorption decreases the value of capacitance generating a hump on the anodic side of the capacitance-potential curves (parts a and b of Figure 4). Spectroscopic investigations8,17 in agreement with the above rationale showed that at moderate positive potential compared to the PZC, imidazolium cation does not leave the surface completely (against the charge repulsion) but may adopt a tilted orientation allowing the ring to interact with the gold electrode. There is an experimental evidence for this60,61 that, at extreme anodic polarizations, aromatic compounds without any ionic group do get desorbed. The adsorption of the imidazolium cation, which is the cause of the generation of a hump, becomes less facilitated with increasing the alkyl chain length. It has been demonstrated that RTILs preserve some solid-like crystalline structure even in their liquid state.50-53 The extent of the crystallinity increases with increasing the alkyl chain length. This solid-like crystalline structure and larger alkyl group in OMIBF4 give the imidazolium ring little room to overlap with the vacant orbital of the gold electrode and consequently the height of the hump decreases with the concomitant shift of its peak potential toward the negative direction of potential (part c of Figure 4), that is, the imidazolium cation is easy to desorb. In the case of OMIBF4 (part c of Figure 4), the PZC and hump shift to the positive and negative direction of potential, respectively, compared to those in EMIBF4 (part a of Figure 4) and BMIBF4 (part b of Figure 4), which is a consequence of the lesser extent of overlapping of the π electron of imidazolium cation with the vacant orbital of the gold electrode. A similar sort of hump was also observed at the gold electrode in EMITFs (Figure 5) where the hump appeared at more negative potential compared to that in EMIBF4 (part a of Figure 4). It is thought that the solid-like crystalline structure of EMITFs with bigger anion impedes the overlapping of the π electron of the imidazolium ring with the vacant orbital of the gold electrode, which in turn favors its desorption from the electrode surface and is responsible for the shift of the hump to the negative direction of potential in EMITFs. On the other hand, such a hump was not observed on the anodic side of the capacitance-potential curve at the gold electrode in N,NDEMMEABF4 (Figure 6), demonstrating that the imidazolium ring is responsible for the appearance of a hump in Figure 4. Capacitance-potential curve at the platinum electrode in EMIBF4 (Figure 7) also has a hump (0.11 V) on the anodic
Glassy Carbon and Gold Electrode Interfaces side of the PZC (-0.67 V) like that at the gold electrode in the same RTIL (part a of Figure 4) suggesting the involvement of a metal electrode in the generation of a hump. This view that the observed hump results from the π-electronic interaction of the imidazolium ring with the metal electrode can be supported by the results shown in Figure 1. Namely, being a nonmetallic electrode, GC does not have the d orbital to interact with the π electron of the imidazolium ring. Thus, the capacitance-potential curves at the GC electrode do not show any hump. The increase in the minimum capacitance with temperature in Figure 8 is consistent with the quasi-lattice concept of the melt structure,41,42 which predicts a loss of the crystallinity (longrange order) with increasing temperature. RTILs are reported to maintain some solid-type crystalline structure even in their liquid state. Thus, at higher temperature the double layer can become more compact with an enhancement of its capacitance at the PZC. Weakening of ion association in RTILs with increasing temperature has been considered to be a probable cause for the enhancement of the capacitance by Lockett et al.11 Bifurcation of the capacitive hump at higher temperatures (50 and 80 °C) in Figure 8 may be a consequence of the enhanced thermal agitation and loosening of the so-called crystalline structure of the RTIL, which assist the desorption of the imidazolium ring from the electrode surface. Thermal vibration at high temperature facilitates the orientational change of imidazolium ring at the interface as well as its desorption. A capacitance-potential curve at room temperature displays an intense hump at 0.39 V with one shoulder at -0.15 V (part b of Figure 4), whereas at 50 and 80 °C two clear humps are observed: ca. -0.15 and 0.470 V at 50 °C and -0.1 and 0.4 V at 80 °C. The first hump at more negative potential (i.e., ca. -0.15, -0.15, and -0.1 V in part b of Figure 4 and in parts a and b of Figure 8, respectively) increases with increasing temperature, whereas the second one at more positive potential (i.e., 0.39, 0.47, and 0.4 V) decreases with increasing temperature. The presence of two humps is considered to be indicative of two types of temperature-dependent interactions (i.e., adsorption-desorption) of the imidazolium ring with the electrode surface depending on the crystallinity and thermal agitation of the RTIL. In this case, it also seems that the capacitance-potential curves with two humps take a camellike shape as per the theory.19-21 A further study on this remarkable observation remains to be conducted. Correlations between PZC and Structures of RTILs. The values of PZC and capacitance at PZC at the GC and gold electrodes in the RTILs studied are compiled in Table 1. Examination of the data reveals some important features of the EDL structure in RTILs. The capacitance at PZC at the gold and GC electrodes in imidazolium-based RTILs decreases in the order of EMIBF4 > PMIBF413 > BMIBF4 > HMIBF4> OMIBF4, which is similar to the decreasing trend of the dielectric constant (ε) of the RTILs. The å values of RTILs have been reported to decrease with increasing the chain length of the alkyl residue of the imidazolium cation.49 This trend also follows the relationship between the capacitance and the distance of the double layer (eq 1), that is, the capacitance decreases with increasing the chain length of the alkyl group. On the other hand, the PZC at the gold electrode shifts to the positive direction of potential with increasing the length of the alkyl chain. As corroborated above, the less overlapping of the π electron of imidazolium ring with the vacant orbital of the gold electrode due to the increasing of the alkyl chain length is responsible for the anodic shift of PZC, whereas at the GC
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16607 SCHEME 1: Structure of the Ions Constituting the RTILs Used in This Study
electrode such a trend was not observed as it does not have the d orbital to interact with the π electron of the imidazolium ring. Conclusions Capacitances were measured at the gold and GC electrode interfaces in a series of RTILs by the impedance technique. Like as in high-temperature inorganic molten salts, parabolic capacitance-potential curves were observed at GC electrode in all of the RTILs studied. ECP has a significant effect on the PZC at the GC electrode. ECO yields an oxide surface on the GC electrode on which the cations of the RTILs are adsorbed favorably, and consequently the PZC shifts to the positive direction of potential. On the other hand, ECR reduces the amount of the oxide on the GC electrode, which allows a lesser number of the cations to be adsorbed on it, and therefore the PZC moves to the negative direction of potential. Such an effect was not observed at the HOPG electrode. A capacitive hump was observed on the anodic side of PZC at the gold electrode and is attributed to the π-electronic interaction of the imidazolium ring with the metal, which was substantiated using a nonmetallic (GC) electrode and changing the ions of RTILs. The differences in the values of PZC and the peak potentials of the hump observed at the gold electrode in the imidazoliumbased RTILs composed of the common anion and the cations with different alkyl substituents could be satisfactorily explained based on the π-electron interaction of imidazolium ring with the gold electrode. It was found that, the larger the interaction is, the more the PZC shifts to the negative direction of potential with a relatively enhanced hump on the anodic side of the PZC. The temperature dependency of capacitance-potential curves reveals that the position and intensity of hump depend on the easiness of interaction of imidazolium ring with the gold electrode, which reflects the crystallinity and thermal agitation of RTILs (Scheme 1). Acknowledgment. The present work was financially supported by Grant-in-Aids for Scientific Research on Priority Areas (No. 417), Scientific Research (A) (No. 19206079) to T.O., from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. M.T.A. gratefully acknowledges the Government of Japan for a MEXT Scholarship. References and Notes (1) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567.
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