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Electrical Double-Layer Structure in Ionic Liquids: A Corroboration of the Theoretical Model by Experimental Results Md. Mominul Islam, Muhammad Tanzirul Alam, 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: July 03, 2008; ReVised Manuscript ReceiVed: August 12, 2008
We experimentally observed for the first time a bell-shaped (convex parabolic) differential capacitance versus potential (Cdl-E) curve, which is expected according to the theory of Kornyshev given for the electrical double layer (EDL) of metal electrode/ionic liquid (IL) interface, at platinum and gold electrodes in four different [quaternary ammonium, imidazolium, and pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion-based] ILs with cations and anions of similar sizes. The Cdl-E curves measured at a glassy carbon (nonmetallic) electrode in the same set of ILs were found to be U-shaped, in contrast to those obtained at platinum and gold electrodes. The present study corroborates the so-called Kornyshev’s model of the EDL at metal electrode/IL interfaces and at the same time demands a theoretical model for the nonmetallic electrode/ IL interface. The EDL formation in ILs is discussed. Introduction Room-temperature ionic liquids (ILs) have attracted a great deal of attention as the excellent media because of not only their different advantages in applications but also the diversity of their tailoring according to what is desired.1-6 Among various advantages of ILs, in general, low vapor pressure, reasonable thermal stability, and a large potential window specially grant them the superiority over the conventional protic and aprotic solvents in the practical applications. Recently, ILs have been increasingly used as media for many electrochemical purposes, for example, fundamental studies,7-16 batteries,17,18 electrosynthesis,19,20 electrodeposition,21 electrochromic devices,22 and capacitors.23-26 The electrical double layer (EDL) structure in ILs may be expected to be different from that in the conventional media containing molecular solvent dipole and supporting electrolyte (i.e., Helmholtz or Gouy-Chapman EDL model)27 because IL is composed of ions and they are directly in contact with the electrode surface (Figure 1). In this regard, the influence of inherent properties [e.g., crystallographic orientation of atom (facet) and work function] of the electrode material, namely compact-layer effect,28,29 on the EDL structure in ILs would also be significant. Thus, in a given IL, the double layer capacitance, interfacial tension and surface charge density may vary from electrode to electrode [i.e., metallic (e.g., gold, platinum, mercury, silver, and copper) or nonmetallic (e.g., carbon and graphite)]. These may ultimately influence the interfacial process, for example, heterogeneous electron-transfer [the standard rate constants of oxygen reduction in imidazolium cation-based ILs (ImILs) have been reported to be considerably smaller than those in conventional solutions8,14 and to be different at platinum, gold, and glassy carbon (GC) electrodes14], electrodeposition and charging capacity. Therefore, the knowledge of EDL structure is of prime importance for a proper understanding of an interfacial process in ILs. * To whom correspondence should be addressed. Tel: +81-45-9245404. Fax: +81-45-9245489. E-mail:
[email protected].
The EDL structure of a system has been usually understood by knowing the surface tension or differential capacitance (Cdl) as a function of electrode potential.27,30-36 In situ spectroscopic techniques37-41 by which the orientation of ions and dipoles at the vicinity of the electrode can be clarified have helped us to understand the interfacial structure of an electrochemical system. Recently, the measurements of a drop time-potential curve called an electrocapillary curve (ECC) at a dropping mercury electrode (DME)30-35,42 and a Cdl-E (capacitance versus potential) curve at a hanging mercury drop electrode (HMDE), platinum, gold, and GC electrodes, and a carbon cloth gas diffusion electrode (GDE) have been carried out in ImILs.30-34 Nanjundiah et al.30 have first measured the ECCs and Cdl-E curves at the DME and the GC electrode and the GDE in various ImILs. We have also studied the ECC35 at DME and Cdl-E curves at HMDE, gold, platinum, and GC electrodes mainly in ImILs.31-35 In analogy to the conventional media,42 the ECC measured in IL possesses a maximum31-35 but no clear valley in the Cdl-E curve at the potential corresponding to the maximum of the ECC,31 which is classically considered as the potential of zero charge (PZC),27 has been found. Baldelli et al.40 have also measured the Cdl-E curve and the IR spectrum using an electrochemical impedance spectroscopic (EIS) technique and sum frequency generation spectroscopy, respectively, at the platinum electrode in ImILs. By combining both results, they have defined the potential at which the value of Cdl is a minimum as the PZC. Also, we have considered the potential corresponding to the observed maximum and minimum in ECC and Cdl-E curve, respectively, measured in various ImILs as the PZC.31-34 In contrast, Kornyshev has recently developed a model for the metal electrode/IL interface based on local density approximation dependent mean-field theory.43 This theory deals with the possible EDL formed with a similar or dissimilar sized cation-anion pair of ILs. From this theory, the parameter gamma, γ ) 2c0/cmax, c0 is the average bulk number density of cations or anions and cmax is the maximal possible local concentration of ions (both cations and anions), respectively, may be recognized as the indicator of the shape of the Cdl-E curve and the position of PZC in the curve.43 For a flat metal
10.1021/jp8058849 CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
Electrical Double-Layer Structure in Ionic Liquids
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Figure 1. Schematic diagrams of EDL structures and C-E curves in conventional solution containing solvent dipole and electrolyte (a and c) and IL (b and d). In panels (a) and (b), the electrode is considered to be negatively polarized.
SCHEME 1: Structures of Cations and Anions of ILs
those often observed in high-temperature inorganic molten salts.46,47 It is noted that Locket et al. have recently observed the camel-shaped Cdl-E curves at GC electrode in ImILs.36 On the basis of the obtained results, a probable structure of the EDL in ILs is discussed. Experimental Section
electrode and ILs with nonadsorbing ions, it has been revealed that the characteristic Cdl-E curve is bell-shaped (part d of Figure 1) when γ > 1/3 or camel-shaped, provided that γ < 1/3. It is noted that the value of γ depends on the degree of porosity of ILs.43-45 For ILs with a rigid, similar-sized cation-anion pair, the value of γ would be 1 that can be rarely the case practically (note: γ ) 1 means that there are no free voids in the liquid and it is completely incompressible).43 Very recently, Oldham45 has supported the Kornyshev’s model by refreshing and modifying simply the principles that Gouy, Chapman, and Stern adopted in their classical treatments of the EDL structure at an electrode/electrolyte solution interface in a particular case when γ ) 1. By a molecular dynamic simulation based on the force fields of charged Lennard-Jones spheres between charged walls, Fedorov and Kornyshev have also verified the Kornyshev’s model and revealed that the EDL in ILs is not one layer thick.43,44 Remarkably, the theoretical Cdl-E curve in ILs is inverted in shape to that classically obtained according to the Gouy-Chapman-Stern theory (parts c and d of Figure 1).27,43-45 According to Kornyshev’s theory, the potential corresponding to the maximum and minimum of the bell-shaped (part d of Figure 1) and camel-shaped curves, respectively, may be regarded as the PZC.43,44 At present, we found that the theoretical Cdl-E curve at a metallic electrode/IL interface (part d of Figure 1) could be obtained with a proper choice of ILs with cations and anions of similar sizes (Scheme 1 and Table 1). Several Cdl-E curves were measured at platinum, gold, and GC electrodes in four different ILs (Scheme 1) using EIS technique. The Cdl-E curves obtained at platinum and gold electrodes were found to be bellor camel-shaped. On the other hand, the Cdl-E curve measured at GC electrode is essentially different in shape from those obtained at platinum and gold electrodes but is comparable with
Reagents. All of the ILs were used as supplied commercially. N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide ([DMOA+][N(Tf)2-], N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([TMPA+][N(Tf)2-]), 1,3-diallylimidazolium bis(trifluoromethanesulfonyl)imide ([DiAlI+][N(Tf)2-]), and N-n-butylN-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMPyr+][N(Tf)2-]) with a purity of more than 99% and less than 0.005% water and halides were obtained from Kanto Chemical Co., Inc. (Japan). Highly pure argon (99.99%) gas was supplied by Nippon Sanso Co., Inc. (Japan). Apparatus and Procedures. The electrochemical cell was a conventional two-compartment Pyrex glass container with a working electrode (Au; φ ) 1.6 mm, Pt; φ ) 1.6 mm, GC; φ ) 1.0 mm), a spiral platinum-wire counter electrode and a silver wire (quasi-) reference electrode. The working electrodes were carefully and gently polished with alumina powder (down to 0.6 µm) with the help of a microcloth and then washed with Milli-Q water by sonication for 15 min. After that, the gold and platinum electrodes were further electrochemically pretreated in argon-saturated 0.05 M H2SO4 solution by repeating the potential scan in the range of -0.2 to 1.5 V until the voltammograms characteristic of the corresponding clean electrodes were obtained. Prior to use, the electrode was washed well with Milli-Q water and dried by blowing air. The EIS measurements were carried out using Solartron SI 1260 Impedance/Gain-Phase Analyzer combined with Solartron SI 1287 Electrochemical Interface. A silver wire was used as a quasireference electrode by dipping a freshly polished silver wire into the cell solution. The time constants of the cell (Rs × Cdl, Rs is solution resistance, Cdl is EDL capacitance) containing ILs have been found to be 0.061-0.29 ms at -1.0 V. Impedance measurements were done at constant frequencies of 321, 426, and 961 Hz by scanning the electrode potential (i.e., dc potential) from negative to positive direction at a scan rate of 5 mV s-1 and the ac potential with 5 mV peak-to-peak amplitude was superimposed on the dc
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TABLE 1: van der Waals Volumes (Vf) of Various Ions of ILs48 cations /anionsa 3
Vf/Å a
[EMI+]
[DMPI+]
[TMEA+]
116
150
110
+
+
[DMEA+]
[TEMA+]
[BMPyr+]
126
143
148
[BF4-]
[N(Tf)2-]
49
147
+
Abbreviations: [EMI ]: 1-ethyl-3-methylimidazolium; [DMPI ]: 1-propyl-2,3-dimethylimidazolium; [TMEA ]: trimethylethylammonium; [DMEA+]: di(methylethyl)ammonium; [TEMA+]: triethylmethylammonium; [BMPyr+]: butylmethylpyrrolodinium; [BF4-]: tetrafluoroborate; [N(Tf)2-]: bis(trifluoromethanesulfonyl)imide.
Figure 2. Typical Cdl-E curve measured at the platinum electrode in argon-saturated [DMOA+][N(Tf)2-]. Inset shows the plots of 1/Cdl versus |E|1/2 derived from the data of the (a) right and (b) left wings of the measured Cdl-E curve.
potential. From the impedance measurement data, the values of capacitance (Cdl) were calculated according to the equation: -Z′′ ) 1/(2π f Cdl), where Z′′ is the imaginary component of impedance and f is the frequency of the measurement.27 It was found that the Cdl-E curves measured by varying f are almost identical in shape, but the Cdl values depend on f. This is known as a capacitance dispersion phenomenon and is commonly observed at polycrystalline and amorphous solid electrodes. Among the various reasons, the roughness of the electrode surface [e.g., atomic scale (steps, kinks, dislocation) and microscopic defects (corrugations, pits, grooves)] and specific adsorption, which has also been proven to be intrinsically coupled with the roughness of the electrode, results in the capacitance dispersion with f.27,36 All of the Cdl-E curves given in this paper were measured at f of 321 Hz. All of the measurements were carried out at room temperature (25 ( 2 °C). Results and Discussion Typical Cdl-E Curve Measured at the Platinum Electrode. Figure 2 represents the typical Cdl-E curve measured at the platinum electrode in argon-saturated [DMOA+][N(Tf)2-]. This curve is a convex parabola (bell-shaped) with a maximum at potential of about -0.2 V and, most interestingly, comparable with that expected from the theoretical model of the EDL structure at the metal electrode/IL interface (part d of Figure 1) proposed by Kornyshev43 and Oldham.45 The plots of 1/Cdl versus | E |1/2 (| E |: the potential difference from the maximum) were found to show roughly linear relationships between 1/Cdl and E1/2 in more negative and positive potential ranges with respect to the maximum (inset in Figure 2). The obtained plots
were found to become actually linear when the values of | E |1/2 reach 0.55 V1/2 (i.e., E ) 0.3 V) and 0.83 V1/2 (i.e., E ) -0.7 V) in the anodic and cathodic wings, respectively. This observation is in good agreement with the charge conservation principle appearing in the Kornyshev’s model.43-45 By comparing the obtained Cdl-E curve with the so-called bell-shaped one (described later), the potential corresponding to the maximum (-0.2 V) was considered as the PZC.43-45 The value of Cdl at the maximum (i.e., at -0.2 V) was estimated to be 11.4 µF cm-2, and the differences in the Cdl values at the maximum and more negative or positive potentials are small [e.g., the difference in the Cdl values at the maximum (-0.2 V) and -1.0 V is 0.81 µF cm-2]. Moreover, the magnitude of the slope of the linear part of the 1/Cdl versus | E |1/2 plot obtained for the left-hand wing is larger (ca. 0.02 µF-1 cm2 V-1/2) than that (ca. 0.01 µF-1 cm2 V-1/2) of the right-hand wing, that is, the obtained Cdl-E curve is unsymmetrical in shape. It is noted that the above-mentioned aspects regarding the Cdl-E curve are very significant to understand the EDL structure in IL (discussed later). Effects of the Type of IL on Cdl-E Curve. Figure 3 shows the Cdl-E curves obtained at Pt electrode in three different ILs. All the measured Cdl-E curves are generally bell-shaped with well-defined maxima in a given small potential range. In a close analysis, it may be seen that the entire shape of the Cdl-E curves and the position of the maximum depend on the type of ILs as summarized in Table 2. The shape of Cdl-E curve measured in [DiAlI+][N(Tf)2-] (part a of Figure 3) is almost identical with that obtained in [DMOA+][N(Tf)2-] (Figure 2), except for the portion of the left-hand wing at potential more negative than -1.0 V (note: it may be associated with the adsorption-desorption phenomenon of IL itself via the allyl group of [DiAlI+] cation as confirmed by cyclic voltammetry). In this case, the maximum was found at -0.25 V with a Cdl value of 11.5 µF cm-2 (Table 2). The Cdl-E curves obtained in [TMPA+][N(Tf)2-] (part b of Figure 3) and [BMPyr+][N(Tf)2-] (part c of Figure 3) were not virtually symmetrical in shape, and an increase of Cdl (capacitance hump) in the right-hand wing (>0 V) took place. The observed capacitance hump is considered to be associated with the reorientation or absorption-desorption of ions of ILs under the application of potential (discussed below). Other characteristics of the measured Cdl-E curves are as follows: A maximum at -0.78 V with a Cdl value of 11.4 µF cm-2 and a capacitance shoulder at -0.43 V were obtained in [TMPA+][N(Tf)2-] (part b of Figure 3), and the maximum was found at a relatively more negative potential (i.e., ca. -0.87 V) with a Cdl value of 8.7 µF cm-2 in [BMPyr+][N(Tf)2-] (part c of Figure 3). It is also noted that the Cdl-E curve containing two maxima in a narrow potential range (-0.4 to -0.8 V) in [TMPA+][N(Tf)2-] may be typed as camel-shaped.43 Similarly to the case of [DMOA+][N(Tf)2-], the potential corresponding to the well-defined maxima of the Cdl-E curves measured in [DiAlI+][N(Tf)2-] and [BMPyr+] [N(Tf)2-] may be considered as the PZC (Table 2). On the contrary, in the case of [TMPA+][N(Tf)2-], the potential corresponding to either the
Electrical Double-Layer Structure in Ionic Liquids
Figure 3. Cdl-E curves measured at the platinum electrode in argonsaturated (a) [DiAlI+][N(Tf)2-], (b) [TMPA+][N(Tf)2-], and (c) [BMPyr+][N(Tf)2-].
maximum at -0.78 V or the minimum at -0.58 V of the Cdl-E curve may be regarded as the PZC (Table 2), depending on whether the obtained Cdl-E curve is bell-shaped or camelshaped,43 respectively (discussed later). Effects of Electrode Material on Cdl-E Curve. To elucidate the effect of the electrode substrate on the EDL structure, the Cdl-E curves at the gold and GC electrodes were measured in [DMOA+][N(Tf)2-], and the obtained results are shown in Figure 4. Except for the capacitance hump at 0.3 V, the Cdl-E curve observed at the gold electrode is essentially bell-shaped as obtained at the platinum electrode (Figure 2). In this case, the maximum was found at -0.45 V with a Cdl of 19.5 µF cm-2 that is greater than that (11.4 µF cm-2) at the platinum electrode.
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16571 A large Cdl at the gold electrode was also observed in other ILs examined in this study. The observed large values of Cdl at the gold electrode in the ILs may be associated with either the specific property of the gold electrode (i.e., metal effect on the compact layer28,29) or a higher degree of roughness, which leads to a substantial error in the measured Cdl value27,36 of the gold electrode surface (described in the Experimental Section). However, in analogy to the platinum electrode, the observed potential corresponding to the maximum (i.e., -0.45 V) at the gold electrode would be considered as the PZC that is by 0.25 V more negative than that at the platinum electrode in the same IL (Table 2). The different values of PZC of gold and platinum electrodes may be ascribed to the so-called metal effect.28,29 In contrast to the platinum and gold electrodes (Figures 2, 3, and part a of 4), the Cdl-E curve obtained at the GC electrode is U-shaped. Similar U-shaped curves were also obtained at the GC electrode in all of the ILs under consideration (not shown) and have been reported to be observed in ImILs,31-34 and in high-temperature inorganic molten salts.46,47 A diagnosis of this curve reveals that the Cdl value at the observed minimum of -0.7 V is 9.6 µF cm-2, and the slopes of the right and left wings of the obtained Cdl-E curve are ca. 3.8 and -4.2 µF cm-2 V-1, respectively. The potential (i.e., -0.7 V) corresponding to the minimum observed at the GC electrode is thought to correspond to the PZC as in the case of ImILs.31-34 The slope of the wing of the Cdl-E curve obtained at GC electrode is significantly larger compared to those obtained at the platinum and gold electrodes. At this stage, such observations at the GC electrode cannot be explained because of the lack of theoretical model for nonmetallic (i.e., carbon, graphite, metal oxide electrodes) electrode/IL interfaces, but the obtained results may be regarded as the advanced information in developing the related EDL theory. Explanation for the Obtained Cdl-E Curves. Thus far we have found bell- and camel-shaped Cdl-E curves, which are expected according to the Kornyshev’s model43 but not found previously,30-34 could be observed experimentally at platinum and gold electrode/IL interfaces. As described in the Introduction Section, the theoretical bell-shaped curve could be typically obtained for the EDL packed with ILs when the parameter γ is greater than 1/3. Generally, this happens when the cation and anion of IL are equally sized, hard spheres (i.e., incompressible). The ILs with dissimilar sizes of cations and anions essentially result in a distorted bell-shaped curve.43 Why we successfully observed the bell-shaped curve can be satisfactorily explained based on the Kornyshev’s model and the morphology of the ions of the ILs used in this study (Scheme 1). The van der Waal’s volumes (Vf) of all the cations of the ILs may be compared with that (147 Å3) of [N(Tf)2-] common anion (Table 1). Here, the Vf values of [DMOA+], [TMPA+], and [DiAlI+] are unknown, but it may be assumed that the Vf value of [DMOA+] is approximately comparable to those of [DMEA+] (126 Å3) and [TEMA+] (143 Å3),48 whereas the Vf value of [TMPA+] is comparable to that of [TMEA+] (110 Å3).48 On the other hand, the Vf value of [DiAlI+] would be larger than that (116 Å3) of [EMI+] and may be roughly assumed to be comparable to that (150 Å3) of [DMPI+] (Table 1). Furthermore, by considering the chain length of the alkyl substituents of the cations of the ILs used (Scheme 1), all of the ions may be assumed to be almost incompressible. These facts may practically result in the value of γ > 1/3. Therefore, the observation of bell-shaped Cdl-E curves especially in [DMOA+][N(Tf)2-], [BMPyr+][N(Tf)2-], and [DiAlI+][N(Tf)2-] is reasonable. In addition, it can be mentioned that, in agreement
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TABLE 2: Results Obtained From the Cdl-E Curves Measured at Different Electrodes in Various ILs at 25 °C characteristics of Cdl-E curves ILs [DMOA+
-
][N(Tf)2 ]
[DiAlI+][N(Tf)2-] [TMPA+][N(Tf)2-] [BMPyr+][N(Tf)2-] [EMI+][BF4-] [EMI+][Cl-] a
electrodes
shape
Pt Au GC Pt Pt
bell-like bell-like U-like bell-like bell-like (camel-like) bell-like
Pt Hg Au GC GC
U-like camel-like
maximum (minimum) or PZC/Va -0.2 -0.45 -0.7 (minimum) -0.25 -0.78 (-0.58) (minimum) -0.87 -0.23 (minimum)b -0.51 (minimum)b 0.09 (minimum)b 0.32 (minimum)c
Cdl at maximum (minimum)/µF cm-2
hump/Va 0.3 0 -0.34b 0.45b
8.7 19.7 12.6 12.8 23
1.0 and 2.0 V). The Cdl-E curve with a maximum at -0.78 V and a shoulder at -0.43 V obtained at the platinum electrode in [TMPA+][N(Tf)2-] may be taken as bell-shaped by neglecting the observed shoulder or camel-shaped by considering the whole curve (part b of Figure 3).43 The size of [TMPA+] (110 Å3) is appreciably smaller than that (147 Å3) of [N(Tf)2-], which may be supposed to result in the packing of cations and anions in the EDL in such a way that the value of γ becomes less than 1/ , resulting in the camel-shaped curve. Recently, Lockett et 3 al.36 have reported that the Cdl-E curve measured at the GC
electrode in chloride anion-based ImILs has two maxima at 1.0 and
