Electrical Double Layer in Mixtures of Room-Temperature Ionic

Mar 25, 2009 - Muhammad Tanzirul Alam , Jahangir Masud , Md. Mominul Islam , Takeyoshi Okajima , and Takeo Ohsaka. The Journal of Physical Chemistry ...
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J. Phys. Chem. C 2009, 113, 6596–6601

Electrical Double Layer in Mixtures of Room-Temperature Ionic Liquids 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: December 10, 2008; ReVised Manuscript ReceiVed: January 16, 2009

The interfacial structure at the Hg electrode in mixtures of room-temperature ionic liquids (1-ethyl-3methylimidazolium tetrafluoroborate (EMIBF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF4)) has been studied for the first time by the measurement of surface tension and differential capacitance. Surface tension, charge density on the electrode surface, and capacitance decrease with increasing the addition of OMIBF4 in EMIBF4, which has been found to be the consequence of the preferential adsorption of the octyl group on the Hg surface and increasing extent of heterogeneity of the mixtures. The continuous widening of the electrocapillary maxima (ECM) with increasing addition of OMIBF4 in EMIBF4 also supports the above reasoning. In accordance with the ECM, the potential corresponding to the minimum of the capacitance-potential curve is assigned as the potential of zero charge (PZC). The probable cause for the appearance of the PZC at the minimum of the capacitance-potential curve has been discussed. Introduction Room-temperature ionic liquids (RTILs) have recently gained increasing attention as environmentally benign alternatives to conventional organic solvents in a variety of synthetic,1,2 catalytic,1,3 and electrochemical applications,4 which is the result of their unique physicochemical properties4-6 and the relative ease with which these properties can be fine-tuned by altering the cationic or anionic moieties comprising the RTILs. For practical application of the RTILs in electrochemical devices, such as lithium ion batteries,7 fuel cells,8 capacitors,9 and solar cells,10 it is important to know the structure of the electrical doublelayer(EDL)attheelectrode/RTILinterfaces.Experimental11-21 and theoretical22-25 work has just started in this regard. A wellaccepted unanimous model of the EDL is yet to be established. Kornyshev22 has shown theoretically that the capacitance (C)-potential (E) curve for the ionic liquids (ILs) with symmetric ion sizes should be bell- or camel-shaped on the basis of the so-called lattice-saturation parameter, and the potential of zero charge (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 predicted22 that, for ILs with asymmetric ion sizes, the maximum of the capacitance will not coincide with the PZC. Later on, Fedorov, Kornyshev,23,24 and Oldham25 have reached the same conclusion separately. In accordance with the theoretical expectation, a bell-shaped C-E curve has recently been reported by our group for the RTIL with almost similar ion sizes (i.e., N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide) at the Pt electrode.11 On the other hand, for the RTILs with dissimilar ion sizes and specific adsorption of ions, the curves were found to be quite different.12-17,19-21 Very recently, Fedorov and Kornyshev24 have performed molecular dynamics simulations for studying the effect of ion size asymmetry on the EDL structure. Their study shows that the C-E curve for the ILs with asymmetric ion sizes should * Corresponding author. Phone: +81-45-9245404. Fax: +81-45-9245489. E-mail: [email protected].

have an asymmetric bell-shaped character, in qualitative agreement with the experimental data.12-17 However, quantitatively, the experimental data are found to be quite different from the theoretical expectations, and this discrepancy could be the consequence of the nonspherical shape of the ions, heterogeneity within the ILs, specific adsorption of ions, and others whose effects are prevailing at the interface and differ in extent from ILs to ILs in real systems but were not taken into account in the theoretical calculation. The effect of the specific adsorption and heterogeneity of RTILs containing long-chain alkyl groups on the C-E curves has been demonstrated by our group in studying the interfacial structure at the Hg electrode.13 Electrocapillary curves (ECCs) at Hg in 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIBF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF4) demonstrate an unusual broadness of the maxima on the anodic side of the PZC, reflecting a unique characteristic of the EDL structure. Structural constraint in orientational change, spatial heterogeneity in their liquid structure, and strong hydrophobic interaction of the larger alkyl group with the Hg surface were found to be responsible for the observed broadness of the ECM.13 In this regard, a model of the EDL structure (Scheme 1) at the Hg interface has been proposed. In this model, the octyl groups have been shown to be preferentially adsorbed on the Hg electrode surface due to their strong hydrophobicity and the charged moieties (imidazolium ring and BF4-) reside away from the electrode surface due to the ease of forming hydrogen bonds and the electrostatic interaction. Thus, the charged moieties cannot get in contact with the electrode surface with a small change of potential, and consequently, there arises a broad maximum in the ECC. It would be interesting if the proposed model could be cross-checked by other means. Unfortunately, spectroscopic investigation is not suitable for probing the interfacial structure at Hg due to its black color and liquidity. Thus, studying the interfacial structure of the mixtures of RTILs has been taken as an alternative for crosschecking the previously proposed structure of the EDL at the Hg electrode in OMIBF4.

10.1021/jp810865t CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009

Electrical Double Layer in Mixtures of Ionic Liquids SCHEME 1: Illustration of the Hg/OMIBF4 Interface at the Potential around the ECM, Where the Lines (|||||||||) and (------) Represent the Electrostatic Interaction and Hydrogen Bonding, Respectively

Two of the main barriers in the application of these RTILs (e.g., OMIBF4) arise from their higher viscosity and lower conductivity, which ultimately result in the reduction of the reaction rate in these media.26 A few attempts have been made in overcoming these barriers, such as addition of a simple electrolyte10,27 or mixing of the highly viscous RTIL with the one that has comparatively lower viscosity.28,29 Mixing of RTILs also provides a means of preparing a reaction medium of desired characteristics. It has been observed recently that the mixed solution of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide provides lower overpotential for the deposition-dissolution of magnesium than every single RTIL system used.28 However, to date, there is no report on the structure of the EDL in mixtures of RTILs. Understanding the EDL structure in mixtures, as well as in a single RTIL, may provide plenty of information needed in their practical utilization, such as in fuel cells and analyzing the effect of the EDL on the electrode reaction kinetics. The purpose of this study is twofold: elucidating the structure of the EDL in mixtures of RTILs and cross-checking the previously proposed structure of the EDL at the Hg electrode in OMIBF4. In this connection, OMIBF4 is mixed with the RTIL of the same group, that is, EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), in different proportions, and the corresponding interfacial structures are investigated by the measurement of C-E curves and ECCs. This is the first report regarding the interfacial structure at the Hg electrode in mixtures of RTILs.

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6597 Electrochemical Measurements. Electrochemical experiments were performed in a three-electrode cell containing a hanging mercury (Hg) drop [model CGME 900, Bioanalytical Systems, Inc. (BAS); area, 0.018 cm2] as a working electrode, a spiral platinum wire as a counter electrode, and a homemade silver/silver chloride (Ag/AgCl) as a reference electrode. The preparation of this reference electrode and the stability of its potential were described elsewhere.15 Each experiment was carried out at a freshly formed Hg drop, unless otherwise noted. Solarton SI 1260 and SI 1287 were used as impedance/gain phase analyzer and electrochemical interface, respectively, for the measurement of capacitances. Impedance measurements were performed 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-1, and the ac potential with 5 mV peak-to-peak amplitude was superimposed on the dc potential. From the impedance measurement data, the value of the capacitance (C) was derived using the equation -Z// ) 1/(2πfC), where Z// is the imaginary component of impedance.30,31 Capacitance was not measured at a potential more positive than 0.2 V as the dissolution of Hg may corrupt the experimental data.32 The drop-time measurement was carried out with a homemade natural dropping Hg electrode in an electrochemical cell with the same arrangement of reference and counter electrodes as described above. In this experiment, the Hg drops were allowed to fall through the capillary from a height of 56.5 cm to N2saturated solution under varied applied potentials. The lifetime of a drop (i.e., drop time) was estimated as an average by counting the total time required for 10 drops to fall at a certain applied potential. The measurement at each applied potential was repeated three times to confirm the reproducibility of the obtained drop time. In a similar way, the drop times were measured in the studied potential range at a potential interval of 0.1 or 0.05 V. Electrolytes were deoxygenated by purging pure N2 gas for about 30 min, and the gas was kept flowing over the liquids during the electrochemical measurements. All the experiments were carried out at room temperature (25 ( 2 °C). Measurement of interfacial tension (γ) from the drop-time experiment at the dropping mercury electrode follows the following relationship.33

tmax )

2πrc γ mg

The weight of the drop at the end of its lifetime is gmtmax, where m is the mass flow rate of the mercury issuing from the capillary, g is the gravitational force, and tmax is the lifetime of the drop. This force is counterbalanced by the γ acting around the circumference of the capillary with radius, rc. Surface charge density (σM) was estimated by differentiating the ECC as per the following Lippmann equation.33

Experimental Section Reagents. RTILs of EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), EMI[N(Tf)2] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), and OMIBF4 (1-octyl-3methylimidazolium tetrafluoroborate) with a purity of more than 99% and less than 0.005% water were obtained from Wako Pure Chemical Industries, Ltd. (Japan). N2 (99.99%) gas of ultrahigh purity was supplied from Nippon Sanso Co., Inc. (Japan).

σM ) -

( ∂E∂γ )

T,P,µ

In this equation, the subscript µ implies that the chemical potentials (activities) of all the components are held constant during each measurement. The slope of the ECC at any point is numerically equal and opposite in sign to the value of σM.

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Figure 1. (a) Interfacial tension versus potential and (b) the corresponding capacitance versus potential curve measured at the N2saturated Hg/EMI[N(Tf)2] interface.

Results and Discussion Differential Capacitance and Electrocapillary Curve at the Hg/EMI[N(Tf)2] Interface. Figure 1 represents the (a) interfacial tension versus potential and the corresponding (b) capacitance versus potential curves measured at the Hg electrode in N2-saturated EMI[N(Tf)2]. The ECC is parabolic with a maximum at -0.55 V, which is assigned as the PZC, in accordance with the theory of electrocapillary.33 The ECC is considered as the “map” of the interfacial structure as the orientational change of the ions, their adsorption-desorption, and the variation of the thickness of the EDL with potential are reflected through the change of its γ. On scanning the potential from negative to positive direction, γ first increases, then passes through a maximum, and at a potential positive to the maximum, γ decreases. This is also the usual trend of the ECC observed in aqueous media.33,34 ECCs with a similar pattern were also observedinotherRTILscontainingshort-chainalkylgroups.13-15,17,18 The ECC measured at Hg in EMIBF4 has the maximum at -0.23 V13 with a γ of 368 dyne cm-1, whereas in this study in EMI[N(Tf)2], the maximum is observed at -0.55 V with a γ of 380 dyne cm-1. The increased γ at Hg/EMI[N(Tf)2] could be the consequence of the higher value of the dielectric constant (ε) of EMI[N(Tf)2] compared to that of EMIBF4. The reported values of ε of EMIBF4 and EMI[N(Tf)2] are 12.8 ( 0.6 and 15.2 ( 0.3, respectively.35 The negative shift of the PZC at Hg/ EMI[N(Tf)2] compared to that at Hg/EMIBF4 could be due to the preferential adsorption of the anion, [N(Tf)2]-, on the Hg surface through its sulfur atom. Sulfur-containing ions and molecules had been shown to possess a quite strong adsorption

Alam et al. affinity toward the Hg.36 Usually, the stronger the adsorption of the anion, the more the PZC shifts to the negative direction of potential. In accordance with the maximum of the ECC, the minimum of the C-E curve (Figure 1b) at -0.55 V is ascribed to the PZC. Differential Capacitance and Electrocapillary Curves at Hg in Mixtures of EMIBF4 and OMIBF4. Figure 2 represents the interfacial tension versus potential curves measured at the dropping Hg electrode in different percent ratios of EMIBF4 and OMIBF4 under N2 atmosphere. The significant features of the curves are (1) the maximum of the ECC in EMIBF4 becomes flat gradually with increasing addition of OMIBF4 and (2) lowering of the value of γ with potential on the cathodic side (i.e., -1.5 to -0.5 V) of the ECC gets retarded with the addition of OMIBF4 in EMIBF4. In accordance with the previous studies,32-34 the sharpness of the ECC, in general, depends on the extent of adsorption of electrolytes. The greater the extent of adsorption of the ions, the more the curve becomes sharper. The ECC in EMIBF4 is sharper than that in OMIBF4 because of the comparatively greater extent of adsorption of the charged moieties of the ions on Hg in EMIBF4. On the other hand, the broad maximum of the ECC in OMIBF4 has been shown to be the consequence of preferential adsorption of the alkyl group on the Hg surface, heterogeneity, and the solid-like crystallinity of the RTIL.13 Long-chain alkyl groups are capable of forming a hydrophobic film around the Hg drop even in low concentration due to their preferential interaction with the hydrophobic Hg surface.37,38 Thus, the addition of a little amount of OMIBF4 in EMIBF4 results in lowering the γ around the ECM, whereas with increasing the addition of OMIBF4, the curves continue to become broader, indicating that the extent of interaction of the charged moieties with the Hg electrode surface is decreasing. For instance, the values of γ at -0.25 V in EMIBF4 containing 0, 2, 6, and 15% (v/v) OMIBF4 and in OMIBF4 are 368, 363, 359, 355 and 346 dyne cm-1, respectively. The only difference in the structures of the ions of the studied RTILs is the length of the alkyl group, that is, octyl in OMIBF4 and ethyl in EMIBF4. The side-chain length of the ions has been known to significantly influence the physical and chemical properties of the ILs, especially liquid crystal formation.39-42 In ILs, the charged moieties of the cation and anion distribute homogeneously because of the strong electrostatic interactions and the ease of forming a hydrogen bond, whereas the neutral tail groups tend to aggregate due to the collective short-range interaction. When the side chain of the cation is long enough, the domain formation of alkyl groups results in a liquid-crystal-like structure and greatly influences the properties of the ILs. Therefore, the increased addition of OMIBF4 increases the heterogeneity and solid-like crystallinity in the mixtures, which is also believed to retard the charged moieties from being in contact with the electrode surface. Retardation of the lowering of the γ on the cathodic side (i.e., -1.5 to - 0.5 V) of the ECC with the addition of OMIBF4 in EMIBF4 is also another sign of the preferential adsorption of the alkyl group on the Hg surface. Increased addition of OMIBF4 increases the percentage of alkyl groups around the Hg surface, which, in turn, decreases the chance of interaction of the positively charged imidazolium ring with the Hg surface, responsible for the relatively higher value of γ on the cathodic side of the mixtures as compared to that in EMIBF4. Figure 3 illustrates the surface charge density (σM) versus potential curves derived from Figure 2. The σM on both sides of the ECM continues to decrease with increasing the addition of OMIBF4 in EMIBF4. This phenomenon is in agreement with

Electrical Double Layer in Mixtures of Ionic Liquids

Figure 2. Interfacial tension versus potential curves measured at the Hg electrode in different percent ratios (v/v) of EMIBF4 and OMIBF4 under N2 atmosphere: (9) 100% EMIBF4, (O) 2% OMIBF4 + 98% EMIBF4, (2) 6% OMIBF4 + 94% EMIBF4, (0) 15% OMIBF4 + 85% EMIBF4, and (b) 100% OMIBF4.

Figure 3. Surface charge density versus potential curves measured at the Hg electrode in different percent ratios (v/v) of EMIBF4 and OMIBF4 under N2 atmosphere: (9) 100% EMIBF4, (O) 6% OMIBF4 + 94% EMIBF4, (2) 15% OMIBF4 + 85% EMIBF4, and (0) 100% OMIBF4.

the previous reasoning; that is, increased addition of OMIBF4 increases the heterogeneity and solid-like crystallinity in the mixtures, and the preferential adsorption of the alkyl group (octyl) of OMIBF4 retards the charged moieties from being in contact with the electrode surface. Consequently, σM on both sides of the ECM decreases, and the region of zero charge density continues to become broader. Figure 4 represents the capacitance (C) versus potential curves measured at the Hg electrode in different percent ratios (v/v) of EMIBF4 and OMIBF4. C at Hg/EMIBF4 diminishes gradually with increasing the addition of OMIBF4. For instance, the values of C at -0.4 V in EMIBF4 containing 0, 2, 6, 15, and 40% (v/v) OMIBF4 are 19.1, 16.5, 14.9, 12.9, and 11.2 µF cm-2, respectively. The hump at the Hg/EMIBF4 interface disappears with the addition of OMIBF4 at a concentration of more than 2% (v/v), and a new hump appears at ca. -0.55 V in 40% OMIBF4. In our previous study, we confirmed the relationship between the hump at -0.55 V and the adsorption of the alkyl group.14 Thus, the appearance of the hump with increasing the addition of OMIBF4 in EMIBF4 is an indication of the preferential adsorption of the alkyl group on the Hg surface. Moreover, disappearance of the hump related to the orientationdependent adsorption of the imidazolium ring at Hg at -0.34 V is a sign of the lesser extent of interaction of the imidazolium

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Figure 4. Capacitance versus potential curves measured at the Hg electrode in different percent ratios (v/v) of EMIBF4 and OMIBF4 under N2 atmosphere: (9) 100% EMIBF4, (O) 1% OMIBF4 + 99% EMIBF4, (2) 2% OMIBF4 + 98% EMIBF4, (∆) 6% OMIBF4 + 94% EMIBF4, (b) 15% OMIBF4 + 85% EMIBF4, (0) 40% OMIBF4 + 60% EMIBF4, and (*) 100% OMIBF4.

Figure 5. Schematic illustration of the Hg interface at the PZC in a mixture of EMIBF4 and OMIBF4, where the lines (|||||||||) and (------) represent the electrostatic interaction and hydrogen bonding, respectively.

ring with the Hg surface in the presence of OMIBF4.14 The same reasoning given above for justifying the cause of the change of γ and σM with different percent ratios of EMIBF4 and OMIBF4 is also applicable for the change of the C with potential. None of the C-E curves have the maximum on the anodic side of the PZC as per the theoretical expectation24 for the IL with larger cations. It could be that the small potential limit (due to the dissolution of Hg32) in the positive direction does not cover the range where the maximum can be observed. The interfacial structure at the Hg electrode at the PZC in a mixture of EMIBF4 and OMIBF4 is depicted schematically in Figure 5. The hydrophobic alkyl groups in the EMIBF4 and OMIBF4 mixture are shown to be adsorbed preferentially on the Hg electrode surface, and the charged moieties reside away from the electrode surface due to the ease of forming hydrogen bonds and the electrostatic interaction. As the heterogeneity in EMIBF4 is smaller and the length of the alkyl group is shorter as compared to OMIBF4, the charged moieties can interact with the electrode surface with a small change of their orientation19-21 in response to the applied potential, and consequently, the values of γ, σM, and C in EMIBF4 are found to be higher. However,

6600 J. Phys. Chem. C, Vol. 113, No. 16, 2009 increasing the addition of OMIBF4 in EMIBF4 increases the heterogeneity and solid-like crystallinity in the mixtures, and the preferential adsorption of the more hydrophobic alkyl group (i.e., octyl) excluded the ions of EMIBF4 from the electrode surface, increasing the average distance between the electrode surface and the charged moieties; consequently, the values of γ, σM, and C continue to decrease. Thus, interfacial structures in mixtures of EMIBF4 and OMIBF4 preserve some intermediate situation of both the RTILs. Moreover, the broadening of the region of the ECM (Figure 2) and zero charge density (Figure 3), as well as the appearance of the hump related to the adsorption of the alkyl group on the Hg electrode surface (Figure 4) with increasing the addition of OMIBF4 in EMIBF4, confirm the explanation given previously13 about the structure of the EDL at the Hg electrode in OMIBF4 and HMIBF4; that is, the preferential adsorption of the larger alkyl group on the Hg surface, heterogeneity, and solid-like crystallinity of OMIBF4 and HMIBF4 impede the charge moieties from being in contact with the Hg electrode surface and are responsible for the broadness of the ECM. Cause of the Appearance of the PZC at the Minimum of the C-E Curve at the Hg Electrode. Theoretically, it has been shown that, for the ILs with symmetric ion sizes, the C-E curve should be bell-shaped and the PZC can be obtained from the maximum of the C-E curve, whereas for the ILs with asymmetric ion sizes, it may shift to the positive or negative direction of potential depending on whether cations are smaller or larger than anions, respectively.22-25 A theoretically expected bell-shaped C-E curve has recently been observed by our group at the Pt electrode in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide with ions of almost similar size.11 However, in this study and others,12-15,17 the potential corresponding to the minimum (or near the minimum) of the C-E curve has been found to be associated with the maximum of the ECC, that is, the PZC. The maximum of ECCs measured at the Hg electrode in EMIBF4, EMI[N(Tf)2], and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate was found to correspond to the minimum of the respective C-E curve.14 The minimum of the C-E curves measured at Hg in HMIBF4 and OMIBF4 also appears within the broad region of the ECM.13 Now, the question is why does the PZC at the Hg electrode appear at the minimum or close to the minimum of the C-E curve? Among probable reasons, for example, different extent of heterogeneity, solid-like crystallinity of the ILs, and adsorption of ions or aggregates, the specific adsorption of the alkyl group on the Hg surface is considered as the main reason. As alkyl groups are electrically neutral and hydrophobic in nature, they prefer to be adsorbed on the Hg electrode surface at the PZC. At the PZC, the charge on the electrode surface is zero, and thus, the charged moieties of the ions do not feel the attraction to interact with the Hg electrode surface, giving some room for the alkyl group to interact with the Hg surface. This adsorption of the alkyl group on the Hg surface decreases C by both lowering the value of ε in the EDL and increasing the effective thickness of the EDL. The fact that the alkyl groups are preferentially adsorbed perpendicularly on the Hg surface allows the EDL to maintain the maximum possible thickness at the PZC. Thus, PZCs at the Hg electrode in RTILs appear at the minimum or close to the minimum of the C-E curves as most of the RTILs examined previously contain alkyl groups of different chain length. The thickness decreases when the potential is changed on both sides of the PZC (i.e., the compressibility of the RTILs is increased) as the ions change

Alam et al. their orientation to cope with the applied potential, which, in turn, results in the generation of a U-shaped C-E curve near the PZC (Figure 4).14 Conclusions The C-E curves and ECCs were measured at the Hg electrode in mixtures of EMIBF4 and OMIBF4 with the aim of understanding the interfacial structure. The γ, σM, and C decrease with increasing the addition of OMIBF4 in EMIBF4, which has been found to be the aftereffect of the preferential adsorption of the octyl group on the Hg surface and the increased extent of the heterogeneity and solid-like crystallinity of the mixtures. The appearance of the maximum in the C-E curve corresponding to the adsorption of the alkyl group on the Hg surface and the broadening of the ECM with increasing the addition of OMIBF4 in EMIBF4 support the above reasoning. ECCs measured in this and some other studies in different RTILs dictate the minimum of the C-E curves as the PZC at the Hg electrode, and the preferential adsorption of the alkyl groups has been discussed to be the probable cause. Adsorption of alkyl groups on the Hg surface decreases C by both decreasing the value of ε in the EDL and increasing the effective thickness of the EDL, and consequently, the PZC appears at the minimum of the C-E curve. A model of the EDL in the mixture of the EMIBF4 and OMIBF4 has also been proposed. Acknowledgment. The present work was financially supported by a Grant-in-Aid for Scientific Research (A) (Grant 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) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Martins, M. A. P.; Frizzo, C. P.; Moreira, D. N.; Zanatta, N.; Bonacorso, H. G. Chem. ReV. 2008, 108, 2015. (3) Rantwijk, F. V.; Sheldon, R. A. Chem. ReV. 2007, 107, 2757. (4) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567. (5) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103. (6) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833. (7) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Kihira, N.; Watanabe, M.; Terada, N. J. Phys. Chem. B 2006, 110, 10228. (8) Souza, R. F. D.; Padilha, J. C.; Goncalves, R. S.; Dupont, J. Electrochem. Commun. 2003, 5, 728. (9) Zhu, Q.; Song, Y.; Zhu, X.; Wang, X. J. Electroanal. Chem. 2007, 601, 229. (10) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Gratzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 13775. (11) Islam, M. M.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16568. (12) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16600. (13) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 2601. (14) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2007, 111, 18326. (15) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. Electrochem. Commun. 2007, 9, 2370. (16) Lockett, V.; Sedev, R.; Ralston, J.; Horne, M.; Rodopoulos, T. J. Phys. Chem. C 2008, 112, 7486. (17) Silva, F.; Gomes, C.; Figueiredo, M.; Costa, R.; Martins, A.; Pereira, C. M. J. Electroanal. Chem. 2008, 622, 153. (18) Nanjundiah, C.; McDevitt, S. F.; Koch, V. R. J. Electrochem. Soc. 1997, 144, 3392. (19) Aliaga, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 18481. (20) Baldelli, S. J. Phys. Chem. B 2005, 109, 13049. (21) Rivera-Rubero, S.; Baldelli, S. J. Phys. Chem. B 2004, 108, 15133.

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