Potential-induced interfacial restructuring of a pyrrolidinium-based ionic liquid on an Au electrode: Effect of polarization of constituent ions

Electrochemistry Communications 100 (2019) 117–120

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Potential-induced interfacial restructuring of a pyrrolidinium-based ionic liquid on an Au electrode: Effect of polarization of constituent ions

T

Kenta Motobayashia, , Yuhei Shibamuraa, Katsuyoshi Ikedaa,b ⁎

a b

Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan Frontier Research Institute for Materials Science (FRIMS), Nagoya Institute of Technology, Nagoya 466-8555, Japan

ARTICLE INFO

ABSTRACT

Keywords: Spectroelectrochemistry Surface-enhanced infrared absorption spectroscopy Ionic liquid/electrode interfaces Electrical double layer Charge delocalization effect Electronic polarization effect

Potential-induced restructuring of 1-propyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide [C3mpyr][TFSA] on an Au electrode has been investigated by using surface-enhanced infrared absorption spectroscopy (SEIRAS). Cation-to-anion exchange in the double layer occurred near the potential of zero charge (pzc) whereas anion-to-cation exchange required much more negative potentials from pzc. This asymmetric behavior was clearly different from those of previously reported imidazolium-based RTILs, which highlighted the effect of charge delocalization and electronic polarization of individual ions on the stability of the multilayered interfacial structure of RTILs.

1. Introduction Room temperature ionic liquids (RTILs), liquid salts at ambient temperature composed entirely of cations and anions, have received significant interest as promising electrolyte materials for electrochemical energy devices such as batteries and supercapacitors. Their fascinating properties, including high thermal stability, wide electrochemical windows, and high ionic conductivity, are expected to improve the performance and safety of electrochemical devices [1–3]. The RTIL/electrode interface, where electrochemical reactions occur, plays crucial roles in such applications, and hence molecular level description of the interface is necessary for maximizing the performance of the devices. The Gouy–Chapman–Stern (GCS) model for the electrical double layer at solid/liquid interfaces with dilute-solution approximation is not suitable for describing RTIL/electrode interfaces that are full of ionic species [4–6]. Alternatively, multiply-ordered layers of ions have been illustrated as an interfacial structure [7–10] based upon various interface-selective analysis techniques such as atomic force microscopy (AFM) [11], X-ray reflectometry (XR) [12], surface force apparatus [13,14] and neutron reflectometry [15]. The RTIL multilayers on the electrode have been reported to be so stable [11,16] that electrochemical reactions can be affected, in the same manner with suppression of reactions in conventional electrolyte solutions due to adsorption of molecules or ions. Therefore, static and dynamic structures of the RTIL multilayers have been investigated by using interface-selective vibrational spectroscopies. Pioneering work

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was conducted by Baldelli et al. using sum frequency generation (SFG), observing the potential-dependent reorientation of alkylimidazolium cations on Pt [17–19], where the imidazolium ring orients relatively parallel to the surface at negative potentials and vice versa. Similar trends have been confirmed by infrared reflection absorption spectroscopy (IRAS) [20], surface-enhanced infrared absorption spectroscopy (SEIRAS) [21–23], and surface-enhanced Raman spectroscopy (SERS) [24,25]. Hysteretic ion replacement is another important feature of the interfaces observed by SFG [26,27] and SEIRAS [22,23,28,29] for various RTILs. This indicated the existence of an activation barrier for ion exchange between the first ionic layer and the overlayers owing to the Madelung-like potential stabilizing the RTIL multilayers. Such interpretation also explains the hysteretic trends in XR [30,31] and electrochemical impedance spectroscopy (EIS) [32]. The interfacial structures and dynamics of RTILs have been understood based on the charged sphere model which successfully simulated differential capacitance curves [5]. What is not considered in this simple model is the electronic polarization of constituent ions, which may affect the ordered structure of RTILs as reported in molecular dynamics (MD) simulation studies of bulk [33,34] and interfaces [35]. This unexamined effect at interfaces can be illuminated experimentally by comparing potential-dependent behavior of aromatic imidazolium cations and non-aromatic pyrrolidinium cations, where delocalized charge due to π orbital leads to pronounced effect of polarization of imidazolium cations against more charge-localized pyrrolidinium ones. We have already reported the behavior of imidazolium-based RTILs

Corresponding author. E-mail address: [email protected] (K. Motobayashi).

https://doi.org/10.1016/j.elecom.2019.02.003 Received 1 December 2018; Received in revised form 26 January 2019; Accepted 4 February 2019 Available online 05 February 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Electrochemistry Communications 100 (2019) 117–120

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Fig. 1. (a) CVs of [C3mpyr][TFSA]/Au at scan rate of 100 mV/s at different potential ranges. (b) The surface charge of an Au electrode as a function of potential in [C3mpyr] [TFSA] measured by the electrode immersion method. The red line is the result of least square fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

during a potential scan from −3.1 to 0.9 V vs Fc/Fc+ where no Faradaic current was observed (Fig. 2). The spectrum collected at −3.1 V before the potential scan was used as the reference to compute the potential difference spectra. Since the SEIRA effect is significant at the surface and sharply fades away within a few monolayer distance from the surface [37], the observed potential difference spectra are ascribed mostly to the first ionic layer in direct contact with the electrode surface and to the overlayers to some extent [37–39]. Positive and negative bands indicate the increase and decrease, respectively, in absorption intensities with respect to the reference spectrum recorded at −3.1 V. Each band in the potential difference spectra (except for that observed at 2847 cm−1) can find its counterpart in the IR spectrum of the bulk [C3mpyr][TFSA] measured with the ATR configuration without the Au film (bottom-most spectra in Fig. 2). Detailed assignments are described in Supplementary Information (Table S1). The significant blue-shift of the CeF stretch modes ν(CF3) in the SEIRA spectra compared with the bulk spectrum originates from the large dispersion of the refractive index of [C3mpyr][TFSA] in this spectral range [40,41], which has been reported for various RTILs containing [TFSA] anion [22,23,29]. Integrated SEIRAS band intensities taken from a series of spectra acquired during a potential cycle are plotted against E in Fig. 3. On the positive-going scan starting from −3.1 V, the intensity of the symmetric CeH stretch modes of the methylene group in the alkyl chain νs(CH2)alkyl (2907 and 2885 cm−1) decreases with potential up to −0.3 V and more steeply at more positive potentials. On the reverse negative going scan, the intensity gradually increases down to −1.7 V and more steeply at more negative potentials to yield a typical hysteresis curve as previously observed for various RTILs [22,23,29]. Similar hysteretic potential-dependent behavior was also observed for the intensity of the asymmetric CeH stretch modes of the alkyl chain

[22,23], and thus, observation of the pyrrolidinium-based RTIL/electrode interface elucidates the effect of polarization of ions on stability of the interfacial structures. Here we report the potential-induced restructuring of 1-propyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)amide [C3mpyr] [TFSA] on an Au electrode surface studied by using SEIRAS, one of the most powerful surface-selective vibrational spectroscopies. In-situ SEIRAS measurements in the spectral range of 3500–1000 cm−1 have allowed us to probe the behaviors of both cations and anions, which showed hysteretic potential dependence. We found that cation-to-anion replacement in the first ionic layer initiates near the potential of zero charge (pzc) during a positive-going potential scan whereas reverse anion-to-cation replacement occurs at much more negative potentials from pzc. This asymmetric behavior which was not observed for imidazolium-based RTILs can be explained by the effect of electronic polarization of ions that can modify the stability of the multi-layered interfacial structure. 2. Results and discussion A CV for an Au electrode in [C3mpyr][TFSA], red line in Fig. 1a, shows double layer region from E = −3.2 to 1.0 V without any Faradaic peak. For wider potential scan, black line in Fig. 1a, shows broad cathodic and anodic peaks at E = −2.0 V and 0.5 V due to the reduction and oxidation of decomposition products of [C3mpyr][TFSA] generated at much positive and negative potentials, respectively. Fig. 1b shows the surface charge q of an Au electrode in [C3mpyr] [TFSA] as a function of potential measured by the electrode immersion method [23,36]. The linear relation between q and E was obtained, which enables us to determine the pzc as −0.12 V vs Fc/Fc+. SEIRA spectra of the [C3mpyr][TFSA]/Au interface were collected

Fig. 2. SEIRA spectra of [C3mpyr][TFSA]/Au at (a) 3080–2780 cm−1 and (b) 1400–1000 cm−1 recorded at various electrode potentials during the positive-going scan at 2 mV/s. The reference spectrum was collected at −3.1 V vs Fc/Fc+. The bottom most spectra are ATR-IR spectra of [C3mpyr][TFSA] reflecting the vibrational features of the bulk.

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previously reported ion exchange behavior: pzc was in-between the threshold potentials for cation-to-anion and anion-to-cation replacement, indicating the requirement of ΔVex for both directions of ion replacement, as observed for [C4mim][TFSA], [C8mim][TFSA], [C4mim][NFSA], and [TOMA][NFSA] [23,29] (TOMA: trioctylmethylammonium, NFSA: bis(nonafluorobutanesulfonyl)amide). ΔVex for anion-to-cation replacement from pzc is much higher for [C3mpyr] [TFSA] than imidazolium-based RTILs. Similar asymmetric ΔVex were observed only for [TBMA][TFSA] among various previous experiments [29] (TBMA: tributylmethylammonium). The origin of hysteretic potential dependence of SEIRA band intensities of RTILs has been discussed and attributed to the activation barrier for the ion replacement in the first ionic layer [22,23,26,27,30,31]. Existence of such activation barrier was visualized by MD simulation calculating a potential energy surface for a point charge along the direction perpendicular to the RTIL/electrode interface [44]. Even on the neutrally charged electrode surface, a doublewell shaped potential energy surface was calculated, and an activation barrier can be found between two local potential minima corresponding to the first and second ionic layers. As demonstrated in the calculation, the activation barrier for ion replacement in the first layer has been understood in terms of the stability of the multi-layered interfacial structure of RTILs [11,12]. Ions in the first layer are stabilized by the charged electrode and also ionic overlayers forming Madelung-like potential. Asymmetric ΔVex of [C3mpyr][TFSA] which is different from those previously observed in imidazolium-based RTILs [23] can be understood in terms of the spatial distribution of charge within ions and effect of electronic polarization of ions on stability of the multi-layered interfacial structures. For the more charge-delocalized cations, such as imidazolium, the spatial distribution of charge can be more drastically modified when polarization is induced by applied electrode potential. The modification of the charge distribution can enhance the screening ability of the ion, which enables effective compensation of the electrode surface charge without changing ion-electrode distance. This can be the reason why pyrrolidinium cation which have less delocalized charge in the present study desorbed at less positive potentials compared with imidazolium-type cations (delocalization of charge of cations are indicated by calculated dipoles and electrostatic potential maps in the Supplementary Information). The effect of polarization of constituent ions of RTILs on their structures has already been discussed through MD simulation studies. Bulk structures of RTILs were calculated based on the models including and omitting polarization and compared each other [33,34]. The results suggested that polarization changes the electrostatic screening condition of ions, leading to relaxation of the long-range structural correlation (within 4 nm) and enhancement of short-range structuring (within second neighboring ion). The distances from next or second neighboring ions (cation-anion and cation-cation distances) were also reported to be shortened. MD simulation study on effects of polarization of ions at the interface [35] also reported similar results: enhanced structuring near the electrode and relaxation of periodicity at distant part from the electrode. These simulation results partly support the above idea of the origin of different potential-dependent behavior of pyrrolidinium and imidazolium-based RTILs. In line with these insights, asymmetric ΔVex of [C3mpyr][TFSA] can be explained by poorer polarization effect of pyrrolidinium cation with more localized charge around N atom than TFSA. For anion-to-cation replacement, polarization effect of TFSA anions in the first layer enhances the stability of the short-range layered structure (with the anionic first layer), which leads to high ΔVex. For cation-to-anion replacement, on the other hand, polarization of TFSA anions in overlayers relaxes the long-range charge ordering of the multi-layered interfacial structure, which reduces ΔVex. This would be the origin of asymmetric ΔVex. As for the imidazolium-based RTILs, high polarization effect of imidazolium cations due to delocalized π orbitals balances the effect of

Fig. 3. SEIRA band intensities as a function of electrode potential recorded during the potential scan at 2 mV/s. νs(CH2)alkyl, νas(CH2)alkyl, νs(CH2)pyr, and ν(CF3) represent the total intensities of the bands at 2907 and 2885; 2970; 2944; 1234 and 1219 cm−1, respectively. The data points affected by the overlap of unassigned bands are indicated by open circles.

νas(CH2)alkyl (2970 cm−1) and symmetric CeH stretch mode of the pyrrolidinium ring νs(CH2)pyr (2945 cm−1) (except for overlaps of an additional unassigned band at 2930 cm−1 found only on the negativegoing scan from 0.9 to −1.0 V as shown in Fig. S1). Intensity of CeF stretch modes ν(CF3) of the anion shows opposite potential dependence: increasing gradually up to −0.3 V and steeply at more positive potentials, and then decreasing gradually down to −1.7 V and steeply at more negative potentials on the reverse scan. In analogy with the results of imidazolium-based RTILs [22,23], gradual and steep slopes of SEIRAS band intensities as a function of potential are interpreted as the changes in the local concentration of the ions in the overlayers and those in the first layer, respectively. Opposite potential dependence of absorption bands for the cation and anion indicates the cation-anion replacement at the first layer or overlayers. It should be noted that the SEIRA intensity is a function not only of the local concentration but also of the orientation of the molecule of interest due to the surface selection rule in SEIRAS (vibrations that yields oscillating dipoles perpendicular to the surface are selectively observable [42]). It enabled us to detect the potential-dependent reorientation of imidazolium cations in various RTILs which showed opposite potential-dependent changes in intensities of CeH stretch modes of imidazolium rings and alkyl chains [22,23]. For [C3mpyr] [TFSA], in contrast, all the ν(CH) of the alkyl chain and pyrrolidinium ring show similar potential-dependent behavior (Fig. 3), whereas νs(CH2)pyr has oscillating dipole parallel to the pyrrolidinium ring. This result indicates that the reorientation of pyrrolidinium ring expected in MD simulation (from a flat orientation to a more vertical one at more positive potentials [43]) was not observed in the present SEIRAS study. A clear difference between [C3mpyr][TFSA] and previously studied 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][TFSA]) can be found in the threshold potentials for the exchange of the first ionic layer relative to pzc. In the present result, threshold potential for the cation-to-anion replacement during the positive potential scan is nearly equal to pzc (Fig. 3), whereas the reverse anion-to-cation replacement starts at the potential much more negative from pzc. The latter means that a certain excess potential from pzc is required for ion replacement, which can be defined as ΔVex. It appears that ΔVex is significant for anion-to-cation replacement but negligible for cation-to-anion replacement. This is in clear contrast to the 119

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polarization of TFSA anions, resulting in rather symmetric ΔVex. A similar scenario can also explain the case of RTILs containing alkylammonium cations with TFSA or NFSA: more charge-localized small alkylammonium such as TBMA results in low polarization effect and asymmetric ΔVex, whereas larger cations with longer alkyl chains such as TOMA results in higher orientational polarizability that leads to symmetric ΔVex [29]. One might expect the effect of specific adsorption of constituent ions on hysteretic behavior of the interface. MD simulation of [C4mpyr] [TFSA] on Au(111) in the literature [45] revealed that TFSA-Au(111) interaction is weaker than C4mpyr-Au(111) interaction. Although detail is still not clear, the effect of specific adsorption on hysteretic potentialdependence of the interface cannot explain our experimental results showing higher activation barrier for anion-to-cation exchange. The effect of aromatic character of RTIL cations on electrochemistry appears rather complicated and controversial. For electrodeposition of Al on Au(111) [46], nanocrystalline and microcrystalline deposits were obtained for pyrrolidinium-based [C4mpyr][TFSA] and imidazoliumbased [C2mim][TFSA], respectively. Underpotential deposition (UPD) of Al and stoichiometric deposition of Ta occurred only for the latter. These phenomena were attributed to stronger cation-electrode Coulombic interaction due to the localized charge of pyrrolidinium cations, which was supported by AFM force curve measurement revealing that greater force is required to rupture the cationic first layer for the pyrrolidinium-based RTIL than imidazolium-based one at the open circuit potential and more negative potentials [47]. As for Co electrodeposition on Pt, on the other hand, sudden drop of diffusion-limiting deposition current at a certain potential was observed during CV measurement for imidazolium-based RTILs, which was attributed to the compact adsorption layer of the imidazolium cations more strongly interacting with the electrode through π interaction [48]. These controversial interpretation of electrochemical behavior can be understood in terms of the electronic polarization of ions which stabilizes short-range structuring and relaxes long-range ordering at the interface. The opposite effects on different domain would result in complex electrochemical behavior. Detailed observation of the correlation between such electrochemical reactions and interfacial restructuring is necessary for fully understanding the mechanism.

Conflict of interest The authors declare no conflicts of interest. Acknowledgment This work was financially supported by JSPS KAKENHI (Grant Number 15K04683 and 18K14188), the Naito Science & Engineering Foundation, and the Hori Sciences & Arts Foundation. We thank Dr. S. Tsuzuki (National Institute of Advanced Industrial Science and Technology) for his kind advices on calculations. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.elecom.2019.02.003. References [1] P. Hapiot, C. Lagrost, Chem. Rev. 108 (2008) 2238–2264. [2] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, Wiley, 2011. [3] Electrochemical properties and applications of ionic liquids, in: A.A.J. Torriero, M.J.A. Shiddiky (Eds.), Nova, 2011. [4] A.A. Kornyshev, J. Phys. Chem. B 111 (2007) 5545–5557. [5] M.V. Fedorov, A.A. Kornyshev, J. Phys. Chem. B 112 (2008) 11868–11872. [6] K.B. Oldham, J. Electroanal. Chem. 613 (2008) 131–138. [7] M.V. Fedorov, A.A. Kornyshev, Chem. Rev. 114 (2014) 2978–3036. [8] A.A. Kornyshev, R. Qiao, J. Phys. Chem. C 118 (2014) 18285–18290. [9] R. Hayes, G.G. Warr, R. Atkin, Chem. Rev. 115 (2015) 6357–6426. [10] K. Dong, X. Liu, H. Dong, X. Zhang, S. Zhang, Chem. Rev. 117 (2017) 6636–6695. [11] R. Hayes, G.G. Warr, R. Atkin, Phys. Chem. Chem. Phys. 12 (2011) 1709–1723. [12] M. Mezger, H. Schröder, H. Reichert, S. Schramm, J.S. Okasinski, S. Schöder, V. Honkimäki, M. Deutsch, B.M. Ocko, J. Ralston, M. Rohwerder, M. Stratmann, H. Dosch, Science 322 (2008) 424–428. [13] K. Ueno, M. Kasuya, M. Watanabe, M. Mizukami, K. Kurihara, Phys. Chem. Chem. Phys. 12 (2010) 4066–4071. [14] S. Perkin, Phys. Chem. Chem. Phys. 14 (2012) 5052–5062. [15] Y. Lauw, M.D. Horne, T. Rodopoulos, V. Lockett, B. Akgun, W.A. Hamilton, A.R.J. Nelson, Langmuir 28 (2012) 7374–7381. [16] Y. Yokota, T. Harada, K.-i. Fukui, Chem. Commun. 46 (2010) 8627–8629. [17] S. Rivera-Rubero, S. Baldelli, J. Phys. Chem. B 108 (2004) 15133–15140. [18] C. Aliaga, S. Baldelli, J. Phys. Chem. B 110 (2006) 18481–18491. [19] S. Baldelli, Acc. Chem. Res. 41 (2008) 421–431. [20] N. Nanbu, Y. Sasaki, F. Kitamura, Electrochem. Commun. 5 (2003) 383–387. [21] N. Nanbu, T. Kato, Y. Sasaki, F. Kitamura, Electrochemistry 73 (2005) 610–613. [22] K. Motobayashi, K. Minami, N. Nishi, T. Sakka, M. Osawa, J. Phys. Chem. Lett. 4 (2013) 3110–3114. [23] K. Motobayashi, N. Nishi, Y. Inoue, K. Minami, T. Sakka, M. Osawa, J. Electroanal. Chem. 800 (2017) 126–133. [24] V.O. Santos, M.B. Alves, M.S. Carvalho, P.A.Z. Suarez, J.C. Rubim, J. Phys. Chem. B 110 (2006) 20379–20385. [25] Y.-X. Yuan, T.-C. Niu, M.-M. Xu, J.-L. Yao, R.-A. Gu, J. Raman Spectrosc. 41 (2009) 516–523. [26] W. Zhou, S. Inoue, T. Iwahashi, K. Kanai, K. Seki, T. Miyamae, D. Kim, Y. Katayama, Y. Ouchi, Electrochem. Commun. 12 (2010) 672–675. [27] W. Zhou, Y. Xu, Y. Ouchi, ECS Trans. 50 (2013) 339–348. [28] K. Motobayashi, M. Osawa, Electrochem. Commun. 65 (2016) 14–17. [29] N. Nishi, K. Minami, K. Motobayashi, M. Osawa, T. Sakka, J. Phys. Chem. C 121 (2017) 1658–1666. [30] A. Uysal, H. Zhou, G. Feng, S.S. Lee, S. Li, P. Fenter, P.T. Cummings, P.F. Fulvio, S. Dai, J.K. McDonough, Y. Gogotsi, J. Phys. Chem. C 118 (2014) 569–574. [31] A. Uysal, H. Zhou, G. Feng, S.S. Lee, S. Li, P.T. Cummings, P.F. Fulvio, S. Dai, J.K. McDonough, Y. Gogotsi, P. Fenter, J. Phys. Condens. Matter 27 (2015) 032101. [32] M. Drüschler, B. Huber, S. Passerini, B. Roling, J. Phys. Chem. C 114 (2010) 3614–3617. [33] D. Bedrov, O. Borodin, Z. Li, G.D. Smith, J. Phys. Chem. B 114 (2010) 4984–4997. [34] J.G. McDaniel, A. Yethiraj, J. Phys. Chem. Lett. 9 (2018) 4765–4770. [35] S. Tazi, M. Salanne, C. Simon, P. Turq, M. Pounds, P.A. Madden, J. Phys. Chem. B 114 (2010) 8453–8459. [36] V. Jendrasic, J. Electroanal. Chem. Interfacial Electrochem. 22 (1969) 157–164. [37] M. Osawa, Bull. Chem. Soc. Jpn. 70 (1997) 2861–2880. [38] E. Johnson, R. Aroca, J. Phys. Chem. 99 (1995) 9325–9330. [39] T. Kamata, A. Kato, J. Umemura, T. Takenaka, Langmuir 3 (1987) 1150–1154. [40] J. Pacansky, C.D. England, R. Waltman, Appl. Spectrosc. 40 (1986) 8–16. [41] T. Buffeteau, J. Grondin, J.-C. Lassègues, Appl. Spectrosc. 64 (2010) 112–119. [42] M. Osawa, K. Ataka, K. Yoshii, Y. Nishikawa, Appl. Spectrosc. 47 (1993) 1497–1502. [43] S. Sharma, H.K. Kashyap, J. Phys. Chem. C 121 (2017) 13202–13210. [44] V. Ivaništšev, M.V. Fedorov, R.M. Lynden-Bell, J. Phys. Chem. C 118 (2014) 5841–5847. [45] R. Wang, S. Bi, V. Presser, G. Feng, Fluid Phase Equilib. 463 (2018) 106–113. [46] F. Endres, O. Höfft, N. Borisenko, L.H. Gasparotto, A. Prowald, R. Al-Salman, T. Carstens, R. Atkin, A. Bund, S. Zein El Abedin, Phys. Chem. Chem. Phys. 12 (2010) 1724–1732. [47] R. Hayes, N. Borisenko, M.K. Tam, P.C. Howlett, F. Endres, R. Atkin, J. Phys. Chem. C 115 (2011) 6855–6863. [48] M. Tułodziecki, J.M. Tarascon, P.L. Taberna, C. Guéry, Electrochim. Acta 134 (2014) 55–66.

3. Conclusions SEIRAS measurements allow us to observe the potential-induced restructuring of [C3mpyr][TFSA] on an Au electrode surface. Potentialdependent reorientation of the pyrrolidinium cation was not observed in contrast to the previous results on imidazolium cations. Hysteretic ion exchange in the first ionic layer were observed as are the cases of various RTILs. The potential of cation-to-anion replacement was found near the pzc, whereas reverse replacement required high ΔVex. Such asymmetric ΔVex are in contrast to more symmetric behavior for imidazolium-based RTILs, which can be explained by the higher effect of electronic polarization of TFSA and imidazolium than pyrrolidinium due to delocalized charges. Polarization of TFSA in overlayers relaxes the long-range charge ordering at the interface resulting in lower ΔVex for cation-to-anion replacement, whereas TFSA in the first layer enhances the short-range structuring that leads to higher ΔVex for anionto-cation replacement. Such an effect would be canceled out by polarization effect of imidazolium cations if contained. The present study indicated that charge delocalization of constituent ions is a decisive property not only for the melting points of RTIL itself but also for the stability of the interfacial structure that may affect electrochemical reactions. For proper design of RTILs as electrolyte materials, charge delocalization should be considered from the view point of both the transport property in the bulk and the electrochemical response at the interface. 120