A comparison of the ultraslow relaxation processes at the ionic liquid|water interface for three hydrophobic ionic liquids

A comparison of the ultraslow relaxation processes at the ionic liquid|water interface for three hydrophobic ionic liquids

Electrochemistry Communications 12 (2010) 1479–1482 Contents lists available at ScienceDirect Electrochemistry Communications j o u r n a l h o m e ...

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Electrochemistry Communications 12 (2010) 1479–1482

Contents lists available at ScienceDirect

Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m

A comparison of the ultraslow relaxation processes at the ionic liquid|water interface for three hydrophobic ionic liquids Yukinori Yasui, Yuki Kitazumi, Hiroyuki Mizunuma, Naoya Nishi, Takashi Kakiuchi ⁎ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

a r t i c l e

i n f o

Article history: Received 5 August 2010 Received in revised form 10 August 2010 Accepted 10 August 2010 Available online 15 August 2010 Keywords: Liquid|liquid interface Ionic liquid|water interface Electrical double layer Ionic liquid Relaxation Ultraslow relaxation Electrocapillarity Electrocapillary curves Trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide Tetradecyltrihexylphosphonium bis(nonafluorobutanesulfonyl)amide Tetradecyltrihexylphosphonium tetrakis(pentafluorophenyl)borate

a b s t r a c t The ultraslow relaxation, on the order of a few seconds or longer, of the structure of the electrical double layer in response to the change in the phase-boundary potential across the ionic liquid (IL)|water(W) interface, which was recently reported for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been confirmed in two new hydrophobic ionic liquids, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. A comparison of the degree of the hysteresis in electrocapillary curves for these ILs with those for trioctylmethylammonium bis (nonafluorobutanesulfonyl)amide demonstrates that the degree of the hysteresis is not correlated with the viscosity of these ILs. The ultraslow relaxation of the electrical double layer seems to be a general feature of ILs at electrified interfaces. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The relaxation of the ionic liquid in the electrical double layer against the modulation of the phase-boundary potential is very slow at the electrified interfaces between the ionic liquid (IL) and water (W) [1,2] and between the IL and metal [3,4]. In the case of a hydrophobic IL, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide ([TOMA+][C4C4N−]), in contact with an aqueous lithium chloride solution, the relaxation transients of the interfacial tension and the charging current are fitted by a double exponential model to give the relaxation times of a few seconds and a minute or longer [2]. Such a slow rearrangement of the double layer structure should have a considerable influence on electrochemical measurements of IL-involved electrochemical interfaces, e.g., voltammetry of charge transfer at the IL|water and IL|metal interfaces [5–9], and also on the performance of electrochemical devices, e.g., the rate of charge– discharge processes of electrical double layer capacitors [10–12] or

that of lithium ion batteries using an IL as the electrolyte in the electrochemical cell [12,13]. So far, we have reported the ultraslow relaxation at the IL|W interface only for [TOMA+][C4C4N−] [1,2,5,14]. Given the possibly penetrating impact of the ultraslow relaxation to electrochemistry of ILs, on the one hand, and the wide and strong interests in fundamentals and applications of ILs, on the other, it is important to examine the generality of the ultraslow relaxation. In this communication, we describe the electrocapillarity at the IL|W interface for two hydrophobic ILs, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide ([THTDP+][C4C4N−]) and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate ([THTDP+][PFPB−])), to demonstrate that the hysteresis is common to these ILs but there is no correlation between the degree of hysteresis and the viscosity of the ILs. 2. Experimental 2.1. Materials

⁎ Corresponding author. E-mail address: [email protected] (T. Kakiuchi). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.011

The methods of the preparation and purification of [TOMA+] [C4C4N−] have been described elsewhere [1]. [THTDP+][C4C4N−] was

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prepared from trihexyltetradecylphosphonium chloride (THTDPCl, Fluka, 95%) and hydrogen bis(nonafluorobutanesulfonyl)amide (HC4C4N, Wako Pure Chem.). [THTDP+][PFPB−] was prepared from THTDPCl and lithium tetrakis(pentafluorophenyl)borate (LiPFPB). LiPFPB was synthesized from bromopentafluorobenzene(99%, Tokyo Chem. Ind.), butyllithium (1.6 mol dm− 3 solution in hexanes, Aldrich), and borontrichloride (1.0 mol dm− 3 solution in hexanes, Aldrich) [15]. Other chemicals used were of reagent grade. 2.2. Measurements of density and viscosity The density of water-saturated [THTDP+][C4C4N−] was measured with an Ostwald-type pycnometer to be 1.218 g cm− 3 at 25.0 °C. The density of [THTDP+][PFPB−] at 50.0 °C was measured with a density/ specific gravity meter (Kyoto Electronics, DA-520) to be 1.226 g cm− 3. The viscosity values of water-saturated ILs measured with a viscometer (Haake, 550) were 6.2 × 102 mPa s for [THTDP+][C4C4N−] at 25.0 °C and 1.4 × 103 mPa s for [THTDP+][PFPB−] at 50.0 °C. 2.3. Electrocapillary curve measurements The electrochemical cell used to control the potential drop across the IL|W interface is represented by,

Ag = AgCl

j

0:05 mol dm3 LiCl + 3

0:05 mol dm LiC4 C4 N ðW2Þ

j



j

j

 3 THTDP þ ½C4 C4 N   0:1 mol dm LiCl Ag = AgCl ðILÞ

ðW1Þ

ð1Þ and

Ag = AgCl

j

0:1 mol dm3 LiCl + 3

0:01 mol dm LiPFPB ðW2Þ

j



THTDP

þ

j

3

j

½PFPB  0:1 mol dm LiCl Ag = AgCl

ðILÞ

ðW1Þ

ð2Þ The cell potential, i.e., the potential of the right terminal referred to that of the left will hereafter be denoted as E. A positive feedback method was employed to compensate the iR drop that remained uncompensated by the four electrode potentiostat (Hokuto Denko, HA1010mA1A). The interfacial tension at the IL|W interface was measured using a pendant drop method. The temperature for the electrocapillarity measurements was 25 °C for [TOMA+][C4C4N−] and [THTDP+][C4C4N−], but was 50 °C for [THTDP+][PFPB−], as the latter IL was highly viscous at 25 °C. The details of drop imaging and the image data analysis to obtain γ values have been described elsewhere [16]. The acquisition time of one frame of a drop image was 5 ms and the sampling interval of the drop images was typically 1.25 for potential-scanning and 1.0 s for potential-step measurements.

Fig. 1. Comparison of electrocapillary curves at IL|W interface recorded at v = 4 mV s− 1. IL: [THTDP+][PFPB−] at 50 °C (■, □); [THTDP+][C4C4N−] at 25.0 °C (▲, Δ); [TOMA+] [C4C4N−] at 25.0 °C (●, ○). Filled symbols: forward scan, open symbols: reverse scan.

the negative direction. Obviously, the largest hysteresis is seen at the [TOMA+][C4C4N−]|0.1 mol dm− 3 LiCl interface and the smallest is at the [THTDP+][PFPB−]|0.1 mol dm− 3 LiCl interface. The equilibrium electrocapillary curves recorded by the potential-step method [2,14] for the same systems were located between the corresponding pair of curves for forward and reverse scans for all three cases (data not shown). A convenient measure of the degree of hysteresis is the difference in the potentials of electrocapillary maxima recorded in the forward and reverse scans, ΔEp . The ΔEp values were 45, 25 and 5 mV for [TOMA+] [C4C4N−], [THTDP+][C4C4N−], and [THTDP+][PFPB−], respectively. Clearly, the magnitude of ΔEp is not directly correlated with the magnitude of the viscosity; 2.0 × 103 ([TOMA+][C4C4N−]) [2], 6.2 × 102 ([THTDP+][C4C4N−]), and 1.4 × 103 ([THTDP+][PFPB−], 50 °C) mPa s. The ultraslow relaxation is therefore not caused simply by the diffusive motion of ions in the electrical double layer in the IL phase. In fact, in an IL having 1 Pa s of viscosity, the diffusion layer thickness of an ion at a relaxation time of 10 s is on the order of μm, which would well exceeds the double layer thickness. A collective motion of IL-constituent cations and anions in response to the change in the electric field across the double layer composed of at least a few ion layers [17–19] should presumably be accompanied with the optimization of three-dimensional ordering of ions in a collective manner and, furthermore, the optimized configuration of the nonpolar parts of the IL-constituent ions.

3. Results and discussion 3.1. Hysteresis in electrocapillary curves The electrocapillary curves at the IL|W interface acquired with linear scanning of the applied voltage showed hysteresis in the forward and reverse scans for both [THTDP+][C4C4N−] and [THTDP+] [PFPB−] as the IL phase. To compare the degree of hysteresis at these interfaces with that at the [TOMA+][C4C4N−]|0.1 mol dm− 3 LiCl interface [1], three electrocapillary curves recorded at the scan rate, v, of 4 mV s− 1 are displayed in Fig. 1. The forward scan of E was made from the positive end of E to

Fig. 2. Potential-step transients of the interfacial tension (a) and charging current (b) at the [THTDP+][C4C4N−]|0.1 mol dm− 3 LiCl interface at 25 °C when E was stepped from − 350 to − 460 mV.

Y. Yasui et al. / Electrochemistry Communications 12 (2010) 1479–1482 Table 1 Time required for 90% relaxation in double potential-step transients of interfacial tension at the [THTDP+][C4C4N−]|0.1 mol dm− 3 LiCl at 25.0 °C.

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Table 2 Time required for 90% relaxation in double potential-step transients of interfacial tension at the [TOMA+][C4C4N−]|0.1 mol dm− 3 LiCl at 25.0 °C.

E1 , E3

E2

t190

t290

E1, E3

E2

t190

t290

mV

mV

s

s

mV

mV

s

s

− 350 − 350 − 350 − 350 − 350

− 240 − 280 − 440 − 460 − 490

12 15 9 10 6

5 4 3 3 3

− 330 − 330 − 330 − 330 − 330

− 210 − 220 − 400 − 420 − 430

28 43 24 24 35

19 17 9 4 7

3.2. Potential-step transients Another quantitative measure of the degree of hysteresis was obtained by potential-step transients of the interfacial tension and charging current. Fig. 2 shows the potential-step transients of the interfacial tension and the charging current at the [THTDP+][C4C4N−]| 0.1 mol dm− 3 LiCl interface when E was stepped from the potential of zero charge, −350 mV, to −460 mV. The time scale of the relaxation appeared in the interfacial tension is similar to that in the charging current. This correspondence is evidence that the ultraslow relaxation is associated with the structural change in the electrical double layer. We recently reported that similar transients at the [TOMA+][C4C4N−]| 0.1 mol dm− 3 LiCl interface were well represented by a double exponential model [2]. Indeed, the interfacial tension transients at the [THTDP+][C4C4N−]|0.1 mol dm− 3 LiCl interface are also well described by a double exponential model (data not shown). A different degree of relaxation time can also be inferred by comparing the times required to relax 90% of the total change, t90, 1 which is illustrated in Fig. 2(a). Table 1 lists t90 values in the interfacial tension transients after the potential was stepped from the pzc to the negative or positive direction of E. After the E was kept at that value for 6 min, E was stepped back to −330 mV. The t90 value for the second step is denoted as t290. The time for 90% relaxation for the first 1 step, t90 , is definitely longer than that for the second step, t290. This trend is seen also at the [TOMA+][C4C4N−]|0.1 mol dm− 3 LiCl interface [2] (see also Table 2). Because this trend is seen in both positive and negative steps, it appears that the transition from the pzc, where no excess surface charge in the double layer, to an ordered state to bring in an excess surface charge density to form the electrical double layer needs more time than the opposite. Another possibility cannot be excluded, that is, after the first step, the structure of the double layer is not completely equilibrated in 6 min and the transition to the next step would be easier and faster. We also reevaluate t190 and t290 for [TOMA+][C4C4N−]|0.1 mol dm− 3 LiCl interface from the interfacial tension transients [2] and listed in Table 2. It is clearly seen that the relaxation is about three times faster in [THTDP+][C4C4N−] than in [TOMA+][C4C4N−]. It is interesting that [THTDP+][C4C4N−] gives a faster relaxation despite a bulkier THTDP+ compared with TOMA+. This difference is presumably related to the different accessibility of the negatively charged C4C4N− to the positively charged center: TOMA+ allows the access of C4C4N− to the close proximity of the charged center, whereas the phosphorus atom in THTDP+ is surrounded by long alkyl groups. The meaningful evaluation of t90 was not possible for [THTDP+][PFPB−] at 50 °C, because of the difficulty of maintaining an accurate positive feedback over a few minutes. But, judging from the ΔEp values (Fig. 1), t90 for [THTDP+][PFPB−] should be one order of magnitude smaller than that for [TOMA+][C4C4N−]. The smallest hysteresis in [THTDP+][PFPB−] suggests the role of the anion symmetry in the relaxation dynamics. 4. Conclusions The ultraslow relaxation of the electrical double layer on the ionic liquid side of the interface is not limited to [TOMA+][C4C4N−], but has

been confirmed in two other hydrophobic ionic liquids, [THTDP+] [C4C4N−] and [THTDP+][PFPB−]. No correlation between the degree of the hysteresis in electrocapillary curves and the viscosity of the ionic liquids suggests that the hysteresis is not simply due to the slow transport of ions in IL but should be ascribed to collective rearrangements of ions inside the electrical double layer, where ions are likely to form multiple layers [17–19]. It seems to be a general feature at a charged interface that the multilayer formation of bulky ions needs a long time, on the order of minutes [20]. The microheterogeneity of the bulk phase in ILs [21–24], whose formation and destruction are slow [25,26], is apparently incompatible with the surface layering and the seamless connection of the two microscopic structures would add additional complexity in the dynamics of an electrical double layer in the ionic liquid at any electrified interfaces. Acknowledgments This research was supported by a Grant-in-Aid for Scientific Research (No. 21245021) from the Ministry of Educations, Sports, Science, and Technology, Japan. Support by the Global COE Program, International Center for Integrated Research and Advanced Education in Materials Science (No. B-09) from the Ministry of Education, Culture, Sports, Science and Technology of Japan is highly appreciated. References [1] Y. Yasui, Y. Kitazumi, R. Ishimatsu, N. Nishi, T. Kakiuchi, J. Phys. Chem. B, 113 (2009) 3273–3276. [2] Y. Yasui, Y. Kitazumi, N. Nishi, T. Kakiuchi, J. Phys. Chem. B, 11 (2010) 2912. [3] V. Lockett, R. Sedev, J. Ralston, M. Horne, T. Rodopoulos, J. Phys. Chem. C, 112 (2008) 7486–7495. [4] M. Drüschler, B. Huber, S. Passerini, B. Roling, J. Phys. Chem. C, 114 (2010) 3614–3617. [5] T. Kakiuchi, Y. Yasui, Y. Kitazumi, N. Nishi, ChemPhysChem, DOI: 10.1002/ cphc.201000314. [6] E.L. Rogers, D.S. Silvester, D.L. Poole, L. Aldous, C. Hardacre, R.G. Compton, J. Phys. Chem. C, 112 (2008) 2729–2735. [7] O. Fontaine, C. Lagrost, J. Ghilane, P. Martin, G. Trippé, C. Fave, J.C. Lacroix, P. Hapiot, H.N. Randriamahazaka, J. Electroanal. Chem., 632 (2009) 88–96. [8] M.A. Vorotyntsev, V.A. Zinovyeva, D.V. Konev, M. Picquet, L. Gaillon, C. Rizzi, J. Phys. Chem. B, 113 (2009) 1085–1099. [9] K.R.J. Lovelock, F.N. Cowling, A.W. Taylor, P. Licence, D.A. Walsh, J. Phys. Chem. B, 114 (2010) 4442–4450. [10] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, G.Q. Wang, Y.S. Yang, J. Power Sources, 158 (2006) 773–778. [11] N. Handa, T. Sugimoto, M. Yamagata, M. Kikuta, M. Kono, M. Ishikawa, J. Power Sources, 185 (2008) 1585–1588. [12] P.A. Johns, M.R. Roberts, Y. Wakizaka, J.H. Sanders, J.R. Owen, Electrochem. Commun., 11 (2009) 2089–2092. [13] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 162 (2006) 658–662. [14] Y. Yasui, Y. Kitazumi, N. Nishi, T. Kakiuchi, J. Chem. Eng. Data, DOI: 10.1021/ je100607w. [15] A.G. Massey, A.J. Park, J. Organometal. Chem., 2 (1964) 245–250. [16] Y. Kitazumi, T. Kakiuchi, Langmuir, 25 (2009) 8062–8068. [17] 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. [18] M. Mezger, S. Schramm, H. Schröder, H. Reichert, M. Deutsch, E.J. De Souza, J.S. Okasinski, B.M. Ocko, V. Honkimäki, H. Dosch, J. Chem. Phys., 131 (2009) 094701-094701. [19] N. Nishi, Y. Yasui, T. Uruga, H. Tanida, T. Yamada, S. Nakayama, H. Matsuoka, T. Kakiuchi, J. Chem. Phys., 132 (2010) 164705.

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