New hydrophobic ionic liquids based on perfluoroalkyltrifluoroborate anions

Journal of Fluorine Chemistry 125 (2004) 471–476

New hydrophobic ionic liquids based on perfluoroalkyltrifluoroborate anions Zhi-Bin Zhou, Masayuki Takeda, Makoto Ue* Battery Materials Laboratory, Mitsubishi Chemical Group Science and Technology Research Center, Inc., 8-3-1 Chuo, Ami, Inashiki, Ibaraki 300-0332, Japan Received 6 October 2003; received in revised form 30 November 2003; accepted 4 December 2003

Abstract New hydrophobic ionic liquids, 1-ethyl-3-methylimidazolium (EMIþ) perfluoroalkyltrifluoroborate ([RfBF3]) (Rf ¼ C2 F5 ; n-C3 F7 , and n-C4F9) were prepared in high yield and purity by facile neutralization of 1-ethyl-3-methylimidazolium (EMIþ) methylcarbonate (MeOCO2) with aqueous Hsolv.[RfBF3]solv. solutions. All the salts prepared were characterized by 19 F, 1 H, 11 B NMR, MS and elemental analysis, and thermal and electrochemical properties of these salts have been measured. [EMI][C2F5BF3] melted at lower temperature (1 8C) than [EMI][BF4] (13 8C), resulting in higher conductivity at low temperature. Its application to double-layer capacitors (DLCs) was examined. # 2003 Elsevier B.V. All rights reserved. Keywords: Hydrophobic ionic liquids; 1-Ethyl-3-methylimidazolium; Perfluoroalkyltrifluoroborate; Electrolyte salts; Double-layer capacitors

1. Introduction Ionic liquids (ILs) have been widely investigated as potential electrolytes for various electrochemical devices including rechargeable lithium cells [1–3], solar cells [4–7], actuators [8–10], and double-layer capacitors (DLCs) [11– 13]. Compared with conventional organic liquid electrolytes, the main advantages of ILs as electrolytes are their nonflammability, nonvolatility, and high thermal stability. We have investigated previously a series of ILs, consisting of the 1-ethyl-3-methylimidazolium (EMIþ) cation and a variety of fluoroanions [14–17] as possible electrolytes for DLCs. The overall performances of all ILs studied were inferior to those of a conventional nonaqueous electrolyte, 1 M [Et3MeN][BF4] in propylene carbonate (PC), mainly due to the high melting points (e.g. [EMI][BF4]) [14], low conductivities (high viscosities) (e.g. [EMI][MF6], M ¼ Nb(V) and Ta(V)) [15], or anodic instability of the ILs (e.g. [EMI][CF3SO3], [EMI][F2.3HF]) [16,17]. However, [EMI][BF4] afforded a good life performance at high temperature, which can be comparable to the conventional nonaqueous electrolyte, whereas all others had very poor life performances. This result stimulated us to develop further *

Corresponding author. Fax: þ81-29-887-3308. E-mail addresses: [email protected] (M. Ue), [email protected] (Z.-B. Zhou). 0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2003.12.003

new ILs as electrolytes related to BF4 to improve the low temperature performances of DLCs. We have prepared lithium and quaternary ammonium salts based on [RfBF3] anions very recently [18]. In this work, we report the preparation and characterization of a new class of ILs [EMI][RfBF3] (Rf ¼ C2 F5 ; n-C3 F7 ;, and n-C4F9), as well as their thermal and electrochemical properties, including the performances as a DLC electrolyte.

2. Results and discussion 2.1. ILs prepared by halide-free preparation ILs, consisting of EMIþ cation with a variety of fluoroanions, are prepared by metathesis reactions between [EMI][X] (X ¼ Cl ; Br ; I ) and silver, alkali metal, ammonium salts or acids of the corresponding fluoroanions [19]. In this case, it is difficult to avoid halide contamination. It has been demonstrated recently that the presence of halide contamination in the resulting ILs can drastically change the physical properties of ILs [20] and may result in catalyst poisoning and deactivation if used as solvents for catalytic reactions [21]. We have developed therefore a halide-free route to prepare ILs by neutralization of a methanol solution of 1-ethyl-3-methylimidazolium (EMIþ) methylcarbonate (MeOCO2) with acids containing the corresponding

472

Z.-B. Zhou et al. / Journal of Fluorine Chemistry 125 (2004) 471–476 O

O

145 0C, 13h N

N

Et

+ MeO

N

N

MeOH

OMe

Et

Me

O

OMe

[EMI][MeOCO2] cation exchange Hsolv.[RfBF3]solv.

K[RfBF3]

[EMI][MeOCO2]

[EMI][RfBF3] 1

a. Rf=C2F5; b. Rf=n-C3F7; c. Rf=n-C4F9 Yields: 1a (90 %); 1b (88 %); 1c (92 %) Scheme 1.

[RfBF3] anion. This unique property facilitated purification by simply washing with water to remove water-soluble impurities.

fluoroanions as shown in Scheme 1, where MeOH and CO2 are the only byproducts [22]. The salts [EMI][RfBF3] (1) prepared by this route were characterized by 1 H, 19 F and 11 B NMR, FAB-MS and elemental analysis. The characterization data are summarized in Table 1, which are in accordance with the expected compositions and structures. Except for F anion (<5 ppm), other halide anions (Cl, Br and I) and alkali metal cation (Kþ) in the resulting salts 1 were not detected. Moreover, the solubility of salts 1 in water was very low <2.5 g (100 ml H2O)1, and the water contents in 1 was lower than 2.5% before drying, while [EMI][BF4] is miscible with water. This hydrophobicity of salts 1 should be attributed to the high water-repellency of Rf group in the

2.2. Thermal properties The thermal behavior of salts 1 was determined by DSC and TGA. The data are collected in Table 2. Fig. 1 shows the DSC curves of salts 1. All quenched samples showed endothermic peaks on heating. Single melting transition was observed for 1a and c at 1 and 20 8C, respectively. It is interesting that 1b showed two melting transition at 12 and 8 8C. A careful examination revealed that the entropy of phase transformation at 12 8C was only 21.1 J/K mol,

Table 1 The characterization data for EMI [RfBF3]a (1) (Rf ¼ C2 F5 (a), n-C3F7 (b), n-C4F9 (c)) 1a 1

H NMR (ppm)

19

1.54 3.99 4.32 7.62 7.69 8.85

1b (t, C–CH3, J ¼ 7:4 Hz) (s, N–CH3) (q, CH2, J ¼ 7:3 Hz) (s, C–CH–N3) (s, C–CH–N1) (s, N–CH–N)

1.55 4.01 4.34 7.65 7.69 8.88

1c (t, C–CH3, J ¼ 7:2 Hz) (s, N–CH3) (q, CH2, J ¼ 7:3 Hz) (s, C–CH–N3) (s, C–CH–N1) (s, N–CH–N)

1.56 4.02 4.36 7.66 7.73 8.85

(t, C–CH3, J ¼ 7:2 Hz) (s, N–CH3) (q, CH2, J ¼ 7:3 Hz) (s, C–CH–N3) (s, C–CH–N1) (s, N–CH–N)

83.6 (s, CF3)

80.9 (s, CF3)

81.4 (s, CF3)

136.4 (s, CF2B)

128.0 (s, CF2)

124.3 (s, CF3CF2)

152.9 (q, BF3, 1 JBF ¼ 39:6 Hz)

134.2 (s, CF2B), 152.5 (q, BF3, 1 JBF ¼ 40:6 Hz)

126.2 (s, CF2), 152.4 (q, BF3, 1 JBF ¼ 38:8 Hz)

1

19.1 (quartet of triplets, JBF ¼ 42:2 Hz, 2 JBF ¼ 19:5 Hz)

19.0 (quartet of triplets, 1 JBF ¼ 40:6 Hz, 2 JBF ¼ 20:2 Hz)

18.6 (quartet of triplets, 1 JBF ¼ 40:6 Hz, 2 JBF ¼ 20:3 Hz)

FABþ: 111 (EMIþ, 100); FAB: 187 (C2F5BF3, 100) C, 32.3 (32.1)

FABþ: 111 (EMIþ, 100); FAB: 237 (C3F7BF3, 100) C, 31.1 (31.1)

FABþ: 111 (EMIþ, 100); FAB: 287 (C4F9BF3, 100) C, 30.2 (30.7)

Calc. (found)

H, 3.7 (3.5) N, 9.4 (9.1) B, 3.6 (3.6)

H, 3.2 (2.8) N, 8.1 (8.0) B, 3.1 (3.1)

H, 2.8 (2.3) N, 7.0 (7.1) B, 2.7 (2.7)

F content (ppm) H2O content (ppm)

<5 15

<5 13

<5 10

11

F NMR (ppm)

B NMR (ppm)

FAB-MS (m/e, %) Elemental analysis (%)

a

NMR measured in acetone-d6.

Z.-B. Zhou et al. / Journal of Fluorine Chemistry 125 (2004) 471–476 Table 2 Thermal properties of ionic liquids

1a 1b 1c

100

Tfra (8C)

Tmb (8C)

DSmc (J/K mol)

Tdd (8C)

40 39

1 8 20

42.3 35.3 36.1

298 292 280

a

Freezing point. Melting point. c Entropy of melting (DSm ¼ DHm =T, where DHm is enthalpy of melting at temperature T (K) and measured by DSC). d Decomposition temperature. b

which was very close to the entropy of melting of a plastic crystal (20 J/K mol), a criterion established by Timmermans [23]. On cooling, the three salts exhibited substantial super cooling effects, 1b and c, freezing at 40 and 39 8C (observed as exothermic peaks), respectively, which were significantly lower than their melting points, while 1a did not undergo phase transition within the temperature range studied. The melting points of salts 1 containing lower symmetry [RfBF3] are lower than that of [EMI][BF4] containing higher symmetry BF4 (mp 13 8C) [24]. However, there is still no clear correlation between the melting points of salts 1 and the chain length of Rf group in [RfBF3], Table 2. Fig. 2 shows the TGA curves of salts 1 from 25–600 8C. All three salts showed similarly stable behavior irrespective of their anion counterparts. They were stable up to 280 8C and decomposed rapidly over 300 8C. As shown in Table 2, the decomposition temperatures of these three salts are similar and stable enough to be used as solvents for organic synthesis or as electrolytes for electrochemical devices.

[EMI][C2F5BF3] [EMI][n-C3F7BF3] [EMI][n-C4F9BF3]

80

Percent of weight / %

ILs

473

[EMI][BF4]

60

40

20

0 100

200

300

400

500

600

0

T/ C Fig. 2. TGA curves of ionic liquids.

2.3. Electrochemical properties Fig. 3 shows the temperature dependence of the electrolytic conductivities of salts 1 in comparison with two representative ILs [EMI][BF4] and [EMI][(CF3SO2)2N]. Owing to the existence of the super cooling, ILs usually show remarkable conductivity below their melting points [13]. This phenomenon was also observed for salts 1. As shown in Fig. 3, the conductivities of all five salts decreased rapidly as the temperature decreased. In salts 1,

[EMI][C2F5BF3] (a)

14

[EMI][C2F5BF3]

heating

[EMI][n-C3F7BF3]

12

cooling

[EMI][n-C4F9BF3] 10 [EMI][n-C3F7BF3] (b)

cooling

[EMI][(CF3SO2)2N]

-1

heating

k / mS cm

endo

[EMI][BF4]

8

6

exo [EMI][n-C4F9BF3] (c)

heating

4

2

cooling 0

-60

-40

-20

0

20

T / 0C Fig. 1. DSC curves of ionic liquids, [EMI][C2F5BF3], [EMI][n-C3F7BF3], [EMI][n-C4F9BF3].

-40

-30

-20

-10

0

10

20

30

0

T / C Fig. 3. Temperature dependences of electrolytic conductivities of ionic liquids (super cooled).

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Z.-B. Zhou et al. / Journal of Fluorine Chemistry 125 (2004) 471–476 15

[EMI][C2F5BF3]

4

[EMI][BF4] 12

I / mA cm

-2

Capacitance / F cm

-3

2

0

-2

9

6

[EMI][C2F5BF3] 3

[EMI][BF4] [EMI][(CF3SO2)2N]

-4

1M [Et3MeN][BF4]/PC

0

-3

-2

-1

0

1 -

2

3

-30

-20

-10

0

-

E / V vs. I3/I

10

20

30

0

T/ C

Fig. 4. Linear sweep voltammogram for [EMI][C2F5BF3] on a Pt electrode at 5 mV s1.

Fig. 5. Temperature dependence of capacitances of DLCs using various ionic liquids.

the conductivities decreased as the Rf group in [RfBF3] (Rf ¼ C2 F5 ; n-C3 F7 , and n-C4F9) became longer; in other words, their conductivities decreased as the sizes of anions [RfBF3] increased (Fig. 3). Among the five salts, 1c, containing the largest anion, showed the lowest conductivity, although it had the lowest melting point (20 8C). This demonstrates that an anion with a very large size is unfavorable for improving the conductivity of ILs, which has also been observed previously [13,14]. The conductivity of 1b was a bit higher than that of [EMI][(CF3SO2)2N] within the temperature range studied (seeing Fig. 3). Unexpectedly, 1a showed a higher conductivity at low temperature (T < 15 8C) and a comparable conductivity at room temperature in comparison with [EMI][BF4] (Fig. 3), although the size of [C2F5BF3] is much larger than that of BF4. It seems that the relatively lower melting point of 1a (mp 1 8C) compared with that of [EMI][BF4] (mp 13 8C) [24] may contribute to its lower viscosity and its improved conductivity behaviors at low temperature. To our knowledge, salt 1a possesses the highest conductivity among the hydrophobic ILs reported [25]. The electrochemical stability of 1a was also investigated by the linear sweep voltammetry (LSV). As shown in Fig. 4, 1a showed similar electrochemical stability to [EMI][BF4] measured in the same conditions [14,16], indicating that both the cathodic and anodic decomposition are limited by EMIþ [16].

[EMI][C2F5BF3] afforded a higher capacitance than [EMI][BF4] and [EMI][(CF3SO2)2N], especially in low temperature region, due to its high conductivity (Fig. 3). Compared with 1 M [Et3MeN][BF4]/PC, it showed a comparable conductivity above 0 8C and a lower capacitance below 0 8C, especially in low temperature regions. Different from rechargeable batteries, the cycling test is not so important for DLC, because the deterioration occurs mainly at maximum operating voltage. Fig. 6 shows the results of the life test of the same cells at 3.0 Vand 70 8C. The capacitance of DLCs using 1a deteriorated rapidly with time, which

2.4. Performances of double-layer capacitors (DLCs) Fig. 5 shows the change of the volumetric capacitances of 2032 coin-type DLCs using various ionic liquids, when operating temperature was varied from 25 to 25 8C.

0

Capacitance losses / %

-20

-40

-60

[EMI][C2F5BF3] [EMI][BF4]

-80

[EMI][(CF3SO2)2N] 1M [Et3MeN][BF4]/PC

-100 0

2

4

6

8

10

12

14

t / day Fig. 6. Capacitance losses of DLCs using various ionic liquids at 3.0 V and 70 8C.

Z.-B. Zhou et al. / Journal of Fluorine Chemistry 125 (2004) 471–476

showed that 1a was not as stable as [EMI][BF4]. It is consistent with our previous results that DLCs using EMI salts with inorganic anions had better life performance than those with organic counterparts [16].

3. Conclusion

475

The onset of melting on heating, and solidifying on cooling were defined as the melting and freezing point, respectively. Thermal gravimetric analysis (TGA) was performed on a Rigaku TG/DTA Thermo plus 8120 thermal analysis system. An average sample weight of 5 mg was placed in a pierced aluminum pan and heated at 10 8C min1 from 25 to 600 8C under a flow of nitrogen. The onset of decomposition was defined as the decomposition temperature (Td).

We have prepared a new class of hydrophobic ILs [EMI][RfBF3] (Rf ¼ C2 F5 ; n-C3 F7 , and n-C4F9) with high yields and purities by a halide-free route. The water contents in salts 1 could be reduced to very low values (<20 ppm) by vacuum under mild conditions due to their immiscibility with water. All these new salts have relatively lower melting points than [EMI][BF4]. Among them, [EMI][C2F5BF3] possessed a higher conductivity at low temperature (T < 15 8C) and a comparable conductivity at room temperature in comparison with [EMI][BF4]. It showed similar electrochemical stability as [EMI][BF4]. Although [EMI][C2F5BF3] showed a higher capacitance than [EMI][BF4], it did not have a good life performance for DLCs.

An aqueous Hsolv.[RfBF3]solv. (70 mmol) solution was prepared by a cation exchange method as described previously [18]. The acid solution was filtered prior to use, and neutralized with equimolar [EMI][MeOCO2]/MeOH solution [22]. After concentrated to about 40 ml at 30–40 8C by evaporation under reduced pressure, the liquid in the lower layer was separated and washed with deionized water (15 ml) and toluene (30 ml) twice. The liquid obtained was dried at 50 8C for 12 h under vacuum to afford the colorless ionic liquid 1 with yields of about 90%.

4. Experimental

4.3. General procedures for electrochemical measurement

4.1. General experimental procedures A methanol solution of 1-ethyl-3-methylimidazolium methylcarbonate ([EMI][MeOCO2]) was prepared by the reaction of 1-ethylimidazole with dimethylcarbonate in methanol at 145 8C for 13 h in an autoclave [22]. [EMI][BF4] [16], and [EMI][(CF3SO2)2N] [16], and potassium perfluoroalkyltrifluoroborate K[RfBF3] (Rf ¼ C2 F5 ; n-C3 F7 , and n-C4F9) [18,26] were prepared according to the reported procedures (Caution: All reactions related to HF and KHF2 solutions were performed in PFA apparatus). NMR spectra were obtained using a JEOL JNM GSX400 spectrometer (19 F at 376.05 MHz, 1 H at 399.65 MHz, and 11 B at 128.15 MHz). Acetone-d6 was used as solvent. Chemical shift values are reported with respect to TMS internal reference for 1 H, and external references CCl3F in CDCl3 for 19 F (d ¼ 0:00 pm) and H3BO3 in D2O for 11 B (d ¼ 0:00 ppm). FAB mass spectra were measured on a JEOL JMS-HX110/ 110A spectrometer. Element analyses were carried out as follows: C, H, and N were determined by CHN elemental analyzer (Elementar, Vario EL); B and K were measured by inductively coupled plasma (ICP, Nippon Jarrell-Ash, IRISAP). Halide ions (F, Cl, Br, and I) were measured by ion chromatography (IC, Dow Chemical). The water content in the ionic liquid was detected by Karl–Fischer titration (Mitsubishi Chemical Corp., CA-06). Melting point (Tm) and freezing point (Tfr) were measured by a differential scanning calorimeter (DSC, Perkin-Elmer, DSC 7). An average sample weight of 5 mg was sealed in an aluminum pan and quenched, initially to 55 8C for 30 min and then heated and cooled at 10 8C min1 between 55 and 30 8C under a flow of nitrogen.

4.2. General synthetic procedures for [EMI][RfBF3] (1)

The water contents of the ILs used for electrochemical measurements were lower than 20 ppm. Electrolytic conductivity (k) was measured by a conductivity meter (Toa Electronics, CM-30S) in a sealed conductivity cell. The cell temperature was controlled by a homemade bath into which the cell was immersed. The conductivity values at low temperature were obtained on cooling agreed well with those obtained on heating. Linear sweep voltammetry (LSV) was performed using an automatic polarization system (Hokuto Denko Corp., HZ3000) in an argon-filled glove box, by using a 5 ml beakertype three-electrode cell equipped with a Pt working electrode (surface area: 0.02 cm2), a Pt wire counter electrode, and an I3/I reference electrode consisting of Pt wire/ 0.05 mol dm3 I2 þ 0:10 mol dm3 [Et4N][I] in the same ionic liquid. 4.4. Procedures for double-layer capacitors (DLCs) evaluation A 2032 coin cell (can size: 2.0 cm in diameter and 0.32 cm in height) was assembled in an argon-filled glove box using a pair of activated carbon electrodes (10 mm in diameter and 0.55 mm thick), a polypropylene separator, and an ionic liquid as electrolyte [15,17]. The capacitance C of the assembled cell was determined by charge–discharge cycling from V ¼ 0 to 2.8 V at a constant current, I, of 5 mA and a given temperature, where it was calculated from discharging time t by C ¼ It=V [27]. The volumetric capacitance was obtained by dividing the above capacitance C by the total volume of a pair of the disk electrodes. The life test

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was carried out by continuously charged at 3.0 V and 70 8C, and its deterioration was monitored by measuring its capacitance at 70 8C at given intervals. All data are the average values of the three same cells.

Acknowledgements We thank Mr. Masahiro Takehara, Ms. Asao Kominato and Ms. Yumi Suzuki for their assistances to this work.

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