Sulfonium Bis(trifluorosulfonimide) Plastic Crystal Ionic Liquid as an

Article pubs.acs.org/JPCC

Sulfonium Bis(trifluorosulfonimide) Plastic Crystal Ionic Liquid as an Electrolyte at Elevated Temperature for High-Energy Supercapacitors Mérièm Anouti,*,† Laure Timperman,† Mostafa el hilali,† Aurélien Boisset,† and Hervé Galiano‡ †

Université François Rabelais, Laboratoire PCM2E, Parc de Grandmont 37200 Tours, France CEA, DAM, Le Ripault, F-37260 Monts, France

‡

ABSTRACT: This study describes the use of the ionic liquid trimethylsulfonium bis(trifluorosulfonimide) [Me3S][TFSI] as an electrolyte for carbon-based supercapacitors at temperatures up to 80 °C. [Me3S][TFSI] is synthesized by a metathesis reaction, possesses a low melting point (Tm = 45.5 °C), and presents crystal plastic behavior indicated by a strong organizational structure with many possible conformations corresponding to several solid−solid phase transitions TS−S. Furthermore, [Me3S][TFSI] is thermally stable up to T = 280 °C and presents a high conductivity up to 20.42 mS cm−1 at 80 °C and a low viscosity, 3 mPa s, at the same temperature. The combination of a good cycling ability with high capacities up to 150 F g−1 at elevated voltage and temperature, i.e., ΔE = 3 V, T = 80 °C, enables the realization of supercapacitors with high specific energies at high temperatures. icochemical properties, these “designer green solvents” have been intensively investigated for various applications.18−25 Ionic liquids containing ammonium cations have been extensively studied; in contrast, those based on phosphonium and sulfonium cations have received less attention. However, sulfonium-based aprotic ionic liquids (AILs) have been devoted great interest as potential substitutes for their ammonium counterparts thanks to practical advantages including high chemical and electrochemical stabilities.26,27 Indeed, sulfonium salts have certain useful properties that can be applied in specialized areas.28 A more recent report indicated that trialkylsulfonium salts based on the dicyanamide anion had low viscosities compared to other ionic liquids, typically in the range 20−60 cP at 20 °C.29 The viscosity is an extremely important parameter in many potential applications of ionic liquids including their use as electrolytes in solar cells.30−32 Sulfonium salts also represent a source of sulfonium ylides that can be generated in situ as synthons for chiral compounds.33 To the best of our knowledge, no studies concerning the use of sulfonium-based ILs as electrolytes for supercapacitors have been carried out as of yet. Recently, we demonstrated that ammonium- and phosphonium-based IL pure or mixed with acetonitrile can be classified as a promising electrolyte for supercapacitor applications.34 The present work reports on the synthesis, physicochemical characterization, and electrochemical characteristics of trimethylsulfonium bis(trifluorosulfonimide) [Me3S][TFSI] as an electrolyte for symmetrical supercapacitors with activated

1. INTRODUCTION Supercapacitors have been devoted great interest because of their high power storage capability, which is highly desirable for applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs).1−3 Supercapacitors can be coupled with fuel cells or batteries to deliver the high power needed during acceleration and to recover energy during braking. Carbonbased capacitors have attracted great attention since these materials possess diversified morphologies with high stability and conductivity.4−9 Activated carbons (ACs) are the most commonly used electrode materials for electrical double-layer capacitor (EDLC) applications. This is due to their relatively low cost and high surface area in comparison with other carbon-based materials. Another factor that influences the properties of supercapacitors is the selected electrolyte. Currently, aqueous solutions are the most utilized.10−13 However, the drawback of aqueous electrolyte-based supercapacitors is a narrow cell voltage and low energy.14 For ACs, typically only about 10−20% of the “theoretical” capacitance was observed due to the presence of micropores that are inaccessible by the electrolyte, wetting deficiencies of electrolytes on the electrode surface, and/or the inability to successfully form a double layer in the pores. In fact, the nature of the electrolyte is essentially responsible for this. The attainable cell voltage of supercapacitors depends largely on the electrolyte breakdown voltage, while ESR depends on the electrode and electrolyte conductivity. The choice of the electrolyte is therefore very influential. Recently, ionic liquids (ILs) have attracted a great deal of attention due to their high thermal stability, good conductivity, nonflammability, suitable polarity, wide electrochemical windows, and recyclability.15−17 Because of their unique phys© 2012 American Chemical Society

Received: February 8, 2012 Revised: April 13, 2012 Published: April 13, 2012 9412

dx.doi.org/10.1021/jp3012987 | J. Phys. Chem. C 2012, 116, 9412−9418

The Journal of Physical Chemistry C

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Figure 1. DSC thermograms of [Me3S][TFSI] from −60 to 250 °C.

Table 1. Molar Mass, Density, Molar Volume, Viscosity, Specific and Equivalent Ionic Conductivity of [Me3S][TFSI] at 50 °C

cell, with platinum as the working electrode, a stainless steel grid as the counter electrode, and a silver wire as the reference electrode. Galvanostatic charge−discharge experiments and cyclic voltammograms (CVs) were conducted using a Teflon Swagelok-type system with a two-electrode cell with activated carbon as the working and counter electrodes. Whatman glass microfiber filter papers were utilized as the separator. The activated carbon electrode material (12 mm in diameter, 7.2 mg, with an active mass of 5.76 mg) was kindly supplied by Batscap. The temperature was controlled from 50 to 80 °C by a thermostatic oven.

carbon as the electrode at elevated temperatures (50 and 80 °C). The results were then compared with those for a reference electrolyte, i.e., neat methyl - butyl pyrrolidinium bis(trifluorosulfonimide),to demonstrate the high energy density of the AC supercapacitor based on [Me3S][TFSI] as electrolyte.

2. EXPERIMENTAL SECTION Trimethylsulfonium iodide (99%) and 1,2-dichloroethane (DCE) (>99%) were purchased from Sigma Aldrich. Bis(trifluorosulfonimide) lithium (LiTFSI) (≥99.0%) was obtained from Solvionic. [Me3S][TFSI] was synthesized by ionic exchange of iodide with LiTFSI salt. Subsequently, this mixture was washed to remove lithium iodide by water and distilled according to a water−DCE heteroazeotropic distillation. Residual DCE was finally evaporated under reduced pressure, giving a white solid compound (Tm = 45.5 °C). The water content was 1000 ppm as analyzed by Karl Fischer titration. The physical properties of synthetized IL are presented in Table 1. A Crison (GLP 31) digital multifrequency conductimeter was utilized to measure the ionic conductivities. The temperature control, from 50 to 80 °C, was realized by a thermostatted bath JULABO F25, with an accuracy of ±0.2 °C. The conductimeter was calibrated using standard solutions of known conductivity (0.1 and 0.01 mol L−1 KCl); the uncertainty for conductivities did not exceed ±2%. Each conductivity was recorded when the stability was superior to 1% within 2 min. Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer DSC 4000 under a nitrogen atmosphere, coupled with an Intracooler SP VLT 100. Samples for DSC measurements were sealed in Al pans. Vapor pressure measurements were performed using an isobaric vapor−liquid equilibrium (VLE). Details of design of the apparatus were previously described by Husson et al.35 Electrochemical measurements were carried out on a Versatile Multichannel Potentiostat (Biologic S.A). The potential window of the IL was measured by linear voltammetry using a three-electrode

3. RESULTS AND DISCUSSION 3.1. Physical Properties of ILs. 3.1.1. Thermal Properties. The thermal behavior of the studied AIL was investigated by DSC from −60 to 250 °C. Figure 1 presents a thermogram recorded at a scan rate of 5 °C min−1 during heating from −20 to 100 °C (step I) followed by cooling from 100 to −60 °C (step II) (Figure 1a). The thermogram shows the melting phase transitions observed at Tm = 45.5 °C (ΔHm = 44.70 J g−1) and numerous peaks corresponding to crystallization phases at Tc from 24.4 to −10.5 °C (ΔHc(tot) = 38.70 J g−1) with a range of supercooling up to 20 °C. The low entropies, ΔSci, calculated for peaks 1 to 6 in the inset (Figure 1a) are from 2 to 19 J K−1 mol−1. The second cycle involved heating from −60 to 250 °C to appreciate the material’s stability at elevated temperature (Figure 1b). No peak was observed in this temperature domain, which means that the ionic liquid was stable between 45 and 250 °C. The many peaks observed for crystallization indicate a strong organizational structure with many possible conformations corresponding to several solid− solid phase transitions TS−S. This behavior is well described for S111FSI and S112FSI salts that are solid at room temperature and which exhibited solid−solid transitions at 8 °C as well as an extremely low melting entropy (ΔSm < 9 J K−1 mol−1).36 These materials are classified as plastic crystals and are interesting for use as ambient-temperature solid electrolytes.37,38 9413



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Table 2. Thermal Properties of the AIL: Crystallization Temperature (Tc), Melting Point (Tm), Isobaric Heat Capacity (Cp), Crystallization Enthalpy (ΔHc), Melting Enthalpy (ΔHm), Crystallization Entropy (ΔSc), and Vapor Pressure (Pv) Tc (°C)

Tm (°C)

Cp (40 °C) (J K−1 mol−1)

Cp (80 °C) (J K−1 mol−1)

ΔHc (kJ mol−1)

ΔHm (kJ mol−1)

ΔSc(i) (i = 1 to 6) (J K−1 mol−1)

Pv (50 °C) (mbar)

−29 to 30

45.5

0.80

0.83

13.82

15.97

2 to 19

12

Figure 2. (a) Evolution of the conductivity, σ, as a function of temperature for [Me3S][TFSI]. (b) Arrhenius plot of the conductivities for [Me3S][TFSI].

Figure 3. (a) Evolution of the viscosity as a function of the temperature for [Me3S][TFSI]. Inset: Shear stress versus shear rate at 20 °C. (b) Arrhenius plot of the viscosities for [Me3S][TFSI]. The solid line represents the Arrhenius fitting.

mS cm−1 for [EMIm][TFSI] and 0.40 mS cm−1 for [BMIm][TFSI])43 or than those based on a phosphonium cation which showed a wide range of conductivities from 0.47 to 4.40 mS cm−1.44 The variation of conductivity was evaluated as a function of temperature as shown in Figure 2a. As expected, the conductivity increased from σ = 10.82 mS cm−1 at 50 °C and reached 20.42 mS cm−1 at 80 °C. It demonstrated an Arrhenius-type behavior with activation energy Ea = 22.03 kJ mol−1 (Figure 2b). It is clear that the highly fluid IL with cations of relatively small volume was favorable for forming highly conductive ILs. For the sulfonium IL homologues, similar values of specific conductivity were obtained at 25 °C: S222[TFSI] (7.1 mS cm−1), S122[FSI] (5.8 mS cm−1), S223[TFSI] (5.03 mS cm−1).45,46 Here only the size of the cation is a determinant parameter. Viscosity. In general, viscosities of sulfonium ionic liquids (with the exception of halide salts) are low and, not surprisingly, dependent on the nature of the anion. At 25 °C, several asymmetric trialkylsulfonium-based ILs have been reported to have a low viscosity.45,46 The viscosities of S112TFSI and S114TFSI have been found to be 52 and 66

The above-mentioned observations will be extended though structural studies with spectral methods in future work. Use of the ionic liquid can be considered over a wide temperature range, between 45.5 and 250 °C. In the remainder of this work, we test the performance of the ionic liquid at 50 and 80 °C. Vapor pressure determines the volatility of a solvent. It governs the exchange rate of ILs across the vap−liq interface. The greatest difficulty and uncertainty arise in the determination of the vapor pressure of low volatile compounds like ILs. In fact, the first published vapor pressures on ionic liquid mixtures, by Wilkes et al., brought the same type of chemical equilibrium.39 In this study, vapor pressure measurements for [Me3S][TFSI] were performed using an isobaric vapor−liquid equilibrium (VLE) (Pv (50 °C) = 12 mbar (Table 2)). This value is higher than an extremely low vapor pressure of 0.1 × 10−12 mbar (100 pPa) for AILs like [C2mim][TFSI] at 25 °C.40 Conductivity. The conductivity (σ) for [Me3S][TFSI] was measured as 10.83 mS cm−1 at 50 °C. This result was significantly higher than values reported for ionic liquid homologues based on ammonium cations with TFSI anions (2 mS cm−1 is thus the highest conductivity found for an N,Ndialkyl-pyrrolidinium-based salt;37,41,42 N-alkyl-imidazolium 0.9 9414



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exhibit a larger electrochemical window. The electrochemical stability of [Me3S][TFSI] was measured at 50 °C by sweep voltammograms recorded with a three-electrode cell configuration using platinum as the working electrode. [Me3S][TFSI] was electrochemically stable in the range of −2.6 to 2.9 V versus Ag as shown in Figure 5, which indicated that its value of

mPa s, and that of S223TFSI was 33 mPa s, which was close to the viscosities of S222TFSI (33 mPa s) and of S221TFSI (36 mPa s) and S123TFSI (39 mPa s). The reduced viscosity of the sulfonium-based salts may be due to a decrease in Coulombic interactions between the sulfonium cation and the anions by comparison with ammonium IL homologues. In this work, the obtained viscosities were lower. According to Figure 3a, the viscosity (η) ranged from 11 to 3 cP when going from 50 to 80 °C; with Newtonian behavior, indeed, shear stress versus shear rate was linear as reported in the inset of Figure 3a. This lower value can be interpreted as elevated in conformational degrees of freedom due to the small cation size. The temperature dependence of the viscosity was investigated for the IL over the temperature range 50 to 80 °C. The viscosity of [Me3S][TFSI] exhibited an Arrhenius-type behavior according to ln(η) = ln(η0) + Ea/RT (Ea = 38.4 kJ mol−1) as shown in Figure 3b. Ea is regarded as the energy barrier that needs to be overcome for the ions in the roomtemperature ionic liquids to move past each other. Moreover, its value can be correlated with the structure of the IL.47 From this viewpoint, the smaller the Ea, the easier it is for the ions to move past each other and the lower the viscosity. Ionicity. One way of assessing the ionicity of ionic liquids is to use the classification diagram based on the classical Walden rule.48,49 The Walden rule relates the ionic mobility represented by the equivalent conductivity Λ to the fluidity η−1 of the medium through which the ions move. The equivalent conductivities Λ are calculated by use of the relation Λ = Vmσ where Vm is the molar volume. Figure 4 shows variation of

Figure 5. Linear voltammograms at a platinum electrode for neat [Me3S][TFSI], v = 10 mV s−1 at T = 50 °C.

electrochemical window is 5.3 V at 50 °C. For the IL, the measured working potential window was similar to that of the quaternary ammonium52 and phosphonium AIL anion,44 ca. 4− 6 V. Fang et al. showed that for a series of sulfonium TFSI ILs ΔE = 5 V.45 Generally, anodic and cathodic limits observed in an ionic liquid are due, respectively, to the oxidation and the reduction of the cations. The mechanism of reduction of alkylsulfonium salts is consistent with a concerted σ sulfur−carbon bond breaking concomitant with electron acceptance.53,54 Oneelectron electrochemical reduction of sulfonium salts is generally irreversible, as a consequence of carbon−sulfur αbond cleavage, providing a sulfide and a carbon. The ratedetermining step for fragmentation has been attributed to the formation of an intermediate sulfuranyl radical55 via direct addition of an electron to sulfur or an intermediate sulfonium cation preceding homolytic cleavage of a σ carbon−sulfur bond.

Figure 4. Equivalent conductivity (Λ) versus fluidity (η−1) for [Me3S][TFSI] (red) and some PILs.50 The solid line is the “ideal” Walden product line fixed with 1 mol L−1 aqueous KCl solutions.

2Me3S+ + 2e− → 2Me• + 2Me2S

(1)

2Me• → (Me)2

(2)

The positive limit can be attributed to the electro-oxidation of sulfonium by the following flow mechanism

ln(Λ) versus ln(1/η) at various temperatures for [Me3S][TFSI] and selected PROTIC ionic liquids (PILS) for comparison.50 The ideal line is obtained on the basis that ions have mobility that are determined only by the viscosity of the medium and that the number of ions present in the equivalent volume is that indicated by salt composition; i.e., all ions contribute equally.51 The position of the ideal line is established using aqueous KCl solutions at high dilution. The results obtained in Figure 4 indicate that [Me3S][TFSI] is a relatively good ionic liquid and becomes more ionic in the Walden classification at low temperature. This observation can be tied to its characteristic as plastic crystals, which maintain good ionicity for use as ambient-temperature solid electrolytes. 3.2. Electrochemical Study. Electrolytes for electrochemical devices should resist reduction and oxidation and

2Me3S+ − 2e− → 2Me2S• + 2Me•

(3)

2Me2S• + 2Me• → Me2S2 + (Me)2

(4)

Both oxidation and reduction reactions are catalyzed at AC after which the working potential window is reduced to 3 V. This domain is used as the operating voltage. The two-electrode cell using a Teflon Swagelok-type system with a two-electrode cell with activated carbon as the working and counter electrodes was applied to evaluate the performance of the AC supercapacitors using neat [Me3S][TFSI] electrolyte at 50 and 80 °C within a voltage limit range from 0 to 3.0 V. Figure 6 shows the cyclic voltammograms recorded at the sweep rate of 20 mV s−1 (T = 80 °C). It is well-known that an ideal capacitance behavior of a carbon material electrode is expressed in the form of a rectangular shape on the 9415



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where m is the weight per electrode of active material (g). The effect of the scan rate thus became visible when comparing the curves from v = 2 to 20 mV s−1 (Figure 7a). In fact, the capacitance decreased from 178 to 100 F g−1, respectively. From Figure 7b, with ΔV = 2 V, it is clear that the temperature effect on the cyclability was lower. The CV curves for 50 and 80 °C were similar, which implies that the capacitance values were not really influenced by this temperature difference (approximately 135 F g−1). The same effect was observed for ΔV = 3 V, with approximately 145 F g−1. However, at 80 °C, the shape of the curve was better suited for charging the capacitor; the effect was opposite that of discharge. This phenomenon can be attributed to the adsorption of sulfonium cations at the carbon for the higher temperature with an adverse effect during discharge. This can be corrected by adapting the surface function on the activated carbon. Galvanostatic charge−discharge cycles were recorded on a two-electrode cell at various tension voltage, i.e., 1.2, 1.6, 2.0, 2.5, and 3.0 V, with current densities of 347 mA g−1 (Figure 8).

Figure 6. Cycling performance of the activated carbon electrode in the [Me3S][TFSI] electrolyte at 80 °C expressed by specific capacitance values obtained from CV at 20 mV s−1 by dividing the specific current by the scan rate.

voltammetry characteristics. As can be seen from the figure, relatively rectangular-shaped CV curves were obtained, which indicates that it possessed good capacitive properties. The capacitance dropped from 130 to 140 F g−1 when going from 50 to 80 °C. As a comparison, Balducci et al. obtained 70 F g−1 at the same scan rate for P14TFSI (T = 60 °C) on activated carbon.56 This performance can be explained by the lower viscosity (3 Cp for [Me3S][TFSI], 100 Cp for P14TFSI), higher conductivity (20 mS cm−1 for [Me3S][TFSI], 6 mS cm−1 for P14TFSI), and smaller volume for the sulfonium IL (1.1 nm for the P14 cation and 0.6 nm for Me3S+). Figure 7 shows the influence of the scan rate and temperature on the cycling stability. The cell capacitance was calculated from the slope of the discharge curve on a twoelectrode cell according to the following equation I C= (5) dV /dt

Figure 8. Galvanostatic charge−discharge curves at 2 mA (347 mA g−1) at 50 °C.

where C is the capacitance of the cell in farad (F), I the discharge current in ampere (A), and dV/dt the slope in volts per second (V s−1). In a symmetrical system, where the active material weight is the same for the two electrodes, the specific capacitance, Csp, in farad per gram of active material (F g−1) is related to the capacitance of the cell by

Csp =

2C m

The discharge cycle was used to determine the specific capacitance (Csc) that was calculated using Csc = 2(IΔt/ mΔV), where I is the constant discharge current, Δt the discharge time, m the mass of one electrode, and ΔV the voltage drop upon discharge (excluding the IR drop). The energy density (E) and power density (P) of an EDLC electrode were calculated with the equations Emax = (1/8) C(ΔVmax)2 and Pmax = (E/Δt), respectively. Here, C is the

(6)

Figure 7. Performace of sulfonium ionic liquid [Me3S][TFSI] electrolyte with an activated carbon electrode. (a) Cyclic voltammograms for twoelectrode cells with active carbon at scan rates of 2, 5, 10, and 20 mV·s−1, at 50 °C. (b) Influence of the temperature on the cyclic voltammograms for two-electrode cells with AC in [Me3S][TFSI]. Scan rate: 5 mV s−1. 9416



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Table 3. Specific Capacitance, Specific Energy, and Specific Power for Neat [Me3S][TFSI], at Two Temperatures (I = 2 mA, i.e., 347 mA g−1) T/°C

voltage tension ΔV (V)

0.7

1.0

1.2

1.4

1.6

1.8

2

2.2

2.5

2.8

3

50

Csp (F g−1) Emax (Wh kg−1) Δt (s) Pmax (Wh kg−1) Csp (F g−1) Emax (Wh kg−1) Δt (s) Pmax (Wh kg−1)

107 1.82 76 86.2 132 2.25 107 75.7

112 3.90 126 111 139 4.83 164 106

115 5.75 161 128.6 139 6.96 203 123.4

119 8.10 198 147.2 138 9.40 241 140.4

122 10.84 235 166 136 12.10 243 179.3

125 14.06 277 182.8 140 15.76 306 185.4

129 17.92 314 205.4 139 19.32 344 202.2

132 22.18 357 223.7 138 23.21 370 225.8

137 29.73 418 256 139 30.19 431 252.2

143 38.93 479 292.6 139 37.87 476 286.4

151 47.19 540 314.6 141 44.10 504 315

80

specific capacitance, Δt the discharge time, and ΔV the voltage drop upon discharge (excluding the IR drop). Values of specific capacitance, energy, and power derived from the AC electrode are presented in Table 3. Activated carbon-based supercapacitors in classical aqueous electrolytes can suffer from the low specific energy and specific power because of their restricted potential window of approximately 1.0 V. This energy increases in more currently studied ILs but at the expense of power. Here, the sulfonium IL electrolyte contributed to simultaneously improve the power and energy density as shown in Table 3. The performances of the supercapacitor microporous activated carbon/[Me3S][TFSI] are very promising. The encouraging capacitance values (up to 140 F g−1) obtained with this microporous carbon were higher than those with ammonium-based ILs like P14TFSI.56 Work is in progress to decrease the resistance by means of specific surface treatments on the carbon electrode material, to increase the power delivered by these systems. This should render them even more attractive for power applications at high temperatures for HEV applications.

ACKNOWLEDGMENTS

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REFERENCES

This research was supported by Conseil Régional de la region Centre through the Sup’Caplipe project. Many thanks are expressed to Batscap for providing the electrode material.

(1) Conway, B. E. Electrochemical supercapacitors: scientific fundamentals and technological applications; Kluwer Academic Plenum: New York, 1999. (2) Pasquier, A. D.; Plitz, I.; Gural, J.; Badway, F.; Amatucci, G. G. J. Power Sources 2004, 136, 160. (3) Cuentas-Gallegos, A. K.; Lira-Cantú, M.; Casañ-Pastor, N.; Gómez-Romero, P. Adv. Funct. Mater. 2005, 15, 1125. (4) Kierzek, K.; Frackowiak, E.; Lota, G.; Gryglewicz, G.; Machnikowski, J. Electrochim. Acta 2004, 49, 515. (5) Li, H.-Q.; Luo, J.-Y.; Zhou, X.-F.; Yu, C.-Z.; Xia, Y.-Y. J. Electrochem. Soc. 2007, 154, A731. (6) Portet, C.; Yushin, G.; Gogotsi, Y. J. Electrochem. Soc. 2008, 155, A531. (7) Xu, B.; Wu, F.; Chen, R.; Cao, G.; Chen, S.; Zhou, Z.; Yang, Y. Electrochem. Commun. 2008, 10, 795. (8) Zeng, X.; Wu, D.; Fu, R.; Lai, H.; Fu, J. Electrochim. Acta 2008, 53, 5711. (9) Xu, B.; Wu, F.; Chen, S.; Zhou, Z.; Cao, G.; Yang, Y. Electrochim. Acta 2009, 54, 2185. (10) Müllier, M.; Kastening, B. J. Electroanal. Chem. 1994, 374, 149. (11) Hu, C.-C.; Tsou, T.-W. Electrochem. Commun. 2002, 4, 105. (12) Toupin, M.; Brousse, T.; Bélanger, D. Chem. Mater. 2004, 16, 3184. (13) Xu, B.; Wu, F.; Chen, R.; Cao, G.; Chen, S.; Yang, Y. J. Power Sources 2010, 195, 2118. (14) Chang, J.-K.; Lee, M.-T.; Tsai, W.-T. J. Power Sources 2007, 166, 590. (15) Lu, W.; Qu, L.; Henry, K.; Dai, L. J. Power Sources 2009, 189, 1270. (16) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (17) Chakrabarty, D.; Seth, D.; Chakraborty, A.; Sarkar, N. J. Phys. Chem. B 2005, 109, 5753. (18) Welton, T. Chem. Rev. 1999, 99, 2071. (19) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391. (20) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (21) Ye, C.; Liu, W.; Chen, Y.; Yu, L. Chem. Commun. 2001, 2244. (22) Hough, W. L.; Rogers, R. D. Bull. Chem. Soc. Jpn. 2007, 80, 2262. (23) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123. (24) Wang, S.-F.; Chen, T.; Zhang, Z.-L.; Pang, D.-W.; Wong, K.-Y. Electrochem. Commun. 2007, 9, 1709. (25) Minami, I. Molecules 2009, 14, 2286. (26) Paulsson, H.; Berggrund, M.; Svantesson, E.; Hagfeldt, A.; Kloo, L. Sol. Energy Mater. Sol. Cells 2004, 82, 345.

4. CONCLUSIONS Trimethylsulfonium bis(trifluorosulfonimide) was synthesized, and the physicochemical properties of this IL were characterized. The [Me3S][TFSI] was found to be both electrochemically and thermally stable. Its high conductivity, i.e., up to 20 mS cm−1 at 80 °C, and low viscosity, i.e., 3 cP at the same temperature, should render it possible to use it as a potential electrolyte in electrochemical devices. The [Me3S][TFSI] has a large electrochemical window, around 5 V, on platinum electrodes and can operate up to 3.0 V on activated carbon in two-electrode cells. At an elevated operating temperature (T = 80 °C), this IL presents a good capacitance of 150 F g−1 and high specific energy and power density. On the basis of its physicochemical properties, as well as its electrochemical performances on activated carbon, the [Me3S][TFSI] IL can be classified as a promising electrolyte for supercapacitor applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (33)247367360. Tel.: (33)247366951. Notes

The authors declare no competing financial interest. 9417



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