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Electrochimica Acta 99 (2013) 108– 116

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

Water-soluble, triflate-based, pyrrolidinium ionic liquids

M. Morenoa, M. Montaninoa, M. Carewskaa, G.B. Appetecchia,∗, S. Jeremiasb, S. Passerinib

a ENEA, Agency for New Technologies, Energy and Sustainable Economic Development, UTRINN-IFC, Via Anguillarese 301, Rome 00123, Italyb Westfälische Wilhelm Universität, Institut für Physikalische Chemie, Corrensstr. 28/30, D48149 Münster, Germany

a r t i c l e i n f o

Article history:Received 7 November 2012Received in revised form 7 March 2013Accepted 9 March 2013Available online 20 March 2013

Keywords:Hydrophilic ionic liquidN-butyl-N-methylpyrrolidiniumtrifluoromethanesulfonateN-methoxyethyl-N-methylpyrrolidiniumtrifluoromethanesulfonate

a b s t r a c t

The physicochemical and electrochemical properties of the water-soluble, N-methoxyethyl-N-methylpyrrolidinium trifluoromethanesulfonate (PYR1(2O1)OSO2CF3) ionic liquid (IL) were investigatedand compared with those of commercial N-butyl-N-methylpyrrolidinium trifluoromethanesulfonate(PYR14OSO2CF3). The results have shown that the transport properties are well correlated with the rheo-logical and thermal behavior. The incorporation of an oxygen atom in the pyrrolidinium cation aliphaticside chain resulted in enhanced flexibility of the ether side chain, this supporting for the higher ionicconductivity, self-diffusion coefficient and density of PYR1(2O1)OSO2CF3 with respect to PYR14OSO2CF3,whereas no relevant effect on the crystallization of the ionic liquid was found. Finally, the presence ofthe ether side chain material in the pyrrolidinium cation led to a reduction in electrochemical stability,particularly on the cathodic verse.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Ionic liquids (ILs) represent a very interesting new class ofroom temperature fluids. Basically, ILs are salts, generally moltenat room temperature (RTILs), formed by an organic cation (e.g.,aliphatic quaternary ammonium, imidazolium, pyrrolidinium andpiperidinium) and an inorganic/organic anion (PF6

−, OSO2CF3−,

BF4− and perfluoroalkylsulfonylimide). The main advantages of ILs

towards common organic solvents are: non-flammability (even ifthe decomposition products of a few of them may be flammable[1]), negligible vapor pressure, high chemical and thermal stabil-ity. In addition, most of them show high ionic conductivity and wideelectrochemical stability.

Among these innovative, neoteric fluid materials, hydrophilicionic liquids based on the pyrrolidinium (PYR1×, where thesubscripts indicate the number of carbon atoms in the linearalkyl chains) cation and the trifluoromethanesulfonate (triflate or(OSO2CF3)−) anion have attracted a large attention as solventsfor various extraction processes. In particular, N-butyl-N-methyl-pyrrolidinium triflate (PYR14OSO2CF3) showed higher selectivity inthe separation of aromatic hydrocarbons from aliphatic ones [2–5]and for the extraction of sulfur compounds from hydrocarbons [4,6]with respect to the heavier and more expensive hydrophobic TFSI-based ILs. In addition, PYR14OSO2CF3 was used in various catalyticprocesses, showing easier processing and feasibility of recycling[7]. PYR14OSO2CF3 was successfully proposed as electrolyte, in the

∗ Corresponding author. Tel.: +39 06 3048 3924; fax: +39 06 3048 6357.E-mail addresses: gianni.appetecchi@enea.it, gianni.appetecchi@alice.it

(G.B. Appetecchi).

place of volatile and hazardous organic compounds, for the real-ization of highly safe, electrochemical devices as super-capacitors[8] and ultra-capacitors [9]. Also, electronically conducting poly-mers of interest as poly(3-methylthiophene) [8] and poly(pyrrole)[10] can be favorably electrodeposited from their monomers usingPYR14OSO2CF3 baths. Finally, no additive is required for electroplat-ing nano-crystalline Cu and Al in water-stable triflate-based ionicliquids as PYR14OSO2CF3 [11].

Previous work [12–15] has shown how the incorporation ofan oxygen atom in the aliphatic side chain of different cations isable to prevent the crystallization of the IL material, this result-ing in more or less drastic lowering of the melting point. In orderto favorably combine this peculiarity with the characteristics ofPYR14OSO2CF3, we have synthesized N-methoxyethyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR1(2O1)OSO2CF3). Thishydrophilic ionic liquid material was previously proved by Rus-sell et al. [16] to be a more suitable media for trans-esterificationof methyl methacrylate, divinyl adipate and 1,4-butanediol withrespect to organic solvents as hexane, acetonitrile, and tetrahydro-furan.

In the present paper we discuss the physicochemical and elec-trochemical properties of PYR1(2O1)OSO2CF3 in comparison withthose of commercial PYR14OSO2CF3.

2. Experimental

2.1. Synthesis of the ionic liquids

The N-butyl-N-methylpyrrolidinium trifluoromethanesulfonate (PYR14OSO2CF3) ionic liquid, purchased from Solvionic,

0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.electacta.2013.03.046

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M. Moreno et al. / Electrochimica Acta 99 (2013) 108– 116 109

Scheme 1. Chemical structure of PYR14OSO2CF3 (A) and PYR1(2O1)OSO2CF3 (B).

was dried under high vacuum (10−5 mbar) at 120 ◦C for at least18 h before the tests. The hydrophilic PYR1(2O1)OSO2CF3 material(Scheme 1) was synthesized at ENEA by properly modifying a pro-cedure route previously developed for hydrophobic ionic liquids[17]. The chemicals N-methylpyrrolidine (97 wt.%), 2-chloroethylmethyl ether (98 wt.%), ethyl acetate (ACS grade, >99.5 wt.%), werepurchased from Aldrich and previously purified (with the excep-tion of ethyl acetate) through the addition of appropriate amountsof activated carbon (Aldrich, Darco-G60) and alumina (acidic,Aldrich Brockmann I). The resulting slurry was stirred at 50 ◦Covernight (at least 18 h). The liquid fraction (purified chemical) wasseparated from the solid phase by vacuum filtering (paper filtermembrane). Silver trifluoromethanesulfonate (Aldrich, >99%) waspreviously dried (in dry-room) under high vacuum (10−5 mbar)at 70 ◦C overnight to remove the salt hydration water. In this waythe correct stoichiometric amount of AgOSO2CF3 was employedin the ionic liquid synthesis route.

The synthesis of PYR1(2O1)OSO2CF3 was performed throughthree steps: (i) synthesis of the precursor PYR1(2O1)Cl; (ii) synthesisof PYR1(2O1)OSO2CF3; (iii) purification of PYR1(2O1)OSO2CF3.

Step (i) N-methoxyethyl-N-methylpyrrolidinium chloride,PYR1(2O1)Cl, was synthesized by reacting N-methylpyrrolidinewith the appropriate amount of 2-chloroethyl methyl ether [17]:

PYR1 + Cl − CH2CH2 − O − CH3ethyl acetate,70◦C−→ PYR1(2O1)Cl(s) (1)

The reagents were firstly dissolved in ethyl acetate and, then,mixed in a reactor. The reaction temperature was set at 70 ◦C forachieving a yield approaching 100% (determined by weighting theobtained precursor after vacuum drying at 100 ◦C overnight) upon5–6 days heating due to the low kinetics of the reaction (1) [15,17].A white, solid precipitate of PYR1(2O1)Cl was obtained whereas theyellowish liquid phase (e.g., containing impurities generally verysoluble in ethyl acetate) was removed by vacuum filtering (paperfilter membrane). Finally, the precursor was rinsed 5 times withethyl acetate.Step (ii) The PYR1(2O1)OSO2CF3, ionic liquid was synthesizedby reacting at room temperature the purified precursor withAgOSO2CF3 (3 wt.% excess for allowing the full reaction ofPYR1(2O1)Cl) for 1 h. The two compounds were previously dissolvedin deionized water:

PYR1(2O1)Cl(aq) + AgOSO2CF3(excess)

RT−→PYR1(2O1)OSO2CF3(aq) + AgCl(s) (2)

The reaction (2) (anion exchange) led to the formation of thehydrophilic ionic liquid (fully soluble in water) and highly insol-uble, AgCl solid precipitate. The latter was removed by vacuumfiltration (0.2 �m Nylon 6,6 filter membrane) to obtain a clearPYR1(2O1)OSO2CF3/water solution.

Step (iii) The presence of silver cations (as AgOSO2CF3) and chlorideanions (as PYR1(2O1)Cl and AgCl) in the PYR1(2O1)OSO2CF3/watersolution was checked with 0.1 N water solution of HCl and AgNO3,respectively (e.g., 1 ml of the ionic liquid/water solution wasadded, respectively, to 1 ml of 0.1 N HCl and AgNO3). A very slightAg+ content was detected by the turning of the resulting solutionfrom transparent to cloudy (precipitation of AgCl) whereas Cl−

was not detected (e.g., the solution hold clear without any precip-itate even after a few day storage followed by centrifugation step).The AgOSO2CF3 excess was removed by slowly adding 0.1 N HClto fully titrate the silver cation, e.g., until no AgCl precipitate wasobserved:

AgOSO2CF3 + HClRT−→HOSO2CF3 + AgCl ↓ (3)

After titration, the solution was vacuum filtered (0.2 �mNylon 6,6 filter membrane) to remove the AgCl solid precipi-tate. Finally, the volatile HCl excess and HOSO2CF3 (by-productfrom the titration with HCl) were massively removed from thePYR1(2O1)OSO2CF3 water solution by vacuum distillation in rotaryevaporator at 90 ◦C until full solvent removal (generally a fewhours). The absence of silver cations and chloride anions was fur-ther confirmed by adding 0.1 N HCl and 0.1 N AgNO3 aqueoussolution to small portions of the ionic liquid, respectively.

Successively, PYR1(2O1)OSO2CF3 (previously dissolved in water)was purified through activated carbon and acidic alumina (car-bon/ionic liquid and alumina/ionic liquid weight ratios equal to 0.6and 0.9, respectively) in order to remove the impurities containedin AgOSO2CF3 and HCl. The liquid fraction (ionic liquid + water)was separated from the solid phase (carbon + alumina) by vac-uum filtering (0.2 �m Nylon 6,6 filter membrane). Successively,the solid fraction was rinsed in situ with deionized water (e.g.,water/(carbon + alumina) weight ratio equal to 2:1) to recoverthe ionic liquid fraction trapped through the purifying materials(but avoiding to extract impurities previously adsorbed). A clear,colorless PYR1(2O1)OSO2CF3 solution in water was obtained. Thewater was removed in rotary evaporator at 90 ◦C under vacuumfor two-three hours. Finally, the ionic liquid was dried with an oil-free, vacuum pump at 60 ◦C for at least 2 h and, then, at 120 ◦Cfor at least 18 h (to promote the full removal of HOSO2CF3). ThePYR1(2O1)OSO2CF3 material was stored in sealed glass tubes withina controlled environment (dry-room, R.H. < 0.1% at 20 ◦C). Table 1lists the weight of all chemicals used for the synthetic route ofPYR1(2O1)OSO2CF3. The yield of each single step and the overallprocess is also reported.

The water content was measured using the standard Karl Fishermethod. The titrations were performed by an automatic Karl Fishercoulometer titrator (Mettler Toledo DL32) located inside the dry-room. The Karl Fisher titrant was a one-component (Hydranal34836 Coulomat AG) reagent provided from Aldrich.

NMR measurements were performed using an Avance III spec-trometer working at 200.13 MHz (1H) (Bruker, Germany). Thespectra were recorded with a broad band probe BBFO (Bruker,Germany). Deuterated DMSO (D6, Aldrich, >99%) was used as thesolvent. Peaks were assigned on the basis of the chemical shifts(chemical shifts are expressed in parts per million, ppm) usingDMSO signal as the reference. Elemental analysis was performedto check the Ag+ and Cl− content.

2.2. Thermal analysis

The thermal measurements were performed using a TA Instru-ments (Model Q100) differential scanning calorimeter (DSC).Hermetically sealed, Al pans were prepared in the dry room. Inorder to allow a complete crystallization [18], the IL materials

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Table 1Weight of all chemicals used in the synthesis route of PYR1(2O1)OSO2CF3. The yield of each single step and of the overall process is also reported.

Chemicals Weight/g Notes

Step i – Synthesis of the PYR1(2O1)Cl precursorpurified PYR1 1.00 0.5 wt% PYR1 excesspurified Cl-CH2CH2-O-CH3 1.11 PYR1/1-Cl-EtOMe weight ratio = 0.90Ethyl acetate (EtAc) 2.22 PYR1/EtAc volume ratio = 0.50Ethyl acetate (rinsing) 2.11 (5× times) Precursor/EtAc weight ratio = 1.00 In total: 10.55 g EtAcPYR1(2O1)Cl 2.11 Step yield: 100%

process yield: 100%Step ii – Synthesis of the PYR1(2O1)OSO2CF3 ionic liquidPYR1(2O1)Cl 2.11 From step iH2O 6.33 PYR1(2O1)Cl/H2O weight ratio = 0.33AgOSO2CF3 (99 wt.%) 3.10 3 wt.% AgOSO2CF3 excess

(1.47 g of AgOSO2CF3 per 1.00 g of PYR1(2O1)Cl)PYR1(2O1)OSO2CF3 (in water) 3.27 Step yield: 95%

(1.55 g PYR1(2O1)OSO2CF3 per 1.00 g of PYR1(2O1)Cl)process yield: 95%

Step iii – Purification of the PYR1(2O1)OSO2CF3 ionic liquidPYR1(2O1)OSO2CF3 (in water) 3.27 From step iiHCl (0.1 N) 0.1–0.2 ml –PYR1(2O1)OSO2CF3 3.27 After removal of Ag+ excessH2O 4.38 IL/H2O volume ratio = 1.00activated carbon 1.96 Carbon/IL weight ratio = 0.6alumina 2.94 Alumina/IL weight ratio = 0.9H2O (rinsing) 9.80 H2O/(C + Al2O3) weight ratio = 2purified PYR1(2O1)OSO2CF3 (in water) 2.94 Step yield: 90%

overall process yield: 85.5%

were thermally annealed in the DSC instrument by repeatedlycycling and/or holding the samples at sub-ambient temperaturesfor varying periods of time. Successively, the samples were cooled(10 ◦C min−1) down to −140 ◦C and, then, heated (10 ◦C min−1) upto 150 ◦C. The appearance of a cold re-crystallization peak duringthe heating scan was taken as test of uncompleted crystallizationof the sample.

2.3. Ionic conductivity

The ionic conductivity was determined by a conductivity meterAMEL 160 in the temperature range from −40 ◦C to 100 ◦C using aclimatic test chamber (Binder GmbH MK53). The entire setup wascontrolled by software developed at ENEA. The ILs were housed(within the dry room) in sealed, glass conductivity cells (AMEL192/K1) equipped with two porous platinum electrodes (cell con-stant equal to 1.0 ± 0.1 cm−1). In order to fully crystallize thematerials [18], the cells were immersed in liquid nitrogen for a fewseconds and, then, transferred in the climatic chamber at −40 ◦C.After a few minutes of storage at this temperature, the solid sam-ples turned again liquid. This route was generally repeated untilthe ionic liquid samples remained solid at −40 ◦C. After a storageperiod at −40 ◦C for at least 18 h the conductivity of the materialswas measured by running a heating scan at 1 ◦C h−1.

2.4. Viscosity measurements

The viscosity measurements were carried out using a rheome-ter (HAAKE RheoStress 600) located in the dry-room. The testswere performed from 20 ◦C to 80 ◦C (1 ◦C min−1 heating rate) in the100 s−1–2000 s−1 rotation speed range. Measurements were takenafter 10 ◦C steps.

2.5. Self-diffusion coefficients

PFG-NMR measurements were performed using an Avance IIIspectrometer, working at 200.13 MHz (1H) and at 188.15 MHz (19F)(Bruker). A Diff30 probe-head was used for the determination ofthe diffusion coefficients with selective RF-inserts for 1H and 19Fand maximum gradient field strength of 1.8 T/m (Bruker, Germany).

In order to minimize convection effects a sample volume of 70 �Lwas chosen. The temperature was adjusted using the gradient cool-ing unit and the digital variable temperature unit (B-VT 3000). Thetemperature in the coil region was additionally checked with anexternal temperature probe connected to a GMH 3710 (Greisingerelectronics), which was inserted into the coil region and, then,removed before sample insertion. The temperature equilibrationtime for each sample was 15 min. From 293 K up to 322 K the sam-ple was heated by solely using the gradient cooling unit to avoidconvection. The maximum temperature of the gradient cooling unitis reached at 322 K. For higher temperatures a constant heated air-flow of 600 L h−1 was used in addition to the gradient cooling unit.Further approaches to suppress convection effects were made byusing the dstegp3s-pulse sequence and always repeating the diffu-sion experiment with a different diffusion time (40 ms and 100 ms)[19,20]. The dstegp3s results differ in diffusion times of less than4%.

2.6. Density measurements

The density measurements were performed from 90 ◦C to 20 ◦Cat 10 ◦C step using a density meter (Mettler Toledo DE40) locatedin the dry-room. The samples were previously degassed undervacuum at 50 ◦C overnight to avoid bubble formation during thecooling scan tests.

2.7. Electrochemical stability

The electrochemical stability window (ESW) was evaluated bylinear sweep voltammetries (LSVs) at 5 mV s−1. A sealed, three-electrode, glass micro-cell, described in details elsewhere [21], wasused for the LSV tests. The cell was loaded with a small amount of ILsample (about 0.5 ml). A glass-sealed, platinum working microelec-trode (active area equal to 0.78 mm2) and a platinum foil counterelectrode (about 0.5 cm2) were used. The reference electrode wasa silver wire immersed in a 0.01 M solution of Ag OSO2CF3 inPYR14TFSI, separated from the cell compartment with a fine glassfrit. This reference electrode was found to be stable for at leastthree weeks. High purity argon (3 ppmv water and 2 ppmv oxy-gen) was flown over the ionic liquid under investigation for 30 min

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Table 2NMR chemical shifts for PYR14OSO2CF3 and PYR1(2O1)OSO2CF3.

PYR14OSO2CF31H NMR: (DMSO D6) ı = 0.92 (t, J = 7.4 Hz, 3H), 1.31 (sext., J= 7.4 Hz, 2H),

1.67 (m, 2H), 2.07 (m, 4H), 2.96 (s, 3H), 3.28 (m, 2H), 3.44 (m, 4H)PYR1(2O1)OSO2CF3

1H NMR: (DMSO D6) ı = 2.20–2.35 (m, 4H), 3.22 (s, 3H), 3.50 (s, 3H),3.60–3.80 (m, 6 H), 3.90–4.00 (m, 2H)

before the start of the test. The gas flow was continued during theexperiment. Separate LSV tests were carried out on each IL sample(previously dried as described in Experimental) to determine thecathodic and anodic electrochemical stability limits. The measure-ments were run scanning the cell potential from the open circuitpotential (OCP) towards more negative (cathodic limit) or positive(anodic limit) potential. Clean electrodes and fresh samples wereused for each test. To confirm the results obtained, the LSV testswere performed at least twice on different fresh samples of eachIL material. The measurements were performed in the dry room at20 ◦C using a Schlumberger (Solartron) Electrochemical Interface(model 1287) controlled by software developed at ENEA.

3. Results and discussion

3.1. Ionic liquid materials

The above reported synthetic route allowed to obtain a clear,colorless and odorless PYR1(2O1)OSO2CF3 ionic liquid material withan overall yield higher than 85% and a water content below 3 ppm,this representing a very low humidity level for hydrophilic mate-rials. NMR measurements (Table 2) and elemental analysis wereperformed to verify the synthesis of the hydrophilic ionic liquids.The latter technique indicated a very high purity level. Silver andchloride content was below the limit of detection of the technique,i.e. 25 ppb and 400 ppb, respectively.

3.2. Thermal behavior

A comprehensive physicochemical characterization were per-formed on hydrophilic PYR14OSO2CF3 and PYR1(2O1)OSO2CF3 ionicliquids. The aim was to find some clues of how the molecular struc-ture (e.g., the two ionic liquids differ only by a heteroatom in thecation main side chain) influences the macroscopic behavior ofthese materials.

The DSC heating traces for PYR14OSO2CF3 andPYR1(2O1)OSO2CF3, reported in Fig. 1 (thermal cycling fea-tures for crystallization are not shown), display pronounced andnarrow endothermic peaks around 5.6 ◦C and 12.9 ◦C (Table 3),respectively for PYR14OSO2CF3 and PYR1(2O1)OSO2CF3, due to themelting of the materials. In addition, endothermic weak featuresare observed at −96.3 ◦C (PYR14OSO2CF3) and −3.7 ◦C and −1.1 ◦C(PYR1(2O1)OSO2CF3) which are likely ascribed to solid-solid phasetransitions occurring below the melting temperature. It is to notethat the absence of exothermal “cold re-crystallization” peaks(during the heating scan) in the DSC traces supports for a completecrystallization of the IL samples [18].

Fig. 1. DSC trace of PYR14OSO2CF3 and PYR1(2O1)OSO2CF3. Scan rate: 10 ◦C min−1.

3.3. Transport properties

The ionic conductivity vs. temperature dependence is plotted inFig. 2. The pure PYR14OSO2CF3 material shows an onset for increas-ing conductivity around 5 ◦C, followed by a steep rise of about fiveorders of magnitude, due to the melting of the IL in good agree-ment with the DSC results depicted in Fig. 1. From −40 ◦C to themelting, the PYR14OSO2CF3 sample exhibits a low conductivity(e.g., around 10−8 S cm−1), typical of a solid crystalline material,with a progressive, even if very modest, increase due to a fewsolid-solid phase transitions. This issue, apparently in contrast withthe DSC data (Fig. 1), is likely ascribed to the much lower heat-ing scan rate of the conductivity tests (1 ◦C h−1) with respect tothe thermal ones (10 ◦C min−1). In addition, it is to account that,for conductivity measurements, the crystallization process is pro-moted by: (i) the much larger amount (> 2 g vs. < 5 mg for DSC) ofIL material under test; (ii) the rough surface of the Pt electrodecell and; (iii) the different procedure followed for crystallizing theIL samples. The PYR1(2O1)OSO2CF3 material show a stable but low(<10−8 S cm−1) conductivity value from −40 ◦C to about −35 ◦C.Above −35 ◦C, the conductivity vs. temperature slope increasessubstantially up to −20 ◦C, this indicating that the ions are ableto move even if the sample is still in the solid phase, e.g., in goodagreement with the DSC data reported in Fig. 1. At −10 ◦C, thesample exhibit a conductivity value of the order of 10−6 S cm−1,which is certainly interesting for solid materials. As a result of

Table 3Physical properties of PYR14OSO2CF3 and PYR1(2O1)OSO2CF3. The melting point (m.p.) of the materials was evaluated from DSC (a) and conductivity (b) measurements. Thedensity (d), viscosity (�), and diffusion coefficient (Dcation, Danion) values are referred to 20 ◦C whereas the conductivity (�) is reported also at −20 ◦C.

m.p. (a)/◦C m.p. (b)/◦C ı/g cm−3 �/m Pa s−1 �(−20 ◦C)/S cm−1 �(20 ◦C)/S cm−1 Dcation (20 ◦C)/m2 s−1 Danion (20 ◦C)/m2 s−1

PYR14OSO2CF3

5.6 ± 0.5 8.4 ± 0.5 1.256 ± 0.002 158 ± 8 (8.0 ± 0.2) × 10−8 (1.3 ± 0.1) × 10−3 (7.37 ± 0.04) × 10−12 (5.87 ± 0.05) × 10−12

PYR1(2O1)OSO2CF3

12.9 ± 0.5 n.d. 1.338 ± 0.002 115 ± 6 3.9 ± 0.1 1.8 ± 0.1 (9.88 ± 0.04) × 10−12 (7.55 ± 0.03) × 10−12

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Fig. 2. Ionic conductivity vs. temperature behavior of PYR14OSO2CF3 (open squares)and PYR1(2O1)OSO2CF3 (solid squares). The error bars, when not reported, fall withinthe data markers.

the substantial gained ion mobility after solid–solid phase tran-sitions, the conductivity increases progressively up to 10−3 S cm−1

prior the IL material is fully molten. Conversely to what observedfor bis(trifluoromethanesulfonyl)imide, TFSI, -based pyrrolidiniumionic liquids [15], the incorporation of an oxygen atom in thecation aliphatic main side chain does not result in hindering or pre-venting the crystallization process (as already previously detectedin bis(fluoromethanesulfonyl)imide, FSI, -based materials) [22],in good agreement with the DSC results. This is likely due tothe much more remarkable tendency of the (OSO2CF3)− anion tocrystallize with fast kinetics [23]. In the molten state, the con-duction value of PYR1(2O1)OSO2CF3 is slightly larger with respectto PYR14OSO2CF3 (Table 3), likely ascribed to the enhanced flex-ibility of the methoxyethyl, CH2CH2OCH3, chain resulting fromthe introduction of an oxygen atom [12–15,24]. Therefore, theentanglement of the side ether group is promoted, this leading toa reduction of the steric hindrance (e.g., higher mobility) of thecation.

Rheological measurements were performed for better under-standing the conduction phenomena. Fig. 3 depicts the temperaturedependence of the viscosity (panel A) and the resistivity (panelB, for comparison purpose) as VTF plots (i.e., the T0 parameterwas obtained from conductivity data). The measurements werecarried out in an interval ranging from 20 ◦C to 80 ◦C, i.e., wherethe IL sample exhibited a Newtonian behavior (not shown). ThePYR1(2O1)OSO2CF3 sample is found to be slightly less viscous withrespect to PYR14OSO2CF3 (Table 3). It is assumed that the insertedoxygen in the main side chain enhances the flexibility, therebyreducing the ion volume of the cation and, therefore, the viscosity.The viscosity and resistivity VTF diagrams exhibit a linear trend forboth IL samples. This is better evidenced in Table 4, which reportsthe fitting parameters of the VTF-fit obtained from conductivityand fluidity (inverse of viscosity) data. Once more, these resultshighlight the strong relationship between the conductivity and theviscosity, especially for PYR1(2O1)OSO2CF3, i.e., the ionic mobilityis mostly affected by the viscous friction of the liquid dragging onthe ion as it attempts to move under an applied DC electric field[25].

The self-diffusion coefficients (D) of PYR14OSO2CF3 andPYR1(2O1)OSO2CF3 are displayed in Fig. 4 (panel A and B) and plottedversus 103 (T−T0)−1 (as K−1). In good agreement with the con-ductivity data, anion and cation of PYR1(2O1)OSO2CF3 have higher

Fig. 3. VTF plot of viscosity (panel A) and resistivity (panel B) of PYR14OSO2CF3 (opensquares) and PYR1(2O1)OSO2CF3 (solid squares). The error bars, when not reported,fall within the data markers.

diffusivities compared to those of PYR14OSO2CF3. For both ionicliquids the cation has a clearly higher diffusion coefficient than theanion. This might be surprising if regarded to the different sizes ofthe ions, however, such a phenomenon was reported for 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) as well [26]. Thecomparison of the data depicted in Figs. 3 and 4 evidences that thediffusion coefficient and the viscosity exhibit an opposite behav-ior with respect to the temperature. As successively discussed inthe text, the investigated OSO2CF3-based ionic liquids are mainlydissociated (Figs. 5 and 6). Therefore, the self-diffusion coefficient,

Table 4VTF fit parameters of conductivity (�) and fluidity (1/�) of PYR14OSO2CF3 andPYR1(2O1)OSO2CF3.

A0 B T0

A = A0 exp[−B/(T − T0)]PYR14OSO2CF3

� 0.20 ± 0.01 530 ± 10 192 ± 11/� 1.9 ± 0.2 600 ± 300 170 ± 40

PYR1(2O1)OSO2CF3

� 0.50 ± 0.04 730 ± 20 164 ± 21/� 1.7 ± 0.8 600 ± 200 160 ± 20

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Fig. 4. VTF plot of self-diffusion coefficients of PYR14OSO2CF3 (panel A) andPYR1(2O1)OSO2CF3 (panel B). The error bars, when not reported, fall within the datamarkers.

Di, can be reasonably correlated with the viscosity (�) through theStokes-Einstein equation [27]:

Di = kBT

6�ri�(4)

where kB is the Boltzmann constant, T is the absolute temperatureand ri is the radius of the ion species i.

The temperature dependent behavior for each ion in both ionicliquids can be described as VTF-like since a straight-line trend isclearly visible. Furthermore, the diffusion coefficients of the anion

Fig. 5. Walden plot of PYR14OSO2CF3 (open squares) and PYR1(2O1)OSO2CF3 (solidsquares). The Walden diagram of PYR14TFSI (open stars) and PYR1(2O1)TFSI (solidstars), obtained from data reported in reference 15, is shown for comparison pur-pose. The error bars, when not reported, fall within the data markers. The solidstraight line, referred to a 0.01 N KCl solution, fixes the position of the ideal Waldenline.

Fig. 6. VTF plot of molar conductivities and r-value (degree of dissociation) ofPYR14OSO2CF3 (panel A) and PYR1(2O1)OSO2CF3 (panel B).

tend to converge to the cation one towards higher temperatures.This tendency is also expressed by the significant different B-values(Table 5). This convergence might indicate the existence of a tem-perature sensitive environment, which allows cation and anion todiffuse with similar speed at high temperatures.

A qualitative approach for investigating the “ionicity” of ILs isrepresented by the Walden rule [12,28]:

�� = k (5)

where � is the molar conductivity, � is the viscosity and k is atemperature dependent constant. Fig. 5 plots the log � vs. log �−1

dependence for hydrophilic PYR14OSO2CF3 and PYR1(2O1)OSO2CF3ionic liquids. The solid straight line (through the origin) ofFig. 5 is referred to a 0.01 N KCl aqueous solution; this system,known to be fully dissociated and to have ions of equal mobil-ity [12], was used as the calibration point (e.g., ideal Waldenline). The PYR14OSO2CF3 and PYR1(2O1)OSO2CF3 samples, as wellas PYR14TFSI and PYR1(2O1)TFSI reported for comparison purpose(from data reported in Ref. [15], are seen to lie just below theideal line, behaving as “good ionic liquids” [12,28,29], e.g., mostlyconsisting of independently mobile ions.

Table 5VTF-fit parameters of diffusion coefficients (D) of PYR14OSO2CF3 andPYR1(2O1)OSO2CF3.

D0 B T0

PYR1(2O1)OSO2CF3

(PYR1(2O1))+ (6 ± 2) × 10−9 760 ± 90 173 ± 9(OSOCF3)− (1.1 ± 0.3) × 10−8 970 ± 80 160 ± 7

PYR14OSO2CF3

(PYR14)+ (4 ± 2) × 10−9 600 ± 100 190 ± 10(OSOCF3)− (1.2 ± 0.2) × 10−9 970 ± 50 165 ± 4

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A different perspective on the “ionicity” can be achieved via theNernst-Einstein Eq. (6), which allows to compare the impedance(7) and PFG-NMR results in terms of molar conductivities.

�NMR = zF2(D+ + D−)RT

(6)

�� = �imp

cion(7)

Impedance and PFG-NMR results contribute unequally to themolar conductivity. The determined diffusion coefficients resultfrom all species the ions can adopt, e.g., single ion, ion pairor micelles. Moreover, impedance measurements only count ionspecies which carry a charge. For the calculation of �NMR, the cationand anion diffusion coefficients were separately fitted with theVTF-equation (Table 5) and the fit parameters were used to cal-culate the molar conductivity. The ratio of both calculated molarconductivities expresses the degree of dissociation (8):

r = ��

�NMR(8)

In Fig. 6 (panels A and B) the molar conductivities are plotted,together with the degree of dissociation (r), as VTF diagrams. Theresults indicated that the IL samples are mainly dissociated, in goodagreement with the Walden plots. For both ionic liquids the dif-ferent molar conductivities diverge towards higher temperatures,which results in an almost linear, decreasing degree of dissociation.This result is contradictory to general understanding that the dis-sociation enhances with the temperature. In agreement with ourresults, Hayamizu et al. have reported a similar trend for PYR13TFSIand PYR13FSI starting from room temperature [30,31]. For otherionic liquids no temperature dependence is discussed [32]. It isstill open and out of scope of this article what is causing this phe-nomenon.

3.4. Density measurements

The properties of PYR14OSO2CF3 and PYR1(2O1)OSO2CF3 in themolten state were also investigated in terms of density at differenttemperatures (Fig. 7). A slight, but progressive density decrease ofabout 4% is observed in passing from 20 ◦C to 90 ◦C. Both hydrophilicionic liquids display a linear density vs. temperature dependencewith similar slope. The density of PYR1(2O1)OSO2CF3 is found to be8% higher than that of PYR14OSO2CF3 (Table 4). This behavior sup-ports, once more, for a decrease of the pyrrolidinium cation sterichindrance due to the better entanglement of the more flexible etherside chain.

3.5. Electrochemical stability

The electrochemical stability window (ESW) of PYR14OSO2CF3and PYR1(2O1)OSO2CF3 (at 20 ◦C) is depicted in Fig. 8 (panel A). Allpotentials are given vs. the Ag/Ag+ reference electrode. The poten-tial vs. Li/Li+ is also indicated. The cathodic (EClimit) and anodic(EAlimit) stability limit potential values, determined by the inter-cept of the step current rise from the X-axis, are listed in Table 6. The

Fig. 7. Density vs. temperature dependence of PYR14OSO2CF3 (open data markets)and PYR1(2O1)OSO2CF3 (solid data markers). The error bars, when not reported, fallwithin the data markers.

Fig. 8. Linear sweep voltammetries (LSVs) of PYR14OSO2CF3 (dotted trace),PYR1(2O1)OSO2CF3 (thick solid trace) and PYR1(2O1)OSO2CF3 containing additionalHOSO2CF3 (thin solid trace) at 20 ◦C (panel A). Panel B reports LSVs of PYR14TFSI(dotted trace) and PYR1(2O1)TFSI (solid trace) for comparison purpose (data takenfrom reference 15). Platinum as the working and counter electrode. The referenceelectrode is a silver wire immersed in a 0.01 M solution of AgOSO2CF3 in PYR14TFSI.Scan rate: 5 mV s−1.

potentials taken when the current density through the cell reached0.05 mA cm−2 (EC1 and EA1) and 0.1 mA cm−2 (EC2 and EA2) are alsoreported in Table 6 for comparison purpose. Both ILs investigatedshow a steep rise in the anodic current density when the oxida-tion of the anion [27] takes place. This process is seen to occur inthe IL with triflate anion at lower potentials (about 200 mV) with

Table 6Cathodic (EC1 and EC2) and anodic (EA1 and EA2) potential values determined from LSV tests run on PYR14OSO2CF3 and PYR1(2O1)OSO2CF3 at 20 ◦C. The potentials were takenwhen the current density through the cell reached 0.05 mA cm−2 (EC1 and EA1) and 0.1 mA cm−2 (EC2 and EA2). The cathodic (ECLimit) and anodic (EALimit) limit potential valueswere determined by the intercept of the step current raises with the X axis. The potentials are given versus the Ag/Ag+ redox couple (Ag wire in a 0.01 M solution of AgOSO2CF3 in PYR14TFSI).

IL sample Cathodic potential/V Anodic potential/V

EC1 EC2 ECLimit EA1 EA2 EALimit

PYR14OSO2CF3 −2.87 ± 0.01 −3.35 ± 0.01 −3.76 ± 0.01 1.86 ± 0.01 2.03 ± 0.01 1.97 ± 0.01PYR1(2O1)OSO2CF3 −0.57 ± 0.01 −0.60 ± 0.01 −1.29 ± 0.01 1.64 ± 0.01 1.75 ± 0.01 1.83 ± 0.01

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M. Moreno et al. / Electrochimica Acta 99 (2013) 108– 116 115

Fig. 9. 1H NMR of PYR1(2O1)OSO2CF3 before (pure, black trace) and after (reduced,red trace) cathodic (low voltage) treatment.

respect to those detected for TFSI-based ionic liquids. This is wellevidenced comparing the anodic curves in panel A with those inpanel B (data taken from Ref. [15]. No other feature is observedduring the anodic scan, thus excluding the presence of impuritieswhich could be oxidized prior to the anodic breakdown potentialof the hydrophilic ILs.

On the cathodic side the incorporation of an oxygen atom in thepyrrolidinium main side chain results in a dramatic decrease, e.g.,about 2.5 V, of the breakdown voltage of the triflate-based ionicliquid (Fig. 8A and Table 6). This might be ascribed to an electrondensity decrease on the oxygen atom in the methoxyethyl group,due to electrostatic interactions (e.g., by lone-pair electrons) withthe positively charged nitrogen in the pyrrolidinium ring, thus pro-moting the electro-reduction of the cation [12]. However, a muchlower decrease of the cathodic stability was observed in the cor-responding TFSI-based ionic liquid (Fig. 8B). A neat explanationof the poor cathodic stability of PYR1(2O1)OSO2CF3 is not easilyavailable. In an effort to identify the cathodic decomposition prod-ucts, NMR measurements were performed on a neat sample anda heavily reduced one. The results are compared in Fig. 9 wherethe 1H NMR spectra of the two samples are illustrated. Unfor-tunately, no significant structural change of the cation could beconfirmed. The reason of this is to be find in the volatility of thedecomposition products (e.g. the decomposition experiments wereperformed under argon flow and, therefore, in situ tests need tobe suitably designed and performed to investigate the cathodicstability of PYR1(2O1)OSO2CF3. The presence of HOSO2CF3 traces(which may affect the cathodic stability) cannot be rejected becauseextremely low contents (e.g., a few ppm) of H3O+ are very difficultto be detected, even if NMR measurements and elemental analysishave proved high purity for PYR1(2O1)OSO2CF3. We have demon-strated that very low amounts of moisture and oxygen are capableto lower the IL cathodic stability [21] but without affecting themassive decomposition of the ionic liquid sample as converselyobserved for PYR1(2O1)OSO2CF3 (Fig. 8A). Nevertheless, with theaim to check the effect of triflic acid on the electrochemical stability,we have performed cathodic LSVs on PYR1(2O1)OSO2CF3 sampleshaving, e.g., upon addition of triflic acid, a HOSO2CF3 content equalto 0.4 wt.%. Also, the measurements were run in an argon atmo-sphere glove-box to avoid any contamination with external oxygen.The results, reported in Fig. 8A (thin solid line), evidenced how thepresence of HOSO2CF3, even in not negligible amounts (4000 ppm),does not affect the massive cathodic decomposition (reduction) ofPYR1(2O1)OSO2CF3. Conversely, the addition of triflic acid leads toremarkable increase of the features appearing prior the massivereduction of the IL sample, mainly due to hydrogen development(H+ reduction) and, secondarily, to parasitic reactions catalyzed by

HOSO2CF3 and other impurities. To summarize, the contempora-neous presence of an oxygen atom in the pyrrolidinium side chainand a strong counter anion as (OSO2CF3)− seems to have a dele-terious effect. This issue, however, needs to be further clarified.Finally, PYR14OSO2CF3 showed an ESW close to 5 V, which makesthis water soluble IL viable for application in high-energy electro-chemical devices.

4. Conclusions

A water-soluble ionic liquid (IL), N-methoxyethyl-N-methyl-pyrrolidinium trifluoromethane sulfonate (PYR1(2O1)OSO2CF3), wassynthesized for use as electrolyte component in high safety,electrochemical devices and as solvent for extraction pro-cesses. The physicochemical and electrochemical properties ofPYR1(2O1)OSO2CF3 were investigated and compared with those of acommercial N-butyl-N-methylpyrrolidinium trifluoromethanesul-fonate (PYR14OSO2CF3) material in order to gain knowledge aboutthe influence of the molecular structure on the macroscopic behav-ior of these materials.

The incorporation of an oxygen atom in the cation aliphatic sidechain was seen to not prevent the crystallization of the IL mate-rial. Conversely, the enhanced flexibility of the ether side chainsupports for the increase in self-diffusion coefficient, density andionic conductivity (e.g., approaching 2 mS cm−1 at 20 ◦C), whichappears well correlated with the rheological and thermal behav-ior. The Nernst-Einstein equation suggested that the “ionicity” istemperature dependent and decreasing towards higher tempera-tures.

Finally, both water-soluble ionic liquids showed anodic elec-trochemical stabilities typical of hydrophobic, TFSI-based, ILs.However, PYR1(2O1)OSO2CF3 showed a poor cathodic electrochem-ical stability. On the other hand, PYR14OSO2CF3 showed an overallESW close to 5 V, which makes this material promising for high volt-age electrochemical devices. Considering that this water-solubleionic liquid presents great advantages in terms of recyclability(it can be extracted by water rinsing), PYR14OSO2CF3 should beconsidered as a good candidate as electrolyte component in super-capacitors and lithium batteries.

Acknowledgment

The authors wish to thank the European Commission STREP“ILHYPOS” project #51307 (FP6) for the financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.electacta.2013.03.046.

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