Study of Ion Diffusional Motion in Ionic Liquid-Based Polymer

Jan 25, 2013 - Department of Physics, Guru Nanak Dev University, Amritsar, 143005, India. ‡. Department of Applied Molecular Chemistry, College of ...
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Study of Ion Diffusional Motion in Ionic Liquid-Based Polymer Electrolytes by Simultaneous Solid State NMR and DTA Dushyant Singh Rajput,† Koji Yamada,‡ and S. S. Sekhon*,†,§ †

Department of Physics, Guru Nanak Dev University, Amritsar, 143005, India Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Narashino, Chiba, 275-8575, Japan § Department of Physics, The University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies ‡

S Supporting Information *

ABSTRACT: Polymer electrolytes containing ionic liquid (IL), 2-methyl-1,3dipropylimidazolium dihydrogenphosphate (MDPImH2PO4) have been studied by 1 H solid state NMR and differential thermal analysis (DTA) simultaneously by using a specially designed probe. To the best of our knowledge, this is the first report of its kind for IL based polymer electrolytes. The variation of NMR line width with temperature for the IL and polymer electrolytes shows line narrowing at the glass transition and melting temperature. The onset of long-range ion diffusional motion also takes place at these temperatures and is accompanied by a sudden increase in ionic conductivity value by 2−3 orders of magnitude. The presence of amorphous and crystalline phases in IL-based polymer electrolytes has been observed from X-ray diffraction (XRD) studies, and the amorphous phase is the high conducting phase in these polymer electrolytes. The IL-based polymer electrolytes have been observed to be thermally stable up to 200 °C. The results obtained from ion transport studies have also been supported by Fourier transform infrared (FTIR), XRD, and cyclic voltammetry (CV) studies.

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INTRODUCTION Ionic liquids (ILs) are primarily composed of bulky and asymmetric cations and large anions. Their physicochemical properties can be tuned by using different combinations of cations and anions. The thermal stability and other properties of hydrophobic and hydrophilic ILs based on imidazolium cations have been reported to depend upon the alkyl chain length of the imidazolium cation and the nature of the anion.1−10 A change in viscosity, density, ion size, and degree of dissociation also affects the ionic conductivity of ILs, but at this stage it is very difficult to estimate the contribution of each parameter separately. ILs generally have higher viscosity due to strong Coulombic forces between the ionic species present in them. A number of ILs containing various fluoroanions have already been widely studied.1−4,11−13 Also, ILs containing acidic counteranions are very important due to the possibility of proton conduction by hopping mechanism, and their potential applications as proton-conducting membranes in polymer electrolyte membrane fuel cells and other devices.14−16 As ILs consist of loosely packed cations and anions, generally both cations and anions are reported to be mobile in ILs as well as in IL-based polymer electrolytes. The nature of mobile species and other ion transport properties of polymer electrolytes are generally studied by solid state NMR.15−19 The glass transition temperatures were observed just below the NMR line narrowing temperatures, i.e., the motional correlation frequency at the glass transition were estimated to be ≤40−60 kHz (full width at half-maximum, fwhm). The ionic conductivity also © 2013 American Chemical Society

shows a sudden increase at the temperature at which line narrowing takes place. The phase change in the materials can also be studied by differential thermal analysis (DTA), and the temperature at which line narrowing takes place can be related to different thermal temperatures (glass transition, melting and crystallization temperature) determined from DTA.19 Generally, both (NMR and DTA) measurements are performed independent of each other. However, a simultaneous recording of solid state NMR and DTA for the same sample under identical conditions can be very helpful in finding a correlation between the NMR and DTA results. In the present study, we have recorded 1H solid state NMR and DTA simultaneously by using a specially designed probe on different samples based on an IL (MDPImH2PO4). This has been done to find a correlation between the motional narrowing, thermal temperatures, and ionic conductivity of different polymer electrolytes. The results obtained from ion transport studies have also been supported by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and cyclic voltammetry (CV).

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EXPERIMENTAL SECTION Polyethylene oxide (PEO) with average molecular weight 5 × 106 (Aldrich), 2-methylimidazole (>99% Merck), sodium Received: November 27, 2012 Revised: January 25, 2013 Published: January 25, 2013 2475

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rings around the glass tubes. The heating rate of the DTA measurement was 0.5 K/min, and the sampling was performed every 10 s using a Keithley 2182 nanovoltmeter. The 1H NMR spectra were observed at 270.2 MHz every 10 K by a single pulse sequence followed by Fourier transformation. The thermal stability of the IL and the polymer electrolytes in the 25−550 °C temperature range was studied by DSC/TGA/ DTA through the use of EXSTAR TG/DTA 6300 system. The thermal studies were carried out under a nitrogen atmosphere at a scan rate of 10 °C/min. The XRD plots of different samples were recorded by Shimadzu 7000 X-ray diffractometer. A cyclic voltammogram for the IL was recorded using a PGSTAT12 potentiostat/galvanostat. FTIR spectra were recorded using a computer-interfaced Hitachi 270-50 FTIR spectrometer.

ethoxide (Merck), 1-bromo-propane (>98% Merck), potassium dihydrogenphosphate (Merck), acetonitrile (Merck), methanol (Sd fine), and absolute ethanol (Sd fine) were used as the starting materials for the preparation of IL and different polymer electrolytes. The IL 2-methyl-1,3-dipropylimidazoliumdihydrogenphosphate (MDPImH2PO4) used in the present study was prepared by using the method reported earlier.3,18,20 First, 2-methylimidazole was dissolved in absolute ethanol at room temperature, and then sodium ethoxide was added. The solution was stirred for 30 min, and then 1bromopropane was added dropwise. The resulting solution was again stirred for 30 min at room temperature. After this, the above solution was refluxed at 80 °C for 10 h. The mixture was then filtered two to three times to remove sodium bromide, and then ethanol was evaporated under reduced pressure to get 1propyl-2-methylimidazole. Further, 1-bromopropane was added to 1-propyl-2-methylimidazole, and the mixture was refluxed at 90 °C for 2 h. The yellowish solid (2-methyl-1,3-dipropylimidazoliumbromide (MDPImBr)) obtained was washed two to three times with ethyl acetate to remove any starting material. The anion exchange reaction was then performed by taking equimolar amounts of 2-methyl-1,3-dipropylimidazoliumbromide and potassium dihydrogenphosphate in acetonitrile. The resulting mixture was then stirred at room temperature for 72 h, and the precipitates of potassium bromide were filtered. This was followed by the removal of acetonitrile by evaporation under reduced pressure and the IL 2-methyl-1,3-dipropylimidazolium dihydrogenphosphate (MDPImH2PO4) was obtained. Polymer electrolytes containing PEO, IL (MDPImH2PO4), and propylene carbonate (PC) in different proportions were prepared by the solution casting method.15,18 In this method, the stoichiometric quantities of IL, polymer, and PC were dissolved in methanol, and the mixture was stirred to obtain a uniform viscous solution, which was then poured into polypropylene dishes. The solvent was allowed to evaporate slowly, and polymer electrolytes in the free-standing film form were obtained. These films were stored under dry conditions and used for different experimental studies. The ionic conductivity of the IL and polymer electrolytes was measured by the impedance method using a cell with platinum electrodes for the IL, and a cell with pressure contact stainless steel electrodes for polymer electrolytes in the film form. The measurements were performed with a computer-interfaced Hioki 3532-50 LCR Hi-Tester (42 Hz-5 MHz) and HP 4284A (20 Hz-1 MHz) precision LCR meter. The viscosity of the IL was measured by Fungilab rotating viscometer (Visco Basic L) using a small sample adapter assembly, and the temperature was controlled within ±0.1 °C with a Julabo circulator (F-12EC). The 1H and 13C NMR spectra of the IL were obtained on a spectrometer (JEOL, AL-300 MHz) at 300 and 75 MHz, respectively, in CDCl3 using tetramethylsilane (TMS) as the internal standard. Chemical shifts are expressed as δ (ppm) downfield from TMS. The 1H solid state NMR and DTA measurements for IL and polymer electrolytes were performed simultaneously by using a homemade pulse NMR spectrometer with a newly designed probe (Supporting Information, Figure S1), in which the reference (Al2O3) and the sample were placed symmetrically in a copper container. A radio-frequency (rf) coil and a dummy coil around the sealed glass tubes (outer diameter 5 mm) were also placed symmetrically. The DTA (sample temperature and the temperature difference, ΔT) were monitored using a pair of sheath thermocouples, which were silver brazing at the stainless

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RESULTS AND DISCUSSION The melting point of the IL is −38 °C, and the density is 1.36 kg per m3. The ionic conductivity and viscosity of the IL was measured at different temperatures in the 30 to 95 °C range, and its variation with temperature is shown in Figure 1. The

Figure 1. Variation of log conductivity and log viscosity with reciprocal temperature of IL.

ionic conductivity increases by 3 orders of magnitude from 8.12 × 10−5 S cm−1 at 30 °C to 3.37 × 10−2 S cm−1 at 150 °C, but there is a sharp decrease in viscosity from 5380 mPa s at 30 °C to 72.8 mPa s at 95 °C. The decrease in viscosity with temperature results in higher mobility (as viscosity (η) and mobility (μ) are inversely related to each other, μ = q/6πrη, where q is the charge and r is the radius of the ion), which leads to an increase in ionic conductivity (σ = nqμ). Hence, a lower viscosity shall lead to higher ionic conductivity. However, care must be taken to differentiate between macro (bulk) viscosity, which is measured experimentally, and the micro (local) viscosity, which is related to the mobility of ions and hence to the ionic conductivity. Due to the self-dissociating nature of the IL, it consists entirely of ions (cations and anions), and generally both contribute to ionic conductivity.4,15 The variation of ionic conductivity with temperature for the neat IL shows a curved plot, which is due to the amorphous nature of the electrolyte and follows Vogel−Tamman−Fulcher (VTF) behavior. The VTF equation21 for ionic conductivity is given as σ = σo exp[−B/(T − To)], where σo is the pre-exponential constant, which is directly proportional to the concentration of mobile ions, B is the pseudo activation energy for conductance, and To is the ideal glass transition temperature. The best fit 2476

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PEO, the ionic conductivity increases with an increase in temperature and shows a sudden jump at ∼65 °C, which corresponds to the melting point of PEO.24 This sudden increase in ionic conductivity is due to a change of the electrolyte from the semicrystalline to the amorphous phase at the melting point and the amorphous phase is generally reported to be the high conducting phase in PEO-based electrolytes.18 Polymer electrolytes containing IL show a maximum conductivity of 3.91 × 10−4 S cm−1 at 120 °C. As this value is not very high, a further increase in ionic conductivity by 1−2 orders of magnitude is required for their potential applications. The ionic conductivity (σ = nqμ, where n is the number of free ions, q is the charge, and μ is the mobility of charge carriers) of different electrolytes can be increased by increasing the number of free ions (n), or by decreasing the viscosity (increasing mobility) of free ions, or by increasing both n and μ. This can be achieved by adding a plasticizer having high dielectric constant and lower viscosity.18,20,25 PC (ε = 64.4, η = 2.53 mPa s), having such properties has been used as a plasticizer in the present study. The addition of PC results in an increase in ionic conductivity by nearly 1 order of magnitude and electrolytes having composition IL + PEO + PC show ionic conductivity of 4.79 × 10−5 at 30 °C, which increases to 1.37 × 10−3 S cm−1 at 120 °C. The higher dielectric constant and lower viscosity of PC, as compared to the IL, helps in enhancing the ionic conductivity of polymer electrolytes. However, large amount of plasticizer could not be added as it leads to deterioration in the mechanical and thermal properties of polymer electrolytes. An optimum amount of plasticizer can thus be chosen to obtain polymer electrolytes with higher ionic conductivity along with good mechanical and thermal properties. The effect of the addition of IL and PC on the presence of amorphous and crystalline phases in different electrolytes was studied by XRD, and the XRD plots for different electrolytes are given in Figure 4. The XRD

parameters for the IL were calculated from the experimental results, and their values are σo = 1.95 S cm−1, B = 443 K, To = 21,1 and R2 = 0.9985. Although ILs possess high ionic conductivity, these are generally not suitable as electrolytes in some electrochemical devices due to their liquid nature. Electrolytes in the film form are generally preferred for such devices so that the drawbacks associated with the use of liquids can be avoided. In the present study, polymer electrolytes containing IL have been prepared in the free-standing film form by incorporating the IL in PEO, which is a semicrystalline polymer. The addition of polymer to the IL generally results in a decrease in ionic conductivity as the polymer provides mechanical support to the electrolytes, and similar results have previously been reported with other ILs. 12,22,23 In the present study, the IL in different concentrations was added to polymer (PEO), and the variation of ionic conductivity with IL concentration is shown in Figure 2. The ionic conductivity at room temperature (30 °C)

Figure 2. Variation of ionic conductivity for polymer electrolytes (PEO+IL) with the concentration of IL.

increases with the concentration of IL, and reaches a value of 3.37 × 10−5 Scm−1 at IL concentration of 50 wt % in polymer. The IL provides free ions for conduction and also has plasticizing properties due to its large size. Polymer electrolytes containing IL, having composition IL + PEO (in equal weight ratio) and IL + PEO + PC (IL + PEO + 20 wt % PC) were also studied. The variation of ionic conductivity with temperature for different electrolytes is shown in Figure 3. For the IL +

Figure 4. XRD plots of IL, PEO, IL + PEO and IL + PEO + PC.

plot for PEO,26 which is semicrystalline in nature, has peaks at 2θ values (in degrees) at 13.30, 14.51, 19.05, and 23.22. The XRD plot for the IL shows a broad halo, which indicates the highly amorphous nature of the IL. The XRD plot for IL + PEO shows the presence of two peaks superimposed on the broad halo. The positions of the sharp peaks correspond to the presence of the crystalline phase of PEO, and it shows that both

Figure 3. Variation of conductivity with temperature for IL, IL + PEO and IL + PEO + PC. 2477

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Figure 5. The 1H NMR spectra and DTA of IL (a,b), IL + PEO (c,d) and IL + PEO + PC (e). The numbers denote the temperature in Kelvin.

motion generally starts at the temperature at which line narrowing takes place, and it should also enhance the ionic conductivity of the electrolyte.19 To verify this, the ionic conductivity of the IL was also measured from 100 to 300 K, the same temperature range in which NMR and DTA measurements have been performed. The motional narrowing has been found to result in long-range translational motion, which should affect the ionic conductivity value of the electrolyte at that temperature. Thus the temperature at which line narrowing takes place and the temperature at which ionic conductivity show a sudden increase should be closely related to each other. Such a correlation has been examined in the present case by combining NMR, DTA, and ionic conductivity results for different electrolytes (IL and polymer electrolytes). For the neat IL, the 1H NMR peak has line width of 36−42 kHz in the temperature range of 100−200 K, which narrows down to less than 2 kHz above 200 K. The line narrowing starts at around 195−200 K, which is very close to the glass transition temperature (198 K) of the IL. The line becomes very narrow (∼2 kHz) above 240−250 K, i.e., above the melting point (245 K) of the IL. This suggests that the onset of long-range translational motion in the IL takes place at the glass transition temperature and changes to the high conducting phase above the melting point of the IL. The onset of ion diffusional motion should also affect the ionic conductivity value, and to verify it, the ionic conductivity of the IL was measured in the same (100−300 K) temperature range. The variation of the ionic conductivity of the IL with temperature has also been included in Figure 6a. The ionic conductivity of the IL is very low and becomes nearly constant in the low temperature region (100−150 K) but shows an increasing trend near the glass transition temperature (at around ∼200 K). A sudden increase in ionic conductivity has also been observed near the melting point of the IL. The ionic conductivity, which is of the order of 10−7 S cm−1 at 100−150 K, increases by 2 orders of magnitude to 10−5 Scm−1 above the melting point of the IL. The temperature at which ionic conductivity shows a jump corresponds to the temperature at which line narrowing was observed. Thus the onset of ion

the crystalline and amorphous phases are present in this sample. This is also reflected in the ionic conductivity results (Figure 3) in which a sharp jump in ionic conductivity value has been observed at the melting point of PEO due to a transition from the semicrystalline to amorphous phase. In the XRD plot for IL + PEO + PC sample, the intensity of the peaks due to crystalline phase have been observed to decrease sharply, which is due to an increase in the amorphous content of polymer electrolyte, as PC acts as a plasticizer. The electrolytes containing PC show higher ionic conductivity at all temperatures. The interactions between polymer, IL, and PC have been studied by FTIR (Supporting Information, Figure S2). The nature of mobile species in IL-based electrolytes, the temperature at which the diffusional motion of ions starts, and its correlation with the glass transition and melting temperature of different electrolytes have been studied simultaneously by solid state 1H NMR and DTA. The 1H solid state NMR and DTA of the IL and polymer electrolytes, having composition PEO + IL and PEO + IL + PC, were recorded simultaneously in the 100−350 K temperature range and are shown in Figure 5. The 1H NMR spectra for all three samples show a single broad line at low temperatures, which is characteristic of the ions being present in the low conducting rigid state. With an increase in temperature, line narrowing takes place, and a single narrow line, which is characteristic of the fluid state, has been observed at higher temperatures. The transformation of the system from the rigid to the diffusive state is normally accompanied by the narrowing of the NMR peaks,25 and it generally takes place at the melting point in the case of salts, and at the glass transition temperature in the case of amorphous materials (glasses, polymers, etc.). These temperatures have been determined from DTA results recorded along with NMR in the same temperature interval. The line width (fwhm) was calculated from the main peaks in the 1H NMR spectra, and its variation with temperature is shown in Figure 6. The variation of line width with temperature for the neat IL is shown in Figure 6a. The glass transition and the melting temperature for the IL, as determined from the DTA, are 198 and 245 K, respectively. The onset of long-range translation 2478

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is lower than that for the neat IL. A sharp component also appeared at temperature lower than those in IL, as shown in Figure 5b. However, above 240 K, which is very close to the melting point of the IL, it shows a line width of only 2 kHz. In addition, above 210 K, two lines (one narrow and other broad) are simultaneously observed, which is due to the presence of both amorphous and crystalline phases of PEO. These results are in agreement with earlier studies on PEO-based polymer electrolytes.27−29 The effect of ion diffusional motion on the ionic conductivity was also studied by measuring the ionic conductivity of this sample at different temperatures in the 100−300 K range, and the variation is shown in Figure 6b. The ionic conductivity of IL + PEO shows a very small value of ∼10−8 S cm−1 in the 100−200 K temperature range, and two sharp jumps in ionic conductivity value have been observed. Ionic conductivity reaches a value of ∼10−5 S cm−1 at temperatures above 300 K. The temperatures at which ionic conductivity shows sharp increases are the same at which line narrowing was observed (Figure 6), and it suggests that the onset of ion diffusional motion at the glass transition and melting temperatures contributes to an enhancement in ionic conductivity. The presence of both crystalline and amorphous phases in this sample IL + PEO was also checked by XRD studies. These plots are given in Figure 4. The XRD plots show a broad halo due to the amorphous phase, which is superimposed with sharp peaks indicating the presence of crystalline phase, and the accepted explanation is that the amorphous phase is high conducting in PEO-based polymer electrolytes.29 However, recently it has been reported that the crystalline phase can also contribute to high ionic conductivity in PEO-based electrolytes,28 and this aspect needs to be further studied to get a clear answer. Due to the low value of ionic conductivity of (IL + PEO), PC was added as a plasticizer, and its effect on the NMR and ionic conductivity results has been studied. The 1H NMR spectrum for the electrolyte having composition PEO + IL + PC (Figure 5) shows line width of 59−50.5 kHz in the 100−189 K temperature range, which narrows down and shows small line width at 209 K, which further narrows down to less than 2 kHz above 230 K. It must be mentioned here that the DTA data for this sample could not be recorded due to some sampling problems. The line narrowing for this sample takes place at a temperature lower than that for the neat IL as well as for IL + PEO. The addition of PC to IL + PEO lowers the glass transition temperature of the electrolyte so the line narrowing takes place at 189 K, which is much lower than the temperature at which line narrowing was observed in IL + PEO (219K) and neat IL (210 K). Despite the addition of PC as a plasticizer, a broad peak is also present in this sample at temperature above 199 K, which is due to the presence of a small amount of the crystalline phase, which was also observed in XRD results (Figure 4). The XRD plot for this sample shows the presence of two small peaks, which indicate the presence of a small amount of the crystalline phase. The effect of the addition of PC on the ionic conductivity value was also studied, and the variation of ionic conductivity with temperature in the range 100−300 K is also included in Figure 6c. The ionic conductivity is ∼10−8 S cm−1 at the 100−180 K temperature range and shows an increasing trend at 180 K, which is the glass transition temperature, and another sharp increase is observed at around 240 K, which corresponds to the melting point of the IL. The ionic conductivity value for this sample is higher than that for the IL + PEO sample at all temperatures in the range 100−300 K.

Figure 6. Variation of line width (fwhm) of 1H NMR lines (filled symbols), and ionic conductivity (open symbols) with temperature for the IL (●,○), IL + PEO (▲, Δ), and IL + PEO + PC (■,□).

diffusional motion is closely related to an increase in ionic conductivity, and it takes place at the glass transition and melting temperature in the case of neat IL. Thus, the onset of long-range translational motion at the temperature at which line narrowing takes place contributes to the ionic conductivity of electrolytes, and a large increase in ionic conductivity has been observed. The relatively low value (36−42 kHz) of the line width for 1H NMR line in IL in the low temperature range (100−150K) as compared to other samples may be due to the fact that in IL, the anions are very loosely bound and lie near the cations. For the polymer electrolytes having composition IL + PEO, the 1H NMR peak has line width of 59−51 kHz in the 100−180 K temperature range, which starts narrowing down at 210 K and decreases to a value of 2 kHz above 240 K. The glass transition temperature of the sample IL + PEO is 188 K, which 2479

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Figure 7. DSC/TGA/DTG plots for (a) IL, (b) IL + PEO, (c) IL + PEO + PC, and (d) PEO.

°C. Polymer electrolyte (IL + PEO + PC, Figure 7c) is thermally stable up to 200 °C with the weight loss of 4.5%. Decomposition of the polymer electrolyte (IL+ PEO + PC) takes place above 250 °C through a single-step weight loss process. Decomposition of polymer electrolyte causes an endothermic peak at 309 °C in the DSC thermogram, and it decomposes totally at around 592 °C, which suggests that polymer electrolytes containing PC are thermally stable up to 200 °C and thus can be used in different devices up to a temperature of 100 °C.

Thus, with the addition of PC, the line narrowing occurs at a lower temperature, and ionic conductivity also shows a jump at lower temperature. The temperature at which line narrowing and a jump in ionic conductivity takes place corresponds to the glass transition and the melting temperature of this sample. The slow decrease of line width in the low-temperature region observed for the polymer electrolytes is due to the reorientation of ions present in the electrolytes.17 The electrochemical window of the IL has been found to be 1.46 V (Supporting Information, Figure S3). The thermal stability of the IL and polymer electrolytes has been studied by TGA/DSC/DTG measurements, and the plots are shown in Figure 7. As observed in the TGA thermogram (Figure 7a), decomposition of the IL takes place above 259 °C through a single-step weight loss process. Upto the temperature of 200 °C, there is a weight loss of 5.26% only, indicating that the IL is thermally stable up to this temperature. The decomposition of the IL is also reflected in the endothermic peaks at 311 and 343 °C in the DSC thermogram. In the case of polymer electrolyte (IL + PEO, Figure 7b), a small endothermic peak was observed at around 69 °C, which is related to the melting of PEO. The peak corresponding to the melting has also been observed in the DSC thermogram (Figure 7d) at 72 °C.26 The TGA thermogram shows that polymer electrolyte (IL + PEO) is thermally stable up to 200 °C with a weight loss of 4.06% only. Decomposition of polymer electrolyte causes an endothermic peak at 297 and 327 °C, and the polymer electrolyte decomposes completely at around 600

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CONCLUSIONS The IL (2-methyl-1,3-dipropylimidazoliumdihydrogenphosphate) containing an acidic counteranion shows high value of ionic conductivity (3.37 × 10−2 Scm−1 at 150 °C). The ion transport properties of polymer electrolytes containing this IL have been studied by simultaneous 1H solid state NMR and DTA by using a specially designed probe. The onset of ion diffusional motion in different electrolytes containing IL takes place at the same temperatures at which line narrowing has been observed in the NMR spectra and corresponds to the glass transition and melting temperature. An increase in ionic conductivity by 2−3 orders of magnitude has also been observed at these temperatures. Thus, the temperature at which line narrowing takes place in 1H solid-state NMR, the glass transition and melting point of the electrolytes observed from DTA, the temperature at which the ion diffusional motion starts and results in an increase in ionic conductivity, are all 2480

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(12) Wilkes, J. S.; Zaworotko, M. J. Air and Water Stable 1-Ethyl-3Methylimidazolium Based Ionic Liquids. J. Chem. Soc., Chem. Commun. 1992, 965−967. (13) Fuller, J.; Breda, A. C.; Carlin, R. T. Ionic Liquid−Polymer Gel Electrolytes from Hydrophilic and Hydrophobic Ionic Liquids. J. Electroanal. Chem. 1998, 459, 29−34. (14) De Souza, R. F.; Padilha, J. C.; Goncalves, R. S.; Dupont, J. Room Temperature Dialkylimidazolium Ionic Liquid Based Fuel Cells. Electrochem. Commun. 2003, 5, 728−731. (15) Sekhon, S. S.; Krishnan, P.; Singh, B.; Yamada, K.; Kim, C. S. Proton Conducting Membrane Containing Room Temperature Ionic Liquid. Electrochim. Acta 2006, 52, 1639−1644. (16) Sun, J.; Forsyth, M.; MacFarlane, D. R. Room-Temperature Molten Salts Based on the Quaternary Ammonium Ion. J. Phys. Chem. B. 1998, 102, 8858−8864. (17) Macfarlane, D. R.; Huang, J.; Forysth, M. Lithium-Doped Plastic Crystal Electrolytes Exhibiting Fast Ion Conduction for Secondary Batteries. Nature 1999, 402, 792−794. (18) Singh, B.; Sekhon, S. S. Polymer Electrolytes Based on Room Temperature Ionic Liquid: 2,3-Dimethyl-1-octylimidazolium Triflate. J. Phys. Chem. B 2005, 109, 16539−16543. (19) Kaur, D. P.; Yamada, K.; Park, J. S.; Sekhon, S. S. Correlation Between Ion Diffusional Motion and Ionic Conductivity for Different Electrolytes Based on Ionic Liquid. J. Phys. Chem. B 2009, 113, 5381− 5390. (20) Lalia, B. S.; Sekhon, S. S. Polymer Electrolytes Containing Ionic Liquids with Acidic Counteranion (DMRImH2PO4, R = Ethyl, Butyl and Octyl). Chem. Phys. Lett. 2006, 425, 294−300. (21) Vogel, H. The Law of the Relationship Between Viscosity of Liquids and the Temperature. Phys. Z. 1921, 22, 645−646. Tammann, G.; Hesse, G. The Molecular Composition of Water. Z. Anorg. Allg. Chem. 1926, 158, 1−16. Fulcher, G. S. Analysis of Recent Measurements of the Viscosity of Glasses. J. Am. Ceram. Soc. 1925, 8, 339−355. (22) Armand, M. B. In Polymer Electrolyte Reviews I; MacCallum, J. R., Vincent, C. A.; Eds.; Elsevier Applied Science Publishers: London, 1987. (23) Fuller, J.; Breda, A. C.; Carlin, R. T. Ionic Liquid−Polymer Gel Electrolytes. J. Electrochem. Soc. 1997, 144, L67−L70. (24) Watanabe, M.; Mizumura, T. Conductivity Study on Ionic Liquid/Polymer Complexes. Solid State Ionics 1996, 86−88, 353−356. (25) Singh, B.; Sekhon, S. S. Physicochemical Studies of PVdF−HFP Based Polymer−Ionic Liquid Composite Electrolytes. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 661−670. (26) Silva, V. P. R.; Silva, G. G.; Caliman, V.; Rieumont, J.; de Miranda-Pinto, C. O. B.; Archanjo, B. S.; Neves, B. R. A. Morphology, Crystalline Structure and Thermal Properties of PEO/MEEP Blends. Eur. Polym. J. 2007, 43, 3283−3291. (27) Susan, M. A. B. H.; Kaneko, T.; Noda, A.; Watanabe, M. Ion Gels Prepared by in Situ Radical Polymerization of Vinyl Monomers in an Ionic Liquid and Their Characterization as Polymer Electrolytes. J. Am. Chem. Soc. 2005, 127, 4976−4983. (28) Zlatka, G.; Andreev, Y. G.; Tunstall, D. P.; Bruce, P. G. Ionic Conductivity in Crystalline Polymer Electrolytes. Nature 2001, 412, 520−523. (29) Fauteux, D. In Polymer Electrolyte Reviews; MacCallum, J. R., Vincent, C. A., Eds.; Elsevier Applied Science: London, 1989; Vol. 2, Chapter 4, pp 121−155.

closely related to each other. The presence of crystalline and amorphous phases has been observed from XRD studies. The transformation of the polymer from the semicrystalline to amorphous phase at the melting temperature has been correlated with the ionic conductivity results. Polymer electrolytes based on IL have been found to be thermally stable up to 200 °C. The proton conduction in electrolytes based on ILs with acidic counteranions can take place by a hopping mechanism, and these electrolytes can be used in proton conducting membranes for high-temperature proton exchange membrane fuel cells. However, further studies are needed to understand the exact nature of the conduction mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

A schematic diagram of the specially designed probe for the measurement of 1H solid state NMR and DTA simultaneously, and a detailed description of 1H and 13C NMR used to determine the formation of the IL, FTIR, and CV results are given in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: 1(868) 662 2002 ext 82591; Fax: 1(868)662 9904. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species. J. Phys. Chem. B 2004, 108, 16593−16600. (2) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Anionic Species. J. Phys. Chem. B 2005, 109, 6103−6110. (3) Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, M.; Gratzel, M. Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inorg. Chem. 1996, 35, 1168−1178. (4) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071−2084. (5) Kragl, U.; Eckstein, M.; Kaftzik, N. Enzyme Catalysis in Ionic Liquids. Curr. Opin. Biotechnol. 2002, 13, 565−571. (6) Kubisa, P. Application of Ionic Liquids as Solvents for Polymerization Processes. Prog. Polym. Sci. 2004, 29, 3−12. (7) Balducci, A.; Bardi, U.; Caporali, S.; Mastragostino, M.; Soavi, F. Ionic Liquids for Hybrid Supercapacitors. Electrochem. Commun. 2004, 6, 566−570. (8) Liu, H.; He, P.; Li, Z.; Liu, Y.; Li, J. A Novel Nickel-Based Mixed Rare-Earth Oxide/Activated Carbon Supercapacitor Using Room Temperature Ionic Liquid Electrolyte. Electrochim. Acta 2006, 51, 1925−1931. (9) Garcia, B.; Lavallee, S.; Perron, G.; Michot, C.; Armand, M. Room Temperature Molten Salts as Lithium Battery Electrolyte. Electrochim. Acta 2004, 49, 4583−4588. (10) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal Properties of Imidazolium Ionic Liquids. Thermochim. Acta 2000, 357−358, 97−102. (11) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170. 2481

dx.doi.org/10.1021/jp3116512 | J. Phys. Chem. B 2013, 117, 2475−2481