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New organic–inorganic crystalline electrolytes synthesized from 12-phosphotungstic acid and the ionic liquid [BMIM][TFSI]
Electrochimica Acta 53 (2008) 7638–7643
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
New organic–inorganic crystalline electrolytes synthesized from 12-phosphotungstic acid and the ionic liquid [BMIM][TFSI] Je-Deok Kim a,∗ , Shigenobu Hayashi b , Mitsuko Onoda a , Akira Sato a , Chikashi Nishimura a , Toshiyuki Mori a , Itaru Honma b a b
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
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Article history: Received 12 December 2007 Received in revised form 26 March 2008 Accepted 29 March 2008 Available online 18 April 2008 Keywords: Organic–inorganic crystal 12-Phosphotungstic acids (PWA) [BMIM][TFSI] Ionic liquid Ion conductor
a b s t r a c t New organic–inorganic hybrid crystalline electrolytes comprised of 12-phosphotungstic acid (PWA) and the ionic liquid [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl)amide] ([BMIM][TFSI]) with high thermal stability and high ion conductivity at high temperatures were obtained. In the new hybrids, there was a strong interaction between [BMIM]+ of the ionic liquid and PWA. The hybrids were very stable up to about 400 ◦ C and showed a high ion jump during heating and cooling processes. Based on results from TG–DTA, DSC, and NMR spectroscopy, the ion jump was due to melting and solidification of the hybrid. The structures of powder and single-crystal samples of the hybrids were also studied. The chemical formula of the hybrid in the single crystal was determined to be PW13 C32 H56 O54 N8 S0.16 F0.26 . This is nearly the same as those determined from the powder samples. By analyzing single-crystal X-ray diffraction (XRD) data, the hybrid was determined to crystallize in the space group Pca21 with the lattice constants a = 18.316(3), ˚ The powder XRD data of the hybrid were assigned. b = 18.327(3), and c = 16.657(3) A. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ionic liquids are salts in which the ions are poorly coordinated, and thus, they are liquids below 100 ◦ C. Sometimes they are liquids even at room temperature, so-called room temperature ionic liquids (RTILs). At least one ion has a delocalized charge, and one component is organic, which prevents the formation of a stable crystal lattice. The chemical and physical properties of ionic liquids can be varied by carefully choosing the cation and anion from among numerous possibilities. Ionic liquids have been receiving increased interest in multidisciplinary areas because of their unique physicochemical properties, such as high thermal stability, negligible vapor pressure, non-flammability, relatively high ionic conductivity, and good electrochemical stability. In addition, they have recently been studied as electrolytes in fuel cells [1–6], lithium batteries [7–9], solar cells [10–11], capacitors [12], and electrochromic [13] and biomimetic [14] applications. Heteropolyacids continue to attract significant attention in the fields of catalysis [15–17], photo- and electrochromism [18],
∗ Corresponding author at: National Institute for Materials Science (NIMS), Fuel Cell Materials Center, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Tel.: +81 29 860 4764; fax: +81 29 860 4667. E-mail address: [email protected] (J.-D. Kim). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.03.085
medicine [19], magnetism [20], fuel cell technology [21–22], and others [23–24]. The primary structure of heteropoly anions of a heteropolyacid, such as 12-phosphotungstic acid (PWA), is a Keggin structure, and the [PW12 O40 ]3− anion has tetrahedral symmetry based on the central PO4 tetrahedron surrounded by 12 WO6 units and high thermal stability (>400 ◦ C). PWA has been extensively studied for use as a proton-conducting electrolyte in low-temperature fuel cells [21,25–28]. However, room temperature applications of PWA with high proton-conductivity (10−1 S/cm) are limited, because the conductivity of PWA is extremely sensitive to the relative humidity of the surrounding atmosphere and because PWA is soluble in water. Attempts have been made to immobilize PWA in silica gel [29] and an ammonium salt [30] and to disperse it in an organically modified electrolyte membrane [31] and organic–inorganic hybrid membranes [32–34]. To overcome the stability problems and to increase the lifetime of the cells, new synthetic routes have continuously been sought to enable fast ionic conduction in the hybrid materials through molecular modification of organic ligands and inorganic structures for PEFC applications. We have previously reported organic–inorganic hybrid electrolytes using PWA ([H3 PW12 O40 ·nH2 O]) and the ionic liquid [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl)amide] ([BMIM][TFSI]) [5]. The hybrids are hydrophilic and include water molecules in their structures. Although the [BMIM][TFSI]–PWA hybrids exhibit high proton-conductivity below 100 ◦ C under
J.-D. Kim et al. / Electrochimica Acta 53 (2008) 7638–7643
an anhydrous N2 atmosphere, the stabilities of the hybrids are poor. In this study, we report an organic–inorganic hybrid crystalline electrolyte having a high thermal stability and a high ion conductivity at high temperatures, prepared by mixing PWA and the ionic liquid [BMIM][TFSI]. The hybrids were very stable up to about 400 ◦ C and showed an ion jump at ∼180 ◦ C. The chemical and electrical properties of the organic–inorganic hybrid crystalline electrolytes were investigated by thermogravimetric (TG) and differential thermogravimetric analysis (DTA), differential scanning calorimetry (DSC), infrared (IR) and NMR spectroscopies, and conductivity measurements. In addition, powder X-ray diffraction (XRD) and single-crystal X-ray diffraction analyses showed that the hybrids have an orthorhombic structure. 2. Experimental PWA (H3 PW12 O40 ·nH2 O) and [BMIM][TFSI] ([C8 H15 N2 ][(CF3 SO2 )2 N]), shown in Fig. 1, were purchased from Wako Pure Chemical and Kanto Chemical Co., Ltd., respectively. Samples of the PWA–[BMIM][TFSI] hybrids were synthesized by mechanically grinding the PWAs in an agate mortar for 5 h at 200 rpm while adding 10–80 wt% of [BMIM][TFSI]. After the mechanical grinding, the hybrids were dissolved in ethanol, and the solutions were filtered through a 0.2 m filter to remove unreacted materials. The samples were kept in a desiccator after drying at room temperature for 3 days to evaporate the ethanol. All of the hybrids were white powders. Element analyses for C, H, N, and O were performed by Atlantic Microlab, and analyses for P, W, S, and F were performed by using inductively coupled plasma-optical emission spectroscopy (ICPOES). Vibrational properties of the molecular structure were characterized by using an attenuated total reflection (ATR) method on an IR spectrophotometer (FT/IR-6200 with ATR PRO 410-S, JASCO). Thermal stabilities of the samples were investigated by TG and DTA with a TG/DTA6200 (SII Co. Ltd.). The samples were heated from room temperature to 500 ◦ C at a rate of 5 ◦ C/min under an O2 atmosphere. In addition, phase-transition temperatures were determined by using DSC on a TA Instruments Q100 differential scanning calorimeter. The Pt pans were exposed to a flow of N2 . The melting, crystallization, and glass-transition temperatures were determined by using heating and cooling cycles from 25 to 250 ◦ C at a rate of 5 ◦ C/min. In order to determine the conduction properties of the PWA–[BMIM][TFSI] hybrids, they were pressed into pellets with a thickness of <1 mm and placed between two circular gold-plated copper blocking electrodes with an area of about 0.2 cm2 . A frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 100 mV were used for the impedance measurements on an SI 1260 Impedance Analyzer (Solatron). All cells were equilibrated for 30 min at each temperature before performing the conductivity measurements. The ion conductivities were measured with a
Fig. 1. Structures of PWA (H3 PW12 O40 ·nH2 O) and [BMIM][TFSI].
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heating and cooling cycle up to 240 ◦ C under a flow of dry nitrogen (non-humidified conditions). 1 H and 31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies were used to observe the proton microenvironment in the PWA–[BMIM][TFSI] hybrids. 1 H and 31 P MAS NMR spectra were obtained at room temperature on a Bruker ASX400 spectrometer with Larmor frequencies of 400.13 and 161.98 MHz, respectively. The 1 H MAS NMR spectra were acquired with a single pulse sequence, whereas the 31 P MAS NMR spectra were acquired using cross polarization from 1 H to 31 P nuclei combined with high-power 1 H decoupling. The chemical shifts of 1 H and 31 P were referenced to the signals of neat tetramethylsilane and 85% H3 PO4 , respectively. 1 H NMR spectra for static samples were acquired on a Bruker ASX200 spectrometer at 200.13 MHz in the range of 296–483 K. A Bruker probe head with a solenoid coil was used. A solid echo pulse ◦ ◦ sequence (90x –1 –90y –2 –echo) was used to obtain the spectra, and the last half of the echo signal was Fourier-transformed. 1 and 2 values were set at 9.5 and 10.0 s, respectively. The frequency scale of the spectra was expressed with respect to neat tetramethylsilane. To determine the crystal structure of the new hybrids, a single crystal was grown from acetone solutions of PWA and [BMIM][TFSI]. Room temperature X-ray diffraction data were collected on a Bruker SMART Apex CCD diffractometer with Mo K␣ ˚ and processed using the Bruker software radiation ( = 0.71073 A) package, which includes SHELX97. Powder XRD data were recorded on a M03XHF22 X-ray diffractometer (MAC Science Co., Ltd.) using ˚ The intensity data for the refinement Cu K␣ radiation ( = 1.5406 A). were collected at room temperature using a 2 step size of 0.02◦ and a data-collection time of 20 s per step.
3. Results and discussion 3.1. Element analyses and molecular structures of the hybrids The chemical formulas of the PWA–[BMIM][TFSI] hybrids with 10, 50, and 80 wt% of [BMIM][TFSI] were determined to be PW13.88 C32.7 H56.10 O55.62 N7.73 S0.01 F0.05 , PW14.15 C34.99 H58.98 O57.51 N8.25 S0.24 F0.55 , and PW14.13 C36.42 H61.32 O58.95 N8.67 S0.45 F1.27 , respectively. Based on element analyses, it seems that there is a strong interaction between the heteropoly anion (PW12 O40 )3− and the cation [BMIM]+ . The excess tungsten may have been due to an error in the measurement. Based on the chemical formulas, it seems that the hybrids basically consist of [BMIM]3 (PW12 O40 ). Fig. 2 shows the IR absorption spectra of (a) pure [BMIM][TFSI], (b) pure PWA, and (c–i) PWA–[BMIM][TFSI] hybrids with different weight percents of [BMIM][TFSI]. All PWA–[BMIM][TFSI] hybrids showed the same absorption properties. Peaks corresponding to [BMIM]+ , the S O groups in [TFSI]− (1160 cm−1 ), and H2 O (1609 cm−1 ), due to the crystal water molecules of PWA, were observed. In addition, a new absorption peak corresponding to W–O–W at 887 cm−1 was observed, indicating that there is a strong interaction between [BMIM]+ and PW12 O40 3− . Fig. 3 shows 1 H and 31 P MAS NMR spectra of the PWA–10 wt% [BMIM][TFSI] hybrid, as well as a 1 H MAS NMR spectrum of pure [BMIM][TFSI]. The 1 H MAS NMR spectrum of the PWA–10 wt% [BMIM][TFSI] hybrid at a spinning rate of 4 kHz showed a sharp signal at 6.4 ppm, which was ascribed to a mobile species exchanging between H3 O+ and H2 O. At a spinning rate of 10 kHz, isotropic peaks were observed at 7.9, 4.2 and 1.5 ppm, which were ascribed to [BMIM]+ . The need for the high spinning rate to observe the peaks for [BMIM]+ indicates that the motion of [BMIM]+ was restricted. On the other hand, in the 31 P MAS NMR spectrum, a single peak
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Fig. 2. FTIR spectra of the hybrids: (a) pure [BMIM][TFSI], (b) pure PWA, (c) PWA–10 wt% [BMIM][TFSI], (d) PWA–20 wt% [BMIM][TFSI], (e) PWA–30 wt% [BMIM][TFSI], (f) PWA–40 wt% [BMIM][TFSI], (g) PWA–50 wt% [BMIM][TFSI], (h) PWA–60 wt% [BMIM][TFSI], and (i) PWA–80 wt% [BMIM][TFSI] ionic liquids.
at −15.2 ppm was observed for the PWA–10 wt% [BMIM][TFSI] hybrid. The 1 H signals of H3 PW12 O40 ·nH2 O (0 ≤ n < 6) have been observed to be between 9 and 10 ppm, whereas the 1 H signal for n = 6 has been reported to be broad [35,36]. The 31 P signal has been observed between −11.0 and −15.6 ppm for H3 PW12 O40 ·nH2 O (0 ≤ n < 6) [35,36]. The 1 H NMR chemical shift of 6.4 ppm suggests that the species exchanging between H3 O+ and H2 O does not coordinate to PW12 O40 3− . In addition, the 31 P NMR chemical shift of −15.2 ppm does not correspond to those of PW12 O40 3− coordinated by H+ and H2 O. It is reasonable that [BMIM]+ coordinates to PW12 O40 3− by exchanging with H+ , and the direct interaction between [BMIM]+ and PW12 O40 3− restricts the motion of [BMIM]+ . 3.2. Thermal stability of the hybrids The thermal stability of the hybrids was investigated by using TG–DTA measurements. TG and DTA curves for pure PWA, [BMIM][TFSI], and PWA–10 wt% [BMIM][TFSI] hybrids are shown in Fig. 4a and b, respectively. The hybrids with PWA–20–80 wt% [BMIM][TFSI] showed the same behavior as the PWA–10 wt% [BMIM][TFSI] hybrid. The thermal stability of the PWA–[BMIM][TFSI] hybrids was higher than that of pure PWA and [BMIM][TFSI]. The PWA–10 wt% [BMIM][TFSI] hybrid showed no weight loss up to 400 ◦ C. In the case of pure PWA, an endothermic peak at 73 ◦ C due to the initial loss of absorbed water was followed by loss of structural water at 184 ◦ C. On the other hand, the PWA–10 wt% [BMIM][TFSI] hybrid showed two endothermic peaks at 172 and 186 ◦ C. The former was due to the evaporation of water from the crystal lattice, and the latter was due to a liquid phase, that is, melting may have occurred. The phase-transition temperatures were determined by using DSC. In Fig. 5a and b, two cycles were observed from 25 to 250 ◦ C. In the first cycle, two endothermic peaks were observed at 172.4 and 189.4 ◦ C during the heating process, and one exothermic peak was observed at 143.9 ◦ C during the cooling process. However, in the second cycle, the exothermic peak at 172.4 ◦ C was not observed, although the other peaks were the same as those in the first cycle. The difference is due to the evaporation of the water from the crystal lattice of the PWA–[BMIM][TFSI] hybrid. In other words, in the DSC of the hybrid, there were two peaks: an endothermic peak at around
Fig. 3. 1 H MAS NMR spectra (a): (i) pure [BMIM][TFSI] at a spinning rate of 2 kHz, (ii) PWA–10 wt% [BMIM][TFSI] at 4 kHz, (iii) PWA–10 wt% [BMIM][TFSI] hybrids at 10 kHz. 31 P MAS NMR spectra (b): PWA–10 wt% [BMIM][TFSI] at 4 kHz. The asterisks indicate spinning side bands.
189 ◦ C and an exothermic peak at around 144 ◦ C. The results from DSC agree with those from DTA. Based on the results of TG–DTA and DSC analyses, the hybrids have high thermal stability and undergo a phase transition. These properties may be due to the strong interaction between the heteropoly anion and [BMIM]+ . 3.3. Conducting properties of the hybrids Fig. 6 shows the ion conductivities of pure [BMIM][TFSI] and the PWA–[BMIM][TFSI] hybrids with different weight percents of the ionic liquid under anhydrous (non-humidified) N2 . The ionic conductivity of pure [BMIM][TFSI] increased with an increase in the temperature, with a maximum conductivity of 2 × 10−2 S/cm at 120 ◦ C. Above 120 ◦ C, the conductivity decreased due to the evaporation of [BMIM][TFSI]. On the other hand, the PWA–[BMIM][TFSI] hybrids showed an ion jump at both 180 and 160 ◦ C and hysteresis with a window of 40 ◦ C during the heating and cooling processes. The difference in the conductivity with different weight percents of
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Fig. 4. (a) TG and (b) DTA results of the hybrids: (i) pure [BMIM][TFSI], (ii) pure PWA, and (iii) PWA–10 wt% [BMIM][TFSI] ionic liquid.
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Fig. 5. DSC traces of PWA–10 wt% [BMIM][TFSI] hybrid: (a) first cycle and (b) second cycle.
ionic liquid was due to the excess ionic liquid. The conductivity of PWA–10 wt% [BMIM][TFSI] hybrid increased nearly four times from 2 × 10−7 at 100 ◦ C to 1 × 10−3 S/cm at 240 ◦ C due to the ion jump, suggesting that the hybrids undergo a phase transition at 180 ◦ C during heating and at 160 ◦ C during cooling. This phase transition is consistent with the results from DTA and DSC. The ion jump was better clarified by using variable temperature 1 H NMR spectroscopy on a static sample. Fig. 7 shows 1 H NMR spectra measured during heating and cooling. The linewidth was broad up to 453 K, where low ion conductivity was observed. The 1 H signal sharpened at 473 K. In the cooling process, the linewidth remained sharp up to 433 K. However, it became broad again below 413 K. The broad linewidth suggests a rigid state, whereas the sharp linewidth indicates a mobile state. Therefore, the ion jump might be a phase transition due to the melting and solidification of the hybrid. 3.4. Crystal properties of the hybrids The hybrids using PWA and [BMIM][TFSI] have similar chemical structures. In addition, the powder diffraction patterns are nearly the same. However, it was not easy to elucidate fully the crystal structure. Therefore, a single crystal of the hybrid (Fig. 8) was grown from an acetone solution of PWA and [BMIM][TFSI]. The chemical formula was determined to be PW13 C32 H56 O54 N8 S0.16 F0.26 , which is similar to that determined from powder samples of the PWA–[BMIM][TFSI] hybrids. The conducting behavior in the
Fig. 6. Anhydrous ion conductivity properties of PWA–[BMIM][TFSI] hybrids with different [BMIM][TFSI] weight percents for the heating and cooling under a flow of non-humidified N2 : (i) PWA–10 wt% [BMIM][TFSI], (ii) PWA–20 wt% [BMIM][TFSI], (iii) PWA–30 wt% [BMIM][TFSI], (iv) PWA–40 wt% [BMIM][TFSI], (v) PWA–50 wt% [BMIM][TFSI], (vi) PWA–60 wt% [BMIM][TFSI], (vii) PWA–80 wt% [BMIM][TFSI], and (viii) pure [BMIM][TFSI].
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J.-D. Kim et al. / Electrochimica Acta 53 (2008) 7638–7643 Table 1 Crystal data, data collection and refinement based on X-ray single-crystal diffraction data Crystal data C24 H42 N6 O40 PW12 Orthorhombic, Pca21 a = 18.316(3) A˚ b = 18.327(3) A˚ c = 16.657(3) A˚ V = 5591.3(15) A˚ 3 Z=4 Mo K␣ radiation Dx = 3.911 Mg m−3 = 25.2 mm−1 T = 298 K 0.35 mm × 0.30 mm × 0.25 mm
Fig. 7. Temperature dependence of 1 H NMR spectra for PWA–30 wt% [BMIM][TFSI] hybrid.
Data collection Bruker SMART diffractometer scans Absorption correction: multi-scan (SADABS; Bruker, 2001) 43669 measured reflections 11353 independent reflections 10240 reflections with I > 2(I) Rint = 0.0415 max = 27.0 h = −23 → 20 k = −22 → 22 l = −21 → 21 Refinement Refinement on F2 R[F > 4(F)] = 0.030 for 10240 reflections R = 0.037 for all unique 11353 reflections wR(F 2 ) = 0.077 S = 1.088 399 parameters H-atom parameters constrained 2 Weight = 1/[ 2 (Fo2 ) + (0.0389P) + 0.00P], where P = (Fo2 + 2Fc2 )/3
Fig. 8. Photograph of a single crystal of the hybrid.
temperature of the single crystal was the same that of PWA–30 wt% [BMIM][TFSI] hybrid. It was also determined that the hybrid crystallized in the orthorhombic space group Pca21 and the lattice ˚ parameters were a = 18.316(3), b = 18.327(3), and c = 16.657(3) A. The chemical formula suggests a ratio of PWA to [BMIM] of 1:3. The positions of S and F proved difficult to assign due to the large W atom and very small amounts of S and F, and thus, more powerful methods, such as neutron diffraction, must be used. A preliminary structure of the hybrid is shown in Fig. 9. Fig. 9b shows the unit cell without O, H, and P atoms. The unit cell has 4 PW12 O40 3− and 12 [BMIM]+ . The crystal data and collection parameters are summarized in Table 1, and more detailed crystallographic
Fig. 9. (a) Atomic structure of the hybrid crystal in the (0 0 1) direction of a unit cell, (b) atomic structure without O, H, and P.
J.-D. Kim et al. / Electrochimica Acta 53 (2008) 7638–7643
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Table 2 ˚ and observed intensities of the X-ray powder diffraction pattern (a = 18.343, b = 18.361 and c = 16.690 A) ˚ Indices, calculated values of d spacings (A) hkl
˚ d (A)
Iobs
hkl
˚ d (A)
Iobs
hkl
˚ d (A)
Iobs
hkl
˚ d (A)
110 111 020 200 002 201 012 121 211 022 221 130 310
12.9766 10.2444 9.1804 9.1714 8.3448 8.0377 7.5970 7.3666 7.3631 6.1750 6.0474 5.8056 5.8010
18 100 18
131 311 222 113 231 321 132 312 203 123 213 040 400
5.4833 5.4795 5.1222 5.1132 4.8693 4.8676 4.7657 4.7632 4.7566 4.6054 4.6046 4.5902 4.5857
12
401 232 322 223 004 042 402 133 313 421 024 204
4.4218 4.3459 4.3447 4.2233 4.1724 4.0219 4.0189 4.0168 4.0152 3.9838 3.7985 3.7979
11 4
233 323 214 341 431 143 413 034 314 052 342 432
3.7557 3.7549 3.7191 3.5852 3.5842 3.4764 3.4746 3.4475 3.3873 3.3611 3.3601 3.3593
4 2 <1 1 4 1 6
14 3 5
13
data and atomic parameters are summarized in the Supplementary information. The crystal structure of the hybrid was compared to that determined from powder XRD data, which are summarized in Table 2. All of the Bragg reflections are consistent with the orthorhombic space group Pca21 and the lattice parameters ˚ The satisfactory fita = 18.343(3), b = 18.361(3) and c = 16.690(3) A. ting results from Rietveld analysis (R = 7.1%, Rwp = 9.1%, program PREMOS [37]) using the preliminary structure obtained from the single-crystal X-ray diffraction analysis show that the crystal structure of the powder specimen is basically the same as that of the single crystal. 4. Conclusions Organic–inorganic hybrid crystalline electrolytes with high thermal stability and high ion conductivity at high temperatures were synthesized by mixing PWA and the ionic liquid [BMIM][TFSI]. The hybrids were very stable up to 400 ◦ C and showed an ion jump during the heating and cooling processes. Based on the NMR, TG–DTA, and DSC results, it was concluded that the ion jump is due to the melting and solidification of the hybrid. In order to better understand the crystal structure of the hybrid, a preliminary structure obtained from single-crystal X-ray diffraction analysis was compared to data obtained from powder XRD analysis. It was verified that the hybrid crystallized in the orthorhombic space group Pca21 . Materials that undergo a high ion jump and have high thermal stability will be useful in a variety of applications, such as fuel cells, super capacitors, and sensor devices. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2008.03.085. References [1] M. Doyle, S.K. Choi, G. Proulx, J. Electrochem. Soc. 147 (2000) 34. [2] A. Noda, M.A.B.H. Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 107 (2003) 4024.
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10 1
Iobs 5 6 19 13 4 7 25
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
New organic–inorganic crystalline electrolytes synthesized from 12-phosphotungstic acid and the ionic liquid [BMIM][TFSI] Je-Deok Kim a,∗ , Shigenobu Hayashi b , Mitsuko Onoda a , Akira Sato a , Chikashi Nishimura a , Toshiyuki Mori a , Itaru Honma b a b
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
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Article history: Received 12 December 2007 Received in revised form 26 March 2008 Accepted 29 March 2008 Available online 18 April 2008 Keywords: Organic–inorganic crystal 12-Phosphotungstic acids (PWA) [BMIM][TFSI] Ionic liquid Ion conductor
a b s t r a c t New organic–inorganic hybrid crystalline electrolytes comprised of 12-phosphotungstic acid (PWA) and the ionic liquid [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl)amide] ([BMIM][TFSI]) with high thermal stability and high ion conductivity at high temperatures were obtained. In the new hybrids, there was a strong interaction between [BMIM]+ of the ionic liquid and PWA. The hybrids were very stable up to about 400 ◦ C and showed a high ion jump during heating and cooling processes. Based on results from TG–DTA, DSC, and NMR spectroscopy, the ion jump was due to melting and solidification of the hybrid. The structures of powder and single-crystal samples of the hybrids were also studied. The chemical formula of the hybrid in the single crystal was determined to be PW13 C32 H56 O54 N8 S0.16 F0.26 . This is nearly the same as those determined from the powder samples. By analyzing single-crystal X-ray diffraction (XRD) data, the hybrid was determined to crystallize in the space group Pca21 with the lattice constants a = 18.316(3), ˚ The powder XRD data of the hybrid were assigned. b = 18.327(3), and c = 16.657(3) A. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ionic liquids are salts in which the ions are poorly coordinated, and thus, they are liquids below 100 ◦ C. Sometimes they are liquids even at room temperature, so-called room temperature ionic liquids (RTILs). At least one ion has a delocalized charge, and one component is organic, which prevents the formation of a stable crystal lattice. The chemical and physical properties of ionic liquids can be varied by carefully choosing the cation and anion from among numerous possibilities. Ionic liquids have been receiving increased interest in multidisciplinary areas because of their unique physicochemical properties, such as high thermal stability, negligible vapor pressure, non-flammability, relatively high ionic conductivity, and good electrochemical stability. In addition, they have recently been studied as electrolytes in fuel cells [1–6], lithium batteries [7–9], solar cells [10–11], capacitors [12], and electrochromic [13] and biomimetic [14] applications. Heteropolyacids continue to attract significant attention in the fields of catalysis [15–17], photo- and electrochromism [18],
∗ Corresponding author at: National Institute for Materials Science (NIMS), Fuel Cell Materials Center, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Tel.: +81 29 860 4764; fax: +81 29 860 4667. E-mail address: [email protected] (J.-D. Kim). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.03.085
medicine [19], magnetism [20], fuel cell technology [21–22], and others [23–24]. The primary structure of heteropoly anions of a heteropolyacid, such as 12-phosphotungstic acid (PWA), is a Keggin structure, and the [PW12 O40 ]3− anion has tetrahedral symmetry based on the central PO4 tetrahedron surrounded by 12 WO6 units and high thermal stability (>400 ◦ C). PWA has been extensively studied for use as a proton-conducting electrolyte in low-temperature fuel cells [21,25–28]. However, room temperature applications of PWA with high proton-conductivity (10−1 S/cm) are limited, because the conductivity of PWA is extremely sensitive to the relative humidity of the surrounding atmosphere and because PWA is soluble in water. Attempts have been made to immobilize PWA in silica gel [29] and an ammonium salt [30] and to disperse it in an organically modified electrolyte membrane [31] and organic–inorganic hybrid membranes [32–34]. To overcome the stability problems and to increase the lifetime of the cells, new synthetic routes have continuously been sought to enable fast ionic conduction in the hybrid materials through molecular modification of organic ligands and inorganic structures for PEFC applications. We have previously reported organic–inorganic hybrid electrolytes using PWA ([H3 PW12 O40 ·nH2 O]) and the ionic liquid [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl)amide] ([BMIM][TFSI]) [5]. The hybrids are hydrophilic and include water molecules in their structures. Although the [BMIM][TFSI]–PWA hybrids exhibit high proton-conductivity below 100 ◦ C under
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an anhydrous N2 atmosphere, the stabilities of the hybrids are poor. In this study, we report an organic–inorganic hybrid crystalline electrolyte having a high thermal stability and a high ion conductivity at high temperatures, prepared by mixing PWA and the ionic liquid [BMIM][TFSI]. The hybrids were very stable up to about 400 ◦ C and showed an ion jump at ∼180 ◦ C. The chemical and electrical properties of the organic–inorganic hybrid crystalline electrolytes were investigated by thermogravimetric (TG) and differential thermogravimetric analysis (DTA), differential scanning calorimetry (DSC), infrared (IR) and NMR spectroscopies, and conductivity measurements. In addition, powder X-ray diffraction (XRD) and single-crystal X-ray diffraction analyses showed that the hybrids have an orthorhombic structure. 2. Experimental PWA (H3 PW12 O40 ·nH2 O) and [BMIM][TFSI] ([C8 H15 N2 ][(CF3 SO2 )2 N]), shown in Fig. 1, were purchased from Wako Pure Chemical and Kanto Chemical Co., Ltd., respectively. Samples of the PWA–[BMIM][TFSI] hybrids were synthesized by mechanically grinding the PWAs in an agate mortar for 5 h at 200 rpm while adding 10–80 wt% of [BMIM][TFSI]. After the mechanical grinding, the hybrids were dissolved in ethanol, and the solutions were filtered through a 0.2 m filter to remove unreacted materials. The samples were kept in a desiccator after drying at room temperature for 3 days to evaporate the ethanol. All of the hybrids were white powders. Element analyses for C, H, N, and O were performed by Atlantic Microlab, and analyses for P, W, S, and F were performed by using inductively coupled plasma-optical emission spectroscopy (ICPOES). Vibrational properties of the molecular structure were characterized by using an attenuated total reflection (ATR) method on an IR spectrophotometer (FT/IR-6200 with ATR PRO 410-S, JASCO). Thermal stabilities of the samples were investigated by TG and DTA with a TG/DTA6200 (SII Co. Ltd.). The samples were heated from room temperature to 500 ◦ C at a rate of 5 ◦ C/min under an O2 atmosphere. In addition, phase-transition temperatures were determined by using DSC on a TA Instruments Q100 differential scanning calorimeter. The Pt pans were exposed to a flow of N2 . The melting, crystallization, and glass-transition temperatures were determined by using heating and cooling cycles from 25 to 250 ◦ C at a rate of 5 ◦ C/min. In order to determine the conduction properties of the PWA–[BMIM][TFSI] hybrids, they were pressed into pellets with a thickness of <1 mm and placed between two circular gold-plated copper blocking electrodes with an area of about 0.2 cm2 . A frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 100 mV were used for the impedance measurements on an SI 1260 Impedance Analyzer (Solatron). All cells were equilibrated for 30 min at each temperature before performing the conductivity measurements. The ion conductivities were measured with a
Fig. 1. Structures of PWA (H3 PW12 O40 ·nH2 O) and [BMIM][TFSI].
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heating and cooling cycle up to 240 ◦ C under a flow of dry nitrogen (non-humidified conditions). 1 H and 31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies were used to observe the proton microenvironment in the PWA–[BMIM][TFSI] hybrids. 1 H and 31 P MAS NMR spectra were obtained at room temperature on a Bruker ASX400 spectrometer with Larmor frequencies of 400.13 and 161.98 MHz, respectively. The 1 H MAS NMR spectra were acquired with a single pulse sequence, whereas the 31 P MAS NMR spectra were acquired using cross polarization from 1 H to 31 P nuclei combined with high-power 1 H decoupling. The chemical shifts of 1 H and 31 P were referenced to the signals of neat tetramethylsilane and 85% H3 PO4 , respectively. 1 H NMR spectra for static samples were acquired on a Bruker ASX200 spectrometer at 200.13 MHz in the range of 296–483 K. A Bruker probe head with a solenoid coil was used. A solid echo pulse ◦ ◦ sequence (90x –1 –90y –2 –echo) was used to obtain the spectra, and the last half of the echo signal was Fourier-transformed. 1 and 2 values were set at 9.5 and 10.0 s, respectively. The frequency scale of the spectra was expressed with respect to neat tetramethylsilane. To determine the crystal structure of the new hybrids, a single crystal was grown from acetone solutions of PWA and [BMIM][TFSI]. Room temperature X-ray diffraction data were collected on a Bruker SMART Apex CCD diffractometer with Mo K␣ ˚ and processed using the Bruker software radiation ( = 0.71073 A) package, which includes SHELX97. Powder XRD data were recorded on a M03XHF22 X-ray diffractometer (MAC Science Co., Ltd.) using ˚ The intensity data for the refinement Cu K␣ radiation ( = 1.5406 A). were collected at room temperature using a 2 step size of 0.02◦ and a data-collection time of 20 s per step.
3. Results and discussion 3.1. Element analyses and molecular structures of the hybrids The chemical formulas of the PWA–[BMIM][TFSI] hybrids with 10, 50, and 80 wt% of [BMIM][TFSI] were determined to be PW13.88 C32.7 H56.10 O55.62 N7.73 S0.01 F0.05 , PW14.15 C34.99 H58.98 O57.51 N8.25 S0.24 F0.55 , and PW14.13 C36.42 H61.32 O58.95 N8.67 S0.45 F1.27 , respectively. Based on element analyses, it seems that there is a strong interaction between the heteropoly anion (PW12 O40 )3− and the cation [BMIM]+ . The excess tungsten may have been due to an error in the measurement. Based on the chemical formulas, it seems that the hybrids basically consist of [BMIM]3 (PW12 O40 ). Fig. 2 shows the IR absorption spectra of (a) pure [BMIM][TFSI], (b) pure PWA, and (c–i) PWA–[BMIM][TFSI] hybrids with different weight percents of [BMIM][TFSI]. All PWA–[BMIM][TFSI] hybrids showed the same absorption properties. Peaks corresponding to [BMIM]+ , the S O groups in [TFSI]− (1160 cm−1 ), and H2 O (1609 cm−1 ), due to the crystal water molecules of PWA, were observed. In addition, a new absorption peak corresponding to W–O–W at 887 cm−1 was observed, indicating that there is a strong interaction between [BMIM]+ and PW12 O40 3− . Fig. 3 shows 1 H and 31 P MAS NMR spectra of the PWA–10 wt% [BMIM][TFSI] hybrid, as well as a 1 H MAS NMR spectrum of pure [BMIM][TFSI]. The 1 H MAS NMR spectrum of the PWA–10 wt% [BMIM][TFSI] hybrid at a spinning rate of 4 kHz showed a sharp signal at 6.4 ppm, which was ascribed to a mobile species exchanging between H3 O+ and H2 O. At a spinning rate of 10 kHz, isotropic peaks were observed at 7.9, 4.2 and 1.5 ppm, which were ascribed to [BMIM]+ . The need for the high spinning rate to observe the peaks for [BMIM]+ indicates that the motion of [BMIM]+ was restricted. On the other hand, in the 31 P MAS NMR spectrum, a single peak
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Fig. 2. FTIR spectra of the hybrids: (a) pure [BMIM][TFSI], (b) pure PWA, (c) PWA–10 wt% [BMIM][TFSI], (d) PWA–20 wt% [BMIM][TFSI], (e) PWA–30 wt% [BMIM][TFSI], (f) PWA–40 wt% [BMIM][TFSI], (g) PWA–50 wt% [BMIM][TFSI], (h) PWA–60 wt% [BMIM][TFSI], and (i) PWA–80 wt% [BMIM][TFSI] ionic liquids.
at −15.2 ppm was observed for the PWA–10 wt% [BMIM][TFSI] hybrid. The 1 H signals of H3 PW12 O40 ·nH2 O (0 ≤ n < 6) have been observed to be between 9 and 10 ppm, whereas the 1 H signal for n = 6 has been reported to be broad [35,36]. The 31 P signal has been observed between −11.0 and −15.6 ppm for H3 PW12 O40 ·nH2 O (0 ≤ n < 6) [35,36]. The 1 H NMR chemical shift of 6.4 ppm suggests that the species exchanging between H3 O+ and H2 O does not coordinate to PW12 O40 3− . In addition, the 31 P NMR chemical shift of −15.2 ppm does not correspond to those of PW12 O40 3− coordinated by H+ and H2 O. It is reasonable that [BMIM]+ coordinates to PW12 O40 3− by exchanging with H+ , and the direct interaction between [BMIM]+ and PW12 O40 3− restricts the motion of [BMIM]+ . 3.2. Thermal stability of the hybrids The thermal stability of the hybrids was investigated by using TG–DTA measurements. TG and DTA curves for pure PWA, [BMIM][TFSI], and PWA–10 wt% [BMIM][TFSI] hybrids are shown in Fig. 4a and b, respectively. The hybrids with PWA–20–80 wt% [BMIM][TFSI] showed the same behavior as the PWA–10 wt% [BMIM][TFSI] hybrid. The thermal stability of the PWA–[BMIM][TFSI] hybrids was higher than that of pure PWA and [BMIM][TFSI]. The PWA–10 wt% [BMIM][TFSI] hybrid showed no weight loss up to 400 ◦ C. In the case of pure PWA, an endothermic peak at 73 ◦ C due to the initial loss of absorbed water was followed by loss of structural water at 184 ◦ C. On the other hand, the PWA–10 wt% [BMIM][TFSI] hybrid showed two endothermic peaks at 172 and 186 ◦ C. The former was due to the evaporation of water from the crystal lattice, and the latter was due to a liquid phase, that is, melting may have occurred. The phase-transition temperatures were determined by using DSC. In Fig. 5a and b, two cycles were observed from 25 to 250 ◦ C. In the first cycle, two endothermic peaks were observed at 172.4 and 189.4 ◦ C during the heating process, and one exothermic peak was observed at 143.9 ◦ C during the cooling process. However, in the second cycle, the exothermic peak at 172.4 ◦ C was not observed, although the other peaks were the same as those in the first cycle. The difference is due to the evaporation of the water from the crystal lattice of the PWA–[BMIM][TFSI] hybrid. In other words, in the DSC of the hybrid, there were two peaks: an endothermic peak at around
Fig. 3. 1 H MAS NMR spectra (a): (i) pure [BMIM][TFSI] at a spinning rate of 2 kHz, (ii) PWA–10 wt% [BMIM][TFSI] at 4 kHz, (iii) PWA–10 wt% [BMIM][TFSI] hybrids at 10 kHz. 31 P MAS NMR spectra (b): PWA–10 wt% [BMIM][TFSI] at 4 kHz. The asterisks indicate spinning side bands.
189 ◦ C and an exothermic peak at around 144 ◦ C. The results from DSC agree with those from DTA. Based on the results of TG–DTA and DSC analyses, the hybrids have high thermal stability and undergo a phase transition. These properties may be due to the strong interaction between the heteropoly anion and [BMIM]+ . 3.3. Conducting properties of the hybrids Fig. 6 shows the ion conductivities of pure [BMIM][TFSI] and the PWA–[BMIM][TFSI] hybrids with different weight percents of the ionic liquid under anhydrous (non-humidified) N2 . The ionic conductivity of pure [BMIM][TFSI] increased with an increase in the temperature, with a maximum conductivity of 2 × 10−2 S/cm at 120 ◦ C. Above 120 ◦ C, the conductivity decreased due to the evaporation of [BMIM][TFSI]. On the other hand, the PWA–[BMIM][TFSI] hybrids showed an ion jump at both 180 and 160 ◦ C and hysteresis with a window of 40 ◦ C during the heating and cooling processes. The difference in the conductivity with different weight percents of
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Fig. 4. (a) TG and (b) DTA results of the hybrids: (i) pure [BMIM][TFSI], (ii) pure PWA, and (iii) PWA–10 wt% [BMIM][TFSI] ionic liquid.
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Fig. 5. DSC traces of PWA–10 wt% [BMIM][TFSI] hybrid: (a) first cycle and (b) second cycle.
ionic liquid was due to the excess ionic liquid. The conductivity of PWA–10 wt% [BMIM][TFSI] hybrid increased nearly four times from 2 × 10−7 at 100 ◦ C to 1 × 10−3 S/cm at 240 ◦ C due to the ion jump, suggesting that the hybrids undergo a phase transition at 180 ◦ C during heating and at 160 ◦ C during cooling. This phase transition is consistent with the results from DTA and DSC. The ion jump was better clarified by using variable temperature 1 H NMR spectroscopy on a static sample. Fig. 7 shows 1 H NMR spectra measured during heating and cooling. The linewidth was broad up to 453 K, where low ion conductivity was observed. The 1 H signal sharpened at 473 K. In the cooling process, the linewidth remained sharp up to 433 K. However, it became broad again below 413 K. The broad linewidth suggests a rigid state, whereas the sharp linewidth indicates a mobile state. Therefore, the ion jump might be a phase transition due to the melting and solidification of the hybrid. 3.4. Crystal properties of the hybrids The hybrids using PWA and [BMIM][TFSI] have similar chemical structures. In addition, the powder diffraction patterns are nearly the same. However, it was not easy to elucidate fully the crystal structure. Therefore, a single crystal of the hybrid (Fig. 8) was grown from an acetone solution of PWA and [BMIM][TFSI]. The chemical formula was determined to be PW13 C32 H56 O54 N8 S0.16 F0.26 , which is similar to that determined from powder samples of the PWA–[BMIM][TFSI] hybrids. The conducting behavior in the
Fig. 6. Anhydrous ion conductivity properties of PWA–[BMIM][TFSI] hybrids with different [BMIM][TFSI] weight percents for the heating and cooling under a flow of non-humidified N2 : (i) PWA–10 wt% [BMIM][TFSI], (ii) PWA–20 wt% [BMIM][TFSI], (iii) PWA–30 wt% [BMIM][TFSI], (iv) PWA–40 wt% [BMIM][TFSI], (v) PWA–50 wt% [BMIM][TFSI], (vi) PWA–60 wt% [BMIM][TFSI], (vii) PWA–80 wt% [BMIM][TFSI], and (viii) pure [BMIM][TFSI].
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J.-D. Kim et al. / Electrochimica Acta 53 (2008) 7638–7643 Table 1 Crystal data, data collection and refinement based on X-ray single-crystal diffraction data Crystal data C24 H42 N6 O40 PW12 Orthorhombic, Pca21 a = 18.316(3) A˚ b = 18.327(3) A˚ c = 16.657(3) A˚ V = 5591.3(15) A˚ 3 Z=4 Mo K␣ radiation Dx = 3.911 Mg m−3 = 25.2 mm−1 T = 298 K 0.35 mm × 0.30 mm × 0.25 mm
Fig. 7. Temperature dependence of 1 H NMR spectra for PWA–30 wt% [BMIM][TFSI] hybrid.
Data collection Bruker SMART diffractometer scans Absorption correction: multi-scan (SADABS; Bruker, 2001) 43669 measured reflections 11353 independent reflections 10240 reflections with I > 2(I) Rint = 0.0415 max = 27.0 h = −23 → 20 k = −22 → 22 l = −21 → 21 Refinement Refinement on F2 R[F > 4(F)] = 0.030 for 10240 reflections R = 0.037 for all unique 11353 reflections wR(F 2 ) = 0.077 S = 1.088 399 parameters H-atom parameters constrained 2 Weight = 1/[ 2 (Fo2 ) + (0.0389P) + 0.00P], where P = (Fo2 + 2Fc2 )/3
Fig. 8. Photograph of a single crystal of the hybrid.
temperature of the single crystal was the same that of PWA–30 wt% [BMIM][TFSI] hybrid. It was also determined that the hybrid crystallized in the orthorhombic space group Pca21 and the lattice ˚ parameters were a = 18.316(3), b = 18.327(3), and c = 16.657(3) A. The chemical formula suggests a ratio of PWA to [BMIM] of 1:3. The positions of S and F proved difficult to assign due to the large W atom and very small amounts of S and F, and thus, more powerful methods, such as neutron diffraction, must be used. A preliminary structure of the hybrid is shown in Fig. 9. Fig. 9b shows the unit cell without O, H, and P atoms. The unit cell has 4 PW12 O40 3− and 12 [BMIM]+ . The crystal data and collection parameters are summarized in Table 1, and more detailed crystallographic
Fig. 9. (a) Atomic structure of the hybrid crystal in the (0 0 1) direction of a unit cell, (b) atomic structure without O, H, and P.
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Table 2 ˚ and observed intensities of the X-ray powder diffraction pattern (a = 18.343, b = 18.361 and c = 16.690 A) ˚ Indices, calculated values of d spacings (A) hkl
˚ d (A)
Iobs
hkl
˚ d (A)
Iobs
hkl
˚ d (A)
Iobs
hkl
˚ d (A)
110 111 020 200 002 201 012 121 211 022 221 130 310
12.9766 10.2444 9.1804 9.1714 8.3448 8.0377 7.5970 7.3666 7.3631 6.1750 6.0474 5.8056 5.8010
18 100 18
131 311 222 113 231 321 132 312 203 123 213 040 400
5.4833 5.4795 5.1222 5.1132 4.8693 4.8676 4.7657 4.7632 4.7566 4.6054 4.6046 4.5902 4.5857
12
401 232 322 223 004 042 402 133 313 421 024 204
4.4218 4.3459 4.3447 4.2233 4.1724 4.0219 4.0189 4.0168 4.0152 3.9838 3.7985 3.7979
11 4
233 323 214 341 431 143 413 034 314 052 342 432
3.7557 3.7549 3.7191 3.5852 3.5842 3.4764 3.4746 3.4475 3.3873 3.3611 3.3601 3.3593
4 2 <1 1 4 1 6
14 3 5
13
data and atomic parameters are summarized in the Supplementary information. The crystal structure of the hybrid was compared to that determined from powder XRD data, which are summarized in Table 2. All of the Bragg reflections are consistent with the orthorhombic space group Pca21 and the lattice parameters ˚ The satisfactory fita = 18.343(3), b = 18.361(3) and c = 16.690(3) A. ting results from Rietveld analysis (R = 7.1%, Rwp = 9.1%, program PREMOS [37]) using the preliminary structure obtained from the single-crystal X-ray diffraction analysis show that the crystal structure of the powder specimen is basically the same as that of the single crystal. 4. Conclusions Organic–inorganic hybrid crystalline electrolytes with high thermal stability and high ion conductivity at high temperatures were synthesized by mixing PWA and the ionic liquid [BMIM][TFSI]. The hybrids were very stable up to 400 ◦ C and showed an ion jump during the heating and cooling processes. Based on the NMR, TG–DTA, and DSC results, it was concluded that the ion jump is due to the melting and solidification of the hybrid. In order to better understand the crystal structure of the hybrid, a preliminary structure obtained from single-crystal X-ray diffraction analysis was compared to data obtained from powder XRD analysis. It was verified that the hybrid crystallized in the orthorhombic space group Pca21 . Materials that undergo a high ion jump and have high thermal stability will be useful in a variety of applications, such as fuel cells, super capacitors, and sensor devices. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2008.03.085. References [1] M. Doyle, S.K. Choi, G. Proulx, J. Electrochem. Soc. 147 (2000) 34. [2] A. Noda, M.A.B.H. Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 107 (2003) 4024.
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10 1
Iobs 5 6 19 13 4 7 25
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