pubs.acs.org/Langmuir © 2010 American Chemical Society
Nanostructure Evolution in High-Temperature Perfluorosulfonic Acid Ionomer Membrane by Small-Angle X-ray Scattering Mayur K. Mistry,† Namita Roy Choudhury,*,† Naba K. Dutta,† and Robert Knott‡ †
Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia, and ‡Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia Received August 2, 2010. Revised Manuscript Received October 26, 2010
The high-temperature morphology of supported liquid membranes (SLMs) prepared from perfluorinated membranes such as Nafion and Hyflon and hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI-TFSI) has been investigated by small-angle X-ray scattering (SAXS). Proton conductivity results of SLMs before and after leaching show an increase in conductivity with temperature up to 160 °C in an anhydrous environment. DSC results show that crystallites within perfluorinated membranes are thermally stable up to 196 °C. Hightemperature SAXS results have been used to correlate structure and morphology of supported liquid membranes with high-temperature conductivity data. The ionic liquid essentially acts as a proton solvent in a similar way to water in hydrated Nafion membranes and increases size of clusters, which allow percolation to be achieved more easily. The cation of the ionic liquid interacts with sulfonate groups within ionic domains through electrostatic interactions and displaces protons. Protons can associate with free anions of the ionic liquid, which are loosely associated with cations and can transport by hopping from anion sites within the membrane. The ionic liquid contributes to proton conductivity at high temperature through achievement of long-range ordering and subsequent percolation.
Introduction Perfluorinated ionomer such as Nafion has outstanding performance as membrane in electrochemical applications due to their unique phase-separated morphology. The unusual transport properties result from their molecular and morphological structure. The material contains a small number of ionic groups, which strongly affects the properties of the fluoropolymer by creating strong intra- and intermolecular secondary bonding forces. Generally, the ionic groups are aggregated in well-ordered domains (clusters) with remainder (multiplets) dispersed in the matrix of low dielectric constant but do not themselves constitute a second phase. On the other hand, the perfluorinated backbone is crystallizable. Thus, such ionomers possess three distinct phases: crystalline, amorphous phases, and ionic aggregates. The aggregates act as labile physical cross-links and influence the stability and reversibility of the crystalline phase over time. The relative proportion of crystalline region depends on the side chain length, equivalent weight, and processing history. Previous small-angle X-ray scattering (SAXS) studies1 on Nafion reveal these membranes have phase-separated microstructure due to hydrophobic and hydrophilic segments of the polymer. The hydrophobic perfluorinated backbone provides mechanical strength while hydrophilic sulfonic acid groups are responsible for the proton conductive properties of the membrane. One of the major problems with Nafion, a long side chain ionomer, is it cannot meet the requirements of a fuel cell due to its poor performance at elevated temperatures greater than 100 °C. A great deal of investigations has been carried out towards the understanding *Corresponding author. E-mail:
[email protected]. (1) (a) Gierke, T. D. Paper presented at 152nd National Meeting, The Electrochemical Society, Atlanta, GA, Oct 1977. (b) Rubatat, L.; Rollet, A. L.; Gebel, G.; Diat, O. Macromolecules 2002, 35(10), 4050–4055. (2) Page, K. A.; Cable, K. M.; Moore, R. B. Macromolecules 2005, 38(15), 6472– 6484. (3) Van der Heijden, P. C.; Rubatat, L.; Diat, O. Macromolecules 2004, 37(14), 5327–5336.
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of morphological structure and mechanism of ion transport in such systems.1-5 While it is well-known that short side chain polymer has significant influence on ion transport property, however, very few investigations report the structure of short side chain ionomer.6-9 Hyflon is another perfluorinated ionomer which has a very similar chemical structure to that of Nafion but has a shorter side chain length.10-13 This short side chain (SSC) perfluorinated ionomer shows improved conductivity over Nafion membranes due to its low equivalent weight (EW) and higher degree of sulfonation.12 Hyflon is characterized by a primary R transition around 160 °C in comparison to conventional Nafion which shows this same transition at 110 °C. This makes the Hyflon membrane more suitable for operation at high temperature.13 However, like Nafion, Hyflon still relies on hydration in order to provide acceptable conductivity values for fuel cell application. There have been many attempts to improve the high-temperature operation of perfluorinated ionomers to develop proton exchange membranes for high-temperature operation.14-18 This (4) Manley, D. S.; Williamson, D. L.; Noble, R. D.; Koval, C. A. Chem. Mater. 1996, 8(11), 2595–2600. (5) Page, K. A.; Landis, F. A.; Phillips, A. K.; Moore, R. B. Macromolecules 2006, 39(11), 3939–3946. (6) Kreuer, K. D.; Schuster, M.; Obliers, B.; Diat, O.; Traub, U.; Fuchs, A.; Klock, U.; Paddison, S. J.; Maier, J. J. Power Sources 2008, 178(2), 499–509. (7) Wu, D.; Paddison, S. J.; Elliott, J. A. Macromolecules 2009, 42(9), 3358– 3367. (8) Gebel, G.; Moore, R. B. Macromolecules 2000, 33(13), 4850–4855. (9) Hristov, I. H.; Paddison, S. J.; Paul, R. J. Phys. Chem. B 2008, 112(10), 2937– 2949. (10) Ghielmi, A.; Vaccarono, P.; Troglia, C.; Arcella, V. J. Power Sources 2005, 145(2), 108–115. (11) Merlo, L.; Ghielmi, A.; Cirillo, L.; Gebert, M.; Arcella, V. J. Power Sources 2007, 171(1), 140–147. (12) Arcella, V.; Troglia, C.; Ghielmi, A. Ind. Eng. Chem. Res. 2005, 44(20), 7646–7651. (13) Arico, A. S.; Baglio, V.; Di Blasi, A.; Antonucci, V.; Cirillo, L.; Ghielmi, A.; Arcella, V. Desalination 2006, 199(1-3), 271–273. (14) Daiko, Y.; Klein, L. C.; Kasuga, T.; Nogami, M. J. Membr. Sci. 2006, 281(1 þ 2), 619–625. (15) Deng, Q.; Moore, R. B.; Mauritz, K. A. J. Appl. Polym. Sci. 1998, 68(5), 747–763.
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alternative approach involves replacing water in perfluorinated membranes with an alternative solvent, which is less volatile than water. Room temperature ionic liquids (IL) are ideal candidates for this purpose as they show high ionic conductivity, excellent thermal and electrochemical stability, and negligible vapor pressure. Several proton exchange membranes that operate under anhydrous conditions have been reported which generally involve incorporating ILs within a polymer membrane. In such a membrane, the IL contributes to proton conductivity, and the polymer membrane provides structural support for the IL and is referred to as supported liquid membranes (SLMs).19-25 Recently, Schmidt et al. incorporated various hydrophilic and hydrophobic ILs into Nafion membranes and achieved conductivities of up to 1 mS cm-1 at 120 °C in anhydrous conditions.26 It was concluded that Nafion containing the hydrophobic IL 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate showed the most promsing result in terms of IL uptake, washing out, plasticization effect, and ion conduction. Martinelli et al.27 have also prepared SLMs for anhydrous conductivity based on aprotic ILs and have achieved conductivity of the order of 10-2 S cm-1 at 115 °C. Perfluorinated ionomer membrane-IL composites have been prepared by Doyle et al. by incorporating the IL 1-butyl-3-methylimidazolium trifluoromethanesulfonate into various perfluorinated ionomers with varying EW and have obtained conductivities of 0.1 S cm-1 at 180 °C under anhydrous conditions.28 It was observed that high proton conductivity can be achieved when sufficient IL absorbs into the membrane, which can be more readily achieved with lower EW membranes. Bennett et al. have also prepared SLMs based on the perfluorinated ionomer Nafion using both a hydrophobic and hydrophilic IL.29 Small-angle X-ray scattering (SAXS) characterization of the SLMs revealed that the structure of the IL has a profound effect on the final morphology of the membrane.24,30 Membranes swollen with the hydrophobic IL exhibited a more homogeneous morphology in comparison with the membrane swollen with hydrophilic IL. Results also indicate that the cation of the IL interacts with sulfonic acid groups of the polymer through an ion exchange process. It is believed that the useful properties of SLMs such as conductivity are closely related to their structure (16) Kannan, A. G.; Choudhury, N. R.; Dutta, N. K. J. Membr. Sci. 2009, 333(1 þ 2), 50–58. (17) Ladewig, B. P.; Knott, R. B.; Hill, A. J.; Riches, J. D.; White, J. W.; Martin, D. J.; Diniz da Costa, J. C.; Lu, G. Q. Chem. Mater. 2007, 19(9), 2372–2381. (18) Mistry, M. K.; Choudhury, N. R.; Dutta, N. K.; Knott, R.; Shi, Z.; Holdcroft, S. Chem. Mater. 2008, 20(21), 6857–6870. (19) Fortunato, R.; Afonso, C. A. M.; Reis, M. A. M.; Crespo, J. G. J. Membr. Sci. 2004, 242(1-2), 197–209. (20) Sekhon, S. S.; Lalia, B. S.; Kim, C. S.; Lee, W. Y. Macromol. Symp. 2007, 249/250 (Advanced Polymers for Emerging Technologies), 216-220. (21) Fortunato, R.; Branco, L. C.; Afonso, C. A. M.; Benavente, J.; Crespo, J. G. J. Membr. Sci. 2006, 270(1-2), 42–49. (22) Neves, L.; Dabek, W.; Coelhoso, I. M.; Crespo, J. G. Desalination 2006, 199 (1-3), 525–526. (23) Kocherginsky, N. M.; Yang, Q.; Seelam, L. Sep. Purif. Technol. 2007, 53(2), 171–177. (24) Sekhon, S. S.; Park, J.-S.; Cho, E.; Yoon, Y.-G.; Kim, C.-S.; Lee, W.-Y. Macromolecules 2009, 42(6), 2054–2062. (25) Mistry, M. K.; Subianto, S.; Choudhury, N. R.; Dutta, N. K. Langmuir 2009, 25(16), 9240–9251. (26) Schmidt, C.; Glueck, T.; Schmidt-Naake, G. Chem. Eng. Technol. 2008, 31(1), 13–22. (27) Martinelli, A.; Matic, A.; Jacobsson, P.; Borjesson, L.; Navarra, M. A.; Panero, S.; Scrosati, B. J. Electrochem. Soc. 2007, 154(8), G183–G187. (28) Doyle, M.; Choi, S. K.; Proulx, G. J. Electrochem. Soc. 2000, 147(1), 34–37. (29) Bennett, M. D.; Leo, D. J.; Wilkes, G. L.; Beyer, F. L.; Pechar, T. W. Polymer 2006, 47(19), 6782–6796. (30) Sekhon, S. S.; Park, J.-S.; Baek, J.-S.; Yim, S.-D.; Yang, T.-H.; Kim, C.-S. Chem. Mater. 2010, 22(3), 803–812. (31) James, P. J.; Elliott, J. A.; McMaster, T. J.; Newton, J. M.; Elliott, A. M. S.; Hanna, S.; Miles, M. J. J. Mater. Sci. 2000, 35(20), 5111–5119. (32) Gebel, G.; Diat, O. Fuel Cells 2005, 5(2), 261–276.
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and morphology.31-38 Since conductivity of proton exchange membranes is strongly dependent on the morphology and structure of the membrane, therefore, in this work we aim to investigate the interaction between the conducting IL and the ionic proton exchange membrane and employ SAXS to investigate precisely the nanostructure evolution of the ionomer composite membranes with temperature. SAXS allows us to probe the structure of novel proton exchange membranes prepared from unique ionic block copolymers and IL (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) inclusions, in which both the polymer and IL can contribute to proton conductivity. The IL was chosen due to its high thermal stability, excellent proton conductivity, and low vapor pressure. The presence of the fluorinated anion makes this IL hydrophobic in nature, which will improve compatibility between IL and perfluorinated membrane. Nafion and Hyflon were selected as the perfluorinated membranes to incorporate the IL for correlation of their microstructure and proton conductivity at various temperatures. We also correlate SAXS results to the proton conducting properties under anhydrous conditions and discuss the effect of side chain length and EW with respect to conductivity and morphology.
Experimental Section Materials. Nafion 112 with an EW of 1100 g/SO3H was purchased from Sigma-Aldrich, and Hyflon with an EW of 890 g/SO3H was provided by Solvay Solexis, Milan, Italy. All membranes were converted to sulfonic acid form using a reported initialization procedure.14 Membranes were purified by boiling in 3% hydrogen peroxide solution for 2 h followed by 1 M H2SO4 for a further 2 h and finally Milli-Q water for another 2 h. The IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMI-BTSI) with a purity of 99.9% was purchased from Solvent Innovation, Germany, and was used without further purification. The properties of this IL are density 1.44 g/mL, melting point 2 °C, molar mass 419.37 g/mol, and conductivity at 25 °C 0.406 S/m. The chemical structures of the ionic liquid, Nafion, and Hyflon are shown in Scheme 1A along with the schematics of ionic clusters and channels of ionomer (Scheme 1B). Methanol and Milli-Q water were used as solvents. Sample Preparation. The membranes were thoroughly dried in a vacuum oven to ensure all traces of water were removed before modification. IL-impregnated membranes were prepared by soaking a weighed piece of the perfluorinated membrane in a solution of the IL, which was diluted with methanol. Membranes were soaked in a 20% solution of IL in methanol over a period of several days at both 60 and 75 °C. Upon removing membranes from the solution after a defined time, they were blotted with KimWipes and dried at 80 °C under vacuum. The mass of the absorbed IL was determined gravimetrically. Leaching Test. The leached membranes were prepared by immersing membranes impregnated with IL in Milli-Q water at room temperature. Membranes were removed from water every 24 h and dried in a vacuum oven until a constant weight was achieved. Loss of IL from membrane was determined gravimetrically. Proton conductivity measurements were carried out on leached samples to determine drop in conductivity in comparison to nonleached membranes. Proton Conductivity. An ESPEC SH-240 (ESPEC Corp, Japan) temperature/humidity chamber and a conductivity cell (33) Rollet, A.-L.; Diat, O.; Gebel, G. J. Phys. Chem. B 2002, 106(12), 3033– 3036. (34) Schmidt-Rohr, K.; Chen, Q. Nature Mater. 2008, 7(1), 75–83. (35) Roche, E. J.; Pineri, M.; Duplessix, R. J. Polym. Sci., Part B: Polym. Phys. 1982, 20(1), 107–116. (36) Roche, E. J.; Pineri, M.; Duplessix, R.; Levelut, A. M. J. Polym. Sci., Part B: Polym. Phys. 1981, 19(1), 1–11. (37) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104(10), 4535–4585. (38) Gebel, G. Polymer 2000, 41(15), 5829–5838.
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Scheme 1. (A) Chemical Structures of Ionic Liquid, Nafion, and Hyflon; (B) Schematics of Cluster and Crystalline Structure of the Perfluorinated Polymer
were used for the measurement of membrane conductivity (σ) under controlled temperature and humidity. Each sample used was approximately 12 10 mm, and σ was calculated using eq 1 l σ ¼ Rhw
ð1Þ
where l is the distance (cm) between the two Pt electrodes, h and w are the thickness (cm) and width (cm) of the membrane, respectively, and R (Ω) is the resistance of the membrane obtained from the complex impedance plot measured by Solartron 1260 (Solartron Analytical, UK). Conductivity measurements on all the samples were made in triplicate, and the error limits associated with conductivity measurements were within 5% as indicated in Table 1. Small-Angle X-ray Scattering (SAXS). SAXS data were collected on a NanoSTAR II instrument (Bruker AXS, Karlsruhe) with pinhole collimation for point focus geometry. The X-ray source is a copper rotating anode (0.3 mm filament) operating at 45 kV and 110 mA, fitted with Montel graded Multilayer X-ray optics, resulting in Cu KR radiation of wavelength λ = 1.54 A˚. The SAXS instrument is fitted with a Vantec 2D detector (effective pixel size 100 100 μm2). Optics and sample chamber were under vacuum to minimize air scatter. For each sample the 2D SAXS data were collected for 1800 s as a function of scattering vector q (where q = 4π sin θ/λ with 2θ the scattering angle) over the range 0.015 < q < 0.3 A˚-1. The membranes were cut into strips ∼2 mm wide. The membrane strips were loaded and sealed in 2 mm i.d. quartz capillaries (Hampton Research) and then mounted in a three-position sample holder with the axis perpendicular to the sample holder. The sample temperature was controlled ((0.5 °C) in the range 30 < T < 190 °C. The SAXS patterns for the Nafion membranes were isotropic and were radially averaged (0 < χ < 360°) with Q = 0 to produce a 1D scattering profile. Many of the patterns for the Hyflon Langmuir 2010, 26(24), 19073–19083
Table 1. Conductivity Values of Unmodified and Modified Membranes conductivity (mS/cm) sample name
100 °C
130 °C
160 °C
Nafion Nafion/IL Nafion/IL leached Hyflon Hyflon/IL Hyflon/IL leached
0.075 ( 5% 1.338 ( 5% 0.171 ( 5% 0.011 ( 5% 0.302 ( 5% 0.078 ( 5%
0.025 ( 5% 2.279 ( 5% 0.546 ( 5%
3.585 ( 5% 1.011 ( 5%
0.82 ( 5% 0.286 ( 5%
1.773 ( 5% 0.746 ( 5%
membranes displayed considerable anisotropy; therefore, all patterns were sector averaged ( χ = (30°) about (i) the detector vertical axis and (ii) the detector horizontal axis. Thermal Analysis. DSC was performed using a TA Q2000 DSC instrument (TA Instruments, New Castle, DE) with a heating rate of 10 °C min-1 under a nitrogen atmosphere. The sample mass was kept between 5 and 10 mg. The unit was fitted with a liquid nitrogen cooling accessory (LNCA). Previous sample history was erased by first heating the samples to 250 °C, holding for 5 min, and then cooling back to ambient temperatures. Dry samples were cooled from room temperature down to -50 °C, kept at the temperature isothermally for 5 min, and then heated to 300 °C.
Results and Discussion Microstructure of Unmodified Membranes by SAXS. Proton conductivity of proton exchange membranes is very much dependent on the structure and morphology of the membrane.39-50 It is well-known that Nafion possesses a distinct (39) Ding, J.; Chuy, C.; Holdcroft, S. Adv. Funct. Mater. 2002, 12(5), 389–394.
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nanophase-separated morphology, which has been studied extensively using SAXS. The morphology of Nafion consists of a fluorinated hydrophobic backbone and hydrophilic domains, which arise from clustering of sulfonic acid groups. The fluorinated backbone of Nafion provides mechanical strength to the membrane while the ionic domains contribute to high proton conductivity. The exact morphology of Nafion is still not well understood after numerous studies, which have attempted to model SAXS data of Nafion in order to describe the morphology of this membrane. Table 1 of the Supporting Information summarizes different models that have been proposed to describe the structure of Nafion as follows: One of the earliest models of Nafion, the cluster network model, proposed by Gierke et al.48 is popular in explaining the microstructure and ionic conductivity of water-swollen Nafion. This model proposes spherical ionic clusters with an inverted micelle structure (3-5 nm in diameter), which are interconnected by narrow water channels (1 nm in width). Although this model received significant acceptance, the cluster-network model is considered too simplistic because of the assumption of periodic distribution of spherical clusters. Another popular model for Nafion is the core-shell model developed by Fujimura et al.45,46 This intraparticle model describes ionic clusters surrounded by a fluorocarbon phase, which is embedded in an intermediate ionic phase consisting of both fluorocarbon polymer and nonclustered ionic sites. However, the model showed a poor fit to experimental data. Moreover, the dimensional and contrast values extracted from best fits were determined to be unrealistic. Dreyfus et al.44 proposed a local-order model, which is an interparticle model used to define the spatial distribution of spherically shaped ionic clusters in Nafion. This model suggested that ionic domains had a tetrahedral-like packing arrangement but with short-range order. Gebel and Lambard47 demonstrated that the local order model provided a better fit to SAXS and SANS profiles of hydrated Nafion than those of the core-shell model. Other models, which do not consider connected spherical shaped ionic clusters, have also been suggested to describe the structure of the hydrophilic phase of Nafion. Litt et al.50 have proposed a lamellar model of Nafion, where the ionic domains are defined as hydrophilic micelles separated by thin, lamellar PTFE-like crystallites. This model was used to explain the shift in the ionomer peak with changing water volume fraction and described Nafion structure as a lamellar organization of planar clusters. As Nafion absorbs water, separation between ionic domains and crystalline regions increases, resulting in an increase in the d-spacing between these ionic domains, and is expected to be proportional to the volume fraction of water in the polymer. The lamellar model provides a convenient and simple explanation for the swelling behavior of (40) Tsang, E. M. W.; Zhang, Z.; Yang, A. C. C.; Shi, Z.; Peckham, T. J.; Narimani, R.; Frisken, B. J.; Holdcroft, S. Macromolecules 2009, 42(24), 9467– 9480. (41) Tsang, E. M. W.; Zhang, Z.; Shi, Z.; Soboleva, T.; Holdcroft, S. J. Am. Chem. Soc. 2007, 129(49), 15106–15107. (42) Kim, J.-S.; Yoshikawa, K.; Eisenberg, A. Macromolecules 1994, 27(22), 6347–57. (43) Hird, B.; Eisenberg, A. Macromolecules 1992, 25(24), 6466–74. (44) Dreyfus, B.; Gebel, G.; Aldebert, P.; Pineri, M.; Escoubes, M.; Thomas, M. J. Phys. (Paris) 1990, 51(12), 1341–54. (45) Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1981, 14(5), 1309–15. (46) Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1982, 15(1), 136–44. (47) Gebel, G.; Lambard, J. Macromolecules 1997, 30(25), 7914–7920. (48) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci., Polym. Phys. Ed. 1981, 19(11), 1687–704. (49) Haubold, H. G.; Vad, T.; Jungbluth, H.; Hiller, P. Electrochim. Acta 2001, 46(10-11), 1559–1563. (50) Litt, M. H. Polym. Prepr. 1997, 38(1), 80–81.
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Nafion; however, it ignores the low angle maximum attributed to the crystalline, interlamellar long-range spacing. The sandwichlike model proposed by Haubold et al.49 describes the structure of Nafion to be a core region sandwiched between shells made of hydrophilic groups and polymer side chains. The lateral dimensions are between 1.5 and 4.5 nm with a total thickness of 6 nm. In a dry state the core remains empty, and with hydrated Nafion the core is filled with water. The model gives information on the basic structural unit while ignoring the mesoscale structure. Gebel et al.47 have investigated the structural evolution of Nafion from dry membrane to a hydrated state. Their structural model is based on polymer rodlike aggregates, which are surrounded by ionic groups that are packed with an orientation ordering in bundles that are randomly orientated in space at the submicrometer scale. Cylindrical polymer aggregates suggested to have a diameter of about 4 nm and length greater than 100 nm and are randomly distributed at the mesoscale. More recently, Schmidt-Rohr have shown that none of the models which have been proposed to explain the structure of hydrated Nafion can match the experimental SAXS data.34 Their proposed model is based on parallel cylindrical water channels having diameters between 1.8 and 3.5 nm. Unlike other models which have ignored crystallinity, this model incorporates 10 vol % of Nafion crystallites that form physical cross-links and are elongated and parallel to water channels. It has been claimed that this new model for the structure of Nafion can effectively explain important features such as fast diffusion of water and protons through Nafion and its persistence at low temperatures. Despite the controversy surrounding the exact structure of Nafion, SAXS profiles (Supporting Information, Figure 1) for Nafion consistently show the following general features: (i) scattering maximum occurring at (q) values in excess of 0.1 A˚-1, which is associated with aggregation of individual ion pairs and is referred to an the ionomer peak; (ii) scattering maximum occurring at q ∼ 0.04 A˚-1, which is associated with crystalline regions of Nafion0 s structure and is called the matrix peak; (iii) demonstrate an upturn in intensity in the very low q region (q < 0.01 A˚-1) and is generally associated with large-scale heterogeneities. For Nafion membranes the majority of the SAXS work has been focused on hydrated Nafion, which requires humidity for high conductivity. Supported liquid membranes containing nonaqueous solvents for high-temperature proton conductivity have not been studied extensively using SAXS. Reported work on Nafion-based supported liquid membranes has mainly focused on the effect of different ionic liquids with different hydrophobicity/ hydrophilicity on the final structure and morphology of membranes under ambient temperature. In this work we have chosen only one hydrophobic ionic liquid and studied its effect on the morphology of perfluorinated membranes with different side chain lengths over a wide range of temperatures. The detailed SAXS investigation of Nafion and Hyflon membranes (Figures 1 and 2) are reported in the Supporting Information. Microstructure of Supported Liquid Membranes by SAXS. 2D images of Hyflon SLMs (Figure 1) like unmodified Hyflon (Figure 2, Supporting Information) show anisotropic scattering indicating strong orientation in Hyflon SLMs from 30 °C up to 190 °C. The 1D SAXS spectrum of Hyflon SLM (Figure 1) integrated in the vertical direction at 30 °C shows the presence of a very diffuse ionomer peak centered at q = 0.2 A˚-1 related to ionic aggregation and corresponds to an intercluster distance of 31.4 A˚. This observation indicates the ion aggregation is present in ionic liquid-swollen membranes in a similar way that ionic clusters are formed in water-swollen membranes. A very weak diffuse matrix peak is also observed centered around q = 0.042 A˚-1, which Langmuir 2010, 26(24), 19073–19083
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Figure 1. 2D images and 1D SAXS spectra of Hyflon SLM.
corresponds to the crystallinity present in Hyflon membranes. However, the SAXS spectrum for Hyflon SLM integrated in the horizontal direction at the same temperature does not show the presence of any ionomer peak. These results confirm that the studied Hyflon SLMs have strong structural orientation in one direction. The formation of a matrix peak and ionomer peak becomes more apparent at higher temperatures. These results suggest that phase separation is induced in Hyflon SLMs with increasing temperature. Similar observations were made for Nafion SLMs. However, the onset temperature at which phase separation is observed occurs at a lower temperature for Hyflon SLMs than that of Nafion SLMs. In the Hyflon SLM system, phase separation seems to be more prominent in the vertical direction and again suggests strong structural orientation in these membranes. It has also been observed that the scattering intensity of Hyflon SLM integrated in the horizontal direction is much higher than that of Hyflon SLM integrated in the vertical direction and further confirms the presence of structural orientation in these membranes. In Nafion SLMs phase separation is observed at 160 °C, and in Hyflon SLMs we start to observe phase separation at 100 °C. The strong orientation observed in 2D SAXS images suggests a lamellae, rodlike, cylindrical, or elongated structured morphology, where the size and shape of scattering Langmuir 2010, 26(24), 19073–19083
objects are different in the vertical and horizontal direction. Conductivity results of Hyflon SLM (Figure 6) later prove orientation in these membranes as conductivity values are higher in one direction than the other. Microstructure of Leached SLMs by SAXS. Figure 2 shows the SAXS 2D images for Nafion SLM leached at various temperatures, which reflects isotropic scattering similar to unmodified Nafion (Figure 1b, Supporting Information). The 2D images show distinct scattering from both ionic domains and crystalline regions, which indicates strong phase separation in leached membranes that was not observed in original SLMs. Figure 2 also shows the corresponding SAXS profiles for leached membranes, which is significantly different to both unmodified Nafion (Figure 1, Supporting Information) and Nafion SLM (Figure 3, Supporting Information). The 1D SAXS spectrum of leached Nafion SLM shows an ionomer peak located at q = 0.22 A˚-1 and a broad matrix peak at q = 0.065 A˚-1. This corresponds to an average intercluster distance of 28.5 A˚. SAXS data show how the intercluster distance between ionic domains changes due to the incorporation of ionic liquid and its leaching. In unmodified Nafion the intercluster distance was calculated to be 27.3 A˚, which increases to 33 A˚ after incorporation of ionic liquid; after the leaching process the intercluster distance is reduced to 28.5 A˚. These findings indicate DOI: 10.1021/la1030763
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Figure 2. 2D images and 1D SAXS spectra of leached Nafion SLM.
that the ionic liquid specifically interacts and interconnects the ionic clusters and has permanently increased the intercluster distance from 27.3 A˚ in unmodified membranes to 28.5 A˚ in leached Nafion SLMs. The result also confirms that ionic liquid has been successfully immobilized within the ionic clusters through electrostatic interactions with sulfonate groups. It has been observed that the scattering intensity of the ionomer peak starts to decrease with increasing temperature up to 190 °C. However, the ionomer peak persists for all temperatures, an observation not seen in Nafion SLMs (Figure 1b, Supporting Information). This can be explained in terms of leaching of excess ionic liquid, which caused complete homogenization of the membrane. For leached membranes both the matrix and ionomer peaks are clearly visible at all temperatures, indicating a return to a distinct phase-separated morphology. 1D SAXS profiles also show that the scattering from the crystalline phase increases with increasing temperature. This observation suggests that long-range ordering in these leached membranes is influenced by temperature. The conductivity results show reduced conductivity for leached samples (Table 1) in comparison to SLMs. However, the remaining IL within the membrane after leaching is still sufficient to allow the percolation threshold to be achieved, and the leached sample demonstrated an increase in conductivity with temperature. At high temperature we observe an increase in phase separation as seen by the presence of a sharp matrix peak at 190 °C, which also contributes to proton transfer in leached membranes. Figure 3 shows the 2D SAXS images for leached Hyflon membranes and continues to exhibit anisotropic scattering as observed in original membrane (Figure 2, Supporting Information). The outer ring related to the ionomer peak is much more distinguishable in leached membranes in comparison to Hyflon SLMs (Figure 1). This has been shown clearly by SAXS results. In Hyflon SLM a very weak and diffuse ionomer peak is observed at elevated temperature; however, in the leached Hyflon SLM we see the presence of distinct ionomer peak at all temperatures. The intensity of the matrix peak increases with temperature up to 190 °C, suggesting strong long-range ordering in the crystalline region. In the corresponding SAXS profiles of leached membranes up to 190 °C (Figure 3), a defined ionomer peak positioned at 19078 DOI: 10.1021/la1030763
q = 0.22 A˚-1 is observed corresponding to an intercluster distance of 28.5 A˚, which does not change with temperature. In leached membranes, phase separation increases with temperature as the density and ordering of crystallites increase. Phase separation is most prominent at 190 °C, and SAXS results suggest that an increase in temperature induces or increases phase separation in leached SLMs. Nevertheless, the high-temperature conductivity results of leached Hyflon membranes indicate sufficient IL remains entrapped within the membrane for the percolation threshold to be achieved. AFM was used to investigate the phase behavior of the blank and IL membranes. Figure 4 shows the phase images of Hyflon, Nafion, and their corresponding SLMs. The images show distribution of light and dark areas corresponding to large-scale fluctuations of the distributions of hydrophilic ionic domains in hydrophobic regions. Although both SLM samples show good connectivity, however, the conductivity value (Table 1) is very low due to decrease in the mobility of the ions at room temperature in the viscoelastic matrix. This observation indicates that the block nature of both the copolymers plays a part in improving IL uptake and proton conductivity by promoting a microphase separation between the fluorinated backbone and hydrophilic sulfonated block. The phase-separated morphology achieved also with IL is beneficial since it assists in the formation of hydrophilic domains and channels required for proton conduction, enhancing the cation interchange of sulfonic acid groups within these domains. This results in an interconnecting ionic domain, which provides inherently good mobility, where the main role of hydrophilic ionic block would be to generate charge carriers by ionization of the acidic groups. The higher conductivity of Nafion supported liquid membranes over corresponding Hyflon membranes is associated with the length of the side chain. A longer side chain allows more segmental mobility of sulfonic acid groups and results in more sulfonic acid groups aggregating together to form larger ionic domains. The short side chain of Hyflon reduces the segmental mobility of sulfonic acid groups, and therefore fewer acidic groups aggregate and form smaller ionic domains in comparison to Nafion. This is also consistent Langmuir 2010, 26(24), 19073–19083
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Figure 3. 2D images and 1D SAXS spectra of leached Hyflon SLM.
with the SAXS results of Nafion and Hyflon and their SLMs, which show better connectivity in the case of Nafion and Nafion SLMs through distinct ionic peak formation, whereas in Hyflon, the ionic domains are more irregularly dispersed with lower connectivity. While the AFM data show room temperature morphology for both systems, the SAXS results of Hyflon systems at higher temperature exhibit strong orientation effect due to long-range ordering. Currently, high-temperature electrochemical mapping is underway to examine the distribution of hydrophilic and hydrophobic domains in each system. Thermal Behavior from Differential Scanning Calorimetry. Figure 5 shows the results obtained from DSC for unmodified membranes, SLMs and leached SLMs, which show endothermic and exothermic peaks from heating and cooling cycles. For all samples, the initial heating scan displays an endothermic peak at ∼196 °C. In agreement with the thermal behavior of semicrystalline polymers with slow crystallization kinetics, this endotherm is assigned to the melting of PTFE-like crystallites. This observation indicates that crystallites within perfluorinated membranes are thermally stable up to temperatures of 196 °C. Work by Page et al.2 also reported similar observation in their study of Naþ and Csþ neutralized forms of Nafion. However, Langmuir 2010, 26(24), 19073–19083
they observed an endothermic peak between 200 and 250 °C, which they assigned to the melting of crystallites, which is dependent on the time and temperature of annealing. The enthalpy associated with these endothermic events has been shown in Table 2. For Nafion-based systems, the heat of fusion (ΔH) has been observed to decrease from 0.23 to 0.19 J/g after incorporation of IL within the Nafion membrane. After leaching of the Nafion SLM, the heat of fusion increases from 0.19 to 0.65 J/g as shown in Table 2. A similar trend was observed for Hyflon-based systems. However, Page et al.2 reported the heat of fusion of initial endothermic event to be 5 J/g for the as-received Nafion sample, which is considerably higher than the values reported in this work. The enthalpy associated with this endothermic event changes with different annealing times, and its position is dependent on the annealing temperature. However, in the present study for unmodified membranes and SLMs only one endothermic peak is observed, which is related to the melting of PTFE crystallites in the presence of an electrostatic network. In the case of leached SLMs, a second endothermic peak is observed. For leached Nafion SLMs the second endothermic peak appears at 145 °C, and for leached Hyflon SLMs it appears at 157 °C. These peaks can be attributed to the cluster transition temperature (Tc). During the DOI: 10.1021/la1030763
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Figure 4. AFM phase images of Hyflon and its SLM (top) and Nafion and its SLM (bottom).
cooling cycle an exothermic peak is observed between 192 and 195 °C and is attributed to crystallization; however, no exotherm is observed due to cluster transition. Anhydrous Proton Conductivity at High Temperature. High-temperature proton conductivity values for unmodified, modified, and leached membranes are listed in Table 1 at 100, 130, and 160 °C. Figure 6A shows Arrhenius plots for all membranes and shows conductivity from 60 up to 160 °C in anhydrous conditions with no external humidification. Figure 6B shows conductivity of Hyflon SLM from 100 to 160 °C in both vertical and horizontal directions, and the inset shows the anhydrous conductivity of Hyflon and Nafion SLMs. These results indicate that Hyflon shows strong orientation in one direction due to a much more ordered structure (due to processing) in comparison to Nafion membranes. Orientation for Hyflon membranes was also observed from SAXS results where 2D SAXS images show anisotropic scattering. Change in conductivity in the horizontal and vertical direction is more apparent at higher temperatures, which indicates that temperature has an influence on the structure and morphology of Hyflon SLM. The results suggest that high temperature induces some kind of ordering in Hyflon, which is discussed in more detail in the SAXS section. Both unmodified Nafion and Hyflon membranes show a decrease in conductivity with increasing temperature; Nafion became resistive after 140 °C, and Hyflon showed resistance after 100 °C. As temperature increases, unmodified membranes start to dehydrate which leads to poor conductivity at high temperatures. Unmodified membranes were in a dry state during conductivity measurements and only contain small amounts of residual water to help facilitate proton transport. At elevated temperatures water starts to evaporate out of membranes, which results in reduced conductivity as there is no longer any proton solvent to assist with proton mobility. Once ionic clusters are dehydrated, they undergo an order-disorder transition above the cluster transition temperature which leads to a collapse of the cluster structure. At this point membranes no longer show any conductivity as the proton transfer pathway has been disrupted and membranes become 19080 DOI: 10.1021/la1030763
resistive. In contrast, supported liquid membranes show an increase in conductivity with temperature up to 160 °C. In supported liquid membranes water has been replaced with an ionic liquid, which acts as the proton solvent in a similar way to water in the case of hydrated membranes. Since the ionic liquid chosen in this work has a relatively low vapor pressure, it does not evaporate out of membranes at high temperature. Therefore, ionic liquid can help contribute to proton conductivity at high temperatures. The conductivity in SLMs is due to the protons contributed by -SO3H groups present in perfluorinated membranes. When swelling unmodified membranes in ionic liquid, the cation of the IL ion exchanges with protons in ionic clusters and interacts with sulfonate groups through electrostatic interactions. Protons can associate with free anions of the IL, which are loosely associated with cations and can transport by hopping among anion sites present in SLMs.30 Nafion SLM shows a conductivity of 3.58 mS/cm at 160 °C, whereas Hyflon only has a conductivity of 1.77 mS/cm at the same temperature. We would normally expect Hyflon SLM to show higher conductivity due to its higher IEC of 1.22 mequiv/g in comparison to Nafion, which only has an IEC of 0.91 mequiv/g; however, this is not the case. In our earlier work25 we have shown through DMA that Hyflon has a higher cluster transition, Tc, than Nafion, which is due to the difference in side chain length between the two membranes. Above Tc the network of hydrophilic clusters made from sulfonic acid groups starts to become extremely mobile before the cluster finally collapses. Below the Tc segmental mobility of sulfonic acid groups is restricted and results in reduced proton mobility, and therefore Hyflon SLMs require higher temperatures to increase segmental and proton mobility, which contributes significantly to proton conduction. However, the higher Tc of Hyflon membranes improves the high-temperature stability in comparison to Nafion. At higher temperatures the long side chain of Nafion is more susceptible to degradation, reducing the thermal stability. Because of this reason, Hyflon SLMs membranes show much more promise for high-temperature operation despite lower conductivities over Nafion SLMs. The effect of the side chain length also has an influence over the ion clustering ability Langmuir 2010, 26(24), 19073–19083
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Figure 5. DSC thermograms of unmodified membranes, SLMs, and leached SLMs. Table 2. Temperature and Enthalpy Data from DSC Study melt
crystallization
sample
temp (°C)
enthalpy (J/g)
temp (°C)
enthalpy (J/g)
Nafion Nafion SLM Nafion SLM leached Hyflon Hyflon SLM Hyflon SLM leached
196.3 196 196.2 196.2 196.1 196.2
0.23 0.19 0.66 0.52 0.26 0.65
192.7 194.5 193.2 194.7 194.3 194.6
0.22 0.59 0.15 3.78 1.24 1.54
and size of ionic clusters. Hyflon with the short side chains forms smaller ionic domains than Nafion membranes. The size of ionic domains has an effect on the proton conductivity; smaller ionic domains with a higher number density could only lead to well connectivity and percolation threshold to be achieved for proton transport. However, in Nafion membranes larger ionic domains can be formed due to the longer side chain and result in improved connectivity and proton transport due to increased percolation. Holdcroft et al.39-41 performed extensive work investigating the effect of side chain length on proton conductivity using various membranes. The authors have shown that membranes based on a longer side chain phase separate to a larger extent than those based on shorter side chains. Short side chains aggregate together Langmuir 2010, 26(24), 19073–19083
to form smaller ionic clusters, these ionic domains being more isolated and conductivity lower, while long side chains form larger ionic domains, which enhances connectivity of ionic domains and thus ionic conductivity.39 Here we observe a decrease in conductivity in the case of leached membranes where excess ionic liquid has been washed out; however, leached membranes show considerably higher conductivities than the corresponding unmodified membranes. Leached membranes continue to show increase in conductivity with temperature, indicating that ionic liquid has been immobilized within membrane and contributes to proton conductivity. Leached membranes show conductivity of 1.01 and 0.75 mS/cm at 160 °C for Nafion and Hyflon, respectively. Since Hyflon has a higher IEC than Nafion, it contains more exchange sites to bind more IL and results in less IL being leached out. While Hyflon SLM still shows a lower conductivity than Nafion SLM after leaching, there is a lower percentage drop in conductivity in the Hyflon SLM, indicating Hyflon has the ability to bind and retain more IL due to its higher IEC. After leaching the membranes excess IL is washed out, which is not bound through electrostatic interactions with sulfonic acid groups. Because of excess IL being removed, the membrane is no longer homogenized and the ionomer peak is more pronounced in leached membranes. However, the cation of the IL is considerably larger and bulkier in comparison to protons and DOI: 10.1021/la1030763
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Figure 6. (A) Arrhenius plots for unmodified, modified, and leached membranes. (B) Conductivity of Hyflon SLM in vertical and horizontal directions, and inset shows high-temperature conductivity of Hyflon and Nafion SLM in anhydrous conditions.
explains the fact that only a limited number could be inserted into the ion cluster of Hyflon, which has short side chain and less ion clustering ability than Nafion. Also, Hyflon has a higher cluster transition temperature than that of Nafion, below which the conductivity is lower. These results indicate that Hyflon (with higher IEC than Nafion) and its SLMs although exhibit lower conductivity than Nafion yet show more long-term stability as well as improved high-temperature stability, which makes Hyflon SLM systems more promising for high-temperature proton exchange membrane application. The general principle that can be derived from this study is that ionic liquid, a superb low vapor pressure dipolar solvent, can act as an excellent charge carrier. While in this study only one type of IL is used, however, the nature of both IL and the polymer will have a significant influence on their interfacial interaction. The IL contributes to anhydrous conductivity; thus, the SLMs can provide conductivity at high temperatures due to low vapor pressure of the IL, eliminating the problem of dehydration as in the water-based system. In general, the nature and structure of the IL have a profound effect on the final morphology of the membrane. Also, a degree of interaction between the IL and polymer is necessary to obtain desired morphology and 19082 DOI: 10.1021/la1030763
conductivity. The key features of these IL are: very good temperature stability, high ionic conductivity, and ability to interact with the counterions while the fluorinated part remains compatible with the fluoropolymer backbone. The proton conductivity in such SLMs occurs through a combined vehicular and structure diffusion mechanism with excess ionic liquid contributing to diffusion of protons through the membrane.
Conclusion In summary, we have investigated the morphology of supported liquid membranes at elevated temperatures by small-angle X-ray scattering (SAXS). Proton conductivity results show that conductivity of SLMs increases with an increase in temperature up to 160 °C. Melting of PTFE-like crystallites has been observed by DSC, and results indicate that crystallites are thermally stable up to temperatures of 196 °C. SAXS results reveal that membranes containing ionic liquid show more pronounced phase separation with increasing temperature, which helps contribute to proton conductivity at high temperatures. The cation interacts with the sulfonate groups within ionic domains through electrostatic interactions and displaces protons from exchange sites. Langmuir 2010, 26(24), 19073–19083
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Protons associate with free anions of the ionic liquid, which are loosely associated with cations, and protons can transport through the membrane by hopping among anion sites within the membrane. The ionic liquid interacts with ionic clusters through an ion exchange process which allows percolation to be achieved and provides a proton transfer pathway through the membrane. The IL contributes to conductivity of the membrane through a vehicular and structure diffusion mechanism. Leached membranes continue to show an increase in conductivity with increasing temperature, indicating sufficient IL remains immobilized within the membrane to provide percolation. Incorporation of ionic liquid within perfluorinated membranes has permanently increased the size of ionic domains which effectively increased the cluster density and helps improve conductivity. SAXS results also show that long-range ordering of crystalline regions increases
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with temperature, which also increases phase separation and improved conductivity of supported liquid membranes. Acknowledgment. The authors gratefully acknowledge the financial support of the Australian Research Council (ARC) through a Discovery grant. The Australian Institute of Nuclear Science and Engineering (AINSE) is also acknowledged for providing access to SAXS instrument at the Australian Nuclear Science and Technology Organization (ANSTO) in Sydney. Thanks are also due to Solvay Solexis for providing Hyflon samples. Supporting Information Available: Details about the microstructures of blank membranes and Nafion SLM. This material is available free of charge via the Internet at http:// pubs.acs.org.
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