942 Chem. Mater. 2010, 22, 942–948 DOI:10.1021/cm901820s
Multilayer Sulfonated Polyhedral Oligosilsesquioxane (S-POSS)-Sulfonated Polyphenylsulfone (S-PPSU) Composite Proton Exchange Membranes† Berryinne Decker,*,‡ Claire Hartmann-Thompson,§ Peter I. Carver,§ Steven E. Keinath,§ and Pasco R. Santurri‡ ‡
Chemsultants International, 9079 Tyler Boulevard, Mentor, Ohio 44060, and §Michigan Molecular Institute, 1910 West St. Andrews Road, Midland, Michigan 48640 Received June 26, 2009. Revised Manuscript Received October 8, 2009
Single-layer proton exchange membranes (PEMs) have been under development for several decades. The current use of conductive polymers in PEM fuel cells has been limited primarily to single layered sulfonated fluoro polymers, e.g. Nafion. Although this architecture has certain positive characteristics, it lacks the robust properties required for use in high-temperature, low=humidity conditions. In this work, a multilayer composite PEM consisting of outer layers of sulfonated polyphenylsulfone (S-PPSU) and an inner layer blend of octa-sulfonated octaphenyl-POSS (S-POSS) and S-PPSU has been developed and shown to exhibit good conductivity, physical and chemical durability, and high strength. The multilayer composite PEM showed improved conductivity at 90 °C and 25% RH relative to analogous single-layer S-POSS-S-PPSU PEMs and to the typical sulfonated fluoro polymers currently in use. Introduction The primary function of a proton exchange membrane (PEM) is to allow proton transport from anode to cathode. Thus, high proton conductivity of the singlelayered membrane or multilayered composite is the first requirement. Several perfluorosulfonic acid polymers have been used to achieve high proton conductivity, e.g., the Nafion and Dow membranes, and Nafion is the most well-studied copolymer membrane for PEMFC. For Nafion membranes, the proton conductivity is about 0.1 S cm-1 but it only exhibits high proton conductivity when it is highly hydrated. At lower water content (less than 50% RH), the conductivity drops significantly.1 Recently, there has been a substantial interest in operating PEMFCs at temperatures higher than 100 °C. Many different materials have been reported as PEMs for high temperature and low relative humidity applications. High degree sulfonated poly (ether ether ketone) (S-PEEK) achieves the proton conductivity required for fuel cell use, but the mechanical properties of the membranes deteriorate.2 Among different polymers tested, partially disulfonated poly(arylene ether sulfone) copolymers, the BPSH (Biphenyl sulfone in H form) series, from the McGrath group, shows some promising results. Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. E-mail:
[email protected]. †
(1) Kordesch, K.; Simader, G. Fuel Cells and Their Applications VCH: Weinheim, Germany, 1996. (2) Mikhailenko, S. D.; Zaidi, S. M. J.; Kaliaguine, S. Catal. Today 2001, 67, 225–236.
pubs.acs.org/cm
They have high proton conductivity (up to 0.17 S cm-1 at 30 °C in water), good mechanical strength, chemical stability, and thermal stability up to 220 °C in air.3,4 These random polymers resemble Nafion in their hydrophilic/hydrophobic phase-separated morphology that varies depending upon the degree of sulfonation. However, Nafion and BPSH differ in several important respects.5 Nafion has a PTFE-like backbone, and is much more flexible than the stiff, aromatic sulfone ether linked backbone of BPSH. The high fluorine content of Nafion may promote phase separation from the polar ionic groups more strongly than the nonfluorinated BPSH. The conductivity of BPSH also increases with the degree of sulfonation. However, once the sulfonation is above 60 mol % (BPSH-60), a semicontinuous hydrophilic phase is formed and the membranes start to swell dramatically, and lose their mechanical strength. Different varieties of multiblock copolymer were designed to obtain optimal morphology. Poly(arylene ether sulfone)-b-polybenzimidazole copolymers6 were produced. An ionic conductivity of 0.047 S cm-1 was achieved for the doped membranes at 200 °C without external humidification. Inorganic/organic hybrid membranes were developed to avoid the trade-off between water swelling and good (3) Wang, F.; Hickner, M.; Kim, Y. S.; Zawondzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231–242. (4) Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Jeffrey, M.; Zawodzinski, T. A.; McGrath, J. E. Macromol. Symp. 2001, 175, 387–396. (5) Hickner, M. A. Transport and Structure in Fuel Cell Proton Exchange Membranes. Ph. D. dissertation. Virginia Polytechnic Institute, Blacksburg, VA, 2003; pp 45-47. (6) Lee, H.; Roy, A.; Lane, O.; McGrath, J.E. Polymer 2008, 49, 5387– 5396.
Published on Web 10/29/2009
r 2009 American Chemical Society
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mechanical properties. Inorganic oxide particles, such as SiO2, ZrO2, and TiO2, have been added to Nafion7-12 in order to enhance water retention. Arico et al. used SAXS to show that silica narrows the hydrophilic channels of Nafion from 7.0 to 6.5 nm, but increases the average ion cluster dimension and water uptake.13 Some reports show that Nafion/SiO2 composite membranes have better conductivity at room temperature, but not at 90 °C or above.14,15 SiO2 was also added to sulfonated poly(fluorinated aromatic ether), but the conductivity at high temperature was not recorded.16 Yoon and Poltarzeski added high-surface-area zeolite to perfluorosulfonyl fluoride resin for the same reason.17,18 This approach lowers gas permeability, but there is no clear evidence as to how these added particles affect the proton transfer properties of the membranes, and more work needs to be done to prove if the water brought in by the ceramic particles is really playing a role in proton transfer. Another approach has been to dope the membranes with inorganic oxide and strong acids. Three examples are membranes containing Al2O3 and poly(vinylidene fluoride)-poly(arcylonitrile) doped with H2SO4,19 membranes containing SiO2 and poly(ethylene glycol) doped with 4-dodeylbenzene sulfonic acid;20 and membranes containing SiO2 and poly(ethylene oxide) doped with monododecylphosphate.21 These have the benefit of low cost compared to Nafion, but the disadvantage of acid loss during membrane humidification. Litt et al. proposed polybezimidazole (PBI)22 and Savinell designed a membrane comprised of H3PO4-doped PBI. This had good proton conductivity around 130-180 °C.23,24 However, phosphoric acid generally tends to wash away during long-term use and multiple operation (7) Mauritz, K. A. Mater. Sci. Eng., C 1998, 6, 121–133. (8) Adjemian, K. T.; Lee, S. J.; Srinivasan, S.; Benziger, J.; Bocarsly, A. B. J. Electrochem. Soc. 2002, 149, A256–A261. (9) Chalkova, E.; Pague, M. B.; Fedkin, M. V.; Wesolowski, D. J.; Lvov, S. N. J. Electrochem. Soc. 2005, 152, A1035–A1040. (10) Arico, A. S.; Creti, P.; Antonucci, P. L.; Antonucci, V. Electrochem. Solid-State Lett. 1998, 1, 66–68. (11) Dimitrova, P.; Friedrich, K. A.; Vogt, B.; Stimming, U. J. Electroanal. Chem. 2002, 532, 75–83. (12) Navarra, M. A.; Croce, F.; Scrosati, B. J. Mater. Chem 2007, 17, 3210–3215. (13) Arico, A. S.; Baglio, V.; Antonucci, V.; Nicotera, I.; Oliviero, C.; Coppola, L.; Antonucci, P. L. J. Membr. Sci. 2006, 270, 221–227. (14) Jalani, N. H.; Dunn, K.; Datta, R. Electrochim. Acta 2005, 51, 553–560. (15) Jung, D. H.; Cho, S. Y.; Peck, D. H.; Shin, D. R.; Kim, J. S. J. Power Sources 2002, 106, 173–177. (16) Kim, Y.; Choi, S.; Lee, H.; Hong, M.; Kim, K.; Lee, H. Electrochim. Acta 2004, 49, 4787–4796. (17) Kwak, S. H.; Yang, T. H.; Kim, C. S.; Yoon, K. H. Solid state Ionics 2003, 160, 309–315. (18) Poltarzewski, Z.; Wieczorek, W.; Przyluski, J.; Antonucci, V. Solid State Ionics 1999, 119, 301–304. (19) Navarra, M. A.; Panero, S.; Scrosati, B. A. Journal of Solid State Electrochemistry 2004, 8, 804–808. (20) Chang, H. Y.; Thangamuthu, R.; Lin, C. W. J. Membr. Sci. 2004, 228, 217–226. (21) Honma, I.; Takeda, Y.; Bae, J. M. Solid State Ionics 1999, 120, 255– 264. (22) Aharoni, S. M.; Litt, M.H. J. Polym. Sci., Part A: Polym. Chem. 1974, 12, 639–650. (23) Wang, J. T.; Savinell, R. F.; Wainright, J.; Litt, M.; Yu, H. Electrochim. Acta 1996, 41, 193–197. (24) Ma, Y. L.; Wainright, J. S.; Litt, M. H.; Savinell, R. F. J. Electrochem. Soc. 2004, 151, A8–A16.
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cycles.25 Also, for automotive applications, a maximum temperature of 120 °C is desired, and PBI- H3PO4 does not have sufficient conductivity at such temperatures. Finally, performance is low because of catalysis limitations. Many other H3PO4 composite membrane systems have been studied, for example, the poly(vinylalcohol)/ H 3PO 4 blend, 26 the poly(ethylenoxide)/poly(methylmethacrylate)/H3PO4 blend,27 and the poly(acrylamide)/poly(ethylenoxide)/H3PO4 blend.28 A comparative study was carried out using poly(phthalazinone ether sulfone ketone)/H3PO429 where the peak power density of single cell operated at 150 °C with dry H2/O2 reached 0.85 W cm-2. The disadvantage of long-term stability is believed to be similar to the PBI- H3PO4 system. A similar system was designed by Bonnet et al., where polymers were replaced with S-PEEK that already carried intrinsic acids. The membrane with SiO2/S-PEEK and H3PO4 had good conductivity up to 130 °C, but only 7 10-3 Scm-1 at 75% RH.30 Adding pure ceramic oxide particles helps reduce membrane swelling, but it also reduces the membrane conductivity. 2.5-5.0 wt.% SO3H-functionalized SiO2 was mixed with Nafion and had 2.7-5.8 times higher conductivity than Nafion at 80 °C at 100%RH.31 Alberti has investigated PEEK or PVDF membranes filled with conductive zirconium phosphonate or zirconium phosphate32 to offset the conductivity loss generally seen when pure inorganic oxide particles are added to membranes. The conductivity for composite PVDF is 0.002 S cm-1 at 120 °C and 90% RH.33 Mendes doped an S-PEEK/PBI membrane with zirconium phosphate, and the conductivity increased 40% compared to S-PEEK.34 Sulfonated PEEK has also been doped with heteropolyacids of the form H3PW12O40.35 The conductivity reached 0.01 S cm-1 at 100 °C when the membranes were soaked in water. Sulfonated PEEK was also doped with WO3. H2O,36 and the conductivity was slightly higher than 0.01 S cm-1 at 100% RH. H3PW12O40 with BPSH gives 0.15 S cm-1 at 130 °C under fully hydrated condition.37 (25) Kerres, J. A. J. Membr. Sci. 2001, 185, 3–27. (26) Petty-Weeks, S.; Zupancic, J. J.; Swedo, J. R. Solid State Ionics 1988, 31, 117–125. (27) Przyluski, J.; Wieczorek, W.; Glowinkowski, S. Electrochim. Acta 1992, 37, 1733–1735. (28) Wieczorek, W.; Such, K.; Florjanczyk, Z.; Stevens, J. R. J. Phys. Chem. 1994, 98, 6840–6850. (29) Li, M.; Zhang, H.; Shao, Z. C. Electrochem. Solid-State Lett. 2006, 9, A60–A63. (30) Bonnet, B.; Jones, D. J.; Roziere, J.; Tchicaya, L.; Alberti, G.; Cassciola, M.; Massinelli, L.; Bauer, B.; Peraio, A.; Ramunni, E. J. New Mater. Electrochem. Syst. 2003, 3, 87–92. (31) Wang, H.; Holmberg, B. A.; Huang, L.; Wang, Z.; Mitra, A.; Norbeck, J. M.; Yan, Y. J. Mater. Chem. 2002, 12, 834–837. (32) Alberti, G.; Casciola, M. Annu. Rev. Mater. Res. 2003, 33, 129–154. (33) Casciola, M.; Alberti, G.; Ciarletta, A.; Cruccolini, A.; Piaggio, P.; Pica, M. Solid State Ionics 2008, 176, 2985–2989. (34) Silva, V.; Weisshaar, S.; Reissner, R.; Ruffmann, B.; Vetter, S.; Mendes, A.; Madeira, L.; Nunes, S. J. Power Sources 2005, 145, 485–494. (35) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M.; Kaliaguine, S. J. Membrane Science 2000, 173, 17–34. (36) Mecheri, B.; D’Epifanio, A.; Vona, M. L. D.; Traversa, E.; Licoccia, S.; Miyayamab, M. J. Electrochem. Soc. 2006, 153, A463–467. (37) Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2003, 212, 263–282.
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Figure 1. Drawing of the mixtures of sulfonated POSS and sulfonated polyphenylsulfone (S-PPSUs).
Heteropolyacids were also doped into Nafion and conductivity was observed to be higher than Nafion up to 90 °C when the membranes were immersed in water.38 The composite membranes made from heteropolyacids and postsulfonated epoxy showed a conductivity 6 10-5 S cm-1 at 165 °C (Herring et al.).39 Membranes with bridged polysilsequioxane and phosphotungstic acid provide a nanoscale interface between inorganic and organic materials40-42 and exhibit a conductivity around 0.001 S cm-1 at 120 °C and 20% RH. Composite membranes made of poly(vinylidene fluoride)cohexafluoropropylene (PVDF-HFP), and silica containing sulfonic acid groups, were prepared via in situ polymerization of tetraethoxysilane and sulfosuccinic acid.43 These membranes take up more water, but do not compensate for the low proton density, leading to lower conductivity. Composite membranes based on (38) Dimitrova, P.; Friedrich, K. A.; Stimming, U.; Vogt, B. Solid State Ionics 2002, 150, 115–122. (39) Sweikart, M. A.; Herring, A. M.; Turner, J. A.; Williamson, D. L.; McCloskey, B. D.; Boonrueng, S. R.; Sanchez, M. J. Electrochem. Soc. 2005, 152, A98–A103. (40) Honma, I.; Nakajima, H.; Nishikawa, O.; Sugimoto, T.; Nomura, S. Solid State Ionics 2003, 162-163, 237–245. (41) Honma, I.; Nakajima, H.; Nomura, S. Solid State Ionics 2002, 154-155, 707–712. (42) Kim, J. D.; Honma, I. Electrochim. Acta 2004, 49, 3429–3433. (43) Kim, D. S.; Park, H. B.; Lee, Y. M.; Park, Y. H.; Rhim, J. W. J. Appl. Polym. Sci. 2004, 93, 209–218.
P
wi = 6, i = 1, 2, ..., 8. x þ y = 0.68.
(3-glycidoxypropyl)trimethoxysilane and oxidized (3mercaptopropyl)trimethoxysilane have a good conductivity of ∼0.1 S cm-1 at 70 °C in fully hydrated conditions.44 The aim of PEM development work is to is to create a smooth continuous path for proton to travel (Figure 1). In our earlier study,45 sulfonated polyhedral oligosilsesquioxane (S-POSS) nanoadditives carrying proton conducting groups were synthesized, characterized and formulated into sulfonated polyphenylsulfone (S-PPSU, Solvay Radel R-5000). These single-layer membranes were demonstrated to have comparable conductivity to Nafion, but superior dimensional stability and strength. S-POSS particles have relatively high IEC value comparing to sulfonated polymer membranes, increasing proton density and contributing to proton conductivity. In similar work, it was demonstrated that S-POSS could contribute to water retention when it was added to proton conducting polymers by Mather46 and Choudury.47 (44) Park, Y. I.; Moon, J.; Kim, H. K. Electrochem. Solid-State Lett. 2005, 8, A191–A194. (45) Hartmann-Thompson, C.; Merrington, A.; Carver, P. I.; Keeley, D. L.; Rousseau, J. L.; Hucul, D.; Keinath, S. E.; Nowak, R. M.; Bruza, K. J.; Thomas, L. S.; Katona, D. M.; Santurri, P. R. J. Appl. Polym. Sci. 2008, 110, 958–974. (46) Choi, J.; Lee, K; Wycisk, R.; Pintauro, P. N.; Mather, P. T. ECS TRansaction 2008, 16, 1433–1442. (47) Subianto, S.; Mistry, M. K.; Choudury, N. R.; Dutta, N. K.; Knott, R. Appl. Mater. Interfaces 2009, 1, 1173–1182.
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In this study, a novel multilayered structure concept has been introduced in order to reduce brittleness and to improve membrane water management to attain high proton conductivity at low relative humidity. Normally, the organic-inorganic composite membranes suffer from brittleness, making MEA hot press preparation difficult, and resulting in cracking and pinholes. A few multilayered membranes have already been developed by Du Pont,48 Dow Chemical Co.,49 Fenton,50 and Tricoli51 to reduce methanol crossover, but this is the first approach to use layers comprised of composite materials for mechanical stability. Two thin outer layers made from sulfonated polyphenylsulfone (S-PPSU) were designed to provide the mechanical strength and increase the ductility of the membranes. The nanoscale size of S-POSS particles brings the benefit of hydrophilic domain connections. S-POSS loading and solvent casting techniques were optimized in order to obtain improved S-POSS dispersion, smaller S-POSS domains, and improved proton conductivity. Experimental Section Materials and Dynamical Mechanical Analyzer (DMA) test. S-POSS and S-PPSU were prepared and DMA was conducted as described in an earlier study.45 Fourier transform infrared (FT-IR). FT-IR was used to study the sulfonation of the S-POSS particles. The scan frequency ranged from 800 cm-1 to 2,000 cm-1. The resolution was 0.2 cm-1. Measurements were recorded using a Perkin-Elmer FT-IR with Pike ATR. The scan number was 64. Membrane Preparation. A mixture of 60% 1-methyl-2-pyrrolidinone (NMP) and 40% dimethylacetamide (DMAc) was used to dissolve the sulfonated polyphenylsulfones (S-PPSU). The S-POSS was dissolved and agitated in water over a 12 h period. 1-Methyl-2-pyrrolidinone (NMP) or dimethyl sulfoxide (DMSO) was added to the aqueous solution. The solution was held at 110 °C to remove water but retain NMP or DMSO. Membranes with different ratios of S-POSS to polymer were cast on flat aluminum foil substrates and dried at 80 °C for 4 h. The membranes were boiled in 0.5 M H2SO4 for 4 h to convert to the proton form used for further membrane characterization For multilayered membranes, the membrane casting was similar to the above procedure, but the drying time was much shorter. The top layer was applied onto the bottom layer after allowing twenty minutes for the bottom layer to dry. The drying time was long enough for the membrane layer to set on the substrate and to cease to be a liquid, but the drying time was also short enough to prevent the bottom layer drying completely, and to retain a semiwet texture. The “semi-wet” surface provides a good basis for bonding to occur between the two layers, and to prevent voids forming at the interface. Conductivity Measurements. Proton conductivity measurements using an AC impedance technique were performed using a CH 604 Impedance Analyzer. A voltage was applied at a frequency varying from 1 to 20 000 Hz, and the impedance (48) Rajendran, G.; Hockessin, D. U.S. Patent 5 981 097, 1999. (49) Plowman, K. R.; Rehg, T. J.; Davis, L. W.; Carl, W. P.; Cisar, A. J.; Eastland, C. S. U.S. Patent 5 654 109, 1997. (50) Si, Y.; Lin, J.; Kunz, H. R.; Fenton, J. M. J. Electrochem. Soc. 2004, 151, A463–A469. (51) Martino, F. D.; Vatistas, N.; Tricoli, V. J. Electrochem. Soc. 2009, 156, B59–B65.
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response was measured. The four-electrode conductivity measurement cell (BekkTech) was immersed in distilled water and the conductivity was measured in liquid water at room temperature. Conductivity at different temperatures and humidity was measured by placing the membrane inside an environmental chamber ZPHS-32 Cincinnati Sub-Zero. Measurements were taken when the membrane reached the equilibrium at the set temperature and humidity. Ion Exchange Capacity (IEC) Determination. In order to determine the acid loading of the membranes, the protons in the membranes were titrated with NaOH solution. Before the titration, all the protons were exchanged with sodium ions. The membranes were immersed in 1 M NaCl solution and stirred overnight. Standard NaOH solution was prepared with a concentration of roughly 1 M. An accurately weighed amount of oxalic acid was used to prepare a 1 N (0.5M) oxalic acid solution. This 1 N oxalic acid solution was used to titrate the standard NaOH solution, and to calculate the precise concentration of the NaOH solution. The standard 1 M NaOH solution was carefully diluted to concentrations of 1 10-1, 1 10-2, and 1 10-3 M, respectively. NaOH solutions were used to titrate the solutions from the ion-exchanged membranes. Both phenolphthalein and a pH meter were used to determine the end point of the titration. Water Uptake Measurements. Membrane water uptake was determined by weighing the membranes before and after immersion in distilled water. Three different single-layered membranes were tested: Nafion, S-PPSU, and composite S-POSS/ S-PPSU. The membranes were dried, weighed, placed in vials filled with the desired concentration of LiCl solution, and weighed again. The time during which membranes were exposed to air was minimized to prevent water loss. Measurements were performed after suspending membranes over LiCl solution for 3 days (enough time to reach equilibrium) in a constant temperature chamber at 90 °C.
Results and Discussion Characterization of S-POSS. In an earlier study, the synthesis and characterization of S-POSS was reported.45 This section describes additional FT-IR and IEC characterization carried out for the S-POSS and S-PPSU selected for this multilayer membrane study. The FT-IR spectra of the POSS particles before and after sulfonation reaction are shown in Figure 2. The broadband at 1080 cm-1 corresponds to the asymmetric stretch of the Si-O-Si groups of silica.52 To determine the surface modification, we subtracted the nonsulfonated POSS spectrum (a) from the spectrum of the sulfonated POSS (b). The result is shown as curve (c). There are five peaks at 1048, 1117, 1170, 1248, and 1351 cm-1. These are assigned as the absorptions of the -SO3H groups.53,54 The bands at 1048 and 1117 cm-1 correspond to the symmetric and asymmetric vibrations of the -SO3- groups. The peaks at 1170 and 1351 cm-1 correspond to the symmetric and asymmetric vibrations of the SO2 groups. These FT-IR spectra indicate that the POSS nanoparticles are successfully sulfonated. (52) Adjemian, K. T.; Lee, S. J.; Srinivasan, S.; Benzigerand, J.; Bocarsly, A. B. J. Electrochem. Soc. 2002, 149, A256–A261. (53) Strasheim, A.; Buijs, K. Spectrochim. Acta 1961, 17, 388–392. (54) Kavc, T.; Kern, W.; Ebel, M. F.; Svagera, R.; Poelt, P. Chem. Mater. 2000, 12, 1053–105.
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Decker et al. Table 1. Proton Conductivity of Membranes Cast from Different Solvents conductivity (S cm-1)
casting solvent NMP 60% NMP/40% DMAc DMSO for particles 60% NMP/40% DMAc for polymers
0.053 0.059 0.071
Figure 2. FT-IR spectra of POSS particles before and after sulfonation. (a) Nonsulfonated POSS. (b) Sulfonated POSS. (c) The difference between sulfonated and nonsulfonated POSS.
Ion Exchange Capacity (IEC) Values. IEC values were measured for both the S-PPSU polymer and S-POSS particles. The S-PPSU polymer in this study has an IEC value 1.5 mmol/g. For Radel R-5000 PPSU, the theoretical IEC when each repeat unit has one sulfonic acid is 2.1 mmol/g. The theoretical IEC when each repeat unit has two sulfonic acid groups is 3.6 mmol/g. Hence the titration result of 1.5 mmol/g suggests that each polymer repeat unit carries 0.68 equivalents of sulfonic acid. The IEC value for the S-POSS particles was 3.5 mmol/g. One octaphenyl-POSS molecule has the capacity to carry eight sulfonic acid groups when all the phenyl rings are successfully sulfonated. The theoretical IEC value associated with complete sulfonation is 4.7 mmol/g. Hence the titration results show that approximately six sulfonic acid groups are present per S-POSS molecule. Steric hindrance was suspected to be the reason for the lower IEC value relative to the theoretical value. Effect of Casting Solvent upon Film Morphology. Several different solvents were used to mix the S-PPSU and S-POSS. The IEC values (1.5 mmol/g for S-PPSU and 3.5 mmol/g for S-POSS) indicate that the S-POSS particles are much more hydrophilic than S-PPSU. To ensure S-POSS particles disperse well inside the polymer matrix, a solvent system with the right polarity is needed. Dielectric constant indicates the polarity of the solvent. Dimethyl sulfoxide (DMSO) has a dielectric constant of 47, higher than N-methyl-pyrrolidone NMP (32) and dimethylacetamide DMAc (38). Hence DMSO is more polar than both NMP and dimethylacetamide DMAc. Table 1 shows the conductivity data for three membranes cast from different solvents, where the membrane cast from the three-solvent system had higher conductivity than the membranes cast from NMP and DMAc respectively. S-POSS particles were stored in DMSO to avoid particle aggregation. In our earlier study,45 NMP was used as the casting solvent. Transmission electron microscopy (TEM) was used to determine the size of the S-POSS domains in the continuous S-PPSU matrix.45 Figure 3 shows an S-POSS
Figure 3. TEM image of a cross-section of 20% S-POSS/80% sulfonated Radel R 5000 film cast from a mixed NMP/DMAc/DMSO solution (scale bar 1 μm). Table 2. Conductivity of the Membranes with Different Loadings of S-POSS Particles S-POSS loading (wt %)
conductivity (S cm-1) at room temperature, immersed in water
0 10 20 30 40
0.053 0.060 0.071 0.056 0.050
(dissolved in DMSO) particle size of 100-500 nm in a membrane cast from DMSO-NMP-DMAc where S-PPSU was dissolved in NMP and DMAc (60/ 40 w/w). This clearly indicates that a highly polar solvent such as DMSO helps the S-POSS particles disperse better in the polymer solution. Effect of S-POSS Loading on Conductivity. To design the optimum multilayer membrane composition, the S-POSS loading giving the best conductivity in a singlelayer system was first determined. Single-layered membranes with different loadings of S-POSS particles were cast and conductivity was measured when the membranes were immersed in deionized water. The results are shown in Table 2. S-POSS loadings of 10 wt % and 20 wt % improve the conductivity relative to an S-PPSU control membrane having 0 wt % S-POSS content. This may be rationalized in terms of the higher IEC value of the S-POSS particles. The conductivity drops form 0.071 S cm-1 to 0.056 S cm-1 when the particle loading increases from 20 wt % to 30 wt %, suggesting that the particles aggregate and block the hydrophilic domains of the polymer matrix. It is known that conductivity is a function of proton mobility and proton density, as shown in eq 1, where σ is conductivity, F is Faraday constant, Zi is charge, μi is mobility and Ci is proton density. The diffusion, where Di is the diffusion coefficient, is a function of mobility.
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Table 3. Water Uptake for the Membranes at 25% RH and 90 °C membrane Nafion 112 S-Radel 20%S-POSS/80%S-PPSU three-layer 20% S-POSS/80% S-PPSU
water IEC (mmol/g) uptake (%) from titration λ 3.0 4.4 6.3 5.5
0.91 1.53 1.82 1.61
1.8 1.6 2.0 1.9
Figure 4. Conductivity for different membranes. The conductivity data were collected at 90 °C at various %RH. (a) Nafion 112. (b) Three-layered membrane. (c) Single-layered membrane with 20% SPOSS and 80% S-PPSU. (d) S-PPSU.
Combining eqs 1 and 2 gives eq 3. Conductivity is a function of diffusion Di, and diffusion includes proton local friction and tortuosity. σ ¼ F2
X
Zi2 μi Ci
Di ¼ μi RT
ð1Þ ð2Þ
ð3Þ
Figure 5. Proposed schematic of membranes with different water contents. (a) Nafion at higher relative humidity. (b) Nafion at lower relative humidity. (c) Composite membrane at higher relative humidity. (d) Composite membrane at lower relative humidity.
There are two competing parameters in the proton conducting mechanism: the proton concentration and the proton diffusion coefficient. The sulfonated POSS particles contribute to the proton density, but they may also act as a barrier for the original hydrophilic domains inside the polymer matrix. When the particles aggregate, this is more likely to cause a barrier and increase the tortuosity of the proton-conducting path. This may explain the conductivity decrease when the particle loading reaches a certain value (Table 2). Effect of Relative Humidity Upon Conductivity and Water Uptake. The membrane with the best conductivity (i.e., with a 20% S-POSS loading) was chosen for further study. The conductivities at 25%, 50%, 75% and 100% relative humidity are shown in Figure 4. S-PPSU has similar conductivity to the membrane mixed with S-POSS particles at 100% RH/fully hydrated conditions. However, when the relative humidity is lowered to 25%RH, S-PPSU show a loss in conductivity, while the 20% S-POSS/80% S-PPSU membrane retains its conductivity. At 25% RH, the conductivity of the composite membrane is the same as the conductivity of single-component Nafion 112. The conductivity achieved via these modifications is around 10-2 S cm-1 at low RH. Thus, the nanoscale modified inorganic/organic composite membrane has great potential for real-world fuel cell applications. The water uptake (%) and of the membranes was calculated from the weight gain of absorbed water relative to the dry membrane, and is reported as a weight percent.
Water uptake can be converted into Lambda (λ) based on IEC values. A consideration of the data in these terms provides more information on the mechanisms, because λ provides a measure if water content that factors out everything except the water/proton ratio. Table 3 shows that all four membranes have similar λ at 25% RH and 90 °C. The ASTM D1042 test for dimensional stability had also previously indicated that membrane swelling is reduced by adding S-POSS particles.45 The three-layered membrane shows a much smaller dimensional change than Nafion 112 and S-Radel membranes, even though the λ values are similar at 25% and 90 °C. This indicates the water molecules are fully utilized in the proton transport process occurring inside the three-layered membrane. When S-PPSU membranes are doped with S-POSS particles, the particles tend to be close to the intrinsic hydrophilic domains. These particles are well dispersed inside the membranes and help to retain water and to improve conductivity (see schematic in Figure 5). The particles themselves tend to absorb water, but they also lower polymer chain mobility and prevent the water absorbing around the polymer chains. Sulfonated POSS particles are surrounded by hydrophilic domains and channels inside the polymer matrix. Furthermore, the high IEC value of S-POSS means that the (strongly hydrated) acid groups on the surface of the particle are closely packed. Figure 5d illustrates the possibility that the channels between polymer hydrophilic domains do not shrink very much at lower relative humidity, because
σ ¼
Di Zi 2 Ci kT
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Decker et al. Table 4. Storage Modulus As a Function of Temperature for Nafion, 100% S-PPSU, Single-Layer S-PPSU with S-POSS, and Three-Layered Membrane with 20% S-POSS in the Middle Layer
Figure 6. Illustration of a three-layered membrane.
the sulfonated POSS aids water retention. This water assists proton mobility, explaining why the conductivity of composite membranes is not a strong function of relative humidity. Effect of Multilayering. Although the composite membrane (S-POSS/S-PPSU) provides better conductivity and water uptake than the control S-PPSU membrane without S-POSS, it does not have optimized mechanical strength and is somewhat brittle. A three-layer structure was developed to overcome this disadvantage. The middle layer contains the composite material that acts as the major conducting component, and two thin layers of S-PPSU were coated as the first and third layers. This concept could also be extended to include five- or sevenlayer structures, where the particle loading is higher in the middle, and then gradually decreases toward the outer layers. For example, a five-layered membrane could be envisaged with an S-POSS content of 0%-10%-20%10%-0% from the first to the fifth layer. Figure 6 is an illustration of the three-layered membrane described in this study, with a 20 wt % S-POSS loading in the middle layer, and pure S-PPSU layers as outside layers. The first and third layers have a thickness of 12 μm, and the middle layer has a thickness of 25 μm. The conductivity was measured at 90 °C at various relative humidities (Figure 4). The three-layered membrane had better conductivity than the single-layered S-POSS/S-PPSU composite membrane. One possible explanation for the threelayered membrane having higher conductivity than the single composite layer is a “void-filling” effect during the casting of the multilayered membranes. For a single-layered composite membrane, it is common to have macro-scale (mm) air bubbles or pin-holes near the particles within the membrane resulting from solvent removal. This phenomenon prevents the successful function of the membrane, and interferes with proton transport. Preparation of multilayered membranes rather than single layer membranes allows each layer to be cast and dried such that the outer surface of the cast layer is not hardened or impervious to additional castings, and allows both internal and surface macro-voids to be filled. Drying times for the first and second castings were controlled so that casting occurred on a semiwet substrate where both internal and surface voids could be filled. This method of filling voids or pinholes from the second or third castings results in higher quality membranes with better interface adhesion of the layers and fewer potential barriers to good conductivity. The three-layer membrane was characterized by DMA, and the data was compared with that of the Nafion, 100%
membrane Nafion 117 single-layer 100% S-PPSU single-layer 20% S-POSS/80% S-PPSU three-layer 20% S-POSS/80% S-PPSU
storage storage storage modulus at modulus at modulus at 30 °C (MPa) 120 °C (MPa) 170 °C (MPa) 600 1954
23 1750
3 884
1426
1120
23
1348
1320
1202
S-PPSU and single layer S-POSS/80% S-PPSU membranes reported previously45 (Table 4). The single-layered composite membrane has a lower storage modulus than single-layered 100% S-PPSU membrane across a range of temperatures, and most notably at the higher temperature of 170 °C. However, the storage modulus for this singlelayered composite membrane is still significantly higher than Nafion 117 at various temperatures. The threelayered membrane has good storage modulus compared to the other membranes. There is no decrease at higher temperature. The higher the storage modulus, the more energy is stored during deformation. This indicates a more “solid” membrane, and the multilayered structure holds the polymer chains and particles well at higher temperature. Good mechanical properties for the multilayer structure at higher temperatures show great promise for fuel cell applications. Conclusion S-PPSU with an IEC 1.5 mmol/g was used as a carrier polymer for sulfonated polyhedral oligosilsesquioxane (S-POSS) to obtain a multilayer PEM with good conductivity at high operating temperatures and low relative humidity, and with good mechanical properties. The S-POSS also reduced membrane swelling. An optimum casting solvent blend and an optimum S-POSS loading were determined in order to achieve maximum particle dispersion. A multilayered structure was developed in order to reduce brittleness, and was demonstrated to have outstanding conductivity at low relative humidity (close to 110-2 S cm-1 at 25% RH and 90 °C). These membranes had excellent physical properties when compared with an analogous single-layer S-POSS/S-PPSU membrane, a 100% S-PPSU membrane control, and a Nafion control. Hence, three-layer S-POSS/S-PPSU composites were shown to be promising proton exchange membrane materials. Acknowledgment. This work was supported by the Department of Energy. We also thank Matthew Stephenson of Impact Analytical (Midland, MI) for carrying out the TEM experiments, and Xiaodong Liu of Impact Analytical (Midland, MI) for carrying out the DMA experiments.
