Chem. Mater. 2005, 17, 387-394
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Photocuring and Photolithography of Proton-Conducting Polymers Bearing Weak and Strong Acids Jennifer Schmeisser and Steven Holdcroft* Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia, V5A 1S6, Canada
Jianfei Yu, Tran Ngo, and Ged McLean Angstrom Power Inc., 980 W. First Street, No. 106, North VancouVer, British Columbia, V7P 3N4, Canada ReceiVed June 8, 2004. ReVised Manuscript ReceiVed NoVember 1, 2004
A series of novel conformable proton-conducting thin films has been prepared from photocurable liquid polyelectrolytes. Films prepared in such a fashion show promise for engineering unconventionally shaped proton-exchange membranes using photolithographic techniques. These films are semi-interpenetrating networks (semi-IPN) comprising a linear proton-conducting guest polymer, sulfonated poly[ether ether ketone] (S-PEEK), in the presence of a statistically cross-linked host polymer matrix comprising divinyl sulfone, vinylphosphonic acid, and acrylonitrile. Film properties ranging from brittle and fragile to robust and flexible have been obtained, depending on the ratio of host/guest composition used. The extent of dissociation of the weak acid component, vinylphosphonic acid, is strongly dependent on the S-PEEK content. Proton conductivity, water content, and lambda values are dependent on the membrane composition and degree of cross-linking. Proton conductivities similar to those of pure S-PEEK, ∼0.07 S/cm, have been observed for a semi-IPN containing as little as 35 wt % S-PEEK.
1. Introduction The proton-exchange membrane (PEM) is a key component of solid polymer electrolyte fuel cells. It acts as both a separator to prevent mixing of reactant gases, and as an electrolyte for transporting protons from anode to cathode.1-3 Nafion is the most widely studied PEM because it exhibits high conductivity, good mechanical strength, and chemical stability, and is commercially available. Nonfluorinated and partially fluorinated hydrocarbon-based membranes are viewed as low-cost alternatives to Nafion.4,5 Sulfonated poly(arylenes) such as sulfonated poly[ether ether ketone] (S-PEEK) are under intense investigation.6-8 They are easily prepared with well-defined ion-exchange capacities (IECs), are generally soluble in organic solvents, possess good proton conductivity, thermal and chemical stability, and exhibit good performance in H2 and methanol/air fuel cells.9,10 Conventionally, Nafion and sulfonated polyarylenes are employed as pre-cast or pre-molded membranes and com* To whom correspondence should be addressed. E-mail:
[email protected].
(1) Appleby, J.; Foulkes, R. L. Fuel Cell Handbook; Van Norstrand: New York, 1989. (2) Prater, K. J. Power Sources 1990, 29, 239-250. (3) Srinivasan, S. J. Electrochem. Soc. 1989, 136 (2), C41-C48. (4) Savadogo, O. J. New Mater. Electrochem. Syst. 1998, 1 (1), 47-66. (5) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463-1502. (6) Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T. A.; McGrath, J. E. Macromol. Symp. 2001, 175, 387-396. (7) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231. (8) Hasiotis, C.; Li, Q. F.; Deimede, V.; Kallitsis, J. K.; Kontoyannis, C. G.; Bierrum, N. J. J. Electrochem. Soc. 2001, 148 (5), A513-A519. (9) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2000, 173, 17-34. (10) Inzelt, G.; Pineri, M.; Schultze, J. W.; Vorotyntsev, M. A. Electrochim. Acta 2000, 45, 2403-2421.
pressed between two catalyzed electrodes under elevated temperature to form membrane-electrode assemblies (MEA) to achieve good interfacial adhesion. However, the requirement of using preformed membranes may restrict new fuel cell processing designs where high temperature and high pressure must be avoided. For instance, novel fuel cell design concepts might require the PEM to be conformable by injection molding, formed as microchannels and unique shapes, or strongly adhering to the catalyst layer without hot pressing. One conceptual route to enhance processability of PEMs is to prepare viscous liquids that can be cast, printed, or sprayed as films or as microstructured designs, and subsequently photocured, thermally cured, or cured by electron beam irradiation. Within this concept, a plausible demonstration is to dissolve a preformed proton-conducting polymer in a vinyl monomer/cross-linking agent, and to polymerize this composition to form a semi-IPN in which one polymer is cross-linked in the presence of a linear polymer (the proton-conducting polymer) as shown in Figure 1.11 The difficulty, however, is finding suitable monomers capable of dissolving the ionic proton-conducting material. In this work a series of semi-interpenetrating networks (semi-IPNs) of proton-conducting membranes are formed by photocuring of polymerizable liquids comprising linear S-PEEK, acrylonitrile (ACN), vinylphosphonic acid (VPA), divinyl sulfone (DVS), and dimethylacetamide (DMA) (Figure 2). S-PEEK was chosen as the proton-conductive medium because of its solubility in a variety of solvents and (11) Klempner, D.; Berkowski, L. Interpenetrating Polymer Networks. In Encyclopedia of Polymer Science and Engineering; Wiley-Interscience: New York, 1987; pp 279-341.
10.1021/cm049083z CCC: $30.25 © 2005 American Chemical Society Published on Web 12/22/2004
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Schmeisser et al. Table 1. Compositions of Liquid Polyelectrolytes (wt %) sample
S-PEEK
S1 S2 S3 S4 S5 S6 S7 S8 S9
VPA
DVS
AN
PI
DMA
0 4 8 17 31
Varying S-PEEK Content 19 44 19 17 43 17 17 41 17 15 37 15 13 31 13
3 3 3 3 2
16 15 15 13 11
26 26 26 26
Varying DVS Content 9 18 33 9 25 25 9 32 18 9 41 9
2 2 2 3
12 12 12 12
2.1 Materials. PEEK, poly[ether ether ketone], (Victrex Mn ≈ 110 000 g/mol), concentrated sulfuric acid (Anachemia), vinylphosphonic acid (Aldrich), and dimethylacetamide (Aldrich) were used as received. Divinyl sulfone (DVS) and acrylonitrile (ACN) were obtained from Aldrich and distilled prior to use. The photoinitiator, bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (IRGACURE 819), was provided courtesy of Ciba Specialty Chemicals Canada Inc. and used as received. 2.2 Preparation of S-PEEK Homopolymer. A series of S-PEEK polymers was prepared by dissolving 18 g of PEEK in
300 mL of concentrated sulfuric acid overnight at room temperature followed by reaction at 50-55 °C for 6.5 h.13 Films of 70-150µm thickness were prepared by dissolving S-PEEK (∼25 wt %) in DMSO and casting onto a glass plate. Cast membranes were dried at 90 °C overnight and subsequently washed in deionized water to remove residual solvent. The degree of sulfonation (DS) of S-PEEK was determined by 1H NMR spectroscopy of 3 wt % S-PEEK/DMSO-d solutions,13 6 and found to be 74%. 2.3 Preparation of Photocurable Polyelectrolytes. Photocurable polyelectrolytes were prepared according to the compositions shown in Table 1. To a 20-mL vial the appropriate amounts of liquid monomer (VPA, DVS, ACN), solvent (DMA), and photoinitiator (PI) were added. All samples were sealed and kept in the dark at room temperature for 12 h to ensure dissolution of the initiator. To another 20-mL vial an appropriate amount of S-PEEK was added and the corresponding solution of monomer/solvent/PI mixture was added. Complete dissolution of the S-PEEK in the monomer mixture took ∼2 days at room temperature. The viscosity of the resulting clear yellow solutions varied from free flowing to honey-like syrup as the S-PEEK content increased. 2.4 UV Photocured Electrolytes. Thin films of each solution were spread with a casting knife onto glass slides fitted with Cu wire (150 µm) spacers. The wires were 75 mm long and separated by 23 mm. The liquids were cured by irradiation using an ELC500 (Electro-Lite Corp., Danbury, CT) light-exposure chamber fitted with four 9-W visible lamps which generate broad spectrum visible light, 410-520 nm, centered at 450 nm. Films were situated inside the chamber at approximately 10 cm from the light source and cured for up to 4 h or until they were no longer tacky. Photocured films were stored in Milli-Q water. Cured film thicknesses ranged from 70 to 150 µm, depending on the Cu wire thickness. 2.5 Characterization. Infrared spectroscopy was performed on a Bomen BM-Series FT-IR spectrometer. UV-visible absorption spectra were recorded on a Cary 3E spectrophotometer. Ion exchange capacities of photocured films were determined by direct titration of triplicate samples to the phenolphthalein endpoint. The titrated ion exchange capacities were then used to estimate λ, typically defined as [H2O]/[SO3-], but in this case defined as [H2O]/ [acid site] to reflect the presence of more than one distinct acid groups that can be associated with water in the membrane. Film thicknesses were measured using a micrometer ((0.001 mm, Mitutoyo) and sample diameters were measured using a caliper ((0.1 mm, Mitutoyo). Thermal decomposition temperatures for the homopolymers and semi-IPNs were determined using thermogravimetric analysis (TGA) on a Shimadzu TGA-50 thermogravimetric analyzer. Samples of dry films of ∼2-5 mg were placed in platinum pans and heated from 25 to 500 °C at a rate of 10 °C/min under ambient atmosphere.
(12) Florjanczyk, Z.; Wielgus-Barry, E.; Poltarzewski, Z. Solid State Ionics 2001, 145, 119-126.
(13) Huang, R. Y. M.; Shao, P.; Burns, C. M.; Feng, X. J. Appl. Polym. Sci. 2001, 82, 2651-2660.
Figure 1. Schematic of semi-IPN.
Figure 2. Chemical structures of (a) S-PEEK, (b) host monomers, and (c) photoinitiator.
because its IEC can be controlled by adjusting reaction parameters.9 These particular monomers were chosen primarily because of their ability to solvate S-PEEK and because of their functionality: ACN provides flexibility and reduces brittleness of photocured films; VPA has been shown to enhance proton conductivity when polymerized into PEMs;12 DVS for its cross-linking ability; and DMA reduces the viscosity of the liquid solution. Thus, a cross-linked matrix comprising VPA, DVS, and ACN is considered the host polymer, and S-PEEK is the guest. The effect of the composition of the photocurable liquid on IEC, proton conductivity, water content, lambda values, thermal decomposition, Tg, and optical clarity of the resultant films are presented. 2. Experimental Section
Photocurable Liquid Polyelectrolytes
Chem. Mater., Vol. 17, No. 2, 2005 389
Figure 3. UV absorption spectra of Sample S4, 17 wt % S-PEEK, liquid, and photocured polyelectrolyte.
Differential scanning calorimetry (DSC) using a DSC Q10 (TA Instruments) was used to determine the glass transition, Tg, of the pure host/guest polymers and photocured semi-IPNs. The instrument was first calibrated against indium. Samples of dry films (∼2-10 mg) were placed in aluminum DSC pans and heated under a nitrogen atmosphere at a rate of 10 °C/min from 20 to 150 °C and held at 150 °C for 10 min to remove residual water. The samples were cooled from 150 to -100 °C and melting thermograms were obtained at a constant heating rate of 10 °C/min from -100 to 250 °C. The data were analyzed using Universal Analysis 2000, version 3.7A. Proton conductivity was determined by a.c. impedance spectroscopy with a Hewlett-Packard 8753A network analyzer (Agilent Technologies) and analyzed with Z-view (Scribner Associates) using a technique previously described.14 The average of three samples is reported. 2.6 Fuel Cell Polarization Curves. Commercial H-TEC (Luebeck, Germany) electrodes were used to evaluate the UV-cured semi-IPN membranes and a sample of Nafion 117 for comparison. The as-received electrodes were used to prepare membraneelectrode assemblies (MEA) simply by clamping the electrolyte of interest between two 16-cm2 electrodes. The MEA was positioned into an H-TEC single cell fixture and installed into an in-house fuel cell (FC) test station. The cell was operated at room temperature and fed with unhumidified H2 at the anode at a flow rate of 50 mL/minute and unhumidified air at the cathode at a flow rate of 100 mL/minute.
3. Results and Discussion 3.1 Spectroscopic Analyses. The UV-Vis absorption spectrum of the photocurable solution extends from the UV to 450 nm as illustrated in Figure 3 for a thin liquid film (70 µm) containing 17 wt % S-PEEK (Sample S4). A broad absorption peak can be seen, as a shoulder between 385 and 415 nm, due to the absorption of the photoinitiator. The transmittance at λmax of the initiator (400 nm) is 19% and hence may be irradiated uniformly throughout the thickness of the liquid film. The incident light source, while having maximum power at 450 nm, emits a broad spectrum of light between 410 and 520 nm, sufficient to photodegrade the (14) Beattie, P. D.; Orfino, F. P.; Basura, V. I.; Zychowska, K.; Ding, J.; Chuy-Sam, C.; Schmeisser, J. M.; Holdcroft, S. J. Electroanal. Chem. 2001, 503, 45-56.
Figure 4. IR spectrum of Sample S4, 17 wt % S-PEEK: (a) composite, and (b) fingerprint region.
initiator. Also shown in Figure 3 is the evolution of the UVVis absorption spectrum as a function of photocure time. The loss of the 385-415 nm shoulder corresponds to photolysis of the photoinitiator. The resulting photocured polyelectrolytes are assumed to be semi-IPNs with S-PEEK playing the role of the linear guest polymer residing in a cross-linked random copolymer of VPA, ACN, and DVS host. To confirm that the polymerization proceeds as expected, changes in the FTIR spectrum of the photocurable solutions were monitored in situ. A small amount of the sample,
