Energy & Fuels 2008, 22, 2539–2542
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Nanocomposite Membranes Made from Sulfonated Poly(ether ether ketone) and Montmorillonite Clay for Fuel Cell Applications Mohammad M. Hasani-Sadrabadi,†,‡ Shahriar H. Emami,*,‡ Reza Ghaffarian,† and Homayoun Moaddel§ Polymer Engineering, and Biomedical Engineering Departments, Amirkabir UniVersity of Technology, Tehran 15875-4413, Iran, and Hydrogen and Fuel Cell, Inc., Claremont, California 91711-1241 ReceiVed NoVember 5, 2007. ReVised Manuscript ReceiVed May 13, 2008
Poly(ether ether ketone) (PEEK) was sulfonated at various degrees with sulfuric acid and dissolved in N,Ndimethylacetamide (DMAc). Montmorillonite (MMT) clay was mixed with this solution and solvent-casted on a glass plate. A Fourier transfer infrared (FTIR) experiment of sulfonated samples showed O-H vibration at 3490 cm-1 and SdO peaks at 1085 and 1100-1300 cm-1. Thermogravimetry analyzer (TGA) experiments revealed thermal degradation above 240 °C. X-ray diffraction (XRD) confirmed almost zero crystallinity for sulfonated PEEK/MMT. When the degree of sulfonation was increased to 80%, ion-exchange capacity, water uptake, and proton conductivity were increased to almost 2.4 meq/g, 75%, and 0.06 S/cm, respectively. Methanol permeability was decreased to 5 × 10-8 cm2/s by the addition of 10 wt % MMT. A sulfonated PEEK/MMT membrane with 62% of sulfonation and 1.0 wt % MMT loading showed membrane selectivity of approximately 8500 compare to 4500 of Nafion 117.
1. Introduction Fuel cells have emerged in the past decade as one of the most promising new technologies to answer global electric power needs. Unlike conventional power generation technologies, fuel cells work without combustion and their environmental side effects. Fuel cells act like continuously fueled batteries, producing direct current (DC) by using an electrochemical process.1 Among the various types of fuel cells, those using a polymer electrolyte membrane (PEM), such as hydrogen fuel cells, direct methanol fuel cells, and biological fuel cells, have received increasing attention in recent times. These fuel cells operate at temperatures close to ambient and are capable of generating high power densities. There are various inherent advantages and disadvantages within all configurations, but all are, to a greater or lesser extent, limited by the performance of the protonconducting polymeric membrane. PEM provides protonic communications between the anode and cathode and acts as a fuel separator.2 Nafion is a well-known PEM polymer, which combines the mechanical strength and chemical/thermal stability with excellent proton conductivity. The high fuel permeability rate across the membrane and its high cost poses a critical limitation in using proton-exchange membrane fuel cells for large-scale applications.3–7 * To whom correspondence should be addressed: Biomedical Engineering Department, Amirkabir University of Technology, Tehran 15875-4413, Iran. Telephone: +98-21-64542367. Fax: +98-21-6495655. E-mail: shahriar16@ yahoo.com. † Polymer Engineering Department, Amirkabir University of Technology. ‡ Biomedical Engineering Department, Amirkabir University of Technology. § Hydrogen and Fuel Cell, Inc. (1) Vielstich, W.; Lamm, A.; Gasteiger, H. A. Handbook of Fuel Cells, Fundamental, Technology and Applications; John Wiley and Sons: Chichester, U.K., 2003; Vol. 1. (2) Gottesfeld, S.; Zawodzinski, T. AdV. Electrochem. Sci. Eng. 1997, 5, 195–301. (3) Wilkinson, S. Auton. Robots 2000, 9 (2), 99–111.
In recent years, there has been an intensive research effort for the development of alternative membranes.8,9 The ideal membranes should not only conduct protons but also act as a fuel barrier. Recently, researchers have been focused on dispersing inorganic fillers in the membrane to overcome fuel crossover.10–13 Clay is a term used to describe a group of hydrous aluminum phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals. Organically treated montmorillonite (MMT) clays are made by inserting low-molecular-weight oligomers or surfactants inside their structural sheets. MMT is a type of layered silicate composed of silica tetrahedral and alumina octahedral sheets. The general formula of MMT is Mx(Al4-xMgx)Si8O20(OH)4, where M and x are the monovalent cation and degree of isomorphous substitution (between 0.5 and 1.3), respectively. The exfoliation of nanoclay in the polymer matrixes can provide high-barrier properties and improves thermal and mechanical properties. Among the nonfluorinated hydrocarbons, ionomer polymers, such as poly(ether ether ketone) (PEEK), because of their lower cost of preparation and good film properties, including chemical, thermal, and mechanical stabilities, were shown to be very (4) Willner, I.; Katz, E.; Patolsky, F.; Buckmann, A. F. J. Chem. Soc., Perkin Trans. 1998, 2, 1817–1822. (5) Allen, R. M.; Bennetto, H. P. Appl. Biochem. Biotechnol. 1993, 39, 27–40. (6) Ren, X.; Zelenay, P.; Thomas, S.; Davey, J.; Gottesfeld, S. J. Power Sources 2000, 86, 111–116. (7) Heinzel, A.; Barragan, V. M. J. Power Sources 1999, 84, 70–74. (8) Savadogo, O. J. New Mat. Electrochem. Systems 1998, 1, 47–55. (9) Savadogo, O. J. Power Sources 2004, 127, 135–161. (10) Reichert, P.; Kressler, J.; Thomann, R.; Mu¨lhaupt, R.; Sto¨ppelmann, G. Acta Polym. 1998, 4911 (2-3), 116–123. (11) Giannellis, E. P. Appl. Organomet. Chem. 1998, 12 (10-11), 675– 680. (12) Lin, Y. F.; Yen, C. Y.; Ma, C. C. M.; Liao, S. H.; Hung, Y. H.; Hsiao, Y. H. J. Power Sources 2007, 14 (165), 692–700. (13) Stangar, U. L.; Groselj, N.; Orel, B.; Schmitz, A.; Colomban, P. Solid State Ionics 2001, 145 (1-4), 109–118.
10.1021/ef700660a CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
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promising for fuel cell applications. Sulfonation is a process of introducing sulfur groups into the polymer system to increase negative charges for better proton conductivity.14–16 Therefore, the goal of this research is to produce polymer membranes with low fuel permeability, low cost of production, and better processability. 2. Experimental Section 2.1. Materials. PEEK for producing membrane sheets was obtained from Poly Science, Inc. (Niles, IL) in the form of extrudates. Organically treated MMT clays (Closite 15A) as a quality enhancer were purchased from Southern Clay Products, Inc. (Gonzales, TX). Sulfuric acid (95-97%, for sulfonation), N,Ndimethylacetamide (DMAc) as a solvent, sodium hydroxide solution (for titration), hydrochloric acid, methanol as fuel, and hydrogen peroxide were purchased from Merck (Darmstadt, Germany). Deionized water as a diluent (purified with Millipore) was used in this work. Nafion 117 (178 µm) membranes from Dupont (Wilmington, DE) were used for comparing the data. 2.2. Nafion Modification. Nafion membranes were boiled in 3 wt % hydrogen peroxide for 30 min, washed (several times), and boiled (1 h) in deionized water. Membranes were boiled again in 1 M sulfuric acid for 30 min and washed 3 times with deionized water. 2.3. Sulfonation Process. Sulfonation of PEEK with various degrees of sulfonation followed the procedure reported in the literature.17,18 PEEK was dried at 100 °C in a vacuum oven overnight. Dried polymer (20 g) dissolved in concentrated sulfuric acid and was stirred vigorously at room temperature for 20-120 h. The sulfonated polymer solution was added gradually to the large excess of ice-cold water under continuous mechanical agitation for 1 h until the polymer suspension was settled down after 12 h. The precipitated particles were filtered and washed several times with deionized water until the pH became neutral. Samples were dried in a vacuum oven at 100 °C for 10 h. 2.4. Membrane Preparation. Dried samples with various degrees of sulfonation were dissolved in DMAc and stirred for 24 h. MMTs were suspended in DMAc solutions at room temperature, stirred for 2 h, ultrasonicated for another 1 h, and mixed with the polymer solutions. The resultant mixtures were ultrasonicated for 30 min, stirred for 8 h at 80 °C, and concentrated in a rotary evaporator. The viscous solutions were casted on a clean glass plate and dried in several steps, at room temperature for one night, 70 °C for 8-10 h, and 120 °C overnight. The membranes were modified by the modification procedure mentioned in the Nafion Modification section. 2.5. Characterization Methods. 2.5.1. Fourier Transfer Infrared (FTIR) Spectroscopy. PEEK sulfonation was detected by FTIR spectroscopy (Nicolet-Magna 560). Scans for samples were recorded at a resolution of 2 cm-1 over the wavenumber region of 500-4000 cm-1. Samples were mixed with KBr grain spectroscopy grade and pressed into a disk by compaction. 2.5.2. ThermograVimetry Analyzer (TGA). The degradation process and the thermal stability of the membranes were investigated by TGA (Perkin-Elmer Pyris1). The approximately 10-20 mg of fully dried samples were kept for 45 min at 200 °C to remove any remaining solvent, and then samples were put under a nitrogen atmosphere using a heating rate of 10 °C/min and heated from 50 to 900 °C. (14) Shan, J.; Vaivars, G.; Luo, H.; Mohamed, R.; Linkov, V. Pure Appl. Chem. 2006, 78 (9), 1781–1791. (15) Mikhailenko, S. D.; Zaidi, S. M. J.; Kaliaguine, S. Catal. Today 2001, 67 (1-3), 225–236. (16) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2000, 173, 17–34. (17) Xing, P.; Robertson, G. P.; Guive, M. D.; Mikhailenko, S. D.; Wang, K.; Kaliaguine, S. J. Membr. Sci. 2004, 229, 95–106. (18) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2000, 173, 17–34.
Figure 1. FTIR spectra of PEEK and sulfonated PEEK.
2.5.3. X-ray Diffraction (XRD). Dispersion of clay particles in membranes was detected by XRD (Siemens XRD-D5000 diffractometer, Cu KR radiation). The 2θ range was converted from 2° to 10°. The typical beam area was smaller than 1 mm2. 2.5.4. Degree of Sulfonation and Ion-Exchange Capacity. Dried samples were soaked in 50 mL of 0.01 N sodium hydroxide solution for 12 h at room temperature. A total of 10 mL of solution was titrated with 0.01 N sulfuric acid. The samples were regenerated with 1 M hydrochloric acid, washed with water, and dried to a constant weight. 2.5.5. Water Uptake of Membranes. Dried membranes were soaked in deionized water at room temperature, quickly weighed in different time intervals by carefully removing the excess water with filter papers, and immersed back in the water tank. This process was repeated for several times until there was no further weight gain. The water uptake was calculated from the following formula: water uptake (%) ) 100 × (Mwet - Mdry)/Mdry where Mwet and Mdry are the weight of wetted and dried membranes in grams. 2.5.6. Proton ConductiVity. Proton conductivity of fully hydrated membranes were measured at room temperature by the AC impedance method using a Solartron Interface 1260 gain phase analyzer, over the frequency range of 1-10 MHz. The conductivity was calculated from the following equation: σ ) L/RA where L is the membrane thickness, A is the surface area of the electrodes, and R is the resistance. 2.5.7. Fuel Permeability. The methanol diffusion coefficient was measured by using homemade two compartmental glass diffusion cells. Methanol (100 mL) was placed on one side of the diffusion cell (cell A), and 100 mL of water was placed on the other side (cell B). The solution in each compartment was continuously stirred to make certain uniformity. The concentration of the methanol in cell B was measured using gas chromatography. The methanol diffusion coefficient was determined as follows: CB(t) )
A DK C(t - t0) VB L
where CB(t) is the concentration of methanol in cell B (in mol/L), DK is the methanol diffusion coefficient (in cm2/s), C is the concentration of methanol in cell A (in mol/L), VB shows the volume of the diffusion reservoir (in cm3), A is the membrane area (in cm2), and L is thickness of the membrane (in cm).
3. Results and Discussion FTIR spectra of the nonsulfonated and sulfonated PEEK samples with 62% degree of substitution is shown in Figure 1. The reaction between benzene rings in PEEK and sulfuric acid molecules occurs through electrophilic aromatic substitution. The sulfonation process produces benzenesulfonic acid and water. The broad bands in sulfonated PEEK samples are
Nanocomposite Membranes for Fuel Cell Applications
Figure 2. Ion-exchange capacity and the degree of sulfonation as a function of the reaction time.
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Figure 4. Effect of the degree of sulfonation on the proton conductivity of sulfonated PEEK membranes.
Figure 5. XRD pattern of MMT, sulfonated PEEK, and sulfonated PEEK/MMT nanocomposite.
Figure 3. Water uptake of the membranes as a function of the sulfonation degree.
observed at 3490 cm-1, which were assigned to the O-H vibration from water molecules. The absorption band at 1085 and 1100-1300 cm-1 (broad peak) in sulfonated PEEK was assigned to sulfur-oxygen (SdO) bonds.19 Figure 2 shows that, by increasing the reaction time with sulfuric acid, ion-exchange capacity (IEC) and degree of sulfonation are increasing to almost 2.4 meq/g and 90%, respectively. Introducing the sulfur groups in the PEEK structure reduce its crystallinty and increase its solubility. The degree of sulfonation of sulfur groups in PEEK alters its solubility from dissolution in hot DMAc (up to 40% sulfonation) to room temperature at up to 70% sulfonation. Above 70% sulfonation, PEEK becomes soluble in methanol and degrades in hot water. As seen in Figure 3, by increasing degree of sulfonation to almost 80%, water uptake is increasing to more than 75%. Water uptake or the ability of the membrane to imbibe a large amount of water enhances the proton conductivity of the membranes.20 Figure 4 shows the effect of the degree of sulfonation on the proton conductivity of sulfonated PEEK membranes. The conductivity was increased gradually as more sulfonic acid groups were introduced to the sample, and the degree of sulfonation reached to 3.2 × 10-2 S/cm for the degree of sulfonation of 82%. Sulfonation of PEEK samples opens up the hard to reach area of crystalline parts and thus accommodates more H2O molecules. When the concentration of sulfuric acid is increased, sulfonation occurs more vigorously and more water molecules form in the membrane. Hydrogen mobility in the membranes is increasing with an augmenting number of water (19) Solomons, T. W. G. Fundamental of Organic Chemistry; John Wiley and Sons: New York, 1990; Chapter 12. (20) Barragan, V. M.; Ruiz Bauza, C.; Villaluenga, J. P. G.; Seoane, B. J. Power Sources 2004, 130, 22–29.
molecules. The high ionic conductivity at elevated sulfonation levels suggests that the water swollen in ionic domains of the pores of the membrane is interconnected to form a network structure. The lower proton conductivity observed in the membranes with a lower degree of sulfonatin may relate to the diffusion limitation caused by segregation in the ionic domains. When the DS increased to a sufficient level, the ionic domains became more interconnected and simultaneously overcame the diffusion limitations and allowed the ionic conductivity to reach a maximum value. According to these results, sulfonated PEEK at 62% of sulfonation was selected as an optimum degree of sulfonation because of good membrane processability and dimensional stability in aqueous environments. Membranes with greater degrees of sulfonation do not have good stability in aqueous environments for long periods of time. Figure 5 illustrates XRD of MMT, sulfonated PEEK, and sulfonated PEEK with MMT. As seen, MMT shows one crystalline peak at around 3°. However, when MMT mixes with sulfonated PEEK, the crystalline peak almost disappears. In other words, MMT nanosized separated particles obtained from crystalline parts of the original material diffuse inside the sulfonated PEEK polymer chains and produce particulate nanocomposite membranes. Figure 6 shows the effect of loading weights of the nanoparticle on the proton conductivity of membranes. With an increasing content of MMT, the proton conductivity of the membranes decreases. The decrease of the proton conductivity was mainly influenced by the presence of silicate plates of MMT into the sulfonated PEEK. This emphasizes the effect of small particles, such as nanoclays, in separating polymer chains from each other and consequently decreasing proton conductivity. In other words, SO3- groups in the polymer chains were decreased per unit volume. Figure 7 shows a decrease in methanol permeability as the MMT content is increased. Because methanol and water molecules have similar properties (e.g., dipole moment),
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Figure 6. Effect of loading weights of nanoparticles on the proton conductivity of sulfonated PEEK membranes.
Figure 7. Methanol permeability of the nanocomposite membranes as a function of the MMT content.
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Figure 9. Membrane selectivity of nanocomposite membranes as a function of MMT loading.
°C.21 The second degradation step is reflected by the decomposition of the main chain. This may be caused by the thermal degradation of sulfonic acid groups at relative low temperatures in comparison to the backbone of the polymer. The thermal decomposition temperature (Td) of the membrane is a function of the sulfonation degree. However, sulfonated PEEK/MMT membranes have sufficient thermal properties for fuel cell applications because of the thermal decomposition above 240 °C. Methanol permeability and proton conductivity are the two transport properties that determine the fuel cell performance in DMFC. Membrane selectivity or ratio of proton conductivity to methanol permeability of the sulfonated PEEK and their nanocomposite membranes at various MMT loading weights are shown in Figure 9. The ideal PEM for a DMFC is expected to have a high proton conductivity and low methanol permeability. The higher selectivity value leads to better membrane performance. We can conclude from selectivity and proton conductivity values that the nanocomposite sulfonated PEEK membrane with 62% of sulfonation and 1.0 wt % MMT loading is approximately 2 times greater than that of Nafion 117. The nanocomposite membrane with this composition is an excellent candidate for fuel cell applications. 4. Conclusion
Figure 8. TGA of PEEK, sulfonated PEEK, and sulfonated PEEK/ MMT nanocomposite.
methanol and water molecules transfer to the cathode by the electro-osmotic drag. At the cathode, methanol causes mixed potential because of the interference of methanol oxidation with the oxygen reduction reaction. To overcome the methanol crossover problem, inorganic fillers, such as MMT, are dispersed between polymer chains. MMT impermeable sheets lead to a tortuous diffusion pathway for methanol across the membrane; thus, methanol crossover is decreased. PEMs may need to exhibit fast proton transport at high temperatures and are required for fuel cell applications. The thermal stability and thermal decomposition temperature (Td) of the PEMs were studied by heating the samples in TGA. The resultant data for the PEEK, sulfonated PEEK, and sulfonated PEEK/MMT composite membranes are displayed in Figure 8. It can be seen that sulfonated membranes have a two-step degradation pattern. As seen, sulfonated PEEK and sulfonated PEEK/MMT expressed similar degradation curves, and they both lost more than 20% of their original weight above 270
In this study, proton-exchange membranes based on sulfonated PEEK with different degrees of sulfonation were prepared; therefore, the optimum degree of sulfonation was determined according to processability, physical and chemical stability, and transport properties of membranes. Afterward, nanocomposites of sulfonated PEEK (at optimum DS) with various loading weights of organically treated MMT were prepared by the solution intercalation method. The nanocomposite membranes exhibit a high conductivity of 1.73 × 10-2 S/cm and low methanol permeability of 2.05 × 10-7 cm2/s. These composite membranes are easy to prepare and much less expensive than the commercial perfluorinated membranes, such as Nafion. Sulfonated PEEK/MMT nanocomposite membranes for their high proton conductivity and low methanol permeability are highly suitable for fuel cell applications. Acknowledgment. The authors express their appreciation for the financial support of this work from the Amirkabir University of Technology and National Foundation for Elites. EF700660A (21) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463–1502.
