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Ionic crosslinking of ionomer polymer electrolyte membranes using barium cations
Journal of Membrane Science 304 (2007) 173–180
Ionic crosslinking of ionomer polymer electrolyte membranes using barium cations Jeffrey V. Gasa a , R.A. Weiss a,b,∗ , Montgomery T. Shaw a,b a
b
Polymer Science Program, University of Connecticut, Storrs, CT 06269-3136, United States Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States Received 27 May 2007; received in revised form 19 July 2007; accepted 20 July 2007 Available online 26 July 2007
Abstract Acidic polymer electrolyte membranes (PEM) based on sulfonated poly(ether ketone ketone) (SPEKK) with relatively moderate to high levels of sulfonation (ion-exchange capacity (IEC) > 1.7 meq/g) have excellent proton conductivities (∼0.1 S/cm), but they absorb excessive amounts of water at elevated temperatures (ca. 60–90 ◦ C). To reduce the water sorption of the membranes and improve their mechanical properties and dimensional stability, the protons in acidic SPEKK membranes were partially exchanged with divalent barium cations to create ionic crosslinks between the sulfonate groups attached to the aromatic rings of the PEKK. The degree of crosslinking was varied by changing the degree of neutralization of the ionomer. The proton conductivity, water uptake, and methanol permeability at various levels of sulfonation and degree of crosslinking were measured. The conductivity was reduced by crosslinking, but the thermal stability, swelling, and barrier properties were improved. Crosslinking reduced the water swelling at room temperature by nearly a factor of two and prevented membrane dissolution at temperatures up to 80 ◦ C. In a water/methanol mixture (72/28, v/v), swelling was reduced by a factor of four. The balance of transport and mechanical properties could be varied to produce a viable PEM for a direct methanol fuel cell by adjusting the crosslink density. © 2007 Elsevier B.V. All rights reserved. Keywords: Ionomers; Proton exchange membranes; Selectivity; Conductivity; Neutralization
1. Introduction Perfluorosulfonic acid membranes, such as Nafion® , are presently considered the standard material for PEM fuel cell (PEMFC) technology because of their high proton conductivities and excellent electrochemical and mechanical stability in the fuel cell environment. However, these membranes possess several properties that are unfavorable when methanol is used as fuel, which most researchers consider to be the main hindrance to the technological and commercial success of direct methanol fuel cells (DMFC). A disadvantage of Nafion® membranes for the DMFC application is the crossover of methanol through the membrane, which adversely affects the performance (cell voltage) and fuel utilization efficiency of the cell. Although the proton conductivity and oxidative resistance of Nafion®
∗ Corresponding author at: Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States. Tel.: +1 860 486 4698; fax: +1 860 486 6048/4. E-mail address: [email protected] (R.A. Weiss).
0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.07.031
membranes are both excellent, the durability and methanol crossover of Nafion® become problematic especially when thinner membranes and high methanol feed concentrations are used to increase power density. As a result, considerable research has been focused recently on developing PEMs with a better balance of transport, mechanical, and aging properties. A key challenge is to improve the resistance of the membrane to methanol permeation without compromising other essential properties, specifically proton conductivity. In our previous work [1], we found that SPEKK with relatively moderate to high levels of sulfonation (ca. IEC > 1.7 meq/g) had proton conductivities comparable to that of Nafion® (ca. 0.1 S/cm at 98% relative humidity and 20 ◦ C). However, these membranes exhibited poor dimensional, mechanical, and barrier properties in an aqueous environment at elevated temperatures (ca. 60–90 ◦ C), which are typical operating conditions in a DMFC. The poor properties were attributed to the excessive water sorption of the membranes at high water activities (ca. > 50 wt% of water at 98% relative humidity). In the work reported in this paper, the objective is to reduce the water and methanol sorption by crosslinking the membrane.
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The idea of crosslinking of PEM is not new, but most of the crosslinking studies done previously pertain to chemical (covalent) crosslinking [2–5]. The crosslinking procedure discussed in this paper is a relatively less studied technique, which is ionic crosslinking. The approach is to exchange with divalent cations some of the protons in acidic membranes. The salt produced by the exchange reaction should have very low water solubility and form a stable crosslink. To the best of the authors knowledge, the application of the abovementioned crosslinking procedure to PEMs is a novel idea. In general, crosslinking alters several important properties of polymers. The more common effects of crosslinking are reduction in the sorption in swelling solvents, increase in transition temperatures, and increase in plateau modulus [2–10]. In some cases, crosslinking can also enhance the tensile strength and toughness of the polymer [11,12]. It can also alter the permeability and permselectivity of the polymer [13,14]. In PEM applications, previous studies have indicated that chemically crosslinked membranes can be superior to Nafion® membranes in terms of properties pertinent to DMFC applications such as methanol permeability, swelling properties, and proton conductivity [2–5]. In the different crosslinked systems that they studied, the common conclusion was that the proton conductivities of their membranes were comparable to Nafion® but the methanol permeabilities were about one order of magnitude lower [2–5]. They also reported that the swelling of their membranes were lower than Nafion® membranes. In this study, it was postulated that the ionic crosslinking of acidic SPEKK would give similar desirable effects such as low swelling and low permeability of SPEKK membranes in polar solvents such as methanol and water. The approach was to use very acidic SPEKK (IEC > 1.5 meq/g) and crosslink some of the sulfonate groups with divalent cations. Barium ions are known to form salts that have extremely low solubility in water, and thus were chosen to be the crosslinking species. Very acidic membranes were used to compensate for the loss of protons due to the ion-exchange process, and therefore maintain high conductivity. The specific objective of this work was to determine the effect of barium crosslinking on the swelling, thermal stability, and transport properties (proton conductivity and methanol permeability) of PEMs. 2. Experimental 2.1. Ionically crosslinked SPEKK membranes SPEKK membranes with IECs of 1.7 and 2.06 meq/g were prepared by sulfonating a 5% (w/v) solution of PEKK with a
53/47 (v/v) mixture of concentrated sulfuric acid and fuming sulfuric acid at room temperature. To crosslink the membranes, the protons in acidic SPEKK membranes were partially exchanged with barium cations from dilute barium acetate solutions. These cations, which are divalent, can link together two sulfonate groups to attain electro-neutrality (Fig. 1). When many sulfonate groups coming from different SPEKK chains are linked together by these divalent cations, a network of SPEKK chains can be formed. The fraction of protons exchanged with barium, denoted here as the exchange fraction, is directly related to the degree of crosslinking. Although the magnitude of the mechanical and transport properties changed with IEC, the effects of Ba-crosslinking of the two different membranes was the same. There are two possible crosslinking mechanisms that can take place in these materials. Ionic crosslinks as shown be Fig. 1 occur as a necessity of achieving electrical neutrality in the material; the effect of these crosslinks is likely to be relatively independent of temperature. Also possible, however, are physical crosslinks due to ion–dipole associations of the Ba-sulfonate groups, which can produce ionic aggregation, often termed ion-clusters in the ionomer literature. The properties of the physical crosslinks are expected to be temperature-dependent. Ion-clusters are distinguished from the ionic crosslinks in that they provide multifunctional crosslinks produced by nano-phase separation of ion-rich domains of the order of 1–5 nm in size in typical highly neutralized ionomers. In the discussion of the effects of Ba-crosslinking in this paper, the crosslinks are simply referred to as ionic, because formation of ionic crosslinks must occur due to the need for electronic neutrality, as shown in Fig. 1. Whether or not physical crosslinks, i.e., ion-clusters, also occurred requires structural analysis of the polymer using techniques such as small angle X-ray scattering, which was not done in this study. To fine-tune the degree of crosslinking, the acidic SPEKK membranes were immersed in dilute solutions (0.1 wt%) of barium acetate at various immersion times at room temperature. After soaking in Ba(OAc)2 solution, the barium-crosslinked membranes were thoroughly washed with copious amounts of water to extract residual barium acetate solution absorbed by the membrane and to remove acetic acid, which is a byproduct of the exchange reaction (Fig. 1). The membranes were preswollen in water prior to immersion in barium acetate solution to remove residual solvent (NMP) from the casting procedure and also to promote the migration of barium ions into the membrane and minimize localization of the crosslinking reaction on the membrane surface. The degree of barium exchange was calculated as the number fraction of sulfonic acid groups substituted with barium.
Fig. 1. Schematic of the barium-crosslinking reaction of acidic SPEKK membranes.
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The amount of sulfonic acid groups was measured by acid–base titration following the previously established procedure for measuring the IEC of SPEKK [1]. The working equation for fractional substitution xE was: xE =
IEC0 − IECE IEC0
(1)
where IEC0 is the IEC of the original sample and IECE is the IEC of the sample after substitution, but based on its original dry weight. 2.2. Characterization of the barium-crosslinked SPEKK membranes 2.2.1. Proton conductivity Impedance spectroscopy has been widely used to measure the proton conductivity of PEMs [15–25]. The conductivity of the membranes was measured using a Hewlett-Packard Agilent® 4284A LCR meter covering a frequency range of 20–106 Hz. The applied voltage was 50 mV. Membrane conductivities were measured using a custom-made cell based on the design of Zawodzinski et al. [15]. Cells of this design measure conductivity along the plane of the membrane. 2.2.2. Water sorption The water sorption of the membranes was measured by placing films in a controlled humidity environment and weighing them every 24 h until the mass of the membranes reached equilibrium, as judged by the point where the changes in mass were merely random weighing errors. The membranes were then dried in vacuum at 100 ◦ C for 24 h and re-weighed. Water sorption is defined here as the mass of the absorbed water divided by the mass of the wet membrane, i.e., the mass fraction of water. 2.2.3. Thermal stability The thermal stability of the polymers was measured with a TA Instruments Hi-Res TGA 2950 thermogravimetric analyzer using a nitrogen atmosphere and a heating rate of 20 ◦ C/min. The samples were first dried for 10 min at 120 ◦ C in the TGA furnace prior to the scan to remove any residual water absorbed by the relatively hydrophilic sulfonated polymers during sample transfer. 2.2.4. Methanol permeability The methanol permeabilities of the Ba-crosslinked SPEKK membranes were measured using the cell shown in Fig. 2. The design of this cell is a variation of that used by Walker et al. to measure methanol permeation through membranes based on sulfonated aromatic hydrocarbons and also perfluorosulfonic acid membranes (Nafion 117® ) [26]. The lower part of the cell, which was a 20-mL glass vial, was filled with 10 mL methanol. Methanol vapor in equilibrium with the liquid diffused along the concentration gradient through the membrane, which was clamped between the mouth of the vial (2 cm in diameter) and the cap. The cap had a 1-cm hole so that the methanol that diffused through the membrane could
Fig. 2. Schematic of the permeation cell.
escape. The cell was placed inside a drying oven, which provided temperature control and an air draft that maintained the methanol concentration and humidity in the environment above the surface of the membrane at a minimum. The mass of the methanol inside the cell was measured as a function of time. For the results reported here, the methanol permeability (P) was calculated by applying Fick’s first law: J = −D
dCm Cb Cb = −DK = −P dx L L
(2)
where J is the molar flux of methanol, D the methanol diffusivity, K the partition coefficient or the solubility of methanol in the membrane, Cm the methanol concentration in the membrane, Cb the methanol concentration in the gas phase, and L is the thickness of the membrane. In this device, it is difficult to quantify accurately the value of Cb . Although Cb at the inner surface of the membrane could be approximated using the vapor pressure of MeOH, it is difficult to estimate Cb at the outer surface of the membrane. It is often assumed that Cb = 0, but a finite methanol concentration near the cap can introduce appreciable error in the calculation of J. To circumvent this problem, a reference sample with known methanol permeability (i.e., Nafion® ) was tested and the methanol permeabilities of the samples were reported relative to the methanol permeability of the reference sample. In this way, the driving force term (Cb ) in Eq. (2) cancels out because all the samples were measured in one batch, and therefore were exposed to the same conditions (same driving force). The resulting relationship for the relative permeability is then Ju Lu Pu = Jr L r Pr
(3)
where u and r refer to the unknown and reference samples, respectively. The reference sample was a Nafion 115® membrane (Dupont de Nemours, Wilmington, DE) with a thickness of 127 m as reported by DuPont.
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Fig. 3. Time evolution of the barium-exchange reaction in SPEKK with an IEC of 1.7 meq/g using 0.1 wt% barium acetate solution.
3. Results and discussion 3.1. Ion-exchange reaction Fig. 3 shows the time evolution of the exchange reaction. It can be seen that a broad range of exchange fractions can be obtained and can almost reach completion (100% exchange). The analysis of the kinetics of exchange reaction is quite complicated because there are two competing rates that have to be deconvoluted, the rate of barium exchange and the diffusion rate of barium ions. The data obtained in this study was not enough to get an accurate model of the kinetics of such ion-exchange process.
Fig. 4. Influence of the degree of barium crosslinking or the exchange fraction on the water sorption of the SPEKK membrane with an IEC of 1.7 meq/g.
3.3. Water sorption vs. temperature
3.2. Water sorption
Fig. 5 shows the water sorption of the SPEKK (IEC = 1.7 meq/g) and barium-exchanged membranes as a function of temperature and exchange fraction. The water sorption of the neat SPEKK membrane with an IEC of 1.7 meq/g exhibited a strong temperature dependence. At temperatures below 45 ◦ C, the temperature dependence of the water sorption was weak, but above 45 ◦ C, there was a sharp upturn in the water sorption. That was probably due to the glass-to-rubber transition of the water-swollen SPEKK. With increasing exchange frac-
The purpose of modifying SPEKK membranes through crosslinking is to reduce the water sorption of these membranes in aqueous environments especially for those membranes with relatively high sulfonation levels. Fig. 4 shows the equilibrium water sorption of barium-exchanged SPEKK membranes (IEC of the original SPEKK was 1.7 meq/g) as a function of the exchange fraction or the degree of crosslinking. These water sorption data were taken at 23.8 ◦ C and 98% relative humidity. The equilibrium water sorption monotonically decreased with increasing exchange fraction. As with other crosslinkable systems [6–10], this result is an expected one because the barium ionic crosslinks reduce the chain mobility, thereby reducing the ability of these membranes to expand and absorb water. If the sorption data is extrapolated to 100% exchange fraction, the water sorption is not zero; it is about 17 wt%. However, this does not imply that the barium crosslinks are dissociated. The membranes were pre-swollen in water prior to immersion in barium acetate solution for crosslinking; thus the energetically most favorable state in equilibrium with water is a somewhat swollen condition even at 100% exchange fraction. This is analogous to the formation of highly swellable hydrogel networks from monomer solutions as opposed to neat monomer [26].
Fig. 5. Influence of temperature and barium-exchange fraction on the water sorption at 25 ◦ C of barium-exchanged SPEKK membranes. The numbers in parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the untreated SPEKK was 1.7 meq/g.
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tion, the position of the upturn shifted to higher temperatures and the magnitude of the sorption values decreased. Above 64% barium exchange, the upturn disappeared and the water sorption became almost temperature-independent. This shows that the barium crosslinking not only reduced the magnitude of the water sorption but also improved the thermal stability of the swelling of these membranes in an aqueous environment. The excessive swelling exhibited by the neat SPEKK membrane at temperatures above 45 ◦ C is undesirable because it compromises the mechanical integrity, dimensional stability, and the barrier properties of the membrane. 3.4. Swelling in water–methanol mixtures In direct methanol fuel cells, the usual practice is to use dilute solutions of methanol in water as fuel to minimize methanol crossover and swelling of the membrane. The limitation on the concentration of methanol in contact with the anode unfavorably limits the performance of the cell. It is desirable that the cell be operated at higher methanol concentrations to increase power density and reduce the fuel volume, which are especially essential for portable applications. Higher methanol concentrations are also desirable for easier water management. Therefore, it is important to gain an understanding of the swelling properties of PEM in methanol–water mixtures. The swelling of the barium-exchanged SPEKK membranes in water–methanol mixtures is shown in Fig. 6. The activity of methanol in water–methanol mixtures varies in a relatively linear fashion with methanol mole fraction over the range shown in Fig. 6 (activity coefficient drops slightly from about 1.6 to around 1.3) [27]. The sorption of neat SPEKK membrane with an IEC of 1.7 meq/g exhibited a strong dependence on methanol concentration, meaning that this polymer prefers methanol over water. The slope of the SPEKK membrane data in Fig. 6 increases monotonically with increasing methanol concentration. As the degree of crosslinking increased, the swelling
Fig. 6. Sorption of water at 25 ◦ C by barium-exchanged SPEKK membranes as a function of methanol concentration and exchange fraction. The numbers in parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the untreated SPEKK was 1.7 meq/g.
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became less dependent on methanol concentration or methanol activity. Above 64% barium exchange, the fluid uptake became virtually independent of methanol activity, which suggests that the barium crosslinking of SPEKK membranes eliminated the favorable interaction with methanol, relative to water. Swelling is reduced to a convenient value of around 20% regardless of the fluid composition. 3.5. Ionic conductivity In the barium-exchange reaction, the mobile protons are replaced by the much less mobile barium ions. The mobility of barium ions is much lower because these ions are bulkier and have far less ionic character. Therefore, the ionic conductivity of the membranes is expected to decrease upon ion-exchange with barium, and that is confirmed in Fig. 7. The conductivity decreased dramatically with increasing degree of crosslinking. If the conductivity of the barium-exchanged SPEKK membranes is plotted against the concentration of the remaining protons (Fig. 8), expressed here in terms of the number of sulfonic acid groups per mole of SPEKK repeat unit, the conductivity of the barium-exchanged membranes overlaps with that of the untreated or the neat SPEKK membranes. It appears that the barium disulfonate groups have very little contribution to the ionic conductivity. This implies that the barium ions indeed have very low mobility and the conductivity is controlled solely by the protons. 3.6. Methanol permeability and permselectivity In direct methanol fuel cells, methanol crossover is an important problem, especially for portable applications where the current densities are relatively low. At low current densities, the rate of methanol oxidation is low and unreacted methanol concentrates at the anode, promoting diffusion of methanol through
Fig. 7. Ionic conductivity of barium-exchanged SPEKK membranes as a function of exchange fraction.
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Fig. 8. Ionic conductivity of barium-exchanged SPEKK membranes and untreated SPEKK membranes as a function of the number of remaining sulfonic acid groups per repeat unit.
the membrane. To reduce crossover, the intrinsic property of the membrane that has to be minimized is the methanol permeability. In this study, the ionic crosslinking of SPEKK using divalent cations such as barium was expected to reduce the methanol permeability by reducing the swelling of the membrane, which will reduce both the solubility and diffusivity contributions to the permeability (see Eq. (2)). The crosslinks themselves may also decrease methanol diffusivity because of attraction between the penetrant and the ionic clusters. Fig. 9 shows the methanol permeability of the bariumexchanged SPEKK membranes relative to that of Nafion® . The methanol permeability of the original SPEKK membrane (IEC = 2.06 meq/g) was only about 40% of that of Nafion® , and the permeability decreased with increasing degree of ionic crosslinking. For a barium exchange of only 20%, the methanol
Fig. 9. Methanol permeability of the barium-exchanged SPEKK (2.06 meq/g) membranes as a function of the exchange fraction.
Fig. 10. Influence of barium-exchange fraction on the proton conductivity of the barium-exchanged SPEKK membranes normalized with respect to the conductivity of Nafion® .
permeability of the SPEKK membrane was about one-fifth that of Nafion® . The proton conductivities of the barium-crosslinked SPEKKs (initial IEC = 2.06 meq/g) relative to the conductivity of Nafion® are shown in Fig. 10. The conductivity of the unneutralized SPEKK was about 40% higher than that of Nafion® . For a barium exchange of 10%, the conductivity of the crosslinked SPEKK was comparable to that of Nafion® ; however, as shown in Fig. 9, the methanol permeability was about 80% lower than that of Nafion® . The ratio of the proton conductivity and the methanol permeability, referred to as permselectivity or selectivity of the membranes is plotted for the SPEKK membranes (initial IEC = 2.06 meq/g) as a function of barium exchange in Fig. 11. There seems to be a maximum in the selectivity at around 14% exchange, but there are not enough data points to make a definitive conclusion about the statistical significance of this maximum. The selectivity at 14% exchange was about five times greater than that of Nafion® . Above 20% exchange, the selectivity dropped to about the same selectivity as for Nafion® .
Fig. 11. Influence of barium-exchange fraction on the permselectivity of the barium-exchanged SPEKK membranes normalized with respect to the permselectivity of Nafion® .
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Fig. 12. Derivative TGA curves of the barium-exchanged SPEKK (1.7 meq/g) membranes for various exchange fractions.
3.7. Thermal stability TGA curves of the barium-exchanged SPEKK membranes (initial IEC = 1.7 meq/g) as a function of the degree of barium exchange are shown in Fig. 12. The ordinate in this plot is the derivative of the sample mass with respect to temperature. A mass loss step in the integral TGA curve (mass vs. temperature) will be manifested as a peak in the derivative plot. The derivative plot simply makes it easier to resolve small mass loss steps and to pin-point the temperature associated with each step. In the TGA derivative curve for the neat SPEKK membrane, the large peak centered around 350 ◦ C is due to desulfonation, while the broad peak around 500 ◦ C is due to degradation of the PEKK backbone. For the barium-exchanged SPEKK membranes, the desulfonation peaks are systematically shifted to higher temperatures, and result in lower mass loss. There is in addition a new peak centered around 750 ◦ C, which is not present in the TGA curve of the neat SPEKK membrane; this may be due to desulfonation of the neutralized sulfonic acid groups. One possible cause of the increase in the first desulfonation temperature is a decrease in diffusion due to the higher polymer viscosity; another is that an increase in pH suppresses the desulfonation reaction. Gaining further information concerning the nature of the degradation reactions would require analysis of the products, which is beyond the scope of this work. 4. Conclusions Crosslinking SPEKK membranes using barium cations was achieved and the degree of crosslinking could be controlled by using dilute aqueous solutions of barium acetate. Although the crosslinking reduced the proton conductivity of SPEKK membranes, the methanol permeability and methanol sorption were reduced significantly. There seemed to be an optimum degree of crosslinking, about 14% barium exchange, where the ratio of the proton conductivity to the methanol permeability, the selectivity, was highest. But, more samples with various degrees of crosslinking need to be tested to confirm the statistical significance of this optimum crosslink density. Statistical analysis of
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the data obtained in this study showed that there is a decreasing relationship between selectivity and the degree of crosslinking. However, when a highly sulfonated PEKK was used (e.g., IEC = 2.06 meq/g), a barium exchange of 10%, still yielded a membrane with a proton conductivity comparable to that of Nafion® , but with a methanol permeability 80% lower than that of Nafion® . Crosslinking with barium ions also greatly improved the thermal stability of the SPEKK membranes under dry conditions, and crosslinking mitigated the problem of excessive water sorption of SPEKK membranes at elevated tempe ratures. It appears from the results presented in this paper that the preparation of viable and robust PEM membranes may be possible by using a highly sulfonated polymer, even one that is close to water-soluble, that is partially neutralized to form a partial ionomer salt. With a divalent cation (e.g., barium), intermolecular crosslinks that form from the ion-bridge due to the metal sulfonate, renders the polymer less swellable by water and also may improve the membranes mechanical properties. However, investigation is needed into the question of the durability of the crosslinks in a working fuel cell. While preliminary experiments indicated that Ba++ can be displaced by H+ in an acidic solutions, the situation in a fuel cell is somewhat different in that all the anions are immobile; thus, the only escape for Ba++ is to move to the cathode. Its presence there could impede the normal cathode reactions, but it is not likely to be washed out. In the materials area, additional work is needed to determine whether other cations also may work to improve membrane selectivity and mechanical properties. In this case, dipole–dipole aggregation of the metal sulfonate groups, similar to what occurs in metal salt ionomers of much lower ion-exchange capacity, may provide the crosslinks. Similarly, other divalent cations or higher valency ions such as Al(III) or Ti(IV) may be of interest. The higher valency cations may provide stronger crosslinking and better water resistance and mechanical properties using less exchange of the protons. Acknowledgment This work was partially supported by a grant from the Dept. of Energy, 10761-001-05. References [1] S. Swier, Y.-S. Chun, J. Gasa, M.T. Shaw, R.A. Weiss, Sulfonated poly(ether ketone ketone) ionomers as proton exchange membranes, Polym. Eng. Sci. 45 (2005) 1081–1091. [2] J. Shin, B. Chang, J. Kim, S. Lee, D. Suh, Sulfonated polystyrene/PTFE composite membranes, J. Membr. Sci. 251 (2005) 247–254. [3] J. Kerres, W. Cui, M. Junginger, Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU crosslinked PSU blend membranes by alkylation of sulfinate groups with dihalogenoalkanes, J. Membr. Sci. 139 (1998) 227– 241. [4] J. Chen, M. Asano, T. Yamaki, M. Yoshida, Preparation of sulfonated crosslinked PTFE-graft-poly(alkyl vinyl ether) membranes for polymer electrolyte membrane fuel cells by radiation processing, J. Membr. Sci. 256 (2005) 38–45.
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Ionic crosslinking of ionomer polymer electrolyte membranes using barium cations Jeffrey V. Gasa a , R.A. Weiss a,b,∗ , Montgomery T. Shaw a,b a
b
Polymer Science Program, University of Connecticut, Storrs, CT 06269-3136, United States Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States Received 27 May 2007; received in revised form 19 July 2007; accepted 20 July 2007 Available online 26 July 2007
Abstract Acidic polymer electrolyte membranes (PEM) based on sulfonated poly(ether ketone ketone) (SPEKK) with relatively moderate to high levels of sulfonation (ion-exchange capacity (IEC) > 1.7 meq/g) have excellent proton conductivities (∼0.1 S/cm), but they absorb excessive amounts of water at elevated temperatures (ca. 60–90 ◦ C). To reduce the water sorption of the membranes and improve their mechanical properties and dimensional stability, the protons in acidic SPEKK membranes were partially exchanged with divalent barium cations to create ionic crosslinks between the sulfonate groups attached to the aromatic rings of the PEKK. The degree of crosslinking was varied by changing the degree of neutralization of the ionomer. The proton conductivity, water uptake, and methanol permeability at various levels of sulfonation and degree of crosslinking were measured. The conductivity was reduced by crosslinking, but the thermal stability, swelling, and barrier properties were improved. Crosslinking reduced the water swelling at room temperature by nearly a factor of two and prevented membrane dissolution at temperatures up to 80 ◦ C. In a water/methanol mixture (72/28, v/v), swelling was reduced by a factor of four. The balance of transport and mechanical properties could be varied to produce a viable PEM for a direct methanol fuel cell by adjusting the crosslink density. © 2007 Elsevier B.V. All rights reserved. Keywords: Ionomers; Proton exchange membranes; Selectivity; Conductivity; Neutralization
1. Introduction Perfluorosulfonic acid membranes, such as Nafion® , are presently considered the standard material for PEM fuel cell (PEMFC) technology because of their high proton conductivities and excellent electrochemical and mechanical stability in the fuel cell environment. However, these membranes possess several properties that are unfavorable when methanol is used as fuel, which most researchers consider to be the main hindrance to the technological and commercial success of direct methanol fuel cells (DMFC). A disadvantage of Nafion® membranes for the DMFC application is the crossover of methanol through the membrane, which adversely affects the performance (cell voltage) and fuel utilization efficiency of the cell. Although the proton conductivity and oxidative resistance of Nafion®
∗ Corresponding author at: Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States. Tel.: +1 860 486 4698; fax: +1 860 486 6048/4. E-mail address: [email protected] (R.A. Weiss).
0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.07.031
membranes are both excellent, the durability and methanol crossover of Nafion® become problematic especially when thinner membranes and high methanol feed concentrations are used to increase power density. As a result, considerable research has been focused recently on developing PEMs with a better balance of transport, mechanical, and aging properties. A key challenge is to improve the resistance of the membrane to methanol permeation without compromising other essential properties, specifically proton conductivity. In our previous work [1], we found that SPEKK with relatively moderate to high levels of sulfonation (ca. IEC > 1.7 meq/g) had proton conductivities comparable to that of Nafion® (ca. 0.1 S/cm at 98% relative humidity and 20 ◦ C). However, these membranes exhibited poor dimensional, mechanical, and barrier properties in an aqueous environment at elevated temperatures (ca. 60–90 ◦ C), which are typical operating conditions in a DMFC. The poor properties were attributed to the excessive water sorption of the membranes at high water activities (ca. > 50 wt% of water at 98% relative humidity). In the work reported in this paper, the objective is to reduce the water and methanol sorption by crosslinking the membrane.
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The idea of crosslinking of PEM is not new, but most of the crosslinking studies done previously pertain to chemical (covalent) crosslinking [2–5]. The crosslinking procedure discussed in this paper is a relatively less studied technique, which is ionic crosslinking. The approach is to exchange with divalent cations some of the protons in acidic membranes. The salt produced by the exchange reaction should have very low water solubility and form a stable crosslink. To the best of the authors knowledge, the application of the abovementioned crosslinking procedure to PEMs is a novel idea. In general, crosslinking alters several important properties of polymers. The more common effects of crosslinking are reduction in the sorption in swelling solvents, increase in transition temperatures, and increase in plateau modulus [2–10]. In some cases, crosslinking can also enhance the tensile strength and toughness of the polymer [11,12]. It can also alter the permeability and permselectivity of the polymer [13,14]. In PEM applications, previous studies have indicated that chemically crosslinked membranes can be superior to Nafion® membranes in terms of properties pertinent to DMFC applications such as methanol permeability, swelling properties, and proton conductivity [2–5]. In the different crosslinked systems that they studied, the common conclusion was that the proton conductivities of their membranes were comparable to Nafion® but the methanol permeabilities were about one order of magnitude lower [2–5]. They also reported that the swelling of their membranes were lower than Nafion® membranes. In this study, it was postulated that the ionic crosslinking of acidic SPEKK would give similar desirable effects such as low swelling and low permeability of SPEKK membranes in polar solvents such as methanol and water. The approach was to use very acidic SPEKK (IEC > 1.5 meq/g) and crosslink some of the sulfonate groups with divalent cations. Barium ions are known to form salts that have extremely low solubility in water, and thus were chosen to be the crosslinking species. Very acidic membranes were used to compensate for the loss of protons due to the ion-exchange process, and therefore maintain high conductivity. The specific objective of this work was to determine the effect of barium crosslinking on the swelling, thermal stability, and transport properties (proton conductivity and methanol permeability) of PEMs. 2. Experimental 2.1. Ionically crosslinked SPEKK membranes SPEKK membranes with IECs of 1.7 and 2.06 meq/g were prepared by sulfonating a 5% (w/v) solution of PEKK with a
53/47 (v/v) mixture of concentrated sulfuric acid and fuming sulfuric acid at room temperature. To crosslink the membranes, the protons in acidic SPEKK membranes were partially exchanged with barium cations from dilute barium acetate solutions. These cations, which are divalent, can link together two sulfonate groups to attain electro-neutrality (Fig. 1). When many sulfonate groups coming from different SPEKK chains are linked together by these divalent cations, a network of SPEKK chains can be formed. The fraction of protons exchanged with barium, denoted here as the exchange fraction, is directly related to the degree of crosslinking. Although the magnitude of the mechanical and transport properties changed with IEC, the effects of Ba-crosslinking of the two different membranes was the same. There are two possible crosslinking mechanisms that can take place in these materials. Ionic crosslinks as shown be Fig. 1 occur as a necessity of achieving electrical neutrality in the material; the effect of these crosslinks is likely to be relatively independent of temperature. Also possible, however, are physical crosslinks due to ion–dipole associations of the Ba-sulfonate groups, which can produce ionic aggregation, often termed ion-clusters in the ionomer literature. The properties of the physical crosslinks are expected to be temperature-dependent. Ion-clusters are distinguished from the ionic crosslinks in that they provide multifunctional crosslinks produced by nano-phase separation of ion-rich domains of the order of 1–5 nm in size in typical highly neutralized ionomers. In the discussion of the effects of Ba-crosslinking in this paper, the crosslinks are simply referred to as ionic, because formation of ionic crosslinks must occur due to the need for electronic neutrality, as shown in Fig. 1. Whether or not physical crosslinks, i.e., ion-clusters, also occurred requires structural analysis of the polymer using techniques such as small angle X-ray scattering, which was not done in this study. To fine-tune the degree of crosslinking, the acidic SPEKK membranes were immersed in dilute solutions (0.1 wt%) of barium acetate at various immersion times at room temperature. After soaking in Ba(OAc)2 solution, the barium-crosslinked membranes were thoroughly washed with copious amounts of water to extract residual barium acetate solution absorbed by the membrane and to remove acetic acid, which is a byproduct of the exchange reaction (Fig. 1). The membranes were preswollen in water prior to immersion in barium acetate solution to remove residual solvent (NMP) from the casting procedure and also to promote the migration of barium ions into the membrane and minimize localization of the crosslinking reaction on the membrane surface. The degree of barium exchange was calculated as the number fraction of sulfonic acid groups substituted with barium.
Fig. 1. Schematic of the barium-crosslinking reaction of acidic SPEKK membranes.
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The amount of sulfonic acid groups was measured by acid–base titration following the previously established procedure for measuring the IEC of SPEKK [1]. The working equation for fractional substitution xE was: xE =
IEC0 − IECE IEC0
(1)
where IEC0 is the IEC of the original sample and IECE is the IEC of the sample after substitution, but based on its original dry weight. 2.2. Characterization of the barium-crosslinked SPEKK membranes 2.2.1. Proton conductivity Impedance spectroscopy has been widely used to measure the proton conductivity of PEMs [15–25]. The conductivity of the membranes was measured using a Hewlett-Packard Agilent® 4284A LCR meter covering a frequency range of 20–106 Hz. The applied voltage was 50 mV. Membrane conductivities were measured using a custom-made cell based on the design of Zawodzinski et al. [15]. Cells of this design measure conductivity along the plane of the membrane. 2.2.2. Water sorption The water sorption of the membranes was measured by placing films in a controlled humidity environment and weighing them every 24 h until the mass of the membranes reached equilibrium, as judged by the point where the changes in mass were merely random weighing errors. The membranes were then dried in vacuum at 100 ◦ C for 24 h and re-weighed. Water sorption is defined here as the mass of the absorbed water divided by the mass of the wet membrane, i.e., the mass fraction of water. 2.2.3. Thermal stability The thermal stability of the polymers was measured with a TA Instruments Hi-Res TGA 2950 thermogravimetric analyzer using a nitrogen atmosphere and a heating rate of 20 ◦ C/min. The samples were first dried for 10 min at 120 ◦ C in the TGA furnace prior to the scan to remove any residual water absorbed by the relatively hydrophilic sulfonated polymers during sample transfer. 2.2.4. Methanol permeability The methanol permeabilities of the Ba-crosslinked SPEKK membranes were measured using the cell shown in Fig. 2. The design of this cell is a variation of that used by Walker et al. to measure methanol permeation through membranes based on sulfonated aromatic hydrocarbons and also perfluorosulfonic acid membranes (Nafion 117® ) [26]. The lower part of the cell, which was a 20-mL glass vial, was filled with 10 mL methanol. Methanol vapor in equilibrium with the liquid diffused along the concentration gradient through the membrane, which was clamped between the mouth of the vial (2 cm in diameter) and the cap. The cap had a 1-cm hole so that the methanol that diffused through the membrane could
Fig. 2. Schematic of the permeation cell.
escape. The cell was placed inside a drying oven, which provided temperature control and an air draft that maintained the methanol concentration and humidity in the environment above the surface of the membrane at a minimum. The mass of the methanol inside the cell was measured as a function of time. For the results reported here, the methanol permeability (P) was calculated by applying Fick’s first law: J = −D
dCm Cb Cb = −DK = −P dx L L
(2)
where J is the molar flux of methanol, D the methanol diffusivity, K the partition coefficient or the solubility of methanol in the membrane, Cm the methanol concentration in the membrane, Cb the methanol concentration in the gas phase, and L is the thickness of the membrane. In this device, it is difficult to quantify accurately the value of Cb . Although Cb at the inner surface of the membrane could be approximated using the vapor pressure of MeOH, it is difficult to estimate Cb at the outer surface of the membrane. It is often assumed that Cb = 0, but a finite methanol concentration near the cap can introduce appreciable error in the calculation of J. To circumvent this problem, a reference sample with known methanol permeability (i.e., Nafion® ) was tested and the methanol permeabilities of the samples were reported relative to the methanol permeability of the reference sample. In this way, the driving force term (Cb ) in Eq. (2) cancels out because all the samples were measured in one batch, and therefore were exposed to the same conditions (same driving force). The resulting relationship for the relative permeability is then Ju Lu Pu = Jr L r Pr
(3)
where u and r refer to the unknown and reference samples, respectively. The reference sample was a Nafion 115® membrane (Dupont de Nemours, Wilmington, DE) with a thickness of 127 m as reported by DuPont.
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Fig. 3. Time evolution of the barium-exchange reaction in SPEKK with an IEC of 1.7 meq/g using 0.1 wt% barium acetate solution.
3. Results and discussion 3.1. Ion-exchange reaction Fig. 3 shows the time evolution of the exchange reaction. It can be seen that a broad range of exchange fractions can be obtained and can almost reach completion (100% exchange). The analysis of the kinetics of exchange reaction is quite complicated because there are two competing rates that have to be deconvoluted, the rate of barium exchange and the diffusion rate of barium ions. The data obtained in this study was not enough to get an accurate model of the kinetics of such ion-exchange process.
Fig. 4. Influence of the degree of barium crosslinking or the exchange fraction on the water sorption of the SPEKK membrane with an IEC of 1.7 meq/g.
3.3. Water sorption vs. temperature
3.2. Water sorption
Fig. 5 shows the water sorption of the SPEKK (IEC = 1.7 meq/g) and barium-exchanged membranes as a function of temperature and exchange fraction. The water sorption of the neat SPEKK membrane with an IEC of 1.7 meq/g exhibited a strong temperature dependence. At temperatures below 45 ◦ C, the temperature dependence of the water sorption was weak, but above 45 ◦ C, there was a sharp upturn in the water sorption. That was probably due to the glass-to-rubber transition of the water-swollen SPEKK. With increasing exchange frac-
The purpose of modifying SPEKK membranes through crosslinking is to reduce the water sorption of these membranes in aqueous environments especially for those membranes with relatively high sulfonation levels. Fig. 4 shows the equilibrium water sorption of barium-exchanged SPEKK membranes (IEC of the original SPEKK was 1.7 meq/g) as a function of the exchange fraction or the degree of crosslinking. These water sorption data were taken at 23.8 ◦ C and 98% relative humidity. The equilibrium water sorption monotonically decreased with increasing exchange fraction. As with other crosslinkable systems [6–10], this result is an expected one because the barium ionic crosslinks reduce the chain mobility, thereby reducing the ability of these membranes to expand and absorb water. If the sorption data is extrapolated to 100% exchange fraction, the water sorption is not zero; it is about 17 wt%. However, this does not imply that the barium crosslinks are dissociated. The membranes were pre-swollen in water prior to immersion in barium acetate solution for crosslinking; thus the energetically most favorable state in equilibrium with water is a somewhat swollen condition even at 100% exchange fraction. This is analogous to the formation of highly swellable hydrogel networks from monomer solutions as opposed to neat monomer [26].
Fig. 5. Influence of temperature and barium-exchange fraction on the water sorption at 25 ◦ C of barium-exchanged SPEKK membranes. The numbers in parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the untreated SPEKK was 1.7 meq/g.
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tion, the position of the upturn shifted to higher temperatures and the magnitude of the sorption values decreased. Above 64% barium exchange, the upturn disappeared and the water sorption became almost temperature-independent. This shows that the barium crosslinking not only reduced the magnitude of the water sorption but also improved the thermal stability of the swelling of these membranes in an aqueous environment. The excessive swelling exhibited by the neat SPEKK membrane at temperatures above 45 ◦ C is undesirable because it compromises the mechanical integrity, dimensional stability, and the barrier properties of the membrane. 3.4. Swelling in water–methanol mixtures In direct methanol fuel cells, the usual practice is to use dilute solutions of methanol in water as fuel to minimize methanol crossover and swelling of the membrane. The limitation on the concentration of methanol in contact with the anode unfavorably limits the performance of the cell. It is desirable that the cell be operated at higher methanol concentrations to increase power density and reduce the fuel volume, which are especially essential for portable applications. Higher methanol concentrations are also desirable for easier water management. Therefore, it is important to gain an understanding of the swelling properties of PEM in methanol–water mixtures. The swelling of the barium-exchanged SPEKK membranes in water–methanol mixtures is shown in Fig. 6. The activity of methanol in water–methanol mixtures varies in a relatively linear fashion with methanol mole fraction over the range shown in Fig. 6 (activity coefficient drops slightly from about 1.6 to around 1.3) [27]. The sorption of neat SPEKK membrane with an IEC of 1.7 meq/g exhibited a strong dependence on methanol concentration, meaning that this polymer prefers methanol over water. The slope of the SPEKK membrane data in Fig. 6 increases monotonically with increasing methanol concentration. As the degree of crosslinking increased, the swelling
Fig. 6. Sorption of water at 25 ◦ C by barium-exchanged SPEKK membranes as a function of methanol concentration and exchange fraction. The numbers in parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the untreated SPEKK was 1.7 meq/g.
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became less dependent on methanol concentration or methanol activity. Above 64% barium exchange, the fluid uptake became virtually independent of methanol activity, which suggests that the barium crosslinking of SPEKK membranes eliminated the favorable interaction with methanol, relative to water. Swelling is reduced to a convenient value of around 20% regardless of the fluid composition. 3.5. Ionic conductivity In the barium-exchange reaction, the mobile protons are replaced by the much less mobile barium ions. The mobility of barium ions is much lower because these ions are bulkier and have far less ionic character. Therefore, the ionic conductivity of the membranes is expected to decrease upon ion-exchange with barium, and that is confirmed in Fig. 7. The conductivity decreased dramatically with increasing degree of crosslinking. If the conductivity of the barium-exchanged SPEKK membranes is plotted against the concentration of the remaining protons (Fig. 8), expressed here in terms of the number of sulfonic acid groups per mole of SPEKK repeat unit, the conductivity of the barium-exchanged membranes overlaps with that of the untreated or the neat SPEKK membranes. It appears that the barium disulfonate groups have very little contribution to the ionic conductivity. This implies that the barium ions indeed have very low mobility and the conductivity is controlled solely by the protons. 3.6. Methanol permeability and permselectivity In direct methanol fuel cells, methanol crossover is an important problem, especially for portable applications where the current densities are relatively low. At low current densities, the rate of methanol oxidation is low and unreacted methanol concentrates at the anode, promoting diffusion of methanol through
Fig. 7. Ionic conductivity of barium-exchanged SPEKK membranes as a function of exchange fraction.
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Fig. 8. Ionic conductivity of barium-exchanged SPEKK membranes and untreated SPEKK membranes as a function of the number of remaining sulfonic acid groups per repeat unit.
the membrane. To reduce crossover, the intrinsic property of the membrane that has to be minimized is the methanol permeability. In this study, the ionic crosslinking of SPEKK using divalent cations such as barium was expected to reduce the methanol permeability by reducing the swelling of the membrane, which will reduce both the solubility and diffusivity contributions to the permeability (see Eq. (2)). The crosslinks themselves may also decrease methanol diffusivity because of attraction between the penetrant and the ionic clusters. Fig. 9 shows the methanol permeability of the bariumexchanged SPEKK membranes relative to that of Nafion® . The methanol permeability of the original SPEKK membrane (IEC = 2.06 meq/g) was only about 40% of that of Nafion® , and the permeability decreased with increasing degree of ionic crosslinking. For a barium exchange of only 20%, the methanol
Fig. 9. Methanol permeability of the barium-exchanged SPEKK (2.06 meq/g) membranes as a function of the exchange fraction.
Fig. 10. Influence of barium-exchange fraction on the proton conductivity of the barium-exchanged SPEKK membranes normalized with respect to the conductivity of Nafion® .
permeability of the SPEKK membrane was about one-fifth that of Nafion® . The proton conductivities of the barium-crosslinked SPEKKs (initial IEC = 2.06 meq/g) relative to the conductivity of Nafion® are shown in Fig. 10. The conductivity of the unneutralized SPEKK was about 40% higher than that of Nafion® . For a barium exchange of 10%, the conductivity of the crosslinked SPEKK was comparable to that of Nafion® ; however, as shown in Fig. 9, the methanol permeability was about 80% lower than that of Nafion® . The ratio of the proton conductivity and the methanol permeability, referred to as permselectivity or selectivity of the membranes is plotted for the SPEKK membranes (initial IEC = 2.06 meq/g) as a function of barium exchange in Fig. 11. There seems to be a maximum in the selectivity at around 14% exchange, but there are not enough data points to make a definitive conclusion about the statistical significance of this maximum. The selectivity at 14% exchange was about five times greater than that of Nafion® . Above 20% exchange, the selectivity dropped to about the same selectivity as for Nafion® .
Fig. 11. Influence of barium-exchange fraction on the permselectivity of the barium-exchanged SPEKK membranes normalized with respect to the permselectivity of Nafion® .
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Fig. 12. Derivative TGA curves of the barium-exchanged SPEKK (1.7 meq/g) membranes for various exchange fractions.
3.7. Thermal stability TGA curves of the barium-exchanged SPEKK membranes (initial IEC = 1.7 meq/g) as a function of the degree of barium exchange are shown in Fig. 12. The ordinate in this plot is the derivative of the sample mass with respect to temperature. A mass loss step in the integral TGA curve (mass vs. temperature) will be manifested as a peak in the derivative plot. The derivative plot simply makes it easier to resolve small mass loss steps and to pin-point the temperature associated with each step. In the TGA derivative curve for the neat SPEKK membrane, the large peak centered around 350 ◦ C is due to desulfonation, while the broad peak around 500 ◦ C is due to degradation of the PEKK backbone. For the barium-exchanged SPEKK membranes, the desulfonation peaks are systematically shifted to higher temperatures, and result in lower mass loss. There is in addition a new peak centered around 750 ◦ C, which is not present in the TGA curve of the neat SPEKK membrane; this may be due to desulfonation of the neutralized sulfonic acid groups. One possible cause of the increase in the first desulfonation temperature is a decrease in diffusion due to the higher polymer viscosity; another is that an increase in pH suppresses the desulfonation reaction. Gaining further information concerning the nature of the degradation reactions would require analysis of the products, which is beyond the scope of this work. 4. Conclusions Crosslinking SPEKK membranes using barium cations was achieved and the degree of crosslinking could be controlled by using dilute aqueous solutions of barium acetate. Although the crosslinking reduced the proton conductivity of SPEKK membranes, the methanol permeability and methanol sorption were reduced significantly. There seemed to be an optimum degree of crosslinking, about 14% barium exchange, where the ratio of the proton conductivity to the methanol permeability, the selectivity, was highest. But, more samples with various degrees of crosslinking need to be tested to confirm the statistical significance of this optimum crosslink density. Statistical analysis of
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the data obtained in this study showed that there is a decreasing relationship between selectivity and the degree of crosslinking. However, when a highly sulfonated PEKK was used (e.g., IEC = 2.06 meq/g), a barium exchange of 10%, still yielded a membrane with a proton conductivity comparable to that of Nafion® , but with a methanol permeability 80% lower than that of Nafion® . Crosslinking with barium ions also greatly improved the thermal stability of the SPEKK membranes under dry conditions, and crosslinking mitigated the problem of excessive water sorption of SPEKK membranes at elevated tempe ratures. It appears from the results presented in this paper that the preparation of viable and robust PEM membranes may be possible by using a highly sulfonated polymer, even one that is close to water-soluble, that is partially neutralized to form a partial ionomer salt. With a divalent cation (e.g., barium), intermolecular crosslinks that form from the ion-bridge due to the metal sulfonate, renders the polymer less swellable by water and also may improve the membranes mechanical properties. However, investigation is needed into the question of the durability of the crosslinks in a working fuel cell. While preliminary experiments indicated that Ba++ can be displaced by H+ in an acidic solutions, the situation in a fuel cell is somewhat different in that all the anions are immobile; thus, the only escape for Ba++ is to move to the cathode. Its presence there could impede the normal cathode reactions, but it is not likely to be washed out. In the materials area, additional work is needed to determine whether other cations also may work to improve membrane selectivity and mechanical properties. In this case, dipole–dipole aggregation of the metal sulfonate groups, similar to what occurs in metal salt ionomers of much lower ion-exchange capacity, may provide the crosslinks. Similarly, other divalent cations or higher valency ions such as Al(III) or Ti(IV) may be of interest. The higher valency cations may provide stronger crosslinking and better water resistance and mechanical properties using less exchange of the protons. Acknowledgment This work was partially supported by a grant from the Dept. of Energy, 10761-001-05. References [1] S. Swier, Y.-S. Chun, J. Gasa, M.T. Shaw, R.A. Weiss, Sulfonated poly(ether ketone ketone) ionomers as proton exchange membranes, Polym. Eng. Sci. 45 (2005) 1081–1091. [2] J. Shin, B. Chang, J. Kim, S. Lee, D. Suh, Sulfonated polystyrene/PTFE composite membranes, J. Membr. Sci. 251 (2005) 247–254. [3] J. Kerres, W. Cui, M. Junginger, Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU crosslinked PSU blend membranes by alkylation of sulfinate groups with dihalogenoalkanes, J. Membr. Sci. 139 (1998) 227– 241. [4] J. Chen, M. Asano, T. Yamaki, M. Yoshida, Preparation of sulfonated crosslinked PTFE-graft-poly(alkyl vinyl ether) membranes for polymer electrolyte membrane fuel cells by radiation processing, J. Membr. Sci. 256 (2005) 38–45.
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