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Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity
Journal of Membrane Science 332 (2009) 63–68
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity Guigui Wang a , Yiming Weng a , Deryn Chu b , Rongrong Chen a , Dong Xie a,∗ a b
Richard G. Lugar Center for Renewable Energy, Indiana University Purdue University, Indianapolis, IN 46202, USA US Army Research Laboratory, Adelphi„ MD 20783, USA
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 21 January 2009 Accepted 25 January 2009 Available online 5 February 2009 Keywords: Polysulfone Chloromethylation Quaternization Alkaline anion exchange membrane Ionic conductivity
a b s t r a c t Alkaline anion exchange membranes of high ionic conductivities were made from polysulfone by adding a chloromethyl pendant group to the polysulfone at different reaction times and temperatures, followed by reacting the chloromethyl group with different amines to form different quaternary ammonium pendant groups which acted as the counterion for hydroxide anion. The effects of temperature and time on chloromethylation of the polymer were investigated and the chloromethylation was optimized. Furthermore, different approaches for quaternization of the synthesized chloromethylated polymer were studied. The results show that both temperature and time exhibited significant impacts on chloromethylation and gelation. It was also found that using an appropriate quaternization approach could significantly improve the ionic conductivity and also could optimize the conductivity of the membrane even though the accessible functional chloromethyl groups were limited. The developed AAEM showed the ionic conductivity up to 3.1 × 10−2 S/cm at room temperature. Increasing temperature increased the ionic conductivity up to 7.33 × 10−2 S/cm. The formed AAEM was stable in a concentrated base up to 8.0 M KOH at room temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ion exchange membranes have many applications. One of the important applications is their uses in fuel cell research. Recently, fuel cell research has become even more important because of the energy crisis. One of the key components of fuel cells is the electrolyte. The use of solid polymer electrolytes represents an interesting path to pursue for these electrochemical devices [1]. Two types of solid polymer electrolytes have been studied in fuel cell research: proton exchange polymer membranes and anion exchange polymer membranes. The former is used for a proton exchange membrane (PEM) fuel cell and the latter for an anion exchange membrane fuel cell. Over the past few decades, much effort has been focused on PEM development [1,2]. Although the PEMs exhibit excellent chemical, mechanical and thermal stability as well as high ionic conductivity, several significant disadvantages have limited their further development when they are applied to fuel cells [2]. The disadvantages in the PEM-constructed fuel cells include slow electrode-kinetics, carbon monoxide poisoning
∗ Corresponding author at: Richard G. Lugar Center for Renewable Energy, Purdue School of Engineering and Technology, Indiana University-Purdue University at Indianapolis, 723 West Michigan Street, SL-220E, Indianapolis, IN 46202, USA. Tel.: +1 317 274 9748; fax: +1 317 278 2455. E-mail address: [email protected] (D. Xie). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.01.038
of non-reusable expensive Pt and Pt-based electrocatalysts at low temperatures, high cost of membrane and high methanol permeability [1]. To overcome these hurdles, alkaline anion exchange membranes (AAEM) were created. AAEMs are designed to provide sufficient hydroxyl ions for ion exchange during electrochemical reactions in alkaline fuel cells. Alkaline fuel cells have numerous advantages over proton exchange membrane fuel cells on both cathode kinetics and ohmic polarization. The inherently faster kinetics of the oxygen reduction reaction in an alkaline fuel cell allows the use of non-noble and low-cost metal electrocatalysts such as silver and nickel. Furthermore, methanol oxidation is more facile in alkaline media than that in acidic media [1]. Currently there is a growing interest in AAEM research [1]. So far only a few AAEMs have been evaluated for use as solid polymer electrolytes for alkaline fuel cells [1,3–8]. These membranes were constructed mainly from polystyrenes, poly(phenylene oxide)s, poly(sulfone-ether)s, etc. [4–6,9–11]. Crosslinked polystyrenes have been widely used as building matrixes for cation and anion exchange resins [12]. Polyphenylene oxide or PPO have excellent electrical properties, unusual resistance to acids and bases as well as other chemicals, excellent dimensional stability, low moisture absorption, and high mechanical and dielectric strength [12,13]. Polysulfones are highperformance polymers that are thermally and chemically stable [12,14]. Although these polymers were modified to prepare the membranes for AAEM applications, none of them have been studied in details regarding the optimization of both chloromethylation and
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quaternization. The preparation of AAEMs involves several chemical reactions: chloromethylation, quaternization, and alkalization [1]. Among them, chloromethylation and quaternization are two key reactions that determine the ionic conductivity [1]. During this study, we found that chloromethylation was not easily controllable and the number and yield of chloromethyl groups attached onto the polymer could be very low, thus affecting the ultimate conductivity. The reaction often caused a gelation if it was not sufficiently controlled. Quaternization is another important reaction in preparation of the AAEM [15–17]. Therefore, in this study, we have demonstrated how we improved chloromethylation by tuning reaction temperature and time that often affect chloromethylation of a polymer and how we improved ionic conductivity in water by using different quaternization approaches. The objective of this study was to synthesize a functionalized polysulfone, investigate the effects of reaction temperature and time on chloromethylation, and study several approaches of quaternization for enhanced ionic conductivity in water of the formed anion exchange membrane. 2. Experimental 2.1. Materials Polysulfone or poly(bisphenol A-co-4-bisphenylsulfone ether) with MW = 30,000, chloroform, zinc chloride, chloromethyl ether, methanol, N,N-dimethylformamide, trimethylamine (TMA), triethylamine (TEA), dimethylethylamine (DMEA), dimethylisopropylamine (DMIPA), tetramethylethylenediamine (TMEDA), bromoethane (BE), potassium hydroxide, and deuterated chloroform were used as received from Fisher Scientific Inc (Pittsburgh, PA) without further purifications. 2.2. AAEM preparation The preparation of the alkaline anion exchange membrane was conducted using three steps: (1) chloromethylation, (2) quaternization and (3) alkalization. The schematic diagram for the preparation of the AAEM is shown in Fig. 1. 2.2.1. Chloromethylation of polysulfone For chloromethylation, 1.2 g polysulfone was dissolved in 7.5 ml chloroform containing 0.06 g zinc chloride. After the polymer was completely dissolved, the temperature was increased to 75 ◦ C and 1 ml chloromethyl ether was added dropwise with stirring. The reaction was allowed for a total of 75 min. After that, the chloromethylated polymer was precipitated in methanol to remove the catalyst, excess chemicals and solvent, followed by washing with methanol and de-ionized water several times prior to drying. 2.2.2. Quaternization of polysulfone Quaternization was conducted via three approaches: (1) quaternizing pre-formed membrane via a tertiary amine [4]; (2) quaternizing the chloromethylated polymer via TMEDA (a difunctional crosslinker), followed by treating with a tertiary amine; and (3) quaternizing the chloromethylated polymer via TMEDA, followed by treating with BE. To Approach 1, the purified chloromethylated polymer was dissolved in N,N-dimethylformamide, followed by pouring the solution into a Petri dish. The polymer in the Petri dish was then dried in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. The formed membrane was then treated with a tertiary amine at 40 ◦ C for 8 h followed by washing with de-ionized water several times. To Approach 2, the purified chloromethylated polymer and the calculated TMEDA were dissolved in N,N-dimethylformamide, followed by pouring the
Fig. 1. Schematic diagram for synthesis: (a) chloromethylation; (b) quaternization; and (c) alkalization.
solution into the Petri dish. The polymer in the Petri dish was then kept at 40 ◦ C for 1 h. After that, a tertiary amine was added and the Petri dish was kept at 40 ◦ C for another 1 h, followed by drying in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. To Approach 3, the purified chloromethylated polymer and different amount of TMEDA were dissolved in N,N-dimethylformamide, followed by pouring the solution into the Petri dish. The polymer in the Petri dish was then kept at 40 ◦ C for 1 h. After that, excess TMEDA was added and the Petri dish was kept at 40 ◦ C for another 1 h, followed by drying in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. The formed membrane was treated with BE at 40 ◦ C for additional 3 h. Finally all the formed membranes from the above three approaches were detached from the Petri dishes before alkalization. 2.2.3. Alkalization of polysulfone The above quaternized membranes were soaked in a 1.0 M potassium hydroxide aqueous solution at room temperature for 24 h, followed by washing with de-ionized water several times and soaking in de-ionized water with numerous exchanges within 24 h prior to evaluation. 2.3. Characterization and ionic conductivity measurements The degree of chloromethylation of polysulfone was characterized by nuclear magnetic resonance (NMR) spectroscopy. Proton NMR (1 H NMR) spectra were obtained on a Varian NMR Spectrometer (Varian-500, Palo Alto, CA) using deuterated chloroform as a solvent. The OH− ionic conductivity of the formed membranes was measured using AC impedance spectroscopy with a Solartron 1250 frequency response analyzer (Solartron Analytical, Farnborough, Hampshire, UK) interfaced with a 1287 potentiostat/galvanostat. The measurement was conducted in the galvanostatic mode over frequencies ranging from 0.1 Hz to 60 kHz with a galvanostatically controlled AC current of 5 mA. A standard four-probe conductivity
G. Wang et al. / Journal of Membrane Science 332 (2009) 63–68
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Fig. 3. Effect of reaction temperature on the number of the tethered chloromethyl group: time = 75 min; CME (chloromethyl ether)/polymer = 2/1 (by mole); polymer conc. = 15%; ZnCl2 = 5%. The number of chloromethyl group was determined by 1 H NMR.
Fig. 2.
1
H NMR spectra: effect of reaction time on chloromethylation.
cell (BekkTech LLC, Loveland, CO) was used to assemble the membrane test sample. The area resistance of the membrane was determined in de-ionized water at 24 ◦ C. Ionic conductivity, (S/cm), was calculated according to the equation [11]: = l/(RTW), where l = the length of the membrane between two potential sensing platinum wires (cm), R = the membrane resistance (), T = the thickness of the membrane (cm) and W = the width of the membrane (cm). 3. Results and discussion 3.1. Chloromethylation Fig. 2 shows the 1 H NMR spectra for polysulfone and chloromethylated polysulfone as well as the effect of reaction time on chloromethylation. The chemical shifts of the polysulfone were found as follows (ppm): a: 6.90–7.86 (multi Hs on phenyl groups) and b: 1.72 (CH3 ). The chemical shifts of the chloromethylated polysulfone are listed below (ppm): a: 6.85–7.86 (multi Hs on phenyl groups), b: 4.56 (CH2 Cl) and c: 1.72 (CH3 ). The characteristic chemical shift at 4.56 confirmed the formation of the chloromethylated
polysulfone. The effect of reaction time on chloromethylation will be discussed below. The most important step in preparing AAEMs is chloromethylation, because the chloromethylated polymer is readily further modified due to the high reactivity of the tethered chloromethyl group [18]. The successful chloromethylation also determines how many functional groups on the polymer chain can be directly quaternized to further transfer to conductive hydroxyl groups, which in turn determines the ionic conductivity. On one hand, we found that both reaction temperature and time could significantly affect the chloromethylation reaction and the number of the tethered chloromethyl group. On the other hand, we noticed that the chloromethylation could easily cause gelation, leading to a lower yield of the chloromethylated polymer. It has been reported that crosslinking often takes place rapidly during chloromethylation of polystyrene resin as the active aromatic ring attacks the chloromethyl group in a Friedel–Crafts alkylation fashion, which in turn causes inter-polymer or intra-polymer crosslinking or gel formation [17,19]. In order to improve chloromethylation without gelation or with less gelation, we studied the effects of both reaction temperature and time on the number of the tethered reactive chloromethyl group. The 1 H NMR spectrum was used as a tool to investigate both effects. Fig. 2 shows a representative diagram containing a set of 1 H NMR spectra for the effect of time on the number of the tethered chloromethyl groups. All the numbers of the chloromethyl groups were obtained by integration of chemical shifts exhibited by chloromethyl and phenyl groups. The following discussion gives the details of the effects of both reaction temperature and time. Reaction temperature is very critical to most chemical reactions. As we know, increasing temperature can either accelerate a reaction by providing sufficient energy to the reactants or it can initiate some unnecessary side reactions that can often be avoided at normal or low temperatures. As shown in Fig. 3, increasing the temperature increased the number of the tethered chloromethyl group. However, when the temperature reached 60 ◦ C, the gelation occurred (Table 1) and 10% of the polymers became the cross-linked gel. The formed gel is not useful for further quaternization and membrane preparation. Fig. 4A shows the chemical scheme for gel formation. At 75 and 85 ◦ C, the gels increased to 35 and 90%, respectively, although the number of the tethered chloromethyl group also significantly increased to 1.65 and 1.98 (close to the theoretical maximum value). Based on both yield and number of the tethered chloromethyl group, we believe that 75 ◦ C might be
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Table 1 Effects of reaction temperature and time on the chloromethylation of the polysulfone. Temperature (◦ C)
Time (min)
Tethered CH2 Cla
Gelationb
23 40 60 75 85 75 75 75 75 75
75 75 75 75 75 15 30 45 60 90
0.49 0.69 1.41 1.65 1.98 0.19 0.38 0.73 1.00 1.00
No No 10% 35% 90% No No No 15% 85%
a b
The number of chloromethyl group was determined by 1 H NMR. Gelation was determined by simple weighing.
the best temperature for chloromethylation of this specific polysulfone. Reaction time is another important factor to many chemical reactions. From Fig. 5, with increasing reaction time, the number of the tethered chloromethyl group increased, indicating that the longer the reaction the more the chloromethyl group would be tethered. However, when the reaction time reached 45 min, the reaction seemed to reach to a plateau and the increase of the number of the tethered chloromethyl groups slowed down. There was no significant difference in the number of the tethered chloromethyl groups (1.56, 1.62, 1.65 and 1.69) among 45, 60, 75 and 90 min, respectively. On the other hand, longer reaction time increased the probability of gel formation (Table 1). At 75 and 90 min, 35% and 85% of the polymers became the gel. Therefore, 75 min seems the optimal reaction time for the chloromethylation to have the highest number of the tethered chloromethylated group but less gel formation. 3.2. Quaternization Formation of quaternary ammonium hydroxide is necessary for the high ionic conductivity of the AAEM membrane [11] and can increase the stability of the membrane [15]. In this study, we have
Fig. 4. Schematic diagrams for both gel formation during chloromethylation and crosslinking reaction during quaternization: (A) gel formation and (B) crosslinking reaction.
Fig. 5. Effect of reaction time on the number of the tethered chloromethyl group: temperature = 75 ◦ C; CME (chloromethyl ether)/polymer = 2/1 (by mole); polymer conc. = 15%; ZnCl2 = 5%. The number of chloromethyl group was determined by 1 H NMR.
applied three approaches to quaternize the chloromethylated polymer. The ionic conductivity test was used as an evaluation tool. The first approach was to follow the published protocol by simply quaternizing the pre-formed membrane via a tertiary amine, followed by alkalization [4]. This approach was simple but the conductivity was low. Five tertiary amines were studied for quaternization. The ionic conductivity of the membranes treated with these five tertiary amines (see Fig. 6) was in the decreasing order: TMA > DMEA > DMIPA > TMEDA > TEA. These tertiary amines have different alkyl chains attached to the amine core. TMA, TEA, DMEA, and DMIPA have three methyl, three ethyl, two methyl and one ethyl, and two methyl and one isopropyl groups, respectively, attached to the amine core. TMEDA has two methyl groups on each end but once reacting with the chloromethyl group on polysulfone only one end can be further quaternized. By comparing the structures of the tertiary amines with the results for conductivity, it is clear that steric hindrance determines the ease of quaternization. The smaller methyl group made the quaternization the easiest, which allowed the membrane quaternized with TMA to have the highest conductivity. The more methyl groups were attached to the
Fig. 6. Conductivity of the AAEMs quaternized using Approaches 1 and 2: (1) Approach 1: the pre-formed membranes treated directly with different tertiary amines in 400% of the tethered chloromethyl groups (by mole); (2) Approach 2: the membranes cross-linked via TMEDA in 100% of the tethered chloromethyl groups (by mole), followed by treating with different tertiary amines in 400% of the tethered chloromethyl groups. The conductivity was determined in de-ionized water at 24 ◦ C.
G. Wang et al. / Journal of Membrane Science 332 (2009) 63–68
Fig. 7. Conductivity of the AAEMs quaternized using Approach 3: the membranes cross-linked with different amount of TMEDA (33%, 50%, 100% and 200% of the tethered chloromethyl groups), followed by treating with excessive TMEDA and BE, respectively. The conductivity was determined in de-ionized water at 24 ◦ C.
amine, the easier the quaternization would be. That is why TEA was the lowest in conductivity and TMEDA was the second lowest to TEA (Fig. 6). The conductivity was in the range of 2.1–5.2 × 10−3 S/cm. The second approach was to use a difunctional crosslinker, TMEDA, to quaternize the chloromethylated polymer when the polymer was still in solution. As a result, the cross-linked polymer network was initially formed. Fig. 4B shows the chemical scheme for crosslinking reaction produced by TMEDA. Following that, the unreacted chloromethyl groups were further quaternized by a mono- or ditertiary amine in order to convert the chloromethyl groups to the quaternized groups as many as possible. In fact, the ionic conductivity was significantly increased with the values ranging from 4.75 × 10−3 to 1.18 × 10−2 S/cm (Fig. 6). As compared to the conductivity values obtained from the first approach, the values from the second approach increased almost one order. The trend for the decreasing order of the ionic conductivity for different amines was similar to that shown for the first approach and the explanation would be the same. The third approach was to cross-link the polymer and then quaternize the cross-linked membrane via BE. This approach was designed to further quaternize the one-end dimethylamino group pendent on the polymer after the membrane was quaternized and cross-linked with difunctional TMEDA. Via this approach, the ionic conductivity was further increased (see Fig. 7) as compared to the second approach. During the study, we examined the effect of the amount of cross-linker (TMEDA) added. It was found that increasing the initial amount of TMEDA significantly increased the ionic conductivity (1.33–2.8 × 10 −2 S/cm). This may be attributed to the following reasons: (1) the post-quaternization with BE might not be as easy as the pre-quaternization with TMEDA, because BE contains an ethyl but not a methyl group. After crosslinking, the steric hindrance may make the quaternization by BE more difficult; (2) TMEDA not only plays a role in polymer crosslinking but also acts as a chemical for quaternization. Because the crosslinking occurred at the very beginning when the polymers were still in solution, the quaternization would be easier. That is why the conductivity at a cross-linker level of 200% (by mole) was the highest (2.8 × 10−2 S/cm). This value was higher than those obtained from the membranes either simply treated with a tertiary amine without cross-linking in Approach 1 or treated with a crosslinker followed by treating with a tertiary amine in Approach 2. The results indicate that both Approach 2 and Approach 3 are better approaches than Approach 1. It appears that using an appropriate quaternization can significantly increase the ionic conductivity of the AAEM. Komkova et al. [16] and Hao et al. [17] also studied
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Fig. 8. Effect of the thickness of the formed AAEM on conductivity: the conductivity was determined in de-ionized water at 24 ◦ C.
the quaternization by treating the membranes with excess different tertiary diamines. However, in addition to adding excess TMEDA diamine, we modified the quaternization by adding BE to further quaternize the remaining tertiary amine groups on attached TMEDA. Although it was hard for us to tell how many remaining tertiary amine groups were converted, by comparing with Approach 2 (Fig. 6) where excess TMEDA was used, the significantly increased conductivities demonstrated in Approach 3 (see Fig. 7) suggest that further quaternization is necessary for improved conductivity. 3.3. Effect of thickness, temperature and KOH on conductivity of the formed membrane Membrane thickness is important to the performance of the AAEM. Generally speaking, a thinner membrane provides a lower resistance of the membrane, whereas a thicker membrane gives a better mechanical stability of the membrane. In order to examine if the thickness would affect the conductivity, we tested the ionic conductivity of the membranes with thickness of approximately 28, 57, 109 and 143 m. The result in Fig. 8 showed that no significant differences in ionic conductivity were found among the membranes with different thickness, meaning that both quaternization and alkalization were relatively uniform throughout each membrane. Fig. 9 shows the effect of temperature on the ionic conductivity of the formed membrane treated with TMEDA/BE. To examine the effect of temperature, the quaternized membrane was treated with 1.0 M KOH at room temperature for 24 h, followed by washing with de-ionized water numerous times and soaking in de-ionized water for 24 h. After complete removal of the free KOH, the conductivity of the membrane was measured at elevated temperatures. The measured conductivity showed a linear increase with increasing temperatures (see Fig. 8). The conductivity values increased from 3.11 to 7.33 × 10−2 S/cm, corresponding to the temperature from 25 to 95 ◦ C. In general, conductivity increases with increasing temperature for ionic conductive materials [4]. Fig. 10 shows the effects of alkaline concentration on the ionic conductivity of the membrane treated with TMEDA/BE at room temperature. To examine the chemical stability of the formed membrane at different concentrations of KOH, we conditioned the membranes individually in KOH from 1.0 to 8.0 M for 24 h. After the free KOH was completely removed, the conductivity of each membrane was measured. There was hardly a change in conductivity for the membranes treated with different concentrations of KOH solutions. The measured conductivity ranged from 2.65 to 2.95 × 10−2 S/cm after conditioning in 1.0–8.0 M KOH solutions. The
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appropriate quaternization approach could significantly improve the ionic conductivity. The developed AAEM showed the ionic conductivity up to 3.1 × 10−2 S/cm at room temperature. Increasing temperature could increase the ionic conductivity up to 7.33 × 10−2 S/cm. The formed AAEM was stable in a concentrated base up to 8.0 M KOH at room temperature. Future study will include evaluating the other properties of this polysulfone-based AAEM, further optimizing its conductivity, studying its stability in base at higher temperatures and applying it to fuel cells. Acknowledgements This work was sponsored by the Army Research Laboratory (W911NF-07-2-0036) and the Richard G. Lugar Center for Renewable Energy at Indiana University-Purdue University Indianapolis. References Fig. 9. Conductivity of the AAEMs with increasing temperature: the conductivity of the membrane was measured in de-ionized water as temperature increased up to 95 ◦ C. The membrane was conditioned in 1.0 M KOH at room temperature for 24 h, followed by complete removal of free KOH prior to conductivity testing.
Fig. 10. Effect of KOH concentration on conductivity of the AAEMs: the conductivity of the membrane was measured after the membrane was conditioned in 1.0–8.0 M KOH at room temperature for 24 h followed by complete removal of free KOH. The conductivity was measured in de-ionized water at 24 ◦ C.
result indicates that the membrane is quite stable at room temperature even when treated with a KOH concentration up to 8.0 M. 4. Conclusions A polysulfone-based alkaline anion exchange membrane with improved ionic conductivity has been developed. The effects of temperature and time on chloromethylation of the polymer were investigated and the chloromethylation was optimized. Furthermore, different approaches for quaternization of the synthesized chloromethylated polymer were studied. The results show that both temperature and time exhibited significant impacts on chloromethylation and gelation. It was also found that using an
[1] J.R. Varcoe, R.C.T. Slade, Prospects for alkaline anion-exchange membranes in low temperature fuel cells, Fuel Cells 5 (2005) 187–200. [2] L.J.M.J. Blomen, M.N. Mugerwa, Fuel Cell Systems, Plenum Press, New York, NY, 1993. [3] Y. Wan, K.A.M. Creber, B. Peppley, V.T. Bui, Chitosan-based electrolyte composite membranes: II. Mechanical properties and ionic conductivity, J. Membr. Sci. 284 (2006) 331. [4] J. Fang, P.K. Shen, Quaternized poly(phthalazinon ether sulfone ketone) membrane for anion exchange membrane fuel cells, J. Membr. Sci. 285 (2006) 317. [5] L. Li, J. Wang, Quaternized polyethersulfone Cardo anion exchange membranes for direct methanol alkaline fuel cells, J. Membr. Sci. 262 (2005) 1. [6] F. Yi, X. Yang, Y. Li, Synthesis and ion conductivity of poly(oxyethylene) methacrylates containing a quaternary ammonium group, Polym. Adv. Technol. 10 (1999) 473. [7] R.C.T. Slade, J.R. Varcoe, Investigations of conductivity in FEP-based radiationgrafted alkaline anion-exchange membranes, Solid State Ionics 176 (2005) 585. [8] J.-J. Kang, W.-Y. Li, Y. Lin, X.-P. Li, X.-R. Xiao, S.-B. Fang, Synthesis and ionic conductivity of a polysiloxane containing quaternary ammonium groups, Polym. Adv. Technol. 15 (2004) 61. [9] T.N. Danks, R.C.T. Slade, J.R. Varcoe, Alkaline anion-exchange radiation-grafted membranes for possible electrochemical application in fuel cells, J. Mater. Chem. 13 (2003) 712–721. [10] G.-J. Hwang, H. Ohya, Preparation of anion-exchange membrane based on block copolymers: Part 1. Quaternization of the chloromethylated copolymers, J. Membr. Sci. 140 (1998) 195–203. [11] J.-S. Park, G.-G. Park, S.-H. Park, Y.-G. Yoon, C.-S. Kim, W.-Y. Lee, Development of solid-state alkaline electrolytes for solid alkaline fuel cells, Macromol. Symp. 249–250 (2007) 174–182. [12] H.G. Elias, Macromolecules, vol. 2: Industrial Polymers and Syntheses, John Wiley & Sons Inc., New York, NY, 2005. [13] R.-Q. Fu, D. Julius, L. Hong, J.-Y. Lee, PPO-based acid–base polymer blend membranes for direct methanol fuel cells, J. Membr. Sci. 322 (2008) 331–338. [14] E. Avram, M.A. Brebu, A. Warshawsky, C. Vasile, Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones, Polym. Degrad. Stab. 69 (2000) 175–181. [15] T. Sata, K. Teshima, T. Yamaguchi, Permselectivity between two anions in anion exchange membranes crosslinked with various diamines in electrodialysis, J. Polym. Sci., Part A 34 (1996) 1475–1482. [16] E.N. Komkova, D.F. Stamatialis, H. Strathmann, M. Wessling, Anion-exchange membranes containing diamines: preparation and stability in alkaline solution, J. Membr. Sci. 244 (2004) 25–34. [17] J.H. Hao, C. Chen, L. Li, L. Yu, W. Jiang, Preparation of solvent-resistant anionexchange membranes, Desalination 129 (2000) 15–22. [18] G.A. Olah, D.A. Beal, S.H. Lu, J.A. Olah, Synthetic methods and reactions. X1 . 1-chloro-4-chloro(bromo)methoxybutane and 1,4-bis-[chloro(bromo)methoxy]butane: new convenient halomethylating agents, Synthesis (1974) 560. [19] R.W. Lenz, Organic Chemistry of High Polymers, Interscience Publishers, New York, NY, 1967.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity Guigui Wang a , Yiming Weng a , Deryn Chu b , Rongrong Chen a , Dong Xie a,∗ a b
Richard G. Lugar Center for Renewable Energy, Indiana University Purdue University, Indianapolis, IN 46202, USA US Army Research Laboratory, Adelphi„ MD 20783, USA
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 21 January 2009 Accepted 25 January 2009 Available online 5 February 2009 Keywords: Polysulfone Chloromethylation Quaternization Alkaline anion exchange membrane Ionic conductivity
a b s t r a c t Alkaline anion exchange membranes of high ionic conductivities were made from polysulfone by adding a chloromethyl pendant group to the polysulfone at different reaction times and temperatures, followed by reacting the chloromethyl group with different amines to form different quaternary ammonium pendant groups which acted as the counterion for hydroxide anion. The effects of temperature and time on chloromethylation of the polymer were investigated and the chloromethylation was optimized. Furthermore, different approaches for quaternization of the synthesized chloromethylated polymer were studied. The results show that both temperature and time exhibited significant impacts on chloromethylation and gelation. It was also found that using an appropriate quaternization approach could significantly improve the ionic conductivity and also could optimize the conductivity of the membrane even though the accessible functional chloromethyl groups were limited. The developed AAEM showed the ionic conductivity up to 3.1 × 10−2 S/cm at room temperature. Increasing temperature increased the ionic conductivity up to 7.33 × 10−2 S/cm. The formed AAEM was stable in a concentrated base up to 8.0 M KOH at room temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ion exchange membranes have many applications. One of the important applications is their uses in fuel cell research. Recently, fuel cell research has become even more important because of the energy crisis. One of the key components of fuel cells is the electrolyte. The use of solid polymer electrolytes represents an interesting path to pursue for these electrochemical devices [1]. Two types of solid polymer electrolytes have been studied in fuel cell research: proton exchange polymer membranes and anion exchange polymer membranes. The former is used for a proton exchange membrane (PEM) fuel cell and the latter for an anion exchange membrane fuel cell. Over the past few decades, much effort has been focused on PEM development [1,2]. Although the PEMs exhibit excellent chemical, mechanical and thermal stability as well as high ionic conductivity, several significant disadvantages have limited their further development when they are applied to fuel cells [2]. The disadvantages in the PEM-constructed fuel cells include slow electrode-kinetics, carbon monoxide poisoning
∗ Corresponding author at: Richard G. Lugar Center for Renewable Energy, Purdue School of Engineering and Technology, Indiana University-Purdue University at Indianapolis, 723 West Michigan Street, SL-220E, Indianapolis, IN 46202, USA. Tel.: +1 317 274 9748; fax: +1 317 278 2455. E-mail address: [email protected] (D. Xie). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.01.038
of non-reusable expensive Pt and Pt-based electrocatalysts at low temperatures, high cost of membrane and high methanol permeability [1]. To overcome these hurdles, alkaline anion exchange membranes (AAEM) were created. AAEMs are designed to provide sufficient hydroxyl ions for ion exchange during electrochemical reactions in alkaline fuel cells. Alkaline fuel cells have numerous advantages over proton exchange membrane fuel cells on both cathode kinetics and ohmic polarization. The inherently faster kinetics of the oxygen reduction reaction in an alkaline fuel cell allows the use of non-noble and low-cost metal electrocatalysts such as silver and nickel. Furthermore, methanol oxidation is more facile in alkaline media than that in acidic media [1]. Currently there is a growing interest in AAEM research [1]. So far only a few AAEMs have been evaluated for use as solid polymer electrolytes for alkaline fuel cells [1,3–8]. These membranes were constructed mainly from polystyrenes, poly(phenylene oxide)s, poly(sulfone-ether)s, etc. [4–6,9–11]. Crosslinked polystyrenes have been widely used as building matrixes for cation and anion exchange resins [12]. Polyphenylene oxide or PPO have excellent electrical properties, unusual resistance to acids and bases as well as other chemicals, excellent dimensional stability, low moisture absorption, and high mechanical and dielectric strength [12,13]. Polysulfones are highperformance polymers that are thermally and chemically stable [12,14]. Although these polymers were modified to prepare the membranes for AAEM applications, none of them have been studied in details regarding the optimization of both chloromethylation and
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quaternization. The preparation of AAEMs involves several chemical reactions: chloromethylation, quaternization, and alkalization [1]. Among them, chloromethylation and quaternization are two key reactions that determine the ionic conductivity [1]. During this study, we found that chloromethylation was not easily controllable and the number and yield of chloromethyl groups attached onto the polymer could be very low, thus affecting the ultimate conductivity. The reaction often caused a gelation if it was not sufficiently controlled. Quaternization is another important reaction in preparation of the AAEM [15–17]. Therefore, in this study, we have demonstrated how we improved chloromethylation by tuning reaction temperature and time that often affect chloromethylation of a polymer and how we improved ionic conductivity in water by using different quaternization approaches. The objective of this study was to synthesize a functionalized polysulfone, investigate the effects of reaction temperature and time on chloromethylation, and study several approaches of quaternization for enhanced ionic conductivity in water of the formed anion exchange membrane. 2. Experimental 2.1. Materials Polysulfone or poly(bisphenol A-co-4-bisphenylsulfone ether) with MW = 30,000, chloroform, zinc chloride, chloromethyl ether, methanol, N,N-dimethylformamide, trimethylamine (TMA), triethylamine (TEA), dimethylethylamine (DMEA), dimethylisopropylamine (DMIPA), tetramethylethylenediamine (TMEDA), bromoethane (BE), potassium hydroxide, and deuterated chloroform were used as received from Fisher Scientific Inc (Pittsburgh, PA) without further purifications. 2.2. AAEM preparation The preparation of the alkaline anion exchange membrane was conducted using three steps: (1) chloromethylation, (2) quaternization and (3) alkalization. The schematic diagram for the preparation of the AAEM is shown in Fig. 1. 2.2.1. Chloromethylation of polysulfone For chloromethylation, 1.2 g polysulfone was dissolved in 7.5 ml chloroform containing 0.06 g zinc chloride. After the polymer was completely dissolved, the temperature was increased to 75 ◦ C and 1 ml chloromethyl ether was added dropwise with stirring. The reaction was allowed for a total of 75 min. After that, the chloromethylated polymer was precipitated in methanol to remove the catalyst, excess chemicals and solvent, followed by washing with methanol and de-ionized water several times prior to drying. 2.2.2. Quaternization of polysulfone Quaternization was conducted via three approaches: (1) quaternizing pre-formed membrane via a tertiary amine [4]; (2) quaternizing the chloromethylated polymer via TMEDA (a difunctional crosslinker), followed by treating with a tertiary amine; and (3) quaternizing the chloromethylated polymer via TMEDA, followed by treating with BE. To Approach 1, the purified chloromethylated polymer was dissolved in N,N-dimethylformamide, followed by pouring the solution into a Petri dish. The polymer in the Petri dish was then dried in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. The formed membrane was then treated with a tertiary amine at 40 ◦ C for 8 h followed by washing with de-ionized water several times. To Approach 2, the purified chloromethylated polymer and the calculated TMEDA were dissolved in N,N-dimethylformamide, followed by pouring the
Fig. 1. Schematic diagram for synthesis: (a) chloromethylation; (b) quaternization; and (c) alkalization.
solution into the Petri dish. The polymer in the Petri dish was then kept at 40 ◦ C for 1 h. After that, a tertiary amine was added and the Petri dish was kept at 40 ◦ C for another 1 h, followed by drying in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. To Approach 3, the purified chloromethylated polymer and different amount of TMEDA were dissolved in N,N-dimethylformamide, followed by pouring the solution into the Petri dish. The polymer in the Petri dish was then kept at 40 ◦ C for 1 h. After that, excess TMEDA was added and the Petri dish was kept at 40 ◦ C for another 1 h, followed by drying in an oven overnight at 70 ◦ C and then at 120 ◦ C for an additional 1 h. The formed membrane was treated with BE at 40 ◦ C for additional 3 h. Finally all the formed membranes from the above three approaches were detached from the Petri dishes before alkalization. 2.2.3. Alkalization of polysulfone The above quaternized membranes were soaked in a 1.0 M potassium hydroxide aqueous solution at room temperature for 24 h, followed by washing with de-ionized water several times and soaking in de-ionized water with numerous exchanges within 24 h prior to evaluation. 2.3. Characterization and ionic conductivity measurements The degree of chloromethylation of polysulfone was characterized by nuclear magnetic resonance (NMR) spectroscopy. Proton NMR (1 H NMR) spectra were obtained on a Varian NMR Spectrometer (Varian-500, Palo Alto, CA) using deuterated chloroform as a solvent. The OH− ionic conductivity of the formed membranes was measured using AC impedance spectroscopy with a Solartron 1250 frequency response analyzer (Solartron Analytical, Farnborough, Hampshire, UK) interfaced with a 1287 potentiostat/galvanostat. The measurement was conducted in the galvanostatic mode over frequencies ranging from 0.1 Hz to 60 kHz with a galvanostatically controlled AC current of 5 mA. A standard four-probe conductivity
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Fig. 3. Effect of reaction temperature on the number of the tethered chloromethyl group: time = 75 min; CME (chloromethyl ether)/polymer = 2/1 (by mole); polymer conc. = 15%; ZnCl2 = 5%. The number of chloromethyl group was determined by 1 H NMR.
Fig. 2.
1
H NMR spectra: effect of reaction time on chloromethylation.
cell (BekkTech LLC, Loveland, CO) was used to assemble the membrane test sample. The area resistance of the membrane was determined in de-ionized water at 24 ◦ C. Ionic conductivity, (S/cm), was calculated according to the equation [11]: = l/(RTW), where l = the length of the membrane between two potential sensing platinum wires (cm), R = the membrane resistance (), T = the thickness of the membrane (cm) and W = the width of the membrane (cm). 3. Results and discussion 3.1. Chloromethylation Fig. 2 shows the 1 H NMR spectra for polysulfone and chloromethylated polysulfone as well as the effect of reaction time on chloromethylation. The chemical shifts of the polysulfone were found as follows (ppm): a: 6.90–7.86 (multi Hs on phenyl groups) and b: 1.72 (CH3 ). The chemical shifts of the chloromethylated polysulfone are listed below (ppm): a: 6.85–7.86 (multi Hs on phenyl groups), b: 4.56 (CH2 Cl) and c: 1.72 (CH3 ). The characteristic chemical shift at 4.56 confirmed the formation of the chloromethylated
polysulfone. The effect of reaction time on chloromethylation will be discussed below. The most important step in preparing AAEMs is chloromethylation, because the chloromethylated polymer is readily further modified due to the high reactivity of the tethered chloromethyl group [18]. The successful chloromethylation also determines how many functional groups on the polymer chain can be directly quaternized to further transfer to conductive hydroxyl groups, which in turn determines the ionic conductivity. On one hand, we found that both reaction temperature and time could significantly affect the chloromethylation reaction and the number of the tethered chloromethyl group. On the other hand, we noticed that the chloromethylation could easily cause gelation, leading to a lower yield of the chloromethylated polymer. It has been reported that crosslinking often takes place rapidly during chloromethylation of polystyrene resin as the active aromatic ring attacks the chloromethyl group in a Friedel–Crafts alkylation fashion, which in turn causes inter-polymer or intra-polymer crosslinking or gel formation [17,19]. In order to improve chloromethylation without gelation or with less gelation, we studied the effects of both reaction temperature and time on the number of the tethered reactive chloromethyl group. The 1 H NMR spectrum was used as a tool to investigate both effects. Fig. 2 shows a representative diagram containing a set of 1 H NMR spectra for the effect of time on the number of the tethered chloromethyl groups. All the numbers of the chloromethyl groups were obtained by integration of chemical shifts exhibited by chloromethyl and phenyl groups. The following discussion gives the details of the effects of both reaction temperature and time. Reaction temperature is very critical to most chemical reactions. As we know, increasing temperature can either accelerate a reaction by providing sufficient energy to the reactants or it can initiate some unnecessary side reactions that can often be avoided at normal or low temperatures. As shown in Fig. 3, increasing the temperature increased the number of the tethered chloromethyl group. However, when the temperature reached 60 ◦ C, the gelation occurred (Table 1) and 10% of the polymers became the cross-linked gel. The formed gel is not useful for further quaternization and membrane preparation. Fig. 4A shows the chemical scheme for gel formation. At 75 and 85 ◦ C, the gels increased to 35 and 90%, respectively, although the number of the tethered chloromethyl group also significantly increased to 1.65 and 1.98 (close to the theoretical maximum value). Based on both yield and number of the tethered chloromethyl group, we believe that 75 ◦ C might be
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Table 1 Effects of reaction temperature and time on the chloromethylation of the polysulfone. Temperature (◦ C)
Time (min)
Tethered CH2 Cla
Gelationb
23 40 60 75 85 75 75 75 75 75
75 75 75 75 75 15 30 45 60 90
0.49 0.69 1.41 1.65 1.98 0.19 0.38 0.73 1.00 1.00
No No 10% 35% 90% No No No 15% 85%
a b
The number of chloromethyl group was determined by 1 H NMR. Gelation was determined by simple weighing.
the best temperature for chloromethylation of this specific polysulfone. Reaction time is another important factor to many chemical reactions. From Fig. 5, with increasing reaction time, the number of the tethered chloromethyl group increased, indicating that the longer the reaction the more the chloromethyl group would be tethered. However, when the reaction time reached 45 min, the reaction seemed to reach to a plateau and the increase of the number of the tethered chloromethyl groups slowed down. There was no significant difference in the number of the tethered chloromethyl groups (1.56, 1.62, 1.65 and 1.69) among 45, 60, 75 and 90 min, respectively. On the other hand, longer reaction time increased the probability of gel formation (Table 1). At 75 and 90 min, 35% and 85% of the polymers became the gel. Therefore, 75 min seems the optimal reaction time for the chloromethylation to have the highest number of the tethered chloromethylated group but less gel formation. 3.2. Quaternization Formation of quaternary ammonium hydroxide is necessary for the high ionic conductivity of the AAEM membrane [11] and can increase the stability of the membrane [15]. In this study, we have
Fig. 4. Schematic diagrams for both gel formation during chloromethylation and crosslinking reaction during quaternization: (A) gel formation and (B) crosslinking reaction.
Fig. 5. Effect of reaction time on the number of the tethered chloromethyl group: temperature = 75 ◦ C; CME (chloromethyl ether)/polymer = 2/1 (by mole); polymer conc. = 15%; ZnCl2 = 5%. The number of chloromethyl group was determined by 1 H NMR.
applied three approaches to quaternize the chloromethylated polymer. The ionic conductivity test was used as an evaluation tool. The first approach was to follow the published protocol by simply quaternizing the pre-formed membrane via a tertiary amine, followed by alkalization [4]. This approach was simple but the conductivity was low. Five tertiary amines were studied for quaternization. The ionic conductivity of the membranes treated with these five tertiary amines (see Fig. 6) was in the decreasing order: TMA > DMEA > DMIPA > TMEDA > TEA. These tertiary amines have different alkyl chains attached to the amine core. TMA, TEA, DMEA, and DMIPA have three methyl, three ethyl, two methyl and one ethyl, and two methyl and one isopropyl groups, respectively, attached to the amine core. TMEDA has two methyl groups on each end but once reacting with the chloromethyl group on polysulfone only one end can be further quaternized. By comparing the structures of the tertiary amines with the results for conductivity, it is clear that steric hindrance determines the ease of quaternization. The smaller methyl group made the quaternization the easiest, which allowed the membrane quaternized with TMA to have the highest conductivity. The more methyl groups were attached to the
Fig. 6. Conductivity of the AAEMs quaternized using Approaches 1 and 2: (1) Approach 1: the pre-formed membranes treated directly with different tertiary amines in 400% of the tethered chloromethyl groups (by mole); (2) Approach 2: the membranes cross-linked via TMEDA in 100% of the tethered chloromethyl groups (by mole), followed by treating with different tertiary amines in 400% of the tethered chloromethyl groups. The conductivity was determined in de-ionized water at 24 ◦ C.
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Fig. 7. Conductivity of the AAEMs quaternized using Approach 3: the membranes cross-linked with different amount of TMEDA (33%, 50%, 100% and 200% of the tethered chloromethyl groups), followed by treating with excessive TMEDA and BE, respectively. The conductivity was determined in de-ionized water at 24 ◦ C.
amine, the easier the quaternization would be. That is why TEA was the lowest in conductivity and TMEDA was the second lowest to TEA (Fig. 6). The conductivity was in the range of 2.1–5.2 × 10−3 S/cm. The second approach was to use a difunctional crosslinker, TMEDA, to quaternize the chloromethylated polymer when the polymer was still in solution. As a result, the cross-linked polymer network was initially formed. Fig. 4B shows the chemical scheme for crosslinking reaction produced by TMEDA. Following that, the unreacted chloromethyl groups were further quaternized by a mono- or ditertiary amine in order to convert the chloromethyl groups to the quaternized groups as many as possible. In fact, the ionic conductivity was significantly increased with the values ranging from 4.75 × 10−3 to 1.18 × 10−2 S/cm (Fig. 6). As compared to the conductivity values obtained from the first approach, the values from the second approach increased almost one order. The trend for the decreasing order of the ionic conductivity for different amines was similar to that shown for the first approach and the explanation would be the same. The third approach was to cross-link the polymer and then quaternize the cross-linked membrane via BE. This approach was designed to further quaternize the one-end dimethylamino group pendent on the polymer after the membrane was quaternized and cross-linked with difunctional TMEDA. Via this approach, the ionic conductivity was further increased (see Fig. 7) as compared to the second approach. During the study, we examined the effect of the amount of cross-linker (TMEDA) added. It was found that increasing the initial amount of TMEDA significantly increased the ionic conductivity (1.33–2.8 × 10 −2 S/cm). This may be attributed to the following reasons: (1) the post-quaternization with BE might not be as easy as the pre-quaternization with TMEDA, because BE contains an ethyl but not a methyl group. After crosslinking, the steric hindrance may make the quaternization by BE more difficult; (2) TMEDA not only plays a role in polymer crosslinking but also acts as a chemical for quaternization. Because the crosslinking occurred at the very beginning when the polymers were still in solution, the quaternization would be easier. That is why the conductivity at a cross-linker level of 200% (by mole) was the highest (2.8 × 10−2 S/cm). This value was higher than those obtained from the membranes either simply treated with a tertiary amine without cross-linking in Approach 1 or treated with a crosslinker followed by treating with a tertiary amine in Approach 2. The results indicate that both Approach 2 and Approach 3 are better approaches than Approach 1. It appears that using an appropriate quaternization can significantly increase the ionic conductivity of the AAEM. Komkova et al. [16] and Hao et al. [17] also studied
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Fig. 8. Effect of the thickness of the formed AAEM on conductivity: the conductivity was determined in de-ionized water at 24 ◦ C.
the quaternization by treating the membranes with excess different tertiary diamines. However, in addition to adding excess TMEDA diamine, we modified the quaternization by adding BE to further quaternize the remaining tertiary amine groups on attached TMEDA. Although it was hard for us to tell how many remaining tertiary amine groups were converted, by comparing with Approach 2 (Fig. 6) where excess TMEDA was used, the significantly increased conductivities demonstrated in Approach 3 (see Fig. 7) suggest that further quaternization is necessary for improved conductivity. 3.3. Effect of thickness, temperature and KOH on conductivity of the formed membrane Membrane thickness is important to the performance of the AAEM. Generally speaking, a thinner membrane provides a lower resistance of the membrane, whereas a thicker membrane gives a better mechanical stability of the membrane. In order to examine if the thickness would affect the conductivity, we tested the ionic conductivity of the membranes with thickness of approximately 28, 57, 109 and 143 m. The result in Fig. 8 showed that no significant differences in ionic conductivity were found among the membranes with different thickness, meaning that both quaternization and alkalization were relatively uniform throughout each membrane. Fig. 9 shows the effect of temperature on the ionic conductivity of the formed membrane treated with TMEDA/BE. To examine the effect of temperature, the quaternized membrane was treated with 1.0 M KOH at room temperature for 24 h, followed by washing with de-ionized water numerous times and soaking in de-ionized water for 24 h. After complete removal of the free KOH, the conductivity of the membrane was measured at elevated temperatures. The measured conductivity showed a linear increase with increasing temperatures (see Fig. 8). The conductivity values increased from 3.11 to 7.33 × 10−2 S/cm, corresponding to the temperature from 25 to 95 ◦ C. In general, conductivity increases with increasing temperature for ionic conductive materials [4]. Fig. 10 shows the effects of alkaline concentration on the ionic conductivity of the membrane treated with TMEDA/BE at room temperature. To examine the chemical stability of the formed membrane at different concentrations of KOH, we conditioned the membranes individually in KOH from 1.0 to 8.0 M for 24 h. After the free KOH was completely removed, the conductivity of each membrane was measured. There was hardly a change in conductivity for the membranes treated with different concentrations of KOH solutions. The measured conductivity ranged from 2.65 to 2.95 × 10−2 S/cm after conditioning in 1.0–8.0 M KOH solutions. The
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appropriate quaternization approach could significantly improve the ionic conductivity. The developed AAEM showed the ionic conductivity up to 3.1 × 10−2 S/cm at room temperature. Increasing temperature could increase the ionic conductivity up to 7.33 × 10−2 S/cm. The formed AAEM was stable in a concentrated base up to 8.0 M KOH at room temperature. Future study will include evaluating the other properties of this polysulfone-based AAEM, further optimizing its conductivity, studying its stability in base at higher temperatures and applying it to fuel cells. Acknowledgements This work was sponsored by the Army Research Laboratory (W911NF-07-2-0036) and the Richard G. Lugar Center for Renewable Energy at Indiana University-Purdue University Indianapolis. References Fig. 9. Conductivity of the AAEMs with increasing temperature: the conductivity of the membrane was measured in de-ionized water as temperature increased up to 95 ◦ C. The membrane was conditioned in 1.0 M KOH at room temperature for 24 h, followed by complete removal of free KOH prior to conductivity testing.
Fig. 10. Effect of KOH concentration on conductivity of the AAEMs: the conductivity of the membrane was measured after the membrane was conditioned in 1.0–8.0 M KOH at room temperature for 24 h followed by complete removal of free KOH. The conductivity was measured in de-ionized water at 24 ◦ C.
result indicates that the membrane is quite stable at room temperature even when treated with a KOH concentration up to 8.0 M. 4. Conclusions A polysulfone-based alkaline anion exchange membrane with improved ionic conductivity has been developed. The effects of temperature and time on chloromethylation of the polymer were investigated and the chloromethylation was optimized. Furthermore, different approaches for quaternization of the synthesized chloromethylated polymer were studied. The results show that both temperature and time exhibited significant impacts on chloromethylation and gelation. It was also found that using an
[1] J.R. Varcoe, R.C.T. Slade, Prospects for alkaline anion-exchange membranes in low temperature fuel cells, Fuel Cells 5 (2005) 187–200. [2] L.J.M.J. Blomen, M.N. Mugerwa, Fuel Cell Systems, Plenum Press, New York, NY, 1993. [3] Y. Wan, K.A.M. Creber, B. Peppley, V.T. Bui, Chitosan-based electrolyte composite membranes: II. Mechanical properties and ionic conductivity, J. Membr. Sci. 284 (2006) 331. [4] J. Fang, P.K. Shen, Quaternized poly(phthalazinon ether sulfone ketone) membrane for anion exchange membrane fuel cells, J. Membr. Sci. 285 (2006) 317. [5] L. Li, J. Wang, Quaternized polyethersulfone Cardo anion exchange membranes for direct methanol alkaline fuel cells, J. Membr. Sci. 262 (2005) 1. [6] F. Yi, X. Yang, Y. Li, Synthesis and ion conductivity of poly(oxyethylene) methacrylates containing a quaternary ammonium group, Polym. Adv. Technol. 10 (1999) 473. [7] R.C.T. Slade, J.R. Varcoe, Investigations of conductivity in FEP-based radiationgrafted alkaline anion-exchange membranes, Solid State Ionics 176 (2005) 585. [8] J.-J. Kang, W.-Y. Li, Y. Lin, X.-P. Li, X.-R. Xiao, S.-B. Fang, Synthesis and ionic conductivity of a polysiloxane containing quaternary ammonium groups, Polym. Adv. Technol. 15 (2004) 61. [9] T.N. Danks, R.C.T. Slade, J.R. Varcoe, Alkaline anion-exchange radiation-grafted membranes for possible electrochemical application in fuel cells, J. Mater. Chem. 13 (2003) 712–721. [10] G.-J. Hwang, H. Ohya, Preparation of anion-exchange membrane based on block copolymers: Part 1. Quaternization of the chloromethylated copolymers, J. Membr. Sci. 140 (1998) 195–203. [11] J.-S. Park, G.-G. Park, S.-H. Park, Y.-G. Yoon, C.-S. Kim, W.-Y. Lee, Development of solid-state alkaline electrolytes for solid alkaline fuel cells, Macromol. Symp. 249–250 (2007) 174–182. [12] H.G. Elias, Macromolecules, vol. 2: Industrial Polymers and Syntheses, John Wiley & Sons Inc., New York, NY, 2005. [13] R.-Q. Fu, D. Julius, L. Hong, J.-Y. Lee, PPO-based acid–base polymer blend membranes for direct methanol fuel cells, J. Membr. Sci. 322 (2008) 331–338. [14] E. Avram, M.A. Brebu, A. Warshawsky, C. Vasile, Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones, Polym. Degrad. Stab. 69 (2000) 175–181. [15] T. Sata, K. Teshima, T. Yamaguchi, Permselectivity between two anions in anion exchange membranes crosslinked with various diamines in electrodialysis, J. Polym. Sci., Part A 34 (1996) 1475–1482. [16] E.N. Komkova, D.F. Stamatialis, H. Strathmann, M. Wessling, Anion-exchange membranes containing diamines: preparation and stability in alkaline solution, J. Membr. Sci. 244 (2004) 25–34. [17] J.H. Hao, C. Chen, L. Li, L. Yu, W. Jiang, Preparation of solvent-resistant anionexchange membranes, Desalination 129 (2000) 15–22. [18] G.A. Olah, D.A. Beal, S.H. Lu, J.A. Olah, Synthetic methods and reactions. X1 . 1-chloro-4-chloro(bromo)methoxybutane and 1,4-bis-[chloro(bromo)methoxy]butane: new convenient halomethylating agents, Synthesis (1974) 560. [19] R.W. Lenz, Organic Chemistry of High Polymers, Interscience Publishers, New York, NY, 1967.