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Synthesis and characterization of a fluorinated cross-linked anion exchange membrane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 0 4 5 e1 1 0 5 2
Available online at www.sciencedirect.com
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Synthesis and characterization of a fluorinated cross-linked anion exchange membrane Wenpin Wang a,b, Shubo Wang b, Weiwei Li b, Xiaofeng Xie b,*, Yafei lv a,** a b
College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
A fluorinated poly(aryl ether oxadiazole)s (FPAEO) was prepared from 2,5-bis(2,3,4,5,6-
Received 5 December 2012
pentafluorophenyl)-1,3,4-oxadiazole
Received in revised form
(TM-BPA) and 4,40 -(hexafluoroisopropylidene)diphenol (6F-BPA). The fluorinated cross-linked
28 March 2013
anion exchange membrane (FCAEM) was synthesized by bromination of FPAEO, quaterni-
Accepted 31 March 2013
zation with 1emethyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-
Available online 12 July 2013
hexanediamine (TMHDA). Electrochemical characteristics, physical characteristics and
(FPOx),
4,40 -isopropylidenebis(2,6-dimethylphenol)
mechanical properties of FCAEM were studied. It exhibited higher conductivity, better meKeywords:
chanical property, and dimensional stability. It showed the hydroxide ion conductivity about
Anion exchange membranes
1.7 102 S/cm, tensile strength about 28.02 MPa, water uptake about 51% and swelling ratio
Alkaline electrolyte fuel cell
about 6% at 20 C. The cross-linked anion exchange membrane could meet the requirement
Imidazolium salts
for alkaline electrolyte membrane fuel cells (AEMFCs).
Fluorinated cross-linked
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Anion exchange membrane fuel cells (AEMFCs) are promising energy conversion devices for stationary and automotive applications because of their integration the advantage of traditional alkaline fuel cells (AFCs) and proton exchange membrane fuel cells (PEMFCs) [1]. Under alkaline conditions, the transfer direction of the hydroxyl groups is different from the Hþ. Due to the influence of the Coulomb interaction, it not only reduce the shortcomings of low power generation efficiency of membrane permeation, but also to improve the reaction [2,3]. The electrode kinetics in AEMFCs is much faster than that in PEMFCs, and non-precious metals can be used in AEMFCs [4,5]. Moreover, they have the ability to provide reduced fuel crossover and great fuels diversity [6] (Schemes 1 and 2).
Anion exchange membranes (AEMs), as a crucial component of AEMFCs, have the duty to transport the hydroxide ion and separate fuel/oxidant simultaneously. The research and development of high-performance AEMs has been receiving more and more recognition in practical application. High conductivity, good stability and good mechanical strength are needed in AEMs. In general, quaternary ammonium or quaternary-phosphonium functional groups are introduced to high-performance engineering polymers for AEMs materials prepared [7e9]. The nucleophilicesubstitution reactions between halogen methyl (chloromethyl or bromomethyl) group and tertiary-amine or tertiary-phosphine molecules have been chosen because those nucleophilicesubstitution reactions have very high reactivity. Now, many kinds of AEMs have been prepared, such as quaternized high-performance
* Corresponding author. A316, INET, Tsinghua University, Beijing, China. Tel./fax: þ86 10 6278 4827. ** Corresponding author. E-mail addresses: [email protected] (X. Xie), [email protected] (Y. lv). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.166
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Scheme 1 e Synthetic route of FPAEO-x, FPAEOBr-x and L-FPAEO-x-MIM (x [ 50%, 75%).
commercial or synthesized polymers (polysulfone [10,11], poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [12,13], polyepichlorohydrin [14], poly(phenylene) [15], poly(aryl ether oxadiazole) [8], etc) and radiation-grafted fluorinated polymers (poly(vinylidene fluoride), poly(ethylene-co-tetrafluoro ethylene) and poly(hexafluoropropylene-co-tetrafluoro ethylene)) [16e18]. Many studies have revealed that the AEMs with quaternary ammonium functional groups showed low conductivity and poor stabilities [19e22]. Compared with PEMs, the problem existing in AEMs is the low conductivity. A direct approach to solve this problem is to increase the functional groups content (typically, quaternary ammonium). But, the membrane with such high functional groups concentrations would suffer from significant swelling. Because of that, the performance of some novel anion exchange membranes based on quaternary ammonium or imidazolium salts functional groups [8,23] prepared by our research group could not achieve what we expected. Cross-linking between polymer chains is a good way to minimize that undesirable effect [2,9,24,25]. In this study, a novel fluorinated cross-linked anion exchange membrane (FCAEM) was synthesized by quaternization using 1-methyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA). A systematic study of the low cross-linked membranes was performed to evaluate the physical and electrochemical properties. The TMHDA is acted as the cross-linking agent. It
not only makes the anion exchange membrane having a network structure to enhance the dimensional stability of the membrane, but also provide ion exchange function points. Due to the higher electro-negativity of fluorine, polyfluoroalkyl polymer as a base membrane material significantly improves the overall performance of the membrane. In addition, an imidazole has the five- member ring heteroaryl structure, the chemical stability is better than the quaternary ammonium group; its alkaline is higher than the quaternary ammonium groups. The anion exchange membrane prepared from it has better chemical stability and the higher ion conductivity.
2.
Experimental
2.1.
Materials
2,5-bis(2,3,4,5,6-pentafluorophenyl)-1,3,4-oxadiazole (FPOx) was synthesized as described in other article [26]; 4,40 -isopropylidenebis(2,6-dimethylphenol) (TM-BPA) and 4,40 -(hexafluoroisopropylidene)diphenol (6F-BPA) were purchased from SigmaeAldrich Ltd; N-methyl imidazole and N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA) were purchased from Shanghai Jingchun Reagent Co., Ltd; Potassium carbonate (purchased from Beijing Chemical Reagent Co.) was dried under vacuum at 140 C before use. N-bromosuccinimide
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Scheme 2 e Synthetic route of C-FPAEO-x-MIM (x [ 50%, 75%).
(NBS) and azodiisobutyronitrile (AIBN) (AR) were provided by Beijing Chemical Company. Methanol, dimethylacetamide (DMAc), dimethylformamide (DMF) and 1,2-dichloroethane were obtained commercially and used as received without further purification.
2.2. Synthesis of fluorinated poly (aryl ether oxadiazole)s containing tetramethyl groups (FPAEO-x) By nucleophilic substitution reaction, the polymers with different molar ratio of TM-BPA to 6F-BPA were synthesized. Taking the typical procedure of FPAEO-50 as an example: TMBPA (2 mmol), 6F-BPA (2 mmol), and FPOx (4 mmol) were dissolved in DMAc (26 mL) in a 100 mL round-bottom flask at
0 C. K2CO3 (12 mmol) was added to the solution. The reaction mixture was kept at 0 C with vigorously stirring for 24 h, and then the mixture was diluted with 10 mL of DMAc. The polymer was precipitated in deionized water and washed with hot water several times. The polymer was dried under vacuum with a yield of 91%.
2.3. Bromination of fluorinated poly (aryl ether oxadiazole)s (FPAEOBr-x) A typical procedure for synthesizing the bromomethylated polymer FPAEO-50 (FPAEOBr-50) was as follow. To a 100 mL round-bottom flask equipped with condenser were added FPAEO-50 (2.0 g) and 50 mL of 1,2-dichloroethane. Under
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stirring adequately, the homogeneous solution was formed. NBS (1.11 g, 6 mmol) and AIBN (0.05 g) were added to the solution. Then the solution was heated and kept reflux for 5 h. The mixture was cooled to room temperature and precipitated in methanol with stirring. The polymer was washed with methanol three times and dried under vacuum to get the resulting product.
2.4. Preparation and membrane casting of linear poly (aryl ether oxadiazole)s containing pendant imidazolium salts (L-FPAEO-x-MIM) The following shows a typical procedure for the preparation of imidazolium based-anion exchange membranes (L-FPAEO-xMIM). Taking L-FPAEO-50-MIM for example: FPAEOBr-50 (1.5 g) was dissolved in DMF (20 mL) at room temperature, and Nmethyl imidazole (2.90 mmol) (calculated by the 80% of DF) was added to the solution with stirring. Then the reaction solution was stirred for 4 h at room temperature. L-FPAEO-50MIM membrane was obtained by casting the solution onto a dust-free flat glass plate. The membrane was first dried at 40 C for 24 h and then vacuum-dried at 60 C for 24 h. Then the membrane was peeled off and kept in deionized water before used.
2.5. Preparation and membrane casting of cross-linking poly (aryl ether oxadiazole)s containing pendant imidazolium salts (C-FPAEO-x-MIM) The following represents a typical procedure for the preparation of cross-linking anion exchange membranes (C-FPAEOx-MIM). Taking C-FPAEO-50-MIM membrane as an example: FPAEOBr-50 (1.5 g) was dissolved in DMF (20 mL) at room temperature, and N-methyl imidazole (2.15 mmol) (calculated by the 80% of DF) was added to the solution with stirring. Then the reaction solution was stirred for 4 h at room temperature. There are 0.75 mmol (calculated by the 80% of DF) residual bromomethyl groups after reaction with N-methyl imidazole. Then 0.38 mmol TMHDA was added to the solution. After 1 min of stirring, the solution was dropped onto a clear flat glass dish where the cross-linked reaction occurred. The membrane was first kept at 35 C for 24 h and then vacuumdried at 60 C for 24 h. The membrane was peeled off from the glass dish and kept in deionized water before used. The content of TMHDA could not be increased because the gelation will happen immediately with the increased TMHDA added. Therefore, the membrane we prepared had low crosslinked structure.
2.6.
then the released chloride ion was titrated with 0.05 mol/L AgNO3 solution using K2CrO4 (10%) as the indicator. Water uptakes of L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes were calculated as following: the membranes were first dried under vacuum at 100 C until the weight remained constant. The dried membranes were accurately measured and then immersed into deionized water at giving temperature. After 24 h, the membranes were quickly wiped with absorption paper and weighed again. The water uptake (Wu) of membranes was calculated according to the following equations: Wu ð%Þ ¼
ðWw Wd Þ 100% Wd
where Ww and Wd are the weight of the wet and dry membranes, respectively. The swelling ratio in water at giving temperature was investigated by measuring the change in length of membrane before and after the swelling. The membrane was immersed into deionized water at giving time for 24 h. The swelling ratio (Sr) of membranes was calculated according to the following equations: Sr ð%Þ ¼
ðLw Ld Þ 100% Ld
(3)
where Ld and Lw are the length of membrane before and after the swelling, respectively. The dimensional stability of the membranes was determined by dimensional change ratio according to the method described in Xu [12]. The dimensional stability (h) was calculated by the following equation: h¼
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi x x0 y y0 x0 y0
(4)
where x, y and xo, yo are the lengths of a membrane and a final membrane in x and y directions, respectively.
Measurement
1
H NMR spectra were measured at a Brucker 600 MHz spectrometer using dimethyl sulfoxide-d6 (DMSO-d6) as a solvent. Ion exchange capacity (IEC) of the AEM was measured by Mohr titration. The L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes in Br form were accurately weighed and immersed in 50 mL 0.5 mol/L NaCl solution for 24 h at room temperature to replace Br with Cl, and then washed with adequate deionized water several times. The membranes were immersed in 50 mL 0.1 mol/L NaNO3 aqueous solution,
(2)
Fig. 1 e 1H NMR spectrum of FPAEO-50 and FPAEO-75 in CDCl3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 0 4 5 e1 1 0 5 2
Fig. 2 e 1H NMR spectrum of FPAEOBr-50 and FPAEOBr-75 in CDCl3.
Tensile measurements were determined with a CMT4204 universal testing machines at a speed of 10 mm min1. Hydroxide conductivity (s) of the membrane was obtained by two-point prode electrochemical impedance technique using a Zahner IM6ex electrochemical working station over the frequency ranging from 100 Hz to 3 MHz according to the method described in Hu [23]. The samples were kept in deionized water for 24 h before testing. The conductivity of the membrane was calculated from the measured resistance and sample dimensions as shown in the following equation: s¼
l RS
(5)
where l is the distance between two electrodes, R is the measured resistance of the membrane and S is the crosssectional area of the membrane perpendicular to this flow. The apparent activation energy (Ea) of conductivity was calculated through the linear Arrhenius relationship between ln s and 1000/T: Ea ¼ b R
(6)
where b is the slope of the linear regression of ln s versus 1000/ T plots, R is the gas constant.
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3.
Results and discussion
3.1.
Synthesis and characterization of the polymers
Fluorinated poly (aryl ether oxadiazole)s (FPAEO) with a controlled degree of ph-CH3 is prepared using various feed ratios of TM-BPA/6F-BPA. As shown in Fig. 1, the structure of the FPAEO-50 and FPAEO-75 were identified by the 1H NMR spectrum. The peaks at d 1.62 and d 2.17 were assigned to the chemical shifts of protons on eC (CH3)2e and ph-CH3 respectively. The peaks at d 6.89, 7.04 and 7.41 were attributed to the chemical shifts of AreH at ortho position of methyl and the mand o-protons (relative to eC (CF3)2e), respectively. The FPAEO-50 and FPAEO-75 spectra showed that the intensity ratio of H (ph-CH3) to H (AreH at ortho position of eC (CF3)2e) is close to 3:1 and 9:1, respectively, as expected for the composition of FPAEO. The bromomethylated Fluorinated poly (aryl ether oxadiazole)s (FPAEOBr-x) was prepared using NBS as the bromination regent. Fig. 2 shows the 1H NMR spectrum of FPAEOBrx. The peaks at about d 4.45 were attributed to the chemical shifts of protons on the bromomethyl. The DF values of bromination, where DF represented the degree of functionalization (the amount of bromomethylbenzyl groups in the proportions of benzyl methyl), were in range of 78e85%. In the membrane preparation, the 80% of DF was considered.
3.2.
IEC, water uptake and swelling ratio
The IEC value, water uptake and swelling ratio of the L-FPAEOx-MIM and C-FPAEO-x-MIM membranes are listed in Table 1. In general, the properties of AEMs, such as hydroxide conductivity, water uptake and swelling ratio, are dominated by ion exchange capacity (IEC) [27]. The IEC value was measured by titration method. In the experiment, benzyl methyl groups were converted to bromomethylbenzyl groups as much as possible. The IEC value was mainly controlled by the amount of ph-CH3 in FPAEO. The IEC value was controlled at about 1.30 and 1.80 mequiv g1. The water uptake and swelling ratio are important parameters that relate to the hydroxide conductivity and mechanical property of AEMs. Because water molecules in membrane facilitate hydroxide transport, higher water uptake leads to higher hydroxide conductivity. However, excessive water uptake in AEMs will result in unacceptable change in dimension and the loss of mechanical property. In general, the membranes which have the relatively similar IEC have similar water uptake. As shown in Table 1, after amination-
Table 1 e Characteristics of L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes. Membranes
L-FPAEO-50-MIM C-FPAEO-50-MIM L-FPAEO-75-MIM C-FPAEO-75-MIM
IEC (mequiv g1)
1.32 1.36 1.80 1.80
Water uptake (%)
Swelling ratio (%)
20 C
60 C
20 C
60 C
9 8 64 51
11 10 95 76
2 2 13 6
5 3 55 20
Conductivity (S cm1)
0.54 0.65 1.70 1.69
102 102 102 102
Ea (kJ mol1)
21.14 18.35 10.69 11.28
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show lower dimensional stability (h) than the cross-linked membranes at the same condition. Especially in higher temperature, the dimensional stability of the cross-linked membranes will be much better. It is because the more compact network structures were formed in the membranes, which reduced the free volume and improved the dimensional stability.
3.4.
Fig. 3 e Effect of temperature on dimensional change ratio of the L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes.
cross-linking treatment, the water uptake of C-FPAEO-x-MIM membrane decreased at some extent when the IEC almost remained unchanged. Besides, as expected, the water uptake and swelling ratio of FPAEO-x-MIM membranes increased with increasing temperature, and the temperature had much greater effect on the water uptake and swelling ratio of LFPAEO-x-MIM membranes than those C-FPAEO-x-MIM membranes. The explanation is that cross-linking increased the interaction of the polymers and decreased the chains mobility. The amination-cross-linked membranes showed reduced water uptake and decreased swelling ratio when the IEC value of cross-linked membranes was almost the same as that of uncross-linked membranes.
3.3.
Dimensional stability
The dimensional stability of AEMs is generally known to have profound effect on mechanical properties. The membrane stability depends on its dimensional change. In applications, a membrane with small dimensional change and high mechanical strength is greatly desired. Fig. 3 demonstrates the effect of temperature on linear dimensional change of the LFPAEO-x-MIM and C-FPAEO-x-MIM membranes. The dimensional stability for all the membranes is decreased with the increasing of IEC and temperature. The linear membranes
Mechanical properties
The mechanical properties of the linear and cross-linked membranes are shown in Table 2. The linear membranes (LFPAEO-50-MIM and L-FPAEO-75-MIM) had tensile strength at maximum load of 26.56 and 17.39 MPa respectively. The crosslinked membranes (C-FPAEO-50-MIM and C-FPAEO-75-MIM) showed slightly improved tensile strength as compared to their corresponding linear membranes. The effect is not very significant because the degree of cross-linking is low, but it is meaningful to maintain the membrane stability. These results indicated that all the membranes were strong and tough enough for potential use as AEMs.
3.5.
Hydroxide conductivities
The hydroxide conductivity of the membrane is particularly important for fuel cells, and higher hydroxide conductivity contributes to higher power density. The hydroxide conductivities of the linear membranes and the cross-linked membranes in water as a function of temperature are listed in Fig. 4. Not surprisingly, the hydroxide conductivities of the membranes were increased with IEC and temperature. In general, the conductivity decreases with the degree of crosslinking. However, when the content of TM-BPA in FPAEO (x in FPAEO-x) is the same, there is no significant difference between the linear membrane and the cross-linked membrane. The reason is that TMHDA as cross-linking agent reacted with AreCH2Br not only cross-linked the linear polymers but also formed quaternary ammonium salt. After crosslinked, IEC remained nearly unchanged. The network
Table 2 e Mechanical properties of the L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes. Sample
L-FPAEO-50-MIM C-FPAEO-50-MIM L-FPAEO-75-MIM C-FPAEO-75-MIM
Tensile strength (MPa)
Tensile modulus (Gpa)
Elongation at break (%)
26.56 28.02 17.39 21.79
0.25 0.12 0.60 0.25
22.97 13.11 17.31 14.00
Fig. 4 e Conductivity curves of the L-FPAEO-x-MIM and CFPAEO-x-MIM membranes at different temperature.
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strength is 28.02 MPa, water uptake is 51% and swelling ratio is 6% at 20 C. The cross-linked membrane is considered to be one of the most promising candidates as AEM for AEMFCs.
Acknowledgments This work was supported by National Basic Research Program of China (973 Program) 2012CB215500, the National High Technology R&D Program of China (2012AA053401) and the National Natural Science Foundation of China (51003053).
references
Fig. 5 e Arrhenius plots for the L-FPAEO-x-MIM and CFPAEO-x-MIM membranes.
structures of the cross-linked membranes did not hinder the ion transport because the degree of cross-linking was low. Only at higher degrees of cross-linking, the diminished conductivity will be appeared because cross-linking may reduce the ion mobility [12]. The conductivity of C-FPAEO-75-MIM membrane showed a value of 1.69 102 S/cm at 20 C and 3.61 102 S/cm at 80 C. The slightly cross-linked network structure formed not only does not limit the ion transmission capacity of the anion exchange membrane, and but also maintain the higher ionic conductivity. Fig. 5 shows the relation between ln s and 1000/T, which determinates the ion transport activation energy (Ea) of the membranes using the Arrhenius equation. The calculated Ea values are listed in Table 1. The prepared membranes showed Ea in the range of 10.69e21.14 kJ mol1. In general, the values of Ea reduce with the increasing of IEC and the hydroxide conductivity. Because the cross-linking degree of the membranes prepared was low, the cross-linked structures had little effect on Ea. The Ea value of C-FPAEO-75-MIM membrane is 11.28 kJ mol1. The membrane with lower Ea indicates that ion transport of the membrane needs lower energy.
4.
Conclusions
A novel fluorinated cross-linked anion exchange membrane (FCAEM) based on imidazolium salts was designed and synthesized via quaternization with N-methyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA). The cross-linked membrane structures were formed by precise control of the amount of imidazolium groups and the cross-linker ratios. These cross-linked membranes were tested as AEMs and compared to their corresponding linear membranes. The results revealed that the membrane (C-FPAEO-75-MIM) exhibited higher conductivity, better mechanical property, and dimensional stability. Its hydroxide ion conductivity is about 1.7 102 S/cm, tensile
[1] Pan J, Lu S, Li Y, Huang A, Zhuang L, Lu J. High-performance alkaline polymer electrolyte for fuel cell applications. Advanced Functional Materials 2010;20:312e9. [2] Robertson Nicholas J, Kostalik IV Henry A, Clark Timothy J, Mutolo Paul F, Abrun˜a He´ctor D, Coates Geoffrey W. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. Journal of the American Chemical Society 2010;132:3400e4. [3] Guo M, Fang J, Xu H, Li W, Lu X, Lan C, et al. Synthesis and characterization of novel anion exchange membranes based on imidazolium-type ionic liquid for alkaline fuel cells. Journal of Membrane Science 2010;362:97e104. [4] Wu Y, Wu C, Xu T, Yu F, Fu Y. Novel anion-exchange organicinorganic hybrid membranes: preparation and characterizations for potential use in fuel cells. Journal of Membrane Science 2008;321:299e308. [5] Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proceedings of the National Academy of Sciences 2008;105:20611e4. [6] Varcoe JR, Slade RCT, Wright GL, Chen Y. Steady-state dc and impedance investigations of H2/O2 alkaline membrane fuel cells with commercial Pt/C, Ag/C, and Au/C cathodes. Journal of Physical Chemistry B 2006;110:21041e9. [7] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angewandte Chemie International Edition 2009;48:6499e502. [8] Liu G, Shang Y, Xie X, Wang S, Wang J, Wang Y, et al. Synthesis and characterization of anion exchange membranes for alkaline direct methanol fuel cells. International Journal of Hydrogen Energy 2012;37: 848e53. [9] Wang J, Wang J, Zhang S. Synthesis and characterization of cross-linked poly (arylene ether ketone) containing pendant quaternary ammonium groups for anion-exchange membranes. Journal of Membrane Science 2012;415, 416:205e12. [10] Gu S, Cai R, Luo T, Jensen K, Contreras C, Yan Y. Quaternary phosphonium-based polymers as hydroxide exchange membranes. ChemSusChem 2010;3:555e8. [11] Wang G, Weng Y, Chu D, Chen R, Xie D. Developing a polysulfone-based alkaline anion exchange membrane for improved ionic conductivity. Journal of Membrane Science 2009;332:63e8. [12] Tongwen X, Zhang FF. Fundamental studies on a new series of anion exchange membranes: effect of simultaneous amination-crosslinking processes on membranes ionexchange capacity and dimensional stability. Journal of Membrane Science 2002;199:203e10.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Synthesis and characterization of a fluorinated cross-linked anion exchange membrane Wenpin Wang a,b, Shubo Wang b, Weiwei Li b, Xiaofeng Xie b,*, Yafei lv a,** a b
College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China
article info
abstract
Article history:
A fluorinated poly(aryl ether oxadiazole)s (FPAEO) was prepared from 2,5-bis(2,3,4,5,6-
Received 5 December 2012
pentafluorophenyl)-1,3,4-oxadiazole
Received in revised form
(TM-BPA) and 4,40 -(hexafluoroisopropylidene)diphenol (6F-BPA). The fluorinated cross-linked
28 March 2013
anion exchange membrane (FCAEM) was synthesized by bromination of FPAEO, quaterni-
Accepted 31 March 2013
zation with 1emethyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-
Available online 12 July 2013
hexanediamine (TMHDA). Electrochemical characteristics, physical characteristics and
(FPOx),
4,40 -isopropylidenebis(2,6-dimethylphenol)
mechanical properties of FCAEM were studied. It exhibited higher conductivity, better meKeywords:
chanical property, and dimensional stability. It showed the hydroxide ion conductivity about
Anion exchange membranes
1.7 102 S/cm, tensile strength about 28.02 MPa, water uptake about 51% and swelling ratio
Alkaline electrolyte fuel cell
about 6% at 20 C. The cross-linked anion exchange membrane could meet the requirement
Imidazolium salts
for alkaline electrolyte membrane fuel cells (AEMFCs).
Fluorinated cross-linked
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Anion exchange membrane fuel cells (AEMFCs) are promising energy conversion devices for stationary and automotive applications because of their integration the advantage of traditional alkaline fuel cells (AFCs) and proton exchange membrane fuel cells (PEMFCs) [1]. Under alkaline conditions, the transfer direction of the hydroxyl groups is different from the Hþ. Due to the influence of the Coulomb interaction, it not only reduce the shortcomings of low power generation efficiency of membrane permeation, but also to improve the reaction [2,3]. The electrode kinetics in AEMFCs is much faster than that in PEMFCs, and non-precious metals can be used in AEMFCs [4,5]. Moreover, they have the ability to provide reduced fuel crossover and great fuels diversity [6] (Schemes 1 and 2).
Anion exchange membranes (AEMs), as a crucial component of AEMFCs, have the duty to transport the hydroxide ion and separate fuel/oxidant simultaneously. The research and development of high-performance AEMs has been receiving more and more recognition in practical application. High conductivity, good stability and good mechanical strength are needed in AEMs. In general, quaternary ammonium or quaternary-phosphonium functional groups are introduced to high-performance engineering polymers for AEMs materials prepared [7e9]. The nucleophilicesubstitution reactions between halogen methyl (chloromethyl or bromomethyl) group and tertiary-amine or tertiary-phosphine molecules have been chosen because those nucleophilicesubstitution reactions have very high reactivity. Now, many kinds of AEMs have been prepared, such as quaternized high-performance
* Corresponding author. A316, INET, Tsinghua University, Beijing, China. Tel./fax: þ86 10 6278 4827. ** Corresponding author. E-mail addresses: [email protected] (X. Xie), [email protected] (Y. lv). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.166
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Scheme 1 e Synthetic route of FPAEO-x, FPAEOBr-x and L-FPAEO-x-MIM (x [ 50%, 75%).
commercial or synthesized polymers (polysulfone [10,11], poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [12,13], polyepichlorohydrin [14], poly(phenylene) [15], poly(aryl ether oxadiazole) [8], etc) and radiation-grafted fluorinated polymers (poly(vinylidene fluoride), poly(ethylene-co-tetrafluoro ethylene) and poly(hexafluoropropylene-co-tetrafluoro ethylene)) [16e18]. Many studies have revealed that the AEMs with quaternary ammonium functional groups showed low conductivity and poor stabilities [19e22]. Compared with PEMs, the problem existing in AEMs is the low conductivity. A direct approach to solve this problem is to increase the functional groups content (typically, quaternary ammonium). But, the membrane with such high functional groups concentrations would suffer from significant swelling. Because of that, the performance of some novel anion exchange membranes based on quaternary ammonium or imidazolium salts functional groups [8,23] prepared by our research group could not achieve what we expected. Cross-linking between polymer chains is a good way to minimize that undesirable effect [2,9,24,25]. In this study, a novel fluorinated cross-linked anion exchange membrane (FCAEM) was synthesized by quaternization using 1-methyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA). A systematic study of the low cross-linked membranes was performed to evaluate the physical and electrochemical properties. The TMHDA is acted as the cross-linking agent. It
not only makes the anion exchange membrane having a network structure to enhance the dimensional stability of the membrane, but also provide ion exchange function points. Due to the higher electro-negativity of fluorine, polyfluoroalkyl polymer as a base membrane material significantly improves the overall performance of the membrane. In addition, an imidazole has the five- member ring heteroaryl structure, the chemical stability is better than the quaternary ammonium group; its alkaline is higher than the quaternary ammonium groups. The anion exchange membrane prepared from it has better chemical stability and the higher ion conductivity.
2.
Experimental
2.1.
Materials
2,5-bis(2,3,4,5,6-pentafluorophenyl)-1,3,4-oxadiazole (FPOx) was synthesized as described in other article [26]; 4,40 -isopropylidenebis(2,6-dimethylphenol) (TM-BPA) and 4,40 -(hexafluoroisopropylidene)diphenol (6F-BPA) were purchased from SigmaeAldrich Ltd; N-methyl imidazole and N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA) were purchased from Shanghai Jingchun Reagent Co., Ltd; Potassium carbonate (purchased from Beijing Chemical Reagent Co.) was dried under vacuum at 140 C before use. N-bromosuccinimide
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Scheme 2 e Synthetic route of C-FPAEO-x-MIM (x [ 50%, 75%).
(NBS) and azodiisobutyronitrile (AIBN) (AR) were provided by Beijing Chemical Company. Methanol, dimethylacetamide (DMAc), dimethylformamide (DMF) and 1,2-dichloroethane were obtained commercially and used as received without further purification.
2.2. Synthesis of fluorinated poly (aryl ether oxadiazole)s containing tetramethyl groups (FPAEO-x) By nucleophilic substitution reaction, the polymers with different molar ratio of TM-BPA to 6F-BPA were synthesized. Taking the typical procedure of FPAEO-50 as an example: TMBPA (2 mmol), 6F-BPA (2 mmol), and FPOx (4 mmol) were dissolved in DMAc (26 mL) in a 100 mL round-bottom flask at
0 C. K2CO3 (12 mmol) was added to the solution. The reaction mixture was kept at 0 C with vigorously stirring for 24 h, and then the mixture was diluted with 10 mL of DMAc. The polymer was precipitated in deionized water and washed with hot water several times. The polymer was dried under vacuum with a yield of 91%.
2.3. Bromination of fluorinated poly (aryl ether oxadiazole)s (FPAEOBr-x) A typical procedure for synthesizing the bromomethylated polymer FPAEO-50 (FPAEOBr-50) was as follow. To a 100 mL round-bottom flask equipped with condenser were added FPAEO-50 (2.0 g) and 50 mL of 1,2-dichloroethane. Under
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stirring adequately, the homogeneous solution was formed. NBS (1.11 g, 6 mmol) and AIBN (0.05 g) were added to the solution. Then the solution was heated and kept reflux for 5 h. The mixture was cooled to room temperature and precipitated in methanol with stirring. The polymer was washed with methanol three times and dried under vacuum to get the resulting product.
2.4. Preparation and membrane casting of linear poly (aryl ether oxadiazole)s containing pendant imidazolium salts (L-FPAEO-x-MIM) The following shows a typical procedure for the preparation of imidazolium based-anion exchange membranes (L-FPAEO-xMIM). Taking L-FPAEO-50-MIM for example: FPAEOBr-50 (1.5 g) was dissolved in DMF (20 mL) at room temperature, and Nmethyl imidazole (2.90 mmol) (calculated by the 80% of DF) was added to the solution with stirring. Then the reaction solution was stirred for 4 h at room temperature. L-FPAEO-50MIM membrane was obtained by casting the solution onto a dust-free flat glass plate. The membrane was first dried at 40 C for 24 h and then vacuum-dried at 60 C for 24 h. Then the membrane was peeled off and kept in deionized water before used.
2.5. Preparation and membrane casting of cross-linking poly (aryl ether oxadiazole)s containing pendant imidazolium salts (C-FPAEO-x-MIM) The following represents a typical procedure for the preparation of cross-linking anion exchange membranes (C-FPAEOx-MIM). Taking C-FPAEO-50-MIM membrane as an example: FPAEOBr-50 (1.5 g) was dissolved in DMF (20 mL) at room temperature, and N-methyl imidazole (2.15 mmol) (calculated by the 80% of DF) was added to the solution with stirring. Then the reaction solution was stirred for 4 h at room temperature. There are 0.75 mmol (calculated by the 80% of DF) residual bromomethyl groups after reaction with N-methyl imidazole. Then 0.38 mmol TMHDA was added to the solution. After 1 min of stirring, the solution was dropped onto a clear flat glass dish where the cross-linked reaction occurred. The membrane was first kept at 35 C for 24 h and then vacuumdried at 60 C for 24 h. The membrane was peeled off from the glass dish and kept in deionized water before used. The content of TMHDA could not be increased because the gelation will happen immediately with the increased TMHDA added. Therefore, the membrane we prepared had low crosslinked structure.
2.6.
then the released chloride ion was titrated with 0.05 mol/L AgNO3 solution using K2CrO4 (10%) as the indicator. Water uptakes of L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes were calculated as following: the membranes were first dried under vacuum at 100 C until the weight remained constant. The dried membranes were accurately measured and then immersed into deionized water at giving temperature. After 24 h, the membranes were quickly wiped with absorption paper and weighed again. The water uptake (Wu) of membranes was calculated according to the following equations: Wu ð%Þ ¼
ðWw Wd Þ 100% Wd
where Ww and Wd are the weight of the wet and dry membranes, respectively. The swelling ratio in water at giving temperature was investigated by measuring the change in length of membrane before and after the swelling. The membrane was immersed into deionized water at giving time for 24 h. The swelling ratio (Sr) of membranes was calculated according to the following equations: Sr ð%Þ ¼
ðLw Ld Þ 100% Ld
(3)
where Ld and Lw are the length of membrane before and after the swelling, respectively. The dimensional stability of the membranes was determined by dimensional change ratio according to the method described in Xu [12]. The dimensional stability (h) was calculated by the following equation: h¼
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi x x0 y y0 x0 y0
(4)
where x, y and xo, yo are the lengths of a membrane and a final membrane in x and y directions, respectively.
Measurement
1
H NMR spectra were measured at a Brucker 600 MHz spectrometer using dimethyl sulfoxide-d6 (DMSO-d6) as a solvent. Ion exchange capacity (IEC) of the AEM was measured by Mohr titration. The L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes in Br form were accurately weighed and immersed in 50 mL 0.5 mol/L NaCl solution for 24 h at room temperature to replace Br with Cl, and then washed with adequate deionized water several times. The membranes were immersed in 50 mL 0.1 mol/L NaNO3 aqueous solution,
(2)
Fig. 1 e 1H NMR spectrum of FPAEO-50 and FPAEO-75 in CDCl3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 0 4 5 e1 1 0 5 2
Fig. 2 e 1H NMR spectrum of FPAEOBr-50 and FPAEOBr-75 in CDCl3.
Tensile measurements were determined with a CMT4204 universal testing machines at a speed of 10 mm min1. Hydroxide conductivity (s) of the membrane was obtained by two-point prode electrochemical impedance technique using a Zahner IM6ex electrochemical working station over the frequency ranging from 100 Hz to 3 MHz according to the method described in Hu [23]. The samples were kept in deionized water for 24 h before testing. The conductivity of the membrane was calculated from the measured resistance and sample dimensions as shown in the following equation: s¼
l RS
(5)
where l is the distance between two electrodes, R is the measured resistance of the membrane and S is the crosssectional area of the membrane perpendicular to this flow. The apparent activation energy (Ea) of conductivity was calculated through the linear Arrhenius relationship between ln s and 1000/T: Ea ¼ b R
(6)
where b is the slope of the linear regression of ln s versus 1000/ T plots, R is the gas constant.
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3.
Results and discussion
3.1.
Synthesis and characterization of the polymers
Fluorinated poly (aryl ether oxadiazole)s (FPAEO) with a controlled degree of ph-CH3 is prepared using various feed ratios of TM-BPA/6F-BPA. As shown in Fig. 1, the structure of the FPAEO-50 and FPAEO-75 were identified by the 1H NMR spectrum. The peaks at d 1.62 and d 2.17 were assigned to the chemical shifts of protons on eC (CH3)2e and ph-CH3 respectively. The peaks at d 6.89, 7.04 and 7.41 were attributed to the chemical shifts of AreH at ortho position of methyl and the mand o-protons (relative to eC (CF3)2e), respectively. The FPAEO-50 and FPAEO-75 spectra showed that the intensity ratio of H (ph-CH3) to H (AreH at ortho position of eC (CF3)2e) is close to 3:1 and 9:1, respectively, as expected for the composition of FPAEO. The bromomethylated Fluorinated poly (aryl ether oxadiazole)s (FPAEOBr-x) was prepared using NBS as the bromination regent. Fig. 2 shows the 1H NMR spectrum of FPAEOBrx. The peaks at about d 4.45 were attributed to the chemical shifts of protons on the bromomethyl. The DF values of bromination, where DF represented the degree of functionalization (the amount of bromomethylbenzyl groups in the proportions of benzyl methyl), were in range of 78e85%. In the membrane preparation, the 80% of DF was considered.
3.2.
IEC, water uptake and swelling ratio
The IEC value, water uptake and swelling ratio of the L-FPAEOx-MIM and C-FPAEO-x-MIM membranes are listed in Table 1. In general, the properties of AEMs, such as hydroxide conductivity, water uptake and swelling ratio, are dominated by ion exchange capacity (IEC) [27]. The IEC value was measured by titration method. In the experiment, benzyl methyl groups were converted to bromomethylbenzyl groups as much as possible. The IEC value was mainly controlled by the amount of ph-CH3 in FPAEO. The IEC value was controlled at about 1.30 and 1.80 mequiv g1. The water uptake and swelling ratio are important parameters that relate to the hydroxide conductivity and mechanical property of AEMs. Because water molecules in membrane facilitate hydroxide transport, higher water uptake leads to higher hydroxide conductivity. However, excessive water uptake in AEMs will result in unacceptable change in dimension and the loss of mechanical property. In general, the membranes which have the relatively similar IEC have similar water uptake. As shown in Table 1, after amination-
Table 1 e Characteristics of L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes. Membranes
L-FPAEO-50-MIM C-FPAEO-50-MIM L-FPAEO-75-MIM C-FPAEO-75-MIM
IEC (mequiv g1)
1.32 1.36 1.80 1.80
Water uptake (%)
Swelling ratio (%)
20 C
60 C
20 C
60 C
9 8 64 51
11 10 95 76
2 2 13 6
5 3 55 20
Conductivity (S cm1)
0.54 0.65 1.70 1.69
102 102 102 102
Ea (kJ mol1)
21.14 18.35 10.69 11.28
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show lower dimensional stability (h) than the cross-linked membranes at the same condition. Especially in higher temperature, the dimensional stability of the cross-linked membranes will be much better. It is because the more compact network structures were formed in the membranes, which reduced the free volume and improved the dimensional stability.
3.4.
Fig. 3 e Effect of temperature on dimensional change ratio of the L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes.
cross-linking treatment, the water uptake of C-FPAEO-x-MIM membrane decreased at some extent when the IEC almost remained unchanged. Besides, as expected, the water uptake and swelling ratio of FPAEO-x-MIM membranes increased with increasing temperature, and the temperature had much greater effect on the water uptake and swelling ratio of LFPAEO-x-MIM membranes than those C-FPAEO-x-MIM membranes. The explanation is that cross-linking increased the interaction of the polymers and decreased the chains mobility. The amination-cross-linked membranes showed reduced water uptake and decreased swelling ratio when the IEC value of cross-linked membranes was almost the same as that of uncross-linked membranes.
3.3.
Dimensional stability
The dimensional stability of AEMs is generally known to have profound effect on mechanical properties. The membrane stability depends on its dimensional change. In applications, a membrane with small dimensional change and high mechanical strength is greatly desired. Fig. 3 demonstrates the effect of temperature on linear dimensional change of the LFPAEO-x-MIM and C-FPAEO-x-MIM membranes. The dimensional stability for all the membranes is decreased with the increasing of IEC and temperature. The linear membranes
Mechanical properties
The mechanical properties of the linear and cross-linked membranes are shown in Table 2. The linear membranes (LFPAEO-50-MIM and L-FPAEO-75-MIM) had tensile strength at maximum load of 26.56 and 17.39 MPa respectively. The crosslinked membranes (C-FPAEO-50-MIM and C-FPAEO-75-MIM) showed slightly improved tensile strength as compared to their corresponding linear membranes. The effect is not very significant because the degree of cross-linking is low, but it is meaningful to maintain the membrane stability. These results indicated that all the membranes were strong and tough enough for potential use as AEMs.
3.5.
Hydroxide conductivities
The hydroxide conductivity of the membrane is particularly important for fuel cells, and higher hydroxide conductivity contributes to higher power density. The hydroxide conductivities of the linear membranes and the cross-linked membranes in water as a function of temperature are listed in Fig. 4. Not surprisingly, the hydroxide conductivities of the membranes were increased with IEC and temperature. In general, the conductivity decreases with the degree of crosslinking. However, when the content of TM-BPA in FPAEO (x in FPAEO-x) is the same, there is no significant difference between the linear membrane and the cross-linked membrane. The reason is that TMHDA as cross-linking agent reacted with AreCH2Br not only cross-linked the linear polymers but also formed quaternary ammonium salt. After crosslinked, IEC remained nearly unchanged. The network
Table 2 e Mechanical properties of the L-FPAEO-x-MIM and C-FPAEO-x-MIM membranes. Sample
L-FPAEO-50-MIM C-FPAEO-50-MIM L-FPAEO-75-MIM C-FPAEO-75-MIM
Tensile strength (MPa)
Tensile modulus (Gpa)
Elongation at break (%)
26.56 28.02 17.39 21.79
0.25 0.12 0.60 0.25
22.97 13.11 17.31 14.00
Fig. 4 e Conductivity curves of the L-FPAEO-x-MIM and CFPAEO-x-MIM membranes at different temperature.
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strength is 28.02 MPa, water uptake is 51% and swelling ratio is 6% at 20 C. The cross-linked membrane is considered to be one of the most promising candidates as AEM for AEMFCs.
Acknowledgments This work was supported by National Basic Research Program of China (973 Program) 2012CB215500, the National High Technology R&D Program of China (2012AA053401) and the National Natural Science Foundation of China (51003053).
references
Fig. 5 e Arrhenius plots for the L-FPAEO-x-MIM and CFPAEO-x-MIM membranes.
structures of the cross-linked membranes did not hinder the ion transport because the degree of cross-linking was low. Only at higher degrees of cross-linking, the diminished conductivity will be appeared because cross-linking may reduce the ion mobility [12]. The conductivity of C-FPAEO-75-MIM membrane showed a value of 1.69 102 S/cm at 20 C and 3.61 102 S/cm at 80 C. The slightly cross-linked network structure formed not only does not limit the ion transmission capacity of the anion exchange membrane, and but also maintain the higher ionic conductivity. Fig. 5 shows the relation between ln s and 1000/T, which determinates the ion transport activation energy (Ea) of the membranes using the Arrhenius equation. The calculated Ea values are listed in Table 1. The prepared membranes showed Ea in the range of 10.69e21.14 kJ mol1. In general, the values of Ea reduce with the increasing of IEC and the hydroxide conductivity. Because the cross-linking degree of the membranes prepared was low, the cross-linked structures had little effect on Ea. The Ea value of C-FPAEO-75-MIM membrane is 11.28 kJ mol1. The membrane with lower Ea indicates that ion transport of the membrane needs lower energy.
4.
Conclusions
A novel fluorinated cross-linked anion exchange membrane (FCAEM) based on imidazolium salts was designed and synthesized via quaternization with N-methyl imidazole and cross-linking with N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine (TMHDA). The cross-linked membrane structures were formed by precise control of the amount of imidazolium groups and the cross-linker ratios. These cross-linked membranes were tested as AEMs and compared to their corresponding linear membranes. The results revealed that the membrane (C-FPAEO-75-MIM) exhibited higher conductivity, better mechanical property, and dimensional stability. Its hydroxide ion conductivity is about 1.7 102 S/cm, tensile
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