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Synthesis and characterization of quaternary ammonium functionalized fluorene-containing cardo polymers for potential anion exchange membrane water electrolyzer applications
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Synthesis and characterization of quaternary ammonium functionalized fluorene-containing cardo polymers for potential anion exchange membrane water electrolyzer applications Dongyang Chen a,b, Michael A. Hickner b,**, Shuanjin Wang a, Jingjing Pan a, Min Xiao a, Yuezhong Meng a,* a
The Key Laboratory of Low-carbon Chemistry and Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, PR China b Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
article info
abstract
Article history:
Soluble quaternary ammonium functionalized poly(fluorenyl ether)s (PFEQAs) with a wide
Received 19 June 2012
range of ion exchange capacities (IECs) were successfully synthesized from a novel tertiary
Received in revised form
amine group containing cardo monomer. Complete conversion of tertiary amine group to
30 July 2012
quaternary ammonium group was established, which enables precise control over IEC of
Accepted 11 August 2012
the resultant polymers by adjusting monomer-loading ratio. The influence of IEC on the
Available online 4 September 2012
thermal stability, mechanical integrity, water uptake and ion conductivity of the PFEQAs were investigated. Furthermore, four counter ions were selected and their influences on
Keywords:
the water uptake and ion conductivity of PFEQAs were studied in detail in order to give
Anion exchange membrane
a comprehensive view of the transport properties of these anion exchange membranes. It
Cardo polymer
was observed that the water uptakes of the membranes with different counter ions fol-
Water electrolyzer
while their ion conductivities followed another lowed the trend: OH > SO2 4 > Cl > I trend: OH > SO2 4 > Cl > I . These membranes exhibited promising characteristics for
anion exchange membrane water electrolyzers working with neutral PH water with or without supporting electrolytes. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has long been accepted as environmentally clean energy source, without release of any pollutant or carbon dioxide upon its use. Wide applications have been established, such as ammonia production and petroleum refining, as well as the ever growing H2/O2 or H2/air fuel cells. Hydrogen is
mainly produced by stream reforming of natural gas currently, however, water electrolysis offers a promising alternative approach for hydrogen production for its clean and high efficiency [1e3]. Alkaline water electrolysis using concentrated alkaline aqueous solutions and a thick porous diaphragm had been well investigated and the drawback of alkaline metal precipitation caused by absorption of CO2 was
* Corresponding author. Tel./fax: þ86 20 84114113. ** Corresponding author. Tel.: þ1 814 867 1847; fax: þ1 814 865 2917. E-mail addresses: [email protected] (M.A. Hickner), [email protected] (Y. Meng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.051
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found, which limited the wide deployment of this technology [4,5]. Proton exchange membrane water electrolysis was thus developed, which was free of metal carbonate precipitation and had higher hydrogen production rate [6,7]. Since the membranes were acidic, such as Nafion, only noble metal catalyst could be adopted, which increased the cost of this technology largely. Recently, Hickner et al. [8] and Zhuang et al. [9] had successfully demonstrated anion exchange membrane water electrolysis technologies which were free of metal carbonate precipitation and free of noble metal catalyst, offering promising strategies for low cost and high performance water electrolysis. Anion exchange membrane (AEM) is one of the key components of anion exchange membrane water electrolyzer. Ideally it should feature high anion conductivity, robust mechanical property and high durability. Good solubility is also required if it is going to be used as catalyst binder. Inspired by the understanding in structureemorphologyeperformance relationship of proton exchange membranes (PEMs) over the last decade, worldwide scientists are exploring enthusiastically high performance AEMs by rational molecular design and microscopic phase control for different applications [10e12]. Various cation groups have been reported, such as quaternary ammonium group [13], pyridinium group [14], imidazolium group [15,16], guanidinium group [17], quaternary phosphonium group [18,19], tertiary sulfonium group [20] and so on. Limited data are existed for comparison of the stability of all these cation groups, however, quaternary ammonium group is the commercialized and most studied cation group due to its ease of preparation and relative high stability [11]. At elevated temperature or high PH, quaternary ammonium groups still tend to be degraded mainly through elimination and nucleophilic substitution mechanisms, which could be influenced by their chemical environments [10]. Furthermore, the existence of b-hydrogen can cause Hofmann degradation [10]. Therefore, the searching of highly stable AEMs and their appropriate operation conditions remains challenging. In anion exchange membrane water electrolysis, neutral aqueous solution with or without supporting electrolyte could be used, which lowers the requirement of alkaline stability of AEMs, giving an opportunity to use quaternary ammonium based membranes. This motivates us to investigate quaternary ammonium functionalized AEMs to accumulate fundamental knowledge for water electrolysis. Diverse polymer backbones have been studied as AEM scaffolds such as polystyrene based aliphatic polymers, poly(arylene ether)s and polysilsesquioxanes [21]. The mechanical properties, solubility and thermal stabilities of AEMs can be adjusted by the scaffold design for different potential applications. Poly(arylene ether)s have high thermal stability and robust mechanical strength for high performance PEMs [22,23], suggesting their possible good performance in AEMs. Generally, quaternary ammonium functionalized poly(arylene ether)s can be synthesized by (a) direct polymerization of tertiary amine containing monomer followed by methylation and (b) post-modification of poly(arylene ether)s by halomethylation and then quaternization [24,25]. The direct polymerization method can precisely control the ion exchange capacity (IEC) of the resultant polymers, therefore,
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is a promising approach for AEM research and development. Herein, we reported firstly the synthesis of quaternary ammonium functionalized fluorene-containing cardo poly(arylene ether)s with a wide range of IECs by a direct polymerization method. The cardo backbone was chosen because of its good solubility and film forming ability. We paid special attention to the influences of IEC and counter ions on the water uptakes and ion conductivities of these solution-cast membranes. Their alkaline stabilities were also studied by the change of IEC and intrinsic viscosity in order to explore best working conditions of these membranes for water electrolyzers.
2.
Experimental
2.1.
Materials
9,90 -Bis(4-hydroxyphenyl)fluorene (BPF) was purchased from Tokyo Kasei Kogyo Co. Ltd., Japan (TCI). Decafluorobiphenyl and cesium fluoride were purchased from SigmaeAldrich Co. Ltd. All the other reagents were obtained from commercial ˚ sources. N,N0 -dimethylacetamide (DMAc) was dried with 4 A molecule sieves prior to use. All the other reagents were used as received.
2.2. Synthesis of 9,90 -bis(3-dimethylaminemethylene-4hydroxyphenyl)fluorene (DABPF) The synthetic route is shown in Scheme 1. To a 500 mL oneneck round-bottom flask equipped with a magnetic stirrer, BPF (35.0 g, 0.1 mol) and ethanol (200 mL) were charged. After BPF was dissolved completely, aqueous solutions of dimethylamine (4.0 equiv., 33%) and formaldehyde (3.0 equiv., 37%) were added. The solution was stirred at room temperature (w25 C) for 20 h. White precipitate formed during the course. The crude product was collected by filtration and subjected to recrystallization twice using ethanol and toluene (volume:volume ¼ 1:3). Yield: 47%. 1H NMR (DMSO-d6, ppm): 2.14 (12 H, s), 3.40 (4 H, s), 6.54e6.59 (2 H, d), 6.71e6.75 (2 H, d), 6.87e6.89 (2 H, d), 7.25e7.31 (2 H, m), 7.33e7.38 (4 H, m), 7.85e7.89 (2 H, d).
2.3. Synthesis of poly(fluorene ether)s containing tertiary amine group (PFETAs) and quaternary ammonium group (PFEQAs) The synthetic route of PFETA-x and PFEQA-x (x denotes the molar percentage of DABPF in the total feeding of DABPF and BPF) are depicted in Scheme 2. Typical procedures (PFETA-60 and PFEQA-60) are as follows. To a flame-dried 100 mL three-necked round-bottom flask equipped with nitrogen inlet, condenser and magnetic stirrer, BPF (1.4016 g, 4 mmol), DABPF (2.7876 g, 6 mmol), decafluorobiphenyl (3.3745 g, 10.1 mmol), DMAc (50 mL) and CsF (4.5570 g, 30 mmol) were charged. The mixture was reacted at 80 C for 24 h under the protection of nitrogen. The resulting viscous mixture was poured into 500 mL deionized water to precipitate out the polymer, which was collected by filtration and purified by procedure of dissolving in chloroform,
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Scheme 1 e Synthesis of 9,90 -bis(3-dimethylaminemethylene-4-hydroxyphenyl) fluorene (DABPF).
filtration and precipitating in ethanol twice. PFETA-60 was obtained with 92% yield. To a 50 mL one-necked round-bottom flask equipped with a magnetic stirrer, PFETA-60 (1.0 g, 1.3 mmol) and DMAc (20 mL) were charged. After PFETA-60 was dissolved, iodomethane (0.5 g, 3.2 mmol) was added quickly and the solution was stirred in the dark for 24 h at room temperature. Then the solution was poured into 200 mL deionized water and the precipitate was filtered out and washed with water and acetone thoroughly before being dried at 80 C under vacuum for 24 h. PFEQA-60 was obtained with 90% yield.
2.4.
Membrane preparation
The PFEQAs were dissolved in DMAc at approximately 8 wt/ vol % and cast on glass plates, dried at 80 C for 12 h, and then dried at 120 C under vacuum for 24 h. The membranes (I form) were removed from the casting plate and immersed in deionized water prior to use. To exchange I form to Cl form, SO2 4 form and OH form, the membranes were immersed in 1 M NaCl, 1 M Na2SO4 and 0.5 M NaOH solution for 24 h, respectively, followed by being immersed in deionized water for 24 h with water changed five times and stored in deionized water prior to use.
Scheme 2 e Synthesis of poly(fluorene ether)s containing tertiary amine group (PFETAs) and quaternary ammonium group (PFEQAs).
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2.5.
Characterization
Nuclear magnetic resonance (NMR) spectra were recorded at 400 MHz in a Bruker DRX NMR instrument and the chemical shifts were listed in ppm downfield from tetramethylsilane (TMS). Gel permeation chromatography (GPC) analysis was performed on a Waters Breeze system equipped with a Waters Styragel column, Waters 515 HPLC pump and Waters 2414 refractive index detector, chloroform as an elution solvent at a flow rate of 1 mL min1 and polystyrene as standards for calibration. Thermal stability was analyzed using a PerkinElmer Pyris Diamond TG/DTA analyzer. The temperature was increased from room temperature to 120 C and maintained for half an hour, then increased to 650 C at a heating rate of 10 C min1 under N2 atmosphere. The tensile properties were determined by SANS (Shenzhen, China) electromechanical universal test machine (model CMT-4014). Samples were cut into dumbbell shape and immersed in room temperature deionized water prior to test. The intrinsic viscosities of the samples were measured in 1-methyl-2pyrrolidinone (NMP) containing 0.05 M LiBr at room temperature using an Ubbelohde viscometer. Ion exchange capacities (IECs) were evaluated using OH form membranes. The membranes (w0.3 g) were immersed in 50 mL of 0.1 M HCl standard solutions for 48 h, and then titrated with 0.1 M NaOH solution using phenolphthalein as indicator. The water uptake of the membranes was defined as weight ratio of the absorbed water to that of the dry membrane. Membranes in different counter ion forms were immersed in deionized water for 24 h before taken out to measure their wet masses. The surface water was wiped out by tissue paper. The dry masses were measured after samples were kept in 80 C oven under vacuum overnight. The water uptakes were calculated using the following equation: Wð%Þ ¼
ðMw Md Þ 100% Md
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nitrogen for half an hour prior to measurements. The ion conductivity was calculated according to the following equation: s¼
d RS
(2)
where d and S are the thickness and area of the sample, respectively, and R is derived from the low intersect of the high frequency semicircle on a complex impedance plane with the Re (Z0 ) axis. The alkaline stability was evaluated by immersing OH form membranes in large excess 1 M NaOH solution at 80 C and recording their IEC and viscosity changes versus immersion time.
3.
Results and discussion
3.1.
Synthesis of monomer and polymers
Through Mannich reaction, the novel tertiary amine functionalized monomer 9,90 -bis(3- dimethylaminemethylene-4hydroxyphenyl)fluorene (DABPF) was successfully synthesized as confirmed by 1H NMR and 13C NMR spectra, Fig. 2. The characteristic peaks of dimethylaminemethylene group in DABPF were observed, which were located at 2.14 ppm (eCH2e) and 3.40 ppm (eCH3) in 1H NMR spectrum and 61.00 ppm (eCH2e) and 45.16 ppm (eCH3) in 13C NMR
(1)
where Md and Mw are the weight of membranes before and after water absorption, respectively. Ion conductivities of membrane in the transverse direction were measured by sandwiching membrane samples between two parallel gold electrode plates and recording their resistances using a Solartron 1255 B frequency response analyzer coupled with a Solartron 1287 electrochemical interface in the frequency range of 100 Hz to 1 MHz. The schematic illustration of the cell for ion conductivity measurement is shown in Fig. 1. Samples were kept in deionized water with purging
Fig. 1 e Schematic illustration of the cell for ion conductivity measurement.
Fig. 2 e 1H NMR (a) and DABPF.
13
C NMR (b) spectra of monomer
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spectrum. All the other peaks were well assigned according to its chemical structure, excluding the possibility of monofunctionalization of 9,90 -bis(4-hydroxyphenyl)fluorene (BPF). Condensation copolymerizations of DABPF with BPF and decafluorobiphenyl were carried out in DMAc using CsF as catalyst. High molecular weight PFETAs were obtained as shown in Table 1. The 1H NMR spectrum of PFETA-80 is shown in Fig. 3 with proper assignment of all the resonance peaks. The characteristic peaks of dimethylaminemethylene group were found to be 2.30 ppm (eCH2e) and 3.68 ppm (eCH3). By controlling the loading molar ratio of DABPF, PFETAs with degree of functionality (DF) varying from 0.4 to 1.6 were obtained. Treating PFETAs with excess iodomethane in DMAc solution gave complete conversion of tertiary amine group to quaternary ammonium group, which was evidenced by 1H NMR spectra of PFEQA-x where the characteristic peaks of tertiary amine group disappeared while the characteristic peaks of quaternary ammonium group (4.65 ppm for eCH2e and 3.09 ppm for eCH3) showed up, Fig. 2. PFEQAs with IECs ranging from 0.6 to 2.0 equiv. g1 were obtained with high intrinsic viscosities as shown in Table 2. The IECs of PFEQAs were closely related to DF of PFETAs, demonstrating the possibility and high efficiency of controlling IEC by tuning the loading amount of tertiary amine containing monomer. The solubility of PFEQAs in different solvents was shown in Table 3. It can be found that the PFEQAs had good solubility in aprotic solvents such as N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N0 -dimethylacetamide (DMAc) and N,N0 -dimethylaceformide (DMF). The good solubility was important because the ionomers can form ion aggregation in solution which was the primary driving force of phase separation in the solution-cast membranes. This well-developed phase separation was found to be critically important for high ion conductivity [24]. On the other hand, the good solubility also allowed PFEQAs to be used as catalyst binder for better system optimization.
3.2.
Water uptake and ion conductivities
The interaction of water with polymers has a significant impact on ion conductivities of the corresponding membranes. Basically the ion conductivities increase with the increasing of water uptake within a specific system, unless too much swelling of membrane is resulted where the ion concentration is decreased. In order to give a fundamental
Table 1 e Properties of PFETAs. Sample PFETA-20 PFETA-40 PFETA-60 PFETA-80
DFa Mn (kg mol1) Mw (kg mol1) PDI Tidb ( C) 0.4 0.8 1.2 1.6
48 55 53 49
101 116 111 103
2.1 2.1 2.1 2.1
268 258 249 240
a Degree of functionalization (number of tertiary amine group per repeat unit). b Initial decomposition temperature (determined from TG/DTG curves as the point of deviation from linearity corresponding to the mass loss due to polymer decomposition).
view of the anion conductivities of PFEQAs for anion exchange membrane water electrolysis, four anions were chosen as counter ions for PFEQAs, namely OH, SO2 4 , Cl and I . Not only OH was studied because the water for electrolysis could contain natural or artificially added inorganic salt as supporting electrolyte to lower the resistance of the membrane medium. The water uptakes of PFEQAs with different counter ions at 20 C are shown in Fig. 4. It can be seen that the water uptake increased with the increasing of IEC regardless of the species of counter ion. More hydrophilic polymer will definitely absorb more water. However, the counter ion has obvious influence on water uptake at any given IEC. The trend for membrane water uptake with different counter ions was: OH > SO2 4 > Cl > I . This trend may be the result of different hydrophilicity and solubility of the corresponding ions. The OH form membranes had very strong water absorbance capability, which is in good agreement with literatures [12,26]. The conductivities of the above four counter ions for PFEQAs at 20 C are shown in Fig. 5. The influence of IEC on ion conductivity was similar to its influence on water uptake. All the ion conductivities increased with the increasing of IEC, as expected. This is because of both the higher concentrations of mobile ions and the larger water uptake at higher IECs. Interestingly, the trend of ion conductivity with those four counter ions was different with the trend of their water uptake. For ion conductivities, OH > SO2 4 > Cl > I . The OH conductivities were the highest among these ions investigated and were comparable or higher than the literature values form membranes had higher water uptakes [27e29]. SO2 4 than Cl form membranes, however, had lower conductivities than the Cl counterparts. This is because the Stokes radius ˚ while that of Cl is only 1.21 A ˚ [10]. The is 2.31 A for SO2 4 smaller the Stokes radius of an ion, the more easily it moves. ion There is also a possibility that higher charged SO2 4 decreased its mobility as compared to mono-charged Cl ion. Therefore, Cl conductivity is higher than SO2 4 conductivity at the same IEC. For I form membranes, their conductivities are the lowest among all these four forms membranes, suggesting that their lowest water uptakes play the dominant effect. The temperature dependence of water uptakes and ion conductivities of PFEQAs were investigated using OH form membranes, Fig. 6. It can be seen that water uptake and OH conductivity increased spontaneously with the increasing of temperature. At 80 C, OH conductivity of as high as around 60 mS cm1 was observed for PFEQA-60. In real anion exchange membrane water electrolysis, elevated temperatures are desired in order to increase the reaction rate and energy efficiency. Furthermore, mixed ion conduction may be the case when the water for electrolysis contains natural or artificially added inorganic salts. This section gives a clear picture of how those four counter ions can contribute to the whole conductivity of the membrane. While OH form membrane exhibited the best performance than the other ion forms membranes, it might bring along a stability issue which will be discussed in next section. Therefore, Cl is a promising candidate to act as supporting electrolyte. In fact, water electrolysis using 0.2 M NaCl solution had been reported with
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Fig. 3 e 1H NMR spectra of PFETA-80 and PFEQA-80.
the benefit of low cell resistance [30]. However, detailed study on different ions’ conductivity of the membranes is essentially important for new membrane/system design.
3.3.
Alkaline stabilities
While high OH conductivity could be achieved at high IEC and high temperature, the membrane stability in these
conditions is of great concern. Usually the stability issue involves the degradation of polymer backbone and decomposition of side chain functional groups [14]. In order to discern how much these two factors contribute to the stability problem of our polymers, PFEQA-60 was chosen to be immersed in 1 M NaOH solution at 80 C for certain times and the changes in IEC and intrinsic viscosity were recorded as shown in Fig. 7. It can be seen that the IEC dropped slowly with
Table 2 e Properties of PFEQAs. Sample
IECa (mequiv. g1)
Intrinsic viscosity (g dL1)
Tidb ( C)
Tensile strength (MPa)
Elongation at break (%)
PFEQA-20 PFEQA-40 PFEQA-60 PFEQA-80
0.6 1.1 1.6 2.0
0.64 0.71 0.69 0.65
229 219 211 205
29 25 22 18
62 48 32 26
a Titrated from OH form membranes. b Initial decomposition temperature.
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Table 3 e Solubility of PFEQAs in different solvents. Sample
H2O
H2O þ n-propanol (1:1 in volume)
n-propanol
NMP
DMSO
DMAc
DMF
CH3Cl
PFEQA-20 PFEQA-40 PFEQA-60 PFEQA-80
a
þb þ þ þ
c
þ þ þ þ
þ þ þ þ
þ þ þ þ
þ þ þ þ
a Insoluble. b Soluble. c Partial soluble.
Furthermore, no measurable degradation could be found in 1 M NaCl or Na2SO4 solutions even at 80 C for a week. Therefore, we are exploring these membranes for water electrolysis under neutral PH conditions with added NaCl as supporting electrolyte currently.
3.4.
Fig. 4 e Water uptake of PFEQAs with different counter ions at 20 C.
the immersion time, however, the viscosity decreased significantly. After three days of immersion, the IEC maintained about 75% of its original value while the viscosity was only about 14% left. These results strongly suggest that the degradation of polymer backbone is more significant than the decomposition of side chain quaternary ammonium groups for PFEQA-60. These results proved against the use of OH form membranes under elevated temperatures in basic conditions. Fortunately, the OH form membranes were relatively stable in neutral PH water at room temperature.
Fig. 5 e Different ions’ conductivities of PFEQAs at 20 C.
Thermal stability and mechanical property
The thermal stabilities of PFETAs and PFEQAs were evaluated by thermogravimetric analysis (TGA). The representative TGA curves of PFETA-80 and PFEQA-80 (OH form) are given in Fig. 8. Two-step decomposition can be found for both types of samples. The first step involved the decomposition of tertiary amine group in PFETAs and quaternary ammonium group for PFEQAs while the second step involved the decomposition of polymer backbone [24]. Obviously the tertiary amine group in PFETAs decomposed at higher temperature than quaternary ammonium group in PFEQAs. The decomposition temperatures of polymer backbone were almost the same for both types of samples because of the same backbone structure. The weight loss caused by release of tertiary amine group was very small, lower than 5% for low DF PFETAs. Therefore, the initial decomposition temperatures (Tid) instead of 5% weight loss temperatures were given in order to reveal the more real thermal stabilities. As shown in Table 1, PFETAs were thermally stable up to as high as 240 C. The Tid decreased with the increasing of DF. For PFEQAs, the Tid decreased with the increasing of IEC but still higher than 205 C, demonstrating good thermal stabilities of PFEQAs for water electrolysis applications, Table 2.
Fig. 6 e OHL conductivity and water uptake of PFEQA-60 as a function of temperature.
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OH > SO2 4 > Cl > I while their ion conductivities followed another trend: OH > Cl > SO2 4 > I . All the samples showed high ion conductivity, high thermal stability and robust mechanical strength, but relative low alkaline stability. They are suitable to work with neutral PH water with or without supporting electrolytes. The water electrolyzer evaluation using PFEQAs as both membrane and catalyst binder is ongoing right now, together with the selection of catalyst and system optimization.
Acknowledgments Fig. 7 e IEC and viscosity changes versus immersion time of PFEQA-60 in 1 M NaOH solution at 80 C.
The authors would like to thank the China High-Tech Development 863 Program (Grant No.: 2007AA03Z217), Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No.: 2003C105004, 2006A10704004, 2006B12401006), and Guangzhou Sci & Tech Bureau (2005U13D2031) for financial support of this work.
references
Fig. 8 e TGA curves of PFETA-80 and PFEQA-80 in OHL form.
The mechanical properties of PFEQAs were listed in Table 2. With the increasing of IEC, both the mechanical strength and elongation at break decreased because of the higher water uptake as discussed above. Water acted as plasticizer in the membrane, the same to the cases in proton exchange membranes [22]. Nevertheless, all the samples still had high mechanical strength because of their robust backbone structure.
4.
Conclusions
New tertiary amine functionalized dihydroxyl monomer DABPF was successfully synthesized via Mannich reaction route. Copolymerizations of DABPF with BPF and decafluorobiphenyl yielded quaternary ammonium functionalized poly(fluorenyl ether)s (PFEQAs) with a wide range of IECs, which were predetermined by the loading ratio of DABPF monomer. Membranes based on PFEQAs were investigated for potential anion exchange membrane electrolysis applications. It was found that the water uptakes of PFEQAs with four different counter ions followed the trend:
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long hydrophobic chain targeting at highly proton conductive and stable membranes. J Mater Chem 2011;21: 12068e77. Li N, Lee S, Liu Y, Lee Y, Guiver MD. A new class of highlyconducting polymer electrolyte membranes: aromatic ABA triblock copolymers. Energy Environ Sci 2012;5:5346e55. Wang J, Zhao Z, Gong F, Li S, Zhang S. Synthesis of soluble poly(arylene ether sulfone) ionomers with pendant quaternary ammonium groups for anion exchange membranes. Macromolecules 2009;42:8711e7. Tanaka M, Koike M, Miyatake K, Watanabe M. Synthesis and properties of anion conductive ionomers containing fluorenyl groups for alkaline fuel cell applications. Polym Chem 2011;2:99e106. Hibbs MR, Hickner MA, Alam TM, McIntyre SK, Fujimoto CH, Cornelius CJ. Transport properties of hydroxide and proton conducting membranes. Chem Mater 2008;20:2566e73. Pan J, Lu S, Li Y, Huang A, Zhang L, Lu J. High-performance alkaline polymer electrolyte for fuel cell applications. Adv Funct Mater 2010;20:312e9. Fang J, Shen P. Quaternized poly(phthalazinon ether sulfone ketone) membrane for anion exchange membrane fuel cells. J Membr Sci 2006;285:317e22. Xiong Y, Liu Q, Zeng Q. Quaternized cardo polyetherketone anion exchange membrane for direct methanol alkaline fuel cells. J Power Sources 2009;193:541e6. Chakrabarty T, Jasti A, Goel NK, Shahi VK, Sahharwal S. Comparative studies on performance of radiation-induced and thermal cross-linked ion-exchange membrane for water electrolysis. Radiat Phys Chem 2011;80:803e9.
Available online at www.sciencedirect.com
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Synthesis and characterization of quaternary ammonium functionalized fluorene-containing cardo polymers for potential anion exchange membrane water electrolyzer applications Dongyang Chen a,b, Michael A. Hickner b,**, Shuanjin Wang a, Jingjing Pan a, Min Xiao a, Yuezhong Meng a,* a
The Key Laboratory of Low-carbon Chemistry and Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, PR China b Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
article info
abstract
Article history:
Soluble quaternary ammonium functionalized poly(fluorenyl ether)s (PFEQAs) with a wide
Received 19 June 2012
range of ion exchange capacities (IECs) were successfully synthesized from a novel tertiary
Received in revised form
amine group containing cardo monomer. Complete conversion of tertiary amine group to
30 July 2012
quaternary ammonium group was established, which enables precise control over IEC of
Accepted 11 August 2012
the resultant polymers by adjusting monomer-loading ratio. The influence of IEC on the
Available online 4 September 2012
thermal stability, mechanical integrity, water uptake and ion conductivity of the PFEQAs were investigated. Furthermore, four counter ions were selected and their influences on
Keywords:
the water uptake and ion conductivity of PFEQAs were studied in detail in order to give
Anion exchange membrane
a comprehensive view of the transport properties of these anion exchange membranes. It
Cardo polymer
was observed that the water uptakes of the membranes with different counter ions fol-
Water electrolyzer
while their ion conductivities followed another lowed the trend: OH > SO2 4 > Cl > I trend: OH > SO2 4 > Cl > I . These membranes exhibited promising characteristics for
anion exchange membrane water electrolyzers working with neutral PH water with or without supporting electrolytes. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has long been accepted as environmentally clean energy source, without release of any pollutant or carbon dioxide upon its use. Wide applications have been established, such as ammonia production and petroleum refining, as well as the ever growing H2/O2 or H2/air fuel cells. Hydrogen is
mainly produced by stream reforming of natural gas currently, however, water electrolysis offers a promising alternative approach for hydrogen production for its clean and high efficiency [1e3]. Alkaline water electrolysis using concentrated alkaline aqueous solutions and a thick porous diaphragm had been well investigated and the drawback of alkaline metal precipitation caused by absorption of CO2 was
* Corresponding author. Tel./fax: þ86 20 84114113. ** Corresponding author. Tel.: þ1 814 867 1847; fax: þ1 814 865 2917. E-mail addresses: [email protected] (M.A. Hickner), [email protected] (Y. Meng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.051
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found, which limited the wide deployment of this technology [4,5]. Proton exchange membrane water electrolysis was thus developed, which was free of metal carbonate precipitation and had higher hydrogen production rate [6,7]. Since the membranes were acidic, such as Nafion, only noble metal catalyst could be adopted, which increased the cost of this technology largely. Recently, Hickner et al. [8] and Zhuang et al. [9] had successfully demonstrated anion exchange membrane water electrolysis technologies which were free of metal carbonate precipitation and free of noble metal catalyst, offering promising strategies for low cost and high performance water electrolysis. Anion exchange membrane (AEM) is one of the key components of anion exchange membrane water electrolyzer. Ideally it should feature high anion conductivity, robust mechanical property and high durability. Good solubility is also required if it is going to be used as catalyst binder. Inspired by the understanding in structureemorphologyeperformance relationship of proton exchange membranes (PEMs) over the last decade, worldwide scientists are exploring enthusiastically high performance AEMs by rational molecular design and microscopic phase control for different applications [10e12]. Various cation groups have been reported, such as quaternary ammonium group [13], pyridinium group [14], imidazolium group [15,16], guanidinium group [17], quaternary phosphonium group [18,19], tertiary sulfonium group [20] and so on. Limited data are existed for comparison of the stability of all these cation groups, however, quaternary ammonium group is the commercialized and most studied cation group due to its ease of preparation and relative high stability [11]. At elevated temperature or high PH, quaternary ammonium groups still tend to be degraded mainly through elimination and nucleophilic substitution mechanisms, which could be influenced by their chemical environments [10]. Furthermore, the existence of b-hydrogen can cause Hofmann degradation [10]. Therefore, the searching of highly stable AEMs and their appropriate operation conditions remains challenging. In anion exchange membrane water electrolysis, neutral aqueous solution with or without supporting electrolyte could be used, which lowers the requirement of alkaline stability of AEMs, giving an opportunity to use quaternary ammonium based membranes. This motivates us to investigate quaternary ammonium functionalized AEMs to accumulate fundamental knowledge for water electrolysis. Diverse polymer backbones have been studied as AEM scaffolds such as polystyrene based aliphatic polymers, poly(arylene ether)s and polysilsesquioxanes [21]. The mechanical properties, solubility and thermal stabilities of AEMs can be adjusted by the scaffold design for different potential applications. Poly(arylene ether)s have high thermal stability and robust mechanical strength for high performance PEMs [22,23], suggesting their possible good performance in AEMs. Generally, quaternary ammonium functionalized poly(arylene ether)s can be synthesized by (a) direct polymerization of tertiary amine containing monomer followed by methylation and (b) post-modification of poly(arylene ether)s by halomethylation and then quaternization [24,25]. The direct polymerization method can precisely control the ion exchange capacity (IEC) of the resultant polymers, therefore,
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is a promising approach for AEM research and development. Herein, we reported firstly the synthesis of quaternary ammonium functionalized fluorene-containing cardo poly(arylene ether)s with a wide range of IECs by a direct polymerization method. The cardo backbone was chosen because of its good solubility and film forming ability. We paid special attention to the influences of IEC and counter ions on the water uptakes and ion conductivities of these solution-cast membranes. Their alkaline stabilities were also studied by the change of IEC and intrinsic viscosity in order to explore best working conditions of these membranes for water electrolyzers.
2.
Experimental
2.1.
Materials
9,90 -Bis(4-hydroxyphenyl)fluorene (BPF) was purchased from Tokyo Kasei Kogyo Co. Ltd., Japan (TCI). Decafluorobiphenyl and cesium fluoride were purchased from SigmaeAldrich Co. Ltd. All the other reagents were obtained from commercial ˚ sources. N,N0 -dimethylacetamide (DMAc) was dried with 4 A molecule sieves prior to use. All the other reagents were used as received.
2.2. Synthesis of 9,90 -bis(3-dimethylaminemethylene-4hydroxyphenyl)fluorene (DABPF) The synthetic route is shown in Scheme 1. To a 500 mL oneneck round-bottom flask equipped with a magnetic stirrer, BPF (35.0 g, 0.1 mol) and ethanol (200 mL) were charged. After BPF was dissolved completely, aqueous solutions of dimethylamine (4.0 equiv., 33%) and formaldehyde (3.0 equiv., 37%) were added. The solution was stirred at room temperature (w25 C) for 20 h. White precipitate formed during the course. The crude product was collected by filtration and subjected to recrystallization twice using ethanol and toluene (volume:volume ¼ 1:3). Yield: 47%. 1H NMR (DMSO-d6, ppm): 2.14 (12 H, s), 3.40 (4 H, s), 6.54e6.59 (2 H, d), 6.71e6.75 (2 H, d), 6.87e6.89 (2 H, d), 7.25e7.31 (2 H, m), 7.33e7.38 (4 H, m), 7.85e7.89 (2 H, d).
2.3. Synthesis of poly(fluorene ether)s containing tertiary amine group (PFETAs) and quaternary ammonium group (PFEQAs) The synthetic route of PFETA-x and PFEQA-x (x denotes the molar percentage of DABPF in the total feeding of DABPF and BPF) are depicted in Scheme 2. Typical procedures (PFETA-60 and PFEQA-60) are as follows. To a flame-dried 100 mL three-necked round-bottom flask equipped with nitrogen inlet, condenser and magnetic stirrer, BPF (1.4016 g, 4 mmol), DABPF (2.7876 g, 6 mmol), decafluorobiphenyl (3.3745 g, 10.1 mmol), DMAc (50 mL) and CsF (4.5570 g, 30 mmol) were charged. The mixture was reacted at 80 C for 24 h under the protection of nitrogen. The resulting viscous mixture was poured into 500 mL deionized water to precipitate out the polymer, which was collected by filtration and purified by procedure of dissolving in chloroform,
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Scheme 1 e Synthesis of 9,90 -bis(3-dimethylaminemethylene-4-hydroxyphenyl) fluorene (DABPF).
filtration and precipitating in ethanol twice. PFETA-60 was obtained with 92% yield. To a 50 mL one-necked round-bottom flask equipped with a magnetic stirrer, PFETA-60 (1.0 g, 1.3 mmol) and DMAc (20 mL) were charged. After PFETA-60 was dissolved, iodomethane (0.5 g, 3.2 mmol) was added quickly and the solution was stirred in the dark for 24 h at room temperature. Then the solution was poured into 200 mL deionized water and the precipitate was filtered out and washed with water and acetone thoroughly before being dried at 80 C under vacuum for 24 h. PFEQA-60 was obtained with 90% yield.
2.4.
Membrane preparation
The PFEQAs were dissolved in DMAc at approximately 8 wt/ vol % and cast on glass plates, dried at 80 C for 12 h, and then dried at 120 C under vacuum for 24 h. The membranes (I form) were removed from the casting plate and immersed in deionized water prior to use. To exchange I form to Cl form, SO2 4 form and OH form, the membranes were immersed in 1 M NaCl, 1 M Na2SO4 and 0.5 M NaOH solution for 24 h, respectively, followed by being immersed in deionized water for 24 h with water changed five times and stored in deionized water prior to use.
Scheme 2 e Synthesis of poly(fluorene ether)s containing tertiary amine group (PFETAs) and quaternary ammonium group (PFEQAs).
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2.5.
Characterization
Nuclear magnetic resonance (NMR) spectra were recorded at 400 MHz in a Bruker DRX NMR instrument and the chemical shifts were listed in ppm downfield from tetramethylsilane (TMS). Gel permeation chromatography (GPC) analysis was performed on a Waters Breeze system equipped with a Waters Styragel column, Waters 515 HPLC pump and Waters 2414 refractive index detector, chloroform as an elution solvent at a flow rate of 1 mL min1 and polystyrene as standards for calibration. Thermal stability was analyzed using a PerkinElmer Pyris Diamond TG/DTA analyzer. The temperature was increased from room temperature to 120 C and maintained for half an hour, then increased to 650 C at a heating rate of 10 C min1 under N2 atmosphere. The tensile properties were determined by SANS (Shenzhen, China) electromechanical universal test machine (model CMT-4014). Samples were cut into dumbbell shape and immersed in room temperature deionized water prior to test. The intrinsic viscosities of the samples were measured in 1-methyl-2pyrrolidinone (NMP) containing 0.05 M LiBr at room temperature using an Ubbelohde viscometer. Ion exchange capacities (IECs) were evaluated using OH form membranes. The membranes (w0.3 g) were immersed in 50 mL of 0.1 M HCl standard solutions for 48 h, and then titrated with 0.1 M NaOH solution using phenolphthalein as indicator. The water uptake of the membranes was defined as weight ratio of the absorbed water to that of the dry membrane. Membranes in different counter ion forms were immersed in deionized water for 24 h before taken out to measure their wet masses. The surface water was wiped out by tissue paper. The dry masses were measured after samples were kept in 80 C oven under vacuum overnight. The water uptakes were calculated using the following equation: Wð%Þ ¼
ðMw Md Þ 100% Md
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nitrogen for half an hour prior to measurements. The ion conductivity was calculated according to the following equation: s¼
d RS
(2)
where d and S are the thickness and area of the sample, respectively, and R is derived from the low intersect of the high frequency semicircle on a complex impedance plane with the Re (Z0 ) axis. The alkaline stability was evaluated by immersing OH form membranes in large excess 1 M NaOH solution at 80 C and recording their IEC and viscosity changes versus immersion time.
3.
Results and discussion
3.1.
Synthesis of monomer and polymers
Through Mannich reaction, the novel tertiary amine functionalized monomer 9,90 -bis(3- dimethylaminemethylene-4hydroxyphenyl)fluorene (DABPF) was successfully synthesized as confirmed by 1H NMR and 13C NMR spectra, Fig. 2. The characteristic peaks of dimethylaminemethylene group in DABPF were observed, which were located at 2.14 ppm (eCH2e) and 3.40 ppm (eCH3) in 1H NMR spectrum and 61.00 ppm (eCH2e) and 45.16 ppm (eCH3) in 13C NMR
(1)
where Md and Mw are the weight of membranes before and after water absorption, respectively. Ion conductivities of membrane in the transverse direction were measured by sandwiching membrane samples between two parallel gold electrode plates and recording their resistances using a Solartron 1255 B frequency response analyzer coupled with a Solartron 1287 electrochemical interface in the frequency range of 100 Hz to 1 MHz. The schematic illustration of the cell for ion conductivity measurement is shown in Fig. 1. Samples were kept in deionized water with purging
Fig. 1 e Schematic illustration of the cell for ion conductivity measurement.
Fig. 2 e 1H NMR (a) and DABPF.
13
C NMR (b) spectra of monomer
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spectrum. All the other peaks were well assigned according to its chemical structure, excluding the possibility of monofunctionalization of 9,90 -bis(4-hydroxyphenyl)fluorene (BPF). Condensation copolymerizations of DABPF with BPF and decafluorobiphenyl were carried out in DMAc using CsF as catalyst. High molecular weight PFETAs were obtained as shown in Table 1. The 1H NMR spectrum of PFETA-80 is shown in Fig. 3 with proper assignment of all the resonance peaks. The characteristic peaks of dimethylaminemethylene group were found to be 2.30 ppm (eCH2e) and 3.68 ppm (eCH3). By controlling the loading molar ratio of DABPF, PFETAs with degree of functionality (DF) varying from 0.4 to 1.6 were obtained. Treating PFETAs with excess iodomethane in DMAc solution gave complete conversion of tertiary amine group to quaternary ammonium group, which was evidenced by 1H NMR spectra of PFEQA-x where the characteristic peaks of tertiary amine group disappeared while the characteristic peaks of quaternary ammonium group (4.65 ppm for eCH2e and 3.09 ppm for eCH3) showed up, Fig. 2. PFEQAs with IECs ranging from 0.6 to 2.0 equiv. g1 were obtained with high intrinsic viscosities as shown in Table 2. The IECs of PFEQAs were closely related to DF of PFETAs, demonstrating the possibility and high efficiency of controlling IEC by tuning the loading amount of tertiary amine containing monomer. The solubility of PFEQAs in different solvents was shown in Table 3. It can be found that the PFEQAs had good solubility in aprotic solvents such as N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N0 -dimethylacetamide (DMAc) and N,N0 -dimethylaceformide (DMF). The good solubility was important because the ionomers can form ion aggregation in solution which was the primary driving force of phase separation in the solution-cast membranes. This well-developed phase separation was found to be critically important for high ion conductivity [24]. On the other hand, the good solubility also allowed PFEQAs to be used as catalyst binder for better system optimization.
3.2.
Water uptake and ion conductivities
The interaction of water with polymers has a significant impact on ion conductivities of the corresponding membranes. Basically the ion conductivities increase with the increasing of water uptake within a specific system, unless too much swelling of membrane is resulted where the ion concentration is decreased. In order to give a fundamental
Table 1 e Properties of PFETAs. Sample PFETA-20 PFETA-40 PFETA-60 PFETA-80
DFa Mn (kg mol1) Mw (kg mol1) PDI Tidb ( C) 0.4 0.8 1.2 1.6
48 55 53 49
101 116 111 103
2.1 2.1 2.1 2.1
268 258 249 240
a Degree of functionalization (number of tertiary amine group per repeat unit). b Initial decomposition temperature (determined from TG/DTG curves as the point of deviation from linearity corresponding to the mass loss due to polymer decomposition).
view of the anion conductivities of PFEQAs for anion exchange membrane water electrolysis, four anions were chosen as counter ions for PFEQAs, namely OH, SO2 4 , Cl and I . Not only OH was studied because the water for electrolysis could contain natural or artificially added inorganic salt as supporting electrolyte to lower the resistance of the membrane medium. The water uptakes of PFEQAs with different counter ions at 20 C are shown in Fig. 4. It can be seen that the water uptake increased with the increasing of IEC regardless of the species of counter ion. More hydrophilic polymer will definitely absorb more water. However, the counter ion has obvious influence on water uptake at any given IEC. The trend for membrane water uptake with different counter ions was: OH > SO2 4 > Cl > I . This trend may be the result of different hydrophilicity and solubility of the corresponding ions. The OH form membranes had very strong water absorbance capability, which is in good agreement with literatures [12,26]. The conductivities of the above four counter ions for PFEQAs at 20 C are shown in Fig. 5. The influence of IEC on ion conductivity was similar to its influence on water uptake. All the ion conductivities increased with the increasing of IEC, as expected. This is because of both the higher concentrations of mobile ions and the larger water uptake at higher IECs. Interestingly, the trend of ion conductivity with those four counter ions was different with the trend of their water uptake. For ion conductivities, OH > SO2 4 > Cl > I . The OH conductivities were the highest among these ions investigated and were comparable or higher than the literature values form membranes had higher water uptakes [27e29]. SO2 4 than Cl form membranes, however, had lower conductivities than the Cl counterparts. This is because the Stokes radius ˚ while that of Cl is only 1.21 A ˚ [10]. The is 2.31 A for SO2 4 smaller the Stokes radius of an ion, the more easily it moves. ion There is also a possibility that higher charged SO2 4 decreased its mobility as compared to mono-charged Cl ion. Therefore, Cl conductivity is higher than SO2 4 conductivity at the same IEC. For I form membranes, their conductivities are the lowest among all these four forms membranes, suggesting that their lowest water uptakes play the dominant effect. The temperature dependence of water uptakes and ion conductivities of PFEQAs were investigated using OH form membranes, Fig. 6. It can be seen that water uptake and OH conductivity increased spontaneously with the increasing of temperature. At 80 C, OH conductivity of as high as around 60 mS cm1 was observed for PFEQA-60. In real anion exchange membrane water electrolysis, elevated temperatures are desired in order to increase the reaction rate and energy efficiency. Furthermore, mixed ion conduction may be the case when the water for electrolysis contains natural or artificially added inorganic salts. This section gives a clear picture of how those four counter ions can contribute to the whole conductivity of the membrane. While OH form membrane exhibited the best performance than the other ion forms membranes, it might bring along a stability issue which will be discussed in next section. Therefore, Cl is a promising candidate to act as supporting electrolyte. In fact, water electrolysis using 0.2 M NaCl solution had been reported with
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Fig. 3 e 1H NMR spectra of PFETA-80 and PFEQA-80.
the benefit of low cell resistance [30]. However, detailed study on different ions’ conductivity of the membranes is essentially important for new membrane/system design.
3.3.
Alkaline stabilities
While high OH conductivity could be achieved at high IEC and high temperature, the membrane stability in these
conditions is of great concern. Usually the stability issue involves the degradation of polymer backbone and decomposition of side chain functional groups [14]. In order to discern how much these two factors contribute to the stability problem of our polymers, PFEQA-60 was chosen to be immersed in 1 M NaOH solution at 80 C for certain times and the changes in IEC and intrinsic viscosity were recorded as shown in Fig. 7. It can be seen that the IEC dropped slowly with
Table 2 e Properties of PFEQAs. Sample
IECa (mequiv. g1)
Intrinsic viscosity (g dL1)
Tidb ( C)
Tensile strength (MPa)
Elongation at break (%)
PFEQA-20 PFEQA-40 PFEQA-60 PFEQA-80
0.6 1.1 1.6 2.0
0.64 0.71 0.69 0.65
229 219 211 205
29 25 22 18
62 48 32 26
a Titrated from OH form membranes. b Initial decomposition temperature.
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Table 3 e Solubility of PFEQAs in different solvents. Sample
H2O
H2O þ n-propanol (1:1 in volume)
n-propanol
NMP
DMSO
DMAc
DMF
CH3Cl
PFEQA-20 PFEQA-40 PFEQA-60 PFEQA-80
a
þb þ þ þ
c
þ þ þ þ
þ þ þ þ
þ þ þ þ
þ þ þ þ
a Insoluble. b Soluble. c Partial soluble.
Furthermore, no measurable degradation could be found in 1 M NaCl or Na2SO4 solutions even at 80 C for a week. Therefore, we are exploring these membranes for water electrolysis under neutral PH conditions with added NaCl as supporting electrolyte currently.
3.4.
Fig. 4 e Water uptake of PFEQAs with different counter ions at 20 C.
the immersion time, however, the viscosity decreased significantly. After three days of immersion, the IEC maintained about 75% of its original value while the viscosity was only about 14% left. These results strongly suggest that the degradation of polymer backbone is more significant than the decomposition of side chain quaternary ammonium groups for PFEQA-60. These results proved against the use of OH form membranes under elevated temperatures in basic conditions. Fortunately, the OH form membranes were relatively stable in neutral PH water at room temperature.
Fig. 5 e Different ions’ conductivities of PFEQAs at 20 C.
Thermal stability and mechanical property
The thermal stabilities of PFETAs and PFEQAs were evaluated by thermogravimetric analysis (TGA). The representative TGA curves of PFETA-80 and PFEQA-80 (OH form) are given in Fig. 8. Two-step decomposition can be found for both types of samples. The first step involved the decomposition of tertiary amine group in PFETAs and quaternary ammonium group for PFEQAs while the second step involved the decomposition of polymer backbone [24]. Obviously the tertiary amine group in PFETAs decomposed at higher temperature than quaternary ammonium group in PFEQAs. The decomposition temperatures of polymer backbone were almost the same for both types of samples because of the same backbone structure. The weight loss caused by release of tertiary amine group was very small, lower than 5% for low DF PFETAs. Therefore, the initial decomposition temperatures (Tid) instead of 5% weight loss temperatures were given in order to reveal the more real thermal stabilities. As shown in Table 1, PFETAs were thermally stable up to as high as 240 C. The Tid decreased with the increasing of DF. For PFEQAs, the Tid decreased with the increasing of IEC but still higher than 205 C, demonstrating good thermal stabilities of PFEQAs for water electrolysis applications, Table 2.
Fig. 6 e OHL conductivity and water uptake of PFEQA-60 as a function of temperature.
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OH > SO2 4 > Cl > I while their ion conductivities followed another trend: OH > Cl > SO2 4 > I . All the samples showed high ion conductivity, high thermal stability and robust mechanical strength, but relative low alkaline stability. They are suitable to work with neutral PH water with or without supporting electrolytes. The water electrolyzer evaluation using PFEQAs as both membrane and catalyst binder is ongoing right now, together with the selection of catalyst and system optimization.
Acknowledgments Fig. 7 e IEC and viscosity changes versus immersion time of PFEQA-60 in 1 M NaOH solution at 80 C.
The authors would like to thank the China High-Tech Development 863 Program (Grant No.: 2007AA03Z217), Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No.: 2003C105004, 2006A10704004, 2006B12401006), and Guangzhou Sci & Tech Bureau (2005U13D2031) for financial support of this work.
references
Fig. 8 e TGA curves of PFETA-80 and PFEQA-80 in OHL form.
The mechanical properties of PFEQAs were listed in Table 2. With the increasing of IEC, both the mechanical strength and elongation at break decreased because of the higher water uptake as discussed above. Water acted as plasticizer in the membrane, the same to the cases in proton exchange membranes [22]. Nevertheless, all the samples still had high mechanical strength because of their robust backbone structure.
4.
Conclusions
New tertiary amine functionalized dihydroxyl monomer DABPF was successfully synthesized via Mannich reaction route. Copolymerizations of DABPF with BPF and decafluorobiphenyl yielded quaternary ammonium functionalized poly(fluorenyl ether)s (PFEQAs) with a wide range of IECs, which were predetermined by the loading ratio of DABPF monomer. Membranes based on PFEQAs were investigated for potential anion exchange membrane electrolysis applications. It was found that the water uptakes of PFEQAs with four different counter ions followed the trend:
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