Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells

Journal of Membrane Science 375 (2011) 204–211

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Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells Xiaoming Yan a , Gaohong He a,∗ , Shuang Gu b , Xuemei Wu a , Liguang Du a , Haiyan Zhang a a

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China Department of Chemical and Environmental Engineering, University of California-Riverside, 900 University Avenue, Riverside, CA 92521, USA

b

a r t i c l e

i n f o

Article history: Received 14 November 2010 Received in revised form 18 March 2011 Accepted 22 March 2011 Available online 29 March 2011 Keywords: Hydroxide exchange membrane Poly (ether ether ketone) Chloromethylation Quaternization Fuel cell

a b s t r a c t Chloromethylated PEEK (CMPEEK) was successfully synthesized for the first time by concentrated sulfuric acid as the solvent and its degree of chloromethylation (DC) was controlled (38–98%) by varying the chloromethylation time. The sulfonation of PEEK was confirmed to be negligible during the chloromethylation by 1 H NMR analysis. CMPEEKs are soluble in high-boiling-point solvents, which makes it possible to prepare CMPEEK membranes. Subsequently, it was quaternized and ion-exchanged by NaOH to form quaternized PEEK hydroxide (QAPEEKOH) membrane. The ion exchange capacity of QAPEEKOH membranes ranges from 0.43 to 1.35 mmol g−1 , and the corresponding degree of quaternization of chloromethyl group increases from 35% to 61%. QAPEEKOH membranes from DC of 38–70% have appropriate water uptake (≤145%) and moderate swelling ratio (≤27%) even at 60 ◦ C. More importantly, their hydroxide conductivities, such as 12 mS cm−1 at 30 ◦ C and 0.95 mmol g−1 of IEC, are higher than those of most other quaternized aromatic polymer membranes. TGA shows that QAPEEKOH has high thermal stability (TOD : 148 ◦ C). All the above properties indicate that QAPEEKOH membrane is very promising for the potential application in hydroxide exchange membrane fuel cells. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) have shown high power density and reasonable energy density. However, the high cost and low durability of the electrocatalysts of PEMFCs have hampered their commercialization [1]. Recently, increasing attention has been drawn to a novel type of power source technology, hydroxide (OH− ) exchange membrane fuel cells (HEMFCs), which have the potential to fundamentally solve the problems of PEMFCs [2–4]. They have demonstrated the ability to offer fast electrode kinetics, desirable applicability of non-precious metals as catalysts, reduced fuel crossover and great fuel diversity [5–7]. As a crucial component of HEMFCs, hydroxide exchange membranes (HEMs) serve as hydroxide conductor and fuel/oxidant separator simultaneously. The research and development of high-performance HEMs has become one of the most challenging works. In general, HEM materials can be prepared by introducing quaternary-ammonium or quaternary-phosphonium [4] functional groups onto high-performance engineering polymers, predominantly through nucleophilic-substitution reaction between halogenomethyl (typically, chloromethyl) group and

∗ Corresponding author. Tel.: +86 411 83637471; fax: +86 411 84707700. E-mail address: [email protected] (G. He). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.03.046

tertiary-amine or tertiary-phosphine molecules. Considering that those nucleophilic-substitution reactions have very high reactivity, the introduction of halogenomethyl group onto polymer matrix has been perceived as a critical step. Strategically, there have been two distinguished routes to introduce the chloromethyl group onto polymer matrix: one is through radiation-grafting chloromethyl-containing vinyl benzyl chloride (VBC) monomer onto fluorinated polymers such as poly(vinylidene fluoride) (PVdF) [8], poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) [9] and poly(ethylene-co-tetrafluoroethylene) (ETFE) [10]; and the other is through direct chloromethylation onto polymer matrix by using chloromethylating agent. Although these two routes can both be used to prepare HEMs successfully, the direct chloromethylation route has been demonstrated to be simpler to conduct and more flexible to choose polymers as matrix than the radiation-graft one. Based on the direct chloromethylation route and following quaternization, many high-performance commercially available and synthesized polymers have been chosen as the matrix to prepare HEMs, including polyethersulfone cardo [11], poly(phthalazinone ether sulfone ketone) [12], polyepichlorohydrin [13,14], polysulfone [5,7,15–17], poly(phthalazinone ether ketone) [18], polyphenylsulfone [19,20], polyetherketone cardo [21], polyetherimide [22,23], tetramethylpoly(phenylene) and Diels-Alder poly(phenylene)tetramethylpoly(phenylene) [24], poly(styrene ethylene butylene

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styrene) [25,26], tetramethyl-polysulfone [27], polyfluorenyl sulfone [28], and so on. As an important high-performance engineering polymer, poly(ether ether ketone) (PEEK) possesses good thermal stability [29], high chemical resistance, excellent mechanical property and many other outstanding properties [30]. It is desirable to prepare a high-performance HEM by the chloromethylation and quaternization of PEEK. However, the HEM based on PEEK has been critically missing, because the very poor solubility of PEEK in organic solvents prevents typical chloromethylation methods from being used [31] in which chlorinated hydrocarbons are as solvents. Encouragingly, a chlorinated hydrocarbon solvent-free chloromethylation method for poly(phthalazinone ether sulfone ketone) was reported, in which inorganic concentrated sulfuric acid was used as reaction solvent and catalyst simultaneously [32]. Meanwhile, PEEK has good solubility in concentrated sulfuric acid. Therefore, in this work, the chloromethylated PEEK (CMPEEK) was synthesized by concentrated sulfuric acid based chloromethylation method, and moreover its membranes were quaternized and ion-exchanged by NaOH to prepare quaternized PEEK hydroxide (QAPEEKOH) membranes. Their properties such as ion exchange capacity, water uptake, swelling ratio, hydroxide conductivity, and thermal stability were investigated. 2. Experimental 2.1. Materials PEEK (VESTAKEEP® 4000G) power was provided by Evonik Degussa (China) Co. Ltd. Chloromethyl octyl ether (CMOE) was synthesized according to Ref. [33]. Methanol, ethanol, acetone, sodium hydroxide, sulfuric acid (92.8%), N,N-dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP) and trimethylamine aqueous solution (33%) were obtained commercially and used as received without further purification. All the chemicals used in the experiments are analytical grade. In order to reduce the effect of CO2 dissolved in deionized water on the prepared membranes, fresh deionized water before used was boiled to remove CO2 . 2.2. Synthesis of CMPEEK Concentrated sulfuric acid was used as solvent to synthesize CMPEEK due to the poor solubility of PEEK in chlorinated hydrocarbons. Meanwhile, CMOE was used as the chloromethylating agent because of its low toxicity and volatility, which is much safer than the generally used chloromethyl methyl ether [33]. Specifically, 2 g PEEK was added into 120 ml 92.8% concentrated sulfuric acid at 0 ◦ C with stirring, and then the temperature was lowered to −10 ◦ C after complete dissolution, followed by addition of 40 ml freshly synthesized CMOE. Subsequently, the reaction was kept for a certain time, typically from 25 to 100 min. The polymer product, CMPEEK, was separated from the reacted mixture by precipitation in ice water, followed by thorough washing with deionized water, and then air-drying at room temperature. In order to completely remove the impurities, the separated CMPEEK was further purified by being dissolved in NMP, precipitated and washed with ethanol, and finally air-dried at room temperature. The chemical structures and synthetic process are shown in Scheme 1. 2.3.

1H

NMR and solubility test

1 H NMR spectroscopy was used to confirm the chloromethylation of PEEK and determine the degree of chloromethylation (DC). 1 H NMR spectra of PEEK and CMPEEKs were recorded by a Varian Unity Inova 400 spectrometer at a resonance frequency of

Scheme 1. Synthesis of the QAPEEKOH.

399.73 MHz. Because of its very poor solubility, PEEK was immersed into DMSO-d6 at 100 ◦ C for 24 h to increase the polymer concentration in the solvent, and then the undissolved polymer was removed and residual solution was used as the PEEK sample for 1 H NMR test. Contrary to PEEK, CMPEEKs have good solubility, and 2 wt.% CMPEEKs solutions were directly prepared by dissolving CMPEEK into DMSO-d6. Tetramethylsilane (TMS) was used as the internal standard in all cases. The solubility of PEEK and CMPEEKs was qualitatively tested by dissolving 0.1 g polymer sample in 5 ml of selected solvents for 48 h at room temperature. Prior to testing, polymer samples were dried for 24 h at 50 ◦ C. 2.4. Preparation of QAPEEK membrane Initially, 0.15 g CMPEEK was dissolved into 3 ml DMAc to prepare a 5 wt.% solution, and then the CMPEEK solution was poured onto a glass plate to cast the membrane. After curing and drying at 40 ◦ C for 3 days, the CMPEEK membrane was peeled off from the glass plate. The quaternization of CMPEEK membrane was conducted by immersing CMPEEK membrane into trimethylamine aqueous solution (33%) at 30 ◦ C for 3 days, and after thorough washing, the quaternized PEEK chloride (QAPEEKCl) membrane was prepared. The quaternized PEEK hydroxide (QAPEEKOH) membrane was obtained by ion-exchange of the QAPEEKCl membrane using 1 M NaOH solution at room temperature for 48 h, followed by washing and immersion with deionized water in a tightly sealed container for additional 48 h to completely remove the residual NaOH. The processes of quaternization and ion-exchange are also shown in Scheme 1. 2.5. Ion exchange capacity (IEC) The IEC of the QAPEEKOH membrane was measured by the back titration method. Initially, 0.2 g QAPEEKOH membrane sample was

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equilibrated with 50 ml 0.01 M HCl standard solution for 24 h, followed by back titration of 0.01 M NaOH standard solution with phenolphthalein as the indicator. The 50 ml 0.01 M HCl standard solution was used as the blank sample for the control experiment. The measured IEC (IECm ) of the QAPEEKOH membrane was calculated by the following equation: IECm =

(Vb − Vs ) · CHCl × 1000 Wdry

where Vb and Vs are the consumed volumes (ml) of the NaOH solution for the blank sample and the QAPEEKOH membrane sample, respectively, CHCl is the concentration (M) of HCl solution, and Wdry is the mass (g) of dry membrane sample. 2.6. Water uptake and swelling ratio Initially, the weight and dimension of dry membrane (dried under vacuum at room temperature for 48 h) were measured, and then the dry membrane was immersed into deionized water and kept in a tightly sealed container for more than 10 h to ensure the full hydration. Subsequently, the weight and dimension of wet membrane were measured. The water uptake and swelling ratio could be calculated by the two following equations, respectively: Water uptake (%) =

Wwet − Wdry Wdry

Fig. 1.

1

H NMR spectra of PEEK and CMPEEKs (DMSO-d6 as solvent).

× 100 3. Results and discussion

Swelling ratio (%) =

lwet − ldry ldry

× 100

where Wwet and Wdry are the weight of wet and dry membrane sample, respectively, lwet and ldry are the average length [lwet = (lwet1 ·lwet2 )1/2 , ldry = (ldry1 ·ldry2 )1/2 ] of wet and dry membrane sample, respectively, in which, lwet1 , lwet2 and ldry1 , ldry2 are the lengths and widths of wet and dry membrane sample, respectively. 2.7. Hydroxide conductivity Hydroxide conductivity of the QAPEEKOH membrane in the longitudinal direction was measured in deionized water by a typical four-electrode AC impedance method. Ivium Technologies A08001 equipment was used as the impedance analyzer, and 1–105 Hz was chosen as the scanning frequency range. The measurement apparatus consists of two platinum foils as the current carriers and two platinum wires as the potential sensors. Prior to the measurement, all the membranes were fully hydrated by deionized water in a tightly sealed container for 24 h. Based on the impedance derived resistance, hydroxide conductivity of the membrane, , was calculated through the following equation: =

L WdR

where L is the distance between the two potential electrodes, d and W are the thickness and width of the membrane sample, respectively, and R was derived from the right-side intersect of semi-circle on the complex impedance plane with the Re(Z) axis. 2.8. Thermogravimetric analysis (TGA) A TGA analyzer (Mettler Toledo TGA/SDTA851e ) was used to test the thermal stability of the PEEK, CMPEEK and QAPEEKOH. About 5–10 mg samples were heated from 100 to 800 ◦ C at a heating rate of 10 ◦ C min−1 under a air atmosphere (air flow rate: 80 ml min−1 ). The samples were dried for 24 h at 50 ◦ C in vacuum to remove moisture prior to the test. Derivative thermogravimetry (DTG) curve is the first order differential of TGA curve on temperature.

3.1. Chloromethylation of PEEK 1 H NMR spectra of PEEK and CMPEEKs are shown in Fig. 1. Before chloromethylation, the pristine PEEK spectrum shows one double-peak at ∼7.8 ppm (4Hc s) and one group of peaks at around 7.0–7.3 ppm (4Hb s and 4Ha s). After chloromethylation, while the Hc s changes to one single-peak due to the slight shift of some Hc s’ positions and overlapping with other Hc s caused by the interferences of introduced –CH2 Cls, the group of Hb and Ha in CMPEEKs continuously diminishes and a new characteristic single-peak at ∼4.7 ppm (Hd of −CH2 Cl) arises, which clearly confirms the CMPEEK has been successfully synthesized. As an electrophilic substitution, the chloromethylation reaction of PEEK would preferentially take place in the bisphenol rings on PEEK (Ha positions), because of the highest electron-density (endowed by the two powerful electron-donating ether groups). In contrast, due to the inactivity of electrophilic substitution arising from the lowest electron-density (adjacent to the strong electron-withdrawing carbonyl group), the Hc s are almost intact during the chloromethylation reaction. Therefore, the degree of chloromethylation (DC) of CMPEEKs can be readily calculated by the following equation: DC = 2A(Hd )/A(Hc ), where A(Hd ) is the integral area of the Hd peak (–CH2 Cl), and A(Hc ) is the one of the Hc peak. The sulfonation of PEEK, which is not desired in our study, possibly happens during the chloromethylation since concentrated sulfuric acid is usually used as the sulfonating agent [34]. However, its reaction activity highly depends on the reaction temperature: at low temperature, the sulfonation reaction is very slow, e.g., the degree of sulfonation (DS) merely reached about 9% at 25 ◦ C of reaction temperature for 180 min (96.3% sulfuric acid and 20 g l−1 polymer concentration) [35]. In this work, an even lower reaction temperature (−10 ◦ C), much shorter reaction time (25–100 min), as well as the reduced concentrations of sulfuric acid and polymer (92.8% and 16.7 g l−1 , respectively) are used; therefore, the sulfonation of PEEK, during the chloromethylation reaction, could be negligible. Indeed, the sulfonated PEEK characteristic peak that locates at ∼7.51 ppm [36,37] (the adjacent aromatic hydrogen to

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Table 1 Solubility of PEEK and CMPEEKs. Sample

Acetone

Methanol

Ethanol

DMF

DMAC

DMSO

NMP

PEEK CMPEEK

− − − − − − − − − − − −

− − − − − − − − − − − −

− − − − − − − − − − − −

− + + + + + + − − − − −

− + + + + + + − − − − −

− + + + + + + − − − − −

− + + + + + + − − − − −

QAPEEKOH

DC: 38% DC: 52% DC: 60% DC: 70% DC: 76% DC: 98% DC: 38% DC: 52% DC: 60% DC: 70% DC: 76%

+: Soluble; −: Insoluble.

sulfonated group), failed to be found among all the CMPEEKs, providing an evidence of the negligible sulfonation above mentioned. The DC of chloromethylated polymers is an important parameter, which determines the IEC of the targeted HEM. In our study, DC of CMPEEK was efficiently controlled by varying the chloromethylation time. As seen in Fig. 2, the DC of CMPEEK increases almost linearly with the chloromethylation time, indicating an excellent DC controllability. In general, the introduction of substituted groups would destroy the regularity of polymer chains and thus improve the solubility of polymers. Although the pristine PEEK is insoluble in all the testing solvents, the CMPEEKs, as expected, show an excellent solubility in DMF, DMAc, DMSO and NMP that are widely used high-boiling-point solvents for the membrane-preparation (Table 1). The excellent solubility of CMPEEKs makes it possible to prepare high-performance HEMs. In addition, the prepared QAPEEKOH membranes are insoluble in all selected solvents. 3.2. IEC In general, IEC dominates the properties of HEMs, such as water uptake, swelling ratio and hydroxide conductivity. The measured IEC (IECm ) and the theoretical one (IECt , assuming the complete

quaternization of chloromethyl groups) of QAPEEKOHs with different DCs of the original CMPEEKs are listed in Table 2. IECm of QAPEEKOH increases remarkably from 0.43 to 1.35 mmol g−1 with DC from 38% to 76%. The CMPEEK with DC of 98% suffered from excessive swelling and lost membrane morphology during the quaternization process. For convenience, the QAPEEKOH membranes are denoted as QAPEEK xx%, where xx% is the DC of original CMPEEK. Those IECm values are apparently lower than the IECt ones, indicating the incomplete conversion of chloromethyl group to the final quaternary ammonium one. Degree of the quaternization of chloromethyl group, DQ, is introduced here to quantitatively describe the quaternization conversion, and the results are also listed in Table 2. The DQ increases considerably from 35% to 61% with increasing the DC. Since the trimethylamine aqueous solution was used as the quaternizing agent, the conversion of hydrophobicity to hydrophilicity of membranes should always accompany the quaternization process. Thus the membranes from higher DC would have better hydrophilicity, and eventually have more chance to react with trimethylamine coming from aqueous solution. The DQ values are at a moderate level, confirming the relatively low reactivity of the amine-aqueous-solution based quaternization method [19,38]. Fortunately, the degree of ammonium (DA) increases with increasing the DC of the original CMPEEK, as shown in Table 2. 3.3. Water uptake and swelling ratio It is well known that the water uptake has a profound effect on the hydroxide conductivity and mechanical property of HEMs. Fig. 3 shows the water uptake of QAPEEKOH membranes at different temperatures. As expected, the water uptake of QAPEEKOH membranes increases with increasing DC and temperature. The temperature has much greater effect on the water uptake of QAPEEKOH membranes with higher DC (or higher IEC) than those with lower DC. The possible explanation is that the higher IEC, at elevated temperature, would make the hydrophilic domains of QAPEEKOH membrane Table 2 IEC and DQ of QAPEEKOH membranes.

Fig. 2. The effect of chloromethylation time on the DC of CMPEEKs.

DC of CMPEEK (%)

IECt a (mmol g−1 )

IECm b (mmol g−1 )

DQc (%)

DAd (%)

38 52 60 70 76 98

1.18 1.55 1.76 2.00 2.13 2.57

0.43 0.85 0.95 1.18 1.35 N/Ae

35 53 52 57 61 N/Ae

13 27 31 40 46 N/Ae

a Theoretical ion exchange capacity, assuming the complete quaternization of chloromethyl groups: IECt = 1000DC/(288.3 + 89.1DC). b Measured ion exchange capacity. c Degree of quaternization, DQ = [(48.5 + 288.3/DC)/(1000/IECm −40.6)] × 100%. d Degree of ammonium, DA = DC·DQ. e Cannot be measured due to the excessive membrane swelling.

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Fig. 3. Water uptake of QAPEEKOH membranes at different temperatures.

easier and better to be continuous, which inevitably expands the overall hydrophilic domains, and then induces more water uptake eventually [39]. Based on the water uptake and corresponding IEC, the numbers of absorbed water molecules per quaternary ammonium hydroxide (QAOH) site, , in those QAPEEKOH membranes are calculated [ = (water uptake/18)/(IECm /1000)] and the results as a function of temperature are shown in Fig. 4.  slightly increases at lower temperatures and rapidly increases at higher temperatures, which attributes to the enhanced plasticization effect of the absorbed water as well as the relaxation effect of the polymer chain themselves at elevated temperature. At lower temperatures (10–50 ◦ C),

Fig. 5. Swelling ratio of QAPEEKOH membranes at different temperatures.

there is no significant difference in  (10–15 H2 O/QAOH site) among QAPEEKOH membranes with the lower DCs (38%−70%) or lower IECs (0.43–1.18 mmol g−1 ), while at higher temperature (60 ◦ C),  apparently increases with increasing the DC or IEC, supporting the improved continuity of hydrophilic domains within the QAPEEKOH membranes mentioned above. For even higher DC or higher IEC, the water uptake of QAPEEKOH 76% drastically increases from 153% to 351% with temperature from 10 to 50 ◦ C, and then the membrane loses the its morphology to form a gel at 60 ◦ C, due to the excessive hydrophilicity arising from the highest IEC (1.35 mmol g−1 ). The swelling ratios of those QAPEEKOH membranes at different temperatures are shown in Fig. 5. Similar to the water uptake, the swelling ratio also increases with increasing both IEC and temperature. Except excessively hydrophilic QAPEEKOH 76%, all the swelling ratios of QAPEEKOH membranes are no more than 27% even at elevated temperature, indicating the good dimensional stability. In addition, those QAPEEKOH membranes (with DC of 38–70%) are thickness-uniform, flexible, and robust. The water uptake (≤145%) and swelling ratio (≤27%) of those QAPEEKOH membranes are appropriate and satisfactory for potential application as HEMs in the HEMFCs. 3.4. Hydroxide conductivity

Fig. 4. Absorbed water molecules per QAOH group of QAPEEKOHs at different temperatures.

The hydroxide conductivity of the QAPEEKOH membranes against the IEC is plotted in Fig. 6. The hydroxide conductivity remarkably increases from 5.6 to 17 mS cm−1 with increasing IEC from 0.43 to 1.35 mmol g−1 (corresponding DC from 38% to 76%). In order to investigate the impact of polymer matrix on the corresponding HEMs, hydroxide conductivities (at ∼30 ◦ C) of reported aromatic polymer based trimethylamine aqueous solution quaternized HEMs with ∼1.0 mmol g−1 of IEC are compared in Table 3. Obviously, the conductivity of HEM is substantially higher for PEEK matrix (12 mS cm−1 ) than for almost all other aromatic polymers, except for the copolymerized DAPP-TMPP that merely has a slightly higher HEM conductivity (13 mS cm−1 ). The possible reason is that the two strongly electron-donating ether bonds, within PEEK main

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209

Fig. 6. Hydroxide conductivity (20 ◦ C) of QAPEEKOH membranes against the IEC.

chain, increase the electron-density of the linked benzyl quaternary ammonium functional group, which enhances the basicity of QAOH, and then improves the hydroxide conductivity of QAPEEKOH [4,5]. The high conductivity and commercial-availability explicitly show that the PEEK is one of the most desirable aromatic polymers as HEM matrix. The hydroxide conductivity of the QAPEEKOH membranes increases with temperature dramatically, as seen in Fig. 7. The QAPEEKOH 70% membrane with IEC of 1.18 mmol g−1 exhibits the highest hydroxide conductivity of 21 mS cm−1 at 60 ◦ C. Although the QAPEEKOH 76% membrane is expected to have even higher hydroxide conductivity due to the highest IEC (1.35 mmol g−1 ), the membrane, unfortunately, swells excessively and loses mechanical strength extremely at elevated temperatures. This reveals a trade-

Fig. 7. Hydroxide conductivity of QAPEEKOH membranes depending on temperature.

off between high hydroxide conductivity and good mechanical property for the QAPEEKOH membranes, analogous to the aromatic polymer matrix based proton exchange membranes [40]. Based on the Arrhenius relationship between ionic conductivity and temperature, the apparent activation energies of hydroxide conductivity (Ea s) were calculated to be 9.5 and 10.2 kJ mol−1 for the QAPEEKOH 60% and QAPEEKOH 70% membranes, respectively, which are comparable to those of other aromatic polymer based trimethylamine aqueous solution quaternized HEMs (9.92 kJ mol−1 of QAPEKCOH [21], 10–12 kJ mol−1 of QAPFSfOH [28] and 11.31 kJ mol−1 of QASEBSOH [25]). This confirms again that the temperature has significant impact on hydroxide conductivity of HEMs.

Table 3 Hydroxide conductivity (∼30 ◦ C, deionized water) of aromatic polymer based trimethylamine aqueous solution quaternized HEMs. Aromatic polymer matrix of HEMs

Poly(ether ether ketone) (PEEK) Polyethersulfone Cardo (PES-C) poly(phthalazinon ether sulfone ketone) (PPESK) poly(phthalazinone ether ketone) (PPEK) Polyphenylsulfone (PPSf) Polyetherketone Cardo (PEK-C) Polyetherimide (PEI) Tetramethylpoly(phenylene) (TMPP) Diels-Alder poly(phenylene)tetramethylpoly(phenylene) (DAPP-TMPP) Poly(styrene ethylene butylene polystyrene) (SEBS) Tetramethyl-polysulfone (TMPSf) Polyfluorenyl sulfone (PFSf) r.m.: room temperature. a 0.1 M NaOH as test medium. b Calculated based on corresponding DC of 130%.

IEC (IECt ) (mmol g−1 )

Hydroxide conductivity (mS cm−1 )

Test conditions

Ref.

Temperature (◦ C)

Impedance technique

0.95 (1.76) 1.25 N/A

12 ∼7.2a 5.2

30 r.m. 30

Four-electrode Four-electrode Two-electrode

This work [11] [12]

(2.43b ) 1.04 0.11 0.983 0.99 0.89

∼1.3 ∼3.6 1.6 2.28 ∼5.7 ∼13

25 30 20 25 30 30

Two-electrode Four-electrode Two-electrode Four-electrode Four-electrode Four-electrode

[18] [19] [21] [23] [24] [24]

0.3

5.12

30

Two-electrode

[25]

0.578 1.48 0.68 1.31

0.69 6.48 (HCO3 − ) ∼2.3 ∼12

r.m. 30 30

Four-electrode Two-electrode Four-electrode

[26] [27] [28]

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X. Yan et al. / Journal of Membrane Science 375 (2011) 204–211 Table 4 Thermal characteristic temperatures (TOD a and TFD b ) of PEEK, CMPEEK 70% and QAPEEKOH 70%. Sample

TOD1 (◦ C)

TFD1 (◦ C)

TFD2 (◦ C)

TFD3 (◦ C)

PEEK CMPEEK 70% QAPEEKOH 70%

565 212 148

580 234 172

– 351 320

– 485 454

a b

The onset decomposition temperature. The fastest decomposition temperature.

rings [41]; the second one (TFD : 351 ◦ C) is assigned to the removal of the methylene groups formed by the previous crosslinking; and the third one (TFD : 485 ◦ C) is due to the main-chain decomposition. Similarly, QAPEEKOH 70% also has three weight loss steps: the first one (TOD : 148 ◦ C, TFD : 172 ◦ C) is probably attributed to the release of the decomposed products (typically, CH3 OH and N(CH3 )3 [42]) of quaternary ammonium groups; the second one (TFD : 320 ◦ C) is assigned to the removal of residual groups (e.g., –CH2 OH and –CH2 N(CH3 )2 ) of the degraded quaternary ammonium groups; and the third one (TFD : 454 ◦ C) is ascribed to the main-chain decomposition. The 148 ◦ C of decomposition temperature of QAPEEKOH is high enough as HEM for HEMFCs.

Fig. 8. TGA curves of PEEK, CMPEEK 70% and QAPEEKOH 70%.

3.5. Thermal stability The TGA and DTG curves of PEEK, CMPEEK 70% and QAPEEKOH 70% are shown in Figs. 8 and 9, respectively. Also, the corresponding thermal characteristic temperatures, the onset decomposition temperatures, TOD s, and the fastest decomposition temperatures, TFD s, are listed in Table 4. There is only one weight loss step for PEEK (TOD : 565 ◦ C, TFD : 580 ◦ C) attributed to the main-chain decomposition of PEEK. As expected, CMPEEK 70% has three weight loss steps: the first one (TOD : 212 ◦ C, TFD : 234 ◦ C) is likely ascribed to the release of chlorine-containing compounds (e.g., HCl and Cl2 ) arising from crosslinking of the chloromethyl groups with aromatic

4. Conclusions CMPEEK was successfully synthesized by chloromethylating PEEK using concentrated sulfuric acid as the solvent and chloromethyl octyl ether as the chloromethylating agent. Its DC was efficiently controlled (38–98%) by varying the chloromethylation time. Equally important, the sulfonation of PEEK during the chloromethylation was confirmed to be negligible. CMPEEKs show excellent solubility in high-boiling-point solvents, which makes it possible to prepare CMPEEK membranes. Subsequently, it was quaternized and ion-exchanged by NaOH to form QAPEEKOH membrane. The measured IEC of QAPEEKOH membranes ranges from 0.43 to 1.35 mmol g−1 , and the corresponding DQ of chloromethyl group in CMPEEKs increases from 35% to 61%. As expected, water uptake and swelling ratio of QAPEEKOH membrane increase with IEC and temperature. QAPEEKOH membranes from DC of 38–70% have appropriate water uptake (≤145%) and moderate swelling ratio (≤27%) even at 60 ◦ C. QAPEEKOH membranes are demonstrated to have high hydroxide conductivity, e.g., QAPEEKOH 70% and QAPEEKOH 76% membranes (IECs of 1.18 and 1.35 mmol g−1 , respectively) exhibit hydroxide conductivity of 13 and 17 mS cm−1 at 20 ◦ C. For comparison, under the similar conditions (∼30 ◦ C, ∼1.0 mmol g−1 of IEC and trimethylamine aqueous solution quaternization method), the hydroxide conductivity of QAPEEKOH membrane (12 mS cm−1 ) is higher than those of almost all the reported quaternized aromatic polymer membranes. The measured apparent activation energies (Ea s) are 9.5 and 10.2 kJ mol−1 for the QAPEEKOH 60% and QAPEEKOH 70% membranes, respectively, which are comparable to those of other aromatic polymers. In addition, QAPEEKOH has high thermal stability (TOD : 148 ◦ C) based on TGA and DTG analysis. All the above properties suggest that QAPEEKOH membrane (especially, with DC of around 60–70%, corresponding IEC of 0.95–1.18 mmol g−1 ) is one of the most promising candidates as HEM for HEMFCs. Acknowledgements

Fig. 9. DTG curves of PEEK, CMPEEK 70% and QAPEEKOH 70%.

The authors thank the support of Program for New Century Excellent Talents in University sponsored by the Education Ministry of China (NCET-06-0272) and National Natural Science Foundation of China (Grant No. 20976027).

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