PTFE composite anion exchange membranes for direct methanol alkaline fuel cells

Journal of Membrane Science 371 (2011) 268–275

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Quaternized polyepichlorohydrin/PTFE composite anion exchange membranes for direct methanol alkaline fuel cells Tian Yi Guo, Qing Hua Zeng, Chun Hui Zhao, Qing Lin Liu ∗ , Ai Mei Zhu, Ian Broadwell Department of Chemical & Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, The College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Received 14 June 2010 Received in revised form 11 December 2010 Accepted 25 January 2011 Available online 1 February 2011 Keywords: Composite membranes Anion exchange membranes Polyepichlorohydrin PTFE membranes Direct methanol fuel cells

a b s t r a c t Novel composite anion exchange membranes have been prepared using polyepichlorohydrin (PECH) to avoid chloromethylation. PECH is first quaternized with N, N, N , N - tetramethyl-1,6-hexane diamine (TMHDA) and then combined with porous polytetrafluoroethylene (PTFE) membranes to form composite anion exchange membranes of quaternized PECH(QPECH)/PTFE. The membranes are characterized via scanning electron microscopy (SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Water uptake, ionic exchange capacity, ionic conductivity and methanol permeability are measured to evaluate their performance in a direct methanol alkaline fuel cell. SEM results indicate that QPECH has penetrated into the pores of the PTFE membranes. The membranes exhibit high ionic exchange capacity and ionic conductivity. The ionic conductivity is in the range 3.60–6.23 × 10−2 S cm−1 at the temperature range 30–80 ◦ C. The membranes have good thermal stability under an air atmosphere and are stable in basic media with KOH concentration up to 8 M at 30 ◦ C. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Direct methanol fuel cells (DMFCs) have several advantages including ease of transportation and storage of the fuel, high energy efficiency and high power density. DMFCs have been considered as a promising power source for portable devices and vehicles. Nafion® perfluorosulfonic acid polymers, a kind of proton exchange membrane (PEM), are commonly used for DMFCs. However, several major technical drawbacks still impede their commercial application, i.e., high methanol crossover from the anode to the cathode and slow methanol oxidation kinetics at the anode [1,2]. It is well known that the reaction kinetics and catalytic activities of the anodic oxidation of fuel and the cathodic reduction of oxygen are significantly higher in alkaline media than those in acidic media, and the methanol permeation in direct methanol alkaline fuel cells (DMAFCs) is also reduced remarkably in alkaline media [3–6]. Consequently, research on DMAFCs has been focused on avoiding the deficiencies of DMFCs in acidic media. To decrease the influence of methanol crossover, anion exchange membranes (AEMs) based on polymers containing quaternary ammonium groups have been developed for applications in alkaline fuel cells. Many are prepared via chloromethylation and quaternization of the benzylchloromethyl groups. However, chloromethylation reaction is not easy to handle. The reaction effi-

∗ Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. E-mail addresses: [email protected], [email protected] (Q.L. Liu). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.01.043

ciency using chloromethyl methyl ether is good but the reagent is highly toxic and carcinogenic, whereas the yield of chloromethylation reaction could be rather low using other chloromethyl reagents [7,8], leading to low ionic conductivity of the membranes. In this study, we adopted polyepichlorohydrin with inherent chloromethyl groups as polymer matrix. Thus, the chloromethylation reaction is avoided, which could reduce the reaction steps and improve the ionic conductivity of the AEMs as expected. However, there is a risk of brittle fracture of the membrane in the presence of water when the ionic exchange capacity of the quaternized polyepichlorohydrin is higher than 1.3 mmol g−1 [9]. To overcome the shortcomings of membrane swelling, porous PTFE membranes have been adopted as supporting materials. PTFE reinforcement technique is considered as one of the most effective methods to increase membrane mechanical strength [10]. For example, PTFE reinforced Nafion composite membranes have been reported for years [11–14]. The composite membranes have many advantages: higher mechanical strength and dimensional stability, lower cost, and availability of thinner membranes. Porous PTFE membranes have also been developed to increase the stability of hydrocarbon proton exchange membranes, such as sulfonated poly(ether ether ketone) (SPEEK) [15], disulfonated poly(arylene ether sulfone) (SPSU) [16], poly(benzimidazole) (PBI) [17], sulfonated polystyrene (SPS) [18], sulfonated polyimide (SPI) [19]. In this work, porous PTFE membranes were used as support to prepare composite membranes to increase the mechanical strength and stability of the anion exchange membranes. To our knowledge, this has not been reported in anion exchange membranes yet.

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Nomenclature A Am b CB0 Ea l lm md mw Mo,HCl Me,HCl P Rm R VA Wu

cross-sectional area of the test membrane (cm2 ) effective area of the membrane (cm2 ) slope of the fitting line initial concentration of methanol in compartment B (mol L−1 ) activation energy of ion transport through the membranes (kJ mol−1 ) distance between the two electrodes (cm) membrane thickness (cm) mass of the dried membranes (g) mass of the wet membranes (g) milliequivalent of HCl required before equilibrium (meq) milliequivalent of HCl required after equilibrium (meq) methanol permeability (cm2 s−1 ) membrane resistance () gas constant (8.314 J K−1 mol−1 ) volume of the solution in compartment A (cm3 ) water uptake (%)

Greek letters  ionic conductivity (S cm−1 )

2. Experimental 2.1. Materials N,N,N ,N -tetramethyl-1,6-hexanediamine (99%, TMHDA) was obtained from International Laboratory. Polyepichlorohydrin (PECH) was purchased from Aldrich. Polytetrafluoroethylene (PTFE) membranes were purchased from Zhejiang Dongyang Sanwei Kongmo Co., Ltd. The average pore size of the PTFE membranes is 0.5 ␮m, the biggest pore size is 2.5 ␮m and porosity is 80%. The thickness of the membranes varies from 30 to 40 ␮m. All other chemicals were supplied from the Shanghai Chemical Reagent Store (China) and used without further purification. 2.2. Preparation of composite anion exchange membranes 2.2.1. Synthesis of quaternized PECH The quaternizaton reaction was performed according to Scheme 1. PECH (1.5 g) was dissolved in 30 mL of N,Ndimethylformamide (DMF) or dimethylsulfoxide (DMSO) and stirred for 2 h to form a homogeneous solution. An amount of TMHDA was added into the solution and stirred to take a reaction

269

Table 1 Results of the quaternization of PECH under different reaction conditions. No. 1 2 3 4 5 6 7 8

PECH:TMHDA (mol:mol) 1:2 1:2 1:2 1:3 1:3 1:3 1:2 1:3

Temperature (◦ C)

Time (h)

Solvent

N content (wt%)

60 80 100 100 100 100 100 100

48 12 12 12 24 12 24 24

DMF DMF DMF DMF DMF DMSO DMSO DMSO

0.19 1.07 1.60 2.40 3.05 3.68 4.15 5.26

for various hours (Table 1). After filtration, the resulting solution was cast onto glass plates, and then dried at 60 ◦ C for 24 h to form films. The obtained membranes were rinsed with deionized water and then dried under vacuum to obtain yellowish products. 2.2.2. Condition for quaternization The quaternization of PECH was influenced by different reaction conditions, such as reactants, temperature, time and solvent, as listed in Table 1. The N content was only 0.19% after reacting at 60 ◦ C for 48 h, and increased to 1.60% when reacting at 100 ◦ C for 12 h. The N content increased with increasing the amount of TMHDA and reaction time. In this study, the effect of different solvents (DMF and DMSO) was investigated. The degree of quaternization was significantly increased when DMSO was used as a solvent. This indicates that the quaternization could be better in DMSO than in DMF. DMSO was therefore adopted as a solvent in the following study. 2.2.3. Preparation of QPECH/PTFE composite membranes The quaternized PECH became brittle in the presence of water. So PTFE membranes were used as matrix to prepare composite membranes with improved physical and chemical properties. The composite anion exchange membranes were prepared as follows. To improve the wettability of PTFE in DMSO, the PTFE membranes were swelled and cleaned in anhydrous ethanol for 30 min. Meanwhile, the same volume of anhydrous ethanol as DMSO was added into the reacting system and stirred at room temperature for 30 min after the quaternization reaction being completed. After that, the above solution was cast onto the swelled PTFE membranes to prepare composite membranes. The membranes were further heated at 50 ◦ C for 2 h and then at 60 ◦ C until the solvent was removed. The composite membranes were peeled off and dried in a vacuum oven at 100 ◦ C for 4 h. As a result, the yellowish translucent membranes were obtained. The composite membranes were then soaked into 1 M KOH solution at 30 ◦ C for 24 h and rinsed with deionized water until pH neutral. The obtained membranes were designated as QPECH/PTFE1, QPECH/PTFE2 and QPECH/PTFE3 corresponding to the reaction nos. 6, 7 and 8 (Table 1), respectively. 2.3. Characterizations 2.3.1. FESEM and EDX The morphology and microstructure of the membranes as well as the element distribution were measured using field emission scanning electron microscope with energy dispersive spectrometry (FESEM, EDX, LEO 1530, Germany). Before observations, the membranes were fractured in liquid nitrogen and then sputtered with gold.

Scheme 1. Synthesis of quaternized polyepichlorohydrin.

2.3.2. FTIR spectroscopy The FTIR spectra were recorded using a Nicolet Avatar 360 spectrophotometer (Thermo Electron Corporation, USA) equipped with

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attenuated total reflectance (ATR) with a resolution of 2 cm−1 and a spectral range 4000–650 cm−1 . 2.3.3. XRD The physical structure of the membranes was studied by Xray powder diffraction (XRD, Panalytical X’pert, Enraf-Nonious Co., Holland) using Cu K␣ radiation with a step size of 0.0167◦ and a scan speed of 0.167◦ s−1 in the range 5–50◦ . 2.3.4. Thermal stability Thermal stability of the membranes was measured using a thermogravimetric analyzer (TGA, TG209F1, NETZSCH, Germany) under an air atmosphere, with a heating rate of 10 ◦ C min−1 from 30 to 600 ◦ C 2.3.5. Water uptake and ionic exchange capacity (IEC) Water uptake was determined by measuring the difference in weight of the membrane before and after being immersed in deionized water. The membrane sample was first placed in deionized water at 30 ◦ C for more than 48 h, and immediately weighed to determine the weight of the wet membrane after removing the surface water. Then the wet membrane was subsequently dried at 60 ◦ C under vacuum until a constant dried weight was achieved. The water uptake Wu (%) can be calculated by: Wu =

mw − md × 100% md

(1)

where mw and md are the mass of the wet and dried membranes (g), respectively. The ionic exchange capacity (IEC) was measured using the classical back titration method. The weight of the dried QPECH membrane was first obtained. Then the membrane was soaked into a 100 mL of 0.1 M HCl solution for 24 h to undergo an ionic exchange process. The solution together with the membrane was back titrated with a 0.1 M NaOH solution. The IEC values (mmol g−1 ) can be calculated via the following relationship: IEC (meq g−1 ) =

Mo,HCl − Me,HCl md

(2)

where Mo,HCl and Me,HCl are the milliequivalents (mmol) of HCl required before and after equilibrium, respectively. md is the mass (g) of the dried membrane. 2.3.6. Ionic conductivity measurements The ionic conductivity of the membranes in the transverse direction was measured by two-probe AC impedance spectroscopy using a Parstat 263 electrochemical equipment (Princeton Advanced Technology, USA) [20]. The measurement of impedance was carried out over a frequency range 0.1–105 Hz within a temperature range 30–80 ◦ C. The ionic conductivity  (S cm−1 ) can be calculated by: =

l ARm

(3)

where l is the distance between the two electrodes (cm), A is the cross-sectional area of the testing membrane (cm2 ), and Rm is the membrane resistance () acquired from a Nyquist plot. 2.3.7. Methanol permeability measurements The methanol permeability was measured within a temperature range 30–80 ◦ C using a home-made diffusion cell which consists of two identical diffusion compartments with volume of approximately 25 cm3 [21]. The increase of methanol concentration with time was measured by gas chromatography (GC-950, Shanghai Haixin Chromatographic Instruments Co., Ltd.). Assuming

Table 2 Weight percent and atomic percent of elements distribution on the cross-section. Element

Weight (%)

Atomic (%)

C O F Cl Au Total

32.31 9.59 1.91 31.30 24.89 100

61.15 13.63 2.28 20.07 2.87 100

a pseudo-steady-state condition and CB  CA , the methanol permeability P (cm2 s−1 ) can be estimated as follows [22]: CA (t) = CB

A  P  m lm

VA

(t − t0 )

(4)

where CA is the concentration (mol L−1 ), VA is the volume of the solution in compartment A (cm3 ), lm is the membrane thickness (cm) and Am is the effective area of the membrane (cm2 ). 3. Results and discussion 3.1. FESEM and EDX Fig. 1(a) shows the surface morphology of the PTFE membrane. The surface of the PTFE membrane is rough and porous. Fig. 1(b) shows a continuous film of QPECH being formed on the surface of the PTFE membrane. The QPECH was found to penetrate into the pore of the PTFE membrane from the bottom surface of the composite membrane (Fig. 1(c)). This can also be observed from the cross-sectional images of the composite membrane (Fig. 1(d and e)). The results suggest that QPECH is compatible with PTFE and the bonding between QPECH and PTFE is effective. Table 2 shows the element distribution on the cross-section of QPECH/PTFE. The chlorine (Cl) content is 20.07 wt% and the oxygen (O) content is 9.59 wt%. These element (not existed in the PTFE matrix) contents demonstrate that QPECH penetrated into the pores of PTFE membranes. 3.2. FTIR The FTIR spectra of PECH, QPECH and QPECH/PTFE3 are shown in Fig. 2. Compared with PECH, two new peaks at 1640 and 3400 cm−1 can be observed in the spectrum of QPECH. The former is associated with the C–N group and the latter belongs to the stretching vibration of O–H group. The observations suggest the quaternary ammonium groups being successfully introduced into PECH. Peaks at 1640 and 3400 cm−1 can be clearly observed on the surface of the QPECH/PTFE3 membrane (Fig. 2(d)), since a film of QPECH was formed on the surface of PTFE from the SEM image. However, a peak at 1210 cm−1 associated with C–F group is observed on the surface of QPECH/PTFE3. This is because the infrared ray can penetrate into the membrane for a certain distance under the attenuated total reflectance mode. The characteristic peak of C–F group can thus be detected on the surface of the QPECH/PTFE3 membrane since just only a thin film of QPECH was formed on the PTFE matrix. Whereas the peaks at 1640 and 3400 cm−1 are rather weak in the bottom surface of QPECH/PTFE3 (Fig. 2(e)). This suggests that there was no QPECH film formed on the bottom surface, and agrees with the results from SEM characterization. 3.3. XRD The physical structure of the membranes was investigated by XRD, as shown in Fig. 3. A broad diffraction peak of PECH and QPECH appeared at 2 = 16–25◦ , which is the amorphous region of the PECH matrix (Fig. 3(a and b)). In the XRD pattern of QPECH, a new sharp

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271

Fig. 1. SEM images of membranes (a) surface of PTFE, (b) top surface of QPECH/PTFE, (c) bottom surface of QPECH/PTFE, (d) cross-section (500×) and (e) cross-section (5000×).

peak appeared at 2 = 44◦ (Fig. 3(b)). This indicates that the quaternization reaction caused an orderly arrangement of the PECH matrix, leading to the formation of a new crystalline region. A sharp peak of PTFE appeared at 2 = 18◦ , which is the crystalline region of the polyfluorocarbon matrix (Fig. 3(c)). The XRD patterns of the top surface and bottom surface of QPECH/PTFE3 are quite different from each other (Fig. 3(d and e)). There is no sharp peak at 2 = 18◦ on the top surface. This is because the PECH matrix can destroy the crystalline structure of PTFE. The bottom surface demonstrates a

similar XRD pattern with PTFE. But the intensity is weaker than the pristine PTFE membrane. This indicates that the penetrated QPECH can affect the crystalline structure of PTFE. 3.4. TGA Fig. 4 shows the TGA curves of PECH, QPECH and QPECH/PTFE3 under an air atmosphere. The weight of PECH decreases rapidly starting from 300 ◦ C due to the decomposition of the chloromethyl

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Fig. 4. TGA curves of the PECH, QPECH and QPECH/PTFE3 membranes.

Fig. 2. FTIR spectra of membranes (a) PECH, (b) QPECH, (c) PTFE, (d) top surface of QPECH/PTFE3,and (e) bottom surface QPECH/PTFE3.

groups. Compared to PECH, QPECH exhibits a three-step degradation process. The first weight loss is from 30 to 200 ◦ C. Before the test, the samples were kept in a vacuum oven under 60 ◦ C. The weight loss is mainly due to the evaporation of the bounded water, as well as some free water absorbed during the preparation of the sample. The second weight loss near 200 ◦ C is attributed to

Fig. 3. XRD patterns of membranes (a) PECH, (b) QPECH, (c) PTFE, (d) top surface of QPECH/PTFE3,and (e) bottom surface of QPECH/PTFE3.

the decomposition of the quaternary ammonium groups. The third weight loss above 300 ◦ C is due to the decomposition of the unreacted chloromethyl groups of PECH. The TGA curve of QPECH/PTFE3 is similar to that of QPECH. The difference is that the QPECH/PTFE3 displays a fourth weight loss at approximately 500 ◦ C. This is attributed to the decomposition of the fluorocarbon of PTFE. Further, the first weight loss of QPECH/PTFE3 is lower than that in QPECH. This indicates that PTFE could depress the hydrophilicity of QPECH. Therefore, the composite membranes would not become brittle in the presence of water. Please be advised that TGA is not a reliable method to probe definitive thermal stabilities since TGA experiments have too short a timescale. The 1st differential graph of the TGA results is shown in Fig. 5. The peaks represent the fastest rate of each decomposition stages. The fastest decomposition rate of the chloromethyl groups shifts from 360 ◦ C for PECH to 280 ◦ C for QPECH, indicating that the intro-

Fig. 5. 1st derivative curves of the TGA curves of the PECH, QPECH and QPECH/PTFE3 membranes.

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Scheme 2. Incomplete reaction of TMHDA.

Fig. 6. Water uptake of the composite membranes.

duction of the quaternary ammonium groups would reduce the thermal stability of PECH. The fastest decomposition rate of the quaternary ammonium groups in QPECH and QPECH/PTFE3 is 230 and 250 ◦ C, respectively; whereas that of the chloromethyl groups in QPECH and QPECH/PTFE3 is 280 and 300 ◦ C, respectively. This is probably that the polyfluorocarbon backbone can increase the stability of the chloromethyl and quaternary ammonium groups, leading to a decrease in the decomposition rate. The observations indicate that the PTFE matrix could improve the thermal stability of the composite anion exchange membranes. 3.5. Water uptake and ionic exchange capacity Fig. 6 shows the water uptake of the three composite membranes. The water uptake increases with increasing the N content in QPECH. The PTFE membrane is hydrophobic, whereas the water uptake of the composite membranes is greater than 70%, indicating that the composite membranes are highly hydrophilic when QPECH was filled in the PTFE membranes. This is because the quaternary ammonium groups in QPECH are rather intimate to water. As a result, the hydrophilic and hydrophobic properties of the composite membranes were changed due to the incorporation of QPECH. IEC was first determined using 13 C NMR, nuclear magnetic resonance and Mohr precipitation titration of Cl− in the presence of potassium chromate and with AgNO3 titration solution. However, the end point of Mohr precipitation titration is hard to judge and NMR cannot be used to quantitively determine IEC values. The backtitration method is finally used in this work to determine the IEC values, as listed in Table 3. The N content of the three composite membranes is different because of the different reaction conditions for the quaternization. Increasing both temperature and time can increase the N content of the composite membranes. The N content of the QPECH/PTFE3 membrane reaches a value of 3.51%. In addition, the IEC acquired from elemental analysis is higher than that from the titration method. The elemental analysis is performed by converting the N element into NO2 gas via combustion in high tem-

perature, and then detecting the NO2 gas via gas chromatography. Therefore, all the N elements in QPECH can be measured. However, in the titration method, hydrochloric acid was used to neutralize the hydroxide ion in the membrane, by which the amount of the hydroxide ion in the membrane was determined by the change in the mole of hydrochloric acid. This suggests that the amount of quaternary ammonium groups in the membrane can be determined by the titration method. The results indicate that the tertiary amines in TMHDA did not completely convert into quaternary ammonium groups (Scheme 2). The IEC value is much higher than that of QSEBS (0.3 mmol g−1 ) which is quaternized polystyrene-blockpoly(ethylene–ranbutylene)-block-polystyrene and reported in our previous study [23]. This is because the chloromethylation process was avoided using PECH as a membrane material. The rate of networking (reticulation) was determined using conductivity meter (inoLab Cond Level 2, WTW, Germany). Before measuring the amount of free chloride ion after the reaction of hexa methylene diamine, we plotted the standard curve of conductivity vs. concentration of chloride ion. The conductivity was measured in the concentration range 0–500 mg L−1 . The experimental results show a good linear relationship between the conductivity and chloride ion concentration within the range 0–500 mg L−1 (R = 0.99995). The linear regression equation is Y = 1.97664X + 4.14055 through determination of standard sample. The conductivity of reaction solutions was determined every 1 h for 26 h. The rate of networking as well as the amount of free chloride ion was calculated by linear regression equation, as shown in Fig. 7.

Table 3 IECs of the three composite membranes. Membranes

N content (wt%)

IECa (mmol g−1 )

IECb (mmol g−1 )

QPECH/PTFE1 QPECH/PTFE2 QPECH/PTFE3

2.67 3.20 3.51

1.91 2.28 2.51

1.10 1.21 1.70

a b

Elemental analysis. Titration method.

Fig. 7. The relation of the concentration of chloride ion with the time of quaternization.

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Fig. 10. Methanol permeability of the QPCEH/PTFE3 membrane vs. temperature.

Fig. 8. Ionic conductivity of the three composite membranes vs. temperature.

3.6. Ionic conductivity The ionic conductivity of the composite membranes as a function of temperature in deionized water is shown in Fig. 8. The ionic conductivity is almost above 10−2 S cm−1 , and increases with increasing temperature and IEC. The QPECH/PTFE3 membrane showed a value of 3.60 × 10−2 S cm−1 at 30 ◦ C and 6.23 × 10−2 S cm−1 at 80 ◦ C. The conductivity is much higher than the QSEBS membrane (5.12–9.37 × 10−3 S cm−1 within a temperature range 30–80 ◦ C) [23]. This is desirable when it is used as an anion exchange membrane for a fuel cell. The high conductivity of the composite membrane is resulting from a high level of quaternization. The grafted quaternary ammonium groups in the membrane are responsible for the formation of ionic transport channels. As mentioned above, the QPECH/PTFE3 membrane showed a high IEC value. Since increasing ion content can lead to an increase in the hydrophilic and ionic nature of the polymer, the high IEC value can help to form continuous ionic transport channels that the OH− anions can pass through. Consequently, the QPECH/PTFE3 membrane demonstrates high ionic conductivity. In addition, the tertiary amine in the membrane can also facilitate the mobility of the OH− anions.

Fig. 9 shows the relation between ln  and 1000/T. The conductivity was assumed to follow Arrhenius behavior, the activation energy of ion transport Ea of the composite membranes can thus be obtained from Arrhenius equation: Ea = −b × R

(5)

where b is the slope of the regression line of ln  vs. 1000/T plots and R is the gas constant (8.314 J K−1 mol−1 ). The Ea value of QPECH/PTFE3 is much lower than the other composite membranes. The lower value shows consistence with the high IEC or water uptake as discussed above. 3.7. Methanol permeability Fig. 10 shows the methanol permeability of the QPECH/PTFE3 membrane in 2 M methanol solution vs. temperature. The methanol permeability increases from 2.44 to 5.23 × 10−6 cm2 s−1 , corresponding to a temperature rise from 30 to 80 ◦ C. As temperature increases, the mobility of both methanol and polymer chains increases, leading to an increase in the methanol permeability. The methanol permeability is comparable with the Nafion membranes. The methanol molecules mainly transport through the ion clusters and the penetrated ion channels in the membranes as the ions do. The high IEC value and water uptake of the composite membrane lead to high swelling of the polymer, resulting in high methanol permeability. 3.8. Stability in KOH solution

Fig. 9. Arrhenius plots for the three composite membranes.

The stability of the QPECH/PTFE3 membranes was investigated by conditioning the membranes in different concentrations of KOH solution at 30 ◦ C for 24 h [24]. Fig. 11 shows the ionic conductivity of the treated membranes after the free KOH was completely removed. The ionic conductivity is in a range 0.034–0.038 S cm−1 after treating the membranes in the concentration range 1–8 M KOH solution. This reveals that the ionic conductivity changed slightly up to 8 M KOH solution. And this indicates that membranes with TMHDA groups are quite stable at 30 ◦ C even being treated with a KOH concentration up to 8.0 M. Komkova et al. [25] investigated the stability in alkaline solution of the quaternary ammonium groups of the anion exchange membranes prepared by different length of the aliphatic chain in the diamine units. The increase in the electron density at the ␤-carbon makes it more difficult for the base to abstract proton. It is shown that increase in electron density at the ␤-carbon can be expected in the quaternary ammonium groups of the TMHDA membrane due to the shift from ␥-carbon, resulting in difficulties for the ␤-proton abstraction. This indicates

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dation of Fujian Province of China Grant nos. 2009J01040 and 2010I0013, and the research fund for the Doctoral Program of Higher Education (no. 20090121110031) in preparation of this article is gratefully acknowledged. References

Fig. 11. Ionic conductivity of the QPECH/PTFE3 membrane vs. KOH concentration.

that TMHDA groups do not slightly decompose with increasing KOH concentration. As a result, the conductivity of membranes with TMHDA groups only slightly changed. Therefore, the membranes with TMHDA groups are one of the most stable membranes in the alkaline solutions, and ionic conductivity can hardly change due to the stability of TMHDA groups. This is one of the main reasons that TMHDA was used as quaternary reagent in this study. To test whether the QPECH will be peeled off, the membrane was soaked and stirred into a 60 mL of 1 M KOH aqueous solution at 80 ◦ C for 48 h, as well as a 60 mL of Fenton reagent, which consists of 3% H2 O2 and 2 ppm FeSO4 . Through the testing process, the QPECH did not peel off. 4. Conclusions PECH with chloromethyl groups was successfully quaternized with TMHDA. The composite anion exchange membranes were prepared by combining the quaternized PECH with porous PTFE membranes. The XRD and SEM results show that quaternized PECH has penetrated into the pores of the PTFE membranes. The water uptake is in the range of 75–100% under different quaternization conditions. The IEC reaches a maximum value of 1.70 mmol g−1 . The ionic conductivity of the composite membranes increases with increasing temperature and IEC. The ionic conductivity of the as-prepared composite membrane in deionized water is 3.60 × 10−2 S cm−1 at 30 ◦ C and 6.23 × 10−2 S cm−1 at 80 ◦ C. The methanol permeability is about an order magnitude of 10−6 cm2 s−1 tested in 2 M methanol solution. Thermal analysis results show that the PTFE membranes could improve the thermal stability of the composite anion exchange membranes, which is stable below 200 ◦ C under an air atmosphere. The ionic conductivity of the composite membranes in deionized water changed slightly when treated with 1–8 M KOH solution at 30 ◦ C for 24 h. Further investigation will focus on the performances applied in direct methanol fuel cells. Acknowledgements Financial support from National Nature Science Foundation of China Grant nos. 20976145 and 21076170, Nature Science Foun-

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