Anion exchange membranes based on quaternized polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene for direct methanol alkaline fuel cells

Journal of Membrane Science 349 (2010) 237–243

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Anion exchange membranes based on quaternized polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene for direct methanol alkaline fuel cells Qing Hua Zeng, Qing Lin Liu ∗ , Ian Broadwell, Ai Mei Zhu, Ying Xiong, Xing Peng Tu 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, Fujian 361005, China

a r t i c l e

i n f o

Article history: Received 17 September 2009 Received in revised form 3 November 2009 Accepted 22 November 2009 Available online 27 November 2009 Keywords: Anion exchange membranes Polystyrene-block-poly(ethylene-ranbutylene)-block-polystyrene (SEBS) Alkaline Direct methanol fuel cells

a b s t r a c t A novel anion exchange membrane has been prepared using polystyrene-block-poly(ethylene-ranbutylene)-block-polystyrene (SEBS) as a starting material. Chloromethyl groups are first introduced into the SEBS and then converted into the quaternary ammonium groups. The membrane is characterized by Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA). Water uptake, oxidative resistance, ionic conductivity and methanol permeability are measured to evaluate its performance in a direct methanol alkaline fuel cell. The ionic conductivity and permeability of the membrane to methanol was found to increase with temperature. The membrane exhibits an ionic conductivity of 9.37 × 10−3 S cm−1 in deionized water at 80 ◦ C. For aqueous methanol solutions between 2 and 10 M at 30 ◦ C, the methanol permeability was observed to range 2.34–4.45 × 10−7 cm2 s−1 . Our novel SEBS membrane also shows high oxidative resistance to Fenton’s reagent and good thermal stability under an air atmosphere. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Direct methanol fuel cells (DMFCs) are considered as promising alternatives to lithium ion batteries in portable devices because of their associated advantages: for example methanol has a high energy density 10 times greater than that of the material used in lithium ion batteries [1] and has ease of handling in its relatively unreactive liquid state. The polymer electrolyte membrane, which can separate the fuels from the transport ions, is one of the key components in a DMFC. Proton exchange membranes (PEMs) are commonly used as polymer electrolyte membranes. Among various PEMs, Nafion® is the state-of-the-art PEM, owing to its high proton conductivity and excellent physical and chemical stability. However, several technical drawbacks such as slow oxidation kinetics of methanol and high methanol crossover from the anode to the cathode [2–4] impede its widespread usage in such applications. To solve the problems, some research groups have used alkaline anion exchange membranes (AEMs) in DMFCs [5–8] instead of PEMs. Several benefits of such fuel cells have been suggested: (1) methanol oxidation in alkaline media is faster than that in acidic media, which will enable the use of non-precious metal catalysts i.e. cost reduc-

∗ 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 © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.11.051

tion; (2) the direction of the ions’ transport in the AEMs is opposite to that of the methanol, which is the reverse of the situation in the PEMs, resulting in a decrease of methanol permeability; (3) the water management regime is simplified since water is produced at the anode and partially consumed at the cathode; (4) corrosion problems are less serious in alkaline than in acid environment [9–12]. Several AEMs with quaternary ammonium groups have been currently developed for applications in direct methanol alkaline fuel cells (DMAFCs), such as polysulfone [13–16], fluorinated polymers including poly(tetrafluoroethene-co-hexafluoropropylene)(FEP), poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVDF) [12,17,18], copolymer of 4vinylpyridin and styrene [19], poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [20], poly(vinyl alcohol) (PVA) and chitosan (CS) [21–23]. However, currently there are no AEM with excellent performances in DMAFC similar to the already established Nafion® in DMFC. This is one area of fuel cell development where further research is needed. The basic requirement of an AEM for use in a DMAFC is that it should have good mechanical, thermal and chemical stability as well as high conductivity. With a thermoplastic phase (styrene) dispersed in an elastomeric matrix (ethylene/butylenes), SEBS is therefore a thermoplastic elastomer with excellent mechanical, chemical and thermal stability [24]. SEBS is a phase-segregated material where the styrene block can be functionalized for further

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applications. The modification of SEBS by sulfonation has been carried out routinely for many years. Many of sulfonation modified SEBSs have already been applied in DMFCs and demonstrate good electrochemical characteristics [25,26]. As the ethylene/butylenes block cannot be sulfonated, the non-ionic block can act as a barrier to methanol. In addition, SEBS is a triblock copolymer and has the ability to form three-phase morphology which may affect the transport properties and subsequently DMFCs performances [27,28]. SEBS is therefore adopted in the present work as a starting material to prepare an AME with the purpose of achieving good performances. 2. Experimental 2.1. Materials SEBS with molecular weight of 118,000 and styrene/ ethylene–butylene (w/w) = 29/71 was purchased from Aldrich. All other chemicals with analytical grade were supplied from the Shanghai Chemical Reagent Store (China) and used without further purification. 2.2. Membrane preparation The anion exchange membrane was prepared via chloromethylation, quaternization and alkalization consecutively. Chloromethyl methyl ether is commonly used as a chloromethylation reagent. Unfortunately, it is highly toxic and carcinogenic. According to the chloromethylation of polystyrene [29] and a report by Fang and Shen [14], we used paraformaldehyde and hydrogen chloride gas as the chloromethylation reagent due to their safety, easy handling and low cost. SEBS was first dissolved in cyclohexane and stirred for 0.5 h to form a homogeneous solution. Dried hydrochloric acid gas was purged into the solution and followed by the addition of an excess of paraformaldehyde (CH2 O)n ((CH2 O)n /SEBS at 3/1 by molar ratio). A given amount of zinc chloride (ZnCl2 ) was then added and stirred to catalyze the reaction for 2 h. The resulting mixture was precipitated and then rinsed with anhydrous ethanol until neutral pH. The product was dried at 60 ◦ C under vacuum for 6 h and a white solid chloromethylated SEBS (CSEBS) was finally obtained. The CSEBS was dissolved in cyclohexane to make a 5 wt% solution. The resulting homogeneous solution was poured onto glass plates and dried at room temperature. The membranes were peeled off and placed in a vacuum oven at 60 ◦ C to completely evaporate the residual solvent (until a constant weight was achieved). The dried membranes were immersed into 30 wt% trimethylamine solution for 48 h to introduce quaternary ammonium groups into the membranes, and then were soaked into 1 M KOH solution for 24 h to complete ionic exchanges (Cl− to OH− ) at room temperature. After rinsing carefully with deionized water until neutral pH was attained, the quaternized SEBS (QSEBS) membranes were lastly stored in deionized water before testing. 2.3. Characterizations 2.3.1. FTIR spectroscopy The FTIR spectra of SEBS, CSEBS and QSEBS in KBr pellets were recorded using a Nicolet IR200 spectrophotometer (Thermo Electron Corporation, USA) with a resolution of 4 cm−1 in a spectral range of 4000–400 cm−1 . 2.3.2. Thermal stability Thermal stability of the membranes was measured by a thermogravimetric analyzer (TG209F1, NETZSCH, Germany) under an air atmosphere, with a heating rate of 10 ◦ C min−1 from 30 to 600 ◦ C.

2.3.3. Oxidative stability The oxidative stability was studied by estimating the weight changes of the membranes in Fenton’s reagent [30]. Small pieces of membrane samples (3 cm × 3 cm) were placed into Fenton’s reagent (4 ppm FeSO4 in 3% H2 O2 ) at 80 ◦ C under stirring. The samples were taken out of the solution at regular intervals and quickly weighed after removing the surface liquid with filter paper. The Fenton’s reagent was refreshed every 10 h. 2.3.4. Water uptake and ionic exchange capacity (IEC) Water uptake was determined by measuring the weight difference of the membrane before and after immersing 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 (%) was 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 performed by the classical back titration method. The weight of the dried QSEBS membrane was first obtained. Then the membrane was soaked into a 100 mL of 0.1 M HCl solution for 48 h to undergo an ionic exchange process. The solution together with the membrane was back titrated with a 0.1 M KOH solution. The IEC values (meq g−1 ) are calculated using the following relationship: IEC (meq g−1 ) =

Mo,HCl − Me,HCl md

(2)

where Mo,HCl and Me,HCl are the milliequivalents (meq) of HCl acquired before and after equilibrium, respectively. md is the mass (g) of the dried membrane. 2.3.5. Ionic conductivity measurements The ionic conductivity of the membranes in the transverse direction was measured in a temperature range 30–80 ◦ C by two-probe AC impedance spectroscopy using a Parstat 263 electrochemical equipment (Princeton Advanced Technology, USA) [22]. The impedance measurements were carried out over a frequency range 0.1–105 Hz. The membranes were equilibrated in deionized water for at least 24 h at room temperature before measurements. The hydrated membrane was then clamped between two stainless steel electrodes which were placed in deionized water to maintain the relative humidity of 100% during each experiment. 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 test membrane (cm2 ), and Rm is the membrane resistance () acquired from a Nyquist plot. 2.3.6. Methanol permeability measurements The methanol permeability was measured in a temperature range 30–80 ◦ C using a home-made diffusion cell which consists of two identical diffusion compartments of approximately 25 cm3 volume [21]. The membrane was clamped between the two compartments. One compartment (A) was filled with 20 cm3 of deionized water, while the other (B) contained the same amount of methanol solution. The cell was stirred and kept at a constant temperature by a water bath. The increase of methanol concentration

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with time (t) in compartment A was measured by a gas chromatography (GC-950, Shanghai Haixin Chromatographic Instruments Co., Ltd). Assuming the pseudo-steady-state and CB  CA , the methanol permeability P (cm2 s−1 ) can be estimated as follows [20]: CA (t) = CB

A  P  m lm

VA

(t − t0 )

(4)

where CA is 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. Preparation of anion exchange membranes There are three consecutive steps in preparation of the anion exchange membrane: chloromethylation, quaternization and alkalization (Scheme 1). In the presence of the catalyst, the

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Table 1 Chloromethylation condition and Cl content of SEBS. Code number

SEBS:(CH2 O)n :Cat (mol:mol:mol)

Temperature (◦ C)

Cl content (wt%)a

1 2 3 4 5 6

1:3:1 1:3:1.5 1:3:3 1:3:1 1:3:1 1:3:1

40 40 40 30 50 60

1.75 1.645 0.77 0.645 1.515 1.16

a

Determined by elemental analysis.

paraformaldehyde and hydrochloric acid first generated –CH2 OH groups and then reacted with the aromatic rings by electrophilic substitution. The hydroxyl groups were replaced by the hydrochloric acid, resulting in the formation of CSEBS. The QSEBS was obtained by the reaction of chloromethyl groups with trimethylamine and subsequently an ion exchange. The chloromethylation of SEBS is strongly affected by different reaction conditions, especially the catalyst and reacting temperature. In this study, the catalyst ZnCl2 has a significant effect (Table 1). When the molar ratio of SEBS to ZnCl2 is at 1:1, the Cl content exhibits a maximum value. This is probably that excessive catalyst could accelerate the crosslinking reaction. At a given reactant concentration and reacting time, the Cl content first increases and then decreases with increasing temperature (Table 1). Increasing temperature can accelerate the electrophilic substitution. However, as temperature increases further, the crosslinking reaction could compete with the chloromethylation reaction, leading to a decrease in Cl content. Therefore, the reaction condition of No. 1 was adopted for the chloromethylation of SEBS. 3.2. FTIR spectroscopy The FTIR spectra of SEBS, CSEBS and QSEBS are shown in Fig. 1. The peaks at 3025 and 3059 cm−1 are associated with the stretching vibration of CH groups of aromatic hydrocarbon. The peaks within ranges 2923–2853 and 1492–1461 cm−1 are attributed to the stretching and bending vibration of –CH2 –groups of the aliphatic chains, respectively. The peaks at 1378 and 966 cm−1 are the bending vibration of CH3 – groups of the aliphatic chains. Two new peaks appearing at 800 and 1265 cm−1 in the spectrum of CSEBS (Fig. 1(b)) are ascribed to the –CH2 Cl groups [31]. This indicates that the SEBS has been successfully chloromethylated. Two characteristic peaks at 1118 and 3439 cm−1 can be observed in the spectrum of QSEBS (Fig. 1(c)). The former represents the stretching

Scheme 1. Chloromethylation, quaternization and alkalization reaction of SEBS.

Fig. 1. FTIR spectra of (a) SEBS, (b) CSEBS and (c) QSEBS.

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Fig. 2. TGA results of the SEBS and QSEBS membranes.

vibration of C–N groups [32] and the latter belongs to the stretching vibration of O–H groups. These observations suggest the quaternary ammonium groups being successfully introduced into the SEBS. 3.3. Thermal stability Fig. 2 shows the TGA curves of the SEBS and QSEBS membranes. The SEBS membrane displays a good thermal stability with a degradation temperature approximately 400 ◦ C under an air atmosphere. The quaternization reaction reduced the thermal stability of the QSEBS membrane. The QSEBS membrane exhibits a three-step degradation process. The first weight loss near 100 ◦ C is attributed to the evaporation of water in the membrane. The second weight loss within a range 190–280 ◦ C is due to the decomposition of the quaternary ammonium groups. At the third stage above 400 ◦ C, the weight of QSEBS losses rapidly. This is attributed to the decomposition of the main chain. Although the introduction of the quaternary ammonium groups decreased the thermal stability of the membranes, the QSEBS membrane still shows great stability below 190 ◦ C, which is comparable to some other AEMs [14,16]. This testing gives an indication of the short-term thermal stabilities of the membranes. 3.4. Oxidative stability It is essential for polymer electrolyte membranes to have a good oxidative stability in the harsh operating environment of fuel cells. During a typical fuel cell operating cycle, oxygen molecules diffuse through the membrane and incompletely reduce at the fuel cell anode, resulting in the formation of HO and HO2 radicals which can cause degradation of the membrane material [33]. Therefore, Fenton’s reagent was adopted to simulate the practical operation environment of fuel cells. Fig. 3 illustrates the weight losses of the membranes as a function of time. An initial increase of weight can be detected at approximately 10 h. This is probably that the swelling of the membranes overwhelmed the membranes degradation at first [34]. The initial swelling is because the membranes were kept in an oxidative solution at 80 ◦ C. Hence, the membranes swelled further with the partially degradation as well as the higher temperature. As time increases, the weight of the membranes decreases gradually. The weight loss of the QSEBS membrane is less than 10% within 120 h (Fig. 3), which is less than some other membranes reported for fuel cells. For example, the quaternized copolymer of vinylbenzyl chloride (VBC) and ␥-methacryloxypropyl trimethoxy silane (␥-MPS) loses a weight of about 70% in 40 h when treated with hot Fenton’s reagent [35]. Further, we measured the ionic

Fig. 3. Chemical stability of the SEBS and QSEBS membranes in a 3% H2 O2 /4 ppm Fe2+ solution at 80 ◦ C.

conductivity of the QSEBS membrane after immersed in the Fenton’s reagent at 80 ◦ C for 120 h. Comparing the ionic conductivity of the QSEBS membrane before and after oxidative experiment (5.12 × 10−3 and 3.34 × 10−3 S cm−1 at 30 ◦ C), the QSEBS membrane exhibits good oxidative stability. 3.5. Water uptake and ionic exchange capacity (IEC) It is well known that water uptake has a significant effect on the transport behavior and mechanical properties of a proton exchange membrane [36]. A membrane with higher water uptake can offer more opportunity for protons to transfer through the membrane, leading to a higher proton conductivity. However, excess water uptake will bring about too high methanol permeability and an unacceptable deformation, thereby even reducing the mechanical stability. Similarly, it is an essential parameter for anion exchange membranes to operate in DMAFCs. Fig. 4 shows the water uptake of the SEBS, CSEBS, and quaternized SEBS before alkalization (SEBSTMA) and QSEBS. Apparently, QSEBS exhibited the highest water uptake among the four kinds of membranes. The reason is that the hydroxyl ions in the QSEBS membrane can make it more hydrophilic than the others. Furthermore, the water uptake of the QSEBS membrane is less than Nafion® 115 (∼36%) [17], which may keep the membrane in good condition under fuel cell operations. However, the low water uptake will affect the ion conductivity of the membrane.

Fig. 4. Water uptake of the SEBS, CSEBS, SEBS-TMA and QSEBS membranes.

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Fig. 5. Ionic conductivity of the QSEBS and N115 membranes vs. temperature.

Fig. 6. Arrhenius plots for the QSEBS and N115 membranes.

The IEC value of the QSEBS membrane is 0.3 meq g−1 which is much lower than a commercial AEM membrane (AHA of ammonium type, Tokuyama Co., Japan) whose IEC is 1.15–1.25 meq g−1 [37]. This is due to the low efficiency of the chloromethylation using the nontoxic reagent paraformaldehyde and hydrogen chloride gas. 3.6. Ionic conductivity Fig. 5 shows the ionic conductivity of the membranes as a function of temperature in deionized water. The ionic conductivity of the QSEBS membrane increases with increasing temperature and reaches a maximum value of 9.37 × 10−3 S cm−1 at 80 ◦ C. As temperature increases, the mobility of both the ions and polymer chains increases, resulting in an increase of the ionic conductivity. For comparison, the membrane of Nafion® 115 (denoted as N115) was chosen as a benchmark. The conductivity of the N115 membrane within a range 2.34–3.94 × 10−2 S cm−1 is similar to the reference [38]. The result demonstrated the conductivity of the QSBES was approximately 4 times lower than that of the N115 membrane at each temperature. There may be three reasons: (1) the lower water uptake of the QSEBS membrane leads to less opportunity for anions to come into contact with water molecules, causing marginally slower anion transportation in the membrane; (2) the IEC value of the QSEBS membrane was 0.3 meq g−1 , indicating less mobile ions than the N115 membrane (0.9 meq g−1 ); (3) the diffusion coefficient of OH− ions is generally less than that of protons, which can affect the conductivities of anion exchange membranes [39]. However, compared with some other AEMs reported for potential fuel cell applications, such as, cross-linked quaternized poly(vinyl alcohol) (2.76–7.34 × 10−3 S cm−1 from 30 to 70 ◦ C) [21], quaternized poly(ether-imide) (2.28–3.51 × 10−3 S cm−1 from 25 to 95 ◦ C) [40], the QSEBS membrane is acceptable at the current stage. It is assumed that the conductivity follows the Arrhenius behavior. Hence, the ion transport activation energy Ea (kJ mol−1 ) of the membranes can be obtained using the regression line (Fig. 6) of ln  vs. 1000/T [12]: Ea = −b × R

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3.7. Methanol permeability The methanol permeability of the membranes was investigated over various methanol concentrations and operating temperatures. Fig. 7 shows the methanol permeability of the membranes in 2 M methanol solution vs. temperature. Methanol permeability increases with increasing temperature. The N115 membrane exhibits value of 2.23 and 5.40 × 10−6 cm2 s−1 at temperature of 30 and 80 ◦ C, respectively. The methanol permeability of the QSEBS membrane is in a range of 2.34–4.21 × 10−7 cm2 s−1 . This reveals that the methanol permeability of the QSEBS membrane is approximately one order of magnitude lower than that of the N115 membrane. It is probably that the SEBS matrix has less methanol affinity compared with perfluorinated hydrocarbon and the nonionic block can act as a barrier to methanol. Fig. 8 presents the methanol permeability of the QSEBS membrane as a function of methanol concentration at 30 ◦ C. The permeability of methanol increases with increasing methanol concentration. However, the methanol permeability exhibits merely a value of 4.45 × 10−7 cm2 s−1 even in 10 M methanol, which is 5 times lower than that of the N115 membrane in 2 M methanol. Increasing methanol feed concentration can improve the kinetics of methanol oxidation, thereby increasing the performance of the fuel cell system [41]. This indicates that the QSEBS membrane can be applied in DMAFCs with a high concentration of methanol. In DMFC applications, the ratio of ionic conductivity to methanol permeability is a characteristic parameter to evaluate the fuel cell

(5)

where b is the slope of the fitting line and R is the gas constant (8.314 J K−1 mol−1 ). The Ea value of the N115 membrane is similar to the report by Li et al. [38]. The Ea of the QSEBS membrane (11.31 kJ mol−1 ) is a little bit larger than that of the N115 membrane (9.81 kJ mol−1 ). This indicates that the mobility of the hydroxide ions in the QSEBS membrane is more temperature sensitive than that of protons in the N115 membrane [12].

Fig. 7. Methanol permeability of the QSEBS and N115 membranes vs. temperature.

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3 and 4 times lower than those of the Nafion® 115 membrane, respectively. The methanol permeability of the anion exchange membrane increases with increasing temperature and/or methanol concentration. The methanol permeability of our membrane shows approximately 10 times less than that of the Nafion® 115 membrane in 2 M methanol solution. In addition, the QSEBS membrane exhibits a low methanol permeability (4.45 × 10−7 cm2 s−1 ) in 10 M methanol solution at 30 ◦ C, which reveals that it may be suitable for application in DMAFCs with a high concentration of methanol. The ratio of ionic conductivity to methanol permeability of the QSEBS membrane is 2–3 times higher than that of the Nafion® 115 membrane. Thermal analysis results show that the QSEBS membrane is stable below 190 ◦ C. The QSEBS membrane demonstrates good oxidative stability under Fenton’s reagent. These results indicate that the membrane could be a potential candidate for direct methanol alkaline fuel cells. Fig. 8. Methanol permeability of the QSEBS membrane as a function of methanol concentration at 30 ◦ C.

Acknowledgements The support of Nature Science Foundation of China (No. 20976145), Nature Science Foundation of Fujian Province of China (No. 2009J01040), and the research fund for the Doctoral Program of Higher Education (No. 2005038401) in preparation of this article is gratefully acknowledged.

Nomenclature A Am b CB0 Ea

Fig. 9. Ratio of ionic conductivity to methanol permeability of the QSEBS and N115 membranes vs. temperature.

performances of membranes. In general, membranes with higher ionic conductivity and lower methanol permeability are more suitable for DMFCs. Fig. 9 shows the ratio of ionic conductivity to methanol permeability of the QSEBS and N115 membranes with increasing temperature. The figure indicates that the ratio of ionic conductivity to methanol permeability of the QSEBS membrane is higher than the N115 membrane. The result is attributed to the relatively low methanol permeability of the QSEBS membrane despite its low ionic conductivity. Also, compared with the ratio of ionic conductivity to methanol permeability of some other AEMs, such as, quaternized poly(vinyl alcohol) (QAPVA)/tetraethoxysilane (TEOS) (0.8 × 104 S s cm−3 at 30 ◦ C) [42], chloroacetylated poly(2,6-dimethyl-1,4-phenylene oxide) (CPPO)/bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) (15.5 × 104 S s cm−3 at 25 ◦ C) [20], the QSBES membrane is a promising candidate for the development of DMAFCs. 4. Conclusions The anion exchange membrane was prepared from polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) via chloromethylation, quaternization and alkalization successively. The FTIR result shows the chloromethyl groups and quaternary ammonium groups being successfully introduced into SEBS. The water uptake and ionic conductivity of the QSEBS membrane are

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 ) ion transport activation energy of 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) milliequivalents of HCl acquired before equilibrium (meq) meq of HCl acquired 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 letter  ionic conductivity (S cm−1 )

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