A novel approach to prepare photocrosslinked sulfonated poly(arylene ether sulfone) for proton exchange membrane

Journal of Membrane Science 463 (2014) 58–64

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A novel approach to prepare photocrosslinked sulfonated poly(arylene ether sulfone) for proton exchange membrane Pushan Wen a,b,c, Zhenxin Zhong d, Lizhong Li a, Fengshan Shen e, Xiang-Dan Li a,n, Myong-Hoon Lee b,nn a Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, PR China b The Graduate School of Flexible and Printable Electronics, Polymer BIN Fusion Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea c Hunan University of Technology, Zhuzhou 412007, PR China d FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, OR 97124, USA e College of Science, Yanbian University, Yanji, Jilin 133002, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 20 August 2013 Received in revised form 12 March 2014 Accepted 17 March 2014 Available online 24 March 2014

Novel sulfonated poly(arylene ether sulfone)s with photo-crosslinkable chalcone moiety in the main chain (SPAESs) are synthesized from 4,40 -dihydroxychalcone, 3,30 -disulfonate-4,40 -difluorodiphenylsulfone and 4,40 -difluorodiphenylsulfone. The resulting polymers are characterized by FT-IR, 1H NMR, UV and TGA techniques. SPAES membranes are photocrosslinked without using a photoinitiator in a hydrated state by UV irradiation (365 nm) to preserve the percolated hydrophilic channels in the hydrated membranes. Untreated SPAES membranes demonstrate high proton conductivity up to 211.61 mS cm  1 at 80 1C for SPAES50. Photocrosslinked SPAES50 membrane maintains proton conductivity up to 191.06 mS cm  1. The photocrosslinking strategy of SPAES membranes in a hydrated state helps reducing the methanol permeability without sacrificing proton conductivity. Methanol permeability of crosslinked SPAES30 membrane was measured to be in the range from 1.23  10  7 cm2 s  1 to 6.48  10  8 cm2 s  1, which is only 1/36 of that of Nafions 117. Photocrosslinked SPAES membranes also exhibit excellent mechanical and thermal properties, and improved oxidative and hydrolytic stability. & 2014 Elsevier B.V. All rights reserved.

Keywords: Poly(arylene ether sulfone) Chalcone Photocrosslink Proton conductivity

1. Introduction Direct methanol fuel cell (DMFC) has attracted considerable attention as a promising candidate power source because of its advantages such as high efficiency, high power density, low emissions, and easy fuel carriage [1–3]. Polymeric membranes play an important role during electricity generation in direct methanol proton-exchange membrane fuel cells. The proton-exchange membrane in DMFC separates the positive and negative electrodes and provides a conduit for proton movement between the electrodes. Currently, the membranes broadly used as proton exchange membranes are perfluorinated copolymers with pendant sulfonic acid groups, such as Nafions (DuPont) and Dow membranes, which exhibit high proton conductivity and excellent chemical stability. However, the high price and high methanol permeability of fluorinated proton-exchange membranes limit their further use in n

Corresponding author. Tel./fax: þ 86 27 6784 2752. Co-corresponding author. Tel.: +82 63 270 2337; fax: +82 63 270 2341. E-mail addresses: [email protected] (X.-D. Li), [email protected] (M.-H. Lee). nn

http://dx.doi.org/10.1016/j.memsci.2014.03.042 0376-7388/& 2014 Elsevier B.V. All rights reserved.

DMFC. The high methanol crossover in Nafions membranes causes catalyst poisoning and unnecessary fuel consumption, which degrades the energy efficiency of fuel cells [4–6]. For these reasons, a number of sulfonated aromatic polymers, such as sulfonated poly(arylene ether sulfone)s, sulfonated poly (arylene ether ketone)s, sulfonated polybenzimidazoles and sulfonated polyimides have been developed [7–10]. Among them, sulfonated poly(arylene ether sulfone)s have been widely accepted as excellent membrane materials in DMFCs because of their excellent material properties, such as low cost, high chemical and thermal stability, satisfying film forming capability, and low methanol crossover [11–13]. Considerable efforts have been devoted to develop polymer electrolyte membranes of high proton conductivity with high proton/methanol selectivity [14–16]. Increasing acid content in the polymer is a common strategy for improving the proton conductivity of PEMs. However, highly sulfonated polymers tend to swell to an unacceptable extent resulting in high methanol diffusion coefficient. Crosslinking is an efficient and simple way to minimize swelling and reduce methanol crossover by forming a crosslinked three dimensional network [17–19]. Photocrosslinking has been demonstrated as an effective way to improve the proton/methanol

P. Wen et al. / Journal of Membrane Science 463 (2014) 58–64

added as an azeotroping agent. The reaction mixture was refluxed at 130 1C for 3 h to dehydrate the system until water was removed from the reaction. The temperature was raised slowly to 150 1C for another 9 h. Then the reaction was cooled to room temperature. The polymer solution was diluted with DMAc, filtered to remove the sodium salt, and precipitated into IPA. The reprecipitation procedure was repeated twice to remove residual oligomer. The resulting polymer was isolated, washed repeatedly with deionized water to completely remove the residual sodium and dried in a vacuum oven at 60 1C for 24 h. FT-IR (KBr, cm  1): 1658 (4CQO), 1581 (4CQCo), 1072 (OQSQO); 1H NMR (DMSO-d6, ppm): 7.1–8.3 (Ar–H), 7.7, 7.9 (–CHQCH–). SPAES40 and SPAES50 containing 40 and 50 mol% of sulfonated monomer SDFDPS, respectively, were synthesized in the same manner.

selectivity and the mechanical properties of PEMs [20,21]. In most of these researches, photocrosslinking of PEMs is performed in a dry-film state and in the presence of a photoinitiator [22,23]. Recently, we have reported a new strategy to crosslink photosensitive PEMs in a hydrated state, which can preserve the percolated hydrophilic channels in a hydrated membrane [24]. Highly fluorinated aromatic monomer, decafluorobiphenyl (DFBP), was copolymerized with photoreactive 4,40 -dihydroxychalcone (4DHC) and sodium 4,40 -difluorodiphenylsulfone-3,30 -disulfonate (SDFDPS) to give a stable and photocrosslinkable polymer membrane with high proton conductivity. However, due to the high cost of DFBP, it is required to develop a less expensive polymer system with similar or better performance for the real application in the DMFC. In the present research, we synthesized and characterized a series of novel crosslinkable sulfonated poly(arylene ether sulfone) (SPAES) in which the photoreactive chalcone groups are copolymerized with sulfonated and unsulfonated diphenylsulfones. SPAES membranes were crosslinked by UV irradiation in a hydrated state without a photoinitiator. The membrane performance of both pristine and crosslinked SPAES membranes was investigated for their potential application in DMFCs.

2.3. Membrane preparation Sodium-salts of copolymers (1.0 g) were dissolved in DMAc (10 mL) and filtered through a membrane filter (ADVANTEC JP020AN). The solutions were cast onto pre-cleaned glass plates. Tough and flexible SPAES films were obtained by drying at 70 1C for 12 h and at 120 1C in vacuum for 24 h. The thickness of membranes was in the range of 90–110 μm. The membranes were immersed in deionized (DI) water for 30 min at room temperature, and irradiated by UV light for 1 min, 5 min, and 15 min, correspondingly, with an intensity of 22 mW cm  2 (365 nm). The UV lamp was a large area light source (M-919X) from Newport Co. with a digital exposure controller (M-68950). The pristine and crosslinked SPAES membranes were transformed to their acid forms by immersing in 1 M H2SO4 for 24 h at room temperature. The acidified films were then soaked in DI water for 24 h, and washed thoroughly.

2. Experimental 2.1. Materials 4,40 -Difluorodiphenylsulfone (DFDPS) was obtained from Aldrich and used as received. 3,30 -Disulfonate-4,40 -difluorodiphenylsulfone (SDFDPS) and 4,40 -dihydroxychalcone (4DHC) were synthesized as reported earlier [25]. N,N-dimethylacetamide (DMAc), toluene, and anhydrous potassium carbonate were obtained from Sinopharm Chemical Reagent Co. Ltd. DMAc was dried overnight over calcium hydride and distilled at reduced pressure and normal pressure. Anhydrous potassium carbonate was dried at 200 1C for 10 h prior to use.

2.4. Characterization 2.4.1. Polymer characterizations UV spectra were recorded on a Perkin-Elmer Lambda BIO 35 UV/VIS spectrophotometer. Infrared spectra were obtained from potassium bromide pellets with a Nicolet NEXUS-470 FT-IR spectrometer. 1H NMR spectra were obtained with a Varian Mercury VX-300 (300 MHz) spectrometer with TMS as an internal reference in DMSO-d6. The thermal behaviors of membranes were investigated on a TA Instruments TGA Q50. The samples were analyzed over the range from 50 1C to 800 1C at a heating rate of 10 1C min  1 in nitrogen atmosphere with an isothermal process at 200 1C for 30 min to remove any residual moisture and solvent.

2.2. Synthesis of polymer A series of polymer containing sulfonated groups and chalcone groups were synthesized via an aromatic nucleophilic substitution reaction with 4DHC, SDFDPS and DFDPS in DMAc with toluene as the azeotroping agent (Scheme 1). The resulting copolymers were abbreviated as SPAES30, SPAES40 and SPAES50, in which numbers denote the mol percent of sulfonated groups in copolymers. The detailed synthesis of the 30 mol% sulfonated copolymer (SPAES30), containing 30 mol% SDFDPS and 70 mol% DFDPS, was as follows. The polymerization was carried out in a 150 mL two-necked flask equipped with a Dean-Stark trap fitted with a condenser and a nitrogen inlet. 4DHC (3.5 mmol), SDFDPS (1.05 mmol), and DFDPS (2.45 mmol) as well as an excess of anhydrous potassium carbonate (10 mmol) were added. DMAc (10 mL) and toluene (5 mL) were then charged into the flask,

2.4.2. Ion exchange capacity The ion-exchange capacity (IEC) was measured by the classical titration method. First, the membrane in acid form was immersed

O (x+y) HO

C

59

O CH CH

+

OH

x F

O F +

S

y F

S

O NaO3S

SO3Na

4DHC

SD FDPS K2CO3

O O

C

O NaO3S

O

S O

DFDPS

DMAc Benzene

O CH CH

F

O

O

C

O CH CH

O

x SO3Na SPAES

Scheme 1. Synthesis of novel crosslinkable sulfonated poly(arylene ether sulfone) (SPAES).

S O

y

P. Wen et al. / Journal of Membrane Science 463 (2014) 58–64

in 50 mL 1.0 M NaCl solution at room temperature for 24 h to replace the protons of sulfonic acid groups with sodium ions. Then, the released protons in solution were titrated with a 0.01 M NaOH solution using phenolphthalein as an indicator. The IEC values were obtained using the following formula: IEC ðmeq g  1 Þ ¼

CV W

where C and V are the concentration and the volume of NaOH solution, respectively. W is the weight of dried membranes. 2.4.3. Water uptake and dimensional change The membrane in acid form was weighed after vacuum-drying at 120 1C for 24 h, and immersed in DI water at 25, 50 and 80 1C for 24 h. The wet membranes were wiped with tissue paper to remove the excess water, and quickly weighed on a balance. The weighing process was continued until a constant weight was obtained. The water uptake of the membranes was calculated in weight percent as Water uptake ð%Þ ¼

W wet  W dry  100 W dry

where Wwet and Wdry are the weights of the wet and dry membranes, respectively. Dimensional changes of membranes were measured by immersing three sheets of samples (3  3 cm2) in DI water at 25, 50 and 80 1C. The change of thickness and length was calculated from the following equations and their average values were obtained.

ΔLð%Þ ¼

Lwet  Ldry T wet T dry  100; ΔTð%Þ ¼  100 Ldry T dry

where Lwet and Twet are the length and thickness of the wet membranes, Ldry and Tdry are the length and thickness of the dry membranes respectively. All the lengths and thicknesses are measured by digital micrometer with a resolution of 1 μm. 2.4.4. Oxidative and hydrolytic stability The oxidative stability and hydrolytic stability of the membranes were measured after immersing the samples into Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 1C, and in DI water at 100 1C, respectively. The oxidative and hydrolytic stabilities were evaluated by recording the time that membranes began to break when they were folded for three times. 2.4.5. Mechanical property The fully hydrated samples (8  1 cm2) were cut into dumbbellshaped specimens and their mechanical properties were measured by using a universal mechanical testing machine (LR10KPlus, Lloyd instruments, UK) at a speed of 2 mm min  1. For each test reported, average values were calculated from at least three samples. 2.4.6. Methanol permeability Glass diffusion cells and Younglin R1750F Refraction Detector were used to obtain the methanol permeability of SPAES membranes. One chamber of the glass cell (VA) was filled with a 2 M (CA) methanol solution in DI water. The other chamber (VB) was filled with DI water. A membrane sample (effective area 3.14 cm2) was clamped between the two chambers. The temperature of the diffusion cell was maintained at 30 1C. Methanol permeates cross the membrane by the concentration difference between the two chambers. The methanol concentration in the receiving chamber as a function of time is given as CBðtÞ ¼

A DK C A ðt t 0 Þ VB L

where A is the membrane area, L is the membrane thickness, D is the methanol diffusivity, and K is the partition coefficient between

the membrane and the adjacent solution. Methanol permeability is represented by the value DK. 2.4.7. Proton conductivity Proton conductivity of the membrane was determined by the four-point probe method using a BT-512 BekkTech membrane conductivity test system (BekkTech LLC, USA) which includes a Keithley 2400 Sourcemeter for electrical measurements. The sample membrane (approximately 1  4 cm2) was connected with four probes: two outside platinum wires to apply the current and two inside platinum wires as reference electrodes. The distance between two inside electrodes was 0.42 cm. Sample was tested at various temperatures between 30 and 80 1C under 100% relative humidity. Nitrogen gas was passed through the conductivity cell to obtain desired relative humidity. The proton conductivity was calculated by the test equipment and defined by the following equation:

s¼

l twR

where s is the proton conductivity, l is the distance between the two electrodes, t and w are the thickness and width of the dried membrane respectively, and R is the resistance value.

3. Result and discussion 3.1. Synthesis and characterization of polymers As shown in Scheme 1, a series of sulfonated poly(arylene ether sulfone) containing various amounts of sulfonated diphenylsulfone monomer, SDFDPS, from 30 to 50 mol% were prepared via aromatic nucleophilic substitution polycondensation of 4DHC, DFDPS and SDFDPS in a DMAc/toluene solvent system. K2CO3 was used as a weak base catalyst. Monomers, 4DHC and SDFDPS, were synthesized as previously reported [25]. The successful introduction of chalcone groups and sulfonated groups was confirmed by FT-IR (Fig. 1) and 1 H NMR (Fig. 2). In FT-IR spectra, the band at 1072 cm  1 was assigned to symmetric OQSQO stretching vibrations of sodium sulfonate group, and the bands at 1658 and 1581 cm  1 suggest the presence of the chalcone group. In 1H NMR spectra (Fig. 2), the signal at 8.3 ppm corresponds to the aromatic proton next to the sulfonate group, and the resonance signals of the chalcone group were observed at 7.7 and 7.9 ppm. Fig. 3 displays the changes of UV–vis spectra of SPAES50 membrane upon UV irradiation in a hydrated state. Before the

SPAES50

Transmittance (%)

60

4000

SPAES40

SPAES30

3500

3000

2500

2000

1500

Wavenumber (cm-1) Fig. 1. FT-IR spectra of SPAESs.

1000

500

P. Wen et al. / Journal of Membrane Science 463 (2014) 58–64

61

100 6

e' d' g k d i b' e b

a' f' h j a c'

Weight (%)

c

SPAES50

80

f

4

70 60

crosslinked SPAES50 2

50 pristine SPAES50

SPAES40

40

pristine (deriv.)

30

Deriv. Weight (wt.%/deg)

90

0

crosslinked (deriv.)

20

SPAES30

0

200

400

600

800

Temperature (oC) 9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0

Fig. 4. TGA of pristine and crosslinked SPAES50.

Chemical Shift (ppm) Fig. 2. 1H NMR spectra of SPAESs.

the solidity of polymer molecules, thereby improving the thermal stabilities of SPAES membranes. 3.3. Water uptake and dimensional change 0s 1s 2s 3s 5s 7s 10s 14s 18s 23s 29s 36s 45s 55s 67s 82s 102s 130s 170s 230s 320s 450s 630s 810s 1080s 1500s 2220s

2.0

λ

Absorbance

1.5

=324 nm

1.0

0.5

0.0

200

300

400

500

600

700

800

Wavelength (nm) Fig. 3. UV spectra of SPAES50.

irradiation, λmax was found at 324 nm corresponding to the π–π* transition of chalcone 4CQCo unit, which also proves the incorporation of the chalcone moiety in the polymer main chain. During the irradiation, the intensity of the absorption band at 324 nm decreased drastically with an isosbestic point at 280 nm, implying the crosslinking of the polymer chain proceeded via a [2þ2] cyclodimerization of the 4CQCo group of chalcone unit in the main chain [24–34].

Water uptake of proton exchange membranes has a direct impact on proton exchange capacity and proton conductivity. On the other hand, high water uptake can cause swelling of proton exchange membranes leading to a decline in the stability. Water uptakes of SPAES membranes were measured at different temperatures, and the results are summarized in Table 1. The pristine membranes were exposed to UV light in a hydrated state for different periods of time, and the resulting samples were abbreviated as SPAESn-1 min, SPAESn-5 min, and SPAESn-15 min, where n denotes for the SDFDPS content. Both water uptake and dimensional change increase with SDFDPS contents and temperature. With increasing the irradiation time, the membrane exhibited a reduced water uptake and lower dimensional changes. This is attributed to a rigid three-dimensional network structure in SPAES membranes generated by the crosslinking. The water uptake, length change and thickness change of pristine SPAES50 are 68.2%, 18.9% and 14.6% at 50 1C, while those of the crosslinked SPAES50-15 min decrease to 40.8%, 12.6% and 9.9%, respectively. It is interesting to note that, when compared with Nafion 117, SPAES membranes show significantly higher water uptake with comparable dimensional changes after the photocrosslinking. For example, while the water uptake and dimensional change (length) of Nafion at 80 1C are 27.9% and 17.2%, those of SPAES50-15 min are 61.3% and 18.6%, respectively. This implies that a three dimensional network structure of SPAES crosslinked under hydrated state can provide a suitable environment for the proton transport with marginal compensation of mechanical properties.

3.2. Thermal properties 3.4. Ion exchange capacity and proton conductivity Both pristine and crosslinked SPAES membranes exhibit excellent thermal stability. As shown in Fig. 4, SPAES50 membrane demonstrates a two-step degradation process upon thermal treatment. The first weight loss in the range from 250 1C to 390 1C is attributed to the degradation of sulfonic acid groups, and the second weight loss occurred at a temperature above 390 1C is due to the decomposition of polymer main chains. The crosslinked SPAES50 membranes exhibit improved thermal stability compared to the corresponding pristine membranes. Td of the pristine SPAES50 at 1% weight loss temperature is 265 1C, which rises to 272 1C for the crosslinked SPAES50 after 15 min of irradiation (SPAES50-15 min). Photocrosslinking of chalcone group in the main chain restricts mobility of polymer chains and increases

Ion exchange capacity (IEC) is an important issue affecting the proton conductivity. The IEC values of the SPAES membranes were listed in Table 1. The IEC values of SPAES membranes have the same trend as the water uptake. Polymers with high SDFDPS contents have high IEC values. IEC of the crosslinked membrane only slightly decreases with increasing UV exposure time. Take SPAES50 as an example, the pristine SAES50 has an IEC value of 1.836 mmol g  1, which decreases to 1.717 mmol g  1 after crosslinking in a hydrated state for 5 min, and to 1.666 mmol g  1 for 15 min. In the hydrated PEM, the hydrophilic/hydrophobic nanoseparation of polymer chains gives rise to form a well-connected hydrophilic domain which is responsible for the transport of

62

P. Wen et al. / Journal of Membrane Science 463 (2014) 58–64

Table 1 Water uptake, dimensional change, IEC and oxidative stability of pristine and crosslinked SPAES membranes. Sample

SPAES30 SPAES30-1 min SPAES30-5 min SPAES30-15 min SPAES40 SPAES40-1min SPAES40-5 min SPAES40-15 min SPAES50 SPAES50-1 min SPAES50-5 min SPAES50-15 min Nafion 117b a b

Water uptake (%)

Dimensional change (length) (%)

Dimensional change (thickness) (%)

25 1C

50 1C

80 1C

25 1C

50 1C

80 1C

25 1C

50 1C

80 1C

8.34 7.73 6.34 5.00 24.75 22.04 17.06 13.31 50.94 40.65 35.70 32.03 18.3

11.42 10.02 8.95 6.65 34.41 31.09 23.92 19.01 71.79 64.82 53.85 45.14 –

13.52 12.03 10.22 7.46 41.95 35.36 29.40 25.70 124.69 113.13 101.97 82.38 27.9

1.4 1.3 1.1 0.9 6.7 5.8 4.6 3.8 15.3 12.6 10.8 9.9 10.6

1.9 1.7 1.4 1.1 8.9 7.9 6.4 5.1 19.5 17.8 15.2 12.6 –

2.3 2.0 1.7 1.3 10.8 9.2 7.7 6.6 29.3 27.1 23.6 19.5 17.2

1.3 1.1 0.9 0.8 5.9 5.1 4.2 3.1 13.1 11.8 10.3 9.2 –

1.7 1.4 1.2 1.0 7.8 6.6 5.3 4.4 18.6 16.3 14.6 12.2 –

2.1 1.8 1.5 1.3 9.3 7.9 6.4 5.3 27.8 25.9 22.2 19.1 –

Oxidative stabilitya (min)

Hydrolytic stability (h)

1.178 1.131 0.997 0.946 1.511 1.481 1.407 1.347 1.836 1.787 1.717 1.666 0.909

120 164 217 290 94 108 140 200 44 55 78 130 –

4600 – – 4600 4600 – – 4600 168 – – 500 –

Fenton's test at 80 1C. Ref. [35] and [36].

250 SPAES30 SPAES40 SPAES50 SPAES30-15min SPAES40-15min SPAES50-15min Nafion 117

200

Conductivity (mS cm-1)

IEC (mmol g  1)

150

100

50

0 20

30

40

50

60

70

80

90

Temperature (oC) Fig. 5. Proton conductivity of Nafions 117 and SPAES membranes.

proton and water. On the other hand, the hydrophobic domain is also created providing the polymer with the mechanical stability and insolubility in water [4]. In this work, the PEM was photoirradiated in the hydrated state, and the photocrosslinking of SPAES is considered to occur predominantly in the hydrophobic domain with minimally consuming the sulfonic acid group in the hydrophilic domain. The slight decrease in IEC values for crosslinked SPAES membranes may be attributed to the denser network structure and the narrower percolated hydrophilic channels in the crosslinked PEMs. It is also interesting to note that the decrease of IEC values with respect to the crosslinking degree was much smaller than those observed in water uptake and dimensional change, suggesting that the crosslinking in a hydrated state is advantageous to achieve a high proton conductivity and low methanol crossover. Proton conductivity is one of the most important parameters of PEMs. The proton conductivities of SPAES membranes were measured at various temperatures and at 100% relative humidity, and the results were compared with that of Nafions 117. As shown in Fig. 5, the proton conductivity of SPAES membranes increases with the increasing test temperature. The proton conductivity of crosslinked SPAES membranes is only slightly (less than 10%) smaller than that of its pristine forms. The conductivity of SPAES30 membrane is relatively low due to the low sulfonate content. Low sulfonate content in PEMs may lead to the formation of dead-ends of proton transporting

channels [37] as well as the reduced numbers of percolated hydrophilic channels, resulting in a low conductivity. Both pristine and crosslinked SPAES50 membranes show a higher proton conductivity than that of Nafions 117 at most of the temperature range. At 80 1C, SPAES50 membrane shows the highest proton conductivity of 211.61 and 191.06 mS cm1 for pristine and crosslinked SPAES membranes, respectively. Proton conductivity of the Nafion 117 at the same condition was measured to be 163.75 mS cm  1. The proton conductivity of all SPAEF membranes decreased slightly (by less than 9.7%) after photocrosslinking as compared with those of pristine membranes. Generally, after crosslinking by other methods such as thermal or ionic, a large decrease of proton conductivity is observed by more than 20% according to the literature [19,38] Park et al. recently reported that sulfonated a poly(arylene ether sulfone) of similar molecular structure was photocrosslinked in a dry state, and the proton conductivity measured at 60 1C was substantially decreased by 27.5% for 10 mol% crosslinked structure after crosslinking [34]. Our results, therefore, suggest that the photocrosslinking of PEMs in a hydrated state can preserve the percolated hydrophilic channels, resulting in a minimal loss of proton conductivity. 3.5. Methanol permeability and selectivity The proton exchange membranes used in DMFC should have good barrier properties of alcohol to reduce the methanol crossover and improve the efficiency of fuel cell. Fig. 6 compares the methanol permeability of both pristine and crosslinked SPAES membranes with that of Nafions 117. Poly(arylene ether sulfone) copolymers are generally known to have much lower methanol permeability than Nafions due to their rigid aromatic backbone. As expected, the methanol diffusion coefficients of pristine SPAES membranes are much lower than that of Nafions 117. The methanol diffusion coefficients of SPAES membranes decrease with increasing SDFDPS contents. After UV treatment in a hydrated state, SPAES membranes showed decreased methanol permeability by nearly 50% compared with those of pristine samples. For example, the methanol permeability of the SPAES30 membrane decreased from 1.23  10  7 cm2 s  1 to 6.48  10  8 cm2 s  1 after 15 min of irradiation, which is only 1/36 of Nafions 117 (2.38  10  6 cm2 s  1). This is attributed to that the crosslinking provides SPAES membrane with a dense network environment and narrower hydrophilic channels, resulting in the suppressed transport of methanol as well as protons. Selectivity, defined as the ratio of proton conductivity to methanol permeability, is one of the important parameters to

Methanol diffusion coefficient 10-7 (cm2 s-1)

P. Wen et al. / Journal of Membrane Science 463 (2014) 58–64

SPAES membranes with various sulfonation degrees (DS) are presented in Table 2. Young's modulus and tensile strength of SPAES membranes decrease with increasing DS, whereas the elongation at break of SPAES membranes shows an opposite trend. Young's modulus and tensile strength of both pristine and crosslinked SPAES membranes (501.96–692.17 MPa and 54.86–86.52 MPa, respectively) are much higher than those of Nafions (234 MPa and 27 MPa, respectively) [39]. The elongation at break of Nafions 117 (327%) is higher than that of SPAES membranes (59.36– 103.37%). As expected, Young's modulus and tensile strength of SPAES membranes increase after crosslinking, whereas the elongation shows a reverse tendency. Photocrosslinking restricts the motion of polymer chains due to the network structure, resulting in increased rigidity and reduced toughness of the membranes.

25

Pristine UV-5min UV-15min

20

63

15

10

5

0

SPAES30

SPAES40

SPAES50

Nafion 117

4. Conclusions

Sample Fig. 6. Methanol permeability of Nafions 117 and SPAES membranes.

Table 2 Mechanical properties of SPAES membranes. Membrane

Young's modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

SPAES30 SPAES40 SPAES50 SPAES30-15 min SPAES40-15 min SPAES50-15 min Nafions 117a

657.93 558.71 501.96 692.17 616.1 550.41 234

78.98 63.19 54.86 86.52 78.9 62.04 27

76.82 94.8 103.37 59.36 72.85 88.33 327

a

Ref. [39].

evaluate the membrane performance in DMFC. Pristine SPAES membranes exhibited high selectivity (at 30 1C) in the range between 2.22  105 and 2.55  105 S s/cm3, which was further increased to the range between 3.83  105 and 4.83  105 S s/cm3 after photocrosslinking. This is approximately 10 times higher than the selectivity of Nafion 117 (0.41  105 S s/cm3) measured at the same condition. 3.6. Oxidative and hydrolytic stability The proton exchange membranes for DMFC should possess good oxidative and hydrolytic stability. Photocrosslinking strategy can not only suppress excess water uptake and methanol permeability but also improve both the oxidative and hydrolytic stability of SPAES membranes. The oxidative stability of pristine and crosslinked SPAES membranes was tested in Fenton's reagent at 80 1C, and the result was tabulated in Table 1. The pristine SPAES30 membrane started to break into pieces in Fenton's reagent after 120 min, whereas the durability was improved to 290 min for the crosslinked SPAES3015 min. Hydrolytic stability of membranes was also investigated by examining the durable time in boiling water. Both SPAES30 and SPAES40 (pristine and crosslinked) remain intact in boiling water for more than 25 days (4600 h). Pristine SPAES50 membrane was stable in boiling water for 168 h, whereas crosslinked SPAES50-15 min was 3 times more stable (500 h). Photocrosslinking of SPAES membranes significantly improved both the oxidative and hydrolytic stability of PEM membranes. 3.7. Mechanical property Good mechanical properties are required for the real application of membranes as PEMs in DMFCs. The mechanical properties of

In summary, a new series of poly(arylene ether sulfone)s containing photosensitive chalcone group in the main chain were successfully synthesized. Crosslinked SPAES membranes were obtained though the irradiation of 365 nm UV light in a hydrated state in the absence of photoinitiator, by which the percolated hydrophilic channels in the PEM were preserved. The photocrosslinked SPAES membranes demonstrated the desired properties of reduced membrane swelling in water, lower methanol diffusion coefficient, and improved dimensional, mechanical, and oxidative stability. Both pristine and crosslinked SPAES membranes show high proton conductivity up to 211.61 mS cm  1 and low methanol permeability down to 6.48  10  8 cm2 s  1. These results indicate that the SPAES membranes crosslinked in a hydrated state can be a promising candidate of PEM materials for fuel cell applications.

Acknowledgments The authors are grateful to the financial support from the National Natural Science Foundation of China (No. 51243005) and the Program of Regional Innovation Center for the Fuel-Cell Technology initiated by Ministry of Knowledge Economy, Republic of Korea. MHL also thanks the National Research Foundation of Korea (NRF) for the financial support from Human Resource Training Project for Regional Innovation.

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