Sulfonated poly(arylene ether sulfone) polymers containing 3,4-difluoro-phenyl moiety as proton exchange membranes

Solid State Ionics 300 (2017) 157–164

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Sulfonated poly(arylene ether sulfone) polymers containing 3,4-difluoro-phenyl moiety as proton exchange membranes RiMing Chen, Guang Li, ShengLin Yang, MengYun Xiong, JunHong Jin ⁎ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University Shanghai, PR China

a r t i c l e

i n f o

Article history: Received 18 June 2016 Received in revised form 21 December 2016 Accepted 26 December 2016 Available online xxxx Keywords: Proton exchange membrane Sulfonated poly(arylene ether sulfone) 3,4-Difluoro-phenyl Phase separation Oxidative stability

a b s t r a c t Novel sulfonated poly(arylene ether sulfone) polymers with 3,4-difluoro-phenyl group (2F-SPAES-xx) were successfully synthesized via the direct copolymerization of 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone, 4,4dichlorodiphenylsulfone and 3,4-difluoro-phenyl hydroquinone. The polymers with different sulfonation degree were prepared by changing the mole ratio (20%–40%) of sulfonated monomer in the polymerization reaction. The proton conductivity, thermal and oxidative stability, mechanical properties, water uptake, swelling ratio, phaseseparated morphology of the membranes were explored. The 2F-SPAES-40 membrane showed higher proton conductivity and better phase separation, oxidative stability than that of sulfonated poly(arylene ether sulfone) membrane (SPAES-6FPA-40). The proton conductivity of 2F-SPAES-40 membrane was up to 0.142 Scm−1 at 80 °C in wet. These results indicated that incorporation of hydrophobic 3,4-difluoro-phenyl side group enhanced the oxidative stability and improved the proton conductivity by promoting the phase separation. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Fuel cells based on proton exchange membranes (PEMFCs) are widely regarded as one kind of promising alternative power sources to the use of fossil fuels as energy generating systems [1]. The proton exchange membrane plays a critical role in PEMFCs. Due to their excellent proton conductive properties, the benchmark PEM remains Nafion membrane. However, the Nafion membrane typically shows limitations such as high cost, high fuel crossover, restricted temperature and humidity operating conditions which prevent a widespread use in PEMFCs [2]. These drawbacks have evoked a worldwide search for alternative membrane materials [3]. Among the aromatic hydrocarbon membrane materials, sulfonated poly(arylene ether sulfone) (SPAES) is considered as a promising candidate for PEM materials owing to its excellent thermal ability, mechanical strength, good longevity under cell condition and so on [4–8]. Although there is increasing interest in the development of block and graft SPAES polymers, industrial-scale production is challenging. In addition, the purity of the monomers is strongly limited to the chain length [9]. Statistical SPAES polymers still need further research. The statistical SPAES polymers often have lower proton conductivity owning to the ionic sulfonic acid groups distributed along the backbone, which lead to poor hydrophilic and hydrophobic phase separation [10]. Herein, to acquire suitable morphological structure for obtaining high proton conductivity, great efforts [11–14] have been made on various ⁎ Corresponding author. E-mail addresses: [email protected] (R. Chen), [email protected] (J. Jin).

http://dx.doi.org/10.1016/j.ssi.2016.12.028 0167-2738/© 2016 Elsevier B.V. All rights reserved.

modification and chemical structure design of SPAES. It has been demonstrated that the most important strategy is to simultaneously improve ion nanochannel and the polarity of hydrophilic/hydrophobic segments [15]. It is reported that the size and shape of the hydrophobic moiety in SPAES influenced the degree of aggregation and the size of ionic domains formed [16,17]. Moreover, the C\\F bond along with its hydrophobic domain protects the polymer backbone from being attacked by oxidising radical species [18,19]. In our previous study, a series of sulfonated polymers with fluorophenyl side groups have been designed to promote the hydrophilic-hydrophobic phase separation [20]. In this paper, 3,4-difluoro-phenyl was introduced into sulfonated poly(arylene ether sulfone) as side chains. We mainly investigated the chemical structure and morphology of the copolymers of 2F-SPAES-40 and SPAES-6FPA-40 to relate to the proton conductivity and water uptake behavior, and to the chemical, thermal stability, which gave a useful insight into the effective molecular design of sulfonated aromatic PEM for fuel cell application. 2. Experimental 2.1. Materials 3,4-Difluoro phenyl hydroquinone was synthesized by the same method as we have previously reported [21]. 4,4′Dichlorodiphenylsulfone (DCDPS) and 3,3′-disulfonate-4,4′dichlorodiphenylsulfone (SDCDPS) were offered by Shanghai Darui Fine Chemical Company. Disulfonated poly(arylene ether sulfone)

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copolymer(SPAES-6FBPA-40) was gained from Yan Jin Technology Co., Ltd. Anhydrous potassium carbonate, toluene, and N,Ndimethylacetamide (DMAc) were purchased from Aldrich. All the chemicals and solvents were used without further purification. 2.2. Measurement 1 HNMR and 19FNMR spectra were measured on a Bruker AV NMR spectrometer (400 MHz) using DMSO-d6 as the solvent and tetramethylsilane as the standard. Inherent viscosities were determined by an Ubbelohde viscometer with a 0.5 g/dL DMAc solution at 25 °C. Fourier transform infrared (FTIR) spectra of the membrane were recorded in the range 4000–400 cm−1 with a resolution of 4 cm−1 (Nicolet 8700 spectrophotometer). Polymers samples were heated under nitrogen at a heating rate of 20 °C/min from 50 °C to 350 °C using TA Q20 differential scanning calorimetry (DSC) measurements. A TA discovery thermal system was employed to assess the thermal stability of the SPAES samples. The samples were equilibrated to 50 °C and then heated to 800 °C at 20 °C/min under nitrogen. The mechanical properties were measured on an Instron 5969 tester with a strain rate of 5 mm/min at room temperature. The samples were cut into a dumbbell shape before the measurement. Atomic force microscopy (AFM) images in the tapping mode were obtained by a Digital Instrument Agilent 5500, using micro-fabricated cantilevers with a force constant of approximately 40 N/m. Smallangle X-ray Scattering (SAXS) data were collected on SAXSess mc2 and corrected for background at room temperature.

2.3. Synthesis of 2F-SPAES-xx polymers 2F-SPAES-xx polymerization procedure was shown in Scheme 1. The synthesis of 2F-SPAES-40 polymer is given as an example. SDCDPS (0.9925 g 2 mmol), DCDPS (0.8747 g 3 mmol), 3,4-difluoro phenyl hydroquinone (1.1337 g 5 mmol), K2CO3 (0.8023 g 5.8 mmol), DMAc (15 mL), and toluene (15 mL) were put into a 50 mL three-necked flask equipped with a magnetic stirrer, a condenser, a Dean-Stark trap, and a nitrogen inlet. The reaction mixture was heated under reflux at

160 °C for 6 h, which stripped off most of the toluene to dehydrate. Then the temperature was raised slowly to 190 °C and maintained until the solution became viscous, after which it was precipitated into 500 mL ethanol with vigorous stirring and then filtered. The resultant was washed thoroughly with hot water and dried at 120 °C under vacuum for 48 h. 2.4. Preparation of SPAES membranes First, salt form of the SPAES polymer (0.6 g) was dissolved in DMAc (15 mL). Then, the solution was filtered using a funnel and cast onto glass dishes (diameter 90 mm), then carefully dried at 60 °C for 48 h under vacuum. Flexible membranes (50–80 μm) were obtained. The resulting membranes were immersed in 1 M HCl solution at room temperature for 48 h to permit the exchange of Na+ with H+. Finally, the membranes were soaked in deionized water for 48 h. 2.5. Ion exchange capacity, water uptake and swelling ratio Ion exchange capacity (IEC, meq g−1) of the membranes was measured using a typical titration method. The membranes (10 mm × 30 mm) in acid form were soaked in 20 mL of a 1 M NaCl solution and equilibrated for at least 48 h to replace the protons with sodium ions. The H+ ions released from the membranes were then titrated with a 0.1 M NaOH solution using phenolphthalein as an indicator. The IEC was gained from the following formula:
IEC meq g−1 ¼ ðVNaOH  CNaOH Þ=Wdry

ð1Þ

where VNaOH is the consumed volume of NaOH, CNaOH is the molar concentration of NaOH, and Wdry is the weight of dried membrane. The IEC values, which also were calculated from the sulfonation degree (DS), were obtained from the following equation [22]: IEC ¼ 1000DS=ðM þ 80DSÞ

ð2Þ

where, DS is calculated from the 1H NMR. M is the molecular weight of the polymer structure unit not containing sulfonic acid.

Scheme 1. Synthesis of 2F-SPAES-xx and the chemical structure of SPAES-6FBPA-40.

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Table 1 Properties of SPAES polymers membrane. Membrane

SDCDPS/DCDPS

ηinh (dLg−1)a

DS(%) from 1H NMR

IEC (meq g−1) from 1H NMR

IEC (meq g−1) from titration

2F-SPAES-20 2F-SPAES-30 2F-SPAES-40 SPAES-6FBPA-40

2/8 3/7 4/6 4/6

2.2 2.1 2.3 1.8

40 60 70 74

0.78 1.18 1.34 1.20

0.77 1.04 1.35 1.25

a

Measured at a concentration of 0.5 g/dL in DMAc solution at 25 °C.

The dimensional stability of the membranes was evaluated by their water uptake (WU) and swelling ratio (SR). A dry membrane was submerged in deionized water in a fully hermetic flask at controlled temperatures for a day. Then, the weight, length (Lx) and width (Ly) of the membrane was immediately measured when the surface water was wiped off carefully. The WU was calculated using the equation:
Water uptake ¼ Wwet –Wdry =Wdry  100%

ð3Þ

where Swet and Sdry are the area of the fully hydrated membrane and of the dry membrane, respectively. 2.6. Proton conductivity The membrane was immersed in 1 M H2SO4 at room temperature for 48 h and then washed to a pH of 7 with deionized water. The proton conductivity of the SPAESs membranes (8 mm × 10 mm) was measured by an AC impedance technique [23]. The proton conductivity was calculated from the following equation:

where Wwet and Wdry are the masses of the fully hydrated membrane and the dry membrane, respectively. The SR of each membrane was calculated using the expressions:

σ ¼ L=RA

S ¼ Lx  Ly
SR ¼ Swet –Sdry =Sdry

where L and A are the thickness and the contact area of the membrane sample, respectively, and R is the membrane resistance.

ð4Þ

Fig. 1. 1H NMR, 19F NMR and FT-IR spectra of 2F-SPAES-xx and SPAES-6FBPA-40 membranes.

ð5Þ

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Fig. 2. Proton conductivity of SPAES membranes at various temperatures in wet.

2.7. Oxidative stability Square pieces of the membrane samples (10 mm × 30 mm) were immersed in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 °C. The oxidative stability was evaluated by recording the retained weights of the membranes after treatment in Fenton's reagent for 1 h, and the time when the membranes wholly disappeared. 3. Results and discussion 3.1. Synthesis of 2F-SPAES-xx polymers The synthesis of 2F-SPAES-xx polymers was represented in Scheme 1. The DS of the polymers was simply varied molar ratio of SDCDPS to

Fig. 4. SAXS spectra of 2F-SPAES-40 and SPAES-6FBPA-40 membranes.

DCDPS. The chemical structure and DS values of gained SPAESs could be determined using 1HNMR analysis [24]. As represented in Table 1, these high intrinsic viscosities indicated that the polymers exhibited high molecular weight. We believe that the amount of strong polar sulfonic groups has affected on viscosity. As shown in Fig. 1(a), the presence of each \\SO3H group resulted in distinct signals for proton at the H3 positions. The aromatic proton of H3 was split at around 8.3 ppm due to the deshielding effect of the sulfonic acid groups. Fig. 1(b) showed the 19F NMR spectrum of 2F-SPAES-40 and SPAES6FBPA-40 polymers. The peak at −63.5 ppm was attributed to the fluorine in the chain of SPAES-6FBPA-40 polymer. The peaks at −139.2 and − 138.5 ppm were credited to the aromatic fluorine atoms of the 2FSPAES-40 polymer. The FT-IR spectra of 2F-SPAES-xx were presented in Fig. 1(c). The copolymers presented absorption band for the aromatic

Fig. 3. AFM topography and phase image SPAES-6FBPA-40 (A, a) and 2F-SPAES-40 (B, b).

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161

Fig. 5. Temperature dependence of water uptake (a) and swelling ratio (b) for SPAES membranes.

sulfone group appears at about 1149 cm−1 and the peak for aryl oxide appears at 1225 cm− 1 [25]. Two absorption peaks appear at around 1070 and 1025 cm−1 are characteristic of the aromatic\\SO3 stretching vibrations [26]. These results confirmed that the target copolymers were synthesized. As seen in Table 1, the IEC is an important parameter for evaluation of DS as it influences the water uptake and proton conductivity. The IEC values of SPAES membranes were calculated from the 1H NMR spectra in the range of 0.78–1.20 meq g−1. The IEC values of SPAES membranes from the titration test were in the range of 0.77–1.25 meq g−1, which coincided with the IEC values of SPAES membranes calculated from 1H NMR spectra. 3.2. Proton conductivity The proton conductivity is an important index of PEM. The proton conductivities of the fully hydrated SPAES-6FBPA-40 and 2F-SPAES-xx membranes were estimated by AC impedance spectroscopy. As expected, the proton conductivities of tested 2F-SPAES-xx membranes increased with DS and temperature as shown in Fig. 2. The proton conductivity of 2F-SPAES-40 membrane was up to 0.142 Scm−1 at 80 °C, which was much higher than that of SPAES-6FBPA-40 at 80 °C (0.0994 Scm−1). The enhanced proton transfer ability of 2F-SPAES-40 membrane could be ascribed to not only the slightly higher IEC value, but also the much more continuous hydrophilic regions (the AFM data).

3.3. The morphology of SPAES membranes Improving the phase separation of membrane is an effective approach to enhance the ionic conductivity [27]. The tapping mode AFM was used to investigate the phase separation between hydrophilic and hydrophobic domain under ambient atmosphere. The height and phase images of 2F-SPAES-40 and SPAES-6FBPA-40 membranes were given in Fig. 3(A), (a) and (B), (b). The bright and dark regions in the images were assigned to the hydrophobic domains and the hydrophilic domains with sulfonic acid containing water, respectively [28]. As showed in Fig. 3(a) and (b), the phase separation of 2F-SPAES-40 and SPAES6FBPA-40 membranes were obvious. The proton-hydrophilic-rich regions of 2F-SPAES-40 membrane were more distributed over nonionic matrix and much more connective ionic domains compared with those of SPAES-6FBPA-40 membrane. Base on the different morphologies, the dependence of proton conductivity between 2F-SPAES-40 and SPAES-6FBPA-40 membranes were explained. The result displayed that introducing 3,4-difluoro-phenyl into the SPAES polymer backbone was in favor of promoting the phase separation of hydrophilic and hydrophobic regions and enhancing proton conductivity. To further explore the microstructure of 2F-SPAES-40 and SPAES6FBPA-40 membranes, SAXS is used to probe ionic clusters/channels. Generally, a broad scattering ionomer peak is observed at a scattering vector (qmax) of approximately 0.2–3 nm− 1 in random SPAES membrane [29–31]. The position (qmax) can be correlated to the

Fig. 6. DSC (a) and TG (b) curves of SPAES membranes.

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Fig. 7. Cross-section SEM images of membranes before acceleration durability tests: (a) 2F-SPAES-20, (b) 2F-SPAES-30, (c) 2F-SPAES-40, (d) SPAES-6FBPA-40 and after acceleration durability tests: (a′) 2F-SPAES-20, (b′) 2F-SPAES-30, (c′) 2F-SPAES-40 and (d′) SPAES-6FBPA-40.

Table 2 Mechanical properties and oxidative stability of SPAES membranes. Polymer membrane

2F-SPAES-20 2F-SPAES-30 2F-SPAES-40 SPAES-6FBPA-40 a b c

Tensile strength (MPa)a

Tensile modulus (GPa)a

Elongation at break (%)a

Oxidative stability RW (%)b

t (min)c

21.52 16.06 20.79 30.10

1.04 0.47 1.00 1.04

3.10 ± 0.53 6.62 ± 2.80 5.06 ± 1.40 21.83 ± 11.30

97.47 93.61 96.21 93.98

7470 4156 222 192

± ± ± ±

0.93 0.93 0.78 4.30

± ± ± ±

0.024 0.077 0.020 0.037

The samples were measured in the hydrous states at room temperature. Retained weights of membranes after treatment in Fenton's reagent for 1 h at 80 °C. The dissolving time of the polymer membrane.

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characteristic distance (d) between the ionic aggregates in the polymerrich matrix [32]. The d was calculated according to the equation: d = 2π/q. The microstructure features of 2F-SPAES-40 and SPAES-6FBPA40 membranes were exhibited in Fig. 4. All the SAXS profiles showed obvious ionic clustering and typical phase separation of 2F-SPAES-40 and SPAES-6FBPA-40 membranes. The qmax values of 2F-SPAES-40 and SPAES-6FBPA-40 membranes were 0.86 and 0.68 nm −1, respectively. The d values were 7.30 nm and 9.23 nm. The average ionic aggregates distance of 2F-SPAES-40 membrane was shorter than that of SPAES6FBPA-40, which is more favorable to proton transport. And the result is in close agreement to the proton conductivity. 3.4. Water uptake and swelling ratio Water uptake and dimensional stability are critical factors in the performance of PEM. In Fig. 5(a) and (b), the water uptake and swelling ratios (SR) of the SPAES membranes increased with DS and temperature. For 2F-SPAES-40, the membrane showed a higher water uptake and SR than that of SPAESE-6FBPA-40 from 30 °C to 80 °C. However, the IEC value of 2F-SPAES-40 (1.35 meq g−1) was slightly higher than that of SPAESE-6FBPA-40 (1.25 meq g− 1). The probably explanation is that the bulky 3,4-difluoro-phenyl group decreases the degree of ordered stacking of polymer chains which lead to more free volume, which is agreed with the DSC analysis in Fig. 6(a). 3.5. Thermal and mechanical properties of SPAES membranes The thermal properties for the SPAES membranes (acid form) were investigated using DSC and TGA. The results were presented in Fig. 6(a) and (b). For the 2F-SPAES-xx membranes, the Tg values of 2FSPAES-20, 2F-SPAES-30 and 2F-SPAES-40 were 205 °C, 218 °C and 228 °C, respectively (in Fig. 6(a)). This was ascribed to the sulfonic acid groups increasing intermolecular interaction and molecular bulkiness and hindering internal rotation [33]. However, the Tg values of SPAES-6FBPA-40 was up to 249 °C. The absence of Tg may reflect with the higher degree of ordered stacking of polymer chains. As shown in Fig. 6(a), the Tg values of 2F-SPAES-xx membranes were above 200 °C which further suggests 2F-SPAES-xx membranes presented the excellent thermal properties in this work. The short-term thermal stabilities of the SPAES membranes were assessed. From the TGA curves (Fig. 7(b)), two typically and major weight loss stages [34] at around 250–340 °C and 460–600 °C displayed in all of the SPAES membranes, which is attributed to the\\SO3H groups removing and the polymer main chain splitting. The slight weight loss under 150 °C of all the samples were explained by the expulsion of the evaporation of residual solvent and water molecules from the polymer matrix or of moisture absorbed from the air. The SPAES-6FBPA-40 had lower weight loss temperature than that of 2F-SPAES-40. The reason could be explained that the IEC value of SPAES-6FBPA-40 was lower than that of 2F-SPAES-40 (in Table 1). Besides, the SPAES6FBPA-40 had higher degree of the ordered stacking ascribed to its linear macromolecule structure. As well as thermal properties, mechanical properties are essential to fabricate membrane electrode assembly for fuel cell applications. The mechanical properties of the SPAES membranes were measured at room temperature and in the hydrated state. These results are listed in Table 2. All the tested SPAES membrane had tensile strength of 16.06– 30.10 MPa, tensile modulus of 0.47–1.04 GPa and elongation at break of 3.10–21.83%. The 2F-SPAES-30 membranes showed the lowest of tensile strength and elongation to break, which may be attributed to the relative low molecular weight of 2F-SPAES-30 polymer. The elongation at break of the 2F-SPAES-xx membrane ranged from 3.10% to 5.06%, whereas, the SPAES-6FBPA-40 membrane had the highest elongation at break. This was ascribed to 3,4-difluoro-phenyl group enlarged the rigidity of the backbone. And it is also believed that mechanical properties of the membranes were significantly influenced by molecular weight.

163

3.6. Oxidative stability The retained weights after treatment in Fenton's reagent for 1 h at 80 °C and the time when the membranes wholly disappeared were used to evaluate the oxidative stability of SPEAS membranes. The result was tabulated in Table 2. In general, a higher IEC leads to lower oxidative stability. However, the retained weights of 2F-SPAES-30 membranes were the lowest among the membrane samples. The relative low molecular weight of 2F-SPAES-30 polymer could be responded for the lowest the retained weights. The wholly disappeared time and retained weight of the 2F-SPAES-40 membrane were higher than that of SPAES-6FBPA-40 membrane. However, the IEC value of the 2FSPAES-40 membrane was slightly higher than that of SPAES-6FBPA-40 membrane (in Table 1). The results indicated that 2F-SPAESE-40 membranes had better oxidative stability than that of SPAES-6FBPA-40 membrane. To investigate the structure change of SPAES membranes before and after acceleration durability tests, the cross-section of the tested SPAES membranes was observed by SEM (Fig. 7). The cross-section SEM images of 2F-SPAES-20 (a), 2F-SPAES-30(b), 2F-SPAES-40 (c) and SPAES6FBPA-40 (d) membranes before acceleration durability tests were displayed in Fig. 7. The cross-section SEM images of (a′), (b′), (c′) and (d′) were the 2F-SPAES-20, 2F-SPAES-30, 2F-SPAES-40 and SPAES6FBPA-40 membranes immersed in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 °C for 1 h, respectively. There were no obvious changes in the cross-section SEM images of the membranes before and after acceleration durability tests. There were no obvious changes in the cross-section SEM images of the membranes before and after acceleration durability tests. The oxidative stability enhancement of 2FSPAES-xx can be attributed to incorporation of 3,4-difluoro-phenyl group. Compared with the hexafluoroisopropyl group, 3,4-difluorophenyl group have greater steric hindrance, which is in favor of increasing the durability of sulfonated aromatic hydrocarbon-based polymer electrolyte membranes [5].

4. Conclusions A series of sulfonated poly(arylene ether sulfone)s copolymers containing 3,4-difluoro-phenyl side chains were synthesized by aromatic nucleophilic polycondensation. The 2F-SPAES-xx membranes had good thermal stability and tensile strength of 16.06–21.52 MPa. The proton conductivities and water uptake of 2F-SPAES-xx were greatly improved by increasing both the sulfonation degree and the temperature. Compared to the SPAES-6FBPA-40 controls, 2F-SPAES-40 achieved higher proton conductivity, better oxidative stability and greater degree of the phase separation. These results indicated that introducing hydrophobic 3,4-difluoro-phenyl side chains was in favor of enhancing the ionic conductivity and oxidative stability. The single cell performance of these membranes will be examined in the future work.

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