Synthesis and Properties of Poly(ether sulfone)s with Clustered

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Synthesis and Properties of Poly(ether sulfone)s with Clustered Sulfonic Groups for PEMFC Applications under Various Relative Humidity Shih-Wei Lee, Jyh-Chien Chen, Jin-An Wu, and Kuei-Hsien Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00919 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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ACS Applied Materials & Interfaces

Synthesis and Properties of Poly(ether sulfone)s with Clustered Sulfonic Groups for PEMFC Applications under Various Relative Humidity Shih-Wei Lee,a Jyh-Chien Chen, a,* Jin-An Wu,b Kuei-Hsien Chenc,d a

Department of Materials Science and Engineering, National Taiwan University of Science and

Technology, Taipei, 10617, Taiwan b

Material and Chemical Research Laboratories Division of Polymer Research, Industrial

Technology Research Institute, Hsinchu, 30011, Taiwan c

Institute of Atomic and Molecular Science, Academia Sinica, Taipei, 10617, Taiwan.

d

Center for Condensed Matter Sciences, National Taiwan University, Taipei, 10617, Taiwan

KEYWORDS: fuel cell, proton exchange membrane, sulfonated poly(ether sulfone), power density, relative humidity

ACS Paragon Plus Environment

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Abstract

Novel sulfonated poly(ether sulfone) copolymers (S4PH-x-PSs) based on a new aromatic diol containing four phenyl substituents at 2, 2’, 6, and 6’ positions of 4,4’-diphenylether were synthesized. Sulfonation was found to occur exclusively on the 4 position of phenyl substituents by NMR spectroscopy. The ion exchange capacity (IEC) values can be controlled by adjusting the mole percent (x in S4PH-x-PS) of the new diol. The fully hydrated sulfonated poly(ether sulfone) copolymers had good proton conductivity in the range of 0.004 to 0.110 S/cm at room temperature. The surface morphology of S4PH-x-PSs and Nafion 212 was investigated by atomic force microscopy (tapping-mode) and related to the percolation limit and proton conductivity. Single H2/O2 fuel cell based on S4PH-40-PS loaded with 0.25 mg/cm2 catalyst (Pt/C) exhibited a peak power density of 462.6 mW/cm2, which was close to that of Nafion 212 (533.5 mW/cm2) at 80 oC with 80 %RH. Furthermore, fuel cell performance of S4PH-35-PS with various relative humidity was investigated. It was confirmed from polarization curves that the fuel cell performance of S4PH-35-PS was not as high as that of Nafion 212 under fully hydrated state due to higher interfacial resistance between S4PH-35-PS and electrodes. While under low relative humidity (53 %RH) at 80 oC, fuel cells based on S4PH-35-PS showed higher peak power density (234.9 mW/cm2) than that (214.0 mW/cm2) of Nafion 212.


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1. Introduction As an alternative energy, proton exchange membrane fuel cells (PEMFC) have been studied extensively for last several decades. Nafion, a perfluorosulfonic acid polymer, developed by Dupont, shows not only good physical and thermal properties but also excellent chemical stability and high proton conductivity in humid conditions as proton exchange membrane (PEM).1 It was reported that the morphology of Nafion observed by AFM, SAXS and TEM showed significant hydrophilic and hydrophobic microphase separation which could facilitate the proton conduction with smaller water uptake and better dimensional stability.2-5 Due to the formation of well-connected and well-distributed hydrophilic ion channels resulted from microphase separation, protons can hop and diffuse with hydronium ion by Grotthuss and Vehicle mechanism in the membranes.6-8 Many alternative membranes such as poly(ether ketone)s (PEK)s,9-10 poly(p-phenylene oxide)s (PPO)s,11 poly(ether sulfone)s (PES)s,12-15 have been synthesized with sulfonic acid group, trying to circumvent the disadvantages of Nafion in terms of high cost, environmental contamination and severe methanol crossover in direct methanol fuel cell.16-17 Among these alternative membranes, poly(ether sulfone) (PES), a widely used engineering thermoplastics,18-19 exhibited excellent thermal stability and good mechanical properties. PES membranes have been exploited in a variety of separation technology.20 The sulfonation of PES was achieved by post-sulfonation on PES or by the copolymerization of sulfonated monomers.21 As the number of sulfonic acid groups per repeat unit increased, sulfonated PES could absorb more water and thus exhibit higher proton conductivity. However, excess water could also lead to poor mechanical strength due to the plasticization effect and severe dimensional swelling.


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Generally, membranes with high ion exchange capacity (IEC) values absorb an excess amount of water that is much higher than the percolation limit of water in the membranes and cannot provide enough mechanical strength for fuel cell application.12-13, 22 For example, McGrath et al. reported that a sulfonated PES with a high IEC value of 2.2 meq./g had more distinct hydrophilic and hydrophobic microphase separation and exhibited higher proton conductivity compared to Nafion or other sulfonated PESs with lower IEC values. However, the fuel cell performance based on this high IEC sulfonated PES was not reported, probably due to its poor mechanical strength resulted from the excess water absorption.13 Therefore, many efforts have been dedicated to the chemical modifications on PES to reduce water uptake and maintain high proton conductivity by forming hydrophilic and hydrophobic microphase separation. These approaches included the introduction of fluorinated moiety to polymer main chain,23 the increase on the density of clustered sulfonic acid per repeat unit,14 the attachment of sulfonic acid on the pendant groups that are away from hydrophobic main chain,24 and the synthesis of block copolymer consisting of sulfonated monomers and hydrophobic monomers.25 Although some of these reports showed improved results with smaller swelling ratio and higher proton conductivity, the discussion on the polarization curves of fuel cells under various relative humidity conditions had less been reported. In this article, we report the synthesis of novel PESs containing sulfonic acid groups on four pendent phenyl substituents of diol monomer. Sulfonated PES copolymers with various IEC values were investigated in terms of water uptake, morphology, proton conductivity and fuel cell performance. Furthermore, the polarization curves of H2/O2 fuel cells based on the sulfonated PES copolymer with an appropriate IEC value (1.94 meq./g) under various relative humidity were analyzed.


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2. Experimental section 2.1. Materials 2,2’,6,6’-Tetraphenyl-4,4’-oxydianiline (4PHODA) was synthesized according to the procedures described in our previous report.26 Commercial reagents bis(4-fluorophenyl)sulfone (BFPS) and potassium carbonate were purchased from ACROS and SHOWA and dried under vacuum at 70 oC overnight. Bis(4-hydroxyphenyl) sulfone (BHPS) from TCI was recrystallized from toluene and dried under vacuum at 70 oC overnight before use. N,N-Dimethylacetamide (DMAc) and toluene were dried overnight over calcium hydride and distilled prior to use. Other reagents and solvents were used as received without further purification.

2.2. Synthesis of monomer 4,4'-Dihydroxy-2,2’,6,6’-tetraphenyldiphenyl ether (4PH-OH) To a 250-mL and single-necked flask were added sulfuric acid solution (10 mL, 70 wt%) and 4PHODA (1.000 g, 1.983 mmol) at room temperature. The reaction mixture was then cooled down by ice to below 5 oC under nitrogen atmosphere. Sodium nitrite (0.300 g, 4.355 mmol) in D.I. water (3 mL) was slowly added into the flask so that the temperature of reaction mixture was controlled below 5 oC. The reaction mixture was added dropwise in to another 250-mL, three-necked and round-bottom flask with a condenser containing 50 wt% sulfuric acid solution (50 mL) at 160 oC within 50 min. After the reaction mixture was cooled to room temperature and poured into ice water, it was extracted with ethyl acetate and washed with statured NaCl aqueous solution several times. The combined organic phase was collected, dried with anhydrous magnesium sulfate and evaporated to obtain the crude product. The crude product was purified by column chromatography with 1/2 (v/v) ethyl acetate/hexane mixture as the eluent and


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recrystallized twice in chloroform to afford 0.301 g (30 % yield) of white crystals; mp: 185-186 o

C, 1H NMR (600 MHz, DMSO-d6, δ, ppm): 6.20 (s, 4H, H1), 7.16 (d, J=8.0 Hz, 8H, H2), 7.30 (t,

J=7.38 Hz, 4H, H4), 7.40 (t, J=7.56 Hz, 8H, H3), 8.93 (s, 2H, H5). 13C NMR (150 MHz, DMSOd6, δ, ppm): 116.6, 126.7, 127.4, 129.0, 132.7, 138.4, 143.2, 151.7. EIMS (m/z): Calcd for 506.19; Found: 506.20 [M] + Anal. Calcd for C36H26O3: C, 84.87; H, 5.11; Found: C, 84.91; H, 5.13.

2.3. Synthesis of poly(ether sulfone) copolymers (4PH-x-PSs) Poly(ether sulfone) copolymers (4PH-x-PS, x: mole percent (%) of 4PH-OH in two diol monomers) were synthesized by the procedures shown below (4PH-30-PS as the example). To a 50-mL, three-necked and round-bottomed flask equipped with a mechanical stirrer, a Dean-Stark trap and a nitrogen inlet were added 4PH-OH (0.4813 g, 0.9500 mmol), BFPS (0.8051g, 3.1665 mmol), BHPS (0.5548 g, 2.2168 mmol) and K2CO3 (0.9626 g, 6.9650 mmol). After Then, distilled DMAc (9.2 mL) and toluene (4.6 mL) were added, the reaction mixture was heated to 135 oC with stirring. Byproduct water was removed form reaction mixture by azeotropic distillation at 135 oC for 5 h. The reaction mixture was further heated at 160 oC for 24 h. The formed viscous solution was then poured into water and the fibrous precipitate was collected and washed with water by Soxhlet extraction for 12 h. After dried at 100 oC overnight, 4PH-30-PS was obtained in a quantitative yield (>95 %).

2.3.1. Sulfonation of poly(ether sulfone) copolymers (S4PH-x-PSs) Sulfonation of poly(ether sulfone) copolymers were conducted using the procedures shown below (S4PH-30-PS as the example). To a 50-mL, three-necked and round bottomed flask


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equipped with a mechanical stirrer were added 4PH-30-PS (1.00 g, 1.84 mmol of average repeat unit) and sulfuric acid (20 mL, 95-97 %). After the reaction mixture was stirred at 10 oC for 24 h, it was then poured into ice water. The fibrous precipitate was collected, washed with D.I. water to pH neutrality, and dried at 80 oC overnight to give 0.92 g of S4PH-30-PS (78 % yield).

2.4. Membrane preparation S4PH-x-PS (1.00 g) was dissolved in DMAc (20 mL). The solution (5 %, w/v) was filtrated and then poured onto a Patri dish. After dried at 70 oC for 12 h and at 120 oC for 12 h, the membrane was peeled off and soaked in 1 N HNO3 for 24 h at room temperature. After being washed with D.I. water several times, the wet membrane was dried at 120 oC in vacuum oven for 6 h.

2.5. Fabrication of membrane electrode assembly Membrane electrode assembly (MEA) was fabricated using two gas diffusion electrodes and proton exchange membrane. The catalyst ink, consisting of 360 µl Nafion® dispersion (5-6 wt%, Dupont Fluoroproducts) and 6.18 mg Pt/C catalyst (40 wt%, Alfa Aesar), was printed onto carbon cloth (CeTech Co., Ltd.) to form gas diffusion electrode. The Pt catalyst loading was 0.25 mg/cm2 for both anode and cathode. The proton exchange membrane was sandwiched between anode and cathode on both side at 125 oC and 250 psi (1.7 MPa) for 120 sec. The effective area of MEAs was 5 cm2.

2.6. Measurements


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Melting point of 4PH-OH was determined on a Mel-Temp capillary melting point apparatus. 1

H and

13

C NMR spectra were measured on a Bruker AVIII spectrometer at 600 MHz and 150

MHz, respectively. Mass spectrometry measurement was performed on a Finnigan TSQ 700 mass spectrometer. Thermal gravimetric analyses (TGA) were obtained in nitrogen with a TA TGA Q500 thermal gravimetric analyzer at a heating rate of 10 oC/min. All of samples were heated at 150 oC to exclude water within polymer for 30 minute and then cooled down to 100 oC before the test started to record. Glass transition temperature (Tg) was determined with differential scanning calorimetry (DSC) using a Perkin Elmer (DSC 4000) at a heating rate of 10 o

C/min under nitrogen. Polymer molecular weights were obtained on a JASCO GPC system (PU-

980) equipped with an refractive index detector (RI-930) and a Shodex GPC KF-804 column, using N,N-dimethylacetamide (DMAc) as the eluent and calibrated with polystyrene standards.

2.6.1 Atomic force microscope (AFM) analysis Microphase-separated morphology of membrane surface can be detected by AFM tappingmode. All of dried membrane (H+ form) were measured with a Bruker Dimension ICON in air, using a commercial Si cantilevers with a force constant of 7.4 N/m and resonance frequency of 160 KHz (Nanosensors, PPP-NCSTR). All of images containing 512 scan lines were performed at scan rate of 1.00 Hz and the height difference of them was below 10 nm which didn’t affect the quality of phase images.

2.6.2. Water uptake and swelling ratio Water uptake was obtained by soaking dry membranes into D.I. water at room temperature for 24 h. The wet membrane was then taken out from water and wiped dry with tissues quickly


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before the wet weight was recorded. The water uptake (WU) was calculated by the following equation (1): WU% = − / × 100 %

(1)

where Ww and Wd are the weights of wet and dried membranes, respectively. Swelling ratio was calculated from the volume change of membrane after soaked in D.I. water at room temperature for 24 h by the following equation (2): Swelling ratio = − / × 100 %

(2)

where Vw and Vd are the volumes of wet and dried membranes, respectively.

2.6.3. Ion exchange capacity (IEC) value measurement Ion exchange capacity (IEC) value was measured by titration. Dry membrane samples were soaked in 1 N HNO3 to convert to H+ form for 24 h at room temperature. They were then washed by D.I. water to neutral pH and dried in vacuum at 100 oC overnight. The weights of dried membrane samples were recorded. The dried membrane samples were soaked into 1 M NaCl solution for 24 h at room temperature to be converted to Na+ form. The NaCl solution was then titrated by 0.01 M NaOH solution. Ion exchange capacity was calculated by the following equation (3): IEC = ×

!"#$ / %&'

(3)

where Vt is the volume of NaOH solution consumed in titration, CNaOH is the concentration of NaOH solution and Wdried is the weight of dried membrane sample.


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2.6.4. Impedance and proton conductivity measurement Impedance was measured using a two-electrode in-plane method by electrochemical impedance spectroscopy (EIS) with a Zahner potentiostat-galvanostat electrochemical workstation model PGSTAT over a frequency range of 1 Hz to 100 KHz with an oscillating voltage of 10 mV. A rectangle membrane (1 cm×2 cm) was fixed in a homemade Teflon cell with two gold electrodes. The distance between two electrodes was 1.0 cm. The cell was set in a chamber with a temperature and humidity controller. Each measurement was conducted at a specific relative humidity after membrane sample was equilibrated at that humidity for 1.5 h. Proton conductivity (σ) was calculated from the impedance data according to equation (4). σ = )/*+,

(4)

where σ is the proton conductivity (S/cm), d is the distance (cm) between the electrodes, t and w are the thickness (cm) and width (cm) of the sample, respectively. R is the resistance (Ω) associated with the ionic conductivity of the sample from the impedance data.

2.6.5. O2/H2 fuel cell performance test In fuel cell test, highly pure hydrogen and oxygen at a flow rate of 200 and 500 standard cubic centimeter per minute (sccm) were fed to anode and cathode, respectively. A single cell was tested in a PEMFC test station (FCED-DD50, Asia Pacific Fuel Cell Technologies Ltd.) with 1 atm of back pressure. The polarization curves (cell voltage vs. current density) were recorded in a steady state. The activation of catalyst was conducted by operating the fuel cell at 0.2 V for 1-2 hours to reach plateau of current density, before the polarization curve was recorded. Humidified gases at different temperatures were used to adjust the relative humidity of fuel cells.


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3. Results and discussion 3.1. Synthesis of monomer The novel diol monomer (4,4'-dihydroxy-2,2’,6,6’-tetraphenyldiphenyl ether, 4PH-OH) was synthesized from 4,4’-diamino-2,2’,6,6’-tetraphenyldiphenyl ether (4PHODA) by diazotization and hydrolysis as shown in Scheme 1. The synthesis of starting compound 4PHODA was reported in our previous publication.26 Compared to traditional diazotization,27-29 more concentrated sulfuric acid solution (70 wt%) was necessary in order to dissolve 4PHODA and facilitate the formation of diazonium compound. The diazonium salt was then converted to diol by hydrolysis in sulfuric acid solution (50 wt%) at 160 oC. The 1H NMR spectrum of 4PH-OH is shown in Figure 1. The peaks appearing in the range of 7.1-7.5 ppm were assigned to the protons (H2, H3 and H4) on the four phenyl substituents of 4PH-OH. Singlet peaks appearing at 6.20 and 8.93 ppm were assigned to aromatic hydrogen (H1) and hydroxyl group (H5), respectively. The 1H NMR spectrum and the results from EI mass and element analysis confirmed that 4PH-OH was successfully synthesized.

Scheme 1. Synthetic route of 4,4'-dihydroxy-2,2’,6,6’-tetraphenyldiphenyl ether (4PH-OH).


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Figure 1. 1H NMR spectrum of 4PH-OH in DMSO-d6.

3.2. Synthesis of sulfonated copolymers Poly(ether sulfone) copolymers (4PH-x-PS, x: mole percent (%) of 4PH-OH in two diol monomers) were synthesized from 4PH-OH, bis(4-fluorophenyl)sulfone (BFPS) and bis(4hydroxyphenyl) sulfone (BHPS) in DMAc by aromatic nucleophilic substitution under basic condition as shown in Scheme 2. The stoichiometric ratio of total hydroxyl groups to fluorides was controlled at 1:1. Four 4PH-x-PSs with different compositions (x: 20, 30, 35 and 40) were prepared. Nevertheless, the solubility of poly(ether sulfone) (4PH-100-PS) derived from 4PHOH and BFPS was so poor that high molecular weight polymer cannot be prepared in DMAc. In addition, 4PH-100-PS after sulfonation is not suitable for PEM applications because the extremely high IEC value might lead to severe swelling and even dissolution in water.


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Ph

x HO

O

OH

O S O

100-x HO

OH

O S O

100 F

F

Ph Ph

BFPS

BHPS

4PH-OH

DMAc/Toluene (2/1, v/v) Solid content = 20 % (w/v) K2CO3

135 oC, 5 h 160 oC, 24 h Ph O

O

O

O S O

Ph Ph

O

x

O S O

4Ph-x-PS

R O R

O R

O S O

O

100-x

x = 20, 30, 35 and 40

Conc. sulfuric acid (95-97 %) Solid content = 5 % (w/v) 10 oC, 24 h R=

SO3H

O

O S O

O

O S O

O

x S4Ph-x-PS Scheme 2. Synthetic route of S4PH-x-PS copolymers.

O S O

SO3H

100-x

Poly(ether sulfone) copolymers 4PH-x-PSs were then sulfonated to form S4PH-x-PSs. The polymer solid content in conc. sulfuric acid was controlled at 5 % (w/v) to provide good solubility and complete sulfonation. On the other hand, temperature during sulfonation also plays an important role. In addition to higher sulfonation rate, it was reported that higher sulfonation temperature would also lead to polymer degradation. However, polymer degradation could be avoided when sulfonation temperature was controlled below 10 oC.30-31 In this study, the sulfonation of 4PH-x-PSs was conducted at 10 oC for 24 h to obtain S4PH-x-PSs with high molecular weights. Table 1 shows the molecular weights (Mn) and polydispersity (PDI) of poly(ether sulfone) copolymers (4PH-x-PSs) before and after (S4PH-x-PSs) sulfonation. The


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molecular weights of 4PH-x-PS copolymers were in the range of 82 to 90 kDa. However, S4PHx-PSs showed GPC molecular weights in the range of 260 to 300 kDa, higher than their unsulfonated predecessors because of the polyelectrolyte effect after sulfonation.32 Although the increase in molecular weight couldn’t absolutely exclude the degradation resulted from sulfonation, it did suggest that the degradation was minimized under the reaction condition in this study. Molecular weight polydispersity decreased from 1.43-1.54 to 1.18-1.29 after sulfonation. It indicated that some sulfonated poly(ether sulfone) copolymers with smaller molecular weights were washed away by D.I. water.

Table 1. Molecular weights of 4PH-x-PSs and S4PH-x-PSs.

a

Mna (Da)

PDIb

4PH-20-PS

90,000

1.49

4PH-30-PS

82,000

1.45

4PH-35-PS

89,000

1.54

4PH-40-PS

84,000

1.43

S4PH-20-PS

300,000

1.27

S4PH-30-PS

281,000

1.22

S4PH-35-PS

267,000

1.29

S4PH-40-PS

272,000

1.18

Measured by gel permeation chromatography with DMAc/LiBr as elution. b PDI = Mw/Mn.


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3.3. Polymer characterization and IEC value calculation The 1H NMR spectra of 4PH-30-PS and S4PH-30-PS are shown in Figure 2. In the case of 4PH-30-PS (Figure 2 (a)), the peak at δ7.97 ppm was assigned to the protons (HA and Ha) ortho to sulfone group on the main chain. The peaks of Hb and Hc ortho to ether group appeared at δ6.74 and δ6.62 ppm, respectively. The peak at 7.24 ppm was contributed by both HB and Hd. The peaks at 7.46 and 7.40 ppm were assigned to He and Hf on phenyl substituents. These peak assignments were also confirmed by COSY and HSQC spectra of 4PH-30-PS as shown in the supporting information (Figure S1 and Figure S2). In addition, the 1H NMR peak integrals of 4PH-30-PS corresponded exactly to the chemical composition of copolymer based on the monomer feed ratio. It indicated that the reactivity of two diols, 4PH-OH and BHPS, was similar so that the monomer feed ratio could directly reflect the chemical composition of the formed copolymer. Figure 2(b) shows the 1H NMR spectrum of sulfonated poly(ether sulfone) copolymer, S4PH-30-PS. The peak corresponding to Hf (Figure (2a)) disappeared after sulfonation. The peak corresponding to He ortho to the sulfonic group appeared at 7.72 ppm distinctly. The peak integrals and the peak assignments combined with the COSY and HSQC spectra of S4PH-30-PS in supporting information (Figure S3 and Figure S4) indicated that sulfonic groups were quantitatively attached only on the para position of phenyl substituents.


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Figure 2. 1H NMR spectra of 4PH-30-PS (a) and S4PH-30-PS (b) in DMSO-d6.

In accordance with the previous results,9, 30 sulfonic groups did not attach on the HB and Hb positions of the phenyl ring connected to strong electron-withdrawing sulfonyl groups (Figure 2(b)). Interestingly, sulfonic groups did not attach on electron-rich Hc and Hd positions, either. It was attributed to the lower reaction temperature (10 oC), shorter reaction time and the steric hindrance effect of the pendant phenyl substituents. In other mono-phenylated PEK/PES cases,9-


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10, 33

sulfonation occurred rapidly at para positions of phenyl rings within several hours at room

temperature and then at the sites ortho to the oxygen of polymer backbone after several days. Therefore, with the specific sulfonation condition in this study, we can attach sulfonic groups at the para positions of the phenyl substituents quantitatively. The theoretical ion exchange capacity values (IECtheory), defined as mili-equivalents of sulfonic groups per gram of copolymer, were calculated from the chemical structures of corresponding sulfonated copolymers. IEC values were also measured by acid-base titration (IECtitra.) or calculated from 1H NMR spectrum (IECNMR). Table 2 shows these IEC values. IEC values calculated theoretically and measured experimentally were quite close. They ranged from 1.36 to 2.15 meq./g and increased when copolymers contained more tetraphenyl-substituted units. The IEC values of these sulfonated copolymers are higher than that of Nafion 212 and are comparable to those of other sulfonated poly(ether sulfone) copolymers reported in the literatures.12-15

Table 2. Ion exchange capacity values of S4PH-x-PSs and Nafion 212. IECtheory

IECNMR

IECtitra.

(meq./g)a

(meq./g)b

(meq./g)c

S4PH-20-PS

1.37

1.34

1.36 ± 0.02

S4PH-30-PS

1.86

1.73

1.67 ± 0.07

S4PH-35-PS

2.10

1.95

1.94 ± 0.06

S4PH-40-PS

2.30

2.12

2.15 ± 0.03

Nafion 212

0.98

-

-

a

Calculated from the chemical structures. b Calculated from 1H NMR spectra. c Determined by titration.


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3.4. Thermal and mechanical properties Figure 3 shows TGA curves of 4PH-x-PS and S4PH-x-PS copolymers. The decomposition temperatures at 5 % weight loss (Td5%) of 4PH-x-PSs were all higher than 420 oC and were typical values among those of poly(ether sulfone)s. After sulfonation, the formed S4PH-x-PSs exhibited two-step degradation in nitrogen atmosphere. The weight loss occurring first above 300 oC was due to the cleavage of attached sulfonic groups and the degradation temperature decreased as more sulfonic groups were attached. The second weight loss up to 480 oC was attributed to the degradation of polymer main chain. Table 3 also lists the DSC glass transition temperatures (Tg) of 4PH-x-PSs and S4PH-x-PSs. The Tgs of 4PH-x-PSs were in the range of 241-244 oC which were higher than those of poly(ether sulfone)s without phenyl substituents on diol monomer (180 oC).19 It was reasonable that four bulky phenyl substituents could increase Tgs by limiting the rotation of polymer chains, but the effect of copolymerization could also reduce Tgs by disturbing the packing of polymer chains. On the other hand, sulfonated S4PH-xPS copolymers exhibited Tgs in the range of 259 to 267 oC which were higher than nonsulfonated ones because bulky and polar sulfonic groups could hinder the rotation of polymer chains. The Tgs were higher as more sulfonic groups were attached.


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Figure 3. TGA curves of 4PH-x-PS and S4PH-x-PS copolymers.

Table 3 also shows the mechanical properties of S4PH-x-PSs. The tensile strength, tensile modulus and elongation were 10.7-28.2 MPa, 0.23-0.32 GPa and 20.3-54.3 %, respectively. As more sulfonic groups were attached on S4PH-x-PSs, tensile strength decreased and elongation increased. The reduced strength and extend elongation were attributed to the increasing amount of water that cannot be excluded completely in the membranes as a plasticizer. All of S4PH-x-PS membranes were mechanically strong enough for MEA fabrication.

Table 3. Thermal and mechanical properties of S4PH-x-PSs and Nafion 212.

a

Tensile strength (MPa)

Elongation at break (%)

Young's modulus (GPa)

Td5% (oC)

Tga (oC)

S4PH-20-PS

28.2 ± 3.9

20.3 ± 0.8

0.23 ± 0.03

344(457)b

259(241)

S4PH-30-PS

27.0 ± 3.3

34.2 ± 1.8

0.25 ± 0.05

320(450)

262(243)

S4PH-35-PS

18.0 ± 2.2

41.8 ± 3.7

0.24 ± 0.02

317(445)

266(244)

S4PH-40-PS

10.7 ± 4.3

54.3 ± 0.3

0.32 ± 0.06

309(420)

267(244)

Nafion 212

25.6 ± 1.4

365.2 ± 63.1

0.12 ± 0.01

-

-

Measured by DSC. b Thermal properties of 4PH-x-PSs in parentheses.

3.5. Water uptake and proton conductivity For low temperature PEMFC application, proton exchange membranes should contain enough water for proton transfer, while maintain mechanical strength for fuel cell operation. As shown in Figure 4 and Table 4, S4PH-x-PS membranes had water uptakes from 19 to 87 % at room temperature after soaking in DI water for 24 h as x increased from 20 to 40. Among these


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membranes, S4PH-20-PS had similar water uptake (19 %) as Nafion 212 at 25 oC. The drastic increase in water uptake (87 %) of S4PH-40-PS was attributed to its high IEC value (2.15 meq./g) which exceeded the percolation limit of sulfonated copolymers.13 Besides, the water uptake of S4PH-50-PS reached above 700 % at room temperature and the mechanical strength were not mechanically strong enough for membrane preparation.

Figure 4. Water uptake and swelling ratio of S4PH-x-PS membranes.

The volume swelling ratio of S4PH-x-PSs ranged from 1.6 to 60.9 % and increased as more sulfonic groups were attached (Table 4 and Figure 4). S4PH-35-PS with a higher water uptake (43 %) exhibited a smaller volume swelling ratio of 18.8 % than that of Nafion 212 (23.4 %) with a less water uptake of 20 %.


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Table 4. Water uptake and proton conductivity (σ) of S4PH-x-PSs and Nafion 212. IECtitra.

a

σ (S/cm)

Swelling ratioa (%)

a

WU (%)

(meq./g)

area

thickness

volume

S4PH-20-PS

1.36

0.004 ± 0.001

19 ± 3

1.3 ± 0.7

0.3 ± 0.2

1.6 ± 0.9

S4PH-30-PS

1.67

0.037 ± 0.005

31 ± 4

14.2 ± 1.9

1.6 ± 0.8

16.0 ± 2.8

S4PH-35-PS

1.94

0.041 ± 0.003

43 ± 3

12.8 ± 1.5

5.4 ± 1.2

18.8 ± 3.0

S4PH-40-PS

2.15

0.110 ± 0.014

87 ± 6

20.4 ± 2.2

33.7 ± 5.3

60.9 ± 9.5

Nafion 212

-

0.066 ± 0.002

20 ± 2

9.5 ± 1.9

12.7 ± 2.5

23.4 ± 4.9

a

Measured at room temperature after membranes were soaked in D.I. water for 24 h at room temperature.

Table 4 also lists the proton conductivity of membranes. S4PH-20-PS with an IEC value of 1.36 meq./g had a proton conductivity of only 0.004 S/cm at room temperature at hydrated state. The proton conductivity of S4PH-30-PS (IEC = 1.67 meq./g) increased to 0.037 S/cm, one order of magnitude higher than that of S4PH-20-PS. This sharp increase in proton conductivity could be resulted from the IEC-related morphology of membranes that is discussed in the following section. As more sulfonic groups were attached, S4PH-35-PS and S4PH-40-PS had proton conductivity of 0.041 and 0.110 S/cm, respectively, in their hydrated state at room temperature. S4PH-35-PH had proton conductivity similar to Nafion 212 (0.066 S/cm) in their hydrated state at room temperature. Figure 5 shows the relative humidity dependence of proton conductivity in the range from 40 to 100 %RH at 80 oC. The proton conductivity increased as the relative humidity increased. Below 80 %RH, all S4PH-x-PS membranes displayed lower proton conductivity than Nafion 212. The proton conductivity of S4PH-20-PS was too small to be measured when the relative


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humidity was below 66 %. All membranes under 100 %RH, except S4PH-20-PS, had proton conductivity (0.20 to 0.25 S/cm) higher than Nafion 212 (0.17 S/cm). In this study, the chemical structures of S4PH-x-PSs were designed so that the sulfonic groups were attached on bulky side phenyl groups to apart from hydrophobic main chains. According to the earlier literature reported by McGrath et al.,13 traditional sulfonated poly (ether sulfone)s (PBPSH-60) containing two sulfonic groups per repeating unit on the main chain with an IEC value of 2.2 meq./g. had a water uptake of 148 %. With similar IEC value (IEC: 2.15 meq./g), S4PH-40-PS successfully reduced water uptake to 87 % while maintaining good proton conductivity.

Figure 5. Proton conductivity of PEM at 80 oC under various relative humidity.

3.6. Morphology of membranes The surface microstructures of S4PH-x-PSs and Nafion 212 were examined by atom force microscopy. Figure 6 shows the tapping-mode images of these membranes. The dark and bright regions in the phase images corresponded to the soft parts of hydrophilic sulfonic acid groups


22

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which could absorb water and to the hard block of hydrophobic main chain segments, respectively. The dark region can thus be regarded as the formed ion channels. The interference by the roughness of the sample surfaces can be excluded by the smooth surfaces observed in the height images of these samples (Figure S5, in supporting information). Figure 6(a) shows the surface phase image of Nafion 212. Hydrophilic/hydrophobic microphase separation containing narrow ion channels with a diameter of approximately 15 nm distributed uniformly and continuously were observed. This morphology provides a beneficial effect on proton transfer. In contrast to Nafion 212, no obvious microphase separation of S4PH-20-PS was observed (Figure 6(b)). As the IEC value reached 1.67 meq./g (S4PH-30-PS, Figure 6(c)), connected and isolated ion clusters co-existed within the membrane with diameters around 19 nm. Compared with S4PH-30-PS, S4PH-35-PS showed better connectivity of ionic clusters and similar channels with diameters of around 22 nm (Figure 6(d)). S4PH-40-PS with an IEC value of 2.15 meq./g showed more and larger ion clusters and ion channels with diameters around 30 nm (Figure 6(e)). This was responsible for the abrupt increase in the water uptake of S4PH-40-PS. According to the proposed Vehicle mechanism and Grotthuss mechanism for proton transfer in the membranes,6-8 the good connectivity of ion channels is essential for higher proton conductivity. From these phase images, S4PH-30-PS and S4PH-35PH showed better connected ion channels than S4PH-20-PS. This could explain that the proton conductivity of S4PH-30-PS and S4PH-35-PS (0.037 and 0.041 S/cm, respectively) was one order of magnitude higher than that (0.004 S/cm) of S4PH-20-PS.


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Figure 6. AFM phase images of membranes (a) Nafion 212, (b) S4PH-20-PS, (c) S4PH-30-PS, (d) S4PH-35-PS and (e) S4PH-40-PS.

3.7. H2/O2 fuel cell performance MEAs of S4PH-x-PS copolymers and Nafion 212 were fabricated and the single cell performances were investigated at 80 oC under 80 % and 100 %RH. Table 5 shows the peak power density and open-circuit voltages (OCV) of single fuel cells. It was noteworthy that S4PH-40-PS showed the highest power density of 462.6 mW/cm2 which was comparable to that of Nafion 212 (533.5 mW/cm2) at 80 oC under 80 %RH. However, under 100 %RH, single fuel cell of S4PH-40-PS failed due to the inferior mechanical strength of its membrane. In contrast, S4PH-20-PS only generated power density of 3.7 mW/cm2 under 80 %RH but 362.5 mW/cm2 under 100 %RH at 80 oC. It indicated that ion clusters within S4PH-20-PS did not connect each other effectively under low RH. Ion clusters could be enlarged to connect together under higher RH condition. In general, the higher relative humidity of the test conditions increases the proton conductivity of membranes. This is beneficial to improve the power density of fuel cell as shown


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in the cases of S4PH-20-PS and S4PH-30-PS. However, if relative humidity is too high, membranes with high IEC values might absorb too much water that might flood the electrodes and deteriorate the mechanical strength of the membranes as shown in the case of S4PH-40-PS.

Table 5. Fuel cell test of S4PH-x-PSs and Nafion 212 at 80 oC. 80 %RH PDpeaka

CD0.73 Vb

OCVc

PDpeak

OCV

(mW/cm2)

(mA/cm2)

(V)

(mW/cm2)

(V)

S4PH-20-PS

3.7

3.7

0.950

362.5

0.925

S4PH-30-PS

394.8

100.1

0.932

463.4

0.932

S4PH-35-PS

445.1

124.1

0.930

372.9

0.922

S4PH-40-PS

462.6

76.8

0.892

broken

-

Nafion 212

533.5

206.6

0.958

847.4

0.953

Membrane

a

100 %RH

Peak power density. b Current density at 0.73 V. c Open circuit voltage.

Figure 7. Polarization curves of S4PH-x-PSs and Nafion 212 at 80 oC under 80 %RH.


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Page 26 of 45

Figure 7 shows the polarization curves of S4PH-x-PSs (x: 30, 35 and 40) and Nafion 212 at 80 o

C under 80 %RH. The effects of activation overpotential, ohmic loss and concentration

overpotential could be analyzed from these polarization curves. In activation region (for example, at 0.73 V), the current density (206.6 mA/cm2) of Nafion 212 was significantly higher than those (3.7-124.1 mA/cm2) of S4PH-x-PSs, due to the lower activation overpotential of Nafion 212 (Table 5). It was also observed that even S4PH-30-PS and S4PH-35-PS had lower activation overpotential than S4PH-40-PS. In fact, the activation overpotential was reported to be inversely related to the catalyst/ionomer active surface area.34-35 Thus, it suggested that the excessive water uptake from S4PH-40-PS membrane might flood the catalyst and reduce the active surface area of Pt, leading to the difficulty of oxygen transport in the catalyst layer.

Figure 8. Polarization curves of S4PH-35-PS and Nafion 212 at 80 oC.


26

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Table 6. Fuel cell test of S4PH-35-PS and Nafion 212 at 80 oC under various relative humidity.

Relative humidity (%)

a

S4PH-35-PS

Nafion 212

PDpeaka

CD0.73 Vb

OCVc

PDpeak

CD0.73 V

OCV

(mW/cm2)

(mA/cm2)

(V)

(mW/cm2)

(mA/cm2)

(V)

53

234.9

68.6

0.900

214.0

88.1

0.988

66

289.1

52.8

0.838

344.7

118.1

0.927

80

445.1

124.1

0.930

533.5

206.6

0.958

100

372.9

117.3

0.922

847.4

255.0

0.953

Peak power density. b Current density at 0.73 V. c Open circuit voltage.

In order to further investigate the appropriate working conditions, the performances of fuel cells based on S4PH-35-PS and Nafion 212 were investigated under various relative humidity at 80 oC as shown in Figure 8 and listed in Table 6. S4PH-35-PS exhibited lower current density at activation region (V=0.73 V) than Nafion 212. But the difference between their current density at 0.73 V reduced as the relative humidity decreased. It indicated that the flooding effect of S4PH35-PS on catalyst layer was reduced at low relative humidity. It was noteworthy that the ohmic loss of S4PH-35-PS increased much faster at ohmic loss region than that of Nafion 212 at 100 %RH. The ohmic resistance are dependent on the proton conductivity of binder and membranes and interfacial resistance between electrodes and membranes. Due to the same binder (Nafion® dispersion) was used and the proton conductivity of S4PH-35-PS and Nafion 212 was very close at 100 %RH, it was thus reasonable that the interfacial resistance of S4PH-35-PS was much larger than that of Nafion 212. Pivovar et al. reported earlier that the interfacial resistances between electrodes and membrane could increase as the difference in water uptake between binder and membrane increased under fully humidified condition.36-37 The increased interfacial


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Page 28 of 45

resistance would lead to the poor performance of H2/O2 fuel cell. They supposed that during the fabrication of MEAs and the operation of fuel cells, binder and PEM experienced shrink/swell process, leading to worse interfacial contact and larger interfacial resistance as the difference of swelling ratio between binders and membranes increased. Thus, at 100 %RH, the larger swelling ratio of S4PH-35-PS, compared to that of Nafion binder, interfered the contact between electrode and PEM and lowered the fuel cell performance. Therefore, the fuel cell composed of Nafion 212 membrane and binder exhibited a peak power density of 847.4 mW/cm2 which was higher than that of S4PH-35-PS fuel cell (372.9 mW/cm2) at 100 %RH. Interestingly, S4PH-35-PS exhibited higher peak power density (234.9 mW/cm2) than Nafion 212 (214.0 mW/cm2) at 53 %RH, although the proton conductivity of S4PH-35-PS was lower than Nafion 212 (Figure 5). It was believed that sulfonated aromatic polymer had slower water diffusion compare to Nafion because of the higher density of sulfonic acid groups.38 Therefore, at low relative humidity, water produced at cathode might diffuse back to anode easier for Nafion 212 and reduced the net flow of proton.

4. Conclusions Novel sulfonated aromatic poly(ether sulfone) copolymers, S4PH-x-PSs, were successfully synthesized by sulfonation on the corresponding poly(ether sulfone)s based on a new diol monomer containing four phenyl substituents at 2, 2’, 6, and 6’ positions of 4,4’-diphenylether. Sulfonation was found to occur exclusively on the 4 position of phenyl substituents by NMR spectroscopy. The ion exchange capacity (IEC) could be controlled by the mole percent (x in S4PH-x-PS) of the new diol. Compared to traditional sulfonated poly(ether sulfone) copolymers with similar IEC values, S4PH-40-PS exhibited good dimensional stability with lower water


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uptake while maintaining good proton conductivity. Among S4PH-x-PSs, S4PH-35-PS exhibited suitable hydrophilic/hydrophobic microphase separation observed by AFM phase images. The dramatic increase in water uptake of S4PH-40-PS indicated that the membrane absorbed water exceeding the percolation limit. Except for S4PH-20-PS, S4PH-x-PSs showed higher proton conductivity than Nafion 212 under 100 %RH at 80 oC. In H2/O2 fuel cell test, the peak power density of S4PH-40-PS under 80 %RH at 80 oC was comparable with that of Nafion 212. Under 100 %RH at 80 oC, due to the higher interfacial resistance resulted from the larger difference in water uptake between binder and S4PH-35-PS, the fuel cell performance of S4PH-35-PS was not as high as that of Nafion 212. The fuel cell based on S4PH-35-PS exhibited better peak power density than that based on Nafion 212 under 53 %RH at 80 oC.

ASSOCIATED CONTENT Supporting information 1

H-1H cosy spectra and 1H-13C HSQC spectra of 4PH-30-PS and S4PH-30-PS, and AFM

height images of Nafion and S4PH-x-PSs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +886-2-27376544; Tel: +886-2-27376526

REFERENCES


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(23) Wiles, K. B.; de Diego, C. M.; de Abajo, J.; McGrath, J. E. Directly Copolymerized Partially Fluorinated Disulfonated Poly(Arylene Ether Sulfone) Random Copolymers for PEM Fuel Cell Systems: Synthesis, Fabrication and Characterization of Membranes and Membrane– Electrode Assemblies for Fuel Cell Applications. J. Membr. Sci. 2007, 294, 22-29. (24) Wang, C.; Young Lee, S.; Won Shin, D.; Rae Kang, N.; Lee, Y. M.; Guiver, M. D. ProtonConducting Membranes from Poly(Ether Sulfone)s Grafted with Sulfoalkylamine. J. Membr. Sci. 2013, 427, 443-450. (25) Lim, Y.-D.; Seo, D.-W.; Lee, S.-H.; Hossain, M. A.; Kang, K.; Ju, H.; Kim, W.-G. Synthesis and Characterization of Sulfonated Poly (Arylene Ether Ketone Sulfone) Block Copolymers Containing Multi-Phenyl for PEMFC. Int. J. Hydrogen Energy 2013, 38, 631-639. (26) Chen, J. C.; Wu, J. A.; Li, S. W.; Chou, S. C. Highly Phenylated Polyimides Containing 4,4′-Diphenylether Moiety. React. Funct. Polym. 2014, 78, 23-31. (27) Burckhalter, J. H.; Tendick, F. H.; Jones, E. M.; Holcomb, W. F.; Rawlins, A. L. Aminoalkylphenols as Antimalarials. I. Simply Substituted α-Aminocresols. J. Am. Chem. Soc. 1946, 68, 1894-1901. (28) Lewis, E. S.; Johnson, M. D. The Reactions of Diazonium Salts with Nucleophiles. VII. pPhenylene-bis-diazonium Ion. J. Am. Chem. Soc. 1960, 82, 5408-5410. (29) Hegarty, A. F. Kinetics and Mechanisms of Reactions Involving Diazonium and Diazo Groups. In Diazonium and Diazo Groups; Patai, S., Eds.; John Wiley & Sons, Ltd.: New York, 1978; pp 511-591.


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(30) Bunn, A.; Rose, J. B. Sulphonation of Poly(Phenylene Ether Sulphone)s Containing Hydroquinone Residues. Polymer 1993, 34, 1114-1116. (31) Unveren, E. E.; Erdogan, T.; Çelebi, S. S.; Inan, T. Y. Role of Post-Sulfonation of Poly(Ether Ether Sulfone) in Proton Conductivity and Chemical Stability of Its Proton Exchange Membranes for Fuel Cell. Int. J. Hydrogen Energy 2010, 35, 3736-3744. (32) Dobrynin, A. V.; Rubinstein, M. Theory of Polyelectrolytes in Solutions and at Surfaces. Prog. Polym. Sci. 2005, 30, 1049-1118. (33) Liu, B.; Robertson, G. P.; Kim, D.-S.; Sun, X.; Jiang, Z.; Guiver, M. D. Enhanced ThermoOxidative Stability of Sulfophenylated Poly(Ether Sulfone)s. Polymer 2010, 51, 403-413. (34) Passalacqua, E.; Lufrano, F.; Squadrito, G.; Patti, A.; Giorgi, L. Nafion Content in the Catalyst Layer of Polymer Electrolyte Fuel Cells: Effects on Structure and Performance. Electrochim. Acta 2001, 46, 799-805. (35) Antolini, E.; Giorgi, L.; Pozio, A.; Passalacqua, E. Influence of Nafion Loading in the Catalyst Layer of Gas-Diffusion Electrodes for PEFC. J. Power Sources 1999, 77, 136-142. (36) Pivovar, B. S.; Kim, Y. S. The Membrane–Electrode Interface in PEFCs: I. A Method for Quantifying Membrane–Electrode Interfacial Resistance. J. Electrochem. Soc. 2007, 154, B739B744. (37) Kim, Y. S.; Pivovar, B. S. The Membrane–Electrode Interface in PEFCs: IV. The Origin and Implications of Interfacial Resistance. J. Electrochem. Soc. 2010, 157, B1616-B1623.


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(38) De Araujo, C. C.; Kreuer, K. D.; Schuster, M.; Portale, G.; Mendil-Jakani, H.; Gebel, G.; Maier, J. Poly(P-Phenylene Sulfone)s with High Ion Exchange Capacity: Ionomers with Unique Microstructural and Transport Features. Phys. Chem. Chem. Phys. 2009, 11, 3305-3312.

For Table of Contents Only


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Scheme 1. Synthetic route of 4,4'-dihydroxy-2,2’,6,6’-tetraphenyldiphenyl ether (4PH-OH). Scheme 1 152x43mm (300 x 300 DPI)


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Scheme 2. Synthetic route of S4PH-x-PS copolymers. Scheme 2 161x117mm (300 x 300 DPI)



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Figure 1. 1H NMR spectrum of 4PH-OH in DMSO-d6. Figure 1 80x41mm (300 x 300 DPI)


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Figure 2. 1H NMR spectra of 4PH-30-PS (a) and S4PH-30-PS (b) in DMSO-d6. Figure 2 161x168mm (300 x 300 DPI)



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Figure 3. TGA curves of 4PH-x-PS and S4PH-x-PS copolymers. Figure 3 80x63mm (300 x 300 DPI)


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Figure 4. Water uptake and swelling ratio of S4PH-x-PS membranes. Figure 4 80x59mm (300 x 300 DPI)



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Figure 5. Proton conductivity of PEM at 80 ℃ under various relative humidity. Figure 5 80x64mm (300 x 300 DPI)


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Figure 6. AFM phase images of membranes (a) Nafion 212, (b) S4PH-20-PS, (c) S4PH-30-PS, (d) S4PH-35PS and (e) S4PH-40-PS. Figure 6 155x84mm (159 x 159 DPI)



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Figure 7. Polarization curves of S4PH-x-PSs and Nafion 212 at 80 ℃ under 80 %RH. Figure 7 80x57mm (300 x 300 DPI)


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Figure 8. Polarization curves of S4PH-35-PS and Nafion 212 at 80 ℃. Figure 8 80x64mm (300 x 300 DPI)


Synthesis and Properties of Poly(ether sulfone)s with Clustered

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