Enhanced Performance of a Sulfonated Poly(arylene ether ketone

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Applications of Polymer, Composite, and Coating Materials

Enhanced Performance of Sulfonated Poly (Arylene Ether Ketone) Block Copolymer Bearing Pendant Sulfonic Acid Groups for PEMFC Operating at 80% RH Kyu Ha Lee, Ji Young Chu, Ae Rhan Kim, and Dong Jin Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03790 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Materials & Interfaces

Enhanced Performance of Sulfonated Poly (Arylene Ether Ketone) Block Copolymer Bearing Pendant Sulfonic Acid Groups for PEMFC Operating at 80% RH Kyu Ha Lee†, Ji Young Chu†, Ae Rhan Kim‡,*, Dong Jin Yoo†,§,* †

Department of Energy Storage/Conversion Engineering of Graduate School,

Hydrogen and Fuel Cell Research Center, and Education Center for Whole Life Cycle R&D of Fuel Cell Systems, Chonbuk National University, Jeollabuk–do 561– 756, Republic of Korea ‡

Department of Bioenvironmental Chemistry and R&D Center for CANUTECH,

Business Incubation Center, Chonbuk National University, Jeollabuk–do 54896, Republic of Korea §

Department of Life Science, Chonbuk National University, Jeollabuk–do 54896, Republic of Korea

1

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ABSTRACT: The series of sulfonated poly (arylene ether ketone) (SPAEK) block copolymers with controlled F–oligomer length bearing pendant diphenyl unit were synthesized via a polycondensation reaction. Sulfonation was verified by 1H NMR analysis to introduce selectively and intensively on the pendant diphenyl unit of polymer backbones. The SPAEK membranes fabricated by the solution casting approach were very transparent and flexible with the thickness of ~50 ㎛. These membranes

with

different

F-oligomer

lengths

were

investigated

to

the

physiochemical properties such as water absorption, dimensional stability, ion exchange capacity, and proton conductivity. As a result, the SPAEK membranes (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) in accordance to increasing the length of hydrophilic oligomer showed excellent proton conductivity in range of 131–154 mS cm–1 compared to Nafion−115 (131 mS cm–1) at 90 ℃ under 100% relative humidity (RH). Among the SPAEK membranes, proton conductivity of SPAEK X9.1Y8.8 (140.7 mS cm–1) is higher than that of Nafion−115 (102 mS cm–1) at 90 ℃ under 80% RH. The atomic force microscopy (AFM) image demonstrated that number of ion transport channels is increased with increase in the length of hydrophilic oligomer in the main chains, and the morphology is proved to be related to the proton conductivity. The synthesized SPAEK membrane exhibited a maximum power density of 324 mW cm–2, which is higher than that of Nafion−115 (291 mW cm–2) at 60 ℃under 100% RH.

KEYWORDS: Pendant sulfonic acid, Proton exchange membrane, Morphology, SPAEK, Fuel cells 2


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1. INTRODUCTION Fuel cells, as energy conversion devices that convert chemical energy directly into electrical energy with the supply of fuel and oxidant, which are considered the promising power generators to replace fossil fuels. Polymer electrolyte membrane fuel cells (PEMFCs) are being extensively explored, owing to their superiority such as satisfactory cell performance, excellent proton transfer rate, and low/zero pollution level. Among the working components of PEMFC, proton exchange membrane (PEM) that can transfer the protons from anode to cathode and obstruct the permeation of fuel gas plays a pivotal role.1–5 Nafion, a commercial perfluorinated ionomer, is the state–of–the

art

polymer

electrolyte

membrane,

which

exhibits

excellent

characteristics of rapid proton conduction, good water transport, excellent mechanical and electrochemical properties. Nevertheless, these advantages come with several drawbacks, including high fuel crossover, low conductivity at above 90 ℃, slow cathode kinetics, and high cost.6–8 Accordingly, many research efforts are being devoted to the development of alternative and cost-competitive PEMs with high proton conductivity under actual working environment. Aromatic hydrocarbon polymers, such as sulfonated poly (ether sulfone)s (SPESs),9–11 sulfonated poly (arylene ether ketone)s (SPAEK)s,12–14 sulfonated poly (ether ether ketone ketone)s (SPEEKK)s,15 sulfonated poly (arylene sulfide sulfone nitrile)s (SPSSN)s,16 sulfonated poly (arylene ether nitrile)s (SPAEN)s,17 and sulfonated polyimides (SPI)s,18,19 were widely exploited for PEM preparation. Kim et al.20 studied the electrochemical performance difference between random copolymer and block copolymer depending on the morphology effect. They 3



demonstrated the phase–separated morphology as the hydrophilic segments and hydrophobic segments are crucial to enhance the proton conductivity and physical stability. Over the past decade, a number of researchers reported the synthesis and characterization of block copolymers with controlled oligomer lengths. The results indicated that the phase separated membranes demonstrate enhanced ion conductivities, and improved chemical stability as the multi-block copolymer has higher local concentration of sulfonic acid groups. However, aromatic hydrocarbon membranes showed dramatic drop of ion conductivity at low humidity because of highly water content. A promising method to improve the performance of the aforementioned PEM mentioned above is to develop a distinctly phase–separated morphology by introducing sulfonic acid groups onto the grafted side chains on the polymer backbone. Bae et al.21 and Guiver et al.22 reported that grafted sulfonic acid groups improve electrochemical properties, chemical stability, and dimensional stability because the sulfonic acid group minimizes the effect of the main chains, and maximizes the water volume fraction at low RH by the ion cluster effect. In line with this fact, high performance PEMs with excellent ion conductivity and dimensional stability can be achieved. In this study, flexible pendant diphenyl–DFB was synthesized via bromination and Suzuki coupling reaction. On the basis of this novel monomer, we designed and synthesized poly (arylene ether ketone) block copolymer with different lengths of hydrophilic precursor, and then the sulfonic acid group was selectively and intensively introduced into the pendant chains by sulfonation. The properties of SPAEK membranes such as dimensional stability, mechanical properties, morphology, proton conductivity, and cell performance were

4


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scrutinized for the corresponding membranes in accordance with the hydrophilic precursor lengths.

2. EXPERIMENTAL 2.1. Materials. Bis (4–fluorophenyl) ketone (DFB), 4,4′–(hexafluoroisopropylidene) diphenol (6F–BPA), bisphenol A (BPA), N–bromosuccinimide (NBS), benzeneboronic acid (PhB(OH)2), tetrakis (triphenylphosphine) palladium (Pd(Ph3)4), anhydrous N,N′– dimethylacetamide (DMAc), anhydrous toluene, potassium carbonate (K2CO3), sodium carbonate (Na2CO3), and concentrated sulfuric acid (H2SO4) 98% were obtained from TCI cooperation and Sigma–Aldrich. Solvents and other reagents were prepared to reagent grade. 2.2. Synthesis of Brominated DFB. The brominated (Br−) DFB was synthesized using DFB and NBS (Scheme 1). For the synthesis sequence, after DFB (20.00 g, 0.092 mol) was completely dissolved in 200 mL H2SO4, NBS (34.26 g, 0.193 mol) which was divided into quarters, was added into the mixture at an interval of 20 minutes, and proceeded with stirring for 20 h at 20–30 ℃. The mixture was poured into 800 mL deionized (DI) water, and the white precipitate was filtered and washed several times. Finally, to obtain high purity Br–DFB, the white powder was recrystallized using toluene. The yield of Br–DFB was 72%. 1. 98% H2 SO4 20-30 oC 2.2 eq NBS

O F

O F

F 2. Toluene/10 wt% Na2 CO 3 110 oC 2.2 eq PhB(OH) 2, 0.04 eq Pd(Ph3 )4 ,

Scheme 1. Synthesis of monomer. 5


F


2.3. Synthesis of Diphenyl–DFB. The diphenyl–DFB was synthesized via Suzuki coupling reaction as follows (Scheme 1). Br–DFB (10.00 g, 0.026 mol) and PhB(OH)2 (7.32 g, 0.066 mol) were dissolved in 300 mL toluene under N2 atmosphere. Then, 120 mL of 10 wt% aqueous Na2CO3 solution and Pd(Ph3)4 (1.22 g, 0.0011 mol) were added slowly into the mixture. After the mixture dissolved completely, the reaction was conducted at 110 ℃ for 20 h. Next, the mixture was cooled to room temperature (RT), and then the viscous solution was poured into co– solvents of methylene chloride/DI water (2/3, v/v), and filtered to remove the suspended catalyst. To extract the organic phase, the mixture was poured into a separating flask, and then the organic phase separated. Finally, to obtain high purity diphenyl–DFB, the white powder was recrystallized using toluene. The yield of diphenyl–DFB was 70.8%. 2.4. Synthesis of Fluorine–/ Hydroxyl– Terminated Oligomers. The fluorine– terminated oligomer (F–oligomer, X) containing diphenyl units was synthesized by aromatic nucleophilic substitution reaction, and the different repeat units (X = 4.8, 7.5, and 9.1) of F–oligomers were prepared by changing the polymerization reaction time. In a typical polymerization (Scheme 2), diphenyl–DFB (1.59 g, 4.29 mmol), 6F–BPA (0.99 g, 3.90 mmol), and K2CO3 (1.08 g, 7.81 mmol) were added into 100 mL roundbottomed flask equipped with a magnetic stirrer and Dean–Stark trap under a N2 inlet. The mixture was stirred at 140 ℃ for 2 h, then the polymerization reaction was conducted at 160 ℃ for 22 h. Afterward, the mixture was poured into co–solvents of methanol/DI water solution (5/5, v/v), the formed polymer was filtered, and washed using methanol and F–oligomer was dried at 70 ℃. The hydroxyl–terminated oligomer (OH–oligomer, Y) was synthesized from DFB (1.00 g, 4.58 mmol) and BPA 6


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(1.15 g, 5.04 mmol). The typical synthetic procedure was performed as described for the preparation of F–oligomer.

F

O

CF3

O F

+ HO

OH

HO

OH

+

F

F

CF3 diphenyl-DFB

6F-BPA

BPA

DFB

DMAc/Toluene K2CO3 160 oC

O F

CF3

O


O

O F + HO X

O

CF3 (X)

O

O (Y)


CF3 O

O

O O

O CF3

OH Y

O Y Z

X 98% H2SO4 R.T SO3H

CF3 O

O

O O

O CF3

X

O Y Z

SO3H

Scheme 2. Synthetic steps to prepared SPAEK copolymers via polycondensation.

2.5. Synthesis of PAEK Copolymer. The synthetic procedure of PAEK copolymer is as follows: F–oligomer (1.00 g, 0.067 mmol), OH–oligomer (0.67 g, 0.067 mmol), K2CO3 (0.018 g, 0.130 mmol), DMAc (20 mL), and toluene (12 mL) were placed in a 100 mL round-bottomed flask. The typical synthetic procedure was performed as 7



described for the preparation of F–oligomer. The yield of PAEK X4.8Y8.8 was approximately 91%. 2.6. Sulfonation of PAEK Copolymer. The sulfonation of PAEK copolymers (1.0 g) were performed using concentrated H2SO4 (20 mL) as sulfonating agent at 20 ℃ for 12 h. Afterward, the mixture was poured into crushed ice to precipitate the powder. The solid polymer was collected by filtration and washed with DI water several times. The product was dried at 70 ℃for 20 h. 2.7. Membrane Preparation. The membrane of SPAEK copolymers were prepared by solution casting method. The photograph reveals that the fabricated SPAEK X4.8Y8.8 membrane is transparent and flexible without any significant visible defects (Figure 1). The thickness of SPAEK membranes of X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8 are 32, 49, and 44 ㎛, respectively. The sulfonated copolymer (0.5 g) was dissolved in DMAc (10 mL). Next, the solution was cast on a glass dish, and then dried at 90 ℃ under vacuum for at least 24 h. To convert to acid form, the membranes were treated with 1 M H2SO4 solution at 95 ℃ for 3 h. Finally, the SPAEK membrane was washed with DI water several times to remove the residual H2SO4.

Figure 1. Photograph of SPAEK (X4.8Y8.8).

8


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2.8 Characterizations. The 1H NMR spectra (600 MHz) was measured using a JNM ECA 600 instrument. The number average molecular weight (Mn), the weight average molecular weight (Mw), the maximum molecular weight (Mz), and polydispersity indices (PDI) of the polymers were measured by HPLC-8320 GPC equipped RI detector with dimethylformamide containing 0.04 mol LiBr as a eluent (flow rate: 1.0 mL min−1) and standard polyethylene oxide as a standard reference. The solubility of the SPAEK copolymers was investigated in a variety of solvents at 60 ℃. The thermogravimetric analysis of the polymers was investigated using a Q 50 (TA instruments) from 30 to 800 ℃ under N2 atmosphere (heating rate: 10 ℃ min–1, flow rate: 60 mL min–1). The oxidative stability of membranes was evaluated by immersing in Fenton’s reagent (4% H2O2 containing 4 ppm FeSO4) at 80 ℃ for 24 h.17, 18 The surface morphology of membranes was obtained in the hydrated state with tapping mode AFM (Veeco instruments, Nanoscope IV). 2.8.1 Ion Exchange Capacity (IEC) Measurement. IEC (meq g–1) of the membranes was measured by acid–base titration method.21,

23, 24

To transform H+

form into Na+ form, 0.1 g of the membranes was immersed in 2 M NaCl aqueous solution at RT for 24 h. The IEC values of membranes were calculated using eq 1: IEC = (VNaOH × CNaOH) / Wdry

(1)

where VNaOH is the consumed volume of NaOH (mL), CNaOH is the concentration of NaOH (N), and Wdry is the dry weight of the membrane. 2.8.2 Water Uptake and Swelling Ratio. The water uptake (WU) was measured using values of membrane weights before/after hydration. All weight of dried membranes were measured, and then immersed in DI water at various temperatures for 24 h. After the surface of wet membrane was wiped with tissue paper, the weight 9



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of wet membranes were measured. The water uptake value of membranes was calculated using eq 2: WU (%) = (Ww – Wd) / Wd × 100%

(2)

where Ww and Wd are wetted and dried membrane weights, respectively. The swelling ratio of SPAEK membranes was measured in comparison of the ratio of thickness and length in the DI water, respectively.24,

25

The swelling ratio was

calculated using eq 3: Swelling ratio (%) = (Sw – Sd) / Sd × 100%

(3)

where Sw and Sd are wetted and dried membrane thickness and length, respectively 2.8.3 Mechanical Behaviors The mechanical behaviors such as tensile strength (TS), young’s modulus (YM), and elongation at break (EB) of the membranes were measured at RT using a universal tensile machine (UTM model LR5K-plus, Lloyd, UK). The membranes were soaked in DI water at RT for 24 h before testing. Prior to the test, the membranes were fabricated by using a cutting machine with a size of 30 × 10 mm. At least three repeated measurements were performed at a rate of 5 mm min−1. 2.8.4 Small Angle X-ray Scattering Measurement (SAXS) The average ionic cluster dimension of the membranes was measured using SAXS (EMPYREAN, Malvern Panalytical, UK). The membranes were prepared with the dimensions of 30 × 10 mm and kept in DI water at RT for 24 h before testing. The average distance between ionic clusters in the fabricated membranes was calculated by the following eq 4:

d = 2π / q

(4)

10


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Where d is the average ion cluster dimension of the membrane (Å) and q is the scattering vector (Å). 2.8.5 Proton Conductivity and Activation Energy. The proton conductivity of membranes was measured by direction of through–plane using a AC impedance spectroscopy (Scitech Korea conductivity test bench, four electrode measurement system), as previously reported.25–27 The evaluation size of membranes used for proton conductivity test was 0.5 cm × 3.0 cm. The proton conductivity (σ) of the membranes was calculated using eq 5:

σ (S/cm) = L / (R × T × W)

(5)

where L (cm) is the distance between the two electrodes, R is the resistance (Ω) of the membrane, T (㎛) is the thickness of the membrane, and W (cm) is the width of the membrane. The activation energy (Ea) of membranes was calculated using eq 6:

lnσ = lnσo – (Ea / R × T)

(6)

where R and T are gas constant and Kelvin temperature, respectively. 2.8.6 Preparation of Membrane Electrode Assembly and Evaluation of PEMFC Performance. To evaluate the single cell performance, the performance was measured using Scitech Korea single cell test station. The membrane electrode assembly (MEA) was fabricated with membrane and commercial gas diffusion electrodes (GDE, loading of 0.3 mg cm–2 Pt catalyst). The electrodes were sandwiched to both sides of the membrane (press condition: 110 ℃, at 1400 psi for 3 min). The single cell active area was 5 cm2, and humidified H2 (anode flow rate: 0.1 L min–1) and O2 (cathode flow rate: 0.4 L min–1) were fed into the test station as fuel without any back pressure. 11



3. RESULTS AND DISCUSSION 3.1 Synthesis and Structural Properties of Monomer and Polymers. The novel monomer (diphenyl–DFB) was successfully synthesized via bromination and Suzuki coupling reaction as shown in Scheme 1. The procedure of bromination reaction was reported in previous reports.22 The Br–DFB was synthesized by a bromination reaction using DFB as the starting material. The structure of Br–DFB was confirmed by 1H NMR in DMSO–d6 (Figure 2). The proton peak of Br–DFB appeared in the range between 7.50 and 8.10 ppm. The proton peak of H1 is related to the shift caused by the electron withdrawing effect of the –Br groups, and the proton peaks of H2 and H3 appeared at 7.76 ppm and 7.55 ppm, respectively, which are related to ketone to ortho and para sites. The diphenyl–DFB was prepared from Br–DFB and PhB(OH)2 in the presence of a catalyst and base via Suzuki coupling reaction. The structure of diphenyl–DFB was characterized by 1H NMR. The peak at 8.05 ppm completely disappeared, and the peaks of H4, 5, 6 appeared at 7.55–7.45 ppm, which is associated to the substituted phenyl groups. The 1H NMR analysis of aromatic protons confirmed that Br–DFB and diphenyl–DFB were successfully synthesized.

12


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Figure 2. The 1H NMR spectra of a) DFB, b) Br−DFB and c) diphenyl−DFB in DMSO−d6. The PAEK copolymers were successfully synthesized using F-oligomers with different length and OH-oligomer via direct polycondensation reaction under basic conditions as shown in Scheme 2. The PAEK copolymers were then sulfonated to obtain SPAEK copolymers. The structure of PAEK and SPAEK copolymer were confirmed by 1H NMR (Figure 3). For PAEK X4.8Y8.8 (Figure 3a), the aromatic proton peak of backbone at 7.10 ppm and 7.30 ppm are assigned to H9 and H1 on the main chain of ortho to ketone and perfluoropropane groups, respectively. The proton peak (H3) at 7.81 ppm corresponding to the ketone containing substituted 13



phenyl groups on the backbone was shifted from 7.05, the peak ascribed to steric hindrance effect of phenyl groups. The proton peak of ortho to isopropane (H11) and ether (H10) was assigned at 7.20 ppm and 7.72 ppm, respectively.

Figure 3. The 1H NMR spectra of a) PAEK (X4.8Y8.8) and b) SPAEK (X4.8Y8.8). Figure 3b showed the 1H NMR spectra of SPAEK X4.8Y8.8. The proton peak of H6 appeared at 7.72 ppm which indicates that the sulfonic acid groups were introduced on the para position of functionalized phenyl groups.28 The molecular weight of F– 14


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oligomers are shown in Table 1 and Figure S2. The Mw of polymers is increased with increasing polymerization reaction time. As shown in Table 1 and Figure S2, the degree of polymerization of synthesized oligomers were adjusted at 8.8 for OHoligomer to be hydrophobic block and at 4.8, 7.5, and 9.1 for F-oligomers to be hydrophilic blocks according to the polymerization reaction time compared to confirm the electrochemical performance according to the length of hydrophilic oligomer.

Table 1. Degree of polymerization, molecular weight (Mn, Mw, Mz), and PDI of oligomers. Oligomer

Obtained Mn length a) (kDa)

Mw (kDa)

Mz (kDa)

Mw/Mn (PDI)

F−oligomer (X4.8)

4.8

3.2

11.5

21.0

3.6

F−oligomer (X7.5)

7.5

5.0

14.9

30.0

2.9

F−oligomer (X9.1)

9.1

6.1

24.5

67.1

4.0

OH−oligomer (Y8.8)

8.8

3.6

10.1

53.9

2.8

a)

Repeat unit was calculated by GPC.

The molecular weights of before/after sulfonation of PAEK copolymers are shown in Table 2. The molecular weights (Mw) of PAEK copolymers were in the range of 31 kDa to 59 kDa, specifying well formation of multiblock copolymers. After sulfonation, the SPAEK copolymers exhibited an increase in molecular weight from 71 kDa to 108 kDa. As a result, it was shown that excessive sulfonic acid groups were introduced into the hydrophilic blocks without degradation in the sulfonation process.

15



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Table 2. Molecular weight (Mn, Mw, and Mz), and PDI of copolymers. Mn (kDa)

Mw (kDa)

Mz (kDa)

Mw/Mn

PAEK (X4.8Y8.8)

5.2

59.4

229.6

8.5

PAEK (X7.5Y8.8)

20.0

50.4

124.1

2.5

PAEK (X9.1Y8.8)

7.0

31.0

146.0

6.0

SPAEK (X4.8Y8.8)

15.2

108.4

559.0

7.1

SPAEK (X7.5Y8.8)

18.4

93.6

333.5

5.1

SPAEK (X9.1Y8.8)

11.5

71.7

322.5

6.3

Polymers

(PDI)

We measured the solubility of SPAEK copolymers in a variety of solvents to choose the suitable casting solvent (Table S1). To evaluate the solubility characteristics of SPAEK copolymers, SPAEK copolymers were dissolved in various solvents at 60 ℃. As a result, the SPAEK copolymers were dissolved easily in polar aprotic solvents (NMP, DMAc, DMF, DMSO, and acetone), and were insoluble in polar protic solvents (methanol and DI water). 3.2 Oxidative and Thermal Stability. The oxidative stability test was conducted by immersing the membrane samples into Fenton’s reagent (4 ppm FeSO4 in 3% H2O2) at 80 ℃ for 24 h to study the long–term stability of membranes. The oxidative stability of membranes was evaluated by comparing the residual weight of the membrane samples after the test. Generally, the degradation of the main chain caused by OH• or OOH• radicals mainly happens in the hydrophilic parts. As summarized in Table 3, the residual weight of SPAEK membranes of X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8 are found to be 91%, 89%, and 83%, respectively. Under identical test condition, the SPAEK X15Y4 membrane, exhibited the residual weight of 54.4%, indicating that the 16


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oxidative resistance of SPAEK X9.1Y8.8 is still 1.52 fold higher than that of SPAEK X15Y4.27 It is noteworthy to mention that sulfonation through phenyl group is more beneficial to improve the oxidative stability rather than main chain sulfonation.29

Table 3. IEC, water uptake, swelling ratio and oxidative stability of Nafion−115 and SPAEK membranes.

Membrane

IEC (meq g−1)

Water uptakea (%) 30 ℃

90 ℃

Swelling ratioa (at 90 ℃, %)

Oxidative stabilityb Length Thickness (%) (△l) (△t)

SPAEK (X4.8Y8.8)

1.12

31

59

19

16

91

SPAEK (X7.5Y8.8)

1.62

33

68

20

25

89

SPAEK (X9.1Y8.8)

2.01

38

70

35

42

83

Nafion−115

0.92

20

35

19

21

99

a b

Measured in water. Oxidative stability test in Fenton’s reagent solution at 80 ℃ up to 24 h.

TGA curves of PAEK and SPAEK copolymers are shown in Figure 4. The decomposition of PAEK (X7.5Y8.8) started at 480 ℃, which is ascribed to the ruining of aromatic polymer backbones. After sulfonation, TGA curves of SPAEK copolymers showed three step degradations. At first, the decomposition of SPAEK copolymers between 60–130 ℃ is due to the removal of free water contained in the sulfonic acid groups on the backbone. The second decomposition between 170 ℃ to 400 ℃ is attributed to the elimination of sulfonic acid groups on the backbone. The third weight loss which started at 460 ℃ is ascribed to the degradation of polymer backbones.

17



The SPAEK copolymers demonstrated good thermal stability below 170 ℃, which is suitable to be applied in PEMFC operated at temperatures below 100 ℃.

Figure 4. Thermogravimetric curves of PAEK and SPAEK membranes.

3.3 Water Uptake, IEC, and Swelling Ratio. The membrane's water uptake, IEC, and dimensional stability are crucial factors in determining the performance of PEM during fuel cell operation.28 As shown in Figure S3 and Table 3, SPAEK membranes with different length of hydrophilic oligomer showed water uptake in the range of 31– 38% at 30 ℃. The water uptake of SPAEK X9.1Y8.8 is higher than Nafion−115 at 90 ℃, which comply with regularity that the water uptake increases with increasing 18


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IEC.25, 30-32 The results of dimensional stability showed similar trends to that of the water uptake values as shown in Table 3, and the swelling ratio of SPAEK membranes was measured according to through-plane (∆t) and in-plane (∆l) of the membranes in the dried and fully wet states. The swelling ratios (∆t and ∆l) of SPAEK membranes are in the range of 16–42%, and is increased with increasing hydrophilic segments. As shown in Table 3, the IEC values of SPAEK membranes ranged from 1.12 to 2.01 meq g–1, revealing its dependency on the sulfonic acid groups on hydrophilic segments.33 3.4 Proton Conductivity and Arrhenius Plot. The proton conductivities of SPAEK membranes were measured at varying temperatures under 100% RH (Figure 5) and different RH in the range of 40−100% at 90 ℃ (Figure 6). As shown in Figure 5, proton conductivity of SPAEK membrane (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) and Nafion−115 found to be 131.2, 139.0, 154.1, and 131.5 mS cm–1 at 90 ℃ under 100% RH, respectively. The high proton conductivity of SPAEK X9.1Y8.8 is ascribed to the increased hydrophilic length in block, which provides wide ion channels, thereby supplying the rapid movement of H+ ions and the enhanced activity of hydronium ions.34–40 Table 5 summarizes the performance of polymer electrolyte membranes with various pendants reported in the literature for comparison with current studies. As shown in Table 5, the SPAES block copolymer membrane has a higher conductivity than the SPAES random copolymer membrane composed of the similar chemical structure. Based on these results, we proved through the AFM image (Figure 8) that block copolymer containing a highly localized and dense sulfonic acid group in the hydrophilic blocks showed high proton conductivity and dimensional 19



stability due to the well-interconnected ion channel formation resulting from the hydrophilic and hydrophobic phase separation.

Figure 5. Proton conductivity of Nafion−115 and SPAEK membranes at various temperatures (under 100% RH).

As shown in Figure 6 and Table 4, the proton conductivity of SPAEK membranes including X4.8Y8.8, X7.5Y8.8, X9.1Y8.8, and Nafion−115 were found to be 88, 102, 140, and 102 mS cm−1 at 90 ℃ under 80% RH. It was confirmed that the increase of the IEC of the fabricated membrane induces an increase in the proton conductivity by providing ion channels.37, 38 However, the proton conductivity of all membranes is found to be decreased sharply below 80% RH, which may be due to the lack of

20


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water content. The detailed comparison of most recently reported PEMs is listed in Table 5.12, 41–44

Table 4. Electrochemical properties of Nafion−115 and SPAEK membranes. Conductivitya (mS cm−1)

Conductivityb (mS cm−1)

(㎛)

30 ℃

90 ℃

60% RH

80% RH

SPAEK (X4.8Y8.8)

32

81.6

131.2

0.07

SPAEK (X7.5Y8.8)

49

62

139

SPAEK (X9.1Y8.8)

44

58.2

Nafion−115

158

61

Membrane

Thickness

Ea (kJ mol−1)

Maximum power densityc (mW cm−2)

88.06

7.26

196.1

0.1

102

12.49

241.7

154.1

0.17

140.7

15.23

323.9

131.5

2.1

102

12.02

291.4

a

Under 100% RH. At 90 °C. c At 60 °C under 100% RH. b

Table 5. IEC and proton conductivity comparison between recently reported PEMs. IEC (meq g−1)

σ (mS cm−1)

Ref.

SPAEK (X4.8Y8.8)

1.12

131.2 (90 ℃)

This work

SPAEK (X7.5Y8.8)

1.62

139.0 (90 ℃)

This work

SPAEK (X9.1Y8.8)

2.01

154.1 (90 ℃)

This work

PS2-PAES-60

1.64

157.0 (80 ℃)

[22]

tsPTPO−100

1.55

92.1 (90 ℃)

[41]

B20V80−SDPA

1.89

115.1 (90 ℃)

[12]

6F−PAEK−SP22

1.77

136.0 (80 ℃)

[42]

Membrane

21



Page 22 of 38

C−SPAEKS/SPVA−10

1.75

85.0 (90 ℃)

[43]

MSBP−14

1.87

131.15 (80 ℃)

[44]

Figure 6. Proton conductivity of Nafion−115 and SPAEK membranes at various relative humidity at 90 ℃.

The activation energy (Ea) of membranes are calculated with the slope of proton conductivity, and is shown in Figure 7 and Table 4. The activation energy values of SPAEK membranes (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) were 7.26 kJ mol–1, 12.49 kJ mol–1, and 15.23 kJ mol–1, respectively. As a result, the activation energy of

22


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Nafion−115 and SPAEK membranes indicates that the vehicular mechanism is the main proton transport mechanism in all the membranes.45–46

Figure 7. The Arrhenius plots of Nafion−115 and SPAEK membranes.

3.5 Morphologies of the Membranes. The surface morphologies of SPAEK membranes were verified through AFM, and the resulting phase and height images are shown in Figure 8 and Figure S4, respectively. The AFM phase image of SPAEK membranes showed a distinct microphase separation represented by dark domains, corresponding to the hydrophilic segments containing sulfonic acid groups and bright domains corresponds to the hydrophobic segments, respectively.47–49 From SPAEK 23



phase images (Figure 8), revealed the coexistence of distinct connections and separations of proton channels. According to the morphology results, SPAEK X9.1Y8.8 showed to have the largest ion cluster region compared with SPAEK X4.8Y8.8 and SPAEK X7.5Y8.8 membranes. It further supports the fact that, the proton conductivity of SPAEK X9.1Y8.8 (154 mS cm–1) is higher than SPAEK X4.8Y8.8 (139 mS cm–1) and SPAEK X7.5Y8.8 (131 mS cm–1). In addition, the ionic cluster dimensions of the SPAEK membranes were verified by SAXS, and the results are given in Figure 9. The SAXS patterns showed the similar morphologies for the SPAEK (X 4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) membranes. The SAXS pattern of the SPAEK X9.1Y8.8 membrane with the longest hydrophilic length demonstrated a characteristic peak at the scattering vector (q) of 0.91 nm−1, corresponding to the ionic cluster dimension of 6.90 nm. Which is larger than that of SPAEK X7.5Y8.8 (d: 6.77 nm, q: 0.93 nm−1) and SPAEK X4.8Y8.8 (d: 6.54 nm, q: 0.96 nm−1). Based on these results, it was confirmed that the large d value can facilitate the distinct the phase separation between hydrophilic and hydrophobic domains to form ionic aggregates, which affects the electrochemical performance of the polymer electrolyte membrane.

24


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Figure 8. AFM phase image of the SPAEK membranes under 100% RH (700 nm x 700 nm): a) SPAEK (X4.8Y8.8), b) SPAEK (X7.5Y8.8), C) SPAEK (X9.1Y8.8).

Figure 9. SAXS profile of SPAEK membranes.

25



Page 26 of 38

3.6 Mechanical behaviors of SPAEK membranes. The mechanical behaviors of SPAEK membranes were measured under ambient conditions. The tensile strength (TS), young’s modules (YM) and elongation at break (EB) were compared to difference length of hydrophilic oligomer, and the mechanical behaviors were listed in Table 6. The TS and YM values of SPAEK membranes gradually decreased from 24.9 to 22.5 MPa and from 1.8 to 1.2 GPa, respectively, as the increased repeat units of hydrophilic oligomer, whereas EB of SPAEK membranes increased from 1.4 to 1.9%, which is probably attributed to the intermolecular force between main chains. In additions, more the pendant sulfonic group substitution in main chain brings a stronger hydrogen bonding between hydrophilic domains, leading to a higher elastic deformation. Therefore, all these results indicate that SPAEK membranes containing pendant sulfonic acid groups could possess sufficient mechanical behaviors for applications.

Table 6. Mechanical behaviors of SPAEK membranes.

Membrane

Tensile strength (MPa)

Young’ modulus Elongation at break (GPa) (%)

SPAEK (X4.8Y8.8)

24.9

1.8

1.4

SPAEK (X7.5Y8.8)

24.6

1.5

1.6

SPAEK (X9.1Y8.8)

22.5

1.2

1.9

3.7 Single Cell Performance. The PEMFC performance of Nafion−115 and SPAEK membranes was investigated at 60 ℃ under 100% RH. For the fabrication of MEAs, the thickness of membranes and gas diffusion layer were kept to be constant (Table 26


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4). The obtained polarization and power density curves are given in Figure 9. All MEAs showed an open circuit voltage (OCV) above 0.9 V, which indicates that the fabricated MEAs have a good fuel (H2) permeation resistance from the anode to cathode through the membrane.50 Figure 10 compares the fuel cell performance of SPAEK (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) and Nafion−115 membranes operated at 60 ℃ under 100% RH. The maximum power density of SPAEK (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) and Nafion−115 membranes exhibit 196.1, 241.7, 323.9, and 291.4 mW cm–2, respectively. Among the SPAEK membranes, the maximum power density of SPAEK X9.1Y8.8 membrane is higher 1.1 times than Nafion-115 membrane (Table 4). Furthermore, the current density of SPAEK X9.1Y8.8 membrane is similar to Nafion−115 membrane at 0.6 V, whereas, the current density of SPAEK X9.1Y8.8 membrane is 1.2 times higher than Nafion−115 at 0.3 V. Under 100% RH, the single cell performance of the SPAEK X9.1Y8.8 membranes is higher than that of Nafion−115, which demonstrates that SPAEK X9.1Y8.8 membrane probably has lower ohmic resistance and mass transfer resistance than that of Nafion−115. Therefore, these results clearly indicate that the length of hydrophilic oligomer in the SPAEK backbone can increase the power density by supplying proton conducting channels and the retaining high amounts of water molecules. Thus, the remarkable power output ability of MEA suggests that X9.1Y8.8 with high conductivity and excellent dimensional stability tend to benefit to improve the performance of PEMFCs. It should also be noted that the performance of the MEA can be improved by optimization of the MEA construction and the proper operating conditions of the fuel cells.

27



Figure 10. Polarization curves of Nafion−115 and SPAEK membranes at 60 ℃ under 100% RH. 4. CONCLUSIONS Novel proton exchange membrane for PEMFC based on the series of SPAEK block copolymers with controlled F–oligomer length bearing pendant diphenyl unit were successfully synthesized via a polycondensation reaction. The new difluorodiphenyl ketone monomer containing two pendant phenyl groups was synthesized via bromination and a Suzuki coupling reaction. When compared with other sulfonated poly (arylene ether ketone) copolymers with similar IEC values, the fabricated membranes showed a reasonable dimensional stability and proton conductivity values (in the range of 131–154 mS cm–1 at 90 ℃ under 100% RH. It appears that microphase separation between the opposing hydrophobic and hydrophilic domains 28


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caused a higher local concentration of sulfonic acid groups, and leads higher proton conductivity and lower change in dimensional stability. The SPAEK (X9.1Y8.8) membrane with the higher IEC by controlled length of hydrophilic oligomer influenced the physiochemical properties, the prepared SPAEK membranes exhibited the clearly phase–separated hydrophilic/hydrophobic domains, caused by increasing the length of hydrophilic oligomer in the main chain and then it was proved that the difference in morphology affects the electrochemical properties of the membrane by confirming the size of the ion domain by SAXS. Furthermore, the SPAEK membranes showed excellent electrochemical properties with mechanical stability, compared to similar random SPAEK membranes with same repeat unit. The H2/O2 fuel cell performance of SPAEK X9.1Y8.8 exhibited a maximum power density of 324 mW cm–2, which is higher than that of Nafion−115 (291 mW cm–2) at 60 ℃ under 100% RH. Based on the results of high electrochemical performance and physiochemical stability, we can speculate that the synthesized SPAEK membranes is more suitable for PEMFC application. ASSOCIATED CONTENT SUPPORTING INFORMATION Supporting Information Available:

1

H NMR spectra of a) F-oligomer and b)

OH−oligomer, GPC data of F−oligomers with different lengths, Water uptake of Nafion−115 and SPAEK membranes at various temperatures, AFM height image of the SPAEK membranes under 100% RH (700 nm × 700 nm): a) SPAEK (X4.8Y8.8), b) SPAEK (X7.5Y8.8), c) SPAEK (X9.1Y8.8), Solubility behavior of the SPAEK membranes in various solvents. 29



Page 30 of 38

AUTHOR INFORMATION * Corresponding authors E–mail addresses: [email protected] (A. R. Kim), +82–63–270–4676 (tel) E–mail addresses: [email protected] (D.J. Yoo), +82–63–270–3608 (tel), +82–63– 270–3908 (fax)

ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No.20164030201070). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF)

funded

by

the

Ministry

of

Science,

ICT

(No.2017R1A2B4005230).

30


and

Future

Planning

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Enhanced Performance of a Sulfonated Poly(arylene ether ketone

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