Thermal stability of proton exchange fuel-cell membranes based on crosslinked-polytetrafluoroethylene for membrane-electrode assembly preparation

Polymer Degradation and Stability 94 (2009) 344–349

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Thermal stability of proton exchange fuel-cell membranes based on crosslinked-polytetrafluoroethylene for membrane-electrode assembly preparation Shin-ichi Sawada a, b, *, Tetsuya Yamaki a, *, Shinpei Kawahito b, Masaharu Asano a, Akihiro Suzuki b, Takayuki Terai b, Yasunari Maekawa a a b

Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2008 Received in revised form 3 December 2008 Accepted 9 December 2008 Available online 24 December 2008

We investigated thermal properties of proton exchange membranes (PEMs) prepared by the radiationinduced grafting of styrene into crosslinked-polytetrafluoroethylene films and the subsequent sulfonation for fuel-cell applications. A conventional thermogravimetric analysis was found to be unreliable because the resulting curve varied greatly with the heating rate. Thus, in order to obtain accurate information, we performed an ex-situ heat-treatment analysis, which involved heating of the PEMs at fixed temperatures of 200–350  C and measurement of their remaining weight, ion exchange capacity (IEC) and proton conductivity (s) after washing in pure water. The IEC and s did not change at any temperature up to 200  C, indicating high thermal stability. At 250  C, however, the PEM properties deteriorated probably via radical cleavage of the C–S bond between a sulfonic acid group and an aromatic ring, and condensation of two sulfonic acid groups. Finally, the PEM was hot-pressed with two electrodes at 200  C to produce a good membrane-electrode assembly for a fuel cell. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Proton exchange fuel cell Membrane-electrode assembly Thermal stability Radiation grafting Crosslinked-PTFE proton exchange membrane

1. Introduction In recent years, polymer electrolyte fuel cells (PEFCs) have been expected to be next-generation power sources for vehicles and other applications due to their low environmental impact and high energy efficiency [1,2]. A key component of the PEFC is a proton exchange membrane (PEM), which transports protons from an anode to a cathode and prevents the reactant gases from mixing. At present, the most widely used PEM is Dupont’s Nafion exhibiting attractive properties such as moderate proton conductivity (s) and excellent chemical stability. Because of its complicated production processes, however, Nafion is still too expensive to be used for practical applications on a worldwide scale. For efficient operation of the PEFC system, the PEMs are hotpressed with porous electrodes to prepare a membrane-electrode assembly (MEA) with low interfacial resistance. Thus, the PEM

* Corresponding authors. Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan. Fax: þ81 27 346 9687. E-mail addresses: [email protected] (S. Sawada), yamaki.tetsuya@ jaea.go.jp (T. Yamaki). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.12.006

needs to endure a thermal load during the hot-pressing normally above its glass transition temperature. Generally, thermal stability of the PEM has been studied by a thermogravimetric analysis (TGA) [3–9], in which the weight of the specimen is recorded while the temperature is raised. One can obtain information regarding thermal decomposition of a PEM from the weight loss occurring simultaneously with volatilization of the products in hightemperature reactions. As a low-cost synthesis technique for Nafion-alternative PEMs, a radiation-induced grafting method has attracted much attention. This consists of the following three steps: (i) g-ray or electron-beam irradiation of a fluoropolymer film; (ii) grafting of a styrene monomer into the irradiated base film; and (iii) sulfonation of the grafted film. The fluoropolymers commonly used as base films are polytetrafluoroethylene (PTFE), poly(tetrafluoroethylene-cohexafluoroethylene) (FEP), poly[tetrafluoroethylene-co-perfluoro (propyl vinyl ether)] (PFA), poly(ethylene-co-tetrafluoroethylene) (ETFE), and poly(vinylidene fluoride) (PVDF). Yamaki et al. recently reported for the first time the use of crosslinked-PTFE films for developing the PEMs by radiation grafting [10,11]. Interestingly, this novel PEM was found to exhibit better s and fuel barrier property compared to Nafion [12,13], which allows us to expect that it will be

S. Sawada et al. / Polymer Degradation and Stability 94 (2009) 344–349

adopted in PEFCs. In fact, Li et al. performed real H2–O2 fuel-cell tests using crosslinked-PTFE based radiation-grafted PEMs, and then reported that their cell performance exceeded that in the case of Nafion [14]. Still, however, little is known about whether these new PEMs have the thermal properties necessary for optimizing the conditions of MEA production. In this study, we clarified the thermal stabilities of the crosslinked-PTFE PEMs by not only the conventional TGA but also by our original ex-situ heat-treatment analysis (EHA). The heat-treated PEM was characterized in terms of the weight, ion exchange capacity (IEC) and s. The EHA was able to count the thermallydecomposed components with low or zero volatility because they were sufficiently washed out after the heat treatment. Subsequently, an MEA of the crosslinked-PTFE PEM was fabricated by hot-pressing at the optimum temperature. 2. Experimental 2.1. PEM synthesis According to previous reports [10,11], we used as a base material a 42-mm thick film of PTFE crosslinked with electron beams at a total dose of 100 kGy. For the grafting, the crosslinked-PTFE film was irradiated again with g-rays at 60 kGy in air at room temperature. The irradiated films were immediately immersed in a styrene monomer (deaerated by Ar bubbling), where they were kept at 60  C for 6 h. After the grafting reaction, the films were washed with toluene to remove any excess styrene. The grafted films were then dried under vacuum at 50  C to a constant weight. The degree of grafting (DOG) was defined as follows:

DOGð%Þ ¼


100 Wg  Wo ; Wo

(1)

where Wo and Wg denote the weights of the original and grafted films, respectively. The grafted films were sulfonated using a mixture of 0.2 mol dm3 chlorosulfonic acid/1,2-dichloroethane at 50  C for 6 h. Finally, the resulting PEMs were rinsed with pure water and then dried in a vacuum oven. 2.2. Characterization of the PEM The IEC of the PEM was determined by acid–base titration. The PEMs in an acid form were immersed in a 3 mol dm3 NaCl solution at 50  C for 6 h. The solution containing protons released from the PEM was titrated with a standardized 0.1 mol dm3 NaOH solution by an automatic titrator (COM-555, HIRANUMA SANGYO Co.) until pH 7 was reached. Based on the volume of the NaOH solution consumed in the titration, the amount of protons in the PEM was calculated, and the IEC was evaluated by:

IEC ¼

1000Ns ; Wd

(2)

where Ns and Wd denote the number of the sulfonic acid groups (mol) and the weight of the dried PEM (g), respectively. The s of the PEM in the fully-hydrated state at 25  C was determined by AC impedance spectroscopy measurements using an LCR meter (LCR HiTESTER 3522-50, HIOKI DENKI Co.) with an AC amplitude of 100 mV. The PEM samples cut into 1.0  2.0 cm were clamped between two Pt electrodes for our impedance spectroscopy experiments. The diameter of a semicircle observed in the Cole–Cole plot corresponds to the PEM resistance, R (U), and so the s was calculated by:

s¼

d ; RS

(3)

345

where d and S denote the distance between the two electrodes (cm) and the cross-sectional area of the PEM (cm2), respectively. 2.3. TGA A TGA was performed by using a sensitive thermobalance (Thermo Plus2/TG-DTA, Rigaku Co.) in an atmosphere of air or nitrogen. Before the measurement, the PEMs were dried under vacuum at 40  C for more than 6 h to remove absorbed water. The specimen with a weight of about 5 mg was located in a Pt pan and heated from room temperature to 700  C at fixed heating rates of 0.5–20  C min1. 2.4. EHA The PEMs with a size of 1  2 cm were heated in an electric muffle furnace (FM37, Yamato Scientific Co.) at fixed temperatures of 100–350  C in air for 6 h. They were then immersed in pure water for 24 h to wash the decomposition products out of the interior of the PEM. The dry weight, IEC and s of the washed PEM were measured by the above-mentioned methods. The remaining weight percentage, RW, was calculated by:

RW ¼

100WdH ; Wd

(4)

where WdH denotes the weight of the dried PEM after the heat treatment. The chemical structures of the PEMs after heat treatment were investigated by Fourier-transform infrared (FT-IR) spectroscopy in a transmission mode. FT-IR spectra were collected by using a spectrometer (FT-710, Horiba Co.) in the wavenumber range of 700–4000 cm1 with a resolution of 4 cm1. 2.5. Fabrication of MEA The dried PEM and two Pt-loaded electrodes (EC-10-05-7, Electrochem. Inc.) with an area of 5 cm2 were hot-pressed under 20 MPa for 30 min at different temperatures. The MEA resistance in the thickness direction, rMEA, was determined by the two-probe AC impedance technique. The MEA was sandwiched between two gold electrodes using an in-house cell, which was placed in an environmental chamber (l-201R, Isuzu Co.) controlled at 60  C and relative humidity of 98%. The measurements were performed using a frequency response analyzer (SI 1255B, Solartron Co.) connected to an electrochemical interface (SI 1287, Solartron Co.) with an AC amplitude of 100 mV over a frequency range of 1– 106 Hz. 3. Results and discussion 3.1. PEM The DOG and IEC of the synthesized PEM were 30% and 1.6 meq g1, respectively. Based on the assumption that a sulfonic acid group is introduced into each styrene unit, the theoretical IEC was calculated to be 1.8 meq g1, which is roughly close to the experimental one. This means that the grafted film was substantially sulfonated. The PEM can be divided into PTFE main chains, polystyrene parts of graft chains, and sulfonic acid groups, whose weight percentages are denoted as R1, R2, and R3, respectively. They are expressed by:

R1 ¼

100m1 x1 ; m1 x1 þ m2 x2 þ m3 x3

(5)

S. Sawada et al. / Polymer Degradation and Stability 94 (2009) 344–349

R2 ¼

100m2 x2 ; m1 x1 þ m2 x2 þ m3 x3

(6)

R3 ¼

100m3 x3 ; m1 x1 þ m2 x2 þ m3 x3

(7)

where mi and xi denote the molecular weight (g mol1) and number (mol) of each component, respectively. Subscripts i ¼ 1, 2, and 3 signify a CF2 unit in PTFE, a styrene part (a sulfonic acid group was excluded from a styrene sulfonic acid (SSA) graft), and a sulfonic acid group, respectively. In addition to equations (5)–(7), the following relationships hold.

100

80

Weight percenage (%)

346

60

40

(b)

(a)

20

DOG ¼

100m2 x2 ; m1 x1

(8) 0

1000x3 : IEC ¼ m1 x1 þ m2 x2 þ m3 x3

From equations (5)–(9), R1, R2, and R3 were expressed as follows:

R1 ¼

10ð1000  m3 IECÞ ; 100 þ DOG

(10)

R2 ¼

DOGð1000  m3 IECÞ ; 10ð100 þ DOGÞ

(11)

R3 ¼

m3 IEC : 10

(12)

By substituting the values of DOG, IEC, m1, m2, and m3 into equations (10)–(12), R1, R2, and R3 were calculated to be 67%, 20%, and 13%, respectively. Fig. 1 shows these values together with the chemical structure of the crosslinked-PTFE PEM. 3.2. TGA Fig. 2 gives a TGA curve for a crosslinked-PTFE PEM heated at a rate of 20  C min1 in nitrogen, which is a typical measurement condition [3,5,8,9]. Similarly to the previous result for the noncrosslinked PTFE based radiation-grafted PEM [8], there were main three weight-loss steps in the temperature ranges of up to 130  C, 290–520  C, and above 520  C. These steps should be attributed to

CF2

CF

1

0

200

(9)

x

CF2

800

Fig. 2. TGA curves for the crosslinked-PTFE PEMs at a heating rate of 20  C min1 in (a) nitrogen and (b) air.

elimination of absorbed water, and decomposition of poly-SSA graft chains and a PTFE matrix, respectively [8]. It is accepted that the thermal behavior of the proton-conductive poly-SSA graft chains is crucial to the stability of a PEM at high temperatures. From this point of view, the crosslinked-PTFE PEM appeared to be stable until the second weight loss started at 290  C. However, we wonder whether the above measurement is sufficient for verifying the suitable conditions of MEA fabrication. What should be considered first is a measurement atmosphere because the MEA is generally produced in air. Fig. 2 also shows a TGA curve obtained in air at the same heating rate of 20  C min1. Apparently, the PTFE chains degraded in air at a lower temperature than in nitrogen, indicating that oxygen accelerates the pyrolysis of the fluoropolymer matrix [15]. On the other hand, the TGA profile below about 500  C was independent of the atmosphere. This agrees well with the fact that the poly-SSA graft chains exhibited similar degradation kinetics both in air and nitrogen [6]. Next, the effect of the heating rate on TGA results was examined. Fig. 3 compares TGA curves obtained at heating rates of 0.5, 2.0, and 20  C min1 in air. For easy comparison, the weight loss due to dehydration was subtracted from the raw data; in other words, the weight percentage was set at 100% up to 200  C, at which the

80

z

60

2 3

SO3H 40

(c) i

600

100

CF2 y

CH2 CH

400

Temperature (°°C)

Component

(b)

(a)

Ri (%)

1

Crosslinked-PTFE main chains

67

2

Grafted polystyrene parts

20

3

Sulfonic acid groups

13

20

0 0

200

400

600

800

Temperature (°C) Fig. 1. Chemical structure of the crosslinked-PTFE PEM. The PEM used in this study showed a DOG of 30% and an IEC of 1.6 meq g1. Calculated weight percentages of the crosslinked-PTFE main chains, grafted polystyrene parts, and sulfonic acid groups are also represented.

Fig. 3. TGA curves for the crosslinked-PTFE PEMs at heating rates of (a) 20, (b) 2.0, and (c) 0.5  C min1 in air. For easy comparison, these curves show the change in the PEM weight alone (i.e., weight loss due to dehydration was subtracted from the raw data).

S. Sawada et al. / Polymer Degradation and Stability 94 (2009) 344–349

2.0 untreated

1.5

IEC (meq g-1)

absorbed water was completely evaporated. The faster the heating rate was, the higher the observed weight-loss onset temperature was. This observation can be explained by considering the following two problems inherent in the TGA. One problem is that the furnace temperature recorded by an instrument could be higher than temperature of the specimen owing to the heat-transfer effect as suggested in previous TGA studies of various polymers (generally having a large heat capacity) [15–20]. Another more likely problem is that the weight loss should not catch up with the actual PEM degradation probably because it takes a certain amount of time to eliminate all the decomposition products from the PEM (this will be suggested in the last paragraph of Section 3.3). These problems become more serious at a faster heating rate, resulting in a large variation in the weight-loss onset temperature as shown in Fig. 3. For this reason, it would be preferable to use an analysis method with no temperature scanning, i.e., involving the heating at fixed temperatures like our EHA.

347

1.0

250 °C

300 °C

0.5

350 °C

0 100

90

80

70

60

RW (%) 3.3. EHA

2.0

0.20

1.5

0.15

1.0

0.10

0.5

0

0.05

0

100

200

300

Let us start to discuss pyrolysis behavior of the poly-SSA graft chains based on Fig. 6. Gupta et al [4] similarly performed a TGA of an FEP-based polystyrene-grafted PEM and analyzed the volatilized substances by FT-IR spectroscopy. According to their study, two neighboring SSA groups formed a crosslinked sulfonate-bridged structure (Structure I) by emitting H2O and SO2. If all the sulfonic acid groups in the PEM are converted to Structure I through Route (A) in Fig. 6, the theoretical relationship between the IEC and RW follows the solid curve in Fig. 5. The experimental result at 250  C, however, deviated from the solid curve, suggesting that the degradation kinetics did not follow Route (A). This should be because Route (A) was proposed without any consideration of the corresponding change in PEM properties such as the IEC. Then, what is another degradation route? In a previous study on the thermal durability of pure poly-SSA [21], a C–S bond was cleaved at 250  C to produce a styrene unit with a phenyl radical (Structure II) yielding a SO3H radical, which would be finally broken into H2O and SO2 gases. Given that only a similar degradation (Route (B) in Fig. 6) takes place in the PEM, the IEC is theoretically varied with the RW as represented by a broken curve in Fig. 5. Since the actual RW value at 250  C lie between the two theoretical curves, thermal-decomposition products from poly-SSA grafts are assumed to have both Structures I and II. Based on the deviation

SO3H HO3S

σ ( S cm-1)

IEC (meq g-1)

The distinctive advantages of our EHA are twofold: (i) immersion of the PEM in pure water after the heat treatment, which is expected to allow any residue to be removed from the PEM, irrespective of its volatility; (ii) parallel characterization of various PEM properties, i.e., the IEC and s as well as the RW, providing us with an experimental insight into the degradation mechanism. Fig. 4 shows the IEC and s as a function of the heat treatment temperature. Neither of them changed up to 200  C, indicating that there was no degradation of the PEM. When the temperature reached 250  C, the IEC decreased to 0.64 meq g1, in other words, 63% of the sulfonic acid groups were eliminated. This significant degradation caused the s to be reduced dramatically from 0.12 to 0.026 S cm1, which is insufficient for PEFC applications. At the maximum temperature of 350  C, the IEC and s reached zero, meaning that all the sulfonic acid groups were eliminated. We plot the RW for the PEM heated at different temperatures against its IEC in Fig. 5. The RW as well as the IEC became lower with a temperature increase, but RW appeared to be insensitive to the PEM deterioration, particularly to the loss of the sulfonic acid groups; for example, the IEC decreased by 61% at 250  C while RW had only a 6% drop. This result undoubtedly demonstrates that RW is not a good measure of thermal characteristics of the PEM. It is more important to track changes in the IEC directly reflecting the amount of the sulfonic acid groups.

Fig. 5. The IEC vs. RW at heat treatment temperatures of 250–350  C. An open square indicates the point when only the PTFE matrix remained (RW ¼ R1, IEC ¼ 0). The solid or broken curve represents the theoretical relationships, which were estimated based on the assumption that degradation occurs through single Route (A) or (B) in Fig. 6, respectively.

0 400

Temperature (°°C) Fig. 4. Plots of (-) the IEC and (B) s as a function of heat treatment temperature. The results of the untreated PEM are also included at room temperature (25  C) as a reference.

Heat treatment Route (A)

Route (B) 2 SO3H

SO2+H2O Structure I

Structure II

SO2 O

Fig. 6. Possible routes of thermal degradation of the poly-SSA graft chains in the crosslinked-PTFE PEM over 250  C.

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S. Sawada et al. / Polymer Degradation and Stability 94 (2009) 344–349

from the theoretical curves, we calculated that each structure is present in equal amounts. Jiang et al. [21] pointed out that the generated phenyl radicals may decompose the graft chains by abstraction of their a-hydrogen atoms. However, this would be possibly ruled out since RW loss was not large. As a result, the thermal degradation of the poly-SSA through the two routes (A) and (B) in Fig. 6 was proposed for the first time in this paper. The proposed degradation mechanism was proved by FT-IR spectroscopy. Fig. 7 shows the FT-IR spectra of the PEMs before and after heat treatment at 250  C for 6 h. In Fig. 7(A), the peak observed at 737 cm1 was due to the PTFE matrices [22]. The 774 and 834 cm1 peaks were assigned to out-of-plane C–H bending vibration of the phenyl rings in polystyrene and poly-SSA, respectively [23]. For the heated PEM, another peak appeared at 795 cm1, representing new chemical species. In the previous paper, the S–O– C bonds in the group of phenyl–SO2–O–phenyl gave rise to the peak at around 800 cm1 [24]. Accordingly, the observed 795 cm1 peak would be due to the S–O–C bond in Structure I in Fig. 6. This is an evidence of the degradation through Route (A). Fig. 7(B) exhibits the 2854 and 2928 cm1 peaks due to the symmetrical and asymmetrical C–H stretching vibrations of the CH2 groups in the polystyrene backbones [23,25]. The 737 cm1 peak from PTFE matrices, which were never affected at 250  C, was taken as an internal standard. The intensity ratio of the 2928 to 737 cm1 peak became higher after the heat treatment. This confirms a relativelyincreased amount of styrene units, supporting the degradation through Route (B) in Fig. 6. When the PEM was heated at 300  C, the RW and IEC were 87% and 0.26 meq g1, respectively. At the same IEC, the RW was lower than the broken line representing the degradation through Route (B), i.e., direct elimination of sulfonic acid groups, indicating the additional decomposition to be necessarily considered. It is said that polystyrene is depolymerized by random scission of its long C– C main chains at 280  C [26]. This follows that a part of polystyrene units would be detached from the graft components, although the main reaction was the H2O and SO2 production. At 350  C, there were no sulfonic acid groups in the PEM as mentioned above. RW was 71%, which is a little higher than R1 (¼67%) plotted as a square in Fig. 5. This suggests that there were some residues, whose weight corresponds to 12% of the original graft chain. Their molecular structures were considered by referring to a study on the thermal stability of poly(1,4-phenylenesulfonate), –(phenyl– SO2–O)n– [27], which contains the same repeat unit as Structure I in Fig. 6. This polymer was reported to degrade at 375  C and yield phenol and SO2. Thus, our PEM also should undergo a similar reaction to afford aromatic rings attached by hydroxyl groups.

Fig. 8 shows the RW obtained at different temperatures. For comparison, the curve (c) in Fig. 3, the most seemingly accurate TGA result at the slowest heating rate, was transcribed into this graph. The TGA data were higher than the RW between 250 and 350  C because they were overestimated owing to the two problems of the TGA as described in Section 3.2. Note that the difference between the EHA and TGA results was much more prominent at 350  C. This can be explained by taking into account the decomposition products as follows. Up to 300  C, the main products are considered to be volatile gases of H2O and SO2 formed through the same routes as at 250  C. On the other hand, the decomposition at 350  C was not only gas formation but also random scission of polystyrene graft components to produce species with widelydistributed molecular weights. Among these molecular products, some would be large in size and less- or non-volatile, thereby apparently restraining weight loss detected by the TGA. In contrast, the EHA has a washing process after heat treatment to surely give lower RW. Recall that the PEM probably had hydroxyl groups even at 350  C as mentioned above. These hydrophilic groups presumably allowed water to penetrate into the PEM and to take away the residues from inside, leading to precise estimation of net PEM weight. It should be noted that the EHA is applicable to any PEM including Nafion and radiation-grafted ones like our crosslinkedPTFE PEM. 3.4. Fabrication and performance of MEA The MEA is generally fabricated by sandwiching a PEM between two electrodes at high temperature and pressure [28–31]. For achieving good interfacial contact, the PEM should be as soft as possible during the hot-pressing [28–31]. In the case of Nafion, the hot-press temperature is typically 120–140  C, at which a Nafion membrane is sufficiently softened and stable. Because the crosslinked-PTFE PEM was found to be stable up to 200  C, the hot-press temperature was set at 200  C, and besides, 250  C as a reference. When the temperature was 200  C, the rMEA was 6.5 U cm2. In contrast, the MEA fabricated at 250  C exhibited much higher rMEA reaching 26 U cm2. The main reason for such high resistivity would be reduction of s (see Fig. 4) and increase of interfacial resistance between the PEM and electrodes. As to the latter, loss of sulfonic acid groups made the PEM less hydrophilic, and so could not maintain an intimate contact with the electrodes. Accordingly, the best temperature for hot-pressing was determined to be 200  C. In fact, we recently constructed a single PEFC using our PEM-based MEA, and then investigated its performance under various operation conditions [32].

A

B

2928

Absorbance (arb. units)

Absorbance (arb. units)

834 774 737

795

950

900

850

800

Wavenumber

750

700

3200

3100

3000

2900

2800

2700

-1)

(cm-1)

Fig. 7. FT-IR spectra of the PEMs (upper) before and (lower) after heat treatment at 250

2854

Wavenumber (cm C

for 6 h. The wavenumber ranges are (A) 700–950 cm1 and (B) 2700–3200 cm1.

S. Sawada et al. / Polymer Degradation and Stability 94 (2009) 344–349

RW (%)

100

80

60

0

100

200

300

400

Temperature (°°C) Fig. 8. Relationship between (,) the RW and heat treatment temperature in the EHA. The TGA curve (c) in Fig. 3 is transcribed into this graph for comparison (broken line).

4. Conclusions We investigated the thermal stability of the crosslinked-PTFE based PEM with an IEC of 1.6 meq g1. The result of the conventional TGA was found to be unreliable because of difficulty in determining the decomposition temperature accurately. Instead, we examined changes in various properties of the PEMs during the course of heat treatment in air after they were washed in pure water. Both the IEC and s did not change at all up to 200  C, indicating that there was no degradation. When the temperature was 250  C, they decreased to 0.64 meq g1 and 0.026 S cm1, respectively. From this relationship between the RW and IEC, the SSA units were considered to degrade through radical cleavage of the C–S bond between an aromatic ring and a sulfonic acid group, and condensation of two sulfonic acid groups. The graft components started to be detached by a C–C bond break of the polystyrene backbone at 300  C, and about 88% of them were lost at 350  C. It was revealed that the above EHA could suitably contribute to elucidation of the PEM degradation mechanism owing to the following two advantages. One is the washing process for complete removal of the thermal-decomposition products in the PEM, the other is to be able to measure the IEC and s in parallel with the weight of the PEM. Finally, the crosslinked-PTFE PEM was hotpressed with electrodes at 200  C to fabricate a low-resistance MEA for a PEFC application. References [1] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sources 2003;114:32–53. [2] Sopian K, Daud WRW. Challenges and future developments in proton exchange membrane fuel cells. Renew Energy 2006;31:719–27. [3] Gupta B, Scherer GG. Proton exchange membranes by radiation grafting of styrene onto FEP films. I. Thermal characteristics of copolymer membranes. J Appl Polym Sci 1993;50:2129–34. [4] Gupta B, Highfield JG, Scherer GG. Proton-exchange membranes by radiation grafting of styrene onto FEP films. II. Mechanism of thermal degradation in copolymer membranes. J Appl Polym Sci 1994;51:1659–66.

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[5] Samms SR, Wasmus S, Savinell RF. Thermal stability of Nafion in simulated fuel cell environments. J Electrochem Soc 1996;143:1498–504. [6] Hietala S, Koel M, Skou E, Elomaa M, Sundholm F. Thermal stability of styrene grafted and sulfonated proton conducting membranes based on poly(vinylidene fluoride). J Mater Chem 1998;8:1127–32. [7] Deng Q, Wilkie CA, Moore RB, Mauritz KA. TGA–FTi.r. investigation of the thermal degradation of Nafion and Nafion/[silicon oxide]-based nanocomposites. Polymer 1998;39:5961–72. [8] Nasef MM. Thermal stability of radiation grafted PTFE-g-polystyrene sulfonic acid membranes. Polym Degrad Stab 2000;68:231–8. [9] Nasef MM, Saidi H, Nor HM. Cation exchange membranes by radiationinduced graft copolymerization of styrene onto PFA copolymer films. III. Thermal stability of the membranes. J Appl Polym Sci 2000;77:1877–85. [10] Yamaki T, Asano M, Maekawa Y, Morita Y, Suwa T, Chen J, et al. Radiation grafting of styrene into crosslinked PTFE films and subsequent sulfonation for fuel cell applications. Radiat Phys Chem 2003;67:403–7. [11] Yamaki T, Kobayashi K, Asano M, Kubota H, Yoshida M. Preparation of proton exchange membranes based on crosslinked polytetrafluoroethylene for fuel cell applications. Polymer 2004;45:6569–73. [12] Sawada S, Yamaki T, Asano M, Terai T, Yoshida M. Proton conduction properties of crosslinked PTFE electrolyte membranes with different graft-chain structures. Trans Mater Res Soc Jpn 2005;30:943–6. [13] Sekine T, Sawada S, Yamaki T, Asano M, Suzuki A, Terai T, et al. Methanol permeation properties of crosslinked-PTFE electrolyte membranes for DMFC applications. Trans Mater Res Soc Jpn 2006;31:871–4. [14] Li J, Matsuura A, Kakigi T, Miura T, Oshima A, Washio M. Performance of membrane electrode assemblies based on proton exchange membranes prepared by pre-irradiation induced grafting. J Power Sources 2006;161:99–105. [15] Conesa JA, Font R. Polytetrafluoroethylene decomposition in air and nitrogen. Polym Eng Sci 2001;41:2137–47. [16] Thomas TJ, Krishnamurthy VN. Thermogravimetric and mass-spectrometric study of the thermal decomposition of PBCT resins. J Appl Polym Sci 1979;24:1797–808. [17] Jenekhe SA, Lin JW, Sun B. Kinetics of the thermal degradation of polyethylene terephthalate. Thermochim Acta 1983;61:287–99. [18] Vasile C, Costea E, Odochian L. The thermoxidative decomposition of low density polyethylene in non-isothermal conditions. Thermochim Acta 1991;184:305–11. [19] Huang JW, Kang CC. Unusual thermal degradation behavior of PEGMA in air at different heating rates. Polym J 2004;36:574–6. [20] Chen X, Yu J, Guo S. Thermal oxidative degradation kinetics of PP and PP/ Mg(OH)2 flame-retardant composites. J Appl Polym Sci 2007;103:1978–84. [21] Jiang DD, Yao Q, Mckinney MA, Wilkie CA. TGA/FTIR studies on the thermal degradation of some polymeric sulfonic and phosphonic acids and their sodium salts. Polym Degrad Stab 1999;63:423–34. [22] Moynihan RE. The molecular structure of perfluorocarbon polymers. Infrared studies on polytetrafluoroethylene. J Am Chem Soc 1959;81:1045–50. [23] Strasheim A, Buijs K. Infra-red spectra of ion-exchangers on polystyrene base. Spectrochim Acta 1961;17:388–92. [24] Choi JH, Lee BC, Lee HW, Lee I. Competitive reaction pathways in the nucleophilic substitution reactions of aryl benzenesulfonates with benzylamines in acetonitrile. J Org Chem 2002;67:1277–81. [25] Liang CY, Krimm S. Infrared spectra of high polymers. VI. Polystyrene. J Polym Sci 1958;27:241–54. [26] Vyazovkin S, Dranca I, Fan X, Advincula R. Kinetics of the thermal and thermooxidative degradation of a polystyrene–clay nanocomposite. Macromol Rapid Commun 2004;25:498–503. [27] Campbell RW, Hill HW. Polymerization of 4-hydroxybenzenesulfonyl chloride. Macromolecules 1973;6:492–5. [28] Huslage J, Rager T, Schnyder B, Tsukada A. Radiation-grafted membrane/electrode assemblies with improved interface. Electrochim Acta 2002;48:247–54. [29] Jung HY, Cho KY, Lee YM, Park JK, Choi JH, Sung YE. Influence of annealing of membrane electrode assembly (MEA) on performance of direct methanol fuel cell (DMFC). J Power Sources 2007;163:952–6. [30] Liang ZX, Zhao TS, Xu C, Xu JB. Microscopic characterizations of membrane electrode assemblies prepared under different hot-pressing conditions. Electrochim Acta 2007;53:894–902. [31] Zhang J, Yin GP, Wang ZB, Lai QZ, Cai KD. Effects of hot pressing conditions on the performances of MEAs for direct methanol fuel cells. J Power Sources 2007;165:73–81. [32] Kawahito S, Sawada S, Yamaki T, Asano M, Ishiguro H, Abdelkareem MA, et al. DMFC performance of crosslinked-polytetrafluoroethylene electrolyte membranes. In: Abstract book of ‘‘47th battery symposium in Japan’’; 2006. p. 158–9.