Enhanced proton conductivity at low humidity of proton exchange membranes with triazole moieties in the side chains

Author’s Accepted Manuscript Enhanced Proton Conductivity at Low Humidity of Proton Exchange Membranes with Triazole Moieties in the Side Chains Min-Kyoon Ahn, Su-Bin Lee, Cheong-Min Min, Yong-Guen Yu, Joseph Jang, Mi-Yeong Gim, JaeSuk Lee www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30821-3 http://dx.doi.org/10.1016/j.memsci.2016.10.018 MEMSCI14804

To appear in: Journal of Membrane Science Received date: 28 June 2016 Revised date: 10 October 2016 Accepted date: 11 October 2016 Cite this article as: Min-Kyoon Ahn, Su-Bin Lee, Cheong-Min Min, Yong-Guen Yu, Joseph Jang, Mi-Yeong Gim and Jae-Suk Lee, Enhanced Proton Conductivity at Low Humidity of Proton Exchange Membranes with Triazole Moieties in the Side Chains, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.10.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced Proton Conductivity at Low Humidity of Proton Exchange Membranes with Triazole Moieties in the Side Chains Min-Kyoon Ahn a,b, Su-Bin Lee a, Cheong-Min Min a, Yong-Guen Yu a,b, Joseph Jang a,b, MiYeong Gim a, Jae-Suk Lee a,b,* a

School of Materials Science and Engineering, Gwangju Institute of Science and Technology

(GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, Republic of Korea b

Research Institute for Solar and Sustainable Energy (RISE), Gwangju Institute of Science and

Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea ABSTRACT Highly conductive, novel sulfonated poly(arylene ether) membranes with different amounts of triazole-introduced monomers

(SPAE-TM)

were prepared via conventional

aromatic

condensation reactions. The triazole-introduced monomer (TM) was used as a proton transfer moiety to increase the proton conductivities of polymer electrolyte membranes (PEMs). To compare the chemical, mechanical and thermal stabilities of non-crosslinked polymer electrolyte membranes (SPAE-TM10, SPAE-TM20), crosslinked polymer electrolyte membranes were prepared via the thermal crosslinking method (cSPAE-TM10, cSPAE-TM20). The SPAE-TM membranes exhibited excellent thermal stabilities, and the non-crosslinked SPAE-TM membranes showed satisfactory glass transition temperatures (Tg = 250 °C). The water uptakes and swelling ratios of the SPAE-TM membranes were successfully suppressed in comparison with the SPAE-TM0 membrane. The proton conductivities of all membranes were higher than those of Nafion 212 over a wide range of temperatures (0.211 – 0.256 S/cm at 80 °C, 100% R.H.). The SPAE-TM20 presented a proton conductivity that was 2.3 times higher than that of SPAE-TM0 at 80 °C and 30% R.H.

1

Graphical abstract

KEYWORDS: polymer electrolyte membrane; triazole; acid-base interaction; crosslinked membrane.

1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have been widely researched in recent decades as potential energy converting devices[1,2]. Proton exchange membranes (PEMs) are important components that efficiently separate fuel gases and transport protons via their ion exchange abilities. The most widely used PEM is Nafion, which was developed by DuPont, Inc. Nafion shows excellent chemical and mechanical stabilities as well as a high proton conductivity due to its perfluorinated backbone and highly hydrophilic sulfonic acid pendant. However, Nafion has low proton conductivity in low humidity conditions, which makes application difficult[3,4]. To overcome this limitation, developing alternative PEMs that maintain their proton conductivities over a wide range of humidities has been strongly recommended. One promising approach is replacing water with N-heterocycles that have high boiling points, such as imidazole,

2

oxadiazole, triazole[5–7]. Because the nitrogens of these N-heterocycles are amphoteric, they can act as proton donors and acceptors. However, combined N-heterocycles often leach out from the membranes, which leads to reductions in proton conductivity[8]. Therefore, covalently linking N-heterocycles with polymer structures is required. Many studies have reported synthesis and conducting properties of polymer electrolytes with N-heterocycles[9,10]. Those polymer electrolytes have shown high proton conductivities without humidity, especially at high temperatures. Despite their abilities to transport protons in anhydrous conditions, their proton conductivities in practical temperature ranges (approximately 80 °C) are very low due to the absence of sulfonic acid groups. An emerging strategy for the development of highly conductive polymer electrolytes for practical applications has been to introduce the N-heterocycles into the main or side chains of sulfonated polymer electrolytes[11–14]. N-Heterocycles within polymer electrolytes always affect not only the proton conductivity but also the mechanical, chemical and water stabilities of the membranes.[15,16] Therefore, appropriately balancing the ratio between sulfonic acid and Nheterocycle groups is very important. We reported the synthesis of various sulfonated poly(arylene ether) electrolytes.[17–19] These polymer electrolytes were prepared via condensation reactions with commercially available monomers. Although they were highly proton conductive (e.q. 0.151 S/cm at 80 °C) in humidified conditions they could not effectively transport protons in low humidity. From this point of view, we need to develop new polymer electrolytes to maintain the proton conducting property especially in low humidity conditions. Here, we present novel sulfonated poly(arylene ether) electrolytes containing triazole-introduced monomers (TM). The TM was introduced via covalent bonding to improve the proton

3

conductivities of the polymer electrolytes without losing their mechanical strengths. We have synthesized polymer electrolyte membranes with varying TM molar ratios. We have also investigated the proton conducting properties of proton exchange membranes to prove the effects of TM at various temperature and humidity conditions as well as thermal, chemical and mechanical properties of the polymer electrolyte membranes.

2. Experimental

2.1. Materials

Potassium-2,5-dihydroxybenzenesulfonate (SHQ), potassium carbonate and sodium azide were purchased from Sigma-Aldrich. 4,4′-Biphenol (BP), 1-(chloromethyl)-2,4-difluorobenzene, 1ethynylbenzene, copper iodide (CuI), and acetonitrile were purchased from Tokyo Chemical Industry (TCI). Decafluorobiphenyl (DFBP) was obtained from Fluorochem. N,NDimethylacetamide (DMAc) and benzene were supplied by Alfa Aesar and used as received.

2.2. Monomer Synthesis

2.2.1. Synthesis of 1-(azidomethyl)-2,4-difluorobenzene 1-(Chloromethyl)-2,4-difluorobenzene (4.87 g, 30.0 mmol), sodium azide (1.95 g, 30.0 mmol), and solvent (90 mL, acetone:water = 2:1) were placed in a one-necked round flask equipped with a magnetic stirrer, thermometer and nitrogen gas inlet tube. The reaction mixture was heated at 60 °C for 12 h. The reaction product was extracted with chloroform and washed with water

4

several times. The extracts were dried over MgSO4. After removing the residual solvent by evaporation, the product was obtained as a clear yellow liquid in 99% yield.

2.2.2. Synthesis of 1-(2,4-difluorobenzyl)-4-phenyl-1,2,3-triazole A solution of 1-(azidomethyl)-2,4-difluorobenzene (3.38 g, 20.0 mmol) and ethynylbenzene (2.04 g, 20.0 mmol) in CH3CN (45 mL) was degassed through freeze thaw cycles at least three times before adding copper(I) iodide (0.38 g, 10.0 mol%). The mixture was stirred at room temperature under a nitrogen atmosphere. After 12 h of reaction, the product synthesized via the click reaction was extracted with ethyl acetate, and the extracts were washed several times with brine. The extracts were dried over MgSO4. Then, the crude solution was filtered through celite. The product was obtained as a white solid in 99% yield by recrystallization with ethyl acetate and hexane.

2.3. Copolymerization of the sulfonated poly(arylene ether)s containing TM (SPAE-TM)

The sulfonated copolymers (SPAE-TM10, SPAE-TM20) were synthesized via an aromatic nucleophilic reaction between the dihydroxyl monomers, SHQ and BP, and the dihalide monomers, DFBP and TM. The molar ratio of the dihydroxyl monomers was fixed (SHQ:BP = 9:1), and the molar ratio of the dihalide monomers was varied by controlling the TM feed ratio. A typical copolymerization procedure will be described using cSPAE-TM10 as follows. SHQ (9.0 mmol, 2.05 g) and BP (1.0 mmol, 0.19 g) were dissolved in a mixture of DMAc (24 mL) and benzene (10 mL) with K2CO3 (1.73 g, 1.25 equiv of hydroxyl monomers). A 100 mL twoneck reaction flask was equipped with a magnetic stirrer, nitrogen inlet, and Dean-Stark trap and

5

heated to 150 °C for 8 h to remove the water completely from the system by azeotropic distillation. After removing benzene, DFBP (3.01 g, 9.0 mmol) and TM (0.27 g, 1.0 mmol) dissolved in DMAc (20 mL) were added and stirred at 110 °C for 24 h. To introduce the crosslinkable groups at the chain ends of the copolymers, 3-hydroxyphenylacetylene (0.18 mL, 1.67 mmol) was added to the reaction mixture. After an additional 3 h of reaction, the reaction mixture was cooled and poured into ethanol. The precipitated polymer was filtered and washed with ethanol several times. Furthermore, the precipitated polymer was collected and dried in a vacuum oven at 60 °C.

2.4. Membrane preparation

All membranes were prepared via a solution casting and evaporation process. The polymers were dissolved in DMAc (10 w/v%) and stirred vigorous until the solution became homogeneous. The solution was filtered to remove any insoluble particles and salts. After filtration, the polymer solution was poured onto a clean glass hot plate with a slowly increasing temperature from 60 to 100 °C over 12 h and was additionally heated at 80 °C for 6 h under vacuum to evaporate residual DMAc completely. In the case of crosslinked polymers, an additional thermal treatment was performed. The crosslinked membrane was placed on a hot plate with a slowly increasing temperature from 100 to 200 °C over 1 h and at 250 °C for 1 h to complete the thermal crosslinking of the ethynyl groups. All of the prepared membranes were immersed in deionized water and peeled off from the glass plate. The membranes were acidified by immersing in 1 N H2SO4 solution at room temperature for 24 h and then washed in deionized water at room temperature for 24 h. All acidified membranes were kept at room temperature in deionized water

6

for at least 2 days before testing.

2.5. Characterization and Measurement

2.5.1. Structural Characterization and Molecular Weight Measurements 1

H and

19

F NMR spectra measured on a JEOL JNM-LA 400 WB FT-NMR were used to

determine the chemical compositions of the monomers and polymers using DMSO-d6 as the solvent. Fourier transform infrared (FT-IR) spectroscopy was utilized to confirm the functional groups of the synthesized copolymers. The molecular weights of the membranes were measured by gel permeation chromatography (GPC, Waters) using a Waters 410 Differential Refractometer with a TSKgel SuperAWM-H column. Polystyrene standards was used for calibration, and DMF containing 0.01 M LiBr was used as the eluent.

2.5.2. Thermal Characterization The thermal properties of the membranes in acid form were determined using a 2100 series TA instrument. The thermal degradations (Td) were determined by TGA in the range from 30 to 800 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The glass transition temperatures (Tg) were determined by thermomechanical analysis (TMA) of the polymer membranes that were equilibrated at 40 °C using an extension force less than 0.05 N on the membranes at a heating rate of 3 °C/min with a 50 mL/min nitrogen flowrate.

2.5.3. Water Uptakes and Swelling Ratios The water uptakes and swelling ratios were determined for all membranes in their acid forms.

7

The acid-form membranes were dried at 80 °C under vacuum until a constant weight was recorded. They were then immersed in deionized water at 30 °C and periodically weighed on an analytical balance until constant water uptake weights were obtained. The water uptake was calculated from the percentage of the weight difference between the wet membrane (Wwet) and the dried membrane (Wdry) and dividing by the dry membrane (Wdry) weight. The swelling ratio was calculated from the percentage of the length difference between the wet membrane (lwet) and the dried membrane (ldry) and dividing by the dry membrane (ldry) length. The equations for the water uptake (%) and swelling ratio (%) are given below:

water uptake (%) =

Wwet -Wdry × 100 Wdry

Swelling ratio (%)=

lwet - ldry × 100 ldry

2.5.4. Ion Exchange Capacity (IEC) The ion exchange capacities (IEC) were determined using the classical titration method. After acidifying and washing the membranes, they were immersed in 0.1 M NaCl solutions for 24 h to replace H+ with Na+. The remaining liquid was titrated with 0.01 M NaOH using phenolphthalein as an indicator. The IEC values were expressed as mequiv of (-SO3H)/g of dry polymer and obtained by the following equation:

IEC (mequiv/g)=

consumed NaOH × molarity NaOH weight of dried membrane

2.5.5. Mechanical Properties

8

The mechanical properties were tested with 70 mm × 5 mm membrane samples at room temperature on an Instron instrument at a strain rate of 10 mm/min, and a 500 N load cell was used.

2.5.6. Proton Conductivity The proton conductivity was measured using a four-point probe method. All the measurements were carried out in a temperature and humidity control chamber (ESPEC, SH-241). The impedance was determined using an electrochemical impedance analyzer (Solatron 1280Z) over a frequency range from 1 Hz to 20 kHz. Using a Bode plot, the frequency region over which the impedance had a constant value was checked, and the resistance was obtained from a Nyquist plot. The proton conductivity was calculated as follows: 𝜎=

𝐿 𝑅𝑆

where σ (S/cm or -1cm-1) is the proton conductivity, L (cm) is the distance between two electrodes, R () is the resistance of the membrane, and S (cm2) is the surface area of the membrane. The impedance of each sample was measured at least five times to ensure reproducibility of the data.

2.5.7. Oxidative stability The oxidative stabilities of the membranes were tested using Fenton’s reagent (3 wt% H2O2 aqueous solution containing 2 ppm Fe2+) at 30 °C and 80 °C, respectively. The residual weight percent was calculated from the membrane weight before and after immersion in Fenton’s reagent.

9

3. Results and Discussion

3.1. Synthesis of Triazole Introduced Monomer (TM)

The TM was synthesized via the Cu-catalyzed “Click” reaction, which is a highly effective reaction method to produce triazole.[20,21] The designed TM have phenyl pendant groups, which can increase the mechanical stability of the polymer membrane.[22] TM was successfully synthesized in two steps from 1-(chloromethyl)-2,4-difluorobenzene as described in Scheme 1. The nucleophilic substitution of 1-(chloromethyl)-2,4-difluorobenzene with NaN3 was carried out at 60 °C for 12 h in an acetone and deionized water co-solvent. The yield was higher than 99%, and the chemical structure of 1-(azidomethyl)-2,4-difluorobenzene was confirmed by 1H NMR. In Fig. 1, the substitution of the azide group is revealed by the appearance of the shifted peak at 4.3 ppm from the methylene. Subsequently, a highly effective click reaction between azide and alkyne was carried out at room temperature for 12 h in CH3CN in the presence of CuI as a catalyst. The chemical structure was confirmed by the appearance of the new peak at 8.5 ppm from the triazole and the shifted methylene peak at 5.7 ppm.

Scheme 1. Synthesis of the triazole-introduced monomer (TM).

10

Fig. 1. 1H NMR spectra of (a) 1-(chloromethyl)-2,4-difluorobenzene (b) 1-(azidomethyl)-2,4difluorobenzene (c) 1-(2,4)-difluorobenzyl)-4-phenyl-1,2,3-triazole.

3.2. Synthesis and Characterization of SPAE-TM

The sulfonated poly(arylene ether)s (SPAEs) with different molar ratios of TM were successfully synthesized via polycondensation as illustrated in Scheme 2. The molar ratio of SHQ and BP was fixed at 9:1 to balance the high proton conductivity and mechanical stability. The chemical structures of SPAE-TM were confirmed by 1H and

19

F NMR spectroscopy in

DMSO-d6, as shown in Fig. S1 and Fig. S2. The 1H NMR spectrum of SPAE-TM0 was inserted for comparison. In the 1H NMR spectra of SPAE-TM, we confirmed that the triazole-introduced monomers and crosslinkable 3-hydroxyphenylacetylene were successfully introduced into the polymer side chains. The

19

F NMR spectra showed two peaks corresponding to ortho and meta

fluorine atoms, which indicates that the desired SPAE-TM polymers were synthesized without any unexpected reactions. The FT-IR spectrum of SPAE-TM shows the characteristic peaks from sulfonate (-SO3) at 1022, 1403 cm-1, corresponding to the symmetric, asymmetric stretching peaks, respectively (Fig. S3). The peak with strong intensity at 1470 cm-1 was assigned to aromatic C=C stretching and the C=N stretching peak of triazole was appeared at 1682 cm -1 with increased intensity according to the TM ratio.

11

Scheme 2. Synthesis of the sulfonated poly(arylene ether)s with triazole-introduced monomers (SPAE-TM).

3.3. Thermal Properties

The thermal stabilities of SPAE-TM were investigated by TGA and are shown in Fig. 2. The 5% weight degradation temperatures (Td5%) of SPAE-TM copolymers were above 305 °C and were listed in Table 1. The polymers presented two-step degradation. The first weight loss observed at round 275 °C is due to the degradation of the sulfonic acid groups attached to the polymer backbone, and the second weight loss observed in the range of 490-530 °C is attributed to the degradation of the polymer backbone.[23–25] The thermal properties of SPAE-TM were not much different, but the crosslinked copolymers showed slightly higher thermal stabilities than the non-crosslinked copolymers. In addition, Td5% of the SPAE-TM0 copolymer was 319 °C,

12

which was higher than those of SPAE-TM10 and SPAE-TM20. When the molar ratio of TM increased, the degradation temperature decreased due to the increase in hydrophilicity.

The TMA curves of the SPAE-TM copolymers are shown in Fig. 3. Below Tg, the thermal expansion behaviors of SPAE-TM were similar. The Tg was calculated as the intersection of two straight lines and listed in Table 1. Considering the fact that the Tg of SPAE-TM0 is 239.9 °C, the Tg of the SPAE-TM copolymers increased as the TM content increased. These trends indicate that the TM of the copolymers can interact with sulfonated groups via acid-base interactions, which restrict the free rotation of the polymer molecules. In addition, the Tg of the crosslinked copolymers did not appear below their degradation temperatures. Therefore, we found that SPAE-TM copolymers showed good thermal stabilities for fuel cell applications.

Fig. 2. TGA curves of the SPAE-TM copolymers.

13

Fig. 3. TMA curves of the SPAE-TM copolymers.

3.4. Ion Exchange Capacity and Water Stability

When considering the proton conductivity and mechanical properties, the ion exchange capacity (IEC) is the one of the most important factors in PEMs. This parameter can predict the performances of polymer electrolytes. Calculated and experimental IEC values of SPAE-TM copolymers are listed in Table 1. The experimental IEC values of SPAE-TM copolymers obtained by the titration method were slightly lower than their calculated IEC values. These results were attributed to the basic property of the triazole groups, i.e., they can accept protons.[26–28] However, all polymer membranes showed reasonably high experimental IEC values in the range of 1.73-1.78 meq/g. In addition, the molecular weights of the SPAE-TM10 and SPAE-TM20 copolymers were 18,600 and 13,200 g mol-1, respectively. In our previous study, SPAE-TM0 copolymers with different degrees of sulfonation showed molecular weights in the range of 15,292 – 19,839 g mol-1.[29] These differences are due to the reactivity difference between TM and DFBP. The water uptakes and swelling ratios of the proton exchange membranes, which directly

14

impact their mechanical stabilities, operating durabilities and proton conductivities, were measured by recording the changes in the weights and dimensions of the membranes. As shown in Fig. 4, the SPAE-TM membranes showed reasonable water uptakes and swelling ratios compared with Nafion. The water uptake of the SPAE-TM membranes increased as the molar ratio of TM increased. For example, SPAE-TM20 showed the highest water uptake of 35 wt% but still maintained a suitable water uptake for fuel cell applications. The water uptake of SPAETM10 was slightly lower than that of SPAE-TM20. This difference is attributed to the different IECs and the hydrophilic nature of the TM in the membranes. The crosslinked membranes showed lower water uptakes than the non-crosslinked membranes. For example, the water uptake of the cSPAE-TM10 and cSPAE-TM20 membranes were 20 wt% and 28 wt%, respectively, while those of the SPAE-TM10 and SPAE-TM20 membranes were 30 wt% and 35 wt%, respectively. These tendencies most likely occur because crosslinking made the polymer chains less flexible and, thus, suppressed the penetration of water molecules into the polymer membranes.[30,31]

Table 1 Molecular weights, thermal properties and ion exchange capacity (IEC) of the SPAE-TM copolymers. IEC (mequiv g-1)

Polymer Mn (g mol-1)

Td5% (°C)

Tg (°C) a

membrane

calcd

Obsd

SPAE-TM0

16,200

319

239.9

1.86

1.86

SPAE-TM10

18,600

312

249.8

1.88

1.75

cSPAE-TM10

25,600

313

-b

1.88

1.73

SPAE-TM20

13,200

305

253.2

1.91

1.78

15

cSPAE-TM20

17,400

-b

311

a

Taken from the intersection of two lines of the TMA curves.

b

Not detected in the TMA curves.

1.90

1.74

Furthermore, all membranes showed lower water uptakes even with higher IEC values than SPAE-TM0 (IEC: 1.86 meq/g). The SPAE-TM copolymer electrolytes can effectively limit the water uptake due to the acid-base interactions between the basic triazoles and the acidic sulfonate groups.[26,32] Similar to the water uptake results, the swelling ratios of the SPAE-TM membranes also exhibited the same tendency. The swelling ratio of the SPAE-TM0 membrane reached 30%, but the other SPAE-TM membranes had swelling ratios of less than 17%.

Fig. 4. The (a) water uptakes and (b) swelling ratios of the SPAE-TM copolymer membranes.

3.5. Proton Conductivity

The temperature dependencies of the proton conductivities of the SPAE-TM were measured from 30 to 80 °C under fully humidified conditions, and the results are depicted in Fig. 5(a).

16

Compared with SPAE-TM0, the proton conductivity of SPAE-TM increased as the molar ratio of TM increased. These results indicate that TM can support proton conduction within polymer chains. The non-crosslinked SPAE-TM10 and SPAE-TM20 showed higher proton conductivities than the corresponding crosslinked cSPAE-TM10 and cSPAE-TM20. These tendencies have been observed in many studies most likely due to the suppressed water uptake and decreased chain mobility via chemical crosslinking.[30,33,34] Although the crosslinked membranes showed slightly decreased proton conductivities, all of the SPAE-TM showed much higher proton conductivities than Nafion 212 over a wide range of temperatures. In particular, the proton conductivity of SPAE-TM20 (0.256 S/cm) was 1.5 times higher than that of Nafion 212 at 80 °C and significantly higher than that of other reported PEMs. Banerjee et al. synthesized polytriazole membranes by incorporating triazole rings in the polymer backbone. Although these membranes showed a high IEC value (2.75 mequiv/g) and a clearly phase separated morphology in the TEM images, the proton conductivity was 0.112 S/cm at 80 °C.[35] Chang et al. reported sulfonated polytriazole-clay nanocomposites, and these nanocomposite membranes exhibited a proton conductivity of 0.122 S/cm at 80 °C.[36] In addition, the side-chain-type PEM with both acidic and basic groups reported by Gong et al. exhibited a proton conductivity of 0.113 S/cm at 80 °C.[13] The proton conductivity as a function of humidity (30-100% R.H.) was also measured at 80 °C. It has been highly recommended for fuel cell applications that the proton exchange membranes maintain their conducting properties in low humidity environments. As shown in Fig. 5(b), the proton conductivity of SPAE-TM0 started to decrease significantly below 50% R.H. Eventually, at 30% R.H., the proton conductivity of SPAE-TM0 dropped to 0.83 mS/cm. Because sulfonated polymer electrolytes transport protons only via hydronium diffusion, they cannot transport

17

protons effectively in low humidity conditions.[37] However, the TM-containing polymer electrolytes showed less sensitive to humidity than SPAE-TM0 due to the transport abilities of TM by proton hopping without water. For example, the proton conductivity of SPAE-TM20 reached 2.02 mS/cm at 30% R.H., which was 2.4 times higher than that of SPAE-TM0.

Fig. 5. Proton conductivities of the SPAE-TM copolymer membranes (a) at different temperatures in 100% R.H. and (b) at different relative humidities at 80 °C. A Nafion 212 membrane was also measured for reference.

3.6. Mechanical Stability and Oxidative Stability

One of the important requirements for PEMs is adequate mechanical properties. The mechanical strength of the membrane affects the long-term stability and durability of a PEMFC. The mechanical properties of crosslinked and non-crosslinked SPAE-TM membranes were measured under ambient conditions. The tensile strength, Young’s modulus and elongation at break are listed in Table 2. All SPAE-TM membranes showed several times higher tensile

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strengths (68 - 114 MPa) than Nafion 212, which is attributed to the acid-base physical interaction and thermal crosslinking as well as the rigid polymer backbone.[38] Furthermore, the introduction of TM greatly increased the Young’s modulus, whereas the elongation property of the SPAE-TM membranes decreased with the introduction of crosslinking due to the limited chain mobility. These results implied that crosslinking makes the membranes brittle. However, the mechanical properties of the SPAE-TM membranes showed reasonable toughness and robustness for practical PEMFC applications. High oxidative stability is an important requirement for polymer membranes for applications because oxidative attack occurs due to radicals (•OH, •OOH) generated during fuel cell operation.[39–41] The oxidative stabilities of the SPAE-TM membranes were measured at 30 °C and 80 °C, as summarized in Table 2. At 30 °C, all of the membranes maintained their original weights, which means that all of the membranes showed high oxidative stabilities. However, at 80 °C, the residual weights of the membranes had a tendency to decrease slowly, which indicates that the polymer backbones began to degrade. After 1 h, the crosslinked membranes still maintained more than 97% of their initial weights, but SPAE-TM10 and SPAE-TM20 lost 12% and 20% of their initial weights, respectively. The difference between SPAE-TM10 and SPAETM20 is attributed to their different IECs and hydrophilicities. Because SPAE-TM20 showed a higher IEC value, it contains more hydrophilic triazole than SPAE-TM10. Therefore, SPAETM20 could easily form water-containing hydrophilic domains, which leads to oxidative attack by radicals.[39] After 7 h, the non-crosslinked membranes were not completely dissolved and existed in a soft gel state. However, the crosslinked membranes kept their shapes and showed high oxidative stabilities at 80 °C, likely due to their suppressed water uptakes and enhanced chemical stabilities due to the chemical crosslinking.[42,43] Consequently, the crosslinked

19

membranes showed high oxidative stabilities comparable to or higher than that of Nafion, which is stable to oxidative attack due to its perfluorinated structure.

Table 2 Mechanical properties and oxidative stabilities of the SPAE-TM copolymer membranes. Tensile

Elongation

Young’s

strength

at break

modulus

(MPa)

(%)

(GPa)

SPAE-TM0

27.3

8.2

SPAE-TM10

68.4

SPAE-TM20

RW a

RW b

Polymer membrane

12 h

24 h

1h

7h

0.9

> 99

> 99

92.5

-

6.6

1.1

> 99

> 99

87.9

-

103.1

6.3

1.5

> 99

> 99

80.1

-

cSPAE-TM10

102.9

6.4

1.1

> 99

> 99

99.3

98.2

cSPAE-TM20

113.8

6.0

1.6

> 99

> 99

97.5

91.8

Nafion 212

27.7 c

157 c

0.25 c

> 99

> 97

97.6

92.6

a

Residual weight percent at 30 °C

b

Residual weight percent at 80 °C

c

The data were taken from reference[18].

4. Conclusions

We have designed and synthesized novel polymer electrolytes with triazole-introduced monomers (SPAE-TM). The SPAE-TM membranes achieved remarkably enhanced proton

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conductivities (0.211 – 0.256 S/cm) compared with SPAE-TM0 as well as improved proton conductivities under low humidity conditions (from 0.83 mS/cm to 2.02 mS/cm). In addition, the proton conductivity of SPAE-TM20 was 1.5 times higher than that of Nafion 212 (at 80 °C, 100% R.H.). These results are mainly due to the amphoteric property of N-heterocyclic triazole, which can transport protons without water. Because the TM was covalently linked with the polymer backbones, the SPAE-TM did not show any mechanical losses. The introduction of TM into the polymer electrolyte induced the acid-base interaction between TM and sulfonate groups, which increased the thermal stability; this interaction could effectively prevent excessive water uptake and swelling. The combination of TM and crosslinking makes SPAE-TM membranes potential alternative polymer electrolyte membranes for hydrogen fuel cells (PEMFCs).

Acknowledgments

The research was supported by the Basic Science Research Program though the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A2A1A01002493); and by the GIST Research Institute (GRI) project through a grant provided by GIST in 2016.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version

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Highlights  Preparation of the proton exchange membranes (PEMs) with triazole-introduced monomer (SPAE-TM)  Higher proton conductivity of SPAE-TM than that of Nafion 212 over a wide range of temperature  Improved proton conductivity of PEMs in low humidity conditions by triazole moieties  Effects on the thermal, mechanical and water stabilities due to acid-base interaction

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