Construction of proton transport channels on the same polymer chains by covalent crosslinking

Author’s Accepted Manuscript Construction of proton transport channels on the same polymer chains by covalent crosslinking Hailan Han, Meiyu Liu, Lishuang Xu, Jingmei Xu, Shuang Wang, Hongzhe Ni, Zhe Wang www.elsevier.com

PII: DOI: Reference:

S0376-7388(15)30143-5 http://dx.doi.org/10.1016/j.memsci.2015.08.047 MEMSCI13934

To appear in: Journal of Membrane Science Received date: 1 June 2015 Revised date: 15 August 2015 Accepted date: 23 August 2015 Cite this article as: Hailan Han, Meiyu Liu, Lishuang Xu, Jingmei Xu, Shuang Wang, Hongzhe Ni and Zhe Wang, Construction of proton transport channels on the same polymer chains by covalent crosslinking, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.08.047 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.

Construction of proton transport channels on the same polymer chains by covalent crosslinking Hailan Hana, Meiyu Liub, Lishuang Xua, Jingmei Xua, Shuang Wangb, Hongzhe Nia, Zhe Wanga,b* a

College of Chemical Engineering,

b

Advanced Institute of Materials Science,

Changchun University of Technology, Changchun 130012, People’s Republic of China *

Corresponding author, Tel.: 86 431 85716155; fax: 86 431 85716155.

E-mail address: [email protected] (Z. Wang)

Abstract A series of novel crosslinked sulfonated poly(arylene ether ketone sulfone)polymers containing amino and carboxyl groups (Am-C-SPAEKS) were prepared by nucleophilic polycondensation reactions. The amino and carboxyl groups were introduced into the same polymer chains. The polymers were characterized by FT-IR and 1H NMR spectroscopy. TEM images of the polymers showed the formation of a continuous proton transport channel after the crosslinking. The proton conductivity of Am-C-SPAEKS was 0.088 S cm1, as high as that of Nafion under the same conditions. Interestingly, the amount of proton transport channels increased with increasing degree of crosslinked. However, the methanol permeability of Am-C-SPAEKS-30 membrane was 1.56 × 107 cm2 s1 at 25 °C, one order of magnitude lower than that of Nafion® 117 membrane. Keywords: Crosslinked, carboxyl group, proton exchange membrane, fuel cell Nomenclature DS degree of sulfonic groups repeating unit of polymer

1

Wwet the weights of wet membranes (g) Wdry the weights of dried membranes (g) Twet the thickness of wet membranes (cm) Tdry the thickness of dry membranes (cm) D water desorption coefficient of membrane l the thickness of membrane (cm) L the distance between the electrodes (cm) R membrane resistance (Ω) S the effective surface area of membrane (cm2) CA the methanol concentration in the methanol reservoir (mol m-3) CB the methanol concentration in the water reservoir (mol m-3) DK methanol diffusion coefficient (cm2 s-1) A the effective thickness of membrane (cm2) L the effective thickness of membrane (cm) VB volume of permeated reservoirs (mL) t time (s) T temperature (K) Greek symbols σ proton conductivity (S cm-1)

1. Introduction Proton exchange membrane fuel cells (PEMFCs) convert chemical energy into

2

electrical energy. Compared to other fuel cells, PEMFC is of great interest because of its high energy efficiency and environmental friendliness. Proton exchange membranes (PEMs) play an important role as the core component of PEMFCs [1, 2]. Nafion [3,5] is the most widely used PEM because of its high thermal stability and proton conductivity. However, some disadvantages such as the high cost and over-reliance on water lead to low proton conductivity and poor thermal chemical properties at high temperatures, thus hindering their future applications. Therefore, the development of new PEM materials with high proton conductivity and low methanol permeability has attracted much interest. Until now alternative electrolytes, aromatic-type polymers such as sulfonated poly(aromatic ether ketones) [6–9], sulfonated polybenzimidazoles [10–12], sulfonated poly(aromatic ether sulfones) [13–15], and sulfonated polimides [16–18] are known for their excellent thermal stabilities and mechanical properties. The proton conductivities of these materials depend on the degree of sulfonation (DS). A high DS of a membrane may lead to higher proton conductivity; however, a high DS may also cause severe swelling of the membrane or even dissolve it in water. Thus, the long-term stability of this type of PEM is not suitable for fuel cell applications. Hence, the challenge for further development of PEM lies in selecting an appropriate DS and then improving the mechanical property and decreasing the methanol permeability while maintaining a high proton conductivity of the membrane [19–23]. Crosslinking may solve these problems. For example, Lee et al. reported a novel crosslinked [24] sulfonated membrane, s-BI as the crosslinker showed higher IEC

3

values and proton conductivities because of the crosslinkers with proton conductivity groups [25]. Ren’s group prepared a series of crosslinked membranes using (3-isocyanatopropyl)triethoxysilane (ICPTES) as the crosslinker that improved the interfacial binding force and water retention capacity [26]. In our previous work, we prepared sulfonated poly( arylene ether ketone sulfone) membranes containing amino groups (Am-SPAEKS) by ionic crosslinking between amino and sulfonic acid groups [27]. The crosslinked membranes exhibited desirable mechanical properties and excellent thermal stabilities. However, the ionic interactions were weak and the crosslinked structure was not stable. Next, we prepared sulfonated poly( arylene ether ketone sulfones) containing carboxyl

groups (C-SPAEKS) and sulfonated

poly( arylene ether ketone sulfones) containing amino groups (Am-SPAEKS) crosslinked membranes (C-SPAEKS/Am-SPAEKS) by covalent crosslinking between amino and carboxyl groups [28]. The covalent crosslinked membranes showed excellent dimensional stability and low methanol permeability, due to the covalent crosslinking was far more stable. However, proton conductivity of these crosslinked membranes was not particularly high. Since the covalent crosslinked membranes were obtained by two polymers. The membranes did not form relatively continuous proton transfer channels because of the interfacial effect. In this paper, unlike other crosslinked membranes, we attempted to introduce amino and carboxyl groups on the same single polymer backbone to prepare a novel sulfonated

poly(

arylene

ether

ketone

sulfone)

crosslinked

membranes

(Am-C-SPAEKS) resulting in a more stable structure of the membrane and

4

eliminating interfacial effect. The amidation of carboxyl and amino groups leading to covalent crosslinking improved the water retention capacity, methanol resistance capacity, and dimensional stability of the membrane. The nitrogen atoms act as both proton acceptor and proton donor, thus shortening the proton transmission distance and increasing the proton conductivity of the membrane. Therefore, proton transport channels may be built by crosslinking because of the interaction between functional groups. The cleavage and formation of ionic bonds among the amino and sulfonic acid groups benefited the proton conductivity of the membrane. The difficulties were that one functional group had to be protected in the reaction of amino and carboxyl groups to form amide bonds during the polymerization reaction. We decided to transform the carboxyl groups into ester groups. After the polymerization reaction, the ester groups in the polymer were hydrolyzed into carboxyl groups before preparing the crosslinked membranes. As shown in Scheme 1, the amount of proton transport channels increased by introducing more crosslinked structures in the matrix. The purpose of this study was to construct a proton transport channel by introducing specific functional groups under low humidity conditions. Therefore, the balance between proton conductivity and methanol permeation was solved to an extent. Scheme 1 Hypothesis of proton transport channel (a) Am-C-SPAEKS-10 and (b) Am-C-SPAEKS-20 2. Experimental 2.1 Materials

5

4-Aminophenyl hydroquinone (4Am-PH), 4-carboxyphenyl hydroquinone (4C-PH), and 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone (SDCDPS) were synthesized inhouse [29–31]. 4,4′-Difluorodiphenylmethanone (DFB) (AR grade) was purchased from Yanbian Longjing Chemical Co., China. Anhydrous K2CO3, tetramethylene sulfone, toluene (AR grade), concentrated H2SO4, methanol, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), NaOH, and HCl were purchased from Beijing Chemical Reagent Co., China. All the organic solvents (Merck) were used as received without further purification. 2.2 Synthesis of 4-esterphenyl hydroquinone (4E-PH) 4C-PH (2 g) and 30 mL of methanol were added to a three-neck round-bottom flask equipped with a mechanical stirrer and condenser. Then, 0.69 g of concentrated H2SO4 was added slowly and reacted for 2 h in an ice bath. Then, the mixture was refluxed for 7 h at 65 °C. After the completion of the reaction, the methanol was removed using a rotary evaporator. The product was dissolved in ethyl acetate (EA) and neutralized with NaHCO3. The EA was removed using a rotary evaporator, and the residue was washed with deionized water several times. Finally, the product was dried in an oven (Scheme 2(a)). 2.3 Synthesis of Am-E-SPAEKS A series of Am-E-SPAEKS polymers were synthesized by nucleophilic substitution polycondensation as shown in scheme 2(b). The DS was 40%, which is the number of sulfonic acid group repeating unit of the polymer. This was determined from the ratio of SDCDPS to DFB. The molar ratio of 4Am-PH to 4E-PH in Am-E-SPAEKS-10,

6

Am-E-SPAEKS-20, and Am-E-SPAEKS-30 were 9:1, 8:2, and 7:3, respectively. The polycondensation reaction of Am-E-SPAEKS-20 was synthesized as follows: 4E-PH (1.098 g), 4Am-PH (2.1105 g), DFB (2.616 g), SDCDPS (1.473 g), and anhydrous K2CO3 (2.531 g) were added to a 100 mL three-neck round-bottom flask equipped with a mechanical stirrer, nitrogen inlet, and reflux condenser. Then, tetramethylene sulfone (14.89 mL), which was used as the solvent, and toluene (14 mL), which was used as the azeotropic agent, were added to the flask. The reaction mixture was heated to 120 °C for 4 h to remove the water from the system. When the toluene was evaporated, the temperature of the mixture was slowly increased to 170 °C and maintained at 170 °C for 10 h. After the completion of the reaction, the solution was cooled and poured into deionized water to obtain the polymers. Finally, the product was washed with deionized water several times to remove the salts and impurities and dried at 80 °C for 24 h. 2.4 Synthesis of Am-C-SPAEKS Am-E-SPAEKS polymer (3 g) was dissolved in a mixture of tetrahydrofuran and water (3:1) and continuously stirred to obtain a homogeneous solution in a nitrogen atmosphere. Next, NaOH (0.324 g) was added slowly in an ice bath. After the mixture was reacted for 2 h, it was slowly heated to 40 °C for 7 h. After the completion of the reaction, the solution was acidified using 0.5 M HCl. After filtration, the product was washed with deionized water several times and dried in an oven (Scheme 2(b)). The monomer ratios of C-SPAEKS and Am-C-SPAEKS polymers are listed in Table 1. Scheme 2 Synthesis of (a) 4E-PH monomer and (b) Am-C-SPAEKS-20 polymer

7

2.5 Preparation of crosslinked membranes First, 1 g of the polymer was dissolved in 10 mL NMP. The polymer solution was stirred vigorously, cast onto glass plates, and placed in a vacuum oven for 24 h at 80 °C to remove the solvent. The dried membranes were acidified with 2.0 M HCl solution for 48 h. The membranes were washed several times with deionized water to remove excess acid. Finally, the crosslinked membranes were obtained after heating at 120 °C for 24 h. 2.6 Characterization Fourier transform infrared spectroscopy (FT-IR) analysis was performed using a Vector-22 (Bruker, Germany). The FT-IR spectra of the samples were measured in the range 4000–300 cm−1 for 128 times at a resolution of 4 cm1. The prepared polymer was further characterized by 1H NMR spectroscopy using a 400 MHz Bruker Avance III spectrometer at 298 K. Deuterated dimethyl sulfoxide (DMSO-d6) was used as the solvent, and tetramethylsilane (TMS) was used as the standard. The thermal stabilities of the membranes were determined by thermogravimetric analysis (TGA) using a Pyris 1 TGA (Perkin Elmer) equipment in a nitrogen atmosphere at a heating rate of 10 °C min1 from 40 to 600 °C. Transmission electron microscopy (TEM) images were obtained using a JEM-1011 electron microscope. Membranes were stained by soaking in a 0.1 M silver nitrate solution at room temperature for 1 day, then rinsed with deionized water and dried under vacuum at room temperature overnight. The stained membranes were cut into small pieces and embedded in Spurr’s epoxy resin and cured overnight at 70 °C. The samples were sectioned to yield slices of 100 nm

8

thickness using a Leica EM UC6 ultramicrotome and placed on copper grids. Small-angle x-ray scattering (SAXS) data were collected on Beamline 4B9A at Beijing Synchrotron Radiation Facility (BSRF). 2.7 Measurements The water uptake and swelling ratio were determined using the following equations: Water uptake (%) =

Wwet  Wdry

Swelling ratio (%) =

Wdry Twet  Tdry Tdry

 100% (1)

 100% (2)

The films were dried in an oven at 80 °C for 24 h, and the weights and lengths (Wdry, Tdry) were measured. The films were placed in deionized water at different temperatures to make sure that the films were fully hydrated. The surface of the film was dried rapidly using a filter paper, and then the weights and lengths (Wwet, Twet) were measured. At a certain temperature, the water content of a membrane changes with time, which is known as the water desorption coefficient of the membrane. This was recorded by TGA at 80 °C for 1 h [32]. The water desorption coefficient was calculated using the following formula: Mt Dt 1  4( 2 ) 2 (3) M l

where D is the water desorption coefficient of the membrane, Mt/M∞ is the change in weight of water with time, and l is the thickness of the membrane. The ion exchange capacity (IEC) values of membranes were determined by 9

classical acid-base titration. Dry membrane was immersed in 1.0 M NaCl soultion for 48 h to replace all of the H+ with Na+. The H+ ions within the solutions were titrated with a 0.01 M NaOH solution using phenolphthalein as the indicator. The IEC values of membranes were measured as follows:

IEC 

VM Wdry

(4)

where V is the volume of NaOH solution used in titration, M is the concentration of NaOH solution, , Wdry is mass of dry membranes. The mechanical properties of the membranes were measured using an Instron 5965 equipment. The size of the membranes was 15 × 4 mm2. The test speed was set at 1 mm min1. Each five group was averaged to obtain the final test results. The sample was cut into a size of 2 × 1 cm2 and then immersed in Fenton’s reagent (3% H2O2 solution containing 4 ppm Fe2+) at 80 °C. After 1 h, the oxidative stabilities were evaluated from the weight change. The degree of crosslinked membranes was characterized by the gel fraction. The gel fraction was measured by solvent extraction [33]. The crosslinked membrane was placed in DMF and extracted using a Soxhlet extractor until no further soluble substance was observed. The rest of membrane was dried to constant mass. The gel fraction Wgel was expressed as equation (5): Wgel 

W1 W0 (5)

where W0 is the original mass of dry crosslinked membrane and W1 is the remaining mass of the dry crosslinked membrane after extraction.

10

The proton conductivity, σ (Scm1), was measured using a four-electrode AC impedance spectrometer, Salton 1260, in the frequency range from 100 kHz to 0.1 Hz, [34]. σ can be calculated using the following equation: σ=

L (6) RS

where σ is the proton conductivity (S cm1), L is the distance (cm) between the electrodes, R is the resistance (Ω), and S is the effective surface area (cm2) of the membrane. The impedance of each sample was measured five times to ensure the accuracy of data. Before the test, each sample was cut into a size of 4 × 1 cm2 and then immersed in water for 48 h. Methanol permeability was measured using a glass diffusion cell separated into two rooms. The rooms were separated by the membrane. One of them was filled with deionized water; the other was filled with methanol. The liquid was stirred during the test. Methanol concentrations in the water cell were determined using a Shimadzu GC-8A gas chromatograph. The methanol permeability was calculated using Eq. (7) [35]. C B (t ) 

A DK C A ( t  t o ) (7) VB L

where A (cm2), L (cm), and VB (mL) are the effective area, thickness of the membranes, and volume of the permeated reservoirs. CA and CB (mol m3) are the methanol concentrations in the methanol and water reservoirs, respectively. DK (cm2 s1) is the methanol diffusion coefficient. 3. Results and discussion 3.1 Structural characterization 11

4E-PH is successfully synthesized as confirmed by 1H NMR shown in Fig. 1 (a). The singlet at δ 3.85 ppm was assigned to methyl proton. The multiplet between  6.50 and 8.0 ppm can be attributed to the protons on benzene rings. The doublets at  8.81 and 8.95 ppm were assigned to the protons of hydroxyl groups. Am-E-SPAEKS and Am-C-SPAEKS polymers were synthesized by nucleophilic substitution and hydrolysis reactions. The structures of the polymers are confirmed by their 1H NMR spectra as shown in Figs. 1 (b) and (c). In Fig. 1 (b), the signals at  3.81 and 5.21 ppm were assigned to the protons of methyl and amino groups, respectively. However, in Fig. 1 (c), the signal at  3.81 ppm disappeared. This indicates that the methyl ester was hydrolyzed to carboxyl group. Thus, Am-C-SPAEKS membranes were successfully synthesized. Fig. 1 1H NMR spectra of 4E-PH (a), Am-E-SPAEKS (b) and Am-C-SPAEKS (c) The FT-IR spectrum of the membrane is shown in Fig. 2. The characteristic sharp bands at 622.38 cm1, 1025.16 cm1, and 1071.77 cm1 were assigned to the S–O stretching vibration and O=S=O asymmetric and symmetric vibrations of the sulfonic acid groups. The absorption bands at 1651.82 and 1403.88 cm1 were assigned to the C=O and CN stretching vibrations of CO–NH groups, respectively. Two broad bands at 3447.07 cm1 and 3358.51 cm1 were assigned to the N–H stretching vibration of amine functionality. Thus, Am-C-SPAEKS crosslinked membranes were successfully synthesized. Fig. 2 FT-IR spectrum of Am-C-SPAEKS crosslinked membrane The gel fraction of the crosslinked membranes indirectly assesses the degree of

12

crosslinked. Am-SPAEKS and C-SPAEKS have a good solubility in DMF, while crosslinked Am-C-SPAEKS membrane is not soluble in it. Therefore, DMF is selected as the solvent to assess gel fraction. The values are listed in Table 2. With the increasing of carboxyl content, the gel fraction gradually increased. This proved that amino groups reacted with carboxyl groups to form crosslinked structure. 3.2 Thermal stability TGA is performed to determine the thermal property of Am-C-SPAEKS as shown in Fig. 3 , all the crosslinked membranes exhibited similar trends. The initial weight loss step was in the vicinity of 100 °C, which can be attributed to the loss of water and solvent. The second weight loss step started around 190 °C, probably because of the degradation of sulfonic acid groups, amide bonds, and unreacted carboxylic acid and amino groups. The third weight loss step was observed at 510 °C, caused by the disintegration of the main chain of the polymers. The membranes showed a higher thermal decomposition temperature with the increase in the degree of crosslinked structure, higher than that of the SPAEKS membrane. The presence of crosslinked structure increased the limitation of rigid polymer backbones, forming a thermally stable structure. The membranes finally maintained more than 62 wt% of the original weight at 600 °C. The results indicate that the crosslinked network structure improved the thermal stability of the membranes, and Am-C-SPAEKS can tolerate high temperatures for PEMFC applications. Fig. 3 TGA curves of SPAEKS and Am-C-SPAEKS crosslinked membranes 3.3 IEC, water uptake and swelling ratio

13

IEC is usually defined as the moles of fixed hydrophilic functional groups per gram of polymer. The total IEC includes sulfonic acid and carboxyl groups in the membranes. Ion exchange capacity plays an important role in determining the water uptake and proton conductivity. The values of membranes were in the range of 0.88-1.07 mmol/g, as listed in Table 3. Although DS was the same, IEC increased with increasing carboxyl groups. It is well known that the water uptake of PEMs plays a very important role in proton transport. Water uptake and swelling ratio significantly affect the proton conductivity and mechanical stability of PEMs. This is because water acts as the carrier in which proton transport depends significantly below 80 °C. However, the uptake of excess water by the membranes also leads to an excess swelling ratio that affects the mechanical stability of PEM, leading to the destruction of the membrane–electrode assembly in PEMFCs. Fig. 4 and 5 show the temperature dependence of water uptake and swelling ratio. The water uptake of all the membranes increased with temperature. Unlike C-SPAEKS, the crosslinked membranes exhibited a lower water uptake. This can be attributed to the hydrophilic sulfonic acid and carboxyl groups that absorb water in C-SPAEKS. However, in Am-C-SPAEKS, the covalent crosslinking between amino and carboxyl groups reduced the free volume and restricted the mobility of the polymer, thus reducing the water absorption [36]. Moreover, the ionic crosslinking between the sulfonic acid and amino groups formed by hydrogen bonds promoted a compact structure; therefore, the hydrophilic channel was narrowed, thus reducing the

14

water uptake of the membranes. SPAEKS membranes have neither additional hydrophilic groups to increase the water uptake nor covalent crosslinked structures and hydrogen bonds to reduce the water absorption; therefore, the water uptake lies somewhere in between. The water uptakes of C-SPAEKS/Am-SPAEKS-10 and C-SPAEKS/Am-SPAEKS-20 were 33% and 28% higher than those of all the Am-C-SPAEKS membranes at 80 °C. [28] This is because the covalent crosslinked structure formed on the same molecular chain is more stable than that obtained by physical blending. Naturally, the mobility of the polymer decreased further, thus decreasing the water uptake. The swelling ratio trend is similar to the water uptake trend of crosslinked membranes. Fig. 5 shows that the swelling ratio of Am-C-SPAEKS membranes are significantly lower than that of other membranes at each temperature. The swelling ratio of Am-C-SPAEKS-30 was 3.8%, which is lower than 7.2% of SPAEKS at 20 °C. The highest swelling ratio was about 9.2% at 80 °C, which is also lower than that of Nafion® 117 (23.7%). This is probably because of the increasing interaction of the polymers resulting from crosslinked structures, which also limits the swelling volume and increases the dimensional stability. The entire swelling ratio of Am-C-SPAEKS membranes was much smaller than C-SPAEKS/Am-SPAEKS membranes [28]. The small swelling ratio can be attributed to the relatively more restriction of the covalent crosslinked network structure than simple physical blending. Fig. 4 Water uptakes of SPAEKS, C-SPAEKS, and Am-C-SPAEKS membranes at different temperatures

15

Fig. 5 Swelling ratios of SPAEKS, C-SPAEKS, and Am-C-SPAEKS membranes at different temperatures 3.4 Water retention capacity The presence of water in membranes is an essential condition to achieve high proton conductivity. Because membranes easily lose water at high temperatures, thus affecting the proton transfer, the water retention capacity is particularly important to the membrane. Water retention capacity is obtained by measuring the water desorption coefficient of membranes by TGA at 80 °C for 1 h. The water desorption coefficient of membranes is measured by Ficker diffusion [37]. The plot of water desorption coefficient vs. time is shown in Fig. 6. As reported in literature, water is retained in membranes by two processes: water binding to the material itself and water retained in pores [38]. In Am-C-SPAEKS membrane, because no pores were present, water adsorption was mainly caused by the membrane itself. The crosslinked Am-C-SPAEKS made the membrane more compact and retained more bound water [39]; therefore, the water retention capacity was significantly better than C-SPAEKS as shown in Fig. 6. Fig. 6 Water diffusion coefficient of C-SPAEKS, Am-SPAEKS, and Am-C-SPAEKS membranes 3.5 Mechanical properties and oxidative stability It is essential for PEMs to possess adequate mechanical properties to support the membrane electrode. The mechanical properties of the membranes at room temperature (25 °C) and 50% RH are listed in Table 2. The Young’s modulus and

16

tensile strength of Am-C-SPAEKS-30 were higher than all other membranes. The tensile strength of Am-C-SPAEKS-10 was 46.52 MPa, which is higher than 43.30 MPa for C-SPAEKS and 37.43 MPa for Nafion® 117. The Young’s modulus of Am-C-SPAEKS-30 increased almost nine times than that of Nafion® 117. Due to the fact that carboxyl and amino groups were introduced into the same single polymer chain, no macroscopic phase separation was observed, and the interfacial effect of the membranes improved. The crosslinked network decreased the free volume of the system and increased the force between the polymers, thus improving the mechanical strength of the membranes. C-SPAEKS/Am-SPAEKS membranes exhibited better tensile strength in the range from 47.35 to 57.12 MPa [28]. This is probably because both the molecular weight and degree of crosslinked affected the mechanical properties [40]. However, crosslinked structure limits the movement of polymer chains, thus reducing the toughness of the membranes. The elongation at break of the crosslinked membrane was much lower than that of Nafion® 117. The oxidative stability and durability of PEMs are extremely important for the performance and long-term use of PEMFCs. The oxidative stability of the membranes was determined by noting their change in weights after using Fenton’s reagent at 80 °C for 1 h. The results are shown in Table 2. The resistance-to-oxidation mainly depends on the water uptake and swelling ratio of membranes [41]. One of the reasons of polymer degradation is free radical attack. High water uptake and swelling ratio decrease the time required for hydroxyl radicals to enter the membranes. The oxidative stability of membranes is reduced naturally. The membrane properties such

17

as transparency and flexibility were maintained even after the test was completed. The resistance-to-oxidation was also affected by crosslinking. After crosslinked structures were formed in the membranes, the polymer chains were less likely to be attacked by hydroxyl radicals. The reserved weight of crosslinked membranes was above 92.36% and increased with increasing degree of crosslinked. The results indicate that the crosslinked structure improves the resistance to degradation. Moreover, the crosslinked membranes can be used in a long-term scale [36]. 3.6 Morphology The

morphology

of

the

membranes

was

analyzed

by

TEM.

The

hydrophilic–hydrophobic microphase separation structure is closely related to the proton conductivity and water uptake of the membranes [42]. The TEM image is shown in Fig. 7. The dark dots in the TEM images represent the hydrophilic domains consisting of sulfonic acid and carboxylic acid clusters stained by Ag+ ions, whereas the light bright area represents the hydrophobic regions [43]. Fig. 7 (a) shows the TEM image of C-SPAEKS; these hydrophilic clusters are evenly dispersed, and no significant phase separation was observed. The protons are transferred through the sulfonic acid groups. Fig. 7 (b) and (c) show that continuous proton transport channels were formed after the crosslinking between amino and carboxyl groups. With increasing degree of crosslinked, the proton transport channels increased. The protons could be conducted by jumping from the sulfonic acid groups to the amino groups, thus shortening the distance between the proton transfer channels. Furthermore, the proton conduction

18

does not depend on water; therefore, the crosslinked membranes possess a high proton conductivity and interconnectivity at high temperatures [44]. When comparing Am-C-SPAEKS-30 (c) to Am-C-SPAEKS-10 (b), a larger amount of transport channels was observed in Am-C-SPAEKS-30, indicating that a higher degree of crosslinked in the polymer resulted in wide proton transfer channels. Fig. 7 TEM images of (a) C-SPAEKS, (b) Am-C-SPAEKS-10, and (c) Am-C-SPAEKS-30 membranes Fig. 8 shows the SAXS spectra of Am-C-SPAEKS crosslinked membranes. SAXS is often used to study the internal structures of the polymers including the ionic clusters [45]. The continuous proton transport channels were formed due to the existence of crosslinked structure. A scattering maximum (ionomer peak) due to the phase separation between hydrophobic separation and hydrophilic separation was observed in crosslinked membranes. Hydrophobic/hydrophilic separation lengths were calculated from peak maximum using the Bragg relation. The Bragg spacing d, referring to the center-to-center distance between two ionic clusters, can be calculated from equation d=2π/q (where q is the scatter vector) [46]. With the degree of crosslinked increasing, the scattering maximum was found to have a value of 0.047, 0.043 and 0.038 Å-1, respectively. Bragg spacing have a value of 13.37, 14.61 and 16.53 nm, respectively. With the degree of crosslinked increasing, the ionic cluster distance increased, further led to the increasing cluster size. It demonstrated that crosslinked membranes will provide larger proton transport channel with increasing degree of crosslinked and coincide with result of TEM.

19

Fig. 8 SAXS spectra of the Am-C-SPAEKS crosslinked membranes 3.7 Proton conductivity Proton conductivity is a key point for PEMFCs and directly affects the performance of fuel cells. The proton conductivity of all the membranes increased with increasing temperature as shown in Table 3 and Fig. 9. Because the water uptake increased, the protons were more easily transferred through enlarged proton channels [36]. In general, protons are transferred by two mechanisms—Vehicle mechanism and Grotthuss mechanism [47,48]. Materials can be used as PEM when the proton conductivity is higher than 102 S cm1. The proton conductivity is shown as Arrhenius plots in Fig. 9. Am-C-SPAEKS-30 showed an activation energy of 6.05 kJ mol1, lower than that of Nafion® 117 (9.1 kJ mol1) and similar to that of C-SPAEKS (6.06 kJ mol1). The proton conductivity of Am-C-SPAEKS-20 (0.058 S cm-1) was similar to that of C-SPAEKS/Am-SPAEKS-30 (0.056 S cm-1) at 25 °C [28] as shown in Table 3. Vehicle mechanism plays a leading role at low temperatures. This mechanism is possible in hydrophilic regions where ions such as H3O+, H5O2+, and H9O4+ are transported between the clustering of sulfonic acid groups. Amino and carboxyl groups formed crosslinked network structures as a storage space by amidation reaction, thus enabling the storage of more bound water. As a result, the proton conductivity of the crosslinked membranes improved. On the other hand, amino and sulfonic groups formed ionic bonds can also store some water. Because the amount of water in membranes is significantly reduced at high

20

temperatures, the conductivity decreases according to Vehicle mechanism. Then, Grotthuss mechanism plays a major role. The introduction of amide bonds in Am-C-SPAEKS shortened the distance of proton transmission, because the nitrogen atoms act as both proton acceptor and proton donor. Protons can be transferred between the nitrogen atoms and sulfonic acid groups, thus significantly improving the transmission channel, as proton transport required less activation energy. And the interaction between sulfonic and amino groups facilitated proton transport according to the formation and cleavage pattern of ionic bonds. The proton transport mechanism in Am-C-SPAEKS crosslinked membranes is shown in Fig. 10. Fig. 9 Proton conductivity of C-SPAEKS and crosslinked membranes at different temperatures Fig. 10 Proposed mechanisms of proton transport in the crosslinked membranes: (a) Vehicle mechanism and (b) Grotthuss mechanism 3.8 Methanol permeability The methanol permeability of PEMs severely affects the battery life. The methanol permeability was measured at 25 °C and 60 °C (Table 3). The methanol permeability of crosslinked membranes increased with increasing water uptake. Therefore, the Am-C-SPAEKS membranes exhibited low permeability in the range from 4.38  107 cm2 s1 to 1.56  107 cm2 s1 at 25 °C, which is much lower than 29.4  107 cm2 s1 for Nafion® 117. This may be closely related to the participation of crosslinked structure, restraining both the mobility and flexibility of polymer chains. The structure makes the polymer matrix more compact, affording the Am-C-SPAEKS membranes

21

with improved methanol barrier properties. Moreover, the methanol permeability of Am-C-SPAEKS-30 was 1.56 × 107 cm2 s1 at 25 °C; this is lower than C-SPAEKS/Am-SPAEKS-30 (2.13 × 107 cm2 s1) [28]. This indicates the introduction of specific functional groups into the same molecular chains results in excellent stability . Membranes for direct-methanol fuel cells (DMFCs) should possess a high proton conductivity and reasonable capacity to hinder methanol permeation. Conclusion A series of novel Am-C-SPAEKS crosslinked membranes were successfully synthesized using 4C-PH and 4Am-PH by typical nucleophilic aromatic substitution reactions. Carboxyl and amino groups were introduced into the same single polymer chain, leading to a highly homogeneous phase structure without macroscopic phase separation. The crosslinked membranes exhibited excellent mechanical properties and dimensional stability. Interestingly, as observed by the TEM results, the number of continuous proton transport channels increased by the introduction of specific functional groups, resulting in enhanced proton conductivity. All the results obtained in this study indicate that these crosslinked membranes have potential for applications in DMFCs. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (Grant No: 51273024; Grant No: 51303015) and Department of Education of Jilin Province (Grant No: 2012103) for financial support for this work.

22

References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352. [2] B. Lafitte, L.E. Karlsson, P. Jannasch, Sulfophenylation of polysulfones for proton-conducting fuel cell membranes, Macromol. Rapid Commun. 23 (2002) 896–900. [3] R.D. Yang, Z.S. Cao, S.W. Yang, I. Michos, Z. Xu, J.H. Dong, Colloidal silicalite-nafion composite ion exchange membrane for vanadium redox-flow battery, J. Membr. Sci. 484 (2015) 1-9. [4] X.G. Teng, J.C. Dai, J. Su, G.P. Yin, Modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow battery, J. Membr. Sci. 476 (2015) 20-29. [5] Q. Zhang, B.J. Liu, W. Hu, W. Xu, Z.H. Jiang, W. Xing, M.D. Guiver, Poly(arylene

ether)

electrolyte

membranes

bearing

aliphatic-chain-linked

sulfophenyl pendant groups, J. Membr. Sci. 428 (2013) 629-638. [6] G. Zhang, H.T. Li, W.J. Ma, L.Y. Zhang, C.M. Lew, D. Xu, Cross-linked membranes with a macromolecular cross-linker for direct methanol fuel cells, J. Mater Chem 21 (2011) 5511-5518. [7] Z.L. Li, X.C. Liu, D.M. Chao, W.J. Zhang, Controllable sulfonation of aromatic poly(arylene ether ketone)s containing different pendant phenyl rings, J. Power Sources 193 (2009) 477-482. [8] J.Y. Park, T.H. Kim, H.J. Kim, J.H. Choi, Y.T. Hong, Crosslinked sulfonated poly

23

(arylene ether sulfone) membranes for fuel cell application, Int. J. Hydrogen Energy 37 (2012) 2603-2613. [9] R.J. Wang, X.M. Wu, X.M. Yan, G.H. He, Z.W. Hu, Proton conductivity enhancement of SPEEK membrane through n-BuOH assisted self-organization, J. Membr. Sci. 479 (2015) 46-54. [10] M.S. Jung, T.H. Kim, Y.J. Yoon, C.G. Kang, J.Y. Lee, H.J. Kim, Y.T. Hong, Sulfonated poly (arylene sulfone) multiblock copolymers for proton exchange membrane fuel cells, J. Membr. Sci. 459 (2014) 72-85. [11] J. Wang, C. Zhao, M. Li, L. Zhang, J. Ni, W. Ma, H. Na, Benzimidazole-cross-linked proton exchange membranes for direct methanol fuel cells, Int. J. Hydrogen Energy 37 (2012) 9330-9339. [12] W. Qian, Y. Shang, M. Fang, S. Wang, X. Xie, J. Wang, W. Wang, J. Du, Y. Wang, Z.

Mao,

Sulfonated

polybenzimidazole/zirconium

phosphate

composite

membranes for high temperature applications, Int. J. Hydrogen Energy 37 (2012) 12919-12924. [13] W.H. Lee, K.H. Lee, D.W. Shin, D.S.Hwang, N.R. Kang, D.H. Cho, J.H.Kim, Y.M. Lee, Dually cross-linked polymer electrolyte membranes for direct methanol fuel cells, J. Power Sources 282 (2015) 211-222 [14] D.S. Kim, G.P. Robertson, M.D. Guiver, Comb-shaped poly (arylene ether sulfone)s as proton exchange membrane, Macromol. 41 (2008) 2126-2134. [15] D.W. Shin, S.Y. Lee, N.R. Kang, K.H. Lee, M.D. Guiver, Y.M. Lee, Durable sulfonated poly (arylene sulfide sulfone nitrile)s containing naphthalene units for

24

direct methanol fuel cells (DMFCs), Macromol. 46 (2013) 3452-3460. [16] K.S. Yoon, J.Y. Lee, T.H. Kim, D.M. Yu, S.K. Hong, Y.T. Hong, Multiblock copolymers based on poly (p-phenylene)-co-poly (arylene ether sulfone ketone) with sulfonated multiphenyl pendant groups for polymer electrolyte fuel cell (PEMFC) application, Eur. Polym. J. 66 (2015) 1-11. [17] H.Y. Yao, Y.H. Zhang, Y. Liu, K.Y. You, N.N. Song, B.J. Liu, S.W. Guan, Pendant-group cross-linked highly sulfonated co-polyimides for proton exchange membranes, J. Membr. Sci. 480 (2015) 83-92. [18] L. Cao, Q.Q. Sun, Y.H. Gao, L.T. Liu, H.F. Shi, Novel acid-base hybrid membrane based on amine-functionalized reduced graphene oxide and sulfonated polyimide for vanadium redox flow battery, Electrochimica Acta 158 (2015) 24-34. [19] H. Li, S. Zhang, Y. Li, Q. Bu, C. Gong, Novel polypyrrolones containing fluorenyl groups in the main chain: synthesis and characterization, High Perform, Polym. 24 (2012) 460-469. [20] P.X. Xing, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, S. Kaliaguine, Synthesis and characterization of poly (aryl ether ketone) copolymers containing (hexafluoroisopropylidene)-diphenol moiety as proton exchange membrane materials, Polymer 46 (2005) 3257-3263. [21] C. Wang, N. Li, D.W. Shin, S.Y. Lee, N.R. Kang, Y.M. Lee, M.D. Guiver, Fluorene-based poly (arylene ether sulfone)s containing clustered flexible pendant sulfonic acids as proton exchange membranes, Macromolecules 44 (2011)

25

7296-7306. [22] H.S. Dang, D. Kim, Cross-linked poly (arylene ether ketone) membranes sulfonated on both backbone and pendant position for high proton conduction and low water uptake, J. Power Sources 222 (2013) 103-111. [23] C.Y. Wang, D.W. Shin, S.Y. Lee, N.R. Kang, Y.M. Lee, M.D. Guiver, Poly (arylene ether sulfone) proton exchange membranes with flexible acid side chains, J. Membr. Sci. 405-406 (2012) 68-78. [24] S. Zhou, S.D. Hai, D. Kim, Cross-linked poly (arylene ether ketone) proton exchange membranes with high ion exchange capacity for fuel cells, Fuel Cells 12 (2012) 589-598. [25] M.Y. Li, G. Z, S. Xu, C.J. Zhao, M.M. Han, L.Y. Zhang, H. Jiang, Z.G. Liu, H. Na, Cross-linked polyelectrolyte for direct methanol fuel cells applications based on a novel sulfonated cross-linker, J. Power Sources 255 (2014) 101-107. [26] J.N. Ren, S.L. Zhang, Y. Liu, Y. Wang, Q.H. Wang, G.B. Wang, A novel crosslinking organic-inorganic hybrid proton exchange membrane based on sulfonated poly (arylene ether sulfone) with 4-amino-phenyl pendant group for fuel cell application, J. Membr. Sci. 434 (2013) 161-170. [27] J.M. Xu, H.L. Cheng, L. Ma, H.L. Han, Y.S. Huang, Z. Wang, Preparation and behavior of “molecular compound” through covalent crosslinking between amino and sulfonic groups in single copolymers, J. Polym. Res. 21 (2014) 1-11. [28] J.M. Xu, H.L. Cheng, L. Ma, H.L. Han, Z. Wang, Construction of a new continuous proton transport channel through a covalent crosslinking reaction

26

between carboxyl and amino groups, Int. J. Hydrogen Energy 38 (2013) 10092-10103. [29] C.J. Zhao, H.D. Lin, H. Na, Novel cross-linked sulfonated poly (arylene ether ketone) membranes for direct methanol fuel cell, Int. J. Hydrogen Energy 35 (2010) 2176-2182. [30] C. Zhang, X.X. Guo, J.H. Fang, H.J. Xu, M.Q. Yuan, B.W. Chen, A new and facile approach for the preparation of cross-linked sulfonated poly(sulfide sulfone) membranes for fuel cell application, J Power Sources 170 (2007) 42-45. [31] Z. Wang, H.Z. Ni, C,J. Zhao, M.Y. Zhang, H. Na, Physical and electrochemical behaviors of directly polymerized sulfonated poly(arylene ether ketone sulfone)s proton exchange membranes with different backbone structures, J. Appl. Polym. Sci. 112 (2009) 858-866. [32] H.L. Cheng, J.M. Xu, L. Ma, L.S. Xu, B.J. Liu, Z. Wang, H.X. Zhang, Preparation and characterization of sulfonated poly (arylene ether ketone) copolymers with pendant sulfoalkyl groups as proton exchange membranes, J. Power Sources 260 (2014) 307-316. [33] H.Y. Pan, Y.Y. Zhang, H.T. Pu, Z.H. Chang, Organic-inorganic hybrid proton exchange membranes based on polyhedral oligomeric silsesquioxanes and sulfonated polyimides containing benzimidazole, J. Power Sources 263 (2014) 195-202. [34] C.L. Gong, Y. Liang, Z.G. Qi, H. Li, Z.Y. Wu, Z.Y. Zhang, S.J. Zhang, X.D. Zhang, Y.F. Li, Solution processable octa(aminopheny)silsesquioxane covalently

27

cross-linked sulfonated polyimides for proton exchange membranes, J. Membr. Sci. 476 (2015) 364-372 [35] J.T. Wang, Y.K. He, L.P. Zhao, Y.F. Li, S.K. Cao, B. Zhang, H.Q. Zhang, Enhanced proton conductivity of nanofibrous composite membranes enabled by acid-base pairs under hydrated and anhydrous conditions, J. Membr. Sci. 482 (2015) 1-12. [36] J.E. Kim, D.J. Kim, Pendant-sulfonated poly (arylene ether ketone) (PAEK) membranes cross-linked with a proton conducting reagent for fuel cells, J. Membr. Sci. 405-406 (2012) 176-184. [37] X.F. Li, C.J. Zhao, H. Lu, Z. Wang, H. Na, Direct synthesis of sulfonated poly (ether ether ketone ketone)s (SPEEKKs) proton exchange membranes for fuel cell application, Polymer 46 (2005) 5820-5827. [38] X.Y. Li, K.H. Nan, S. Shi, H. Chen, Preparation and characterization of nano-hydroxyapatite/chitosan cross-linking composite membrane intended for tissue engineering, In. J. Biol. Macromol. 50 (2012) 43-49 [39] L. Geng, Y. He, D. Liu, X. Dai, C.L. Lv, Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells, Microporous Mesoporous Mater. 148 (2012) 8–14. [40] M.Y. Li, G. Zhang, H.P. Zuo, M.M. Han, C.J. Zhao, H. Jiang, Z.G. Liu, L.Y. Zhang, H. Na, End-group cross-linked polybenzimidazole blend membranes for high temperature proton exchange membrane, J. Membr. Sci. 423-424 (2012) 495-502.

28

[41] J.Y. Lee, J.H. Lee, S. Ryu, S.H. Yun, S.H. Moon, Electrically aligned ion channels in cation exchange membranes and their polarized conductivity, J. Membr. Sci. 478 (2015) 19-24. [42] L.M. Robeson, H.H. Hwu, J.E. Mcgrath, Upper bound relationship for proton exchange membranes: empirical relationship and relevance of phase separated blend, J. Membr. Sci. 302 (2007) 70-77. [43] W. Li, A. Manthiram, M.D. Guiver, B.J. Liu, High performance direct methanol fuel cells based on acid-base blend membranes containing benzotriazole, Electrochem. Commun. 12 (2010) 607-610. [44] Q. Zhang, F.X. Gong, S.B. Zhang, S.H. Li, Novel side-chain-type cardo poly (aryl ether sulfone) bearing pendant sulfoalkyl groups for proton exchange membranes, J. Membr. Sci. 367 (2011) 166-173. [45] C.J. Zhao, H.D. Lin, K. Shao, X.F. Li, H.Z. Ni, Z. Wang, H. Na, Block sulfonated poly(ether ether ketone)s (SPEEK) ionomers with high ion-exchange capacities for proton exchange membranes, J Power Sources 162 (2006) 1003-1009. [46] C.P. Liu, C.A. Dai, C.Y. Chao, S.J. Chang, Novel proton exchange membrane based on crosslinked poly(vinyl alcohol) for direct methanol fuel cells, J Power Sources 249 (2014) 285-298. [47] M. Eikerling, A.A. Kornyshev, A.M. Kuznetsov, J. Ulstrup, S. Walbran, Mechanisms of proton conductance in polymer electrolyte membranes, J. Phys. Chem. B, 105 (2001) 3646-3662. [48] C.L. Fang, X.N. Toh, Q.F. Yao, D. Julius, L. Hong, J. Y. Lee,

29

Semi-interpenetrating polymer network proton exchange membranes with narrow and well-connected hydrophilic channels, J Power Sources 226 (2013) 289-298.

30

Table 1 Monomer ratios of C-SPAEKS and Am-C-SPAEKS polymers

31

Bisphenol Sample

DS

4C-PH 4Am-PH

SDCDPS

DFB

x/y

+

+

+

+

+

+

8:2

A SPAEKS

40%

C-SPAEKS

40%

+

Am-C-SPAEKS-10

40%

+

+

+

+

9:1

Am-C-SPAEKS-20

40%

+

+

+

+

8:2

Am-C-SPAEKS-30

40%

+

+

+

+

7:3

x/y: Molar ratio of 4Am-PH (Bisphenol A) to 4C-PH +: This monomer was added. Table 2 Mechanical properties, oxidative stability and gel fraction of the crosslinked membranes, C-SPAEKS, and Nafion® 117 Young’s

Tensile

Oxidative

Gel

stability

Fraction

(RW %)

(%)

297.60

99

-

6.81 ± 0.17

91.70

-

4.87 ± 0.12

92.36

74.3

4.03 ± 0.10

94.19

80.6

2.64 ± 0.07

95.27

86.5

Elongation at Membranes

modulus

strength break (%)

Nafion® 117

(MPa)

(MPa)

185.00

37.43

1302.20 ±

43.30 ±

C-SPAEKS 37.76

1.30

1279.65 ±

46.52 ±

Am-C-SPAEKS-10 38.38

1.39

1323.78 ±

47.82 ±

Am-C-SPAEKS-20

Am-C-SPAEKS-30

40.56

1.44

1653.28 ±

52.39 ±

32

49.56

1.83

Table 3 Proton conductivity, methanol permeability and IEC of the membranes Proton conductivity Samples

1

Methanol permeability 7

(S cm )

IEC

2 1

(10 cm s ). (mmol/g)

25 °C

80 °C

25 °C

60 °C

Nafion® 117

0.057

0.087

29.40

-

0.90

C-SPAEKS

0.055

0.084

10.91

24.12

0.89

Am-C-SPAEKS-10

0.052

0.080

4.38

8.26

0.88

Am-C-SPAEKS-20

0.058

0.087

3.86

5.77

0.94

Am-C-SPAEKS-30

0.061

0.088

1.56

3.34

1.07

33

34

35

36

37

38

39

40

41

Graphical abstract

42

The series of novel crosslinked membrane showed continuous proton transport channels. Interestingly, with increasing degree of crosslinked, the proton transport channels increased. The membrane exhibited high proton conductivity and low methanol permeability due to the special structure. Highlight 

The amino and carboxyl groups were introduced into same chains by polycondensation.



We prepared novel crosslinked proton exchange membranes.



TEM images showed proton transport channels were formed after crosslinking.



Proton transport channels increase with increasing degree of crosslinked.

43



The membrane exhibited high proton conductivity and low methanol permeability.

44