Preparation, characterization and enhanced performance of functional crosslinked membranes using poly(vinyl alcohol) as macromolecular crosslinker for fuel cells

Abstract

Functional cross-linked membranes based on sulfonated poly(arylene ether ketone sulfone) containing pendant carboxyl groups (C-SPAEKS) and poly(vinyl alcohol) (PVA) as macromolecular crosslinker were fabricated. The covalent cross-linking reaction was realized by thermal treatment between carboxyl and hydroxyl groups. The cross-linked membranes exhibited favorable thermal stability, good mechanical property, positive dimensional stability and desirable single cell performance. Moreover, the cross-linked membranes show low methanol permeability coefficients ranged from 4.01 × 10⁻⁷ cm⁻² s⁻¹ to 8.71 × 10⁻⁷ cm⁻² s⁻¹ with the PVA weight ratio decreased from 50% to 10%, which were much lower than that of C-SPAEKS membrane (15.97 × 10⁻⁷ cm⁻² s⁻¹). The proton conductivities of all the cross-linked membranes were above 10⁻² S cm⁻¹ at 25 °C. Moreover, the cross-linked membranes exhibited higher relative selectivity than that of composite membranes, which indicated that this series of cross-linked membranes had potential application value and were expected to be used in fuel cells.

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Introduction

As an energy conversion device, proton exchange membrane fuel cells (PEMFCs) have received widespread attention.^1–3 During the past few decades, PEMFCs have been applied in vehicular transportation, portable power field and other application fields.⁴ In fuel cell system, polymer electrolyte membrane is one of the core components.^5,6 At present, the widely commercialized proton exchange membrane (PEM) is the perfluorosulfonic acid membranes such as Nafion manufactured by DuPont. In general, Nafion membranes show copious merits such as high proton conductivity under low temperature as well as hydrated conditions, desirable chemical property and favorable electrochemical stability.^7–9 However, some shortcomings have seriously hindered its further application.^10–12 For example, (i) the synthesis process is very complex which leads to high cost, (ii) the proton conductivity decreases sharply at high temperature and low humidity which cause the reduction of cell efficiency, (iii) the methanol permeability is serious at high temperature which results in impairing of cell performance. Therefore, it is urgent to develop such membrane materials which have low cost, desirable proton conductivity and suitable methanol permeability.

In recent years, numerous sulfonated aromatic polymer electrolyte membranes have been developed as alternative materials to Nafion. For example, sulfonated poly(arylene ether ketone sulfone)s,^7,10,13 sulfonated polyimides,^14,15 sulfonated poly(ether ketone)s,^16–18 sulfonated poly(ether sulfone)s,^19–22 and polybenzimidazoles^23–25 etc. have been developed and studied in detail. Kim et al. demonstrated the high proton conductivity of sulfonated poly(ether sulfone) with hydroxyl groups in their work.²⁶ McGrath et al., Watanabe et al. and Na et al. exhibited the excellent thermal and chemical stability as well as low methanol permeability of sulfonated aromatic polymer in their paper.^27–29 Wang et al. reported a series of sulfonated poly(arylene ether ketone sulfone) membranes which exhibited very low methanol diffusion and good prospects in PEMFC usages.^9,10 For sulfonated aromatic polymers, high degree of sulfonation (DS) is necessary to achieve abundant proton conductivity. Unfortunately, high DS always leads to bad dimensional stability, poor mechanical property and undesirable methanol crossover, which impede its practical applications.^30,31 Cross-linking is an effective method to solve the above problem. Especially, covalent cross-linking makes a great contribution to the improvement of the membranes' properties such as dimensional stability, thermal property and methanol crossover etc.^32,33 Na et al. and Lee et al.^32,34 demonstrated that the cross-linked membranes containing sulfonic acid groups show desirable dimensional stability, wonderful methanol prevention and suitable mechanical property. However, in cross-linking reaction, sulfonic acid groups may be consumed actually which leads to sharp decline in proton conductivity. Therefore, new cross-linking functional groups should be introduced to reduce the consumption of sulfonic acid groups.

In this work, we introduce carboxylic acid groups as cross-linking functional groups into polymer chain of sulfonated poly(arylene ether ketone sulfone). Poly(vinyl alcohol) (PVA) is selected as a macromolecular cross-linker due to its numerous functional hydroxyl groups, desirable methanol diffusion resistance properties, good solubility and film-forming properties.³⁴ Moreover, macromolecular cross-linker has a high molecular weight and contains large amounts of cross-linking points on its backbone. Therefore, the membranes can get high degree of cross-linking.³⁵ We studied the effect caused by different weight ratio of PVA on the properties of the membranes. The DS of the polymer was controlled by adjusting the feed ratio between 3,3′-disulfonated-4,4′-dichlorodiphenylsulfone (SDCDPS) and 4,4′-difluorodiphenylmethanone. The cross-linked membranes were obtained by thermal treatment at 120 °C. The properties of the cross-linked membranes including dimensional stability, mechanical property, water uptake, oxidative stability, methanol permeability, proton conductivity and the morphology were studied in detail.

Experimental

Materials

SDCDPS and 4-carboxylphenyl hydroquinone (4C-PH) were synthesized in our lab.^7,10 Other chemicals used in this work such as 4,4′-difluorobenzophenone, 2,2-bis(4-hydroxyphenyl) propane (bisphenol A) (AR grade), toluene (≥99.5%), N-methyl-2-pyrrolidinone (NMP) (99%), dimethyl sulfoxide (DMSO) (AR grade), HCl, PVA, and anhydrous K₂CO₃ (AR grade) were obtained from commercial sources.

Synthesis of sulfonated poly(arylene ether ketone sulfone) with carboxylic acid groups (C-SPAEKS)

Scheme 1 shows the synthesis route of C-SPAEKS polymer. The DS of the system was always kept in 80%. 4,4′-Difluorobenzophenone (4.140 g, 18 mmol), SDCDPS (5.892 g, 12 mmol), 4C-PH (1.38 g, 6 mmol), bisphenol A (5.472 g, 24 mmol), K₂CO₃ (4.140 g, 30 mmol), NMP (40.14 mL) and toluene (20 mL) were carefully added to a 250 mL three-necked flask equipped with a mechanical stirrer, reflux condenser under N₂ protection. The reaction mixture was refluxed for 4 h at 120 °C. When toluene was discharged, the reaction temperature was raised slowly and kept at 190 °C for several hours. The mixture was poured into 500 mL HCl (1 M) solution to precipitate the polymer when the system reaches a certain viscosity. The precipitates were ground to fine powders and washed with boiled deionized water for several times to remove the inorganic salts. Eventually, the polymer was dried at 60 °C for 48 h.

Scheme 1 Synthesis route of C-SPAEKS polymer.

Membrane preparation

C-SPAEKS and PVA were used for fabricating composite membranes. The C-SPAEKS/PVA composite membranes were prepared by changing the PVA weight ratio from 10% to 50%. For example, C/P-40 was fabricated by dissolving C-SPAEKS (1.2 g) and PVA (0.8 g) in 12 mL and 8 mL of DMSO, respectively, and stirred to form a homogeneous solution. Then, the two kinds of solution were mixed, cast onto a glass plate. The prepared membrane was placed in a vacuum oven at 60 °C for 48 h. The cross-linked membranes denoted as Cr-C/P-xx were prepared by thermal treatment at 120 °C for 8 h. The thickness of the membranes ranged from 50 μm to 80 μm. The preparation process of cross-linked membranes is shown in Scheme 2.

Scheme 2 The preparation process of cross-linked membranes.

Characterization

Fourier transform infrared spectroscopy (FT-IR) spectra were obtained by using a Vector-22 spectrometer equipped with the attenuated total reflectance (ATR) device. The ¹H NMR experiment were performed using a 400 MHz Bruker Avance III spectrometer. Deuterated dimethyl sulfoxide (DMSO-d₆) was used as the solvent. Tetramethylsilane (TMS) was used as the internal reference standard. Thermogravimetric analysis (TGA) was performed by using Pyris 1TGA (Perkin Elmer) analyzer (N₂ atmosphere). The heating rate (10 °C min⁻¹) was performed during the test accompanying heating range from 40 °C to 600 °C. JEM-1011 transmission electron microscope (TEM) and Agilent 5100 atomic force microscope (AFM) were performed to investigate the morphology of the membranes. In order to obtain the Ag⁺ forms membranes, the membranes were stain with silver ions by exchanging H⁺ of –SO₃H in 0.1 M AgNO₃ aqueous solution. And then the stained samples were washed with deionized water, and dried under vacuum at room temperature overnight. The Ag⁺ forms samples were embedded in Spurr's epoxy resin and cured overnight at 70 °C and then sectioned to yield slices of 100 nm thickness using an ultramicrotome and placed on copper grids.

Measurement

Mechanical property

In this paper, the method of mechanical property test was the same as our previous work.⁷ Each sample is required for at least three measurements and then an average was recorded.

Ion exchange capacity (IEC)

The IEC values were obtained by traditional titration method. In order to replace all H⁺, the specimens were soaked in aqueous NaCl (1.0 mol L⁻¹) for 24 h. Then the solution was titrated with 0.05 mol L⁻¹ NaOH, using phenolphthalein as an indicator. The IEC was calculated as follows:where V is the exhausted volume of NaOH; M equals 0.05 mol L⁻¹; W_dry is the weight of dry membranes.

Water uptake (WU) and swelling ratio (SR)

The WU and SR were determined by measuring the changes in weight and thickness, respectively. The experiment steps were reported in our previous work.⁷ The WU and SR were calculated as follows:where W_dry and W_wet represent the dry and wet weight of the membranes, respectively. T_dry and T_wet respectively represent the thickness of dry and wet membranes.

The λ represents the number of water molecules per sulfonic groups which is calculated by WU and IEC via the following relationship:³³

where n(H₂O) and n(SO₃⁻) are the mole number of H₂O and SO₃⁻ respectively, and 18 represents molecular weight of H₂O.

Gel fraction

The samples were immersed in DMSO at 80 °C for 24 h. Then the residual membranes were taken out, dried and weighed. Gel fraction was obtained from the ratio between residual weight and initial weight.

Oxidative stability

The specimens were immersed in Fenton's reagent (3% H₂O₂ solution containing 2 ppm FeSO₄) at 80 °C. The standard of oxidative stability was evaluated by recording the time that the membranes began to break when shaking the vials vigorously.

Proton conductivity

The test method of proton conductivity is the same as our previous work.⁷ Four-electrode AC impedance method was carried out in this work. The samples (immersed in deionized water for 24 h before the test) were sandwiched between Teflon molds. The proton conductivity (σ) was measured at different temperature (under 100% relative humidity) as follows:where L (cm) is the distance between two electrodes, R (ohm) is the membrane resistance, and S (cm²) is the membrane cross-sectional area.

Methanol permeability

An iron diffusion cell device was used to measure the methanol concentration. The device composed of two reservoirs (A and B), one side stored deionized water, and the other side stored methanol. The reservoirs were separated by membranes. Before the test, the membranes were soaked in deionized water for 24 h. The solutions were magnetically stirred invariably. The apparatus diagram is shown in Scheme 3. Agilent-6890N gas chromatograph was used to obtain the methanol concentration (in water side). The DK which represents methanol permeability coefficient was calculated as follows:where A (cm²) and L (cm) is the effective area and thickness of the membrane; C_B and C_A (mol L⁻¹) are the methanol concentrations of B and A, respectively; V_B (mL) is the volume of the permeated reservoir.

Scheme 3 The methanol diffusion apparatus diagram.

Results and discussion

Structure characterization of monomer and polymers

Fig. 1 shows the ¹H NMR spectra of 4C-PH monomer and C-SPAEKS polymer. The peak at δ 12.91 ppm was attributed to the proton of –COOH. The peaks between δ 6.62 and 7.94 ppm were attributed to the aromatic protons. The protons in phenolic hydroxyl groups were observed at δ 8.81 and 8.90 ppm. For C-SPAEKS polymer, the peaks at δ 8.26 and 7.74 ppm were assigned to the protons adjacent to –SO₃H and –COOH, respectively.

Fig. 1 ¹H NMR spectra of 4C-PH monomer and C-SPAEKS polymer.

The FT-IR spectrum of Cr-C/P-20 membrane is shown in Fig. 2. The peak at 1081.65 and 1020.23 cm⁻¹ were assigned to the O=S=O and S=O symmetrical stretching in –SO₃H groups. The peak at 1157.50 cm⁻¹ can be assigned to the sulfone groups stretching vibrations in polymer main chain. The absorption peaks at 1585.78 and 1494.91 cm⁻¹ can be attributed to the C=C stretching vibrations of benzene ring. The absorption band at 1651.62 cm⁻¹ can be assigned to the C=O stretching vibration of –COOH and ester groups. The peak at 1237.50 cm⁻¹ was assigned to C–O–C of ester groups. The absorption observed at 3399.59 cm⁻¹ was assigned to –OH stretching vibration (in PVA, –COOH and –SO₃H). The band at 2961.19 cm⁻¹ can be assigned to asymmetric –CH₂ stretching.

Fig. 2 FT-IR spectrum of Cr-C/P-20 membrane.

Thermal and mechanical properties

The thermal stability of the membranes is very important for its application in PEMFC, and the loss temperature of functional groups directly influences the performance of the membranes. TGA curves of PVA, C-SPAEKS and cross-linked membranes are shown in Fig. 3. For PVA, two weight loss stages were obvious: (i) the first stage is the evaporation of moisture around 100 °C, (ii) the second stage around 178 °C is the degradation of PVA polymer backbone. C-SPAEKS membrane underwent three weight loss steps, (i) firstly, evaporation of moisture and residual solvents (100–170 °C), (ii) secondly, the shedding of –SO₃H (at ∼261 °C), (iii) thirdly, the decomposition of C-SPAEKS backbone (around 489 °C). Cr-C/P membranes exhibited three weight loss stages. The first weight loss was ascribed to the loss of moisture. The second weight loss around 231 °C was corresponded to the loss of –SO₃H and ester groups. The third stage around 463 °C was assigned to the decomposition of polymer chain of PVA and C-SPAEKS. It is obvious in Fig. 3 that the residual weight of the cross-linked membranes decreased with increasing PVA content. The main reason is that the thermal stability of aliphatic PVA is poor, which directly leads to the decrease of residual weight of the cross-linked membranes. Moreover, Table 1 shows temperature leading to 5% weight loss (T_d5%) of PVA, C-SPAEKS and cross-linked membranes. The T_d5% values of cross-linked membranes (176–192 °C) are higher than that of PVA (152 °C), lower than that of C-SPAEKS (225 °C). Although the thermal property of Cr-C/P membranes decreased slightly, it can still meet the needs of the PEMFC.

Fig. 3 TGA curves of PVA, C-SPAEKS and cross-linked membranes.

Table 1 Mechanical property data and T_d5% of C-SPAEKS, PVA and cross-linked membranes

Membranes	Td5% (°C)	Young's modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
a Ref. 3.
C-SPAEKS	225	1922.24	60.25	9.63
PVA	152	—	—	—
Nafion 117^a	—	180.00	38.00	301.50
Cr-C/P-10	190	1445.58 ± 29.21	42.62 ± 1.22	27.01 ± 0.31
Cr-C/P-20	192	2936.34 ± 42.51	75.34 ± 2.21	43.23 ± 0.47
Cr-C/P-30	187	1082.81 ± 21.26	28.19 ± 0.53	68.55 ± 1.19
Cr-C/P-40	189	839.47 ± 8.96	23.34 ± 0.42	32.98 ± 0.58
Cr-C/P-50	176	813.24 ± 7.22	18.68 ± 0.19	17.99 ± 0.19

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Mechanical property is one of the most important factors in PEM applications. From a certain aspect, good mechanical property means a longer service life.³⁰ Fig. 4 shows the stress vs. strain curves of Cr-C/P-10, Cr-C/P-20, Cr-C/P-30, Cr-C/P-40 and Cr-C/P-50. Young's modulus, tensile strength and elongation at break of C-SPAEKS, Nafion and cross-linked membranes are exhibited in Table 1. The Cr-C/P series of membranes showed excellent mechanical stability accompany with high Young's modulus (813.24–2936.34 MPa) and tensile strength (18.68–75.34 MPa). All the cross-linked membranes showed much higher Young's modulus than that of Nafion 117 (Young's modulus = 180 MPa). The Young's modulus of Cr-C/P-20 is almost 16 times higher than that of Nafion 117. The mechanical property data and curves showed that the cross-linked membranes have desirable mechanical properties.

Fig. 4 Stress vs. strain curves of cross-linked membranes at 25 °C.

IEC, water uptake and swelling ratio

IEC is the key factor to determine the proton conductivity, and it plays an important role in the application of PEMFC. Table 2 exhibits the IEC values of C-SPAEKS, composite membranes and cross-linked membranes. The IEC values decreased with increasing PVA weight ratio. The IEC values of the cross-linked membranes were almost the same as that of the composite membranes. This proved that the number of –SO₃H did not change much after cross-linking reaction.

Table 2 IEC, gel fraction, λ and oxidative stability values of the membranes^a

Samples	IEC (mmol g^−1)		Gel fraction (%)		λ (H2O molecule/SO3^−)	Oxidative stability (min)
	a	b	a	b	b	a	b
a a: composite membranes, b: cross-linked membranes, ●: massive dissolved.
C-SPAEKS	1.82	—	0	—	—	110	—
C-SPAEKS/PVA-10	1.67	1.62	0	●	0.7	94	172
C-SPAEKS/PVA-20	1.53	1.49	0	34.6	2.2	83	163
C-SPAEKS/PVA-30	1.30	1.27	0	41.4	3.1	71	154
C-SPAEKS/PVA-40	1.09	1.04	0	60.0	4.7	58	139
C-SPAEKS/PVA-50	0.93	0.89	0	68.4	11.5	47	130

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The presence of water molecules has a vital role for proton transfer.³⁰ In vehicle mechanism, the water molecules are used to form hydronium ions such as H₃O⁺, H₅O₂⁺ and H₉O₄⁺. In Grotthuss mechanism, the formation of hydrogen bond network as the proton transfer jumping sites is also dependent on water molecules.³⁶ However, excessive water can cause the mechanical property to be poor, and the dimensional stability become worse, directly affect the application of PEM. Water uptake and swelling ratio of the cross-linked membranes are shown in Fig. 5. Both WU and SR of the Cr-C/P membranes were increased with elevated temperature and increasing PVA content. When the PVA content reached 50%, the WU and SR of Cr-C/P-50 membrane increased drastically, and reached 60.15% and 21.99% respectively. This is mainly because that PVA has strong hydrophilic property. There are a large number of hydrophilic hydroxyl groups in PVA main chain which leads to higher water absorption. Therefore, the membranes have high water uptake. In addition, from Table 2, the cross-linked membranes had low λ values which are cause by the low water absorption. The λ values increased with the increasing PVA weight ratio. This trend is consistent with the trend of water uptake. Although the WU and SR of the cross-linked membranes increased with increasing PVA content, the membranes still maintained desirable dimensional stability.

Fig. 5 Water uptake and swelling ratio of cross-linked membranes.

Gel fraction and oxidative stability test

Gel fraction data and oxidative stability values are shown in Table 2. It is obvious that all the composite membranes were completely dissolved in DMSO (in Fig. 6(a)). However, the cross-linked membranes were only partially dissolved in DMSO (in Fig. 6(b)). Moreover, the gel fraction values increased with increasing PVA weight ratio. This is because that the cross-linked points provided by PVA increase with increasing PVA content. At the same time, the degrees of cross-linking increase and cross-linked networks also increase. Thus, the gel fraction values exhibit a rising trend.

Fig. 6 Photographs of (a) gel fraction of composite membranes; (b) gel fraction of cross-linked membranes; (c) composite membranes immersed in Fenton's reagent at 80 °C for 2 h; (d) cross-linked membranes immersed in Fenton's reagent at 80 °C for 2 h.

The oxidative stability was evaluated by recording the time that the membranes began to break in Fenton's reagent. From Table 2, with the increscent PVA content, the times that the membranes began to break into pieces become shorter. This is mainly because that the oxidative stability is directly related to the water absorption and swelling ratio.⁷ Generally speaking, the membranes with high water absorption are easily attacked by free radicals because the water can decrease the density of the polymer chain.³⁷ Moreover, the instability of PVA aliphatic backbone directly leads to the low oxidative stability of the cross-linked membrane. From the photographs of Fig. 6(c) and (d), it can be seen that after the membranes were immersed in Fenton's reagent for 2 h, the cross-linked membranes can still maintain good shape; however, the composite membranes have been completely broken into pieces. Although the cross-linked membranes show decreased breaking time with increasing PVA content, they still have a much higher oxidative stability than that of the composite membranes.

Morphology

The micro-morphology of the membranes is closely related to their physical property, chemical property and electrochemical property. It is helpful to explore the mechanism of proton transfer in the membrane by illuminating the relationship between the microstructure and property.^7,38 Eisenberg's theoretical hypothesis is that sulfonic acid groups may gather into ion clusters, thus forming ionic transport channels.³⁹ In order to explore the relationship between the microstructure and property, both AFM and TEM were carried out, respectively.

For AFM test, tapping mode was performed in this work. Fig. 7 shows the microstructure of Cr-C/P-30. In AFM images, the black areas represent the soft region containing hydrophilic –SO₃H. The yellow areas represent hard structure of hydrophobic polymer backbone. As shown in Fig. 7, the obvious phase separation structures are conducive to the transmission of the protons.³⁹

Fig. 7 AFM tapping phase images of Cr-C/P-30 (2D view and 3D view).

Fig. 8 shows the TEM images of C/P-30 membrane and Cr-C/P-30 membrane. The black dots represent the hydrophilic domain containing sulfonic acid and carboxyl acid groups. The bright areas represent the hydrophobic polymer main chain.⁷ In TEM images, large hydrophilic ion clusters were observed. It can be seen from Fig. 8 that the morphology of the composite membranes and cross-linked membranes are almost the same. However, there are subtle differences between composite membranes and cross-linked membranes. After cross-linking reaction, the ion clusters decreased slightly. This may because of two reasons. One reason is that a number of –OH and –COOH groups are consumed in the cross-linking reaction, resulting in a decrease of ion clusters. Another reason is that the consumed –OH groups directly result in the reduction of hydrogen bonds in the system. Thus the formation of ion clusters is affected at a certain extent.⁶

Fig. 8 TEM images of (a) C/P-30 membrane and (b) Cr-C/P-30 membrane.

Proton conductivity, methanol permeability and relative selectivity

Proton conductivity is one of the most important application parameters of the membranes. Table 3 shows the proton conductivities of Nafion 117, composite membranes and cross-linked membranes. As commercial PEM materials, Nafion 117 possessed the highest proton conductivity. At 25 °C, the proton conductivities between composite membranes and cross-linked membranes were not significantly different. However, when the temperature elevated to 80 °C, the composite membranes began to dissolve, and the proton conductivities of composite membranes can not be measured. In contrast, the cross-linked membranes still kept in good shape, and the proton conductivities of cross-linked membranes were obtained. Fig. 9 shows the proton conductivity curves of Cr-C/P-10, Cr-C/P-20, Cr-C/P-30, Cr-C/P-40 and Cr-C/P-50 at different temperature. Proton conductivities showed an upward trend with increasing temperature. However, the proton conductivities decreased gradually with the increasing PVA weight ratio. This is because PVA itself does not contain conductive –SO₃H groups. With the increase of PVA content, –SO₃H groups in the chain of C-SPAEKS are diluted, which leads to the reduction of proton conductivities. In spite of a slight decrease in proton conductivity, it can still meet the requirements of PEM materials for fuel cells.⁷

Table 3 Experimental data of C-SPAEKS, composite and cross-linked membranes^b

Sample	Proton conductivity (S cm^−1)		Methanol permeability coefficient (cm^−2 s^−1)	Relative selectivity (S scm^−3)
	25 °C	80 °C	25 °C	25 °C
a Ref. 30.b ■: the membrane dissolved partially after several hours at 80 °C, ○: the membranes broken into pieces after several hours at 80 °C.
C-SPAEKS	0.036	0.082	15.97 × 10^−7	2.25 × 10^4
Nafion 117^a	0.059	0.110	24.1 × 10^−7	2.45 × 10^4
C/P-10	0.031	■	15.29 × 10^−7	2.03 × 10^4
C/P-20	0.027	○	14.02 × 10^−7	1.93 × 10^4
C/P-30	0.021	○	12.47 × 10^−7	1.68 × 10^4
C/P-40	0.017	○	10.95 × 10^−7	1.55 × 10^4
C/P-50	0.015	○	9.41 × 10^−7	1.59 × 10^4
Cr-C/P-10	0.030	0.076	8.71 × 10^−7	3.44 × 10^4
Cr-C/P-20	0.026	0.070	7.95 × 10^−7	3.27 × 10^4
Cr-C/P-30	0.021	0.061	6.82 × 10^−7	3.08 × 10^4
Cr-C/P-40	0.016	0.054	5.02 × 10^−7	3.59 × 10^4
Cr-C/P-50	0.014	0.047	4.01 × 10^−7	3.49 × 10^4

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Fig. 9 Proton conductivities of cross-linked membranes.

Methanol permeation is an important factor affecting the application of PEMFC. In the process of fuel cell operation, methanol infiltrate from anode to cathode which not only lead to the waste of fuel, but also make the catalyst poisonous. As a result, the performance of the fuel cells dropped dramatically which seriously affected its application.^30,40 From Table 3, Nafion 117 showed the highest methanol permeability coefficients (24.1 × 10⁻⁷ cm⁻² s⁻¹). The methanol permeability coefficients of cross-linked membranes were much lower than that of composite membranes and Nafion 117. Cr-C/P-50 exhibited the lowest methanol permeability coefficient which is only 1/6 of Nafion 117. The reason why the cross-linked membranes had such a low methanol permeation coefficient was that the cross-linked network structures formed by cross-linking reaction effectively inhibited the infiltration of methanol. In addition, hydrogen bonds also played an important role in hindering methanol crossover. Moreover, the introduction of PVA also makes a great contribution to the low methanol permeability coefficient. PVA main chain curled in methanol solution, resulting in the transmission channel of methanol blocked.⁷ The schematic diagram of hindering methanol crossover is shown in Scheme 4.

Scheme 4 Schematic diagram of blocking methanol crossover.

Relative selectivity is a crucial parameter to evaluate the comprehensive performance of the membranes. Generally, high selectivity implies good overall performance.³⁸ Fig. 10 exhibits the relative selectivity values of Nafion 117, composite membranes and cross-linked membranes. The cross-linked membranes showed higher selectivity (range from 3.08 × 10⁴ to 3.59 × 10⁴ S scm⁻³) than that of both Nafion 117 (2.45 × 10⁴ S scm⁻³) and composite membranes (range from 1.55 × 10⁴ to 2.03 × 10⁴ S scm⁻³). This is mainly because cross-linked membranes have desirable proton conductivity and much lower methanol permeability coefficient. According to the selectivity, it can be speculated that the cross-linked membranes possess desirable comprehensive performance.

Fig. 10 Relative selectivity of composite membranes and cross-linked membranes.

Single cell performance

It is necessary to measure and evaluate the fuel cell performance of the membranes before a large scale application.^41,42 The fuel cell performance of Cr-C/P-10, Cr-C/P-30 and Cr-C/P-50 are evaluated by single cell test. Fig. 11 shows the polarization curves and power density curves of Cr-C/P-10, Cr-C/P-30 and Cr-C/P-50 membranes. In the test, the cell temperature is 85 °C and the gas temperature is controlled at 90 °C. The maximum power densities of Cr-C/P-10, Cr-C/P-30 and Cr-C/P-50 membranes are 63.14 mW cm⁻², 54.02 mW cm⁻² and 39.94 mW cm⁻², respectively. From Fig. 11, the power density of the membranes showed an obvious downward trend with increasing PVA content. This may be due to the decreased proton conductivity.^41,42 The result shows that the cross-linked membranes have desirable single cell performance and are expected to be used in fuel cells.

Fig. 11 Polarization curves and power density curves for PEMFC with Cr-C/P-10, Cr-C/P-30 and Cr-C/P-50 membranes.

Conclusion

C-SPAEKS-based cross-linked membranes using PVA as macromolecular crosslinker were prepared. This series of cross-linked membranes show positive thermal property, good mechanical property, favorable dimensional stability and desirable single cell performance. It is noteworthy that Cr-C/P-40 exhibited the highest relative selectivity (3.59 × 10⁴ S scm⁻³) which is due to its much lower methanol permeability coefficient (5.02 × 10⁻⁷ cm⁻² s⁻¹) at 25 °C. Moreover, this series of cross-linked membranes exhibited desirable single cell performance which reflected their potential application value as PEM materials. According to all the experiment results in this work, this series of cross-linked membranes show very good prospects and are expected to be applied in fuel cells.

Acknowledgements

We are thankful for the financial support from the National Natural Science Foundation of China (Grant no. 51273024 and 51303015) and Program for New Century Excellent Talents in University of Ministry of Education of China.

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