Preparation and properties of branched sulfonated poly(arylene ether ketone)/polytetrafluoroethylene composite materials for proton exchange membranes

Abstract

Branched sulfonated poly(arylene ether ketone)s (BSPAEKs) exhibit excellent oxidative stability and solubility, making them suitable for proton exchange membranes (PEMs). However, the mechanical properties of branched membranes cannot fully satisfy the requirements of PEMs. In this work, BSPAEK/polytetrafluoroethylene (PTFE) composite membranes are prepared by casting a BSPAEK solution onto porous PTFE films that contain different concentrations of Triton surfactant to reinforce their mechanical properties. The properties of the composite membranes, including their mechanical properties, proton conductivities, oxidative stabilities, water uptake, thermal stabilities and swelling ratios, are investigated experimentally. The tensile strength of BSPAEK/PTFE-7 (7 wt% Triton as a surfactant) is 26.0 MPa, which is 2.1 times higher than that of a pristine membrane. In addition, the BSPAEK/PTFE composite membranes exhibit excellent dimensional and oxidative stabilities. The BSPAEK/PTFE-5 (5 wt% Triton as a surfactant) composite membrane is tested in a direct methanol fuel cell (DMFC), and it can yield a peak power density of 69.70 mW cm⁻² at 60 °C, which is somewhat comparable to those using Nafion membranes.

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1. Introduction

Proton exchange membrane fuel cells (PEMFCs) have been widely investigated for use as automotive, stationary, and portable power sources because of their low emissions and high conversion efficiency.^1–4 As one of the key components in PEMFCs, the proton exchange membrane (PEM) must possess excellent chemical and physical properties as well as high proton conductivity.^5,6 Over the past two decades, one of the most suitable PEM materials has been perfluorinated copolymers (e.g., DuPont's Nafion). However, their difficult synthesis and processing, high cost and low conductivity under low-humidity or high-temperature hinder their widespread applications.^7–9 Hence, considerable efforts have been devoted to developing new generation of membranes for use in PEMFCs. Among the many alternative membrane materials, sulfonated poly(arylene ether)s are promising because of their acceptable cost, high thermal stability and simplicity in synthesis and processing.^10,11 However, most of these materials not only become excessively water swollen in the wet state but also possess short lifetimes when subjected to a combination of hydrolysis and oxidative degradation.^12,13 Therefore, improving the durability of inexpensive membranes has become an active area of research on PEMs.

Cross-linking is widely accepted to be an effective method for improving the oxidative stability of membranes.^14–17 However, cross-linked membranes are generally insoluble in common organic solvents and are difficult to reprocess, which has restricted their development as commercial membranes. In recent years, our group has been dedicated to researching highly branched proton exchange membranes.^18–20 Studies have shown that branching is also an excellent method for improving the properties of membranes.^19–24 Compared to cross-linked membranes, branched membranes not only effectively improve the oxidative stability and conductivity but also exhibit good solubility in common organic solvents.¹⁹ Recently, the highest branched PEM with a 10% degree of branching (DB) was prepared by our group.²⁰ The oxidative stability and proton conductivity of the branched membranes increased with increasing DB. However, the mechanical properties of the highly branched membranes clearly decreased with increasing DB, and the tensile strength of the membrane with a 10% DB was quite low (11.5 MPa) and could not satisfy the requirements of PEMs. This might result from the decrease in chain entanglement with increasing branching due to the hard arms and short chains between the branching points.

A viable strategy for overcoming the shortcomings of branched PEMs is to introduce flexible side chains in the branched polymer, which could help increase the entanglement of the polymer chains and thereby improve the mechanical properties. Recently, a series of branched polymers with pendent acid groups on the side chains was successfully synthesized.²⁵ However, the method for preparing these polymers was relatively complicated, and the improvement in mechanical properties was not obvious. Considering the performance and cost of various strategies, a promising approach for solving this problem is preparing PTFE-based composite membranes. The porous PTFE reinforcement technique is an effective method for increasing the mechanical strength of membranes.^26,27 Reinforced composite membranes exhibit better mechanical properties and dimensional stability.^28,29 Nevertheless, the low surface energy and chemical inertness of PTFE, inhibit the combination of PTFE and non-fluorinated main-chain polymer, which are detrimental to the properties of composite membranes because the performance of the film strongly depends on the combination of the polymer and porous PTFE substrate.³⁰ To alleviate this problem, adding surfactant to the polymer solution is proven to be an effective way.³¹

Branched sulfonated poly(arylene ether ketone)s (BSPAEKs)/PTFE composite membranes were produced and investigated in this study. A BSPAEK with a 10% branching agent was synthesized through direct polycondensation reactions. The composite membranes were fabricated by casting the BSPAEK solution with various concentrations of the surfactant Triton® X-100, onto the porous PTFE membrane. The resulting functionalized PEMs were expected to possess excellent chemical and dimensional stabilities and improved mechanical strength. The properties of the composite membranes, including their proton conductivity, water uptake and single cell performance, were investigated systematically.

2. Experiments

2.1 Materials

Porous PTFE membranes (thickness of 15 ± 2 μm, mean pore size of 0.40 ± 0.05 μm) from Shanghai Minglie (China) were used as supporting materials for the composite membranes. Triton® X-100 (polyethylene glycol mono-4-octylphenyl ether) was purchased from Energy Chemical (China). 1,3,5-Triphenylbenzene, 4,4′-difluorodiphenyl sulfone (DFDS), 4-fluorobenzenesulfonyl chloride, sulfonated 4,4′-difluorobenzophenone (SDFBP) and bisphenol fluorene were purchased from commercial sources and were used as received. Toluene and DMAc were dried respectively over sodium wire and 4 Å molecular sieves prior to use. Anhydrous potassium carbonate was dried at 300 °C for 24 h in a furnace prior to use.

2.2 Synthesis of the highly branched polymers (BSPAEK)

1,3,5-Tris[4-(4-fluorophenyl sulfonyl)phenoxy] benzene (B₃) was synthesized from 1,3,5-triphenylbenzene and 4-fluorobenzenesulfonyl chloride using a method previously reported in the literature.³² Based on the B₃ monomer, BSPAEK with 10% branching agent was synthesized via direct polycondensation reactions. The polymerization procedure is described as follows. Bisphenol fluorene (1.40 g, 4.00 mmol), SDFBP (1.013 g, 2.40 mmol), DFDS (0.254 g, 1.00 mmol), B₃ (0.312 g, 0.40 mmol), and potassium carbonate (0.8278 g, 6.0 mmol) were introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (10 mL) and toluene (10 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the reaction temperature was increased to 170 °C, and the reaction continued for 3.5 h. After cooling to room temperature, the mixture was slowly poured into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h.

¹H NMR (400 MHz, DMSO, ppm): 8.14 (s, 0.6H), 8.04 (br, 0.3H), 7.84–7.96 (m, 2.1H), 7.60 (d, 0.6H), 7.34–7.49 (m, 3.0H), 7.15 (m, 2H), 7.04 (d, 0.5H), 6.95 (d, 2H), 6.84 (d, 0.6H).

2.3 Preparation of BSPAEK/PTFE composite membranes

A porous PTFE membrane was cut to a size of 15 cm × 15 cm on glass after cleaning by soaking in ethanol for 30 min at room temperature. The salt from the BSPAEK (2.5 g) was dissolved in DMAC (20 mL) with stirring at room temperature. Moreover, the surfactant Triton was added to the BSPAEK solution with different weight ratios to improve the wettability of the BSPAEK in the porous PTFE. The BSPAEK/PTFE composite membrane was prepared by casting the homogenous solution onto the PTFE membrane and then drying at 60 °C for 24 h under a vacuum. The resulting membranes were acidified with 1 M H₂SO₄ for 24 h to exchange Na⁺ with H⁺. Finally, the membranes were immersed in deionized water overnight to eliminate excess H₂SO₄ and stored in deionized water for testing purposes. The BSPAEK/PTFE composite membranes were denoted by BSPAEK/PTFE-x (x = 0, 3, 5, 7), where x represents the surfactant concentration (0, 3, 5, and 7 wt%) in the branched polymer.

2.4 Physical and chemical characterizations

The ¹H NMR spectra, reported in ppm, were recorded on a Varian 400 MHz NMR instrument using tetramethylsilane (TMS) as the internal standard. The thermal stability of the polymers was investigated at a heating rate of 10 °C min⁻¹ over the temperature range from 50 °C to 600 °C using a Q50 TGA instrument under a nitrogen atmosphere with a flow of 50 mL min⁻¹. The mechanical properties of the membranes were evaluated at room temperature using an electromechanical universal testing machine (CMT4204, MTS systems, China) at a strain rate of 2 mm min⁻¹. The samples were cut into a 15 × 4 mm² dumbbell shape. Surface and cross-sectional images of the membranes were obtained using a scanning electron microscope (SEM, SU-70, Hitachi) after freeze-fracturing the membranes in liquid N₂ and then coating them with a thin layer of gold (∼10 nm) using a sputter coater, which increased their surface conductivity and thereby improved the resolution and quality of the SEM images.

2.5 Water uptake and dimensional stability

The membranes were first dried under vacuum at 80 °C for 24 h to determine their masses and then immersed in deionized water at different temperatures for 24 h to determine the mass after absorption. The water uptake of the membranes was reported in weight percent, as determined using eqn (1):

WU = (W_s − W_d)/W_d × 100% (1)

where W_d and W_s are the masses of the membranes before and after water absorption, respectively.

The swelling ratio is described as the linear expansion rate of the wet membrane and can be calculated using eqn (2):

Swelling ratio (%) = (L_s − L_d)/L_d × 100% (2)

where L_d and L_s are the thicknesses of the membranes before and after water absorption, respectively.

2.6 Proton conductivity measurement

The proton conductivity of each hydrated film was measured using an impedance analyzer (Solartron 1260A) with a perturbation of 10 mV and a frequency range of 10 MHz to 500 Hz. The proton conductivity measurement of fully hydrated membranes was carried out with the cell immersed in liquid water. The testing temperature range of the impedance measurements was varied from 30 °C to 90 °C. The proton conductivity was calculated using eqn (3):

σ = d/RS (3)

where σ is the proton conductivity (S cm⁻¹), d is the distance between the two electrodes (d = 1.88 cm), R is the resistance of the membrane, and S is the cross-sectional area of the membrane (cm²).

2.7 Oxidative and hydrolytic stabilities

A small piece of the membrane sample was soaked in Fenton's reagent (3% H₂O₂ containing 2 ppm FeSO₄) at 80 °C. The oxidative stability was evaluated by recording the time when the membranes began to disappear and noting their change in weight after using Fenton's reagent at 80 °C for 1 h. The hydrolytic stability was also investigated by placing a membrane sample in boiling water.

2.8 Fuel cell test

To test the practical performance of the membranes in house, a direct methanol fuel cell (DMFC) fixture with an active area of 4 cm² was used. The anode and cathode electrodes were purchased from Johnson Matthey® with catalog numbers of 45 374 (2.7 mg cm⁻² Pt and 1.35 mg cm⁻² Ru) and 45 375 (2 mg cm⁻² Pt), respectively. The membrane electrode assembly was formed by the hot-press method at a temperature of 80 °C and a pressure of 4.0 MPa for 3.5 minutes. The polarization curves of the DMFC were recorded by an Arbin® BT-5HC multiple independent channel test station. During the fuel cell test, the operating temperature of the DMFC was controlled to be 60 °C; a 2 M methanol solution with a flow rate of 2 mL min⁻¹ and air with a flow rate of 50 sccm were fed to the anode and cathode, respectively.

2.9 Measurement of limiting methanol-crossover current

The limiting methanol-crossover current was measured by voltammetric method originally proposed by Ren et al.³³ To carry out this measurement, the cathode of the DMFC was supplied by deionized water at a flow rate of 1 mL min⁻¹ to generate an inert environment and to provide sufficient water for the methanol oxidation reaction (MOR) on the cathode. The flow rate of methanol solution on the anode and the operating temperature are the same as those in the polarization tests. During the measurement, the cell was charged at a voltage of 0.85 V to ensure the oxidation of permeated methanol from the anode and the charge current, known as limiting methanol-crossover current, was recorded until the current was stable.

3. Results and discussion

3.1 Preparation and morphology of the composite membranes

Highly branched polymers with a 10% DB were synthesized in this work. These membranes exhibit excellent properties for use as PEMs, except for their mechanical properties.²⁰ PTFE-reinforced BSPAEK composite membranes were successfully prepared by casting the BSPAEK and surfactant solution onto the porous PTFE to enhance the mechanical strength of the membranes. The porous PTFE film had a mean pore size of approximately 0.40 ± 0.05 μm, as shown in Fig. 1. The incompatibility of hydrophobic PTFE and partially hydrophilic BSPAEK hinders the penetration of the polymer solution into the support film. The addition of a surfactant can be a good solution to overcome this problem. The surfactant Triton possesses a chemical structure that consists of hydrophobic t-C₈H₁₇–C₆H₄ and hydrophilic –(OCH₂CH₂)₁₀OH, which causes Triton to act as a bridge between the hydrophobic PTFE and hydrophilic polymer, thereby improving the compatibility of these materials during preparation. BSPAEK/PTFE composite membranes were successfully prepared, and the thicknesses of the samples ranged from 64 to 74 μm. The cross-sectional morphologies of the composite membranes were observed using an SEM, as shown in Fig. 2. The resulting images showed that the Triton concentration had a direct impact on the preparation of the BSPAEK cast onto the PTFE membrane. When no Triton was present in the BSPAEK solution, the pores of the porous PTFE were not completely filled by BSPAEK. By increasing the Triton concentration in the BSPAEK solution, the degree of BSPAEK infiltration into PTFE increased. Moreover, the surface morphologies of the composite membrane are shown in Fig. 3. Compared to the surface of BSPAEK/PTFE-0, BSPAEK/PTFE-7 exhibited a relatively smooth surface after the addition of surfactant, indicating that the Triton can improve the compatibility of the polymer with PTFE. A corresponding pure polymer membrane (BSPAEK) was also prepared for later comparison.

Fig. 1 SEM image of PTFE.

Fig. 2 SEM images of the cross-sectional morphologies of the composite membranes.

Fig. 3 SEM images of the surface morphologies of BSPAEK/PTFE-0 and BSPAEK/PTFE-7.

3.2 Mechanical properties

It is essential for PEMs to possess adequate mechanical properties to support the electrodes. The mechanical properties of the BSPAEK/PTFE and BSPAEK membranes are shown in Fig. 4 and listed in Table 2. The BSPAEK membrane exhibited a tensile strength of 12.1 MPa. Compared to pristine BSPAEK, the BSPAEK/PTFE membranes exhibited improved mechanical properties with a tensile strength that ranged from 20.2 to 26.0 MPa. The high strength of the porous PTFE network structure might effectively bind to the polymer and hold the polymer chains. Moreover, the tensile strength of the composite membranes gradually increased as the concentration of Triton increased because the surfactant might have improved the connection between the BSPAEK and porous PTFE, thereby increasing the interfacial interaction (Fig. 2). The tensile strength of BSPAEK/PTFE-7 was 26.0 MPa, which is 2.1 times higher than that of the membrane that formed from pristine branched polymers under the same conditions. The tensile strength of the composite membranes was similar to that of Nafion-117 (25.7 MPa). These data indicate that using PTFE as a supporting material for the composite membranes can effectively improve the mechanical properties of BSPAEK.

Fig. 4 Tensile strengths of the membranes.

3.3 Water uptake and swelling ratio

The water uptake of PEMs plays an important role in proton transport. The water uptake and swelling ratio significantly affect the proton conductivity and mechanical stability. However, excess absorption by a film can create a high swelling ratio, which is detrimental to the mechanical stability of the PEM. Fig. 5 and Table 1 present the water uptakes of all the samples at varying testing temperatures. The water uptakes of all the membranes increased with temperature, and all the membranes had relatively high water uptakes because of the unique branched structure of BSPAEK.¹⁹ The pristine BSPAEK membrane exhibited a higher water uptake than the BSPAEK/PTFE composite membrane. This result could be attributed to the presence of the hydrophobic PTFE film on the composite membrane, which had difficulty in absorbing water molecules. In addition, when the Triton content was higher, the combination of BSPAEK and PTFE would be more compatible, thereby increasing the degree to which BSPAEK impregnated the PTFE and decreasing the water uptake (Fig. 6). At 80 °C, BSPAEK exhibited the highest water uptake of 46.3%, whereas the BSPAEK/PTFE-7 showed a water uptake of 34.3%.

Fig. 5 Water uptakes of the membranes.

Table 1 Properties of the membranes

Samples	Thickness (μm)	Water uptake (%)		Swelling ratio (%)		Proton conductivity (S cm^−1)
		30 °C	80 °C	30 °C	80 °C	30 °C	80 °C
BSPAEK/PTFE-0	64	34.1	40.6	11.6	13.8	0.17	0.36
BSPAEK/PTFE-3	70	32.2	38.3	10.6	12.9	0.15	0.32
BSPAEK/PTFE-5	74	31.0	37.0	10.3	12.4	0.14	0.31
BSPAEK/PTFE-7	67	28.9	34.3	9.5	11.5	0.11	0.27
BSPAEK	72	38.1	46.3	13.0	16.0	0.23	0.43

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Fig. 6 Diagrams of BSPAEK and BSPAEK/PTFE.

The swelling ratio of the branched membranes was determined at different temperatures to evaluate the dimensional stability of the membranes. Fig. 7 shows the swelling ratios of all the membranes, and the trends were consistent with those of the water uptakes. The swelling of BSPAEK/PTFE-7 was 9.58, which was lower than the 13.0% of BSPAEK at 30 °C. This result can be probably explained by the increasing amount of Triton from the compatibility of BSPAEK and PTFE, which limits the swelling volume and increases the dimensional stability. Furthermore, the swelling ratios of all the samples were lower than that of Nafion-117 (23.7%) at 80 °C because of the branched structure.

Fig. 7 Dimensional stabilities of the membranes.

3.4 Proton conductivity

Proton conductivity is a key characteristic of PEMFCs and directly affects the performance of fuel cells. The conductivity values were obtained from impedance spectroscopy. The proton conductivity of all the samples increased with increasing temperature, as shown in Fig. 8 and Table 1. Because the water uptake increased, protons were more feasibly transferred through enlarged proton channels.³⁴ In addition, the proton conductivity of the composite membranes was reduced compared to the pristine BSPAEK sample due to the poor conductivity of the porous PTFE film.^35,36 The proton conductivity of BSPAEK was 0.43 S cm⁻¹ at 80 °C, whereas that of BSPAEK/PTFE-7 was 0.27 S cm⁻¹.³⁷ In addition, it was found that increasing the amount of Triton in the composite membranes from 0 to 7% resulting in a decrease in the proton conductivity, which was consistent with the water absorption values. The presence of proton vehicles relatively decreased in the composite membranes, which caused the proton conductivity to gradually decrease. In addition, the proton conductivities of all the samples were higher than 0.10 S cm⁻¹, indicating that these membranes can be used as PEMs.

Fig. 8 Proton conductivities of the membranes.

3.5 Thermal properties

The thermal properties of the membranes were investigated via TGA under a nitrogen atmosphere, as shown in Fig. 9 and listed in Table 2. All the samples were preheated at 150 °C for 30 min in the TGA furnace to remove any moisture. The pristine PTFE porous film was thermally stable and displayed no degradation below 500 °C. The fluorocarbon bond was destroyed primarily above 550 °C.³⁸ The BSPAEK membrane exhibited two consecutive weight-loss stages. The first weight loss occurred from the decomposition of sulfonic acid groups, whereas the second weight loss step was caused by the breakdown of the main polymer backbone. The BSPAEK/PTFE film exhibited an extra stage of weight loss at approximately 530–590 °C compared to the BSPAEK film. This stage was related to thermal decomposition. Furthermore, the 5% weight loss temperatures of the BSPAEK/PTFE membranes were lower than those of the BSPAEK membrane, which may have been affected by the PTFE and surfactant. In general, all the BSPAEK/PTFE membranes had excellent thermal stabilities and were suitable for low temperature (<100 °C) fuel cells.

Fig. 9 TGA curves of BSPAEK, PTFE and BSPAEK/PTFE-5.

Table 2 Mechanical and oxidative stabilities of the membranes

Samples	Tensile strength (MPa)	Oxidation stability (h)	Weight residue^a (%)
a In Fenton's reagent for 1 h at 80 °C.
BSPAEK/PTFE-0	20.2	>8	99
BSPAEK/PTFE-3	22.3	>8	100
BSPAEK/PTFE-5	23.8	>8	99
BSPAEK/PTFE-7	26.0	>8	100
BSPAEK	12.1	5.35	100
Nafion-117	25.7	>8	100

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3.6 Oxidative and hydrolytic stabilities

The oxidative stability and durability of PEMs are extremely important for the performance and long-term use of PEMFCs. Free radicals, such as oxygen, hydroxide and peroxide, typically attack the hydrophilic domains of polymer membranes, degrading the polymer chain. The oxidative stability of the branched membranes was evaluated by measuring the time that elapsed before the membrane began to break after immersion in Fenton's reagent at 80 °C to determine whether the samples could withstand strong oxidizing environments during fuel cell operation. The results are presented in Table 2. It can be seen in Table 2 that all the composite membranes exhibited excellent oxidative stability (>8 h) in Fenton's reagent at 80 °C as compared to the pristine BSPAEK membrane (5.35 h). More importantly, it worth mentioning that the oxidative stability of the composite membranes is comparable to that of the Nafion-117 membrane (>8 h), which is one of the most commonly used proton-conducting membranes. Furthermore, the oxidative stability was also evaluated by recording the weight change after immersion in Fenton's reagent at 80 °C for 1 h. It is observed in Table 2 that all the samples exhibited nearly no weight changes: the weight residues of the composite membranes, pristine BSPAEK membrane and the Nafion-117 membrane are within the range of 99–100% suggesting the strong chemical stability of the in-house fabricated membranes. The hydrolytic stability was investigated by treating the membrane samples in boiling water for more than 8 days. The membranes did not change in shape or appearance after the treatment, implying that no hydrolysis had occurred during the treatment.

3.7 Single cell performance

BSPAEK/PTFE-5 and BSPAEK were used to fabricate MEAs, which were tested in a DMFC. The polarization curves of these MEAs are shown in Fig. 10. The DMFC with BSPAEK/PTFE-5 exhibited a peak power density of 69.70 mW cm⁻², which was somewhat comparable to those when using Nafion membranes,^39–42 suggesting the potential application of the composite BSPAEK/PTFE membranes in fuel cells. In addition, Fig. 10 shows that the performance of the DMFC with BSPAEK/PTFE-5 was considerably higher than that of the DMFC with BSPAEK, although the proton conductivity of BSPAEK/PTFE-5 was lower, as shown in Fig. 8. To understand this phenomenon, we measured the limiting methanol-crossover currents of these two DMFCs and the data are shown in Table 3. It is found that the limiting methanol-crossover current density of the BSPAEK membrane is 132.1 mA cm⁻², higher than that of the BSPAEK/PTFE-5 membrane (114.3 mA cm⁻²). This may be because the BSPAEK/PTFE composite membrane is based on the hydrophobic PTFE skeleton, which possesses a uniform surface morphology whereas the pristine BSPAEK membrane shows some cracks on the surface, which can intensify the methanol permeation. In addition, it should be mentioned that the actual methanol crossover current should be higher than this limiting crossover current as methanol in the cathode would be dragged to the anode by electro-osmosis.^33,43,44 As the electro-osmosis is proportional to current, the higher current means more methanol is dragged to the anode. As a result, the actual difference in the methanol-crossover rates between the BSPAEK/PTFE-5 and BSPAEK membranes would be larger when the electro-osmosis is taken into account. Another reason leading to the lower performance of the BSPAEK membrane may be its poor contact with the electrodes: only a small part of the BSPAEK membrane was stuck to the electrodes after hot pressing. The measured internal resistances of the DMFC with the BSPAEK/PTFE-5 membrane and BSPAEK membrane were 122 mΩ and 154 mΩ (Table 3), respectively, though the conductivity of the BSPAEK/PTFE-5 membrane was lower. This result suggested that the contact resistance of the electrode/membrane interface was relatively high for the DMFC based on the BSPAEK membrane. In addition, the poor contact between the membrane and electrode may lead to the loss of triple phase boundaries, which further decreased the cell performance of the DMFC with the BSPAEK membrane. In summary, the increased methanol crossover and the relatively poor contact could be the reasons for the lower performance of the DMFC with the BSPAEK membrane.

Fig. 10 Performance of the DMFCs with BSPAEK/PTFE-5 and BSPAEK membranes at 60 °C (anode/cathode: 2 M methanol/air).

Table 3 Internal resistances and limiting methanol-crossover current densities of the DMFCs based on the composite membrane and the pristine membrane

Samples	Internal resistance (mΩ)	Limiting methanol-crossover current density (mA cm^−2)
BSPAEK/PTFE-5	122	114.3
BSPAEK	154	132.1

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4. Conclusions

Composite membranes (BSPAEK/PTFE) were successfully prepared by casting the BSPAEK solution onto PTFE membranes that contained different concentrations of the surfactant Triton. Surface and cross-sectional characterizations were performed using an SEM, and the images revealed that the surfactant effectively improved the compatibility of porous PTFE and BSPAEK. TGA indicated that the composite membranes possessed excellent thermal stability, allowing their qualification in medium-temperature fuel cells. The tensile strength of the BSPAEK/PTFE composite membranes was considerably higher (20.2–26.0 MPa) than that of pristine BSPAEK. The tensile strength of BSPAEK/PTFE-7 was 26.0 MPa, which is 2.1 times higher than that of pristine membranes. Using PTFE as a supporting material to prepare composite membranes could significantly improve the shortcomings of BSPAEK. Although the presence of PTFE and a surfactant decreased the water uptake and proton conductivity of the composite membranes compared to pure BSPAEK, the swelling ratio of the composite membranes was lower than that of BSPAEK, and the composite membranes exhibited excellent oxidative stability. Moreover, the performance of the BSPAEK/PTFE-5 membrane in a DMFC was evaluated, and a peak power density of 69.70 mW cm⁻² was achieved, demonstrating that BSPAEK/PTFE composite membranes hold promise for use as practical proton exchange membranes in fuel cells.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51306125), the Natural Science Foundation of Guangdong Province (2015A030313546), Shenzhen Sci & Tech research grant (JCYJ20130329105010137, JCYJ20150331142303052 and ZDSYS201507141105130), Shenzhen Science and Technology Foundation (No. KQCX20140519105122378), Natural Science Foundation of SZU (No. 827-000015), Nanshan District special funds (FG2013JNYF0015A) and the Special Fund of the Central Finance for the Development of Local Universities (No. 000022070140).

References

1. J. Ni, M. Hu, D. Liu, H. Xie, X. Xiang and L. Wang, J. Mater. Chem. C, 2016, 4, 4814–4821.
2. B. Bae, T. Yoda, K. Miyatake, H. Uchida and M. Watanabe, Angew. Chem., Int. Ed., 2010, 49, 317–320.
3. T. J. Peckham and S. Holdcroft, Adv. Mater., 2010, 22, 4667–4690.
4. H. Ni'mah, W. Chen, Y. Shen and P. Kuo, RSC Adv., 2011, 1, 968.
5. S. Wang, F. Dong and Z. Li, J. Mater. Sci., 2012, 47, 4743–4749.
6. Y. Di, W. Yang, X. Li, Z. Zhao, M. Wang and J. Dai, RSC Adv., 2014, 4, 52001–52007.
7. H. Hou, M. L. Di Vona and P. Knauth, J. Membr. Sci., 2012, 423, 113–127.
8. B. P. Tripathi and V. K. Shahi, Prog. Polym. Sci., 2011, 36, 945–979.
9. Y. Zhao, Y. Fu, Y. He, B. Hu, L. Liu, J. Lü and C. Lü, RSC Adv., 2015, 5, 93480–93490.
10. X. Liu, X. Meng, J. Wu, J. Huo, L. Cui and Q. Zhou, RSC Adv., 2015, 5, 69621–69628.
11. R. Mukherjee, S. Banerjee, H. Komber and B. Voit, RSC Adv., 2014, 4, 46723–46736.
12. A. El-Kharouf, A. Chandan, M. Hattenberger and B. G. Pollet, J. Energy Inst., 2012, 85, 188–200.
13. J. Lawrence and T. Yamaguchi, J. Membr. Sci., 2008, 325, 633–640.
14. S. Lee, N. Kang, D. Shin, C. Lee, K. Lee, M. Guiver, N. Li and Y. Lee, Energy Environ. Sci., 2012, 5, 9795–9802.
15. X. Chen, P. Chen, Z. An, K. Chen and K. Okamoto, J. Power Sources, 2011, 196, 1694–1703.
16. H. Pan, S. Chen, Y. Zhang, M. Jin, Z. Chang and H. Pu, J. Membr. Sci., 2015, 476, 87–94.
17. L. Zhang, D. Qi, G. Zhang, C. Zhao and H. Na, RSC Adv., 2014, 4, 51916–51925.
18. H. Xie, D. Tao, X. Xiang, Y. Ou, X. Bai and L. Wang, J. Membr. Sci., 2015, 473, 226–236.
19. L. Wang, D. Wang, G. Zhu and J. Li, Eur. Polym. J., 2011, 47, 1985–1993.
20. H. Xie, D. Wang, D. Tao and L. Wang, J. Power Sources, 2014, 262, 328–337.
21. Y. Li, M. Xie, X. Wang, D. Chao, X. Liu and C. Wang, Int. J. Hydrogen Energy, 2013, 38, 12051–12059.
22. H. S. Park, D. W. Seo, S. W. Choi, Y. G. Jeong, J. H. Lee, D. Il Kim and W. G. Kim, J. Polym. Sci., Part A-1: Polym. Chem., 2008, 46, 1792–1799.
23. S. Matsumura, A. R. Hlil, C. Lepiller, J. Gaudet, D. Guay, Z. Shi, S. Holdcroft and A. S. Hay, Macromolecules, 2008, 41, 281–284.
24. K. Matsumoto, T. Higashihara and M. Ueda, Macromolecules, 2008, 41, 7560–7565.
25. D. Liu, D. Tao, J. Ni, X. Xiang, L. Wang and J. Xi, J. Mater. Chem. C, 2016, 4, 1326–1335.
26. B. Wang, C. Tseng, C. Shih, Y. Pai, H. Kuo and S. Lue, J. Membr. Sci., 2014, 464, 43–54.
27. H. Lin, J. Huang, Y. Chen, P. Su, T. Yu and S. Chan, J. Polym. Res., 2012, 19, 1–8.
28. H. Lin, Y. S. Hsieh, C. Chiu, T. Yu and L. Chen, J. Power Sources, 2009, 193, 170–174.
29. D. Xing, B. Yi, F. Liu, Y. Fu and H. Zhang, Fuel Cells, 2005, 5, 406–411.
30. C. Bi, H. Zhang, Y. Zhang and S. Xiao, J. Power Sources, 2009, 194, 838–842.
31. S. Lu, R. Xiu, X. Xu, D. Liang, H. Wang and Y. Xiang, J. Membr. Sci., 2014, 464, 1–7.
32. S. Matsumura, A. R. Hlil, C. Lepiller, J. Gaudet, D. Guay, Z. Q. Shi, S. Holdcroft and A. S. Hay, Macromolecules, 2008, 41, 281–284.
33. X. Ren, T. Springer, T. Zawodzinski and S. Gottesfeld, J. Electrochem. Soc., 2000, 147, 466–474.
34. J. Kim and D. Kim, J. Membr. Sci., 2012, 405–406, 176–184.
35. L. Chen, T. L. Yu, H. Lin and S. Yeh, J. Membr. Sci., 2008, 307, 10–20.
36. L. Huang, L. Chen, T. Yu and H. Lin, J. Power Sources, 2006, 161, 1096–1105.
37. K. D. Kreuer, Chem. Mater., 1996, 8, 610–641.
38. Y. Zhao, H. Yu, D. Xing, W. Lu, Z. Shao and B. Yi, J. Membr. Sci., 2012, 421, 311–317.
39. M. Higa, S. Feng, N. Endo and Y. Kakihana, Electrochim. Acta, 2015, 153, 83–89.
40. K. Dutta, S. Das and P. P. Kundu, J. Membr. Sci., 2015, 473, 94–101.
41. N. Bogolowski and J. Drillet, Chem. Eng. J., 2015, 270, 91–100.
42. X. Yan, T. Zhao, L. An, G. Zhao and L. Zeng, Int. J. Hydrogen Energy, 2015, 40, 16540–16546.
43. X. Yan, T. Zhao, L. An, G. Zhao and L. Zeng, Electrochim. Acta, 2014, 139, 7–12.
44. X. Zhao, W. Yuan, Q. Wu, H. Sun, Z. Luo and H. Fu, J. Power Sources, 2015, 273, 517–521.

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