Advances in Polymer Technology
Volume 2020 (2020), Article ID 4563860, 13 pages
https://doi.org/10.1155/2020/4563860

Research Article

3D Network Structural Poly (Aryl Ether Ketone)-Polybenzimidazole Polymer for High-Temperature Proton Exchange Membrane Fuel Cells

Junqiao Jiang,¹ Erli Qu,¹ Min Xiao,¹ Dongmei Han,1,2 Shuanjin Wang,¹ and Yuezhong Meng¹

¹The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
²School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China

Correspondence should be addressed to Shuanjin Wang; wangshj@mail.sysu.edu.cn and Yuezhong Meng; mengyzh@mail.sysu.edu.cn

Received 4 June 2020; Revised 20 July 2020; Accepted 29 July 2020; Published 14 August 2020

Academic Editor: Sagar Roy

Copyright © 2020 Junqiao Jiang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Poor mechanical property is a critical problem for phosphoric acid-doped high-temperature proton exchange membranes (HT-PEMs). In order to address this concern, in this work, a 3D network structural poly (aryl ether ketone)-polybenzimidazole (PAEK-cr-PBI) polymer electrolyte membrane was successfully synthesized through crosslinking reaction between poly (aryl ether ketone) with the pendant carboxyl group (PAEK-COOH) and amino-terminated polybenzimidazole (PBI-4NH₂). PAEK-COOH with a poly (aryl ether ketone) backbone endows superior thermal, mechanical, and chemical stability, while PBI-4NH₂ serves as both a proton conductor and a crosslinker with basic imidazole groups to absorb phosphoric acid. Moreover, the composite membrane of PAEK-cr-PBI blended with linear PBI (PAEK-cr-PBI@PBI) was also prepared. Both membranes with a proper phosphoric acid (PA) uptake exhibit an excellent proton conductivity of around 50 mS cm⁻¹ at 170°C, which is comparable to that of the well-documented PA-doped PBI membrane. Furthermore, the PA-doped PAEK-cr-PBI membrane shows superior mechanical properties of 17 MPa compared with common PA-doped PBI. Based upon these encouraging results, the as-synthesized PAEK-cr-PBI gives a highly practical promise for its application in high-temperature proton exchange membrane fuel cells (HT-PEMFCs).

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) have drawn much attention as clean power generation devices due to their many attractive features, such as high efficiency, high power density, and environmental friendliness for potential application as power sources in stationary transportation and portable devices [1–5]. Proton exchange membranes (PEMs) are key components of the PEMFCs, which play the main role of proton transport and fuel isolation. Compared with conventional low-temperature PEMFCs, high-temperature PEMFCs (HT-PEMFCs) operate at above 100°C, which made it possess many advantages, such as simple water and heat management, high CO tolerance, and less dependence on platinum catalysts [6–8].

Among all the types of HT-PEMs, phosphoric acid-doped polybenzimidazole (PA-PBI) membranes have been considered to be the most promising candidates for a high-temperature proton exchange membrane owing to its good chemical and thermal stability and excellent proton conductivity under anhydrous conditions at high temperature [9–12]. In order to obtain high proton conductivity, it is necessary to dope an excess amount of phosphoric acid (PA) in the membrane [13]. However, due to the strong plasticization of PA molecules, high proton conductivity is usually at the expense of the mechanical strength of the membrane [14–17]. Besides, the preparation of a high-strength PA-PBI membrane requires high molecular weight of PBI, which exhibits poor solubility in organic solvents and makes it fairly hard to prepare uniform casting solution [18]. Therefore, to realize high-performance HT-PEMs and high-efficiency HT-PEMFCs, it is one of the important issues to settle the balance between proton conductivity and mechanical properties of phosphoric acid-doped polymer membranes.

In fact, several strategies have been proposed to deal with the issue. Crosslinking is considered an effective strategy to enhance the properties of PA-doped membranes for HT-PEMFCs [19–23]. In the past few decades, a variety of crosslinkers have been widely studied, including small molecular and macromolecular crosslinkers. Small molecular crosslinkers such as epoxides [24, 25] and halides [26–28] are more frequently used for the improvement of the membrane properties. Compared with small molecular crosslinkers, macromolecular crosslinkers will give higher overall performance to HT-PEMs. Recently, Yang et al. [29] studied the covalently crosslinking PBI membrane using chloromethyl polysulfone as the macromolecular crosslinker. Their results show that the properties of the crosslinked membranes with high PA doping level are almost unchanged during the test. In addition, other macromolecular crosslinkers such as poly (vinyl benzyl chloride) [30] and bromomethylated poly (aryl ether ketone) [31] have also been adopted to prepare crosslinked PBI membranes. Unfortunately, most of these methods are based on the sacrifice of N-H sites on the imidazole ring of the PBI main chain, which ultimately impedes the absorption of phosphoric acid and proton transport [32]. All in all, it is an urgent demand to design a reasonable polymer framework and a suitable covalent crosslinker to satisfy the actual requirements of HT-PEMs. To our knowledge, poly (aryl ether ketone) (PAEK) is extensively used in the preparation of various electrolyte membranes in fuel cells due to its good thermal, mechanical, and chemical stability [33–36].

In this work, we prepared a 3D network structural poly (aryl ether ketone)-PBI (PAEK-cr-PBI) through crosslinking reaction between PAEK with the pendant carboxyl group (PAEK-COOH) and amino-terminated imidazole (PBI-4NH₂). For the purpose of enhancing the mechanical strength of a PA-doped membrane without the sacrifice of proton conductivity, a new crosslinker (PBI-4NH₂, polybenzimidazole terminated with amino groups) was designed and synthesized. This kind of crosslinked membrane gets rid of the binding of PBI as a polymer framework. Besides, poly (aryl ether ketone) has good thermal stability to meet the test requirements of HT-PEMFCs, and the 3D network structure of the membrane can maintain good mechanical properties after phosphoric acid doping. In addition, we also prepared the composite membrane PAEK-cr-PBI@PBI, which was obtained by blending PAEK-cr-PBI and PBI polymers. In order to better evaluate the performance of the as-synthesized PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes, we compared the well-documented PBI membrane with it. The properties of these membranes, including their solubility in common solvent, morphology analysis, thermal stability, oxidative stability, proton conductivity, and mechanical strength, were studied and compared.

2. Experiment

2.1. Materials and Chemicals

2.1.1. Materials

Poly (4,4-(diphenyl ether)-5,5-bibenzimidazole) (PBI, 6000 Pa·S) was obtained from Shanghai Shengjun Plastic Technology Co., Ltd. Figure 1 shows the molecular structure of PBI.

Figure 1: The molecular structure of poly (4,4-(diphenyl ether)-5,5-bibenzimidazole) (PBI).

2.1.2. Chemicals

4,4-Bis(4-hydroxyphenyl)-valeric acid (98%), phosphorus pentoxide (P₂O₅), polyphosphoric acid (PPA), phosphoric acid solution (85 wt%), and 1-methyl-2-pyrrolidinone (NMP) were obtained from Aladdin Chemistry Co. Ltd. 4,4-Difluorobenzophenone (99%), isophthalic acid (IPA), and 3,3-diaminobenzidine (DAB, 97%) were obtained from Shanghai Macklin Biochemical Co. Ltd. Potassium carbonate (K₂CO₃), toluene, isopropyl alcohol, tetrahydrofuran (THF), and hydrochloric acid (HCl) were purchased from Guangzhou Chemical Reagent Factory. Ammonia solution (25 wt%) was purchased from Tianjin Fuyu Fine Chemical Co. Ltd.

2.2. Synthesis

2.2.1. Synthesis of PAEK-COOH

Poly (arylene ether ketone) with the pendant carboxyl group (PAEK-COOH) was synthesized via the step condensation polymerization [33, 37, 38]. The synthetic pathway of PAEK-COOH is shown in Scheme 1(a); in a 250 mL three-neck round-bottom flask equipped with a magnetic stirrer, a condenser, a Dean-Stark trap, and a nitrogen inlet, 4,4-bis(4-hydroxyphenyl)-valeric acid (2.86 g, 0.01 mol) and 4,4-difluorobenzophenone (2.18 g, 0.01 mol) were dissolved in 40 mL DMSO and 45 mL toluene. Then, K₂CO₃ (3.46 g, 0.025 mol) was added to the reaction flask. Under an atmosphere of nitrogen, the mixture was heated to 145°C and maintained at this temperature for 4 h until the water was totally separated by means of a Dean-Stark trap using toluene. Then, the system temperature was elevated to 170°C to render polymerization for 22 h.

(a)

(b)

(c)

Scheme 1: Synthesis of (a) PAEK-COOH, (b) PBI-4NH₂, and (c) PAEK-cr-PBI.

Purification of the polymers was done by decantation of the supernatant from the reaction mixture after cooling to room temperature; then, HCl and THF were added at a ratio of 1 : 4; thus, the mixture was stirred until the solid polymer was completely dissolved. After the stirring, the orange-colored polymer layer floating on the top of the liquid phase was collected. The PAEK-COOH solution in THF (orange-colored polymer layer) was precipitated by dropping it into 500 mL of isopropyl alcohol in a 1 L beaker. The obtained white polymer was washed with isopropyl alcohol, followed by deionized water. Then, the purified product, PAEK-COOH, was dried in a 100°C dry oven for 24 h.

2.2.2. Synthesis of the Crosslinker (PBI-4NH₂)

The amine-terminated polybenzimidazole (PBI-4NH₂) was synthesized by the condensation reaction of 3,3-diaminobenzidine (DAB) with isophthalic acid (IPA) in polyphosphoric acid (PPA) [39, 40], as shown in Scheme 1(b). In a 100 mL dry three-neck flask equipped with a nitrogen inlet and a nitrogen outlet, 27 g of PPA and 7 g of phosphorus pentoxide were heated at 120°C under nitrogen flow and mechanically stirred about 4 h till a clear solution was observed. After cooling to room temperature, DAB (1.2856 g, 6 mmol) and IPA (0.4985 g, 3 mmol) were added; then, the reaction mixture was mechanically stirred, slowly heated to 190°C, and kept at this temperature for 20 h. After cooling to about 80°C, the viscous solution was slowly poured into the ice deionized water and then was neutralized to pH 7 with dilute solution of ammonium hydroxide solution (5 wt%). The product was filtered and washed with deionized water several times and dried in a freeze dryer for 2 days at minus 60°C to remove the residual solvents completely.

2.3. Preparation of Membranes

2.3.1. Preparation of the PAEK-cr-PBI Membrane

The synthesis route of PAEK-cr-PBI is shown in Scheme 1(c), and the PAEK-cr-PBI membrane was fabricated by a solution casting method, as shown in Scheme 2. The calculated result of the carboxyl equivalent in PAEK-COOH is 2.15 mmol/g. Firstly, PAEK-COOH (300.0 mg) was dissolved in 2-3 mL 1-methyl-2-pyrrolidinone (NMP) to form a uniformly transparent solution. Then, the PBI-4NH₂ (168.6 mg, the molar ratio of diamino to carboxyl is 1 : 1) was dissolved completely in 2 mL NMP. The total polymer concentration was controlled to be 8~15% (/). After that, the solution of PBI-4NH₂ was added dropwise into the solution of PAEK-COOH in NMP and then stirred for 4 h to obtain a uniform solution. The resulting polymer solution was cast onto a clean glass plate followed by heating at 80°C for 5 h to remove residual solvent and then a further thermal treatment at 150°C for 2 h and then put in a vacuum at 190°C for 12 h for further crosslinking. The obtained membrane was then peeled off, washed thoroughly with deionized water, and dried at 100°C for 24 h.

Scheme 2: Schematic diagram of the preparation procedure of the PA-doped 3D network structural PAEK-cr-PBI membrane.

2.3.2. Preparation of the PAEK-cr-PBI@PBI Membrane

Similar to the PAEK-cr-PBI membrane, the composite PAEK-cr-PBI@PBI membrane was also fabricated by a solution casting method. What needs to be explained here is that the mass ratio between PAEK-cr-PBI and PBI is 4 : 1; the detail of the preparation process is as follows: PAEK-COOH (300.0 mg) and PBI (117.2 mg) were dissolved in 3 mL 1-methyl-2-pyrrolidinone (NMP) to form a uniformly transparent solution. Then, the PBI-4NH₂ (168.6 mg) was dissolved completely in the 1-2 mL NMP. The rest of the procedure is the same as the preparation process of the PAEK-cr-PBI membrane.

2.3.3. Preparation of the PBI Membrane

The preparation method of the PBI membrane is the most convenient: 0.5 g PBI was dissolved in 5 mL NMP and stirred for 24 h at room temperature to obtain a homogeneous solution. Then, the polymer solution was cast on a flat glass plate and then dried at 80°C for 24 h.

2.4. Characterizations

2.4.1. Structural Characterization

¹H NMR spectra of the PAEK-COOH and PBI-4NH₂ were recorded on a Bruker DRX-500 NMR spectrometer with deuterated DMSO (DMSO-d₆) as the solvent and tetramethylsilane (TMS) as the standard at room temperature. The molecular weight of synthesized PAEK-COOH was measured using a gel permeation chromatography (GPC) system (Waters 515 HPLC Pump, Waters 2414 detector), and the measurement was carried out at 30°C with the THF solvent. The surface and cross-section morphologies of the membranes were recorded on scanning electron microscopy (FE-SEM, Quanta 400F).

2.4.2. Solubility Test

The crosslinking degree was characterized by measuring the solubility of the PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes in common solvents. The membranes were immersed in NMP for 12 h at 90°C. Then, the residual samples in the NMP solution were collected, dried, and weighed. The weight residue of the membranes was calculated by the following equation:

where is the weight of membranes after the test and is the weight of membranes before the test.

2.4.3. PA Uptake and Swellings

The PBI, PAEK-cr-PBI, and PAEK-cr-PBI@PBI membranes were dried in a vacuum oven at 80°C for 12 h. After measuring the weight, length, width, and thickness, they were immersed in 85 wt% phosphoric acid solution at 120°C for 15 h. The membranes were taken out and wiped with tissue wipes. After drying at 100°C for 12 h, PA uptake and the length, width, and thickness of each sample were measured immediately. Each type of membranes was tested for three samples, and the average value was reported. The PA uptake ratio of the membrane was calculated by the following equation:

where is the weight of the PA-doped membrane and is the weight of the undoped dry membrane. The dimensional swelling ratio of the membrane was calculated by the following equations:

where and represent the area and volume of the dry membrane, respectively, and and represent the area and volume of the wet membrane, respectively, after being immersed in 85 wt% phosphoric acid solution at 120°C.

2.4.4. Thermal Stability

The thermogravimetric (TGA) curves of membranes were obtained by Pyris 1 TGA (PerkinElmer) under the nitrogen atmosphere. All the samples were preheated at 100°C for 2 h to eliminate the absorbed water and organic solvents before testing. Then, the samples were tested from 50 to 800°C with a heating rate of 10°C min⁻¹.

2.4.5. Oxidative Stability

To test oxidative stability, the undoped membrane samples were immersed into Fenton’s reagent (3% H₂O₂ solution containing 4 ppm Fe²⁺) at 80°C for 40 h; then, the samples were removed from the solution and dried in a vacuum oven at 120°C for 12 h. Each type of membranes was tested for 3 samples, and the average value was reported. The residual weight of the samples represented the oxidative stability [18, 41].

2.4.6. Proton Conductivity

The proton conductivities of PBI, PAEK-cr-PBI, and PAEK-cr-PBI@PBI membranes were measured by electrochemical impedance spectroscopy (EIS) using a PGSTAT204 electrochemical workstation (AUT50992, AUTOLAB). The membrane sample was immersed in 85 wt% PA solution for 12 h at room temperature before testing. The impedance measurement was performed under anhydrous conditions from 120°C to 170°C in the frequency range of 0.1 Hz to 100 kHz. The conductivity is calculated by the following equation:

where (cm) is the distance between two electrodes, (Ω) is the membrane resistance obtained by a Nyquist plot, and (cm) and (cm) are the width and thickness of the membrane, respectively.

2.4.7. Mechanical Properties

The mechanical properties of PA-doped membranes were evaluated on a tensile tester (New SANS, Shenzhen, China) with a stretching speed of 10 mm/min under an ambient atmosphere. The size of all the samples is . Before testing, the PA-doped membranes were dried in a vacuum oven at 100°C for 24 h to remove any absorbed water or organic solvents. Each type of membranes was tested for 4 samples, and the average value was reported.

3. Results and Discussion

3.1. Chemical Structure Identification

From the GPC analysis, the number and weight average molecular weights of PAEK-COOH were 40,000 and 92,000 g mol⁻¹, respectively. This result suggests that the polymer possesses a high enough molecular weight to form a membrane.

The synthesized PAEK-COOH and PBI-4NH₂ were analyzed using ¹H NMR (500 MHz, DMSO-d6), and the presence of carboxyl and amino groups was confirmed as shown in Figure 1. As it can be seen in Figure 2(a), the protons in the benzene ring of the polymer main chain show chemical shifts at 7.06, 7.27, and 7.73 ppm; the NMR peak at 12.05 ppm comes from the proton in the carboxyl group of PAEK-COOH. In Figure 2(b), the broad peak at 13.10 ppm is attributed to the -NH proton of the benzimidazole groups. Peaks of 9.05-9.16 ppm are assigned to protons (H1) near the two benzimidazole groups. Peaks in the range of 6.93-6.50 ppm are ascribed to aromatic protons of benzene rings (H3, H4, and H5) at both ends. Peaks in the range of 8.28-7.41 are attributed to the remaining aromatic protons (H2-6). The broad peak at 3.55 ppm is attributed to protons of the terminal amino group. The results of peak distribution indicate that the desired PBI-4NH₂ was prepared successfully.

(a)

(b)

Figure 2: ¹H NMR spectra of (a) PAEK-COOH and (b) PBI-4NH₂.

The schematic representation of the possible crosslinking structures of the crosslinked membrane is shown in Figure 3. Because of the insolubility of the crosslinked membrane (PAEK-cr-PBI), it is difficult to confirm the imidazolization between PAEK-COOH and PBI-4NH₂ by the conventional analysis.

Figure 3: Schematic representation of the possible crosslinking structures of the PAEK-cr-PBI membrane.

The solubility test was carried out to ascertain the crosslinking degree of PAEK-cr-PBI and PAEK-cr-PBI@PBI successfully. It is easy to know that if a complete dissolution of the membrane occurs, there is no crosslinking in the membrane. On the other hand, if the membrane has a residual sample after a period of immersing, the membrane is approved to be crosslinked. As shown in Figure 4, PAEK-cr-PBI and PAEK-cr-PBI@PBI membrane samples maintained the structural integrity after immersing in NMP at 90°C for 12 h. The weight of the PAEK-cr-PBI and PAEK-cr-PBI@PBI sample was over 97% after treatment. These results confirm that the crosslinking occurred between PAEK-COOH and PBI-4NH₂. In addition, there was no significant dissolution of PBI in the PAEK-cr-PBI@PBI membrane, which may be owing to the network structure of PAEK-cr-PBI suppressing PBI molecular chain movement. On the contrary, the noncrosslinked membranes (PBI) completely dissolved in NMP in 2 h.

(a)

(b)

Figure 4: (a) Photographs and (b) gel content of membranes after the solubility test in NMP at 90°C for 12 h. (1) PBI, (2) PAEK-cr-PBI, and (3) PAEK-cr-PBI@PBI.

3.2. Morphology Analysis

The surface and cross-section morphologies of the prepared membranes were studied using SEM micrography. In Figure 5, it is observed that PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes show a completely homogenous and compact structure without pores as well as the PBI membrane, which is essential for separating the fuel and oxidant during fuel cell operation and benefits the fuel cell performance.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5: SEM micrographs of the surface (up) and cross-section (down) of PA-undoped membranes: (a, d) PBI, (b, e) PAEK-cr-PBI, and (c, f) PAEK-cr-PBI@PBI.

3.3. Thermal Stability

Thermal stability of polymer electrolytes is one of the most critical features for long-term durability of fuel cells, especially in high-temperature operation. The thermal stability of PA-undoped and PA-doped membranes was studied by TGA under the nitrogen atmosphere. As shown in Figure 6(a), two-step degradation can be observed in both undoped PAEK-cr-PBI and PAEK-cr-PBI@PBI membrane samples. The first weight loss beginning at around 250°C was attributed to the degradation of imidazole groups and carboxyl groups on the side chains, which suggested that the amino and carboxyl groups do not react completely. The second step observed at about 470°C was due to the degradation of the polymer main chain, including poly (aryl ether ketone) (PAEK) and poly (4,4-(diphenyl ether)-5,5-bibenzimidazole) (PBI). On the other hand, the undoped PBI membrane decomposed only in one step at around 500°C. After doping with PA, as shown in Figure 6(b), for all the membranes, weight loss appeared earlier at about 160°C because of the dehydration of phosphoric acid. Nonetheless, the degradation of polymer membranes itself was observed in the temperature range similar to that of PA-undoped polymer membranes. The TGA results show that both PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes exhibit excellent thermal stabilities and can be used for HT-PEMFCs.

(a)

(b)

Figure 6: TGA curves of (a) PA-undoped and (b) PA-doped membranes under the N₂ atmosphere.

3.4. Oxidative Stability

To estimate the long-term stability of PEMs, the oxidative stability of the membranes was measured by the Fenton test. During the fuel cell operation, the incomplete reduction of oxygen produces HO∙ and HO₂∙ radicals. Subsequently, these lively radicals attack the skeleton of the polymer membrane, which causes membrane degradation. The degradation of the membranes was investigated by determining the weight loss. As shown in Figure 7, the PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes show superior oxidative stabilities as compared to the PBI membrane. It is because the densely crosslinked structure reduces the possibility of radical species to attack the polymer chain. After 40 h of the Fenton test, only 79.9% of the initial weight of the linear PBI membrane remained. However, the residual weight values of PAEK-cr-PBI and PAEK-cr-PBI are both more than 95%. The results indicated that the polymer PAEK-cr-PBI exhibited excellent oxidative stability.

(a)

(b)

Figure 7: (a) Photographs and (b) residual weight of the membranes after the Fenton test: (1) PBI, (2) PAEK-cr-PBI, and (3) PAEK-cr-PBI@PBI.

3.5. PA Uptakes and Swellings

The proton transport of HT-PEMs depends on the dissociation of phosphoric acid, so the content of free phosphoric acid has a direct impact on proton conductivity of HT-PEM materials. The PA-doped membranes were obtained by immersing the dried membranes into 85 wt% phosphoric acid solution at 120°C for 15 h, and the PA uptake and swelling ratio of the membrane are summarized in Table 1. To make it easier to follow, the photographs of the PA-doped membranes and histogram of PA uptake of the PBI, PAEK-cr-PBI, and PAEK-cr-PBI@PBI membranes are shown in Figure 8.

Table 1: PA doping behavior of the membranes.

Membrane
PA uptake (%)
(%)
(%)

PBI

PAEK-cr-PBI

PAEK-cr-PBI@PBI

(a)

(b)

Figure 8: (a) Photographs of the PA-doped membrane and (b) PA uptake of the membranes: (1) PBI, (2) PAEK-cr-PBI, and (3) PAEK-cr-PBI@PBI.

As it can be seen in Figure 8, the highest PA uptake was obtained by the PBI membrane. However, the PA uptakes of the membranes synthesized in this study are not disappointing. As it can be seen in Table 1, the PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes have a PA uptake of 193% and 224%, respectively. Obviously, the PAEK-cr-PBI@PBI membrane, which is the membrane PAEK-cr-PBI blended with some PBI, did absorb more phosphoric acid than the PAEK-cr-PBI membrane without PBI.

In respect to the swelling of the membrane caused by doping PA molecules, the PAEK-cr-PBI membrane showed a volume swelling of below 80% and an area swelling of below 40%. The low swelling of the PA-doped PAEK-cr-PBI membrane probably benefited from its crosslinked structure and the skeleton stability of the poly (aryl ether ketone) matrix. However, the swelling of the membrane with PBI (PAEK-cr-PBI@PBI) was higher than that of the pristine PAEK-cr-PBI membrane. That may be due to the enhancement of PA uptake of the membrane or the plasticizing effect of phosphoric acid on the PAEK-cr-PBI framework.

3.6. Proton Conductivities

The proton conductivity measurement was carried out from 120 to 170°C in anhydrous condition. In order to evaluate the proton conductivity of crosslinked PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes, the well-documented PBI membrane was selected as the comparison. In Figure 9, the proton conductivities of all the membranes increase with the increase in the testing temperature as we expected. It is interesting to see that these three kinds of membranes have almost the same proton conduction ability, which demonstrates that the proton conductivity of the PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes is comparable to that of the traditional PBI membrane. However, there is a change observed in the slope above 150°C for the proton conductivity curves of all membranes, which may be due to the transformation of a proton conducting mechanism, because the condensed phosphoric acids such as the dimer (pyrophosphoric acid (H₄P₂O₇)), trimer (meta- or tripolyphosphoric acid (H₅P₃O₁₀)), and larger fused structures were formed when the temperature is over 150°C [1]. Obviously, when the temperature is over 150°C, the PAEK-cr-PBI membrane with the lowest PA uptake exhibits the highest proton conductivity, which illustrates that the 3D network structure of the membrane is conducive to proton conduction.

Figure 9: Proton conductivity of PA-doped membranes.

3.7. Mechanical Properties

It is well known that the excellent mechanical properties of HT-PEMs are essential for practical application. The typical stress-strain curves of the membranes at room temperature are shown in Figure 10, and the mechanical properties are displayed in Table 2. As shown in Figure 10, the PA-doped PAEK-cr-PBI membrane exhibits the best tensile strength of 17 MPa among the three kinds of membranes as expected. In the meanwhile, the PA-doped PAEK-cr-PBI@PBI membrane also showed fairly excellent mechanical strength of over 10 MPa. However, the tensile strength of the PA-doped PBI membrane is only about 3.5 MPa. These results show that the PA-doped 3D network structural PAEK-cr-PBI membrane has outstanding mechanical properties; moreover, it can be used as a support material to improve the mechanical properties of PA-doped PBI membrane without reducing the proton conductivity.

Figure 10: Mechanical properties of PA-doped membranes at room temperature.

Table 2: Summary of proton conductivities and mechanical properties of PA-doped membranes.

Membrane
PA uptake (%)
Proton conductivity (mS cm⁻¹)a
Tensile strength (MPa)b
Elongation (%)b

PBI

50

PAEK-cr-PBI

52

PAEK-cr-PBI@PBI

51

aMeasured at 170°C and under anhydrous condition. bMeasured at room temperature.

4. Conclusions

In this work, a 3D network structural PAEK-cr-PBI membrane was prepared and investigated as HT-PEMs for HT-PEMFCs. The 3D network structural membrane PAEK-cr-PBI and the composite membrane PAEK-cr-PBI@PBI both showed excellent thermal stability which is suitable for operation in the high-temperature range of 120~180°C. Moreover, the PAEK-cr-PBI and PAEK-cr-PBI@PBI membranes exhibit excellent proton conductivities of about 50 mS cm⁻¹ at 170°C, which are comparable to those of the traditional PBI membrane. As an encouraging result, the PA-doped PAEK-cr-PBI membrane showed superior mechanical properties of 17 MPa than the PA-doped linear PBI membrane, which is only 3.5 MPa. These results show that the as-synthesized PAEK-cr-PBI gives a highly practical promise for its application in HT-PEMFCs.

Data Availability

All of the data used to support the findings of this study are included within the article.

Conflicts of Interest

There is no conflict of interest.

Acknowledgments

This work was supported by the National Key Research and Development Program (2018YFA0702002), the Link Project of the National Natural Science Foundation of China and Guangdong Province (Grant No. U1601211), the National Key Research and Development Program (Japan-China Joint Research Program) (2017YFE9127900), the National Natural Science Foundation of China (51573215, 21506260, 21706294, and 21376276), the Natural Science Foundation of Guangdong Province (2016A030313354), Guangdong Science and Technology Department (Grant Nos. 2016A050503001, 2016B010114004, and 2017B090901003), the Guangzhou Science and Technology Plan Project (Grant Nos. 201804020025 and 201707010424), and the Fundamental Research Funds for the Central Universities (Grant Nos. 17lgjc37, 18lgpy32, and 19lgpy07).

References

W. Ma, C. Zhao, J. Yang et al., “Cross-linked aromatic cationic polymer electrolytes with enhanced stability for high temperature fuel cell applications,” Energy & Environmental Science, vol. 5, no. 6, pp. 7617–7625, 2012.

J. Yang, Q. Li, L. N. Cleemann et al., “Crosslinked hexafluoropropylidene polybenzimidazole membranes with chloromethyl polysulfone for fuel cell applications,” Advanced Energy Materials, vol. 3, no. 5, pp. 622–630, 2013.

S. Wang, C. Zhao, W. Ma et al., “Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells,” Journal of Materials Chemistry A, vol. 1, no. 3, pp. 621–629, 2013.

Z. Guo, X. Xu, Y. Xiang, S. Lu, and S. P. Jiang, “New anhydrous proton exchange membranes for high-temperature fuel cells based on PVDF–PVP blended polymers,” Journal of Materials Chemistry A, vol. 3, no. 1, pp. 148–155, 2015.

K. J. Kallitsis, R. Nannou, A. K. Andreopoulou et al., “Crosslinked wholly aromatic polyether membranes based on quinoline derivatives and their application in high temperature polymer electrolyte membrane fuel cells,” Journal of Power Sources, vol. 379, pp. 144–154, 2018.

S. S. Araya, F. Zhou, V. Liso et al., “A comprehensive review of PBI-based high temperature PEM fuel cells,” International Journal of Hydrogen Energy, vol. 41, no. 46, pp. 21310–21344, 2016.

A. Chandan, M. Hattenberger, A. el-kharouf et al., “High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) - A review,” Journal of Power Sources, vol. 231, pp. 264–278, 2013.

F. Liu, S. Wang, H. Chen et al., “Cross-linkable polymeric ionic liquid improve phosphoric acid retention and long-term conductivity stability in polybenzimidazole based PEMs,” ACS Sustainable Chemistry & Engineering, vol. 6, no. 12, pp. 16352–16362, 2018.

J. Fang, X. Lin, D. Cai, N. He, and J. Zhao, “Preparation and characterization of novel pyridine-containing polybenzimidazole membrane for high temperature proton exchange membrane fuel cells,” Journal of Membrane Science, vol. 502, pp. 29–36, 2016.

J. A. Asensio, E. M. Sánchez, and P. Gómez-Romero, “Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest,” Chemical Society Reviews, vol. 39, no. 8, pp. 3210–3239, 2010.

F. J. Pinar, P. Cañizares, M. A. Rodrigo, D. Úbeda, and J. Lobato, “Long-term testing of a high-temperature proton exchange membrane fuel cell short stack operated with improved polybenzimidazole-based composite membranes,” Journal of Power Sources, vol. 274, pp. 177–185, 2015.

S. Gao, H. Xu, T. Luo et al., “Novel proton conducting membranes based on cross-linked sulfonated polyphosphazenes and poly(ether ether ketone),” Journal of Membrane Science, vol. 536, pp. 1–10, 2017.

L. Vilčiauskas, M. E. Tuckerman, G. Bester, S. J. Paddison, and K. D. Kreuer, “The mechanism of proton conduction in phosphoric acid,” Nature Chemistry, vol. 4, no. 6, pp. 461–466, 2012.

J. Kerres and V. Atanasov, “Cross-linked PBI-based high-temperature membranes: stability, conductivity and fuel cell performance,” International Journal of Hydrogen Energy, vol. 40, no. 42, pp. 14723–14735, 2015.

Y. Cai, Z. Yue, X. Teng, and S. Xu, “Phosphoric acid doped crosslinked polybenzimidazole/modified graphene oxide composite membranes for high temperature proton exchange membrane applications,” Journal of The Electrochemical Society, vol. 165, no. 11, pp. F914–F920, 2018.

Y. Cai, Z. Yue, X. Teng, and S. Xu, “Radiation grafting graphene oxide reinforced polybenzimidazole membrane with a sandwich structure for high temperature proton exchange membrane fuel cells in anhydrous atmosphere,” European Polymer Journal, vol. 103, pp. 207–213, 2018.

Y. Cai, Z. Yue, Q. Jiang, and S. Xu, “Modified silicon carbide whisker reinforced polybenzimidazole used for high temperature proton exchange membrane,” Journal of Energy Chemistry, vol. 27, no. 3, pp. 820–825, 2018.

J. Jiang, X. Zhu, H. Qian et al., “Cross-linked sulfonated poly (ether ether ketone) electrolytes bearing pendent imidazole groups for high temperature proton exchange membrane fuel cells,” Sustainable Energy & Fuels, vol. 3, no. 9, pp. 2426–2434, 2019.

D. Joseph, N. N. Krishnan, D. Henkensmeier et al., “Thermal crosslinking of PBI/sulfonated polysulfone based blend membranes,” Journal of Materials Chemistry A, vol. 5, no. 1, pp. 409–417, 2017.

C.-H. Shen and S. L.-C. Hsu, “Synthesis of novel cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells,” Journal of Membrane Science, vol. 443, pp. 138–143, 2013.

Z. Yue, Y.-B. Cai, and S. Xu, “Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide- benzimidazole) for high temperature proton exchange membrane fuel cells,” International Journal of Hydrogen Energy, vol. 41, no. 24, pp. 10421–10429, 2016.

Z. Yue, Y.-B. Cai, and S. Xu, “Phosphoric acid-doped cross-linked sulfonated poly(imide-benzimidazole) for proton exchange membrane fuel cell applications,” Journal of Membrane Science, vol. 501, pp. 220–227, 2016.

Z. Yue, Y.-B. Cai, and S. Xu, “Proton conducting sulfonated poly (imide-benzimidazole) with tunable density of covalent/ionic cross-linking for fuel cell membranes,” Journal of Power Sources, vol. 286, pp. 571–579, 2015.

J. Yang, Y. Xu, P. Liu, L. Gao, Q. Che, and R. He, “Epoxides cross-linked hexafluoropropylidene polybenzimidazole membranes for application as high temperature proton exchange membranes,” Electrochimica Acta, vol. 160, pp. 281–287, 2015.

H.-L. Lin, Y.-C. Chou, T. L. Yu, and S. W. Lai, “Poly(benzimidazole)-epoxide crosslink membranes for high temperature proton exchange membrane fuel cells,” International Journal of Hydrogen Energy, vol. 37, no. 1, pp. 383–392, 2012.

C.-H. Shen, L. C. Jheng, S. L. C. Hsu, and J. Tse-Wei Wang, “Phosphoric acid-doped cross-linked porous polybenzimidazole membranes for proton exchange membrane fuel cells,” Journal of Materials Chemistry, vol. 21, no. 39, pp. 15660–15665, 2011.

Q. Li, C. Pan, J. O. Jensen, P. Noyé, and N. J. Bjerrum, “Cross-linked polybenzimidazole membranes for fuel cells,,” Chemistry of Materials, vol. 19, no. 3, pp. 350–352, 2007.

M. Han, G. Zhang, Z. Liu et al., “Cross-linked polybenzimidazole with enhanced stability for high temperature proton exchange membrane fuel cells,” Journal of Materials Chemistry, vol. 21, no. 7, pp. 2187–2193, 2011.

J. Yang, D. Aili, Q. Li et al., “Covalently cross-linked sulfone polybenzimidazole membranes with poly (vinylbenzyl chloride) for fuel cell applications,” Chem Sus Chem, vol. 6, no. 2, pp. 275–282, 2013.

S. Wang, C. Zhao, W. Ma et al., “Macromolecular cross-linked polybenzimidazole based on bromomethylated poly (aryl ether ketone) with enhanced stability for high temperature fuel cell applications,” Journal of Power Sources, vol. 243, pp. 102–109, 2013.

L. Wang, Z. Liu, Y. Liu, and L. Wang, “Crosslinked polybenzimidazole containing branching structure with no sacrifice of effective N-H sites: towards high-performance high-temperature proton exchange membranes for fuel cells,” Journal of Membrane Science, vol. 583, pp. 110–117, 2019.

C. M. Tuan and D. Kim, “Anion-exchange membranes based on poly(arylene ether ketone) with pendant quaternary ammonium groups for alkaline fuel cell application,” Journal of Membrane Science, vol. 511, pp. 143–150, 2016.

D. D. Tham and D. Kim, “C2 and N3 substituted imidazolium functionalized poly(arylene ether ketone) anion exchange membrane for water electrolysis with improved chemical stability,” Journal of Membrane Science, vol. 581, pp. 139–149, 2019.

G. Li, C. Zhao, C. Zhu et al., “Side-chain-type quaternized naphthalene-based poly (arylene ether ketone) s for anhydrous high temperature proton exchange membranes,” RSC Advances, vol. 6, no. 101, pp. 98854–98860, 2016.

L. Lin, Z. Chen, Z. Zhang et al., “New comb-shaped ionomers based on hydrophobic poly(aryl ether ketone) backbone bearing hydrophilic high concentration sulfonated micro-cluster,” Polymer, vol. 96, pp. 188–197, 2016.

D. Liu, Y. Xie, S. Li et al., “High dimensional stability and alcohol resistance aromatic poly (aryl ether ketone) polyelectrolyte membrane synthesis and characterization,” ACS Applied Energy Materials, vol. 2, no. 3, pp. 1646–1656, 2019.

N. Lee, D. T. Duong, and D. Kim, “Cyclic ammonium grafted poly (arylene ether ketone) hydroxide ion exchange membranes for alkaline water electrolysis with high chemical stability and cell efficiency,” Electrochimica Acta, vol. 271, pp. 150–157, 2018.

S. Zhu, Z. Chen, B. Han, G. Wang, Z. Jiang, and S. Zhang, “Novel nanocellular poly (aryl ether ketone) foams fabricated by controlling the crosslinking degree,” RSC Advances, vol. 5, no. 64, pp. 51966–51974, 2015.

S. Maity and T. Jana, “Polybenzimidazole block copolymers for fuel cell: synthesis and studies of block length effects on nanophase separation, mechanical properties, and proton conductivity of PEM,” ACS Applied Materials & Interfaces, vol. 6, no. 9, pp. 6851–6864, 2014.

W. Li, X. Guo, and J. Fang, “Synthesis and properties of sulfonated polyimide–polybenzimidazole copolymers as proton exchange membranes,” Journal of Materials Science, vol. 49, no. 7, pp. 2745–2753, 2014.

J. Jang, D.-H. Kim, M.-K. Ahn et al., “Phosphoric acid doped triazole-containing cross-linked polymer electrolytes with enhanced stability for high-temperature proton exchange membrane fuel cells,” Journal of Membrane Science, vol. 595, p. 117508, 2020.