Crosslinkable polymeric ionic liquid improve phosphoric acid retention

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Crosslinkable polymeric ionic liquid improve phosphoric acid retention and long-term conductivity stability in polybenzimidazole based PEMs Fengxiang Liu, Shuang Wang, Hao Chen, Jinsheng Li, Xue Tian, Xu Wang, Tiejun Mao, Jingmei Xu, and Zhe Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03419 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Sustainable Chemistry & Engineering

Crosslinkable polymeric ionic liquid improve phosphoric acid retention and long-term conductivity stability in polybenzimidazole based PEMs

Fengxiang Liu,a Shuang Wang,*ab Hao Chen,a Jinsheng Li,b Xue Tian,a Xu Wang,a Tiejun Mao,a Jingmei Xuab and Zhe Wang*ab

a. College of Chemical Engineering, Changchun University of Technology, no. 2055 Yanan avenue, Changchun 130012, China. b. Advanced Institute of Materials Science, Changchun University of Technology, no. 2055 Yanan avenue, Changchun 130012, China. *Corresponding authors, e-mail addresses: [email protected] (S. Wang), tel.: 86 431 85716155; fax: 86 431 85716155.

Abstract A series of composite cross-linked membrane based on fluorine-containing polybenzimidazole (6FPBI) and a crosslinkable polymeric ionic liquid (cPIL) have been prepared for high temperature proton exchange membrane (HT-PEM) applications. Particularly, the obtained composite cross-linked membranes showed excellent phosphoric acid doping ability and proton conductivity. Based on the trade-off between mechanical strength and proton conductivity of composite membranes, the optimal content of cPIL is 20 wt.% (6FPBI-cPIL 20 membrane). For instance, the 6FPBI-cPIL 20 membrane with a PA doping level of 27.8 exhibited a proton conductivity of 0.106 S cm-1 at 170 °C, which is much higher than that of pristine 6FPBI membrane. The most outstanding contribution of this work is that the 6FPBI-cPIL membranes showed improved phosphoric acid retention and long-term conductivity stability under harsh conditions (80 °C / 40 % RH) for 96h. In particular, the proton conductivity and PA doping level of the 6FPBI-cPIL 20 membrane remained at a high level of 0.064 S cm-1 and 8.5 after 96 h of the test, respectively.

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Keywords: polybenzimidazole; crosslinkable polymeric ionic liquid; phosphoric acid retention;

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long-

term conductivity stability;

Introduction Proton exchange membrane fuel cells (PEMFCs) have garnered significant attention as green energy supply devices for stationary and mobile applications.1–6 The core component of PEMFCs is proton exchange membranes (PEMs) that separate the anode from the cathode and conduct protons. Nafion, the most commercially successful PEM product, exhibits excellent proton conductivity only at low temperatures and high humidity. Therefore, the PEMFCs that use Nafion requires humidified inlet streams and large radiators to dissipate waste heat,7,8 especially its high cost hinder the large-scale applications. In recent years, PEMs research works mainly focus on obtaining high proton conductivity under low-humidity or non-humidified conditions.8–11 C. H. Park and co-workers have previously reported that a polymer membrane can improve the water retention by nanometer-scale cracks in a hydrophobic surface coating to challenge low-humidity operating conditions.12 However, these PEMs could not operate under higher temperature (above 100 °C). High-Temperature proton exchange membrane fuel cells (HT-PEMFCs) were developed for operation at elevated temperature (100–200 °C) and anhydrous environment. Compared to the well-developed PEMFCs technology typically operating at 80 °C, the HT-PEMFCs have many advantages,13,14 such as a simple water and cooling system, higher catalytic activity and enhanced tolerance to fuel impurities (e.g. CO). Despite these benefits, PEMFCs operating at elevated temperatures need to meet some technical challenges for PEMs. Firstly, elevated operating temperature means a higher thermal, chemical and oxidation stability requirement for polymer materials. Secondly, PEMs play a critical role of separating the anode from the cathode, which requires the adequate mechanical strength. Finally and probably most importantly, water have a negligible contribution to proton conductivity since water evaporates above 100 °C. Therefore, PEMs with high proton conductivity are necessary even under anhydrous conditions.


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Acid-base polyelectrolytes for fuel cells have been recognized as a promising solution to meet these technical challenges,15 and one of the most successful examples is phosphoric acid-doped polybenzimidazole (PBI).16–19 It is worth noting that neat phosphoric acid (H3PO4) is the compound with the highest intrinsic proton conductivity, which is mainly due to the combination of polarizable hydrogen bonds and a dense hydrogen bond network.19–21 In phosphoric acid-doped polybenzimidazoles (PA/PBI) membranes, PBI acting as a strong Bronsted base bonded with neat PA through acid-base interaction. The proton conduction process of PA/PBI doesn’t rely on H2O molecular, which makes PA/PBI membranes can exhibit excellent proton conductivity even under elevated temperature (above 100 °C) and anhydrous conditions.16–19 PA/PBI membranes for HT-PEMFCs applications could be traced back to the work of Savinell et al.,22 and these membranes exhibited relatively high proton conductivity , excellent oxidative and thermal stability. Despite these attractive properties, the electrochemical performance of PA/PBI membranes is relatively lower than that of Nafion membranes. Various reports aim to enhance the electrochemical performance of PA/PBI membranes have been widely carried out.23–28 As a potential method, introduction of ionic liquids (ILs) is particularly advantageous to enhance the electrochemical performance of PEMs. Ionic liquids (ILs) are room-temperature molten salts, which have recently attracted considerable attention for application in PEMFCs due to their excellent electrochemical properties.29–31 PEMs based on PA/PBI and ILs exhibit high proton conductivity at elevated temperatures.30,32–34 However, the proton conductivity of such membranes deteriorates after long-term usage due to ILs leaching out from the membranes.32,33 This issue can be addressed if a portion of the ILs are immobilized in membranes, and many groups, including our group, have made some attempts to prepare functional ILs on the silica particles.35–38 However, the incorporation amount of the IL groups is constrained by the aggregation of silica particles. In addition, the exudation of ILs can be avoided by polymeric ionic liquids (PILs), which are composed of polymer cations and functional anions. PILs hold some of the unique properties, for instance, PILs are anticipated to provide a continuous fast pathway for proton transmission, because the anions on the PILs act as proton acceptors. When PIL is introduced into the polymer matrix, a double or multiple proton transport channels will be



formed and the defects of IL leakage will be addressed.39–42 Moreover, the synergistic effect of PILs and PBI with covalent cross-linking interaction is promising to obtain PEMs with enhanced physical and electrochemical properties. In this study, we report the preparation of a composite cross-linked membrane based on fluorinecontaining PBI (6FPBI) and a crosslinkable PILs, a copolymer of allyl glycidyl ether and [ViBuIm][TFSI] (cPIL). The polymer chains of 6FPBI and cPIL are linked to each other by the covalent bonds. We predict that cPIL will provide continuous pathways for proton transport. By controlling the amount of cPIL in the 6FPBI based matrix, we have prepared a series of 6FPBI-cPIL composite membranes for further systematically study, including the thermal and chemical stability, mechanical property, micromorphology and electrochemical properties of these composite membranes. To simulate conditions of actual application, we tested the long-term conductivity stability of the membranes under harsh conditions.

Experimental Materials 2, 2-bis-(4-carboxyphenyl) hexafluoro propane was purchased from Apollo scientific. 3, 3’diaminobenzidine (DAB), allyl glycidyl ether, 1-chlorobutane, 1-vinylimidazole, deuterated dimethyl sulfone (DMSO-d6) were obtained from Aladdin. Lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI), azobisisobutyronitrile (AIBN) and polyphosphoric acid (PPA) were provided by Macklin. Other reagents were purchased from local suppliers. All the reagents were used without further purification.

Polymerization of the 6FPBI As shown in Scheme 1, 6FPBI was synthesized by a typical polycondensation reaction. The preparation details are as follows: 173.28 g of PPA was heated and stirred at 140 °C in a 250 mL four-necked flask, equipped with a mechanical stirrer, a thermo-couple, a drying tube and a N2 inlet. 4.29 g (20 mmol) of 3, 3’-diaminobenzidine was dissolved in the PPA. The solution was stirred until it became


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homogeneous. Subsequently, 7.86 g (20 mmol) of 2, 2-bis-(4-carboxyphenyl) hexafluoro propane and 3.79 g of P2O5 were added into the solution. Then, the mixture was further heated to 200 °C for 8 h. The resulting viscous solution was slowly poured into 1 L deionized water, and a tough filament polymer was obtained. The resulting filament polymer was cut and repeatedly boiled for several times in deionized water, and then the polymer was dried in vacuum oven at 100 °C for 24 h. The detailed synthetic procedure follows the previous work.43

Scheme 1 Chemical structure of 6FPBI.

Preparation of the cPIL The preparation of cPIL could be divided into three steps: synthesis of [ViBuIm]Cl, synthesis of [ViBuIm]TFSI by anion exchange reaction and polymerization of cPIL through free radical polymerization, as shown in Scheme 2. The synthesis of [ViBuIm]Cl and [ViBuIm]TFSI was accomplished by a typical reported reaction.44–47 Firstly, a mixture of 1-vinylimidazole (9.1 mL, 0.1 mol) and 1-chlorobutane (12.5 mL, 0.12 mol) was heated and stirred at 60°C for 72 h under nitrogen protection. The resulting liquid product [ViBuIm]Cl was extracted with toluene and acetone to remove the unreacted 1-vinylimidazole and 1-chlorobutane, then, dried at 50 °C for 24 h under vacuum. The chemical structures of the synthesized [ViBuIm]Cl was confirmed by 1H NMR analysis. Secondly, the obtained [ViBuIm]Cl and lithium salt of bis-(trifluoromethanesulfonyl) imide (LiTFSI) were dissolved in deionized water at a molar ratio of 1:1 and carried out the anion exchange reaction at room temperature. After 3 hours, the lower oily liquid product [ViBuIm]TFSI was extracted and dried at 50 °C for 24 h under vacuum. Finally, the monomer [ViBuIm]TFSI (0.09 mol), allyl glycidyl ether (0.01 mol) and initiator (0.3 wt.%), azobisisobutyronitrile (AIBN), were added into a 50 mL two-neck flask with a condenser and a vent of N2. The mixture was magnetically stirred in an oil bath of 70 °C for 24



h. After the copolymerization completed, the obtained dark viscous liquid was dried in a vacuum oven at 50 °C for 24 h.

Scheme 2 The synthesis process of cPIL

Membranes preparation The 6FPBI was dissolved in DMAc with continuous stirring until the solution became homogeneous, and then the cPIL was added and stirring continued. The dense composite membranes were obtained by the solution casting method on a glass plate in a 90 °C vacuum oven for 24 h. The resulting membranes were then allowed to react at 150 °C in an oven for 9 h to cause cross-linking. Finally, the formed membranes were dried at 100 °C for 5 days to remove the residual solvent. The reaction process is illustrated in Scheme 3. This series of 6FPBI-cPIL composite membranes were prepared at a weight ratio of 10 %, 20 %, 30 % and 40 % in the obtained composite membranes and designated as 6FPBIcPIL 10, 6FPBI-cPIL 20, 6FPBI-cPIL 30 and 6FPBI-cPIL 40, respectively. Besides, a pristine 6FPBI membrane was prepared as a contrast.


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Scheme 3 Preparation of 6FPBI-cPIL membranes.

Characterization and measurements Structure Characterization The Fourier transforms infrared spectra (FT-IR) of the membranes and cPIL were performed on a Vector-22 (Bruker, Germany) spectrometer. The cPIL and all the composite membrane samples were dried at 50 °C in a vacuum before testing. All the samples were prepared by dispersing the membranes powder in dry KBr disk and measurement was carried out in the range of 4000–400 cm-1. The 1H NMR spectra of [ViBuIm]TFSI was characterized by AVANCE Ⅲ 400 MHz (Bruker, Germany) spectrometer, using DMSO-d6 as solvent and tetramethylsilane (TMS) as the standard. 1H NMR (400 MHz, DMSO-d6) (ppm): 0.93 (m, 3H), 1.31 (m, 2H), 1.83 (m, 2H), 4.20 (m, 2H), 5.43 (t, 1H), 5.96 (t, 1H), 7.28 (m, 1H), 7.93 (d, 1H), 8.18 (d, 1H), 9.50 (d, 1H).47

Morphology analysis The scanning electron microscopy (SEM) images were taken with a JSM6510 scanning electron microscope. All the samples were obtained by fracturing the membranes in liquid nitrogen and sputtercoated with Pt prior to measurement.



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X-ray diffraction characterization The wide-angle X-ray diffraction (WAXD) spectra of all the membrane samples were recorded using a Smart Lab X-ray diffractometer with Cu Kα radiation in 2θ range of 5-40°. The average d-spacing (dsp) was calculated per the amorphous peak maxima using Bragg's Equation (1) as follows: 𝑑𝑠𝑝 = 𝑛λ 2sin 𝜃

(1)

where n is an integral number, λ denotes the X-ray wavelength (1.540 Å), dsp refers to the average intersegmental spacing between polymer chains and θ represents the diffraction angle.

Thermal characterization The thermogravimetric analysis (TGA) curves were obtained under nitrogen flow by Pyris1 TGA (Perkin Elmer). Before measurement, all the samples were dried at 100 °C for several hours to remove the residual solvent and any absorbed water. The samples were heated from 100 to 800 °C at a heating rate of 10 °C min-1.

Oxidative stability and solubility test The oxidative stability is important for fuel cell applications. Before being tested, all the membrane samples were cut into 30 mm × 10 mm small pieces and immersed in deionized water at 80 °C for 24 h in order to remove the residual solvent. The membrane samples were dried and immersed in Fenton’s reagent (3 wt.% H2O2 solution with 4 ppm Fe2+) at 80 °C. Periodically the membrane samples were taken out from the solution and dried until constant weight to record the remaining weight and the time of membranes begin to break. The remaining weight and broken time were recorded to evaluate the oxidative stability. The solubility in common solvents was used to testify the cross-linking extent of the obtained cross-linked membranes. All the membrane samples were cut into small pieces and immersed in DMAc for 24 h to observe if the membranes were dissolved completely or swelling.

Mechanical properties


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The mechanical properties of the membranes were measured by using an AGS-X equipment (Shimadzu, Japan) at room temperature. Test speed of undoped and doped membrane samples were 2 mm min-1 and 5 mm min-1, respectively. The size of membrane samples was 3 mm × 10 mm and each membrane were tested at least 5 times to reach an average value.

PA doping level and proton conductivity All the membrane samples were cut into 50 mm × 10 mm and dried to avoid the influence of residual solvents and absorbed water. Then, the membrane samples were immersed in 85 wt.% H3PO4 for 24 h at 80 °C. The weight of the membrane samples before (Wundoped) and after (Wdoped) doping PA is recorded and used to calculate the doping level by the following equation: (𝑊doped ― 𝑊undoped)

PA doping level = (𝑊 undoped(1 ― 𝑋%))

𝑀PA

𝑀PBI

(2)

where X % is the weight percentage of cPIL in the membrane samples, MPA and MPBI represent the molecular weight of phosphoric acid and repeat unit of 6FPBI, respectively. The proton conductivity of the PA/PBI membranes was measured by a four-electrode ac impedance method over a frequency range of 1-106 Hz from 100 to 170 °C using Metrohm Autolab PGSTAT302N. For the acid doped membranes, the ambient humidity affected the proton conductivity significantly. In order to remove the any absorbed water, the proton conductivity values were obtained at a cooling process. The measurements were carried out under anhydrous conditions and the proton conductivity was calculated by the following equation: 𝜎 = 𝐿/(𝐴 × 𝑅)

(3)

where σ is the proton conductivity in S cm-1, L corresponds to the distance between the two electrodes, R represents the resistance of the membrane, and A refers to the cross-sectional area of the membrane. To further investigate the proton conduction mechanism of membrane, the activation energy (Eα) for proton conduction was calculated by the Arrhenius equation:



𝐸𝛼

ln(𝜎) = ln𝜎0 ― 𝑅𝑇

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(4)

where T is the absolute temperature (K), R is the gas constant (8.314 J mol-1 K-1), and σ0 is a preexponential factor.

Results and discussion Characterization of chemical structure The polymeric ionic liquid monomer, [ViBuIm]TFSI, was obtained via the anion exchange of [ViBuIm]Cl, which was synthesized from 1-vinylimidazole and 1-chlorobutane. To confirm the chemical structure of the PIL monomer, the 1H NMR spectra of the [ViBuIm]TFSI in DMSOd6 are characterized and shown in Fig. 1. The chemical shifts of 7.93, 8.18 and 9.50 ppm were assigned to the attached imidazolium cations. It could confirm that [ViBuIm]TFSI was successfully prepared.47

Fig. 1 The 1H NMR spectra of the [ViBuIm]TFSI.

The resulting FT-IR spectra of cPIL, 6FPBI and all the composite membranes were shown in Fig. 2. The typical absorption peak at 2800–3400 cm-1 was assigned to the stretching vibration of N-H bond in the imidazole ring of the 6FPBI. For cPIL and all the composite membranes, the absorption peak at 3152 cm-1 may on account of imidazolium ring vibration of cPIL. Due to the introduction of the cross-linkable PILs, the vibrations at 918 cm-1 and 953 cm-1 were attributed


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to the epoxy group. The absence of these peaks suggested that the epoxy groups were opened and completely reacted with the imidazole rings of 6FPBI.

Fig. 2 FTIR spectra of cPIL, 6FPBI and 6FPBI-cPIL membranes.

Morphology analysis The internal microscopic morphologies of membranes were studied by SEM images and WAXD spectra. Fig. 3 a, b and c showed the SEM images of cryo-fractured cross-section morphology of 6FPBI (a), 6FPBI-cPIL 10 (b) and 6FPBI-cPIL 20 (c), respectively. It could be distinctly observed that the rough cross-section microscopic morphologies of 6FPBI-cPIL 10 and 20 membranes due to the incorporation of cPIL enhances the toughness of the membrane samples, resulting in a rough fracture cross section.36,48 It was worth noted that this change of morphologies means the closer chain packing and was confirmed in the WAXD spectra. The polymer interchain spacing (dsp) could be determined by wide angle X-ray diffraction (WAXD) spectra of pristine 6FPBI membrane and 6FPBI-cPIL membranes (Fig. 3 d). The WAXD spectra of all the membranes implied the amorphous nature, and these amorphous peaks in WAXD spectra towards higher 2θ for cPIL component. The dsp was found to decrease from 5.90 Å to 5.53 Å after cross-linking between cPIL and 6FPBI, which also indicated that the closer chain packing and more compact chemical structure of 6FPBI-cPIL membranes. Such structure was sure for enhancing the mechanical strength.



Fig. 3 SEM of cross-section morphology of 6FPBI (a), 6FPBI-cPIL 10 (b) and 6FPBI-cPIL 20 (c); WAXD spectra of 6FPBI and 6FPBI-cPIL (d).

Thermal stability The thermal stability of undoped and doped 6FPBI-cPIL and pristine 6FPBI membranes were investigated by thermogravimetric analysis (TGA) and shown in Fig. 4. The derivative of weight was calculated and shown in Fig. 4 for a clearer observation of the weight loss. The pristine 6FPBI membrane showed the first weight loss region started from 200 to 300 °C, which can be attributed to the loss of residual solvent. While the second weight loss began at around 500 °C corresponded to the decomposition of the main polymer chain. For the 6FPBI-cPIL composite membranes, a four-step degradation pattern was observed (Fig. 4 a). The first degradation at about 250 °C, which likewise appeared in the PBI curve, and the degradation beginning temperature increase could be attributed to the cross-linking structure of 6FPBI-cPIL


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membranes. The appearance of two new weight loss in the range of 350–530 °C, which were considered to be the decomposition of the TFSI anions of cPIL and cPIL backbone, respectively.30,40 Comparing with the undoped membranes, the weight loss curves for the doped membranes presented a similar trend. The first step began to lose weight at about 160 °C due to the dehydration of PA and the formation of pyrophosphoric acid and higher ordered phosphate species at higher temperature. The results are similar to the previous literatures.36,49,50 Furthermore, all the composite membranes showed similar Td5% to the pristine 6FPBI membrane (the Td5% of PA/6FPBI was 245 °C and that of PA/6FPBI-cPIL X was in the range of 220–257 °C). From these results, it can be concluded that all the membranes exhibit good thermal stability below 200 °C for potential applications in HT-PEMFCs.

Fig. 4 TGA curves of undoped (a) and PA doped (b) membranes.

Oxidative stability and solubility test



During the operation of PEMFCs, the polymer chain will be attacked by ·OH and ·OOH radicals resulting in membranes degradation, so the oxidative stability is necessary to be evaluated.51,52 In the present work, the polymer degradation was obtained by recording the weight loss and breaking time of stability as shown in Fig. 5. It could be seen from Fig. 5 a, all membranes had a sustained weight loss, and the weight losses membranes in the Fenton’s reagent to assess the oxidative slightly increased with higher cPIL content, which may be attributed to the anion exudation of cPIL. As shown in Fig. 5 b, the breaking time of 6FPBI-cPIL membranes remained around 288 to 360 h, which was notably longer than that of pristine 6FPBI membrane. It indicated that the cross-linking structure based on a nucleophilic addition reaction between the epoxy groups of cPIL and the imidazole rings on the polymer backbone could make the chemical structures more compact (which was confirmed by WAXD spectra in Fig. 3 d), resulting in significantly improved oxidative stability of the composite membranes.


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Fig. 5 The membrane degradation (a) and breaking time (b) in 3% H2O2 containing 4 ppm Fe2+ at 80 °C.

Fig. 6 Remaining weight of membrane samples and photographs before (a) and after (b) immersed in DMAc for 24 h.

Furthermore, the cross-linking extent of 6FPBI-cPIL membranes was characterized by the chemical stability test in N, N-dimethylacetamide (DMAc) for 24 h at room temperature. The changes could be distinctly observed from the photographs of the membranes before and after the test as shown in Fig. 6. After a 24 h immersion time, the pristine 6FPBI membrane samples were completely dissolved in DMAc, while all the 6FPBI-cPIL membrane samples were insoluble. It could demonstrate that cross-linked network was successfully introduced into the 6FPBI-cPIL membranes. As the cPIL incorporation increases, the residual mass also decreases, due to the fact that cPIL is more soluble.

Mechanical properties The mechanical strength of PEMs directly effects on the performance of PEMFCs. Fig. 7 a and Fig. 7 b showed the actual tensile curves of the undoped and doped membranes at room temperature, respectively. For the undoped 6FPBI-cPIL membranes, the content of cPIL and covalent cross-linking between the epoxy groups on the cPIL and the –NH groups in the imidazole ring of PBI were key factors controlling the mechanical properties of the membranes. As shown in Table 1, undoped pristine 6FPBI and 6FPBI-cPIL 20 membranes possessed tensile strengths of 116 MPa and 119 MPa, respectively.



However, the tensile strengths of the 6FPBI-cPIL membranes decreased with increasing content of cPIL, because the pure cPIL with weaker tensile strengths and higher elongation at failure than that of 6FPBI matrix. But at the same time there was covalent cross-linking between cPIL and 6FPBI, which could enhance the tensile strengths of composite membranes. The results above further confirmed by the more the amount of cPIL in the membranes, the lower the Young’s modulus and the higher the elongation at failure. When the content of cPIL reached 20 wt.%, the 6FPBI-cPIL 20 membrane exhibited the best mechanical properties. As shown in Fig. 7 b and Table 1, the membranes had poorer mechanical properties after doping with phosphoric acid higher doping levels indicated free acid in the polymer matrix, which caused the destruction of hydrogen bonds between the 6FPBI polymer chains, resulting in the deterioration of mechanical properties. Therefore, the higher phosphoric acid doping levels usually led to poor mechanical. For instance, the PA/6FPBI-cPIL 10 membrane in this work with the highest phosphoric acid doping level (33.9 ± 1.7 mol/mol) exhibited the lowest tensile strength (1.46 ± 0.08 MPa). Benicewicz et al. reported that PA/PBI membranes with a tensile strength of 1.5 MPa or more could be fabricated into MEAs that could be fuel cell performance tested.53 It should be noted that all the composite membranes except PA/6FPBI-cPIL 10 in this work exhibited a tensile strength of above 2.0 MPa, which was strong enough.


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Fig. 7 Stress-strain curves of undoped (a) and doped (b) membranes.

Table 1 Mechanical property of pristine 6FPBI and 6FPBI-cPIL membranes before and after doping with phosphoric acid. Undoped Sample

Young’s modulus (GPa)

Ultimate tensile strength (MPa)

Doped Elongation

Young’s

at failure

modulus

(%)

(MPa)

Ultimate tensile strength (MPa)

Elongation at failure (%)

6FPBI

3.78±0.05

116±6

5.0±0.8

117±8

4.14±0.17

270±19

6FPBI-cPIL 10

3.68±0.12

118±4

5.4±0.3

32±3

1.46±0.08

38±6

6FPBI-cPIL 20

3.54±0.07

119±7

6.3±1.1

83±4

2.98±0.17

119±11

6FPBI-cPIL 30

3.35±0.02

107±1

6.4±0.6

71±2

2.28±0.04

162±8

6FPBI-cPIL 40

2.21±0.04

66±2

7.3±0.7

72±7

2.32±0.07

219±9

PA doping level



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The prerequisite for these membranes to achieve high proton conductivity for HT-PEMFCs applications is being doped with phosphoric acids. The transport of protons is accomplished by dissociation of the phosphoric acid molecules, so the phosphoric acid doping level determines the proton conductivity of the PEMs. In the present work, the phosphoric acid doping ability of the 6FPBI-cPIL membranes was accomplished by immersing all the samples in 85 wt.% PA solutions. In general, it is well known that cross-linking result in more compact chemical structures, which will reduce the phosphoric acid doping ability of membranes. However, the incorporation of cPIL led to an enhancement of the phosphoric acid doping ability for 6FPBIcPIL membranes. As shown in Table 2, the phosphoric acid doping level of 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes could reach 33.9 and 27.8, respectively, which was much higher than that of pristine 6FPBI membrane. This result was attributed to the ionic liquid group of cPIL acting as a strong Bronsted base to effectively adsorb PA. However, further incensement of cPIL caused more compact chemical structures (which can be confirmed from Fig. 3 d), leading to decreased phosphoric acid doping ability, as well as the phosphoric acid doping level. For instance, the phosphoric acid doping levels of 6FPBI-cPIL 30 and 6FPBI-cPIL 40 membranes reduced to 6.9 ± 1.0 and 5.4 ± 0.5, respectively. Table 2 The PA doping level, proton conductivity and activation energy of membranes. Sample

PA doping level

Proton conductivity σ (S cm-1)

Activation energy Eα

(mol/mol)

110°C

170°C

(kJ/mol)

6FPBI

6.4±0.6

0.030 ± 0.005

0.059 ± 0.007

16.29

6FPBI-cPIL 10

33.9±1.7

0.077 ± 0.004

0.116 ± 0.008

9.52

6FPBI-cPIL 20

27.8±2.2

0.068 ± 0.004

0.106 ± 0.003

10.64

6FPBI-cPIL 30

6.9±1.0

0.040 ± 0.005

0.069 ± 0.007

12.28

6FPBI-cPIL 40

5.4±0.5

0.014 ± 0.003

0.030 ± 0.005

16.98

Proton conductivity In general, the proton conductivity is the most important performance to assess the PEMs. It must be noted that the proton conduction is actually the process of hydrogen ions directional


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movement, and there are two kinds of conduction mechanisms in the 6FPBI-cPIL membranes. The protons could be transported by hopping between the benzimidazole, cPIL and phosphoric acid (Grotthuss mechanism) or by free phosphoric acid molecules as vehicles (vehicle mechanism)20,54–56. The ionic liquid groups of the cPIL were presented in membrane as anions and cations, which could conduct protons faster and adsorb more phosphate molecules than that of 6FPBI matrix. As can be seen, the ionic liquid groups adsorbed phosphoric acid molecules and were tightly arranged on the cPIL chain, resulting in efficient and continuous proton transport channels to facilitate proton conduction. In this work, the proton conductivity of the membranes was carried out from 110 to 170 °C under anhydrous conditions, as shown in Table 2 and Fig. 8. As expected, all the membranes exhibited higher proton conductivity with elevating temperature. At the same temperature, the 6FPBI-cPIL membranes showed higher conductivity than that of pristine 6FPBI membrane, and the proton conductivity of the 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes reached to 0.116 S cm-1 and 0.106 S cm-1 at 170 °C, respectively. However, the 6FPBI-cPIL 10 membrane exhibited poor mechanical properties after doping with phosphoric acid, so the content of cPIL had an optimum value of 20 wt.%. All the results above indicated that the cPIL could effectively improve proton conductivity of the 6FPBI membrane, and the proton conductivity of the 6FPBIcPIL 10 and 6FPBI-cPIL 20 membranes reached to 0.116 S cm-1 and 0.106 S cm-1 at 170 °C, respectively. However, the 6FPBI-cPIL 10 membrane exhibited poor mechanical properties after doped with phosphoric acid, so the content of cPIL had an optimum value of 20 wt.%. All the results above indicated that the cPIL could effectively improve proton conductivity of the membranes by increasing phosphoric acid doping level and constructing proton transport channels. In order to further study proton conduction mechanism of the membranes, the Arrhenius plots of all the membranes and their proton conduction activation energy values (Eα) was analysed, as shown in Table 2 and Fig. S 3. It could be clearly seen that the proton conductivity of all the membranes displayed a rising trend with elevating temperature. This was because high



temperature could effectively enhance the Brownian motion and attributed to faster proton transferring, resulting in an overall increment in proton conductivity. In the PA doped membranes, the proton conduction depends on the conduction of phosphoric acid molecules and the interaction of polymers and the phosphoric acid.57 Moreover, the phosphoric acid molecules could protonate the imidazole ring, allowing protons to jump between PBI backbones and promote acid through hydrogen bond formation and cleavage processes. Notably, the ionic liquid groups of cPIL as anions and cations present in membrane, could more easily complete hydrogen bond formation and cleavage with phosphoric acid molecules. As shown in Table 2 and Fig. S 3, due to the high PA doping level, the activation energies (Eα) of the 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes during proton conduction are as low as 9.52 kJ mol-1 and 10.64 kJ mol-1, respectively. However, the 6FPBI-cPIL 30 membrane exhibited an activation energies (Eα) of 12.28 kJ mol-1, which was higher than that of 6FPBI membrane (16.29 kJ mol-1) at similar PA doping level.

Fig. 8 The proton conductivity of the pristine 6FPBI membrane and 6FPBI-cPIL membranes.

Long-term conductivity stability and phosphoric acid retention


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The long-term stable proton conductivity is necessary to be applied in PEMFCs, so the Longterm conductivity stability and phosphoric acid retention of pristine 6FPBI, 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes were recorded in Fig. 9. In order to simulate the PEMs real operating environment, the membrane samples were exposed at 80 °C / 40 % RH and weighed after wiping the surface-absorbed PA and water. Under this testing condition, we can observe the PA loss through water condensation.9 Within the first 24 h, a noticeable decrease in the phosphoric acid doping levels of all the membrane samples occurred, which was attributed to the exudation of free PA molecules from the membranes. In the following long-term testing, nearly constant phosphoric acid doping levels were observed of all the samples. After the stability test for 96 h, the 6FPBI-cPIL 20 membrane exhibited a phosphoric acid doping level of 8.5 higher than that of the 6FPBI (2.6) and 6FPBI-cPIL 10 (8.1), which was also reflected in their proton conductivity at 170 °C (proton conductivity decreased at 96 h: from 0.059 S cm-1 to 0.032 S cm-1 for 6FPBI, from 0.116 S cm-1 to 0.062 S cm-1 for 6FPBI-cPIL 10 and from 0.106 S cm-1 to 0.064 S cm-1 for 6FPBI-cPIL 20). This is due to the incorporation of cPIL to introduce a large amount of ionic liquid groups into the system. Because of the ionic interaction between these ionic liquid groups and the phosphate molecules, a large amount of bound phosphoric acid is formed. In the process of phosphoric acid loss, the free phosphoric acid molecules were rapidly lost and the bound phosphoric acid molecules were retained. Therefore, the 6FPBI-cPIL composite membranes exhibit enhanced phosphoric acid retention ability and long-term proton conductivity stability.



Fig. 9 The PA doping levels of the 6FPBI, 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes as a function of time at 80 °C / 40 % RH, and the proton conductivity at 0 h and 96 h is shown in the panel.

Conclusion Herein, a novel type of cross-linked composite membranes based on crosslinkable polymeric ionic liquid (cPIL) was successfully prepared for applications in HT-PEMFCs. The cross-linking reaction of the 6FPBI-cPIL membranes was completed by a facile thermal cross-linking method. Compared to the pristine 6FPBI membrane, the 6FPBI-cPIL membranes displayed excellent chemical and oxidative stability, as well as the relative good mechanical properties. Moreover, the 6FPBI-cPIL membranes could reach extremely high phosphoric acid doping levels to achieve appreciable proton conductivity. For instance, the 6FPBI-cPIL 20 membrane with a phosphoric acid doping level of 27.8 exhibited a proton conductivity of 0.106 S cm-1 at 170 °C. Especially, the incorporation of cPIL raised the phosphoric acid doping level retention ability and enhanced the long-term proton conductivity stability, where the 6FPBI-cPIL 20 membrane possessed a proton conductivity value of 0.064 S cm-1 at 80 °C / 40 % RH after 96 h.

Acknowledgements The authors gratefully acknowledge the financial support of this work by Natural Science Foundation of China (grant no.s 51603017 and 51673030), Jilin Provincial Science &


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Technology Department (grant no.s 20180101209JC, 20160520138JH, 20160519020JH), and ChangBai Mountain Scholars Program of Jilin Province.

Associated content Supporting Information. The SEM images of cryo-fractured cross-section morphology of 6FPBI-cPIL 30 and 6FPBI-cPIL 40 membranes and the Arrhenius plots of the pristine 6FPBI membrane and 6FPBI-cPIL membranes can be found in the supporting information.

Corresponding Author * E-mail: [email protected] (S. Wang).

Notes The authors declare no competing financial interest.

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Table of Contents

Synopsis:

A novel polybenzimidazole / crosslinkable polymeric ionic liquid membranes with

excellent Long-term phosphoric acid doping level and conductivity stability for HT-PEM.


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Scheme 1 Chemical structure of 6FPBI. 19x4mm (300 x 300 DPI)



Scheme 2 The synthesis process of cPIL. 29x5mm (300 x 300 DPI)


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Scheme 3 Preparation of 6FPBI-cPIL membranes. 82x78mm (300 x 300 DPI)



Fig. 1 The 1H NMR spectra of the [ViBuIm]TFSI. 82x57mm (300 x 300 DPI)


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Fig. 2 FTIR spectra of cPIL, 6FPBI and 6FPBI-cPIL membranes. 82x58mm (300 x 300 DPI)



Fig. 3 SEM of cross-section morphology of 6FPBI (a), 6FPBI-cPIL 10 (b) and 6FPBI-cPIL 20 (c); WAXD spectra of 6FPBI and 6FPBI-cPIL (d). 171x137mm (300 x 300 DPI)


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Fig. 4 TGA curves of undoped (a) and PA doped (b) membranes. 82x115mm (300 x 300 DPI)



Fig. 5 The membrane degradation (a) and breaking time (b) in 3 % H2O2 containing 4 ppm Fe2+ at 80 °C. 82x113mm (300 x 300 DPI)


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Fig. 6 Remaining weight of membrane samples and photographs before (a) and after (b) immersed in DMAc for 24 h. 58x41mm (300 x 300 DPI)



Fig. 7 Stress-strain curves of undoped (a) and doped (b) membranes. 82x114mm (300 x 300 DPI)


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Fig. 8 The proton conductivity of the pristine 6FPBI membrane and 6FPBI-cPIL membranes.



Fig. 9 The PA doping levels of the 6FPBI, 6FPBI-cPIL 10 and 6FPBI-cPIL 20 membranes as a function of time at 80 °C / 40 % RH, and the proton conductivity at 0 h and 96 h is shown in the panel.


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Crosslinkable polymeric ionic liquid improve phosphoric acid retention

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