Construction of High-Performance, High-Temperature Proton

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Construction of High Performance High-Temperature Proton Exchange Membranes through Incorporating SiO2 Nanoparticles into Novel Cross-Linked Polybenzimidazole Networks Xiaobai Li, Hongwei Ma, Peng Wang, Zhenchao Liu, Jinwu Peng, Wei Hu, Zhenhua Jiang, and Baijun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06808 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 2, 2019

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ACS Applied Materials & Interfaces

Construction of High Performance High-temperature Proton Exchange Membranes through Incorporating SiO2 Nanoparticles into Novel Cross-linked Polybenzimidazole Networks Xiaobai Li,† Hongwei Ma,† Peng Wang,† Zhenchao Liu,† Jinwu Peng,† Wei Hu,‡ Zhenhua Jiang,† and Baijun Liu*,† † Key

Laboratory of High Performance Plastics, Ministry of Education. National & Local Joint

Engineering Laboratory for Synthesis Technology of High Performance Polymer. College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China. ‡

College of Chemical Engineering, Changchun University of Technology, 2055 Yan’an Street,

Changchun 130012, P.R. China KEYWORDS: High-temperature proton exchange membrane; Polybenzimidazole; Crosslinking; Nanocomposite; Porous polyhydroxy SiO2

ABSTRACT: The practical applications of phosphoric acid-doped polybenzimidazole (PA-PBI) as high-temperature proton exchange membranes (HT-PEMs) are mainly limited by their poor dimensional-mechanical stability at high acid doping levels (ADLs) and the leaching of PA from

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membranes during fuel cell operation. In this work, to overcome these issues, novel cross-linked PBI networks with additional imidazole groups are fabricated by employing a newly synthesized bibenzimidazole-containing dichloro compound as cross-linker and an arylether-type Ph-PBI as matrix. Ph-PBI featured by good solubility under high molecular weight offers satisfactory filmforming ability and mechanical strength using for the matrix. Importantly, the additional imidazole moieties in BIM-2Cl endow the cross-linked PBI membranes improved dimensionalmechanical stability with simultaneously enhanced ADLs and proton conductivity. Furthermore, superior acid retention capability is obtained by incorporating porous polyhydroxy SiO2 nanoparticles into these cross-linked networks. As a result, it is delighted to find that the SiO2/cross-linked PBI composite membranes are suitable to manufacture membrane electrode assemblies (MEAs), and an excellent H2/O2 cell performance, that is a peak power density of 497 mW·cm-2 at 160 oC under anhydrous conditions, can be achieved.

1. Introduction High-temperature proton exchange membrane fuel cells (HT-PEMFCs) operating at 100−200 °C and low humidity conditions have attracted increasing attention because of many advantages, including suppression of catalyst poisoning by CO, enhanced catalyst efficiency and overall resistance to fuel impurities, as well as simplified thermal/water management systems.

1-2

High-

temperature proton exchange membranes (HT-PEMs), as a key component in HT-PEMFCs, have become an active research field. The extensively investigated phosphoric acid-doped polybenzimidazole (PA-PBI) holds great promise as a candidate for HT-PEMs owning to excellent chemical and thermal stability, and good proton conducting capability at elevated temperatures and low humidity. 3-5 High acid doping levels (ADLs) are highly necessary for PA-


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PBI membranes to obtain high conductivity, but it always results in the sacrifice of dimensionalmechanical stability of PBI membranes. Furthermore, excess PA molecules in membranes tend to leach out during fuel cell operation. The problems caused by poor mechanical properties and acid leakage of PA-PBI membranes result in severe deterioration of HT-PEMFC performance and durability. 6-10 Increasing the molecular weight of linear PBIs could endow the corresponding membranes improved mechanical strength to sustain higher ADLs. 11-12 But, high molecular weight generally causes a poor solubility and even a loss of film-forming ability of PBIs.

13

Improving the free

volume of PBI membranes is another way to enhance their dimensional-mechanical stability at high ADLs. 14-15 However, due to the linear structure of PBIs, the improvement by this approach is still limited. Cross-linked PBI membranes had reinforced stability than linear ones at similar ADLs.

4, 16-17

Cross-linked PBI membranes could be fabricated by ionic cross-linking between

basic PBIs and acidic polymers,

18-19

or by more stable covalent cross-linking reactions

20-23.

A

highly efficient N-substitution reaction between the imidazole groups and electrophilic active groups such as halogen

4, 16, 24-25

or epoxy

26-27

was widely applied to build highly stable PBI

networks. However, with the introduction of typical cross-linking agent, the relative content of imidazole groups, which play an important role in PA doping and proton conduction in PBI membranes, is inevitably reduced. Simultaneously, a more compact structure of cross-linked PBI is produced, resulting in decreased capacity of acid adsorption and proton conduction. 28 In addition, to alleviate the acid loss of PA-PBI membranes and enhance their stability during fuel cell operation, modification of PBI matrix by introducing some hygroscopic nanoparticles, such as silica (SiO2), titanium dioxide (TiO2), zeolites, montmorillonites and sulfonated polyhedral oligosilsesquioxane (S-POSS), has been proposed as well.

13, 29-32

As is evident from


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the above discussion, although these attempts have indeed yielded the enhancements in some certain properties (e.g. proton conductivity) of PA-PBI membranes, some other properties (e.g. tensile strength and stability) have been compromised. Therefore, the creation of simple and efficient strategies for the developments of reliable PA-PBI based HT-PEMs having comprehensive performance, including high conductivity, satisfactory dimensional-mechanical stability, and superior acid retention capability, is still a significant challenge. In this investigation, in order to overcome these critical issues, novel cross-linked PBI networks were constructed. To prevent the decrease of relative amount of imidazole groups in the cross-linked membranes and enhance membrane dimensional-mechanical stability without sacrificing ADLs and conductivity, a new cross-linker containing additional imidazole units was designed and synthesized. An arylether-type PBI (Ph-PBI) prepared in our previous work was selected as the polymer frameworks.14-15 Ph-PBI showed good solubility under high molecular weight, which is beneficial to simplify the processability of membranes and improve the stability of polymer matrix. Moreover, nanocomposite membranes were fabricated by dispersing porous polyhydroxy SiO2 nanoparticles into the cross-linked Ph-PBI membranes to enhance the ability of membranes to hold the PA and water molecules and alleviate the loss of acid. By this way, the resultant nanocomposite cross-linked PBI membranes were anticipated to exhibit enhanced overall performance as HT-PEMs. 2. Experimental Section 2.1 Materials. Chloroacetic acid (99.5%, Shanghai Chemical Reagent, China) and 3,3’diaminobenzidine (DAB, 97%, Aladdin Reagent) were used as purchased without purification. Porous polyhydroxy SiO2 nanoparticles with average size of 20 nm were obtained from Nanjing Crown Industry Co. Ltd., China. The N2 adsorption isotherm of this SiO2 particles exhibits


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similar type-I sorption behavior (Figure 1) with a Brunauer-Emmett-Teller (BET) surface area of 426 m2/g and a total pore volume of 2.69 cm3/g. The polymerization solvent PPMA was prepared by a previously reported method.33 Hydrochloric acid (HCl), sulfuric acid (H2SO4) and sodium bicarbonate (NaHCO3) were purchased from Beijing Chemical Reagent. Dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,Ndimethylacetamide (DMAc) were obtained from Tianjin Tiantai Fine Chemicals Co. Ltd., China.

Figure 1 N2 adsorption and desorption isotherms of the SiO2 nanoparticles. 2.2 Synthesis of the cross-linker 2,2'-bis(chloromethyl)-5,5'-bibenzimidazole. As shown in Scheme 1a, the cross-linker, 2,2'-bis(chloromethyl)-5,5'-bibenzimidazole (BIM-2Cl), was synthesized from chloroacetic acid and DAB by a dehydration condensation reaction. The synthesis procedure is as follows: DAB (0.02 mol) and chloroacetic acid (0.05 mol) were added into a three-necked flask containing 160 mL HCl (5 M), and refluxed the mixture in a nitrogen atmosphere under mechanical stirring for 5 h. The reaction solution was cooled to room temperature (RT) after filtering out solid residue, and then poured into ice water and neutralized


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with NaHCO3. After the pH of the solution reached 7, the precipitated product was filtered out and washed with cold water for several times. The yield was about 85%. IR (KBr, cm-1): 752 (C-Cl), 1616 (C=N), 1292 (C-N), and 3160 (N-H). 1H

NMR (500 MHz, DMSO-d6, ppm): 12.80 (s, 2H, -NH), 7.81 (s, 2H), 7.65 (d, 2H), 7.56 (d,

2H), and 4.96 (s, 4H, -CH2). Scheme 1 Synthesis of the cross-linker BIM-2Cl (a) and cross-linked Ph-PBI (b).

2.3 Preparation of the cross-linked Ph-PBI membranes. Ph-PBI with high molecular weight (Mw=93 kDa) was synthesized as reported in our previous work.14 The cross-linked PBIs (cPBIs, Scheme 1b) membranes were fabricated by a solution casting method, as shown in Figure 2a. The homogenous solutions were obtained by stirring for 5 h after mixing the 8 wt% BIM2Cl/DMAc and 8 wt% Ph-PBI/DMAc solutions at RT. In the mixtures, the molar ratio of -CH2Cl in the cross-linker to -NH in Ph-PBI were 5%, 10% and 20%, corresponding to the resultant cPBI membranes, i.e., c-PBI-5, c-PBI-10 and c-PBI-20, respectively. The reference Ph-PBI and c-


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PBI membranes were gained after a heat treatment process, followed by a boilling process by deionized water as reportd in our earlier work.15

Figure 2 The description of the procedure for preparing c-PBI membranes (a) and SiO2/c-PBI composite membranes (b). 2.4 Preparation of SiO2/c-PBI nanocomposite membranes. The SiO2/c-PBI composite membranes were prepared by employing c-PBI-20 as the polymer matrix. The contents of SiO2 nanoparticles in the resulting SiO2/c-PBI composite membranes were 2 wt%, 5 wt%, 8 wt% and 10 wt%, respectively. A detailed fabrication procedure of the SiO2/c-PBI membrane containing 5 wt% SiO2 nanoparticles is described as follows (Figure 2b): SiO2 particles (0.05 g) were firstly dispersed in DMAc (3 mL) to form a homogenous SiO2/DMAc dispersion. Then the SiO2/DMAc dispersion was added dropwise to a mixed solution of BIM-2Cl and Ph-PBI (0.85 g Ph-PBI and 0.10 g BIM-2Cl in 12 mL DMAc), and a homogeneous mixture was obtained after ultrasonicating this solution at RT for 2 h. Other mixtures with different SiO2 contents were also


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prepared by this method. The corresponding SiO2/c-PBI-20 composite membranes were prepared by the same solution casting procedure as that for preparing c-PBI membranes. The resulting membranes are termed as c-PBI-20-SiO2-2, c-PBI-20-SiO2-5, c-PBI-20-SiO2-8 and c-PBI-20SiO2-10, respectively. The PA-PBI membranes were obtained by soaking Ph-PBI, c-PBI and SiO2/c-PBI-20 membranes in PA aqueous solution (85 wt%) at 120 °C for 72 h, respectively. All these membranes before PA doping had a thickness of 60−80 μm. 2.5 Measurements. The 1H NMR spectrum of BIM-2Cl was performed on a Bruker 510 spectrometer (500 MHz) using tetramethylsilane (TMS) as an internal standard and DMSO-d6 as solvent. FTIR spectra of BIM-2Cl powders and PBI membranes were carried out on a Nicolet Impact 410 Fourier-transform infrared spectrometer over the wavenumber range of 400−4000 cm-1. The cross-sectional images of SiO2/c-PBI-20 composite membranes were observed on a Nova NanoSEM 450 scanning electron microscopy (SEM, FEI Company). Thermogravimetric analysis (TGA) was conducted on a Perkin Elmer Pyris-1 thermogravimetric analyzer from 50 °C to 800 °C under N2 or air atmosphere at a heating rate of 10 °C·min-1. The mechanical properties of the PA-doped and undoped membranes were evaluated on a SHIMADZU AG-I 1KN instrument at a strain rate of 2 mm·min-1 at RT under ambient humidity of ~50% RH. The membranes were cut into 15 mm × 5 mm rectangular samples. The PA uptake (wt%) of PBI membranes was measured by titration using 0.1 M sodium hydroxide solution (NaOH). A pH-meter was used to continuously monitor the changes in pH value of the solution. According to the dissociation reactions of PA, the first equivalence point was empolyed to determine the volume of NaOH required for neutralization.3, 34 The PA uptake was calculated from equation (1). The PA uptake was also measured by weighing the membranes


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before and after acid doping and calulated using equation (2). ADL and dimensional swelling of PBI membranes were evalulated according to the following equations (3) and (4), respectively. (1) PA uptake (wt%) = (CNaOH×VNaOH×MPA)/Wdry ×100% (2) PA uptake (wt%) = [(Wd−Wu)/Wu]×100% (3) ADL = [(Wd−Wu)/MPA]/[(Wu−Wc−Wsio2)/MPBI] (4) Volume swelling (%) = [(Vd−Vu)/Vu]×100% where CNaOH and VNaOH are the concentration and volume of NaOH solution consumed, and Wdry is the weight of dry membranes; Wu and Vu are the weight and volume of the undoped PBI membranes; Wd and Vd are the corresponding weight and volume of the PA-doped membranes; Wc and Wsio2 are the weight of BIM-2Cl and SiO2 nanoparticles in the membranes; MPBI and MPA are the molecular weights of PBI repeat unit and PA molecules, respectively. The stability of acid in the membranes was evalulated by putting the PA-doped membranes over the water vapor condition for 6 h.31 The weight of these membranes at different test times (Wi) was measured after the water absorbed in the membranes was dried at 120 °C for 3 h. The remaining weight ratio (R%) of PA in the membrane samples was calculated using the following equation (5): (5) R (%) = (Wi–Wu)/(Wd–Wu)×100% The proton conductivity of PA-PBI membranes was tested by a four-electrode ac impedance method from 0.1 Hz to 105 Hz on a Princeton Applied Research Model 273A Potentiosta. The measurement was conducted under anhydrous conditions over the temperature range of


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80−200 °C using rectangular membrane samples (1 cm × 5 cm). The conductivity  (S·cm-1) was calculated from equation (6): (6) σ = L/RA where R (Ω), A (cm2) and L (cm) are the membrane resistance, the cross-sectional area of the membranes and the distance between the two electrodes, respectively. 2.6 Fuel cell test. The HT-PEMFC performance of PA-PBI membranes was tested at 160 oC under atmospheric pressure. The catalyst ink was prepared as follows: Acid-doped PBI was empolyed as the ionomer. A mixture of PA and formic acid containing 0.5 wt% PBI (the molar ratio of PBI to PA was 1:10) served as the solvent for the preparation of catalyst ink. The catalyst powders (60 wt% Pt/C) were mixed with the above PBI solution forming the catalyst ink. The ratio of Pt to PBI in this ink was about 13:1. The gas diffusion electrodes were fabricated by spraying the ink onto non-woven. The Pt loading in the catalyst layer was about 0.6 mg·cm-2. The membrane-electrode assemblies (MEAs) with active areas of 9 cm2 were prepared by sandwiching the PA-PBI membranes between two pieces of gas diffusion electrodes and hotpressing at 200 ◦C under a pressure of 100 kg F/cm2. EPDM rubber and PTFE were used as gasket and sealing ring, respectively. Non-humidified O2 and H2 were supplied to the cathode and anode of fuel cell with flow rates of 0.15 L·min-1 and 0.3 L·min-1, respectively. Both the anode and cathode flow fields are comprised of a single serpentine channel which is rectangular with 1 mm width and depth. Polarization curves were received by a current step potentiometry. 3. Results and discussion


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3.1 Design and synthesis of a cross-linker having a bibenzimidazole chemical structure. The chemical reactivity of N-H groups in PBIs has been exploited for N-substitution reactions to fabricate covalently cross-linked membranes. However, this normally resulted in the consumption of N-H groups, which contribute to acid doping and proton transport.9, 21 In this study, to overcome this problem, a cross-linker, BIM-2Cl, containing bibenzimidazole groups was synthesized by a simple and efficient condensation reaction between chloroacetic acid and DAB. This special cross-linker is expected to provide additional functional imidazole moieties to the cross-linked PBI and simultaneously make an enhancement of mechanical stability and proton conductivity of PA-doped membranes. Figure 3a shows the 1H NMR spectrum of BIM2Cl. The two singlet signals at 12.80 ppm and 4.96 ppm are identified as the protons from imidazole and methylene, respectively. The proton peaks at 7.56–7.81 ppm are assigned to the protons of phenyl. All signals are well in agreement with their suggested molecular structure, indicating that BIM-2Cl was successfully synthesized. For the cross-linker, BIM-2Cl, an interesting phenomenon of solubility changes was found when it was placed at different temperatures. The freshly prepared BIM-2Cl exhibited good solubility in many solvents, such as DMAc, DMSO, NMP and PA. However, after keeping BIM-2Cl at RT or at higher temperature like 80 °C for a while, some insoluble residues were produced. But, BIM-2Cl maintained good solubility when it was stored below -10 °C. FTIR spectral analyses of BIM-2Cl kept at -10 °C, RT and 80 °C, termed as CL-10, CLRT and CL80, were performed. As observed from Figure 3b, the characteristic absorption bands centered at 1453 cm-1 are ascribed to the in-plane deformation of benzimidazole rings.

35

The peaks at

1616 cm-1 and 1292 cm-1 are identified as the stretching vibrations of C=N and C-N in the imidazole rings, respectively.4, 16 The strong absorption bands at 2500–3600 cm-1 are attributed


11


to the stretching vibration of N-H in imidazole rings.

36

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Compared with CL-10, the N-H

absorption bands of CLRT and CL80 were much weaker. In addition, the bands at 752 cm-1 corresponding to C-Cl groups

37

of CLRT and CL80 were weaker than that of CL-10. The FTIR

results demonstrate that the N-substitution reactions occurred between the molecules of BIM-2Cl at RT and 80 °C, and as a consequence, a presumed cross-linked structure formed, as shown in Figure 3c, which led to the production of insoluble substances.

Figure 3. (a) 1H NMR spectrum of BIM-2Cl; (b) FTIR spectra of BIM-2Cl after being kept at different temperatures; (c) presumed molecular structures of the insoluble BIM-2Cl and (d) TGA curves of the soluble and insoluble BIM-2Cl. The thermal stability of the soluble BIM-2Cl and insoluble BIM-2Cl (c-BIM-2Cl) was investigated by TGA under N2. As shown in Figure 3d, following the weight losses of water


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absorbed in the materials (below 120 °C), three decomposition steps were observed for the soluble BIM-2Cl and c-BIM-2Cl. The first degradation (~15%) for the soluble BIM-2Cl occurring at 180–350 °C was attributed to the loss of HCl caused by the N-substituted reactions. However, c-BIM-2Cl exhibited higher onset decomposition temperature (200–350 °C) and less weight loss (~12%) at this step. This is because the self-cross-linking reactions of c-BIM-2Cl occurred before the TGA test, which consumed part of the -Cl in CH2-Cl groups and imidazole protons. The second weight losses observed at 370–450 °C for both soluble BIM-2Cl and cBIM-2Cl were attributed to the decomposition of CH2-Cl groups, unreacted BIM-2Cl molecules, as well as oligomers of BIM-2Cl with small molecular weights.37-39 The weight losses above 550 °C were assigned to the degradation of larger molecules resulted by the N-substituted reactions. At 800 °C, the residual masses of the two samples were as high as ~60%. The TGA results further confirm that, self-cross-linking reactions occurred during the heating process, producing polymers which increased the thermal stability of BIM-2Cl. 3.2 Construction of the nanocomposite membranes based on c-PBI networks and porous polyhydroxy SiO2 nanoparticles. The c-PBI membranes were prepared by N-substitution reactions of BIM-2Cl with imidazole groups of PBI. Solvent-soluble Ph-PBI with high Mw was employed as the polymer backbone to improve the stability of the polymer matrix. Considering the high reactivity of the cross-linker, the freshly prepared BIM-2Cl with good solubility was firstly dissolved in DMAc before being added to the Ph-PBI/DMAc solution, to avoid the occurrence of cross-linking reactions before membrane forming. The c-PBI membranes were prepared by solution casting, followed by a heat treatment process to evaporate the solvent and complete the cross-linking reaction.16 In addition, in the present work, porous polyhydroxy SiO2 nanoparticles were incorporated into the c-PBI membranes to enhance the acid retention


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capability, leading to the resultant SiO2/c-PBI composite membranes with both enhanced ADLs and acid stability. Solubility changes in c-PBI and the SiO2/c-PBI membranes offer an intuitive way to measure the extent of cross-linking. As shown in Figure 4a-f, in contrast to the Ph-PBI membrane which was completely dissolved in DMAc at RT within about 12 h, no significant change was observed for c-PBI and SiO2/c-PBI membranes after being immersed in DMAc at RT for 12 h. The solubility test results of the obtained membranes showed that cross-linked networks had formed in c-PBI and SiO2/c-PBI membranes. The gel fractions of c-PBI-10, c-PBI-20, c-PBI-20-SiO2-2, c-PBI-20-SiO2-5 and c-PBI-20-SiO2-10 membranes were 97.1%, 99.2%, 98.5%, 98.4% and 97.7%, respectively, measured by soaking these membranes in DMAc at 100 °C for 24 h. 40-41 Figure 4g shows the FTIR spectra of the soluble BIM-2Cl, pure SiO2 particles, Ph-PBI, c-PBI and SiO2/c-PBI membranes. The absorption bands centered around 1616 cm-1, 1486 cm-1 (1453 cm-1 for BIM-2Cl) and 1292 cm-1 for all samples are identified as the characteristic peaks of the benzimidazole rings. Compared with Ph-PBI membrane, the c-PBI and SiO2/c-PBI membranes showed enhanced intensity of the characteristic bands (806 cm-1) of -CH2- from BIM-2Cl.35 As reported in other work on cross-linked PBI membranes similarly prepared by N-substitution reactions, an obviously decreased intensity of the absorption band of N-H in imidazole rings would be observed with the improvement of the degrees of cross-linking.16, 25 However, in this work, no remarkable reduction of the peak intensity of N-H (2900–3600 cm-1) was observed. This is because the cross-linker BIM-2Cl contains additional imidazole units. For SiO2 nanoparticles, the peaks at 3338 cm-1 and 1633 cm-1 are assigned to the asymmetric stretching vibration of -OH and the bending vibration of H-O-H of water. The absorption bands at around 1106 cm-1 and 955 cm-1 are ascribed to the asymmetric stretching vibration of Si-O-Si and the


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bending vibration of Si-OH, and the peaks at 802 cm-1 and 474 cm-1 are attributed to the symmetrical stretching and bending vibration of Si-O.42 Due to the introduction of SiO2, the absorption bands at 1633 cm-1, 1106 cm-1 and 474 cm-1 of the SiO2/c-PBI composite membranes are obviously enhanced. The FTIR results further confirm that the c-PBI and the SiO2/c-PBI membranes were prepared successfully.

Figure 4 Photographs of (a) Ph-PBI membrane and (b-f) c-PBI-10, c-PBI-20, c-PBI-20-SiO2-2, c-PBI-20-SiO2-5 and c-PBI-20-SiO2-10 membranes after being soaked in DMAc at RT for 12 h; (g) FTIR spectra of BIM-2Cl, pure SiO2 particles, Ph-PBI, c-PBI and SiO2/c-PBI membranes. The compatibility of polymer matrix and inorganic particles affects the properties of the composite membranes significantly. Figure 5a-d illustrates the cross-sectional images of SiO2/cPBI membranes with different SiO2 contents determined by SEM. As observed, there are almost no aggregated SiO2 particles appeared in the SEM images, indicating a well dispersion of SiO2 in the c-PBI-20 matrix. EDS mapping of Si element of c-PBI-20-SiO2-2 and c-PBI-20-SiO2-8 membranes (Figure 5e and f) also demonstrates that the distribution of SiO2 particles in the membranes is homogeneous. This good dispersion is obviously attributed to the chemical affinity


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of the hydroxyl groups on SiO2 particle surface with the PBI matrix. Meanwhile, all membranes have no large size pores and cracks. This preferred dense membrane structure would minimize the fuel gases crossover and benefit the fuel cell performance.

Figure 5. SEM images of the cross-section of c-PBI-20 membranes with different SiO2 contents: (a) 0 wt%, (b) 2 wt%, (c) 5 wt% and (d) 8 wt%; EDS mapping of Si of (e) c-PBI-20-SiO2-2 and (f) c-PBI-20-SiO2-8 membranes. Figure 6 shows the TGA results of the Ph-PBI based membranes tested under air. All the samples exhibited typical TGA curves of PBI-type membranes, including a loss of trace moisture absorbed in the membranes (80–150 oC), loss of the residual DMAc solvent bonded to PBI molecular chains (220–350 oC), and decomposition of PBI backbones (≥ 500oC).15, 29, 43 The high decomposition temperatures of PBI backbones demonstrate that these membranes have excellent thermal stability under the HT-PEMFC operating conditions. Compared with Ph-PBI, the DMAc removal temperatures of c-PBI membranes moved to higher ones with the increase of crosslinking degrees (Figure 6a). This may be due to the increased numbers of imidazole groups in the


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c-PBI membranes, which form stronger interaction with DMAc. The mass losses of moisture of SiO2/c-PBI-20 composite membranes were greater than that of c-PBI-20 membrane (Figure 6b). This indicates that the composite membranes are more hygroscopic due to the introduction of polyhydroxy SiO2 nanoparticles. At 800 °C, the residual masses of the composite membranes had a good agreement with the initial doping amounts of SiO2 particles (Figure 6b).

Figure 6. TGA curves of the Ph-PBI based membranes: (a) c-PBI and pure Ph-PBI membranes; (b) SiO2/c-PBI-20 composite membranes. The insets of (a) and (b) are the corresponding amplified TGA curves below 600 °C. 3.3. PA uptake, dimensional swelling and proton conductivity. PA uptake and membrane dimensional swelling were evaluated by immersion of Ph-PBI based membranes in PA solution at 120 °C for 72 h, and the results are listed in Table 1. As previously reported, the cross-linked PBI membranes normally exhibited a reduced PA doping ability than the linear ones due to their reduced relative content of N-H groups and the more compact cross-linked structure.

41, 44, 45

In

this study, in contrast to those reported cross-linked membranes, the c-PBI membranes using BIM-2Cl as cross-linker achieved improved ADLs than the pure Ph-PBI membrane, because more basic imidazole sites, which had affinity for PA molecules, were incorporated into PBI


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system. For example, after doping for 72 h, Ph-PBI obtained a PA uptake of 232 wt%, while, the PA uptakes of c-PBI membranes increased from 267 wt% to 316 wt% when the degrees of crosslinking increased from 5% to 20%. The PA uptakes of PBI membranes tested by weighing the membranes before and after PA adsorption are slightly higher than those measured by titration, as shown in Table 1. This is probably related to the water adsorbed in membranes. The existence of traces of water favors the proton transport of PA-PBI membranes.1 Here, it should be mentioned that due to the ease of self-cross-linking, some self-cross-linked BIM-2Cl would form in c-PBI membranes especially at high BIM-2Cl additions (Figure 7), thereby reducing the degrees of cross-linking of Ph-PBI and endowing the c-PBI membranes improved acid uptake ability. More importantly, the PA uptake of c-PBI-20 membrane was 84% higher than that of PhPBI; but its volume swelling was just 30% larger than the swelling of Ph-PBI membrane (Table 1). This much better dimensional stability was attributed to the more stable cross-linked networks of c-PBI membranes.

Figure 7. A depiction of the chain packing and PA doping in SiO2/c-PBI-20 membranes.


18

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As a result, all c-PBI membranes exhibited higher conductivities than Ph-PBI membrane, and the conductivities increased with an increase of the degrees of cross-linking as shown in Figure 8a. For instance, c-PBI-20 membrane exhibited a conductivity of 232 mS·cm-1 at 200 °C, which was about two times higher than that (126 mS·cm-1) of Ph-PBI membrane (Table 1). In addition, due to the enhanced relative concentration of imidazole groups, the present c-PBI membranes also exhibited higher PA uptake and proton conductivity than many other notable cross-linked analogues (Table 1, Ref. [4, 7, 41, 45, 47-49]). The conductivities of all reference PA-PBI membranes listed in Table 1 were measured at high temperatures (140–180 oC) under low RH conditions (dry air condition or 5% RH). Moreover, the c-PBI membranes showed lower volume swelling at even higher ADL (c-PBI-5, ADL: 16.9, volume swelling: 150%) than that of some analogues (Ref. [4, 45, 48], ADLs: ~13, volume swelling: > 170%). For the most reported m-PBI, to avoid excessive dimensional swelling and maintain sufficient tensile strength for MEA fabrication, moderate ADLs (~10) were usually preferred, which would cause to a relatively low conductivity (< 100 mS·cm-1) even at 180 °C (Table 1, Ref. [50, 55]). Although Nafion exhibits high proton conductivity (~100 mS·cm-1) at lower temperatures (≤ 80 oC) under high humidity (~100% RH), it shows low efficiency when operating above 100 oC.46 Table 1. PA uptake, swelling, tensile properties and conductivity of PBI based membranes. Samples

PA uptake

ADL

(wt%) Wa

Tb

Ph-PBI

253

232

c-PBI-5

281

c-PBI-10

Volume

Tensile stress

Tensile strain

Tensile modulus

Conductivity

swelling

(MPa)

(%)

(GPa)

(mS·cm-1)

(%)

UDc

Dd

UDc

Dd

UDc

Dd

160 °C

200 °C

14.7

147

114±6

12±3

10±4

35±9

3.04±0.19

0.15±0.04

97

126

267

16.9

150

151±12

18±4

10±3

17±6

3.12±0.25

0.29±0.06

155

180

320

299

19.7

165

154±10

14±2

14±4

19±8

3.08±0.21

0.21±0.05

164

215

c-PBI-20

339

316

22.0

177

158±13

14±2

12±5

32±10

3.31±0.32

0.14±0.04

185

232

c-PBI-20-SiO2-2

350

329

23.1

181

137±7

15±4

19±7

40±13

3.14±0.28

0.12±0.05

199

244


19


Page 20 of 47

c-PBI-20-SiO2-5

355

335

24.2

186

135±10

13±3

13±3

29±10

3.39±0.30

0.13±0.03

180

228

c-PBI-20-SiO2-8

342

314

24.1

180

137±6

12±2

14±5

27±9

3.22±0.17

0.10±0.02

157

206

c-PBI-20-SiO2-10

328

307

23.7

174

139±7

12±3

9±3

25±8

2.93±0.22

0.11±0.03

145

201

eSC-B-OPBI-10[7]

223.0

--

--

76.6

72.7

13.9

4.0

35.1

--

--

--

44k

72.9

--

--

--

--

11.5

--

73.9

--

0.02

--

~100h

--

--

13.1

186

--

10.4

--

~35

--

--

93i

--

eCrL-4.6%F PBI[4] 6

--

--

13.5

178

118.1

11.8

5.2

63.0

4.2

0.18

115i

--

ePBI-30%Ph[45]

--

--

13.6

304

--

~4.8

--

--

--

--

--

130k

ePBI/BADGE[41]

--

--

10.3

--

114

--

26

--

--

--

--

66k

eb-PBI-90[49]

--

--

7.9

--

103.0

10.9

5.4

26.0

--

--

--

32j

392

--

15.1

149

--

~6

--

~56

--

~0.12

98il

--

f10wt%S-POSS[50]

--

--

11.4

136

--

3.4

--

54

--

0.05

--

110k

fPy-PBI/1.5%PGO[51]

--

--

9.9

--

--

4.6

--

139.2

--

--

--

76g

fPBI-GO-1[52]

--

--

13

--

--

--

--

--

--

--

--

170k

fPBI-BaZrO [53] 3

--

--

13

--

--

--

--

--

--

--

--

125kl

fdA5MIL[54]

--

--

6.3

--

--

--

--

--

--

--

--

67h

m-PBI55]

--

--

7.6

195

--

7.4

--

26.1

--

0.18

--

0.075k

m-PBI[50]

--

--

11

--

--

15

--

149

--

0.05

--

0.07k

ePpF-co-ABPBI-65[47] eCL-6.6%SO

[48] 2PBI

efc-6-sTiO -PBI-OO[6] 2

aPA

uptake tested by weighing; bPA uptake tested by titration; cUndoped membranes; dDoped membranes; eCross-linked membranes; membranes; g140 °C; h150 °C; i160 °C; j170 °C; k180 °C; l5% RH.

fNanocomposite

The incorporation of SiO2 nanoparticles had a further effect on the PA uptake and conductivity of the composite membranes (Table 1 and Figure 8b). The SiO2/c-PBI-20 membranes with SiO2 contents of 2–8 wt% showed higher PA uptakes than the neat c-PBI-20 membrane, but the PA uptake decreased slightly for higher SiO2 concentration. This is mainly because the porous structure and acidophilic hydroxyl groups of SiO2 particles play a positive role in PA adsorption (Figure 7) at low SiO2 loading. However, as the amount of SiO2 increased, dense H-bonding networks between -OH of SiO2 and -NH of PBI would form (Figure 7), resulting in decreased number of active basic sites for acid doping and thus reduced PA uptake.29,

56

The c-PBI-20-

SiO2-2 membrane with a PA uptake of 329 wt% exhibited the highest conductivity of 244


20

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mS·cm-1 at 200 °C. Furthermore, compared with other nanocomposite PBI membranes (Table 1, Ref. [50-54]), these SiO2/c-PBI-20 membranes showed significantly improved PA uptake and proton conductivity, along with acceptable volume swellings of about 180%.

Figure 8. Proton conductivity of PA-doped PBI membranes: (a) the c-PBI membranes and (b) the SiO2/c-PBI-20 composite membranes. 3.4 The stability of acid and proton conductivity. It is essential for the PA-PBI based HTPEMs to reduce the leaching of acid from membranes during the fuel cell operation, in order to minimize the performance degradation of HT-PEMFCs.31, 53 In this study, the acid stability test was carried out by recording the weight change of the PA-doped membranes under water vapor condition for 6 h. The PA remaining percentages at different test times are shown in Figure 8a. It was found that, all membranes exhibited a noticeable weight loss rate during the initial 2 h period followed by a reduced acid weight loss rate. Compared with Ph-PBI and c-PBI-20 membranes, the SiO2/c-PBI-20 composite membranes exhibited enhanced PA stability with increasing the contents of SiO2 nanoparticles in the membranes. After 6 h, the c-PBI-20-SiO2-10 membrane showed a PA loss of 36 wt%, which was much lower than the loss of 56 wt% of pure Ph-PBI membrane. This is because the hygroscopicity SiO2 nanoparticles improve the capability


21


Page 22 of 47

of the composite membranes to retain acid and water, 57,58 and also the porous structure of SiO2 further provides reservoirs for PA molecules (Figure 7). These characteristics endow the composite membranes an improved equilibrium partition composition of PA and water, as the new acid loss mechanism proposed by Kim et. al, resulting in less PA loss.10

Figure 9. The PA stability of the membranes under the water vapor at ~100 °C (a); Conductivity stability of the membranes at 160 oC without humidification (b). The proton conductivity stability of PA-doped c-PBI-20-SiO2-2, c-PBI-20-SiO2-10, c-PBI-20 and Ph-PBI membranes was evaluated at 160 oC for 108 h. The results are shown in Figure 8b. Before testing, these membranes were pre-dried at 160 oC for 6 h. The initial conductivities of Ph-PBI, c-PBI-20, c-PBI-20-SiO2-2 and c-PBI-20-SiO2-10 membranes reduced by 17%, 14%, 12% and 9% respectively (Figure 9b), compared with their original conductivity values without heat treatment (Table 1). This is ascribed to the removal of water and some unstable PA in the membranes. Thereafter, a slight decline in conductivities was observed. At the end of 108 h measurement, the conductivity decreases of Ph-PBI, c-PBI-20, c-PBI-20-SiO2-2 and c-PBI-20SiO2-10 membranes were 22%, 19%, 15% and 11%, respectively, demonstrating that the SiO2/cPBI-20 composite membranes had a higher conductivity stability due to their better acid and


22

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water retention ability at high temperature. This greatly enhanced acid and proton conductivity stability would improve the lifetime of the PA-doped SiO2/c-PBI-20 composite membranes and make them promising candidates for HT-PEMs. 3.5 Mechanical properties. Good mechanical properties are desired for PA-PBI membranes to be applied in HT-PEMFCs. As listed in Table 1, the c-PBI and SiO2/c-PBI-20 composite membranes before acid doping exhibited much improved tensile strengths than that of the undoped Ph-PBI membrane. As widely reported, the tensile strength is dramatically decreased with the improvement of ADLs because of the plasticizing effect of acid, resulting in significantly reduced fuel cell performance and durability.35,

59

Therefore, the mechanical

stability, especially at high PA loading, is a vital parameter for evaluating the properties of the PA-PBI type HT-PEMs. As observed from Table 1, Ph-PBI membrane with a PA uptake of 232 wt% had a tensile strength of 12 MPa. In contrast, the c-PBI based membranes with higher PA uptakes still showed higher tensile strengths due to their tougher cross-linked frameworks. For example, the c-PBI-20-SiO2-2 membrane had a higher mechanical strength of 15 MPa with a much improved PA uptake 329 wt%. Furthermore, the PA-doped c-PBI membranes and SiO2/cPBI-20 composite membranes showed comparable tensile stresses to many other cross-linked and composite PBI membranes with lower PA uptakes (Table 1, Ref. [4, 7, 45, 47-51]). Thus, the PA-doped membranes fabricated in this work show useful mechanical properties suitable for HTPEM applications. 3.6 Fuel Cell Performance. The power density and polarization curves of PA-doped c-PBI-20, c-PBI-20-SiO2-2, c-PBI-20-SiO2-5 and Ph-PBI membranes with the thicknesses of 125 μm, 122 μm, 129 μm and 118 μm, respectively, are shown in Figure 10. A peak power density of 262 mW·cm-2 and a cell voltage of 0.59 V at a current density of 0.2 A·cm-2 were obtained for Ph-


23


Page 24 of 47

PBI membrane. The c-PBI-20, c-PBI-20-SiO2-2 and c-PBI-20-SiO2-5 membranes exhibited much improved fuel cell performances. It should be primarily attributed to their superior tradeoff between proton conductivity and dimensional-mechanical stability, and good acid stability. Among all the samples, the c-PBI-20-SiO2-2 membrane showed the highest peak power density of 497 mW·cm-2, and a high voltage of 0.67 V at 0.2 A·cm-2. And, the c-PBI-20-SiO2-2 membrane had lower cell resistance (0.105 Ω cm2) than that of c-PBI-20 (0.123 Ω cm2) and PhPBI (0.196 Ω cm2) membranes. The high open circuit voltages (OCVs) above 0.9 V of the HTPEMFCs for these membranes indicating their low gas permeability.60 Furthermore, as compared in Table 2, the c-PBI-20 and SiO2/c-PBI-20 composite membranes exhibited comparable or even better HT-PEMFC performance than PA-doped commercially available m-PBI membranes and some notable analogues measured at similar conditions, indicating that the present c-PBI based HT-PEMs have promising application prospect.

Figure 10. HT-PEMFC performance of c-PBI-20, c-PBI-20-SiO2-2, c-PBI-20-SiO2-5 and PhPBI membranes at 160 oC without humidification: polarization curves (open symbols) and power density curves (filled symbols).


24

Page 25 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Comparison of fuel cell performances based on PA-PBI membranes. aTensile

Conductivity

OCV

bVoltage

Peak power density

(MPa)

(mS·cm-1)

(V)

(V)

(mW·cm-2)

c-PBI-20-SiO2-2

14.7

199

0.95

0.67

497

H2/O2

160

c-PBI-20

13.6

180

1.02

0.67

467

H2/O2

160

g-PBI-20[15]

6.5

171

0.92

0.65

443

H2/O2

160

m-PBI[55]

7.4

75

~1.00

~0.70

442

H2/O2

180

HB-PBI[61]

--

168

0.82

0.64

346

H2/O2

150

12.3

~85

0.91

~0.65

460

H2/O2

160

PBI-Ep-151[44]

--

15

0.90

~0.60

358

H2/O2

160

Be3Br-7.5%[25]

8.5

95

~0.95

~0.65

~340

H2/O2

160

F6PBI-10%R3[26]

9.7

~60

~0.90

~0.68

--

H2/O2

160

PA-PBI Membranes

PDA-PBI[62]

aTensile

stress

Fuel gas

cTemp.

(oC)

stress of PA-PBI membranes tested at RT; bVoltage at 0.2 A·cm-2; cTest temperature for conductivity and cell performance

4. Conclusions In this work, novel SiO2/c-PBI composite membranes were investigated for HT-PEMs. For the purpose of enhancing the mechanical strength and PA uptake (i.e. conductivity) of PBI membranes simultaneously, a new cross-linker, BIM-2Cl, containing additional imidazole units was designed and synthesized, and an interesting self-cross-linking phenomenon at relatively low temperature was found by BIM-2Cl. High-molecular-weight Ph-PBI having good solubility was employed as the polymer backbone to provide good processability and mechanical properties for the matrix membrane. Moreover, porous polyhydroxy SiO2 nanoparticles were introduced into the c-PBI membranes to improve their acid retention. The successful fabrication of the SiO2/cPBI composite membranes was proved by comparing their FTIR spectra and the solvent solubility with that of Ph-PBI membrane. As an encouraging result, the resulting SiO2/c-PBI


25


Page 26 of 47

composite membranes simultaneously exhibited much enhanced acid doping capability, proton conductivity, dimensional-mechanical and acid stability, as well as HT-PEMFC performance than Ph-PBI membrane. The c-PBI-20-SiO2-2 membrane with a high PA uptake of 329 wt% showed a high conductivity of 244 mS·cm-1 at 200 oC without humidification, and it had a peak power density of 497 mW·cm-2 with non-humidified H2 and O2 at 160 oC. In summary, these SiO2/c-PBI composite membranes possess many distinguished features for potential applications in HT-PEMFCs. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Baijun Liu: 0000-0002-9553-0658 ACKNOWLEDGMENT Financial support for this project was provided by the National Natural Science FoundationChina (No.: 51873076 and 21404013), the Science and Technology Development Plan of Jilin Province-China (20180201076GX and 20180201075GX), the Industrial Technology Research and Development Funds of Jilin Province (No.: 2019C042-4), and the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (No.: 201826). REFERENCES


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(22) Li, X.; Liu, C.; Zhang, S.; Zong, L.; Jian, X. Functionalized 4-Phenyl PhthalazinoneBased Polybenzimidazoles for High-Temperature PEMFC. J. Membr. Sci. 2013, 442, 160-167. (23) Kim, S.-K.; Kim, K.-H.; Park, J. O.; Kim, K.; Ko, T.; Choi, S.-W.; Pak, C.; Chang, H.; Lee, J.-C. Highly Durable Polymer Electrolyte Membranes at Elevated Temperature: Crosslinked Copolymer Structure Consisting of Poly(benzoxazine) and Poly(benzimidazole). J. Power Sources 2013, 226, 346-353. (24) Yue, Z.; Cai, Y.-B.; Xu, S. Phosphoric Acid-Doped Cross-linked Sulfonated Poly (imidebenzimidazole) for Proton Exchange Membrane Fuel Cell Applications. J. Membr. Sci. 2016, 501, 220-227. (25) Yang, J.; Jiang, H.; Gao, L.; Wang, J.; Xu, Y.; He, R. Fabrication of Crosslinked Polybenzimidazole Membranes by Trifunctional Crosslinkers for High Temperature Proton Exchange Membrane Fuel Cells. Int. J. Hydrogen Energ. 2018, 43, 3299-3307. (26) Yang, J.; Xu, Y.; Liu, P.; Gao, L.; Che, Q.; He, R. Epoxides Cross-linked Hexafluoropropylidene Polybenzimidazole Membranes for Application as High Temperature Proton Exchange Membranes. Electrochim. Acta 2015, 160, 281-287. (27) Ngamsantivongsa, P.; Lin, H.-L.; Yu, T. L. Crosslinked Ethyl Phosphoric Acid Grafted Polybenzimidazole and Polybenzimidazole Blend Membranes for High-Temperature Proton Exchange Membrane Fuel Cells. J. Polym. Res. 2016, 23, DOI: 10.1007/s10965-015-0911-3. (28) Nasef, M. M. Radiation-Grafted Membranes for Polymer Electrolyte Fuel Cells: Current Trends and Future Directions. Chem. Rev. 2014, 114, 12278-12329.


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Figure 1. N2 adsorption and desorption isotherms of the SiO2 nanoparticles 279x195mm (150 x 150 DPI)


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Scheme 1. Synthesis of BIM-2Cl (a) and cross-linked Ph-PBI (b). 200x110mm (150 x 150 DPI)



Figure 2. The illustration for the preparation of c-PBI and SiO2/c-PBI composite membranes. 266x162mm (150 x 150 DPI)


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Figure 3. (a) 1H NMR spectrum of BIM-2Cl; (b) FTIR spectra of BIM-2Cl after being kept at different temperatures; (c) presumed molecular structures of the insoluble BIM-2Cl and (d) TGA curves of the soluble and insoluble BIM-2Cl. 200x163mm (150 x 150 DPI)



Figure 4. Photographs of (a) Ph-PBI membrane and (b-f) c-PBI-10, c-PBI-20, c-PBI-20-SiO2-2, c-PBI-20SiO2-5 and c-PBI-20-SiO2-10 membranes after being immersed in DMAc at RT for 12 h; (g) FTIR spectra of BIM-2Cl, pure nano-SiO2, Ph-PBI, c-PBI and SiO2/c-PBI membranes. 262x133mm (150 x 150 DPI)


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Figure 5. SEM images of the cross-section of c-PBI-20 membranes with different SiO2 contents: (a) 0 wt%, (b) 2 wt%, (c) 5 wt% and (d) 8 wt%; EDS mapping of Si of (e) c-PBI-20-SiO2-2 and (f) c-PBI-20-SiO2-8 membranes. 230x123mm (150 x 150 DPI)



Figure 6. TGA curves of the Ph-PBI based membranes: (a) pure Ph-PBI and c-PBI membranes; (b) SiO2/cPBI-20 composite membranes. 262x108mm (150 x 150 DPI)


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Figure 7. A depiction of the chain packing and PA doping in SiO2/c-PBI-20 composite membranes. 214x167mm (150 x 150 DPI)



Figure 8. Proton conductivities of PA-doped PBI membranes: (a) the c-PBI membranes and (b) the SiO2/cPBI-20 composite membranes. 254x107mm (150 x 150 DPI)


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Figure 9. The PA stability of the membranes under the water vapor at 100 °C (a); Conductivity stability of the membranes at 160 oC without humidification (b). 276x109mm (150 x 150 DPI)



Figure 10. Fuel cell performance of Ph-PBI, c-PBI-20, c-PBI-20-SiO2-2 and c-PBI-20-SiO2-5 membranes at 160 oC without humidification: polarization curves (open symbols) and power density curves (filled symbols). 289x223mm (150 x 150 DPI)


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TOC 164x85mm (150 x 150 DPI)


Construction of High-Performance, High-Temperature Proton

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