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Functionalized 4-phenyl phthalazinone-based polybenzimidazoles for high-temperature PEMFC
Journal of Membrane Science 442 (2013) 160–167
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Functionalized 4-phenyl phthalazinone-based polybenzimidazoles for high-temperature PEMFC Xiuping Li a,b, Cheng Liu a,b,c,n, Shouhai Zhang a,b,c, Lishuai Zong a,b, Xigao Jian a,b,c,n a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People's Republic of China Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116024, People's Republic of China c Liaoning Province Engineering Center of High Performance Resins, Dalian 116024, People's Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 November 2012 Received in revised form 14 March 2013 Accepted 18 April 2013 Available online 30 April 2013
A series of novel polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups (PPBIOH) were synthesized from 4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC), 2,5-dyhydroxyterephthalic acid (TPA-OH) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA). The resultant polymers showed good solubility in 98% H2SO4 and NMP. The inherent viscosities of the polymers in 98% H2SO4 at 25 1C were in the range of 0.33–2.10 dL/g, increasing obviously with decreasing hydroxyl content in the main chain. The corresponding acid doped membranes were prepared directly from the polymerization solution by the PPA process. The chemical structures of PPBIOH polymers and membranes were characterized by FT-IR. These membranes showed reasonable doping levels (10.1–12.2 mol H3PO4), high proton conductivities (0.10–0.21 S cm−1 above 120 1C), good mechanical properties (stress of 1.7–2.9 MPa and strain of 28.1–68.0%) and high oxidative stability (breaking time in the range of 52–155 h). The structures of PPBIOH membranes were analyzed by the Wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). The WAXD and POM results indicate the acid doped PPBIOH membranes exhibit semi-crystalline character, from which the mechanism of proton conduction can be further investigated. The combination performance makes these PPBIOH membranes promising candidates for potential applications in high-temperature proton exchange membrane fuel cells (PEMFCs). & 2013 Elsevier B.V. All rights reserved.
Keywords: Functionalized polybenzimidazole Phthalazinone Proton exchange membrane Acid doped
1. Introduction The conventional proton exchange membrane fuel cells (PEMFCs) are based on perfluorosulfonic acid type membranes, such as DuPont's Nafions or similar polymers [1–4]. However, their high cost, environmental inadaptability and susceptibility to catalyst poisoning by impurities such as CO have limited their application. In recent years, much effort has been focused on the development of novel proton exchange membrane (PEM) to substitute the perfluoropolymers and improve the performance of PEMFCs [5–9]. Proton exchange membrane fuel cells operated at high temperatures (4100 1C) can offer many significant benefits, such as better fuel cell CO tolerance, better heat and water management, faster electrode electrochemical reaction kinetics and higher fuel impurity resistance [10,11]. For high performance at elevated temperatures, high-temperature PEMFCs should possess the increased demands for
n Corresponding authors at: Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116024, People's Republic of China. Tel.: +86 411 84986191; fax: +86 411 84986109. E-mail addresses: [email protected], [email protected] (C. Liu), [email protected] (X. Jian).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.04.044
new electrolyte materials such as follows: low-cost materials, high proton conductivity over 100 1C and long-time durability [11]. Therefore, high-temperature polymer electrolyte membrane materials have become promising candidates for application in PEMFC field. Phosphoric acid (PA) doped PBI membranes were investigated as promising high-temperature PEMFC candidates for their good mechanical property, excellent chemical and thermal stability, particularly since the landmark study by Wainright et al. in 1995 [12]. Additionally, the PA doped PBI membranes can maintain high proton conductivity under anhydrous conditions. The PPA process is an effective method to prepare PA doped PBI membranes with high doping levels, which usually lead to high proton conductivity. The PPA process [13–15] is a sol–gel process to fabricate PBI-H3PO4 membranes directly from PBI polycondensation solution in PPA at around 200 1C, without isolation or re-dissolution of the polymer after synthesis. The PPA in the membranes is transformed into PA by hydrolysis of PPA in the air, resulting in PA doped PBI membranes. Therefore, the PBI membranes with high doping levels of 20–50 mol of PA per PBI repeat unit can be obtained by the PPA process, which have expected high proton conductivities at high temperatures. Though high doping levels may lead to high proton conductivity, too much PA loading in PBI membrane could result in poor mechanical
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stability and oxidative stability. Crosslinking has been reported as a feasible method to improve the chemical stability as well as the mechanical properties of the polymer [16–20]. More recently, dihydroxy functional groups in the PBI main chain were incorporated and used as handles for further crosslinking reactions [21]. In this paper, we preliminarily focus on the effect of functional hydroxyl groups and twisted phthalazinone moieties on the structures of polybenzimidazoles and the properties of PA doped membranes by chemical modification. Thus, a novel series of PPBIOH copolymers containing 4-phenyl phthalazinone moieties and functional hydroxyl groups were synthesized. The corresponding PPBIOH PA doped membranes were prepared by the PPA process. The functional hydroxyl groups were expected to form the crosslinked structure of the membranes and enhance the interaction between polymer chains, by which the mechanical properties and the oxidative stabilities could be enhanced. Meanwhile, 4-phenyl phthalazinone moiety exhibits a heterocyclic, twisted, non-coplanar and asymmetrical structure, which can cumber the close packing of the polymer chains and increase the free space between the molecular chains. In addition, the N atom and carbonyl group of 4-phenyl phthalazinone moiety could also enhance the acid absorbing and proton conductivity. This special structure may afford interspaces to concentrate PA and form a particular proton conduction pathway through these high PA concentration interspaces in the membranes, which makes the membranes present high proton conductivity. The relationship of polymer structure and properties of PPBIOH membranes has been investigated and discussed in detail.
2. Experimental details 2.1. Materials 3,3′-Diaminobenzidine (DAB, 99%) and 2,5-dihydroxyterephthalic acid (TPA-OH, 99%) were obtained from Sigma-Aldrich Co. Inc and used as received. 4-(4-(4-(4-Carboxyphenoxy)phenyl)1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC) was synthesized according to the method reported previously [22]. Polyphosphoric acid (PPA, 116%) was prepared by heating 1:1.8 weight ratio of ortho phosphoric acid (85%) and phosphorus pentoxide for 6 h at 120 1C. All other solvents and chemicals were purchased from commercial sources and used without further purification. 2.1.1. Polymer synthesis and film formation Polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups were synthesized from various stoichiometric mixtures of CPPBC and TPA-OH with DAB by solution copolycondensation in PPA. These copolymers are referred as PPBIOH-x, where x is the molar fraction of TPA-OH in the feed. Taking the synthesis of PPBIOH-5 as an example, DAB (0.642 g, 3 mmol), TPA-OH (0.0297 g, 0.15 mmol) and CPPBC (1.3623 g, 2.85 mmol) were placed into a 100 mL three-neck round bottom flask equipped with an overhead mechanical stirrer, a water cooled condenser and nitrogen purge inlet and outlet, followed by the addition of 20 mL PPA. The reaction mixture was stirred in N2 at 90 1C for 1 h, 170 1C for 7 h and 200 1C for 7 h. During the polymerization at 200 1C, the reaction mixture became more and more viscous and 10 mL PPA was added in this stage. After the final stage of polymerization, the mixture was heated to 220 1C and another 15 mL PPA was added into the reaction mixture to lower the solution viscosity, aiding the membrane casting process. After stirring for 30 min, part of the polymer solution was directly cast onto clean glass substrates with a doctor blade. The obtained membrane was placed in a humiditycontrolled environment (60–70% RH) at room temperature for a
161
week, absorbing moisture from the environment. The samples were transformed into gel membranes and the viscous phosphoric acid solution resulting from PPA hydrolysis could be observed from the membranes. Then the membranes were wiped dry and dried in vacuum before further measurements. The membrane preparation procedure described above is the PPA process. The membranes after hydrolysis were dried in vacuum at 100 1C for 12 h before further measurements. The thickness of final acid doped PPBIOH gel membrane was in the range of 100–300 μm. The remaining reaction solution was poured into hot water with stirring. The fiber-like precipitate was filtered off and washed with water until neutral. The obtained polymer was dried in vacuum at 100 1C for 12 h. The resultant PPBIOH polymers were insoluble in any solvents due to the crosslink structure of PA ester, which were discussed in detail in the results and discussion section. In order to further investigate their solubilities and inherent viscosities, the PPBIOH polymers were treated in DMSO/H2O mixture with 0.02 M of NaOH to remove the PA ester as described in the previous literature [23]. 2.1.2. Characterization The solubility of the polymers after the base-treatment was carried out by dissolving polymer in solvent at a concentration of 0.01 g/mL. Inherent viscosities of the polymers after the basetreatment were obtained by Ubbelohde capillary viscometer at 25 1C using 98% concentrated sulfuric acid as solvent. FT-IR spectra were performed on a Thermo Nicolet Nexus 470 Fourier transform infrared spectrometer. The membranes were measured directly using attenuated total reflection (ATR) method. The polymer samples were prepared by pressing the mixture of KBr powder and polymer powder into one pellet. Wide-angle X-ray diffraction (WAXD) was recorded at room temperature on a Rigaku D/max 2400 automatic X-ray diffractometer with Ni-filtered Cu Kα radiation (40 V, 100 mA). Polarized optical microscopy (POM) observations were performed at room temperature on a Nikon OPTIPHOT2-POL optical microscope equipped with a digital camera. The phosphoric acid doping level of the membrane was measured by titration with 0.5 M sodium hydroxide. The acid doping level, DL, expressed as the moles of phosphoric acid per mole of PBI repeat unit was calculated from the following equation: DL ¼
ðV NaOH C NaOH Þ ðW dry =M polymer Þ
where VNaOH is the volume of sodium hydroxide solution, CNaOH is concentration of sodium hydroxide solution, Wdry is the weight of dry membrane sample measured by a Sartorius BSA224S-CW electronic balance and Mpolymer is the molecular weight of the polymer repeat unit. The proton conductivity (s) of the acid doped membranes was measured by two-probe electrochemical impedance spectroscopy technique as described in the previous literature [24]. The tested membrane was sandwiched between two stainless steel electrodes in the test cell. The test cell was placed in a temperature controlled sealed oven, in which the air relative humidity was in the range of 5–10%. The resistance of the tested membrane was obtained using CHI600C in the frequency range of 1–105 Hz with amplitude of 5 mV. The tests were recorded at the temperatures from 20 1C to 180 1C with both increasing temperature and decreasing temperature. For each sample, at least three pieces of membranes were tested and their average value was calculated. The proton conductivity (s) of the membranes was calculated using the following equation: s ðS=cmÞ ¼
L SR
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where R is the resistance of the membrane, L represents the distance between two electrodes which is also the thickness of the membrane and S is the area of the interface between the membrane and the electrodes. The oxidative stability was evaluated by immersing the acid doped membrane samples into Fenton's reagent (30 ppm FeSO4 in 30% H2O2) at 25 1C. The procedure was repeated with fresh Fenton solutions every 12 h. During the test, membranes were touched gently with a glass stick every hour, observing if the membrane start to break into pieces. The oxidative stability of the membranes was evaluated by recording the time of membrane start breaking. For each sample, at least four samples were tested and their average value was calculated. The mechanical properties of membrane samples were measured at 25 1C using Instron 5567 instrument with a stretching rate of 2 mm/min. The width of the samples was 4 mm and the distance between the two clips was 25 mm. For each sample, at least four samples were used and their average value was calculated.
3. Results and discussion 3.1. Polymer synthesis and membrane preparation Polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups (PPBIOH) were synthesized via copolycondensation of DAB, TPA-OH and CPPBC in PPA, as described in Scheme 1. The typical monomer concentration in the polymerization of the homo-PBI containing 4-phenyl phthalazinone moieties was 7.61 wt% and the obtained polymer concentration was 7.02 wt% [25]. Because of the low solubility of TPA-OH in PPA solution, the initial monomer concentrations for polymerization of PPBIOH polymers were modified. In the modified polymerization conditions, the monomer concentration was lowered to 3.79–4.62 wt%, resulting the polymer concentration of 3.55– 4.37 wt% (summarized in Table 1). PPBIOHs can be identified as random copolymers of PBI-OH and PPBI. The PBI-OH and PPBI chain segments in PPBIOH main chains played different roles on the properties of polymers and membranes due to the difference in structure characteristics. The PBI-OH polymer was demonstrated to present inter- and intramolecular hydrogen-bonded structures resulted from the hydroxyl groups [26,27] and the remaining hydroxyl groups were expected to be beneficial to PA absorbing and proton conduction. Additionally, it has been suggested that the phosphoric acid esters can be formed by heating hydroxyl compounds with phosphoric acid at temperatures above 100 1C [21]. Thus hydroxyl groups in 116% PPA are expected to form phosphoric acid esters at the high-temperature polymerization conditions of PPBIOH, which were 90 1C for 1 h, 170 1C for 7 h
and 200 1C for 7 h. The PPBIOHs and their corresponding membranes are expected to possess cross-linked structures with phosphoric acid esters. The suggested cross-linked chemical structures of PPBIOHs with phosphoric acid ester are shown in Fig. 1. The cross-linked feature should enhance the mechanical property and the dimensional stability of the PPBIOH membranes. Meanwhile, the PPBI chain segments in PPBIOH polymers can increase the free space between the molecular chains and reduce the interaction of polymer chains because of the introduction of unsymmetrical, twisted and non-coplanar 4-phenyl phthalazinone structures, which could impede the formation of phosphoric acid ester and restrict the degree of crosslinking. The structure of acid doped PPBIOH membranes will be further demonstrated by the following discussion.
3.2. Solubility and inherent viscosity The original PPBIOH polymers before the base-treatment were insoluble in any solvents even on heating at 80 1C. After the basetreatment, the polymers were soluble in H2SO4 (98 wt%), NMP and partially soluble in DMAc, as presented in Table 2. The solubility behavior indicates that the resultant PPBIOH polymers were crosslinked through phosphoric acid ester bridges, which were removed by the base-treatment afterwards. By comparison, p-PPBIs showed excellent solubility in H2SO4 (98 wt%), NMP, DMAc and DMSO, due to the incorporation of twisted non-coplanar 4-phenyl phthalazinone structure as we reported previously [28]. Although both of p-PPBIs and PPBIOHs contain p-phenylene linkages and 4-phenyl phthalazinone moieties, PPBIOHs showed obviously worse solubility than p-PPBIs. This phenomenon indicates the strong interaction between PPBIOH main chains due to the intermolecular hydrogen bonding formed by hydroxyl groups. The solubility results confirm the cross-linked structures of the PPBIOH membranes and the strong interaction of hydrogen bonding in PBIOH chain segments formed by hydroxyl groups. The inherent viscosities of PPBIOH polymers ranged from 2.10 dL/g to 0.33 dL/g, which declined almost an order of Table 1 Reagent ratios and monomer/polymer concentrations for polymerization of PPBIOH Polymers. Polymer
DAB, TPA-OH, g (mmol) g (mmol)
CPPBC, g (mmol)
Monomer conc. (wt%)
Polymer conc. (wt%)
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
0.642(3) 0.642(3) 0.642(3) 0.642(3)
1.3623(2.85) 1.2906(2.70) 1.0038(2.10) 0.7170(1.50)
4.62 4.53 4.16 3.79
4.37 4.28 3.92 3.55
0.0297(0.15) 0.0594(0.30) 0.1782(0.90) 0.2970(1.50)
conc. is short for concentration.
Scheme 1. Synthesis of PPBIOHs.
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Fig. 1. Cross-linked structure of PPBIOH membranes.
Table 2 Inherent viscosities and solubility of PPBIOH. Name
I.V. (dL/g)
H2SO4（98 wt%）
NMP
DMAc
DMSO
DMF
THF
CHCl3
HCOOH
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
2.10 1.30 0.75 0.33
++ ++ ++ ++
++ ++ ++ +−
+− +− +− +−
− − − −
− − − −
− − − −
− − − −
− − − −
++: solubility in room temperature; +−: partially soluble; −: insoluble. NMP: N-methyl pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethyformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.
magnitude with increasing hydroxyl content in PPBIOH, suggesting the rapidly decreased polymer molecular weights. The results are reasonable taking the reactivity of monomers into account. The steric hindrance and the electron donating effect of the hydroxyl groups could reduce the reactivity of carboxyl group in the TPAOH diacid monomer, which could mainly account for the lower reactivity of TPA-OH than that of CPPBC. 3.3. FT-IR spectra The FI-IR spectra of base-treated PPBIOH polymers and acid doped PPBIOH membranes by the PPA process are shown in Figs. 2 and 3, respectively. The spectra in Fig. 2 present characteristic peaks of the PPBIOH polymers after base-treatment. The broad peaks at around 3406 cm−1 are ascribed to the hydrogen-bonded hydroxyl groups and N–H stretch. The benzimidazole characteristic bands can be clearly observed at 1634 cm−1 (C ¼N/C ¼C stretching), 1438 cm−1 (in-plane ring vibration of 2,6-disubstituted benzimidazole), 1285 cm−1 (imidazole ring breathing mode) and 805 cm−1 (the heterocylic ring vibration), similar with that reported previously [29]. The peaks at 1663 cm−1 and 1230 cm−1 are ascribed to the lactam (C ¼O) and aromatic ether linkage (−O−) stretching of 4-phthalazinone moiety, respectively. Meanwhile, they become more intense gradually with the increasing of 4-phenyl phthalazinone moiety content in the polymer backbone, as described in the literature [28]. Moreover, the FT-IR spectra of acid doped PPBIOH membranes prepared by the PPA process are exhibited in Fig. 3. The resultant membranes were so thick (100–300 μm) that their FT-IR test were carried out by the ATR method, which made the characteristic bands of the polymer structures not that clear. However, the characteristic bands of the aromatic phosphate stretch (peaks at
Fig. 2. FT-IR spectra of PPBIOH polymers after base-treatment.
1190 cm−1, 990 cm−1 and 885 cm−1) are so distinct which confirms the formation of phosphoric acid esters and phosphate bridges between polymer chains in the membranes. The FT-IR results indicate the hydrogen-bonded hydroxyl structure in the PPBIOH copolymers after base-treatment and confirm the formation of phosphate bridges in the corresponding acid doped membranes.
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Fig. 3. FT-IR spectra of acid doped PPBIOH membranes.
Fig. 4. WAXD curves of PPBIOH polymers after base-treatment (A: PPBIOH-5; B: PPBIOH-10; C: PPBIOH-30; D: PPBIOH-50).
3.4. X-ray diffraction The WAXD curves of the base-treated PPBIOH polymer powder and the acid doped PPBIOH membranes by the PPA process are illustrated in Figs. 4 and 5, respectively. The PPBIOH polymers without PA ester present a broad peak around 241 and several sharp peaks at 301 and 471, which indicates semi-crystalline characteristic of the polymers. The results are quite different from other PBIs containing 4-phenyl phthalazinone moieties, which all exhibit amorphous nature [25,28]. Meanwhile, for most PA doped PBI membranes, doping with acid could destroy the crystalline order of the membranes, which makes the films more amorphous [30]. However, as shown in Fig. 5, all the acid doped PPBIOH membranes have a broad peak around 241 and several sharp peaks at 71, 151, 301, 461 indicating semi-crystalline characteristic of the membranes. This observation is different from the previous results of other acid doped PBI membranes with amorphous feature [30,31]. The semi-crystalline characteristic of both PPBIOH polymers and the corresponding acid doped membranes may be mainly ascribed to the partially ordered arrangement of the polymer chains. The broad large angle around 241 could be assigned to
Fig. 5. WAXD curves of acid doped PPBIOH membranes (A: PPBIOH-5; B: PPBIOH-10; C: PPBIOH-30; D: PPBIOH-50).
the π–π stacking of aromatic heterocyclic rings [32]. The peaks around 301 agree with the previous results of poly-p-phenylenebenzimidazole [33], which could be ascribe to para-phenylene chains packing of the PBI-OH segments. As aforementioned, the PBI-OH segments in the PPBIOH polymer have strong interaction due to the hydrogen bond with hydroxyl groups, which is favorable for the aggregation of PBI-OH segments and thus the ordered arrangement of molecular chains. Meanwhile, the acid doped PPBIOH membranes by the PPA process present a bit more comlicated structure than the polymers. The PPBIOH membranes are supposed to have cross-linked structure, but the twisted and non-coplanar 4-phenyl phthalazinone structure in PPBIOH main chains may hinder the crosslinking to some extent, resulting in the limited degree of crosslinking. Chen et al. [34] have demonstrated that the limited crosslinking could be favorable to a more regular arrangement between polymer chains during membrane formation. Thus, the acid doped PPBIOH membranes present more crystalline than the base-treated polymers do. Compared with the results of the polymers, the additional peaks in the membrane WAXD curves around 7o and 15o could correspond to the chains laterally packing along the in-plane direction in ordered domains [35] and interchain ordered packing in the direction of membrane thickness [32,35], respectively. The limited cross-linked structure of PPBIOH could contribute to the formation of partially ordered arrangement of PBI-OH segments, resulting in the semi-crystalline characteristic of the PPBIOH membranes. 3.5. Polarized optical microscopy observations The morphology of acid doped PPBIOH membranes was further investigated by polarized optical microscopy. Fig. 6 shows the POM image of acid doped PPBIOH-5 membrane. A few bright crystalline regions can be detected in the POM image, indicating partially crystalline in the membrane. This phenomenon is in accordance with the inference in the X-ray diffraction section, which may due to the formation of partially ordered arrangement of PBI-OH segments. Furthermore, the crystalline regions are about 30– 60 μm wide and separately distributed, corresponding to the low content of PBI-OH segments and aggregation of PBI-OH segments. Meanwhile, the PPBI segments may aggregate in the amorphous regions, where H3PO4 could concentrate. Otherwise, the PPBIOH-10, PPBIOH-30 and PPBIOH-50 membranes were too thick (100–300 μm) and non-transparent, which
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Table 3 Doping level and mechanical properties of PPBIOH membranes. Membrane
Doping level (mol H3PO4)
Young's modulus (MPa)
Tensile strength (MPa)
Elongation at break (%)
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
12.2 11.3 10.1 10.6
29.3 27.8 21.7 9.1
2.9 2.5 2.5 1.7
28.1 48.6 55.9 68.0
Fig. 6. Polarized optical microscope image of acid doped PPBIOH-5 membrane.
impede the light transmission through the membrane. Thus the polarized optical microscopy observation of PPBIOH-10, PPBIOH30 and PPBIOH-50 membranes cannot be carried out. Finally, peculiar morphologies were observed in POM to confirm the semi-crystalline character of the PPBIOH membranes. 3.6. Acid doping level and proton conductivity The acid doping levels of PPBIOH membranes ranged from 10.1 to 12.2 mol H3PO4 per PBI units (shown in Table 3), which are much lower as compared to other acid doped PBI membranes (20– 40 mol H3PO4) by the PPA process [36,37]. The comparably low acid doping levels may be attributed to the cross-linked structure of the PPBIOH membranes. It has been demonstrated in the previous literature that the PA absorption is depressed for the cross-linked membrane [38]. From another point of view, the low doping levels of PPBIOH membranes can be another evidence for the cross-linked structure of PPBIOH membranes. Moreover, as mentioned in the WAXD and POM section above, it has been inferred that the PBI-OH chain segments could aggregate and form the partially ordered arrangement of molecular chain. The formation of hydrogen bond (OH–OH or OH–NH) makes the PBI-OH chain segments aggregate together as well as destroys the PA absorbing ability of –OH and –NH in these chain segments. This could be another reason for the relatively low doping levels of PPBIOH membrane besides their cross-linked structure. This explanation is also in consistent with the decline trend of PA doping levels with increasing hydroxyl content in PPBIOH main chains. The proton conductivities of the dry PPBIOH membranes with the increasing temperature and the decreasing temperature are illustrated in Figs. 7 and 8, respectively. The measurement was carried out in anhydrous condition. All membranes, regardless of the test temperature increasing or decreasing, showed high proton conductivities ( 40.1 S cm−1 at temperatures above 120 1C) and PPBIOH-5 membrane exhibited the highest proton conductivity of 0.21 S cm−1 at 160 1C. For most acid doped PBI membranes reported previously, the proton conductivity is highly dependent on the PA doping level [39]. The PA doped membranes with the proton conductivity of 0.2 S cm−1 usually carry more than 20 mol H3PO4 per polymer repeat unit [40,41]. However, it is worth noting that PPBIOH membranes with just 10.1–12.2 mol H3PO4 exhibited proton conductivity around 0.2 S cm−1 at the temperature of 160 1C, which implies that the proton conductivities of PPBIOH membranes are
Fig. 7. Proton conductivity curves of the PPBIOH membranes with increasing temperature.
Fig. 8. Proton conductivity curves of the PPBIOH membranes with decreasing temperature.
not only dependent on the PA loading. Furthermore, the proton conductivity of membrane can also be affected by its chemical structure or morphology differences induced by the chemical structure as reported previously [21]. Thus, it can be conjectured the structure of PPBIOH membrane could be beneficial for proton conduction. As aforementioned, the PBI-OH chain segments could aggregate and form the crystalline domains, which means the PPBI chain segments aggregate in the amorphous domains. Moreover, the PPBI chain segments could form much larger interspaces between polymer chains than PBI-OH chain segments do because of the introduction of unsymmetrical, twisted and non-coplanar 4-phenyl phthalazinone structures. Thus, the absorbed PA might
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be aggregated in the interspaces generated by 4-phenyl phthalazinone structure. Besides, the N atoms and carbonyl group in 4phenyl phthalazinone moieties are also favorable for absorbing PA [30,42]. As a result, high-efficiency proton exchange pathways may be formed in the amorphous domains through these interspaces with high PA concentration, which could mainly account for the high proton conductivities of the PPBIOH membranes with relatively low doping levels. These results agree very well with the discussion reported previously, the crystalline regions contain low acid content, which forces the excess acid into the amorphous phase, thus increasing the overall conductivity [43]. 3.7. Mechanical properties The mechanical property results of PPBIOH membranes were summarized in Table 3. The membranes displayed tensile strength in the range of 1.7–2.9 MPa and elongation at break in the range of 28.1– 68.0%. The mechanical property values of PPBIOH suggest the membrane exhibit sufficiently high mechanical strength for PEMFC use. It is well known that the mechanical properties of polymer membranes mainly depend on the inherent viscosity, chemical structure of the polymers and macromolecular orientation or crystallinity [32]. For these PPBIOH membranes, the inherent viscosities decreased sharply from 2.1 to 0.33 g/dL with increasing hydroxyl content. However, the tensile strength values reduce slightly from 2.9 to 1.7 MPa, which were comparable with other PA doped PBI membranes [11]. The results could be mainly attributed to the cross-linked structure of PPBIOH membranes. Specially, the elongations at break of PPBIOH membranes (28.1– 68.0%) are much lower than those of other acid doped PBI membranes (150–500%) [13,14,21,29]. The increasing hydroxyl groups could enhance the interaction of polymer chains, which may reduce the elongations at break of the membranes. However, the elongations at break of PPBIOH membranes increased with increasing hydroxyl content in polymers, which agreed well with the declining trend of the inherent viscosity. This may be attributed to the sharply decreased inherent viscosities of PPBIOH with increasing hydroxyl content. The polymers with higher inherent viscosities have much longer molecular chains, which could wrap each other and form chain entanglement, restricting the movement of the polymer chains. Meanwhile, the lower molecular weight polymer chains would serve as plasticizers, which contribute to the elastic deformation and thus the higher elongation at break. On the basis of the analysis above, the inherent viscosity should be the key factor which mainly account for the variation of the elongations at break of PPBIOH membranes. 3.8. Oxidative stability The oxidative stability of membranes in Fenton's reagent is recognized as an indication of long-term durability of PEM in PEMFC. Fig. 9 presents the time that the membrane samples with different hydroxyl content started to break into pieces. The PPBIOH-5, PPBIOH10, PPBIOH-30 and PPBIOH-50 membranes broke into pieces after 155 h, 136 h, 80 h and 52 h, respectively. Commercial m-PBI membrane started to break into pieces after 30 min test [44]. Many aromatic proton exchange membranes with good durability dissolved completely after such measurement for less than 36 h [45,46]. Thus, the PPBIOH membranes present excellent oxidative stability for PEMFC use. It was anticipated that the cross-linked structure formed by hydroxyl groups of PPBIOH would enhance the oxidative stability. However, the oxidative stability of these membrane samples declined with increasing hydroxyl content of the polymer. A tentative explanation for this phenomenon was represented as follows. Because the hydroxyl group with oxygen atom can be easily attacked by the Fenton's reagent, the increased hydroxyl groups could reduce the
Fig. 9. Oxidative stability of acid doped PPBIOH membranes.
oxidative stability. Furthermore, the molecular weight of the polymer, which seriously depressed with increasing hydroxyl content, may partly account for the decreased oxidative stability. Thus, the membrane (PPBIOH-50) with the highest dihydroxy content exhibited the poorest oxidative stability. Nevertheless, the acid doped PPBIOH-50 membrane with such low inherent viscosity (0.33 dL/g) still present so excellent oxidative stability (starting break at 52 h), suggesting that the structure of PPBIOH could enhance the membrane oxidative stability.
4. Conclusion A series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups with expected cross-linked structures were successfully synthesized. The related phosphoric acid doped membranes were prepared by direct casting method using PPA as casting solvent (the PPA process). PPBIOHs exhibit cross-linked structures with phosphoric acid ester confirmed by the solubility behavior, the FT-IR spectra and the results of mechanical properties. Meanwhile, the WAXD results and polarized optical microscopy observations show semicharacter of the PPBIOH membranes, suggesting the aggregation of PBI-OH chain segments in the ordered domains. At the same time, PPBI chain segments containing 4-phenyl phthalazinone moieties could form large interspaces between polymer chains and aggregate in the amorphous domains, which facilitate the aggregation of phosphoric acid. The membranes with relatively low acid doping level had high proton conductivities. From the analysis of the polymer structures and the membrane properties, the possible formation of high-efficiency proton exchange pathway promoted by the aggregation of PPBI chain segments may account for it, demonstrating PPBIOHs with 4-phenyl phthalazinone moieties and hydroxyl groups could enhance the proton conduction of the membranes. Furthermore, the reasonable mechanical strength and oxidative stability can be attributed to the cross-linked PPBIOH structures. In summary, all these excellent properties above make PPBIOH polymers and the corresponding membranes promising potential candidates for use in high-temperature fuel cell application.
Acknowledgment The authors thank the National Natural Science Foundation of China (NSFC, No. 20604004) for its support of this research.
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Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Functionalized 4-phenyl phthalazinone-based polybenzimidazoles for high-temperature PEMFC Xiuping Li a,b, Cheng Liu a,b,c,n, Shouhai Zhang a,b,c, Lishuai Zong a,b, Xigao Jian a,b,c,n a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People's Republic of China Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116024, People's Republic of China c Liaoning Province Engineering Center of High Performance Resins, Dalian 116024, People's Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 November 2012 Received in revised form 14 March 2013 Accepted 18 April 2013 Available online 30 April 2013
A series of novel polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups (PPBIOH) were synthesized from 4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC), 2,5-dyhydroxyterephthalic acid (TPA-OH) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA). The resultant polymers showed good solubility in 98% H2SO4 and NMP. The inherent viscosities of the polymers in 98% H2SO4 at 25 1C were in the range of 0.33–2.10 dL/g, increasing obviously with decreasing hydroxyl content in the main chain. The corresponding acid doped membranes were prepared directly from the polymerization solution by the PPA process. The chemical structures of PPBIOH polymers and membranes were characterized by FT-IR. These membranes showed reasonable doping levels (10.1–12.2 mol H3PO4), high proton conductivities (0.10–0.21 S cm−1 above 120 1C), good mechanical properties (stress of 1.7–2.9 MPa and strain of 28.1–68.0%) and high oxidative stability (breaking time in the range of 52–155 h). The structures of PPBIOH membranes were analyzed by the Wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). The WAXD and POM results indicate the acid doped PPBIOH membranes exhibit semi-crystalline character, from which the mechanism of proton conduction can be further investigated. The combination performance makes these PPBIOH membranes promising candidates for potential applications in high-temperature proton exchange membrane fuel cells (PEMFCs). & 2013 Elsevier B.V. All rights reserved.
Keywords: Functionalized polybenzimidazole Phthalazinone Proton exchange membrane Acid doped
1. Introduction The conventional proton exchange membrane fuel cells (PEMFCs) are based on perfluorosulfonic acid type membranes, such as DuPont's Nafions or similar polymers [1–4]. However, their high cost, environmental inadaptability and susceptibility to catalyst poisoning by impurities such as CO have limited their application. In recent years, much effort has been focused on the development of novel proton exchange membrane (PEM) to substitute the perfluoropolymers and improve the performance of PEMFCs [5–9]. Proton exchange membrane fuel cells operated at high temperatures (4100 1C) can offer many significant benefits, such as better fuel cell CO tolerance, better heat and water management, faster electrode electrochemical reaction kinetics and higher fuel impurity resistance [10,11]. For high performance at elevated temperatures, high-temperature PEMFCs should possess the increased demands for
n Corresponding authors at: Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116024, People's Republic of China. Tel.: +86 411 84986191; fax: +86 411 84986109. E-mail addresses: [email protected], [email protected] (C. Liu), [email protected] (X. Jian).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.04.044
new electrolyte materials such as follows: low-cost materials, high proton conductivity over 100 1C and long-time durability [11]. Therefore, high-temperature polymer electrolyte membrane materials have become promising candidates for application in PEMFC field. Phosphoric acid (PA) doped PBI membranes were investigated as promising high-temperature PEMFC candidates for their good mechanical property, excellent chemical and thermal stability, particularly since the landmark study by Wainright et al. in 1995 [12]. Additionally, the PA doped PBI membranes can maintain high proton conductivity under anhydrous conditions. The PPA process is an effective method to prepare PA doped PBI membranes with high doping levels, which usually lead to high proton conductivity. The PPA process [13–15] is a sol–gel process to fabricate PBI-H3PO4 membranes directly from PBI polycondensation solution in PPA at around 200 1C, without isolation or re-dissolution of the polymer after synthesis. The PPA in the membranes is transformed into PA by hydrolysis of PPA in the air, resulting in PA doped PBI membranes. Therefore, the PBI membranes with high doping levels of 20–50 mol of PA per PBI repeat unit can be obtained by the PPA process, which have expected high proton conductivities at high temperatures. Though high doping levels may lead to high proton conductivity, too much PA loading in PBI membrane could result in poor mechanical
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stability and oxidative stability. Crosslinking has been reported as a feasible method to improve the chemical stability as well as the mechanical properties of the polymer [16–20]. More recently, dihydroxy functional groups in the PBI main chain were incorporated and used as handles for further crosslinking reactions [21]. In this paper, we preliminarily focus on the effect of functional hydroxyl groups and twisted phthalazinone moieties on the structures of polybenzimidazoles and the properties of PA doped membranes by chemical modification. Thus, a novel series of PPBIOH copolymers containing 4-phenyl phthalazinone moieties and functional hydroxyl groups were synthesized. The corresponding PPBIOH PA doped membranes were prepared by the PPA process. The functional hydroxyl groups were expected to form the crosslinked structure of the membranes and enhance the interaction between polymer chains, by which the mechanical properties and the oxidative stabilities could be enhanced. Meanwhile, 4-phenyl phthalazinone moiety exhibits a heterocyclic, twisted, non-coplanar and asymmetrical structure, which can cumber the close packing of the polymer chains and increase the free space between the molecular chains. In addition, the N atom and carbonyl group of 4-phenyl phthalazinone moiety could also enhance the acid absorbing and proton conductivity. This special structure may afford interspaces to concentrate PA and form a particular proton conduction pathway through these high PA concentration interspaces in the membranes, which makes the membranes present high proton conductivity. The relationship of polymer structure and properties of PPBIOH membranes has been investigated and discussed in detail.
2. Experimental details 2.1. Materials 3,3′-Diaminobenzidine (DAB, 99%) and 2,5-dihydroxyterephthalic acid (TPA-OH, 99%) were obtained from Sigma-Aldrich Co. Inc and used as received. 4-(4-(4-(4-Carboxyphenoxy)phenyl)1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC) was synthesized according to the method reported previously [22]. Polyphosphoric acid (PPA, 116%) was prepared by heating 1:1.8 weight ratio of ortho phosphoric acid (85%) and phosphorus pentoxide for 6 h at 120 1C. All other solvents and chemicals were purchased from commercial sources and used without further purification. 2.1.1. Polymer synthesis and film formation Polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups were synthesized from various stoichiometric mixtures of CPPBC and TPA-OH with DAB by solution copolycondensation in PPA. These copolymers are referred as PPBIOH-x, where x is the molar fraction of TPA-OH in the feed. Taking the synthesis of PPBIOH-5 as an example, DAB (0.642 g, 3 mmol), TPA-OH (0.0297 g, 0.15 mmol) and CPPBC (1.3623 g, 2.85 mmol) were placed into a 100 mL three-neck round bottom flask equipped with an overhead mechanical stirrer, a water cooled condenser and nitrogen purge inlet and outlet, followed by the addition of 20 mL PPA. The reaction mixture was stirred in N2 at 90 1C for 1 h, 170 1C for 7 h and 200 1C for 7 h. During the polymerization at 200 1C, the reaction mixture became more and more viscous and 10 mL PPA was added in this stage. After the final stage of polymerization, the mixture was heated to 220 1C and another 15 mL PPA was added into the reaction mixture to lower the solution viscosity, aiding the membrane casting process. After stirring for 30 min, part of the polymer solution was directly cast onto clean glass substrates with a doctor blade. The obtained membrane was placed in a humiditycontrolled environment (60–70% RH) at room temperature for a
161
week, absorbing moisture from the environment. The samples were transformed into gel membranes and the viscous phosphoric acid solution resulting from PPA hydrolysis could be observed from the membranes. Then the membranes were wiped dry and dried in vacuum before further measurements. The membrane preparation procedure described above is the PPA process. The membranes after hydrolysis were dried in vacuum at 100 1C for 12 h before further measurements. The thickness of final acid doped PPBIOH gel membrane was in the range of 100–300 μm. The remaining reaction solution was poured into hot water with stirring. The fiber-like precipitate was filtered off and washed with water until neutral. The obtained polymer was dried in vacuum at 100 1C for 12 h. The resultant PPBIOH polymers were insoluble in any solvents due to the crosslink structure of PA ester, which were discussed in detail in the results and discussion section. In order to further investigate their solubilities and inherent viscosities, the PPBIOH polymers were treated in DMSO/H2O mixture with 0.02 M of NaOH to remove the PA ester as described in the previous literature [23]. 2.1.2. Characterization The solubility of the polymers after the base-treatment was carried out by dissolving polymer in solvent at a concentration of 0.01 g/mL. Inherent viscosities of the polymers after the basetreatment were obtained by Ubbelohde capillary viscometer at 25 1C using 98% concentrated sulfuric acid as solvent. FT-IR spectra were performed on a Thermo Nicolet Nexus 470 Fourier transform infrared spectrometer. The membranes were measured directly using attenuated total reflection (ATR) method. The polymer samples were prepared by pressing the mixture of KBr powder and polymer powder into one pellet. Wide-angle X-ray diffraction (WAXD) was recorded at room temperature on a Rigaku D/max 2400 automatic X-ray diffractometer with Ni-filtered Cu Kα radiation (40 V, 100 mA). Polarized optical microscopy (POM) observations were performed at room temperature on a Nikon OPTIPHOT2-POL optical microscope equipped with a digital camera. The phosphoric acid doping level of the membrane was measured by titration with 0.5 M sodium hydroxide. The acid doping level, DL, expressed as the moles of phosphoric acid per mole of PBI repeat unit was calculated from the following equation: DL ¼
ðV NaOH C NaOH Þ ðW dry =M polymer Þ
where VNaOH is the volume of sodium hydroxide solution, CNaOH is concentration of sodium hydroxide solution, Wdry is the weight of dry membrane sample measured by a Sartorius BSA224S-CW electronic balance and Mpolymer is the molecular weight of the polymer repeat unit. The proton conductivity (s) of the acid doped membranes was measured by two-probe electrochemical impedance spectroscopy technique as described in the previous literature [24]. The tested membrane was sandwiched between two stainless steel electrodes in the test cell. The test cell was placed in a temperature controlled sealed oven, in which the air relative humidity was in the range of 5–10%. The resistance of the tested membrane was obtained using CHI600C in the frequency range of 1–105 Hz with amplitude of 5 mV. The tests were recorded at the temperatures from 20 1C to 180 1C with both increasing temperature and decreasing temperature. For each sample, at least three pieces of membranes were tested and their average value was calculated. The proton conductivity (s) of the membranes was calculated using the following equation: s ðS=cmÞ ¼
L SR
162
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where R is the resistance of the membrane, L represents the distance between two electrodes which is also the thickness of the membrane and S is the area of the interface between the membrane and the electrodes. The oxidative stability was evaluated by immersing the acid doped membrane samples into Fenton's reagent (30 ppm FeSO4 in 30% H2O2) at 25 1C. The procedure was repeated with fresh Fenton solutions every 12 h. During the test, membranes were touched gently with a glass stick every hour, observing if the membrane start to break into pieces. The oxidative stability of the membranes was evaluated by recording the time of membrane start breaking. For each sample, at least four samples were tested and their average value was calculated. The mechanical properties of membrane samples were measured at 25 1C using Instron 5567 instrument with a stretching rate of 2 mm/min. The width of the samples was 4 mm and the distance between the two clips was 25 mm. For each sample, at least four samples were used and their average value was calculated.
3. Results and discussion 3.1. Polymer synthesis and membrane preparation Polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups (PPBIOH) were synthesized via copolycondensation of DAB, TPA-OH and CPPBC in PPA, as described in Scheme 1. The typical monomer concentration in the polymerization of the homo-PBI containing 4-phenyl phthalazinone moieties was 7.61 wt% and the obtained polymer concentration was 7.02 wt% [25]. Because of the low solubility of TPA-OH in PPA solution, the initial monomer concentrations for polymerization of PPBIOH polymers were modified. In the modified polymerization conditions, the monomer concentration was lowered to 3.79–4.62 wt%, resulting the polymer concentration of 3.55– 4.37 wt% (summarized in Table 1). PPBIOHs can be identified as random copolymers of PBI-OH and PPBI. The PBI-OH and PPBI chain segments in PPBIOH main chains played different roles on the properties of polymers and membranes due to the difference in structure characteristics. The PBI-OH polymer was demonstrated to present inter- and intramolecular hydrogen-bonded structures resulted from the hydroxyl groups [26,27] and the remaining hydroxyl groups were expected to be beneficial to PA absorbing and proton conduction. Additionally, it has been suggested that the phosphoric acid esters can be formed by heating hydroxyl compounds with phosphoric acid at temperatures above 100 1C [21]. Thus hydroxyl groups in 116% PPA are expected to form phosphoric acid esters at the high-temperature polymerization conditions of PPBIOH, which were 90 1C for 1 h, 170 1C for 7 h
and 200 1C for 7 h. The PPBIOHs and their corresponding membranes are expected to possess cross-linked structures with phosphoric acid esters. The suggested cross-linked chemical structures of PPBIOHs with phosphoric acid ester are shown in Fig. 1. The cross-linked feature should enhance the mechanical property and the dimensional stability of the PPBIOH membranes. Meanwhile, the PPBI chain segments in PPBIOH polymers can increase the free space between the molecular chains and reduce the interaction of polymer chains because of the introduction of unsymmetrical, twisted and non-coplanar 4-phenyl phthalazinone structures, which could impede the formation of phosphoric acid ester and restrict the degree of crosslinking. The structure of acid doped PPBIOH membranes will be further demonstrated by the following discussion.
3.2. Solubility and inherent viscosity The original PPBIOH polymers before the base-treatment were insoluble in any solvents even on heating at 80 1C. After the basetreatment, the polymers were soluble in H2SO4 (98 wt%), NMP and partially soluble in DMAc, as presented in Table 2. The solubility behavior indicates that the resultant PPBIOH polymers were crosslinked through phosphoric acid ester bridges, which were removed by the base-treatment afterwards. By comparison, p-PPBIs showed excellent solubility in H2SO4 (98 wt%), NMP, DMAc and DMSO, due to the incorporation of twisted non-coplanar 4-phenyl phthalazinone structure as we reported previously [28]. Although both of p-PPBIs and PPBIOHs contain p-phenylene linkages and 4-phenyl phthalazinone moieties, PPBIOHs showed obviously worse solubility than p-PPBIs. This phenomenon indicates the strong interaction between PPBIOH main chains due to the intermolecular hydrogen bonding formed by hydroxyl groups. The solubility results confirm the cross-linked structures of the PPBIOH membranes and the strong interaction of hydrogen bonding in PBIOH chain segments formed by hydroxyl groups. The inherent viscosities of PPBIOH polymers ranged from 2.10 dL/g to 0.33 dL/g, which declined almost an order of Table 1 Reagent ratios and monomer/polymer concentrations for polymerization of PPBIOH Polymers. Polymer
DAB, TPA-OH, g (mmol) g (mmol)
CPPBC, g (mmol)
Monomer conc. (wt%)
Polymer conc. (wt%)
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
0.642(3) 0.642(3) 0.642(3) 0.642(3)
1.3623(2.85) 1.2906(2.70) 1.0038(2.10) 0.7170(1.50)
4.62 4.53 4.16 3.79
4.37 4.28 3.92 3.55
0.0297(0.15) 0.0594(0.30) 0.1782(0.90) 0.2970(1.50)
conc. is short for concentration.
Scheme 1. Synthesis of PPBIOHs.
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163
Fig. 1. Cross-linked structure of PPBIOH membranes.
Table 2 Inherent viscosities and solubility of PPBIOH. Name
I.V. (dL/g)
H2SO4（98 wt%）
NMP
DMAc
DMSO
DMF
THF
CHCl3
HCOOH
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
2.10 1.30 0.75 0.33
++ ++ ++ ++
++ ++ ++ +−
+− +− +− +−
− − − −
− − − −
− − − −
− − − −
− − − −
++: solubility in room temperature; +−: partially soluble; −: insoluble. NMP: N-methyl pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethyformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.
magnitude with increasing hydroxyl content in PPBIOH, suggesting the rapidly decreased polymer molecular weights. The results are reasonable taking the reactivity of monomers into account. The steric hindrance and the electron donating effect of the hydroxyl groups could reduce the reactivity of carboxyl group in the TPAOH diacid monomer, which could mainly account for the lower reactivity of TPA-OH than that of CPPBC. 3.3. FT-IR spectra The FI-IR spectra of base-treated PPBIOH polymers and acid doped PPBIOH membranes by the PPA process are shown in Figs. 2 and 3, respectively. The spectra in Fig. 2 present characteristic peaks of the PPBIOH polymers after base-treatment. The broad peaks at around 3406 cm−1 are ascribed to the hydrogen-bonded hydroxyl groups and N–H stretch. The benzimidazole characteristic bands can be clearly observed at 1634 cm−1 (C ¼N/C ¼C stretching), 1438 cm−1 (in-plane ring vibration of 2,6-disubstituted benzimidazole), 1285 cm−1 (imidazole ring breathing mode) and 805 cm−1 (the heterocylic ring vibration), similar with that reported previously [29]. The peaks at 1663 cm−1 and 1230 cm−1 are ascribed to the lactam (C ¼O) and aromatic ether linkage (−O−) stretching of 4-phthalazinone moiety, respectively. Meanwhile, they become more intense gradually with the increasing of 4-phenyl phthalazinone moiety content in the polymer backbone, as described in the literature [28]. Moreover, the FT-IR spectra of acid doped PPBIOH membranes prepared by the PPA process are exhibited in Fig. 3. The resultant membranes were so thick (100–300 μm) that their FT-IR test were carried out by the ATR method, which made the characteristic bands of the polymer structures not that clear. However, the characteristic bands of the aromatic phosphate stretch (peaks at
Fig. 2. FT-IR spectra of PPBIOH polymers after base-treatment.
1190 cm−1, 990 cm−1 and 885 cm−1) are so distinct which confirms the formation of phosphoric acid esters and phosphate bridges between polymer chains in the membranes. The FT-IR results indicate the hydrogen-bonded hydroxyl structure in the PPBIOH copolymers after base-treatment and confirm the formation of phosphate bridges in the corresponding acid doped membranes.
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Fig. 3. FT-IR spectra of acid doped PPBIOH membranes.
Fig. 4. WAXD curves of PPBIOH polymers after base-treatment (A: PPBIOH-5; B: PPBIOH-10; C: PPBIOH-30; D: PPBIOH-50).
3.4. X-ray diffraction The WAXD curves of the base-treated PPBIOH polymer powder and the acid doped PPBIOH membranes by the PPA process are illustrated in Figs. 4 and 5, respectively. The PPBIOH polymers without PA ester present a broad peak around 241 and several sharp peaks at 301 and 471, which indicates semi-crystalline characteristic of the polymers. The results are quite different from other PBIs containing 4-phenyl phthalazinone moieties, which all exhibit amorphous nature [25,28]. Meanwhile, for most PA doped PBI membranes, doping with acid could destroy the crystalline order of the membranes, which makes the films more amorphous [30]. However, as shown in Fig. 5, all the acid doped PPBIOH membranes have a broad peak around 241 and several sharp peaks at 71, 151, 301, 461 indicating semi-crystalline characteristic of the membranes. This observation is different from the previous results of other acid doped PBI membranes with amorphous feature [30,31]. The semi-crystalline characteristic of both PPBIOH polymers and the corresponding acid doped membranes may be mainly ascribed to the partially ordered arrangement of the polymer chains. The broad large angle around 241 could be assigned to
Fig. 5. WAXD curves of acid doped PPBIOH membranes (A: PPBIOH-5; B: PPBIOH-10; C: PPBIOH-30; D: PPBIOH-50).
the π–π stacking of aromatic heterocyclic rings [32]. The peaks around 301 agree with the previous results of poly-p-phenylenebenzimidazole [33], which could be ascribe to para-phenylene chains packing of the PBI-OH segments. As aforementioned, the PBI-OH segments in the PPBIOH polymer have strong interaction due to the hydrogen bond with hydroxyl groups, which is favorable for the aggregation of PBI-OH segments and thus the ordered arrangement of molecular chains. Meanwhile, the acid doped PPBIOH membranes by the PPA process present a bit more comlicated structure than the polymers. The PPBIOH membranes are supposed to have cross-linked structure, but the twisted and non-coplanar 4-phenyl phthalazinone structure in PPBIOH main chains may hinder the crosslinking to some extent, resulting in the limited degree of crosslinking. Chen et al. [34] have demonstrated that the limited crosslinking could be favorable to a more regular arrangement between polymer chains during membrane formation. Thus, the acid doped PPBIOH membranes present more crystalline than the base-treated polymers do. Compared with the results of the polymers, the additional peaks in the membrane WAXD curves around 7o and 15o could correspond to the chains laterally packing along the in-plane direction in ordered domains [35] and interchain ordered packing in the direction of membrane thickness [32,35], respectively. The limited cross-linked structure of PPBIOH could contribute to the formation of partially ordered arrangement of PBI-OH segments, resulting in the semi-crystalline characteristic of the PPBIOH membranes. 3.5. Polarized optical microscopy observations The morphology of acid doped PPBIOH membranes was further investigated by polarized optical microscopy. Fig. 6 shows the POM image of acid doped PPBIOH-5 membrane. A few bright crystalline regions can be detected in the POM image, indicating partially crystalline in the membrane. This phenomenon is in accordance with the inference in the X-ray diffraction section, which may due to the formation of partially ordered arrangement of PBI-OH segments. Furthermore, the crystalline regions are about 30– 60 μm wide and separately distributed, corresponding to the low content of PBI-OH segments and aggregation of PBI-OH segments. Meanwhile, the PPBI segments may aggregate in the amorphous regions, where H3PO4 could concentrate. Otherwise, the PPBIOH-10, PPBIOH-30 and PPBIOH-50 membranes were too thick (100–300 μm) and non-transparent, which
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Table 3 Doping level and mechanical properties of PPBIOH membranes. Membrane
Doping level (mol H3PO4)
Young's modulus (MPa)
Tensile strength (MPa)
Elongation at break (%)
PPBIOH-5 PPBIOH-10 PPBIOH-30 PPBIOH-50
12.2 11.3 10.1 10.6
29.3 27.8 21.7 9.1
2.9 2.5 2.5 1.7
28.1 48.6 55.9 68.0
Fig. 6. Polarized optical microscope image of acid doped PPBIOH-5 membrane.
impede the light transmission through the membrane. Thus the polarized optical microscopy observation of PPBIOH-10, PPBIOH30 and PPBIOH-50 membranes cannot be carried out. Finally, peculiar morphologies were observed in POM to confirm the semi-crystalline character of the PPBIOH membranes. 3.6. Acid doping level and proton conductivity The acid doping levels of PPBIOH membranes ranged from 10.1 to 12.2 mol H3PO4 per PBI units (shown in Table 3), which are much lower as compared to other acid doped PBI membranes (20– 40 mol H3PO4) by the PPA process [36,37]. The comparably low acid doping levels may be attributed to the cross-linked structure of the PPBIOH membranes. It has been demonstrated in the previous literature that the PA absorption is depressed for the cross-linked membrane [38]. From another point of view, the low doping levels of PPBIOH membranes can be another evidence for the cross-linked structure of PPBIOH membranes. Moreover, as mentioned in the WAXD and POM section above, it has been inferred that the PBI-OH chain segments could aggregate and form the partially ordered arrangement of molecular chain. The formation of hydrogen bond (OH–OH or OH–NH) makes the PBI-OH chain segments aggregate together as well as destroys the PA absorbing ability of –OH and –NH in these chain segments. This could be another reason for the relatively low doping levels of PPBIOH membrane besides their cross-linked structure. This explanation is also in consistent with the decline trend of PA doping levels with increasing hydroxyl content in PPBIOH main chains. The proton conductivities of the dry PPBIOH membranes with the increasing temperature and the decreasing temperature are illustrated in Figs. 7 and 8, respectively. The measurement was carried out in anhydrous condition. All membranes, regardless of the test temperature increasing or decreasing, showed high proton conductivities ( 40.1 S cm−1 at temperatures above 120 1C) and PPBIOH-5 membrane exhibited the highest proton conductivity of 0.21 S cm−1 at 160 1C. For most acid doped PBI membranes reported previously, the proton conductivity is highly dependent on the PA doping level [39]. The PA doped membranes with the proton conductivity of 0.2 S cm−1 usually carry more than 20 mol H3PO4 per polymer repeat unit [40,41]. However, it is worth noting that PPBIOH membranes with just 10.1–12.2 mol H3PO4 exhibited proton conductivity around 0.2 S cm−1 at the temperature of 160 1C, which implies that the proton conductivities of PPBIOH membranes are
Fig. 7. Proton conductivity curves of the PPBIOH membranes with increasing temperature.
Fig. 8. Proton conductivity curves of the PPBIOH membranes with decreasing temperature.
not only dependent on the PA loading. Furthermore, the proton conductivity of membrane can also be affected by its chemical structure or morphology differences induced by the chemical structure as reported previously [21]. Thus, it can be conjectured the structure of PPBIOH membrane could be beneficial for proton conduction. As aforementioned, the PBI-OH chain segments could aggregate and form the crystalline domains, which means the PPBI chain segments aggregate in the amorphous domains. Moreover, the PPBI chain segments could form much larger interspaces between polymer chains than PBI-OH chain segments do because of the introduction of unsymmetrical, twisted and non-coplanar 4-phenyl phthalazinone structures. Thus, the absorbed PA might
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be aggregated in the interspaces generated by 4-phenyl phthalazinone structure. Besides, the N atoms and carbonyl group in 4phenyl phthalazinone moieties are also favorable for absorbing PA [30,42]. As a result, high-efficiency proton exchange pathways may be formed in the amorphous domains through these interspaces with high PA concentration, which could mainly account for the high proton conductivities of the PPBIOH membranes with relatively low doping levels. These results agree very well with the discussion reported previously, the crystalline regions contain low acid content, which forces the excess acid into the amorphous phase, thus increasing the overall conductivity [43]. 3.7. Mechanical properties The mechanical property results of PPBIOH membranes were summarized in Table 3. The membranes displayed tensile strength in the range of 1.7–2.9 MPa and elongation at break in the range of 28.1– 68.0%. The mechanical property values of PPBIOH suggest the membrane exhibit sufficiently high mechanical strength for PEMFC use. It is well known that the mechanical properties of polymer membranes mainly depend on the inherent viscosity, chemical structure of the polymers and macromolecular orientation or crystallinity [32]. For these PPBIOH membranes, the inherent viscosities decreased sharply from 2.1 to 0.33 g/dL with increasing hydroxyl content. However, the tensile strength values reduce slightly from 2.9 to 1.7 MPa, which were comparable with other PA doped PBI membranes [11]. The results could be mainly attributed to the cross-linked structure of PPBIOH membranes. Specially, the elongations at break of PPBIOH membranes (28.1– 68.0%) are much lower than those of other acid doped PBI membranes (150–500%) [13,14,21,29]. The increasing hydroxyl groups could enhance the interaction of polymer chains, which may reduce the elongations at break of the membranes. However, the elongations at break of PPBIOH membranes increased with increasing hydroxyl content in polymers, which agreed well with the declining trend of the inherent viscosity. This may be attributed to the sharply decreased inherent viscosities of PPBIOH with increasing hydroxyl content. The polymers with higher inherent viscosities have much longer molecular chains, which could wrap each other and form chain entanglement, restricting the movement of the polymer chains. Meanwhile, the lower molecular weight polymer chains would serve as plasticizers, which contribute to the elastic deformation and thus the higher elongation at break. On the basis of the analysis above, the inherent viscosity should be the key factor which mainly account for the variation of the elongations at break of PPBIOH membranes. 3.8. Oxidative stability The oxidative stability of membranes in Fenton's reagent is recognized as an indication of long-term durability of PEM in PEMFC. Fig. 9 presents the time that the membrane samples with different hydroxyl content started to break into pieces. The PPBIOH-5, PPBIOH10, PPBIOH-30 and PPBIOH-50 membranes broke into pieces after 155 h, 136 h, 80 h and 52 h, respectively. Commercial m-PBI membrane started to break into pieces after 30 min test [44]. Many aromatic proton exchange membranes with good durability dissolved completely after such measurement for less than 36 h [45,46]. Thus, the PPBIOH membranes present excellent oxidative stability for PEMFC use. It was anticipated that the cross-linked structure formed by hydroxyl groups of PPBIOH would enhance the oxidative stability. However, the oxidative stability of these membrane samples declined with increasing hydroxyl content of the polymer. A tentative explanation for this phenomenon was represented as follows. Because the hydroxyl group with oxygen atom can be easily attacked by the Fenton's reagent, the increased hydroxyl groups could reduce the
Fig. 9. Oxidative stability of acid doped PPBIOH membranes.
oxidative stability. Furthermore, the molecular weight of the polymer, which seriously depressed with increasing hydroxyl content, may partly account for the decreased oxidative stability. Thus, the membrane (PPBIOH-50) with the highest dihydroxy content exhibited the poorest oxidative stability. Nevertheless, the acid doped PPBIOH-50 membrane with such low inherent viscosity (0.33 dL/g) still present so excellent oxidative stability (starting break at 52 h), suggesting that the structure of PPBIOH could enhance the membrane oxidative stability.
4. Conclusion A series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups with expected cross-linked structures were successfully synthesized. The related phosphoric acid doped membranes were prepared by direct casting method using PPA as casting solvent (the PPA process). PPBIOHs exhibit cross-linked structures with phosphoric acid ester confirmed by the solubility behavior, the FT-IR spectra and the results of mechanical properties. Meanwhile, the WAXD results and polarized optical microscopy observations show semicharacter of the PPBIOH membranes, suggesting the aggregation of PBI-OH chain segments in the ordered domains. At the same time, PPBI chain segments containing 4-phenyl phthalazinone moieties could form large interspaces between polymer chains and aggregate in the amorphous domains, which facilitate the aggregation of phosphoric acid. The membranes with relatively low acid doping level had high proton conductivities. From the analysis of the polymer structures and the membrane properties, the possible formation of high-efficiency proton exchange pathway promoted by the aggregation of PPBI chain segments may account for it, demonstrating PPBIOHs with 4-phenyl phthalazinone moieties and hydroxyl groups could enhance the proton conduction of the membranes. Furthermore, the reasonable mechanical strength and oxidative stability can be attributed to the cross-linked PPBIOH structures. In summary, all these excellent properties above make PPBIOH polymers and the corresponding membranes promising potential candidates for use in high-temperature fuel cell application.
Acknowledgment The authors thank the National Natural Science Foundation of China (NSFC, No. 20604004) for its support of this research.
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