Acid doped polybenzimidazoles containing 4-phenyl phthalazinone moieties for high-temperature PEMFC

Journal of Membrane Science 423–424 (2012) 128–135

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Acid doped polybenzimidazoles containing 4-phenyl phthalazinone moieties for high-temperature PEMFC Xiuping Li a,b, Cheng Liu a,b,c,n, Shouhai Zhang a,b,c, Guipeng Yu 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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 April 2012 Received in revised form 30 July 2012 Accepted 1 August 2012 Available online 9 August 2012

A novel series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages (p-PPBI) were synthesized from 4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)yl)benzoic acid (CPPBC), terephthalic acid (TPA) and 3,30 -diaminobenzidine (DAB) in polyphosphoric acid (PPA). The chemical structures of p-PPBI polymers were characterized by FT–IR, 1H NMR, 13C NMR and WAXD. The resultant polymers have inherent viscosities from 1.78 to 3.43 dL/g in 96% sulfuric acid at 30 1C and show good solubility in aprotic polar solvents, such as N-methyl-2-pyrrolidone (NMP), N,Ndimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). These p-PPBI polymers exhibit high glass transition temperatures (Tg) in the range of 399–426 1C and excellent thermal stability with the 5% (Td5%) and 10% (Td10%) weight loss temperatures in the range of 516 to 594 1C and 560 to 672 1C, respectively. These amorphous polymers were cast into membranes and further investigated as proton exchange membranes. Membranes with high acid doping level ( 15.2 mol H3PO4) and high proton conductivity (  0.13 S/cm) were obtained after doping with phosphoric acid. These obtained membranes present good oxidative stability and mechanical property even with high doping levels. The p-PPBI polymers are promising candidates for high-temperature proton exchange membranes. & 2012 Elsevier B.V. All rights reserved.

Keywords: Polybenzimidazole Phthalazinone Soluble Acid doped Proton exchange membrane

1. Introduction Proton exchange membrane fuel cell (PEMFC) is considered to be a promising technology for clean and efficient power generation [1–5]. For a high-efficiency PEMFC system, proton exchange membranes (PEMs) are the key elements. Compared with the perfluorosulphonic acid (PFSA) polymers, e.g., Nafions, which is the most used PEM material currently, phosphoric acid doped polybenzimidazoles are considered promising candidates operating at high temperature (100–200 1C) for their excellent thermal and chemical stability as well as high proton conductivity [6–8]. High temperature fuel cells operating at temperatures higher than 100 1C can provide many benefits, such as high CO tolerance, fast electrochemical reaction kinetics, and high fuel impurity tolerance, which allow for simplification of the fuel processing system and possible integration of the fuel cell stack with fuel processing unit [1,9–12]. From the proton conducting mechanism points of view, phosphorus and phosphoric acid are amphoteric, with both proton donor (acidic) and proton acceptor (basic) n Corresponding authors at: Dalian University of Technology, Dept of Polymer Material, State Key Laboratory of Fine Chemicals, Box 42, Zhongshan Road 158, Dalian 116012, China. Tel.: þ86 411 84986191; fax: þ 86 411 84986109. E-mail addresses: [email protected] (C. Liu), [email protected] (X. Jian).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.08.005

groups to form dynamic hydrogen bond networks, in which protons can readily transfer by hydrogen bond breaking and forming processes [13,14]. For acid doped PBI membranes, there is a tradeoff between the desired properties of the membranes, such as high proton conductivity and good mechanical properties. For example, poly(2,20 (m-phenylene)-5,50 -bibenzimidazole) (m-PBI) membrane exhibits high proton conductivity at high doping level of 13.0–16.0 mol H3PO4, but the mechanical property of m-PBI membrane with the same doping level is too worsen to use for PEMFC. Bjerrum et al. [7] suggested the maximal doping level around 5.0 mol H3PO4 to maintain reasonable mechanical strength of m-PBI membranes for PEMFC application, corresponding to a conductivity of 0.03 S/cm. Comparatively, poly(2,20 -(1,4-phenylene)-5,50 -bibenzimidazole) (p-PBI) shows superior mechanical strength because of the paraordered molecular orientation and high molecular weight of the polymer [15–17]. Thus, p-PBI might retain its mechanical strength with high doping level. However, the application of p-PBI in PEMFC is limited by its extremely poor processibility, as p-PBI is only soluble in strong acids. Much effort has been exercised to improve the solubility of p-PBI, such as incorporation of aryl ether linkage or m-phenylene linkage into p-PBI backbone [9,18]. However, these reports still compromise the thermal stability or the mechanical stability of p-PBI.

X. Li et al. / Journal of Membrane Science 423–424 (2012) 128–135

4-phenyl phthalazinone moiety presents a heterocyclic, twisted, non-coplanar and asymmetrical structure, which can reduce the regularity of the polymer main chain, encumber the close packing of chains and increase the free space between the molecular chains. A series of aromatic polymers containing 4-phenyl phthalazinone moieties [19–23] have been developed, which exhibit improved solubility, excellent thermal stability and chemical resistance. Introduction of 4-phenyl phthalazinone structure into p-PBI backbone may lead to amorphous structure of the polymer, which could be beneficial for both solvent solubility and proton transfer. Additionally, N atoms of 4-phenyl phthalazinone moiety in PBI mainchain is expected to absorb more phosphoric acid and consequently enhance the proton conductivity [24]. From above, it is inferable that polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages may possess both good solubility and high mechanical properties. Thus, the related membranes can be expected to retain good mechanical property with high doping level. In this paper, a novel series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages were synthesized, and the related acid doped membranes were prepared afterwards. The relationship of polymer structures and membrane properties was investigated systematically, and the properties of polymers and the corresponding acid doped membranes were characterized and discussed in detail.

2. Experimental details 2.1. Materials Terephthalic acid (TPA, 99%) and 3,30 -diaminobenzidine (DAB, 99%) were purchased from Sigma-Aldrich Co. Inc. and purified by recrystallization from ethanol and water, respectively. 4-(4-(4-(4Carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC) was synthesized according to the method reported previously [25]. Polyphosphoric acid (PPA, 116%) was prepared by heating 1:1.8 weight ratio of ortho phosphoric acid (85%) and phosphorus pentaoxide for 6 h at 120 1C. All other solvents and chemicals were obtained from commercial sources and used as received.

2.3. Membrane preparation and acid doping PPBI and p-PPBI-X membranes were prepared by the solution casting technique using NMP as solvent. A typical procedure for the membrane fabrication is as follows. The polymer sample was dissolved in NMP with the concentration of 5 wt% to obtain a homogeneous solution. Then the solution was filtered and cast onto a clean flat glass plate. The solvent was evaporated slowly at 60 1C for 12 h under a ventilated hood. After cooling to room temperature, the obtained brown membrane was soaked in water and peeled off from the substrate. The membrane was then dried at 100 1C under vacuum for 12 h. The thickness of solution cast membranes in dry state was in the range of 30–50 mm. The membranes were doped into 85 wt% phosphoric acid (PA) at 150 1C for 72 h. After acid doping, the membranes were taken out from PA, wiped dry with tissue, and dried under vacuum at 100 1C for 12 h. 2.4. Characterization FT–IR spectra were conducted on a Thermo Nicolet Nexus 470 Fourier transform infrared (FT–IR) spectrometer. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured with a Brucker spectrometer at an operating temperature of 25 1C using H2SO4-d6 as solvent. Inherent viscosities of the polymers were measured by Ubbelohde capillary viscometer at 30 1C using 96% concentrated sulfuric acid as solvent. The glass transition temperature (Tg) was determined with a Mettler DSC822 differential scanning calorimetry (DSC) in flowing nitrogen at a heating rate of 10 1C/min from 100 to 450 1C. Thermogravimetric analysis (TGA) of the polymers was performed on a Mettler TGA/SDTA851 thermogravimetric analysis instrument under nitrogen atmosphere at a heating rate of 10 1C/min from 150 to 800 1C. Wideangle X-ray diffraction (WAXD) was performed at room temperature on a Rigaku D/max 2400 automatic X-ray diffractometer with Ni-filtered Cu Ka radiation (40 V, 100 mA). The acid doping level was calculated as the difference in weight of the membrane with and without phosphoric acid, thereby determining the moles of phosphoric acid per mole of PBI repeat unit from the following equation: Doping level ¼

2.2. Polymer synthesis The PBI copolymers containing 4-phenyl phthalazinone moieties and p-phenylene linkages (p-PPBIs) were synthesized by varying the feed ratio of CPPBC to TPA from 20 to 80 mol%. These copolymers were referred as p-PPBI-X (where X refers to the mol% of CPPBC). The 4-phenyl phthalazinone based homo-PBI (PPBI) [22] was also synthesized for membrane preparation and doping. The typical procedure for the synthesis of p-PPBI-20 is as follows. To a 100 mL three-neck round bottom flask equipped with an overhead mechanical stirrer, a water-cooled condenser and nitrogen purge inlet and outlet, was charged with 1.2840 g (6 mmol) DAB, 0.7968 g (4.8 mmol) TPA, 0.5736 g (1.2 mmol) CPPBC and 15 mL PPA. The reaction mixture was mechanically stirred at 90 1C for 1 h, 150 1C for 8 h and 190 1C for 7 h. During the polymerization, the reaction mixture became more and more viscous, and developed a light yellow color. The obtained solution was poured into 400 mL distilled water with stirring. The yellowish fiberlike precipitate was washed with 10% aqueous sodium bicarbonate solution, and immersed in sodium bicarbonate solution for 120 h to eliminate the residue phosphoric acid. After washing with water until neutral, the polymer was filtered off and dried in vacuum at 100 1C for 24 h to obtain the dry fibrous p-PPBI-20. Yield of the polymer was quantitative.

129

½ðW 2 W 1 Þ=MH3 PO4  ½W 1 =MPBI 

where M H3 PO4 and MPBI present the molecular weight of phosphoric acid and the repeat unit of polymers, respectively, W1 and W2 are the weight of undoped membrane and acid doped membrane, respectively. The proton conductivity (s) of the acid doped membranes was carried out by two-probe electrochemical impedance spectroscopy technique. The tested membrane was sandwiched between two electrodes in the test cell, thus the two probes were on the opposite sides of the membrane. 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 different temperatures from 20 to 180 1C. The proton conductivity (s) of the membranes was calculated using the following equation:

s ðS=cmÞ ¼

L SR

where R is the measured resistance of the membrane, L represents the thickness of the membrane, and S is the cross-sectional area of the membrane. The oxidative stability test was carried out 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

130

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126.9 ppm (C3, C6) could be assigned to the chemical shifts of carbons of 4-phenylphthalazinone moiety, which vary with the content of 4-phenylphthalazinone moiety in the polymer mainchain. The FT–IR, 1H NMR and 13C NMR results confirm the structures of PBIs containing 4-phenyl phthalazinone groups and p-phenylene linkages.

fresh Fenton solution every 12 h. The oxidative stability results were evaluated by recording the time of membrane breaking and weight changes before and after immersing for 120 h. The mechanical properties of membrane samples were measured on Instron 5567 instrument at 25 1C 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 membrane, at least four samples were tested and their average value was calculated.

3.2. Inherent viscosity and solubility

3. Results and discussion 3.1. Synthesis and structure characterization As shown in Scheme 1, the polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages (p-PPBI) were synthesized by the polycondensation of 3,30 -diaminobenzidine (DAB) with a mixture of terephthalic acid (TPA) and 4-(4-(4(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC). The structure characterization of these resultant polymers was carried out by FT–IR, 1H NMR and 13C NMR. FT–IR spectra of p-PPBI-20, p-PPBI-40, p-PPBI-60, and p-PPBI-80 are shown in Fig. 1. It is known that PBI is highly hygroscopic and can absorb moisture up to 15–19% of its weight readily [8,26]. So O–H stretching at about 3600 cm  1 and N–H stretching of imidazole ring at about 3400–3200 cm  1 are expected. The benzimidazole characteristic bands can be clearly observed at 1630 cm  1 (C ¼N/ C¼C stretching), 1605 cm  1 (ring vibration of conjugation between benzene and imdazole rings), 1435 cm  1 (in-plane ring vibration of 2,6-disubstituted benzimidazole), 1287 cm  1 (imidazole ring breathing mode) and 806 cm  1 (the heterocylic-ring vibration), similar with that reported previously [9,27]. Additionally, the strong absorption bands at 1230 cm  1 and 1663 cm  1 are ascribed to the –O– stretching and lactam of phthalazinone moiety, respectively, and they become more intense gradually with the increasing of 4-phenyl phthalazinone moiety content in the polymer backbone. 1 H NMR spectra are shown in Fig. 2. The protons at around 9 ppm can be attributed to the Hn of 4-phenyl phthalazinone structure. Other aromatic protons from 7.9 to 9.6 ppm with expected multiples and integrations are consistent with the proposed chemical structures of p-PPBIs. As shown in Fig. 3, the carbon atoms of p-PPBI in 13C –NMR spectra are in the range of 117.4–158.6 ppm and assigned in detail. The signals at 158.3 (C1), 157.6 (C12), 147.1 (C2), 132.9 (C4, C5), 132.0 (C10), 130.8 (C9),

COOH + n HOOC

m HOOC

The inherent viscosities (I.V.) of the polymers were measured by Ubbelohde capillary viscometer at a concentration of 0.5 g/dL at 30 1C using 96% concentrated sulfuric acid as solvent. As shown in Table 1, the inherent viscosities are in the range of 1.78 to 3.43 dL/g, indicating high enough molecular weight of the polymers for membrane formation. Moreover, the inherent viscosities of p-PPBIs (2.86–3.43 dL/g) are much higher than that of PPBI (1.78 dL/g), due to the para oriented backbone structure. Polymers with such inherent viscosities are desirable to achieve mechanically stable membranes with high acid doping levels. The solubility of the obtained polymers is tested at a concentration of 0.01 g/mL, as summarized in Table 1. The original p-PBI with inherent viscosity of 3.22 dL/g is only soluble in strong acids [28]. The p-PPBIs exhibit better solubility in polar organic solvents such as NMP, DMAc, DMF and DMSO compared with p-PBI and

Fig. 1. FT IR spectra of p-PPBIs.

O

O

+

N N COOH

PPA

N N H

H N

H2N

NH2

H2N

NH2

m+n

N2

N

NH

N H

N

O N

m

O N N n

Scheme 1. Synthesis of p-PPBIs.

X. Li et al. / Journal of Membrane Science 423–424 (2012) 128–135

*

H N

N

131

N

NH

N H

N

O N

N H

O N N

Fig. 2. 1H NMR spectra of p-PPBIs.

4 N

34

33

32 32

33 34

N

28

17 17 N 35 37 37 35 N H H 36 36

29 30 31

N 27

26

25 25

26

17 N 24 H 22 23 24 23

Fig. 3.

13

11 15 14 13 12 16 O 17 22 N 27 NH

C NMR spectra of p-PPBIs.

3 10 8 9 2

5 6 7 1

N N 18 19

O 21 20

132

X. Li et al. / Journal of Membrane Science 423–424 (2012) 128–135

Table 1 Inherent viscosity and solubility of p-PBI, PPBI, and p-PPBIs. Polymer I.V. (dL/g)

H2SO4 (96%)

NMP DMAc DMSO DMF H3PO4 (85%)

THF CH3Cl

p-PBI [28] p-PPBI20 p-PPBI40 p-PPBI60 p-PPBI80 PPBI

3.22

þþ















2.86

þþ

þþ

þþ

þþ

þ







3.43

þþ

þþ

þþ

þ

þ







2.94

þþ

þþ

þþ

þþ

þ







3.24

þþ

þþ

þþ

þ

þ







1.78

þþ

þþ

þþ

þþ

þ







þþ: solubility –>þþ: solubility in room temperature; þ  : partially soluble;  : insoluble.NMP: N-methyl pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,Ndimethyformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.

Fig. 5. DSC curves of p-PPBIs.

Table 2 Thermal properties of PPBI and p-PPBIs. Polymer

Tga (1C)

Td5%b (1C)

Td10%b (1C)

Cyc (%)

p-PPBI-20 p-PPBI-40 p-PPBI-60 p-PPBI-80 PPBI

399 409 426 415 402

586 545 503 542 535

668 604 625 594 572

81.4 79.5 81.9 74.8 66.0

a Glass transition temperature tested by DSC at a heating rate of 10 1C/min in nitrogen. b Temperature for 5% and 10% weight loss in nitrogen, heating rate of 10 1C/min. c Char yield calculated as the percentage of solid residue after heating from 100 to 800 1C in nitrogen.

Fig. 4. TGA curves of p-PPBIs in N2.

most of other PBIs [29,30]. The introduction of the non-coplanar 4-phenyl phthalazinone structure into the PBI main chain reduces the intermolecular forces between polymer chains thus enhances the solubility. The improved solubility of these polymers makes the membranes be easily fabricated from the PBI solution in organic solvents. From the results above, it can be concluded that PBIs containing 4-phenyl phthalazinone moieties and p-phenylene linkages possess high molecular weight as well as excellent solubility. 3.3. Thermal properties The thermal properties of p-PPBIs were characterized by TGA and DSC, as shown in Figs. 4 and 5 and Table 2. The glass transition temperatures (Tg) of these polymers are from 399 to 426 1C, which are much higher than that of p-PBI (361 1C) [18]. The Tg values of p-PPBIs are higher than that of p-PBI because of the introduction of 4-phenyl phthalazinone moieties into polymer mainchain. Although the flexible aromatic ether linkage introduced by 4-phenyl phthalazinone moiety decreases the rotation barrier of the PBI main chain, the rigid aromatic heterocyclic structure of 4-phenyl phthalazinone hinders the free segmental motions and hence imparts p-PPBI polymers with high Tg values [31]. Therefore, the effect of 4-phenyl phthalazinone moiety seems to be dominant and eventually should account for the high Tg values of these p-PPBIs. These resultant PBIs have

sufficiently high Tg values for high temperature proton exchange membranes. The TGA curves of p-PBIs was recorded in flowing nitrogen at a 10 1C/min heating rate from 150 up to 800 1C, as shown in Fig. 4. To eliminate the initial weight loss of loosely bound absorbed water molecules at around 100 1C, the samples were preheated to 150 1C, and the measurements were performed from 150 to 800 1C. The decomposition temperature for 5% and 10% weight loss (Td5% and Td10%) under nitrogen were 503–586 1C and 572– 668 1C, respectively. The polymers maintain 66.0–81.9% of their original weight at 800 1C. These results indicate the remarkable thermal stability of these polymers. The excellent thermal properties suggest the p-PPBIs are excellent candidates for further investigation as high-temperature proton exchange membranes.

3.4. Polymer crystallinity To study the polymer morphology, the undoped membranes prepared from PPBI and p-PPBIs were examined using WAXD measurement. Pure p-PBI exhibits semi-crystalline character [28]. However, as shown in Fig. 6, the diffraction peaks of all polymers appear at 101–351 (2y) widely, which indicates amorphous in nature. The introduction of the unsymmetrical, twisted and non-coplanar 4-phenyl phthalazinone moiety into the polymer backbone may destroy the crystalline structure of p-PPBI. The amorphous structure of p-PPBIs is expected to be beneficial for solvent solubility, acid doping and proton transfer.

X. Li et al. / Journal of Membrane Science 423–424 (2012) 128–135

133

3.6. Proton conductivity

Fig. 6. WAXD curves of undoped p-PPBI membranes.

The proton conductivities of the resultant acid doped membranes measured at different temperatures are shown in Fig. 8, which increase with both temperature and doping level. The conductivity of acid doped PPBI membrane with the highest doping level (15.2 mol H3PO4) reaches 0.13 S/cm at 160 1C. The values are sufficiently high, which are even similar with some membranes at doping levels of 30–50 mol H3PO4 [33,34]. Otherwise, the decrease of proton conductivity above 160 1C can be due to the dehydration of phosphoric acid, which became H4P2O7 [35]. As discussed in the WAXD section of this paper, the incorporation of 4-phenyl phthalazinone structure could lead to the amorphous structure of the PBI, which should be beneficial for proton transfer. However, all the p-PPBI membranes present comparatively lower proton conductivities and show a limited increase with temperature. It has been concluded that the conductivity results from proton migration along the anionic chain rather than from proton exchange along the polymer chain between the protonated and non-protonated nitrogen [7]. The nitrogen atoms and carbonyl groups in 4-phenyl phthalazinone structure should facilitate proton transfer because of comparatively weak hydrogen bonding, which helps to exchange protons due to less friction [24]. Thus, the 4-phenyl phthalazinone structure in PPBI and p-PPBI main chain may form a special anion pathway for proton conduction. The p-phenylene linkages in p-PPBI polymer somehow hinder the anion pathway. The relatively low proton conductivity of p-PPBI membranes in contrast with that of PPBI membrane may confirm this conjecture. The accurate proton transfer mechanism of PPBI and p-PPBIs still needs further investigating. 3.7. Mechanical properties

Fig. 7. Doping levels of p-PPBIs. (PRU: polymer repeat unit)

Mechanical properties of the doped and the undoped membranes are summarized in Table 3. The variation curves of stress and strain as a function of acid doping level are illustrated in Fig. 9. The undoped membranes present excellent tensile strength in the range of 80.8–98.8 MPa and the elongation at break from 9.2% to 30.6%. After doping at high temperature, the tensile strength decreases (26.1–8.5 MPa) while the elongation at break increases (46.7–267.1%) with the increasing of doping level (Fig. 9). The absorption of phosphoric acid leads to reduction of

3.5. Doping study The PPBI and p-PPBI membranes were easily fabricated by casting polymer solution using NMP as solvent. All membranes were soaked into a H3PO4 solution (85 wt%) at 150 1C for 72 h, obtaining the doping levels ranging from 7.58 to 15.20 (shown in Fig. 7). Acid doping level is defined as the number of moles of absorbed phosphoric acid (mol H3PO4) per mole of PBI repeat unit. It is well known that high doping level can result in high proton conductivity for phosphoric acid doped PBI membranes, due to the excess H3PO4 which could form anion pathway for proton conduction [32]. The doping level increases with the increasing content of 4-phenyl phthalazinone moiety, which indicates the introduction of 4-phenyl phthalazinone moiety could enhance the acid absorbing ability of PBI. The twisted non-coplanar 4-phenyl phthalazinone structure could destroy the crystalline structure and reduce the intra- and intermolecular forces. The looser polymer chain packing of PBI containing 4-phenyl phthalazinone moieties and p-phenylene linkages is beneficial for the absorption of phosphoric acid.

Fig. 8. Proton conductivities of PPBI and p-PPBI membranes with different doping levels at different temperatures.

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X. Li et al. / Journal of Membrane Science 423–424 (2012) 128–135

Table 3 Mechanical properties of PPBI and p-PPBI membranes. Membrane

p-PPBI-20 p-PPBI-40 p-PPBI-60 p-PPBI-80 PPBI

Doping Level (mol H3PO4)

7.58 8.61 9.86 10.42 15.20

Table 4 Oxidative stability of acid doped PPBI and p-PPBI membranes.

Tensile strength (MPa)

Elongation at break (%)

Undoped

Acid doped

Undoped

Acid doped

87.3 89.8 98.8 84.8 80.8

26.1 23.3 18.3 16.0 8.5

15.7 30.6 20.4 17.8 9.2

46.7 53.3 101.3 179.0 267.1

Membrane

Doping level (mol H3PO4)

Time at break (h)

120 h residual weight(%)

p-PPBI-20 p-PPBI-40 p-PPBI-60 p-PPBI-80 PPBI

7.58 8.61 9.86 10.42 15.2

80 81 76 75 70

63.2 59.5 58.0 38.7 23.0

PPBI and p-PPBI membranes with such high doping levels still exhibit excellent oxidative stability in the rigorous conditions, which are sufficient for PEMFC use.

4. Conclusion

Fig. 9. Mechanical properties of membranes with different doping levels. (PRU: polymer repeat unit; ’: tensile strength; m: Strain at maximum)

tensile strength and enhancement in elongation at break for all membranes. This could be due to the plasticizing effect from phosphoric acid, which is also a common phenomenon observed from many other polybenzimidazoles [35,36]. The tensile strengths of these acid doped p-PPBI membranes are higher than those of other membranes at the same doping levels [37]. This could be due to the increased para oriented monomers which can lead to changes in chain packing, membrane morphology and mechanical strength. The tensile strength of PPBI (1.78 dL/g) membrane with the highest doping level of 15.2 mol H3PO4 was 8.5 MPa, which is comparable with membrane with doping level of 6.0–8.0 mol H3PO4 [7,38]. These mechanical results are even comparable with the crosslinked PBIs [39], indicating remarkable mechanical properties of these polymers. The excellent mechanical stability of these acid doped PBI membranes is more than sufficient for high temperature PEMFC use. 3.8. Oxidative stability The oxidative stability of PEM is important to the lifetime of PEMFC. Proton exchange membranes are known to undergo degradation resulting from hydroxyl or peroxyl free radicals formed by the decomposition of H2O2 generated at cathode during operational conditions of fuel cells [40]. The oxidative stability (the resistance to hydroxyl and peroxyl free radicals) of these acid doped membranes was measured by the Fenton test. As summarized in Table 4, the membrane samples start to break into pieces after 70–81 h test, and the residual weight of the membranes after 120 h is in the range of 63.2–23.0%. Commercial m-PBI starts to break into small pieces after 30 min test [40]. Many aromatic PEM with good durability dissolved completely after such measurement for less than 36 h [41,42]. Therefore, the

A novel series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages were synthesized successfully, and the related acid doped membranes were prepared. The structure of these PBIs was characterized by FT–IR, 1 H NMR and 13C NMR. These polymers exhibit high inherent viscosities from and better solubility than commercial p-PBI and most of other PBIs. The introduction of twisted non-coplanar 4-phenyl phthalazinone group into p-PBI backbone reduces the intermolecular forces between polymer chains and destroys the crystalline structure, which enhances the solubility and results in the amorphous structure of polymers. Additionally, the nitrogen atoms in 4-phenyl phthalazinone groups could facilitate proton transfer. Proton conductivity membranes with high doping levels were obtained by soaking in the phosphoric acid at high temperature. These membranes present high proton conductivity at high temperature, good mechanical properties and excellent oxidative stability. All these superior properties above make these polymers and corresponding membranes promising potential candidates for use in high temperature fuel cell operation.

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