Fabrication of crosslinked polybenzimidazole membranes by trifunctional crosslinkers for high temperature proton exchange membrane fuel cells

Fabrication of crosslinked polybenzimidazole membranes by trifunctional crosslinkers for high temperature proton exchange membrane fuel cells

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Fabrication of crosslinked polybenzimidazole membranes by trifunctional crosslinkers for high temperature proton exchange membrane fuel cells Jingshuai Yang, Haoxing Jiang, Liping Gao, Jin Wang, Yixin Xu, Ronghuan He* Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, China

article info

abstract

Article history:

Two trifunctional bromomethyls containing crosslinkers, 1,3,5-tris(bromomethyl)benzene

Received 15 September 2017

(B3Br) and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (Be3Br), are employed to cova-

Received in revised form

lently crosslink polybenzimidazole (PBI) membranes for the high temperature proton ex-

11 December 2017

change membrane fuel cell. The presence of three bromomethyl groups in each crosslinker

Accepted 25 December 2017

molecule is expected to create more free volume for acid doping while enhancing the adhesive strength of the PBI chains. In addition, the influence of the two crosslinker structures on the property of the crosslinked membranes is compared and analyzed. All

Keywords:

the crosslinked PBI membranes exhibit longer morphology durability over the pristine PBI

Crosslinking

membrane toward the radical oxidation. Moreover, the crosslinked PBI membranes with

Polybenzimidazole

the crosslinker Be3Br containing three ethyl groups display superior acid doping level, high

Mechanical strength

conductivity and excellent mechanical strength simultaneously, over those with the

Conductivity

crosslinker B3Br and the pristine PBI membrane. Single cell measurements based on the

High temperature

acid doped membrane with a crosslinking degree of 7.5% by Be3Br demonstrate the tech-

Fuel cell

nical feasibility of the prepared membranes for high temperature proton exchange membrane fuel cells. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction High temperature proton exchange membrane fuel cells (HTPEMFCs) have attracted much attention as clean energy conversion devices for automotive, stable and portable applications because of suppressed CO poisoning, simple water and better thermal managements, resulting from their operating temperature above 100  C [1]. The widely investigated phosphoric acid (PA) doped polybenzimidazole (PBI) membrane has been developed to be one of the most promising electrolytes

for HT-PEMFC [2,3]. Furthermore, the PBI-based HT-PEMFCs exhibit several advantages, including high CO or SO2 tolerance, nearly anhydrous working conditions, better heat utilization and possible integration with fuel processing units [1,3,4]. So far, one of the critical issues for attaining a superior performance of the PBI-based HT-PEMFCs is the trade-off between the conductivity and mechanical strength. As a class of heterocyclic polymers, PBI contains two imide groups per repeat unit. These alkaline groups can be easily doped with phosphoric acid (PA) molecules. The abundant doped PA

* Corresponding author. E-mail address: [email protected] (R. He). https://doi.org/10.1016/j.ijhydene.2017.12.141 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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molecules could bring in the PBI membrane superiorly high proton conductivity. The sol-gel method is an alternative approach to fabricate PBI membranes with acid doping levels (ADLs) as high as 20e40, as proposed by Benicewicz et al. [5,6]. The obtained membranes exhibited quite high conductivities of around 0.2 S cm1 above 160  C. As an expense, however, the mechanical strength of PA doped PBI membranes dramatically deteriorates as the increase of ADL, due to the decreased interaction force between PBI polymer chains [2,7]. Therefore, various modified approaches for PBI electrolytes have been developed in order to improve the mechanical strength without or with less sacrifice of the proton conductivity. Up to now, various kind of explorations have been employed such as increasing molecular weight of the PBI polymer [8], fabricating composite membranes with nano inorganic compounds such as TiO2 [9], HfO2 [10], silica [11], clay [12] and heteropolyacids [13,14], synthesis of PBI variants [5,15e18] or highly branched PBIs [19], crosslinking the PBI ionically or covalently [20e26], and thermal curing of PBI membranes [27,28]. Since the N-H groups in the benzimidazole rings of PBIs are chemically reactive, the covalent crosslinking of PBI membranes can be achieved via a SN2 reaction between the PBI and a cross-linker containing two or more electrophilic active groups, such as halides [2] and epoxides [25]. As a result, covalent crosslinking has been demonstrated as an effective method to reinforce the PBI membranes with high ADLs. The macromolecule cross-linkers of chloromethylated polysulfone (CMPSU) [22] and poly(vinylbenzyl chloride) [23] have been employed to fabricate covalently multi-crosslinked hexafluoropropylidene containing PBI (F6PBI) and sulfone PBI (SO2PBI) membranes, respectively. The macromolecule multicrosslinked F6PBI and SO2PBI membranes display better fuel cell performance and long term fuel cell durability, resulting from the superior mechanical strength at elevated temperatures while sustaining high PA contents. Besides, other macromolecule cross-linkers including polybenzoxazine [21] and bromomethylated poly (aryl ether ketone) [24] have been used to prepare the multi-crosslinked PBI membranes as well. Due to more compact structures, multi-crosslinked PBI membranes generally displayed lower ADLs than the neat PBI membranes under the same acid doping condition [22,24e26]. Previously, we observed that the silane multi-crosslinked PBI

membranes showed superior ADLs comparing to the pristine PBI membrane [29]. This phenomenon probably resulted from the introduction of the siloxane network, which might create a more free volume for adoption of PA molecules [11,20,30]. For better understanding the influence of the crosslinking structure on the properties of the membrane, we tried to crosslink PBIs by using novel cross-linkers with adjustable structures. In the present work, two typical crosslinkers, 1,3,5tris(bromomethyl)benzene and 1,3,5-tris(bromomethyl)-2,4,6triethylbenzene, were used to prepare covalently crosslinked PBI. Each crosslinker molecule has three bromomethyl groups, which are expected to create more free volume for acid doping and increase the conductivity of membranes without or less weaken their mechanical properties. Through a facile SN2 reaction as mentioned previously [22,31], the crosslinkers with and without the flexible groups of triethyl were grafted onto the PBI backbones, respectively. The degree of cross-linking was optimized by controlling the additive amount of the crosslinker in order to make a comparison on the properties of the fabricated membranes simply and effectively. The physicochemical properties and premier fuel cell performance were investigated.

Experimental Fabrication of crosslinked PBI membranes The PBI polymer was synthesized by condensation polymerization of 3,30 -diaminobenzidine tetrahydrochloride dehydrate (Applichem, USA) and isophthalic acid (Sigma-Aldrich), with a molar ratio of 1:1 in polyphosphoric acid (Sinopharm Chemical Reagent Co., Ltd), as reported previously [8]. The obtained polymer has an inherent viscosity of around 0.73 dL g1 (5 g L1 in 98 wt% sulfuric acid at 30  C). As shown in Fig. 1, the crosslinked PBI membranes were fabricated from PBI and two different crosslinkers, i.e. 1,3,5-tris(bromomethyl)benzene (B3Br, Sigma-Aldrich) and 1,3,5-tris(bromomethyl)-2,4,6triethylbenzene (Be3Br, Sigma-Aldrich), respectively. The detailed procedure is as follows. A PBI solution of 2 wt% in N,N-dimethylacetamide (DMAc, J&K Scientific) was prepared by dissolving PBI in DMAc at 160  C under refluxing. The crosslinker, i.e B3Br or Be3Br, was added to the PBI solution. The resulted mixture was ultrasonicated for 1 h at room

Crosslinking

N

H N

N H

N

or B3Br

Be3Br

PBI Fig. 1 e Schematic illustration of the crosslinked PBI membranes.

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temperature (RT) to obtain a homogenous solution. The obtained solution was poured onto a Petri dish, followed by drying at 80  C for 16 h and at 120  C for 8 h. The generated membrane was peeled off, washed thoroughly with demineralized water at 80  C for 1 h and further dried at 100  C for 10 h. The nucleophilic reaction between PBI and the crosslinker was accomplished during the membrane formation simultaneously. By assuming complete conversion, the theoretical cross-linking degree was estimated and expressed as x% according to the initial mole ratio of the bromomethyl groups of the crosslinker to the benzimidazole groups of PBI in the present work. The finally obtained membranes, denoted as B3Br-x% and Be3Br-x%, were all uniform and flexible with a thickness ranged from 50 to 70 mm measured by a micrometer.

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100  C for about 1 h until a stable value of the conductivity was reached. The membrane-electrode assemblies (MEAs) with an active electrode area of 6.25 cm2 were fabricated at 150  C with a pressure of 1 MPa for duration of 5 min according to the previous work [30]. The platinum loading was 0.58 mg cm2 for each electrode. Hydrogen and oxygen at flow rates of 120 and 60 mL min1, respectively, were supplied to the fuel cell without any pre-humidification. Polarization curves were obtained using a current step potentiometry.

Results and discussion Fabrication of crosslinked PBI membranes by trifunctional crosslinkers

Acid doping and swelling The acid doping of the membranes were achieved by immersing membranes in 85 wt% PA solutions at RT for around 5e7 days. The ADL of a membrane, defined as the mole number of PA molecules per mole PBI repeat unit, was calculated based on the mass gains of the membrane samples during the acid doping as described elsewhere [32]. The membrane swellings, in terms of area and volume increase, were determined accordingly by measuring the membrane dimension changes.

Characterizations The crosslinking of the membranes was determined by measuring their solubilities in DMAc at 80  C within 24 h. The dissolution level of the membranes in DMAc was calculated from the weight losses of the membranes during the measurement. Fourier transform infrared spectra (FT-IR) of membranes were carried out on a Bruker VERTEX70 spectrometer equipped with a DTGS detector and a ZnSe crystal as attenuated total reflection (ATR) accessory. Thermogravimetric analysis (TGA, HT/808, METTLER-TOLEDO) was performed in inert atmosphere (N2) at a heating rate of 10  C min1. The Fenton test was used to evaluate the chemical stability of membranes. Pre-weighed dry membranes were immersed in a 50 mL Fenton aqueous solution consisting of 3 wt% H2O2 and 4 ppm Fe2þ (added as (NH4)2Fe(SO4)2$6H2O) for a certain time at 68  C. Then the samples were taken out, washed thoroughly with demineralized water and dried at 120  C for at least 12 h before weighing. As a consequence, the weight loss of the membranes was determined. The Fenton solution was replaced with freshly prepared one for each test cycle. Tensile stress-strain curves of membranes were collected using an instrument (CMT6502, SANS Company, China) with a constant separating speed of 5 mm min1 in the ambient atmosphere. The initial dimension of dumb-bell shaped membrane samples was 25 mm in length and 4 mm in width. A heating oven was used to perform the measurements at 120  C. The through-plane conductivities of acid doped membranes were measured using a four-probe conductivity cell with a frequency about 3 kHz under ambient air without any humidification. The whole cell was kept in an oven for controlling the temperature. To avoid the influence of moisture on the proton conductivity, all the membrane samples were pre-heated at

All the obtained crosslinked PBI membranes are homogeneous and transparent with brown color. As reported, the cross-linking occurrence of the membranes could be demonstrated by their solubility in strong polar organic solvents under elevated temperatures [22e25,29]. The pristine PBI membrane was completely dissolved in DMAc at 80  C within about 24 h as shown in Fig. 2, which is in agreement with reported literature [24,25,29]. On the contrary, all the crosslinked PBI membranes survived in DMAc and maintained a weight of more than 80% after solubility measurements under the same conditions. These results confirm the covalent crosslinking occurred between PBI and the crosslinker. Moreover, the membranes crosslinked with Be3Br swelled significantly compared to those with B3Br, especially having a low crosslinking degree of 7.5%. The multi-branched structure of Be3Br at a low crosslinking degree presumably resulted in the increased free volume and less compact polymer chains for easy entrance of the solvent molecules. As the crosslinking degree was estimated according the initial ratio of the reactants by assuming complete conversion, the results shown in Fig. 2 might also resulted from the less efficient crosslinker of Be3Br due to the steric hindrance of the ethyl groups on the full conversion of the bromomethyl groups.

FT-IR The FTIR bands are highly sensitive to changes in molecular structure as shown in Fig. 3. All featured bands in the pure PBI spectrum have been assigned [2,15,16]. The characteristic absorption at 3437 cm1 is assigned to the stretching vibration of N-H groups in benzimidazole rings of PBI [1,2], which became weak or even unobservable as the increase in the crosslinking degree of the crosslinked B3Br-x% and Be3Br-x% membranes. Meanwhile, the characteristic bands at 578 or 584 cm1 attributed to C-Br in B3Br or Be3Br compounds [33], were not found in the spectra of the crosslinked membranes. These results indicate the successful crosslinking reaction between the crosslinker and the PBI. In addition, a noticeable absorption peak at around 2930 cm1 is observed in the spectra of both B3Br-x% and Be3Br-x% membranes. That absorption peak should be assigned to the stretching vibration of C-H groups in saturated alkanes [22,29], which further confirmed the occurrence of triad-crosslinking between the B3Br or Be3Br and PBI.

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Fig. 2 e Residue weight and photographs of PBI (1), B3Br-7.5% (2), B3Br-15% (3), B3Br-30% (4), Be3Br-7.5% (5), Be3Br-15% (6) and Be3Br-30% (7) membranes after the solubility test in DMAc at 80  C for 24 h.

1.0 0.9

0.8

0.6 0.3

0.6

2960

2970

0.4 Be3Br 1.0

578

B3Br

1.0

584

2930 2930

0.9 1.0

a.u.

a.u.

0.9 B3Br-7.5% 1.0

0.8

0.9

Be3Br-15%

B3Br-15%

1.0

1.0

0.9 Be3Br-30% 1.0

0.9 B3Br-30% 1.0

0.9 PBI

Be3Br-7.5%

3437

0.9 PBI

4000 3500 3000

1800

1500

1200

900

600

-1

Wavenumber / cm

(A)

3437

4000 3500 3000

1800

1500

1200

900

600

-1

Wavenumber / cm (B)

Fig. 3 e FTIR spectra of PBI, its crosslinked counterparts and cross-linkers of B3Br (A) and Be3Br (B).

Fenton test It is of great importance to fabricate the PEMs with good chemical stability, which is usually estimated by the Fenton

test [1,2]. During Fenton test, radicals such as HO$ and HOO$, which are produced via the decomposition of H2O2 in the presence of catalyst Fe2þ or Cu2þ, tend to attack the heteroatom containing polymeric chains and result in degradation of

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phosphoric acid doped in the membranes might suppress the attack of Fenton reagent [34]. Thus, we couldn't judge the durability of the HT-PEMFC merely according to any stability tests of the membranes including the Fenton test. The failure of a HT-PEMFC might result from mechanical degradations of the PEMs, leaching of the doping acids and loss of catalyst activities [4,8].

100 95

85 PBI

80

TGA

B3Br-7.5% B3Br-15%

75

B3Br-30% Be3Br-7.5%

70

Be3Br-15% Be3Br-30%

65 0

50

100

150

200

250

300

350

Time / h

Fig. 4 e Fenton test results of PBI and crosslinked PBI membranes in 3 wt% H2O2 solution containing 4 ppm Fe2þ at 68  C. the polymer membranes. In the present work, the oxidative stability was investigated by submersing the membranes in Fenton's reagent (3% H2O2 þ 4 ppm Fe2þ) at 68  C. As shown in Fig. 4, after submersed in the Fenton reagent for 144 h, the neat PBI membrane displayed a weight loss of around 16 wt% and disintegrated into pieces, which is in a good agreement with previous reports [16,22,25]. Compared to the pristine PBI membrane, the B3Br possessing rigid aromatic rings in the structure displayed a lower weight loss during the Fenton test. For example, the weight losses of the B3Br-15% and B3Br-30% membranes are only around 14 wt% and 11 wt%, respectively, meanwhile no fragmentation is observed for any of them after 310 h of the Fenton test. However, the cross-linker Be3Br with three flexible ethyl chains made the membrane a higher weight loss than the PBI membrane within the earlier period of the Fenton test, say about 150 h. Nevertheless, the crosslinked membranes possessed significantly longer morphology durability over the neat PBI membrane. For instance, the crosslinked membranes broken to pieces after the Fenton test of nearly 190 h for Be3Br-7.5% and Be3Br-15%, and 310 h for Be3Br-30%, respectively. This result indicates that the rigid structure cross-linker allowed the crosslinked PBI membranes to exhibit higher oxidative stability than the cross-linker with flexible alkyl chain structure [24,25,29]. The presence of the alkyl chain could not protect the polymer from the attack of the oxidative radicals via such as the effective sterichindrance. Inversely, the much branched structure of the crosslinker Be3Br apparently brought about more free volume for redicals to carry out the oxidation. In addition, bulky ethyl groups in Be3Br would hinder the reaction with PBI and the crosslinking reaction might be less than the one estimated, which would be another reason for the higher weight loss of Be3Br-x% membranes in the Fenton test. Hence, it is necessary to design a suitable chemical structure of multi-functional cross-linkers, which can benefit the crosslinked membrane to have superior oxidative resistance properties. It is worth noting that the effect of the doping phosphoric acid on the radical stability of membranes was not considered in the above discussion. It has been reported that the

The thermal stability of pristine PBI, B3Br-7.5%, B3Br-30%, Be3Br-7.5% and Be3Br-30% membranes were evaluated by using TGA recorded in N2 flow at a heating rate of 10  C min1. As shown in Fig. 5, the initial weight losses below 200  C for all the membranes are due to the loosely bound absorbed solvent or water, as previously reported [27,35]. Based on the above results, it indicates that the crosslinked membranes seemed to retain more solvent or water than PBI membranes, which is in agreement with the acid doping results mentioned in following section. Moreover, the weight loss of crosslinked membranes increased as the increase of crosslinked component content. The slight difference in mass loss between B3Br-x% and Be3Brx% membranes apparently resulted from the effect of the crosslinker structure. When the temperature reached above 510  C, there is a dramatic weight loss, obviously resulting from the decomposition of the PBI chain. The TGA results indicate that all the investigated crosslinked membranes possess enough thermal stability for operation at temperatures below 200  C for polymer electrolyte based HTPEMFCs.

Acid doping and swelling Table 1 illustrates the acid doping in 85 wt% PA results of PBI and crosslinked PBI membranes at room temperature. As previously reported, the crosslinked PBI membranes normally displayed lower ADLs than the neat PBI membrane under the same acid doping condition [22,25]. In order to achieve higher ADLs, the crosslinked PBI membranes normally perform the acid doping at elevated temperatures. In

100 90 80

Mass / %

Mass / %

90

PBI

70

B3Br-7.5% B3Br-15%

60

Be3Br-7.5% 50

Be3Br-15%

40 100

200

300

400

500

600

700

800

o

Temperature / C Fig. 5 e TGA curves of PBI and its crosslinked membranes in N2 at a heating rate of 10  C min¡1.

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Table 1 e Comparisons of the changes in weight and dimensions of various membranes in 85 wt% PA solutions at room temperature. Membrane PBI B3Br-7.5% B3Br-15% Be3Br-7.5% Be3Br-15%

ADL 11.1 ± 0.1 11.1 ± 0.2 11.8 ± 0.3 11.6 ± 0.2 12.8 ± 0.5

Mass% 352.0 ± 353.5 ± 371.7 ± 367.1 ± 402.7 ±

24.2 28.8 30.7 29.4 33.4

Area% 89.2 ± 75.8 ± 79.7 ± 76.8 ± 88.3 ±

8.2 4.7 8.3 5.3 7.6

Volume% 273.3 247.1 268.2 265.3 317.4

± 27.6 ± 21.7 ± 23.2 ± 12.8 ± 20.6

the present work, the crosslinked membranes exhibited comparable or higher ADLs than pristine PBI membranes under the same acid doping conditions, especially having a high crosslinking degree. For example, Be3Br-7.5% and Be3Br15% membranes display ADLs of around 11.6 and 12.8, while the ADL of pristine PBI membrane is about 11.1 after immersing in 85 wt% PA solutions at RT. In our previous work, we observed similar phenomenon for silane compounds crosslinked PBI membranes [29]. These results might result from the increased free volume for adopting phosphoric acid [11,20]. Especially, Be3Br-15% membrane with extra three ethyl groups has a higher ADL than B3Br-15% membrane. However, the crosslinked membranes with further higher crosslinking degree, i.e 30%, became brittle and were easy to break during acid doping procedure. Thus, membranes with crosslinking degrees of 7.5% and 15% were investigated in the following sections. Table 1 also shows the average dimensional swellings of the crosslinked PBI membranes by the acid doping. As seen from the table, crosslinking of the PBI membrane decreased dimensional swellings of the crosslinked membranes at a similar ADL. Similar results were also observed by other groups [20,24,27]. For instance, the average volume swelling of B3Br-15%/11.8 PA membrane was 268.2%, while that of PBI/ 11.1PA was as high as 273.3%. These results reveal that the structure of cross-linkers including their sizes and stereostructure might affect the compaction of the polymer chains. A suitable structure design of crosslinkers is necessary to achieve high ADLs with improved dimensional stability at the same time.

0.11

Conductivity / S cm-1

0.10 0.09 0.08 PBI/11.1PA

0.06

B3Br-7.5%/11.1PA B3Br-15%/11.8PA

0.05

Conductivity Fig. 6 shows the anhydrous proton conductivity ranging from 100  C to 180  C for the acid doped crosslinked PBI membranes. In a temperature range of 100e150  C, the conductivity of the membranes increased with the increase in temperature. Above 150  C, the dehydration of the membrane could occur under dry conditions, which would strongly affect the conductivities. Therefore, high ADLs of membranes were normally performed to sustain the high conductivities of HTPEMs, as evidenced in the previous work [25,31]. As seen in Fig. 6, the Be3Br-15.0%/12.8PA membrane exhibited the conductivity as high as 0.095 S cm1 at 180  C due to its high ADL. As previously reported, the covalent crosslinking might result in lower conductivity of the crosslinked membrane due to the decrease in acid doping content and the more restricted polymer chains [22e25]. The B3Br-7.5%/11.1PA membrane indeed exhibited slightly lower conductivities than the PBI/ 11.1PA membrane, as shown in Fig. 6. However, all the Be3Br-x % membranes displayed higher ADLs than the PBI membrane and meanwhile obtained higher conductivities. Nevertheless, the precise proton conducting mechanism is needed to investigate in the future work.

Mechanical strength The mechanical integrity of a HT-PEM is essential for preparing the MEA and long-term use in PEMFCs. Fig. 7 and Table 2 shows the results of mechanical properties at RT and 120  C of acid doped PBI and its crosslinked counterparts with different ADLs. Generally, polymer membranes exhibit reduced tensile stress at break at elevated temperatures compared with that at room temperature, especially to acid doped membranes. For example, the tensile strength of PBI membrane with an ADL of 11.1 was 6.0 MPa at room temperature, and it only maintained 1.2 MPa at 120  C due to the significant thermoplastic deformation [2,7]. This result indicates the failure of the PA doped PBI membrane as electrolyte at high temperatures. As seen from the figure, the B3Br7.5% and Be3Br-7.5% membranes exhibited obviously higher tensile strengths of 3.9 and 3.5 MPa at 120  C, respectively, than that PBI without crosslinking when they had similar ADLs, indicating the achievement of enhanced tensile strength as expected by triad-crosslinking, especially at the elevated temperature. From the above results, it can be found that B3Br-x% and Be3Br-x% membranes exhibited similar tensile strength when they had similar ADLs. In addition, both acid doped B3Br-15% and Be3Br-15% membranes exhibited lower tensile strength values than their membranes with a low crosslinking degree of 7.5%, obviously resulting from different acid doping levels. Thus, the results suggest that the doped acid content plays a dominated role on the tensile strength of the membranes [7,36].

Be3Br-7.5%/11.6PA Be3Br-15.0%/12.8PA

0.04 80

100

120 140 160 Temperature / oC

180

Fuel cell performance 200

Fig. 6 e Conductivities of acid doped PBI and crosslinked PBI membranes as a function of the temperature.

To demonstrate the technical feasibility of the acid doped crosslinked membranes as potential high temperature PEMs, efforts were made to assemble the MEA and test its performance at 140, 160 and 180  C using un-humidified hydrogen

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10 PBI/11.1PA

4

B3Br-7.5%/11.1PA B3Br-15%/11.8PA

8

Be3Br-7.5%/11.6PA Be3Br-15%/12.8PA

Stress / MPa

Stress / MPa

3

6

4

PBI/11.1PA

2

B3Br-7.5%/11.1PA B3Br-15%/11.8PA

2

1

Be3Br-7.5%/11.6PA Be3Br-15%/12.8PA

0

0 0

30

60

90

120

0

150

30

60

90

120

150

Strain / %

Strain / %

(B)

(A)

Fig. 7 e Tensile stress-strain curves of PA doped PBI and crosslinked PBI membranes at RT (A) and 120  C (B).

Table 2 e Conductivities and mechanical properties of acid doped membranes. Membrane

Conductivity @ 160  C/S cm1

Strength/MPa RT

B3Br-7.5%/11.1PA B3Br-15%/11.8PA Be3Br-7.5%/11.6PA Be3Br-15%/12.8PA PBI/11.1PA

0.083 0.090 0.095 0.101 0.088

± 0.003 ± 0.003 ± 0.006 ± 0.005 ± 0.004

and oxygen. Comprehensively considering the mechanical property and conductivity, the Be3Br-7.5%/11.6PA membrane was chosen to perform the fuel cell test. As shown in Fig. 8, the open circuit voltage is around 0.94 V for the Be3Br-7.5%/11.6PA based fuel cell, which is comparable to that of other

Fig. 8 e Polarization curves (open symbols) and power densities (solid symbols) of fuel cells using un-humidified hydrogen and oxygen at 140  C (circle), 160  C (square) and 180  C (triangle) based on Be3Br-7.5%/11.6PA membrane (thickness of 90 mm) with a catalyst loading of 0.58 mg Pt cm¡2 for each electrode.

9.2 7.2 8.5 5.7 6.0

± 0.3 ± 0.2 ± 0.4 ± 0.2 ± 0.1

Elongation/% 

RT

120  C

± 0.2 ± 0.1 ± 0.2 ± 0.2 ± 0.1

94.7 ± 7.6 72.0 ± 5.1 140.3 ± 11.8 78.2 ± 4.5 87.4 ± 8.2

101.9 ± 9.1 69.7 ± 4.8 134.4 ± 9.4 95.6 ± 10.7 46.9 ± 5.3

120 C 3.9 2.7 3.5 2.2 1.2

crosslinked PBI membranes. For instance, an OCV of around 0.95 V has been reported for the MEA based on the PA doped CMPSU cross-linked F6PBI membranes [22]. The high OCV value indicated dense membranes with reasonable low gas permeability [27,37]. As previously reported, the hydrogen permeability of a membrane is influenced by its chemical structure. As an example, the polymer with extra bulk pendant groups normally exhibits higher hydrogen permeability than the polymers without grafted branches [38]. Therefore, the crosslinking might improve the gas crossflow of the membranes and further investigations should be carried out as well. In addition, a higher performance of the MEA is achieved at a higher temperature as shown in Fig. 8. For example, the peak power density of the MEA is 374 mW cm2 at 180  C, while it is only 274 mW cm2 at 140  C. The increased power density apparently resulted from both increased membrane conductivity and faster electrode kinetics as the increase in the temperature [6]. It should be noted that the mechanical strength of the PBI/11.1PA membrane is too weak to prepare the MEA. As previously reported, the PBI/7.6PA based MEA exhibited a peak power density of 442 mW cm2 at 180  C [39], which is higher than that of the Be3Br-7.5%/11.6PA membrane based MEA. However, it should be remarked that neither gas diffusion electrodes nor the hot-pressing procedure were optimized in the present study. Thus the improved overall fuel cell performance could be expected after the associated optimizations.

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Conclusions [6]

Two types of covalently crosslinked PBI membranes were fabricated and employed as electrolytes for PEMFCs after doping phosphoric acids. Two crosslinkers containing three bromomethyl groups in each molecule, i.e. 1,3,5-tris(bromomethyl)benzene (B3Br) and 1,3,5-tris(bromomethyl)2,4,6-triethylbenzene (Be3Br), were used to crosslink the PBI backbones through the SN2 reaction, respectively. The formation of crosslinked structures was determined by comparing the solubility difference in the solvent DMAc between the crosslinked membrane and the neat PBI, as well as by analyzing FTIR-ATR spectra, respectively. Additionally, the Fenton test reveals that the crosslinked PBI membranes exhibited much longer morphology durability over the pristine PBI membrane toward the attack of radicals. All the crosslinked membranes displayed lower dimensional swellings than the neat PBI membrane at the comparable acid doping level. The crosslinker Be3Br with extra three ethyl groups allowed the crosslinked membranes to exhibit higher ADLs than pristine PBI membranes under the same acid doping conditions, especially at a high crosslinking degree. The Be3Br-7.5%/11.6PA membrane achieved a high conductivity of 0.095 S cm1 at 180  C without humidifying. Meanwhile, the crosslinked membranes showed higher tensile strength (3.5 MPa) than the pristine membrane (1.2 MPa) at 120  C when they had a comparable ADL of around 11. Fuel cell tests demonstrated the technical feasibility of the crosslinked membrane as the high temperature proton exchange membrane electrolyte in fuel cells.

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Acknowledgements The authors are grateful for the financial supports from the National Natural Science Foundation of China (51603031 and 51572044), the Scientific Research Funds of Liaoning Provincial Education Department (LZ2015031) and the Fundamental Research Funds for the Central Universities of China (N160504006).

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