Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells

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Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells Zhouying Yue a, Yang-Ben Cai a, Shiai Xu a,b,* a

Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China b The Chemical Engineering College of Qinghai University, Xining 810016, China

article info

abstract

Article history:

Silane-cross-linked sulfonated poly(imide benzimidazole) (CSiSPIBI) membranes were

Received 19 June 2015

prepared using g-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560) as a cross-linker, and

Received in revised form

the cross-linked structure was characterized using Fourier transform infrared spectroscopy

13 October 2015

and solubility test. The resulted cross-linked membranes were further doped with phos-

Accepted 14 October 2015

phoric acid (PA) by means of the acidebase interaction with the alkaline imidazole ring and

Available online xxx

the electrostatic interaction with siloxane. The results show that PA-doped CSiSPIBI membranes have high proton conductivity due to the formation of a new proton transport

Keywords:

pathway between PA and sulfonic acid. Under high temperature and low humidity con-

Phosphoric acid-doping

ditions, the proton conductivity of PA-doped sulfonated membranes is one to two orders of

Sulfonated poly(imide benzimid-

magnitude higher than that of non-PA-doped membranes and PA-doped non-sulfonated

azole)

membranes. The silane-cross-linked membranes display improved chemical stability and

Organic-inorganic cross-linking

mechanical strength, especially the oxidative stability. The complete dissolution time in

Proton exchange membrane

Fenton's reagent increases from 510 min for the sulfonated polyimide/polybenzimidazole blend membrane to 1450 min for the silane-cross-linked membrane. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Fuel cells have emerged as a promising clean alternative to the traditional energy sources in use today [1], among which

proton exchange membrane fuel cells (PEMFCs) are considered to be the most promising one [2]. However, the lack of appropriate and cost-effective materials used for proton transport, especially proton exchange membranes (PEMs), has become a major obstacle to the development of PEMFCs. Much

* Corresponding author. Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China. Tel.: þ86 21 64253353. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.ijhydene.2015.10.057 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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research has been conducted on the development of new materials such as sulfonated aromatic hydrocarbon polymers [3,4]. The performance and properties of these newly developed materials are generally evaluated using Nafion (DuPont), a well-known perfluorosulfonic acid polymer, as the standard of reference [5]. Protons in the sulfonated membranes are transported through water by the vehicle mechanism. As the presence of such carriers is essential for high proton conductivity, the operating conditions of fuel cells are constrained by the physical properties of water. Recent studies have found that operating PEMFCs under relatively low humidity (<50%) and high temperature conditions offers several advantages from the thermodynamic, kinetic and engineering perspectives [6]. Therefore, there is an increasing interest in developing proton-transporting materials that can be used under low humidity and high temperature conditions [7,8]. A better understanding of the proton transport mechanisms is essential for the development of PEMs with high proton conductivity under low humidity and high temperature conditions. Two principal mechanisms have been proposed for proton transport in PEMs: vehicle mechanism and Grotthuss (proton hopping) mechanisms. In the vehicle mechanism, hydrated proton (H3Oþ) diffuses through the aqueous medium in the conventional PEM such as Nafion or sulfonated aromatic polymers, While the Grotthuss mechanism mainly takes place in the PA-doped PBI membranes as the proton hopping between one phosphoric acid molecule to another. Due to the physical properties of water and phosphoric acid, the sulfonated PEMs are mainly used under the hydrated condition at 30e80  C and the PA-doped PBI membranes are mainly operated under the anhydrous condition at 100e200  C. According to the above mechanisms, two methods have been proposed to prepare membranes for higher temperature conditions: (i) incorporation of hydrophilic inorganic compounds such as hygroscopic oxides (e.g.TiO2) [3], and nano fillers such as graphite oxide [9], microcapsules [10] and heteropolyacids [11] into sulfonated polymer membranes to enhance the binding capacity of water; (ii) replacement of water by a less volatile proton solvent, such as ionic liquids [12] or PA [13]. PA-doped polybenzimidazole (PA-PBI) membrane is considered as the most promising candidate for high temperature PEMFCs [14]. However, although an increased PA content can significantly improve the proton conductivity, it also results in a reduction in the mechanical strength as a result of the strong plasticization of PA [15]. Several approaches have been developed to improve the properties of PA-PBI membranes, including preparation of PBI composites such as ionically cross-linked membranes from the mixture of PBI and acidic polymers like sulfonated poly(ether ether ketone) [16] and sulfonated polyimide [17]. The blend membranes have better mechanical strength and proton conductivity than PA-doped non-sulfonated PBI membranes [18], but weak ionic cross-linking bond and composition range due to the miscibility problems between the two polymers [19]. In recent years, the covalent cross-linking method has also been used to modify the polymer backbones. We have previously synthesized a series of covalently cross-linked sulfonated poly(imide benzimidazole)s (SPIBI) through the reaction

between halogen and imidazole, and the resulted covalently cross-linked membranes displayed higher mechanical strength but lower proton conductivity due to the microstructural changes [20]. In this study, covalently cross-linked PA-doped sulfonated PEMs were prepared using g-(2,3epoxypropoxy)propyltrimethoxysilane (KH560) as a crosslinker. The imidazole ring in SPIBI reacted with the epoxy group in KH560, whereas the Si-(OCH3)3 group in cross-linker self-condensed after hydrolysis, resulting in the formation of silane-cross-linked structure in the SPIBI. After PA-doping, PA molecules were absorbed by the alkaline imidazole group, siloxane and sulfonic acid. The results show that the incorporation of silane-cross-linked structure and PA molecules has a significant effect on the mechanical properties, thermal/ chemical stability and proton conductivity of SPIBI membranes.

Experimental Materials 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA) and PA were obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. The cross-linker KH560 was purchased from Shanghai Resin Factory Co., Ltd. and used without further purification. 3,30 -Bis(4-sulfophenoxy) benzidine (BSPOB) and 6,60 -bis [2-(4-aminophenyl) benzimidazole] (BAPBI) were synthesized as described previously [21,22]. Triethylamine (Et3N) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. and purified by distillation under a reduced pressure and dehydrated with 4
A molecular sieves prior to use. Benzoic acid and dimethyl suloxide (DMSO) were also purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. and used as catalyst and solvent, respectively. Other common reagents were obtained from commercial sources as reagent grade and used as received.

Synthesis of SPIBIx SPIBI70 was synthesized using BSPOB (2.8 mmol, 1.4804 g), BAPBI (1.2 mmol, 0.4998 g) and NTDA (4.0 mmol, 1.0720 g) by a solution-thermal imidization method as previously reported [20]. In the nomenclature of SPIBIx, x (80, 70 and 60) represents the molar fraction of the sulfonated diamine monomer.

Fabrication of cross-linked sulfonated poly(imide benzimidazole)s (CSiSPIBIx) CSiSPIBIx membranes were fabricated using 6 wt.% SPIBI solution in DMSO and a certain amount of KH560 (the molar ratio of the epoxy group to the amino group was 1) by a solution-casting and evaporation method. The mixture was stirred at room temperature and casted on a clean glass plate, and dried in an oven at 80  C for 24 h and then at 160  C for 1 h in a vacuum oven to remove the solvent and proceed the reaction between imidazole and KH560. These dry membranes were peeled off and immersed in 1.0 M H2SO4 at 80  C for 24 h to obtain the siloxane hydrolysis-condensation structure [23],

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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and then immersed in 1.0 M HCl solution at room temperature for 2 d to convert the membranes from the Et3N salt form to the protonated form. The thickness of resulted CSiSPIBIx membranes ranged from 40 to 60 mm.

Doping procedure PA-doped membranes were obtained by soaking the crosslinked membranes in 85 wt.% PA aqueous solution at 80  C for 48 h. Then the membranes were taken out and wiped, and the amount of PA doped was determined by titration using standard 0.1 M NaOH solution with thymolphthalein as the titration indicator. The membranes were then rinsed with water and dried in a vacuum oven at 130  C for 24 h to obtain the dry weight of membrane. The PA level was calculated from: PA wt:% ¼

VNaOH CNaOH MPA =2  100% Wdry þ VNaOH CNaOH MPA =2

(1)

where Wdry is the weight of dry membrane, VNaOH and CNaOH are the volume and molar concentration of NaOH solution consumed, and MPA is the molecular weight of PA.

Characterization and measurements The formation of the covalently cross-linked structure was determined using a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer in the range of 400e4000 cm1 and solubility change after immersing in DMSO for 24 h at 80  C. The ion exchange capacity (IEC, meq g1) was determined through the classical titration method. The dried membranes were soaked in 1 M NaCl solution for 2 d to release Hþ ions and titrated with 0.01 M NaOH and phenolphthalein (titration indicator). The thermal degradation of membranes was measured using a TG 209 thermogravimetric analyzer (Netzsch, Germany) under a nitrogen atmosphere. Before measurements, samples were preheated to 120  C and maintained at this temperature for 15 min to remove absorbed moisture and residual solvent, and then cooled to 80  C and reheated to 750  C at a heating rate of 10  C min1. To determine the oxidative stability of these membranes, samples (0.5 cm  1 cme50 mm) were immersed in Fenton's reagent (3 ppm Fe2þ, 3% H2O2) at 80  C, and the elapsed time when these membranes dissolved completely (t) and weight changes after soaking for 1 h were recorded. To determine the hydrolyte stability, small pieces of membranes were soaked in deionized water at 80  C, and the hydrolyte stability was evaluated by the elapsed time when these membranes started to lose mechanical strength. The proton conductivity of membranes was measured by using a CHI660E electrochemical analyzer over the frequency range of 106 to 10 Hz. The membrane was held between two electrodes of PTFE mould. The in-plane proton conductivity (s, S cm1) was calculated according to the following equation: s ¼ L/RS, where L (cm), R (U) and S (cm2) are the distance between the two electrodes, the resistance and cross-sectional area of the membrane, respectively. The experiment was performed at a given RH and temperature.

3

Results and discussion Fabrication of membranes and their solubility in dimethyl sulfoxide Side-chain-type SPIBI was used as the base material due to the easy formation of microphase separated structure and the reactive amine groups in the imidazole ring. Chemically, the N-alkylation of the imidazole ring can be achieved by the reaction between the amine groups and halogen [24] or epoxide [25]. The chemical structure of the cross-linker has a significant effect on the properties of the resulted cross-linked membranes. In the present work, the cross-linked structure is formed by using KH560 as a cross-linker, and the reaction between amine in SPIBI and epoxy at an elevated temperature and the hydrolysis-condensation of siloxane are shown in Scheme 1. The obtained membranes are homogeneous and transparent. The FT-IR spectra of SPIBI80, KH560 and CSiSPIBIx membranes are shown in Fig. 1a. The absorption band at 910 cm1, which can be assigned to the epoxy group of KH560, disappears completely in the CSiSPIBI membranes, and the wide absorption band of eNH (3250e3500 cm1) becomes weaker after cross-linking, indicating the successful reaction between the epoxy group and the imidazole ring. It is difficult to observe the vibration absorption band of SieOeSi, which generally occurs at 1040 cm1 [23], because of the disturbance of O]S]O at 1020 and 1090 cm1. The solubility change of the membranes is an intuitive way to examine the formation of cross-linked network. Fig. 1b shows the residual mass of membranes immersed in DMSO at 80  C after 24 h. Pristine SPIBI80 membrane is completely dissolved in DMSO within 24 h. SPIBI70 membrane breaks down into pieces with the residual weight lower than 50%. After covalent cross-linking, CSiSPIBI70 membrane remains undissolved after 24 h, with the residual mass higher than 88.7%. However, CSiSPIBI80 membrane is almost completely dissolved due to the low cross-linking density. The solubility in DMSO decreases dramatically as the degree of cross-linking increases, which indicates a high resistance of these crosslinked membranes to DMSO. PA-doped cross-linked membranes, PAy-CSiSPIBIx (y represents the PA-doping level), were successfully prepared by immersing CSiSPIBIx membranes in 85% PA aqueous solution at 80  C for 48 h. After immersion, all membranes remain intact and tough. The sulfonation level and PA-doping level are expected to have a significant influence on the proton conductivity of resulted membranes. In order to elucidate the effects of PA and sulfonic acid (SA) on the proton conductivity of PAy-CSiSPIBIx membranes, the IEC and PA-doping level were measured by titration (Table 1). Typically, PA can be absorbed by alkaline groups such as imidazole and pyridine in the polymer chain. However, the imidazole content in the present study is low (20e40 mol%), resulting in a low PA-doping level. In order to enhance the PAdoping level, KH560 is used as a cross-linker and a site to absorb PA due to the electrostatic interactions. The PA level of PA-CBrSPIBI is ca. 45 wt.%, which is still lower than that of conventional PA-PBI membranes (typically 6e10 mol of PA per

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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loss of moisture absorbed by hygroscopic PA at about 100  C, and the second step starting at about 170  C is due to the condensation reaction of PA [23]. The thermal degradation of membranes at above 500  C is assigned to the decomposition of SPIBI main chain and the dehydration of pyrophosphoric acid to polyphosphoric acid. Fig. 2 also shows that PACSiSPIBI70 membrane exhibits a poorer thermal stability than the corresponding blend membrane. Td10% is 223  C for PA-CSiSPIBI70 and 235  C for PA40-SPI(70)/PBI(30), respectively. Although the introduction of flexible aliphatic chain of KH560 reduces the thermal stability of cross-linked membranes, all of these membranes show sufficient thermal stability at temperature below 200  C, which is within the temperature range of fuel cell applications.

mole polymer repeat unit [moles PA/PBI]), that is 65e76 wt %) [26,27]), but higher than that of our previously synthesized dihalomethyl cross-linked SPIBI membranes (ca. 30 wt.%). SPI100 (based on NTDA/BSPOB) and PA-doped SPI/PBI (PASPI(70)/PBI(30)) blend membranes, where the number represents the weight ratio) were prepared and compared.

Thermal stability The thermal properties of both undoped and PA-doped crosslinked membranes were investigated by thermogravimetric analysis (TGA). As shown in Fig. 2, pristine CSiSPIBI70 membrane has excellent thermal stability with two distinct degradation stages, which is similar to that of SPI100. The weight loss is mainly attributed to the decomposition of SA and main polymer chain. The first weight loss stage of CSiSPIBI70 and SPI100 membranes at about 280  C is due to the cleavage of eSO3H [28], while the second one at about 500  C is associated with the degradation of main polymer chain [29]. The thermal properties of all PA-doped membranes decline with four degradation steps. The first step is due to the weight

Mechanical properties Covalent/ionic cross-linking density and PA-doping level are key factors controlling the mechanical behavior of membranes. It is known that the impregnation of membranes with PA results in significant deterioration of the mechanical

HO3S O

O

OO

O

N

N

O

O

x

H N

N

N

N H

O

O

N

N

O

O

O KH560

DMSO,stirring y

n homogeneous

OCH3 Si OCH3 OCH3

casting,80°C/24h,160°C/1h

SPIBI SO3H HO3S

O

OO

O

N

N

O

O

x

H N

N

N

N

O

O

N

N

O

O

CH2 O CH

SO3H

O

O

N

N

O

O

x

H N

N

N

N CH2 CH O O

SO3H HO3S

OO

O

SO3H

n

80°C/24h

OCH3 Si OCH3 OCH3

O

HO3S OO

immersed in 1 M H2SO4 y

O

O

N

N

O

O

O Si O O O Si O CH2

N

O

O

n

O Si CH2 O Si O

O

N

y

x

O O CH CH2 N

N

N

N H

O

O

N

N

O

O

y

n

CSiSPIBI

Scheme 1 e Preparation of silane-cross-linked CSiSPIBI membranes. Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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(a)

(b)

910cm 2930,2860cm

CSiSPIBI60

CSiSPIBI70

Transmittance(a.u.)

CSiSPIBI80

100

94.5

95.0

2

3500

3000

2500

2000

1500

1000

500

Wavenumbers (cm-1)

0.0 5

6

SPIBI80

SPIBI70

46.3

20

Almost completely dissolved

1

40

CSiSPIBI70

CSiSPIBI60

60

SPIBI60

Remaining Weight (%)

80

KH560

4000

CSiSPIBI80

88.7 SPIBI80

0 3

4

Fig. 1 e (a) FT-IR spectra of KH560, SPIBI80 and cross-linked CSiSPIBIx membranes; and (b) solubility of SPIBIx and crosslinked CSiSPIBIx membranes in DMSO at 80  C for 24 h.

properties and the increase in maximum elongation, and cross-linking is an effective approach to improve the mechanical strength. The mechanical properties of membranes are shown in Table 2, and the actual stressestrain curves of undoped (group A) and PA-doped (group B) membranes at room temperature are shown in Fig. 3. The undoped SPI(70)/ PBI(30) blend membrane has a tensile strength of 81.5 MPa and elongation at break of 6.6%. CSiSPIBI70 membrane has a higher tensile strength and elongation at break than SPI(70)/ PBI(30), indicating that introduction of covalently cross-linked structure improves the mechanical properties of membranes. CSiSPIBI60 shows the highest tensile strength (106.5 MPa) because of high cross-linking density, whereas CSiSPIBI80 shows the lowest tensile strength because of less crosslinking sites. The molar fraction of the benzimidazole ring in SPIBI80 is only 20%, leading to more side reaction such as the grafting reaction rather than covalent cross-linking reaction.

Furthermore, the grafting structure disrupts the ionic crosslinking in the pristine SPIBI80, and the introduction of KH560 reduces the tensile stress of SPIBI80 membrane. Thus, the introduction of appropriate covalent cross-linking density can significantly improve the mechanical strength of membranes. The doping acid is known to have a plasticizing effect, resulting in poor tensile stress of PA-doped membranes. The tensile stress of all PA-doped membranes decreases to half of that of non-PA-doped ones. For example, the tensile stress of PA-SPI(70)/PBI(30) and PA-CSiSPIBI70 is 39.6 and 41.7 MPa, respectively; whereas the tensile stress of PA-CSiSPIBI80 is only 18.0 MPa due to the grafting structure. However, the introduction of cross-linked inorganic siloxane network allows the tensile stress to be maintained at a moderate level.

100

CSiSPIBI70 PA40-SPI(70)/PBI(30) PA45-CSiSPIBI70 SPI100

Table 1 e IEC and PA contents of polymer membranes. IEC/meq g1 Calculateda Measuredb PA42-CSiSPIBI80 PA45-CSiSPIBI70 PA48-CSiSPIBI60 CSiSPIBI80 CSiSPIBI70 CSiSPIBI60 PA40-SPI(70)/PBI(30)c SPI100d a b

c

d

1.48 0.93 0.47 1.48 0.93 0.47 0.52 2.63

1.55 1.02 0.54 1.55 1.02 0.54 0.48 2.58

90

PA content (%)

42.3 44.9 48.4 e e e 40.7 e

IEC calculated from the content of SA per polymer weight. IEC measured using 0.01 M NaOH with phenolphthalein as the indicator. SPI and PBI blend membrane, where the number represents the weight ratio. NTDA/BSPOB membrane.

Weight (%)

Membranes

80

70

60

50 100

200

300

400

500

600

700

o

Temperature C

Fig. 2 e Thermal stability of CSiSPIBI70, PA45-CSiSPIBI70, PA40-SPI(70)/PBI(30) and SPI100.

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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Table 2 e The mechanical properties of cross-linked membranes before and after doping in 85% PA solution. Samples

Tensile strength (MPa) CSiSPIBI60 CSiSPIBI70 CSiSPIBI80 SPI(70)/PBI(30) a

106.5 101.9 57.7 81.5

Elongation at break (%)

± 11.5 ± 3.6 ± 7.6 ± 4.5

10.4 9.18 3.73 6.6

Tensile strength (MPa)

± 2.9 ± 1.7 ± 0.5 ± 1.1

36.3 ± 41.7 ± 18.0 ± 39.6 ±

3.5 1.7 4.1 2.4

Elongation at break (%) 22.7 17.8 3.2 11.7

± 3.2 ± 2.8 ± 0.7 ± 2.7

48.4 44.9 42.3 40.7

Doped membranes were obtained by immersing the membranes in 85% PA at 80  C for 48 h.

For example, the tensile stress of PA-PBI membranes at an acid doping level of 5e9 is typically only 3e4 MPa [15,30], which is much lower than that of cross-linked membranes in the present study. Covalent cross-linking is clearly an effective approach to improve the mechanical stability of aciddoped membranes.

Hydrolytic and oxidative stability Stability is critical for the long-term durability of PEMs. Although the commonly used six-membered naphthalenic polyimides are hydrolytically more stable than their fivemembered analogs, further improvement should be made for wider applications. The hydrolytic stability was determined by the elapsed times when the hydrated membranes began to lose their mechanical strength (broken after lightly bent). Table 3 shows that both doped and undoped CSiSPIBI80 membranes have very poor hydrolytic stability because of the side reaction. It is also noted that the hydrolytic stability of CSiSPIBI70 membrane is better than that of CSiSPIBI80 membranes, but poorer than that of blend ones. This may be because the introduction of inorganic siloxane disturbs the ionic cross-linked structure in the pristine SPIBI70 membrane, resulting in a decrease in tensile stress. CSiSPIBI60 membrane shows the best hydrolytic stability due to high cross-linking density, which is tough enough to withstand the attack of water.

A-1 CSiSPIBI60 A-2 CSiSPIBI70 A-3 CSiSPIBI80 A-4 SPI(70)/PBI(30)

120

100 A-2

Stress (MPa)

PA-content 80  C (%)

Dopeda

Undoped

80

A-1

B-1 PA48-CSiSPIBI60 B-2 PA45-CSiSPIBI70 B-3 PA42-CSiSPIBI80 B-4 PA40-SPI(70)/PBI(30)

A-4

Radical-induced degradation of polymer membranes has also been recognized as a critical issue for the durability of PEMFCs. The chemical stability was evaluated by the weight changes of undoped membranes immersed in Fenton's reagent (3 ppmFe2þ, 3 wt.% H2O2) at 80  C for 1 h and the elapsed time (t) when one membrane was dissolved completely. The results of the Fenton tests are summarized in Table 3. The pristine SPI(70)/PBI(30) membrane exhibits better chemical stability than SPI100 membrane with the dissolution time higher than 450 min, indicating that the introduction of benzimidazole groups improves the resistance to radical attack during the Fenton test. The oxidative stability of the membranes increases as the cross-linking degree increases. For example, the oxidative stability of PA-CSiSPIBI70 membrane is superior to that of blend ones. The dissolution time of PA-CSiSPIBI70 is increased to 1450 min, indicating that covalent cross-linking is another important factor affecting the oxidative stability of membranes [31,32].

Proton conductivity Proton conductivity of PA-CSiSPIBI, PA-SPI/PBI, CSiSPIBI70, SPI100 and Nafion 115 membranes was measured from 30 to 150  C under 30% RH condition, and the results are shown in Fig. 4. For the acid doped membranes, the testing temperature, ambient humidity, types of acid used, and acid doping level have a significant influence on the proton conductivity. The proton conductivity of PA-doped sulfonated membranes is superior to that of non-PA-doped and PA-doped non-sulfonated membranes. For example, the proton conductivity of

Table 3 e Hydrolytic and oxidative stability of CSiSPIBIx and PA-CSiSPIBIx membranes. Polymer

60

B-4

B-2 B-1

20

Residual weight (%)

Time (day)

CSiSPIBI60 CSiSPIBI70 CSiSPIBI80

1185 915 780

>60 10 4

SPI(70)/PBI(30) PA48-CSiSPIBI60 PA45-CSiSPIBI70 PA42-CSiSPIBI80 PA40-SPI(70)/PBI(30) SPI100

450 1335 1450 810 510 50

97 97 Break down into pieces 94 e e e e 0

B-3

0 0

5

10

15

20

25

Strains (%)

Fig. 3 e Stress-strain curves of undoped and doped membranes.

30

Hydrolytic stability at 80  C

t (min)

A-3

40

Oxidative stability at 80  C

30 >60 33 5 45 1.5

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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polyimide under humidified conditions is attributed to the hydrophobic/hydrophilic micro-phase separation structure and well-contacted hydrophilic domains formed by the aggregation of SA groups. However, the channels responsible for proton and water transport become narrower after crosslinking [20]. Scheme 2 shows that the incorporation of PA molecules into the polymer matrix influences the characteristics of the hydrophilic and hydrophobic phases. On the one hand, SA groups absorb PA molecules, forming a new proton transport pathway between PA and SA, which provides additional proton conduction sites inside the hydrophilic channels of SPI. On the other hand, alkaline imidazole groups and silane-cross-linked structure immobilize PA molecules in the hydrophobic phase. Since the phosphoric acid groups can simultaneously act as proton acceptors and donors, a continuous proton transport pathway forms in the hydrophobic phase of polymer matrix. Both hydrophobic and hydrophilic phase contain high density of proton transfer sites, leading to wide-range transfer pathways and higher proton conductivity. The sulfonation degree has a small influence on the proton conductivity of PA-CSiSPIBI membranes. For example, the proton conductivity of PA48-CSiSPIBI60 (7.1 mS cm1) is higher than that of PA42-CSiSPIBI80 (50  C, 3.9 mS cm1), indicating that the higher the sulfonation degree is, the lower the proton conductivity will be. This is because when the sulfonation degree is low, there are more imidazole sites and silane-crosslinked structures in the polymer chain, leading to a higher amount of PA. The conductivity of all membranes at 30e150  C increases with an increase of temperature except for the non-PA-doped membranes. The proton conductivity of sulfonated membranes decreases at temperatures above 100  C due to

Fig. 4 e Temperature dependence of conductivity of PACSiSPIBI, PA-SPI/PBI, CSiSPIBI70, SPI100, PA-PBI and Nafion 115 at 30% RH.

PA45-CSiSPIBI70 is 0.0031 S cm1, which is approximately two orders of magnitude higher than that of PA-PBI (30  C/30% RH, 6.5  105 S cm1). The similar phenomenon is also found in PA-doped PBI and sulfonated PBI [33,34]. According to the proton transport mechanisms, the conductivity is strongly dependent on the density of proton-conducting sites of the membrane. Scheme 2 shows the complex interaction among the sulfonic groups, phosphoric acid, imidazole moiety and water. The proton conductivity of Nafion or sulfonated

HO3S

H N

N

NH

N H

O

O

OO

N

N

N

N

O

O

O

O

x

O

O yn

H2PO4H2PO4 H

HO3S OO O SO3H

O

N

N

O

O

H N x

H2PO4-

N

N H

H3PO4

SO3H

N CH2 CH O O -

O3S

H3 O+

O

O

O

N

N

O

O

O Si O O O Si O CH2

OO

O

N

N

O

H3PO4

O

x

y n

O Si CH2 O Si O OH2

H2PO4-

O O CH CH2 N

N

NH

N H

O

O

N

N

O

O

y

n

H2PO4SO3H

Scheme 2 e Possible interactions between sulfonic acid, phosphoric acid, H2O and imidazole ring. Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9

dehydration. Protons transport as hydrated compounds such þ as H3Oþ, H5Oþ 2 , H9O4 by the diffusion mechanism in the sulfonated membranes. Hence, the conductivity of SPI100, Nafion 115 and CSiSPIBI membranes are decreased at temperatures above the boiling point of water. In general, PA-doped PBI is the most widely used material for high temperature PEMFCs and membranes with a higher PA-doping level exhibit higher conductivity but poorer mechanical stress. However, despite the low PA-doping level, the proton conductivity of PA-doped sulfonated membranes is much higher than that of PA-PBI. All these results indicate that although PA has a significant effect on the conductivity at high temperature, SA also plays an important role.

Conclusions In this study, a series of silane-cross-linked membranes have been synthesized and further doped with PA due to the acidebase interaction with the imidazole rings and the electrostatic interaction with siloxane. The introduction of silanecross-linked structure can not only increase the PA-doping level, but also improve the chemical and mechanical properties of resulted PA-CSiSPIBI membranes. PA45-CSiSPIBI70 membrane shows a proton conductivity of 0.018 S cm1 at 90  C/30%RH, which is approximately one to two orders of magnitude higher than that of normal membranes due to the formation of a new proton transport pathway. These membranes also exhibit improved mechanical strength and chemical stability because of the silane-cross-linked structure. PA45-CSiSPIBI70 membrane shows a tensile stress of 41.7 MPa, and can resist hydrolytic and free radical attack for more than two months and 1450 min, respectively. All these results suggest that PA-CSiSPIBI membranes have the potential for use as PEMs.

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

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Acknowledgments [17]

This research is financially sponsored by the Fundamental Research Funds for the Central Universities (WD1315012) and Shanghai Pujiang Program (D200-2R-1181). [18]

references [19] [1] Wang H, Xu X, Johnson NM, Dandala NK, Ji HF. High proton conductivity of water channels in a highly ordered nanowire. Angew Chem Int Ed Engl 2011;50:12538e41. [2] Steele BC, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [3] Wu H, Cao Y, Li Z, He G, Jiang Z. Novel sulfonated poly (ether ether ketone)/phosphonic acid-functionalized titania nanohybrid membrane by an in situ method for direct methanol fuel cells. J Power Sources 2015;273:544e53. [4] Yang L, Tang B, Wu P. A novel proton exchange membrane prepared from imidazole metal complex and nafion for low humidity. J Membr Sci 2014;467:236e43. [5] Park CH, Lee CH, Guiver MD, Lee YM. Sulfonated hydrocarbon membranes for medium-temperature and low-

[20]

[21]

[22]

humidity proton exchange membrane fuel cells (PEMFCs). Prog Polym Sci 2011;36:1443e98. Zhou F, Andreasen SJ, Kær SK. Experimental study of cell reversal of a high temperature polymer electrolyte membrane fuel cell caused by H2 starvation. Int J Hydrogen Energy 2015;40:6672e80. Bose S, Kuila T, Nguyen TXH, Kim NH, Lau K-t, Lee JH. Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges. Prog Polym Sci 2011;36:813e43. Soberats B, Yoshio M, Ichikawa T, Taguchi S, Ohno H, Kato T. 3D anhydrous proton-transporting nanochannels formed by self-assembly of liquid crystals composed of a sulfobetaine and a sulfonic acid. J Am Chem Soc 2013;135:15286e9. Xu C, Cao Y, Kumar R, Wu X, Wang X, Scott K. A polybenzimidazole/sulfonated graphite oxide composite membrane for high temperature polymer electrolyte membrane fuel cells. J Mater Chem 2011;21:11359. Li Z, Yue X, He G, Li Z, Yin Y, Gang M, et al. Enhanced water retention and low-humidity proton conductivity of sulfonated poly(ether ether ketone) hybrid membrane by incorporating ellipsoidal microcapsules. Int J Hydrogen Energy 2015;40:8398e406. Lin C, Haolin T, Mu P. Periodic nafion-silica-heteropolyacids electrolyte t for PEM fuel cell operated near 200  C. Int J Hydrogen Energy 2012;37:4694e8. Thanganathan U, Nogami M. Investigations on effects of the incorporation of various ionic liquids on PVA based hybrid membranes for proton exchange membrane fuel cells. Int J Hydrogen Energy 2015;40:1935e44. van de Ven E, Chairuna A, Merle G, Benito SP, Borneman Z, Nijmeijer K. Ionic liquid doped polybenzimidazole membranes for high temperature proton exchange membrane fuel cell applications. J Power Sources 2013;222:202e9. Wainright J, Wang JT, Weng D, Savinell R, Litt M. Acid-doped polybenzimidazoles: a new polymer electrolyte. J Electrochem Soc 1995;142. L121eL3. He R, Li Q, Bach A, Jensen JO, Bjerrum NJ. Physicochemical properties of phosphoric acid doped polybenzimidazole membranes for fuel cells. J Membr Sci 2006;277:38e45. € ring T. Synthesis and Kerres J, Ullrich A, Meier F, Ha characterization of novel acidebase polymer blends for application in membrane fuel cells. Solid State Ionics 1999;125:243e9. Iizuka Y, Tanaka M, Kawakami H. Preparation and proton conductivity of phosphoric acid-doped blend membranes composed of sulfonated block copolyimides and polybenzimidazole. Polym Int 2013;62:703e8. Suzuki K, Iizuka Y, Tanaka M, Kawakami H. Phosphoric aciddoped sulfonated polyimide and polybenzimidazole blend membranes: high proton transport at wide temperatures under low humidity conditions due to new proton transport pathways. J Mater Chem 2012;22:23767. Malinowski M, Iwan A, Parafiniuk K, Gorecki L, Pasciak G. Electrochemical properties of PEM fuel cells based on nafionepolybenzimidazoleeimidazole hybrid membranes. Int J Hydrogen Energy 2015;40:833e40. Yue Z, Cai Y-B, Xu S. Proton conducting sulfonated poly (imide-benzimidazole) with tunable density of covalent/ionic cross-linking for fuel cell membranes. J Power Sources 2015;286:571e9. Fang J, Zhai F, Guo X, Xu H, Okamoto K-i. A facile approach for the preparation of cross-linked sulfonated polyimide membranes for fuel cell application. J Mater Chem 2007;17:1102. Yue Z, Cai Y-B, Xu S. Facile synthesis of a symmetrical diamine containing bis-benzimidazole ring and its thermally stable polyimides. J Polym Res 2014;21:1e8.

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9

[23] Wang S, Zhao C, Ma W, Zhang N, Zhang Y, Zhang G, et al. Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells. J Mater Chem A 2013;1:621. [24] Tang T-H, Su P-H, Liu Y-C, Yu TL. Polybenzimidazole and benzyl-methyl-phosphoric acid grafted polybenzimidazole blend crosslinked membrane for proton exchange membrane fuel cells. Int J Hydrogen Energy 2014;39:11145e56. [25] Lin H-L, Chou Y-C, Yu TL, Lai S-W. Poly(benzimidazole)epoxide crosslink membranes for high temperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2012;37:383e92. [26] Chen C-Y, Lai W-H. Effects of temperature and humidity on the cell performance and resistance of a phosphoric acid doped polybenzimidazole fuel cell. J Power Sources 2010;195:7152e9. [27] Matar S, Higier A, Liu H. The effects of excess phosphoric acid in a polybenzimidazole-based high temperature proton exchange membrane fuel cell. J Power Sources 2010;195:181e4. [28] Park J, Han SY, Kim D. Synthesis and properties of crosslinked poly(arylene ether ketone) electrolyte membranes containing hygroscopic proton conductors. Int J Hydrogen Energy 2015;40:8160e71.

9

 [29] Alvarez-Gallego Y, Ruffmann B, Silva V, Silva H, Lozano AE, de la Campa JG, et al. Sulfonated polynaphthalimides with benzimidazole pendant groups. Polymer 2008;49:3875e83. [30] Plackett D, Siu A, Li Q, Pan C, Jensen JO, Nielsen SF, et al. High-temperature proton exchange membranes based on polybenzimidazole and clay composites for fuel cells. J Membr Sci 2011;383:78e87. [31] Zhang G, Guo X, Fang J, Chen K. Okamoto K-i. Preparation and properties of covalently cross-linked sulfonated copolyimide membranes containing benzimidazole groups. J Membr Sci 2009;326:708e13.  P, Bjerrum NJ. Cross-linked [32] Li Q, Pan C, Jensen JO, Noye polybenzimidazole membranes for fuel cells. Chem Mater 2007;19:350e2. [33] Bae J-M, Honma I, Murata M, Yamamoto T, Rikukawa M, Ogata N. Properties of selected sulfonated polymers as proton-conducting electrolytes for polymer electrolyte fuel cells. Solid State Ionics 2002;147:189e94. re J. Synthesis and [34] Glipa X, El Haddad M, Jones DJ, Rozie characterisation of sulfonated polybenzimidazole: a highly conducting proton exchange polymer. Solid State Ionics 1997;97:323e31.

Please cite this article in press as: Yue Z, et al., Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2015), http:// dx.doi.org/10.1016/j.ijhydene.2015.10.057