High performance blend membranes based on sulfonated poly(arylene ether sulfone) and poly(p-benzimidazole) for PEMFC applications

Journal of Industrial and Engineering Chemistry 29 (2015) 104–111

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High performance blend membranes based on sulfonated poly(arylene ether sulfone) and poly(p-benzimidazole) for PEMFC applications Mihee Won, Sohyun Kwon, Tae-Hyun Kim * Organic Material Synthesis Lab. Department of Chemistry, Incheon National University, 119 Academy-ro, Songdo-dong, Yeonsu-gu, Incheon 406-772, Korea

A R T I C L E I N F O

Article history: Received 9 December 2014 Received in revised form 12 February 2015 Accepted 5 March 2015 Available online 1 April 2015 Keywords: Polymer electrolyte membranes Blend membranes Ionic crosslinking Poly(p-benzimidazole) Sulfonated poly(arylene ether sulfone)

A B S T R A C T

Novel ionically crosslinked blend polymer membranes were developed based on sPES and p-PBI. p-PBI was used as the key component in polymeric blends with sPES to produce stable but yet still conductive and miscible membranes. Specific interactions between the acid moiety and the amine moiety contributed to the miscibility of the sPES–PBI blend polymers in DMSO. The properties of the membranes were better than those of bare membranes, and the conductivity of a 5 wt.% PBI blend membrane exceeded 0.13 S/cm at 80 8C. The influence of the p-PBI content on chemophysical properties of the blend membranes was investigated systematically. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction The development of novel proton exchange membranes (PEMs) with a high proton conductivity, good stability under humidified and heated conditions, low fuel permeability, and low-cost preparation conditions remains an important challenge to the realization of practical PEM fuel cells (PEMFCs). Perfluorinated copolymers, including Nafion1, are the most commonly used PEMs because they offer excellent chemical, mechanical, and oxidative stabilities, while yielding a high proton conductivity when fully hydrated. The high cost of preparation, high methanol permeability, relatively low performance, and low stability under high operating temperatures, necessitate a search for alternative PEMs [1–4]. Chemically modified and fully aromatic polymers have received significant attention because they appear to meet the operating requirements for fuel cell applications. These polymers may be easily converted to sulfonated polymers via a sulfonation process, and the proton conductivity may be enhanced by the addition of sulfonic acid groups. Sulfonated poly(arylene ether sulfone)s (sPES)s are reasonably good choices for PEMFCs. They are economical and provide adequate mechanical properties, thermal stability, and a high chemical resistance [5–7]. At high water

* Corresponding author. Tel.: +82 32 835 8232; fax: +82 32 835 0762. E-mail address: [email protected] (T.-H. Kim).

content levels, water permeation and electro-osmotic drag are maintained, yielding a high proton conductivity [8]. As the degree of PES sulfonation (DS) increases, the water uptake increases, resulting in a high proton conductivity at the cost of the membrane’s mechanical and dimensional stability. Polymer blending is a common and potentially versatile approach to enhancing the chemical and/or physical properties of one or multiple components [9]. Blending composite acidic and basic polymers, for example, produces ionically crosslinked systems that minimize undesired swelling and enhance the material stability. The development of new useful blends has been severely limited by the incompatibility of many polymers of interest [10]. The entropy of mixing tends to be small, and the enthalpy of mixing and DG tend to be positive, leading to phase separation in the blend. An efficient method of overcoming this problem involves inducing specific interactions between the two polymers that promote miscibility. The prerequisite for such specific interactions is the existence or the introduction of appropriate functional groups into the macromolecular chains. Aromatic polybenzimidazole (PBI) is a macromolecule that possesses both donor and acceptor hydrogen bonding sites that can participate in specific interactions. PBI also exhibits a high thermal stability, high chemical resistance, and an outstandingly high glass transition temperature [11,12]. This material displays ionic conductivity upon doping with phosphoric acid [13]. PBI has been used as a component in polymeric blends involving high-Tg polymers, including sPES [14,15], to produce high-performance

http://dx.doi.org/10.1016/j.jiec.2015.03.022 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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7.18 (40H, d, J = 8.0, ArH4), 7.12 (24H, d, J = 8.0, ArH1), 7.00 (8H, d, J = 7.5, ArH5); GPC (DMF, RI)/Da Mn 9.48  104, Mw 1.31  105 and Mw/Mn 1.38.

blends between sulfonic acid in sPES and basic amines in PBI. In this case, the miscibility of the polymer blend may be assessed in terms of specific interactions. The use of PBIs has been limited due to its poor solubility in organic solvents. For this reason, structural variations of PBIs involving meta-substituted linkages [16–18], aryl ether linkages [19,20], or main-chain alkyl substituents on the monomers prior to polymerization [21] have been designed in an effort to improve solubility. The poly[2,20 -(m-phenylene)-5,50 bibenzimidazole] (m-PBI) derivatives are the most frequently studied among the PBIs, whereas poly[2,20 -(1,4-p-phenylene)-5,50 bibenzimidazole] (p-PBI) has rarely been utilized due to its poor solubility in organic solvents. In this work, we prepared ionically crosslinked systems using blend membranes based on sPES and p-PBI for use as novel PEMs (Structure 1). The main aim of the present work involves the development of stable yet conductive miscible blend systems consisting of the easily available acidic polymer sPES and the basic polymer p-PBI. Although several studies have examined the use of PBI derivatives as crosslinkers, to our knowledge, this is the first instance of a systematic investigation of the influence of the p-PBI content on the chemical and physical properties, as well as proton conductivity, of sPES/p-PBI blend membranes.

Poly(phosphoric acid) (PPA) (100 cm3) was added to a twonecked round bottom flask equipped with a condenser and a nitrogen inlet. The PPA solution was allowed to stir at 100 8C for 2 h under a nitrogen atmosphere. 3,30 -Diaminobenzidine (2.58 g, 12.04 mmol) and terephthalic acid (2 g, 12.04 mmol) were added to the PPA solution with stirring. The temperature of the reaction mixture was then increased to 200 8C and maintained there for 20 h. After the reaction time, the viscous solution was poured into deionized water. The brown powder product was filtered and washed with deionized water several times. The powder polymer was dried at 80 8C under vacuum for 12 h. The polymer was then stirred in a 10% potassium carbonate solution for 24 h. Finally, the product was filtered and washed with deionized water and heated at 80 8C in vacuum oven for 12 h to give p-PBI 2 as a brown powder (3.4 g, 74.2%); dH (400 MHz, d6-DMSO) 13.15 (2H, s, imidazole – NH5), 8.41 (4H, s, ArH1), 7.52–8.05 (6H, m, ArH2,3,4); GPC (DMF, RI)/ Da Mn 1.05  104, Mw 2.43  104 and Mw/Mn 2.30.

Experimental

Fabrication of the crosslinked membranes

Materials

All membranes were prepared in a DMSO solution of the corresponding polymers using a solution-casting method, and the film thickness was controlled using the doctor blade technique. The sPES polymer 1 was first dissolved in DMSO to form a 12 wt.% solution. To this solution was added the PBI crosslinker 2 (3, 5, and 7 wt.% relative to the sPES) dissolved in hot DMSO, and the solution was allowed to stir until a clear solution was obtained. The solution was then poured onto a clean glass plate, and the film was cast and dried in a vacuum oven at 100 8C for 12 h. The membrane on the glass was soaked in water at room temperature to remove residual solvent and was peeled away by immersion in deionized water. The prepared membranes were treated first in a 0.5 M H2SO4 solution at 80 8C for 4 h and then immersed in deionized water at room temperature for 24 h prior to drying in a vacuum oven at 80 8C for 12 h. For comparison, the non-crosslinked membrane was obtained according to the same procedure, except for the addition of the PBI crosslinker.

4,40 -Dichlorodiphenylsulfone (DCDPS), terephthalic acid (TPA), and poly(phosphoric acid) (PPA) were obtained from Aldrich Chemical Co. 4,40 -Biphenol (BP) and 3,30 -diaminobenzidine (DAB) were purchased from TCI. Potassium carbonate, DCDPS, and BP were dried under vacuum at 40 8C for 12 h prior to polymerization. Nafion-117 membrane was purchased from Aldrich Chemical Co. All other chemicals were obtained from commercial sources and were used without further purification. Synthesis of sulfonated poly(arylene ether sulfone) (sPES) 1 4,40 -Dichlorophenylsulfone was added to a two-necked round bottom flask equipped with a condenser and two gas traps. While the flask was immersed in an ice bath, fuming sulfuric acid was injected. The reaction mixture was heated at 110 8C overnight, after which the mixture was cooled to room temperature. The mixture was then poured over ice, and an excess of NaCl was added until the product was separated out. The product was filtered, washed with ethanol several times, and redissolved in deionized water. The solution was neutralized to pH 7 with 2 M Na2CO3 and then recrystallized. 4,40 -Biphenol (3.22 g, 17.29 mmol), 4,40 dichlorodiphenylsulfone (2.53 g, 8.81 mmol), 3,30 -disulfonated4,40 -dichlorodiphenyl sulfone (4.25 g, 8.65 mmol) and potassium carbonate (2.92 g, 21.13 mmol) were added to a mixture of DMAc (30 cm3) and toluene (25 cm3) in a two-necked round bottom flask equipped with a Dean–Stark apparatus, a condenser, and a nitrogen inlet. The reaction mixture was heated at 150 8C for 4 h. Once the water had been essentially removed from the reaction mixture by azeotropic distillation, the toluene was distilled away. The temperature of the reaction mixture was then increased to 170 8C, and the solution was stirred at this temperature for another 16 h under a nitrogen atmosphere. The mixture was ten cooled to room temperature and added to MeOH. The product was collected by filtration and washed with deionized water several times before drying at 80 8C under vacuum for at least 48 h to give the random copolymer 1 as brown flakes (9.5 g, 95%); dH (400 MHz, d6-DMSO) 8.30 (8H, s, ArH7), 7.94 (24H, d, J = 8.0, ArH2), 7.85 (8H, d, J = 7.5, ArH6), 7.72 (40H, d, J = 7.5, ArH3),

Synthesis of polybenzimidazole 2

Characterization and measurements 1

H NMR spectra were obtained on an Agilent 400-MR (400 MHz) instrument using d6-DMSO as a reference or internal deuterium lock. FT-IR spectra were recorded on a Nicolet MAGNA 560-FTIR spectrometer. Molar masses were determined by Gel Permeation Chromatography (GPC) using two PL Gel 30 cm  5 mm mixed C columns at 30 8C running in DMF and calibrated against polystyrene (Mn = 600–106 g/mol) standards using a Knauer refractive index detector. The X-ray diffraction patterns of the dry membranes were recorded using a Rigaku HR-XRD smartlab diffractometer by employing a scanning rate of 0.1˚/min in a 2u range from 0˚ to 1.5˚ with a Cu-Ka X-ray (l = 1.54 A˚). The dried membranes were placed under vacuum at 80 8C for 24 h prior to the measurement. Thermal stability of the membranes was analyzed by the thermogravimetric analysis measurements on a Shimadzu TGA2950 instrument at a heating rate of 10 8C/min in a nitrogen flow.

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Tapping mode AFM was performed using a Bruker MultiMode instrument. A silicone cantilever with an end radius <10 nm and a force constant of 40 N/m (NCHR, nanosensors, f = 300 kHz) was used to image the samples at ambient temperature. The samples were equilibrated with 50% RH at least 24 h prior to the imaging. The measurements were conducted under the same conditions for each sample to keep consistency. Tensile properties were measured on a Shimadzu EZ-TEST E2-L instrument benchtop tensile tester using a crosshead speed of 1 mm/min at 25 8C under 50% relative humidity. The membranes have a thickness between 30 and 50 mm. Engineering stress was calculated from the initial cross sectional area of the sample and Young’s modulus (E) was determined from the initial slope of the stress–strain curve. The membrane samples were cut into a rectangular shape with 80 mm  8 mm (total) and 30 mm  8 mm (test area). The oxidative stability was investigated by immersing small pieces (5 mm  5 mm) of the membranes into Fenton’s reagent (3% H2O2 containing 2 ppm FeSO4) at 80 8C and evaluated by recording the time that membranes began to break into pieces. Ion exchange capacity (IEC) of the membranes were determined by the titration method. A membrane in a H+-form was first equilibrated in 2.0 M NaCl solution for 24 h to fully exchange the protons with sodium cations. The membrane was then rinsed with deionized water. The acidity in the NaCl solution was titrated by 0.01 M NaOH. The membrane was then re-acidified, washed and dried to measure the dry weight (Wdry, g). IEC (in meq/g) was calculated as the moles of exchangeable protons per gram of the dry weight according to the following equation:

ion exchange capacity ðIECÞ ¼

DV NaOH C NaOH W dry

;

where DVNaOH is the volume of NaOH consumed, CNaOH is the concentration of NaOH and Wdry is the weight of the dry membrane. Dimensional change of the membranes was evaluated from measuring the swelling ratio of the membranes, which was investigated by immersing the round-shaped membranes into water at room temperature and 80 8C, respectively, and the changes of both in-plane and thickness direction (through-plane) were calculated using the following equations:


Dt ð%Þ ¼

Dl ð%Þ ¼

t  t dry  100 t dry


l  ldry  100; ldry

where tdry and ldry are the thickness and diameter of the dried membranes, respectively, and t and l refer to those of the membranes immersed in water for 24 h. The dried membranes were prepared by placing membranes under vacuum at 60 8C for 24 h prior to the measurement. Conductivity. Proton conductivity (s) of each membrane coupon (size: 1 cm  4 cm) was obtained using s = l/RA (l: distance between reference electrodes, A: cross-sectional area of a membrane coupon). Here, ohmic resistance (R) was measured by four-point probe alternating current (ac) impedance spectroscopy using an electrode systems connected with an impedance/ gain-phase analyzer (SI-1260) and an electrochemical interface (SI-1287) over the frequency range from 10 to 200 kHz. The proton conductivity value was obtained by the average of at least

3 different tests. The conductivity measurements were performed from 20 8C to 80 8C, and the film samples were placed in a closed cell to keep the relative humidity at 100%. The conductivity value was obtained by the average of at least 3 trials with same time intervals. Total water uptake (%) was measured as follows: after soaking the membranes in distilled water for more than 24 h, they were wiped with a filter paper and weighed immediately (Wwet). The membranes were then dried under a vacuum condition until a constant weight was obtained (Wdry). The water uptake (%) by weight is the ratio of the hydrated membrane to the dried membrane. The value was calculated using the following equation:

WU ð%Þ ¼



W wet  W dry W dry




 100:

Results and discussion Synthesis of the sulfonated poly(arylene ether sulfone) copolymer and poly(p-benzimidazole) crosslinker The sulfonated poly(arylene ether sulfone) containing 40 mol% hydrophilic sulfonate groups (sPES-40, 1) was chosen as the pristine polymer due to its ease of preparation and well-known properties [22,23]. Despite having a high conductivity, the water uptake and swelling ratios at high temperatures were too high for use in PEM applications. 1 was synthesized via an addition–elimination-type polycondensation between 4,40 -dichloro diphenylsulfone (DCDPS, 6 mol) and 3,30 -disulfonated-4,40 -dichlorodiphenyl sulfone (SDCDPS, 4 mol), in the presence of 4,40 -biphenol [BP, (6 + 4) mol] (Scheme 1). A high molecular weight of Mn = 9.48  104 g/mol, as confirmed by GPC, was obtained, and the copolymer composition (6:4) was confirmed by comparing the relative 1H NMR absorption peak areas of H7 (8.30 ppm) and H2 (7.94 ppm), as shown in Fig. 1a. The p-PBI crosslinker 2 was prepared according to the literature procedure in PPA, which acted as a solvent as well as a catalyst and dehydrating agent (Scheme 1) [24]. A molecular weight of 1.05  104 g/mol (Mn), as confirmed by GPC, was obtained, and 1 H NMR spectroscopy confirmed the structure (Fig. 1b). Preparation and characterization of the ionically crosslinked sPES–PBI blend membranes A DMSO solution of the sPES (sPES-40, 1) was mixed with 3%, 5%, or 7% (by weight) PBI (2) in DMSO to yield homogeneous solutions. The selection of DMSO as a solvent was found to be important for obtaining clearly miscible blend solutions, and other solvents, including DMF or DMAc, were not efficient for this purpose. Each solution was cast into films on glass plates and dried under vacuum at 100 8C for 12 h. During the drying process, ionically crosslinked membranes having different PBI contents were produced, and the three products were designated sPES–PBI3, sPES–PBI-5, and sPES–PBI-7, respectively (Scheme 1). All ionically crosslinked blend sPES–PBIs and non-crosslinked pristine sPES-40 compositions produced transparent and flexible membranes 30–50 mm thick. The blend membranes resulting from the sPES–PBI-3, sPES–PBI-5, or sPES–PBI-7 compositions were insoluble in common organic polar solvents, including DMF and DMSO, whereas sPES-40 was completely soluble, indicating the successful formation of a crosslinked network (Table 1). The gel fraction, measured from the ratio of the weight of the crosslinked membranes after extraction from DMSO to the initial weight, and considered an indirect measurement of crosslinking density, was

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Scheme 1. Syntheses of sPES-40 (a) and p-PBI (b).

98.1%, 98.5% and 99.0% for sPES–PBI-3, sPES–PBI-5 and sPES–PBI-7, respectively, indicating that the crosslinking efficiency was high (Table 1). The crosslinking density increased with the macromolecular crosslinker (PBI 2) content. The ionically crosslinked structures were verified by comparing the FT-IR spectra obtained from the pristine sPES 1 with those of the physically blended sPES–PBIs (Fig. 2). The peaks at 1102 cm1 and 1021 cm1, which corresponded to the vibrational modes of the O5 5S5 5O moiety in the sulfonic acid groups of sPES, shifted to slightly higher wavenumbers upon blending with the PBI crosslinker,

whereas the other peaks remained stationary (compare Fig. 2b to d) [19,20]. These results verify the presence of ionic crosslinks between the acid groups in sPES and the basic nitrogen moiety in the PBI crosslinker. SAXS and morphological analysis SAXS experiments revealed that the ionic cluster size, calculated from the first-order scattering peaks observed in Fig. 3 using the relationship, 2P/q (Fig. 3 and Table 2), decreased as the PBI content

Fig. 1. 1H NMR spectra of sPES-40 1 in d6-DMSO (a) and p-PBI in d6-DMSO (b).

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Table 1 Solubility and gel fraction data obtained from the pristine sPES-40 and ionically crosslinked sPES–PBI blend membranes. Membrane

sPES-40 sPES–PBI-3 sPES–PBI-5 sPES–PBI-7

Solubility

(a)

Gel fraction (%)

DMF

DMSO

H X X X

H X X X

– 98.1 98.5 99.0

(b) increased due to the formation of a more densely packed crosslinked network. A morphological analysis of AFM images revealed less distinctive phase separation and narrower hydrophilic domains as the PBI content increased, indicating the formation of smaller ionic clusters (Fig. 4). Both the SAXS and AFM data suggested that the interactions between the polymer chains were enhanced by ionic crosslinking; hence, a higher PBI crosslinker content hindered proton ion transport.

(c)

(d)

0.0

0.1

0.2

0.3

0.4

Q-value, nm-1 Fig. 3. SAXS patterns obtained from the sPES-40 (a), sPES–PBI-3 (b), sPES–PBI-5 (c), and sPES–PBI-7 (d) membranes in the dry state.

IEC and proton conductivity The IEC, defined as the milliequivalents (meq) of conducting groups per gram of polymer, plays a crucial role in the water uptake and conductivity of a membrane. As expected, the IECs of the ionically crosslinked sPES–PBI blend membranes decreased as the amount of PBI crosslinker 2 increased (Table 3). The proton conductivities of all three blend membranes were measured at temperatures ranging from 20 8C to 80 8C in liquid water and were compared with the conductivities of the pristine

Table 2 Scattering vector and ionic cluster size of the pristine sPES-40 and the sPES–PBI blend membranes calculated from the SAXS data.

Fig. 2. FT-IR spectra of sPES-40 (a), sPES–PBI-3 (b), sPES–PBI-5 (c), and sPES–PBI-7 (d).

Membrane

Scattering vector (A˚1)

Ionic cluster size (A˚)

sPES-40 sPES–PBI-3 sPES–PBI-5 sPES–PBI-7

– 0.0142 0.0212 0.0216

– 442.5 296.4 290.9

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Fig. 4. AFM images obtained from the sPES-40 (a), sPES–PBI-3 (b), sPES–PBI-5 (c), and sPES–PBI-7 (d) membranes in dry states.

polymer sPES-40 and Nafion-117. As shown in Fig. 5 and Table 3, the conductivity decreased with increasing PBI content. Again, this behavior resulted from the interactions between sPES and the PBI polymer chain. As the PBI content increased, the hydrophilic channel size decreased to inhibit the proton conductivity, as confirmed by the AFM and SAXS results. The conductivities of all three blend membranes revealed Arrhenius-type behavior with a high temperature dependence (Fig. 5). In fact, all of the ionically crosslinked sPES–PBI blend membranes exhibited conductivities exceeding 0.13 S/cm at 80 8C, higher than the values measured from typical chemically crosslinked PEMs, even if the PEMs had high IEC values, and these conductivity values were comparable to that of Nafion-117. The apparent activation energies (DEa) of proton conduction, as estimated from the slope of the Arrhenius plot, revealed that the activation energy of sPES–PBI-5 (12.6 kJ/mol) was significantly

higher than that of sPES–PBI-3 (11.5 kJ/mol) but similar to that of sPES–PBI-7 (12.7 kJ/mol, Table 3). This result indicated that a percolation threshold of ion channels through which protons were effectively transported was reached at 5 wt.% PBI crosslinker. Indeed, the difference between the conductivities of sPES–PBI-5 and sPES–PBI-7 was less than the corresponding difference between sPES–PBI-5 and sPES–PBI-3 at each temperature measured (Table 3). These results suggested that only a 5 wt.% PBI crosslinker was sufficient to achieve a maximum enhancement in the properties of the blend membranes without sacrificing the conductivities. Water uptake and swelling For most PEMs, water uptake in the membranes was closely related to the proton conductivity, as the water acted as a transport

Table 3 IEC, conductivity and activation energy values. Membrane

sPES-40 sPES–PBI-3 sPES–PBI-5 sPES–PBI-7 Nafion-117

IEC (meqg1)

Proton conductivity (Scm1)

Ea (kJ/mol)

calcd

expt

20 8C

40 8C

60 8C

80 8C

1.53 1.45 1.28 1.14 –

1.66 1.46 1.34 1.21 0.91

0.13 0.09 0.06 0.05 0.08

0.19 0.14 0.10 0.09 0.10

0.24 0.18 0.13 0.12 0.17

0.28 0.21 0.15 0.13 0.17

10.5 11.5 12.6 12.7 9.21

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ln (σ )

Nafion-117

1000/T, K-1 Fig. 5. Arrhenius plots of the conductivity versus temperature for the sPES-40 (black), sPES–PBI-3 (blue), sPES–PBI-5 (green), sPES–PBI-7 (red), and Nafion-117 (pink) membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

medium for proton ions. Excessive water uptake, however, caused significant swelling, which rendered the membranes too mechanically weak for use as PEMs. We found that, although higher than the water uptake of Nafion-117, ionic crosslinking effectively reduced swelling. The water uptake of the crosslinked sPES–PBI membranes was quite low, even at 80 8C, indicating that the membrane flexibility was retained in water, even at high temperatures, due to the formation of a 3-D network (Table 4). The dimensional stability of the crosslinked sPES–PBI membranes was evaluated in greater detail because large changes in dimensions can complicate the fabrication of fuel cell devices. As expected, all crosslinked sPES–PBI membranes showed much lower swelling ratios (higher dimensional stabilities), even compared to those of Nafion-117, in both the in-plane and through-plane directions (Table 4). The low water uptake and dimensional changes of the ionically crosslinked sPES–PBI blend membranes indicated that these ionic crosslinks effectively suppressed membrane swelling.

Fig. 6. TGA graphs of the sPES-40 (black), sPES–PBI-3 (blue), sPES–PBI-5 (green), and sPES–PBI-7 (red) membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

As expected, the thermal stability increased with increasing crosslinking density. PEMs must also possess mechanical strength sufficient to withstand the conditions used in the fabrication of the MEAs. The mechanical properties of crosslinked sPES–PBI membranes were tested at 50% RH (Fig. 7 and Table 5). The Young’s modulus increased as the crosslinking density increased (1.7 GPa for sPES– PBI-3, 1.9 GPa for sPES–PBI-5, and 2.1 GPa for sPES–PBI-7), indicating that the interactions between the polymer chains were enhanced by crosslinking, leading to a greater mechanical strength. In addition to a good thermomechanical stability, membranes in fuel cells must also be stable to oxidation. Oxidation stability was analyzed here using Fenton’s test at 80 8C. All crosslinked membranes (sPES–PBIs) withstood the solution up to 360 min, and a much lower oxidative stability (180 min) was observed for the non-crosslinked pristine sPES-40 membrane (Table 5). Although Nafion-117 showed much higher stability, this oxidation stability was very high for a polymer electrolyte material based on a nonfluorinated hydrocarbon skeleton. Membranes based on

Thermal and mechanical stability The thermal stability of the ionically crosslinked sPES–PBI blend membranes in their acid form was evaluated using TGA techniques (Fig. 6). The initial slight weight loss in the membranes of less than 9% probably arose from the evaporation of water or residual solvent. All membranes subsequently displayed two-stage weight loss behavior. The first stage, at around 300 8C, most likely arose from the loss of sulfonic acid groups, whereas the second stage at 460 8C was due to the decomposition of the polymer main chains. Table 4 Water uptake and swelling ratio of the pristine sPES-40 and sPES–PBI blend membranes. Membrane

sPES-40 sPES–PBI-3 sPES–PBI-5 sPES–PBI-7 Nafion-117

Water uptake (%)

Swelling ratio (%)

Dt

Dl

20 8C

80 8C

20 8C

80 8C

20 8C

80 8C

47.1 33.0 26.0 20.7 16.1

58.5 42.9 34.3 27.8 18.0

12.8 8.3 3.3 2.7 12.9

37.1 12.1 11.8 11.1 18.9

14.3 5.0 4.8 4.8 9.0

14.3 7.3 7.3 4.8 13.5

Fig. 7. Stress–strain curves of the sPES-40 (black), sPES–PBI-3 (blue), sPES–PBI-5 (green), sPES–PBI-7 (red), and Nafion-117 (pink) membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Won et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 104–111 Table 5 Young’s modulus and dissolution time of sPES-40 and the sPES–PBI blend membranes at 80 8C in Fenton’s reagent. Membrane

Young’s modulus (GPa)

tdissolve (min)

sPES-40 sPES–PBI-3 sPES–PBI-5 sPES–PBI-7 Nafion-117

1.0 1.7 1.9 2.1 0.12

180 360 360 360 >500

polyethers typically dissolve into Fenton’s reagent within several hours under milder conditions at 25 8C [25]. Conclusions We describe the preparation of an ionically crosslinked system using blend membranes based on sPES and p-PBI for use as a novel PEM. The miscibility of the polymer blend originated from the specific interactions between sulfonic acid in the sPES and the basic amine in PBI. The ionic crosslinking introduced in the sPES–PBI blend system enhanced the physical and chemical properties of the corresponding membranes while providing a conductivity exceeding 0.13 S/cm at 80 8C, with only 5 wt.% PBI (relative to sPES). Unlike most PBI blend systems, in which structurally modified PBI derivatives were required due to the poor solubility of p-PBI in organic solvents, our blend system was based on the easily available unmodified p-PBI. Although several studies have reported the use of PBI derivatives as crosslinkers, to our knowledge, this is the first example of a systematic investigation of the influence of the p-PBI content on the chemical and physical properties, as well as proton conductivity, of sPES/p-PBI blend membranes.

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