Article pubs.acs.org/Macromolecules
Durable Sulfonated Poly(benzothiazole-co-benzimidazole) Proton Exchange Membranes Gang Wang,†,‡ Kang Hyuck Lee,† Won Hyo Lee,† Dong Won Shin,† Na Rae Kang,† Doo Hee Cho,† Doo Sung Hwang,† Yongbing Zhuang,† Young Moo Lee,*,† and Michael D. Guiver*,†,§ †
Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea College of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, P. R. China § National Research Council, Ottawa, Ontario K1A 0R6, Canada ‡
S Supporting Information *
ABSTRACT: Two series of random sulfonated poly(benzothiazole-co-benzimidazole) polymers (sPBT-BI) with 70% and 60% degree of sulfonation were evaluated as proton exchange membranes. sPBT was also prepared for a comparative study. The mechanical properties of sPBT-BI were greatly enhanced by incorporation of benzimidazole (BI); sPBT-BI70-10 showed a tensile strength of 125 MPa and elongation at break of 38.9%, an increase of 56.5% and 145%, respectively, compared with sPBT. The solubility, dimensional stability, thermal properties, and oxidative stability of sPBT-BI were also improved. The ionic clusters of sPBT-BI membranes in both AFM phase images and TEM images became narrower with increasing amounts of BI while containing the same molar amount of sulfonic acid groups. This resulted in lower dimensional swelling and higher mechanical strength, but the proton conductivity decreased. However, high proton conductivity was achieved by incorporating an appropriate content of BI. PEMFC H2/air single cell performances and durabilities were improved by incorporation of 5% of BI units in sPBT.
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INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have been intensively studied in recent years for clean energy conversion. A central component of PEMFCs is the proton exchange membrane (PEM).1 The most widely used commercial PEMs are perfluorosulfonic acid membranes such as Nafion, which have high proton conductivity and good durability but also have some limitations such as low operating temperature, high methanol permeability, and high cost.1−3 Consequently, there have been many studies on nonfluorinated aromatic polymers in attempts to improve these properties.1−23 Sulfonated polybenzimidazoles (sPBI) are a class of high performance aromatic polymers which show high thermal and oxidative stability, outstanding dimensional stability, and excellent mechanical properties.13,17−19,27 However, sPBI PEMs have low proton conductivity because of acid−base interactions between the sulfonic acid and benzimidazole.17−19 Sulfonated polybenzothiazoles (sPBT) are a related class of high performance and high temperature polymers, which have high proton conductivity and are regarded as promising matrices for PEMs.24−26 Only a few sPBTs have been reported for PEMs because many sPBT structures could not be obtained with high molecular weight or good solubility.20−22 sPBTs have rigid structures from strong intermolecular interactions, and they exhibit no softening or glass transition before thermal © 2014 American Chemical Society
degradation. Since sPBT membranes can only be fabricated by solution casting,20−23,25 it is important that the polymers have high molecular weight, good solubility, and good mechanical properties for long-term durability in PEMFCs.20−22 To address this, an appropriate amount of benzimidazole (BI) could be incorporated into the backbone of sPBTs.23,28−34 BI units, when protonated by the protons of sulfonic acid groups, reduce intermolecular attraction and disrupt the compact packing of polymer chains to improve the solubility.20−23 Conversely, incorporation of BI could also form hydrogen bonding and ionic cross-linking, which can improve the dimensional stability, mechanical properties, and PEMFC durability for low-temperature PEMFC.20−23,28−34 In the present work, two series of sulfonated poly(benzothiazole-co-benzimidazole)s (sPBT-BI) were synthesized by random polycondensation from 2,5-diamino-1,4-benzenedithiol dihydrochloride and 3,3′-diaminobenzidine with bis(2sulfonate-4-carboxyphenyl) ether and 2,2-bis(4-carboxyphenyl)hexafluoropropane. Sulfonated polybenzothiazole (sPBT-70) was also prepared from 2,5-diamino-1,4-benzenedithiol dihydrochloride with bis(2-sulfonate-4-carboxyphenyl) ether and 2,2Received: July 9, 2014 Revised: August 26, 2014 Published: September 12, 2014 6355
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Ion exchange capacity (IEC) was determined by standard titration methods. The membranes were immersed into saturated NaCl solutions for 48 h to transfer the proton into solution. After removing the membranes, the solutions were titrated to neutral with 0.01 M NaOH, using phenolphthalein as indicator. Thermogravimetric analysis (TGA) was performed using a TGA Q500 (TA Instruments, New Castle, DE) in N2 flow. The acid form polymers were heated at 150 °C for 30 min to remove water and then heated to 800 °C at a heating rate of 20 °C/min. Differential scanning calorimetry (DSC) was measured on a DSC Q20 (TA Instruments, New Castle, DE). The acid form samples, which were preheated at 150 °C for 30 min in N2 flow, were heated from 90 to 350 °C at a heating rate of 10 °C/min. Dynamic mechanical analysis (DMA) was performed by using a DMA Q800 (TA Instruments, New Castle, DE). The membrane samples (area: 0.9 × 4 cm2) (thickness: ∼40 μm) were cut by a punch die and tested in the tensile mode at 1 Hz. The test temperature was in the range of 150−350 °C at a heating rate of 10 °C min−1. The proton conductivity was determined by measuring the resistance value over the frequency range from 100 mHz to 100 kHz using an impedance test analyzer (Model VSP and VMP3B, Biologics, Gainesville, VA) at 30, 50, 70, and 80 °C in water. Conductivity was calculated using the equation
bis(4-carboxyphenyl)hexafluoropropane for comparison. The incorporation of BI into sPBT, which contained the flexible ether and hexafluoroisopropylidene linkages, was expected to improve the solubility, dimensional stability, mechanical properties, proton conductivity, PEMFCs single cell performances, and long-term durabilities. The effect of incorporating BI on the properties of sPBT was explored in detail.
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EXPERIMENTAL SECTION
Materials. 2,5-Diamino-1,4-benzenedithiol dihydrochloride (DABDT) was purchased from TCI (Japan). Silicone oil was purchased from Shin-Etsu. Chemical. Co. Ltd. (Japan). 3,3′-Diaminobenzidine (DAB), 2,2-bis(4-carboxyphenyl)hexafluoropropane (6FA), bis(4carboxyphenyl) ether (BCPE), phosphorus pentoxide, poly(phosphoric acid) (PPA), methanesulfonic acid (MSA), 1-methyl-2-pyrrolidinone (NMP), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), sulfolane, methanol, sulfuric acid (30% free SO3), sodium carbonate (Na2CO3), and all other chemicals were obtained from Sigma-Aldrich. Bis(2-sulfonate-4-carboxyphenyl) ether (SCPE) was synthesized according to previously published work.18,22,35 All the commercial reagents were used without further purification. Synthesis of Sulfonated Poly(benzothiazole-co-benzimidazole) (sPBT-BI). sPBT-BI was synthesized by polycondensation, and the preparation of sPBT-BI70-5 is described as an example. The polycondensation reaction was performed in a 250 mL three-necked round-bottom flask heating by a silicone oil bath and equipped with a mechanical Teflon stirrer and a nitrogen inlet/outlet. A mixture composed of DABDT (1.615 g, 6.586 mmol) and PPA (48 g) was stirred sequentially at 25 °C for 12 h and at 70 °C for 24 h, at which point no further hydrogen chloride was evolved. After cooling for 1 h, DAB (0.0723 g, 0.3467 mmol), SCPE (2.2437 g, 4.8531 mmol), and 6FA (0.8159 g, 2.080 mmol) were added to the reaction mixture, and stirring was continued at 100 °C for 8 h. After cooling to room temperature, phosphorus pentoxide (6.4 g) was added to the mixture, which was then heated sequentially in the following steps: 120 °C for 12 h, 150 °C for 12 h, 170 °C for 12 h, 190 °C for 12 h, and 210 °C for 12 h. The resulting viscous solution was cooled to 140 °C, and MSA was added into the system with stirring for 1 h. The product was then precipitated slowly into deionized water to obtain a dark-green fibrous polymer and washed with deionized water several times to remove residual acid. The product was soaked in 5wt% Na2CO3 solution for 24 h to obtain the sodium sulfonate salt form of the polymer, which was washed until pH ∼ 7, followed by drying in vacuo at 120 °C for 24 h. The structural characterization data of the sodium form polymer are as follows. Yield: 94%. 1H NMR (DMSO-d6): 13.18 (s), 13.21 (s), 8.95 (d), 8.91 (s), 8.60−8.64 (s), 8.31−8.38 (t), 8.16 (d), 7.99−8.03 (d), 7.78−7.81 (t), 7.65−7.70 (m), 7.59−7.64 (d), 7.04−7.10 (d). FT-IR (film, cm−1): 1464, 1404, 1314 (benzothiazole ring stretching), 962 (benzothiazole ring bending), 1090, 1030, 622 (sulfonate).20−23 Proton Exchange Membrane Preparation. Polymer solutions were prepared by dissolving sPBT-70 and sPBT-BIs into DMSO over a 24 h period. After filtration through a 0.1 μm filter, polymer solutions were cast onto clean glass plates and then dried sequentially at 70 °C for 24 h, in vacuo at 100 °C for 1 h, and at 120 °C for 1 h to remove residual solvent. After cooling to room temperature, the resulting membranes in the sodium sulfonate salt form were removed from the glass plates by immersion into deionized water. Membranes in the acid form were prepared by soaking salt form membranes in 1 M HCl for 48 h at room temperature, followed by several washing cycles to remove residual acid.20−23 Measurements. 1H NMR was measured by a Mercury Plus 300 MHz spectrometer (Varian) using dimethyl-d6 sulfoxide (DMSO-d6) as a solvent. FT-IR spectra were obtained from an Infrared Microspectrometer (IlluminatIR, SensIR Technologies, Danbury, CT). Molecular weights, estimated from poly(methyl methacrylate) (PMMA) standards, were measured by gel permeation chromatography (GPC, Waters, Milford, MA) using Styragel columns, a Waters 2414 refractive index detector, and NMP containing 0.05 M LiBr as the eluent.
σ = L /(RWd)
(1)
where R, L, W, and d represent the resistance, length, width, and thickness of the samples, respectively. The proton conductivity of each membrane sample (size: 1 cm × 4 cm) was also characterized at 80 °C under 100% RH, 1.5 atm, at 100 °C under 85% RH, 1.5 atm, and at 120 °C under 35% RH, 1.5 atm in a two-probe type conductivity cell by using an impedance/gain-phase analyzer (Solartron 1260) and an electrochemical interface (Solartron 1287, Farnborough Hampshire, ONR, UK). The temperature and RH were controlled by Fuel cell station (CNL, Seoul, Korea) under nitrogen flow. Furthermore, the conductivity cell was back-pressurized at 1.5 atm to maintain hydrated status. H2/air single cell performances of sPBT-70 series, sPBT-BI, and Nafion 212 membranes were performed on a test station (Won-A-Tech, SMART1, Seoul, Korea). H2 and air were fed on membrane electrode assemblies (MEAs) which were fabricated by a catalyst-coated substrate (CCS) method at a flow rate of 100 mL min−1 at 80 °C under 100% RH and at 100 °C under 85% RH, respectively. The operation conditions were back-pressurized at 1.5 atm, in the same way as conductivity measurements. The PEMFCs durability test was conducted by measuring the voltage drop during a constant current load density (0.2 A cm−2) at 100 °C and 85% RH, 1.5 atm to investigate the effect of incorporation of BI on the PEMFCs membrane stability during fuel cell operation.45 The water uptake, in-plane dimensional swelling and through-plane dimensional swelling of membranes were measured by the change ratio of weight, length, and thickness from the wet to dry state. The dry samples were made by drying in vacuo at 100 °C for 24 h. The basic weights (Wd) of dried membranes were measured by an electronic balance (Sartorius, 0.1 mg), the basic average thicknesses (td) of four fixed places on dried membranes were measured by a thickness gauge (Mitutoyo, 0.001 mm), and the basic lengths (ld) of dried membranes were calculated from the area result tested by ICAMSCOPE. The weight (Ww), length (lw), and thickness (tw) of the wet membranes were measured after immersion in water for 24 h by the same way. The water uptake and dimensional swelling were calculated by the following equations:
water uptake = (Ww − Wd)/Wd × 100%
(2)
in‐plane dimensional swelling = (l w − ld)/ld × 100%
(3)
through‐plane dimensional swelling = (tw − td)/td × 100%
(4)
Water uptake and dimensional swelling were measured three times for each sample. The average data were taken, and the standard deviation from the mean was below 7%. 6356
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Scheme 1. Synthesis of sPBT-BIs Random Polymers Having Four Types of Repeat Units and Photographs of the SPBT-BI70-5 Membranes
Scheme 2. Synthesis of sPBT-70
Table 1. Monomer Feed Ratios, Molecular Weight, Mechanical Properties, and Yield of sPBT-70, sPBT-BIs, and Nafion 212 DABDT/DAB/SCPE/ 6FA
molecular weight
mechanical properties
ionomers
molar ratio
Mn (kg mol−1)
Mw (kg mol−1)
PDI
sPBT70 sPBT-BI70-5 sPBT-BI70-10 sPBT-BI70-20 sPBT-BI60-5 sPBT-BI60-10 sPBT-BI60-20 Nafion 212
100/0/70/30 95/5/70/30 90/10/70/30 80/20/70/30 95/5/60/40 90/10/60/40 80/20/60/40
52 61 58 82 51 54 54
167 198 190 222 161 178 176
3.23 3.28 3.30 2.71 3.14 3.29 3.29
tensile strength (MPa)
Young’s modulus (GPa)
elongation at break (%)
yield (%)
79.9 ± 9.0 95.1 ± 5.0 125 ± 5.0 111 ± 5.0 105 ± 6.6 72.5 ± 4.5 123 ± 6.0 19.8 ± 0.3
1.84 ± 0.04 1.67 ± 0.12 1.96 ± 0.04 1.93 ± 0.03 1.72 ± 0.11 1.29 ± 0.05 2.02 ± 0.05 0.105 ± 0.001
15.9 ± 4.1 33.9 ± 2.7 38.9 ± 1.8 39.0 ± 5.2 29.5 ± 3.4 35.5 ± 6.0 36.6 ± 3.2 255.9 ± 6.4
96 95 96 95 95 96 96
surface morphology. The membrane morphology was obtained in the tapping mode at ambient temperature and humidity conditions. The sodium salt form polymer solution in DMSO was dissolved, filtered, and cast on a dust-free glass plate. The membrane sample was obtained by heating at 70 °C for 24 h and then at 120 °C for 12 h in a vacuum to remove residual solvent. Transmission electron microscopy (TEM) was also used to investigate the morphology of membranes by a Carl Zeiss LIBRA 120 energy-filtering transmission electron microscope operating at an accelerating voltage of 120 kV. The samples were prepared by immersing the membranes in 0.5 M lead(II) acetate aqueous solution to obtain stained samples with lead ion (Pb2+) and rinsed several times with deionized water. After drying them in a vacuum, the stained samples were sectioned to 70 nm slices with a RMCMTX Ultra microtome and placed on copper grids.
The oxidative stability of membranes was measured following a typical procedure.36 Dried membranes (∼0.02 g) were soaked in 25 mL of Fenton’s reagent (3% H2O2 containing 2 ppm FeSO4) at 80 °C. The residual weight after 1 h and the elapsed time when the membrane started to dissolve (τ1) and dissolve completely (τ2) were recorded. The mechanical properties of dried membranes were measured at a crosshead speed of 1 mm min−1 using a universal testing machine (Shimazu, AGS-500NJ, Tokyo, Japan) following ASTM (ISO37-4). Five specimens of each sample were tested. Atomic force microscopy (AFM) was used to measure the microscopic morphology of membranes, which was conducted using a Digital Instruments Multimode 8 (Veeco, Plainview, NY) with a diNanoScope V controller (Veeco) to investigate phase-separation behavior of the membrane samples. A silicon probe (Nanosensors, Switzerland) with a force constant of 1.2−29 N m−1 was used to scan 6357
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RESULTS AND DISCUSSION Synthesis of Sulfonated Poly(benzothiazole-cobenzimidazole)s (sPBT-BI). The system being studied is a random polymer composed of four types of repeat units: sulfonated benzothiazole, nonsulfonated benzothiazole, sulfonated benzimidazole, and nonsulfonated benzimidazole. The degree of sulfonation was set by the monomer ratio of SCPE to 6FA, while the benzimidazole content was set by the monomer ratio of DAB to DABDT. As shown in Scheme 1, two series of random sPBT-BI polymers were synthesized by polycondensation of DABDT and DAB with SCPE and 6FA. The first series containing 70mol% sulfonated monomer SCPE, denoted as sPBT-BI70-xx, where “70” refers to the molar fraction of monomer units SCPE and “xx” refers to the molar fraction of BI in the feed. The second series containing 60% sulfonated monmers (SCPE) was denoted as sPBT-BI60-xx. Sulfonated polybenzothiazole was also prepared for a comparative study by polycondensation of DABDT with SCPE and 6FA, which contained 70% molar fraction of sulfonated monomer SCPE and was denoted as sPBT-70, as shown in Scheme 2. All the sPBT-70 and sPBT-BI polymers were obtained as green fiber-like solids. The polymer molecular weights were evaluated by gel permeation chromatography (GPC). As shown in Table 1, high molecular weights were achieved, with weight-average molecular weights in the range of (161−222) × 103 g mol−1 and polydispersity indices (PDI) in the range of 2.71−3.30. The membranes were also tough and flexible, as shown by the mechanical properties listed in Table 1. The 1H NMR spectrum of sPBT-BI70-20 was used as an illustrative example for structural analysis of the polymer series. Figure 1 shows the spectral signals of sPBT-BI70-20, which were
The structures of sPBT-70 and sPBT-BIs were further confirmed by FT-IR spectroscopy. Figure 2 shows absorption
Figure 2. FT-IR spectra of sPBT-70 and sPBT-BI membranes.
bands at 1462, 1404, 1314, and 962 cm−1, which are assigned to the characteristic vibration of benzothiazole rings; the intensity of these absorption bands increased with increasing amounts of BI in the copolymer. The characteristic bands related to −SO3Na are located at 1082 and 1021 cm−1.20−22 The TGA curves of sPBT-70, sPBT-BI, and Nafion 212 membranes in N2 are shown in Figure 3a,b, and the 5% weight-
Figure 1. 1H NMR spectra of sPBT-BI70-20 in the sodium salt form.
assigned based on previously published work.22 The hydrogen atoms H10 and H10′ are the active N−H protons of the BI rings in the sulfonated and nonsulfonated repeat units, which exhibit slightly different chemical shifts of 13.18 and 13.21 ppm, respectively. This can be used to confirm that BI units were successfully incorporated into the sPBT polymer chain.22 Furthermore, the hydrogen atoms H5, H5′, H6, H6′, and H4′ in hexafluoroisopropylidene-containing repeat units appear at 8.31−8.38, 7.65−7.70, and 8.95 ppm, respectively, which is close to the chemical shifts of sPBT with a similar chemical environment published previously.20−23
Figure 3. TGA curves of (a) sPBT-70, sPBT-BI70s and (b) sPBT-BI60s, Nafion 212. 6358
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Table 2. Tg, Td5, IEC, and Oxidative Stability of sPBT-70, sPBT-BIs, and Nafion 212 Membranes IEC (mequiv g−1) ionomers
Tg (°C)
Td5 (°C)
calculated
measured
thickness (μm)
residuea (%)
τ1b (h)
τ2b (h)
sPBT-70 sPBT-BI70-5 sPBT-BI70-10 sPBT-BI70-20 sPBT-BI60-5 sPBT-BI60-10 sPBT-BI60-20 Nafion 212
235 248 250 254 243 247 253 115
372 377 389 391 378 395 401 377
2.74 2.73 2.71 2.68 2.35 2.34 2.32 0.92
2.65 2.64 2.62 2.59 2.28 2.26 2.23 0.96
53 59 48 54 60 48 43 55
95 96 97 98 99 99 99 −c
2.0 3.0 3.2 4.0 3.0 3.2 4.2 −c
3.0 4.0 4.2 7.0 4.5 4.8 7.0 −c
Residual weight of membranes after treatment in Fenton’s reagent for 1 h. bτ1 and τ2 refers to the time when the membrane began to dissolved and dissolved completely at 80 °C, respectively. cNafion 212 did not dissolve in Fenton’s reagent.
a
Table 3. Solubility of sPBT-70 and sPBT-BI Ionomersa
loss temperatures (Td5) are also summarized in Table 2. Both sPBT-70 and sPBT-BI series present a two-step degradation process; the first weight loss of sPBT-70 and sPBT-BI70s started at about 320 °C, and the first weight loss of sPBT-BI60s started at about 345 °C, which is attributed to the degradation of sulfonic acid groups. The second weight loss of all polymers started at about 450 °C, attributed to the degradation of polymer backbone.37−42 The first and second weight-loss temperatures of both series of polymers increased with increasing amounts of BI groups incorporated into the copolymer, indicating that the BI units were beneficial in improving thermal stability. This was possibly caused by the increased acid−base interactions between the basic BI and sulfonic acid groups, as reported previously.23 The thermal properties of sPBT-70 and sPBT-BIs membranes were also investigated by DSC and DMA. No glass transition or exothermal/endothermal peaks were detected in the temperature range of 90−350 °C, which was probably the result of high chain rigidity of the rigid-rod structure and intermolecular interactions hindering the movement of molecular segments, as observed in other aromatic sulfonated polymers.17−23 Glass transition temperatures (Tg)s of sPBT-70 and sPBT-BI membranes were measured using DMA (Figure S1), and the results are compared with Nafion 212 and listed in Table 2. sPBT-70 and sPBT-BI membranes showed Tgs in the range of 235−254 °C, which was much higher than that of Nafion 212 (115 °C). The sPBT-70 series with the higher degree of sulfonation showed higher Tgs compared with the sPBT-60 series, similar to the behavior of other reported sPBI and sPBT.22,47 Tg increased as BI units were incorporated into the backbone of sPBT. For example, sPBT-70 had a Tg of 235 °C, while that of sPBT-BI70-20 was 254 °C. The Tg of sPBT-BI60-20 also increased from 243 to 253 °C compared with sPBT-BI60-5. Since these sPBT-70 and sPBT-BI polymers exhibited neither glass transitions nor melting behavior prior to thermal degradation, homogeneous membranes could not be fabricated by extrusion or blow molding. Thus, solubility is very important for membrane fabrication.20−23 The solubility data for sPBT-70 and sPBT-BI ionomers in the salt form at a concentration of 0.01 g/mL are listed in Table 3. sPBT-70, sPBT-BI70-5, and sPBTBI60-5 polymers were soluble in DMSO, NMP, DMAc, and DMF on heating, sPBT-BI70-10, -70-20, -60-10, and -60-20 were soluble in DMSO and NMP at room temperature, and sPBTBI70-20 and -60-20 were soluble in DMAc at room temperature. This illustrates the solubility enhancement effect of incorporating BI into the copolymers. All the sPBT-BI copolymers exhibited good solubility in organic solvents, which ensured that they could be used to fabricate PEMs by solution casting.
ionomers
DMSO
DMAc
NMP
DMF
sulfolane
methanol
sPBT-70 sPBT-BI70-5 sPBT-BI70-10 sPBT-BI70-20 sPBT-BI60-5 sPBT-BI60-10 sPBT-BI60-20
+ + ++ ++ + ++ ++
+ + + ++ + + ++
+ + ++ ++ + ++ ++
+ + + + + + +
± ± ± ± ± ± ±
− − − − − − −
a
Symbols: (++) soluble at room temperature, (+) soluble on heating, (±) partially soluble or swelling on heating, (−) insoluble even on heating.
In order to obtain high proton conductivity while controlling dimensional swelling, two ranges of IEC values were investigated. The IEC of sPBT-70 and sPBT-BI70 series was in the range of 2.68−2.74 mequiv g−1, and for the sPBT-BI60 series, it was the range of 2.32−2.35 mequiv g−1. The theoretical and experimental IEC of sPBT-70, sPBT-BI, and Nafion 212 membranes were obtained by calculation and by the titration method, respectively, and are listed in Table 2.19−23 A reasonable correlation between the experimental and theoretical values is observed, indicating that the sulfonated monomer was completely incorporated into the polymer backbone. Among the key properties of proton exchange membranes, they should have high proton conductivity and good mechanical properties to allow proton transport, act as a catalyst support, and isolate the oxygen and hydrogen gas in fuel cells.20−23 Highly conductive aromatic PEMs often exhibit excessive swelling, which compromises mechanical properties.2 The water uptake, in-plane dimensional swelling, and through-plane dimensional swelling of PEMs have a great influence on mechanical properties, proton conductivity, PEMFC single cell performances, and durability results. Therefore, it is important for PEM to have appropriate water uptake and good dimensional stability. The wt% water uptake results are shown in Figure 4. At a given temperature, the water uptake of the sPBT-BI70 series was higher than that of the sPBT-BI60 series because they have a higher degree of sulfonation. The water uptake of sPBT-70 was 55.3 wt % at 80 °C, which was higher than that (32.5%) of Nafion 212. The water uptake of sPBT-BI membranes decreased with an increase in the content of BI groups. For example, the water uptake of sPBT-BI70-20 membrane incorporating 20% BI decreased about 15 wt% compared with the sPBT-70 membrane. Normally, lower water absorption results in membranes having greater dimensional stability, which is illustrated by in-plane dimensional swelling and through-plane dimensional swelling in 6359
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Nafion 212. Similar to water uptake, the in-plane and throughplane dimensional swelling of sPBT-BI also decreased with increasing BI group content. For instance, in-plane and throughplane dimensional swelling of sPBT-70 was 17.9% and 18.9% at 80 °C, which decreased to 15.1% and 14.3%, respectively, by incorporating 20% BI. Similarly, in-plane dimensional swelling and through-plane dimensional swelling of sPBT-BI60-20 membrane was 13.6% and 7.1% at 80 °C, which decreased to 11.3% and 5.9%, respectively, compared with a sPBT-6F60 membrane reported previously having a similar IEC.22 This illustrates that the sPBT-BI membranes had improved dimensional stability by incorporation of BI into the polymer. The temperature-dependent proton conductivities of sPBT70 and sPBT-BI membranes were evaluated under two different conditions. Conductivities in the lower temperature range of 30−80 °C were measured in water (Figure 6a), while those in the
Figure 4. Temperature dependence of water uptake of sPBT-70, sPBTBIs, and Nafion 212 membranes.
Figure 5. Therefore, the incorporation of BI allows the water uptake of the membranes to be adjusted, if the trade-off with proton conductivity is not too great. The temperature dependence of in-plane dimensional swelling and through-plane dimensional swelling for sPBT-70, sPBT-BI membranes, and Nafion 212 are shown in Figure 5. The in-plane swelling of sPBT-70 and sPBT-BI membranes increased much more slowly with temperature compared with Nafion 212. They also showed a lower through-plane swelling value than that of
Figure 6. Proton conductivity of sPBT-70, sPBT-BI series, and Nafion 212 membranes: (a) in water at various temperatures; (b) 80 °C, 100% RH, 1.5 atm; 100 °C, 85% RH, 1.5 atm; and 120 °C, 35% RH, 1.5 atm.
higher temperature range of 80−120 °C were measured in a controlled relative humidity environment of 100−35% (Figure 6b), corresponding to the dehydrating effects of elevated temperature. As expected, the proton conductivities increased with increasing degree of sulfonation and temperature in the hydrated lower temperature range, while they decreased with decreasing RH in the higher temperature range, similar to the behavior of Nafion 212. Furthermore, the proton conductivities decreased with increasing BI content, as observed in earlier
Figure 5. Temperature dependence of (a) in-plane dimensional and (b) through-plane dimensional swelling of sPBT-70, sPBT-BIs, and Nafion 212 membranes. 6360
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Figure 7. AFM phase images and TEM images of Nafion 212:5 (a) sPBT70, (b) sPBT-BI70-5, (c) sPBT-BI70-10, (d) sPBT-BI70-20, (e) sPBT-BI60-5, (f) sPBT-BI60-10, and (g) sPBT-BI60-20.
work;23 this was probably the result of acid−base interactions between sulfonic acid and BI units that bound some of the protons.23 However, sufficiently high proton conductivities could be obtained by incorporating modest amounts of BI units into the copolymer. For instance, copolymer sPBT-BI70-5 containing 5% BI units had proton conductivities of 197 mS cm−1 at 80 °C measured in water, 178 mS cm−1 at 80 °C under 100% RH, 68 mS cm−1 at 100 °C under 85% RH, and 15 mS cm−1 at 120 °C under 35% RH. On the other hand, for the copolymers containing greater amounts of BI units, the conductivities of sPBT-BI70-20, -60-10, and -60-20 at 120 °C under 35% RH (Figure 6b) were below the measurement threshold of our equipment. Therefore, 5% of BI incorporated into sPBT was judged to be an appropriate amount to achieve high proton conductivity. The microscopic morphology of the membranes is one important aspect to study, since it influences the macroscopic properties.2,46 Both AFM and TEM were used to visualize the microscopic morphologies of sPBT-70 and sPBT-BI membranes and compare them with that of Nafion 212. All of the membranes presented a bright/dark nanophase-separated morphology in the AFM phase images and TEM images, with the dark regions representing the hydrophilic domains, which absorbed water and facilitated the transport of water and protons, and the bright regions corresponding to the hydrophobic domains, which
provided mechanical strength and dimensional stability in the membranes.2 The AFM images presented in Figure 7 show that the connectivity of the ionic clusters in the sPBT-BI70 series, which has a higher degree of sulfonation, was greater than that of sPBT-BI60 series. The diameters of the ionic clusters in the sPBT-BI series were much narrower than that of Nafion 212, with the clusters becoming narrower with the increase in BI content. For instance, the diameter of ionic clusters in sPBTBI70-20 was about 8 nm, which was much smaller than that of sPBT70 (18 nm), which did not contain BI units. Because narrower ionic clusters could be separated more distinctly by hydrophobic domains compared with the wider clusters when hydrated, the outcome could be improved dimensional stability and higher mechanical strength of sPBT-BI membranes across similar IEC values.21,22 The TEM images of ionic clusters in sPBT-70 and sPBT-BI membranes shown in Figure 7 also showed a parallel trend with AFM phase images, with the sPBT-BI70 series having apparently more ionic cluster connectivity compared with the sPBT-BI60 series. Similar to the observations from AFM imagery, the TEM images also suggested a narrowing of the diameter of ionic clusters with increasing content of BI. The ionic cluster sizes of sPBT-BI membranes in the TEM images were much smaller than that of Nafion 212.5 For example, the ionic cluster size of sPBTBI70-20 was about 3.4 nm in TEM images, while that of Nafion 6361
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2125 was about 13 nm. The smaller ionic cluster could improve the dimensional stability and mechanical strength of PEMs, as reported for other aromatic ionomers.6,38,39 It is important for PEMs to have good oxidative stability for practical fuel cell applications. Accelerated oxidative stability tests of sPBT-70, sPBT-BI, and Nafion 212 membranes were evaluated using Fenton’s reagent, as shown in Table 2. Residual weights of sPBT-BI membranes were all above 96% after 1 h exposure, indicating notable improvement by incorporation of BI groups. Furthermore, τ1 and τ2 of sPBT-BI70-20 were 4.0 and 7.0 h, respectively, which was more than double that of sPBT-70. The sPBT-BI membranes exhibited higher oxidative stability compared with many aromatic sulfonated membranes treated under the same conditions.20−23,43,44 Proton exchange membranes require good mechanical properties because they support the catalyst and isolate the oxygen and hydrogen gas in fuel cells.16−23 In addition, good mechanical properties are crucial for withstanding the stresses arising from operating cycles over the long term. The membranes of sPBT-70, sPBT-BI, and Nafion 212 were dried at 120 °C for 12 h in vacuum before testing and the mechanical properties are compiled in Table 1. The sPBT-70 membrane exhibited a tensile strength of 79.9 MPa, a Young’s modulus of 1.84 GPa, and an elongation at break of 15.9%. Incorporating BI groups in the sPBT-BI membranes greatly improved the mechanical properties. The tensile strength, Young’s modulus, and elongation at break of sPBT-BI70-10 membranes were 125 MPa, 1.96 GPa, and 38.9%, respectively. These values were 56.5%, 6.5%, and 145% higher, respectively, compared with those of sPBT-70. For the lower degree of sulfonation, the sPBT-BI60 series exhibited a tensile strength in the range of 72.5−123 MPa, a Young’s modulus of 1.29−2.02 GPa, and an elongation at break of 29.5− 36.6%, which were significantly improved compared with previous work.22 All the sPBT-70 and sPBT-BI membranes showed much higher tensile strength, Young’s modulus, and lower elongation at break than those of Nafion 212. The PEMFC single cell performances of sPBT and sPBT-BI membranes were tested at two different conditions: at 80 °C under fully humidified conditions and at 100 °C under 85% RH conditions to determine the effect on fuel cell performance of the amount of BI groups incorporated into sPBT, as shown in Figure 8. Single cell performances of Nafion 212 membrane were also tested as a benchmark for sPBT and sPBT-BI membranes at the same condition and shown in Figure S2 of the Supporting Information. The single cell performances were improved by incorporation of 5% of BI groups. The maximum power densities of sPBT-BI70-5 membrane were 273 mW cm−2 at 80 °C, under 100% RH, and 279 mW cm−2 at 100 °C, under 85% RH, which were higher than those of sPBT-70 membrane. It is probable that because the connectivity of the ionic clusters of the sPBT-BI70-5 membrane in the microscopic morphology is narrower than that of sPBT-70 membrane, the resulting improved dimensional stability and mechanical properties were responsible for improving the cell performances. However, the maximum power densities of sPBT-BI70-10 membrane containing 10% BI groups were lower than that of the sPBT-70 membrane, which is probably due to the increased acid−base interactions between sulfonic acid and BI units reducing available protons for transport.17,19,23 Nafion 212 membrane showed current density of 560 mA cm−2, and that of sPBT-BI70-5 membranes was 450 mA cm−2 measured at 0.6 V at 80 °C under fully humidified conditions. However, the current density of sPBT-BI70-5 (460 mA cm−2) at 100 °C and 85% RH has been maintained even at
Figure 8. H2/air single cell performances of sPBT-70 and sPBT-BI membranes: (a) 80 °C, 100% RH, 1.5 atm; (b) 100 °C, 85% RH, 1.5 atm.
elevated temperature, whereas that of Nafion 212 membrane measured at 0.6 V at 100 °C under 85% RH conditions has been reduced to 506 mA cm−2 compared with the current density of Nafion 212 at 80 °C under 100% RH (560 mA cm−2). A durability test was conducted by measuring the voltage drop during a constant current load density (0.2 A cm−2) at 100 °C and 85% RH, 1.5 atm to compare with the voltage drop between sPBT-70 and -70-5 membranes under the same conditions,45 as shown in Figure 9. The voltage drop of sPBT-70 and -BI70-5 membranes were about 0.2 and 0.1 V, respectively, in the first 100 h. A rapid voltage drop for the sPBT-70 membrane occurred after ∼350 h, while that for the sPBT-BI70-5 was over 500 h, which was about 1.5 times longer than that of sPBT-70 membrane. These comparative results for the sPBT-70 and -70-5 membranes indicated an improvement in the membrane durability by incorporating 5% of BI groups.
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CONCLUSIONS Two series of random sulfonated poly(benzothiazole-cobenzimidazole) were synthesized by polycondensation. Sulfonated polybenzothiazole was also prepared for a comparative study. All of the sPBT-BI copolymers exhibited good solubility in DMSO and NMP due to the flexible linkages and incorporation of BI units. sPBT-BI membranes showed a significant enhancement in mechanical properties compared with the sPBT membranes. The dimensional stability, thermal properties, and 6362
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support of the National Natural Science Foundation of China (51403054).
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Figure 9. Single cell durabilities of sPBT-70 and -BI70-5 membranes at 100 °C, 85% RH, 1.5 atm during constant load (0.2 A cm−2) measurements.
oxidative stability in sPBT-BI polymers were also improved by incorporation of BI. All of the sPBT-BI membranes exhibited distinct bright/dark nanophase-separated morphology in the AFM phase images. The connectivity of the ionic clusters in the higher IEC sPBT-BI70 series in both the AFM phase images and TEM images was greater than that of sPBT-BI60 series, while the ionic clusters became narrower with increasing amounts of BI, for a similar degree of sulfonation. For example, the ionic cluster diameter of sPBT-BI70-20 was only about half that of sPBT70, which may be responsible for the observed higher dimensional stability and mechanical strength of the sPBT-BI membranes. The proton conductivity of sPBT-BI membranes showed a decreasing trend with increasing BI, which was believed to be the result of acid−base interactions between sulfonic acid and BI reducing the available protons for transport. However, a 5% content of BI did not compromise the proton conductivity of sPBT-BI membranes to a significant extent, since the sPBT-BI membranes had comparable conductivity to sPBT and modestly lower than Nafion 212. The H2/air single cell performances and long-term durabilities were improved by incorporation of 5% BI units into the sPBT backbone. Therefore, the incorporation of minor amounts of BI into sPBT has an overall beneficial effect on PEM properties and fuel cell performance.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*Tel +82-2-2220-0525; fax +82-2-2291-5982; e-mail ymlee@ hanyang.ac.kr (Y.M.L.). *Tel +1-613-993-9753; fax +1-613-991-2384; e-mail michael.
[email protected] (M.D.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3A7B4049745). G.W. appreciates the 6363
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