Effect of Alkanediol Additives on the Properties of Polyphenylene

May 22, 2019 - Novel sulfonated and carboxylated polyphenylene (SPP-QP-BA) containing additives were synthesized as proton exchange membranes...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 9915−9920

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Effect of Alkanediol Additives on the Properties of PolyphenyleneBased Proton Exchange Membranes Zhi Long,† Yaojian Zhang,‡ Junpei Miyake,‡ and Kenji Miyatake*,‡,§ †

Interdisciplinary Graduate School of Medicine and Engineering, ‡Clean Energy Research Center, and §Fuel Cell Nanomaterials Center, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan

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S Supporting Information *

ABSTRACT: Novel sulfonated and carboxylated polyphenylene (SPP-QP-BA) containing additives were synthesized as proton exchange membranes. As the additives, linear and flexible alkanediols with various chain lengths (Cn: n = 4 for butyl, n = 8 for octyl, and n = 12 for dodecyl) were investigated. SPP-QP-BA(Cn) composite membranes possessed nanoscale phase-separated morphologies. As the alkyl chain length increased, the ionic domain size, water uptake, and proton conductivity slightly increased. Although the effect of the chain length on these properties was rather minor, the elongation property was affected much by the alkyl chain length and significantly increased with decreasing the alkyl chain length. The SPP-QP-BA(C4) membrane having the shortest alkyl chain length displayed the best mechanical property (yield stress: 43 MPa; maximum strain: 99%), which was much higher than that (yield stress: 39 MPa; maximum strain: 68%) of our previous SPP-QP. In addition, SPP-QP-BA(Cn) membranes showed excellent chemical stability similar to the SPP-QP.

1. INTRODUCTION

showed high elongation among the polyphenylene-based PEMs reported thus far. Additives have widely been used to modify properties of PEMs. For example, Kim et al. reported that heteropolyacid (H3PW12O40)/sulfonated poly(arylene ether sulfone) composite membranes showed improved mechanical strength, higher proton conductivity, and lower water uptake than the unfilled membranes.15 Li et al. reported that benzimidazole grafted PEEK/SPEEK composite membranes exhibited improved mechanical properties, oxidative stability, reduced water uptake, and methanol permeability compared with the pristine SPEEK membrane.16 Okamoto et al. reported that polyester diol/poly(lactic acid) blends exhibited higher elongation at break than the poly(lactic acid).17 These results suggested that aliphatic-based additives might improve flexibility of our SPPQP membranes.14 Herein, we present a series of composite PEMs (SPP-QPBA(Cn), Figure 1), sulfonated polyphenylenes modified with alkanediols, to improve the mechanical robustness of the membranes. As the additive, linear alkanediols with various chain lengths (Cn: n = 4 for butyl, n = 8 for octyl, and n = 12 for dodecyl) were investigated. To reinforce interaction between the diols and the polymer, we introduced −COOH groups onto the polymer. The effect of the alkyl chain length of the additive segment on the membrane properties such as morphology, water uptake, proton conductivity, mechanical properties, and chemical stability was investigated.

As a next-generation clean energy system, proton exchange membrane (PEM) fuel cells have received much attention because of high energy efficiency, fast start-up, and low pollution emission.1−3 PEM is an important component of the fuel cells as solid electrolyte and separator between cathode and anode.4,5 Perfluorosulfonic acid (PFSA) membranes (for example, Nafion membrane from Dupont) are state-of-the-art because of the excellent membrane properties such as high proton conductivity as well as mechanical and chemical stabilities and act as a benchmark to be compared with newly designed membranes with novel structures. PFSA membranes have some drawbacks such as high cost, complicated synthetic process, high gas permeability, low thermal stability, and poor durability in open-circuit voltage (OCV) conditions, which may impede widespread dissemination of fuel cells.6,7 Fluorine-free aromatic ionomer membranes have been considered as a candidate of nextgeneration PEM to overcome these issues.8,9 We have made many efforts in designing fluorine-free aromatic ionomer membranes.10−12 Among them, sulfonated polyphenylene ionomer seems to be one of the most promising candidates because the main chain is composed of chemically stable phenylene groups without vulnerable heteroatom linkages, such as ether bonds.9,13,14 Recently, we have reported the design and preparation of a sulfonated polyphenylene membrane (SPP-QP), which showed high proton conductivity, low gas permeability, and extremely high chemical stability.14 However, the mechanical properties (Young’s modulus = 1.3 GPa; strain at the break point = 68%; tensile strength = 34 MPa) still need to be improved although the membrane © 2019 American Chemical Society

Received: Revised: Accepted: Published: 9915

March 21, 2019 May 15, 2019 May 22, 2019 May 22, 2019 DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920

Article

Industrial & Engineering Chemistry Research

Figure 1. Synthesis of SPP-QP-BA precursor and SPP-QP-BA(Cn) composite membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethyl sulfoxide (DMSO) (KANTO Chemical), toluene (dehydrated) (KANTO Chemical), hydrochloric acid (KANTO Chemical), potassium carbonate (K2CO3) (KANTO Chemical), 2,2′-bipyridyl (TCI), bis(1,5cyclooctadiene)nickel(0) (Ni(cod)2) (KANTO Chemical), lead(II) acetate (Pb(OAc)2) trihydrate (KANTO Chemical), sodium chloride (NaCl) (KANTO Chemical), 2,5-dichlorobenzenesulfonic acid dihydrate (SP) (TCI), 2,5-dichlorobenzoic acid (BA) (TCI), 1,4-butanediol (TCI), 1,8-octanediol (TCI), and 1,12-dodecanediol (TCI) were purchased and used as received. Quinquephenylene (QP) monomer was prepared according to our previous report.14 2.2. Synthesis of Precursor SPP-QP-BA. A 100 mL flask equipped with a Dean−Stark trap, a stirring bar, and an N2 purge was charged with SP monomer (0.789 g, 3.00 mmol), QP monomer (0.650 g, 1.44 mmol), BA monomer (0.057 g, 0.30 mmol), K2CO3 (0.547 g, 3.96 mmol), 2,2′-bipyridyl (1.777 g, 11.38 mmol), DMSO (12.5 mL), and toluene (12.5 mL). After dehydration at 170 °C for 2 h, the mixture was cooled to 80 °C, and then Ni(cod)2 (2.925 g, 11.38 mmol) was add to the mixture. After stirring for another 3 h at 80 °C, the mixture was cooled to room temperature and diluted with DMSO. The mixture was poured into 6 M HCl to precipitate out the product, which was then washed with 6 M HCl and water several times to provide SPP-QP-BA quantitatively. 2.3. Synthesis of Composite Membrane SPP-QPBA(Cn). In all cases, the same precursor polymer (SPP-QPBA, titrated ion exchange capacity (IEC) = 2.4 mequiv g−1) was used, and the amount of the additives (alkanediols) was set to be 5 wt % of SPP-QP-BA. First, the alkanediol was dissolved in DMSO to obtain the solution of 3 mg mL−1. Two milliliters of the alkanediol solution was mixed with 120 mg of SPP-QP-BA, and additional DMSO was added to dilute the solution to be 3−5 wt %. The solution was casted onto the glass plate of 4 × 4 cm2 and dried at 80 °C for 12 h to obtain a membrane, followed by thermal treatment at 100 °C for 1 h and at 120 °C for 2 h in a vacuum. The prepared composite membrane was washed with 2 M HCl and water several times.

3. RESULTS AND DISCUSSION 3.1. Preparation of Precursor Terpolymer (SPP-QPBA). In consideration of the interaction between the polymer and additives, BA monomer containing carboxylic acid group (−COOH) was used as the third component in addition to the SP and QP monomers (Figure 1). For the chemical composition of the precursor terpolymer (SPP-QP-BA), we set the feed molar ratio of SP (−SO3H) to BA (−COOH) as 1 to 0.1 so as not to deteriorate the membrane properties (e.g., proton conductivity). In addition, the m-/p-phenylene ratio of the SPP-QP-BA backbone was set to be the same as that in our previous SPP-QP membrane (Table S1). Overall, the novel SPP-QP-BA and our previous SPP-QP were designed to have similar molecular structures, except for the presence or absence of the −COOH groups. The synthesis of the precursor terpolymer (SPP-QP-BA) was performed under similar polymerization conditions to those of our previous SPP-QP via Ni-mediated coupling reaction under alkaline conditions. The obtained SPP-QP-BA was soluble in polar aprotic solvents such as dimethyl sulfoxide (DMSO) but not in water. The chemical structure of SPP-QPBA was analyzed by the 1H NMR spectrum, in which all of the peaks were well-assigned (Figure 2a), and a few differences were found at 7.83 ppm assignable to benzoic acid groups compared with SPP-QP. The FT-IR spectrum further

Figure 2. (a) 1H NMR spectra in DMSO-d6 at 80 °C. (b) FT-IR spectra of SPP-QP-BA precursor and our previous SPP-QP. 9916

DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920

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Industrial & Engineering Chemistry Research

Figure 3. 1H NMR spectra of (a) SPP-QP-BA(C4), (b) SPP-QP-BA(C8), and (c) SPP-QP-BA(C12) in DMSO-d6 at 80 °C (soluble part). FT-IR spectra of (d) SPP-QP-BA(C4), (e) SPP-QP-BA(C8), and (f) SPP-QP-BA(C12).

confirmed the chemical structure of SPP-QP-BA (Figure 2b). The absorption bands at 1597, 1577, 1520, and 1463 cm−1 were assigned to stretching vibrations of C−C bonds in phenylene groups. The stretching vibration of OSO bonds in the sulfonic acid groups appeared at 1016 and 1078 cm−1. In addition, characteristic peaks assigned to −COOH groups were confirmed. The absorption bands at 1705 cm−1 was assigned to stretching vibrations of CO, and a slightly broad peak at 1402 cm−1 was assigned to stretching vibrations of O−H in −COOH groups. Although the molecular weight of SPP-QP-BA could not be measured by our gel permeation chromatography (GPC) because of the strong interaction between −COOH groups and our GPC column, SPP-QP-BA had sufficient membrane-forming capability to provide a transparent, tough, and flexible membrane. The ion exchange capacity (IEC) of the SPP-QP-BA membrane was titrated to be 2.4 mequiv g−1, which was the same as that of our previous SPP-QP membrane.14 3.2. Preparation of Composite Membranes (SPP-QPBA(Cn)). SPP-QP-BA(Cn) composite membranes were prepared via casting from DMSO solution containing both the precursor polymer (SPP-QP-BA) and alkanediols, followed by thermal treatment (see the Experimental Section for details). In all cases, the feed molar amount of −OH groups (from additives) was set to be higher than that of the −COOH groups (from the SPP-QP-BA precursor) (Table S2). The SPP-QP-BA(Cn) composite membranes were homogeneous and transparent, indicating the good compatibility between SPP-QP-BA and the alkanediol additives. To analyze the chemical structure in more detail, Figure 3a−c shows the 1H NMR spectra for the SPP-QP-BA(Cn) membranes. In all cases, some new peaks assigned to aliphatic protons of the additives appeared at higher magnetic field (ca. 1−4 ppm) for the composite membranes. In the FT-IR spectra (Figure 3d− f), an obvious broad peak at 3000−3600 cm−1 was assigned to stretching vibrations of O−H in alkanediols. The IEC values of those composite membranes were 2.2 mequiv g−1, which were slightly lower than that of the SPP-QP-BA precursor (IEC = 2.4 mequiv g−1) because −OH groups of alkanediols as additives did not function as ion exchangeable groups. 3.3. Properties of the Composite Membranes (SPPQP-BA(Cn)). Figure 4 shows transmission electronic microscopic (TEM) images of the SPP-QP-BA(Cn) composite membranes stained with Pb2+ ions. The dark and bright areas

Figure 4. TEM images of (a) SPP-QP-BA, (b) SPP-QP-BA(C4), (c) SPP-QP-BA(C8), and (d) SPP-QP-BA(C12) membranes.

represent hydrophilic and hydrophobic domains, respectively.18 All membranes exhibited nanoscale phase-separated morphologies based on the hydrophilic/hydrophobic differences in the polymer components. The phase-separated morphology and the ionic domain size (ca. 3 nm) were similar for SPP-QP-BA (2.4 mequiv g−1) and SPP-QP (2.4 mequiv g−1) membranes. The ionic domain size tended to increase slightly as increasing the alkyl chain length of the additives for the composite membranes; ca. 3 nm for SPP-QPBA(C4), ca. 4.3 nm for SPP-QP-BA(C8), and ca. 4.8 nm for SPP-QP-BA(C12), indicating that the additives would be incorporated in the ionic domains. To understand the morphology in more detail, small-angle X-ray scattering (SAXS) analysis was conducted for the SPPQP-BA(Cn) composite membranes at 80 °C and various relative humidity (RH) conditions (Figure S1). In all cases, the membranes showed minor and broad scattering peaks with negligible humidity dependence. The d spacing of SPP-QP-BA was ca. 7 nm. It increased slightly with increasing the alkyl 9917

DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920

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Industrial & Engineering Chemistry Research chain length of the additives for SPP-QP-BA(Cn); d = ca. 7 nm for SPP-QP-BA(C4), d = 7.5−8.5 nm for SPP-QP-BA(C8), and d = 10−12 nm for SPP-QP-BA(C12). This tendency was coincident with the results of TEM images, indicating that the observed scattering peaks in SAXS curves would be associated with hydrophilic domains. The d spacings obtained from the SAXS curves were larger than the ionic domain sizes in the TEM images because of the swelling with the absorbed water. Compared with our previous SPP-QP membrane (IEC = 2.7 mequiv g−1), the scattering peak intensity of SPP-QP-BA-based membranes (IEC = 2.2−2.4 mequiv g−1) was smaller probably because of the lower IEC. In addition, the d spacing (ca. 8.4 nm) of our previous SPP-QP membrane (IEC = 2.7 mequiv g−1) was comparable with SPP-QP-BA(C8) (IEC = 2.2 mequiv g−1) (ca. 7.5−8.5 nm) and clearly larger than that of SPP-QP-BA (IEC = 2.4 mequiv g−1) (ca. 7 nm), indicating that the alkyl chain length of the additives as well as IEC affected the morphology of the membranes. Water uptake of SPP-QP-BA and SPP-QP-BA(Cn) composite membranes was measured at 80 °C as a function of RH (Figure 5a). The water uptake of SPP-QP-BA was lower

conductivity of the SPP-QP-BA precursor and the SPP-QPBA(Cn) composite membranes was measured under the same conditions as those for the water uptake measurement (Figure 5b). The proton conductivity of SPP-QP-BA was lower than that of SPP-QP because of lower acidity and lower water absorbability. SPP-QP-BA(Cn) membranes showed slightly lower proton conductivity than that of SPP-QP-BA membrane in particular at low RH because of the formers’ lower IEC values. In addition, alkanediols probably did not contribute to the proton conduction. The storage modulus (E′), loss modulus (E″), and tan δ (= E″/E′) were also measured at 80 °C as a function of RH by dynamic mechanical analyses (DMA) (Figure 6). Similar to

Figure 6. Viscoelastic properties of membranes at 80 °C as a function of RH: (a) E′, (b) E″, and (c) tan δ. As a comparison, data of our previous SPP-QP membrane (IEC = 2.4 mequiv g−1) are also included.14

Figure 5. (a) Water uptake and (b) proton conductivity of SPP-QPBA(C4), SPP-QP-BA(C8), and SPP-QP-BA(C12) at 80 °C as a function of relative humidity. As a comparison, data of our previous SPP-QP membrane (IEC = 2.4 mequiv g−1) are also included.14

the SPP-QP membrane, E′, E″, and tan δ of the SPP-QP-BA precursor and the SPP-QP-BA(Cn) composite membranes were not so affected by humidity; high E′ (>1 GPa) and no transitions in the E″ and tan δ curves were observed under the tested conditions, indicating that the SPP-QP-BA precursor and the SPP-QP-BA(Cn) composite membranes possessed excellent mechanical stability at a wide range of humidity. Detailed comparison provided the possibility that E′ and E″ increased with increasing the alkyl chain length of the diols. However, the difference was rather minor so that the DMA was not effective to reveal the effect of the additives on the mechanical properties. Therefore, a tensile test was conducted to further investigate the effect of the alkanediol additives on the mechanical properties. Figure 7 shows stress−strain curves of the

than that of SPP-QP at high RH despite the same IEC value (2.4 mequiv g−1). Under all humidity conditions, the SPP-QPBA(Cn) (IEC = 2.2 mequiv g−1) membranes exhibited lower water uptake than that of the SPP-QP-BA precursor (IEC = 2.4 mequiv g−1) membrane because of the formers’ lower IEC values. Among the SPP-QP-BA(Cn) membranes, SPP-QPBA(C12) membrane having the longest alkyl chain length showed the highest water uptake. The higher water absorbability would be related to the more developed, larger ionic domains. A similar trend was observed in dimensional change (in deionized water at 60 °C for 6 h), in which both inplane and through-plane dimensional changes increased with increasing the alkyl chain length (Table S3). The proton 9918

DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920

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Industrial & Engineering Chemistry Research

Figure 7. Stress−strain curves of the membranes at 80 °C and 60% RH. As a comparison, data of our previous SPP-QP membrane (IEC = 2.4 mequiv g−1) are also included.14

Figure 8. Hydrogen and oxygen permeability of membranes at 80 °C as a function of RH. As a comparison, data of our previous SPP-QP (IEC = 2.4 mequiv g−1)14 and Nafion 212 membranes are also included.

membranes at 80 °C and 60% RH. Young’s modulus, yield stress, and maximum strain are summarized in Table 1. Pristine and the composite membranes showed similar stiffness (i.e., Young’s modulus and yield stress), whereas the elongation property strongly depended on the chain length of the diols. SPP-QP-BA showed the lower maximum strain (30%) compared with SPP-QP (68%) possibly because of the hydrogen bonding of the −COOH groups. The maximum strain significantly increased with decreasing the alkyl chain length of the diols and reached 99% for the SPP-QP-BA(C4) membrane, which was ca. 1.5 times higher than that (68% of maximum strain) of the SPP-QP membrane. SPP-QP-BA(C12), however, showed the smallest maximum strain (24%) among those membranes. The larger additive may lower interpolymer interactions of SPP-QP-BA. The hydrogen and oxygen gas permeabilities of SPP-QPBA(C4) membrane were measured at 80 °C as a function of RH and compared with our previous SPP-QP (IEC = 2.4 mequiv g−1)14 and Nafion 212 membranes (Figure 8). The SPP-QP-BA(C4) membrane showed low hydrogen and oxygen permeability over a wide range of humidity. As the humidity increased, the hydrogen and oxygen permeabilities of SPP-QPBA(C4) membrane decreased, which was inverse behavior with our previous SPP-QP membrane probably because of the lower water absorbability of the SPP-QP-BA(C4) membrane at high humidity. At 80 °C and 90% RH, the hydrogen and oxygen permeabilities of the SPP-QP-BA(C4) membrane were 58% and 46% that of the SPP-QP membrane and 17% and 9% that of Nafion 212 membrane. The oxidative stability of SPP-QP-BA(Cn) membranes was evaluated by immersing the samples in Fenton’s reagent (3% H2O2 containing 2 ppm Fe2+) at 80 °C for 1 h (Table 1). The SPP-QP-BA membrane showed high retention in weight (99%) and IEC (100%) similar to the SPP-QP membrane,

indicating that carboxyl groups did not deteriorate the chemical stability. SPP-QP-BA(Cn) composite membranes also exibited high retention (>90%) in weight and IEC. A slightly larger loss in IEC for the composite membranes may be indicative that the additive promoted oxidative degradation of the sulfonic acid groups. FT-IR spectra did not practically change after the Fenton’s test (Figure S2a), further supporting the oxidative stability of the SPP-QP-BA(Cn) composite membranes under the harsh oxidative conditions. The intensity of the aliphatic protons, however, decreased in the 1H NMR spectra after Fenton’s test (Figure S2b). The peak integral ratio of aromatic protons to aliphatic protons was 0.26 for the pristine and 0.24 after the Fenton’s test for the SPP-QPBA(C12) membrane. The results are in accordance with the slightly lower remaining weight of the SPP-QP-BA(Cn) composite membranes than those of SPP-QP-BA and SPPQP membranes (Table 1). Furthermore, the phase-separated morphology (Figure S3) and viscoelastic property (Figure S4) of the SPP-QP-BA(Cn) membranes did not change after the Fenton’s test, indicating high chemical stability of the SPP-QPBA(Cn) composite membranes. A membrane electrode assembly (MEA) with an SPP-QPBA(C4) membrane was prepared and subjected to the fuel cell performance tests. Figure S5 shows the ohmic drop-included polarization curves and ohmic resistances at 80 °C and 100% RH. The SPP-QP-BA(C4) (IEC = 2.2 mequiv g−1) cell exhibited high open circuit voltage (OCV) of 0.99 V for O2 supply and 0.97 V for air supply, indicating the good gas barrier properties of the SPP-QP-BA(C4) membrane. The fuel cell performance of the SPP-QP-BA(C4) cell was slightly lower than those of our previous SPP-QP (IEC = 2.6 mequiv g−1) cell mostly because of the higher ohmic resistance (0.08 Ω cm2) of the SPP-QP-BA(C4) cell than that (0.06 Ω cm2) of

Table 1. Tensile Property and Oxidative Stability of Membranes tensile propertya

oxidative stabilityb

membrane

titrated IEC (mequiv g−1)

Young’s modulus (GPa)

yield stress (MPa)

maximum strain (%)

retention of weight (%)

retention of IEC (%)

SPP-QP14 SPP-QP-BA SPP-QP-BA(C4) SPP-QP-BA(C8) SPP-QP-BA(C12)

2.4 2.4 2.2 2.2 2.2

1.30 1.22 1.0 1.22 1.31

39 37 43 40 42

68 30 99 84 24

99 99 98 95 97

100 100 100 92 90

At 80 °C and 60% RH. bAfter the Fenton’s test at 80 °C for 1 h.

a

9919

DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920

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Industrial & Engineering Chemistry Research

(4) Miyake, J.; Kusakabe, M.; Tsutsumida, A.; Miyatake, K. Remarkable Reinforcement Effect in Sulfonated Aromatic Polymers as Fuel Cell Membrane. ACS Appl. Energy Mater. 2018, 1, 1233. (5) Zhang, H.; Shen, P. K. Recent Development of Polymer Electrolyte Membranes for Fuel Cells. Chem. Rev. 2012, 112, 2780. (6) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535. (7) Kusoglu, A.; Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 2017, 117, 987. (8) Miyake, J.; Miyatake, K. Fluorine-free Sulfonated Aromatic Polymers as Proton Exchange Membranes. Polym. J. 2017, 49, 487. (9) Adamski, M.; Skalski, T. J. G.; Britton, B.; Peckham, T. J.; Metzler, L.; Holdcroft, S. Highly Stable, Low Gas Crossover, ProtonConducting Phenylated Polyphenylenes. Angew. Chem. 2017, 129, 9186. (10) Miyake, J.; Sakai, M.; Sakamoto, M.; Watanabe, M.; Miyatake, K. Synthesis and Properties of Sulfonated Block Poly(arylene ether)s Containing m-Terphenyl Groups as Proton Conductive Membranes. J. Membr. Sci. 2015, 476, 156. (11) Miyake, J.; Mochizuki, T.; Miyatake, K. Effect of the Hydrophilic Component in Aromatic Ionomers: Simple Structure Provides Improved Properties as Fuel Cell Membranes. ACS Macro Lett. 2015, 4, 750. (12) Miyake, J.; Hosaka, I.; Miyatake, K. Effect of Sulfonated Triphenylphosphine Oxide Groups in Aromatic Block Copolymers as Proton-exchange Membranes. Chem. Lett. 2016, 45, 33. (13) Skalski, T. J. G.; Britton, B.; Peckham, T. J.; Holdcroft, S. Structurally-Defined, Sulfo-Phenylated, Oligophenylenes and Polyphenylenes. J. Am. Chem. Soc. 2015, 137, 12223. (14) Miyake, J.; Taki, R.; Mochizuki, T.; Shimizu, R.; Akiyama, R.; Uchida, M.; Miyatake, K. Design of Flexible Polyphenylene Protonconducting Membrane for Next-generation Fuel Cells. Sci. Adv. 2017, 3, No. eaao0476. (15) Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E. Fabrication and Characterization of Heteropolyacid (H3PW12O40)/Directly Polymerized Sulfonated Poly(arylene ether sulfone) Copolymer Composite Membranes for Higher Temperature Fuel Cell Applications. J. Membr. Sci. 2003, 212, 263. (16) Li, H.; Zhang, G.; Ma, W.; Zhao, C.; Zhang, Y.; Han, M.; Zhu, J.; Liu, Z.; Wu, J.; Na, H. Composite Membranes Based on a Novel Benzimidazole Grafted PEEK and SPEEK for Fuel Cells. Int. J. Hydrogen Energy 2010, 35, 11172. (17) Okamoto, K.; Ichikawa, T.; Yokohara, T.; Yamaguchi, M. Miscibility, Mechanical and Thermal Properties of Poly(lactic acid)/ Polyester-diol Blends. Eur. Polym. J. 2009, 45, 2304. (18) Zhang, Y.; Miyake, J.; Akiyama, R.; Shimizu, R.; Miyatake, K. Sulfonated Phenylene/Quinquephenylene/Perfluoroalkylene Terpolymers as Proton Exchange Membranes for Fuel Cells. ACS Appl. Energy Mater. 2018, 1, 1008.

the SPP-QP cell. The result is reasonable taking into account slightly lower proton conductivity of the SPP-QP-BA(C4) membrane (Figure 5).

4. CONCLUSIONS SPP-QP-BA(Cn) composite membranes containing various alkanediol additives (C4, C8, and C12) were successfully synthesized. The ionic domain size, water uptake, and proton conductivity slightly increased with increasing the aliphatic chain length of the additives; however, the difference was rather minor. The alkanediol additive was effective in improving the elongation property of the membranes, and the effect was in the order of C4 (99%) > C8 (84%) > C12 (24%) in terms of the maximum strain under the heated and humidified conditions. The SPP-QP-BA(Cn) membranes exhibited excellent chemical stability under the highly oxidizing conditions (i.e., Fenton’s test); the post-test membranes retained the molecular structure, morphology, and viscoelastic property.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01564. Measurement methods, m-/p-phenylene ratio, feed molar ratio, IEC, solubility, dimensional change and water uptake of the membranes, SAXS profiles of SPPQP-BA and composite membranes, FT-IR, 1H NMR, TEM, and viscoelastic properties of SPP-QP-BA and composite membranes after Fenton’s test (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.M.). ORCID

Kenji Miyatake: 0000-0001-5713-2635 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the SPer-FC Project and by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan through a Grant-in-Aid for Scientific Research (KAKENHI JP18K04746, JP18H02030, JP18H05515, and JP18K19111).

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REFERENCES

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DOI: 10.1021/acs.iecr.9b01564 Ind. Eng. Chem. Res. 2019, 58, 9915−9920