Chemically Stable, Highly Anion Conductive Polymers Composed of

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Article Cite This: Macromolecules 2019, 52, 2131−2138

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Chemically Stable, Highly Anion Conductive Polymers Composed of Quinquephenylene and Pendant Ammonium Groups Ryo Akiyama,† Naoki Yokota,§ and Kenji Miyatake*,†,‡ Fuel Cell Nanomaterials Center and ‡Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan § Takahata Precision Co. Ltd., 390 Maemada, Sakaigawa, Fuefuki, Yamanashi 406-0843, Japan Macromolecules 2019.52:2131-2138. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/05/19. For personal use only.

†

S Supporting Information *

ABSTRACT: To evaluate the effect of five consecutive phenylene units on the properties of anion conductive polymer membranes, a series of quaternized copolymers (QP-QAF) composed of quinquephenylene and fluorene groups functionalized with pendant hexyltrimethylammonium groups were designed and synthesized. Precursor copolymers, QP-AF, with controllable copolymer compositions were synthesized by Yamamoto coupling reaction as high molecular weights. Quaternization of QP-AF using dimethyl sulfate followed by solution casting provided bendable and tough QP-QAF membranes with the ion exchange capacity (IEC) ranging from 0.80 to 2.78 mequiv g−1. The QPQAF membranes contained phase-separated morphology due to the hydrophilic/hydrophobic differences in the components as confirmed by TEM images. High hydroxide ion conductivity (up to 134 mS cm−1 in water at 80 °C) was obtained with the membrane (IEC = 2.25 mequiv g−1). The membranes were stable in strongly alkaline conditions (∼4 M KOH at 80 °C) for 1000 h. An H2/O2 alkaline fuel cell using the QP-QAF membrane exhibited 248 mW cm−2 of the maximum power density at 60 °C under fully humidified conditions.

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INTRODUCTION Polymer electrolyte fuel cells (PEFCs) have been commercialized for automobiles and residential (cogeneration) applications; however, strongly acidic proton exchange membranes (PEMs) require use of costly and unabundant platinum as electrocatalysts. In addition, perfluorosullfonic acid-based polymers as state-of-the-art PEMs are also expensive. To address the issues associated with PEMFCs, anion exchange membrane fuel cells (AEMFCs) seem attractive options because of the possible use of abundant transition metals such as iron, cobalt, and nickel as alternative electrocatalysts under basic conditions.1−4 In the past decade, there have been a considerable effort to develop high-performance and durable AEMs.5−7 In particular, the chemical stability of AEMs in alkaline media is an issue for long-term, reliable operation of AEMFCs.8,9 Many cationic head groups such as cyclic and acyclic ammonium, phosphonium, and other onium groups and polymer main-chain structures have been investigated.10−16 Some were claimed to survive in strongly alkaline conditions for hundreds of hours. In the pursuit of chemically stable AEMs and PEMs, we have recently reported some insightful results on the polymer structures. Combination of lack of heteroatom linkages such as ether, sulfide, and sulfone in the main chain and employing pendant ammonium groups contributed to the alkaline stability of AEMs. One of the typical examples is QPAF-4 (Figure 1a) copolymers composed of perfluoroalkylene/phenylene main chain and hexylammonium head groups, which exhibited high hydroxide ion conductivity (>80 mS cm−1) and alkaline © 2019 American Chemical Society

Figure 1. Chemical structures of (a) QPAF-4 and (b) SPP-QP polymers.

stability (>1000 h in 1 M KOH at 80 °C).11 For PEMs, we recently found that quinquephenylene groups with controlled m-/p-composition functioned well in sulfonated polyphenlene to provide highly oxidative stability and good membraneforming capability. In fact, the polyphenylene ionomer or SPPQP (Figure 1b) membrane composed of quinquephenylene Received: October 13, 2018 Revised: February 14, 2019 Published: February 20, 2019 2131

DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138

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Macromolecules Scheme 1. Synthesis of QP-AF and QP-QAF Copolymers

a magnetic stirrer bar were added QP-AF-3 (652 mg, 1.55 mmol of dimethylamino groups) and 6.5 mL of DMAc. To the solution, dimethyl sulfate (737 μL, 7.77 mmol) was added. Then, the mixture was reacted for 4 days at rt and diluted with 6.5 mL of DMAc. The resulting mixture was poured into 150 mL of deionized water to precipitate a crude product as pale yellow powder. The precipitate was washed with deionized water and dried at 60 °C in a vacuum for 8 h to obtain QP-QAF-3 in methyl sulfate ion (MeOSO3−) form as a pale orange powder (763 mg, 93% yield). The obtained QP-QAF-3 was redissolved in 12 mL of DMSO, filtered, and cast onto a flat glass plate. Drying the solution at 50 °C gave a QP-QAF-3 membrane. The membrane was soaked in 1 M KOH at 80 °C for 48 h. The membrane was then placed in an excess of degassed deionized water for 1 day to give the QP-QAF-3 membrane in hydroxide ion (OH−) form. Preparation of Catalyst-Coated Membrane and Operation of Alkaline Fuel Cell. Pt/CB (50 wt % of Pt, TEC10E50E, Tanaka Kikinzoku Kogyo) (0.80 g), water (3.3 g), and ethanol (3.3 g) were mixed with a planetary ball mill at 270 rpm for half an hour. After the addition of 6.80 g of 5 wt % QPAF-4 (IEC = 2.10 mequiv g−1) solution in ethanol, the mixture was mixed with a planetary ball mill at 270 rpm for another half an hour and with a pot mill for 12 h. The ratio of QPAF-4 and carbon black in the slurry was set to be 0.8 (by weight). The obtained slurry was sprayed onto both sides of the QPQAF-3 membrane (22 μm thick) and pressed at 10 kgf cm−2 at rt for 3 min. In the resulting catalyst-coated membrane (CCM), the catalyst area was 4.4 cm2. The loading amount of Pt was ca. 0.20 mg/cm2 for both cathode and anode. The CCM was soaked in 1 M KOH for 48 h to transform the membrane and ionomer into hydroxide ion forms and then washed thoroughly with deionized water to remove residual KOH. The CCM was set in a single cell with gas diffusion layers and a gasket. The gas diffusion layers were wet-proof carbon cloth with microporous layer (PANEX30 PW03, Zoltek Companies) for the anode and carbon paper (29BC GDL, SGL) for the cathode,

and sulfophenylene did not decompose in accelerated oxidative stability tests (e.g., Fenton’s test).17 The objective of the present research is to combine the structures of QPAF-4 and SPP-QP into a single polymer (QP-QAF) for better performing AEMs and evaluate the effect of five consecutive phenylene groups as hydrophobic component on the membrane properties. Synthesis and characterization of QP-QAF polymer membranes are described. Properties and alkaline fuel cell performances of QP-QAF membranes were investigated to evaluate the effect of quinquephenylene groups on the AEMs.

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EXPERIMENTAL SECTION

Synthesis of QP-AF. QP monomer 1 and AF monomer 2 were prepared according to the literature.11,17 A typical procedure for QPAF-3 is as follows (m:n = 1:1). To a three-neck round-bottomed flask (100 mL) equipped with a magnetic stirrer bar, a Dean−Stark trap, a reflux condenser, and a nitrogen inlet/outlet, 1 (372 mg, 0.825 mmol), 2 (404 mg, 0.825 mmol), 2,2′-bipyridine (600 mg, 3.84 mmol), 10 mL of DMAc, and 5 mL of toluene were added. The mixture was heated at 170 °C for 2 h to remove water. After removal of toluene, the mixture was cooled to 80 °C. To the mixture was added bis(1,5-cyclooctadiene)nickel(0) (Ni(cod)2) (1.00 g, 3.64 mmol). The copolymerization reaction was performed at 80 °C for 3 h. Then, the mixture was cooled to room temperature (rt), diluted with 10 mL of DMAc, and poured into large excess of hydrochloric acid to precipitate a crude product as pale yellow powder. The precipitate was washed with deionized water, K2CO3 aqueous solution, deionized water, and methanol several times. Drying at 60 °C in vacuum for 8 h gave QP-AF-3 as a pale yellow solid (652 mg, 99% yield). Preparation of QP-QAF Membranes. A typical procedure for QP-QAF-3 is as follows (m:n = 1:1). To a vial (50 mL) equipped with 2132

DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138

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Macromolecules Table 1. Physical Properties of QP-AF and QP-QAF

IECd (mequiv g−1)

QP-AF a

b

no.

m:n

m:n

yield (%)

1 2 3 4

5:1 2:1 1:1 1:2

5.6:1 2.0:1 1.0:1 1.1:2

100 97 99 96

Mnc

(kDa) 5.2 9.8 8.6 7.5

c

b

Mw (kDa)

Mw/Mn

QP-QAF m:n

NMR

titration

131 147 187 149

25 15 22 20

4.6:1 2.2:1 1.2:1 1.1:2

0.89 1.50 2.21 2.85

0.80 1.60 2.25 2.78

Feed comonomer ratio. bDetermined by 1H NMR spectra. cDetermined by GPC measurements. dCalculated as QP-QAF (OH− form).

a

respectively. Before the fuel cell measurement, fully humidified hydrogen and nitrogen were supplied to the anode and cathode, respectively. The fuel cell was operated at 60 °C with hydrogen and oxygen (both fully humidified) at a flow rate of 100 mL min−1 to the anode and the cathode, respectively.

quaternized QP-QAF were soluble in some polar organic solvents such as DMF, DMAc, and DMSO but not in abovementioned halogenated solvents (Table S1). The chemical structure of QP-QAF was analyzed by 1H NMR spectra (Figure S3), in which the methylene and methyl protons (i and j) α to nitrogen atoms were shifted down to lower magnetic field compared to those of QP-AF (Figure S2). The protons of methyl sulfate ions were observed at 2.33 ppm. The results suggest successful quaternization reaction with dimethyl sulfate. The copolymer compositions of QP-QAF calculated from the integral ratios of the proton peaks were in fair agreement with those of the parent QP-AF (Table 1). Casting from the DMSO solution gave light yellow and transparent membranes from the four QP-QAF polymers. The result suggests that quinquephenylene groups composed of one pphenylene and four m-phenylene groups as hydrophobic components contributed to good membrane-forming capability of pendant-type ammonium containing polymers, similarly to the sulfonated polyphenylene SPP-QP ionomers. The ion exchange capacities (IECs) were estimated by titration, which were also in good agreement with those calculated from the 1H NMR spectra (hereafter, we refer to the IEC values obtained by titration). Morphologies of the Membranes. In Figure 2, crosssectional TEM images of QP-QAF membranes (IEC = 0.80, 1.60, 2.25, and 2.78 mequiv g−1) stained with PtCl42− are shown. Regardless of large differences in their IEC values (or ammonium concentration), the four QP-QAF membranes exhibited very similar phase-separated morphologies, where the white areas represent hydrophobic domains and the black areas represent hydrophilic domains containing ammonium groups (and their aggregates). The hydrophilic and hydrophobic domains were both spherical with ca. 2 nm of the average diameters. The morphologies of QP-QAF membranes were similar to those of QPAF-4 membranes, 11 and thus, quinquephenylene groups of successive five phenylene units were highly hydrophobic contributing to the developed morphology. Properties of the Membranes. Figure 3a shows water uptake (at rt) and Figure 3b shows hydroxide ion conductivity (in water at 30 °C) of QP-QAF and QPAF-4 membranes as a function of IEC. Compared with QPAF-4 membranes, QPQAF membranes exhibited much smaller water uptake but similar ion conductivity taking their IEC differences into account. Recently, Bae et al. reported AEMs (PFB+, IEC = 3.59 mequiv g−1) containing the same hydrophilic components (pendant ammoniohexyl groups on fluorenyl groups).10 Despite its much lower IEC value, QP-QAF (IEC = 2.25 mequiv g−1) showed slightly higher water uptake and higher conductivity than those of PFB+ probably due to the five sequenced phenylene rings as the hydrophobic component. In Figure 4 is replotted hydroxide ion conductivity as a function of λ (number of absorbed water molecules per ammonium

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RESULTS AND DISCUSSION Synthesis of QP-QAF Polymers. The synthetic route for the title quaternized polymers, QP-QAF, is summarized in Scheme 1. Quinquephenylene (QP) monomer 1, one pphenylene group capped with four m-phenylene groups, which was designed to provide the resulting polymers with good membrane-forming capability, was synthesized by double Suzuki−Miyaura coupling reactions.17 Amine-functionalized fluorene (AF) monomer 2 was prepared from fluorene via chlorination, bromoalkylation, and amination reactions according to the literature.11 Monomers 1 and 2 were copolymerized by nickel-mediated Yamamoto coupling reaction to obtain precursor copolymers, QP-AF. A series of QP-AF-(1−4) copolymers with different copolymer compositions (m and n) were obtained as high molecular weights (Mn = 5.2−9.8 kDa and Mw = 131−187 kDa) (Table 1). Polydispersity indexes (PDI) were M w /M n = 15−25, substantially high for polycondensation reactions. GPC curves (Figure S1) indicated the formation of oligomeric products at long retention time, possibly containing larger content of quinquephenylene units. These oligomers were likely to be precipitated out of the reaction mixture at the early stage of the polymerization reaction causing high PDI values. The obtained QP-AF were soluble in some common organic solvents such as halogenated solvents (chloroform and dichloromethane) and THF (Table S1). The chemical structure of QP-AF was analyzed by 1H NMR spectra (Figure S2). All proton peaks were well-assigned to the supposed polymer structure. It is noticeable that the methylene and methyl protons (i and j) α to nitrogen atoms appeared at different chemical shifts, depending on the copolymer composition (or the concentration of the amine groups). The copolymer compositions estimated from the integral ratios of the proton peaks were in good accordance with the feed comonomer compositions. Quaternization of QP-AF was performed with dimethyl sulfate to obtain QP-QAFs in methyl sulfate ion (MeOSO3−) forms. In our previous study of QPAF-4, iodomethane was used for the quaternization reactions.11 However, ion exchange reaction of iodide ions to other anions (such as hydroxide ions) was not very efficient, and trace amount of iodide ions often remained in the quaternizaed membranes even after careful and repeated ion exchange reactions, which could affect some important membrane properties (data not shown). Therefore, in this study, we replaced iodomethane with dimethyl sulfate as the quaternization reagent. The reaction proceeded well in a homogeneous solution (DMAc as solvent), and the products were recovered as a pale yellow powder. The 2133

DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138

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Figure 4. Hydroxide ion conductivity of QP-QAF and QPAF-4 membranes as a function of λ.

Figure 2. Cross-sectional TEM images of the membranes stained with tetrachloroplatinate ions: (a) QP-QAF-1 (IEC = 0.80 mequiv g−1), (b) QP-QAF-2 (IEC = 1.60 mequiv g−1), (c) QP-QAF-3 (IEC = 2.25 mequiv g−1), and (d) QP-QAF-4 (IEC = 2.78 mequiv g−1). Figure 5. Temperature dependence of hydroxide ion conductivity of QP-QAF membranes.

30 to 80 °C. QP-QAF-3 membrane with IEC = 2.25 mequiv g−1 achieved very high ion conductivity (134 mS cm−1) at 80 °C. The activation energy (Ea) for the hydroxide ion conduction estimated from the slopes was ca. 1012 kJ mol−1, which were more or less similar to the reported values for aromatic polymer-based AEMs,11,18−25 implying the similar conduction mechanism involving the hydrated ions. The hydrophobic quinquephenylene groups were unlikely to affect the hydroxide ion conducting properties since the ion conduction was more related to the hydrophilic domains. The dynamic mechanical analyses (DMA) were performed for the QP-QAF membranes in methyl sulfate ion forms. The storage moduli (E′), loss moduli (E″), and tan δ (= E″/E′) are shown as a function of relative humidity at 80 °C in Figure 6a and as a function of temperature at 60% RH in Figure 6b. In the humidity dependence, the initial E′ was lower and decreased more for higher IEC membranes because of the larger water absorbability. In E″ and tan δ curves, no peaks were observed for all the membranes up to ca. 90% RH. In the temperature dependence, the four membranes exhibited stable DMA properties with minor changes in E′, E″ and tan δ up to ca. 95 °C. Compared to QPAF-4 membranes which exhibited a thermal transition peak at ca. 70−80 °C depending on the copolymer composition and IEC,11 QP-QAF membranes were thermomechanically more stable because of rigid quinquephenylene groups as the hydrophobic component. Stress versus strain curves were obtained for the QP-QAF membranes at 80 °C and 60% RH (Figure 7), and the results are summarized in Table 2. The Young’s modulus was 13 GPa for QP-PAF-1 and decreased as increasing the IEC. Similarly, the maximum stress and strain were lower for higher IEC membranes. The trend was opposite from what was obtained

Figure 3. (a) Water uptake and (b) hydroxide ion conductivity of QP-QAF and QPAF-4 membranes as a function of IEC.

group). QP-QAF membranes exhibited much higher conductivity than that of QPAF-4 membrane, suggesting better utilization of water for the hydroxide ion conduction. In other words, rigid-rod-like structure of quiquephenylene groups suppressed excess swelling. The effect seems more pronounced than that for sulfonated SPP-QP ionomer membranes. The hydroxide ion conductivity of QP-QAF membranes at different temperatures is shown in Figure 5. All membranes showed nearly Arrhenius-type temperature dependence from 2134

DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138

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Figure 6. DMA curves of QP-QAF membranes in MeOSO3− forms: (a) humidity dependence at 80 °C and (b) temperature dependence at 60% RH.

Table 2. Mechanical Properties of QP-QAF Membranes (Data Taken from Figure 7) membrane

Young’s modulus (GPa)

maximum stress (MPa)

maximum strain (%)

QP-QAF-1 QP-QAF-2 QP-QAF-3 QP-QAF-4

13 11 6 5

53 46 35 27

50 20 28 18

strength. In addition, higher IEC membranes absorbed more water, which also weakened the mechanical properties. Compared to the quinquepehylene groups, the perfluorohexylene groups contributed to large elongation at break (ca. >200% under the same conditions). The alkaline stability of QP-QAF-1, -2, and -3 membranes was evaluated in 1 M KOH at 80 °C (Figure 8). While the conductivity fluctuated to some extent due to the swelling in particular for the high IEC membrane, three QP-QAF membranes showed small changes in the conductivity within acceptable errors up to 1000 h. The remaining conductivities (defined as the conductivity after 1000 h divided by the

Figure 7. Stress versus strain curves of the QP-QAF membranes in MeOSO3− forms at 80 °C and 60% RH.

for QPAF-4 membranes, whose Young’s modulus and maximum stress became higher as increasing IEC.11 Because these two series of AEMs share the same hydrophilic components, the differences in the elongation properties must be ascribed to the hydrophobic components. In the QP-QAF membranes, the higher IECs corresponded to lower quinquephenylene contents causing smaller mechanical 2135

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Figure 8. Changes of the hydroxide ion conductivity (at 40 °C) of QP-QAF-1, -2, and -3 membranes in 1 M KOH at 80 °C.

maximum conductivity) were 95% (QP-QAF-1), 94% (QPQAF-2), and 85% (QP-QAF-3). The post-test membranes were bendable and soluble in organic solvents. The 1H NMR spectra of the post-test QP-QAF membranes did not show evidence in structural changes and chemical degradation (Figure S4). The alkaline stability of QP-QAF membranes were similar to that of the QPAF-4 membrane, indicating that both quinquephenylene and perfluoroalkylene groups were highly alkaline stable.11 Then, QP-QAF-3 membrane was evaluated in severer alkaline conditions (in 4 and 8 M KOH, Figure 9). In 4 M

Figure 10. (a) Cell voltage and power density and (b) ohmic resistance of hydrogen/oxygen fuel cell with QP-QAF-3 membrane operated at 60 °C and 100% RH.

presumably because of the lower conductivity in humidified conditions than in water. The maximum power density was 248 mW cm−2. The catalyst and the catalyst layers need to be optimized for better fuel cell performance. In the literature, much higher maximum power density (ca. 1400 mW cm−2) was reported with the newly developed anion exchange polymer membranes.26,27 However, such high fuel cell performance was achievable only with high gas flow rates (1000 mL min−1 for hydrogen and oxygen with 4.5 cm2 of the electrode area) or low gas utilization (ca. 3.4% and 1.7% for hydrogen and oxygen, respectively, at 1.0 A cm−2 of the current density). The cell performance decreased significantly as decreasing the flow rate (or increasing the gas utilization).27 In the present work, the gas flow rate was set at 100 mL min−1 both for hydrogen and oxygen (or gas utilization was set at 30% and 15% for hydrogen and oxygen, respectively) taking the practical fuel cell operating conditions into account.

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Figure 9. Changes of the hydroxide ion conductivity (at 40 °C) of QP-QAF-3 membrane in 4 and 8 M KOH at 80 °C.

CONCLUSIONS Novel quaternized copolymers (QP-QAF) containing quinquephenylene and pendant hexyltrimethylammonium groups were synthesized. The quinquephenylnene groups with m-/p= 4/1 composition provided the resulting copolymer AEMs with dimensional, chemical, and mechanical stabilities. Compared to the other AEMs sharing the same head ammonium groups (e.g., QPAF-4 containing perfluoroalkylene groups in the main chain), QP-QAF membranes exhibited smaller water absorbability and comparable (or higher) hydroxide ion conductivity in water. The five consecutive phenylene groups as rigid hydrophobic component also contributed to mechanical stability of the QP-QAF membranes with no glass transition behavior at least up to 95 °C under humidified conditions, while the elongation of QP-PAF membranes was not as large as that of QPAF-4 membranes. Because of the lack of the heteroatom linkages such as ether, sulfide, and sulfone groups in the polymer main chain, QPQAF membranes were alkaline stable in 4 M KOH at 80 °C for 1000 h. In 8 M KOH, the hydroxide ion conductivity decreased as the testing time. The post-mortem analyses of the membranes suggested that the alkaline degradation was likely to involve the decomposition of the head ammonium groups

KOH, the membrane was stable for 1000 h with minor changes in the conductivity. In 8 M KOH, however, the conductivity decreased with the testing time to 41 mS cm−1 (48% of the remaining conductivity) after 1000 h. In the 1H NMR spectrum of the post-test QP-QAF-3 membrane (Figure S5), the peaks assigned to methyl and methylene groups attached to the ammonium groups were smaller. The IEC value was smaller (1.67 mequiv g−1) than the initial IEC (2.25 mequiv g−1). Because the post-test membrane was still bendable, the degradation mechanism of QP-QAF membranes in strongly alkaline solution seemed to involve the decomposition of the head ammonium groups rather than the polymer main chains. Fuel Cell Evaluation. The catalyst-coated membrane (CCM) prepared using a QP-QAF-3 membrane (22 μm thick) and Pt/C-based catalyst layers was evaluated in hydrogen/oxygen fuel cell at 60 °C (Figure 10). The open circuit voltage (OCV) was 1.0 V, indicative of low gas permeability of the membrane. The ohmic resistance was ca. 0.15 Ω cm2 and higher than that (0.02 Ω cm2) calculated from the hydroxide ion conductivity of the membrane (Figure 5), 2136

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(9) Kreuer, K.-D.; Jannasch, P. A Practical Method for Measuring the Ion Exchange Capacity Decrease of Hydroxide Exchange Membranes during Intrinsic Degradation. J. Power Sources 2018, 375, 361−366. (10) Lee, W.-H.; Mohanty, A. D.; Bae, C. Fluorene-Based Hydroxide Ion Conducting Polymers for Chemically Stable Anion Exchange Membrane Fuel Cells. ACS Macro Lett. 2015, 4, 453−457. (11) Ono, H.; Kimura, T.; Takano, A.; Asazawa, K.; Miyake, J.; Inukai, J.; Miyatake, K. Robust Anion Conductive Polymers Containing Perfluoroalkylene and Pendant Ammonium Groups for High Performance Fuel Cells. J. Mater. Chem. A 2017, 5, 24804− 24812. (12) Olsson, J. S.; Pham, T. H.; Jannasch, P. Poly(N,Ndiallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N-Spirocyclic Quaternary Ammonium Cations. Macromolecules 2017, 50, 2784−2793. (13) Womble, C. T.; Coates, G. W.; Matyjaszewski, K.; Noonan, K. J. T. Tetrakis(dialkylamino)phosphonium Polyelectrolytes Prepared by Reversible Addition- Fragmentation Chain Transfer Polymerization. ACS Macro Lett. 2016, 5, 253−257. (14) Kwasny, M. T.; Zhu, L.; Hickner, M. A.; Tew, G. N. Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity. J. Am. Chem. Soc. 2018, 140, 7961−7969. (15) Fan, J.; Wright, A. G.; Britton, B.; Weissbach, T.; Skalski, T. J. G.; Ward, J.; Peckham, T. J.; Holdcroft, S. Cationic Polyelectrolytes, Stable in 10 M KOHaq at 100 °C. ACS Macro Lett. 2017, 6, 1089− 1093. (16) Lee, W.-H.; Park, E. J.; Han, J.; Shin, D. W.; Kim, Y. S.; Bae, C. Poly(terphenylene) Anion Exchange Membranes: The Effect of Backbone Structure on Morphology and Membrane Property. ACS Macro Lett. 2017, 6, 566−570. (17) 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, eaao0476. (18) Tanaka, M.; Fukasawa, K.; Nishino, E.; Yamaguchi, S.; Yamada, K.; Tanaka, H.; Bae, B.; Miyatake, K.; Watanabe, M. Anion Conductive Block Poly(arylene ether)s: Synthesis, Properties, and Application in Alkaline Fuel Cells. J. Am. Chem. Soc. 2011, 133, 10646−10654. (19) Shimada, M.; Shimada, S.; Miyake, J.; Miyatake, K.; Uchida, M. Anion Conductive Aromatic Polymers Containing Fluorenyl Groups: Effect of the Position and Number of Ammonium Groups. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 935−944. (20) Miyake, J.; Watanabe, M.; Miyatake, K. Ammonium-functionalized Poly(arylene ether)s as Anion-Exchange Membranes. Polym. J. 2014, 46, 656−663. (21) Yokota, N.; Shimada, M.; Ono, H.; Akiyama, R.; Nishino, E.; Asazawa, K.; Miyake, J.; Watanabe, M.; Miyatake, K. Aromatic Copolymers Containing Ammonium-Functionalized Oligophenylene Moieties as Highly Anion Conductive Membranes. Macromolecules 2014, 47, 8238−8246. (22) Akiyama, R.; Yokota, N.; Nishino, E.; Asazawa, K.; Miyatake, K. Anion Conductive Aromatic Copolymers from Dimethylaminomethylated Monomers: Synthesis, Properties, and Applications in Alkaline Fuel Cells. Macromolecules 2016, 49, 4480−4489. (23) Mahmoud, A. M. A.; Elsaghier, A. M. M.; Otsuji, K.; Miyatake, K. High Hydroxide Ion Conductivity with Enhanced Alkaline Stability of Partially Fluorinated and Quaternized Aromatic Copolymers as Anion Exchange Membranes. Macromolecules 2017, 50, 4256−4266. (24) Ozawa, M.; Kimura, T.; Akiyama, R.; Miyake, J.; Inukai, J.; Miyatake, K. Copolymers Composed of Perfluoroalkyl and Ammonium-Functionalized Fluorenyl Groups as Chemically Stable Anion Exchange Membranes. Bull. Chem. Soc. Jpn. 2017, 90, 1088− 1094. (25) Akiyama, R.; Yokota, N.; Otsuji, K.; Miyatake, K. Structurally Well-Defined Anion Conductive Aromatic Copolymers: Effect of the Side-Chain Length. Macromolecules 2018, 51, 3394−3404.

rather than the polymer main chain. These properties of the QP-QAF membranes seem promising for the application in energy devises such as alkaline fuel cells and water electrolysis. A hydrogen/oxygen fuel cell using the QP-QAF membrane exhibited reasonable performance while the electrode materials and structures need further optimization.

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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.macromol.8b02199.

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General method, solubility, GPC charts, and 1H NMR spectra of the polymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel +81 55 220 8707; 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 Japan Science and Technology Agency (JST) through CREST (JPMJCR12C3) and by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan through a Grant-in-Aid for Scientific Research (18H02030, 18H05515, and 18K19111). K.M. acknowledges the Ogasawara Foundation for the Promotion of Science and Engineering for financial support.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138

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DOI: 10.1021/acs.macromol.8b02199 Macromolecules 2019, 52, 2131−2138