Article pubs.acs.org/Macromolecules
Anion Conductive Aromatic Copolymers from Dimethylaminomethylated Monomers: Synthesis, Properties, and Applications in Alkaline Fuel Cells Ryo Akiyama,† Naoki Yokota,§ Eriko Nishino,∥ Koichiro Asazawa,∥ and Kenji Miyatake*,†,‡ †
Fuel Cell Nanomaterials Center and ‡Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan Takahata Precision Japan Co. Ltd., 390 Maemada, Sakaigawa, Fuefuki, Yamanashi 406-0843, Japan ∥ Frontier Technology R&D Division, Daihatsu Motor Co. Ltd., 3000 Ryuo, Gamo, Shiga 520-2593, Japan §
S Supporting Information *
ABSTRACT: A novel series of anion conductive aromatic copolymers were synthesized from preaminated monomers (2,5-, 3,5-, or 2,4-dichloro-N,N-dimethylbenzylamine), and their properties were investigated for alkaline fuel cell applications. The targeted copolymers (QPE-bl-11a, -11b, and -11c) were synthesized via nickel-mediated Ullmann coupling polymerization, followed by quaternization and ion exchange reactions. Unlike the conventional method involving chloromethylation or bromination, this method provided copolymers with well-defined chemical structure. The hydrophilic components of the copolymers were composed of chemically stable phenylene main chain modified with high-density ammonium groups. Oligo(arylene ether sulfone ketone)s were employed as a hydrophobic block. QPE-bl-11a gave tough and bendable membranes by solution casting. The obtained membrane with the highest ion exchange capacity value (IEC = 2.47 mequiv g−1) showed high hydroxide ion conductivity (130 mS cm−1) in water at 80 °C. The QPE-bl-11a membrane showed reasonable alkaline stability in 1 M KOH aqueous solution for 1000 h at 60 °C. A platinum-free fuel cell was successfully operated with hydrazine as a fuel, the QPE-bl-11a as a membrane, and an electrode binder. The maximum power density of 380 mW cm−2 was achieved at a current density of 1020 mA cm−2 with O2.
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INTRODUCTION As an efficient and environmentally benign energy device, fuel cells have received increasing attention.1 Especially, polymer electrolyte fuel cells (PEFCs) using proton exchange membranes (PEMs) are operable at moderate temperatures and thus suitable for applications in stationary cogeneration systems, portable devices, and electric vehicles.2 However, use of strongly acidic PEMs requires expensive, exhaustible precious metals (Pt) as electrocatalysts. To address this issue, replacement of PEMs with anion exchange membranes (AEMs) seems an attractive option. Under the basic conditions of anion exchange membrane fuel cells (AEMFCs), abundant metals, for example, Ni, Co, and Fe, can be used as electrocatalysts.3−5 Moreover, the kinetics of the oxygen reduction reaction is more favorable under alkaline than under acidic conditions. In the past decade, large numbers of highly conductive and chemically stable AEMs were developed.6−11 Similar to PEMs composed of aromatic block copolymers that exhibit high proton conductivity due to their well-developed hydrophilic/ hydrophobic phase-separated morphology,12 we have attempted several investigations and developed multiblock poly(arylene ether)s having hydrophilic blocks with high density ammonium groups to achieve high hydroxide ion conductivity.13−15 In contrast, further improvement is still required in © XXXX American Chemical Society
chemical stability of AEMs under alkaline conditions. Kim et al.16 and Ramani et al.17 reported independently that degradation of quaternized poly(arylene ether)s main chain was caused by the reaction with hydroxide ions on ether linkages close to electron-withdrawing groups such as tethered benzyltrimethylammonium groups. To solve this problem, Hickner et al.18 have designed and synthesized comb-shaped aromatic copolymers containing long alkyl side chains pendant to the nitrogen-centered cationic group and revealed that these membranes kept high ion conductivity in alkaline solution. Holdcroft et al.19,20 also reported that AEMs containing sterically hindered polybenzimidazolium showed high hydroxide ion conductivity and stability. More recently, we have developed aromatic copolymer (QPE-bl-9) membranes composed of ammonium-functionalized phenylene (carbon−carbon bonded) groups as the hydrophilic component. The QPE-bl-9 membrane showed high hydroxide ion conductivity (138 mS cm−1) in hot water (80 °C) and reasonable alkaline stability for 1000 h in 1 M KOH aqueous solution at 40 °C.21 However, chloromethylation reaction using chloromethyl methyl ether (CMME) was required for the synthesis of QPE-bl-9. CMME is Received: February 25, 2016 Revised: June 6, 2016
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DOI: 10.1021/acs.macromol.6b00408 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
2H), 7.33 (m, 1H). 13C NMR (125 MHz, CDCl3): δ 34.6, 37.8, 127.8, 128.6, 130.1, 130.7, 133.2, 137.7, 166.9. 2b was synthesized from 1b in a manner similar to that of 2a. 3,5-Dichloro-N,N-dimethylbenzamide (2b): brown solid (85% yield). 1H NMR (500 MHz, CDCl3): δ 2.98 (s, 3H), 3.10 (s, 3H), 7.30 (m, 2H), 7.40 (m, 1H). 13C NMR (125 MHz, CDCl3): δ 35.1, 39.1, 125.3, 129.3, 135.0, 138.9, 168.2. In the case of 2c, although the procedure was similar to that of 2a and 2b, the product was obtained as a liquid. 2,4-Dichloro-N,N-dimethylbenzamide (2c): Orange liquid (96% yield). 1H NMR (500 MHz, CDCl3): δ 2.87 (s, 3H), 3.13 (s, 3H), 7.24 (d, J = 8.1 Hz, 1H), 7.29−7.33 (m, 1H), 7.42 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 34.7, 38.0, 127.6, 128.7, 129.4, 131.2, 134.8, 135.3, 167.4. Synthesis of 2,5-Dichloro-N,N-dimethylbenzylamine (3a). A 500 mL one-neck round-bottomed flask equipped with a condenser, a nitrogen purge, and a magnetic stirrer bar was charged with LAH (4.14 g, 109 mmol) and THF (250 mL). To this gray suspension, 2a (23.8 g, 109 mmol) in limited amounts was added. After stirring for 26 h under reflux conditions, the reaction mixture was cooled to room temperature and quenched by the Fieser method.22 Specifically, after continuous additions of deionized water (4 mL), 15% aqueous sodium hydroxide (4 mL), and deionized water (20 mL), and stirring for a while, the resulting light gray precipitate was filtered and washed with THF several times. Then, the filtrate was concentrated in a vacuum, and the residue was purified by silica gel column chromatography (eluent: EtOAc/hexane = 9/1) to afford desired amine 3a (17.3 g, 77% yield) as a light yellow liquid. 1H NMR (500 MHz, CDCl3): δ 2.30 (s, 6H), 3.49 (s, 2H) 7.16 (dd, J = 2.5, 8.4 Hz, 1H), 7.27 (d, J = 8.4 Hz, 1H), 7.46 (d, J = 2.5 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 45.5, 60.4, 128.1, 130.4, 130.5, 132.3, 132.6, 138.4. 3b and 3c were synthesized from 2b and 2c, respectively, in a similar manner to that of 3a. 3,5-Dichloro-N,N- dimethylbenzylamine (3b): colorless liquid (61% yield). 1H NMR (500 MHz, CDCl3): δ 2.24 (s, 6H), 3.36 (s, 2H), 7.22 (d, J = 1.7 Hz, 2H), 7.25 (t, J = 1.7 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 45.3, 63.2, 127.1, 134.7, 142.7. 2,4-Dichloro-N,Ndimethylbenzylamine (3c): light yellow liquid (76% yield). 1H NMR (500 MHz, CDCl3): δ 2.28 (s, 6H), 3.49 (s, 2H), 7.22 (dd, J = 2.1, 8.2 Hz, 1H), 7.370 (d, J = 8.1 Hz, 1H), 7.372 (d, J = 2.1 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 45.4, 60.1, 126.8, 129.1, 131.6, 133.1, 134.8, 135.1. Synthesis of PE-bl-11. ClPS-terminated telechelic oligomers 4 were synthesized according to the literature23 (see Supporting Information). A typical procedure for PE-bl-11 is as follows (X = 4, 3a, m:n = 1:12). A 100 mL three-neck round-bottomed flask equipped with a condenser, a nitrogen purge, a Dean−Stark trap, and a magnetic stirrer bar was charged with oligomer 4 (482 mg, 0.123 mmol), 2,2′bipyridine (600 mg, 3.84 mmol), dichloromonomer 3a (311 mg, 1.52 mmol), toluene (5 mL), and DMAc (10 mL). The mixture was stirred at 170 °C for 2 h to remove water. Then, after removal of toluene and cooling to 80 °C, Ni(cod)2 (1.00 g, 3.64 mmol) was added, and the reaction was continued at the same temperature for 19 h After cooling to room temperature and diluting with additional DMAc (10 mL), the resulting mixture was poured into a 200 mL of concentrated hydrochloric acid to afford the light yellow precipitate. The resulting solid was collected by filtration, washed with deionized water, potassium carbonate aqueous solution, deionized water, and methanol successively and dried at 60 °C under reduced pressure overnight to afford PE-bl-11 as a light yellow solid (423 mg, 84% yield). From 1H NMR spectrum, the ratio of m to n was calculated as 1.0 to 7.6. Preparation of QPE-bl-11. A typical procedure for PE-bl-11 is as follows (X = 4, 3a, m:n = 1:12). A 20 mL vial equipped with a magnetic stirrer bar was charged with obtained PE-bl-11 (359 mg, 0.801 mmol; amount of nitrogen) and DMAc (3.5 mL). To this solution, iodomethane (249 μL, 4.00 mmol) was added. After stirring for 48 h, the reaction mixture was diluted with additional DMAc (3 mL) and poured into a 100 mL of deionized water to afford the light yellow precipitate. The obtained crude product was washed with deionized water several times and dried at 60 °C under reduced pressure overnight to afford QPE-bl-11 in iodide ion form as a light orange solid (428 mg). From the 1H NMR spectrum, the ratio of m to n was calculated as 1.0 to 4.8. The obtained QPE-bl-11 (300 mg) was
known to possess carcinogenic activity and often accompany unfavorable side reactions such as cross-linking. Furthermore, it is rather difficult to achieve high chloromethylation degree (defined as number of chloromethyl groups per phenylene unit). In this article, we report an advanced aromatic copolymer AEMs synthesized from preaminated monomers (three dichlorodimethylbenzylamines) as scaffold for ammonium groups. The hydrophilic components are designed to have high ammonium densities without linkages by heteroatoms such as ether, sulfone, and ketone groups. By using dichlorodimethylbenzylamine monomers, quaternized ammonium groups could be introduced on each phenylene ring (100% degree of quaternization in the hydrophilic component). Sequenced (arylene ether sulfone ketone) units were employed as the hydrophobic block to afford good film forming capability. Synthesis, hydroxide ion conductivity, mechanical and alkaline stability, and platinum-free fuel cell performance of the AEMs are described.
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EXPERIMENTAL SECTION
Materials. 2,5-Dichlorobenzoic acid (>98.0%), 3,5-dichlorobenzoic acid (>98.0%), 2,4-dichlorobenzoic acid (>95.0%), dimethylamine hydrochloride (>99.0%), 4,4′-dichlorodiphenyl sulfone (ClPS, >98.0%), 4,4′-dihydroxybenzophenone (DHBP, >98.0%), and 2,2′bipyridine (>99.0%) were purchased from TCI Co., Ltd., and used as received. Dichloromethane (>99.5%), N,N-dimethylformamide (DMF, >99.5%), trimethylamine (>99.0%), chloroform-d1 with 0.03% TMS (CDCl3, 99.8% D), lithium aluminum hydride (LAH, >92.0%), tetrahydrofuran (THF, >99.5%), sodium hydroxide (>97.0%), N,Ndimethylacetamide (DMAc, >99.0%), toluene (>99.5%), potassium carbonate (>99.5%), bis(1,5-cycloocatadiene)nickel(0) (Ni(cod)2, >95.0%), 35−37 wt % hydrochloric acid, 1,1,2,2-tetrachloroethane (TCE, >97.0%), iodomethane (>99.5%), dimethyl sulfoxide (DMSO, >99.0%), potassium hydroxide (>86.0%), chloroform (>99.0%), Nmethyl-2-pyrrolidone (NMP, >99.0%), ethanol (>99.5%), 2-propanol (>99.7%), dimethyl-d6 sulfoxide with 0.03% TMS (DMSO-d6, 99.9% D), and potassium tetrachloroplatinate(II) (>95.0%) were purchased from Kanto Chemical Co., Inc., and used as received. Oxalyl chloride (>95.0%) was purchased from Wako Pure Chemical Industries, Ltd., and used as received. 1,1,2,2,-Tetrachloroethane-d2 (TCE-d2, 99% D) was purchased from Across Organics and used as received. 50 wt % Ni/C was purchased from Cataler Corporation and used as received. NPC-2000 (Fe−N−C catalyst) was purchased from Pajarito Powder, LLC, and used as received. 60 wt % hydrazine hydrate aqueous solution was purchased from Otsuka-MGC Chemical Company, Inc., and used as received. Synthesis of 2,5-Dichloro-N,N-Dimethylbenzamide (2a). A 500 mL one-neck round-bottomed flask equipped with a magnetic stirrer bar was charged with 2,5-dichlorobenzoic acid (1a; 22.4 g, 117 mmol) and dichloromethane (200 mL). To this suspension was added a solution of oxalyl chloride (16.8 g, 132 mmol) in dichloromethane (66 mL) followed by DMF (10 drops). After stirring for 6 h, full conversion of starting carboxylic acid was confirmed by the 1H NMR spectrum. Next, after cooling with an ice bath, to this acyl halide solution was added dimethylamine hydrochloride (18.9 g, 232 mmol) followed by triethylamine (50 mL, 359 mmol) slowly. After the ice bath was removed, the mixture was stirred at room temperature for a further 24 h and diluted with deionized water. After separation of the two layers, the aqueous layer was extracted with dichloromethane. Then, the combined organic layers were washed with 1 M hydrochloric acid, saturated sodium hydrogen carbonate aqueous solution, and deionized water and concentrated in vacuo. By addition of hexane to the residue, a light brown solid was formed. The resulting solid was filtered, washed with hexane, and dried at 60 °C under reduced pressure to afford desired amide 2a (23.8 g, 93% yield). 1H NMR (500 MHz, CDCl3): δ 2.88 (s, 3H), 3.13 (s, 3H), 7.24−7.31 (m, B
DOI: 10.1021/acs.macromol.6b00408 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of Dichloromonomers 3
Scheme 2. Synthesis of Oligomer 4, PE-bl-11(4)a and QPE-bl-11(4)a
dissolved in DMSO (3 mL) and then cast onto a flat glass plate. Drying the solution at 50 °C gave a QPE-bl-11 membrane (50−60 μm thick). The membrane was treated with 1 M KOH aqueous solution at 40 °C for 48 h. The membrane was washed and soaked in degassed deionized water for 1 day to obtain the QPE-bl-11 membrane in hydroxide ion form. Measurements. 1H and 13C NMR spectra were recorded using CDCl3, TCE-d2, or DMSO-d6 as a solvent. Apparent molecular weights were estimated by gel permeation chromatography (GPC) calibrated with standard polystyrene (PS) samples. DMF containing 0.01 M LiBr was used as eluent. For transmission electron microscopic (TEM) observation, the membranes were ion-exchnaged with tetrachloroplatinate ions. The stained membranes were embedded in epoxy resin, sectioned to 50 nm thickness, and placed on a copper grids. Ion exchange capacity (IEC) was determined by titration. The membrane samples in iodide ion forms were treated with 1 M KOH aqueous solution at 40 °C for 48 h, and the resulting membranes in hydroxide ion forms were neutralized with 1 M hydrochloric acid. The membrane samples in chloride ion forms (ca. 50 mg) were immersed in 12.5 mL of 0.1 M sodium nitrate aqueous solution for 24 h. The amount of Cl− released from the membranes was measured by titration with 0.01 M silver nitrate aqueous solution using potassium chromate as an indicator and sodium hydrogen carbonate as a pH adjuster. The hydroxide ion conductivity of the membranes was measured in degassed and deionized water using a four-probe
conductivity cell attached to an ac impedance spectroscopy system. Water uptake was measured at room temperature. The oxidative stability of the membrane was evaluated by treating the sample with hot Fenton’s reagent (3.5% H2O2 containing 2 ppm FeSO4 at 80 °C) for 8 h. Temperature dependence of the storage moduli (E′), loss moduli (E″), and tan δ (E″/E′) of QPE-bl-11(4)a membranes in iodide ion forms was carried at 60% relative humidity (RH) from room temperature to 95 °C at a heating rate of 1 °C min−1. RH dependence of E′, E″, and tan δ was carried out at 80 °C from ca. 0 to 90% RH at a humidification rate of 1% RH min−1. Details on these measurements are described in the Supporting Information. Preparation of Catalyst-Coated Membrane (CCM) with QPEbl-11(4)a and Fuel Cell Operation. A mixture of 50 wt % Ni/C (0.060 g) and DMF/2-propanol (1/1 by weight, 1.5 g) was sonicated with a TAITEC VP-50 ultrasonic homogenizer for 10 min. A 2.0 wt % QPE-bl-11(4)a (IEC = 1.95 mequiv g−1, 0.33 g as polymer weight) solution in DMF/2-propanol (1/1 by weight) was added to the mixture. The mixture was sonicated for 3 min and stirred with an IKA VIBRAX VXR basic shaker for 15 min. The slurry obtained was sprayed onto one side of QPE-bl-11(4)a membrane (IEC = 1.95 mequiv g−1, 80 μm thick, 16 cm2) at 50 °C to form the anode catalyst layer. The coated area was 4.0 cm2. Then, NPC-2000 (0.050 g) and DMF/2-propanol (1/1 by weight, 2.0 g) were sonicated with a Sharp UT-206 (37 kHz) for 60 min. A 2.0 wt % QPE-bl-11a (IEC = 1.95 C
DOI: 10.1021/acs.macromol.6b00408 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. 1H NMR spectra of (a) oligomer 4 (X = 4), (b) PE-bl-11(4)a, and (c) QPE-bl-11(4)a (m:n = 1:12). mequiv g−1, 1.3 g as polymer weight) solution in DMF/2-propanol (1/ 1 by weight) was added to the mixture and stirred with an IKA VIBRAX VXR basic shaker for 15 min. The slurry obtained was sprayed onto the other side of the membrane in a similar manner to the anode catalyst layer to form the cathode catalyst layer. The loaded amounts of the catalysts were 2.6 mg cm−2 for Ni as the anode and 1.3 mg cm−2 for NPC-2000 as the cathode. The CCM was pressed at 3.3 MPa at room temperature for 30 s. The CCM was treated with 1.0 M KOH aqueous solution at room temperature overnight for the ion exchange to the hydroxide ion form. The CCM was sandwiched by two gas diffusion layer (Freidenburg H1410 carbon cloth for the anode, and GTI carbon cloth with microporous layer for the cathode) and two bipolar plates. Point symmetric serpentine flow fields were used. A fuel cell was operated at 60 °C supplying a mixture of 1.0 M KOH and 10 wt % hydrazine aqueous solution to the anode at a flow rate of 1.3 × current (A) mL min−1 (minimum flow rate was 1.0 mL min−1) and humidified oxygen or air to the cathode at a flow rate of 500 mL min−1. The operating pressure was set at 10 kPa for the cathode.
the desired dichloromonomers 3 in high yields. All these compounds were well-characterized by 1H NMR and 13C NMR spectra (Figures S1−S6). Synthesis of QPE-bl-11. The quaternized block copolymers QPE-bl-11 were synthesized as shown in Scheme 2. Initially, the ClPS-terminated telechelic hydrophobic oligomers 4 were synthesized by nucleophilic substitution polymerization of DHBP and slight excess of ClPS using potassium carbonate as a base in DMAc solution. The chain lengths were controlled by the feed monomer ratios. The chemical structure of 4 was characterized by 1H NMR spectra (Figure 1a and Figure S7). The numbers of repeat units of 4 calculated from the integral ratios in the 1H NMR spectra were in good agreement with those expected from the feed monomer ratios (Table 1). On Table 1. Molecular Weight of Oligomers 4
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RESULTS AND DISCUSSION Synthesis of Dichloromonomers 3. Three types of dichloromonomers 3 in which substitution positions were different were synthesized as shown in Scheme 1. First, commercially available dichlorobenzoic acids 1 were converted to amides 2 via chlorination24 followed by the reaction with dimethylamine hydrochloride in one pot. Then, the resulting amides 2 were reduced by LAH under reflux conditions to give
Xa
Xb
Xc
Mnd (kDa)
Mne (kDa)
Mwe (kDa)
Mw/Mn
2 4 8
2.5 5.1 8.2
3.9 7.1 12.5
1.4 2.5 3.8
1.9 3.3 5.7
4.1 7.3 13.9
2.1 2.2 2.5
a
Calculated from the feed comonomer ratio. bDetermined by 1H NMR spectra. cCalculated from Mn (GPC). dCalculated from X (NMR). eDetermined by GPC analyses (calibration with polystyrene standards). D
DOI: 10.1021/acs.macromol.6b00408 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Properties of PE-bl-11 and QPE-bl-11 PE-bl-11
a
Xa
monomer
m:na
m:nb
yield (%)
Mnc (kDa)
Mwc (kDa)
Mw/Mn
QPE-bl-11 m:nb
2 2 4 4 4 4 4 4 8 8
3a 3a 3a 3a 3a 3b 3c 3c 3a 3a
1:3 1:6 1:6 1:12 1:18 1:6 1:6 1:9 1:12 1:24
1:1.6 1:4.9 1:3.7 1:7.6 1:9.6 1:4.7 1:3.0 1:4.7 1:5.9 1:15.5
90 86 70 84 79 92 83 87 86 70
10.4 18.1 29.4 27.5 19.2 26.5 22.4 11.9 20.6 18.7
77.0 126 148 129 235 262 82.5 44.6 96.9 74.2
7.4 7.0 5.0 4.7 12.2 9.9 3.7 3.8 4.7 4.0
1:1.9 1:4.4 1:3.7 1:5.3 1:9.7 −f 1:3.2 1:3.2 1:5.7 1:9.6
IECb,d (mequiv g−1) 1.19 2.15 1.29 1.62 2.47 1.77 1.09 1.32 1.23 1.81
(1.06)e (2.12) (1.79) (2.56) (2.62) (1.54) (1.15) (−f) (1.36) (1.96)
Calculated from the feed ratio. bDetermined by 1H NMR spectra. cDetermined by GPC analyses (calibrated with polystyrene standards). Calculated as QPE-bl-11 (OH− form). eValues in parentheses were determined by titration. fNot determined.
d
Figure 2. Structure of PE-bl-9 and QPE-bl-9 containing similar hydrophilic structure as that of QPE-bl-11.
dimethyl groups was used for estimation of the composition (ratio of m to n) in PE-bl-11s. When 3a and 3c were used, the composition of these monomers incorporated in the copolymers was relatively low compared with the feed amounts. Only ca. 50−60% of the monomers used were introduced in the copolymers. This was presumably because of the steric hindrance of ortho-substituents in the monomers. On the other hand, for less hindered 3b, the copolymer composition was higher (ca. 80%). Compared with trisubstituted dichloromonomers 3, simple m- and p-dichlorobenzenes were more reactive and the introduction of those monomers was quantitative. In fact, when Ullmann coupling reaction of oligomer 4 and dichlorobenzenes was carried out under the same reaction conditions as for PE-bl-11, the ratio of oligophenylene moieties was the same as the feed ratio (Scheme S1 and Figure S11). While the quaternization of PE-bl-11 was first conducted using iodomethane in DMAc solution, it was revealed that there were big differences in the solubility among the QPE-bl-11s. In the case of PE-bl-11(4)a derived from 3a, the light yellow precipitate was obtained after the addition of the quaternization reaction mixture into a deionized water. Isolated QPE-bl11(4)a could be analyzed by 1H NMR spectrum (Figure 1c), and the quaternization was confirmed from the integration of the methyl groups. The molecular weight measurement of the resulting QPE-bl-11 was unavailable because of the strong interactions with the GPC column. In the case of PE-bl-11(4)b derived from 3b, the reaction with iodomethane in DMAc solution provided gel-like insoluble product. Therefore, PE-bl11(4)b membrane was immersed in 5% (v/v) iodomethane ethanol solution. The resulting QPE-bl-11(4)b was insoluble in both DMSO-d6 and TCE-d2. In the case of QPE-bl-11(4)c derived from 3c, the product could not be recovered because of no precipitation after the addition of the reaction mixture into a deionized water. In this case, the reaction mixture was directly cast onto a flat glass plate. This quaternized QPE-bl-11(4)c membrane was analyzed by 1H NMR spectrum (Figure S12).
the other hand, the numbers of repeat units of 4 calculated from Mn determined by gel permeation chromatography (GPC) analyses (Figure S8, calibrated with polystyrene standards) were higher than those calculated from the comonomer compositions and the 1H NMR spectra, probably because of the rigid-rod-like main chain structure as discussed in our previous report.23 PE-bl-11(4)a was prepared from oligomer 4 (X = 4) and dichloromonomer 3a by nickel-mediated Ullmann coupling reaction.21,23,25−31 First, the effects of solvent were investigated in Ullmann coupling reaction using 3a (Table S1) to find that DMAc gave better results (slightly higher molecular weight of the product with closer copolymer composition to the feed ratio) than DMSO. Then, using DMAc as the solvent, several types of PE-bl-11s were synthesized by changing the reaction conditions, such as chain length of 4 (X = 2, 4, or 8), monomers 3a−c and feed amount (Table 2). As a result, PE-bl11s were obtained in reasonable yields (Table 2). The PE-bl11s obtained were soluble in several organic solvents such as chloroform, TCE, NMP, DMF, and DMAc. The molecular weights of PE-bl-11s determined by GPC were Mn = 10.4−29.4 kDa and Mw = 74.2−262 kDa (Table 2). Polydispersity indexes were relatively high (Mw/Mn = 3.7−12.2). Mw of PE-bl-11s were smaller than those of PE-bl-9s which contained no dimethylaminomethylene groups (Figure 2).21 The results indicate that the dichloromonomers 3 are less reactive in the Ullmann coupling reaction than m- and p-dichlorobenzenes. On the other hand, whereas PE-bl-9s afforded the bimodal peaks in the GPC curves, GPC profiles of PE-bl-11s were unimodal as shown in Figure S9, suggesting the formation of the copolymers with no or little oligomers composed solely of the dichloromonomers 3. The chemical structure of PE-bl-11s was characterized by 1H NMR spectra (Figure 1b and Figure S10). In the 1H NMR spectra, although dimethylamine moiety was confirmed, the peak of benzylic methylene groups was very broad and not suitable for quantitative analyses. Therefore, the peak of E
DOI: 10.1021/acs.macromol.6b00408 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. TEM images of (a) QPE-bl-11(2)a (IEC = 1.19 mequiv g−1), (b) QPE-bl-11(4)a (IEC = 1.29 mequiv g−1), (c) QPE-bl-11(8)a (IEC = 1.23 mequiv g−1), (d) QPE-bl-11(4)b (IEC = 1.77 mequiv g−1), and (e) QPE-bl-11(4)c (IEC = 1.09 mequiv g−1) membranes stained with tetrachloroplatinate ions.
tion and that the morphology was dependent on the hydrophobic block length. The phase separation was larger for QPE-bl-11a(4) than for QPE-bl-9 probably due to the differences in the quaternization procedure; QPE-bl-11 was quaternized in solution and then cast to the membrane, while QPE-bl-9 was quaternized as a membrane.21 It is noticeable that the morphology was not practically affected by the IEC value as confirmed in the three QPE-bl-11(4)a membranes with different IEC values (1.29, 1.62, and 2.47 mequiv g−1; see Figures S14a, S14b, and S14c). Compared to QPE-bl-11(4)a, QPE-bl-11(4)b and QPE-bl-11(4)c membranes having the same number of hydrophobic repeat units (X) exhibited less developed phase separation containing smaller spherical domains (ca. 1−2 nm in diameter, Figures 3d and 3e), implying that the substitution positions of ammonium groups and the main chain in the hydrophilic component affected the self-aggregation properties. Water uptake and hydroxide ion conductivity of the QPE-bl11a, -11b, and -11c membranes were measured at room temperature and 30 °C, respectively, and are plotted in Figure 4 as a function of the IEC. The general trend is that the water uptake and conductivity both increase as increasing IEC value. QPE-bl-11a membranes exhibited higher water uptake than that of QPE-bl-9 membranes, reflecting the above-mentioned morphology. QPE-bl-11b membrane exhibited much lower water uptake and hydroxide ion conductivity than those of QPE-bl-11a membranes presumably because of the different quaternization procedure for QPE-bl-11b membrane (see
Casting from a DMSO solution of QPE-bl-11(4)a derived from 3a afforded yellow, transparent, and bendable membranes (Figure S13). While QPE-bl-11(4)b and QPE-bl-11(4)c membranes (m:n = 1:6) were also transparent and flexible, QPE-bl-11(4)b membrane was orange. QPE-bl-11(4)c with high IEC (1.32 mequiv g−1) was unsuccessful and gave a brittle membrane. The IEC values were also measured by titration, which were comparable to or slightly higher than those estimated from the 1H NMR spectra (Table 2). Morphology and Properties of QPE-bl-11 Membrane. QPE-bl-11 membranes were ion exchanged from iodide ions to tetrachloroplatinate ions for TEM observation. The membranes were reddish brown and somewhat less flexible in PtCl42− forms. Figure 3 shows the cross-sectional TEM images of selected samples of QPE-bl-11a, -11b, and -11c membranes. The dark areas represent hydrophilic domains containing stained (ion-exchanged) ammonium groups, and the bright areas represent hydrophobic domains composed of the polymer main chains. The QPE-bl-11 membranes showed hydrophilic/ hydrophobic phase-separated morphology. While QPE-bl11(2)a membrane contained minute spherical domains (ca. 1−2 nm in diameter, Figure 3a), QPE-bl-11(4)a and QPE-bl11(8)a membranes with longer X exhibited more developed phase separation containing larger hydrophilic domains (ca. 5− 10 nm in width, Figures 3b and 3c). (Note that they share a similar IEC value.) The results suggest that the hydrophilic components were sequenced to a certain extent although the dichloromonomer 3 was used for the semiblock copolymerizaF
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membrane preparation and quaternization procedures, the existence of polar functional groups (sulfone and ketone) and the lack of fluorine groups would be accountable for high water absorbability of QPE-bl-11a membranes. The QPE-bl-11a with longer hydrophobic chain length exhibited higher water absorbability probably because of more developed phase separated morphology. The hydroxide ion conductivity and its dependence on the IEC value were similar between QPE-bl9 and QPE-bl-11a membranes. Compared to QPE-bl-9 membranes which exhibited a jump in the conductivity at λ = ca. 18 (Figure 6), QPE-bl-11a membranes exhibited approx-
Figure 4. (a) Water uptake (room temperature) and (b) hydroxide ion conductivity (30 °C) of QPE-bl-11 and QPE-bl-9 membranes as a function of IEC.
above). The water uptake was then replotted as the number of absorbed water molecules per ammonium group (λ) in Figure 5, which further supports the higher water absorbability of QPE-bl-11a membranes than QPE-bl-9 membranes. The λ value increased as increasing IEC for QPE-bl-11a membranes, while the λ value was nearly constant and independent of IEC for QPE-bl-9 membranes. In addition to the differences in the
Figure 6. Hydroxide ion conductivity of QPE-bl-11 and QPE-bl-9 membranes at 30 °C as a function of λ at room temperature.
imate linear increase of the conductivity to the IEC value. The results imply that the absorbed water was more efficiently utilized for the hydroxide ion conductivity for QPE-bl-9 membranes than for QPE-bl-11a membranes. This discussion is not contradictory to higher water uptake behavior of the QPE-bl-11a membranes (vide supra) derived from less hydrophobic (non-fluorinated) main chain structure. Unlike the water uptake behavior, the hydroxide ion conductivity seemed less dependent on the hydrophobic chain length. The hydroxide ion conductivity of the QPE-bl-11 membranes showed Arrhenius-type temperature dependence at least up to 80 °C as shown in Figure 7. The apparent activation energies calculated from the slopes were ca. 12−17 kJ mol−1, parallel to the reported values for AEMs,14,15,30,32,33 indicating that hydrated hydroxide ions are involved in the ion conducting mechanism. The QPE-bl-11(4)a membrane (IEC = 1.62 mequiv g−1) showed high conductivity comparable to that of QPE-bl-9 membrane with similar IEC (1.76 mequiv g−1). The hydroxide ion conductivity of QPE-bl-11a membranes increased with increasing IEC value, and the QPE-bl-11(4)a with the highest IEC (2.47 mequiv g−1) achieved a value of 130 mS cm−1 at 80 °C. This conductivity is among the highest for aromatic polymer based anion exchange membranes. One of the major problems associated with anion exchange membranes is a poor alkaline stability. The alkaline stability of the QPE-bl-11(4)a (IEC = 1.29, 1.62, and 2.47 mequiv g−1, I−
Figure 5. λ of QPE-bl-11 and QPE-bl-9 membranes as a function of IEC. λ values were calculated from the data in Figure 4a and IEC values. G
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slight chemical and morphological degradations could have caused significant loss in the hydroxide ion conductivity. In contrast, 1H NMR spectrum of the higher IEC (2.47 mequiv g−1) QPE-bl-11(4)a membrane showed decrease in methyl peaks of the trimethylammonium groups (Figure S17). From the 1H NMR spectrum in Figure S17, it was estimated that ca. 28% of the ammonium groups was lost. This severer chemical degradation of the higher IEC membrane must be related with higher water uptake and swelling. It should be noted that QPE-bl-11(4)a membranes (1.62 and 2.47 mequiv g−1) showed higher hydroxide ion conductivity than that of QPE-bl-9 membrane (1.97 mequiv g−1) after 500 h alkaline stability test. Oxidation stability is critical for anion exchange membranes to the applications in fuel cells. As an accelerated test, QPE-bl11(4)a (IEC = 1.95 mequiv g−1) was treated with hot Fenton’s reagent. After 8 h, the post-test membrane retained its shape and bendability. The remaining weight, IEC, and hydroxide ion conductivity were 105%, 95%, and 96%, respectively, indicating high oxidative stability of the QPE-bl-11 membrane. As a mechanical stability test, dynamic mechanical analysis (DMA) was carried out for the membranes in iodide ion form. The storage moduli (E′), loss moduli (E″), and tan δ (E″/E′) of QPE-bl-11(4)a membranes in I− forms was measured at 60% RH as a function of temperature up to 95 °C (Figure 9). The storage moduli and loss moduli of QPE-bl-11(4)a membranes were nearly constant from room temperature to 95 °C. The IEC seemed a minor factor to affect these properties. Compared to QPE-bl-9 membrane, QPE-bl-11 membranes exhibited slightly higher storage and loss moduli. Three QPEbl-11(4)a membranes did not show detectable peaks in the E″and tan δ curves, implying that they did not exhibit a glass transition behavior under the tested conditions. Humidity dependence of E′, E″, and tan δ of QPE-bl-11(4)a was also measured at 80 °C (Figure S19). Similar to the temperature dependence, QPE-bl-11a membranes showed higher storage and loss moduli than those of QPE-bl-9 membrane. The loss moduli decreased as increase in the humidity for all the membranes due to the increased water absorption. An MEA (membrane electrode assembly) was prepared using QPE-bl-11(4)a (IEC = 1.95 mequiv g−1) as membrane and electrode binders (note that this copolymer was specially prepared in a larger scale for fuel cell experiments and thus is not included in Table 2 and the above discussion). The fuel cell was operable with hydrazine aqueous solution as a fuel at 60 °C (Figure 10). The open circuit voltage (OCV) was relatively high at 0.78 V (with air) and 0.79 V (with O2), indicating low hydrazine permeability of the membrane. The fuel cell exhibited the maximum power density of 380 mW cm−2 at a current density of 1020 mA cm−2 with O2 and 172 mW cm−2 at 452 mA cm−2 with air. The ohmic resistance was ca. 200 mΩ cm2 for both conditions, which was higher than that (100 mΩ cm2) calculated from the hydroxide ion conductivity (80 mS cm−1 at 60 °C in water) and the thickness (80 μm) of the membrane, presumably because of the contact resistance between QPE-bl-11(4)a membrane and the catalyst layers. The results imply that the optimization of the CCM preparation method could result in better fuel cell performance.
Figure 7. Temperature dependence of hydroxide ion conductivities of QPE-bl-11 derived from 3a and QPE-bl-9 membranes in water.
form) membranes were evaluated at 60 °C in 1 M KOH aqueous solution (Figure 8). The conductivity of lower IEC
Figure 8. Change of ion conductivity of QPE-bl-11(4)a (IEC = 1.29, 1.62, and 2.47 mequiv g−1) and QPE-bl-9 (IEC = 1.97 mequiv g−1) membranes at 60 °C in 1 M KOH aqueous solution. The membranes were used in iodide ion form to avoid the effect of CO2 from the air.
(1.29 mequiv g−1) QPE-bl-11(4)a membrane jumped from 1.8 to 68 mS cm−1 within 24 h due to the ion exchange reaction from iodide ion to more conductive hydroxide ion. Then, the conductivity decreased gradually with time and maintained 23 mS cm−1, 34% of the peak conductivity, after 1000 h. The posttest membrane was not so ductile as the pristine membrane. Since there were only minor changes confirmed in the 1H NMR spectrum and TEM image (Figures S15 and S18), the degradation during the alkaline stability test involved the main chain scission and decomposition of the ammonium groups. The same tendency was also observed for the middle IEC (1.62 mequiv g−1) QPE-bl-11(4)a membrane. In this case, the conductivity dropped after 500 h, and only 1.6% of the peak conductivity was maintained after 1000 h. However, the 1H NMR spectra of the post-test QPE-bl-11(4)a membranes did not imply serious chemical degradation (Figure S16). These results indicate that the lack of heteroatom linkages in the hydrophilic components improved the alkaline stability and that
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CONCLUSIONS We have developed a series of quaternized aromatic copolymers (QPE-bl-11s) from chlorine-terminated oligo(arylene ether H
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Figure 10. (a) Cell voltage, power density, and (b) ohmic resistance as a function of current density of direct hydrazine fuel cell with QPE-bl11(4)a as membrane and electrode binders at 60 °C.
water. In addition, it was revealed that QPE-bl-11(4)a membranes were more stable than QPE-bl-9 membrane under alkaline stability test. Lack of heteroatoms linkages contributed to improving the chemical stability of the polymer main chains, however, trimethylbenzylammonium groups decomposed under harsh alkaline conditions. A platinum-free fuel cell with hydrazine as a fuel, the QPE-bl-11(4)a as a membrane, and electrode binder was successfully operated. The maximum power density of 380 mW cm−2 was achieved at a current density of 1020 mA cm−2 with O2.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 9. DMA of QPE-bl-11(4)a (IEC = 1.29, 1.62, and 2.47 mequiv g−1) and QPE-bl-9 (IEC = 1.97 mequiv g−1) membranes at 60% RH.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00408. 1 H and 13C NMR spectra of compounds 2 and 3, 1H NMR spectra and GPC profiles of oligomer 4, 1H NMR spectra of PE-bl-11 and QPE-bl-11, TEM images of QPE-bl-11(4)a, DMA of QPE-bl-11(4)a and QPE-bl9 membranes at 80 °C as a function of RH (PDF)
sulfone ketone)s and three types of dimethylamine-containing dichloromonomers 3 in which substitution positions were different. Bendable membranes were obtained from the copolymers of well-controlled chemical structure and high ammonium density. From the TEM observation, it was found that the hydrophilic components of QPE-bl-11a were sequenced to a certain extent although the dichloromonomer 3a was used for the semiblock copolymerization and that the morphology was dependent on the hydrophobic block length rather than the IEC value. The water uptake and hydroxide ion conductivity of QPE-bl-11a membranes both increased as increasing IEC value. This trend was different from that of QPE-bl-9 membranes containing similar hydrophilic structure but less ammonium density. The high local ion concentration (or high ammonium density) of QPE-bl-11a contributed to high hydroxide ion conductivity. As a result, QPE-bl-11(4)a membrane with highest IEC (2.47 mequiv g−1) exhibited significantly high conductivity of 130 mS cm−1 at 80 °C in
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel +81 552208707; Fax +81 552208707 (K.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by CREST, Japan Science and Technology Agency (JST) and the Ministry of Education, I
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Culture, Sports, Science and Technology (MEXT) Japan through a Grant-in-Aid for Scientific Research (26289254).
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