Article pubs.acs.org/Macromolecules
Aromatic Copolymers Containing Ammonium-Functionalized Oligophenylene Moieties as Highly Anion Conductive Membranes Naoki Yokota,†,∥,⊥ Manai Shimada,†,∥,⊥ Hideaki Ono,†,⊥ Ryo Akiyama,‡,⊥ Eriko Nishino,⊥,# Koichiro Asazawa,⊥,# Junpei Miyake,‡,⊥ Masahiro Watanabe,‡ and Kenji Miyatake*,‡,§,⊥ †
Interdisciplinary Graduate School of Medicine and Engineering, ‡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 ⊥ Japan Science and Technology Agency, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan # Frontier Technology R&D Division, Daihatsu Motor Co. Ltd., 3000 Ryuo, Gamo, Shiga 520-2593, Japan S Supporting Information *
ABSTRACT: The synthesis and properties of anion conductive aromatic copolymers containing oligophenylene moieties as a scaffold for quaternized ammonium groups are reported. Our new hydrophilic components consist of a chemically robust oligophenylene main chain modified with a high density of ionic groups. A partially fluorinated oligo(arylene ether) was employed as a hydrophobic block. The targeted copolymers (QPE-bl-9) were synthesized via nickel-mediated coupling polymerization, followed by chloromethylation, quaternization, and ion exchange reactions. QPE-bl-9 provided tough, bendable membranes by solution casting. The resulting membrane with the highest ion exchange capacity (IEC = 2.0 mequiv g−1) exhibited high hydroxide ion conductivity (138 mS cm−1) in water at 80 °C. Reasonable alkaline stability of QPE-bl-9 membrane was confirmed in 1 M KOH aqueous solution for 1000 h at 40 °C. A noble metal-free fuel cell with QPE-bl-9 used as the membrane and electrode binder was successfully operated. A maximum power density of 510 mW cm−2 was achieved at a current density of 1.20 A cm−2 with hydrazine as the fuel and O2 as the oxidant.
■
INTRODUCTION Fuel cells are regarded as highly efficient energy devices due to their direct conversion of chemical energy to electricity.1 Among several types of fuel cells, polymer electrolyte fuel cells (PEFCs) using proton exchange membranes (PEMs) are operable at moderate temperatures and are thus suitable for applications in cogeneration system, portable devices, and electric vehicles.2 However, the use of strongly acidic PEMs require expensive, exhaustible precious metals (Pt) as electrocatalysts. Replacing PEMs with anion exchange membranes (AEMs) solves this problem in principle and enables the use of abundant transition metals such as Ni, Co, and Fe as electrocatalysts.3−5 Moreover, it is well-known that the kinetics of the oxygen reduction reaction are more favorable under alkaline conditions. In the past decade, there have been a number of attempts to develop highly anion conductive, chemically stable AEMs.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 revealed that multiblock poly(arylene ether)s containing a high density of ammonium groups in hydrophilic blocks exhibited a high hydroxide ion conductivity, 144 mS cm−1, at 80 °C in water.13 However, there appear to be stability issues for © XXXX American Chemical Society
ammonium-functionalized poly(arylene ether)s under alkaline conditions. Kim et al.14 and Ramani et al.15 reported that phenylene ether groups are vulnerable to attack by hydroxide ions, leading to main chain degradation of the poly(arylene ether)s. To improve the chemical stability, Hickner et al.16 proposed that comb-shaped aromatic copolymers containing long alkyl side chains pendant to the nitrogen-centered cationic group are highly stable in alkaline solution. Holdcroft et al.17,18 reported that AEMs based on sterically hindered polybenzimidazolium possessed high hydroxide ion conductivity and stability. In this paper, we report an advanced version of our aromatic copolymer AEMs, which contain oligophenylene moieties as a scaffold for ammonium groups. The hydrophilic components are designed to have high ammonium group densities without hetero linkages such as ether, sulfone, or ketone groups so as to achieve high hydroxide ion conductivity and chemical stability. The hydrophobic blocks used were partially fluorinated oligo(arylene ether)s in order to provide good film forming capability. Received: September 25, 2014 Revised: November 19, 2014
A
dx.doi.org/10.1021/ma5019878 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
several times. After drying in a vacuum oven at 60 °C for overnight, QPE-bl-9 membrane (in chloride form) was obtained. Then, the membrane was treated with 1 M potassium hydroxide aqueous solution at room temperature for 48 h. The membrane was washed and soaked in degassed pure water for 1 day to obtain the hydroxide form of the QPEbl-9 membrane. Measurements. 1H and 19F NMR spectra were recorded using CDCl3, TCE-d2, or DMSO-d6 as a solvent. Apparent molecular weights were measured with gel permeation chromatography. DMF containing 0.01 M lithium bromide was used as eluent. For transmission electron microscopic (TEM) observation, the membrane was stained with tetrachloroplatinate ions. The stained membrane was embedded in epoxy resin, sectioned to 50 nm thickness, and placed on a copper grid. Hydroxide ion conductivity of the membranes was measured in degassed, deionized water using a four-probe conductivity cell attached to an ac impedance spectroscopy system. Water uptake was measured at room temperature. Dynamic mechanical analysis (DMA) of the membranes was carried out at 60% relative humidity (RH) from room temperature to 95 °C at a heating rate of 1 °C min−1. Details on these measurements were described in our previous paper.20 Preparation of Catalyst Coated Membrane (CCM). A mixture of NiZn powder (0.15 g), DMF (1.5 mL), and 2-propanol (0.82 mL) was sonicated with a TAITEC VP-50 ultrasonic homogenizer for 10 min. A 2 wt % CMPE-bl-9 (IEC = 1.2 mequiv g−1, 0.83 g as polymer weight) solution in DMF/2-propanol (1/1 by weight) was added to the mixture, which 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 a CMPE-bl-9 membrane (IEC = 1.2 mequiv g−1, 16.0 cm2) at 55 °C to form the anode catalyst layer. The coated area was 7.3 cm2. Then, iron phenanthroline (0.050 g), DMF (0.91 mL), and 2-propanol (1.1 mL) were mixed with an ITOH LA-PO.1 ball mill at 200 rpm for 2 h and sonicated for 60 min. A 2 wt % CMPE-bl-9 (IEC = 1.2 mequiv g−1, 0.84 g as polymer weight) solution in DMF/2-propanol (1/1 by weight) was added to the mixture, which was sonicated for 3 min. A 60 wt % polytetrafluoroethylene dispersion (Daikin, D-210C, 0.0034 g) was added to the mixture and stirred for 15 min. The slurry obtained was sprayed onto the other side of the membrane, in a similar way to that described above, to form the cathode catalyst layer. The loaded amounts of the catalysts were 2.6 mg cm−2 for NiZn as the anode and 1.0 mg cm−2 for iron phenanthroline as the cathode. The catalyst-coated membrane (CCM) was pressed at 13 MPa at room temperature for 30 s. Fuel Cell Operation. The CCM was treated with 30 wt % trimethylamine aqueous solution at room temperature for 48 h for quaternization of the membrane and binder and with 1.0 M potassium hydroxide aqueous solution overnight for the ion exchange to the hydroxide ion form. The quaternized CCM was mounted in a single cell with a gas diffusion layer (anode: cloth, Zoltek; cathode: cloth, Zoltek with microporous layer) and a gasket. A fuel cell was operated at 80 °C, supplying a mixture of 1.0 M potassium hydroxide and 5.0 wt % hydrazine aqueous solution to the anode at a flow rate of 2 mL min−1 and humidified oxygen or air (26% RH) to the cathode at a flow rate of 500 mL min−1. The operating pressure was set at 20 kPa for both electrodes. Point symmetric serpentine flow fields were used for both electrodes.
Synthesis, hydroxide ion conductivity, mechanical and alkaline stability, and fuel cell performance of the AEMs are described.
■
EXPERIMENTAL SECTION
Materials. Decafluorobiphenyl (DFBP, >98.0%, TCI Inc.), hexafluorobisphenol A (HFBPA, >98.0%, TCI Inc.), 4-chlorophenol (CP, >98.0%, TCI Inc.), 1,4-dichlorobenzene (>99.0%, TCI Inc.), 1,3dichlorobenzene (>98.0%, TCI Inc.), potassium carbonate (>99.5%, Kanto Chemical), 2,2′-bipyridine (>99.0%, TCI Inc.), bis(1,5cyclooctadiene)nickel(0) (Ni(cod)2, >95.0%, Kanto Chemical), chloromethyl methyl ether (CMME, >94.0%, Kanto Chemical), 0.5 mol L−1 zinc chloride tetrahydrofuran solution (Sigma-Aldrich), 45 wt % trimethylamine aqueous solution (Sigma-Aldrich), methanol (>99.8%, Kanto Chemical), 35−37 wt % hydrochloric acid (Kanto Chemical), and potassium hydroxide (>86.0%, Kanto Chemical) were commercial products and used as received. Dimethyl sulfoxide (DMSO, >99.0%, Kanto Chemical), N-methyl-2-pyrrolidinone (NMP, >99.0%, Kanto Chemical), N,N-dimethylacetamide (DMAc, >99.0%, Kanto Chemical), toluene (>99.5%, Kanto Chemical), and 1,1,2,2-tetrachloroethane (TCE, >97.0%, Kanto Chemical) were dried over 4 Å molecular sieves (Kanto Chemical) prior to use. Chloroform-d1 with 0.03% TMS (CDCl3, 99.8 atom % D, Acros Organics), 1,1,2,2-tetrachloroethane-d2 (TCE-d2, 99 atom % D, Acros Organics), dimethyl-d6 sulfoxide with 0.03% TMS (DMSO-d6, 99.9 atom % D, Acros Organics), N,Ndimethylformamide (DMF, >99.7%, Kanto Chemical), anhydrous lithium bromide (>95.0%, Kanto Chemical), and potassium tetrachloroplatinate(II) (>95.0%, Kanto Chemical) were commercial products and used as received. Synthesis of CP-Terminated Telechelic Oligomers 2. A typical procedure is as follows (X = 4). DFBP-terminated telechelic oligomers 1 were synthesized according to the literature.19,20 A 100 mL roundbottomed flask equipped with a mechanical stirrer and a nitrogen inlet/ outlet was charged with oligomer 1 (3.0 g, 0.89 mmol), CP (0.29 g, 2.2 mmol), and DMAc (30 mL). The mixture was heated up to 50 °C to obtain a homogeneous solution. After cooling to room temperature, potassium carbonate (0.43 g, 3.0 mmol) was added to the mixture. The reaction was carried out at 40 °C for 3 h. The mixture was cooled to room temperature and poured dropwise into a large excess of pure water to precipitate a white powder. The crude product was washed with hot, ultrapure water and hot methanol several times. After drying in a vacuum oven at 60 °C for overnight, oligomer 2 was obtained in 87% yield. Synthesis of PE-bl-9. A typical procedure is as follows (X = 4, p:q:r = 1:2:8). A 100 mL round-bottomed flask equipped with a mechanical stirrer and a nitrogen inlet/outlet was charged with oligomer 2 (0.60 g, 0.14 mmol), 1,4-dichlorobenzene (0.040 g, 0.27 mmol), 1,3dichlorobenzene (0.16 g, 1.1 mmol), 2,2′-bipyridine (0.57 g, 3.6 mmol), and DMAc (20 mL). After heating at 80 °C to obtain a homogeneous solution, Ni(cod)2 (1.0 g, 3.6 mmol) was added to the solution. After heating at 80 °C for 3 h, the mixture was cooled to room temperature and diluted with additional DMAc (10 mL). The mixture was poured dropwise into a large excess of diluted hydrochloric acid to precipitate a white powder. The crude product was washed with hot ultrapure water and hot methanol several times. After drying in a vacuum oven at 60 °C for overnight, PE-bl-9 was obtained in 92% yield. Preparation of QPE-bl-9 Membranes. A typical procedure is as follows (CMPE-bl-9 (X = 4, p:q:r = 1:2:8)). A 100 mL pressure bottle equipped with a magnetic stirring bar was charged with PE-bl-9 (0.60 g, 0.080 mmol), 0.5 M zinc chloride tetrahydrofuran solution (2.5 mL, 1.3 mmol), CMME (16 mL, 0.20 mol), and TCE (27 mL) in a glovebox under argon. After heating at 80 °C for 120 h, the mixture was cooled to room temperature and diluted with additional TCE. The mixture was poured dropwise into a large excess of methanol to precipitate a white powder. The crude product was washed with methanol several times. After drying in a vacuum oven at 60 °C for overnight, CMPE-bl-9 was obtained. The CMPE-bl-9 (0.50 g) obtained was dissolved in TCE (5.0 mL) and cast onto a flat glass plate. Drying the solution at 50 °C gave a membrane (60−90 μm thick). The membrane was immersed in 45 wt % trimethylamine aqueous solution at room temperature for 48 h. The membrane was washed with 1 M hydrochloric acid and ultrapure water
■
RESULTS AND DISCUSSION
Synthesis of Model Oligomers. The targeted quaternized aromatic copolymers, QPE-bl-9, consist of fluorinated arylene ether-based hydrophobic segments and rigid, phenylene-based hydrophilic segments. Since oligo- and poly(p-phenylene)s are highly symmetric and thus show very limited solubility in common organic solvents, we first investigated the cooligomerization of 1,4-dichlorobenzene with 1,3-dichlorobenzene via an Ni-catalyzed cross-coupling reaction to tune the solubility and the molecular weight of the oligophenylene compounds (Scheme S1). The oligomerization reactions were carried out with varying feed comonomer composition (x = m/ (m + n) × 100 mol %). While the oligomer obtained with x = 20 was soluble in TCE, other feed comonomer compositions gave B
dx.doi.org/10.1021/ma5019878 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Scheme 1. Synthesis of Oligomers 1 and 2, PE-bl-9, CMPE-bl-9, and QPE-bl-9
oligomers sparingly soluble in TCE (Table S1). A high molecular weight (Mw = 1.2 kDa, Mn = 3.3 kDa) oligomer with reasonable polydispersity (2.8) was obtained with x = 20. The oligomers contained a somewhat higher composition of 1,4-phenylene moieties than that of the feed composition, as suggested by 1H NMR spectra, presumably due to the higher reactivity of 1,4dichlorobenzene than 1,3-dichlorobenzene under the inves-
tigated conditions. From these results, we have determined that it was best to maintain x = 20 for the subsequent polymerization reactions. Synthesis of QPE-bl-9. The quaternized block copolymers, QPE-bl-9, were synthesized as shown in Scheme 1. Nucleophilic substitution polycondensation of DFBP and HFBPA was carried out for 1 using potassium carbonate as a base in DMAc solution. C
dx.doi.org/10.1021/ma5019878 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Figure 1. 1H NMR spectra of (a) oligomer 1 (X = 4), (b) oligomer 2 (X = 4), and (c) PE-bl-9 (X = 4, p:q:r = 1:2:8) in CDCl3 at room temperature.
The chain lengths and terminal groups were controlled with feed comonomer composition. The chemical structure of 1 was characterized by 1H and 19F NMR spectra, in which all peaks were well-assigned to the intended chemical structure (Figure 1a and Figure S3a). Detailed discussion on the synthesis and characterization of 1 can be found in our previous report.19 The numbers of repeat units of 1 were estimated from the 19F NMR spectra and GPC analyses (Figure S4, calibrated with polystyrene standards), which were in fair agreement with those expected from the feed comonomer ratios (Table 1). Precursor oligomers 2 for the hydrophobic blocks were prepared by end-capping 1 with CP under reaction conditions similar to those for 1. The end-capping reactions were carried out at low temperature (40 °C) in order to selectively favor reactions at the terminal fluorine groups. The chemical structure of 2 was characterized by 1H and 19F NMR spectra (Figure 1b and Figure
Table 1. Molecular Weight of Oligomers 1 and 2 1
2
xa
xb
xc
Mnd (kDa)
Mwd (kDa)
Mw/Mn
3 4 7 3 4 7
3.7 4.8 5.7 4.2 5.9 5.7
4.7 5.8 6.9 4.2 5.5 6.7
3.3 4.0 4.7 3.2 4.0 4.8
5.6 7.2 8.6 5.6 6.9 8.4
1.7 1.8 1.8 1.7 1.7 1.7
a
Calculated from the feed comonomer ratio. bDetermined by 19F NMR spectra. cCalculated from Mn. dDetermined by GPC analyses (calibrated with polystyrene standards).
S3b). In the 1H NMR spectra of 2, new peaks (peaks 3 and 4) appeared at 7.3 and 7.0 ppm assignable to the terminal chlorophenoxy groups. In the 19F NMR spectra of 2, no peaks D
dx.doi.org/10.1021/ma5019878 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
chloromethyl groups per phenylene ring in the oligophenylene moieties) was estimated from the 1H NMR spectra (Table 2). The DC values were 0.8 or 0.9, meaning that the CMPE-bl-9 obtained had nearly one chloromethyl group per phenylene ring in the hydrophilic segments. In the GPC profiles of CMPE-bl-9, the peak at longer retention time (lower molecular weight portions), which was observed for the parent PE-bl-9, as mentioned above, disappeared since chloromethylated oligophenylene compounds were soluble in methanol and were removed readily during the purification process (Figure S5). Casting from a TCE solution of CMPE-bl-9 provided colorless, transparent, and bendable membranes. CMPE-bl-9 membranes were quaternized by treating with trimethylamine aqueous solution. The QPE-bl-9 membranes obtained were brown, translucent, and less flexible compared with the CMPEbl-9 membranes. QPE-bl-9 was slightly soluble in DMSO, and recasting from solution was not available. In the 1H NMR spectra of QPE-bl-9, new peaks (peak 11) appeared at 2.9 ppm assignable to the methyl groups on the quaternary ammonium groups (Figure 2b). Peaks 8 and 8′ shifted to lower magnetic field compared to those of CMPE-bl-9 and were broad singlets. These results indicate complete quaternization of the chloromethyl groups. The molecular weight of QPE-bl-9 could not be measured because of a strong interaction with our GPC columns. Properties of QPE-bl-9 Membranes. Figure 3 shows crosssectional TEM image of QPE-bl-9 membrane (in tetrachloroplatinate form), in which the dark areas represent ionic clusters composed of ammonium tetrachloroplatinate groups and the bright areas represent hydrophobic domain composed of the polymer main chains. QPE-bl-9 membrane exhibited phaseseparated morphology despite its random structure of the hydrophilic component. Compared with our block copolymer QPE-bl-5 membrane (Figure 4),20 which shares the same hydrophobic blocks with QPE-bl-9 but contains ether linkages in the hydrophilic blocks, the domain size of QPE-bl-9 membrane was smaller (
