Elastic Long-Chain Multication Cross-Linked Anion Exchange

Apr 3, 2017 - With excellent properties, such as high proton conductivity, high mechanical properties, and good chemical stability, Nafion has been de...
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Elastic Long-Chain Multication Cross-Linked Anion Exchange Membranes Juanjuan Han,†,‡ Liang Zhu,† Jing Pan,*,†,‡ Tawanda J. Zimudzi,† Ying Wang,‡ Yanqiu Peng,‡ Michael A. Hickner,*,† and Lin Zhuang‡ †

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: Anion exchange membranes (AEMs) are a promising class of materials that enable non-noble metals to be used as catalysts in fuel cells. Compared to their acidic counterparts, typically Nafion and other perfluorosulfonatebased membranes, the low OH− conductivity in AEMs remains a concern as these materials are developed for practical applications. Cross-linked macromolecular structures are a popular way to optimize the trade-off between the ionic conductivity and the water swelling of AEMs with high ion exchange capacities (IECs). However, common cross-linked AEMs (e.g., x(QH)QPPO) that have high degrees of cross-linking with low molecular weight between cross-links are usually mechanically brittle. Moreover, the cross-links in AEMs can hinder the transport of OH−, leading to unsatisfactory conductivities. Here we report a series of elastic and highly conductive poly(2,6-dimethylphenylene oxide) (PPO)-based AEMs (x(QH)3QPPO) containing flexible, long-chain, multication cross-links. The strength and flexibility of the x(QH)3QPPO samples are significantly improved as compared to the conventional x(QH)QPPO membranes and multication un-cross-linked materials reported previously. The high conductivities in these new materials (x(QH)3QPPO-40, IEC = 3.59 mmol/g, σOH− = 110.2 mS/cm at 80 °C) are attributed to the distinct microphase separation observed in the x(QH)3QPPO membranes by SAXS and TEM analyses. Furthermore, the x(QH)3QPPO samples exhibit good dimensional (swelling ratio of x(QH)3QPPO-40 is 25.0% at 80 °C) and chemical (22% and 25% decrease in IEC and OH− conductivity in 1 M NaOH at 80 °C for 30 days, respectively) stabilities, making this cross-linking motif suitable for potential membrane applications in fuel cells and other electrochemical devices.

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INTRODUCTION Modern fuel cell and electrochemical reactor technology is replacing conventional liquid electrolyte solutions with solid polymer electrolytes (SPEs) to yield higher power density devices, simplified operation, and easier maintenance compared to conventional flooded cells.1−4 The most popular SPEs currently used are Nafion, a perfluorinated sulfonic acid (PFSA)-based membrane, and other PFSA variants. With excellent properties, such as high proton conductivity, high mechanical properties, and good chemical stability, Nafion has been demonstrated in many types of devices with excellent performance and robust long-term durability.5−7 Albeit the archetypical material, the strongly acidic nature of Nafion and other PEMs restrict highly active and stable catalysts to just several noble metals and alloys, principally those containing platinum. The scarcity and the high cost of these catalysts and the potential environmental impacts of perfluorinated polymers have become major obstacles preventing the widespread application of PEMFCs.8−12 Using non-platinum group metal catalysts that are efficient and stable in acidic environments13,14 © 2017 American Chemical Society

and employing chemically stable aromatic proton-conducting membranes15−17 are possible solutions to alleviate these problems with current PEMFC materials, but such efforts on new catalysts and membranes remain in the initial stages of research for future PEMFC systems. As an alternative to acidic systems, anion exchange membranes (AEMs) that operate at high pH have emerged as promising materials in the pursuit of lower-cost, nextgeneration materials for fuel cells. AEMs create an alkaline environment in the device where many nonprecious metal and nonmetal catalysts are stable, including low-cost metals and their oxides.18−22 Usually, AEMs are composed of rigid backbone polymers, such as polysulfone,23−25 poly(phenylene oxide),26−29 polystyrene,30−32 poly(ether ketones),33−36 or poly(phenylene),37 and employ cationic groups, such as quaternary ammonium (QA),38,39 imidazolium,40,41 phosphoReceived: June 1, 2016 Revised: July 21, 2016 Published: April 3, 2017 3323

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Figure 1. Preparation of (a) quad-cation cross-linked QPPO, x(QH)3QPPO, (b) quaternized poly(2,6-dimethylphenylene oxide) (QPPO), and (c) double-cation cross-linked QPPO, x(QH)QPPO.

nium,9 or guandinium,42,43 as the backbone-tethered organic base. Compared to their acidic PEM counterparts, AEMs generally display lower ionic conductivity. The intrinsic lower mobility of OH− and the less-developed hydrophilic/hydrophobic microphase-separated morphology of aromatic polymers compared to perfluorinated structures naturally lead to lower ionic conductivity and higher activation energies for ion conduction in AEMs compared to well-known PEM systems.44−48 Increasing the ion exchange capacity (IEC) of the material is a common strategy to increase the ion conductivity of AEMs. However, AEMs with high IECs always exhibit high water uptake and high swelling, which is a detriment to their application in fuel cells and other devices where mechanically robust, dimensionally stable materials are desired.24,47 To restrain the membrane swelling, various cross-linking strategies have been used in AEM systems.49−54 Among them, ditertiary amines (DTA), especially N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), is a popular reagent to produce crosslinked AEMs. TMHDA reacts with the benzylic halide groups on the polymer backbone, producing both cations and crosslinks in AEMs.55−57 Previous studies have shown that AEMs cross-linked by TMHDA exhibited lower water swelling than un-cross-linked samples, and the quaternary ammonium groups linked by the hexyl chain were more chemically stable than bare benzyltrimethylammonium (BTMA) cations.57,58 However, TMHDA cross-linked AEMs showed low OH− conductivity, and even with high IECs (above 1.5 mmol/g), their OH− conductivities (around 10 mS/cm at room temperature) were significantly lower than the proton conductivity of Nafion (∼100 mS/cm in liquid water at 30 °C).58,59 In addition to lower ion conductivity, we also found that the flexibility and toughness of such cross-linked AEMs was unsatisfactory. While TMHDA cross-linked AEMs have low swelling due to their high degree of cross-linking (DC), the membranes were brittle

and were difficult to integrate as separators in electrochemical devices. To circumvent brittleness in polymeric AEMs, we were inspired by the successful example of elastic gels and networks in which long, flexible cross-links are often employed to improve the mechanical strength and flexibility of the materials.60,61 Our plan was to design a long, flexible DTA linker as the cross-linking reagent to form elastic, cross-linked PPO-based AEMs. Simultaneously, to guarantee high IEC materials, multiple cations were introduced into the cross-links. Specifically, a long-chain, multication, cross-linked AEM with four cations (Figure 1a, x(QH)3QPPO) on each cross-link chain was developed. Since the un-cross-linked multication AEMs studied previously in our group62 showed hydrophilic/ hydrophobic phase separation and enhanced ion conduction, but poor mechanical properties, the proposed cross-linked structure is expected to exhibit similar phase separation and ion conductivity effects, while simultaneously demonstrating enhanced mechanical properties compared to our previously reported materials. To demonstrate the performance improvement of the x(QH)3QPPO film, two other AEMs, i.e., un-crosslinked quaternized PPO (Figure 1b, QPPO) and TMHDA cross-linked PPO (Figure 1c, x(QH)QPPO), were prepared to serve as control samples.

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EXPERIMENTAL SECTION

Materials. Poly(2,6-dimethylphenylene oxide) (PPO powder, Mw (typical) ∼ 30 000 g/mol, Mn (typical) ∼ 20 000 g/mol) was purchased from Sigma-Aldrich (St. Louis, MO) and dried under vacuum at room temperature overnight. N-Bromosuccinimide (NBS), 2,2′-azobis(2-methylpropionitrile) (AIBN), 1,6-dibromohexane, N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), and trimethylamine (TMA, 33% w/w in ethanol) were obtained from Sigma-Aldrich and used as received. The solvents and other chemicals used in this work were obtained from VWR International (Radnor, PA) and used as received. 3324

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Macromolecules Synthesis and Characterization of 1,18-(N′,N′-Dimethylamino)-6,12-(N,N-dimethylammonium)octodecane Bromine. N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHDA, 5.25 mL) and 1,6-dibromohexane (0.63 mL) were dissolved in ethanol to form dilute solutions with a concentration of 2% v/v, respectively. The 1,6dibromohexane solution was added dropwise into the TMHDA solution under stirring at room temperature. The temperature of the mixture was elevated to 60 °C and stirred for 5 h to complete the reaction. After removing the solvent, a white powder was produced. The raw product was washed with ethanol and anhydrous ether sequentially, and the washing process was repeated several times to remove residual TMHDA. The final product with a yield of 87% was obtained by drying under vacuum at 60 °C for 24 h. N(CH3)2(CH2)6-N+(CH3)2Br−-(CH2)6-N+(CH3)2Br−-(CH2)6-N(CH3)2: 1H NMR (300 MHz, in D2O): N(CH3)2-: δ 2.14−2.22 ppm, 12H; N(CH3)2−CH2−: δ 2.28−2.41 ppm, 4H; N(CH3)2−CH2− CH2−: δ 1.40−1.50 ppm, 4H; −N+(CH3)2−CH2−CH2−(CH2)2−: δ 1.39 ppm, 12H; −N+(CH3)2−CH2−CH2−: δ 1.75−1.85 ppm, 8H; −N+(CH3)2−CH2−: δ 2.95−3.10 ppm, 8H; −N+(CH3)2−: δ 3.23− 3.35 ppm, 12H (Figure S1a). Electrospray ionization−mass spectrometry (ESI-MS): A peak at m/z = 507.2 with a relative abundance of 100 corresponded to [ N(CH 3 ) 2 (CH 2 ) 6 -N + (CH 3 ) 2 Br − -(CH 2 ) 6 -N + (CH 3 ) 2 -(CH 2 ) 6 -N(CH3)2]+. Synthesis of Quaternized PPO (QPPO). Brominated PPO (BrPPO) samples with different degrees of bromination (DB) of 20, 30, 40, 60, and 80 mol % were prepared following the literature.63 To synthesize QPPO, BrPPO with the desired DB was dissolved in NMP to form a 5 wt % solution, into which TMA was added and stirred for 4 h at 45 °C to produce QPPO solution. The QPPO solutions produced with the BrPPO with DBs of 40, 60, and 80% were cast onto clean, flat glass plates and dried in a convection oven (1370 GM gravity oven, VWR, Radnor, PA) at 80 °C for 24 h and then further dried under vacuum at 80 °C overnight to form the QPPO-40, QPPO60, and QPPO-80 AEMs, respectively. To replace the Br− anion with OH−, each membrane was immersed in 1 M NaOH solution at room temperature for 10 h. This process was repeated four times with fresh NaOH to ensure complete displacement of Br−. Finally, the AEMs in OH− form were repeatedly rinsed with deionized water until the pH of residual water was neutral. Synthesis of Short Chain Cross-Linked Quaternized PPO (x(QH)QPPO). BrPPO samples with DBs of 40, 60, and 80% were dissolved in DMSO to form 0.2 wt % solutions, into which TMHDA (the equivalents of TMHDA (degree of functionality of two) to the benzyl bromo group in BrPPO was 0.5 to ensure a 1:1 cross-linking reaction) was added and stirred for 2 h at 40 °C to produce un-crosslinked AEM solutions. The solutions were cast onto clean, flat glass plates and dried in a convection oven at 80 °C for 24 h and then further dried in a vacuum oven at 80 °C overnight to form the x(QH)QPPO-40, x(QH)QPPO-60, and x(QH)QPPO-80 membranes. The ion exchange procedure described above was used to produce OH− form AEMs. Synthesis of Long-Chain Multication Quaternized PPO (x(QH)3QPPO). BrPPOs with DBs of 20, 30, and 40% were dissolved in DMSO to form 0.2 wt % solutions, into which 1,18-(N′,Ndimethylamino)-6,12-(N,N-dimethylammonium)octodecane bromide (the equivalents of cross-linker (degree of functionality of two) to the benzyl bromo group in BrPPO was 0.5 to ensure a 1:1 cross-linking reaction) was added and stirred for 2 h at 40 °C to produce un-crosslinked AEM solutions. Membrane casting and ion exchange were analogous to the other samples previously described. Characterization and Measurements. 1H NMR spectra were measured at 300 MHz on a Bruker AV 300 (Bruker Co., Billerica, MA) spectrometer using D2O as the solvent. Electrospray ionization mass spectrometry (ESI-MS) measurements were performed on a Finnigan LCQ advantage instrument (Thermo Finnigan LLC, San Jose, CA). Xray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos XSAM-800 spectrometer (Kratos Analytical Ltd., Manchester, UK) with an Mg Kα radiator. The N(2p) signal was collected and analyzed using XPS Peak software (XPSpeak 4.0). FTIR

spectra were obtained using a Bruker Vertex 70 FTIR spectrometer (Billerica, MA) equipped with a liquid nitrogen cooled MCT detector. Transmission spectra were collected on 7 mm KBr pellets prepared by mixing 2.5 mg of polymer with 60 mg of anhydrous KBr. The spectra were signal averaged over 100 scans at 4 cm−1 resolution with a 0.5 mm aperture size and a nitrogen purge at ambient temperature. All spectra were processed using Bruker OPUS 6.5 software. Small-angle X-ray scattering measurements were obtained using a Rigaku instrument (Rigaku Co., Tokyo, Japan) equipped with a pinhole camera with an Osmic microfocus Cu Kα source and a parallel beam optic. Typical counting times for integration over a multiwire area detector were 45 min with typical membrane thicknesses of ∼100 μm. Measurements were taken under vacuum at room temperature on dry samples. Scattered intensities were normalized for background scattering and beam transmission. Transmission electron microscopy (TEM) samples were prepared using a LeicaUltracut UC6 ultramicrotome (Leica Biosystems Inc. Wetzlar, Germany) with an EMFC6 cryo attachment. The sections were collected on carbon/Formvarcoated grids. Transmission electron images were obtained using a JEOL JEM 1200 EXII microscope (JEOL Ltd., Akishima, Tokyo, Japan) equipped with a tungsten emitter operating at 80 kV. Images were captured on unstained Br− form samples recorded on a CCD camera using TCL software. Tensile measurements were obtained for samples equilibrated in liquid water and ambient air at room temperature. The membrane samples for tensile testing were cut into a dumbbell shape (12 mm × 3 mm in the test area), and tensile measurements were performed using an Instron-5866 (Norwood, MA) instrument at a crosshead speed of 5 mm/min at room temperature (25 °C). Thermogravimetric analysis (TGA) was performed on a TGA Q50 (TA Instruments, New Castle, DE) using samples (5 mg) placed in an Al2O3 crucible. The samples were heated from 30 to 650 °C at a rate of 5 °C/min under flowing nitrogen (40 mL/min). To obtain the titrated gravimetric IEC values, AEMs in the OH− form were immersed in 50 mL of 0.01 M HCl standard solution for 24 h. The solutions were then titrated with a standardized NaOH (0.01 M) solution to pH = 7. The membrane was washed and immersed in deionized water for 24 h to remove any residual HCl and then dried under vacuum at 45 °C for 24 h and weighed to determine the dry mass (in Cl− form). The IEC of the membrane was calculated with eq 1: n i(H+) − n f(H+) IEC = mdry(Cl) (1) The swelling ratio was obtained by measuring the dimensions of membranes in both dry (in Br− form) and wet (in OH− form) states and calculated from the ratio of the change of dimension in the wet state to the membrane dimension in the dry state. Water uptake (WU) was measured after drying the membrane in hydroxide form at 60 °C under vacuum for 12 h. The dried membrane was immersed in water and periodically weighed on an analytical balance until a constant mass was obtained, giving the mass-based water uptake. The hydration number (λ) was calculated from eq 2:

λ=

1000 × WU M H2O × IEC

(2)

where MH2O is the molecular mass of water (18.015 g/mol), and IEC is the ion exchange capacity by titration. In-plane conductivity (σ) of membrane samples was obtained using eq 3:

σ=

d R × Ls × Ws

(3)

where d is the distance between the electrodes, R is the membrane resistance, and Ls and Ws are the thickness and width of the membrane, respectively. The membrane impedance was measured over the frequency range from 100 kHz to 100 mHz by two-point probe alternating current (ac) impedance spectroscopy using an 3325

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Macromolecules impedance/gain-phase analyzer (Solartron 1260A, Solartron Analytical, Farnborough Hampshire, ONR, UK). The hydroxide conductivity measurements in the longitudinal direction at temperatures ranging from 30 to 80 °C under fully hydrated conditions were carried out with the cell immersed in water which was degassed and blanketed with flowing ultrahigh-purity (UHP) argon to preserve the OH− form of the sample. The effective diffusion coefficient (D) of the mobile ion in AEMs was calculated from a form of the Nernst−Einstein equation: D=

σRT cz 2F 2

The presence of different types of nitrogen-based amine and ammonium functional groups (quaternary ammonium (QA-1, the cation attached to the PPO backbone, and QA-2, the cation within the cross-linking chain, and TA) were identified by X-ray photoelectron spectroscopy (XPS). The degree of cross-linking (DC) in the cross-linked AEMs was also obtained from XPS measurements. The cross-linking reaction converted the TA groups into QA; thus, the molar ratio of QA to TA can be used to show the efficiency of the cross-linking reaction. As displayed in Figure 2a, in the x(QH)QPPO membrane the area of the

(4)

where σ is the measured conductivity, R is the ideal gas constant, T is temperature, c is the computed concentration of ions calculated by eq 5, z is valence charge, and F is Faraday’s constant:64

c = 0.001 ×

ρ × IEC 1 + 0.01X v‐H2O

(5)

where ρ is the density of the tested polymer, which was measured by a buoyancy method,62 and Xv‑H2O is the volume-based water uptake. To determine the barrier to ion transport, the calculated D was compared to the ion diffusivity in dilute solution (D0, calculated from eq 6), the maximum diffusivity of an ion in water:64

D0 =

μkBT q

(6)

where μ is the dilute solution ion mobility, kB is the Boltzmann constant, T is temperature, and q is the ion charge.64 Gel fraction was used to assess the extent of cross-linking in the membrane. To calculate the gel fraction, the cross-linked membrane was immersed in DMSO at 80 °C for 1 day. The gel fraction was calculated from the ratio of the remaining mass of the polymer after immersion in solvent to its initial mass before immersion. Chemical stability tests were conducted by immersing an AEM sample (50 ± 3 μm in thickness) in 1 M NaOH solution maintained at 80 °C for 30 days. Periodically during the stability test, the IEC and ionic conductivity of the AEMs were measured to evaluate the changes in the materials under strongly alkaline degradation conditions. Fuel Cell Device Measurements. Pt/C catalyst (60 wt % Pt on carbon, Johnson Matthey Co. West Chester, PA) was mixed with QPPO-60 ionomer solution and sprayed on a carbon cloth gas diffusion electrode (GDL, CeTech W1S1005, Cetech Co., Ltd., Taiwan). The Pt loading on both the anode and the cathode was 0.4 mg/cm2, and the area of the electrodes was 4 cm2. The weight percentage of QPPO-60 ionomer in both electrodes was calculated to be 12 wt %. QPPO-40 and x(QH)3QPPO-40 membranes with a thickness of 55 ± 5 μm were sandwiched between two catalyst-coated GDLs, respectively, to form the membrane electrode assembly (MEA). The assembled H2−O2 fuel cells were tested (850e Multi Range, Scribner Associates Co., Southern Pines, NC) under galvanostatic conditions using fully humidified H2 and O2 gases flowing at a rate of 250 mL/min at 60 °C with 0.1 MPa of back-pressure.

Figure 2. XPS signals for N 1s electron in cross-linked (a) x(QH)QPPO and (b) x(QH)3QPPO.

peak that corresponds to the N 1s-electron in the QA-1 group (with the higher binding energy (BE) of 402.3 eV)65 was 10 times larger than that of the peak corresponding to the TA group (with the lower BE of 399.6 eV).65 For the long-chain multication cross-linked AEM, x(QH)3QPPO, the N in QA-1 (with the lower BE of 402.3 eV) and QA-2 (with the lower BE of 401.8 eV)65 were the only peaks corresponding to QA groups, and no obvious peak was found for the N in a TA group. These results shows that there was no or only trace amounts of un-cross-linked TA groups remaining in the crosslinked films, indicating a high DC for the AEMs in this work. The gel fraction tests (Table S1) for these membranes were consistent with the XPS results demonstrating high degrees of cross-linking in these materials. Mechanical Properties. The comparison of mechanical properties between AEMs with different structures and IECs is shown in Figure 3. For the un-cross-linked QPPO, the sample with an IEC of 2.27 mmol/g (QPPO-40) showed both good strength (tensile strength of 18.9 MPa) and flexibility (elongation at break of 22.5%). When the IEC of QPPO was increased to 3.22 mmol/g (QPPO-60), very high water uptake was observed (Table S1). High water uptake weakened the ionic interactions between polymer chains, resulting in poor

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of uncross-linked QPPO samples followed our previous work (see Figure S1b for the 1H NMR data).62 In order to produce the cross-linked AEMs (x(QH)QPPO and x(QH)3QPPO) with different IECs (Table S1), the desired DTA was added into 0.2 wt % DMSO solutions of BrPPO with different degrees of bromination (DB). In dilute DMSO solution, none or only one of the tertiary amine (TA) groups in the DTA reacted with the benzyl bromide (BBr) of the BrPPO and produced an un-crosslinked AEM precursor. After evaporating the DMSO at 80 °C, the TA groups fully reacted with the benzyl bromo moieties to form cross-links in the AEM. 3326

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elastic and also maintained materials with high strength by limiting the water swelling of the AEMs. Swelling Ratio and OH− Conductivity. The trade-off between conductivity and swelling properties is one of the common challenges in AEM development. As shown in Figure 4, QPPO-40 with an IEC of 2.27 mmol/g showed a low

Figure 3. (a) Tensile strength and (b) elongation at break for wet AEMs in OH− form tested at room temperature.

mechanical properties of the AEM. In our previous work, a series of multication side chain AEMs (having two or three cations per side chain site) that showed microphase separation were developed to enhance the conductivity of PPO-based AEMs.62 Although having superior ion conductivity to BTMA PPO (that is, QPPO in the present work) AEMs, the mechanical properties (with a strength of ∼15 MPa and an elongation to break of ∼15%) of the multication side chain membranes were similar to that of BTMA PPO films. As mentioned previously, the common short-chain crosslinked x(QH)QPPO exhibited poor mechanical properties. With high DC, the x(QH)QPPO membranes with different IECs were brittle. Such membranes had low tensile strengths of 11.4, 12.9, and 8.8 MPa for the x(QH)QPPO films with IECs of 2.41, 3.30, and 4.01 mmol/g, respectively (Figure 3a). The flexibilities of these samples were less than that of the un-crosslinked QPPO-60 (Figure 3b) with elongation at breaks of 4.5, 5.1, and 3.6% for the x(QH)QPPO films with IECs of 2.41, 3.30, and 4.01 mmol/g, respectively. Replacing the short cross-link in the AEM with a long, flexible cross-linker was an effective method to enhance the mechanical properties of the membranes. As is shown in Figure 3, both the strength and the flexibility of the x(QH)3QPPO membranes were significantly enhanced compared to the x(QH)QPPO films and even better than that of the un-crosslinked QPPO and multication side chain AEMs. The tensile strengths of the x(QH)3QPPO-20, -30, and -40 samples with IECs of 2.14, 3.05, and 3.59 mmol/g were all approximately 20 MPa, and their elongations at break were 34.6, 38.7, and 34.4%, respectively. As expected, even with IECs above 2.0 mmol/g, the long, flexible cross-linker rendered the x(QH)3QPPO films

Figure 4. (a) Swelling ratio and (b) OH− conductivity of PPO-based AEMs (QPPO, x(QH)QPPO, and x(QH)3QPPO) with different IECs measured at room temperature.

swelling ratio at room temperature, and even at 80 °C, the swelling ratio was maintained below 15.7% (Figure S3a). However, low OH− conductivities (22.5 and 49.9 mS/cm at 30 and 80 °C) usually accompany low swelling ratios. A further increase of the IEC can improve the conductivity of QPPO. For example, with an IEC of 3.22 mmol/g, QPPO-60 showed a conductivity of 33.6 mS/cm at room temperature, which was increased to 45.4 mS/cm at 50 °C (Figure S3a). Unfortunately, enhancing ionic conductivity by increasing the IEC of the material leads to a significant increase in water swelling. The swelling ratios of QPPO-60 were as high as 67.5% at 30 °C and 95.2% at 50 °C. Introducing cross-links into AEM systems is a proven method of restraining the swelling of high IEC materials. As depicted in Figure 4a, cross-linked x(QH)QPPO-40 with a relatively low IEC (2.41 mmol/g) showed extremely low swelling ratios (just 5.1% at 80 °C (Figure S3a)). The x(QH)QPPO-60 and -80 samples with IECs as high as 3.30 and 4.01 mmol/g have swelling ratios of 10.2% and 32.1% at room temperature and 20.8% and 45.0% at 80 °C (Figure S3a). Although the x(QH)QPPO samples with high IECs showed low swelling, their conductivity was unsatisfactory. The conductivities of x(QH)QPPO samples were significantly lower than those of the QPPO films with similar IECs. Even when the IEC was increased to 4.01 mmol/g, the OH− 3327

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Macromolecules conductivity of x(QH)QPP-80 was 26.8 mS/cm at room temperature, which was slightly greater than that of the QPPO40 (IEC = 2.27 mmol/g) and much lower than that of the QPPO-60 (IEC = 3.22 mmol/g). Despite the low conductivity at room temperature, x(QH)QPPO showed its advantage under elevated temperature. Specifically, the conductivity of x(QH)QPP-80 reached 69.6 mS/cm at 80 °C, which was 1.4 times higher than that of the QPPO-40 sample (Figure S3b). In common with the x(QH)QPPO sample, x(QH)3QPPO also exhibited low swelling (Figure 4a), and the swelling ratios of the x(QH)3QPPO membranes were close to that of the x(QH)QPPO samples with similar IECs. For x(QH)3QPPO40, even though the IEC was as high as 3.59 mmol/g, the swelling ratio was controlled at 18.6% and 25.0% at 30 and 80 °C (Figure S3a), respectively. In addition to low swelling ratios, x(QH)3QPPO showed excellent conductivity. With IECs of 2.14, 3.05, and 3.59 mmol/g, the OH− conductivities of x(QH)3QPPO-20, -30, and -40 were 21.1, 38.1, and 58.5 mS/ cm at 30 °C and increased to 41.9, 67.6, and 110.2 mS/cm, when the temperature was elevated to 80 °C, respectively (Figure S3b). Ion Transport and Microphase Morphology. It is important to note that the chemical structures of the crosslinks have an important effect on the ion transport in AEMs. Although the IECs of the x(QH)3QPPO series of membranes were lower, the conductivities of these samples were significantly greater than that of the x(QH)QPPO series. The metric of normalized ionic diffusion coefficient (D/D0) for OH− transport was employed here to elucidate the intrinsic ion conduction performance of different AEMs. As shown in Figure 5, for a specific type of AEM, higher IEC always increased λ and led to greater D/D0.

multication cross-links showed improved ion conduction in x(QH)3QPPO. The values of D/D0 for x(QH)3QPPO were greater than that of the other two AEMs at similar λ. Specifically, x(QH)3QPPO-20, -30, and -40 with λs of 8.6, 14.9, and 22.4 showed D/D0 values of 0.06, 0.11, and 0.20, which were 6.0, 2.8, and 2.0 times higher than that of the x(QH)QPPO-40, -60, and -80 membranes, respectively. To better understand the relationship between the AEM chemical structures and their ion transport attributes, the microphase-separated morphologies of the materials were studied by TEM and SAXS. As can be identified from Figure 6, the hydrophilic species were distributed rather evenly in the QPPO and x(QH)QPPO films, suggesting no obvious microphase-separated morphology in these materials. For the x(QH)3QPPO sample, the long multication cross-link induced clear microphase separation between ionic and nonionic phases. The hydrophilic species aggregated to form ion-rich clusters, which were beneficial for building effective ion conduction pathways, and guaranteed high ion conductivity in x(QH)3QPPO membranes. Furthermore, quantitative information on the size of the ionic clusters was obtained from SAXS results. In Figure 7, no ionic peak was observed in the SAXS data of QPPO and x(QH)QPPO, consistent with their features displayed in TEM images (Figure 6a,b). For the x(QH)3QPPO, a distinct ionic peak emerged at 1.15 nm−1 (corresponding to an interdomain Bragg spacing of 5.5 nm), indicating the aggregation of ionic clusters in the cross-linked multication AEM system. However, despite showing characteristic hydrophilic/hydrophobic phase separation, it is apparent from the TEM and the SAXS data that no long-range, regular order of the ionic phase exists in this type of material. Alkaline Stability. The lack of predictable long-term chemical stability is currently preventing the demonstration of long-lived alkaline membrane electrochemical devices under desired operating conditions for real-world applications. The overall chemical stability of AEMs is influenced by the degradation resistance of both the cation and the polymer backbone under alkaline conditions, and in some operational cases, electrochemical oxidative stress. At elevated temperatures, the groups with a strongly electrophilic nature (e.g., quaternary ammonium) are more likely to be attacked by OH−.1,10 To evaluate the alkaline stability of the AEMs in this work, QPPO-40, x(QH)QPPO-60, and x(QH)3QPPO-40 were immersed in 1 M NaOH solution at 80 °C for 30 days (with replacement of the 1 M NaOH every 3 days during the testing period). As shown in Figure 8, the cross-linked AEMs (x(QH)QPPO-60 and x(QH)3QPPO-40) showed much better cation stability compared to the QPPO-40 sample under alkaline conditions. After 30 days of immersion, the IEC and OH− conductivity of QPPO-40 at 80 °C in 1 M NaOH decreased by 56.8% and 61.2%, respectively. In contrast, the IECs of cross-linked AEM x(QH)QPPO-60 and x(QH)3QPPO-40 films decreased to 2.75 and 2.80 mmol/g, respectively. The OH− conductivities were maintained at 37.9 and 82.6 mS/cm at 80 °C, which were 18.1 and 25.0% lower than the values measured prior to the stability test. Fuel Cell Testing. High conductivity, robust mechanical and low swelling properties, and reasonable chemical stability rendered the x(QH)3QPPO-40 sample a good candidate for AEM fuel cell applications. Figure 9 summarizes the performance of assembled H2−O2 single fuel cells at 60 °C using a 55 ± 5 μm thick x(QH)3QPPO-40 membrane with Pt catalyst loadings of 0.4 mgPt/cm2 on the anode and cathode (ionomer

Figure 5. Ratio of the effective OH− diffusion coefficient, D, to the dilute solution OH− diffusivity, D0, as a function of hydration number (λ) for AEMs at room temperature.

A moderate D/D0 was observed for QPPO-40 under relatively low λ (D/D0 = 0.08 at λ = 17.7). With an extremely high hydration number (λ = 78.8), the QPPO-60 showed a high D/D0 of 0.30. Compared to that of the QPPO, the x(QH)QPPO membranes with low λ exhibited lower ion conductivity, with a D/D0 of the x(QH)QPPO-40 (λ = 9.3) and x(QH)QPPO-60 (λ = 15.0) of 0.01 and 0.04, respectively. Even when the λ of x(QH)QPPO-80 was increased to 28.3, the D/D0 was 0.10. The lower D/D0 observed for the x(QH)QPPO sample suggests that the short cross-links posed barriers within the ion conduction channels that hindered ion transport in the AEMs. Different from the case of x(QH)QPPO, longer 3328

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Figure 6. Transmission electron microscopy (TEM) images in Br− form for (a) QPPO-80 (IECideal = 4.25 mmol/g), (b) x(QH)QPPO-80 (IECideal = 4.68 mmol/g), and (c) x(QH)3QPPO-40 (IECideal = 3.97 mmol/g). The dark contrast in the images represents the hydrophilic domains where the Br− anions reside.

Figure 9. Discharge performance of H2−O2 AEMFCs based on QPPO-40 (55 ± 5 μm thick, IEC = 2.27 mmol/g) and x(QH)3QPPO40 membranes (55 ± 5 μm thick, IEC = 3.59 mmol/g). The catalyst in both the anode and the cathode of the AEMFCs was Pt/C (60%, Johnson Matthey Co.), with a Pt loading of 0.4 mg/cm2. The weight percentage of ionomer (QPPO-60 solution) in the catalyst layers was 20 wt %. Pure H2 and O2 gases were fully humidified and fed at a rate of 250 mL/min with 0.1 MPa back-pressure.

Figure 7. SAXS patterns for QPPO-80 (IECHNMR = 3.91 mmol/g), x(QH)QPPO-80 (IECHNMR = 3.98 mmol/g), and x(QH)3QPPO-40 (IECHNMR = 3.47 mmol/g).

binder: QPPO-60 with a weight percentage of 20%). Meanwhile, an AEMFC based on QPPO-40 working under the same operational conditions was chosen as a reference (the x(QH)QPPO series membranes were too brittle to be used as a separator in fuel cells). For an AEMFC, the discharge performance is closely related to the ionic conductivity of the employed AEM material. Higher conductivity of the membrane leads to smaller voltage drop, which ensures greater power density of the device. The peak power density of the AEMFC based on the highly conductive x(QH)3QPPO-40 membrane (with an ionic conductivity of 86.9 mS/cm at 60 °C) was 0.302 W/cm2 at a current density of 0.7 A/cm2, which was 3 times greater than that of the cell based on QPPO-40 (with a conductivity of 36.9 mS/cm at 60 °C).

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CONCLUSIONS In summary, to address the brittleness and the poor conductivity issues that are common in short-chain TMHDA cross-linked PPO AEMs (x(QH)QPPO), a novel long-chain multication cross-linked PPO AEM (x(QH)3QPPO) has been designed and synthesized. The long, flexible cross-link between PPO backbone chains rendered x(QH)3QPPO samples highly flexible, while the multiple cations in the cross-linking chain not only enabled high IECs but also induced obvious hydrophilic/ hydrophobic microphase separation in the material, leading to high conductivity. Additionally, like the x(QH)QPPO sample, the x(QH)3QPPO material exhibited sufficient dimensional

Figure 8. Chemical stability of the quaternary ammonium cation of QPPO-40, x(QH)QPPO-60, and x(QH)3QPPO-40 in 1 M NaOH solution at 80 °C: (a) OH− conductivity measured at 80 °C and (b) IEC as a function of time. 3329

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and chemical stabilities, which were better than BTMAfunctionalized PPO (QPPO). Typically, the x(QH)3QPPO-40 sample with an IEC of 3.59 mmol/g showed the best performance within the x(QH)3QPPO series where the tensile strength and elongation at break were 18.6 MPa and 34.4%, respectively. The OH− conductivity of the material was 110.2 mS/cm at 80 °C with the swelling ratio being controlled to as low as 25.0%. In comparison, all of the produced x(QH)QPPO AEMs were rather brittle films exhibiting poor mechanical strengths and flexibilities. The conductive performance of the x(QH)QPPO AEM was also unsatisfactory. Even when the IEC of x(QH)QPPO-80 was as high as 4.01 mmol/g, its OH− conductivity was just 69.6 mS/cm at 80 °C. In addition, the cross-linked quaternary ammonium groups in the x(QH)3QPPO sample exhibited good chemical stability when the material was degraded in 1 M NaOH solution at 80 °C for 30 days. The IEC and conductivity of the x(QH)3QPPO-40 sample were maintained at 2.80 mmol/g and 82.6 mS/cm (at 80 °C), which were 22.0% and 25.0% lower than the values obtained from the samples before the stability test. Employing the high-performance x(QH)3QPPO-40 membrane as a separator, an AEMFC using Pt catalysts was successfully assembled and tested showing a peak power density of 0.302 W/cm2 at a current density of 0.7 A/cm2 at a device operation temperature of 60 °C.

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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.6b01140. Figures S1−S5 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.P.). *E-mail [email protected] (M.A.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge our industrial sponsors. This work was funded in part by the Advanced Research Projects Agency Energy (ARPA-E), U.S. Department of Energy, under Award DE-AR0000121, the United States−Israel Binational Science Foundation (BSF) through Energy Project No. 2011521, the U.S. National Science Foundation DMREF program via Grant CHE-1534326, the National Natural Science Foundation of China (21303124, 21303123), and the China Scholarship Council (201506270046). Infrastructure support was also provided by The Pennsylvania State University Materials Research Institute and the Penn State Institutes of Energy & the Environment. M.A. H. acknowledges the Corning Foundation and the Corning Faculty Fellowship in Materials Science and Engineering for support.

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REFERENCES

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