Backbones for Chemically Stable and Conductive Anion Exchange

May 15, 2017 - on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones and ...... L.; Lu, J.; Zhuang, L. Constructing ionic highway in alkaline poly...
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Cationic Side-Chain Attachment to Poly(Phenylene Oxide) Backbones for Chemically Stable and Conductive Anion Exchange Membranes Jing Pan,†,‡ Juanjuan Han,†,‡ Liang Zhu,† and Michael A. Hickner*,† †

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 ABSTRACT: A series of chemically stable and ionically conductive side-chain anion exchange membranes (AEMs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones and alkyltrimethylammonium cations are reported in this work. Two alkyltrimethylammonium groups with n-propyl (C3) and n-pentyl (C5) alkyl chains were tethered onto PPO backbones through secondary amine moieties, resulting in two side-chain AEMs, that is, NC3Q-PPO and NC5Q-PPO and NC5 Q-PPO. In comparison to benzylic QA groups (e.g., benzyltrimethylammonium cations in quarternized PPO (QPPO) and benzylalkyldimethylammonium cations in comb-shaped PPOs (i.e., QC3-PPO and QC6-PPO)), the alkyltrimethylammonium cations of the side-chain PPOs, which do not possess highly reactive benzylic protons adjacent to both the aromatic ring and the cation, showed superior alkaline stability. After 30 days of aging in 1 mol/L NaOH solution at 80 °C, the retention of the conductivities of NC3Q-PPO (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), and NC5Q-PPO-60 (IEC = 2.57 mmol/g) were 73.1%, 89.9%, and 81.2% compared with 39.8%, 41.2%, and 56.5% for the QPPO-40 (IEC = 2.27 mmol/g), QC3-PPO-40 (IEC = 2.22 mmol/g), and QC6PPO-40 (IEC = 2.13 mmol/g) samples, respectively. In addition to good stability, the side-chain NC5Q-PPO-40 and NC5QPPO-60 with longer spacers between the aromatic polymer backbone and the cation exhibited high conductivities of 73.9 and 96.1 mS/cm at 80 °C in liquid water, while the swelling ratios were limited to 15% and 28%. The flexible linear spacer in NC5QPPO membranes induced distinct hydrophilic/hydrophobic microphase separation, which enhanced the physical properties of the membranes. Thus, we believe that the NC5Q-PPO-type AEMs derive their superior performance from both their unique chemical structures with n-pentyl cationic tethers and the microphase-separated morphologies of the materials driven by the side chain architecture.

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INTRODUCTION For a future hydrogen-based economy,1 fuel cells, as a category of highly efficient energy conversion devices, have witnessed a surge in research interest, great advances in performance, and a decline in costs over the last 10 years. Among various types of fuel cells under development today, polymer electrolyte fuel cells (PEFCs) have been recognized as the most promising power sources for zero-emission vehicles and portable devices.2−4 As one of the key materials in PEFCs, the solid polymer electrolyte (SPE) plays dual roles: one being a media for conducting ions, the other being a separator that keeps the anode and the cathode from direct contact. Compared with conventional liquid electrolytes, SPEs can not only boost the power density of the cell by compacting the architecture and lightening the fuel cell devices, but solid polymer membranes also simplify operation and maintenance of the system due to the absence of liquid electrolyte handling systems.2,5 As the most developed SPE used in PEFCs, Nafion, a perfluorosulfonic acid proton exchange membrane (PEM), shows remarkable conductivity, stability, and mechanical properties.6−8 Although the structure of Nafion is © 2017 American Chemical Society

well-designed and its performance in electrochemical systems is superior to other cation exchange membranes, the strong acidic nature of Nafion restricts highly active and stable catalysts in PEMFCs to just several noble metals and alloys. Consequently, the high cost and the scarcity of fuel cell catalysts become the main drawbacks that hinder widespread application of this technology.9−13 To resolve the issues surrounding use of expensive catalysts in acidic fuel cells while maintaining the advantages of SPEs, anion exchange membranes (AEMs) with polymer-tethered cationic groups to facilitate OH− conduction are considered a leading candidate for next-generation, low-cost fuel cell systems.14−18 Many conventional AEMs employ an aromatic polymer backbone, such as polysulfone, 19−21 poly(phenylene oxide),22−25 polystyrene,26−28 poly(ether ketones),29−32 or poly(phenylene),33 and a cationic group as the organic base Received: April 11, 2017 Revised: May 13, 2017 Published: May 15, 2017 5321

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Chemistry of Materials which is usually closely tethered onto the polymer backbone through a benzylic group. This type of benzylic tethering chemistry is often employed in AEM synthesis because it is easy to modify arylene-based polymers through chloromethylation, and the amination of the benzylic halides is a facile reaction. The most common cationic group used in AEMs is benzyltrimethylammonium (BTMA).14,17−20,34,35 However, the chemical stability of the BTMA group under alkaline conditions is not sufficient for long-term device operation, especially at elevated temperatures.5,11,36,37 To improve the alkaline stability of BTMA-based AEMs, other cationic species and tethering groups have been studied.10,13,25,38−46 Among them, aromatic cations, such as imidazolium38,39 and guandinium,40,41 have attracted the most attention. The proponents of these resonance stabilized cations assert that the density of the positive charge can be delocalized throughout the aromatic framework surrounding the cationic center, which has enabled such AEMs to show longer stability lifetimes than BTMA-based materials.39−41 In contrast to these reports, researchers have conducted FT-Raman spectroscopic studies and measurements of membrane properties on the degradation of benzylmethylimidazolium (BMI) and BTMA-based AEMs, and found that the BMI cation is intrinsically less chemically stable compared to the BTMA group.47,48 These seemingly conflicting reports point to the importance of the details of the cationic group structure, the strategies used to incorporate cations into polymers, and methods to measure their degradation. Recently, Marino and Kreuer49 performed a comprehensive study on the degradation kinetics of different quaternary ammonium (QA)-containing molecules in 6 M NaOH solution at 160 °C. One of the most important conclusions reached in this work is that avoiding placement of the benzylic group adjacent to the cation is crucial to improve the alkaline stability of the molecules. Attaching a cationic group to an aromatic polymer via a linear carbon spacer, for example, an alkyl chain, to make sidechain AEMs is an effective approach to achieve QA-based AEMs without a direct benzylic tether adjacent to the cation.44,45 Additionally, cyclic aliphatic cations may have enhanced stability compared to other ammonium architectures. However, compared to degradation studies of BTMA-based AEMs with benzylic cation attachments, the degradation of side-chain AEMs, which have an aliphatic spacer between the aromatic group and the cation and require advanced synthetic methods, has not been investigated in detail. In the present work, two alkyltrimethylammonium groups with different alkyl chain lengths were tethered to PPO backbones through a secondary amine group, producing two side-chain PPO-based AEMs, that is, NC3Q-PPO (Figure 1b) and NC5Q-PPO (Figure 1c) with n-propyl (C3) and n-pentyl (C5) spacers. To demonstrate the superior properties of the sidechain AEMs, three benzylic QA-based AEMs, QPPO (Figure 1a) and two comb-shaped PPOs (QC3-PPO (Figure 1d) and QC6PPO (Figure 1e)), were prepared to serve as control samples. The effect of the structure of the cationic groups in regards to the alkaline stability of the AEMs is highlighted. In addition, measurements of the conductivity of these different structures were important for evaluating their performance as state-of-theart AEMs.

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Figure 1. Chain schematics and chemical structures of (a) QPPO, (b) NC3Q-PPO, (c) NC5Q-PPO, (d) QC3-PPO, and (e) QC6-PPO. temperature overnight. N-Bromosuccinimide (NBS), 2,2′-azobis(2methylpropionitrile) (AIBN), trimethylamine (TMA, 33% w/w in ethanol), ammonium hydroxide solution (28% NH3 in H2O), (3bromopropyl)trimethylammonium bromide, (5-bromopentyl)trimethylammonium bromide, N,N-dimethylpropylamine, and N,Ndimethylhexylamine 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. Synthesis and Characterization of N,N,N-Trimethylpropane1,3-diammonium bromide and N,N,N-Trimethylpentane-1,5diammonium bromide. (3-Bromopropyl)trimethylammonium bromide (1.3 g) or (5-bromopentyl)trimethylammonium bromide (1.4 g) was dissolved in ethanol to form dilute solutions with a concentration of 2 v/v %. The (3-bromopropyl)trimethylammonium bromide or (5bromopentyl)trimethylammonium bromide solution was added dropwise into 15 mL of ammonium hydroxide 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 reactants. The final product with a yield of 81% for N,N,N-trimethylpropane-1,3diammonium bromide or 86% for N,N,N-trimethylpentane-1,5diammonium bromide was obtained by drying under vacuum at 60 °C for 24 h. NH3+Br−−CH2−CH2−CH2−N+(CH3)3Br−: 1H NMR (300 MHz, in D2O) NH3+− δ 8.13 ppm, 3H; NH3+−CH2− δ 3.18−3.25 ppm, 2H; NH3+−CH2−CH2− δ 2.34−2.40 ppm, 2H; NH3+−CH2−CH2−CH2− N+(CH3)3− δ 3.01−3.12 ppm, 11H. NH3+Br−−CH2−CH2−CH2−CH2−CH2−N+(CH3)3Br−: 1H NMR (300 MHz, in D2O) NH3+− δ 8.18 ppm, 3H; NH3+−CH2− δ 3.25−3.40 ppm, 2H; NH3+−CH2−CH2−CH2−CH2− δ 1.52−1.80 ppm, 4H; NH3+−CH2−CH2−CH2− δ 1.20−1.40 ppm, 2H; −N+(CH3)2−CH2− δ 2.80−2.92 ppm, 2H; −N+(CH3)3− δ 3.10 ppm, 9H. Electrospray ionization-mass spectrometry (ESI-MS): A peak at m/z = 197.1 with a relative abundance of 100 corresponded to [NH3+Br−− CH2−CH2−CH2−N+(CH3)3]+. And the peak at m/z = 225.1 with a relative abundance of 100 corresponded to [NH3+Br−−CH2−CH2− CH2−CH2−CH2−N+(CH3)3]+.

EXPERIMENTAL SECTION

Materials. Poly(2,6-dimethyl phenylene 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 5322

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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)

Synthesis of Quaternized PPO (QPPO). Brominated PPO (BrPPO) samples with degrees of bromination (DB) of 40 or 60 mol %, depending on the desired final IEC of the sample, were prepared following the literature.50 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 and 60% were filtered and 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, QPPO-60, 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 Side-Chain Quaternized PPO (NC3Q-PPO and NC5Q-PPO). One gram of BrPPO with a DB of 40 or 60% was dissolved in DMSO to form a 0.2 wt % solution, into which 0.05 g of K2CO3 was added and stirred for 1 h. The cation precursor (N,N,Ntrimethylpropane-1,3-diammonium bromide or N,N,N-trimethylpentane-1,5-diammonium bromide) which was dissolved in DMSO at 0.5 wt % was then added dropwise into the polymer solution (the equivalents of cation precursor to the bromobenzyl group in BrPPO was 1.5:1 mol:mole) and stirred for 24 h at 60 °C to produce the side-chain AEM solution. When the concentration of BrPPO was increased above 0.5 wt %, the mixture of BrPPO and difunctional N,N,Ntrimethylpentane-1,5-diammonium bromide gelled after 30 min of reaction at 60 °C. Thus, we maintained the BrPPO at 0.2 wt % in dilute solution to ensure there was no formation of tertiary amines which would cause cross-linking. After filtering out the insoluble inorganic salt, the product (NC3QPPO or NC5Q-PPO) was precipitated into methanol, washed several times with deionized water, and then dried in a vacuum oven for 24 h at 60 °C. To fabricate the AEM samples, NC3Q-PPO or NC5Q-PPO powders were dissolved in DMSO to form 5 wt % solutions. The solutions were filtered and 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 NC3Q-PPO-40, NC3Q-PPO-60, NC5Q-PPO-40, and NC5Q-PPO-60 membranes, respectively. The ion exchange procedure described above was used to produce OH− form AEMs. Synthesis of Comb-Shaped Quaternized PPO (QC3-PPO and QC6-PPO). Comb-shaped quaternized PPOs (QC3-PPO and QC6PPO) were prepared following the literature.50 BrPPOs with DBs of 40 or 60% were dissolved in DMSO to form 10 wt % solutions, into which N,N-dimethylpropylamine or N,N-dimethylhexylamine (the molar ratio of the tertiary amine-bearing molecule to the bromobenzyl group in BrPPO was 1.5) was added and stirred for 48 h at room temperature. Membrane casting and ion exchange were analogous to the other samples described above. 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). 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/Formvar-coated 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. 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

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 × ICE

(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, 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 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 exclude CO2 and 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

(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 constant51

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,46 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 water51

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.51 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.

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RESULTS AND DISCUSSION Synthesis and Characterization. The chemical structures of the synthesized AEMs were characterized by 1H NMR 5323

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Figure 2. 1H NMR spectra of (a) QPPO, (b) NC3Q-PPO, (c) NC5Q-PPO, (d) QC3-PPO, and (e) QC6-PPO.

Table 1. Physical Properties of AEMs Tested at Room Temperature sample

IECa (mmol/g)

IECb (mmol/g)

IECc (mmol/g)

OH− conductivity (mS/cm)

swelling ratio (%)

water uptake (%)

λ

QPPO-40 QPPO-60 NC3Q-PPO-40 NC3Q-PPO-60 NC5Q-PPO-40 NC5Q-PPO-60 QC3-PPO-40 QC3-PPO-60 QC6-PPO-40 QC6-PPO-60

2.68 3.52 2.45 3.25 2.22 2.86 2.54 3.41 2.30 2.99

2.36 3.34 2.30 2.82 1.94 2.67 2.35 2.84 2.06 2.77

2.27 3.22 2.17 2.79 2.03 2.57 2.22 2.81 2.13 2.74

22.5 33.6 18.4 25.1 24.5 35.9 15.7 21.5 23.7 39.2

12 68 10 45 7 20 11 59 7 34

72 457 61 201 50 166 55 275 39 206

17.7 78.8 15.6 40.0 13.4 35.9 13.6 54.3 9.2 41.7

a

Ideal IEC calculated from polymer composition and the degree of functional group. bExperimental IEC calculated from 1H NMR. cExperimental IEC measured by titration.

spectroscopy. As shown in Figure 2a, for the conventional QPPO, the peaks between δ = 6.0 and 7.5 ppm corresponded to the H atoms in the aromatic rings. The peaks that emerged at a δ

of 2.0 ppm were assigned to the H in the methyl group attached directly to the benzene ring and the appearance of the peaks from δ = 4.0 to 5.0 ppm identified the H atoms of the benzyl group. 5324

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Chemistry of Materials The peaks at δ = 3.1 ppm demonstrated the existence of the QA cationic group in the polymer structure. The 1H NMR spectra of the comb-shaped (QC3-PPO and QC6-PPO) and the side-chain (NC3Q-PPO and NC5Q-PPO) AEMs were more detailed than those of QPPO because of the resonances emerging from the H atoms of the alkyl chains. Specifically, for the comb-shaped and the side-chain PPOs, the peaks at δ = 2.0 ppm corresponded to the H atoms of the alkyl chain, except for the methylene groups attached to N atoms, which were found at δs of 3.0 to 3.2 ppm (Figure 2b to e). For the side-chain NC3Q-PPO and NC5Q-PPO, the peaks that emerged at δ = 2.4 ppm were for the H atoms in the methylene group tethered to the secondary amine (Figure 2b and c). The experimental IECs measured by both 1H NMR and titration methods, were close to those calculated from the chemical structures with a specific DF (Table 1), demonstrating that the related reactions can be conducted effectively under the reported conditions. OH− Conductivity and Swelling Ratio. High OH− conductivity and low swelling ratio are considered crucial requirements for AEMs in fuel cells. As shown in Figures 3 and 4, the conductivities and swelling ratios of AEMs were controlled by the chemical structures and IECs of the samples. Among the

Figure 4. Comparison of swelling ratio of PPO-based AEMs with different IECs measured at different temperatures. (a) Data for QPPOs (QPPO-40 (IEC = 2.27 mmol/g) and QPPO-60 (IEC = 3.22 mmol/g)) and NC3Q-PPOs (NC3Q-PPO-40 (IEC = 2.17 mmol/g) and NC3QPPO-60 (IEC = 2.79 mmol/g)) and QC3-PPOs (QC3-PPO-40 (IEC = 2.22 mmol/g) and QC3-PPO-60 (IEC = 2.81 mmol/g)) with shorter alkyl chain length. (b) Data for QPPOs (QPPO-40 (IEC = 2.27 mmol/ g) and QPPO-60 (IEC = 3.22 mmol/g)) and NC5Q-PPOs (NC5QPPO-40 (IEC = 2.03 mmol/g) and NC5Q-PPO-60 (IEC = 2.57 mmol/ g)) and QC6-PPOs (QC6-PPO-40 (IEC = 2.13 mmol/g) and QC6PPO-60 (IEC = 2.74 mmol/g)).

studied AEMs, NC5Q-PPO membranes exhibited the best overall performance. With the IECs of 2.03 and 2.57 mmol/g, the conductivities of NC5Q-PPO-40 and NC5Q-PPO-60 samples were 24.5 and 35.9 mS/cm at room temperature, and increased to 73.9 and 96.1 mS/cm at 80 °C, respectively (Figure 3b). Meanwhile, low water swelling of these membranes was observed. At room temperature, the swelling ratios of NC5QPPO-40 and NC5Q-PPO-60 were 7% and 20%, and at 80 °C, the swelling ratios were maintained at 15% and 28%, respectively (Figure 4b). In comparison, for all the other AEM structures reported in this work, the combination of high conductivity and low swelling ratios were difficult to achieve. For example, although the conductivities of QPPO-60 (IEC = 3.22 mmol/g) and QC6PPO-60 (IEC = 2.74 mmol/g) were comparable to that of the NC5Q-PPOs (Figure 3b), these samples swelled considerably in water, especially at elevated temperatures. Specifically, the swelling ratios of QPPO-60 (IEC = 3.22 mmol/g) and QC6PPO-60 (IEC = 2.74 mmol/g) were as high as 95% at 50 °C, and 57% at 60 °C, respectively (Figure 4b). With lower IECs, QPPO40 (IEC = 2.27 mmol/g) and QC6-PPO-40 (IEC = 2.13 mmol/ g) showed low swelling ratios (Figure 4b); however, their

Figure 3. Comparison of OH− conductivity of PPO-based AEMs with different IECs measured at different temperatures. (a) Data for QPPOs (QPPO-40 (IEC = 2.27 mmol/g) and QPPO-60 (IEC = 3.22 mmol/g)) and NC3Q-PPOs (NC3Q-PPO-40 (IEC = 2.17 mmol/g) and NC3QPPO-60 (IEC = 2.79 mmol/g)) and QC3-PPOs (QC3-PPO-40 (IEC = 2.22 mmol/g) and QC3-PPO-60 (IEC = 2.81 mmol/g)) with shorter alkyl chain length. (b) Data for QPPOs (QPPO-40 (IEC = 2.27 mmol/ g) and QPPO-60 (IEC = 3.22 mmol/g)) and NC5Q-PPOs (NC5QPPO-40 (IEC = 2.03 mmol/g) and NC5Q-PPO-60 (IEC = 2.57 mmol/ g)) and QC6-PPOs (QC6-PPO-40 (IEC = 2.13 mmol/g) and QC6PPO-60 (IEC = 2.74 mmol/g)). 5325

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number (λ = 78.8), the QPPO-60 showed a high D/D0 of 0.30. The intrinsic ion transport in the NC3-PPO and QC3-PPO membranes was poorer than that of the QPPOs. With λ values of 15.6 and 13.6, the D/D0 values of NC3-PPO-40 and QC3-PPO40 were 0.06 and 0.05, respectively. Even when the λ values of the NC3-PPO-60 and QC3-PPO-60 were increased to 40.0 and 54.3, the D/D0 values were as low as 0.13 and 0.14. Increasing the length of the alkyl chain enhanced ion transport in the side-chain and the comb-shaped PPOs. Although the λs of NC5-PPOs and QC6-PPOs were quite low, the ion transport in such AEMs was reasonably facile. The D/D0s of NC5-PPO-40 (λ = 13.4) and QC6-PPO-40 (λ = 9.2) were 0.08 and 0.07, respectively, which were close to that of the QPPO-40 (λ = 17.7), and higher than that of the NC3-PPO-40 (λ = 15.6) and QC3-PPO-40 (λ = 13.6). For the NC5-PPO-60 (λ = 35.9) and QC6-PPO-60 (λ = 41.7), their D/D0s were 0.18 and 0.22, which were 3.0 and 4.4 times larger than that of the NC3-PPO-60 (λ = 40.0) and QC3-PPO-60 (λ = 54.3) (Figure 5b). TEM was used to study the microphase-separated morphologies of different AEMs to understand the differences in their properties. As identified from Figure 6, there is no significant contrast in the micrograph of the QPPO film, suggesting no obvious microphase-separated morphology or ionic clustering in this material, as reported previously. Introducing hydrophobic alkyl side chains in AEMs induced hydrophilic/hydrophobic microphase separation, and the qualitative structure of the microphase-separated morphologies were determined by the structures of the alkyl chain-tethered QA groups. In the sidechain PPOs (NC3Q- and NC5Q-PPO), the hydrophilic species aggregated to form isolated ionic clusters with larger sizes than in the other samples. In contrast, in the comb-shaped PPOs (QC3and QC6-PPO), the smaller sized ionic clusters tended to have good interconnectivity. In addition, increasing the length of the alkyl chain inducted more distinct microphase separation in the side-chain and the comb-shaped PPOs. As depicted in Figures 6b−e, the sizes of the ionic clusters in NC5Q- and QC6-PPO with longer side chains were significantly larger than those in the NC3Q- and QC3-PPO samples with shorter side chains. Quantitative information on the size of the ionic clusters can be obtained from small-angle X-ray scattering (SAXS) analysis. In Figure 7, no ionic peak was observed in the SAXS signal of QPPO-60 (IEC = 3.22 mmol/g), which is in line with its TEM image (Figure 6a). For NC3Q-PPO-60 (IEC = 2.79 mmol/g) and NC5Q-PPO-60 (IEC = 2.57 mmol/g), scattering peaks emerged at 3.7 and 1.8 nm−1 (corresponding to Bragg spacings of 1.7 and 3.5 nm), respectively, indicating the aggregation of ionic clusters in the side-chain PPO system. In comparison to that of the side-chain PPOs, uniform microphase-separated morphologies were observed in the comb-shaped PPOs. The hydrophilic species tended to disperse evenly in such AEMs, rather than aggregate to form larger-sized ionic clusters. Specifically, there was no ionic peak detected in the QC3-PPO-60 (IEC = 2.84 mmol/g) sample with a propyl linker. A broad and low intensity peak with a qmax-value of 3.6 nm−1 (corresponding to a Bragg spacing of 1.7 nm) was observed for the QC6-PPO-60 (IEC = 2.74 mmol/g) sample. Inducing microphase separation in AEMs is known as a general method for enhancing ion conduction, and indeed, the distinct phase separation in NC5Q- and QC6-PPO samples were beneficial for increasing the ion transport in these materials. However, phase separation observed in the NC3Q-PPO membrane lead to poor ion conductive performance in this membrane. The low D/D0 (Figure 5b) for NC3Q-PPO and QC3-

conductivity was significantly lower than that of the NC5Q-PPO samples (Figure 3b). It is known that the length of the alkyl chain in side-chain and comb-shaped PPOs has a significant influence on AEM properties. With longer side chains, both the conductive and antiswelling performance of the NC5Q-PPOs and QC6-PPOs were greater than or similar to that of the QPPOs with similar IECs. However, with shorter side chains, the conductivities of NC3Q-PPOs and QC3-PPOs were notably poorer than the conductivities of the QPPOs with similar IECs (Figure 3a). Although the swelling ratios of NC3Q-PPO and QC3-PPO samples were lower than or close to that of the QPPOs (Figure 4a), in general, these low conductivity values will lead to poor fuel cell performance. Ion Transport and Morphology. The intrinsic ion conduction performance of different AEMs, which was elucidated by the metric of normalized ionic diffusion coefficient (D/D0) for OH− transport, was employed here to demonstrate the effect of the side chain structure on the ion transport in AEMs. As shown in Figure 5, AEMs with higher IECs always

Figure 5. Hydration number (λ, a) and ratio of the effective OH− diffusion coefficient, D, to the dilute solution OH− diffusivity, D0 (D/D0, b), as a function of alkyl chain length for AEMs at room temperature.

exhibited larger λs, which led to more effective ion transport, and thus, greater D/D0. The hydrophobic alkyl chain in the side-chain and comb-shaped AEMs restricted the water uptake of these samples, which decreased the λs of these samples compared to that of the QPPOs with similar IECs. For a specific type of AEM (i.e., side-chain or comb-shaped PPO), the use of longer alkyl chains resulted in lower λ values due to the increased hydrophobicity of the sample (Figure 5a). QPPO-40 showed a moderate D/D0 under relatively low λ (D/D0 = 0.08 at λ = 17.7). With an extremely high hydration 5326

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Figure 6. Transmission electron microscopy (TEM) images in Br− form for (a) QPPO-60 (IEC = 3.22 mmol/g), (b) NC3Q-PPO-60 (IEC = 2.79 mmol/g), (c) NC5Q-PPO-60 (IEC = 2.57 mmol/g), (d) QC3-PPO-60 (IEC = 2.84 mmol/g), and (e) QC6-PPO-60 (IEC = 2.74 mmol/g), respectively. The dark contrast in the images represents the hydrophilic domains where the Br− anions reside.

microphase separation displays good D/D0 under relatively low water uptake, high conductivity will be achieved. In the present study, NC5Q-PPO-60 with distinct microphase separation exhibited good D/D0 at a low hydration number, and this sample showed superior conductivity to the other AEMs in this work. In comparison, even though the hydration number and the D/D0 of the QPPO-60 sample were 2.2 and 1.6 times larger than that of the NC5Q-PPO-60 membrane, the conductivity of QPPO-60 was lower than that of NC5Q-PPO-60 under the same testing conditions, likely because of its low ion concentration in the swollen state and poorly developed morphology. Alkaline Stability. The relative alkaline stability of the QPPO-40, NC3Q-PPO-40, NC5Q-PPO-40, NC5Q-PPO-60, QC3-PPO-40, and QC5-PPO-40 membrane samples was tested 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. Stability data of QPPO-60, NC3Q-PPO-60, QC3-PPO-60, and QC5PPO-60 AEMs could not be achieved under the test conditions due to their solubility in hot alkaline solutions. As shown in Figure 8, the loss of OH− conductivity and IEC for the different AEMs was dependent on the structure of QA groups. The conventional QPPO and the comb-shaped PPO with different benzylic QA groups showed poor stability in alkaline conditions. Specifically, after 30 days of testing, the OH− conductivity of QPPO-40 (IEC = 2.27 mmol/g) with the BTMA cation and QC3-PPO-40 (IEC = 2.22 mmol/g) with the N-benzyl-N,N-dimethylpropyl-1-ammonium (QC3) cation decreased by 61.2% and 58.8%, respectively, and the loss of their IECs were as high as 56.8%. Increasing the length of the alkyl chain substituent led to enhanced stability in comb-shaped PPOs. After the stability test, the QC6-PPO-40 (IEC = 2.13 mmol/g) with a N-benzyl-N,N-dimethylhexyl-1-ammonium (QC6) cation showed a conductivity loss of 43.5%, while its IEC loss was 40.4%. As demonstrated by Marino and Kreuer, the existence of the benzylic group in QA moieties could not only

Figure 7. SAXS of QPPO-60 (IEC = 3.22 mmol/g), QC3-PPO-60 (IEC = 2.84 mmol/g), QC6-PPO-60 (IEC = 2.74 mmol/g), NC3Q-PPO-60 (IEC = 2.79 mmol/g), and NC5Q-PPO-60 (IEC = 2.57 mmol/g) membranes in Br− form at dry state.

PPO samples suggested that the short alkyl chain posed barriers within the ion conduction channels that hindered ion transport. Generally, there is a trade-off between D (the computed ion diffusion coefficient from conductivity measurements) and c (the ion concentration in the hydrated sample) in most AEM materials that we have studied. The D/D0 (D0 = ion diffusivity in dilute solution) is greatly affected by the water uptake and microphase-separated structure of the membrane. A large amount of water and obvious microphase separation could lead to low tortuosity of the ionic channels, which is good for efficient ion transport. As for the concentration of the ions in the hydrated material, c, high IEC and/or low water uptake bring about large c. As a result, if a high D/D0 of the AEM was caused only by high water uptake, the membrane usually will not show very good conductivity because of the low c. Ideally, if an AEM with obvious 5327

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carbon−carbon bond formation, which is difficult to achieve at scale. For high performance fuel cell applications, AEMs should possess high conductivity and excellent stability under a wide variety of operating conditions. High conductivity ensues low internal cell resistance and high discharge performance, while high stability can lead to long-term device operation, including under dynamic start−stop conditions and rest periods. To comprehensively evaluate these two critical qualities, we plotted the initial OH− conductivity (measured at 80 °C) of different AEMs against the retention of OH− conductivity after 30 days of stability testing in 1 mol/L NaOH solution. As depicted in Figure 9, there is an evident trend correlating these properties for this set

Figure 8. Chemical stability of the quaternary ammonium cation of QPPO-40 (IEC = 2.27 mmol/g), NC3-PPO-40 (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), NC5Q-PPO-60 (IEC = 2.57 mmol/g), QC3−PPO-40 (IEC = 2.22 mmol/g), and QC6-PPO-40 (IEC = 2.13 mmol/g) in 1 M NaOH solution at 80 °C: (a) OH− conductivity measured at 80 °C and (b) IEC as a function of time.

Figure 9. Comprehensive evaluation on the critical qualities of AEMs (QPPO-40 (IEC = 2.27 mmol/g), NC3-PPO-40 (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), NC5Q-PPO-60 (IEC = 2.57 mmol/g), QC3-PPO-40 (IEC = 2.22 mmol/g), and QC6-PPO-40 (IEC = 2.13 mmol/g)) by plotting the initial OH− conductivity measured at 80 °C against the retention of OH− conductivity after 30 days of stability test in 1 mol/L NaOH solution.

enhance the electrophilic character of QA group, but also stabilize the intermediates (i.e., radicals and carbanions at benzylic site) of the degradation reactions, leading to rapid decomposition of the QA groups in alkaline solutions.49 Placing a spacer between the QA and the aromatic backbone in side-chain PPOs ensured that the benzylic methylene group was not attached directly to a cation. As a result, the side-chain PPOs exhibited higher alkaline stability in comparison to the other two AEMs. Specifically, the IECs of NC3Q-PPO-40 (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), and NC5QPPO-60 (IEC = 2.57 mmol/g) films decreased to 1.54, 1.84, and 2.13 mmol/g, respectively. The OH− conductivities of these samples were maintained at 25.6, 66.4, and 76.9 mS/cm at 80 °C, which were 26.9, 10.1, and 19.8% lower than the values measured prior to the stability test. Additionally, in common with the comb-shaped PPOs with longer spacers, NC5Q-PPO samples showed better performance than that of the NC3Q-PPO membrane. The steric hindrance of the longer alkyl chain was likely to reduce the rate of Hoffman elimination, and thus, improve the AEM stability. Meanwhile, the more obvious phase separated morphology in NC 5Q-PPOs could have also contributed to the higher stability of the AEM in alkaline solution.36 Remarkably, there was no clear signature of hydroxide attack at the secondary nitrogen linker in the side chain PPO samples. This is an interesting observation that may point to new tethering chemistries for cationic side chains in addition to

of samples, which shows progress in both increasing conductivity and stability simultaneously. The conventional QPPO-40 containing the BTMA moiety exhibited moderate conductivity, but this sample showed the lowest stability in alkaline conditions. Introducing a propyl amine chain between the aromatic backbone and the QA group in NC3Q-PPO-40 improved the stability compared to the BTMA-based sample. The retention of the conductivity of NC3Q-PPO-40 was 73.1%, which was double that of the QPPO-40 sample. Although NC3Q-PPO-40 had higher stability, the conductivity of this AEM was low, which is not advantageous for application in a high-performance fuel cell device. Increasing the alkyl chain length of the side-chain cationic group not only further improved the alkaline stability, but also brought about high conductivity. Specifically, the retention of conductivity of NC5Q-PPO-40 and NC5Q-PPO-60 were 89.9% and 80.2%, while their initial conductivities were 73.9 and 96.1 mS/cm at 80 °C in liquid water, respectively. Replacing one of the methyl groups in BTMA by a propyl chain to give QC3-PPO40 did not increase the stability greatly, and worse, resulted in an AEM with extremely low conductivity. QC6-PPO-40 samples with a tethered hexyl chain exhibited significantly better chemical stability and ionic conductivity than the QC3-PPO membrane. However, the performance of the QC6-PPO-40 was still unsatisfactory, and significantly lower than the performance of the NC5Q-PPO series membranes. 5328

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(3) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Alternative polymer systems for proton exchange membranes (PEMs). Chem. Rev. 2004, 104, 4587−4612. (4) Steele, B. C. H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, 345−352. (5) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T.; Zhuang, L. Anion-exchange membranes in electrochemical energy systems. Energy Environ. Sci. 2014, 7, 3135−3197. (6) Liu, D.; Kyriakides, S.; Case, S. W.; Lesko, J. J.; Li, Y.; McGrath, J. E. Tensile behavior of Nafion and sulfonated poly(arylene ether sulfone) copolymer membranes and its morphological correlations. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1453−1465. (7) Choi, P.; Jalani, N. H.; Datta, R. Thermodynamics and proton transport in Nafion II. Proton diffusion mechanisms and conductivity. J. Electrochem. Soc. 2005, 152, E123−E130. (8) Kreuer, K.-D. Proton conductivity: materials and applications. Chem. Mater. 1996, 8, 610−641. (9) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Pt−Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect? Energy Environ. Sci. 2015, 8, 177−181. (10) Gu, S.; Cai, R.; Luo, T.; Chen, Z.; Sun, M.; Liu, Y.; He, G.; Yan, Y. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew. Chem., Int. Ed. 2009, 48, 6499−6502. (11) Varcoe, J. R.; Slade, R. C. T. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005, 5, 187−200. (12) Li, G.; Pan, J.; Han, J.; Chen, C.; Lu, J.; Zhuang, L. Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications. J. Mater. Chem. A 2013, 1, 12497−12502. (13) Pan, J.; Li, Y.; Han, J.; Li, G.; Tan, L.; Chen, C.; Lu, J.; Zhuang, L. A strategy for disentangling the conductivity−stability dilemma in alkaline polymer electrolytes. Energy Environ. Sci. 2013, 6, 2912−2915. (14) Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 20611−20614. (15) Hu, Q.; Li, G.; Pan, J.; Tan, L.; Lu, J.; Zhuang, L. Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode. Int. J. Hydrogen Energy 2013, 38, 16264−16268. (16) Gu, S.; Sheng, W.; Cai, R.; Alia, S. M.; Song, S.; Jensen, K. O.; Yan, Y. An efficient Ag−ionomer interface for hydroxide exchange membrane fuel cells. Chem. Commun. 2013, 49, 131−133. (17) Varcoe, J. R.; Slade, R. C. T.; Wright, G. L.; Chen, Y. Steady-state dc and impedance investigations of H2/O2 alkaline membrane fuel cells with commercial Pt/C, Ag/C, and Au/C cathodes. J. Phys. Chem. B 2006, 110, 21041−21049. (18) Poynton, S. D.; Kizewski, J. P.; Slade, R. C. T.; Varcoe, J. R. Novel electrolyte membranes and non-Pt catalysts for low temperature fuel cells. Solid State Ionics 2010, 181, 219−222. (19) Pan, J.; Chen, C.; Li, Y.; Wang, L.; Tan, L.; Li, G.; Tang, X.; Xiao, L.; Lu, J.; Zhuang, L. Constructing ionic highway in alkaline polymer electrolytes. Energy Environ. Sci. 2014, 7, 354−360. (20) Pan, J.; Lu, S.; Li, Y.; Huang, A.; Zhuang, L.; Lu, J. Highperformance alkaline polymer electrolyte for fuel cell applications. Adv. Funct. Mater. 2010, 20, 312−319. (21) Chen, C.; Pan, J.; Han, J.; Wang, Y.; Zhu, L.; Hickner, M. A.; Zhuang, L. Varying the microphase separation patterns of alkaline polymer electrolytes. J. Mater. Chem. A 2016, 4, 4071−4081. (22) Zhu, L.; Pan, J.; Christensen, C. M.; Lin, B.; Hickner, M. A. Functionalization of poly(2,6-dimethyl-1,4,-phenylene oxide)s with hindered fluorene side chains for anion exchange membranes. Macromolecules 2016, 49, 3300−3309. (23) Zhu, L.; Zimudzi, T. J.; Li, N.; Pan, J.; Lin, B.; Hickner, M. A. Cross-linking of comb-shaped polymer anion exchange membranes via thiol-lene click chemistry. Polym. Chem. 2016, 7, 2464−2475. (24) Pan, J.; Zhu, L.; Han, J.; Hickner, M. A. Mechanically tough and chemically stable anion exchange membranes from rigid-flexible semiinterpenetrating networks. Chem. Mater. 2015, 27, 6689−6698.

CONCLUSIONS In summary, to avoid the use of a benzylic tether or a methylene moiety that is positioned directly between an aromatic ring and a cation, which is commonly used to attach a quaternary ammonium (QA) cation to aromatic backbone-based AEMs, novel side-chain PPO AEMs (NC3Q-PPO and NC5Q-PPO) with alkyltrimethylammonium cationic groups have been designed and synthesized. Superior alkaline stability was observed in the side-chain PPOs compared to the benzylic QA-based AEMs, QPPO and two comb-shaped PPOs (QC3PPO and QC6-PPO). NC5Q-PPOs samples with an n-pentyl amine spacer were more stable than that of the NC3Q-PPO npropyl amine side-chain materials. Specifically, after 30 days of degradation in 1 M NaOH solution at 80 °C, the conductivities of NC3Q-PPO, NC5Q-PPO-40, and NC5Q-PPO-60 were maintained at 25.6, 66.4, and 76.9 mS/cm (at 80 °C), which were 26.9%, 10.1%, and 19.8% lower than the values obtained for the samples before the stability test, respectively. In comparison, QPPO-40, QC3-PPO-40, and QC6-PPO-40 were rather unstable in hot alkaline solution. Although QC6-PPO-40 showed the highest stability among these three samples, the IEC and conductivity decreased significantly. The retention of the conductivity of QC6-PPO-40 was just 56.5%, after the stability test. In addition to the improvement in degradation resistance, introducing a long and flexible spacer induced distinct hydrophilic/hydrophobic microphase separation in NC5Q-PPO membranes, leading to high conductivity and low water swelling. Specifically, with the IECs of 2.03 and 2.57 mmol/g, NC5QPPO-40 and NC5Q-PPO-60 showed hydroxide conductivities of 73.9 and 96.1 mS/cm at 80 °C, with limited swelling ratios of 15% and 28%, respectively.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jing Pan: 0000-0003-4324-4761 Liang Zhu: 0000-0003-4551-0963 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge our industrial sponsors. This work was funded in part by the U.S. Department of Energy, EERE Fuel Cell Technologies Office under award number: DEEE0006958, 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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