Poly(olefin)-Based Anion Exchange Membranes Prepared Using

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Poly(olefin)-Based Anion Exchange Membranes Prepared Using Ziegler−Natta Polymerization Liang Zhu,† Xuedi Yu,† Xiong Peng,‡ Tawanda J. Zimudzi,† Nayan Saikia,† Michael T. Kwasny,§ Shaofei Song,∥ Douglas I. Kushner,† Zhisheng Fu,∥ Gregory N. Tew,§ William E. Mustain,‡ Michael A. Yandrasits,⊥ and Michael A. Hickner*,†

Downloaded by UNIV AUTONOMA DE COAHUILA at 17:36:11:516 on May 29, 2019 from https://pubs.acs.org/doi/10.1021/acs.macromol.8b02756.

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Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States § Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States ∥ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ⊥ Corporate Research Laboratory, Electrochemical Components Lab, 3M Center, St. Paul, Minnesota 55144-1000, United States S Supporting Information *

ABSTRACT: Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1butene with 11-bromo-1-undecene using Ziegler−Natta polymerization. The resulting bromoalkyl-functionalized poly(olefin)s were converted to quaternary ammonium-containing anion-conductive copolymers by reacting the pendant bromoalkyl group with trimethylamine or a custom-synthesized tertiary amine containing pendant quaternary ammonium moieties. Poly(olefin)-based AEMs with three cations per side chain showed considerably higher hydroxide conductivities, up to 201 mS/cm at 80 °C in liquid water, compared to that of samples with only one cation per bromoalkyl site (68 mS/cm, 80 °C, liquid water), likely due to phase separation in the triple-cation structure. More importantly, triple-cation side-chain poly(olefin) AEMs exhibited higher hydroxide conductivity under relative humidity conditions (50%−100%) than typical AEMs based on benzyltrimethylammonium cations. The triple-cation the triple-cation side-chain poly(olefin)-based AEM exhibited an ionic conductivity as high as 115 mS/cm under 95% RH at 80 °C and 11 mS/cm under 50% RH at 80 °C. In addition to high ionic conductivity, the triple-cation side-chain poly(olefin) AEMs exhibited good chemical and dimensional stability. High retention of ionic conductivity (>85%) was observed for the samples in 1 M NaOH at 80 °C over 1000 h. Based on these high-performance poly(olefin) AEMs, a fuel cell with a peak power density of 0.94 W cm−2 (1.28 W cm−2 after iR correction) was achieved under H2/O2 at 70 °C. The results of this study suggest a new, low-cost, and scalable route for preparation of poly(olefin)-based AEMs for anion exchange membrane applications.

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INTRODUCTION By converting energy-dense chemical fuels into electricity without emission of harmful pollutants, such as SOx and NOx, fuel cells can provide clean, efficient, and reliable power to stationary and mobile electronic devices.1−4 In a fuel cell, electricity is generated by means of an electrochemical reaction that is promoted by an overall negative Gibbs free energy between chemical fuel and the oxidant.5 In a proton exchange membrane fuel cell (PEMFC), one of the key materials is the solid ionic polymer membrane which serves as both the proton transport medium and the separator for isolating the cathode and anode.6,7 The most frequently used membrane in PEMFCs is Nafion, a perfluorinated sulfonic acid resin with a perfluorinated backbone and pendant perfluorosulfonic acid side groups.8,9 Nafion variants, and its related structures, are © XXXX American Chemical Society

the state-of-the-art proton exchange membranes (PEM) with high proton conductivity, excellent chemical stability, and good mechanical properties, ensuring high performance and durability in PEMFCs.10−13 The widespread application of PEMFCs based on perfluorosulfonic acid membranes, however, has been hindered by significant drawbacks including their high cost and the environmental hazards of processing perfluorinated materials. Additionally, most PEMFCs require precious metals as electrocatalysts which greatly increases the cost of the device.6,14 Received: December 31, 2018 Revised: February 21, 2019

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DOI: 10.1021/acs.macromol.8b02756 Macromolecules XXXX, XXX, XXX−XXX

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and potential cost of preparation.56 In contrast, poly(olefin)based AEMs prepared by Ziegler−Natta polymerization have become a target in the community due to their high chemical stability and low cost of preparation.56,62,63 While the previously reported poly(olefin)-based AEMs showed high alkaline stability, they exhibited moderate ionic conductivity. In addition, the cell performance based on poly(olefin) AEMs via Ziegler−Natta catalyzed copolymerization in the previous report was extremely low (peak power density ≤40 mW cm−2), which is far removed from practical application.56,62,63 Thus, more direct routes to stable, conductive materials that show high performance in devices are desired. In the current approach reported here, we synthesized a series of new poly(olefin)-based AEMs with two kinds of cationic side chains via a one-pot olefin copolymerization and subsequent Menshutkin reaction to study the relationship between the structure of the side chain and the AEM performance in terms of ionic conductivity, dimensional stability, and alkaline stability of the materials. When the number of cations on the side chain was increased from one to three, the poly(olefin)-based AEMs exhibited lower swelling, much higher ion conductivity, and significantly better chemical stability than single cation side-chain variantshighlighting the importance of phase separation in these materials. This report details the structure−property relationships of poly(olefin)-based AEMs and provides a new, low-cost, and scalable route for preparation of poly(olefin)-based AEMs for practical alkaline fuel cell applications.

To break the reliance of fuel cell technology on noble metals, especially Pt, as electrocatalysts, solid polymer membrane electrolyte fuel cells with cationic membranes, i.e., anion exchange membrane fuel cells (AEMFCs), have been suggested.6,15,16 Compared to PEMFCs, AEMFCs have the potential to employ non-noble metals as electrocatalysts and may also have enhanced kinetics for the cathode reaction since these fuel cells operate under alkaline conditions,14−18 resulting in performance improvements of the device and a reduction in fuel cell cost.19−22 Because of these advantages, in the past decade an increasing effort has been placed on developing advanced AEM materials, membrane-electrode assembly fabrication technologies, and device operational strategies for AEMFCs.6 Among recent research activities focused on alkaline-based devices, research on developing high-performance AEM materials has gained a lot of attention.6 The major challenge in developing high-performance AEM materials for AEMFCs is designing and synthesizing highly ion conducting polymer membranes which show good chemical and thermal stability and maintain low degrees of swelling to promote sufficient mechanical strength under the cell operating conditions.19−24 A variety of polymers with aromatic backbone main chains ranging from poly(arylene ethers)25−30 to poly(sulfone)s31−35 to poly(phenylene oxide)s5,6,24,36−43 have been employed as AEM materials. However, arylene ether bonds in aromatic AEMs are likely to degrade under alkaline conditions at elevated temperature.44−46 AEMs with polymer backbones containing no ether bonds, such as poly(phenylene)s,44,47,48 poly(styrene)s,49−53 spiroionene,54 poly(benzimidazolium),55 and poly(olefin)s,56−58 have been shown to be relatively chemically stable under alkaline conditions. For example, poly(phenylene)-based hydroxide ion-conducting AEMs synthesized by Hibbs et al.59 demonstrated excellent chemical stability after treatment in 1 M KOH at 90 °C for 336 h. In contrast, Arges and Ramani46 revealed that poly(sulfone)-based AEMs suffered a rapid failure (168 h in 2 M KOH at 60 °C) due to polymer backbone degradation in alkaline conditions via polymer backbone hydrolysis. The previous studies showed that the polymer backbone has a major influence on the chemical stability of AEM materials. Recently, inspired by the previous studies, the design and development of poly(olefin)-based AEMs have garnered increased attention because of the relatively stable chemical structures afforded by an all aliphatic backbone design.56−58 Among the possible designs for aliphatic backbone AEMs, ring-opening metathesis polymerization (ROMP) has been employed as a common method to prepare poly(olefin)-based AEMs. For example, Kostalik et al.60 reported a solvent processable, tetraalkylammonium-functionalized polyethylenebased AEM synthesized via ROMP from a tetraalkylammonium-functionalized monomer and cyclooctene by using Grubbs second-generation catalyst.60 These types of AEMs exhibited hydroxide conductivity as high as 40 mS/cm in deionized water at 20 °C with IEC = 1.29 mmol/g.60 In the development of highly chemically stable AEMs, Noonan et al.58 reported a cation-bearing monomer based on Schwesinger’s work61 to enable tetrakis(dialkylamino)phosphoniumfunctionalized polyethylene. The resulting AEM demonstrated only 18% loss of hydroxide conductivity after exposure to 15 M KOH for 20 weeks at room temperature.58 However, the synthetic route employed to achieve these AEMs was multistep, Ru-catalyzed, and low yield, resulting in complexity

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EXPERIMENTAL SECTION

Materials. All manipulations of air- and/or moisture-sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk techniques. 11-Bromo-1-undecene was purchased from Sigma-Aldrich and distilled over calcium hydride (CaH2) under vacuum before further use. TiCl3·AA, AlEt2Cl (1.0 M in heptane), allylmagnesium bromide solution (1.0 M in diethyl ether), 4methylbenzyl bromide, N,N,N′,N′-tetramethyl-1,6-hexanediamine, and (5-bromopentyl)trimethylammonium bromide were obtained from Sigma-Aldrich and used as received. PTFE plate stock was purchased from McMaster-Carr, and the PTFE mold was machined in-house. Synthesis of 4-(4-Methylphenyl)-1-butene. To a stirring solution of allylmagnesium bromide (1.20 L, 1.20 mol), 4methylbenzyl bromide (185.06 g, 1.00 mol) in anhydrous tetrahydrofuran (400 mL) was added dropwise and stirred overnight at room temperature. The reaction was quenched with saturated aqueous ammonium chloride and extracted with dichloromethane. The combined organic layer was dried over MgSO4 overnight and concentrated under reduced pressure. The crude product was purified by column chromatography over silica gel with hexane as the eluent to give a colorless liquid (yield 83%). Synthesis of 1-(N′,N′-Dimethylamino)-6,11-(N,N,Ntrimethylammonium)undecane Bromide. 1-(N′,N′-Dimethylamino)-6,11-(N,N,N-trimethylammonium) undecane bromide was synthesized according to a previous report.6 To 400 mL of chloroform, 200 mmol of N,N,N′,N′-tetramethyl-1,6-hexanediamine and 20 mmol of (5-bromopentyl)trimethylammonium bromide were dissolved and heated at 60 °C for 12 h. After evaporation of the chloroform, the residual liquid reagent was removed under vacuum at elevated temperature. The crude product was further purified through recrystallization in methanol. The recrystallized product was dried in a vacuum oven at room temperature with a yield of 89%. The chemical structure and purity of the product were characterized by 1H NMR in DMSO-d6. Synthesis of Poly(4-(4-methylphenyl)-1-butene-co-11bromo-1-undecene).64 In a typical copolymerization reaction, 50 mL of toluene was introduced into a 100 mL glass bottle equipped B

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Macromolecules with a magnetic stirrer. The reactor was injected with 11-bromo-1undecene (2.0 g, 8.58 mmol) as a comonomer and 4-(4methylphenyl)-1-butene (6.4 g, 43.83 mmol). 0.05 g of TiCl3·AA and 1.0 mL of AlEt2Cl (1.0 M in heptane) were added to the glass bottle to initiate copolymerization. The reactivity ratios of 4-(4methylphenyl)-1-butene (r1) and 11-bromo-1-undecene (r2) evaluated by the Fineman−Ross equation were 1.09 and 1.06, respectively.63 The results indicated a tendency for statistical copolymerization.63 After 3 h of reaction at 70 °C, the polymerization was quenched by methanol, and the polymer solution was precipitated in acidic methanol solution. The produced polymer was washed with acidic methanol several times and then dried in a vacuum oven at 60 °C overnight with a yield of 67%. The resulting copolymer was bromoalkyl-functionalized poly(olefin) (M20C9, where the M stands for methyl substituent on the aromatic ring; the 20 refers to 20 mol % bromoalkyl-functionalized units as determined by 1H NMR of the isolated polymer product; the C9 means the spacer between the main chain and the bromo moiety is nine methylenes). Fabrication of Membranes. The copolymer M20C9 (0.40 g) was dissolved in tetrahydrofuran (10 mL) to yield a 4 wt % solution. The solution was then cast onto a leveled PTFE mold and dried at 50 °C under ambient pressure for 24 h followed by vacuum drying for another 24 h at 50 °C to give a transparent, tough film (60 ± 5 μm in thickness). Subsequently, the membrane was immersed in trimethylamine solution at 35 °C for 72 h to obtain quaternized poly(olefin)based anion exchange membranes. The resulting AEM is M20C9N (where the M stands for methyl substituent on the aromatic ring, the 20 refers to 20 mol % bromoalkyl-functionalized units, and the C9 means the spacer between the main chain and N+C4 is nine methylenes). For the preparation of the triple-cation poly(olefin)based AEMs, the copolymer M20C9 (0.80 g) was dissolved in tetrahydrofuran (20 mL) to yield a 4 wt % solution. The solution was then cast onto a leveled PTFE mold and dried at 50 °C under ambient pressure for 24 h followed by vacuum drying for another 24 h at 50 °C to give a transparent, tough film (60 ± 5 μm in thickness). Subsequently, the membrane was immersed in 1-(N′,N′-dimethylamino)-6,11-(N,N,N-trimethylammonium)undecane bromide (30% w/v) ethanol solution at 50 °C for 5 days to obtain triple-cation poly(olefin)-based AEMs. The resulting AEM was M20C9NC6NC5N, where the M stands for methyl substituent on the aromatic ring, the 20 refers to 20 mol % bromoalkylfunctionalized units, the C9 means the spacer between the main chain and first N+C4 is nine methylenes, the C6 represents that the spacer between the first N+C4 and the second N+C4 is 6 methylenes, the second N indicates the second N+C4 in the side chain, C5 means the spacer between the second and third ammonium group is 5 methylenes, and the third N refers to the third N+C4 with a CH3 terminal group from the cation. Characterization. 1H nuclear magnetic resonance (NMR) spectra were recorded at 300 MHz on a Bruker (Bruker Co., Billerica, MA) AV 300 spectrometer, and chemical shifts were listed in parts per million (ppm) downfield from tetramethylsilane (TMS). Ionic conductivity (σ) was measured by impedance spectroscopy on a Solartron 1260 A impedance/gain-phase analyzer (Solartron Analytical, Farnborough, Hampshire, UK) with a two-point, in-plane geometry at frequencies ranging from 100 kHz to 100 Hz.65 During the ionic conductivity measurements, temperature and humidity were controlled with an ESPEC SH-241 environmental chamber. The temperature was held at 80 °C, while the relative humidity (RH) was controlled from 50% to 100%. The membrane resistance was acquired from the real value of the impedance where the imaginary response was zero. The ionic conductivity, σ (mS/cm), of each membrane was calculated from the equation σ = L/RA, where L is the distance between reference electrodes, R is the resistance of the membrane, and A is the crosssectional area of the sample. Bicarbonate conductivities were measured by exchanging the bromide form membranes in 1 M sodium bicarbonate for 24 h followed by rinsing to remove excess salt. Chloride conductivities were measured by exchanging the bromide form membranes in 1 M NaCl at room temperature for 24 h followed

by extensive rinsing in fresh deionized water to remove excess salt. Under an argon atmosphere, hydroxide conductivities were measured by exchanging the bromide form membranes in 1 M NaOH for 24 h followed by rinsing to remove excess salt with degassed and deionized water. The membranes were subsequently placed into conductivity cells and immersed in deionized water that was degassed and blanketed with flowing argon gas to avoid atmospheric CO 2 contamination. Water uptake was measured after drying the membranes in their corresponding counterion forms at 60 °C under vacuum for 24 h. The dried membrane was immersed in deionized water and periodically weighed on an analytical balance until a constant mass was obtained, giving the mass-based water uptake. Water uptake was calculated via (WU = (mhyd − mdry)/mdry), where mhyd is the hydrated membrane mass and mdry is the dry sample mass. The hydration number (λ), or the number of water molecules per ionic group, was calculated from

ji mhyd − mdry zyzijjj 1000 yzzz zzjj λ = jjj j zj M H OIEC zzz m0 k {k 2 {

(1)

where MH2O is the molecular mass of water (18 g mol−1) and IEC is the ion exchange capacity with units of milliequivalents of ion per gram of polymer. The swelling ratio (SR) was characterized by the linear expansion ratio of the membrane sample, which was determined using the difference between wet and dry dimensions of a membrane sample (3 cm in length and 1 cm in width). The calculation was based on the following equation: SW (%) =

X wet − Xdry Xdry

× 100% (2)

where Xwet and Xdry are the lengths of the wet and dry membranes, respectively. To determine the titrated gravimetric IEC values, membranes in the OH− form were immersed in 50 mL of 0.01 M HCl standard solutions for 24 h. Then, the solutions were titrated with a standardized NaOH (0.01 M) solution to pH = 7. Subsequently, the samples were washed and immersed in deionized water for 24 h to remove the residual HCl and then dried under vacuum at 50 °C overnight and weighed to calculate the dry masses in the Cl− form. The IEC of the membranes was calculated using eq 3:

IEC =

ni(H+) − n f(H+) mdry(Cl)

(3)

where mdry(Cl) is the mass of dry membranes, ni(H+) is the initial amount of H+ in the HCl solution, and nf(H+) is the final amount of H+ in the HCl solution. FTIR measurements were performed in attenuated total reflection (ATR) geometry using a Bruker Optics (Billerica, MA) Vertex 70 instrument equipped with a Harrick Scientific (Pleasantville, NY) MVP Pro ATR accessory with a diamond ATR crystal set at an incident angle of 45°. Spectra were collected at a resolution of 4 cm−1 collected at a scan rate of 5 kHz using a room temperature deuterated triglycine sulfate (DTGS) detector, and 400 scans averaged to produce a spectrum. The DTGS detector was used to access peaks at frequencies below 500 cm−1. All spectra were processed using Bruker Optics Opus software. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Physical Electronics (Chanhassen, MN) VersaProbe II instrument equipped with a monochromatic Al Kα X-ray source (hν = 1486.7 eV) and a concentric hemispherical analyzer. Charge neutralization was performed using both low-energy electrons ( 2.0 mmol/g could not be obtained since the higher IEC samples were highly water swollen and became too brittle to characterize. Usually, increased water content improves conductivity, but AEMs with higher IECs exhibit lower ionic conductivity where the membrane is too water swollen and ionic dilution occurs due to the high water uptake.6 When comparing the hydroxide conductivity of MxC9NC6NC5N and MxC9N membranes, the MxC9NC6NC5N samples demonstrated much higher hydroxide conductivity than that of the MxC9N samples at similar IECs. We attributed the higher hydroxide conductivity of MxC9NC6NC5N to the microphase-separated morphology formation discussed in the next section, which can greatly facilitate ion transport in AEM materials, thus enhancing the hydroxide conductivity.6 As shown in Figure 4a, the M20C9NC6NC5N sample exhibited a

the bromoalkyl moiety at 3.4 ppm demonstrated a successful coordination polymerization reaction. The weight-average molecular weights (Mw) of poly(4-(4-methylphenyl)-1-butene-co-11-bromo-1-undecene)s were in the range of (342− 395) × 103 g/mol (Table S1). As shown in Figure 2a, the reaction of poly(4-(4methylphenyl)-1-butene-co-11-bromo-1-undecene) (M30C9)

Figure 2. Evidence of the Menshutkin reaction between M30C9 and NC6NC5N. (a) FTIR for M30C9 and M30C9NC6NC5N. (b) XPS for M30C9 and M30C9NC6NC5N. (c) 13C solid-state NMR of M20C9NC6NC5N and M20C9N.

with 1-(N′,N′-dimethylamino)-6,11-(N,N,N-trimethylammonium)undecane bromide (NC6NC5N) was confirmed by the appearance of the three quaternary ammonium group v(CN+) stretch vibrational peaks at 914, 956, and 967 cm−1.66 The appearance of the δ(CN+) bending vibration at 1485 cm−1 also confirms the inclusion of NC6NC5N in the polymer network.65,69 The high intensity of these peaks is consistent with three cations per chain. The complete disappearance of E

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Macromolecules Table 1. Properties of the Poly(olefin)-Based Membrane Samples sample

IECa (mmol/g)

IECb (mmol/g)

M30C9N M20C9N M13C9N M30C9NC6NC5N M20C9NC6NC5N M13C9NC6NC5N M5C9NC6NC5N

1.90 1.23 0.86 3.55 2.76 2.02 0.91

1.76 1.15 0.76 3.07 2.41 1.69 0.73

WUc (wt %) (OH−) 237 129 53 357 193 94 30

± ± ± ± ± ± ±

15 11 9 21 14 7 3

WUc (wt %) (HCO3−) 213 107 46 237 146 63 25

± ± ± ± ± ± ±

WUc (wt %) (Cl−)

10 8 3 12 11 3 2

197 90 41 175 110 53 23

± ± ± ± ± ± ±

10 5 3 10 9 2 1

σc (mS/ cm) (OH−) 34 25 14 45 66 42 24

± ± ± ± ± ± ±

4 3 2 3 4 2 2

σc (mS/cm) (HCO3−) 8 6 3 9 14 9 5

± ± ± ± ± ± ±

1 1 1 1 1 1 1

σc (mS/ cm) (Cl−)

λ (OH−)

in-plane swellingc (%)

± ± ± ± ± ± ±

75 58 36 51 36 25 18

49 32 15 65 41 22 9

11 9 5 14 21 14 8

1 1 1 1 2 2 1

a

Calculated from the polymer composition and the degree of functionalization. bTitrated values. cMeasured at room temperature in water.

Figure 4. Hydroxide conductivity of poly(olefin)-based membranes in liquid water at room temperature as a function of (a) IEC and (b) hydration number (λ).

Figure 3. (a) Liquid water uptakes and (b) swelling ratios of poly(olefin)-based membranes in OH− form as a function of IEC at room temperature.

displayed a hydroxide conductivity up to 18.9 mS/cm at room temperature (IEC = 2.17 mmol/g). For a better comparison among the samples with different IECs, the hydroxide conductivity was plotted as a function of λ (the number of absorbed water molecules per ammonium group) in Figure 4b. The approximate trend was that the hydroxide conductivity increased with λ because of the increased number of water molecules per ionic site, facilitating the transport of hydroxide ions.24 Under a given λ value, as shown in Figure 4b, the MxC9NC6NC5N samples displayed much higher hydroxide conductivities than the MxC9N series. For example, M20C9NC6NC5N with λ = 36 showed a hydroxide conductivity of 66 mS/cm, while the M13C9N sample with the same λ value displayed a much lower hydroxide conductivity of 14 mS/cm. Compared to the homogeneous MxC9N membranes, microphase separation occurred between the hydrophobic poly(olefin) backbone and hydrophilic triple-cation side chain of MxC9NC6NC5N

maximum hydroxide conductivity of 66 mS/cm at room temperature in liquid water, which is significantly higher than that of previous reported poly(olefin)-based AEMs. For example, the polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS)-based AEMs synthesized by Lin et al.75 exhibited hydroxide conductivity up to 25 mS/cm at room temperature in liquid water. Zhang et al.56 reported poly(olefin)-based AEMs with bulky poly(4-methyl-1-pentene) moieties demonstrating hydroxide conductivity of 43 mS/cm at room temperature with IEC = 1.92 mmol/g. The polybutadiene-b-poly(4-methylstyrene) (PB-b-P4MS) AEMs reported by Li et al.76 exhibited hydroxide conductivity of 24 mS/cm at 23 °C. Zhang et al.62 reported quaternized polypropylene (PP)-based membranes exhibiting hydroxide conductivity of 19 mS/cm at room temperature in water with IEC = 1.66 mmol/g. The poly(olefin) AEM with bulky poly(4phenyl-1-butene) moieties synthesized by Zhu et al.63 F

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Macromolecules samples, resulting in an increased local ion concentration and enhanced ion mobility.24,77,78 Thus, the triple-cation side-chain poly(olefin)-based AEMs demonstrated significantly higher ionic conductivities at similar λ values compared to the MxC9N samples.24,77,78 One of the obstacles to develop advanced AEMs is the tradeoff between hydroxide conductivity and swelling ratio. The hydroxide conductivity of the MxC9N membranes was enhanced by increasing the IEC of the samples. As depicted in Figure 5, the M30C9N sample with an IEC of 1.76 mmol/g

Figure 6. Hydroxide conductivity of poly(olefin)-based and PPOBTMA40 membranes in liquid water as a function of temperature.

AEMs reported by Li et al.72 exhibited hydroxide conductivity of 73 mS/cm at 60 °C with IEC = 1.92 mmol/g in water. Compared to PEMs, AEMs are thought to be inferior in terms of ionic conductivities, especially under relative humidity (RH) conditions. Typically, high proton conductivity (≥200 mS/cm) under 100% RH at 80 °C can easily be achieved in PEMs due to the higher mobility of protons compared to hydroxide.76 In contrast, the hydroxide conductivity of AEMs under 100% RH at 80 °C is much lower than that of PEMs, typically in the range 50−100 mS/cm.79,80The hydroxide conductivity of AEMs under operating humidity in the cell is important to fuel cell device performance. Figure 7 shows the RH dependence of the hydroxide conductivity in the range

Figure 5. Hydroxide conductivity of poly(olefin)-based membranes in liquid water at room temperature as a function of in-plane swelling ratio.

exhibited a hydroxide conductivity of 34 mS/cm at room temperature, which was much higher than that of the M13C9N sample (14 mS/cm in liquid water, room temperature). However, the enhanced hydroxide conductivity of the MxC9N series through increasing the IEC of the materials was accompanied by a large increase in swelling ratios. In contrast, the triple-cation side-chain poly(olefin)-based AEMs maintained a good balance between hydroxide conductivity and swelling ratio. As shown in Figure 5, the M13C9NC6NC5N sample exhibited a hydroxide conductivity of 42 mS/cm with a swelling ratio of 22% in liquid water, while M30C9N with similar IEC showed lower hydroxide conductivity (34 mS/cm) but suffered from severe swelling (49%). This result is attributed to the formation of microphase-separated structures in the triple-cation side-chain poly(olefin)-based sample that helps to boost the hydroxide conductivity of membranes without excessive hydration and large swelling degrees.5,6 The hydroxide conductivity of the AEMs at the operating temperature of the fuel cell impacts the performance of the device. Figure 6 illustrates the temperature dependence of the hydroxide conductivity of the poly(olefin)-based and PPOBTMA40 (Scheme S1) membranes in liquid water. The M20C9NC6NC5N membrane demonstrated the highest hydroxide conductivity (201 mS/cm) at 80 °C, which is almost 5 times higher than that of PPO-BTMA40 under the same conditions and significantly higher than in previous reported results. For example, Zhang et al.56 reported that poly(olefin)-based AEMs with bulky poly(4-methyl-1-pentene) moieties displayed hydroxide conductivity of 81 mS/ cm at 80 °C with IEC = 1.92 mmol/g. The polystyrene-bpoly(ethylene-co-butylene)-b-polystyrene (SEBS)-based AEMs synthesized by Lin et al.75 demonstrated a hydroxide conductivity up to 56.4 mS/cm at 80 °C in liquid water. The polybutadiene-b-poly(4-methylstyrene) (PB-b-P4MS)

Figure 7. Hydroxide conductivity of Nafion 212, M20C9NC6NC5N, M30C9N, and PPO-BTMA40 membranes as a function of relative humidity at 80 °C. G

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ing to an interdomain spacing of ∼6.5 nm. In contrast, no ionic peak was observed for M20C9N, suggesting no microphase separation was present in the sample. We attributed the increased ion conductivity and enhanced dimensional stability of the triple-cation side-chain poly(olefin)-based AEMs to the well-developed hydrophilic/hydrophobic microphase separation in these materials.6,19,38,41,74 Alkaline Stability. The chemical stability of the cations and polymer backbones is of importance in new materials because AEMs with quaternary groups are known to degrade under alkaline conditions. Examples of possible degradation pathways for the cations with alkyl spacers include β-hydrogen (Hofmann) elimination, direct nucleophilic substitution at the α-carbon, and nitrogen ylide formation.81−85 To evaluate the long-term alkaline stability of the AEMs, M20C9NC6NC5N, M20C9N, and PPO-BTMA40 membrane samples were immersed in argon-saturated 1 M NaOH solution at 80 °C for 1000 h. The degradation of the samples was investigated by measuring the change in the hydroxide conductivity of the samples as a function of time. As shown in Figure 9, the

50−100% RH at 80 °C. The M20C9NC6NC5N sample demonstrated a comparable RH conductivity to Nafion 212. For example, the ionic conductivities at 95% RH and 80 °C of Nafion 212 and M20C9NC6NC5N were 124 and 115 mS/cm, respectively. In contrast, PPO-BTMA40 with tethered benzyltrimethylammonium cations showed much lower ionic conductivity than that of Nafion 212 at the same conditions. For example, the ionic conductivity of the PPO-BTMA40 membrane at 95% RH and 80 °C is 23 mS/cm, which is 6 times lower than that of Nafion 212 under the same conditions. Formation of a hydrophobic/hydrophilic microphase-separated morphology is likely the reason for the enhanced RH conductivity in the triple-cation side-chain poly(olefin) AEMs.24,77,79 Morphology Characterization. Higher conductivity in polymeric membranes for fuel cell applications usually results from better-developed ion transport pathways as the materials is hydrated.6,41 Generally, for different membranes with similar IECs and water uptake, it is desirable to form microphaseseparated ionic domains, which facilitate ionic channels for ion transport.6,41 To investigate the microphase-separated morphologies of the poly(olefin)-based AEMs in this study, SAXS patterns were recorded for the M20C9NC6NC5N and M20C9N samples. As shown in Figure 8, an obvious scattering

Figure 9. Conductivity loss of M20C9NC6NC5N, M20C9N, and PPO-BTMA40 membranes in 1 M NaOH solution at 80 °C. Hydroxide conductivity measured at room temperature as a function of aging time.

M20C9NC6NC5N sample displayed the greatest alkaline stability during the testing period. The hydroxide conductivity of the M20C9NC6NC5N membrane decreased by 15.8% over 1000 h. Compared to the M20C9NC6NC5N, M20C9N exhibited moderate cation stability where the hydroxide conductivity of M20C9N decreased by 20% after 1000 h testing. We attributed the enhanced alkaline stability of M20C9NC6NC5N to the microphase separation of hydrophobic and hydrophilic domains, which is in good agreement with previous reports.6,41 In contrast, PPO-BTMA40 with tethered benzyltrimethylammonium cations showed poor alkaline stability and lost 60% of its initial hydroxide conductivity after only 500 h immersion in 1 M NaOH at 80 °C. Compared to the PPO-BTMA40 sample based on poly(phenylene oxide), the significantly enhanced chemical stability of poly(olefin)-based AEMs was attributed to the chemical stability of poly(olefin) backbones under nucleophilic conditions.44−46 Fuel Cell Performance. Polarization and power density curves for an AEMFC with a PtRu/C anode, Pt/C cathode, and the M20C9N6NC5N or M20C9N AEMs operating at 70 °C under optimized reacting gas dew points are shown in Figure 10. The AEMFC based on M20C9N6NC5N achieved a

Figure 8. (a) Schematic structures of the poly(olefin)-based AEMs and (b) SAXS profiles of the dry membranes in the bromide form.

peak was observed for M20C9NC6NC5N, indicating microphase separation between the nonpolar poly(olefin) backbone and the hydrophilic triple-cation side chains. The absence of a second-order scattering peak for the poly(olefin)-based AEMs demonstrates that no long-range ordered structures were present, but only locally correlated arrangement of microphaseseparated domains existed.6,41The location of the ionic peak for the M20C9NC6NC5N sample was 0.97 nm−1, correspondH

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cations per side chain, enabling a study of how the arrangement of the cations on the side chain impacts poly(olefin)-based AEM properties. Compared with the typical poly(olefin)-based AEMs with a single cation on the side chain, the membranes with triple-cation side chains demonstrated considerably higher hydroxide ion conductivities but lower swelling ratios due to the to the formation of microphase-separated structures. More importantly, triplecation side-chain poly(olefin)-based AEMs displayed high RH conductivity from 50% RH to 100% RH. In addition to high ion conductivity, the triple-cation side-chain poly(olefin)based AEMs exhibited good chemical and dimensional stabilities, likely due to the phase separation in these materials. High retention of ionic conductivity was observed for the triple-cation side-chain poly(olefin)-based AEMs during alkaline stability testing under 1 M NaOH at 80 °C for 1000 h. The results of this study suggest a new, low-cost, and scalable route for preparation of poly(olefin)-based AEMs for practical alkaline fuel cell applications.

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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.8b02756. Chemical structures of BTMA40, 1H NMR of 4-(4methylphenyl)-1-butene in CHCl3-d1, GPC results for MxC9, tabulated XPS results for M30C9 and M30C9NC6NC5N (PDF)

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Figure 10. i−V and i−power curves: (a) H2/O2 AEMFC performance without back-pressure; (b) H2/air AEMFC performance 0.05 MPa back-pressure for both anode and cathode. Catalyst loading: anode 0.60 mgPtRu cm−2; cathode 0.60 mgPt cm−2. −2

AUTHOR INFORMATION

Corresponding Author

*Tel +1 814 867 1847; Fax +1 814 865 2917; e-mail mah49@ psu.edu (M.A.H.).

−2

peak power density of 0.94 W cm (1.28 W cm after iR correction) under H2/O2 at 70 °C (Figure 10a), ranking as one of the best AEM demonstrations in terms of single cell performance to date.67,68,86−89 The peak power density is 23 times higher than that of AEMFC based on copolymer prepared by Ziegler−Natta polymerization in the previous report.56 In contrast, the AEMFC based on the M20C9N sample only demonstrated a peak power density of 0.16 W cm−2 under the same conditions, which is 6 times lower than that of AEMFC based on the M20C9N6NC5N membrane. The performance of this cell with the M20C9N6NC5N membrane was mainly achieved through the membrane’s high ionic conductivity, and reasonable cell water management, in addition to excellent electrode performance and tuned operational conditionsall key factors together needed for achieving high AEMFC performance. To further explore the AEMFC performance of this membrane, the same cell configuration was subjected to reacting gases of H2/air (CO2 free) and was able to achieve peak power density of 0.66 W cm−2 (0.78 W cm−2 after iR correction) (Figure 10b) at 70 °C, which further verified the excellent performance of this membrane. In contrast, the AEMFC based on the M20C9N membrane exhibited a peak power density of 0.12 W cm−2 under the same conditions, which is 5 times lower than that of AEMFC based on the M20C9N6NC5N sample.

ORCID

Liang Zhu: 0000-0003-4551-0963 Xiong Peng: 0000-0001-8737-5830 Douglas I. Kushner: 0000-0002-3020-7737 Gregory N. Tew: 0000-0003-3277-7925 William E. Mustain: 0000-0001-7804-6410 Michael A. Hickner: 0000-0002-2252-7626 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is acknowledged from the U.S. Department of Energy through Advanced Research Projects Agency-Energy (ARPA-E) Award DE-AR0000776 and the Fuel Cell Technologies Office Award DE-EE0008433.0001. M.A.H acknowledges the Corning Foundation for fellowship support and the Penn State Materials Research Institute and Penn State Institutes for Energy and the Environment for infrastructure support.

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REFERENCES

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CONCLUSIONS In summary, we have designed and synthesized new poly(olefin)-based side-chain-containing AEMs with one or three I

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DOI: 10.1021/acs.macromol.8b02756 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02756 Macromolecules XXXX, XXX, XXX−XXX