Stable Elastomeric Anion Exchange Membranes Based on Quaternary

Sep 17, 2015 - Department of Chemistry and Chemical Biology, New York State .... Miles Page , Chulsung Bae , Yushan Yan , Piotr Zelenay , Yu Seung Kim...
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Stable Elastomeric Anion Exchange Membranes Based on Quaternary Ammonium-Tethered Polystyrene‑b‑poly(ethylene-cobutylene)‑b‑polystyrene Triblock Copolymers Angela D. Mohanty,† Chang Y. Ryu,† Yu Seung Kim,‡ and Chulsung Bae*,† †

Department of Chemistry and Chemical Biology, New York State Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, United States ‡ MPA-11: Materials Physics and Application, Sensors and Electrochemical Device Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *

ABSTRACT: A chemically stable and elastomeric triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), was functionalized with various benzyl- and alkyl-substituted quaternary ammonium (QA) groups for anion exchange membrane (AEM) fuel cell applications. Synthetic methods involving transition metal-catalyzed C−H borylation and Suzuki coupling were utilized to incorporate six different QA structures to the polystyrene units of SEBS. Changes in AEM properties as a result of different QA moieties and chemical stability under alkaline conditions were investigated. Anion exchange polymers bearing the trimethylammonium pendants, the smallest QA cation moiety, exhibited the most significant changes in water uptake and block copolymer domain spacing to offer the best ion transport properties. It was demonstrated that incorporating stable cation structures to a polymer backbone comprising solely C−H and C−C bonds resulted in AEM materials with improved longterm alkaline stability. After 4 weeks in 1 M NaOH at 60 and 80 °C, SEBS-QA AEMs remained chemically stable. Fuel cell tests using benzyltrimethylammonium-containing SEBS (SEBS-TMA) as an AEM demonstrated excellent performance, generating one of the best maximum power density values and lowest ohmic resistance with low Pt catalyst loaded electrode reported thus far. Both polymer backbone and cation functional group remained stable after 110 h lifetime test at 60 °C.

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INTRODUCTION

mechanical stabilities under strong alkaline environment and low anion conductivity. It is generally recognized that the stability of both polymer backbone and cation functional group play a crucial role in device durability. Aromatic polymers continue to be the most widely used base materials in AEM applications because they boast good thermal and mechanical stability and are easy to synthesize and modify.9,10 So far poly(arylene ether)s, such as Udel polysulfone (PSU), are the most commonly used polymer backbone for chemical modification to introduce ionic functionalities. Unfortunately, researchers have discovered that such aryl ether-containing polymers undergo cleavage of

A variety of energy storage and conversion electrochemical devices, such as polymer electrolyte membrane fuel cells, redox flow batteries, and water electrolysis, rely on ion-conducting polymer electrolyte membranes to separate and transport ions between the anode and cathode.1−4 Among these membranes, anion exchange membranes (AEMs) continue to receive increased attention because of their advantages of fast liquid fuel oxidation reaction in alkaline media, efficient water management, and the ability to use nonprecious metal electrocatalysts for oxygen reduction reaction.5−7 Additionally, AEM-based electrochemical devices (as opposed to the liquid alkaline system) prevent leakage of corrosive fuels and carbonate precipitation.6,8 However, the most significant challenges currently preventing the advancement of AEMs in clean energy conversion technology are their poor chemical and © XXXX American Chemical Society

Received: June 24, 2015 Revised: September 4, 2015

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DOI: 10.1021/acs.macromol.5b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of QA-Functionalized SEBS via C−H Borylation and Suzuki Coupling Reactions

optimum tethered cation structures is important not only for improved long-term AEM durability but also for greater fuel cell performance because hydroxide ion conductivity is heavily dependent on the interactions of tethered cation and water. Other researchers have found similar enhancements in chemical stability by introducing a long alkyl chain between QA center and polymers.23,24 Herein, we report postfunctionalization of polystyrene-bpoly(ethylene-co-butylene)-b-polystyrene (SEBS), a commercial thermoplastic elastomer, with various benzyl- and alkylsubstituted QA groups for AEM fuel cell applications. SEBS was chosen as the polymer backbone since it can provide good chemical stability in alkaline media owing to its absence of aryl ether bonds. Additionally, SEBS has advantage of being a high molecular weight (>100 kg/mol) triblock copolymer with rubbery ethylene−butylene units, which can afford good mechanical properties and enhanced anion conductivity via tailoring nanoscale phase separated morphology. Previously, there have been several attempts to prepare SEBS-based AEMs; however the adopted synthetic process (i.e., chloromethylation) did not fully utilize the great potential of this elastomeric material for electrochemical applications. For example, a number of research groups have prepared benzyltrimethylammonium-functionalized SEBS via chloromethylation synthetic procedures.25,26 The chloromethylation of SEBS resulted in low levels of functionalization and gelation issues arose if a higher degree of functionalization (DF) was attempted. To incorporate a greater amount of ionic groups, SEBS with a higher styrene content was attempted.27 Yet their materials exhibited low conductivity and again faced gelation issues of the chloromethylation process. In this article, we report the synthesis of six different examples of benzyl- and alkyl-substituted SEBS-QAs using transition metal-catalyzed aromatic C−H bond functionalization and subsequent Suzuki coupling synthetic methods28−33 and systematic study of the relationships between tethered QA structures and alkaline fuel cell membrane properties. Because

the C−O bonds via SNAr attack of hydroxide anion when electron-withdrawing cation functional groups are in close proximity.11 For example, Arges and Ramani utilized 2D NMR spectroscopy to exemplify the ether bond hydrolysis of quaternary ammonium (QA)-functionalized PSU.12 Since pristine PSU and chloromethylated PSU remained stable under the same alkaline conditions, they concluded that the tethered cations trigger the backbone degradation. In 2014, Amel et al. further demonstrated that the electron-withdrawing sulfone linkage in PSU has a profound influence on its alkaline stability.13 They found that QA-functionalized PSU degraded much faster than QA-functionalized poly(phenylene oxide), a poly(arylene ether) without an electron-withdrawing sulfone linkage, under similar alkaline treatment. More recently, Choe et al. showed by computational modeling and experimental methods that when an AEM polymer backbone contains aryl ether bonds, the C−O bond cleavage is more favored (i.e., it has a lower energy barrier toward OH− attack) over the degradation of tethered QA group.14 But when the polymer backbone contains no aryl ether bonds, such as poly(phenylene)s,15 the polymer backbone remains stable. Although polymer backbone degradation has become an increased topic of interest in recent AEM research, instability of the cationic functional group continues to remain a major technical challenge.16 Among tethered cation structures, benzyltrimethylammonium cation is the most frequently employed QA in AEMs because it can be tethered to aromatic polymers readily. Recently, Pivovar et al. provided computational and experimental evidence that the benzyltrimethylammonium cation may be more stable than previously believed.17−19 Our group has recently demonstrated with small molecule QA stability studies that alkyl-tethered cation moieties also have comparable or greater stability in alkaline media than benzyl-substituted ones.20,21 Hibbs and Tomoi et al. have independently provided evidence suggesting improved cation stability when QAs are separated from the polymer backbone via an alkyl spacer.15,22 The identification for B

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Macromolecules

original 30 mol % aromatic rings was borylated; thus the total percentage of borylated styrene units in SEBS-Bpin was 26 mol %. To replace the boronate ester on the polymer chain with an amine group, palladium-catalyzed Suzuki cross-couplings with amine-containing aryl halides (labeled as An in Scheme 1) were conducted. Various structures of An (where n = 1−5) were synthesized as described in Supporting Information (Scheme S1). The Suzuki coupling reactions reliably gave full conversion of the boronate ester to the desired amines, yielding SEBS-An. As evidenced in Figure 2, new resonances from the

all six SEBS-QAs have the same DF, the effects of different QA structures on polymer membrane properties can be systematically studied for the development of ionic channels at the molecular level. The six QAs chosen were demonstrated to have good alkaline stability in our previous small molecule model QA studies.20,21 More importantly, because a variety of SEBS starting materials with various levels of styrene content are readily available, selective incorporation of these QA moieties to a range of SEBS triblock copolymers will lead to the development of AEMs having diverse ranges of morphology, mechanical properties, and fuel cell membrane performance. In this article, we will first focus on the chemical modification of SEBS containing 30 mol % styrene content.

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RESULTS AND DISCUSSION Synthesis of Anion-Conducting SEBS Polymers. Commercially available SEBS triblock copolymer containing total 30 mol % styrene was functionalized via our group’s optimized transition metal-catalyzed synthetic method.28 By this synthetic approach, amines of various structures were efficiently incorporated to the styrene units of SEBS (Scheme 1). First, iridium-catalyzed aromatic C−H borylation was conducted to yield borylated polymer, SEBS-Bpin. The borylation is known to convert the C−H bonds of aromatic sp2 carbons selectively to C−B bonds while leaving the C−H bonds of alkyl sp3 carbons unaffected.34 Both 1H and 13C NMR spectra confirmed successful borylation of the aromatic rings (Figure 1). In 13C NMR spectrum a new resonance at 24.9 ppm Figure 2. Representative 1H NMR spectra for amine-functionalized SEBS in CDCl3: (A) SEBS-A1, (B) SEBS-A2, (C) SEBS-A3, (D) SEBS-A4, and (E) SEBS-A5.

incorporated amines were clearly visible in the 1H NMR spectra. Additionally, the 13C NMR resonance at 24.9 ppm from the Bpin methyl group disappeared after the Suzuki coupling. The DFs of the amines were estimated based on the relative intensity of the methyl group in the 1,2-butylene unit of the polymer chain (peak c at 0.78−0.88 ppm of Figure 2) with respect to the aromatic-tethered CH2 groups of the amines (peaks e, g, k, o, and u at 2.50−3.75 ppm of Figure 2) and were confirmed to match within 5% of the borylation degree. SEBSAn polymers were fully soluble in THF, CHCl3, and toluene at room temperature, as well as in hexanes when heated. Molecular weights of the polymers were measured before and after borylation using GPC (Figure S16). The C−H borylation resulted in negligible changes in molecular weight and PDI of SEBS (Mw = 105 kg/mol and PDI = 1.04 for SEBS; Mw = 114 kg/mol and PDI = 1.06 for SEBS-Bpin). Unfortunately, after Suzuki coupling to exchange the boronate ester for the desired amine, GPC of the amine-containing polymers could not be analyzed due to interaction of the amine functional groups with the GPC column. However, our group’s previous study of similar functionalization of polysulfone demonstrated that Suzuki coupling of borylated polymers does not alter the polymer’s molecular weight.28 Since the final QA-functionalized SEBS polymers were not soluble in any solvents, the tertiary amines of SEBS-An were converted to QAs via the methylation of precast SEBS-An films under heterogeneous conditions. First, thin films (approximately 30−50 μm thick) of SEBS-An were cast from a toluene solution onto Teflon plates. These light-yellow colored films

Figure 1. Representative 1H NMR spectra for (A) SEBS and (B) SEBS-Bpin, and zoomed-in 13C NMR spectra for (C) SEBS and (D) SEBS-Bpin in CDCl3.

appeared (peak d in Figure 1D) due to the four methyl groups of the pinacolboronate ester (Bpin). No other changes were found in 20−45 ppm suggesting that the ethylene−butylene units of SEBS remained intact during the C−H borylation. In 1 H NMR spectrum, the aromatic region of SEBS-Bpin was split into three broad resonances in 6.0−7.8 ppm (peaks a and e of Figure 1B). The proton resonance of the Bpin methyl groups (peak d at 0.9−1.5 ppm) overlapped with the methylene resonance of SEBS backbone (peak b of Figure 1B). Therefore, the DF was estimated based on the relative intensity of the methyl group in 1,2-butylene unit of the polymer chain (peak c at 0.78−0.88 ppm) with respect to the increased integral ratio of the overlapping SEBS-methylene and Bpin methyl resonance (peaks b and d at 0.9−1.5 ppm). It was found that 89% of the C

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Macromolecules were flexible, elastomeric, and transparent (Figure S1). Then, these films were immersed in a N,N-dimethylacetamide solution of dimethyl sulfate at 50 °C for 48 h to methylate the nitrogen atoms, giving the five different SEBS-QAs shown in Scheme 1. A sixth QA-functionalized SEBS, SEBS-DMHA, was additionally prepared via alkylation of SEBS-A1 with 1bromohexane (Scheme 2). To ensure complete quaternization

determine the conversion of the amines to QAs, all SEBS-QA membranes were analyzed with elemental analysis for their chlorine-to-nitrogen molar ratio. Elemental analysis confirmed complete quaternization for SEBS-TMA, SEBS-DMP, SEBSTMHA, and SEBS-DMHA (Table 1). Note that the chlorineto-nitrogen ratio for SEBS-DMP is expected to be 1-to-2 since methylation will only occur at terminal nitrogen atom (this was confirmed with a small molecule DMP structure and will be reported in future publications). The chlorine content, however, was lower than expected for SEBS-MCH and SEBSMiPr, thus indicating that incomplete quaternization occurred for these membranes. Because MCH and MiPr contain bulky and hydrophobic moieties, we suspect that complete quaternization of their amines was not achieved. IEC values were also measured from titration of the final QA form polymers using two different titration methods (Table S1). Unfortunately, the titrated IECs deviate significantly from the elemental analysis IEC values for certain AEMs. Apparently, our titration methods do not accurately measure the quantity of QAs present. Unreliable IEC titrations have frequently been reported in AEM literature and have been attributed to several factors including systematic errors during titration, incomplete ion exchange, incomplete drying, or inaccessible cation groups due to polymer sterics.11,15,35−43 It is unclear at this point why titrated IEC values of certain polymers were lower than expected. To avoid confusion, only IEC values obtained from elemental analysis are reported in Table 1, while a summary of titrated IEC values are reported in Supporting Information (Table S1). As a result of incomplete conversions to QA forms for SEBSMCH and SEBS-MiPr, greater focus of this report will be directed toward analysis of SEBS-TMA, SEBS-DMP, SEBSTMHA, and SEBS-DMHA, which yielded complete quaternization based on elemental analysis. Thermal and Mechanical Properties. Thermal and mechanical properties of SEBS before and after postfunctionalization were analyzed. The thermal degradation of these polymers was examined with thermogravimetric analysis (TGA), as shown in Figure S3. Pristine SEBS exhibited a sharp one-step degradation at 416 °C, whereas all SEBS-QA polymers in Cl− form showed a two-step degradation process. SEBS-QA polymers had a first weight loss around 200 °C due to the degradation of the cationic group and a second weight loss around 400 °C due to the degradation of polymer main chain, which are consistent with previous reports.25,26,44 Note that some SEBS-QAs showed 5−10% weight loss at 50−150 °C

Scheme 2. Synthesis of SEBS-DMHA via Alkylation of SEBSA1

that might arise from the steric hindrance of polymer, this reaction was accomplished using a N,N-dimethylacetamide solution of 1-bromohexane at 100 °C. Once quaternized, polymers were initially obtained in either methyl sulfate or bromide anion forms; they were then ion exchanged to Cl−, HCO3−, and OH− counteranion forms by immersing in 1 M NaCl, 1 M NaHCO3, and 1 M NaOH solutions, respectively. Since all SEBS-QA AEMs were derived from the same borylated precursor polymer, they should have similar theoretical ion-exchange capacity (IEC) values ranging 1.77− 2.41 mequiv/g (Table 1). The theoretical IEC values were determined from the amine concentrations of the precursor polymers, SEBS-An, in 1H NMR spectra assuming that all amines were converted to the corresponding QAs. To

Table 1. Ion-Exchange Capacity, Elemental Analysis, Water Uptake, and Ion Conductivity Data of SEBS-QA Membranes OH− σ (mS/cm)f SEBS-QA SEBS-TMA SEBS-DMP SEBS-MCH SEBS-MiPr SEBS-TMHA SEBS-DMHA

IECa (mequiv/g) 2.41 2.06 1.77 2.07 2.01 2.01

b

(2.19) (1.96)b (0.71)b (0.54)b (1.95)b (1.91)b

elem. analysisc (Cl/N ratio)

WUe (%)

HCO3− σ (mS/cm) 30 °C

Cl− σ (mS/cm) 30 °C

30 °C

60 °C

0.91:1.0 0.95:2.0d 0.40:1.0 0.26:1.0 0.97:1.0 0.95:1.0

211 249 68 49 236 194

12 9 2 2 8 7

13 10 3 3 9 9

45 19 8 7 39 28

89 (102)g 33 10 9 59 (63)g 34

a

Theoretical IEC values calculated from the concentrations of amine precursors in 1H NMR spectra. bIECs estimated from elemental analysis of Clto-N ratios. cCl-to-N ratio measured from elemental analysis data (all membranes were ion exchanged to chloride counteranion form after methylation). dDMP theoretically should have a 1:2 ratio of Cl/N since the functional group contains two N atoms with methylation occurring at only one N. eWater uptake was measured at rt in OH− form (average of three measurements). fAll OH− σ were measured in water under argon atmosphere and the values are an average of two measurements. gOH− σ measured at 80 °C. D

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Macromolecules due to the release of trapped water molecules in the hygroscopic membranes. To probe the effects of chemical modification on thermal transition behaviors, the chemically modified polymers were investigated using differential scanning calorimetry (DSC). Two or three thermal transitions are expected for SEBS: (1) a low glass transition temperature (Tg1) corresponding to the ethylene-co-butylene block, (2) a high glass transition temperature (Tg2) corresponding to the styrene block, and (3) a broad endothermic transition at the melting temperature (Tm) near 20 °C, depending on the degree of crystallinity of the ethyleneco-butylene block.45,46 Figure S4 shows the DSC curves for SEBS, SEBS-A1, and SEBS-TMA. The first transition around −55 °C represents Tg1 of the flexible ethylene-co-butylene elastomeric domains in SEBS. As expected, the low glass transition temperature did not change after the functionalization because the ethylene-co-butylene block of SEBS-A1 and SEBS-TMA was not chemically modified. The anticipated high glass transition corresponding to Tg2 of the styrene block was not clearly visible in DSC, possibly due to the low styrene content.45 A broad endothermic peak in the vicinity of 20 °C was observed, which is consistent with Tm as a result of crystal formation of the ethylene chains.45 Because the changes in Tg2 of the functionalized styrene domains could not be visualized with DSC, we sought an alternative method of thermal analysis. Dynamic mechanical thermal analysis (DMTA) tests were performed for SEBS and SEBS-TMA in Cl− form (Figure S5). The DMTA data displays two glass transitions of pristine SEBS with tan delta peak maxima at −34 and 106 °C for Tg1 and Tg2 of elastomeric ethylene and rigid styrene phases, respectively, which agree with the literature observations.45,46 After functionalization of the aromatic rings, the first tan delta peak for the rubbery block in the SEBS-TMA slightly shifted to −43 °C. The second peak corresponding to the glassy block of TMA-modified PS became broadened with reduced intensity because of the plasticization of the glassy domains by water molecules bound to TMA segments via the ionic interactions. The mechanical properties of an AEM are important for fabrication of robust membrane electrode assemblies (MEAs) and long-term stability of membranes during fuel cell operations. The mechanical properties of SEBS-TMA and SEBS-TMHA were measured at 50 °C under controlled relative humidity (RH) conditions (Figure 3). Due to their high elastomer-like nature, we could not accurately measure the elongation at break of the membranes at high RH: both samples reached the instrumental limit without breaking. This remarkable enhancement of elongation at break for the elastomeric SEBS-QAs at high RH is a result of the pronounced plasticization of the glassy domains, offering improved ductility via craze plasticity.47,48 However, the elongation at break under dry conditions (0% RH) dramatically decreased while the modulus and tensile strength increased. Because SEBS-TMA has shorter ionic side chain and higher IEC than SEBS-TMHA (see Table 1), the former membrane had slightly higher strength at 0% RH but became more flexible as RH increased. Membrane Transport Properties. AEM properties, such as IEC, water uptake (WU), and ion conductivity of the six SEBS-QA polymers are summarized in Table 1. Note that all QA-functionalized polymers were prepared from the same batch of SEBS-Bpin precursor having 26 mol % borylated styrene units; thus all SEBS-QA polymers have the same DF.

Figure 3. Stress−strain curves of (A) SEBS-TMA and (B) SEBSTMHA measured at 50 °C with 0%, 50%, and 90% relative humidity (RH).

Ion conductivities were measured in HCO3−, Cl−, and OH− forms at 30 °C for each membrane. All films were observed to have OH− conductivity in the range of 1−3 times greater than HCO3− and Cl− conductivities, which agrees with the anion mobility in dilute solution.15,49 Additionally, OH− conductivity significantly increased when the measurement temperature was raised to 60 °C. All OH− conductivity values correlated well with their WUs, suggesting that the ion conductivity is strongly dependent on the water contents. SEBS-TMA, SEBS-DMP, SEBS-TMHA, and SEBS-DMHA exhibited high WUs of 211%, 249%, 236%, and 194%, respectively. SEBS-DMP showed the highest WU overall probably because its QA moiety contains two nitrogen atoms, thus allowing greater interactions with water molecules. Despite the highest WU, it did not translate to the highest conductivity. This reduced conductivity of SEBS-DMP may have resulted from poor contact between the membrane surface and electrode (it has a great tendency to fold) or dilution of the ion concentration due to its exceptionally high WU. In contrast, SEBS-MCH and SEBS-MiPr exhibited significantly lower WUs of 68% and 49%, respectively, as a result of the shielding of nitrogen cation with sterically hindered hydrophobic dicyclohexyl and diisopropyl moieties. This shielding effect additionally resulted in incomplete quaternization of the amines (refer to elemental analysis data in Table 1). The lower WUs negatively impacted anion conductivity in comparison to the other SEBS-QA AEMs. SEBS-DMHA and SEBS-TMHA are constitutional isomers with almost identical theoretical IECs; they have the same numbers of C, H, and N atoms but different arrangement of the E

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ening of the high-order scattering peaks may suggest that longrange periodically ordered structure is not well developed, possibly due to insufficient time for ordering of domains in solvent-cast membranes.51,52 All SEBS-QA polymers retained nanoscale phase separation after functionalization with a correlation peak q* between 0.01 and 0.02 Å−1; however depending on the structure of tethered cations, they displayed morphology structures that differed from that of parent SEBS. SEBS-QA polymers containing one methyl and two bulky hydrophobic pendant groups (−N+CH3R2; that is, SEBS-MiPr and SEBS-MCH) displayed the first-order scattering peak at q* = 0.02 Å−1, virtually unchanged position from that of SEBS (Figure 4A). It has been shown that morphologies can also be influenced by choice of solvent and solvent-annealing techniques;53 however in our case, attempts to improve the long-range ordering development of the block copolymer morphology by solvent annealing with toluene were not successful. The significantly different morphology of SEBS-MCH and SEBS-MiPr in comparison to all others may have been influenced by the incomplete quaternization of the sterically hindered amines. In contrast, SEBS-QA polymers containing two or more methyl pendant groups (i.e., SEBS-TMA, SEBS-DMP, SEBSDMHA, and SEBS-TMHA) displayed peaks that were shifted to lower q* values suggesting that significant hydration of the QA moieties during the solvent casting occurred. Compared with pristine SEBS, the loss of high-order peaks in these SEBSQA polymers is also indicative of the lack of periodic nanoscale structure, which is different from the SAXS profile observed in the pristine SEBS. The d-spacing values from the new q* peak maxima were calculated as 44, 40, 38, and 34 nm for SEBSTMA, SEBS-DMP, SEBS-DMHA, and SEBS-TMHA, respectively. The correlation between d-spacing and water uptake is shown in Figure 4B to highlight the effectiveness of molecular design for the pendant groups of QA structure with respect to the hydration ability and domain spacing of SEBS-QA polymers. The quaternized polymers containing two bulky aliphatic chains made the pendant QAs too hydrophobic to effectively interact with water and to undergo d-spacing changes. However, the QA polymers with at least two methyl substituents were more hydrophilic and exhibited significantly higher water uptake (>200%) and noticeable changes in domain spacing. Alkaline Stability Analysis. All six QAs chosen for incorporation to SEBS were found to be quite stable in our small model compound studies.20,21 Among them, TMHA demonstrated to have the best alkaline stability, while TMA showed the lowest stability in the model QA studies conducted at 120 °C. To investigate whether the stability of these cations in small molecules can be translated into stability of polymer membranes, the alkaline stability of SEBS-TMA and SEBSTMHA was evaluated by immersing the membranes in 1 M NaOH solutions at 60 and 80 °C up to 4 weeks. These two membranes also have the highest ion conductivity and seem to be the most promising AEMs. Because the QA-functionalized membranes were insoluble in any solvents, their stability could not be confirmed by NMR spectroscopy. Monitoring changes in IECs after the alkaline treatment cannot be used either because we found that the titrated IEC values do not agree well with the theoretical IEC values despite complete quaternization for certain SEBS-QAs. To confirm the presence of tetheredQAs, the alkaline test-treated and untreated SEBS-TMA and

atoms in the ionic side chains. Between the two polymers, alkyltethered SEBS-TMHA showed a significantly higher WU and OH− conductivity than benzyl-tethered SEBS-DMHA. This is probably because the hydrophobic hexyl tail of SEBS-DMHA shields the ammonium cation and reduces the interactions with water, thus lowering WU and conductivity. SEBS-TMA showed the highest OH− conductivity (reaching up to 89 mS/cm at 60 °C and up to 102 mS/cm at 80 °C) due to its highest IEC and relatively high water uptake among the six SEBS-QA AEMs. From the data of Table 1, we concluded that compact cation structures located at the end of side chains (e.g., trimethylammonium of SEBS-TMA and SEBS-TMHA) yield the best anion transport properties of AEMs regardless of how they are tethered to the polymer backbone (a plot of OH− conductivity vs experimental IEC from elemental analysis data is shown in Figure S2). Morphology Characterization by SAXS. Figure 4A shows the small-angle X-ray scattering (SAXS) profiles of

Figure 4. (A) SAXS profiles for SEBS and SEBS-QAs measured at room temperature. The data is plotted as logarithm of thickness normalized intensity (I) as a function of q. To avoid overlaps of the SAXS profiles, the data is vertically shifted. The solid arrows represent first- and higher-order scattering peaks. The dashed vertical line was inserted as a guide for the eye. (B) Correlation between d-spacing and water uptake of SEBS-QAs.

solvent-cast SEBS and SEBS-QA membranes. For comparison of the changes in q*, a dotted line was inserted at the q* peak position of SEBS. Pristine SEBS displayed three broad scattering peaks at q*, √3q*, and √4q*, suggesting a cylindrical morphology. With the equation d = 2π/q*, the domain spacing (d) at q* = 0.02 Å−1 was calculated to be 31 nm for SEBS before the QA functionalization. This selfassembled morphology of the SEBS triblock copolymer is driven by the inherent chemical incompatibility between the styrene blocks and ethylene-co-butylene blocks.50 The broadF

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Macromolecules SEBS-TMHA membranes were studied with Cl− conductivity, TGA, and FT-IR spectroscopy. Although benzyltrimethylammonium is the most widely used cation structure in AEMs, prior evaluations of the cation’s stability in AEMs have been hampered by concurrent degradation of aryl ether bonds in polymer backbones. Since SEBS does not contain such cleavable ether bonds, the alkaline stability results of SEBS-TMA will provide an informative assessment of the stability of benzyltrimethylammonium when it is attached to a polymer for AEM applications. As shown in Figure 5, FT-IR analysis revealed no changes in the C−N

Figure 5. Zoomed FT-IR spectra for (A) SEBS-A1, (B) SEBS-TMA before alkaline treatment, (C) SEBS-TMA after 1 week, and (D) SEBS-TMA after 4 weeks in 1 M NaOH at 80 °C. Spectra were referenced to the aromatic CC bend at 1612 cm−1. To avoid overlaps the graphs are vertically shifted.

Figure 6. TGA curves of (A) SEBS-TMA series polymers and (B) SEBS-TMHA series polymers (before and after 4 weeks in 1 M NaOH at 60 and 80 °C). Analysis was conducted in Cl− form.

stretch band at 1221 cm−1 after 1 week and 4 weeks of 1 M NaOH treatment at 80 °C (also refer to Figure S7). This suggests that the benzyltrimethylammonium functional group of SEBS-TMA remained intact. Additionally, the TGA curves of SEBS-TMA after alkaline treatment closely matched the original two-step degradation curve of the initial untreated membrane: the first weight loss around 200 °C of SEBS-TMA after 4 weeks of alkaline treatments at 60 and 80 °C was virtually unchanged from that of untreated SEBS-TMA (Figure 6A). Again, this data strongly supports that the TMA-functional groups remained intact after the alkaline treatments. Figure S6 further highlights similarities observed for the initial onset point of degradation of the QA-functional group before and after alkaline treatment. Anion conductivity of SEBS-TMA after alkaline stability test was also evaluated by ion exchanging the membrane to Cl− form after alkaline treatment. Cl− rather than OH− conductivity was measured because OH− conductivity testing can be challenging sometimes due to possible CO2 contamination. As shown in Figure 7A, the Cl− conductivity of SEBS-TMA remained unchanged after 4 weeks in 1 M NaOH solution at 80 °C. SAXS data was utilized to investigate whether any morphology changes occurred during the alkaline tests. As seen in Figure 7B, the SAXS profile of SEBS-TMA did not show any noticeable change after 4 weeks of 1 M NaOH treatment at 80 °C. Altogether, these data support that the tethered cation group and morphology structure of SEBS-TMA remained intact during the alkaline test at 80 °C. SEBS-TMHA also showed no changes in Cl− conductivity after 4 weeks of 1 M NaOH treatment at 60 °C. However,

Figure 7. (A) Changes in Cl− conductivity and (B) SAXS profiles of SEBS-TMA and SEBS-TMHA before and after 4 weeks in 1 M NaOH at 80 °C. Error bars in panel A indicate data collected from three measurements. SAXS profiles are vertically shifted to avoid overlaps.

G

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Macromolecules when the alkaline test temperature was raised to 80 °C, we discovered that the membrane exhibited a slight drop in Cl− conductivity (Figure 7A), whereas its TGA curve (Figure 6B) and FT-IR spectra (Figure S8) remained unchanged. Initially, the drop in Cl− conductivity led us to suspect that minor cation degradation had occurred; however negligible change in the TGA curve (Figure 6B) and no loss in C−N stretch of FT-IR (Figure S8) indicated that the TMHA cation remained intact. To investigate further, characterization with SAXS was conducted for SEBS-TMHA membranes before and after alkaline treatment at 80 °C. The SAXS data revealed that the primary scattering peak of SEBS-TMHA shifted to a lower q* value, resulting in change of d-spacing from 34 to 37 nm, after 4 weeks of alkaline treatment (Figure 7B). Additionally, the intensity of the high-order peak was reduced, which indicates minor morphological changes had occurred for SEBS-TMHA. This morphology change is likely caused by gradual disintegration of the physically cross-linked polystyrene domains under alkaline stability test conditions. Water molecules can act as a plasticizer and decrease the Tg2 of the styrene blocks of SEBS-TMHA (possibly below 80 °C) under hydrated conditions. Because the rigid polystyrene blocks can no longer serve as physically cross-linked domains, a different morphology structure could result. Thus, we believe the reduced Cl− conductivity of SEBS-TMHA AEMs after 4 weeks alkaline test at 80 °C was ascribed to factors related to morphology change rather than actual cation degradation. To enhance durability of AEM fuel cells, we suggest that one should employ more rigid polymer backbone (without cleavable C−O bonds) as an AEM base material to prevent morphological changes. Fuel Cell Performance. The polarization and power density curves of SEBS-TMA measured at 60 and 80 °C are shown in Figure 8A. The power density increased up to current density of 0.4 A/cm2 and gradually decreased. The high frequency resistance (HFR) of the cell was very low (0.088 Ω· cm2) indicating that the SEBS-TMA membrane is highly conductive. Stable HFR over the entire current density range suggests good water management of the hydrated membrane under the cell operating conditions. The MEAs made of SEBSTMA membrane exhibited excellent fuel cell performance with peak power density of 223 mW/cm2 at 60 °C. When the temperature was raised to 80 °C, the maximum power density of 240 mW/cm2 could be obtained. Given that only 0.2 mg/ cm2 cathode Pt catalyst loading was used, SEBS-TMA demonstrates one of the best performances among AEM fuel cells reported in literature.11,54−60 A 110 h life test at 60 °C was performed for SEBS-TMA (Figure 8B). Although the cell voltage slightly decreased over time (probably due to the electrode performance loss), no sign of membrane degradation was observed: the open circuit voltage maintained above 1.0 V and the cell HFR also decreased from 0.088 to 0.062 Ω·cm2. The reduced HFR after 110 h life test was probably due to factors related to changes in morphology.61 TGA and FT-IR analysis of the membrane after the life test of fuel cells confirmed the presence of the QAfunctional groups, thus further suggesting that SEBS-TMA remained chemically stable during fuel cell tests (Figure S10). Additionally, after ion exchanging to Cl− form, elemental analysis revealed negligible changes in the Cl-to-N ratio from the membrane of 110 h life test, indicating no loss in cation functional groups occurred during fuel cell testing (Table S2).

Figure 8. (A) I−V polarization, HFR, and power density curves of SEBS-TMA (membrane thickness = 35 μm) at 60 °C (■) and 80 °C (●). (B) I−V polarization curves of SEBS-TMA before and after 110 h life test at 60 °C. Test conditions: Pt loading = 0.2 mg/cm2 for anode and cathode, back pressure 15 psig.

The fuel cell performance of SEBS-TMHA was also evaluated. Although it showed poorer performance than SEBS-TMA (ca. peak power densities 77 mW/cm2 at 60 °C, Figure S9A), probably due to the higher HFR of the MEAs, the HFR values of the SEBS-TMHA MEAs still remained stable even after 110 h life test at 80 °C, which strongly supports that the QA group of SEBS-TMHA has excellent chemical stability under the fuel cell conditions (Figure S9C). Similar to SEBSTMA, SEBS-TMHA showed negligible changes in the Cl-to-N ratio as measured by elemental analysis after the 110 h life test, suggesting that the tethered QA functional groups remained intact (Table S2).

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CONCLUSIONS In summary, aromatic rings of SEBS were functionalized with various benzyl- and alkyl-substituted QA groups for AEM fuel cell applications. Transition metal-catalyzed C−H borylation and Suzuki coupling synthetic methods were utilized to incorporate six different structures of QAs selectively to the styrene units of SEBS. The water uptakes and ion conductivity of SEBS-QA AEMs were heavily dependent on the structure of pendent QAs. The ionic polymers bearing compact trimethylammonium cation moieties, SEBS-TMA and SEBS-TMHA, exhibited the best hydroxide ion conductivity. They additionally demonstrated improved morphology over the QA polymers with bulky cation structures, as evidenced by SAXS. The bulky pendant groups on cation structures of SEBS-MCH and SEBSMiPr resulted in incomplete quaternization, poorly defined H

DOI: 10.1021/acs.macromol.5b01382 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

atmosphere, then stirred in a 75 °C oil bath for 14 h. After cooling to room temperature, the reaction solution was diluted with CHCl3 (25 mL) and filtered through a short plug of silica gel to remove the catalyst. The filtrate was concentrated using a rotary evaporator and poured into methanol to precipitate the polymer. The dissolution and precipitation process was repeated one more time. The borylated polymer was isolated as white fibers after drying under vacuum at rt for 16 h (6.00 g). The mol % of borylated aromatic units was estimated based on the relative intensity of the methyl group in the 1,2-butylene unit of the polymer main chain (at 0.78−0.88 ppm) with respect to the increased integral ratio of the overlapping SEBS-methylene and boronate ester methyl resonance (at 0.9−1.5 ppm). Mw = 114 kg/mol, PDI = 1.06 (measured from GPC). Pd-Catalyzed Suzuki Coupling of SEBS-Bpin for SEBS-An. The following represents a typical procedure for the synthesis of N,Ndimethylbenzylamine-containing SEBS (SEBS-A1). In a nitrogen-filled glovebox, SEBS-Bpin (0.50 g, 0.80 mmol of Bpin), K2CO3 (0.33 g, 2.41 mmol, 3 equiv based on the amount of boryl group of SEBSBpin), Pd(dppf)Cl2−CH2Cl2 (19.6 mg, 3 mol % based on the amount of boryl group of SEBS-Bpin), anhydrous THF (4 mL), and a magnetic stirring bar were placed into a two-neck 50 mL roundbottom flask. The flask was removed from the glovebox and fitted with a reflux condenser under nitrogen atmosphere. N2-bubbled amine A1 (0.50 g, 2.41 mmol, 3 equiv based on the amount of boryl group of SEBS-Bpin) followed by distilled water (0.40 mL) was added to the reaction mixture via a syringe. After stirring at 75 °C for 12 h, the reaction was cooled and diluted with CHCl3 (15 mL), then filtered through a short plug of silica gel to remove the catalyst. The filtrate was concentrated using a rotary evaporator and poured into methanol to precipitate the polymer. The dissolution and precipitation process was repeated one more time. The SEBS-A1 was isolated as light-brown fibers after drying under vacuum at rt for 16 h (0.46 g). The DF of the amine was estimated based on the relative intensity of the methyl group in the 1,2-butylene unit of the polymer main chain (at 0.78− 0.88 ppm) with respect to the aromatic-tethered −CH2− group of the amine (peak e in Figure 2A at 3.3−3.5 ppm). Refer to the Supporting Information for the synthesis of SEBS-A2 through SEBS-A5. Quaternization of Amine-Functionalized SEBS for Synthesis of SEBS-QA. The following is a representative procedure for the heterogeneous methylation and ion exchange reaction of SEBS-A1. Thin films (approximately 30−50 μm thick) of SEBS-A1 were cast from toluene onto Teflon plates about 3 × 3 cm2 in size. The films were dried in an oven at 60 °C for 12 h with a positive air flow to slowly evaporate the toluene. Films were removed from Teflon plates by immersing in water. For methylation, the films were immersed in a mixture of dimethyl sulfate (8 equiv based on amount of amine) and anhydrous N,N-dimethylacetamide (30 mL) at 50 °C for 48 h. The films were then washed with and soaked in water for an additional 8 h. To ion exchange into hydroxide ion form, the films were soaked in degassed 1 M NaOH for 48 h at rt in an argon-filled glovebox after which they were washed with degassed deionoized water until pH became neutral. Ion exchange into Cl− and HCO3− forms was conducted in a similar way with 1 M NaCl and 1 M NaHCO3, respectively, in open atmosphere. Synthesis of SEBS-DMHA. Thin films of SEBS-A1 were cast from toluene according to the method described above. For alkylation, the films were immersed in a mixture of 1-bromohexane (8 equiv based on amount of amine) and N,N-dimethylacetamide (30 mL) at 100 °C for 48 h. The films were then washed and soaked in water for additional 8 h. Ion exchange reaction was performed as described above. Characterization. 1H NMR spectra were obtained with a Varian Unity 500 MHz spectrometer, and chemical shifts were referenced to TMS (δ 0.00 ppm). 13C NMR spectra were obtained with a Bruker 600 MHz (14.1 T) spectrometer. All NMR spectra were recorded at 25 °C and were processed with MestReNova 8.1 (Mestrelab Research SL) software. Molecular weight measurement was performed using a VISCOTEK GPC equipped with three Viscotek general mixed-bed columns in series and tetra detector array, set at 30 °C with a flow rate of 1.0 mL/min of THF as the mobile phase. The instrument was calibrated using polystyrene standards.

morphology, and lower water uptakes, which ultimately reduced their hydroxide ion conductivity. Alkaline stability studied by FT-IR and TGA revealed no degradation of cationic functional groups for SEBS-TMA and SEBS-TMHA after 4 weeks at 80 °C. They remained physically strong and flexible after the alkaline treatments, suggesting that the SEBS polymer backbone remained chemically stable. Although SEBS-QA AEMs are chemically stable, a noticeable change observed in the SAXS data of SEBS-TMHA after alkaline treatment at 80 °C over 4 weeks indicates that rubbery alkaline membranes may not maintain stable morphology for a prolonged period of time under high temperature hydrated conditions. MEAs made of SEBS-TMA membrane exhibited excellent fuel cell performance reaching high peak power density (223 mW/cm2 at 60 °C). Additionally, both SEBS-TMA and SEBSTMHA membranes remained chemically and mechanically stable under 60−80 °C AEM fuel cell test conditions. These results demonstrate that (i) flexible polymer materials bearing solely C−C and C−H bonds, such as SEBS, offer great potential for use in high-performance AEM-based energy storage and conversion devices and (ii) benzyl- or alkyltethered trimethylammoniums are the best QA candidate structure providing good ion conductivity and stability, if attached to backbones composed of all C−C bonds, in typical operating conditions of AEM fuel cells. Although sterically bulky QAs (e.g., MCH, MiPr) were found to have comparable or better stability than benzyltrimethylammonium under alkaline conditions in model compound studies, when they are tethered to polymers, they tend to result in incomplete quaternization and lower ion conductivity. This study of structure−property relationships between QA cation groups and their fuel cell membrane properties will guide further development of advanced AEM materials. Incorporation of these chemically stable QA moieties to a wide range of SEBS triblock copolymers may lead to the development of AEMs having diverse ranges of morphology, mechanical properties, and fuel cell membrane performance.

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EXPERIMENTAL METHODS

Materials. Polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene [SEBS; Mw = 105 kg/mol, polydispersity index (PDI) = 1.04 measured from gel permeation chromatography (GPC) using THF as eluent] and 4,4′-di-tert-butyl bipyridine (dtbpy) were purchased from SigmaAldrich. Bis(pinacolato)diboron (B 2 Pin 2 ) and chloro(1,5cyclooctadiene)iridium(I) dimer ([IrCl(COD)]2) were donated by Frontier Scientific Co. and Sinocompound Technology Co., respectively. [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II) complex with dichloromethane (Pd(dppf)Cl2− CH2Cl2), sodium hydroxide (NaOH), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and dimethyl sulfate were purchased from Alfa Aesar. Anhydrous tetrahydrofuran (THF) and anhydrous N,N-dimethylacetamide were obtained from Acros Organics and stored in a nitrogenfilled glovebox. Methanol, toluene, and chloroform were reagent grade and used as received. Ir-Catalyzed C−H Borylation of SEBS. The following is a representative procedure for the synthesis of borylated SEBS having 26 mol % borylation degree (SEBS-Bpin). In a nitrogen-filled glovebox, SEBS (5.00 g, 13.9 mmol of styrene units), B2Pin2 (12.4 g, 48.7 mmol, 3.5 equiv), [IrCl(COD)]2 (0.49 g, 1.5 mol % based on the amount of B2Pin2), dtbpy (0.39 g, 3 mol % based on the amount of B2Pin2), anhydrous THF (50 mL), and a magnetic stirring bar were placed into a 100 mL round-bottom flask. The flask was removed from the glovebox and fitted with a reflux condenser under nitrogen I

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Water Uptake (WU) Measurements. In an argon-filled glovebox, fully hydrated AEMs in OH− form were taken out of water, blotted quickly with a KimWipe to remove surface liquid, and weighed immediately. The membranes were then dried at rt under vacuum for 24 h and weighed again. WU % was calculated from WU (%) = [(Wwet − Wdry)/Wdry] × 100%, where Wwet and Wdry are the weight of the water-swollen and the corresponding dry membranes, respectively. Anion Conductivity Measurements. In-plane ion conductivity measurements (σ in mS/cm) of each membrane (approximate size 3 cm × 0.6 cm) were taken using a four-point electrode method with BT-512 membrane conductivity test system (BekkTech LLC). Measurements were carried out under fully hydrated conditions at 30 and 60 °C. The cell was immersed in deionized degassed water and blanketed with a flow of argon gas. At a given temperature, the samples were equilibrated at least 90 min before recording measurements. The ion conductivity was calculated according to σ = L/(RWT), where L is the distance between the two inner platinum wires (0.425 cm), R is the resistance of the membrane in mΩ, and W and T are the width and the thickness of the membrane in centimeters, respectively. Morphology Study with Small-Angle X-ray Scattering (SAXS). SAXS profiles were collected using a Bruker Nanostar-U instrument equipped with a turbo (rotating anode) X-ray source. The sample-to-detector distance was 105 cm. Measurements of the dried films were obtained under vacuum at ambient temperature. Typical collection time was 30 min, and the membrane thickness was on the order of 100−200 μm. The scattering profiles were normalized based on sample thickness and are represented in logarithm of thickness normalized intensity (I) as a function of scattering vector, q. To avoid overlaps of the SAXS profiles, the data was vertically shifted. Mechanical Testing. Stress−strain measurements were carried out by dynamic mechanical thermal analyzer (TA Q800-RH). Temperature and humidity were controlled in an environmental chamber. The chamber temperature was equilibrated at 50 °C with 0%, 50%, and 90% RHs for 40−60 min. Tensile test was performed using 0.5 in. × 1.0 in. rectangular test strips with membranes in OH− form. A load ramp of 0.5 MPa/min was used. Each sample was tested twice. Fuel Cell Performance Testing. Fuel cell performance of SEBSTMA and SEBS-TMHA was characterized in a single cell under 100% RH. Carbon-supported Pt catalyst (20 wt % Pt/C BASF) was used for the cathode and anode. For catalyst ink, hexyltrimethylammonium functionalized poly(phenylene) ionomer (IEC = 2.0 mequiv/g) was dispersed in alcohol (solid content 1 wt %), followed by mixing with the Pt/C catalyst. [Note, the properties of this ionomer were previously reported15]. The catalyst inks were painted on a gas diffusion layer (GDL). The nominal catalyst loading for anode and cathode was 0.2 mg/cm2 with the geometric active cell area of 5 cm2. Initial H2/O2 polarization curves and online HFR for the MEAs were obtained simultaneously after 1 h break-in at 60 °C. The long-term fuel cell test was performed at 60 and 80 °C at a constant voltage of 0.3 V. After the long-term test, MEAs were immersed into a 0.5 M NaOH solution for 30 min and thoroughly rinsed with deionized water. This process was necessary to remove any possible (bi)carbonation formation from fuel cell humidifiers. The fuel cell performance after long-term test was measured without break-in at 60 and 80 °C.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Rensselaer Polytechnic Institute (startup for C.B. and Slezak Fellowship for A.D.M.) and NSF (CAREER DMR-0747667 for C.B.) are greatly appreciated. The authors thank Joel Morgan for his assistance with SAXS, Sinocompound Technology Co. for a donation of Ir complex, and Frontier Scientific Co. for a donation of B2Pin2. Y.S.K. thanks the US DOE FCTO program, Technology Development Manager Dr. Nancy Garland, for financial support. The authors thank Dr. Cy Fujimoto (Sandia National Lab.) for kind supply of poly(phenylene) ionomer.

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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.5b01382. Synthesis of amine-containing aryl halides (An), synthesis of SEBS-An (n = 2−5), thermal and mechanical testing of SEBS-QAs, TGA and FT-IR data after alkaline stability tests, SEBS-TMHA fuel cell performance test, and NMR spectra of An (n = 1−5) (PDF) J

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