Alkaline Stability of Benzyl Trimethyl Ammonium Functionalized

Sep 17, 2014 - Sensors and Electrochemical Devices Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States. #...
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Alkaline Stability of Benzyl Trimethyl Ammonium Functionalized Polyaromatics: A Computational and Experimental Study Yoong-Kee Choe,† Cy Fujimoto,‡ Kwan-Soo Lee,§ Luke T. Dalton,# Kathy Ayers,# Neil J. Henson,⊥ and Yu Seung Kim*,§ †

National Institute of Advanced Industrial Science & Technology, Tsukuba, 305-8568, Japan Organic Materials Science, Sandia National Laboratory, Albuquerque, New Mexico 87185, United States § Sensors and Electrochemical Devices Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States # Proton Onsite, Wallingford, Connecticut 06492, United States ⊥ Physics and Chemistry of Materials, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

S Supporting Information *

ABSTRACT: Alkaline stability of benzyl trimethylammonium (BTMA)-functionalized polyaromatic membranes was investigated by computational modeling and experimental methods. The barrier height of hydroxide initiated aryl-ether cleavage in the polymer backbone was computed to be 85.8 kJ/mol, a value lower than the nucleophilic substitution of the αcarbons on the benzylic position of BTMA cationic functional group, computed to be 90.8 kJ/mol. The barrier heights of aryl− aryl cleavage (polymer backbone) are 223.8−246.0 kJ/mol. The computational modeling study suggests that the facile aryl−ether cleavage is not only due to the electron deficiency of the aryl group but also due to the low bond dissociation energy arising from the ether substituent. Ex situ degradation studies using Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) spectroscopy indicated that 61% of the aryl−ether groups degraded after 2 h of treatment in 0.5 M NaOH at 80 °C. BTMA cationic groups degraded slowly over 48 h under the same conditions. In situ degradation studies validate the calculated results: anion exchange membrane fuel cells and water electrolyzer using poly(arylene ether) membranes exhibit a catastrophic, premature failure during lifetime tests, while no sudden performance loss is observed with an ether-free poly(phenylene) membrane. Despite the gradual performance loss due to the degradation of BTMA cation functional group, the membrane electrode assembly using the poly(phenylene) membrane exhibited a lifetime of >2000 h in the alkaline water electrolyzer mode at 50 °C.

1. INTRODUCTION

reductive stability, good mechanical properties, low gas permeability, and solvent resistance. The flexibility in tailoring the chemistry of polyaromatics makes them even more attractive in various electrochemical devices. The most common synthetic method to prepare cationfunctionalized polyaromatics is chloromethylation of polyaromatics via Friedel−Crafts alkylation with chloromethyl methyl ether and subsequent conversion of the chloromethyl groups into quaternary ammonium.3 A viable alternative to the

Hydroxide-conducting ionomer membranes have received attention for anion exchange membrane fuel cells (AEMFCs) and solid-state alkaline water electrolysis, since inexpensive metal catalysts can potentially be employed. The advantages of a membrane-based system include no carbonate precipitation, easy startup/shutdown, high differential pressure operation, and simple system design. 1 However, the harsh operating conditions of alkaline electrochemical devices require good chemical stability of the membrane.2 One of the most common polymeric structures of hydroxide-conducting membranes is polyaromatics. Cation-functionalized polyaromatic membranes have been known for their excellent thermal oxidative and © 2014 American Chemical Society

Received: July 3, 2014 Revised: September 16, 2014 Published: September 17, 2014 5675

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chloromethyaltion is bromination of methylated polyaromatics and subsequent amination.4 Other preparation methods via activated fluorine−amine reaction5 or direct co-polymerization of dihalogenated and bis-dimethylaminomethyl diphenol monomers6 have also been employed for the preparation of cation-functionalized polyaromatics. By far, the most utilized cation functional group is the benzyl trimethylammonium (BTMA), although potentially more stable cations (such as phosphonium,7 guanidinium,8 and imidazolium9) have been suggested. The alkaline stability of cation-functionalized polyaromatic membranes has mainly been discussed in the context of cation stability. The recognized cation degradation routes under high pH conditions include Hofmann elimination,10 nucleophilic substitution,11 and ylide formation.12 Thus far, lifetime studies of cation-functionalized polyaromatics indicate that cation stability is of paramount importance for stable operation of alkaline electrochemical devices.13 However, equally important is the stability of the polyaromatic backbone, since facile hydrolysis of aryl−ether linkage and bisphenol A of cationfunctionalized polyaromatics can take place under high pH conditions.6,14 This is a rather surprising result, since polyaromatics typically have excellent resistance to hydrolysis under basic conditions.15 However, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) studies indicated that the hydrolysis of cation-functionalized polyaromatics occurs by the nucleophilic attack of hydroxide ions to the cation-functionalized polyaromatic backbone. While the degradation of cationic functional groups and polyaromatic backbones can simultaneously occur, one degradation route is likely more predominant over the other, depending on the structure of hydroxide-conducting polyaromatics and impacts the performance durability of alkaline electrochemical devices. Therefore, it is important to know the relative stability between the cation functional group and polymer backbone of hydroxide-conducting polyaromatics. In this paper, the degradation barrier heights of BTMA-functionalized polyaromatics are computed using density functional theory (DFT) to compare the relative stability of the cationic functional group and polymer backbone. The BTMA cationic functional group is selected for this study since it is most commonly employed and has reasonably good stability.16 In situ stability tests of two model polyaromatic membranes, i.e., poly(arylene ether) and poly(phenylene) were performed in AEMFCs and alkaline water electrolysis. These experimental studies verify the DFT results and provide further insight into the relationship between membrane property and device performance.

Figure 1. Chemical structure of F-PAE and ATM-PP.

Table 1. Electrochemical Properties of Polymersa

F-PAE ATMPP

number average molecular weight (Mn) (g/mol)

weight based ion exchange capacity (IECW) (mequiv/g)

40 000 51 000

2.7 1.7

water uptake

b

(wt %)

99 ± 10 70 ± 6

hydroxide conductivity (σ) (mS/cm) 46c 37d

Data taken from ref 6. bMeasured at 30 °C. cCl− form, measured at 80 °C. dBr− form, measured at 80 °C.

a

2.2. Computational Methods. Density functional theory (DFT) calculations were performed using the Gaussian 09 suite of programs.18 Geometry optimization and harmonic vibrational frequency calculations were undertaken, employing the ωB97XD functional, where empirical dispersion and long-range corrections were taken into account19 with a 6-311++G(2d, 2p) basis set.20 Effects of water molecules were taken into consideration by the polarizable continuum model (PCM).21 Vibrational frequency calculations were carried out to check the nature of transition states and the presence of a single imaginary frequency and we observed one imaginary frequency associated with reaction coordinates. Thermochemistry data such as free energies of reactions were evaluated from the results of vibrational frequency calculations where a temperature of 80 °C was considered. Figure 2 shows the model compounds employed in the present study.

Figure 2. Model compounds for DFT calculations. 2.3. Membrane Characterization. The FTIR spectra were recorded using a Thermoscientific FTIR 8700 system at a resolution of 4 cm−1 in the range of 4000−600 cm−1. All samples were prepared as a membrane form and characterized after drying at 80 °C in a convection oven for 30 min. Changes in FTIR peak heights and positions were compared after the normalization with the aromatic CC peaks of benzene at 1477 cm−1 for F-PAE and 1490 cm−1 for ATM-PP. The 1H NMR spectra in deuterated DMSO (DMSO-d6) were recorded using a Bruker Avance 500 spectrometer (500 MHz). A small amount of tetramethylsilane (TMS) served as a zero-frequency reference for 1H NMR spectra. The mechanical properties of quaternized membranes were characterized by dynamic mechanical thermal analyzer (TA Q800RH). The temperature and humidity were controlled in an environmental chamber. The tensile test was performed using 0.5 in. × 1 in. rectangular test strips at a load ramp of 0.5 MPa min−1 at 50 °C and 50% relative humidity (RH). Hydroxide form of samples were prepared before and after 2-h degradation test in 0.5 M NaOH at 80 °C right before the mechanical tests. The stress−strain behavior of the

2. EXPERIMENTAL SECTION 2.1. Polymer Synthesis. Two BTMA-functionalized polyaromatic membranes were prepared. Partially fluorinated poly(arylene ether) (F-PAE) compounds were synthesized by nucleophilic aromatic substitution (SNAr) using decafluorobiphenyl, 4,4′-biphenol, and 2,2′-bisdimethylaminomethyl-4,4′-biphenol.6 After the reaction, the polymer was quaternized by iodomethane at room temperature. The poly(phenylene) having benzylic methylammonium groups (ATMPP) was synthesized by an irreversible Diels−Alder reaction.17 The benzylic methyl groups were converted to bromomethyl groups, which reacted with trimethylamine in the solid state. Detailed synthetic procedures and characterizations of BTMA-functionalized polyaromatics have been described in previous work.6 Figure 1 and Table 1 shows the chemical structure and electrochemical properties of hydroxide-conducting polyaromatics employed in the present study. 5676

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height for the SN2 reaction, in terms of ΔG⧧, has been computed to be 90.8 kJ/mol. As seen in Figure 3, the leaving group of the SN2 reaction is TMA, and OH− attacks the carbon atom on the benzylic position. A previous DFT study on the same reaction showed a lower ΔG⧧ value (i.e., 79.1 kJ/mol)11 because the present calculation based on the ωB97XD functional considers long-range interactions as well as van der Waals interactions whose effects are not completely considered in the previous B3LYP functional. Also, the basis set employed in the present study (6-311++G(2d,2p)) are slightly more accurate than the one employed in the previous study (6-311+ +G(2d,p)).The other degradation pathway, SNAr, occurs in two consecutive steps. The first step of SNAr is the addition of OH− to the phenyl group, which results in an intermediate complex (shown in Figure 3). The barrier height of this process is computed to be 127.2 kJ/mol, which is higher than that of SN2. The relatively higher barrier can be attributed to the loss of the aromatic character on the phenyl group, which is induced upon the addition of the nucleophile. The following transition state shows a much higher barrier (201.7 kJ/mol) in comparison, indicating a very strong bond between the phenyl group and methyl ammonium. In cases of typical SNAr, where the leaving group has a strong electron-withdrawing nature (such as NO2), a barrier height for the second step tends to be lower than that of the first one, because the aromatic character on the phenyl group is regained in a final product.22 More-detailed discussions on SNAr will be given in a later section. 3.1.2. Aryl−Ether Cleavage. Figure 4 shows the detailed reaction pathways for the degradation occurring at the

membranes was measured after obtaining equilibrium RH of 50% at least for 60 min. 2.4. Device Performance Evaluation. AEMFC performance using F-PAE and ATM-PP were characterized in a single cell. Platinum black catalysts (Johnson Matthey) and double-sided hydrophobic carbon cloths (E-TEK, Inc.) were used for the fuel cell electrodes. FPAE was used in the electrode binder. For the catalyst ink, the soluble portion of the ionomer was dispersed in NMP (solid content = 2.5 wt %), followed by mixing with a platinum catalyst. The catalyst inks were painted on the membrane surface. The nominal catalyst loading for anode and cathode were 3 mg cm−2 with the geometric active cell area of 5 cm2. High catalyst loading was used in order to minimize catalystrelated degradation during the cell test. Initial polarization curves and online high-frequency resistance (HFR) for the membrane electrode assemblies (MEAs) were obtained simultaneously after 3 h break-in at 60 °C. The extended-term fuel cell test was performed at 60 °C under full hydrations. The current density and HFR of the cell at 0.3 V were measured every minute. We have tested multiple samples to ensure the reproducibility of the extended-term test. Electrolysis performance was measured in Proton OnSite’s commercial cell stack hardware with material modifications for the anion exchange membrane chemistry. Gas diffusion electrodes were fabricated by spray deposition of catalyst inks on gas diffusion materials at similar loadings to the fuel cell. Platinum black on carbon paper was used as the cathode and iridium oxide on titanium was used as the anode. Iridium is not the optimal catalyst in alkaline solution, but served as an appropriate baseline in the ionomer-based system. Continued understanding of the interaction between nickel and other non-noble-metal catalysts with anion exchange polymers is needed to improve performance. The cell active area was 28 cm2. Testing was performed at a differential pressure of 100 psi and a temperature of 50 °C.

3. RESULTS AND DISCUSSION 3.1. Computational Studies. 3.1.1. BTMA Degradation. The cation degradation of 2-tetramethylammonium diphenyl ether (1-orth) is examined. Figure 3 shows the degradation

Figure 4. Free-energy profiles on the aryl−ether cleavage.

backbone part of 1-orth or 1-meta, which are the basic building compounds of BTMA-functionalized poly(arylene ether)s. This reaction is also categorized as SNAr, where the nucleophile is OH− and the leaving group is OAr. Free-energy profiles indicate that the C−O bond formation between the oxygen atom of OH− and the carbon atom of the phenyl group tethered to the cationic group is rate-limiting. The barrier height of aryl-ether cleavage depends on the position of the cationic group; the barrier height of the 1-orth degradation reaction is computed to be 85.8 kJ/mol, which is lower than that of the 1-meta degradation (i.e., 111.3 kJ/mol). The transition state is followed by intermediates (2-orth, 2-meta), which correspond to the Meisenheimer complex.23 The carbon atom on which OH− is attached in the intermediates has sp3 hybridization, since it is now tetra-coordinated, resulting in the

Figure 3. Free-energy profiles of the BTMA cation.

pathway of 1-orth. The cation degradation is a chemical reaction where a cationic functional group is detached from the phenyl group or is modified to some extent, resulting in the loss of its cationic moiety. Detachment of trimethylammonium (TMA) can occur via two degradation pathways: an SN2 reaction or a nucleophilic aromatic substitution (SNAr). Other reactions that lead to cation degradation, such as elimination reactions, were not taken into account in this study, because there are no β-hydrogen atoms for elimination.11 A barrier 5677

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ortho or para position, with respect to a leaving group, exhibits a faster SNAr compared with that located at the meta position.24 In fact, BTMA itself is a very strong electron-withdrawing group, where its group electronegativity is computed to be 10.7 eV. This value is larger than that of typical electronwithdrawing groups such as NO2 (7.8 eV) or F (10.0 eV).25 Therefore, it is not surprising that SNAr of 1-orth or 1-meta follows the general trend of SNAr. 3.1.3. Aryl−Aryl Cleavage. Although the barrier height of aryl−ether cleavage via SNAr reaction pathway becomes lower with more electron-withdrawing substituents on the phenyl group,26 one should note that such a trend is only observed with electron-withdrawing heteroatom substituents. In general, the leaving group abilities of SNAr are follows:26 F > NO2 > OSO2Ar > Cl >Br > I > OAr > OH ≫ NH2.22 Scheme 1

loss of the aromatic nature on the phenyl ring. The C−O bond between the phenyl group and the ether oxygen is 1.37 Å for both 1-orth and 1-meta, while their bond lengths are elongated to 1.51 and 1.52 Å for 2-orth and 2-meta, respectively (see Figure 5), which can be interpreted as the nature of the C−O bond in the former retaining sp2 hybridization, whereas, in the latter, it is transformed to sp3. These intermediates then undergo aryl−ether C−O bond cleavage, resulting in 3-orth and 3-meta as the final products. The barrier height of the second transition state, with respect to the intermediates, is very low, which, in part, arises from restoration of aromaticity on the phenyl group. The overall reactions are exothermic, by 69.0 and 72.8 kJ/mol for 1-orth and 1-meta, respectively. In comparing the degradation of the backbone to that of the cation, the degradation of the poly(arylene ether) backbone of 1-orth is likely to occur prior to the degradation of its cationic group, since the barrier height (85.8 kJ/mol) of the ratedetermining step of 1-orth degradation is lower than that (90.8 kJ/mol) of the BTMA cationic group degradation. The computational result suggests that 1-meta is more robust, so polymer electrolyte membranes containing such a structure are less likely to decompose under alkaline environments. To gain further insight into the backbone degradation, in Figure 5, we present the molecular structures of 1-orth, 1-meta,

Scheme 1. SNAr Reaction with Grignard Reagent

Figure 5. Molecular structure and selected Mulliken charge of 1-orth, 1-meta, 2-orth, and 2-meta.

describes one example of SNAr where OCH3 is replaced by C2H5.26 CN is strongly electron-withdrawing and has a relatively large group electronegativity (8.6 eV), compared with that of the OCH3 group (5.7 eV),25 and could be considered as a good leaving group for SNAr. However, what is actually replaced by the Grignard reagent in Scheme 1 is OCH3 not CN, which stems from the strong nature of the chemical bond between the carbon of Ar and the carbon atom of CN, where the bond dissociation energy between the CN group and Ar is computed to be 134.7 kJ/mol at the ωB97XD/6-311+ +G(2d,2p) level of theory, whereas the bond strength between the OCH3 group and Ar is computed to be 92.5 kJ/mol at the same theory level. This experimental result parallels the computational results discussed in the previous section, which indicates that BTMA is a very poor leaving group in SNAr, although its group electronegativity is very high. Also, it suggests the idea of replacing OAr by Ar (replacement of C−O by C−C) should be a reasonable strategy to improve the backbone stability of polyaromatics. Figure 6 shows free-energy profiles of the backbone degradation of 4-meta and 4-para, which are model

2-orth, and 2-meta, along with the Mulliken charges on selected atoms. It can be seen in this figure that, for both 1-orth and 1-meta, the carbon atoms adjacent to the ether group or BTMA have substantial positive charge, while other carbon atoms on the phenyl ring have negative charges. In going from the reactant to the intermediate, it is observed that a total negative charge on the phenyl ring changes significantly. For example, the overall charge on the carbon atoms of the phenyl group in 1-orth is −0.11 and this value in 2-orth becomes −0.85, indicating that a negative charge of the OH− is transferred partially to the phenyl ring. It is notable that the atomic charge on the C1 atom, which is the ether-connected carbon atom, in 1-orth is 0.46, whereas that in 1-meta is 0.28. Thus, the C1 atom in 1-orth is likely to interact more strongly with OH− than that in 1-meta, which could be a reason why 1orth shows the lower barrier height for OH− addition. Such a trend is consistent with the general observation of SNAr reactions in which an electron-withdrawing group located at the

Figure 6. Free-energy profiles on the aryl−aryl cleavage. 5678

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compounds of ATM-PP. The reactions are also categorized as SNAr. The first step is an addition of OH− to the phenyl group, resulting in the formation of the Meisenheimer complex (5meta, 5-para). The complex then undergoes the aryl−aryl cleavage steps whose barrier heights are computed to be 223.8 and 246.0 kJ/mol for 4-para and 4-meta, respectively. In the aryl−aryl cleavage step, the phenyl moiety of 5-para and 5meta is detached as a phenyl anion. Since the phenyl moiety loses aromaticity upon alteration to the phenyl anion, 6-para and 6-meta shows relatively higher energy with respect to reactants, which are endothermic by 171.1 and 189.1 kJ/mol for 6-para and 6-meta, respectively. The phenyl anions abstract a hydrogen atom of neighboring the phenol moiety in a barrierless manner, leading to the final products (7-para and 7meta). It is evident that the lowering of the energetics, in going from 6-para/6-meta to 7-para/7-meta, arises from the restoration of aromatic character in the final products while the energetics of 7-para and 7-meta are still high, with respect to those of the reactants (4-para and 4-meta), indicating that the forward reactions are thermodynamically unfavorable. Through the discussed computational analysis, we conclude that the BTMA-functionalized wholly aromatic poly(phenylene) backbone is more stable compared to BTMAfunctionalized partially fluorinated poly(arylene ether). The barrier height of the BTMA degradation (90.8 kJ/mol) is much lower than that of the poly(phenylene) backbone degradation (223.8−246.0 kJ/mol), indicating that the backbone is more stable than the cationic functional group. This result is in agreement with the experimental observation as will be discussed in the following section. 3.2. Experimental Verification. 3.2.1. Ex Situ Degradation Studies. Figure 7 shows the chemical structures and barrier

Figure 8. FTIR spectra of F-PAE in the phenolic O−H bending region, and C−O−C and C−O stretching regions, as a function of NaOH treatment time.

peak at 1320 cm−1 corresponds to the O−H in-plane bending vibration of fluorophenol,27 which increases with intensity after 2 h of NaOH treatment. The evolution of the phenolic structure is further confirmed by the C−OH stretching peaks at 994 cm−1 and the loss of intensity at 1003 cm−1 which corresponds to the C−O−C symmetric stretching peak of the aromatic rings. These peak changes suggest that a phenolic structure is generated as a result of aryl−ether cleavage, as suggested by the DFT results. Rapid structural change within 2 h, were followed by less prominent change, indicates that substantial backbone degradation occurred within this time frame. The main chain degradation of F-PAE was confirmed by 1H NMR. Figure 9 shows the 1H NMR before and after 2 h of the

Figure 9. 1H NMR spectra of F-PAE (a) before and (b) after stability test in 0.5 M NaOH at 80 °C for 2 h.

Figure 7. Chemical structure of F-PAE (top) and ATM-PP (bottom) and the energy barrier heights of hydroxide-conducting polyaromatics.

stability test with 0.5 M NaOH at 80 °C. The peak at 7.65 ppm corresponding to the protons in the backbone was shifted upfield, ca. 7.43 ppm after the stability test, because of the main chain aryl−ether cleavage. In addition, the peaks at 7.25 and 7.08 ppm appear probably due to biphenyl ether cleavage. The quaternized aryl−ether cleavage is relatively large (ca. 61%), compared to the non-quaternized biphenyl ether cleavage (ca. 4%). A relatively small peak change, i.e., 12% of the α-hydrogen on the benzylic position of the BTMA group at 4.7 ppm, was observed (see Figure S1 in the Supporting Information), indicating that the rate of backbone degradation was > 5 times faster during 2 h of NaOH treatment than that of cationic group degradation as suggested by the modeling study. For ATM-PP, on the other hand, no backbone degradation was

heights for the SN2 reaction of two BTMA-functionalized polyaromatics. For F-PAE, the barrier height of aryl−ether cleavage, “3” is computed to be the lowest (85.8 kJ/mol) and the barrier height of TMA α-carbon degradation, “1”, follows close behind (90.8 kJ/mol). The ATM-PP has much higher barrier heights for aryl−aryl cleavage, so the barrier height of TMA α-carbon degradation has the lowest barrier height. FTIR spectroscopy was used to examine the degradation behavior of F-PAE and ATM-PP under high pH conditions. Membrane treatment of BTMA-functionalized polyaromatics was performed in 0.5 M NaOH at 80 °C as a function of treatment time. Figure 8 shows the FTIR peaks of F-PAE at 1300−1340 cm−1 and 980−1020 cm−1 during membrane treatment. The 5679

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The mechanical properties of the hydroxide form of F-PAE and ATM-PP membranes were measured before and after 2 h of treatment in 0.5 M NaOH at 80 °C. In order to prevent possible degradation during the conversion of the membrane from the salt form, mild conversion conditions (ca. 0.01 M NaOH at room temperature for 3 h) were employed. Both the F-PAE and ATM-PP membranes have a tough and ductile nature before the 2 h of treatment. After 2 h of treatment, the color of F-PAE membranes slightly changed from dark brown to light yellow and became brittle (see Figure S3 in the Supporting Information), while no physical change was observed with ATM-PP. Figure 11 shows the stress−strain

observed after the stability test (see Figure S2 in the Supporting Information). Figure 10 shows the FTIR C−N stretching peaks of F-PAE and ATM-PP peak at 1187 and 1341 cm−1, respectively. The

Figure 10. FTIR spectra of the C−N stretching region in (a) F-PAE and (b) ATM-PP, as a function of NaOH treatment time.

C−N stretching peak of ATM-PP at ∼1200 cm−1 overlaps with the product C−O stretching peaks, so another C−N stretching peak range (1330−1355 cm−1) is shown here. The C−N stretching peaks of both polymers gradually decrease as the treatment time increases, indicating BTMA cation degradation. Unlike the aryl−ether cleavage, the decrease of the C−N stretching peak takes place over the full 48-h time range. This suggests that the BTMA cation degradation occurs more slowly than aryl−ether cleavage. This is consistent with the DFT results that the barrier height for cation degradation is higher than those of aryl−ether cleavage. It is also noted that the C−N stretching peaks of F-PAE is slightly shifted to higher wave numbers (blue shift). This is probably due to the cationic group degradation of the BTMA attached to the phenolic structure occurs before that of the BTMA attached to the un-degraded aryl−ether backbone. More-electron-deficient BTMA groups (product of aryl ether cleavage) are in excess (and, thus, blueshifted) while the less-electron-deficient BTMA groups (before aryl−ether cleavage) have been consumed (preferable cation degradation pathway “1” over pathway “2” in Scheme 2). The

Figure 11. Stress−strain curves of (a) F-PAE and (b) ATM-PP before and after 2 h of treatment in 0.5 M NaOH at 80 °C.

behaviors of the membranes; the elongation at break, as well as the modulus and strength, of F-PAE significantly deteriorated after the base treatment, confirming that backbone degradation occurred. On the other hand, the elongation at break of ATMPP increased while the modulus and tensile strength decreased after the base treatment. As a result, the tough and ductile nature of the membrane was maintained. These changes are likely due to the hydrothermal effect29 during the base treatment. 3.2.2. In Situ Degradation Studies. Figure 12 compares the 130 h AEMFC life test of MEAs using F-PAE and ATM-PP. The same catalyst (Pt black) and ionomer (F-PAE) were used for the MEA fabrications and constant voltage (0.3 V) was applied with several intermittent restarts denoted by arrows. Both MEAs shows notably high current density loss within the first ∼10 h. We believe that this current density loss is primarily due to cationic adsorption onto the Pt catalyst in the anode.30 After intermittent restarts, both MEAs recovered their performance. F-PAE MEA shows more performance recovery, accompanied by restoration of cell resistance. This result suggests that cumulative dehydration of the F-PAE membrane and insufficient removal of anode water during continuous AEMFC operations give further impact on performance durability. Besides the impacts originated from water management, the HFR of both MEAs gradually increases after the first 60 h. The gradual increase of HFR indicates membrane conductivity loss primarily due to the degradation of BTMA cationic functional group. Compared to the ex situ test results at 80 °C, the rate of degradation of BTMA functional groups in the MEAs (in situ tests) was relatively slow. This is likely attributed to the fact that the concentration of OH− for the in situ test is less than that for the ex situ test, which employed 0.5 M NaOH. The major difference between the two MEAs is the catastrophic failure of the F-PAE cell at ∼60 and 82 h (denoted

Scheme 2. Degradation Sequence for F-PAE

faster degradation of the BTMA group adjacent to the hydroxyl group is likely attributed to the fact that water access to the BTMA groups adjacent to the hydroxyl group is easier than BTMA groups adjacent to the fluorinated phenyl group (before cleavage), since fluorinated groups are very hydrophobic, which hinders the access of water molecules.28 We do not see the peak shift in ATM-PP which does not have a fluorinated moiety and maintains its backbone structure until the end of the ex situ test. 5680

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membrane33 was also measured. The cell voltage of all water electrolysis cells increases rapidly during the initial few hours of testing time (∼30 h); after that, the cell voltage stabilizes and increases slightly before significant cell voltage increase or the durability test was completed. The water electrolysis cell using the commercial hydrocarbon membrane shows substantial cell voltage increases at ∼450 and 830 h. The cell voltage of the ATM-PP cell increases slightly, yet the cell operation was continued without catastrophic failure until the test was stopped before it reached a failure criterion (i.e., 3.0 V). The excellent durability of the ATM-PP cell, despite gradual cell performance loss, indicates that the poly(phenylene) backbone is stable to maintain the mechanical integrity of the cell. The durability of water electrolysis cells using Radel-based polyaromatic anion exchange membranes were reported in a previous report.32 While the Radel-based polyaromatic membrane electrolysis cell exhibited various lifetimes, depending on ionomer type and water-feed mode, shown in Figure 13 it is the best Radel-based polyaromatic MEA. All Radel-based electrolyzer cells showed a catastrophic failure between 200 h and 535 h of operation. The similarity of catastrophic performance loss for poly(arylene ether) cells in the fuel cell and electrolyzer modes during life tests suggests that the failure is originated from the same cause, most likely from mechanical issues or breaches of the membranes, although direct evidence for this is not provided. The in situ test results of F-PAE and ATM-PP MEAs indicate further important aspects of anion exchange polyaromatic membranes. First, current BTMA functional group can be used for prolonged operation if the operating temperature is low enough (ca. < 60 °C). Second, brittleness of ionomer in the catalyst layer (note that F-PAE ionomer is used for both F-PAE and ATM-PP AEMFC MEAs) may give negligible impact on device performance and long-term stability. Third, even with backbone degradation of poly(arylene ether), good initial device performance and reasonable performance longevity can be obtained if no caustic solution is used. Fourth, the polymer backbone stability, which is related to membrane mechanical toughness, is the key issue for longterm durability of the alkaline electrochemical devices.

Figure 12. Current density and HFR changes of (a) F-PAE and (b) ATM-PP cell during AEMFC life test. The life test was performed at 80 °C at a constant voltage of 0.3 V. Arrows denote the intermittent restarts of the test; light gray shading denotes the regions where significant performance loss of F-PAE cell occurred.

in gray shadows). The current density of the F-PAE MEA abruptly decreased with additional HFR gain after the breakdown. For the ATM-PP MEA, no substantial change was found until the end of the life test. The substantial losses exclusively found in the F-PAE MEA seem to be associated with the mechanical failure of the membrane. In the ex situ test, the backbone degradation of F-PAE cell occurred prior to the BTMA degradation. However, for the in situ test, the impact of the backbone degradation of the F-PAE becomes evident only after a certain level of polymer backbone degradation. The catastrophic failure was followed by intermittent restarts where the cell potential, current density, and membrane hydration changed. This suggests that the cell intermittent restarts may assist the membrane breakdown. This type of failure has been observed with PEMFCs using hydrocarbon membranes during wet−dry cycling accelerated stress test (AST), where mechanical breakdown of hydrocarbon membranes occurs.31 Figure 13 compares the durability of alkaline water electrolysis cells using ATM-PP and a poly(arylene ether) (Radel).32 For comparison purposes, the durability of alkaline water electrolysis cells using a commercial anion exchange

4. CONCLUSIONS This study compares hydroxide nucleophilic degradation on benzyl trimethylammonium (BTMA)-functionalized polyaromatics by computational modeling and experimental methods. Computational studies indicate that the barrier height of aryl− ether cleavage of the polyaromatic backbone is lower than that of BTMA degradation. This is confirmed by ex situ experiments, where most polymer backbone degradation is observed within 2 h while cation degradation is much slower over the 48 h in 0.5 M NaOH at 80 °C. In situ AEMFC and water electrolysis tests indicate that the impact of backbone degradation can be delayed up to a certain point, after which the stress level of the cell exceeds the mechanical integrity of the anion exchange membrane. In contrast, the degradation of the cationic group results in an increase in gradual cell resistance over time. Presented in this research is the first demonstration of long-term cell performance using an etherfree BTMA-functionalized poly(phenylene) membrane in alkaline electrochemical devices, although obtaining better cationic group stability remains a major technical challenge.

Figure 13. Voltage change during water electrolysis test at 50 °C; the voltage change of Radel poly(sulfone) was redrawn from ref 32. 5681

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra is provided as Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Y.-K.C. and N.J.H. performed DFT calculations. C.F. and K.S.L. characterized the polymer stability. L.T.D. and K.A. measured the performance durability of the devices. Y.S.K. advised and oversaw the research. All authors contributed to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. Dae-Sik Kim (Lotte Chemical) and Michael Hibbs (SNL) for synthesizing polymer membranes. This work was supported, in part, through the Japan-US Cooperation on Clean Energy Technology Program. Y.-K.C. acknowledges financial support from the Ministry of Economy, Trade and Industry of Japan. Other co-authors thank U.S. Department of Energy ARPA-E and Fuel Cell Technology Programs for financial support. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under Contract No. DEAC52-06NA25396, and Sandia National Laboratory is a multiprogram laboratory operated by Sandia Corporation, which is a wholly owned subsidiary of Lockheed Martin Company for the U.S. Department of Energy’s National Nuclear Security Administration, under Contract No. DEAC04-94AL85000.

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