Highly Conducting Anion-Exchange Membranes Based on Cross-Linked Poly(norbornene): Ring Opening Metathesis PolymerizationWanting Chen,†,‡ Mrinmay Mandal,‡ Garrett Huang,‡ Xuemei Wu,† Gaohong He,†

and Paul A. Kohl*,‡

†State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School ofChemical Engineering, Dalian University of Technology, Dalian 116024, China‡School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States

ABSTRACT: A series of cross-linked (XL) anion-exchangemembranes (AEMs) were synthesized on the basis of the ringopening metathesis polymerization (ROMP) of norbornenemonomers (rPNB). Poly(bromopropyl norbornene)-block-poly(butyl norbornene) diblock copolymers and poly-(bromopropyl norbornene) homopolymers have an all-hydrocarbon backbone and a high ion-exchange capacity(IEC), up to 4.73 mequiv/g. N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHDA) was used as a cross-linking agentto control the water uptake and mechanical instability. Thecross-linked (20 mol % concentration) membrane made fromhigh molecular weight poly(bromopropyl norbornene)(XL20-rPNB-LY100) had the highest conductivity of 99mS/cm at 25 °C and 195 mS/cm at 80 °C. The alkaline stability of the best membrane was excellent with no detectabledegradation in conductivity after 792 h in 1 M NaOH at 80 °C. Membranes were successfully tested as the polymer electrolytein an AEM fuel cell.

KEYWORDS: anion-exchange membranes (AEMs), ring opening metathesis polymerization (ROMP), poly(norbornene),cross-linking, fuel cells

■ INTRODUCTION

Alkaline anion-exchange membrane electrochemical devices,including fuel cells (AEMFCs), electrolyzers, and flowbatteries, have gained increased interest due to their use ofnonplatinum catalysts and facile reaction kinetics.1−3 The solidanion-exchange membrane is a key component of theelectrochemical device. AEMs have been investigated, andseveral notable advances have been made in recent years.4−7

The requirements for AEMs include (i) high ionicconductivity, (ii) chemical and thermal stability at high pH,and (iii) adequate mechanical toughness and durability fordevice fabrication and operation.8,9 However, optimization ofall the properties can be difficult because the polar moieties inthe polymer backbone or elsewhere in the structure have led toissues with long-term alkaline stability.10−12

A common strategy to increase the ionic conductivity is tosynthesize polymers with high ion-exchange capacity (IEC).However, this can lead to high water uptake (WU), which mayreduce mechanical toughness and flood the ion conductionchannels.13,14 Cross-linking is a simple and effective way tolimit the WU and membrane swelling. However, a high degreeof cross-linking can inhibit polymer flexibility and lead to poorion mobility and inferior mechanical properties.15−17 The long-term alkaline stability of AEMs is a critical issue for anionicdevices. Electrokinetics improve with temperature, and high

water vapor pressure can assist in water management; however,nucleophilic hydroxide attack is accelerated at high temper-ature.18 Benzyl-attached trimethylammonium is known to bean unstable means of cation attachment (i.e., fixed quaternaryammonium cation). The electron withdrawing nature of thearomatic ring makes the benzyl-attached quaternary ammo-nium cation susceptible to nucleophilic attack.19 To mitigatethis degradation, long alkyl tethers have been used to replacethe methylene groups between the polymer backbone and thefixed cation headgroup.20−22 Miyatake et al. optimized thelength of the pendant chain and found that side-chains withthree carbon atoms lead to a balance between high ionconductivity and low water uptake in AEMs.23 Moreover,electron withdrawing groups, such as sulfones and aryl ethers,have stability problems in alkaline solutions at a typical deviceoperating temperature (e.g., 80 °C).24,25 Thus, AEMscomposed of an all-hydrocarbon backbone with long alkyltethered side-chains are promising structures in terms ofchemical stability.26,27

Poly(norbornene) is an attractive polymer backbone forAEMs because a variety of monomers can be synthesized by

Received: November 28, 2018Accepted: February 11, 2019Published: February 11, 2019

Article

www.acsaem.orgCite This: ACS Appl. Energy Mater. 2019, 2, 2458−2468

© 2019 American Chemical Society 2458 DOI: 10.1021/acsaem.8b02052ACS Appl. Energy Mater. 2019, 2, 2458−2468

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Diels−Alder reactions.28 The low molecular weight of thenorbornene monomer allows for high IEC. Norbornenes canbe polymerized through vinyl addition polymerization or ringopening metathesis polymerization (ROMP). Coates et al.carried out the ROMP of dicyclopentadiene and tetraalky-lammonium-functionalized norbornene to synthesize AEMswith a hydroxide conductivity of 18 mS/cm at 20 °C and IECof 1.4 mequiv/g.29 The polymer was cross-linked via a metal-cation based route using a bis(terpyridine)Ru(II) complex.The resulting hydroxide conductivity was 28.6 mS/cm at 30°C with IEC of 1.4 mequiv/g.30 In an effort to increase theconductivity, the IEC was increased and cross-linking wasintroduced to enhance the properties. Wang et al. reported analkyl cross-linked AEM with an IEC of 2.89 mequiv/g resultingin a hydroxide conductivity of 64.79 mS/cm at 25 °C;however, long-term alkaline stability was not shown.31 Thediphenyloxide cross-linked AEM with IEC of 2.79 mequiv/gand OH− conductivity of 40 mS/cm at 30 °C had poor alkalinestabililty (34% conductivity loss after immersion in 2 M NaOHfor 16 days at 50 °C) along with material brittleness.32 Anotherhydrogenated poly(norbornene) membrane with an ether-linkage and flexible tether was prepared by Price et al.33 Theresulting conductivity was 69 mS/cm at 20 °C and 133 mS/cmat 80 °C. However, the conductivity degraded by about 50%after soaking in 0.1 M NaOH for 239 h at 90 °C. In summary,the state-of-the-art AEMs based on ROMP prepared poly-(norbornene)s have generally had low conductivity and/orpoor long-term alkaline stability.

In this study, AEMs with all-hydrocarbon backbones usingbromopropyl norbornene (BPNB) and butyl norbornene(BuNB) have been created by two different synthetic routes.Both vinyl addition polymerization and ROMP routes havebeen explored. In this paper, the ROMP of norbornenehomopolymers and diblock copolymers is disclosed. In thisand in the preceding paper, the vinyl addition synthetic routewas used to prepare the all-hydrocarbon polymer backboneAEMs.34 In both papers, a flexible alkyl tether, trimethylam-monium cation, was used. The combination of a hydrocarbonpoly(norbornene) backbone and tethered quaternary ammo-nium headgroup is shown to produce excellent alkalinestability and high ionic conductivity. In this paper, polymerswith high IEC, up to 4.73 mequiv/g, were synthesized and castinto membranes using N,N,N′,N′-tetramethyl-1,6-hexanedi-amine (TMHDA) as a cross-linking agent. Consequently, aconductivity of 99 mS/cm at 25 °C and 195 mS/cm at 80 °Cwas achieved with no loss in conductivity after 792 h in 1 MNaOH at 80 °C. This is higher than that of previously reportedhydroxide conductivity membranes at 80 °C, for chemicallystable AEMs.

■ EXPERIMENTAL SECTIONMaterials. Dicyclopentadiene, 5-bromo-1-pentene, and 1-hexene

were purchased from Alfa Aesar and used as-received. Thefunctionalized monomers, i.e., bromopropyl norbornene (BPNB)and butyl norbornene (BuNB), were synthesized, as describedpreviously.35,36 The polymerizations were conducted in a glovebox(dry nitrogen atmosphere) to avoid moisture and air. Prior to

Scheme 1. Synthesis of the XL-rBPN-Y100 Homopolymer and XL-rBPN-Xm-Yn Diblock AEMs

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polymerization, the monomers were purified by distillation oversodium and degassed by three freeze−pump−thaw cycles. Tetrahy-drofuran (THF), dichloromethane (anhydrous, DCM), sodiumbicarbonate (NaHCO3), methanol, tosyl hydrazide, sodium hydrox-ide, N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), ethyl vinylether, trimethylamine solution (TMA, 50 wt %), and toluene werepurchased from Sigma-Aldrich and used as-received. The Grubbs’third-generation catalyst (Grubbs’ third, G3) was synthesized fromGrubbs’ second-generation catalyst (purchased from Sigma-Aldrich)and pyridine.37

Synthesis of the Diblock Copolymers and Poly(BPNB)Homopolymers. The poly(BuNB-b-BPNB) diblock copolymerswere synthesized by the sequential addition of the monomers atroom temperature in a glovebox, as shown in Scheme 1. The materialsare designated rPNB-Xm-Yn, where rPNB stands for ROMPnorbornene, and X and Y are the hydrophobic BuNB and hydrophilicBPNB monomers in the polymer, respectively; m and n are the mol %of the BuNB and BPNB monomers, respectively, as calculated by 1HNMR. The homopolymers are designated as rPNBY100 and rPNB-LY100, where Y100 stands for the homopolymer of BPNB and LY100stands for the homopolymer of BPNB with a higher molecular weight.All monomers were purified by distillation and degassed by three

freeze−pump−thaw cycles before the polymerization. Grubbs’ thirdwas dissolved in DCM to make a 0.01 g/mL solution and then stirredfor 10 min. The monomer solution (0.01 M) was obtained bydissolving BuNB or BPNB in DCM and stirring for 10 min. Thecatalyst solution was injected into the BPNB monomer solution undervigorous stirring to prepare the first block, poly(BPNB). After 10 minof reaction time, the BPNB polymerization was complete. A smallaliquot was removed and quenched with ethyl vinyl ether for gelpermeation chromatography (GPC) analysis. A BuNB solution wasthen added to the reaction solution and stirred for 5 min to add theBuNB block onto the poly(BPNB). After the reaction was complete,ethyl vinyl ether was added to the reaction mixture to quench thepolymerization. The mixture was stirred for 30 min. Excess solventwas allowed to evaporate in air, and the concentrated polymersolution was precipitated in methanol. The resulting product wasprecipitated in methanol twice and dried at room temperatureovernight. The procedure for poly(BPNB) homopolymers was similarto that for the diblock polymer without the addition of the secondBuNB block.Hydrogenation of Double Bonds. The unsaturated ROMP

polymer (0.4 g) was dissolved in toluene (50 mL) at roomtemperature in a two-necked round-bottom flask with a refluxcondenser.38,39 A stoichiometric amount of tosyl hydrazide (1:6 ratioof the double bond to hydrazide) was added to the polymer solution.The solution was purged with nitrogen gas for 30 min to removedissolved oxygen. The mixture was stirred at 110 °C for 24 h undernitrogen gas. The solution was then cooled to room temperature andwashed three times with a saturated NaHCO3 solution (500 mL eachtime) until the reaction solution was transparent. The polymersolution was concentrated by removing toluene at room temperatureovernight, and the product was precipitated in methanol.Preparation of the Cross-Linked AEMs. The hydrogenated

polymer (0.15 g) was dissolved in 6 mL of toluene. The cross-linkingreagent, TMHDA, was dissolved in 1.5 mL of toluene and added tothe polymer solution. The mixture was stirred for 30 min at roomtemperature and then filtered through a 0.45 μm poly-(tetrafluoroethylene) membrane syringe filter into an aluminumdish. The solution was dried at 60 °C for 24 h, by which time thecross-linking reaction was completed. The term “cross-linkerconcentration” is the mol % of cross-linking compound (i.e.,TMHDA) added to the polymer mixture with respect to the numberof cross-linkable sites within the polymer. It is noted that while thecross-linking agent may fully react with sites on the polymer, it wasnot determined what fraction of the cross-linking reaction occurredbetween intramolecular sites vs intermolecular sites. The membraneswere peeled off the aluminum dish and immersed in TMA at roomtemperature for 48 h to quaternize the bromopropyl headgroups. Thequaternized membrane in bromide form was washed thoroughly with

deionized (DI) water and soaked in 1 M NaOH for 24 h undernitrogen to exchange the Br− ions for OH− ions. The cross-linkedAEMs are denoted XLp-rPNB-Xm-Yn and XLp-rPNB-Y100 for thediblock and homopolymer membranes, respectively, where XL meanscross-linking and p is the cross-linker concentration.

Material Characterization. The same characterization techni-ques were used here as described in the accompanying paper for thisstudy.34 A brief description is provided here in this paper. 1H NMRspectroscopy was performed using a Bruker Avance 400 MHz NMRspectrometer, with tetramethylsilane as an internal standard andCDCl3 as the solvent. The number-average molecular weight (Mn)and polydispersity index (Mw/Mn) of the polymers were obtained byGPC analysis (Shimadzu) equipped with a LC-20 CE HPLC pumpand a refractive index detector (RID-20 A, 120 V). All themeasurements were performed in THF with the eluent flow rate of1.0 mL min−1 at 30 °C. A polystyrene standard was used. The in-plane hydroxide conductivity was measured using a four-electrodeprobe and an electrochemical impedance spectrometry (1 Hz to 1MHz) with a PAR 2273 potentiostat. HPLC-grade water with anitrogen purge was used. Water uptake (WU), swelling ratio (SR),and hydration number (λ) were determined, as described in theaccompanying paper.34 The numbers of bound water (nonfreezable,Nbound) and freezable water (Nfree) were determined by differentialscanning calorimetry (DSC). DSC measurements were carried out ona Discovery DSC with autosampler (TA Instruments). The fullyhydrated membrane without surface water (5−10 mg) was sealed inan aluminum pan, cooled to −50 °C, and then heated to 30 °C at arate of 5 °C/min under 20 mL/min N2 flow to determine Nbound andNfree.

34 DSC was also used to measure the glass transition temperature(Tg) of the AEMs at a heating rate of 20 °C/min. Each membranewas temperature cycled between 20 and 200 °C to obtain Tg.

Thermogravimetric analysis (TGA) was conducted on a TAInstruments Q50 analyzer with 5−10 mg of dry membrane inbromide form. The temperature was ramped from room temperatureto 800 °C at a heating rate of 10 °C/min in a nitrogen atmosphere.The mechanical properties of the fully hydrated and dry membraneswere measured with a SANS CMT8102 stretching tester (XinsansiCo., China) at a stretch rate of 5 mm/min. Alkaline stability wastested by soaking the AEMs in 1 M NaOH solution at 80 °C in aTeflon-lined Parr reactor. The conductivity of the treated AEMs wasmeasured at 25 °C vs time after the complete removal of the residualNaOH. Moreover, the AEMs were also studied by FT-IR before andafter alkali treatment to further confirm the chemical structures.

Transmission Electron Microscopy (TEM). TEM was per-formed on dry AEM samples in Br− form to qualitatively observe thephase segregated microstructure of the membranes. Each membranewas embedded in epoxy and sectioned by Microtone. The membraneswere stained with I− by immersion in a NaI solution (2.0 M) for 2days at 80 °C and measured by a JEOL JEM-2000EX microscope.16

Small Angle X-ray Scattering (SAXS). SAXS was also used toanalyze the phase segregation of block copolymer AEMs. Hydratedmembranes in bromide form were tested in air using the NSLS-IIbeamline at the Center for Functional Nanomaterials (BrookhavenNational Laboratory, Upton, NY). The wave vector (q) wascalculated using eq 1, where 2θ is the scattering angle.

πθ

=l

q4

sin 2 (1)

The characteristic separation length, or interdomain spacing (d) (i.e.,the Bragg spacing), was calculated using eq 2.

π=dq

2(2)

H2/O2 Fuel Cell Measurements. The AEM anode and cathodewere fabricated via a previously used slurry method and were identicalin composition.40 A poly(norbornene) ionomer powder with lowermolecular weight (20.5 kDa) was first synthesized according to theprevious work.36 The dry ionomer powder and 50% platinum onVulcan XC-72 (carbon) catalyst were ground together with a mortar

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and pestle in isopropyl alcohol (IPA) to achieve the proper slurry forspraying. After sonicating for 30 min, the homogenized catalyst andionomer slurry was sprayed onto 1% water-proofed Toray TGPH-060carbon paper and dried for 24 h at room temperature. The metalloadings on electrodes were ∼2.1 mg/cm, and an ionomer/carbonratio of 40% was used. The electrodes were converted to the OH−

form by soaking in 1 M NaOH for 2 h with a N2 purge. The NaOHsolution was refreshed every 20 min. The electrodes and ROMPpoly(norbornene) diblock AEMs were placed into Fuel CellTechnologies hardware for the test in a Scribner 850e Fuel CellTest Station at 60 °C. The dew points of the H2 and O2 gas feeds (0.5L min−1) were adjusted throughout the testing to optimize the waterbalance within the fuel cell.

■ RESULTS AND DISCUSSIONrPNB-Xm-Yn diblock copolymers and rPNB-Y100/rPNB-LY100(poly(BPNB)) hydrophilic homopolymers were synthesized inthis study. The monomers (BuNB and BPNB) were preparedaccording to previous reports.35,36 The Grubbs’ third-generation catalyst (G3) was synthesized from Grubbs’second-generation catalyst and pyridine.37 BPNB ([M]0/[G3] = 100:1) was reacted for 10 min, whereas BuNB([M]0/[G3] = 20:1) was reacted for 5 min to grow the diblockcopolymers. The polymerization time was optimized for theindividual monomer additions and their feed ratios. Therepresentative GPC traces of rPNB-X60-Y40 are shown inFigure 1 to demonstrate the sequential growth of each block

during the formation of the diblock copolymer. Aftercompletion of the first block, the Mn was 18.34 kDa andMn/Mw was 1.82. The addition of the second monomer led toa higher Mn, 24.68 kg/mol, and Mn/Mw of 2.19, showing thesuccessful addition of the second block. The GPC traces of thediblock copolymer before and after hydrogenation had asimilar trend and Mn (24.68 vs 25.96 kg/mol). This shows thatno side reactions occurred during hydrogenation. The Mn

values of the synthesized polymers were kept constant in orderto study the relationship between the different copolymers.The mechanical properties of the homopolymer (rPNB-Y100)were poor. Thus, a higher Mn homopolymer (rPNB-LY100)was also synthesized, as shown in Table 1. For [BPNB]/[G3]= 200:1, the length consisted of 129 repeat units (25 minreaction time). A higher molecular weight homopolymer wasproduced by using a higher BPNB:G3 ratio (i.e., 500:1) andextending the reaction time to 2 h. The resulting homopolymerhad 198 repeat units.The 1H NMR spectra of the diblock copolymer before and

after hydrogenation are shown in Figure 2. The signal at 5.33ppm (Ha) is from the protons in the double bond on thepolymer backbone. The characteristic signal of the protons inthe terminal methyl group of the butyl side-chain is at 0.89ppm (Hc), and the methylene protons (−CH2Br) adjacent tothe bromine atom in bromopropyl side-chain are at 3.40 ppm(Hb). The mole ratio (R) of the BuNB:BPNB block wascalculated by comparing the NMR integration ratio for the Hc

and Hb protons (R = 2I(Hc)/3I(Hb)). The IEC values for thepolymers in the OH− form were determined by IECNMR =1000/(150R + 213), where 150 and 213 are the molecularweights of the BuNB and quaternized BPNB (in OH− form)repeat units, respectively. The IECNMR for the synthesizedpolymers ranged from 2.31 to 4.73 mequiv/g (Table 1).The carbon−carbon double bond in the poly(norbornene)

backbone was hydrogenated to improve the chemical stabilityof the polymer.41 After hydrogenation, the NMR peakcorresponding to the protons at the double bond (Ha) wasabsent, which shows complete hydrogenation of the doublebonds.

Microphase Segregation. The segregation of the hydro-philic and hydrophobic regions of the cross-linked polymer wasinvestigated by SAXS and TEM (Figures 3 and 4, respectively).The interdomain spacing (d) was established to be 2π/q andlisted in Table 2. Well-resolved peaks were observed in thecross-linked diblock rPNB-X60-Y40 membranes. The SAXSpeak shifted from 0.08 to 0.10 nm−1 when the cross-linkerconcentration was changed from 10 to 45 mol %. Thiscorresponds to a decrease in domain size (d) from 78.64 to62.71 nm (Table 2). This suggests that the higher cross-linkerconcentration slightly narrows the size of the ionic channels.Close inspection of XL35-rPNB-X60-Y40 shows that there aretwo peaks and the q vector ratio is 1:2, which indicates alamellar polymer morphology. The XL-rPNB-X22-Y78 mem-brane had only one peak at a higher q value (∼0.30 nm−1),corresponding to a smaller domain size (∼21 nm) and lessmicrophase segregation. These results are consistent with theTEM images shown in Figure 4. In Figure 4, the dark regionscorrespond to the hydrophilic domains and lighter regionscorrespond to hydrophobic domain.

Figure 1. GPC-RI traces of the rPNB-X60-Y40 for the first block anddiblock aliquot (before and after hydrogenation).

Table 1. Molecular Weight and IECNMR of the Synthesized Polymers

polymer expected Y/X monomer ratioa actual Y/X monomer ratiob mol % of BPNB/BuNBc Mn (kDa)d Mw/Mn

d IECNMR (meq/g)c

rPNB-X60-Y40 100/100 85/42 40/60 24.68 2.20 2.31rPNB-X22-Y78 100/20 98/22 78/22 24.08 2.37 3.95rPNB-Y100 200/0 129/0 100/0 27.73 1.63 4.73rPNB-LY100 500/0 198/0 100/0 42.47 2.54 4.73

aCalculated from the feed ratio of monomers. bGPC results. cMol % and IEC determined by 1H NMR. dGPC data in THF versus polystyrenestandards.

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As expected, the homopolymer, XL-rPNB-LY100, showedvery little sign of phase segregation. Only one weak peak(forming a shoulder in the SAXS) was observed. These smallpeaks are due to isolated ionic clusters, as shown in Figure 3.This lack of obvious phase segregation is in contrast to theresults for block copolymers which can be highly effective increating phase segregated, high mobility ion conductionchannels. This was also concluded in the accompanyingpaper concerning vinyl-addition poly(norbornene).34 Whenthe cross-linker concentration was increased from 15 to 25 mol%, the size of ionic clusters decreased from 25.68 to 23.05 nm,which is larger than those in the AEMs with low IECvalues.42,43

Ion-Exchange Capacity (IEC) and Water Uptake (WU).High IEC can contribute to high anion conductivity within themembrane, assuming the anion−cation pairs are ionized andthe anions have high mobility. Unfortunately, high IEC canalso cause high, unacceptable water uptake and ion channelflooding, which decreases hydroxide mobility. The IEC of thepolymers in this study varied from 2.31 to 4.73 mequiv/g,Table 1. The relatively low molecular weight of bromopropylnorbornene allowed for the synthesis of an all-hydrocarbonpolymer backbone with exceptionally high IEC. There is ageneral trend of higher WU with a lower degree of cross-linking. Polymer with high IEC and low cross-linker

Figure 2. 1H NMR spectra of the diblock copolymer before and after hydrogenation.

Figure 3. SAXS spectra of the dry membranes in Br− form (the numbers are the cross-linker concentrations).

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concentration was soluble in aqueous TMA during amination.Thus, the focus of this study is on the cross-linked polymers.WU is necessary for ion solvation and conduction. There is a

general correlation between WU and swelling ratio for aparticular polymer backbone at a specific cross-linkerconcentration, as shown here in Figure 5. The WU moreinversely correlates with cross-linking. WU also correlates withIEC (at the same cross-linker concentration) but to a lesserextent than cross-linking. For each series of AEMs, WU valueswere similar, for the same cross-linker concentration. Forexample, at 20 mol % cross-linker concentration, the highestwater uptake was 157% for XL20-rPNB-X22-Y78 (3.78mequiv/g), while the lowest value was 115% for XL20-rPNB-LY100 (4.51 mequiv/g). However, for AEMs based on

the same polymer, WU dramatically changed with the cross-linker concentration. For instance, XL-rPNB-LY100 showed thelowest WU of 79% at 25 mol % cross-linker concentration,whereas the highest WU was 224% at 15 mol % cross-linkerconcentration. For all prepared AEMs, WU varied from 40% to400%, while the swelling ratio varied from 18% to 53% at roomtemperature. It was not possible to test the swelling ratio ofXL-rPNB-Y100 AEMs due to membrane brittleness.The hydration number (λ), the average number of water

molecules per ion pair, for each polymer is tabulated in Table2. Figure 5c shows the degree of solvation of each ion pairwithin the polymer versus the cross-linker concentration. Atthe similar molecular weight and cross-linker concentration,the λ value for the different AEMs (i.e., XL-rPNB-X60-Y40, XL-

Figure 4. TEM micrographs of the XL35-rPNB-X60-Y40 (a), XL20-rPNB-X22-Y78 (b), and XL20-rPNB-LY100 (c) membranes in Br− form.

Table 2. Properties of the XL-rPNB Membranesa

σOH, mS/cm

AEM XL, mol % IECNMR, meq/g WU, % λ Nfree Nbound σ/IEC (80 °C) 25 °C 80 °C SR, % d, nm

XL10-rPNB-X60-Y40 10 2.26 216 53 5.64 47.45 30.97 36 70 40 78.64XL35-rPNB-X60-Y40 35 2.20 100 25 5.76 19.49 49.55 53 109 28 66.07XL45-rPNB-X60-Y40 45 2.17 57 15 2.50 12.09 31.34 33 68 20 62.71XL20-rPNB-X22-Y78 20 3.78 157 23 6.59 16.48 45.24 92 171 46 21.64XL30-rPNB-X22-Y78 30 3.71 103 15 3.95 11.47 43.40 81 161 27 21.09XL40-rPNB-X22-Y78 40 3.63 93 14 2.60 11.63 42.70 77 155 26 20.57XL15-rPNB-LY100 15 4.56 224 27 4.98 22.31 31.14 75 142 45 25.68XL20-rPNB-LY100 20 4.51 115 14 5.68 8.48 43.24 99 195 28 25.42XL25-rPNB-LY100 25 4.45 79 10 3.72 6.14 33.71 74 150 18 23.05

aXL is cross-linker concentration. IEC was calculated on the basis of 1H NMR (including the mass of the TMHDA). λ is hydration number. Nfree isthe number of freezable water molecules. Nbound is the number of bound, nonfreezable water molecules. SR is swelling ratio. d-spacing (nm) wascalculated from SAXS data.

Figure 5. Water uptake (a), swelling ratio (b), and λ (c) of the membranes as a function of cross-linker concentration at 25 °C.

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rPNB-X22-Y78, and XL-rPNB-Y100) is analyzed. The diblockXL-rPNB-X60-Y40 AEM with the lowest IEC had the highest λvalue compared to the other two AEMs, showing that well-ordered hydrophilic lamellar domains in the XL-rPNB-X60-Y40aided in water absorption. The highest IEC polymer, XL-rPNB-Y100, had the lowest λ, showing that the ion channels didnot swell as easily as the others.The hydration numbers were further parsed into the number

of freezable (or free) waters (Nfree) and bound water molecules(Nbound) by DSC freezing point measurements. A higherdegree of cross-linking caused both the Nfree and Nbound watersto decrease, which is consistent with established trends. Foreach polymer, there is an optimum cross-linker concentrationto achieve the highest mobility. Insufficient WU (i.e., highcross-linker concentration) leads to low mobility whileexcessive WU (low cross-linker concentration) enables ionchannel flooding.44 For example, XL15-rPNB-LY100 (15 mol% cross-linker concentration) had Nfree = 4.98 and Nbound =22.31 waters. High WU caused flooding of the ion channelsand dilution of the hydroxide resulting in only 75 mS/cmconductivity and relatively low ion mobility, as measured by σ/IEC = 31.14. Increasing the cross-linker concentration from 15to 20 mol % resulted in about the same Nfree (5.68); however,Nbound was lower (8.48), which gave higher conductivity, 99mS/cm at 25 °C. The conductivity increase is due to higherion mobility, σ/IEC = 43.24. Further increasing the cross-linker concentration to 25 mol % resulted in insufficient WUwith Nfree = 3.72 and Nbound = 6.14. This level of WU was lesseffective in terms of mobility, and σ/IEC decreased to 33.71.The result is similar to that of the 15 mol % cross-linkerconcentration sample.The diblock AEMs with the highest conductivity, XL35-

rPNB-X60-Y40 and XL20-rPNB-X22-Y78, had similar Nfree (5.76and 6.59, respectively) and Nbound (19.49 and 16.48,respectively). However, the homopolymer (XL20-rPNB-LY100) with the lowest Nfree (5.68) and Nbound (8.48) hadthe highest conductivity among all the AEMs. The mobilitywas high as evidenced by σ/IEC = 43.24. This shows that theROMP homopolymer with 20 mol % cross-linker concen-tration made effective use of the WU with adequate water forhigh mobility without channel flooding.The conductivity vs cross-linker concentration data is

summarized in Figure 6a. The hydroxide mobility (andhydroxide conductivity) is highly influenced by the amountof cross-linker. As the amount of cross-linker increased in thesamples, the conductivity peaked at 103 mS/cm (at 25 °C)

with a cross-linker concentration of 20 mol % for the XL20-rPNB-Y100. Although the XL20-rPNB-Y100 had the highestconductivity among all AEMs reported in this paper, itsmechanical properties were poor. The mechanical propertieswere improved by increasing the molecular weight of thepolymer, XL20-rPNB-LY100, which had excellent conductivity(99 mS/cm at 25 °C and 195 mS/cm at 80 °C). Theconductivities reported here are higher than the ROMP AEMsreported in the literature, mostly due to the high IEC value ofpoly(norbornene).31−33,45

The hydroxide transport activation energy (Ea)46,47 was

calculated and is shown in Figure 6b. For XL-rPNB-X60-Y40, asthe cross-linker concentration increased from 10 to 45 mol %,the Ea increased from 9.46 to 13.25 kJ/mol. For high IECAEMs (i.e., XL20-rPNB-LY100 and XL20-rPNB-X22-Y78),their Ea values are about the same (10.94 and 10.40 kJ/mol,respectively), indicating similar hydroxide environments in theAEMs.A different way to examine the data is to plot conductivity as

a function of λ at 25 °C, Figure 7. Figure 7 shows the region(low λ) where there is insufficient water, and the region ofexcess swelling (high λ). High λ values lead to ion channelflooding and a decline in conductivity. At λ < 20, thehomopolymer AEMs had relatively high conductivity com-pared to the diblock AEMs (XL-rPNB-X22-Y78 and XL-rPNB-

Figure 6. Hydroxide conductivity of the membranes as a function of cross-linker concentration (a) and Arrhenius plots of the XL-rPNBmembranes (b).

Figure 7. Hydroxide conductivity of the XL-rPNB AEMs as afunction of hydration number (λ) compared with AEMs reported inthe literature (20−30 °C).

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X60-Y40). For example, XL20-rPNB-LY100 (λ = 14) hadhydroxide conductivity of 99 mS/cm, while the XL40-rPNB-X22-Y78 had conductivity of 77 mS/cm. Compared with otherAEMs reported in the literature,11,20,21,36,48−56 the XL-rPNBAEMs had higher conductivity at moderate λ (10 to 30),suggesting XL-rPNB AEMs are promising candidates for theelectrochemical applications.Thermal Stability. The thermal stability of the ROMP

AEMs was assessed using thermogravimetric analysis (TGA),Figure 8a. There are three mass-loss steps in the TGA. Thefirst is the loss below 100 °C which corresponds to the loss ofwater and any remaining organic solvent in the mem-branes.48,49 The second step at about 200 °C corresponds tothe loss of the quaternary ammonium groups (N+(CH3)3, QA)because the weight loss is close to the QA mass in the AEMs.For example, the weight loss for the XL35-X60-Y40 is about 12wt % at the second step, and the QA groups have about 13 wt%. The third mass loss is most likely polymer backbonedecomposition. Thus, ROMP poly(norbornene) backbone hassufficient stability for electrochemical devices at 80 °C, such asfuel cells. In addition, Figure 8b shows that the Tg of the XL35-rPNB-X60-Y40, XL20-rPNB-X22-Y78, and XL20-rPNB-LY100materials is 145, 165, and 170 °C, respectively. The highernumber of ionic repeat units corresponds to a higher Tg.Mechanical Stability. The mechanical properties of the

fully hydrated and dry XL-rPNB-X60-Y40, XL-rPNB-X22-Y78,and XL-rPNB-LY100 membranes are shown in Figure 9.Elongation-to-break values for the hydrated AEMs are greaterthan 14.0%, while the tensile strength of each is relatively low,<2.6 MPa. The elongation-to-break for hydrated XL-rPNB-X60-Y40 (tensile stress = 2.5 MPa, strain = 51.7%) was thehighest value obtained. It is high in comparison to that of thehydrated XL-rPNB-X22-Y78 (1.5 MPa, 14.0%). This suggeststhat higher ionic content within the AEM decreases themechanical properties. XL20-rPNB-LY100 (1.9 MPa, 24.5%)had better mechanical properties than XL20-rPNB-Y100 due toits higher molecular weight. The dry AEMs had higher tensilestrength (>10 MPa) than the hydrated membranes showingthat the presence of absorbed water lowers the mechanicalstrength.Alkaline Stability. The alkaline stability of the XL20-

rPNB-LY100 (4.51 mequiv/g), XL20-rPNB-X22-Y78 (3.78mequiv/g), and XL35-rPNB-X60-Y40 (2.20 mequiv/g) mem-branes was evaluated by soaking them in 1 M NaOH at 80 °C

for over 500 h, Figure 10a. All the tested AEMs maintainedtheir mechanical properties, flexibility, and strength aftersoaking in NaOH. There was a slight increase in conductivityafter 24 h which may be due to a more complete conversion tothe hydroxide form from either bromide or carbonate. In mostmembranes, there was little or no decrease in conductivityfrom the initial value. Even XL20-rPNB-LY100, which had ahigh IEC (4.51 mequiv/g), retained its initial conductivityvalue after 792 h in 1 M NaOH at 80 °C (within experimentalerror), confirming excellent chemical stability. For the diblockpolymers (e.g., XL35-rPNB-X60-Y40 which had lower IEC),the conductivity dropped 1.44% after 576 h. The XL20-rPNB-X22-Y78 membrane showed a decrease of 4.77% after 672 h. Asshown in Figure 10b, FT-IR analysis showed no change in theC−N stretch at 1490, 1260, and 1142 cm−1 after the alkalinetreatment.50,57,58 This suggests that the AEM chemicalstructures remained intact and the loss of conductivity maybe caused by cracks along the edges of the AEMs.

Fuel Cell Tests. The ROMP AEMs were tested in AEMfuel cells to ensure that the membranes were sufficientlyrobust, conductive, and stable so that they can be used in amembrane electrode assembly (MEA) and electrochemicaldevice. An MEA was constructed from each of the XL20-rPNB-LY100, XL20-rPNB-X22-Y78, and XL35-rPNB-X60-Y40membranes and operated in a H2/O2 fuel cell at 60 °C,Figure 11. The open-circuit voltage of the cell with the XL20-

Figure 8. TGA (a) and endothermic DSC thermogram (b) curves of the XL35-rPNB-X60-Y40, XL20-rPNB-X22-Y78, and XL20-rPNB-LY100 in Br−

form. To avoid overlaps, the graphs are vertically shifted in part b.

Figure 9. Stress−strain curves of the fully hydrated and dry XL-rPNBmembranes.

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rPNB-X22-Y78 membrane was 0.60 V, which is modest. Thepoor mechanical properties of the membrane caused smallcracks resulting in fuel crossover and/or cell shorting.3

Although the freestanding XL20-rPNB-LY100 membrane wasmore robust, its open-cell voltage was still somewhat low (0.70V), most likely due to the high gas crossover originating fromthe high water uptake. XL35-rPNB-X60-Y40 had a lower IECand the best mechanical properties. The open-circuit voltagewas 0.83 V. The maximum power density for the XL20-rPNB-LY100 and XL35-rPNB-X60-Y40 fuel cells was 126 mW/cm2 at333 mA/cm2, and 172 mW/cm2 at 401 mA/cm2, respectively.These tests show that conductivity and mechanical propertieshave to be optimized for the intended application of themembrane. The variables to be considered include operatingtemperature and cross-membrane differential pressure. Opti-mization and further testing of these materials may be thesubject of future reports.

■ SUMMARYA series of ROMP cross-linked poly(norbornene)s weresynthesized and evaluated for possible use in electrochemicaldevices. The polymers were composed of an all-hydrocarbonbackbone and flexible alkyl side-chain with quaternaryammonium headgroup. The polymers with exceptionally highIEC (4.73 mequiv/g) were synthesized by choosing a properratio between hydrophobic and halogenated monomers withrespect to G3. Modest cross-linking (20 mol % cross-linker

concentration) was used to achieve high hydroxide con-ductivity (XL20-rPNB-LY100), 99 mS/cm at 25 °C and 195mS/cm at 80 °C, which are higher than those for previouslyreported ROMP AEMs. Moreover, the XL20-rPNB-LY100membrane had excellent alkaline stability. Further optimizationof properties and reinforcement of the membrane may lead tohigher fuel cell performance.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: kohl@gatech.edu. Phone: 404-894-2893.

ORCIDPaul A. Kohl: 0000-0001-6267-3647NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of theARPA-E IONICS program, China Scholarship Council, andthe helpful discussions of Dr. Edmund Elce. We would also liketo thank Dr. Sungmin Park for his invaluable help with SAXSmeasurements at Brookhaven National Laboratory.

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