Highly conductive and chemically stable alkalineanion exchange membranes via ROMP oftrans-cyclooctene derivativesWei Youa, Elliot Padgettb, Samantha N. MacMillana, David A. Mullerb, and Geoffrey W. Coatesa,1

aDepartment of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853; and bSchool of Applied and Engineering Physics,Cornell University, Ithaca, NY 14853

Contributed by Geoffrey W. Coates, March 15, 2019 (sent for review January 18, 2019; reviewed by Jeffrey Scott Moore and Timothy M. Swager)

Alkaline anion exchange membranes (AAEMs) are an importantcomponent of alkaline exchange membrane fuel cells (AEMFCs),which facilitate the efficient conversion of fuels to electricity usingnonplatinum electrode catalysts. However, low hydroxide conduc-tivity and poor long-term alkaline stability of AAEMs are the majorlimitations for the widespread application of AEMFCs. In this paper,we report the synthesis of highly conductive and chemically stableAAEMs from the living polymerization of trans-cyclooctenes. Atrans-cyclooctene–fused imidazolium monomer was designed andsynthesized on gram scale. Using these highly ring-strained mono-mers, we produced a range of block and random copolymers. Sur-prisingly, AAEMs made from the random copolymer exhibited muchhigher conductivities than their block copolymer analogs. Investiga-tion by transmission electron microscopy showed that the blockcopolymers had a disordered microphase segregation which likelyimpeded ion conduction. A cross-linked random copolymer demon-strated a high level of hydroxide conductivity (134 mS/cm at 80 °C).More importantly, the membranes exhibited excellent chemical sta-bility due to the incorporation of highly alkaline-stable multisubsti-tuted imidazolium cations. No chemical degradation was detectedby 1H NMR spectroscopy when the polymers were treated with 2 MKOH in CD3OH at 80 °C for 30 d.

alkaline anion exchange membrane | trans-cyclooctene | block and randomcopolymer | cross-linked polymer | transmission electron microscopy

Alkaline anion exchange membranes (AAEMs) are a class ofpolymer electrolytes constructed from immobilized cations

with hydroxide counteranions (1–3). AAEMs have been widelyused for a variety of electrochemical applications, such as redoxflow batteries, electrodialysis, and water electrolysis (4–7). Oneof the most important applications for AAEMs is their use aspolyelectrolytes in alkaline exchange membrane fuel cells(AEMFCs). These devices can efficiently convert chemical en-ergy in fuels (e.g., H2, hydrazine, and direct alcohols) into elec-tricity. In comparison with proton exchange membrane fuel cellsand aqueous alkaline fuel cells, AEMFCs are advantageous be-cause they allow for rapid reduction of oxygen at the cathode, limitfuel cross-over, prevent the formation of carbonate precipitates,and are compatible with nonnoble electrocatalysts (8–12). Becauseof their essential role in AEMFCs, AAEMs have been extensivelystudied to optimize their performance and, in particular, to im-prove their hydroxide conductivity and alkaline stability.Hydroxide conductivity is one of the most important parameters

used to evaluate AAEMs, because it is directly related to the ohmicresistance and power density of an AEMFC (9). There are severalstrategies to enhance the hydroxide conductivity of AAEMs. Themost straightforward method is to increase the ion exchange ca-pacity (IEC), such that more ions are present in AAEMs. However,a higher IEC typically results in higher water uptake. While thepresence of water facilitates hydroxide transportation, excess wateruptake decreases the membrane’s mechanical strength and dilutesthe ions, reducing the overall conductivity (13). However, AAEMconductivity is significantly influenced by hydrophobic–hydrophilic

microphase separation, because a well-defined polymer morphology(e.g., bicontinuous hydrophobic–hydrophilic channels) facilitateswater/hydroxide transportation in solid-state membranes (14). Inprevious reports, modifying polymer morphology with multiblocksequences (15–18), long flexible side chains (18–21), or microporousstructures (22) has improved hydroxide conductivity. For instance,Watanabe and coworkers (15) observed that multiblock poly(aryleneether)s showed significantly higher hydroxide conductivity thantheir random copolymer analogs [σ (OH−, 60 °C) = 126 mS/cmand 35 mS/cm for multiblock and random copolymers, respec-tively. These polymers had a similar IEC (∼ 1.9 mmol Cl−/g)].Although AAEM hydroxide conductivity has significantly im-

proved in the past decade and several AAEMs exhibit a hydroxideconductivity above 100 mS/cm at 80 °C, long-term alkaline stabilityremains a major problem (3). Since AEMFCs usually operate athigh temperatures in environments with low water contents (23),high concentrations of hydroxide ions can cause degradation ofboth cations and polymer backbones via deprotonation and nu-cleophilic attack (3, 9). Cation decomposition in AAEMs leads toa reduction in ionic conductivity; consequently, numerous studieshave focused on increasing the alkaline stability of cations (2).Quaternary ammonium cations are commonly used in AAEMs.

Significance

Alkaline exchange membrane fuel cells (AEMFCs) can generateelectricity from chemical energy in renewable fuels (e.g., H2) usinginexpensive nonplatinum electrode catalysts. The alkaline anionexchange membrane (AAEM) is a major component in AEMFCs, asits hydroxide conductivity and alkaline stability are directly re-lated to the power density and durability of the AEMFC. In thiswork, we synthesized AAEMs consisting of polyethylene back-bones and alkaline-stable imidazolium cations via the living ring-opening metathesis polymerization of trans-cyclooctene mono-mers, wherein either block or random copolymer sequences canbe achieved. The polymer morphology was studied by trans-mission electron microscopy to understand the difference in con-ductivity. High hydroxide conductivities and alkaline stabilitieswere observed for the cross-linked random copolymer AAEMs.

Author contributions: W.Y. and G.W.C. designed research; W.Y., E.P., and S.N.M. per-formed research; W.Y., E.P., S.N.M., D.A.M., and G.W.C. analyzed data; and W.Y., E.P.,S.N.M., D.A.M., and G.W.C. wrote the paper.

Reviewers: J.S.M., University of Illinois at Urbana–Champaign; and T.M.S., MassachusettsInstitute of Technology.

The authors declare no conflict of interest.

Published under the PNAS license.

Data deposition: The atomic coordinates and structure factors have been deposited in theCambridge Structural Database, Cambridge Crystallographic Data Centre, Cambridge CB21EZ, United Kingdom, www.ccdc.cam.ac.uk (CSD reference no. 1878813).1To whom correspondence should be addressed. Email: coates@cornell.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1900988116/-/DCSupplemental.

Published online April 29, 2019.

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Recent studies have shown that cyclic ammonium (24–27) andtetraalkyl ammonium cations, specifically those with long alkylchains (18–20, 28–30), have better alkaline stability than tradi-tional benzyltrimethyl ammonium cations (31–33). Other cations,such as imidazolium (16, 21, 34–43), phosphonium (44–49), andmetal cations (50–54), have also been widely applied in variousAAEMs. More specifically, pentasubstituted imidazolium (34–38),tetrakis(dialkylamino)phosphonium (44–46), and cobaltocenium(50, 51) have shown superior alkaline stability. Among thesealkaline-stable cations, pentasubstituted imidazolium is particularlyintriguing because its charge is delocalized throughout an aromaticring, and, more importantly, all five substituents on imidazoliumcan be easily tuned for optimized conductivity and alkaline stability(36). For example, Holdcroft and coworkers (37) reported theincorporation of multisubstituted imidazolium cations directly intopolymer backbones. Despite its excellent alkaline stability [stable at100 °C in 10 M KOH (aq) for 7 d], the polymer dissolved in waterabove 80 °C. More recently, our group described the synthesis ofalkaline-stable AAEMs by incorporating multisubstituted imida-zolium cations into chemically inert polyethylene backbones (38).Sequential ring-opening metathesis polymerization (ROMP) andhydrogenation were performed with cis-cyclooctene and cis-cyclooctene–fused imidazolium monomers, but oligomer macro-cycles were obtained rather than the desired high-molecular-weightpolymers. As a result, a bifunctional cross-linker was required toproduce high-performance AAEMs [σ (OH−, 22 °C) = 37 mS/cmand σ (OH−, 80 °C) = 71 mS/cm for the optimized AAEM] (38).It should be noted that AAEMs with alkaline-stable imidazo-

lium, phosphonium, and metal cations usually have lower hydroxideconductivity than AAEMs containing ammonium cations [σ (OH−,80 °C) < 100 mS/cm] (3). This is likely because enhanced cationalkaline stability is usually achieved by increasing the cation’ssteric hindrance, which often decreases hydrophilicity, therebyreducing hydroxide conductivity (54). Several AAEMs contain-ing randomly distributed alkaline-stable cations on polyethylenebackbones have shown excellent alkaline stability (38, 44, 51). Asa result, we were interested in synthesizing block copolymer an-alogs to modify the microphase separation of these materials andpotentially enhance their conductivity (55). Previously, function-alized polyethylene AAEMs were prepared via a ROMP/hydro-genation method using cis-cyclooctene derivatives, which made itdifficult to synthesize block copolymers due to the uncontrollable

secondary metathesis during the ROMP of cis-cyclooctene. In2009, Grubbs and coworkers (56) reported living ROMP of trans-cyclooctene and showed its ability to form well-defined, high-molecular-weight block copolymers with polyethylene backbonesafter hydrogenation. Inspired by this strategy, we report thepreparation of alkaline-stable AAEMs using the living ROMP oftrans-cyclooctene and its derivative (Fig. 1, 1 and 2). These high-ring-strain monomers not only significantly increase the rate ofROMP but also permit the synthesis of cation-functionalizedpolyethylene block copolymers with controlled block lengths.To our surprise, when control experiments were performed toprepare random copolymers with the same IEC, the randomcopolymers showed much higher conductivity than the blockcopolymers. The microstructures and morphologies of themembranes were investigated by transmission electron micros-copy (TEM) to explain this unexpected observation. The prop-erties of the random copolymer AAEMs can be furtherimproved by cross-linking with dicumyl peroxide (DCP) beforehydrogenation (57), and the resultant membrane showed excellenthydroxide conductivity and alkaline stability.

Results and DiscussionMonomer Synthesis. Monomer 2 was synthesized in three stepsfrom its cis-derivative 3 by adapting literature procedures (Fig. 2)(58). Alkaline-stable imidazolium-fused cis-cyclooctene 3, whichwas prepared on a 20-g scale in our previous work (38), was firstreacted with m-chloroperoxybenzoic acid (m-CPBA) to form thecorresponding epoxide 4. The moderate isolated yield was causedby partial product loss during the removal of epoxidation by-products (3-chlorobenzoic acid and its corresponding anion; seeSI Appendix for more details). The purified intermediate 4 wasthen treated with LiPPh2 and AcOH/H2O2 to generate intermediate5 as a white solid, which precipitated from the THF/water reactionmixture and was purified by simple filtration on a 10-g scale.Deprotonation and elimination with NaH in DMF followed byanion exchange with Amberlite (chloride form) generated thetrans-cyclooctene–fused imidazolium chloride monomer 2 in 89%isolated yield. No column chromatography purification was re-quired during the synthesis of 2. Notably, although strong bases(LiPPh2 and NaH) and strong oxidants (m-CPBA and H2O2) wereused to synthesize monomer 2, no degradation of the imidazoliummoiety was observed, which suggests that the multisubstitutedimidazolium cation has excellent alkaline and oxidant stability (36).The structure of monomer 2 was further confirmed by single-

crystal X-ray diffraction (Fig. 2B). The bond length of the trans-alkene C=C bond was determined to be 1.323 Å with a distortionof the two C(sp3) substituents from the plane of the trans-alkene(C–C=C–C dihedral angle = 135°). These results are consistentwith previous reports of trans-cyclooctene derivatives (59, 60).Notably, the orange solid 2 is stable and may be stored in a vialon the benchtop for at least 3 mo without any alkene isomeri-zation or other decomposition (SI Appendix, Fig. S7).

Polymer Synthesis.According to reports by Grubbs and coworkers(56), living ROMP of trans-cyclooctene 1 and its derivatives can

NN

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Et EtX

+

OH

co

1 2trans-cyclooctenes

NN

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Et Et

x y

Fig. 1. Proposed preparation of AAEMs through ROMP of trans-cycloocteneand its cation-functionalized derivative.

N

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Et

BF4

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Et

EtBF4

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49% yield

i) LiPPh2

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PhN

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i) NaH

ii) Amberlite89% yield

ClN

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3 4

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A B

Fig. 2. Multigram synthesis (A) and single-crystalstructure of monomer 2 (B). The molecular struc-ture of 2 was determined by single-crystal X-ray dif-fraction. Atoms are drawn at the 50% probabilitylevel. Hydrogen atoms (except for the two on thetrans-alkene), chloride anions, solvent (dichloro-methane), and minor disorder are omitted for clarity(see SI Appendix for more details).

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be achieved using Grubbs’ first-generation catalyst (G1) andexcess PPh3. The concentration of the active Ru complex isminimized as it binds with excess PPh3. As a result, only highlystrained monomers react with G1/PPh3, and the competing sec-ondary metathesis is significantly suppressed (56). The poly-merization is living in THF; however, in CH2Cl2, secondarymetathesis occurs before complete monomer consumption andhigh-molecular-weight contaminates are observed by gel per-meation chromatography (GPC) (56). Imidazolium-functionalizedROMP monomers and polymers have poor solubility in THF, andtherefore we optimized the polymerization conditions in CH2Cl2.We noticed that the competing secondary metathesis was signifi-cantly inhibited if a solution of 1 in CH2Cl2 was injected into avigorously stirring catalyst/PPh3 solution at 0 °C. After the initialaddition, the reaction was warmed to 22 °C, and GPC analysisshowed a narrow unimodal peak with no detectable high-molecular-weight contaminants (SI Appendix, Fig. S9). The polymerization isbetter controlled at 0 °C (61), and thus subsequent polymerizationsof trans-cyclooctenes were performed under these conditions.It is relatively difficult to monitor the polymerization of ionic

monomer 2 by GPC, likely due to strong interchain ionic inter-actions within these charged polymers (62). Nevertheless, wehave evidence to support that block copolymers with well-defined structures have been synthesized (Fig. 3A). (i) Grubbsand coworkers (56) have reported the successful synthesis offunctionalized polyethylene block copolymers via ROMP andsubsequent hydrogenation of trans-cyclooctene derivatives whereinhigh-molecular-weight block copolymers were obtained with nar-row dispersity (up to 215 kDa, Ð = 1.05). So, we proposed thatwell-defined block copolymers could be obtained by sequentialadditions of monomers 1 and 2 under similar conditions. (ii) Thehomopolymerization of 2 by G1/PPh3 was monitored by 1H NMRanalysis. The monomer consumption was found to follow first-order kinetics (SI Appendix, Fig. S11), which were also observedfor the homopolymerization of 1 (SI Appendix, Fig. S10) (63). Incontrast, when cis-cyclooctene was polymerized with G1 in theabsence of PPh3, the monomer conversion rate showed an obviousdeviation from first-order kinetics (SI Appendix, Fig. S12) becausethe polymerization was not living due to secondary metathesis(56). (iii) The polymerization of 2 reached >95% monomer con-version within 15 min at 22 °C. This is in distinct contrast to thehomopolymerization of 3 in our previous report, in which an

equilibrium of macrocyclic oligomers was reached after 6 h withonly ca. 60%monomer conversion due to extensive intramolecularsecondary metathesis (38). In addition, the 1H NMR spectrum ofhomopolymer B-[2]200 (SI Appendix, Fig. S15) clearly showedpeaks corresponding to the desired structures with backbone al-kenes in close to a 1:1 E/Z ratio, which was different from themacrocyclic oligomers of 3 (38). (iv) The block copolymer(B-[1]x[2]y) also showed unique solubility compared with the cor-responding homopolymers. B-[2]200 is soluble in CH2Cl2, MeOH,and water but insoluble in CHCl3, Et2O, and THF, while poly-cyclooctene (B-[1]800) is soluble in CH2Cl2 and precipitates fromMeOH. B-[1]x[2]y does not precipitate from MeOH, and it is bestsolubilized in CH2Cl2, demonstrating that the block copolymer hasdistinctive solubility that resembles the solubilities of both homo-polymers. (v) After hydrogenation, block copolymer HB-[1]x[2]yhas a melting temperature above 130 °C, which also unambiguouslysuggests the existence of a high-molecular-weight crystalline poly-ethylene block (SI Appendix, Fig. S24). However, the hydrogenatedblock copolymers showed poor solubility in any solvent, whichprevented characterization by solution-phase 1H NMR spectros-copy and membrane casting by solvent evaporation methods. As aresult, HB-[1]x[2]y was processed by a melt-pressing method toproduce thin films for AAEMs (see SI Appendix for more details).Random copolymers with similar feed ratios of monomers 1

and 2 were prepared as controls (Fig. 3B), and 1H NMR analysiswas used to monitor monomer consumption during the randomcopolymerization. The polymerization exhibits first-order de-pendency in both monomers (SI Appendix, Figs. S13 and S14).The ionic monomer 2 has a higher polymerization rate than 1during both homopolymerization and random copolymerization.Preliminary density functional theory calculations using the B3LYPdensity functional method were performed, and the relative ring-opening energies of 1 and 2 were estimated (38). Ionic monomer2 has a higher ring-opening energy than 1 (−28.9 kcal/mol ver-sus −20.4 kcal/mol, respectively). In contrast, the cis-monomer3 has a lower ring-opening energy than cis-cyclooctene (−7.5 kcal/molversus −11.5 kcal/mol, respectively) (SI Appendix, Table S2) (38).This observation is consistent with the report by Fox and co-workers (64) that conformationally strained trans-cyclooctenederivatives are more reactive than trans-cyclooctene.Along with linear copolymers, we prepared cross-linked

AAEMs, because cross-linking is an effective strategy to control

1 eq. Grubbs I cat.60 eq. PPh3

x y

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Et EtCl

bPh

+

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1) Dicumyl peroxide (DCP) cross-link (10 wt%)

Crabtree's cat.40 atm H2

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B

C

Fig. 3. Synthesis of AAEMs through living ROMP of trans-cyclooctene derivatives 1 and 2. (A) Synthesis of block copolymers through sequential monomeraddition and hydrogenation. (B) Synthesis of hydrogenated random copolymers. (C) Synthesis of hydrogenated cross-linked random copolymers.

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membrane swelling in water and enhance a polymer’s mechanicalstrength (38, 65–68). To maintain the AAEM’s ionic conductivityand alkaline stability, the structures of the cross-linkers need to bejudiciously designed to balance the hydrophobicity and chemicalstability. In 2002, Mather, Coughlin, and coworkers reported theuse of DCP to radically cross-link polycyclooctene to produceshape memory materials (57). This is an ideal method to cross-linkpolycyclooctene-based AAEMs, as the polymer backbones are di-rectly connected through chemically stable C–C bonds (69). Inspiredby this work, the random copolymers R-[1]x[2]y were cross-linkedbefore hydrogenation (Fig. 3C). After the curing process, the re-sultant polymer membranes were no longer soluble in CH2Cl2,suggesting that the cross-linking was successful. Model compoundstudies indicated that the imidazolium cations were stable under thecross-linking conditions (see SI Appendix for more details). Thecross-linked membranes were relatively brittle and sticky, so theywere hydrogenated in a solution of Crabtree’s catalyst ([(1,5-cyclo-octadiene)(pyridine)Ir(PCy3)][PF6]) in CH2Cl2 under 40 atm H2.

Membrane Properties and Morphologies. Table 1 summarizes theproperties of the AAEMs prepared from monomers 1 and 2. Thehydroxide conductivity [σ (OH−)] is one of the most importantparameters used to evaluate AAEM performance, while wateruptake (WU) and dimensional change (ΔL) represent a mem-brane’s capability to control water swelling. To our surprise, therandom copolymer AAEM (HR-[1]1056[2]200) showed muchhigher ionic conductivity and water uptake than the diblock co-polymer AAEM (HB-[1]1072[2]200), although their IEC valueswere very similar. Diblock copolymer AAEMs with higher imi-dazolium content (e.g., HB-[1]390[2]200) became overswelled inwater at 22 °C and their hydroxide conductivity was unable to bemeasured. The ionic conductivity of random copolymer wasfurther improved by increasing the IEC (HR-[1]390[2]200), but themembrane overswelled at higher temperature as well. Cross-linking significantly reduced the membrane’s water uptake anddimensional changes after hydration. A moderate improvementin conductivity occurred when 10 wt % DCP was used to cure theAAEM, although its IEC decreased (HC-[1]498[2]200). The con-ductivity increased with temperature, and all membranes fol-lowed Arrhenius behavior (SI Appendix, Figs. S32 and S33). Theconductivity-temperature slopes were used to estimate the apparentactivation energies (11.7 kJ/mol, 16.5 kJ/mol, and 15.0 kJ/mol forHB-[1]1072[2]200, HR-[1]1056[2]200, and HC-[1]498[2]200, respectively),which were comparable to findings reported in the literature(15, 20, 55, 70). The cross-linked membrane showed hydroxideconductivity above 130 mS/cm at 80 °C (134 ± 2 mS/cm forHC-[1]498[2]200). This hydroxide conductivity is much higher thanpreviously reported AAEMs with alkaline-stable cations, such asthe ones with macrocyclic imidazolium [σ (OH−, 80 °C) = 71 mS/cm](38), tetrakisaminophosphonium [σ (OH−, 50 °C) = 32 mS/cm](44), and cobaltocenium [σ (OH−, 80 °C) = 74 mS/cm] (51). Some

ammonium-functionalized AAEMs have been reported to havehigher hydroxide conductivity (up to 200 mS/cm at 80 °C), butthese AAEMs are not as chemically stable as HC-[1]498[2]200(discussed below), likely due to degradation of ammonium cationsunder alkaline conditions (3, 15, 19, 65, 66).To understand why the random copolymers outperformed the

analog block copolymers, the AAEMs were characterized by TEM(Fig. 4). The counteranions were exchanged with iodide ions toprovide stronger dark contrast for ionic groups in the TEM images.As shown in Fig. 4 A and B, the block copolymer sample displaysaggregation of ionic domains, creating a disordered microphaseseparation. Dark hydrophilic domains with irregular shapes andsizes from 10 to 60 nm are visible, separated by similar-sized, lighterhydrophobic domains (see SI Appendix, Fig. S34 for another TEMimage with a lower magnification). This microphase separation inHB-[1]1072[2]200 likely causes the membrane’s poor hydroxide con-ductivity, as the large hydrophobic domains separate the hydrophilicdomains and impede hydroxide transport. We propose that underthe membrane melt-pressing conditions the block copolymers areunable to achieve the ordered equilibrium microphase separation(16, 71). We attempted to optimize the melt-pressing conditions byincreasing processing temperature (up to 160 °C) or time (up to 1h), but no significant changes in ionic conductivity were observed.In contrast to the block copolymer, the random AAEM

(HR-[1]1056[2]200) shows an amorphous and generally uniformmorphology, although they were processed under the same casting

A

C D

B

Fig. 4. TEM images of (A and B) HB-[1]1072[2]200 and (C and D) HR-[1]1056[2]200,with close-up images (B and D) at right. Dark contrast shows iodine atomsexchanged onto ionic groups.

Table 1. Summary of AAEM properties

Samples* Polymer architecture [1]/[2]†IECNMR,

mmol Cl−/g†

IECTitrate,mmol Cl−/g‡

σ (OH−, 22 °C),mS/cm§

σ (OH−, 80 °C),mS/cm§ WU,{ % ΔL,# %

HB-[1]1072[2]200 Diblock 5.36 1.10 1.03 5 ± 1 11 ± 1 28 10HR-[1]1056[2]200 Random 5.28 1.11 1.05 28 ± 1 83 ± 3 67 13HR-[1]390[2]200 Random 1.95 1.88 1.87 48 ± 1 n.d.ǁ 257 46HC-[1]498[2]200 X-linked random 2.49 1.69 1.60 49 ± 1 134 ± 2 115 17

*See Fig. 3 for AAEM structures.†Determined by 1H NMR analysis before hydrogenation/cross-linking.‡Determined by back-titration.§Determined from the average of three trials ± SD.{Water uptake at 22 °C = 100 × [masswet – massdry]/massdry %.#Dimensional change at 22 °C = 100 × [lengthwet – lengthdry]/lengthdry %.kNot determined due to sample overswelling.

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conditions (Fig. 4 C and D). The ionic clusters present in therandom copolymer samples appear to be well mixed at nanometerlength scales, without large regions showing lighter or darkercontrast. A few dark ionic clusters are visible, although these aresparse and unlikely to impact the conductivity. The cross-linkedrandom AAEM (HC-[1]498[2]200) shows morphology similar tothat of the random AAEM (HR-[1]1056[2]200) (SI Appendix, Fig.S35). Based on these TEM images, we propose that the amor-phous random copolymers have higher hydroxide conductivitythan the block copolymers due to their homogeneous micro-structures, which provide more continuous paths for hydroxidetransport. The domain segregation was also characterized bysmall-angle X-ray scattering (SAXS) (SI Appendix, Fig. S36),which showed broad peaks corresponding to ca. 13-nm and 9-nmdomain sizes for HR-[1]1056[2]200 and HC-[1]498[2]200, respectively.No peak is visible forHB-[1]1072[2]200, and the segregated domainsobserved in TEM imaging are too large for a corresponding peak,if present, to be resolved in these SAXS profiles. As stated inother reports (71–73), the synthesis of block copolymers alone isnot enough to ensure high ionic conductivity in AAEMs. It is alsonecessary to optimize membrane processing conditions to produceAAEMs with microphase separations that provide continuous iontransport channels.

Alkaline Stability. Both 1H NMR analysis and electrochemical im-pedance spectroscopy were used to evaluate the alkaline stability ofthe prepared AAEMs. Two soluble polymer samples, hydroge-nated homopolymer of 2 (HB-[2]200) and hydrogenated randomcopolymer (HR-[1]148[2]200), were subjected to 1H NMR stabilityanalysis by heating in 2 M KOH in CD3OH at 80 °C in sealedNMR tubes for 30 d (see SI Appendix for more details). These arethe same conditions that we were using to evaluate the alkalinestability of cationic model compounds (36, 38). No degradationwas observed as measured by the integration ratio of the polymerto an internal standard, indicating that the polymers have excellentchemical stability (Fig. 5A and SI Appendix, Figs. S39 and S40).We soaked the hydrogenated membranes in 1 M KOH (aq) in

air at 80 °C for 30 d, which are common conditions to test theconductive stability of solid-state AAEMs (19, 28, 38, 44, 51, 66).Unfortunately, non-cross-linked random copolymer AAEMswere mechanically unstable despite their moderate ionic con-ductivities (e.g., HR-[1]1056[2]200). The samples became brittleand cracked into small pieces after 7 d of stability testing. Em-brittlement upon aging was not observed for previously reportedrandom copolymer AAEMs with polyethylene backbones (44,51). Likely, the random copolymer AAEMs prepared in this

work contain fused imidazolium rings in the polyethylene backbones,rather appended to it. Cations within the backbone may absorbmore water at elevated temperature, which would ultimately lead tomembrane overswelling and mechanical instability. However, thecross-linked AAEMs showed excellent alkaline and mechanicalstabilities and no embrittlement was observed (Fig. 5B). The hy-droxide conductivity (at 22 °C) of HC-[1]498[2]200 was determinedto be 46 ± 2 mS/cm after treatment in 1 M KOH at 80 °C for 30 d,compared with 48 ± 1 mS/cm initially. We hypothesize that theinsignificant decrease in conductivity over time is possibly caused byslight membrane swelling at 80 °C instead of chemical degradation.

ConclusionIn conclusion, a series of AAEMs with polyethylene backbonesand imidazolium cations were prepared by the living co-polymerization of trans-cyclooctene (1) and trans-cyclooctene-fused imidazolium monomer (2). The polymers showed excellentalkaline stability and no chemical degradation was detected by 1HNMR spectroscopy when they were treated with 2 M KOH inmethanol at 80 °C for 30 d. Cross-linked random copolymers fromthese trans-cyclooctene monomers demonstrated the highest hy-droxide conductivity (49 ± 1 mS/cm at 22 °C and 134 ± 2 mS/cm at80 °C) and long-term alkaline stability. To understand why its con-ductivity is over five times lower than its random copolymer analogHR-[1]1056[2]200, the morphology of the block copolymer sampleHB-[1]1072[2]200 was studied by TEM imaging, which showed dis-ordered microphase separation in the block copolymers and a ho-mogeneous morphology in the random copolymers. The disorderedstructure of the block copolymer membranes likely leaves the ionicdomains disconnected and unable to efficiently conduct hydroxideanions. We propose that the membrane processing conditions (meltpressing) led to the disordered microphase separation in the blockcopolymers. This study highlights the importance of microphasecontinuity for AAEM conductivity.

ACKNOWLEDGMENTS. We thank I. Keresztes and A. Condo for assistancewith NMR and mass spectrometry, J. Grazul and M. S. Ramos for TEM samplepreparation and facility support, H. Abruna for help with electrochemical im-pedance spectroscopy, X. Liu for assistance with SAXS, and K. Noonan forhelpful discussions. This work was supported as part of the Center forAlkaline-based Energy Solutions, an Energy Frontier Research Center fundedby the US Department of Energy, Office of Science, Basic Energy SciencesAward DE-SC0019445. This study made use of the NMR facility supported byNSF Grant CHE-1531632 and the Cornell Center for Materials Research SharedFacilities supported by NSFMaterials Research Science and Engineering CentersGrant DMR-1719875. E.P. was supported by NSF Graduate Research FellowshipDGE-1650441.

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