ChemComm
Communication
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scale of d = 2p/qmax, where qmax is the peak maximum. The for 30 days, while under similar conditions the typical benzyl
corresponding d-spacing values fall into the range of 1.7–2.5 nm trimethyl quaternary ammonium-based PPO membranes became
ꢀ1
(
2.5–3.6 nm ), which roughly correspond to the length of the very brittle due to severe degradation. Additionally, non-cross-linked,
extended aliphatic side chains. An ordered morphology was comb-shaped membranes became opaque under these degradation
9
observed in the case of the lightly cross-linked X60Y15 sample which conditions. No significant loss in conductivity or IEC for XxYy AEMs
did not show any order in its uncross-linked form, Fig. 2. We were observed after alkaline stability testing, which supports their
hypothesize that the cross-linking induced order in the X60Y15 excellent long-term stability, as shown in Fig. S3 and S4 (ESI†).
sample by coalescing domains of alkyl chains due to the metathesis Unlike the previously reported TMHDA cross-linked AEMs, in which
linking of the terminal alkenes. The ionic domain peak of the highly the b-hydrogens of the quaternary ammonium pendant alkyl chains
cross-linked membrane X60Y60 shifted to higher q (lower d spacing) induced the rapid degradation of quaternary ammonium groups by
upon cross-linking likely indicating compression of the ionic Hoffmann elimination, the longer alkyl side chain protected the QA
domains or further exclusion of other moieties with cross-linking, groups from degradation by hydroxide. Moreover, improved alkaline
although the cross-linked X60Y60 peak breadth did increase slightly. stability was observed for all cross-linked XxYy AEMs compared to
The relatively large width of the peaks indicates a weak separation non-cross-linked AEMs with one long alkyl side chain (Fig. S3
between the two components, and the fact that that no second order and S4, ESI†), likely due to the steric hindrance of the coupled,
14
peak was observed suggests that the arrangement of the phase longer alkyl chains after cross-linking.
separated domains was only locally correlated and no long-range
ordered structures were formed.
In summary, we have designed and demonstrated a new class of
cross-linkable, comb-shaped cationic copolymers for stable anion
The phase-separated, cross-linked membranes exhibited hydroxide exchange membranes. The cross-linked AEMs were achieved by
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1
conductivities of 18–40 mS cm at 20 1C which was much greater Grubbs-catalyzed olefin metathesis and the degree of cross-linking
than the conductivities of typical PPO AEMs based on trimethyl- could be controlled readily by tuning the amount of alkene groups
ꢀ1
ꢀ
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amine (IEC = 1.39 meq. g , OH conductivity: 5 mS cm ), and pendant to the polymer chain. Outstanding dimensional stability,
meets the basic requirement for membranes in AMFCs even at high temperature (80 1C) was observed for all cross-linked
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2
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(410
S cm ). If bicarbonate species should form in the AEMs. The cross-linked AEMs exhibited nanoscale organized phase-
12
membrane under operating conditions, the conductivities of our separated morphology, resulting in high ion conductivity compared
system are still comparable to other cross-linking-free comb-shaped to typical PPO AEMs containing benzyltrimethyl ammonium cationic
9
AEMs. The cross-linked comb-shaped membranes showed a much groups. In addition, the cross-linking of alkyl side chains improved
lower swelling ratio (o10%) but comparable conductivity to those the alkaline stability of the AEMs relative to the non-cross-linked
reported for quaternary ammonium AEMs in the hydroxide form. AEMs. After 30 days immersion in 1 M NaOH at 80 1C, no obvious
Specifically, quaternary ammonium poly(sulfone) (QAPSU) AEMs changes in IEC and conductivity were observed. The combination of
10c
reported by Zhang and co-workers exhibited a swelling ratio of excellent solubility of the precursor polymer in low-boiling-point
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ꢀ1
1
7% and OH conductivity of 40 mS cm at 20 1C, a commercial solvents makes the olefin metathesis cross-linking of comb-shaped
FAA-QAOH sample had a swelling ratio of 26% and hydroxide con- copolymers attractive as a method for producing high-performance
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ductivity 17 mS cm , and a cross-linked quaternary phosphonium AEM materials for fuel cell applications.
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1
(
CL-TQPQOH, IEC = 1.23 meq. g ) AEM possessed a swelling ratio
This work was funded by the Advanced Research Projects
Agency – Energy (ARPA-E), U.S. Department of Energy, under
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8
of 15% and OH conductivity of 38 mS cm reported by Yan, et al.
Overall, the cross-linked XxYy membranes showed higher hydroxide Award No. DE-AR0000121.
conductivity but lower swelling ratios than these samples, as shown
in Fig. 1b. To quantitatively describe the trade-off behaviour between
swelling and conductivity, the hydroxide conductivity-normalized
s
swelling ratio was calculated (SR , Table 1). Generally, a high
Notes and references
1
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membrane swelling ratio at a given IEC value will dilute the
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2
1
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3
4
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1
conductivity-normalized swelling ratio (0.16–0.21% cm mS
reported, which is much lower than other cross-linked or non-
)
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cross-linked AEMs (0.47–3.85% cm mS ). These results suggest
that our olefin metathesis cross-linking concept mitigates AEM
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2
008, 41, 9130–9139.
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ionic conductivity of these materials, cross-linked comb-shaped
membranes displayed good alkaline stability which is critical for
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and flexible after immersion in 1 M NaOH (pH B 14) at 80 1C
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4094 | Chem. Commun., 2014, 50, 4092--4095
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