A series of copoly(arylene ether sulfone)s that have precisely two, three, or four quaternary ammonium (QA) groups clustered directly on single phenylene rings along the backbone are studied as anion-exchange membranes. The copolymers are synthesized by condensation polymerizations that involve either di-, tri-, or tetramethylhydroquinone followed by virtually complete benzylic bromination using N-bromosuccinimide and quaternization with trimethylamine. This synthetic strategy allows excellent control and systematic variation of the local density and distribution of QA groups along the backbone. Small-angle X-ray scattering of these copolymers shows extensive ionic clustering, promoted by an increasing density of QA on the single phenylene rings. At an ion-exchange capacity (IEC) of 2.1 meq g(-1), the water uptake decreases with the increasing local density of QA groups. Moreover, at moderate IECs at 20 °C, the Br(-) conductivity of the densely functionalized copolymers is higher than a corresponding randomly functionalized polymer, despite the significantly higher water uptake of the latter. Thus, the location of multiple cations on single aromatic rings in the polymers facilitates the formation of a distinct percolating hydrophilic phase domain with a high ionic concentration to promote efficient anion transport, despite probable limitations by reduced ion dissociation. These findings imply a viable strategy to improve the performance of alkaline membrane fuel cells.

Polymeric anion-exchange membranes (AEMs) are critical components for alkaline membrane fuel cells (AMFCs) which offer several attractive advantages including the use of platinumfree catalysts and a wide choice of fuel. The development of AMFCs and other electrochemical energy systems is currently severely limited by the lack of AEMs with suffi cient alkaline stability. Still, signifi cant advances have been made in recent years and one of the most promising approaches to emerge is the design and synthesis of cationic polymers with various side chain arrangements. Especially, synthetic strategies where the cationic ionexchange groups are placed on pendant alkyl spacer chains along the backbone seem to signifi cantly improve microphase separation, hydroxide ion conductivity, and alkaline stability in relation to standard AEMs with cations placed in benzylic positions directly on the backbone. This article reviews recent approaches to high-performance cationic membrane polymers involving different side chain designs, and discusses some possible future directions.

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A series of fully aromatic polymers having only sulfone bridges linking the aromatic rings have been synthesized via polycondensations and studied as protonexchange membranes. Mixtures of tetrasulfonated 4,4′-bis[(4chlorophenyl)sulfonyl]-1,1′-biphenyl (BCPSBP), non-sulfonated BCPSBP, and 4,4′-thiobisbenzenethiol were copolymerized by nucleophilic aromatic substitution reactions to obtain sulfonated poly(arylene thioether sulfone)s (SPATSs) with ion exchange capacities (IECs) between 2.0 and 4.0 mequiv g −1 . The thioether bridges of the SPATSs were quantitatively oxidized to sulfone bridges to obtain the corresponding sulfonated poly(arylene sulfone)s (SPASs). Small-angle Xray scattering of dry SPATS and SPAS membranes showed that the tetrasulfonated segments promoted a distinct phase separation of the ionic groups already at quite low ionic contents. The SPAS polymers degraded between 300 and 340 °C in air, which was significantly above the degradation temperature of the corresponding SPATSs polymers. Moreover, SPAS membranes showed a significantly lower water uptake than the corresponding SPATS membranes. SPATS and SPAS membranes with IEC values of 2.4 and 2.2 mequiv g −1 , respectively, maintained high proton conductivity at low relative humidity (RH). At 30% RH and 80 °C, these membranes reached 8 and 10 mS cm −1 , respectively. The latter value coincided with that recorded for the stateof-the-art perfluorinated NRE212 membrane under the same conditions. Thus, the SPAS materials combine a straightforward synthetic pathway with a very robust polymer structure giving high proton conductivity at reduced RH.

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