Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) to convert chemical energy efficiently into electrical energy via redox reactions, have been attracting considerable attention as one of the potentially clean, quiet, and portable future power sources. 1-3 The proton exchange membrane (PEM), which acts as an electrolyte to transport protons from the anode to the cathode, is the key component of fuel cell systems. Perfluorosulfonic acid membranes, such as Nafion, are the current state-of-the-art PEM materials, because of their superior chemical and electrochemical stability, in addition to high proton conductivity with relatively low ion exchange capacity (IEC). However, perfluorosulfonic acid membranes suffer from critical drawbacks of high cost, high methanol crossover, and low proton conductivity as well as poor mechanical stability at elevated temperatures (T > 80°C). This has stimulated extensive research into the investigation of promising alternatives. 2,4 The majority of current research on PEMs is based on sulfonated high-performance aromatic polymers, because of the high thermal and chemical stabilities, as well as excellent mechanical properties of the parent polymers. 5,6 These aromatic polymers, including sulfonated poly(ether-etherketone), one of the membranes studied extensively for PEM, usually contain sulfonic acid groups randomly distributed along polymer main chains. The model suggested by Kreuer 3 for a sulfonated poly(ether-ketone) shows less pronounced ionic/ nonionic separation than that of Nafion, i.e., a morphology with narrower channels than those in Nafion, but with highly branched channels and many dead-end channels. Therefore, in general, the sulfonated polymers that have been synthesized to date require a much higher IEC in comparison with Nafion to compensate for this and to obtain conductivities comparable to that of Nafion. For example, sulfonated poly(ether-etherketone) required IEC of 2.45 mequiv/g to attain conductivity of 51 mS/cm, 7 and IEC of 1.52 mequiv/g for conductivity of 0.4 mS/cm. 8 High IEC usually results in high water uptake (WU) for membranes and loss of their mechanical properties.We have previously reported that linear poly(sulfide-ketone) (PSK) bearing six sulfonic acid groups on each end group were successfully synthesized by regioselective post-sulfonation, and the membrane displayed relatively high proton conductivity (16 mS/cm) at very low IEC (0.48 mequiv/g). 9 The results suggested that the PSK membrane possessed better microphase-separated morphology. In these linear polymers, at the IECs required for good conductivities, the molecular weights would be too low and the required mechanical properties in the membranes would not be attainable. The required balance of IECs vs molecular weights can be attained by introducing branches into the endcapped polymers. As in the linear end-capped polymers, we would expect to have a favorable situation for segregated microdomain formation of the core polymer and the ionic end groups.In t...
Branched wholly aromatic poly(ether‐ketone)s (PEKs) bearing clusters of sulfonic acid groups on a novel end‐group, 3,6‐ditrityl‐9H‐carbazole, were synthesized by the polycondensation of activated aromatic difluorides and 4,4′‐dihydroxybenzophenone in the presence of the above end‐group and trifunctional branching agent, 1,3,5‐tris(4‐(4‐fluorophenylsulfonyl)phenyl)benzene, followed by postsulfonation to introduce up to 8 sulfonic acid groups on each endgroup in PEKs. PEMs of these sulfonated branched PEKs with the same level of IECs (0.99–1.25 mequiv./g) as Nafion® (0.91 mequiv./g) showed proton conductivities (66–95 mS/cm) comparable to that (98 mS/cm) of Nafion®®. TEM analysis showed significantly phaseseparated, and worm‐like, highly connected morphology in the PEK membranes.
Fuel cells, electrochemical devices converting chemical energy efficiently into electrical energy via redox reactions, are expected to be one of the clean future power sources. 1,2 Proton exchange membranes (PEM), which act as an electrolyte to transport protons from the anode to the cathode, are the key component of PEM fuel cell systems. Perfluorosulfonic acid membranes such as Nafion are the current choice for PEMs because of their superior chemical and electrochemical stability, in addition to high proton conductivity with relatively low ion exchange capacity (IEC). However, their critical drawbacks of high cost, high methanol crossover, and low proton conductivity as well as poor mechanical stability at elevated temperatures (T > 80 °C) have led researchers to investigate promising alternatives. 2,3 There has been considerable effort on PEMs based on sulfonated polystyrenes and their derivatives, 4,5 acid complexes of basic polymers, 6,7 and organic-inorganic hybrids. 8,9 However, the majority of current research on PEMs is based on sulfonated high-performance aromatic polymers because of the high thermal and chemical stabilities as well as excellent mechanical properties of the parent polymers. 10,11 Two general approaches are available for the synthesis of sulfonated high-performance aromatic polymers: (1) post-sulfonation of existing aromatic polymers usually leading to random functionalization along polymer main chain; 12 (2) direct copolymerization of sulfonated monomers to afford random copolymers. However, sulfonated random copolymers prepared by these approaches can generally achieve conductivities comparable to that of Nafion only with high IECs, resulting in high water uptake (WU) and loss of mechanical properties. For example, randomly sulfonated poly(ether-ether-ketone), one of the membranes studied extensively for PEMs, required an IEC of 1.52 mequiv/g to attain conductivity of 0.4 mS/cm 13 and an IEC of 2.45 mequiv/g for a conductivity of 51 mS/cm. 14 It is widely recognized that the superior proton conductivity of Nafion is attributed to the extensive nanoscale phase separation of ionic
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