Ionomers for Proton Exchange Membrane Fuel Cells with Sulfonic Acid Groups on the End Groups:  Novel Branched Poly(ether−ketone)s

Abstract

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...

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

“…These designs included linear [136], branched [137,138], and dendritic multiblock [139,140] structures. Since the authors used the post-polymerization sulfonation method, electron-withdrawing groups, such as ketone and/or sulfone groups, were introduced onto the PEM backbone to avoid main chain sulfonation and enhance the solubility of the resulting polymers.…”

“…For the former strategy, sulfonated branched PEKs were synthesized in accordance with the concept of PEMs with sulfonatable end groups, as shown in Fig. 35(b) [137,138]. Hexaphenylbenzene moieties and 3,6-ditrityl-9H-carbazole moieties were used as end groups for sulfonation, where six and eight sulfonic acid groups, respectively, were successfully introduced.…”

Polymer Electrolyte Membranes Derived from New Sulfone Monomers with Pendent Sulfonic Acid Groups

“…It has been found that the morphological structure of the copolymers was comparable to that of Nafion 117, which explains their high proton conductivity. 34,35 Another approach to enhance PEM performance is with high IEC values, but to design the highly proton conducting PEM to have less susceptibility to water swelling or solubility. Here, the selection of a suitable mechanically stable polymer as a platform is very important.…”

Designing a regenerable stimuli-responsive grafted polymer-clay sorbent for filtration of water pollutants

“…The presence of pyridine is also reflected by the appearance of an indicative peak at 292 eV corresponding to C-π* satellite electrons. Polymerization-induced clay exfoliation (suggested by XRD measurements, Figure S1B) as reported for poly(ethyl acrylate) brushes grafted to montmorillonite [43]. …”