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

Abstract

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

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.…”

“…Fig. 35(a) shows sulfonated linear aromatic poly(sulfide ketone)s (PSK) with six sulfonic acid groups on each hexaphenylbenzene end group [136]. When using the analogous aromatic dithiol monomers, the resulting non-sulfonated polymer was insoluble in any organic solvent due to its rigidity and regularity.…”

Ionomers for proton exchange membrane fuel cells with sulfonic acid groups on the end‐groups: Novel branched poly(ether‐ketone)s with 3,6‐ditrityl‐9H‐carbazole end‐groups

“…Other commercially available materials and solvents were used without further purification. 1-(4-Hydroxyphenyl)-2,3,4,5,6-pentaphenylbenzene 20 (1) and 1,3,5-tris(4-(4-fluorophenylsulfonyl)phenyl)benzene 16 (3) were syn- Figure 2. TEM micrographs of PEK and Nafion…”

Fluorene-Based Poly(arylene ether sulfone)s Containing Clustered Flexible Pendant Sulfonic Acids as Proton Exchange Membranes

“…Hay and co-workers reported the synthesis of linear poly(sulfide ketone)s and branched poly-(ether ketone)s bearing clusters of six sulfonic acid moieties situated on the chain ends, by regioselective postsulfonation. 38,39 These ionomers possessed good microphase-separated morphology and displayed relatively high proton conductivity (0.091 S/cm at room temperature with 100% RH (relative humidity)) at very low IEC (1.09 mequiv/g). However, it is difficult to further increase their IEC values and proton conductivity based on this methodology because the sulfonated groups are located only on the chain ends.…”