The sulfonation selectivity of seven poly(ether ether ketone)s (PEEKs) was investigated, and several possessed targeted single-or double-substituted sites per repeated unit on pendant phenyl groups via the postsulfonation approach. The presence of the various pendant groups enabled postsulfonation to occur under mild reaction conditions, in much shorter times than required for the sulfonation of commercial PEEK. A series of poly(ether ketone)s (PEKs) with ion exchange capacity of 2.23-0.84 mequiv/g could be realized by controlling the length of unsulfonated segments of both homopolymers and copolymers. These side-group sulfonation polymers had excellent mechanical properties, good thermal and oxidative stability, and good dimensional stability in hot water. The methanol permeability values of Me-SPEEKK, Me-SPEEKDK, Ph-SPEEKK, and Ph-SPEEKDK at room temperature were in the range 3.31 × 10 -7 -9.55 × 10 -8 cm 2 /s, which is several times lower than that of Nafion 117. Me-SPEEKK and Ph-SPEEKK also exhibited high proton conductivity of 0.15 S/cm at 100°C, which is higher than that of Nafion 117. Transmission electron microscopy analysis was used to observe their microstructure for evidence of microphase separation of ionic and hydrophobic domains. The results showed these side-group-acid materials are possible inexpensive candidate materials for proton exchange membranes in fuel cell applications.

Three sulfonated aromatic polymers with different sequence lengths were studied in order to better understand the relationship between molecular structure, morphology, and properties of proton exchange membranes as a function of relative humidity. A random copolymer with a statistical distribution of sulfonic acid groups had very small domain sizes, whereas an alternating polymer with sulfonic acid groups spaced evenly along the polymer chain was found to have larger, but quite isolated, domains. The multiblock copolymer studied herein showed highly phase-separated hydrophilic and hydrophobic domains, with good long-range connectivity. Scanning force microscopy as a function of relative humidity was used to observe water absorption and swelling of the hydrophilic domains in each of the three membranes. The conductivity, water sorption kinetics, and fuel cell performance, especially at low relative humidity, were found to be highly dependent upon the morphology. The multiblock copolymer outperformed both the random and alternating systems at 100°C and 40% RH fuel cell operating conditions and showed similar performance to Nafion.

Phosphoric acid-doped polybenzimidazole (PA-PBI) used in high-temperature proton exchange membranes (HT-PEMs) frequently suffers from a serious loss of mechanical strength because of the “plasticizing effect” of the dopant acid. Conventional cross-linking approaches generally enhance membrane stability. However, acid doping levels (ADLs) and consequently proton conductivity inevitably decrease. This is due to the formation of more compact molecular structures and a reduced amount of functional imidazole units, caused by their consumption in introducing the cross-linker. To resolve the common problems of current PA-PBI-based HT-PEMs, herein, a highly acidophilic imidazole-rich cross-linked network with superior “antiplasticizing” ability is constructed based on a novel multifunctional cross-linker. This unique bischloro/bibenzimidazole (“A2B2-type”) molecular structure has extremely high reactivity, including “self-reaction” among the cross-linkers and “inter-reaction” between the cross-linker and PBI molecules. The resulting imidazole-rich cross-linked membranes exhibit the desired combination of high ADLs, high conductivity, outstanding dimensional–mechanical stability, and excellent fuel cell performance. In comparison to a corresponding linear PBI membrane, one membrane with a high content of the cross-linker of 30% has a 100 wt % increased acid uptake, a doubling in proton conductivity at 200 °C, and a maximum power density of 533 mW·cm–2 at 160 °C without humidification.