Ion‐Pair Interactions in Polyphenylene‐Based Quaternized Membranes Designed for Phosphoric Acid‐Doped Proton Exchange Membranes for High‐Temperature Fuel Cells

Phosphoric acid (PA)-doped proton exchange membranes (PEMs) face significant challenges owing to the loss of PA, particularly under high humidity conditions. Ion-pair interactions between PA and quaternary ammonium (QA) groups can effectively mitigate PA loss. Herein, polyphenylene-based quaternized membranes (BAF-QAF and C7-QAF) comprising distinct hydrophobic moieties [BAF = (perfluoropropane-2,2-diyl)dibenzene and C7 = 1,1-diphenylcycloheptane] and fluorenyl groups with pendant QA head groups are designed and used as PA-doped PEMs with low or no fluorine contents to realize high-temperature and low-humidity operability. The resulting membranes exhibited excellent PA retention, maintaining >85% of their initial proton conductivities at 90% relative humidity after 10 humidity cycles. PA-doped membranes PA-C7-QAF and PA-BAF-QAF exhibit superior proton conductivities of 60.3 and 58.4 mS cm^-1 at 160 °C, respectively. PA-C7-QAF and PA-BAF-QAF fuel cells deliver peak power densities of 0.579 and 0.537 W cm^-2 at 140 °C and 0.706 and 0.640 W cm^-2 at 160 °C, respectively, under dehumidified conditions. The PA-C7-QAF cell also exhibits impressive durability with an average voltage decay of 30 µV h^-1 (140 °C, 0.15 A cm^-2) after an initial voltage drop. These findings underscore PA-C7-QAF and PA-BAF-QAF membranes as promising components in high-temperature fuel cells.