Use of a High-Performance Poly(p‑phenylene)-Based Aromatic Hydrocarbon Ionomer with Superacid Groups in Fuel Cells under Low Humidity Conditions

Aromatic ionomers with perfluoroalkyl sulfonic acid groups for fuel cell applications have been prepared mostly by the post-functionalization method. Herein, we present a direct polymerization method using a novel monomer with a perfluorosulfonic acid group to control the amount and position of the sulfonic acid groups. A poly­(p-phenylene)-based aromatic hydrocarbon ionomer bearing a pendant perfluorosulfonic acid group in a substituent at the 2-position is synthesized by Ni(0)-catalyzed coupling polymerization. The direct polymerization provides M_n values of up to 169 000 with a highly controlled molecular structure and allows the formation of thin membranes. These ionomers were found to combine the positive features of perfluorinated and aromatic hydrocarbon ionomers, and these thin membranes with a relatively high ion exchange capacity showed high proton conductivity and excellent fuel cell performance (907 mW cm^–2 even at 80 °C and 30% RH) under low humidity conditions compared with other reported aromatic hydrocarbon ionomers.