Sulfonated Phenylene/Quinquephenylene/Perfluoroalkylene Terpolymers as Proton Exchange Membranes for Fuel Cells

A novel series of terpolymers (SQF) containing sulfophenylene, quinquephenylene, and perfluoroalkylene groups in the polymer main chain were designed and synthesized as proton exchange membranes for fuel cells. The terpolymers with high molecular weight (M_w = 179-207 kDa, M_n = 41-50 kDa) and different ion exchange capacity (IEC) values (1.70, 2.56, and 3.34 mequiv g^-1) gave flexible self-standing membranes by solution casting. Compared to the two-component (sulfophenylene and quinquephenylene segments) copolymer membranes, the incorporation of the third component, perfluoroalkylene groups, resulted in better water utilization for the proton conduction, while it did not alter the other properties such as gas permeability and mechanical strength. The selected membrane (SQF-3 with IEC = 2.56 mequiv g^-1) exhibited high fuel cell performance under high- and low-humidity conditions with maximum power density reaching 0.97 W cm^-2 at 100% RH (relative humidity) and 0.82 W cm^-2 at 30% RH, respectively, at a current density of 1.51 A cm^-2 with oxygen. A good interfacial compatibility between the SQF-3 membrane and catalyst layers resulted in mass activity of the cathode catalyst comparable to that obtained with the Nafion membrane NRE 211. During the open circuit voltage (OCV) hold test with air and hydrogen at 80 °C and 30% RH for 1000 h, the OCV showed a slight decrease from 0.97 to 0.88 V. Post-test analyses revealed that the SQF-3 membrane retained its initial high fuel cell performance due to its high chemical stability as well as low gas permeability.