Partially Fluorinated Polyphenylene Ionomers as Proton Exchange Membranes for Fuel Cells: Effect of Pendant Multi-Sulfophenylene Groups

The six kinds of sulfonated poly(arylene perfluoroalkylene) (SPAF) ionomers with pendant multi-sulfophenylene groups were designed and synthesized to apply to fuel cells as proton exchange membranes. The SPAF polymers possessed high ion exchange capacity (IEC) values (2.07-2.15 mequiv g^-1), good solubility in organic solvents, and high molecular weight, providing the flexible membranes by solution casting. Compared with our previous SPAF-MM membrane (with no pendant sulfophenylene groups), the introduction of the pendant sulfophenylene groups resulted in the significant improvement of proton conductivity, whereas it did not deteriorate the other favorable membrane properties, such as gas impermeability and mechanical properties. The SPAF-BM, as the chosen membrane, exhibited higher fuel cell performance than that of our previous SPAF-MM membrane under low humidified conditions. During the open circuit voltage (OCV) hold test, the SPAF-BM cell showed the low average decay of 40 μV h^-1 and kept high OCV even after 1000 h. Post-test analyses proved that the SPAF-BM membrane after the OCV hold test retained the original fuel cell performance without practical changes in the molecular structure and molecular weight due to the high chemical stability of SPAF-BM.