Highly Gas Permeable Proton-Conducting Ionomers Containing Cyclohexyl Groups: Synthesis, Properties, and Applications in Fuel Cell Electrodes

Aromatic polymer-based ionomers (SBAFC6) with high gas permeability and high proton conductivity were designed as electrode binders in proton exchanged membrane fuel cells. The target terpolymers containing sulfophenyelene (SP), hexafluorobiphenylne (BAF), and cyclohexyl (C6) groups were synthesized by the Ni-promoted polycondensation reaction of the corresponding chlorinated monomers, in which composition of the C6 component was varied (17, 30, and 50 mol %). The terpolymers were obtained as high-molecular weight (Mn = 19.2–95.0 kDa and Mw = 64.8–159.0 kDa), and their properties were evaluated as thin membranes obtained by solution casting. SBAFC6 membranes exhibited comparable proton conductivity and much higher gas permeability compared with the sulfonated copolymer (SBAF) without C6 groups and benchmark proton conductive perfluorinated ionomer (Nafion) membranes. The highest gas permeability was achieved with SBAFC650 with 50 mol % C6 groups; hydrogen and oxygen permeability at 80 °C was 8.58 × 10–9 cm3 cm cm–2 s–1 cmHg–1 and 4.49 × 10–9 cm3 cm cm–2 s–1 cmHg–1, respectively. A fuel cell was fabricated using SBAFC650 as a binder in the cathode catalyst layer, which showed larger activation overpotential (or lower mass activity of the platinum catalyst) compared to the Nafion binder but smaller oxygen diffusion overpotential. With air and hydrogen, the SBAFC650 binder cell showed 1.5 A cm–2 of the current density at 0.32 V of the cell voltage while that of the Nafion cell was lower than 0.3 V, proving the effectiveness of the highly gas-permeable SBAFC650 ionomer in the cathode catalyst layer for high current density operation of fuel cells.