Highly Gas\nPermeable Proton-Conducting Ionomers Containing\nCyclohexyl Groups: Synthesis, Properties, and Applications in Fuel\nCell Electrodes

Aromatic polymer-based ionomers (SBAFC6) with high gas\npermeability\nand high proton conductivity were designed as electrode binders in\nproton exchanged membrane fuel cells. The target terpolymers containing\nsulfophenyelene (SP), hexafluorobiphenylne (BAF), and cyclohexyl (C6)\ngroups were synthesized by the Ni-promoted polycondensation reaction\nof the corresponding chlorinated monomers, in which composition of\nthe C6 component was varied (17, 30, and 50 mol %). The terpolymers\nwere obtained as high-molecular weight (<i>M</i><sub>n</sub> = 19.2–95.0 kDa and <i>M</i><sub>w</sub> = 64.8–159.0\nkDa), and their properties were evaluated as thin membranes obtained\nby solution casting. SBAFC6 membranes exhibited comparable proton\nconductivity and much higher gas permeability compared with the sulfonated\ncopolymer (SBAF) without C6 groups and benchmark proton conductive\nperfluorinated ionomer (Nafion) membranes. The highest gas permeability\nwas achieved with SBAFC6<sub>50</sub> with 50 mol % C6 groups; hydrogen\nand oxygen permeability at 80 °C was 8.58 × 10<sup>–9</sup> cm<sup>3</sup> cm cm<sup>–2</sup> s<sup>–1</sup> cmHg<sup>–1</sup> and 4.49 × 10<sup>–9</sup> cm<sup>3</sup> cm cm<sup>–2</sup> s<sup>–1</sup> cmHg<sup>–1</sup>, respectively. A fuel cell was fabricated using SBAFC6<sub>50</sub> as a binder in the cathode catalyst layer, which showed larger activation\noverpotential (or lower mass activity of the platinum catalyst) compared\nto the Nafion binder but smaller oxygen diffusion overpotential. With\nair and hydrogen, the SBAFC6<sub>50</sub> binder cell showed 1.5 A\ncm<sup>–2</sup> of the current density at 0.32 V of the cell\nvoltage while that of the Nafion cell was lower than 0.3 V, proving\nthe effectiveness of the highly gas-permeable SBAFC6<sub>50</sub> ionomer\nin the cathode catalyst layer for high current density operation of\nfuel cells.