Effects of Humidity and Produced Water on Specific Adsorption of High Oxygen Permeability Ionomers Composed Entirely of Cyclic Monomers on Cathode Performance for Polymer Electrolyte Fuel Cells

To enhance polymer electrolyte fuel cell (PEFC) performance, it is necessary to improve cathode ionomer performance, with attention to the development of high oxygen permeability ionomers (HOPIs) to mitigate the oxygen reduction reaction rate-limiting process of oxygen transport. We developed a new synthetic route for a cyclic monomer with a fluorosulfonyl group and synthesized a novel ionomer composed entirely of cyclic monomers. Preliminary investigations showed that it exhibited the expected HOPI performance while maintaining basic electrolyte performance. Cell evaluation of the MEA with the HOPI as a cathode ionomer confirmed improvements, especially under low humidity and high current density conditions. Overvoltage component analysis verified activation overvoltage lowering due to improved catalytic activity and concentration overvoltage decrease due to improved oxygen permeability. This was attributed to the ability of HOPI to avoid specific adsorption, to improve oxygen solubility, to improve oxygen transport due to increased ionomer permeability, and to improve Knudsen diffusion due to pore volume preservation in the catalyst layer. Careful humidity dependence examinations revealed a characteristic step in the Tafel plot under low humidity conditions, which was sensitive to ionomer differences. While the step occurrence mechanism remains debatable, we hypothesize that the step reflects the specific adsorption of the ionomer on Pt. Specific adsorption is not a static phenomenon but rather a dynamic one, which can vary according to the amount of water present near the catalyst surface. The difference in current density around the step could reflect the loss of Pt active surface area due to specific adsorption, enabling quantitative analysis of ionomer-induced performance degradation in MEAs. Cell evaluations combined with a high-performance membrane showed that an MEA using the HOPI as the cathode ionomer exhibited improved performance and high robustness versus humidity variations.