High Oxygen Permeability Ionomer Composed Entirely of Cyclic Monomers: Synthesis, Properties, and Application to Polymer Electrolyte Fuel Cell Cathodes

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Polymer Electrolyte Fuel Cells (PEFCs) are key devices, producing electricity from hydrogen and oxygen, pivotal for sustainable energy systems with no CO2 emissions. They are considered for various uses, including power sources for mobility, stationary cogeneration and off-grid power generation. However, the transportation sector, especially Heavy-Duty Vehicles (HDVs) and aviation, which have high emissions, experiences difficulties with decarbonization and cannot rely on existing batteries due to high load, high output, and long usage time. This scenario places high expectations on the broader application of PEFCs. To improve the performance of PEFCs, it is necessary to improve the performance of cathode ionomers, and the development of high oxygen permeability ionomers (HOPIs), which mitigate the oxygen transport rate-limiting process of the oxygen reduction reaction, has attracted much attention.1-5 In this study,6 we developed a new synthesis method for a cyclic monomer with a fluorosulfonyl group (SMD-E4) and succeeded in synthesizing a new HOPI composed entirely of cyclic monomer by copolymerization with a hydrophobic cyclic monomer (MMD). The basic electrolyte properties of the HOPI were compared with those of Nafion, and HOPI showed comparable proton transport performance with excellent oxygen permeability, suggesting its potential use as a PEFC material. By employing the HOPI as a cathode ionomer in MEAs, improved performance was observed, especially under low humidity and high current density operating conditions. Improvements were observed in both activation overvoltage and concentration overvoltage, as well as in electrochemical surface area, mass activity, specific activity, and oxygen transport-related factors. Analysis of the catalyst layers revealed that the property of the HOPI to maintain a relatively high pore volume within the catalyst layers may mitigate the direct contact between Pt and ionomer and contribute to the lowering of activation overvoltage. Furthermore, the separation of the oxygen transport resistance component suggests that the HOPI not only lowers the oxygen transport resistance of the ionomer itself, but also contributes to lowering the oxygen transport resistance derived from Knudsen diffusion by maintaining a high pore volume in the catalyst pores.Moreover, the performance of MEAs with the HOPI applied as a cathode ionomer was evaluated in a structure similar to that of a practical PEFC with a high-performance membrane and low Pt usage, and it was found that a power density improvement of more than 140% compared to that of Nafion was achieved under low humidity conditions (20% RH). Furthermore, the unique property of the HOPI that concentration overvoltage decreases under low humidity suggests the possibility of achieving MEAs with high robustness to humidity changes.These results indicate that HOPIs can fully demonstrate their performance under the harsh operating conditions of next-generation PEFCs, such as low humidity and high current density, and will play an important role in improving the performance of PEFCs. Acknowledgement This work was partially based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References 1) K. Yamada et al., ECS Trans., 50, 1495 (2013).2) R. Shimizu et al., J. Electrochem. Soc., 165, F3063 (2018).3) R. Jinnouchi et al., Nat. Commun., 12, 4956 (2021).4) J. P. Braaten et al., J. Power Sources, 522, 230821 (2022).5) N. Macauley, et al., Adv. Energy Mater., 12, 2201063 (2022).6) T. Hirai et al., ACS App. Energy Mater. (2024), https://doi.org/10.1021/acsaem.4c00177. Figure 1