In situ investigation of moisture sorption mechanism in fuel cell catalyst layers

Research focusing on catalyst layers is critical for enhancing the performance and durability of proton exchange membrane fuel cells. In particular, the role of the ionomer is pivotal but remains poorly explored due to the difficulty to access complex electrode structures. Moreover, perfluorosulfonic acid (PFSA) polymers are usually employed in catalyst layers but their drawbacks have spurred interest in aromatic compounds, which promise improved conductivity and performance. Here we investigated the structure-to-function relationship and interactions in novel catalyst layers using non-perfluorinated sulfonic acid ionomers, e.g. multiblock poly(arylene ether sulfones) bearing perfluorosulfonic acid side chains. By combining dynamic vapor sorption, small-angle neutron scattering and synchrotron humidity-controlled infrared spectroscopy, we accessed the water uptake, nanostructures, and molecular structures in a series of catalyst layers prepared with different loadings of aromatic polymer, as well as reference compounds, e.g. pure membrane and polymer-carbon systems. Our measurements show that the water sorption mechanism in catalyst layers differs from pure ionomers due to catalyst-induced structural changes. We observed that most of the formed ionic species interact primarily with the platinum catalyst and probably locate at the particle-ionomer interface. These results emphasize the need for continued research to advance aromatic-type ionomers in fuel cell technology under realistic conditions.