The Formation-Structure-Functionality Relationship of Catalyst Layers in Proton Exchange Membrane Fuel Cells

Understanding the relationship between the formation, structure, and functionality of catalyst layers is crucial for designing catalyst layers with specific high-current-density operations. In this study, we investigated the impact of the ionomer-to-carbon (I/C) ratio and solid content on transport properties. We conducted fuel cell performance and diagnostic measurements to demonstrate the combined effects of the I/C ratio and solid content on the mass transport, particularly oxygen transport. To elucidate the roles of the I/C ratio and solid content in catalyst layer formation, we utilized dynamic light scattering and rheological measurements. By analyzing the local and global structure of ionomer-Pt/C assemblages in the catalyst inks, we observed that the I/C ratio and solid content influence the competition between homo-aggregation and hetero-aggregation, the strengths of inter- and intra-cluster bonds, and the rigidity and connectivity of the particulate structure. Additionally, high-shear-application simulations tend to reduce the connectivity of the particulate network and induce cluster densification, unless the global structure is mechanically stable and resilient. Based on this understanding, we established the formation-structure-functionality relationship for catalyst layers, thereby providing fundamental insights for designing catalyst layers tailored to specific functionalities.