Impact of Platinum/Carbon Aggregate Size on Agglomerate Structure and Performance of Polymer Electrolyte Fuel Cells

Abstract The aggregate size of Pt/C plays a crucial role in determining the agglomerate structure formed in catalyst inks and the porous structure of catalyst layers (CLs); consequently, it strongly influences the performance of polymer electrolyte fuel cells (PEFCs). To elucidate the underlying mechanism, we controlled the Pt/C aggregate size by varying the number of ball-milling cycles and evaluated the resulting agglomerate structures and CL properties. Large aggregates, with limited outer surface area for ionomer adsorption that allows hydrophobic interactions among Pt/C aggregates to dominate, promoted flocculation and the formation of coarse agglomerates, leading to non-uniform CLs and poor proton-conduction pathways within the agglomerates. As a result, platinum utilization decreased, and cell performance deteriorated. In contrast, smaller aggregates provided sufficient ionomer adsorption, balancing electrostatic repulsion and hydrophobic attraction. This facilitated colloidal gel formation and yielded uniform CLs. Improved ionomer coverage increased platinum utilization and mitigated oxygen transport resistance, leading to enhanced performance. Overall, these findings highlight that precise control of aggregate size is essential not only for stabilizing catalyst inks but also for designing CL microstructures that enable high-performance PEFC electrodes.