Effect of the Pore Size of Mesoporous Carbon Supports for Cathode Catalysts on the Cell Performance of Polymer Electrolyte Fuel Cells

Mesoporous carbon has attracted increasing attention as an effective material for cathode catalyst supports in polymer electrolyte fuel cells (PEFCs), contributing to enhanced cell performance. However, a structural design strategy for mesoporous carbon has not yet been established. In this study, we investigated the influence of mesopore size on cell performance, focusing on ionomer penetration into the mesopores. Monodispersed mesoporous carbon spheres (MMCSs) with similar particle sizes and mode pore sizes ranging from 3 to 10 nm were synthesized using a template method with mesoporous silica and were applied as supports for cathode Pt catalysts. Electrochemical measurements and ionomer distribution analyses revealed that as the mode pore size increased, a greater amount of ionomer penetrated into the mesopores. This led to more pronounced ionomer-induced catalyst poisoning within the pores and disruption of ionomer pathways on the external surface of the carbon spheres. As a result, MMCSs with larger pore sizes exhibited reduced catalytic activity and increased proton transport resistance. Conversely, decreasing the pore size effectively suppressed ionomer penetration and its associated negative effects. However, when the pore size was too small (3.0 nm in this study), the suppression of catalyst poisoning was less effective, likely because a significant amount of catalyst was deposited on the external surface of the mesoporous carbon rather than inside the pores. Considering these combined effects, carbon supports with intermediate pore sizes (approximately 4–5 nm) were found to achieve a favorable balance, suppressing ionomer-induced catalyst poisoning while avoiding a significant increase in proton transport resistance within the catalyst layer. These mesoporous carbons also demonstrated superior oxygen transport properties and better performance at high current densities compared to commercial solid-core and porous carbon supports. In conclusion, the use of mesoporous carbon has been shown to be effective in enhancing cell performance, and mesopore size─by affecting the extent of ionomer penetration─has been identified as a critical factor in determining overall cell performance.