Dual pore network modeling of coupled gas transport and electrochemical reactions in the cathode catalyst layer considering oxygen diffusion barriers at void–ionomer connections

Proton exchange membrane fuel cells (PEMFCs) are promising clean energy-conversion devices, but their performance and durability are strongly governed by transport and reaction processes in the cathode catalyst layer (CCL). Here, a dual-network pore network model coupling oxygen diffusion, proton conduction, and electrochemical reaction was developed to clarify ionomer-dependent transport–reaction behavior in the CCL. Gas diffusion resistance is dominated by ionomer-related pathways, especially void–ionomer connections, while resistance in void pores is negligible. With increasing ionomer content, void–ionomer resistance decreases and ionomer–ionomer resistance increases. At low ionomer contents (<25%), the void pathway remains relatively open, but poor ionomer connectivity limits proton access and effective triple-phase-boundary formation, resulting in low catalyst utilization. Excessive ionomer (>30%) further improves proton conduction but leads to excessive Pt coverage by ionomer and increases local oxygen-diffusion resistance toward the Pt/ionomer interface, reducing oxygen-accessible active sites and aggravating concentration polarization. An intermediate ionomer content of 25–30% therefore provides the best balance among oxygen diffusion, proton conduction, and active-site availability, leading to enhanced reaction activity and catalyst utilization. The Damköhler-number analysis further suggests a shift toward active-site-utilization limitation in ionomer-rich structures. These results provide theoretical guidance for the efficient design of catalyst-layer microstructures.