Humidity-Dependent Oxygen Transport in Carbon-Supported Catalysts for PEMFCs Revealed by Nanoscale X-ray Tomography

Optimizing oxygen transport in polymer electrolyte membrane fuel cell (PEMFC) catalyst layers requires understanding how hierarchical nanoscale pore morphology, ionomer swelling, and water condensation interact under humidified operating conditions. Here, we directly visualize humidity-dependent changes in pore saturation and quantify porosity and tortuosity evolution in two commercial Pt/C catalysts using synchrotron-based X-ray nanocomputed tomography, complemented by high-resolution electron microscopy. Compared with a conventional solid carbon support, the mesoporous carbon exhibits a hierarchical nanoscale pore network that delays water accumulation, maintains higher accessible pore volume, and moderates tortuosity increases across a wide RH range. In contrast, the solid-carbon support experiences early pore blockage and severe loss of gas-phase pathways. These structural trends correlate with electrochemical measurements, indicating that mesoporous frameworks mitigate ionomer-induced oxygen transport resistance by preserving continuous diffusion channels. Our findings demonstrate that controlling the nanoscale pore hierarchy of carbon supports is essential for improving water management and oxygen transport in PEMFC cathodes, particularly under high current density operation.