Molecular Engineering of Membrane Electrode Assemblies With Superior Humidity Robustness for Fuel Cells

It remains a substantial challenge to achieve stable performance of polymer electrolyte membrane fuel cells (PEMFCs) under fluctuating ambient humidity, largely owing to the difficulty of sustaining oxygen diffusion across the full humidity range. Here, we incorporate a spirobifluorene‐containing microporous ionomer (QTPSp) into the cathode catalyst layer to regulate local oxygen transport in PEMFC electrodes. The rigid, contorted spirobifluorene moieties restrict backbone conformational freedom and frustrate chain packing, generating persistent sub‐nanometer free volume and a hierarchical porous architecture with bicontinuous domains and abundant voids. Quantitative analysis reveals a 54% reduction in local oxygen transport resistance, demonstrating that the microporous ionomer architecture effectively alleviates local oxygen‐transport limitations while maintaining sufficient ion transport. The fuel cell achieves a peak power density of 1.04 W cm −2 under fully dry H 2 /Air. The humidity robustness parameter increases 2.5‐fold, showing that the QTPSp‐4%‐based MEA maintains high power density and low sensitivity to humidity.