In situ observation of water evaporation in gas diffusion layers of polymer electrolyte fuel cells

During the shutdown processes of polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles, effective gas purging is essential to minimize residual water within the fuel cells and ensure safe cold starts under subzero temperatures. High-frequency resistance (HFR) measurements are commonly used to confirm the completion of the gas purging process; however, these measurements reflect only the water content in the proton exchange membrane (PEM) and overlook water accumulation in the gas diffusion layer (GDL). In this study, we employed in situ synchrotron X-ray radiography with HFR monitoring to investigate the effects of the gas flow rate and purging duration on the water evaporation behavior within the cathode GDL. The results indicated that the water accumulation in the under-rib region of the cathode GDL was evaporation resistant, even with high-flow-rate gas purging. Quantitative analysis revealed that evaporation proceeded mainly via vapor-phase diffusive transport, with convective transport playing a minor role. Importantly, water slugs remained in the flow channels acted as diffusion barriers, limiting vapor transport. Immediately after purging, residual water migrated from the GDL to the PEM, contributing to HFR relaxation. These insights support the development of more effective purging protocols to ensure reliable cold starts in PEFC systems.