Temperature dependent water transport mechanism in gas diffusion layers revealed by subsecond operando X-ray tomographic microscopy

The product water of polymer electrolyte fuel cells (PEFCs) has to pervade the gas diffusion layer (GDL) and can lead to shortages of educt gas diffusion pathways to the catalyst layer hence to significant performance losses. Here we report on efforts made to enable subsecond and submicron operando X-ray tomographic microscopy (XTM) over a wide range of cell operating temperatures. The short XTM scan times allow to quantify the contributions of capillary-fingering and phase-change-induced flow on the overall water transport inside the cathode GDL at two different cell operating temperatures of 40 °C and 80 °C during a current ramp-up process. The results suggest that phase-change-induced water transport dominates the initial increase of saturation levels at typical automotive PEFC operating temperatures of about 80 °C, whereas capillary-fingering driven transport dominates the development of saturation from the beginning to stagnating stage at 40 °C.