Operando Monitoring of Oxygen Drop in Catalyst Layers Accompanied by a Water Increase in Running Proton Exchange Membrane Fuel Cells

[Image: see text] Accurate understanding of internal phenomena and their feedback is intrinsically important for improving the performance and durability of proton exchange membrane fuel cells. The oxygen partial pressure (p(O(2))) at 10 μm from the cathode catalyst layer (CL) in the gas diffusion layer was measured by using an optical fiber with an oxygen-sensitive dye applied to the apex, when the current density was abruptly increased. p(O(2)) decreased with increasing current density at constant air utilization. This decrease in oxygen partial pressure is attributed to the increased amount of water at the CL at the cathode due to the oxygen reduction reaction and electro-osmotic drag, as previously proposed. A shortage of oxygen in the CL for 1 s was also detected by using this p(O(2)) monitoring. These results are consistent with the previous results obtained by operando time-resolved measurements of the water distribution in the electrolyte membranes of a running fuel cell.