The graphical abstract shows that liquid water as well as water vapor is present inside the catalyst layer (CL) and the gas diffusion layer (GDL) even at temperature above 100 °C, especially near the outlet as well as at the edges of the rib and under the rib. The liquid water and water vapor are reducing the diffusion of oxygen to CL through the GDL reducing performance and causing very high p(O₂) at the surface of the GDL alongside the gas flow channel, which was visualized using the nondestructive, real-time/space visualization system of the University of Yamanashi.

Physical and chemical parameters, such as temperature, water/hydrogen/oxygen partial pressures, are distributed inhomogeneous inside a polymer electrolyte fuel cell during the operation and have a large influence on its performance and durability. In this study, the oxygen partial pressure (p(O₂)) is visualized in real-time/space using an oxygen-sensitive dye on the surface of the gas diffusion layer (GDL) during power generation at temperatures of 80, 100, and 110 °C using a 20 mm × 20 mm single cell with ten straight gas flow channels. p(O₂) on the surface of the GDL is visualized for the first time at temperatures higher than 100 °C, desired especially for heavy-duty vehicle application, due to advantages such as less susceptibility to catalyst poisoning and the option to use smaller and lighter radiators. The oxygen partial pressure on the surface of the GDLs is monitored to be higher than the values expected from a simple model and decreased only slightly along the gas flow channel with increasing current densities. The work shows that high p(O₂) on the surface of the GDL is due to the short gas flow channels and accumulating water/vapor inside the GDL and the catalyst layer limiting the gas diffusion.