Observation of water transport in the micro-porous layer of a polymer electrolyte fuel cell with a freezing method and cryo-scanning electron microscope

• The freezing method can be used to elucidate details of the water transport in MPLs.
• Produced water passes through the MPL as vapor at ordinary temperatures, above 35 °C.
• Liquid water accumulates in the MPL at the low operating temperature of 5 °C.

Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte fuel cells (PEFCs), however, the detailed mechanism of how the produced water is drained from these layers is not well understood. This paper observed the cross-sectional distribution of liquid water inside the cathode MPL to elucidate details of the phase state of the water transported through the MPL. The freezing method and cryo-scanning electron microscope (cryo-SEM) are used for the observations; the freezing method enables immobilization of the liquid water in the cell as ice forms by the freezing, and the cryo-SEM can visualize the water distribution in the vicinity of the MPL at high resolution without the ice melting. It was shown that no liquid water accumulates inside the MPL in operation at 35 °C, while the pores of the MPL are filled with liquid water under very low cell temperature operation, at 5 °C. These results indicate that the produced water passes through the MPL not as a liquid but in the vapor state in usual PEFC operation. Additionally, liquid water at the interface between the MPL and a catalyst layer (CL) was identified, and the effect of the interfacial contact on the water distribution was examined.