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

Abstract

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 35ºC, while the pores of the MPL are filled with liquid water under very low cell temperature operation, at 5ºC. 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.

Modelling water intrusion and oxygen diffusion in a reconstructed microporous layer of PEM fuel cells

“…This is consistent with the observation of Aoyama et al [24], where the liquid water accumulated in the CL but was unable to move into the MPL. Therefore, there must be other pathways for the liquid water to exit.…”

“…Since the MPL was assumed to be more hydrophobic than the CL, these models predicted that with an increase in MPL thickness, the water would gradually flood the part of MPL adjacent the CL. Recent experiments, however, have casted doubts over this hypothesis [24]. For example, the observation of Aoyama et al [24] showed that their MPL remained dry even after liquid water had accumulated at the CL-MPL interface, indicating that the water did not move into the MPL and must have found alternative exits.…”

“…Recent experiments, however, have casted doubts over this hypothesis [24]. For example, the observation of Aoyama et al [24] showed that their MPL remained dry even after liquid water had accumulated at the CL-MPL interface, indicating that the water did not move into the MPL and must have found alternative exits. Since water movement in a medium is controlled by its pore geometry and hydrophobicity, understanding pore-scale processes is essential to help MPL design and has attracted increased interest, ranging from its structure characterization [25], process visualization [26], to numerical modelling [27].…”

“…One possibility is cracks that might have developed in their MPL [33][34][35][36]. Although Aoyama et al [24] hypothesized that the water could have entered the MPL as water vapour, there must also be an exit for the liquid water condensed in the CL.…”

Impact of micro-porous layer on liquid water distribution at the catalyst layer interface and cell performance in a polymer electrolyte membrane fuel cell

“…The freezing method immobilizes the liquid water in the cell as ice forms by decreasing the temperature in a short time, and the cryo-SEM visualizes the ice distribution in the MPL at high resolutions at −150 °C [15]. The research has however not resolved a number of issues, including the effects of the MPL on in-plane distribution of liquid water at the CL surface and the overall cell performance, a better understanding of which would enable further advances in the water management.…”