No 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.

In polymer electrolyte membrane fuel cells, a gas diffusion layer (GDL) with a micro-porous layer (MPL) gives better anti-flooding performance than GDLs without an MPL. To investigate the function and mechanism of the MPL to suppress water flooding, the liquid water distribution at the cathode catalyst layer (CL) surface are observed by a freezing method; in the method liquid water is immobilized in ice form by rapid freezing, followed by disassembling the cell for observations. The ice covered area is quantified by image processing and cells with and without an MPL are compared. The results show that the MPL suppresses water accumulation at the interface due to smaller pore size and finer contact with the CL, and this results in less water flooding. Investigation of ice formed after −10 °C cold start shutdowns and the temporary performance deterioration at ordinary temperatures also indicates a significant influence of the liquid water accumulating at the interface. The importance of the fine contact between CL and MPL, the relative absence of gaps, is demonstrated by a gas diffusion electrode (GDE) which is directly coated with catalyst ink on the surface of the MPL achieving finer contact of the layers. Highlights• Liquid water distribution at the cathode catalyst layer (CL) surface is observed.• Performance deteriorates due to the liquid water accumulated on the CL surface.• The MPL reduces liquid water accumulation between the CL and the MPL.• Cold startup induces much water accumulation and temporary performance deterioration.• A gas diffusion electrode with fine CL to MPL contact mitigates the flooding.

For further improvements in the startup ability below freezing and the durability of polymer electrolyte fuel cells, understanding the ice formation mechanism during cold startup is particularly significant. This study observes cross-sectional ice distributions in a catalyst layer (CL) during isothermal galvanostatic operation at −20 • C using a cryo-scanning electron microscope. The effects of current density, cathode gas conditions, initial water content of the membrane, and cell temperature on the cold start characteristics and the ice formation process in the CL are evaluated. The observational results show that at higher current densities, the region with active ice formation moves from the membrane to the gas diffusion layer sides during the freezing period and vacant pores remain near the membrane even after cell shutdown, while the pores are completely filled with nearly-uniformly growing ice at lower current density operation. This is consistent with the experimental finding from the cold start characteristics that the estimated amount of ice accumulated in the cell until the shutdown decreases as the current density increases. Contrary to expectations, these changes are largely independent of cathode gas conditions, even with pure oxygen. Additional factors controlling the ice formation process are discussed based on the experimental results. The polymer electrolyte fuel cell (PEFC) is a potential candidate for automotive power sources and portable electricity generators because of its characteristics of high efficiency, high power density, low operating temperature, and other advantageous characteristics. In cold climates, however, ensuring startup and durability of PEFCs at subfreezing conditions is a critical issue for the practical use of PEFC. It has been reported that freezing of water produced by the cathode reaction induces shutdown (voltage failure) during startup at subfreezing temperatures, and this freezing causes various kinds of degradation of the cell performance.1-3 Therefore, a number of practicable strategies based on thermal management and control of the residual water present in the cell before cooling and starting have been developed. However, a basic understanding of details of the ice formation process at cold startup, which is particularly-significant for further improvements in the cold startup ability and durability, remains insufficiently addressed in the literature.Some modeling studies have been conducted to describe the heat and mass transfer, and ice formation during cold startup. [4][5][6][7] The research group of the authors has investigated the performance of a PEFC at temperatures below the freezing point by both simulation and experiments.4 That study identified the initial temperature at which self-starting becomes possible, and this is determined by the balance of the produced heat and water generated due to the reaction. With regard to freezing at temperatures closer to zero, like −10• C, it was reported that the produced water is present in a supercooled state, a...

PVL volume, which could be affected by hemodynamic conditions, was significantly associated with the minimal cross-sectional gap area between the aortic annulus and the stent frame. These data may improve our understanding of the mechanism of the occurrence of post-TAVR PVL.