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Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte membrane fuel cells (PEFCs), but details of the mechanism to suppress water flooding has not been fully understood. In this study, we investigated water distribution at the interface between MPL and catalyst layer (CL) at the cathode side to clarify the effect of MPL on discharge of produced water. Freezing method was applied to observe the distribution of the condensed water, and the distribution of ice on the CL surface was quantified by a binary image processing. In addition, comparing normal operation with start-up temperature -10*C, there is large amount of condensed water on the CL surface after operation at -10*C. Based on these results, the MPL has a role to prevent accumulation of water on the surface of CL, and suppresses water flooding.

In polymer electrolyte fuel cells (PEFCs), the freezing of produced water induces the extreme deterioration of the cell performance below zero. This phenomenon is serious problem in cold regions and is needed to be solved to achieve the practical use of PEFCs. In this study, we investigated the ice formation and the reaction rate in the cathode catalyst layer (CL) using a CRYO-SEM and a three-phase boundary model to clarify the freezing mechanism in the cold start at -20oC. The observation results showed that the ice distribution formed in the CL is changed by the operation time and the current density. In particular, in a case of large current density, it was shown that ice grows from the membrane side to the gas diffusion layer (GDL) side. This process can also be analyzed in detail by the model that calculates the reaction rate in the CL.

The porous separator has recently been proposed as an alternative to cells with gas flow channels in PEFCs. In this study, basic characteristics of a PEFC with porous separator were identified using a cell for the direct observation of phenomena at the surface of porous separator and measurements of the local current density. The transport behavior of liquid water inside the porous separator was also investigated using another cell for the direct observation of the porous cross-section. Because of the higher contact resistance, the cell voltage with porous separator is lower than that with serpentine separator, but a stable operation can be maintained with the porous separator under conditions at higher current density. The cross-sectional observation showed that the porous separator is superior in the drainage performance from the surface of gas diffusion layer, and the hydrophilic property of porous separator is important for the better cell performance.

Because electron, proton, and oxygen are necessary for the cathode reaction in polymer electrolyte fuel cell, achieving the optimum structure of electrode catalyst layer and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. In this study, we presented evaluation equations to investigate the relationship between the structure of cathode catalyst layer and the cell performance. The evaluation equations are based on the analysis using the three-phase boundary model by the authors, and can estimate the effects of cathode catalyst layer structures on the cell performance easily. This paper discusses optimum structure of the catalyst layer using these evaluation equations. Furthermore, the dominant parameter of the catalyst layer structure such as the amount of polymer electrolyte and the oxygen solubility in the polymer was confirmed experimentally.

In startups of polymer electrolyte fuel cells at temperatures close to 0 °C below freezing, water produced by the cathode reaction is present in a supercooled state and may freeze. This study investigates the ice formation processes during isothermal operation at -10 °C. The observations using a cryo-scanning electron microscope show that an ice layer is formed at the interface between the cathode catalyst layer (CL) and the micro-porous layer (MPL), and that the layer becomes thicker during the startup. This suggests the possibility for produced water to be transported from the CL to the ice layer through the ionomer. To evaluate water transport phenomena, changes in water content of the polymer electrolyte membrane in contact with supercooled water are estimated by measurements of the high frequency resistance. The results show that the membrane resistance, corresponding to the water content, increases after freezing of the supercooled water, and the difference increases with the degree of supercooling. This indicates that the driving force of the water transport is the concentration gradient caused by the freezing, supporting the above suggestion. A hydrophilic MPL is introduced to enhance water removal from the interface into the MPL, resulting in improvements in the cold startup.