Cold start characteristics of a polymer electrolyte membrane fuel cell are investigated experimentally, and microscopic observations are conducted to clarify the freezing mechanism in the cell. The results show that the freezing mechanism can be classified into two types: freezing in the cathode catalyst layer at very low temperature like −20 °C, and freezing due to supercooled water at the interface between the catalyst layer and the gas diffusion layer near 0 °C like −10 °C. The amount of water produced during the cold start is related to the initial wetness condition of the polymer electrolyte membrane, because water absorption by the membrane due to back diffusion plays an important role to prevent the water from freezing. It is also shown that after the shutdown of the cold start the cell performance of a subsequent operation at 30 °C is temporarily deteriorated after the freezing at −10 °C, but not after the freezing at −20 °C. The ice formed at the interface between the catalyst layer and the gas diffusion layer is estimated to cause the temporary deterioration, and the function of a micro porous layer coating the gas diffusion layer for the ice formation is also discussed. Highlights• Two freezing types at cold start in and on the surface of a cathode catalyst layer • Direct observation of the ice formed on the catalyst layer surface • Temporary performance deterioration at 30 °C caused by ice on the surface

Numerical simulations using the lattice Boltzmann method (LBM) are developed to elucidate the dynamic behavior of condensed water and gas flow in a polymer electrolyte membrane (PEM) fuel cell. Here, the calculation process of the LBM simulation is improved to extend the simulation to a porous medium like a gas diffusion layer (GDL), and a stable and reliable simulation of two-phase flow with large density differences in the porous medium is established. It is shown that dynamic capillary fingering can be simulated at low migration speeds of liquid water in a modified GDL, and the LBM simulation reported here, which considers the actual physical properties of the system, has significant advantages in evaluating phenomena affected by the interaction between liquid water and air flows. Two-phase flows with the interaction of the phases in the two-dimensional simulations are demonstrated. The simulation of water behavior in a gas flow channel with air flow and a simplified GDL shows that the wettability of the channel has a strong effect on the two-phase flow. The simulation of the porous separator also indicates the possibility of controlling two-phase distribution for better oxygen supply to the catalyst layer by gradient wettability design of the porous separator.Keywords: PEM fuel cell, Lattice Boltzmann method, Two-phase flow, Large density difference, Gas diffusion layer, Wettability

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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.

Transport of electrons, protons, and oxygen are necessary for the cathode reactions in polymer electrolyte membrane fuel cells, and achieving the optimum structure of the electrode catalyst layer and the efficient transport of reactants is an effective avenue to reduce the use of platinum catalyst. This study applied three-phase boundary and cathode catalyst layer models to understand details of optimally efficient structures for the transport of reaction components. The factors dominating the effects of the catalyst layer structure and the properties identified in this manner are investigated using the models. Additionally, equations of evaluation are developed to evaluate the effects of the structure and the properties on the cell performance, and the effectiveness of the developed equations is confirmed by a comparison of the results calculated by the equations with the model simulations. From these results, the structure of the porosity, the catalyst layer, and the polymer electrolyte thicknesses, that are optimum for the gas transport and proton conduction, are determined. It is found that the solubility of oxygen in the polymer is one of the dominant factors in the processes of the cathode catalyst layer, and that increasing the solubility is highly effective to reduce the need for platinum