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...

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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.

In Polymer electrolyte membrane fuel cells (PEFCs), freezing of produced water induces the extreme deterioration of cell performance below zero. This phenomenon is a serious problem in cold regions and is needed to be solved to achieve the practical use of PEFCs. In this study, we investigated ice distribution at the cold start in a PEFC using an optical microscope and a CRYO-SEM to clarify the freezing mechanism. The observation results showed that the cold start at −10°C makes ice at the interface between the cathode catalyst layer (CL) and the micro porous layer of gas diffusion layer. Little ice was, however, observed in the cold start at −20°C, which indicated the ice formation inside the CL. The CRYO-SEM observation was conducted at −20°C to investigate the ice formation inside the CL, and this identified the effects of the current density and the cathode gas species on the ice distribution.

In PEM 釦 el cell belQw zero ， 廿 eezlng under the cold start sometimes causes a deterioration of ccll perfomiance in normal operation ． ］ nparticular ， this phenomenon is especialiy prominent a 負er the − 10 °C s觚 than aner the − 20 °C that． This is because the ice forrned at the interface between the cathode catalyst layer and GDL at the −10 °C start． This paper fbcused on the effect of current density and tightening pressure ofthe cell， and investigated the mechanism of the − 10 °C start effect on norma ［operation ． The results showed that the perfbrmanGe deterioration is large as current density at the −1 ゜ C start is small ． This phenomenon became prorninent in caso of smaU tightening pressure of the cell because ofwide ice fbrmation on the catalyst layer surface ． Key Mords ： PEM Fuel Cell ， Cold Start ， Microscope Observation ， Water Management