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
Cold start characteristics of a polymer electrolyte fuel cell were investigated experimentally, and microscopic observations were conducted to clarify the freezing mechanism in the cell. The results shows that the freezing mechanisms are classified into two types: freezing in catalyst layer at very low temperature like −20C, and freezing of supercooled water at the interface between cathode catalyst layer and micro porous layer at near 0C like −10C. The amount of produced water in each the processes is related to the initial wet condition of the membrane because there is a period of back diffusion of produced water into the membrane. It is also shown that the performance of a subsequent normal temperature operation at 30C after the shutdown in the cold start is temporarily deteriorated after the freezing at −10C, but not after the freezing at −20C. The ice formed at the interface between the catalyst and the micro porous layers is estimated to cause the temporal deterioration, and the function of micro porous layer on the gas diffusion layer is also discussed.
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