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In startups of polymer electrolyte fuel cells at temperatures close to 0 degrees 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 degrees 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.

In startups of polymer electrolyte fuel cells at temperatures close to 0 degrees 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 degrees 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.

Polymer electrolyte membrane fuel cells using a hydrophilic micro-porous layer (MPL) consisting of carbon fiber (CF) and ionomer show better performance than those using conventional hydrophobic MPL with carbon black (CB) under a wide range of humidity conditions. This study investigates the effects of the wettability and structure of the MPL on cell performance and discusses the mechanism for improving cell performance by the hydrophilic CF-MPL at high humidity conditions. The water distribution inside the MPL is evaluated by a freezing method and cryo-SEM observations, and a comparison of the distribution and the cell performance of hydrophilic and hydrophobic MPL cells with various structures is made. The results show that the performance improvement is caused by preventing increases in concentration over-voltage at high current densities; where the fiber structure and thicker layers are important with hydrophilic MPL. In hydrophilic CF-MPL with better performance, liquid water accumulates in pores at the catalyst layer (CL) side, while there is no liquid water at the gas diffusion layer side. These results indicate that the hydrophilic CF-MPL enhances the liquid water transport from the CL due to the hydrophilic properties, and promotes water evaporation due to the large pores in the thick fiber structure.

Polymer electrolyte membrane fuel cells using a hydrophilic micro-porous layer (MPL) consisting of carbon fiber (CF) and ionomer show better performance than those using conventional hydrophobic MPL with carbon black (CB) under a wide range of humidity conditions. This study investigates the effects of the wettability and structure of the MPL on cell performance and discusses the mechanism for improving cell performance by the hydrophilic CF-MPL at high humidity conditions. The water distribution inside the MPL is evaluated by a freezing method and cryo-SEM observations, and a comparison of the distribution and the cell performance of hydrophilic and hydrophobic MPL cells with various structures is made. The results show that the performance improvement is caused by preventing increases in concentration over-voltage at high current densities; where the fiber structure and thicker layers are important with hydrophilic MPL. In hydrophilic CF-MPL with better performance, liquid water accumulates in pores at the catalyst layer (CL) side, while there is no liquid water at the gas diffusion layer side. These results indicate that the hydrophilic CF-MPL enhances the liquid water transport from the CL due to the hydrophilic properties, and promotes water evaporation due to the large pores in the thick fiber structure.

For interfaces between micro-porous layers (MPL) and catalyst layers (CL) made by the gas diffusion electrode (GDE) method, a seamless interface without gaps, shows better performance than that of cells with an interface made by the decal transfer method. With the decal transfer method, the MPL is simply hot-pressed to the CL-membrane assembly. This study investigates the effect of interface structure on cell performance and water transport in the MPL. Water distribution in cross sections of multiple layers were observed by a freezing method, where the cell is cooled below freezing temperature in short time and the water was observed in ice form by Cryo-SEM. The results show that a membrane electrode assembly (MEA) using the GDE method improves cell performance at high current densities. Direct observations by the freezing method and cryo-SEM show that there is no water accumulation at the MPL/CL interface made by the GDE method, while water accumulates at the interface made by the decal method. Other observations show that the water amount inside the MPL increases similarly in the two types of MEA when lowering the temperature, and the difference between the two types of MEA was only the water amount in the interface.

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 degrees 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.

For interfaces between micro-porous layers (MPL) and catalyst layers (CL) made by the gas diffusion electrode (GDE) method, a seamless interface without gaps, shows better performance than that of cells with an interface made by the decal transfer method. With the decal transfer method, the MPL is simply hot-pressed to the CL-membrane assembly. This study investigates the effect of interface structure on cell performance and water transport in the MPL. Water distribution in cross sections of multiple layers were observed by a freezing method, where the cell is cooled below freezing temperature in short time and the water was observed in ice form by Cryo-SEM. The results show that a membrane electrode assembly (MEA) using the GDE method improves cell performance at high current densities. Direct observations by the freezing method and cryo-SEM show that there is no water accumulation at the MPL/CL interface made by the GDE method, while water accumulates at the interface made by the decal method. Other observations show that the water amount inside the MPL increases similarly in the two types of MEA when lowering the temperature, and the difference between the two types of MEA was only the water amount in the interface.

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 degrees 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.