Ice Formation from a Supercooled State and Water Transport through Ionomers during PEFC Cold Startup

Abstract

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.