Durability of polymer electrolyte membrane fuel cells operated at subfreezing temperatures

The structure, composition, and interfaces of membrane electrode assemblies (MEA) and gas-diffusion layers (GDLs) have a significant effect on the performance of single-proton-exchange-membrane (PEM) fuel cells operated isothermally at subfreezing temperatures. During isothermal constant-current operation at subfreezing temperatures, water forming at the cathode initially hydrates the membrane, then forms ice in the catalyst layer and/or GDL. This ice formation results in a gradual decay in voltage. High-frequency resistance initially decreases due to an increase in membrane water content and then increases over time as the contact resistance increases. The water/ice holding capacity of a fuel cell decreases with decreasing subfreezing temperature (-10�C vs. -20�C vs. -30�C) and increasing current density (0.02 A cm^-2 vs. 0.04 A cm^-2). Ice formation monitored using in-situ high-resolution neutron radiography indicated that the ice was concentrated near the cathode catalyst layer at low operating temperatures (≈-20�C) and high current densities (0.04 A cm^-2). Significant ice formation was also observed in the GDLs at higher subfreezing temperatures (≈-10�C) and lower current densities (0.02 A cm^-2). These results are in good agreement with the long-term durability observations that show more severe degradation at lower temperatures (-20�C and -30�C).