Rapid self-start of polymer electrolyte fuel cell stacks from subfreezing temperatures

Polymer electrolyte fuel cell (PEFC) systems for light-duty vehicles must be able to start unassisted and rapidly from temperatures below −20 °C. Managing buildup of ice within the porous cathode catalyst and electrode structure is the key to self-starting a PEFC stack from subfreezing temperatures. The stack temperature must be raised above the melting point of ice before the ice completely covers the cathode catalyst and shuts down the electrochemical reaction. For rapid and robust self-start it is desirable to operate the stack near the short-circuit conditions. This mode of operation maximizes hydrogen utilization, favors production of waste heat that is absorbed by the stack, and delays complete loss of effective electrochemical surface area by causing a large fraction of the ice to form in the gas diffusion layer rather than in the cathode catalyst layer. Preheating the feed gases, using the power generated to electrically heat the stack, and operating pressures have only small effect on the ability to self-start or the startup time. In subfreezing weather, the stack shut-down protocol should include flowing ambient air through the hot cathode passages to vaporize liquid water remaining in the cathode catalyst. Self-start is faster and more robust if the bipolar plates are made from metal rather than graphite.