Water transport during startup and shutdown of polymer electrolyte fuel cell stacks

A dynamic three-phase transport model is developed to analyze water uptake and transport in the membrane and catalyst layers of polymer electrolyte fuel cells during startup from subfreezing temperatures and subsequent shutdown. The initial membrane water content (lambda, the number of water molecules per sulfonic acid site) is found to be an important parameter that determines whether a successful unassisted self-start is possible. For a given initial subfreezing temperature at startup, there is a critical lambda (lambda(h)), above which self-start is not possible because the product water completely engulfs the catalyst layers with ice before the stack can warm-up to 0 degrees C. There is a second value of lambda (lambda(1)), below which the stack can be self-started without forming ice. Between lambda(1) and lambda(h), the stack can be self-started, but with intermediate formation of ice that melts as the stack warms up to 0 degrees C. Both lambda(1) and lambda(h), are functions of the initial stack temperature, cell voltage at startup, membrane thickness, catalyst loading, and stack heat capacity. If the stack is purged during the previous shutdown by flowing air in the cathode passages, then depending on the initial amount of water in the membrane and gas diffusion layers and the initial stack temperature, it may not be possible to dry the membrane to the critical lambda for a subsequent successful startup. There is an optimum lambda for robust and rapid startup and shutdown. Startup and shutdown time and energy may be unacceptable if the lambda is much less than the optimum. Conversely, a robust startup from subfreezing temperatures cannot be assured if the lambda is much higher than this optimum. (C) 2010 Elsevier B.V. All rights reserved.