An effective PEMFC system shutdown purge strategy for improving the purging effect of liquid water and the dehydration of stack

The shutdown process is one of the critical stages when the proton exchange membrane fuel cell system runs as an actual application. Finding the effect of system operating conditions on shutdown purging could improve the purging effect of liquid water and the dehydration of the stack. In this study, an automotive fuel cell system of 350 single cells was developed on the vehicle scale and tested for factors that influence shutdown process. Seven shutdown purge cases for about 150 s were studied to investigate the shutdown purge strategy. It was found that 100% bypass valve opening reduced the purge time and improved the purge efficiency. The HCP speed should not be too slow or too fast in the shutdown process. The higher temperature of coolant water favored rapid shutdown and purge, and a large stoichiometric ratio of the cathode inlet improved the purge effect but enhanced the energy consumption of the air compressor. Neither too high nor too low purge pressure could blow out the liquid water more efficiently and improve the ohmic resistance. During purging, current density mainly influenced the water production rate and the maintenance state of stack temperature. With a higher stack temperature, purge process with a small current density could quickly reduce the stack humidity and enhance the ohmic resistance. Accelerating the opening rate of the anode purge valve could not effectively accelerate purge effect of the stack, while extending the valve closing time could improve hydrogen utilization. Future studies to investigate the combined impact of these factors on the fuel cell system shutdown purge will be carried out to develop the most effective strategy.