Magnetic resonance imaging investigation of water accumulation and transport in graphite flow fields in a polymer electrolyte membrane fuel cell: Do defects control transport?

Water management remains a leading challenge in the implementation of polymer electrolyte membrane (PEM) fuel cells. At present there are many excellent models for the distribution of water within PEM fuel cells, but little quantitative data on the water distribution that can be compared to models. In this paper magnetic resonance imaging (MRI) is used to examine and quantify the flow of water in graphite coated flow fields in a miniature PEM (hydrogen) fuel cell. It was found as with Teflon® flow fields, that the water accumulated in waves along the bottom of the flow field. The water waves moved very slowly through the flow channels and seem to get stuck on tiny defects in the flow field. The water accumulated at the defect, until the wave nearly bridged the gap between the cathode and the bottom of the flow field. Then the water wave was pushed along to the next defect. Surprisingly, the current out of the cell was nearly constant as waves accumulated and were swept away, even though the flow was clearly not at steady state. These results show that small defects in the wall of the flow field play a critical role in water transport in the flow fields.