(171a) Drops, Slugs and Flooding in Pem Fuel Cells
Flooding in Polymer Electrolyte Membrane (PEM) fuel cells has been examined with a single channel cell oriented horizontally and vertically. Flooding occurs due to liquid water accumulation in the gas flow channel inhibiting mass transport between the gas flow channel and the membrane/electrode interface. Water formed at the cathode/membrane interface flows through the largest pores in the GDL when driven by a hydraulic pressure sufficient to overcome the hydrophobic surface energy of the GDL's pores. Liquid water drops grow on the surface of the GDL until gravity or shear forces overcome the interfacial tension between the water in pore and the drop on the surface. After drops detach from the pores they aggregate and form slugs. When the cathode flow channel is horizontal with the cathode facing down the liquid water falls away from the GDL and collects on the wall of the flow channel and is pushed out with minimal effect on the local current density in the fuel cell. When the cathode faces up the water slugs accumulate on the GDL surface downstream in the flow channel causing the local current density to periodically fluctuate as water slugs accumulate and are pushed from the flow channel. Liquid water droplets fall by gravity when the fuel cell is vertically oriented. When the gas flows are downward the local fuel cell currents are stable. In contrast, when the gas flow opposes gravity it creates a pulsating flow of liquid slugs in the flow channels. The slugs block the gas flow and cause periodic local current fluctuations. Flooding in PEM fuel cells is gravity dependent and the local current densities depend on dynamics of liquid slugs moving through the flow channels.