Water Flow in, Through, and Around the Gas Diffusion Layer

The gas diffusion layer (GDL) is an essential element of the electrodes in polymer electrolyte membrane (PEM) fuel cells. The GDL makes electrical contact with the catalyst layers to provide a larger area to carry the electronic current from the catalyst to the bipolar plates reducing the resistance for current flow. The GDL must be porous to permit the flow of reactant gas from the gas flow channel to the catalyst layer and flow of water from the catalyst layer to the gas flow channel. The GDL must be electrically conductive, but it is also desirable for it to be hydrophobic to keep liquid water from filling the pores and blocking gas flow to the catalyst layer. Figure 1 is a schematic of the flow paths for the reactant gases and liquid water in the GDL coupled with transport in the gas flow channels, proton transport in the PEM and electron transport through the external load. There have been numerous studies characterizing the pore structure of GDL materials [1–12]. Gostick et al. have recently written a very comprehensive review of studies characterizing gas diffusion media [7]. Most studies of the GDL have focused on water flooding the GDL [1, 5, 8, 13–18]. Several investigators have reported saturation curves from imbibing and draining water into the gas diffusion media; this data has been analyzed to determine pore volumes, pore radii and the internal contact angle of GDL materials. Several studies have also examined the hysteresis between the imbibition and draining curves attributing it to contact angle hysteresis and geometrical effects [3, 4, 6, 10, 19]. Benziger et al. [20, 21] and coworkers introduced water penetration experiments to characterize GDL materials. Their results showed that carbon fiber GDL materials are hydrophobic and liquid water penetrates the largest pores. They suggested that liquid water only penetrates a few large pores and the smaller pores, that comprise the majority of the pore volume, remain free of liquid water and allow gas to be transported from the gas flow channel to the catalyst layer. Other researchers have employed NMR and neutron scattering imaging to confirm that liquid water penetrates only the largest pores [22–24]. The incipient water penetration results indicate that the tail of the pore size distribution is critical for controlling water transport.