characterization and visualization of two-phase flow properties of gas diffusion layers used in a PEM fuel cell

Abstract

Due to the low-temperature operation of Polymer Electrolyte Membrane fuel cell (PEMFC), liquid water can build up in either flow channels or gas diffusion layers (GDL). Better understanding of the effect of two-phase transport properties on liquid water transport in these porous media is crucial for PEMFC performance improvement. Capillary curves representing two-phase flow properties of porous media are not readily available for the porous media used in a PEMFC because of the minute length scales and complex materials, and no clear relationship between the GDL properties and transport characteristics has been established. This thesis work was designed to address these issues. Volume displacement method was applied to measure the relationships between the capillary pressure and liquid water saturation for two commercial gas diffusion materials, Toray TGP-H-090 and Toray TGP-H-060. The impact of channel-rib structure on capillary properties of GDLs was investigated by two different configurations with different rib-channel designs. The saturation level was found to decrease as the rib-to-channel width ratio increased since less area of the GDL was available for liquid water penetration into the GDL. The effects of perfluorotetrafluoroethylene (PTFE) loadings and compression were also studied. PTFE addition to the GDL above 20wt% had little effect on the capillary curves because the fluoropolymer was not uniformly distributed on the carbon surface but thickened the existing coat of PTFE. Inhomogeneous compression distributions caused by rib/channel designs created different local physical properties. Furthermore, the capillary pressure properties in GDL were affected by unevenly distributed compression. Consequently, capillary pressure curves using average liquid saturation level and capillary pressure may not accurately describe the transport properties in porous media. Neutron imaging was used to study water distribution in both in-plane and through-plane directions in a GDL. It was shown that the liquid water saturation level in the GDL above the ribs was less than that above the channels, illustrating the role of the flow field elements on the local water distribution in the GDL. The difference in the liquid saturation level proved that higher compression level of the GDL above the ribs led to different morphological properties and, consequently, transport properties. Once liquid water breakthrough was reached, water was observed to flow through a single pathway. These results demonstrated that the assumption of isotropic transport properties of the GDLs in PEM fuel cell models needed to be reconsidered.