|About the Book|
Although substantial research efforts have been directed to investigate the liquid water transport in gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC), the current understanding of this important issue is unable to address the roleMoreAlthough substantial research efforts have been directed to investigate the liquid water transport in gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC), the current understanding of this important issue is unable to address the role of GDL microstructure and surface wettability on liquid water transport. In addition to the normal operation, startup of a PEFC from subzero temperatures, commonly referred as cold-start, is of great importance especially for automotive applications. The water present in the membrane and porous layers, i.e. catalyst layer (CL), micro porous layer (MPL) and GDL, may freeze at a subzero temperature and makes the startup of a PEFC difficult or impossible. To alleviate the cold-start problem, a fuel cell is typically purged with gas to remove residual water prior to engine shutdown. Liquid water transport in a PEFC GDL during gas purge sets up the initial condition of the water distribution in PEFC for cold start, and hence significantly affects the cold-start characteristics of a PEFC. In spite of its importance, liquid water transport in a GDL during gas purge is scarcely researched.-In this thesis, multi-scale investigations are carried out to obtain a fundamental understanding of liquid water transport in PEFC GDL, taking GDL microstructure and wettability into account, during flooding and gas purge. Extensive pore-network models are developed to elucidate the role of microstructure and wettability distribution on GDL flooding. The role of capillary fingering in GDL liquid water transport is discussed. The ensuing concave shaped-saturation profiles can not be explained by two-phase Darcys law based model. The need to modify the present macroscopic models based on two-phase Darcys law based models to incorporate capillary fingering phenomena in hydrophobic GDLs. X-ray microtomography is deployed to quantify liquid water in a PEFC GDL, and three-dimensional liquid water distribution during GDL drying is imaged for the first time. Detailed macroscopic purge model is developed to elucidate the gas purge mechanisms, and key factors influencing purge effectiveness are highlighted. The model predictions are compared with experimental results under various purge conditions. A good match with experiments is obtained at higher purge temperatures, whereas some differences in the HFR profiles are observed at lower temperatures. Drying front evolution in a hydrophobic GDL is studied, and the role of drying front morphology in addressing the observed differences between numerical and experimental results is presented.