Investigation of the Representative Area of the Water Saturation in Gas Diffusion Layers of Polymer Electrolyte Fuel Cells

During operation of polymer electrolyte fuel cells, water condenses in the porous structure of the gas diffusion layer (GDL). The condensed water limits efficiency and durability of the fuel cell. For optimization of the porous materials, understanding of the structure and characteristic length scale of the liquid water distribution is of crucial interest. X-ray tomographic microscopy was employed to image in situ the condensed water in GDLs of the type Toray TGP-H-060. It was found that the local water distribution pattern, created by the electrochemical reactions in the fuel cell, is mainly driven by the substrate structure on the micrometer scale, as repeatedly generated water patterns in the same structure have a local correlation. The concept of the representative equivalent area (REA) was employed to characterize the dry GDL structure and to identify the characteristic length scale of the liquid water phase. The dry fiber structure was found to have a representative area of 0.50 mm(2). A similar area of 0.35-0.60 mm(2) is necessary for representing the water distribution characteristics with an error of 10% in a GDL with a liquid saturation of 42-49%. However, at a lower liquid saturation of 22-25% the area increases to 1.35-1.60 mm(2) which indicates that the REA of the liquid saturation cannot be derived from the dry structure only.