Water Transport in the Gas Diffusion Layer of PEM Fuel Cells: An Energy Approach

Abstract

The design of a fibrous gas diffusion layer (GDL) that manages water effectively over the full range of fuel cell operating conditions has to account for the morphology of the gas-liquid interfaces under complex geometrical and wetting configurations, encountered within the fiber matrix. An energy minimization approach is taken to capture the interface evolution of water breaking through the pores. Water in contact with fibers can take the shape of an axisymmetric or non-axisymmetric droplet depending on the fiber diameter, the droplet volume and the contact angle. The energy barrier between the two morphologies as well as the percolation pressure of water through the pores, are determining factors in the process of liquid fingers formation. We consider a droplet breakthrough model which sets the contact line delimiting the protruding gas-liquid interface along both wetting and non-wetting fibers, yielding mixed wetting conditions. The breakthrough pressure of water through a mixed wetting pore is determined to be 40% larger than through a wetting pore, a ratio which is consistent with experimental results. ESEM images indicate that the model captures accurately the interface evolution while accounting for the specific geometrical features and wetting properties of the GDL. It is advantageous to corroborate a realistic pore model such as the one proposed in this study, with existing models including the pore network model to characterize and design optimal transport features of the porous medium.