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Capillary and multiphase mass transfer properties of fuel cell electrode materials

Jeffrey Gostick, Chemical Engineering, University of Waterloo, Canada

The formation of liquid water inside polymer electrolyte membrane fuel cells is an ongoing technical challenge. Water is produced by the electrochemical reaction in proportion to the current density, so efforts to increase fuel cell power density must provide for effective removal of product water. Failure to do so results in excessive accumulation of liquid water in the porous electrode which severely hinders gas phase transport of reactants, leading to concentration polarization and even mass transfer limited currents. Understanding the behavior and impact of liquid water inside the fuel cell electrode has been hampered by a lack of experimental and modeling techniques appropriate for these unique porous materials. The fibrous gas diffusion layer materials, for instance, are very thin (< 400 microns), highly porous (> 80%), anisotropic, compressible and chemically heterogeneous. Numerous experimental approaches have been developed to measure properties such as the permeability tensor and water-air capillary pressure curves. The experimental findings have been used to develop a pore network percolation model of the fuel cell electrode in an effort to predict multiphase transport properties such as relative permeability and effective gas diffusivity. The pore network model has been extended to simulate fuel cell operation and estimate limiting current as a function of electrode water content. Many important findings have resulted from this investigation, such as the role of anisotropy on liquid water distribution, the importance of high aspect ratio of percolation properties, and the dominating effects of neutral wettability on capillary behavior.

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