Gas Diffusion Requirement

In an operating SOFCs, gases (air or fuel) diffuse through the porous electrodes to reach electrode/electrolyte interfaces. The gas diffusion rate must be fast enough to minimize the diffusion loss. Three primary processes are involved with gas diffusion in porous electrodes ordinary diffusion, Knudsen diffusion, and surface diffusion. The surface diffusion is negligible at elevated temperatures (e.g., 600-800°C). The question remains as to whether ordinary diffusion or Knudsen diffusion is the limiting step of the diffusion process. The answer resides in the pore 

The Knudsen diffusion coefficient (D ) is determined by the pore radius (r), temperature (K), and gas molecular weight (M)  

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