Concentration Polarization or Mass Transport Losses

The oxygen transport is based on several mechanisms. The gaseous reactants enter the fuel cell typically through the gas channels, and then diffuse through the GDL and into the catalyst layer. The gases diffuse into the catalyst layer either to the catalyst surface where they react or are dissolved into the ionomer in the catalyst layer. The gas phase mass transport in the GDL is due to diffusion governed by Pick s law, in which the rate of diffusion is a function of the concentration gradient, the thickness of the GDL, and the diffusion coefficient for the species [21]. 

As the pore size decreases, the Knudsen diffusion model may become important. This model describes diffusion when the gas molecules collide more often with the pore walls than with each other, and it is these molecule-pore wall interactions that start to dominate the rate of diffusion. Knudsen diffusion should be considered when pore diameters are typically less than 100 nm under atmospheric conditions, and is important in the catalyst layer and possibly in the GDL [21]. 

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