In this paper. The effect of overflow on PEM fuel cell is analyzed. The model includes essential processes such as physical principles and electrochemical processes within the membrane, catalyst layer, electrode and the flow channels. Results obtained by this method show a well agreement with those of the experiments. Like gases, water is assumed to diffuse into the cathode and the catalyst layer. Overpotential has been ignored in reactions occurred in the catalyst layer of the anode side. The present model assumes a reasonable amount of water within the catalyst layer and electrode void fraction, which is explained herein. The existing water within the gas diffusion layer (in void parts of the diffusion layer) makes diffusion difficult for the oxygen, and to contribute in the reaction. Lack of oxygen results in and decrease in electrochemical reactions rate and consequently, reduces in the produced electrical current in the cell. Therefore, the oxygen molecules should diffuse through the liquid water instead of diffusing through gases in voids. There is no direct experimental method to evaluate the amount of liquid water existing within voids. The only way of prediction is by modeling, which takes into account the affect of overflow and relates that into the increases in loses. When these results well agree to each other, a polarization curve can be produced, which can be expected the amount of over flow in the model is in good agreement with the real data.
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