oxygen electrode, thus creating a zero-flux condition for the water. The other part of the model was designed to represent the transfer in the gas phase in the hydrogen compartment. Here a two-dimensional stationary model was used, since the characteristic reaction time for this part of these system is much shorter than for the electrolyte. Polar coordinates were used because of the circular geometry of the cell. The gas compartments both contain plastic grids that can be treated as porous media, so the gas flow was assumed to follow Darcy's law; this relates the gas flow pattern to the pressure distribution, with a source term to account for hydrogen consumption and water flux through the electrode. The transfer through the compartment of water and hydrogen in the gas phase was represented by diffusion-convection equation. The two parts of the model are joined together by the value of the of the water flux through the hydrogen electrode. At each of the grid cells of the gas model, there is a one-dimensional model for the electrolyte; thus the electrolyte transfer parallel to the electrode is neglected. The overall model allows the time-dependent behavior of the system to be calculated; it is found that the cell should reach stationary operation after a matter of minutes. This model allows the distribution of hydrogen humidity across the electrode surface to be calculated for different operating conditions, particularly as a function of hydrogen flow rate. One surprising result of these calculations was the low value for the mass transfer coefficient representing gas phase transfer through the electrode. Otherwise, the results of the calculations do show a slight drying of the electrode near the gas inlet that seems to be physically realistic.
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