stable over a long period. On the other hand, when the gas flow is lower or higher than the optimum value, we observe a rapid decrease in the cell voltage. When the hydrogen flow rate is 40 I/h (i.e. above the optimum value), the voltage fall-off is due to the electrode drying out, with a brief voltage rise that could be due to an increase in KOH concentration preceding saturation. When the flow rate is below the optimum value, there is an electrolyte-dilution effect and electrode flooding. Gas and water management: Main features of the theoretical model A numerical model has been developed to represent the mass transfer phenomena within the fuel cell. It is aimed particularly at optimizing the excess hydrogen flow (see Figure 5). This model is in two main parts. Firstly, a one-dimensional transitory model describes the behavior of the electrolyte: the transfer of potassium hydroxide and water by migration, diffusion and convection, with gradual formation of a concentration gradient between the electrodes. The limiting conditions for this model are given by the conditions at the electrodes, i.e. electrochemical reactions consuming or producing water and hydroxide ions, and the gas transfer through the electrode by diffusion. It has so far been assumed that the oxygen flow is slow enough for a gas- liquid equilibrium to be maintained at the
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