Within sight of the operating temperature of PEMFC, an appropriate diphasic water management must be ensured. Indeed, water is needed in the cell to decrease the membrane resistance. On the other hand, water production and the low operating temperatures lead to water condensation in the Gas Diffusion Layers (GDL). This phenomenon, commonly called flooding, involves drastic voltage drops, in particular at high current density. Diphasic water transport in the cell must hence be characterized in order to anticipate such problems.

In that way, a transient pseudo-2D diphasic mass transport model is developed in order to analyse vapor condensation and water evaporation in the GDL, for any operating conditions. The cell voltage is also computed to simulate the voltage drops caused by water flooding. Two phenomena are involved: water accumulates at the electrodes and makes some active sites ineffective; at the same time, liquid water in the GDL hinders reactive gas access to the electrodes, so that the reactive gas pressure at the electrodes decreases.

The key conclusion is that critical voltage drops are observed before the electrode was fully flooded (Figure 1). Actually, the increase in the effective gas diffusion resistance caused by water accumulation in the GDL makes the reactive gas pressure decrease to zero, before the GDL was fully filled with liquid water. So, the voltage lost caused by the flooding of the active area is negligible compared to that caused by the reduced diffusion of the reactive gas to the electrodes.