Purges are essential in fuel cells to avoid voltage drops caused by water flooding and/or CO poisoning. Indeed, liquid water accumulation in the Gas Diffusion Layers leads to reduce the cell voltage by hindering reactive gas access to the electrodes. On the other hand, inert gas also accumulates rapidly at the anode, especially in dead-end mode. A very low concentration of CO is sufficient to poison the electrodes.
The need and the effects of the anodic and cathodic purges on voltages are highlighted experimentally in figure 1. Different cells in the stack are affected by the anodic and cathodic purges respectively. Thus, both purges are needed to keep the cell in optimal conditions.

A complete model of mass transfer in the cell is developed in order to simulate the transient gas distribution and the cell voltage. Time responses of CO and water accumulations are computed, respectively at the anode and cathode sides. Transient water balance in the membrane is also studied to characterize the membrane hydration influence on cell voltage.

Purges frequencies are almost linearly dependent on the current density, which impose the reactive gas consumption and the water production in the cell. As a result, the purges must be done frequently to avoid CO poisoning at the anode. Even though the water balance transient phase duration is longer, the membrane water content reaches 75% of its steady state value after a few seconds. Purges at the cathode are also needed, but less frequently than at the anode side.