Water in PEMFC originates from two

Water in PEMFC originates from two main sources. It is either produced by ORR at the catalyst cathode or fed externally into the cell through the humidification or cooling gases. The amount of byproduct water can be calculated directly from the current density of the cell. The amount of external water is, however, more difficult to estimate since it varies depending on the temperature, pressure and flow rate of the humid/coolant gas used. In general, water transport in PEMFC involves a complex interplay of processes. In the membrane, for example, there are three water fluxes during the operation of the cell known as electroosmotic drag, back diffusion and water generation. The effect of electro-osmotic drag on the water distribution is considered to be the dominant as many hydrogen ions drag water molecules when travelling from the anode to cathode. Meanwhile, the high concentration of the water generated at the cathode forces the water stream to diffuse back into the anode. The balance between these two fluxes is critical for maintaining a uniform water distribution across the membrane and hence an optimum ionic conductivity of the membrane. The effect of back diffusion becomes more significant at high current densities and particularly when the extraction rate of water from the cathode region is extremely low.