A numerical model predicting liquid water saturation within the cathode electrode of a proton exchange membrane fuel cell

A two-phase, half cell, model of a proton exchange membrane fuel cell cathode has been developed with emphasis on the liquid water saturation within the three porous layer structure, i.e. the catalyst layer, the micro-porous layer and the gas-diffusion layer. The model was run under varying current density, operating temperature, relative humidity and gaseous flow rate. The results show that the calculated liquid saturation profiles strongly depends on the local distribution of these variables which are highly coupled. For example, an under-humidified gas feed entering into the cell, first undergoes through the evaporation regime at low current densities, followed by liquid-water generation at moderate current densities, and finally evaporation of the liquid-water at higher current densities due to increase in the cell temperature. The increase in air flow rate enhances the liquid-water saturation within the catalyst layer, near the channel area, and reduces its amount under the ribs. The reduced liquid-saturation under the ribs is due to higher local temperature because of increased reactant transport that results in its evaporation. In summary, prediction of liquid-water saturation within the cathode is a highly complicated phenomenon that is strongly coupled to the above variables.