| Electrode flooding is one of the major causes of poor performance of Proton-Exchange-Membrane (PEM) fuel cells. Understanding the factors controlling electrode flooding is critical for further improvement. In this study, a two-phase flow model was developed to study the effects of liquid water in the porous backing layer on the performance of a cathode of a PEM fuel cell using interdigitated flow fields. By using parameters fitted from one set of experimental data, the two-phase flow model developed in this work is capable of predicting experimental results at other flow rates and temperatures with good agreement. The model was then used to evaluate two flow field design parameters: the number of channels and shoulders, and the ratio between the channel and shoulder widths. For constant pressure drop operation, more channels and shorter shoulder widths are beneficial due to better oxygen transport and water removal caused by higher gas flow rates.; Next, the effects of misalignment in the main Membrane-Electrode-Assembly (MEA) on the measurement of reference electrode were investigated. The potential that the reference electrode measures is greatly affected by the misalignment of the two edges of the main MEA, because the potential probe is not in the main current flow path due to geometric restrictions by the thin electrolyte. With only a slight misalignment, the reference potential is overwhelmingly masked by the edge potential of the protruding electrode. Slow electrode kinetics can significantly magnify edge effects. The reference sensing point varies significantly with the misalignment factor, and cell voltage/current, which makes it not suitable for quantitative analysis. With special configuration of the main MEA, the reference electrode can be used to measure the internal electronic resistance of the electrode, and also as a qualitative tool to detect electrode flooding.; Finally, using the pressure drop associated with interdigitated flow field as a diagnostic tool, the effects of electrode flooding were investigated. The pressure drop between inlet and outlet channels can be used to monitor the liquid water content in the porous electrodes because of the strong dependence of the gas permeability of the porous electrodes on the liquid water content. By using this approach, concrete evidence is provided to show that inadequate water removal causes liquid water buildup in the cathode, and excessive flooding can severely reduce the cell performance. It was also found that liquid water transport processes are slow and hysteresis of water content exists during water imbibition and drainage cycles. |
