(Invited) Limiting Current As a Tool to Study Oxygen Transport in PEM Fuel Cells
The study of oxygen transport in PEM fuel cells requires consideration of water transport as well. Because water can appear in both vapor and liquid form, heat transport must also be considered. The flow-field channels, the diffusion medium, the micro-porous layer, and the electrode all influence transport in different ways, and it is desirable to be able to understand transport limitations in each of these components separately. We will explain how oxygen transport limiting current can be used to study transport in all of these components and briefly outline a model of oxygen, water, and heat transport that arose from the study of limiting current.
Fig. 1 shows polarization curves taken with a 5cm2differential cell, using different dilute concentrations of oxygen in nitrogen for the cathode gas feed. At each concentration of oxygen, the polarization curve reaches a limiting current at which the curve becomes vertical. (The current increases occurring below 0.1 V are due to hydrogen evolution and can be ignored in the determination of limiting currents.)
Fig. 2 shows a plot of oxygen transport resistance versus limiting current for the polarization curves shown in Fig. 1. Three different regions are apparent. The dry region occurs at low current densities when water production is small enough so that all water can diffuse through the DM/MPL in vapor form. In the transition region, water production becomes large enough so that condensation starts to take place; the liquid water obstructs oxygen transport and increases transport resistance. At large enough current densities, in the wet region, oxygen transport resistance has stabilized at a higher value, indicating that beyond this point the cell is able to manage increasing water production without further impeding oxygen transport towards the cathode.
It will be shown that different DM/MPL combinations exhibit different plots of the type shown in Fig. 2. Modeling shows that oxygen transport resistance is determined by a combination of the materials’ ability to transport both heat and water away from the electrode, and a procedure will be described for extracting oxygen transport parameters from limiting current data. A model of water transport in both vapor and liquid form in hydrophobic gas diffusion layers is also formulated, and the physical motivation for the model will be discussed.
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