Diagnostic Analysis of the Oxygen Fraction at the Boundary Between Cathode Flowfield and Gas Diffusion Layer in the PEMFC

Abstract

Enhancement of the fuel cell performance at higher current densities is important to improve the power density and reduce the cost of proton exchange membrane fuel cell (PEMFC) system. Mass transport overpotential is the major barrier to achieving high performance at high current density. The overpotential at the cathode is significantly large and the oxygen partial pressure in the oxygen reduction reaction (ORR) area is vital. Condensed water in the flowfield and the gas diffusion layer (GDL) reduces oxygen transport to the ORR area. Direct investigation of oxygen transport has been limited by an inability to resolve water saturation dependent properties. A novel diagnostic method to analyze the boundary between flowfield and GDL surface is required. A measurement of the oxygen partial pressure in the flowfield while the fuel cell is operating was introduced [1-3]. This method uses oxygen sensitive fluorophore materials whose fluorescent luminescence is a function of oxygen quenching as described by the Stern-Volmer equation. In the previous work, the oxygen quenching rate was modified in order to enable in-situ oxygen partial pressure measurement on the surface of the cathode GDL with typical operating conditions of an automotive PEMFC [3]. In this work, a computational fluid dynamic (CFD) based two-phase fuel cell model was newly developed to be validated with measured oxygen fractions. Figure 1 shows the geometric configuration of this tested single cell. Figure 2 shows that simulation of oxygen partial pressure distribution shows similarity to the previous experimental data. Figure 3 shows measured oxygen partial pressure on the cathode GDL surface along with fraction of flowfield channel length. This oxygen fraction is deviated from the average stoichiometric line. The spatial variability of current density, which is proportion to the oxygen consumption rate, is considered to be a large contributor. Combined empirical and modeling approaches enable to gain the mechanistic understanding of two-phase fluid flow in the flowfield and the surface of GDL, including water condensation and implication with the fuel cell operating conditions. Further analysis will be discussed.