The catalyst layer of a polymer electrolyte fuel cell is commonly represented in mathematical models as an agglomerate structure of carbon catalyst-support particles. There are two prevailing assumptions for the structure of the agglomerates. The first is that the pores are filled with perfluorosulfonated-ionomer (PFSI). The second is that the pores are hydrophilic and are flooded only with liquid water during operation. The objective of this work is to develop numerical models for single water-filled and ionomer-filled agglomerates in a cathode catalyst layer of a polymer electrolyte membrane fuel cell (PEMFC), and investigate the properties of oxygen transport, proton transport, and reaction kinetics. The two models provide different solutions for the distribution of oxygen and protons, and produce a different reaction profile within the agglomerate. Previous numerical water-filled ionomer models in the literature have neglected the effect of the ionomer thin film. Therefore, the results obtained for both ionomer and water-filled models could not be easily compared. In this article, the equations developed relate the assumed structure of the agglomerates to the structure of the catalyst layer (CL). Results compare the effect of the thin film thickness in the two different types of agglomerates and relate the phenomena occurring within the agglomerates to overall catalyst layer performance.

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