An improved agglomerate model with adsorption isotherm for oxygen transport in CCLs with low Pt loading

Abstract

Understanding of oxygen transport in cathode catalyst layers with low Pt loading is of great importance to the proton exchange membrane fuel cells. The resistance to oxygen transport within the catalyst layers (CCLs) increases significantly as the Pt loading decreases. Thus, it is necessary to develop an accurate continuum-scale model for the oxygen transport and reaction in catalyst layers with low Pt loading so as to guide the optimization of catalyst layers. To address this issue, a modified agglomerate model is developed in the present study. In the model, the CCLs are conceptualized as packings of agglomerate particles composed of the carbon, ionomer, and catalyst. Each of agglomerate particle is covered by a thin layer of ionomer. The main features of our model are two folds: (1) adsorption isotherm is employed instead of Henry's law, commonly used the previous agglomerate models, to depict the oxygen adsorption at the surface of ionomer; (2) non-uniform oxygen transfer flux across the ionomer layer surrounded each agglomerate is considered. The results predicted by our developed model are in good agreement with the experimental data for the polarization curve and the variations of the oxygen transport resistance with the Pt loadings. Especially, our model is superior to the previous agglomerate models in regarding to the variation of oxygen transport resistance with mass fraction of Pt in the catalyst. The modified agglomerate model developed in this work provides an effective tool for investigation of oxygen transport and reaction in CCLs with low Pt loading.