Model for investigation of oxygen transport limitation in a polymer electrolyte fuel cell

A model for oxygen transport in the catalyst layer of polymer electrolyte fuel cells was developed. The model includes oxygen transport resistance for dissolution from the gas phase into the ionomer, which is typically assumed to be negligible in conventional models. The dissolution kinetics were experimentally assessed by measuring the diffusion-limited current density, id, in a planar Pt electrode covered with thin ionomer films as a function of film thickness. The extrapolation of i(d)(-1) to zero thickness reflects the dissolution resistance. The effect of dissolution resistance is added to a conventional agglomerate model of the catalyst layer as an additional cause of transport loss. The agglomerate size was determined so that the model predicted the same diffusion-limited current density as the experimental results. The agglomerate size was close to the particle size observed in scanning electron micrographs of the cross-section of the catalyst layer prepared by focused ion beam milling. In contrast, the conventional agglomerate model must adopt an agglomerate size that is larger by one order-of-magnitude than that experimentally observed. As a result, it is reasonable to attribute the hindrance of oxygen transport in the catalyst layer to the slow oxygen dissolution at the gas-ionomer interface, rather than to the presence of large agglomerates. (c) 2012 Elsevier B.V. All rights reserved.