Analysis of Oxygen-Transport Diffusion Resistance in Proton-Exchange-Membrane Fuel Cells

The quantification procedure of oxygen-transport resistances for different fuel-cell layers and phenomena is described. The total transport resistance is obtained from limiting-current measurements under conditions where oxygen diffusion is dominant (i.e., high flow rates, small cell size, humidified but subsaturated feeds, and low feed oxygen partial pressure). By systematically varying the experimental conditions, the contributions of molecular and Knudsen diffusion and permeation through the ionomer film covering the catalyst-layer agglomerates are determined. It is found that the ionomer-film resistance is dominant, especially at lower temperatures and lower Pt loadings. The calculated film properties through the ionomer hint that it is much more resistive than the bulk membrane for state-of-the-art cells.