Unexpected over-linear transport loss in low-Pt PEM fuel cells has been a subject of numerous studies and discussions in literature. Most of the authors agree that these losses are due to oxygen transport in the Nafion film covering Pt/C agglomerates in the cathode catalyst layer (CCL). We develop a model for PEM fuel cell impedance, which takes into account oxygen transport through the film. The model is fitted to experimental impedance spectra of a low-Pt PEM fuel cell. Fitting gives the film thickness in the range of 10 to 13 nm, and the film transport resistivity decreasing from 1.2 s cm-1 to 0.2 s cm-1 as the cell current density increases from 50 to 800 mA cm-2. Fitting returns low value of the through-plane oxygen diffusivity in the CCL, indicating that the low-Pt electrode is partially flooded.
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