  A single--pore model for performance of the cathode catalyst layer (CCL) in a
PEM fuel cell is developed. The model takes into account oxygen transport
though the CCL depth and through the thin Nafion film, separating the pore from
Pt/C species. Analytical solution to model equations reveals the limiting
current density $j_N^{\rm lim}$ due to oxygen transport through the Nafion
film. Further, $j_N^{\rm lim}$ linearly depends of the CCL thickness, i.e., the
thinner the CCL, the lower $j_N^{\rm lim}$. This result may explain unexpected
lowering of low--Pt loaded catalyst layers performance, which has been widely
discussing in literature.
