  A model for impedance of a PEM fuel cell cathode catalyst layer under
simultaneous application of potential and oxygen concentration perturbations is
developed and solved. The resulting expression demonstrates dramatic lowering
of the layer impedance under increase in the amplitude of the oxygen
concentration perturbation. In--phase oscillations of the overpotential and
oxygen concentration lead to formation of a fully transparent to oxygen
sub--layer. This sub--layer works as an ideal non polarizable electrode, which
strongly reduces the system impedance.
