  Analytical model for impedance of oxygen transport in the gas--diffusion
layer (GDL) and cathode channel of a PEM fuel cell is developed. The model is
based on transient oxygen mass conservation equations coupled to the proton
current conservation equation in the catalyst layer. Analytical formula for the
"GDL+channel" impedance is derived assuming that the oxygen and proton
transport in the cathode catalyst layer (CCL) are fast. In the Nyquist plot,
the resulting impedance consists of two arcs describing oxygen transport in the
air channel (low-frequency arc) and in the GDL. The characteristic frequency of
GDL arc depends on the CCL thickness: large CCL thickness strongly lowers this
frequency. At small CCL thickness, the high-frequency feature on the arc shape
forms. This effect is important for identification of peaks in distribution of
relaxation times spectra of low--Pt PEMFCs.
