A Model for PEM Fuel Cell Impedance: Oxygen Flow in the Channel Triggers Spatial and Frequency Oscillations of the Local Impedance

A physical model for impedance of a PEM fuel cell cathode side is developed. The model takes into account oxygen flow in the cathode channel. Analytical solution to model equations reveals an effect of spatial oscillations of the local impedance Z loc along the channel coordinate z . The oscillations arise due to interference of the local and transported down the channel perturbations of the oxygen concentration. With the growth of the frequency of the exciting signal, the domain of spatial oscillations of Z loc shrinks toward the oxygen channel inlet; a process, analogous to the skin–effect in classic electrodynamics. An expression for the characteristic width of the skin–layer is derived. The spatial oscillations are accompanied by the oscillations of Z loc along the frequency axis ω. The amplitude of Z loc oscillations decreases exponentially along z and ω, with the characteristic scale of the exponent dependent on the oxygen diffusion coefficient D b in the gas–diffusion layer. This effect enables determination of D b from the local cell impedance measured close to the channel outlet at a real operating stoichiometry of the oxygen flow.