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. Electrochemical impedance spectroscopy (EIS) is, perhaps, the most powerful tool for in situ characterization of fuel cells.1,2 In spite of impressive progress in developing physical models for PEMFC impedance, 3-15 cell and stack developers still routinely use simple transmission line models (TLMs) for understanding the impedance spectra.16,17 A great advantage of TLM is simplicity and fast operation, which enables rapid characterization of the cell components in terms of TLM resistivities and capacitances.18 However, relation of TLM parameters to the physical transport and kinetic coefficients of the cell is beyond the scope of this approach.A much more informative physical characterization of the cell can, in principle, be done with the physical impedance models; however, these models are typically slow for accurate least-squares (LS) fitting of experimental spectra. A core element of numerical impedance models is a strongly nonlinear transient model for the cathode catalyst layer performance.19-22 A linearized and Fourier-transformed version of this model leads to the complex-valued boundary-value problem, which in the general case of arbitrary cell current can only be solved numerically. 23 The presence of the boundary-value problem solver makes the respective fitting code slow for massive processing of experimental results.An alternative approach is based on analytical solutions for the impedance.22,24-26 Based on ideas developed in Refs. 27-29 this approach aims at deriving analytical solutions for the linearized version of the cell performance model. If the cell current density is small, these solutions can be obtained and a closed-form expression for the system impedance Z can be derived. Analytical relations for Z typically include the Tafel slope of the oxygen reduction reaction (ORR), the proton conductivity of the cathode catalyst layer (CCL), and the oxygen diffu...

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We report a model for HT‐PEMFC with the detailed description of transport processes in the cathode catalyst layer (CCL). The model is compared to our previous model, which assumed ideal oxygen and proton transport in the CCL. Calculations show that close to the oxygen channel inlet, the CCL works in the regime limited by poor proton transport, while close to the outlet, the CCL operation is determined by poor oxygen transport. The model is validated by fitting the experimental polarization curve; the CCL transport and kinetic parameters resulting from fitting are discussed.

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