Characterization of polymer electrolyte fuel cells using AC impedance spectroscopy

The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large de currents. The impedance spectrum of the air cathode is shown to contain two features: a higher frequency loop or are determined by interfacial charge-transfer resistance and catalyst layer properties and a lower frequency loop determined by gas-phase transport limitations in the backing. The lower frequency loop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass-transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.