Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells

A macrohomogeneous model that was studied in a previous publication under stationary conditions is used to calculate the small-signal dynamic response of the cathode catalyst layer in polymer electrolyte fuel cells. Within this approach the effects of reaction kinetics and double layer capacitance at the dispersed catalyst I electrolyte interface, proton conductivity of the electrolyte network within the layer and oxygen diffusion through the gas-pore space are studied. The analytical expressions derived reveal relationships between the structure of the layer and impedance spectra. Particularly strong dependences of the differential resistivity on the electrode composition appear if either proton transport or oxygen diffusion dominate the voltage losses. This happens for compositions that are close to the percolation thresholds of either proton conductivity in the electrolyte network or gas-pore diffusivity. Due to proton transport limitations, a linear branch is seen in impedance spectra in the high frequency limit, whereas in the low frequency domain a semicircular part arises. These results may help to distinguish the contribution of the catalyst layer from the contribution of other fuel cell components and characterize it quantitatively. (C) 1999 Elsevier Science S.A. All rights reserved.