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

A macrohomogeneous model that was studied in a previous publication under s tationary conditions is used to calculate the small-signal dynamic response of the cathode catalyst layer in polymer electrolyte fuel cells. Within th is approach the effects of reaction kinetics and double layer capacitance a t the dispersed catalyst I electrolyte interface, proton conductivity of th e electrolyte network within the layer and oxygen diffusion through the gas -pore space are studied. The analytical expressions derived reveal relation ships between the structure of the layer and impedance spectra. Particularl y strong dependences of the differential resistivity on the electrode compo sition appear if either proton transport or oxygen diffusion dominate the v oltage losses. This happens for compositions that are close to the percolat ion 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 lo w frequency domain a semicircular part arises. These results may help to di stinguish the contribution of the catalyst layer from the contribution of o ther fuel cell components and characterize it quantitatively. (C) 1999 Else vier Science S.A. All rights reserved.