Gas diffusion impedance in characterization of solid oxide fuel cell anodes

Impedance spectra obtained on high-performance (<0.1 Ohm cm(2)) Ni/yttria stabilized zirconia (YSZ) cermet anodes at 1000 degrees C in H(2)/H(2)O/inert gas mixtures are considered. An impedance are with a characteristic frequency of about 10 to 100 Hz is related to finite diffusion limitation by considering the dependence on water partial pressure, apparent thermal activation, and effect of the inert gas component. By experiments in different test geometries the diffusion zone is demonstrated to be a stagnant gas layer outside the anode structure. An estimated stagnant gas layer thickness of about 1 mm is arrived at. The diffusion impedance is demonstrated to be avoidable by using a setup geometry with two coupled electrodes in direct electrical contact through opposing sides of a contacting mesh. (C) 1999 The Electrochemical Society. S0013-4651(98)12-054-2. All rights reserved.