Gas-diffusion process in a tubular cathode substrate of a SOFC - II: Identification of gas-diffusion process using AC impedance method

The effects of cathodic dc bias, bulk p(O2), and effective O-2- diffusivity on ac impedance spectra of Siemens Westinghouse Power Corporation's cathode- supported solid oxide fuel cells were systematically studied over a temperature range of 800 to 1000degreesC. It was found that the activation process dominated the overall electrode kinetics at 800degreesC, by which the applied dc bias reduced the electrode resistance considerably. With increasing the temperature to above 900degreesC, the activation process became effectively activated, leading to a visible arc at the lowest frequency on the impedance spectrum, which is relevant to the pore gas- diffusion process. Under this circumstance, lower bulk p(O2) and lower effective O-2 diffusivity were shown in ac impedance spectra to increase pore gas- diffusion polarization as predicted by the gas- diffusion theory shown in part I of this paper. DC bias was also found to greatly affect the gas- diffusion process as a result of increased dc current. (C) 2004 The Electrochemical Society.