Measurement and Modeling of the Impedance Characteristics of Porous La1-xSrxCoO3-δ Electrodes

Porous La1-xSrxCoO3-delta (LSC) electrodes having Sr composition x=0.4 (LSC-64) and x=0.2 (LSC-82) were fabricated on Sm-doped ceria electrolytes, and studied using electrochemical impedance spectroscopy between T=650-750 degrees C and p(O2)=0.01-1.0 atm. The faradaic portion of the impedance was found to exhibit Gerischer or Gerischer-like characteristics, indicating colimitation by kinetics and transport. The characteristic resistance and frequency response was analyzed using a transport and reaction model that considers parallel bulk and surface diffusion of oxygen, as well as three-dimensional transport effects near the electrode electrolyte interface. In the case of LSC-64, measured characteristics appear to be largely consistent with a bulk transport path, based on independent measurements of the thermodynamic, kinetic, and transport properties of LSC-64. In the case of LSC-82, which has a much lower bulk vacancy concentration under the same conditions, results were inconsistent with an entirely bulk transport path. Results for LSC-82 could be rationalized assuming a parallel surface transport path, where surface mobility is governed by some kind of interstitial or adatom diffusion mechanism.