ELECTRODE-KINETICS OF POROUS MIXED-CONDUCTING OXYGEN ELECTRODES

In this paper we use continuum modeling to analyze the mechanism of th e oxygen reduction reaction at a porous mixed-conducting oxygen electr ode. We show that for La-0.6(Ca, Sr)(0.4)Fe0.8Co0.2O3-delta at 700 deg rees C, solid-state oxygen diffusion and O-2 surface exchange dominate the electrochemical behavior, producing effective ''chemical'' resist ances and capacitances. This behavior can be explained both qualitativ ely and quantitatively in terms of the known bulk and surface properti es of the materials. This mechanism appears to be generally valid for mixed conductors with high rates of internal mass transfer, but breaks down for mixed conductors that have poor ionic transport. Our analysi s also suggests that, for the best electrode materials, extension of t he reaction zone beyond the three-phase boundary is limited to a few m icrometers. We also show that gas phase diffusion resistance can contr ibute significantly to cell impedance at P(o2)less than or equal to 0. 1 atm.