Electrode kinetics of porous mixed-conducting oxygen electrodes

Continuum modelling was used to analyse the mechanism of the oxygen reduction reaction at a porous mixed-conducting oxygen electrode. For La0.6(Ca, Sr)0.4Fe0.8Co0.2O3-delta at 700 C, solid-state oxygen diffusion and O2 surface exchange dominate the electrochemical behaviour, producing effective "chemical" resistances and capacitances. This behaviour can be explained in terms of the known bulk and surface properties 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. For the best electrode materials, extension of the reaction zone beyond the three-phase boundary is limited to a few micron. Gas phase diffusion resistance can contribute significantly to cell impedance at oxygen partial pressures up to 0.1 atm. 26 refs.