Electric and electrochemical properties of catalytically active oxygen electrode materials

The electrical conductivity has been investigated of some oxygen ion and mixed conducting materials. Electrodes are prepared from thin sputtered layers of these oxides combined with a small Au or Pt strip. The kinetics of the oxygen reaction has been studied for temperatures of 820–1020 K and PO2 values of 10−4 - 1 atm. respectively. Highest oxygen ion conductivities are found in solid solutions of 0.7 Bi2O3−0.3 Tb2O3.5 followed by 0,7 CeO2 - 0.3Tb2O3.5 (CT-30) and (TbxGd1−x) with pyrochlore structure and x=0 (TGZ-0). Highest, p-type, electronic conductivities are found in CT-30 and TGZ with x=1. TGZ-0 was used in all experiments as the solid electrolyte. Combined current-overvoltage (ν) and impedance measurements show that with Au strips the nature of the oxidic materials has a pronounced effect on especially the cathodic polarization, the effect being larger with larger ν. Introduction of p-type conductivity does not decrease the electrode resistance. The Butler-Volmer equation is obeyed with effective cathodic and anodic transfer coefficients close to 0.5 and 1.5 respectively while the effective exchange current varies as the 0.5 power of PO2. These results can be interpreted by a mechanism where charge transfer in the metal-oxide region is rate controlled by surface diffusion on the oxide.