KINETICS OF OXYGEN, PLATINUM STABILIZED ZIRCONIA AND OXYGEN, GOLD/STABILIZED ZIRCONIA ELECTRODES UNDER EQUILIBRIUM CONDITIONS/

The kinetics of electrodes of the type oxygen, noble metal/stabilized zirconia were investigated electrochemically under equilibrium conditi ons. Single-crystal cubic, polycrystalline cubic, and polycrystalline tetragonal zirconia were chosen as electrolyte materials; platinum and gold were employed as metal components. An electrode setup including a working electrode with a massive metal contact was developed and fou nd to be suitable for well-reproducible long-term measurements. Impeda nce spectra were recorded in the temperature range from 773 to 1173 K with an oxygen partial pressure range from 1 to 101,325 Pa using a low excitation voltage. For platinum, the results indicate a uniform reac tion mechanism which involves surface diffusion of dissociatively adso rbed oxygen on the metal surface as rate-determining step. From this, the determination of characteristic thermodynamic data is possible. Fo r gold, the results indicate a change in the reaction mechanism. The r esults are consistent with one mechanism, in which diffusion limitatio n of dissociatively adsorbed oxygen on the metal surface occurs, and a competing mechanism, in which transfer limitation of oxygen adsorbed in direct vicinity of the three-phase boundary takes place. The compar ison of the different electrolytes does not exhibit significant differ ences concerning the electrode kinetics.