THE OXYGEN-TRANSFER PROCESS ON SOLID OXIDE NOBLE-METAL ELECTRODES, STUDIED WITH IMPEDANCE SPECTROSCOPY, DC POLARIZATION AND ISOTOPE-EXCHANGE

The electrochemical oxygen transfer process at the yttria stabilized z irconia (YSZ) and Fe-implanted YSZ, and at the erbia stabilized bismut h oxide (BE25) surface is studied with dc polarization and impedance s pectroscopy using gold electrodes, and with O-18 gas phase exchange. T he surface modification by Fe-implantation increases the exchange curr ent density up to a factor of 50, but analysis of the impedance spectr a at different polarization levels indicates that the type of electrod e reaction is not changed by the implantation. Inductive effects at ca thodic polarizations are interpreted with a stepwise transfer of elect rons. Isotope exchange experiments show an increase in adsorption/reac tion sites at the surface after implantation. The high exchange curren t density, I0, for BE25 is independent of type of electrode but does d epend on electrode.morphology. I0 can be equated to the surface oxygen exchange rate, indicating that the entire electrolyte surface is acti ve in the electrode exchange process. Qualitative interpretation of th e impedance spectra measured at different levels of polarization resul ts in a model where adsorbed oxygen species diffuse over the oxidc sur face, while charge transfer occurs across the surface.