IN-SITU PHOTOELECTRON-SPECTROSCOPY OF OXYGEN ELECTRODES ON STABILIZED ZIRCONIA

Evaporated silver films as an oxygen electrode material were studied in-situ and ex-situ by XPS and UPS under UHV conditions. The experimental set-up consisted of a miniaturised solid stare galvanic cell with yttria stabilised zirconia as solid electrolyte and a solid oxide reference electrode. The oxygen activity at the working electrode was changed by controlling the electrode potential versus the reference electrode consisting of a metal/metal oxide system Me/MeO (with Me = Pd or Fe). The electrode surface was investigated by XPS and UPS under cathodic polarisation with low oxygen activity as well as under anodic polarisation with steady-state oxygen evolution. The measurements have been performed between 720 K and 1100 K. With XPS, several oxygen species have been detected at the silver surface. Their concentration depends on the applied electrode potential. A species with low XPS binding energy was found whose intensity increased with anodic polarisation. It is attributed to oxygen species at the surface and at subsurface sites. The latter corresponds to oxygen atoms dissolved in the bulk. Under cathodic polarisation, the oxygen species at low binding energy disappeared, but a second oxygen species at higher XPS binding energy grew in intensity with increasing cathodic polarisation. In view of the reducing conditions and the presence of water, this latter oxygen signal is attributed to a steady state concentration of hydroxyl species at the surface. With UPS, the work function of electrons at the silver surface was determined as a function of the applied overpotential. The work function of silver increased by about 1.6 eV in changing from a cathodic polarisation of -1.5 V to an anodic polarisation of +1.5 V. This pronounced change is caused by a concentration decrease of adsorbed OH and a corresponding increase of adsorbed O atoms leading to a higher surface electron affinity. Applying high cathodic as well as anodic polarisation led to the formation of insulated silver particles within open pores of the silver layer on the surface of the zirconia. These particles which are electronically coupled to the Fermi level of the underlying zirconia cause a superposed but slightly shifted second UPS spectrum with the shift depending on the effective overvoltage. The as determined overvoltage at the silver electrode depends linearly on the cell voltage.