IN-SITU PHOTOELECTRON-SPECTROSCOPY OF OXYGEN ELECTRODES ON STABILIZEDZIRCONIA

Evaporated silver films as an oxygen electrode material were studied i n-situ and ex-situ by XPS and UPS under UHV conditions. The experiment al set-up consisted of a miniaturised solid stare galvanic cell with y ttria stabilised zirconia as solid electrolyte and a solid oxide refer ence electrode. The oxygen activity at the working electrode was chang ed by controlling the electrode potential versus the reference electro de consisting of a metal/metal oxide system Me/MeO (with Me = Pd or Fe ). The electrode surface was investigated by XPS and UPS under cathodi c polarisation with low oxygen activity as well as under anodic polari sation with steady-state oxygen evolution. The measurements have been performed between 720 K and 1100 K. With XPS, several oxygen species h ave been detected at the silver surface. Their concentration depends o n the applied electrode potential. A species with low XPS binding ener gy 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 ca thodic polarisation, the oxygen species at low binding energy disappea red, but a second oxygen species at higher XPS binding energy grew in intensity with increasing cathodic polarisation. In view of the reduci ng 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 surfac e was determined as a function of the applied overpotential. The work function of silver increased by about 1.6 eV in changing from a cathod ic polarisation of -1.5 V to an anodic polarisation of +1.5 V. This pr onounced change is caused by a concentration decrease of adsorbed OH a nd a corresponding increase of adsorbed O atoms leading to a higher su rface electron affinity. Applying high cathodic as well as anodic pola risation led to the formation of insulated silver particles within ope n pores of the silver layer on the surface of the zirconia. These part icles which are electronically coupled to the Fermi level of the under lying zirconia cause a superposed but slightly shifted second UPS spec trum with the shift depending on the effective overvoltage. The as det ermined overvoltage at the silver electrode depends linearly on the ce ll voltage.