Work function changes of polarized electrodes on solid electrolytes

The work function of a metal used as an electrode on a solid electrolyte has been reported to be closely related to the catalytic activity of this metal toward a variety of chemical reactions. A work function change (Delta phi), and in such cases also a modification of the catalytic activity of the electrode (NEMCA), can be achieved by polarization, which probably leads to ionic species spilled over the metal surface. The quantitative relation between Delta Phi and the overpotential h of the electrode, however, is still a matter of debate. In this contribution a well-defined model is presented that allows us to quantitatively calculate the influence of the overpotential on Delta Phi for different spillover species and kinetic situations. Parameters are (i) the surface potential step for complete coverage of the spillover sites, (ii) the equilibrium surface coverage of the spillover species, and (iii) the charge of the spillover species. For O- as the spillover species it is shown that frequently S-shaped curves occur for the Delta Phi shd relation and even Delta Phi approximate to 2e eta may result for certain parameters. In the case of O2- spillover a moderate equilibrium surface coverage and a reasonable number of surface sites indeed yield the often discussed relationship Delta Phi e eta. (c) 2005 The Electrochemical Society. All rights reserved.