ELECTRODE POLARIZATION AT THE AU, O2(G)/FE IMPLANTED YTTRIA-STABILIZED ZIRCONIA INTERFACE

Ion implantation has been applied to modify the surface properties of yttria-stabilized zirconia (YSZ). A three-electrode cell was used for measuring steady state current-overpotential curves and for determining the electrode impedance. An increase of the equilibrium exchange current density at the Au, O2(g)/yttria stabilized zirconia interface with a factor 10-50 has been observed after the implantation of 15 kV Fe-56 up to a dose of 8 X 10(16) at. cm-2. This increase results from a broadening of the active surface area due to an increase in the electronic conductivity of the Fe implanted YSZ surface and from an increase of the fraction of coverage of the adsorbed oxygen molecules. The double layer capacitance of the Au, O2,g/YSZ interface increases with a factor 10-100 after the Fe implantation. This is most likely due to the variable oxidation state of the implanted Fe ions, thus providing an additional way for charge accumulation. In comparison with the not implanted Au, O2,g/YSZ interface no changes in the rate-determining steps of the oxygen exchange mechanism occur after Fe implantation. Similar apparent charge-transfer coefficients have been determined. A slight decrease in the oxygen partial pressure dependence of I0 is observed. The experimental results can still be explained with a reaction model where the charge-transfer process is in competition with the surface diffusion of molecular adsorbed oxygen species along the noble metal-solid electrolyte interface. At cathodic and anodic overpotentials inductive effects appear at low frequencies in the impedance diagram. The inductive effects result from a charge-transfer mechanism where a stepwise transfer of electrons to adsorbed oxygen species takes place.