ELECTRODE POLARIZATION AT THE AU,O2(G) YTTRIA STABILIZED ZIRCONIA INTERFACE .2. ELECTROCHEMICAL MEASUREMENTS AND ANALYSIS

The impedance of the Au, O2(g)/yttria stabilized zirconia interface has been measured as function of the overpotential, temperature and oxygen partial pressure. At large cathodic overpotentials (eta < - 0.1 V) and large anodic overpotentials (eta > + 0.1 V) inductive effects are observed in the impedance diagram at low frequencies. Those inductive effects result from a charge transfer mechanism where a stepwise transfer of electrons towards adsorbed oxygen species occurs. A model analysis shows that the inductive effects at cathodic overpotentials appear when the fraction of coverage of one of the intermediates increases with more negative cathodic overpotentials. The steady state current-voltage characteristics can be analyzed with a Butler-Volmer type of equation. The apparent cathodic charge transfer coefficient is close to alpha-c = 0.5 and the apparent anodic charge transfer coefficient varies between 1.7 < alpha-a < 2.8. The logarithm of the equilibrium exchange current density (I0) shows a positive dependence on the logarithm of the oxygen partial pressure with a slope of m = (0.60 +/- 0.02). Both the apparent cathodic charge transfer coefficient and the oxygen partial pressure dependence of I0 are in accordance with a reaction model where a competition exists between charge transfer and mass transport of molecular adsorbed oxygen species along the electrode/solid electrolyte interface. The apparent anodic charge transfer coefficients deviate from the model prediction.