Electrode polarization at the Au, O₂(g)/yttria stabilized zirconia interface: Part I: theoretical considerations of reaction model

Three different reaction models are discussed which describe the oxygen exchange reaction at the Au, O₂(g)/yytria stabilized zirconia interface. The first model assumes the charge transfer process to be rate determining. If the electron transfer to the adsorbed oxygen species occurs in a stepwise fashion low frequency inductive effects can be simulated in the frequency dispersion of the electrode impedance. If the charge transfer process is in competition with mass transport of oxygen along the Au, O₂(g)/stabilized zirconia interface the second model can predict “apparent” Tafel behaviour of the current-overpotential curve. The real charge transfer coefficients change from α_c = α_a = 1 to apparent values of α_c = 0.5 and α_c = 1.5. Due to a gradient in the fraction of coverage of the molecular adsorbed oxygen species along the Au, O2(g)/stabilized zirconia interface, the oxygen partial pressure dependence of the equilibrium exchange current density changes from I₀ ∝ P_O2¹⁴ to I0 ∝ P_O2⁵⁸. Depending on the basic charge transfer mechanism inductive effects at the electrode remain possible. The electrode impedance derived from this model under equilibrium conditions thus far revealed only capacitive effects. This makes this reaction model difficult to distinguish from the electrode impedance of a pure charge transfer process with an adsorbed intermediate. In case the mass transport process is rate determining limiting currents are predicted at moderate values of the applied overpotential. The electrode impedance then consists of a finite-length Warbung diffusion element and inductive effects cannot be predicted.