Oxygen transfer properties of ion-implanted yttria-stabilized zirconia

The influence of surface modification by ion implantation on oxygen transfer in yttria-stabilized zirconia (YSZ) has been studied. Implantation of 15 keV 56Fe in YSZ with a maximum dose of 8×1016 atoms cm−2 yields reproducible surface layers of approximately 20 nm deep with a maximum Fe cation fraction of 0.5 at the surface. After annealing these layers are stable up to 700–800°C. The exchange current densities for the Fe-implanted layers, measured using porous gold electrodes, are a factor of 10–50 larger than observed for not-implanted YSZ. 18O isotope exchange experiments show that for Fe-implanted samples the surface oxygen exchange rate is at least a factor 30 larger than for normal YSZ. The electrode kinetics has been studied for normal and implanted YSZ using current-overvoltage measurements and impedance measurements under bias. An electrode reaction model for the transfer of oxygen has been developed. This model is able to explain the low frequency inductive loop in the impedance diagram which is observed at high cathodic and anodic polarizations for implanted as well as normal YSZ.\u

Electrode polarization at the Au, O2(g)/yttria stabilized zirconia interface. Part II: 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 (η < −0.1 V) and large anodic overpotentials (η > +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 c=0.5 and the apparent anodic charge transfer coefficient varies between 1.7< a<2.8. The logarithm of the equilibrium exchange current density (Io) 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 Io 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.\u

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 56Fe up to a dose of 8 × 1016 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 Io 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 step-wise transfer of electrons to adsorbed oxygen species takes place.\u

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 c = a = 1 to apparent values of c = 0.5 and 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 ∝ PO21/4 to I0 ∝ PO25/8. 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

Electrochemical characterisation of 3Y-TPZ-Fe2O3 composites

The influence of the addition of ferric oxide to 3Y-TZP on the conductivity and microstructure of sintered Y-stabilised tetragonal zirconia ceramics (3Y-TZP) was investigated. A comparison was made between two different dense 3Y-TZP¿¿-Fe2O3 composites. Compacts were made by pressureless sintering at 1150 °C or by sinterforging at 1000 °C and 100 MPa. The sinterforging process resulted in smaller zirconia and hematite grains and a higher monoclinic zirconia content as compared to the compact that was sintered pressureless. The high monoclinic content led to loss of ionic conductivity. The addition of ferric oxide caused electronic conductivity. The sinterforging resulted in a high concentration of metastable defects in the zirconia¿hematite composite, leading to a relatively high electronic conductivity. Heating above 380 °C caused irreversible loss of these defects and a large decrease in electronic conductivity

The oxygen transfer process on solid oxide/noble metal electrodes, studies with impedance spectroscopy, polarization and isotope exchange

The electrochemical oxygen transfer process at the yttria stabilized zirconia (YSZ) and Fe-implanted YSZ, and at the erbia stabilized bismuth oxide (BE25) surface is studied with dc polarization and impedance spectroscopy using gold electrodes, and with 18O gas phase exchange. The surface modification by Fe-implantation increases the exchange current density up to a factor of 50, but analysis of the impedance spectra at different polarization levels indicates that the type of electrode reaction is not changed by the implantation. Inductive effects at cathodic polarizations are interpreted with a stepwise transfer of electrons. Isotope exchange experiments show an increase in adsorption/reaction sites at the surface after implantation. The high exchange current density, I0, for BE25 is independent of type of electrode but does depend on electrode morphology. I0 can be equated to the surface oxygen exchange rate, indicating that the entire electrolyte surface is active in the electrode exchange process. Qualitative interpretation of the impedance spectra measured at different levels of polarization results in a model where adsorbed oxygen species diffuse over the oxide surface, while charge transfer occurs across the surface

Surface oxygen exchange properties of bismuth oxide-based solid electrolytes and electrode materials

The surface oxygen exchange coefficient, ks, has been measured for the solid solution (Bi2O3)0.75(Er2O3)0.25 and (Bi2O3)0.6(Tb2O3)0.4 (abbreviated BE25 and BT40), using gas-phase 18O exchange techniques. The activation enth alpy of ks amounts to ΔE=110 kJ/molforBT40 andΔE=130 kJ/molforBE25. The magnitude of ks for the purely ionic conducting BE25 is comparable with values obtained from electrode polarization (I−V) measurements (ΔE=140 kJ/mol.) The comparatively high ks values show a (PO2)n dependence on oxygen pressure with values of n close to 0.5, indicating surface control in the oxygen transport process. Bismuth oxide containing solid solutions show a large activity in the oxygen exchange reaction with the gas phase

Three-electrode current-voltage measurements on erbia-stabilized bismuth oxide with sputtered noble metal electrodes

The anodic and cathodic polarization behaviour of sputtered porous gold electrodes on (Bi2O3)0.75(Er2O3)0.25 (abbreviated BE25) was studied as function of temperature and oxygen partial pressure using a three-electrode cell. The anodic polarization is smaller than the cathodic polarization, allowing current densities of 2 × 103 A m−2 at 0.1 V and 1041 K in oxygen for oxygen evolution. Comparison with the polarization behaviour of similar sputtered platinum electrodes on BE25 shows little effect of the electrode material on the exchange current densities. This indicates that the electrolyte surface is active in the oxygen transfer process, while the noble metal electrode serves merely as current collector. Analysis of the electrode impedance shows strong influence of surface diffusion on the electrode reaction(s). For the exchange current density an unusual Po2 dependence is observed.\u

The electrochemical influence and oxygen exchange properties of mixed conducting electrode materials on solid oxide electrolytes

Electrolyte electrode materials combinations of Er2O3 stabilized Bi2O3 and mixed i.e. both electronic and ionic conducting materials are investigated in order to lower electrode polarization losses and operating temperatures of electrochemical sensors, pumps and SOFC reactors made of them. 18O exchange rates are correlated with exchange current densities