A package for impedance/admittance data analysis

An outline is given of a Basic computer program which facilitates the analysis of frequency dispersion data. With this program an equivalent circuit, and starting values for the corresponding circuit parameters, can be extracted from the dispersion data. A circuit description together with crude parameter values form an essential requirement for a subsequent NLLSF procedure. A brief description is given of a frequency dispersion simulation program, also written in Basic, which can be used to compare measured data with a calculated response. Both programs employ the Circuit Description Code (CDC), thus allowing the use of a variety of equivalent circuits. The use of both programs is demonstrated with the analysis of a dispersion measurement performed on a sample of Sn-doped AgCrS2, which is a pure ionic conductor

A Nonlinear Least Squares Fit procedure for analysis of immittance data of electrochemical systems

A Nonlinear Least Squares Fit (NLLSF) program is described, with which frequency dispersion data of electrochemical systems can be analyzed in terms of an equivalent circuit. The NLLSF procedure uses a combination of an analytical and gradient search according to the Marquardt algorithm. Through the use of an unique Circuit Description Code (CDC) different equivalent circuits may be used with the program. The use of an analytical derivatives routine enhances the execution speed. The power of such a fit procedure is demonstrated in multi parameter fits, on synthetic and real data, performed by the program “EQIVCT”

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

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

Alternatives to Kronig-Kramers Transformation and Testing, and Estimation of Distributions

Two alternatives to Kronig-Kramers analysis of small-signal ac immittance data are discussed and illustrated using both synthetic and experimental data. The first, a derivative method of approximating imaginary-part response from real-part data, is found to be too approximate in regions where the imaginary-part varies appreciably with frequency. The second, a distribution of relaxation-times fitting method, is shown to be valuable for testing whether a data set satisfies the Kronig-Kramers relations and so is associated with a system whose properties are time-invariant. It also is valuable for estimating real- or imaginary-part response from the other part, usually with small error. Unlike Kronig-Kramers analysis, the second method usually requires no extrapolation outside the range of the measured data. Finally, this discrete-function method also allows one to estimate the distribution of relaxation times or activation energies associated with a given set of frequency-response data. This application is described and illustrated for both synthetic and experimental data and is shown to yield good but somewhat approximate results for the estimation of continuous distributions. It is particularly valuable for identifying response regions arising from a continuous distribution and distinguishing them from those associated with discrete time-constant response

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

Oxygen surface exchange kinetics of erbia-stabilized bismuth oxide

The surface oxygen exchange kinetics of bismuth\ud oxide stabilized with 25 mol% erbia (BE25) has been studied\ud in the temperature and pO2 ranges 773–1,023 K and 0.1–\ud 0.95 atm, respectively, using pulse-response 18O–16O isotope\ud exchange measurements. The results indicate that BE25\ud exhibits a comparatively high exchange rate, which is rate\ud determined by the dissociative adsorption of oxygen. Defect\ud chemical considerations and the observed pO2\ud 1=2 dependence\ud of the rate of dissociative oxygen adsorption suggest\ud electron transfer to intermediate superoxide ions as the rate\ud determining step in surface oxygen exchange on BE2

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

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