Hebb-Wagner Polarization Method Research Articles

(Invited) Measurement of Electronic Conductivity in 8YSZ Using an Embedded Electrode

Predominantly ionic conductors and mixed ion and electron conductors (MIEC) have numerous applications in gas permeable membranes, solid oxide fuel cell (SOFC), solid oxide electrolysis cell (SOEC) to name a few. Both ionic and electronic conductivities of MIEC are of interest. A traditional way to measure partial conductivity is the Hebb-Wagner polarization method [1]. In the original work by Hebb, electronic conductivity of Ag2S was measured as a function of position by measuring position-dependent electric potential. Application of the Hebb-Wagner method to materials such as YSZ involves fitting to the polarization equation. Since in most studies position-dependent electric potential cannot be readily measured. Most measurements are made under applied voltage large enough so that parts of the sample exhibit n-type and p-type conduction as well as the intrinsic region in which the electronic conductivity is exceptionally low. This may lead to an underestimate of electronic conductivity under certain conditions. We recently investigated a steady state permeation technique to measure the electronic conductivity in YSZ [3]. A YSZ disc sample with embedded Pt probe is made using die-pressing and sintering at 1500°C. Pt paste is applied on both surfaces of the sample, and the rest of the bare YSZ surface is covered by a sealant glass. Measurements are made under very low applied voltages, as low as 0.01 V. Under such a low applied voltage, the entire sample is under a nearly uniform oxygen chemical potential and thus exhibits a nearly constant electronic conductivity. In that study, electronic conductivity of YSZ was measured in air, which corresponds to p-type conduction. Here we report a transient technique for the measurement of electronic conductivity of YSZ using a sample with an embedded electrode and a cavity. The electrical circuit used in the transient technique is shown in Figure 1. During charging, oxygen ions migrate through YSZ electrolyte from electrode II to electrode I, and electrons migrate through external circuit from electrode I to electrode II. Effectively, neutral oxygen gas is pumped into YSZ (changing its stoichiometry slightly) and into cavity between YSZ and Pt probe. The kinetics of this process is dictated by oxygen ion conductivity, and the corresponding ionic resistance can be obtained. During discharge, both oxygen ions and electron migrates through the YSZ electrolyte, and neutral oxygen dissipates until its partial pressure inside the sample reaches the same value outside the sample. The kinetics of this process is limited by electronic conductivity, which can be obtained from the time dependence of the potential across the embedded electrode and the surface electrodes. Figure 2 shows the time dependence of the applied voltage, the measured current,the measured Nernst voltage, and the calculated electronic resistance of the YSZ sample at 650°C. Figure 3 shows an Arrhenius plot of the electronic resistance of YSZ. The corresponding activation energy is measured to be 1.9 eV. The results of the transient technique described here agree well with our previous steady state permeation technique. Funded by in part DOE EFRC Grant Number DE-SC0001061 as a flow-through from the University of South Carolina and in part by DOE under Grant DE-FG02-06ER46086.

Microstructure and ionic conductivity of SrTiO3 heterogeneously doped YSZ composite ceramics

To improve the ionic conductivity of ZrO 2 stabilized by 8 mol% of Y 2 O 3 (8YSZ), the SrTiO 3 heterogeneously doped 8YSZ composite ceramics with different volumetric fractions of SrTiO 3 were prepared through powder sintering at 1400 °C and 1450 °C in air. The phase compositions, microstructures, electrical conductivities and ion transference numbers of the composites were investigated by X-ray diffraction, scanning electron microscopy, complex impedance and Hebb–Wagner polarization method, respectively. The composite ceramics only consist of cubic YSZ and SrTiO 3 . The deviations of the typical diffraction peak positions in XRD spectra and the variations in unit cell parameters of both SrTiO 3 and YSZ in composites have occurred due to the elemental inter-diffusion between both phases. The grain boundaries between these two heterogeneous phases do not exhibit any presence of a segregated impurity phase, and the heterogeneous doping of SrTiO 3 can suppress the growth of YSZ grains at higher doping levels. The apparent conductivities of the composites show a peak at 5 vol.% of SrTiO 3 addition. The activation energies for the oxygen ion transport of 8YSZ doped with 0–15 vol.% of SrTiO 3 all fall into the range of 101.71–104.93 kJ/mol. The ion transference numbers of the composites with 1–15 vol.% of SrTiO 3 are all higher than 0.99, demonstrating that the electrical conductivity is almost pure ionic in nature. By analyzing the effects of SrTiO 3 additions on the normalized conductivities of the composites, the SrTiO 3 /YSZ heterogeneous interfacial effects on the conductivity improvement can be proposed. • SrTiO 3 heterogeneously doped 8YSZ composites are prepared by sintering in air. • Elemental diffusions between SrTiO 3 and YSZ occur after powder sintering. • The SrTiO 3 /YSZ heterogeneous interface can enhance the conductivity of composites. • Electrical conductivities of YSZ doped with 1–15 vol.% of SrTiO 3 are ionic in nature.

Proton conductors based on alkaline-earth substituted La28−xW4+xO54+3x/2

Lanthanum tungstates, "La6WO12", are mixed ion proton-electronic conductors with very interesting properties for technological applications and better phase stability compared to alkaline earth perovskites. A new series of compounds La(27.04-x)M(x)W(4.96)O(55.44-x/2□8.56+x/2) (M = Ca(2+), Sr(2+) and Ba(2+)) are investigated with the aim of increasing the concentration of oxygen vacancies and studying their effects on the structure and transport properties. The materials have been studied by high-resolution laboratory X-ray powder diffraction and scanning electron microscopy combined with energy dispersive spectroscopy (EDS). High temperature X-ray powder diffraction and thermal analysis in wet and dry N2 gas did not show any evidence of phase transition up to 800 °C. The total conductivity was studied by impedance spectroscopy under dry and wet atmospheres and as a function of the oxygen partial pressure. The electronic contribution to the conductivity was determined by the Hebb-Wagner polarization method. The generation of extrinsic vacancies in the lattice with alkaline earth doping leads to a decrease of the ionic conductivity for high doping level, suggesting a proton trapping mechanism.

High temperature electrical behavior of CaTi1−xFexO3−δ (x=0–0.5). Oxygen-ion, electronic and proton conductivity

Abstract Total electrical conductivity and contributions of different charge carriers such as oxygen ions, protons, electrons and holes in CaTi1 − xFexO3 − δ (x = 0–0.5) were measured as functions of temperature, oxygen partial pressure and iron content. Total electrical conductivity measurements were made by means of the four-probe dc technique. Partial electronic conductivity was studied by the Hebb–Wagner polarization method. Transport numbers of oxygen ions and protons in CaTi1 − xFexO3 − δ were measured by the EMF method. It has been shown that CaTi1 − xFexO3 − δ is a mixed oxygen-ion and electronic conductor at small iron concentrations, and it is a predominantly electronic conductor at large iron contents. Proton conductivity of CaTi1 − xFexO3 − δ is negligibly small. Ambipolar conductivity of CaTi1 − xFexO3 − δ was calculated. With known ambipolar conductivity a profile of the oxygen chemical potential inside CaTi1 − xFexO3 − δ under applied oxygen partial pressure gradient can be figured out.

Structural and electrical properties of La2–xSrxMo2O9–δ

Sr-doped La2Mo2O9 were prepared by solid state reaction and characterized by XRD, impedance spectroscopy and Hebb- Wagner polarization method. XRD patterns of the samples indicated that the solubility limit of Sr2+ in La2–xSrxMo2O9–δ was in the range of 7 mol.% to 7.5 mol.%, i.e., the maximum stoichiometric coefficient x in La2–xSrxMo2O9–δ was larger than 0.14 and less than 0.15. The cubic lattice parameter of La2–xSrxMo2O9–δ (0<x≤0.14) increased linearly with the content of dopant Sr2+ increasing and the relationship could be expressed as: a=0.7154+0.00451x (mm). The total conductivities of Sr-doped La2Mo2O9 were measured by impedance spectroscopy method in argon and air atmosphere, respectively. The results indicated that substituting Sr2+ for La3+ could suppress the first order phase transform of La2Mo2O9 and stabilize the cubic structure to room temperature, and the conductivities of Sr-doped La2Mo2O9 were more sensitive to the change of atmosphere than that of undoped La2Mo2O9. The conductivity variation of Sr-doped La2Mo2O9 with the dopant content exhibited similar trend in air and argon atmosphere, firstly decreased and then increased with the dopant content increasing, and a minimum value appeared at x=0.1. The electronic conductivities of Sr-doped La2Mo2O9 were measured using Hebb-Wagner polarization method in argon atmosphere. It could be found that Sr-doping caused the increase in electronic conductivity and decrease in oxide-ion transport numbers, but the conducting species were predominantly oxide ions. And the oxide-ion transport numbers of some Sr-doped La2Mo2O9 samples could also reach 0.99.

Electrolytic properties of the MeLn2S4-Ln′2 S3 phases synthesized using the sol-gel and cryochemical technologies

The possibility of synthesizing sulfide-conducting solid electrolytes based on the ternary sulfides MeLn2S4 doped with the binary sulfides Ln′2S3 has been approved and the influence of dopants on the electrolytic properties of the basic compounds has been investigated for the first time. The influence of the specific features of the method used for synthesizing complex sulfide phases from nanometer-sized oxide precursors on the important functional properties of the solid electrolytes has been analyzed. The samples have been examined using the X-ray powder diffraction analysis, electron scanning microscopy, and electron microprobe analysis. The region of the existence of the solid solutions has been determined, the total electrical conductivity has been studied, and the activation energy for electrical conduction for samples with different dopant contents has been calculated. The ionic and electronic transfer numbers have been determined using the modified versions of the emf method and the Hebb-Wagner polarization method. A possible mechanism of defect formation has been proposed.

Partial electronic conductivity and electrolytic domain of bilayer electrolyte Zr 0.84Y 0.16O 1.92/Ce 0.9Gd 0.1O 1.95

Partial electronic conductivity and total conductivity have been determined by Hebb–Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr 0.84Y 0.16O 1.92(YSZ)/Ce 0.9Gd 0.1O 1.95(GDC) with thickness ratios 10 − 3 /1 and 10 − 4 /1 at 800°, 900° and 1000 °C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the “continuity hypothesis” that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed.

Synthesis of complex sulfide phases on the CaYb2S4-Y2S3 quasibinary section and investigation of their structural and electrolytic properties

The possibility of synthesizing sulfide-conducting solid electrolytes in the CaYb2S4-Y2S3 quasibinary system is tested for the first time. The system is characterized by the X-ray diffraction analysis and the electron microscopy. The total conductivity of specimens is found by the conductometric method. The ionic and electronic transport numbers are determined based on the modified versions of the emf method and the Hebb-Wagner polarization method. The possible mechanism of defect formation is discussed.

Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes

Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La 9.333 + x Si 6O 26 + 1.5 x (LSO) were investigated in the oxygen-excess region ( x > ca. 0.3). We have found that the oxygen excess-type LSO (OE-LSO), namely La 10Si 6O 27 on weighted basis, exhibited high conductivity, and substitution of the Si-site of LSO with some dopants ( M n+ ) had a positive effect toward the conducting property. Furthermore, it was also found that addition of a very small amount of iron ions into the M-doped OE-LSO, La 10(Si 6- y M n+ y )O 27-(2–0.5 n) y , improved its conductivity. On the other hand, replacement of the La-site with various ions for La 10(Si 6- y M n+ y )O 27-(2–0.5 n) y did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1- α){La 10(Si 5.8Al 0.2)O 26.9}- α(FeO γ ), evaluated with the Hebb–Wagner polarization method were very low: i.e., 1.1 × 10 − 3 and 2.9 × 10 − 3 under P(O 2) = 1.1 × 10 4 Pa at 1073 K for α = 0.00 and 0.005, respectively. Conductivity for each sample was unchanged under humidified atmosphere at 1073 K sustained for over 50 h, revealing that both compositions were chemically stable. It was concluded that 0.995{La 10(Si 5.8Al 0.2)O 26.9}–0.005(FeO γ ) is suitable for the fuel cell electrolytes because of its high and almost pure ionic conductivity, and its good chemical stability under humidified as well as reducing conditions.

Electronic conductivity measurement of gadolinia doped ceria by DC-partial-polarization

A simple technique is presented to measure the partial electronic conductivity of solid electrolytes. It is based on the Hebb-Wagner polarization method but does not require complete blocking of the electrode. A major advantage of the method is the applicability over a much wider temperature range, especially measurements at higher temperatures. The technique needs to satisfy only the condition that the oxygen partial pressure in the reference gas is sufficiently low in order to decrease the ionic current to such an amount that the electronic conduction will be predominant at higher polarization voltages. The simple experimental arrangement is applied to measure the conductivity of the electronic minority charge carriers and to calculate the EDB (electrolytic domain boundary) values of 20 mol-% gadolinia doped ceria.

Electrical conductivity of iron-doped calcium titanate

Both total and partial electronic conductivity of iron-doped calcium titanates, of general formula CaTi 1− x Fe x O 3−δ ( x=0, 0.1,…0.5), were investigated as a function of oxygen partial pressure at high temperatures (900, 1000°C) by means of the four-probe technique and the Hebb–Wagner polarization method, respectively. The oxygen partial pressure interval extended from 1 to 10 −18 atm. Ti substitution by Fe results in a considerable increase of both ionic and electronic conductivity and in appearance of a wide region of primarily ionic conductivity, independent of the oxygen partial pressure. Of these compositions, the x=0.2 sample has the highest oxygen ion conductivity. A model of defect equilibrium is discussed.

A.C. behavior of proton-conducting solid electrolytes based on sulphosalicylic acid in hydrogen atmosphere

The conductivity ( σ) of sulphosalicylic acid and its composites with iron(III) sulphosalicylate (1:4 molar) is measured using impedance spectroscopy and the Hebb–Wagner polarization method. The conductance is found to be purely protonic with typical values of σ(25°C)=4.1×10 −4 and 3.0×10 −4 S cm −1, respectively. The exchange currents at TiH x | H +-solid electrolyte interphase boundaries increase with increasing partial pressure of H 2 in mixtures with argon.

Review of the limitation of the Hebb-Wagner polarization method for measuring partial conductivities in mixed ionic electronic conductors

Review of the limitation of the Hebb-Wagner polarization method for measuring partial conductivities in mixed ionic electronic conductors

Review of the limitation of the Hebb-Wagner polarization method for measuring partial conductivities in mixed ionic electronic conductors

The limitation of the use of partial current blocking for determining partial conductivities in mixed ionic electronic conductors, MIECs, is reviewed. The discussion refers to the Hebb-Wagner polarization method for determining the electronic conductivity and to its analogue for determining the ionic conductivity. These two methods suffer from severe problems that may render results useless because of false interpretation. The problems are discussed. New experimental evidence for some problems is presented. A four point, Hebb-Wagner type, measurement is discussed. It allows one to detect whether the correct experimental conditions prevail.

Cathodic reaction mechanism for dense Sr-doped lanthanum manganite electrodes

Abstract Dense La0.81Sr0.09MnO3 (LSM) electrode was prepared by a laser ablation method and electrochemical measurements were carried out. The electrode impedance was proportional to the film thickness and almost independent of the oxygen partial pressure. Steady-state polarization characteristics were measured and oxygen ionic conductivity of LSM as calculated using Hebb-Wagner polarization method. The slopes of log i − log a0 curves were calculated to be −1 2 . These results suggest that the cathodic reaction mechanism of the dense LSM electrode is quite different from that of porous electrode.

Electrical properties of modified TZP(+TiO2,FeOx)

1 to 24 mol% TiO2 and FeOX were added to 3.2 mol% Y2O3 doped ZrO2 (Z3Y) to obtain tetragonal zirconia polycrystals (TZP) with modified electronic properties. The materials were prepared by coprecipitation which allows to obtain fine, homogeneous and sinteractive powders. The solubility of TiO2 in Z3Y can reach 24 mol%, while the maximum solubility of FeOx is about 8 mol%. The impedance results show a decrease of the bulk and total conductivity of titania doped TZP with increasing titania concentration, while those of FeOx doped TZP show only minor changes. The Hebb-Wagner polarization method was applied to evaluate the partial hole and electron conductivities. Three models are presented to interpret the polarization curves. 1.6 mol% FeOx doped TZP has higher hole conductivity in air, while titania doped TZP has higher electronic conductivity at low oxygen partial pressures as compared to pure Z3Y.

Four point Hebb-Wagner polarization method for determining the electronic conductivity in mixed ionic-electronic conductors

A four point Hebb-Wagner polarization method is analyzed. The original analysis by Wagner applies to a two-electrode configuration. The purpose of the present method is to allow the determination of the partial electronic (electron/hole) conductivity, σ el, of a mixed ionic-electronic conductor, MIEC, excluding the adverse effect of electrode overpotential. We discuss both a linear configuration, and a van der Pauw-type configuration with cylindrically shaped samples. The method of determining σ el is presented. It is shown that the nature of the electrodes, whether ion blocking or non-blocking, is important, and that a four-electrode arrangement eliminates the adverse effect of overpotential at the current-carrying electrodes only by proper choice of all electrodes. The nature of the dominant electronic defects (electrons or holes) is determined from the ratio of the voltage drop between the various electrodes.

Recent investigations into the properties of mixed ionic electronic conductors

The ionic conductivity in mixed ionic electronic conductors (MIECs) is usually determined by a polarization technique analogous to the Hebb-Wagner polarization method used for determining the electronic (electron/hole) conductivity. However, this technique has some experimental difficulties which may render the results useless. These difficulties are discussed and examples are presented. A new method is presented, which is denoted as the “zero driving force” technique. In this method the electronic current is eliminated by short circuiting, thus eliminating the driving force acting on the electrons/holes. This new technique does not suffer from the drawbacks of the polarization method. It is shown that MIEC probes can be used to transfer μ e and μ i information. Two voltemeters connected to MIEC probes applied to an MIEC sample can be used to measure Δ μ e and Δ μ i . This measurement is simultaneous and refers to the same two contact points. However, reversible probes cannot generally be tolerated on MIECs in the one-dimensional configuration.

Mixed Ionic Electronic Conduction in Solids

AbstractChemical diffusion in mixed ionic electronic conductors (MIECs) occurs via ambipolar motion. It finds many applications which are reviewed. The partial ionic and electronic conductivities are also of interest. Methods for determining these quantities are reviewed emphasizing problems with the Hebb-Wagner polarization method and suggesting an alternative method. The use of MIECs for probing simultaneously the ionic and electronic driving forces and for producing p-n junctions is discussed.