Oxygen Ion Diffusion Coefficients Research Articles

Effect of Cr 2O 3 on the 18O tracer incorporation in SOFC materials

Investigations of the impact of Cr 2O 3 overlayers on the oxygen self diffusion in two SOFC materials were conducted to gain insight into the Cr poisoning mechanism at the cathode side of solid oxide fuel cells (SOFCs) with stainless steel interconnects. High density Y 0.15Zr 0.85O 2 (YSZ) and La 0.6Sr 0.4Co 0.2Fe 0.8O 3 (LSCF) sintered pellets were covered with 3 to 30 nm Cr overlayers that were subsequently oxidized, forming Cr 2O 3. Standard 18O tracer diffusion experiments at 800 °C were performed and ToF-SIMS profiling revealed that the oxygen ion diffusion coefficients were unaffected by the thin Cr 2O 3 overlayers, which is predictable since they are a bulk property, but the extracted effective surface exchange coefficients varied with Cr 2O 3 overlayer thickness. Solid-state reaction measurements and electronic structure considerations concerning the surface exchange, led to the conclusion that the observed oxygen uptake hindrance for Cr 2O 3 capped LSCF and the slight increase of the surface exchange coefficient for Cr 2O 3 capped YSZ can be attributed to the electronic properties of Cr 2O 3. A critical thickness for Cr 2O 3 was determined to be 12 nm where the transition from decreasing cathode-performance to a Cr 2O 3-property-governed regime occurs.

ReaxFF Reactive Force Field for Solid Oxide Fuel Cell Systems with Application to Oxygen Ion Transport in Yttria-Stabilized Zirconia

We present the ReaxFF reactive force field developed to provide a first-principles-based description of oxygen ion transport through yttria-stabilized zirconia (YSZ) solid oxide fuel cell (SOFC) membranes. All parameters for ReaxFF were optimized to reproduce quantum mechanical (QM) calculations on relevant condensed phase and cluster systems. We validated the use of ReaxFF for fuel cell applications by using it in molecular dynamics (MD) simulations to predict the oxygen ion diffusion coefficient in yttria-stabilized zirconia as a function of temperature. These values are in excellent agreement with experimental results, setting the stage for the use of ReaxFF to model the transport of oxygen ions through the YSZ electrolyte for SOFC. Because ReaxFF descriptions are already available for some catalysts (e.g., Ni and Pt) and under development for other high-temperature catalysts, we can now consider fully first-principles-based simulations of the critical functions in SOFC, enabling the possibility of in silico optimization of these materials. That is, we can now consider using theory and simulation to examine the effect of materials modifications on both the catalysts and transport processes in SOFC.

Crystal Structure, Disorder, and Diffusion Path of Oxygen Ion Conductors Y1-xTaxO1.5+x(x= 0.215 and 0.30)

We report the results of a neutron powder diffraction study of the yttrium tantalates Y0.785Ta0.215O1.715 and Y0.7Ta0.3O1.8. In the orthorhombic fluorite-related Y0.7Ta0.3O1.8 compound, the Y and Ta atoms were occupationally ordered, and the oxygen ions were localized at around the stable positions. On the contrary, in the defect fluorite-type cubic Y0.785Ta0.215O1.715 material, the Y and Ta atoms were occupationally disordered, and the oxygen ions exhibited a large spatial distribution and positional disorder. At a higher temperature of 808 K, the spatial distribution of the oxygen ions was larger, and the diffusion paths of the oxygen ions were observed along the 〈100〉 directions. The high diffusion coefficient and conductivity of oxygen ions in cubic Y0.8Ta0.2O1.7 are attributable to the occupational and positional disorder.

A Study of the Oxygen Transport Kinetics in SrFeO3-x

ABSTRACTExperimental results from electrical conductivity relaxation (ECR) measurements on SrFeO3-x are described. Values of Dchem and kchem were obtained by monitoring the variation of the time dependence of the electrical conductivity after an abrupt change in the oxygen partial pressure. Values for the oxygen ion and vacancy diffusion coefficients were calculated from the measured thermodynamic factors and compared with those of other compositions in the series La1-xSrxFeO3-x. The surface exchange coefficients, kchem and kex were also determined.

Determination of the oxygen ion diffusion coefficients in mixed conductors by the volumetric measurement of the reoxidation kinetics

Abstract Mixed conducting ceramics (ionic and electronic conducting oxides) have a high application potential for the oxygen separation from oxygen containing gas mixtures (mostly air). Because the ion diffusion controls frequently the rate of the total permeation flux, a simple and fast determination of the oxygen diffusion coefficient is the prerequisite for an efficient material screening. In this work, the volumetric measurement of the reoxidation of chemically reduced mixed conductors was used for the determination of oxygen ion diffusion coefficients. The described method has advantages compared to the frequently used mass relaxation technique, e.g. a higher measuring effect, the suitability for fast processes (by use of large samples) and a high resistance to abrupt pressure changes. The checkup of the measurement method on the reference materials SrFe 1− x Co x O z ( x =0.6) and La 0.79 Sr 0.2 MnO 3+ z indicates a sufficient agreement with previous published values. The experimental results show a high internal consistency, as it is essential for a screening method. Diffusion coefficients are independently on the oxygen concentration over a wide range of the oxygen stoichiometry for all tested materials. Low activation energies in the range 0.10–0.5 eV were found. These latter results are opposed to the majority of publications on mixed conducting oxides. Additionally, new promising perovskite compositions in the system Sr 1− x Ca x Mn 1− y Fe y O 3− z were presented.

Molecular dynamics simulation of enhanced oxygen ion diffusion in strained yttria-stabilized zirconia

The application of strain to yttria-stabilized zirconia (YSZ), which can be realized by sandwiching a thin YSZ film epitaxially between layers of a material with larger lattice constants, is proposed as a means to enhance oxygen ion mobility. The possible mechanism of such an enhancement was investigated by molecular dynamics using a CeO2–YSZ superlattice. The calculated diffusion coefficient of oxygen ions in the superlattice is some 1.7 times higher than in YSZ alone due to a decreased activation barrier from the strain of the YSZ structure.

Structure and Dynamics in Calcium Aluminate Liquids: High‐Temperature 27Al NMR and Raman Spectroscopy

27Al NMR spectra of CaO–Al2O3 liquids at 2500 K have been obtained using a recently developed technique that involves CO2 laser heating and sample levitation inside the NMR magnet. The spectra consist of single, narrow lines (100 to 200 Hz FWHM), indicating that rapid chemical exchange among molecular species occurs in the liquids. Isotropic chemical shifts vary linearly with composition, becoming more shielded with increasing Al2O3content. This is most likely due to an increase in the average nearest‐neighbor coordination of Al, as the proportion of higher coordinate Al species (AlO5 and AlO6) has been observed to increase with alumina content in glasses along this join. This is also in agreement with ion dynamics simulations of liquids along the CaO–Al2O3 join, which show the average Al coordination to range from 4 at low alumina content to 5 for Al2O3liquid. Correlation times for 27Al spin–lattice relaxation estimated from the NMR line widths are in agreement with shear relaxation times determined from experimental viscosity measurements. This may indicate that oxygen exchange between neighboring aluminate polyhedra is a key component in the mechanism for viscous flow. Viscosities determined from oxygen ion diffusion coefficients via ion dynamics simulation agree well with extrapolation of experimental data, in further support of this. We have also obtained high‐temperature Raman spectra of CaAl2O4 and Ca12Al14O33 glasses and liquids up to 1928 K. The liquid spectra contain structure similar to those of glasses, indicating that the aluminate liquids consist of AlOn polyhedral units on the vibrational time scale, rather than simple ionic species.

A possibility of the oxygen diffusion enhancement in YSZ: a molecular dynamics study

A constant volume molecular dynamics calculation has been carried out for an yttria stabilized zirconia (YSZ) with composition (ZrO 2) 0.8(Y 2O 3) 0.2, in which Y ions are artificially arranged in layers. The oxygen ion diffusion coefficient in the artificial YSZ is larger than that in YSZ with a random distribution of Y ions. A way of synthesizing YSZ with a larger value of the oxygen ion diffusion coefficient is suggested.

Diffusion Coefficient of Oxygen Ions in Molten Calcium Chloride

Calciothermic reduction of metal oxides in a molten salt media is a commercial process for the production of certain actinides and lanthanides which produces a calcium oxide saturated calcium chloride salt mix as the process waste. Electrolytic reduction of calcium oxide (CaO) has been carried out in a molten calcium chloride (CaCl2) medium to produce calcium metal cathodically and to regenerate the chloride. A porous ceramic anodic sheath has been used which allows the diffusion of ions from the catholyte into the anolyte. Diffusion of oxygen ions has been found to be the rate controlling step in the electrowinning of calcium metal. The diffusion coefficient of oxygen ions in molten CaCl2 has been measured in the temperature range of 800 – 900°C. The data has been used to design the cell for optimizing the cell efficiency.

The role of oxygen ion diffusion in the electrode reactions of β″ alumina

A model has been proposed to explain the current-time curves obtained by applying fixed voltages to polycrystalline β alumina at elevated temperatures using gold contacts. It is proposed in the model that the oxygen electrode reaction can occur at the gold “blocking” contacts. The observed current-time curves can be explained in terms of the diffusion and migration of oxygen ions to the gold anode and sodium ion transport within the bulk of the β alumina. The model provides a new method for determining the oxygen ion diffusion coefficient within the polycrystalline β alumina.

Influence of water content on the rates of crystal nucleation and growth in lithia-silica and soda-lime-silica glasses

Two sets of glasses were studied with compositions close to Li2O·2SiO2 and Na2O·2CaO·3SiO2, and with water contents ranging from 0.019 to 0.136 wt% and 0.007 to 0.040 wt%, respectively. The crystal nucleation and growth rates increased markedly with increase in water content, whereas the viscosities of the glasses decreased. For the lithia glasses, increases in nucleation rates at various temperatures closely corresponded to reductions in viscosity, indicating that the main effect of water was to lower the kinetic barrier to nucleation (ΔGD), rather than to alter the thermodynamic barrier to nucleation (W*). For the soda-lime glasses, ΔGD was also lowered by water content but additional effects due to differences in base compositions were observed. The kinetic barriers to growth were lowered by water content for both sets of glasses, increases in growth rates corresponding closely to reductions in the viscosities. It is suggested that the large effects of water on nucleation and growth may be due to an increase in the oxygen ion diffusion coefficient. In the soda-lime glasses addition of sodium fluoride produced similar effects to the addition of water. Liquidus temperature measurements, and the results of DTA, optical microscopy and electron microscopy are also reported.

The influence of the particle size distribution on the measurement of oxygen ion diffusion coefficients in La0.50Sr0.50CoO3−y

The influence of the particle size distribution on an earlier electrochemical measurement of the oxygen ion diffusion coefficient at 76°C in La 0.50 Sr 0.50 CoO 3− y was examined. A value of 1.55×10 −13 cm 2 s −1 was found which compares favorably with the earlier value of 1.23×10 −13 cm 2 s −1 determined on the basis of a single characteristic particle size. The measured current differs only by a factor 1.3 from the calculated one, which factor can be attributed to the use of an idealized model with spherical particles.

The influence of the particle size distribution on the measurement of oxygen ion diffusion coefficients in La 0.50Sr 0.50CoO 3− y

The influence of the particle size distribution on an earlier electrochemical measurement of the oxygen ion diffusion coefficient at 76°C in La 0.50Sr 0.50CoO 3− y was examined. A value of 1.55×10 −13 cm 2 s −1 was found which compares favorably with the earlier value of 1.23×10 −13 cm 2 s −1 determined on the basis of a single characteristic particle size. The measured current differs only by a factor 1.3 from the calculated one, which factor can be attributed to the use of an idealized model with spherical particles.

The electrochemical determination of oxygen ion diffusion coefficients in La0.50Sr0.50CoO3−y Experimental results and related properties

The electrochemical determination of oxygen ion diffusion coefficients in La0.50Sr0.50CoO3−y Experimental results and related properties

The electrochemical determination of oxygen ion diffusion coefficients in La 0.50Sr 0.50CoO 3−y: Experimental results and related properties

Results of an electrochemical method for the determination of oxygen ion diffusion coefficients D O 2− in porous pellets of La 0.50Sr 0.50CoO 3−y are presented. Log D O 2− can be expressed as log D O 2− =−(2.3±0.2) 10 3/ T−(6.6±0.6) cm 2 s −1. The activation energy E a for the diffusion process equals 44 kJ mol −1. Further the related electrochemical measurement and the adjustment of the oxygen deficiency y are described. At 75°C the following empirical Nernst relation between the electrode potential E (vs. a Pt/H 2 (1 atm) electrode in the same solution) and Δ y is found: E=627–108 ln (Δ y + 0.0054) mV. (Δ y= y−y 0 ; y 0=mole fraction of vacancies at the reversible O 2 potential of 1190 mV). The use of La 0.50Sr 0.50CoO 3−y as a solid solution electrode in practical storage cells seems to be excluded for thermodynamic and kinetic reasons.

An electrochemical method for the determination of oxygen ion diffusion coefficients in La1-xSrxCoO3-y compounds Theoretical aspects

Theoretical aspects of a novel electrochemical method for the determination of O2− diffusion coefficients in porous pellets of La1−xSrxCoO3−y are discussed. This method is based on a bounded 3-dimensional diffusion model.

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO3 by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D= 0.33 exp (-22.5 ± 5.0 kcal/RT) cm2/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D= 5.2 × 10−6 exp (-26.1 ± 5.0 kcal/RT) cm2/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10−5 by virtue of the relation between the oxygen self-diffusion coefficient, D02-, and the oxygen vacancy diffusion coefficient, Do□; Do2-= [O□]Do□ where the oxygen ion concentration [O2-] is taken as unity.

Effect Of Grain Boundaries on Diffusion‐Controlled Processes in Aluminum Oxide

Oxygen ion diffusion coefficients in single‐crystal Al2O3 are several orders of magnitude less than alminum ion diffusion coefficients in polycrystalline Al2O3. In polycrystalline Al2O3, oxygen ion diffusion is enhanced by the presence of grain boundaries as in the chloride ion diffusion in the alkali halides. Creep and sintering of polycrystalline Al2O3 occur at a faster rate than is possible through control by lattice diffusion of oxygen; the rates are in fair quantitative agreement with cation diffusion. It is tentatively concluded that enhanced oxygen diffusion in regions adjacent to boundaries allows aluminum ion bulk diffusion to be rate controlling for these processes. The electrical conductivity in Al2O3 is too high to be related to either anion or cation diffusion and is predominantly electronic.