Oxygen Chemical Diffusion Coefficient Research Articles

Pr‐ and Co‐substitution in rare earth nickelates: Application as SOEC air electrodes

AbstractIn this work, fundamental material properties of compounds in the system (La,Pr)2(Ni,Co)O4+δ as well as their performance as air electrodes in solid oxide electrolysis cells were investigated. Nickelates co‐doped with Pr and Co were characterized on a material basis by means of X‐ray diffraction and thermogravimetry. Conductivity and conductivity relaxation measurements were performed in order to obtain the electronic conductivity as well as the chemical surface exchange coefficient and the chemical diffusion coefficient of oxygen as a function of temperature and oxygen partial pressure. These parameters can be regarded as the most essential properties at the material level required to assess the suitability of mixed ionic‐electronic conducting ceramics for application as air electrode in solid oxide cells. The electrode performance of the materials was then tested on fuel electrode‐supported button cells at 800°C. The electrodes were applied by screen‐printing and the effect of varying the Pr‐content and Co‐content of the electrode powder was investigated. Cell tests were performed by means of current‐voltage measurements in electrolysis mode. While no significant impact of Pr‐doping on the investigated material properties was observed, the electrode performance of Pr‐containing materials was significantly better than for the Pr‐free compound, which has been discussed in detail.

Synergistic effects of A-site deficiency and anion doping on Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode for H2O electrolysis and CO2 electrolysis

In this work, a strategy of A-site deficiency and F-doping was proposed to prepare Pr0.4Sr0.55Ni0.2Fe0.7Mo0.1F0.1O2.9 (PSNFMF) cathode. Compared with the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 (R-PSNFM), the reduced PSNFMF facilitate the exsolution reaction and generate more oxygen vacancies. Moreover, at 800 °C, the chemical oxygen surface exchange coefficient (kchem) of R-PSNFMF is 4.31 × 10−4 cm s−1, 21.8% higher than the value of 3.54 × 10−4 cm s−1 for R-PSNFM, and the oxygen chemical bulk diffusion coefficient (Dchem) of R-PSNFMF is 34.21 × 10−5 cm2 s−1, 24.3% higher than the value of 27.53 × 10−5 cm2 s−1 for R-PSNFM, suggesting faster oxygen transport kinetics are obtained for R-PSNFMF. Meanwhile, the synergistic effect of A-site deficiency and anion doping can also promote the electrolysis of H2O and CO2. For H2O electrolysis, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.75 A cm−2, 31.6% higher than the value of −0.56 A cm−2 for R-PSNFM based cell. And for CO2 electrolysis in harsh condition at 800 °C, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.43 A cm−2, 38.7% higher than the value of −0.31 A cm−2 for R-PSNFM based cell.

Doping effects of alkaline earth element on oxygen reduction property of high-entropy perovskite cathode for solid oxide fuel cells

The nearly infinite arrangement and combination of multiple components in high entropy materials provide a huge platform for the modification and performance regulation of new materials. It is an attractive idea to introduce the concept of high entropy into material design and improve its electrochemical performance through component regulation. In this paper, cobalt-based high-entropy perovskite oxides with different alkaline earth metal contents were successfully synthesized. The phase structure, direct current conductivity, oxygen mobility characters and electrochemical properties of these materials are studied. It is found that with the continuous substitution of rare earth elements by alkaline earth elements in high-entropy materials, the content of oxygen vacancies increases and the electrochemical performance is enhanced. The chemical oxygen diffusion coefficient (Dchem) and surface exchange coefficient (Kchem) of the high-entropy oxide La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 are 2.8 × 10−6 cm2 s−1 and 9.1 × 10−3 cm s−1 at 700 °C, respectively. The electrochemical impedance spectroscopy measurement results show that La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 has the minimum polarization resistance, which is 0.03 Ω cm2 at 700 °C in air. The anode-supported fuel cell with the high entropy oxide reaches a peak power density of 792 mW cm−2 at 700 °C and exhibits attractive long-term stability and excellent CO2 resistant properties.

Oxygen exchange and transport properties of the first-order Ruddlesden-Popper phase La2Ni0.9Co0.1O4+δ

The rare earth nickelate La2Ni0.9Co0.1O4+δ (LNCO) was synthesized via the citrate/EDTA method. X-ray powder diffraction confirmed that the material is single-phase and crystallizes in the orthorhombic K2NiF4-type structure. In-situ dc-conductivity and conductivity relaxation measurements on a bar-shaped sample were applied to determine the electronic conductivity as well as the chemical surface exchange coefficient and chemical diffusion coefficient of oxygen from 600 °C to 850 °C and 0.01 ≤ pO2 / bar ≤ 0.1. The results indicate that substitution of nickel with cobalt in La2Ni0.9Co0.1O4+δ leads to significantly higher values of the surface exchange coefficient compared to La2NiO4+δ, while the electronic conductivity is somewhat reduced. The oxygen non-stoichiometry of LNCO was studied by thermogravimetry in the same temperature and oxygen partial pressure range. The measured thermal expansion coefficients of LNCO fit well with those of common solid electrolytes such as GDC and YSZ. Self-diffusion coefficients of oxygen and ionic conductivities were calculated from the experimentally determined diffusivities and the thermodynamic factor of oxygen by using the Nernst-Einstein relation. The results indicate that LNCO offers an attractive option for application as air electrode in solid oxide cells.

Considerable oxygen reduction activity and durability of BaO nanoparticles-decorated Ln0.94BaCo2O5+δ electrocatalysts

Owing to unique catalytic properties and durability, nanoparticles-containing heterostructured catalysts have attracted much interest in energy storage and conversion fields. Here we report a multi-composite electrocatalyst consisting of BaO nanoparticles and Ln0.94BaCo2O5+δ (Ln = La, Pr, Nd, Sm, and Gd) (e-Ln0.94BCO) though in situ exsolution route. As-obtained e-Ln0.94BCO catalysts exhibit desirable oxygen reduction reaction (ORR) activity and durability toward solid oxide fuel cells (SOFCs) cathode application. When varying Ln3+ from Gd3+ to La3+, the phase transformation of orthorhombic-tetragonal-cubic can be identified in Ln0.94BaCo2O5+δ, significantly demonstrating improved electrochemical performance as well as electrical conductivity, oxygen surface exchange and chemical diffusion coefficients. Resultantly, the e-L0.94BCO cathode exhibits the lowest area-specific resistance (ASR) of 0.03 Ω cm2 at 700 °C, reduced by ≈ 43% relative to pristine La0.94BaCo2O5+δ (L0.94BCO) cathode. Under realistic operating conditions, the e-L0.94BCO cathode-based button cell delivers a peak power density (PPD) 0f 1.05 W cm−2 at 700 °C, along with outstanding long-term stability. Furthermore, an exceptional CO2 durability is achieved in e-L0.94BCO with negligible degradation rate. Density-functional theory (DFT) calculations imply that interfacial BaO makes a movement of O 2p-band center to the Fermi level, which is closely related to promoted electrochemical performance.

Enhanced electrochemical redox kinetics of La0.6Sr0.4Co0.2Fe0.8O3 in reversible solid oxide cells

Due to its high conversion efficiency in fuel-to-power and power-to-fuel modes, reversible solid oxide cell (RSOC) is widely regarded as a promising device. In this study, La0.6Sr0.4Co0.2Fe0.7M0.1O3 (M=Fe, Mo, Nb) oxides are prepared and composited with Sm0.2Ce0.8O2-(Li0.67Na0.33)2CO3 to form reversible single component cell (RSCC). Among them, La0.6Sr0.4Co0.2Fe0.7Mo0.1O3 (LSCFM) shows the highest oxygen vacancy concentration, further promoting catalytic activity towards oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). Besides, Mo or Nb doping enhances the surface reaction rate constant (kchem) and oxygen chemical bulk diffusion coefficient (Dchem) at the temperature range of 550–700 °C. LSCFM shows the highest kchem of 39.3 × 10−5 cm s−1, approximate 3.5 times as 11.1 × 10−5 cm s−1 for La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), and Dchem for LSCFM is 31.1 × 10−6 cm2 s−1, approximate 2.5 times as 12.4 × 10−6 cm2 s−1 for LSCF at 700 °C. Furthermore, the rate determining steps (RDS) of ORR and HOR for LSCFM-based cell are the reduction of oxygen atoms to O− and the charge transfer reaction, respectively. Under SOFC/SOEC reversible operation, the RSCC composed of LSCFM exhibits the best cell performance. In the solid oxide fuel cell mode, the maximum power density reaches 236.2 mW cm−2 and in the solid oxide electrolysis cell mode the current density at 1.3 V is -312.4 mA cm−2.

Oxygen diffusion in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2

This study evaluates the self-diffusion and chemical diffusion coefficients of oxygen in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2 using point defect chemistry (oxygen vacancies and interstitials). The self-diffusion coefficient changed in proportion to the 1/n power of oxygen partial pressure, similar to the defect concentration. All parameters used to represent the diffusion coefficients were determined, and the experimental data were accurately stated. The defect formation and migration energies of the oxides were compared, and the change in Frenkel defect concentration was found to affect the high-temperature heat capacities of CeO2 and ThO2. The oxygen chemical diffusion was evaluated in the oxides, excluding the line compound ThO2, and the coefficients increased dramatically around the stoichiometric composition, i.e., the chemical diffusion coefficient was much higher at stoichiometric composition, with the oxygen-to-metal ratio equal to 2.00, than in low oxygen-to-metal oxides. This difference altered the mechanism of the reduction and oxidation processes. In the reduction process, the chemical diffusion control rate was dominant and a new phase with the oxygen-to-metal ratio equal to 2.00 was formed, which then expanded from the surface in the oxidation process from a low oxygen-to-metal ratio to the stoichiometric composition.

Oxygen potential, oxygen diffusion, and defect equilibria in UO2±x

Since the oxygen potential and the oxygen diffusion coefficient of UO2 have a significant impact on fuel performance, many experimental data have been obtained. However, experimental data of the oxygen potential and the oxygen diffusion coefficient in the high temperature region above 1673 K are very limited. In the present study, we aimed to obtain these data and analyze them by defect chemistry. the oxygen potentials and the oxygen chemical diffusion coefficient of UO2 were measured by the gas equilibrium method in the near stoichiometric region at temperatures ranging from 1673 to 1873 K. A data set of oxygen potentials was made together with literature data and analyzed by defect chemistry. The oxygen potential of UO2 was determined as a function of O/U ratio and temperature, and an equation representing the relationship was derived. The oxygen chemical diffusion coefficient values obtained in this study were reasonably close to the literature values. The oxygen partial pressure dependence of the oxygen chemical diffusion coefficients was predicted from the evaluated results of the oxygen potential data, but no clear dependence was observed.

Enhancing catalytic activity of CO2 electrolysis via B-site cation doped perovskite cathode in solid oxide electrolysis cell

Solid oxide electrolysis cell (SOEC) is considered as a high energy-efficient CO2 conversion method and a promising technology to achieve carbon neutralization. Thus, in this work, the reduced Co or Ni doped Pr0.4Sr0.6Fe0.9Mo0.1O3 perovskite oxides are used as the cathode of SOEC. After reduction, Co-Fe alloy nanoparticles are in situ exsolved from Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3 and Pr0.4Sr0.6Co0.45Fe0.45Mo0.1O3, and NiFe10.8 alloy nanoparticles are attached on the Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 matrix. The reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest oxygen vacancy concentration and CO2 adsorption ability, which further promote oxygen surface exchange and electrochemical surface exchange reaction. Besides, the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest surface reaction rate constant (kchem) of 3.54 × 10-4 cm s-1 and highest oxygen chemical bulk diffusion coefficient (Dchem) of 27.53 × 10-5 cm2 s-1, suggesting that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the best catalytic activity for CO2 reduction reaction. The symmetrical cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the lowest polarization resistance (Rp) value, and at 800 °C, the electrolysis cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode also exhibits the best performance of 0.81 A cm-2 in 50%CO-50%CO2 and 1.05 A cm-2 in pure CO2 at 1.5 V, indicating that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 is a promising cathode for CO2 electrolysis.

In-situ segregation of A-site defect (La0.6Sr0.4)0.90Co0.2Fe0.8O3-δ to form a high-performance solid oxide fuel cell cathode material with heterostructure

As a potential cathode material for solid oxide fuel cells, the commercial application of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has to face the challenges in insufficient oxygen reduction reaction (ORR) activity, segregation of Sr element, and CO2 poisoning. Therefore, the effect of A-site non-stoichiometry on the electrochemical performance of (LS)1-xCFs (x = −0.05, 0, 0.05, 0.10, 0.15) from the aspect of microstructure/elemental surface chemical environment evolution is investigated in this paper. The results show that (LS)0.90CF has the best ORR activity and the highest electrochemical performance. The excellent electrochemical performance is attributed to the (LS)0.90CF/Co2FeO4/CoFe2O4-type heterostructure, formed by in-situ segregation induced by A-site defects, which improves the surface oxygen diffusion coefficient and chemical bulk diffusion coefficient of the cathode. At the same time, the A-site defect can effectively inhibit the segregation of Sr element in (LS)0.90CF, thereby improving the tolerance of CO2. At 800 °C, the area-specific resistance (ASR) of (LS)0.90CF (0.023 Ω cm2) is 66.7% lower than that of LSCF (0.069 Ω cm2), and the peak power density (1.57 Wcm−2) is 1.8 times higher than that of LSCF (0.87 Wcm−2). Therefore, perovskite A-site defect-induced B-site segregation to form heterostructures provides an effective strategy for the preparation of high-performance cathode materials.

Surface kinetics and bulk transport in La2Ni0.5Cu0.5O4+δ membranes from conductivity relaxation

This work reports conductivity relaxation measurements on both uncoated (1.2 mm thick) and coated (2.0 mm thick) La2Ni0.5Cu0.5O4+δ membranes in the temperature range between 550 and 850 °C and oxygen partial pressures from 0.01 to 1.0 atm. The results show that surface kinetics has a significant effect on the relaxation profiles, especially at low temperatures and should not be neglected when extracting transport parameters. Oxygen chemical diffusion and surface exchange coefficients have been determined by transient conductivity with surface modification. Higher activation energy of surface exchange compared to bulk diffusion is observed for La2Ni0.5Cu0.5O4+δ, similar to that for La2NiO4+δ. Based on the oxygen partial pressure dependence of the surface exchange coefficient, it has been revealed that oxygen dissociative adsorption rate-limits the surface exchange.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

Perovskite Oxyfluoride Ceramic with In Situ Exsolved Ni-Fe Nanoparticles for Direct CO2 Electrolysis in Solid Oxide Electrolysis Cells.

Solid oxide electrolysis cell (SOEC) is a potential technique to efficiently convert CO2 greenhouse gas into valuable fuels. Thus, there is significant interest in developing highly active and stable electrocatalysts for the CO2 reduction reaction (CO2RR). Herein, a Ni and F co-doping strategy is proposed to facilitate the exsolution reaction and form a new cathode, Ni-Fe alloy nanoparticles embedded in ceramic Sr2Fe1.5Mo0.5O6-δ (SFM) doped with fluorine. F-doping and Ni-Fe exsolution enhance CO2 adsorption by a factor of 2.4 and increase the surface reaction rate constant (kchem) for CO2RR from 6.79 × 10-5 to 18.1 × 10-5 cm s-1, as well as the oxygen chemical bulk diffusion coefficient (Dchem) from 9.42 × 10-6 to 19.1 × 10-6 cm2 s-1 at 800 °C. Meanwhile, the interfacial polarization resistance (Rp) decreases by 52%, from 0.64 to 0.31 Ω cm2. At 800 °C and 1.5 V, an extremely high current density of 2.66 A cm-2 and a stability test over 140 h are achieved for direct CO2 electrolysis in the SOEC.

Conversion Kinetics and Ionic Conductivity in Na-β"-Alumina + YSZ (Naβ"AY) Sodium Solid Electrolyte via Vapor Phase Conversion Process.

Sodium ion batteries have been receiving increasing attention and may see potential revival in the near future, particularly in large-scale grid energy storage coupling with wind and solar power generation, due to the abundant sodium resources, low cost, and sufficiently high energy density. Among the known sodium ion conductors, the Na-β”-alumina electrolyte remains highly attractive because of its high ionic conductivity. This study focuses on the vapor phase synthesis of a Na-β”-Alumina + YSZ (Naβ”AY) composite sodium electrolyte, which has higher mechanical strength and stability than conventional single phase β”-Alumina. The objectives are the measurement of conversion kinetics through a newly developed weight-gain based model and the determination of sodium ionic conductivity in the composite electrolyte. Starting samples contained ~70 vol% α-Alumina and ~30 vol% YSZ (3 mol% Y2O3 stabilized Zirconia) with and without a thin alumina surface layer made by sintering in air at 1600 °C. The sintered samples were placed in a powder of Na-β”-alumina and heat-treated at 1250 °C for various periods. Sample dimensions and weight were measured as a function of heat treatment time. The conversion of α-Alumina in the α-Alumina + YSZ composite into Naβ”AY occurred by coupled diffusion of sodium ions through Na-β”-alumina and of oxygen ions through YSZ, effectively diffusing Na2O. From the analysis of the time dependence of sample mass and dimensions, the effective diffusion coefficient of Na2O through the sample, , was estimated to be 1.74 × 10−7 cm2 s−1, and the effective interface transfer parameter, , was estimated as 2.33 × 10−6 cm s−1. By depositing a thin alumina coating layer on top of the bulk composite, the chemical diffusion coefficient of oxygen through single phase Na-β”-alumina was estimated as 4.35 × 10−10 cm2 s−1. An AC impedance measurement was performed on a fully converted Naβ”AY composite, and the conductivity of the composite electrolyte was 1.3 × 10−1 S cm−1 at 300 °C and 1.6 × 10−3 S cm−1 at 25 °C, indicating promising applications in solid state or molten salt batteries at low to intermediate temperatures.

Breakthrough in the reduction of oxygen-annealing time for REBCO melt-textured bulks under an oxygen atmosphere containing water vapor

Since REBCO melt-textured bulks are produced in an extremely carrier underdoped state after melt-growth, oxygen-annealing is required for a few hundred hours to control the oxygen composition throughout the bulks due to their slow oxygen-diffusivity. In this study, we discovered that the oxygen-annealing time can be largely reduced with the introduction of water vapor to the oxygen-annealing process for Ag-added YBCO melt-textured bulks without any degradation of the superconducting properties. It was revealed that highly dense stacking faults were induced to the matrix during this wet oxygen-annealing, which are considered to act as fast diffusion paths of oxygen. The estimated chemical diffusion coefficients of oxygen at 400 °C for the bulk annealed under wet O2 was ∼4.3 × 10−11 m2s−1, which is almost twice as high as that annealed under standard dry O2 atmosphere. High Jc without a decrease in Tc was confirmed for the sample annealed under wet O2, when the wet O2 annealing was performed for a moderate time. In addition, a combined oxygen-annealing process, where wet O2 was supplied for the first 24 h followed by a dry O2 process for the remaining period, was found to be more effective to achieve both enhanced oxygen-diffusion and maintain high field trapping properties. These improvements are probably because moderate amounts of defects were introduced to the bulk as a function of the duration of the wet O2 annealing. The introduction of water vapor to the oxygen-annealing process will become a versatile technique to reduce the total processing time of REBCO melt-textured bulks.

Improved activity of oxygen in Ni–Ce0.8Sm0.2O2-δ anode for solid oxide fuel cell with Pr doping

Ni–Ce0.8PrxSm0.2-xO2-δ (x = 0–0.15) is studied as an anode of solid oxide fuel cells. The doping of Pr facilitates the chemical oxidation of CH3OH on the ceramic phase because the strength of Pr–O bond is weaker than that of Ce–O bond. With the addition of Pr, the electronic conductivity of the ceramic phase increases in an oxidizing atmosphere but decreases in a reducing atmosphere. The electrochemical conductivity relaxation results indicate that Pr improves oxygen surface exchange coefficient and oxygen chemical diffusion coefficient of Ce0.8Sm0.2O2-δ simultaneously. The doping of Pr shows negligible influence on the performance of the cell fed with H2 due to the high reactivity of H2. However, the maximum power density of the cell with methanol as the fuel increases significantly with the addition of Pr in the anode, demonstrating the high importance of the oxygen activity of the anode when a less active fuel is used. A Ce0.8Sm0.2O2-δ-carbonate electrolyte-supported single cell with Ni–Ce0.8Pr0.1Sm0.1O2-δ anode shows a maximum power density of 792 mW cm−2 at 650 °C with methanol as the fuel.

Investigation of structural, electrical and electrochemical properties of La0.6Sr0.4Fe0.8Mn0.2O3-δ as an intermediate temperature solid oxide fuel cell cathode

La0.6Sr0.4Fe0.8Mn0.2O3 (LSFM) compound is synthesized by sol-gel method and evaluated as a cathode material for the intermediate temperature solid oxide fuel cell (IT-SOFC). X-ray diffraction (XRD) indicates that the LSFM has a rhombohedral structure with R-3c space group symmetry. The XRD patterns reveal very small amount of impurity phase in the LSFM and Y2O3-stabilized ZrO2 (YSZ) mixture powders sintered at 600, 700, 800 and 850 °C for a week. The maximum electrical conductivity of LSFM is about 35.35 S cm−1 at 783 °C in the air. The oxygen chemical diffusion coefficients, DChem, are increased from 1.39 × 10−6 up to 1.44 × 10−5 cm2 s−1. Besides, the oxygen surface exchange coefficients, kChem, are obtained to lie between 2.9 × 10−3 and 1.86 × 10−2 cm s−1 in a temperature range of 600–800 °C. The area-specific resistances (ASRs) of the LSFM symmetrical cell are 7.53, 1.53, 1.13, 0.46 and 0.31 Ω cm2 at 600, 650, 700, 750 and 800 °C respectively, and related activation energy, Ea, is about 1.23 eV.

Crystal structure, oxygen nonstoichiometry, and mass and charge transport properties of the Sr-free SOFC/SOEC air electrode material La0.75Ca0.25FeO3-δ

The mixed ionic-electronic conducting perovskite La0.75Ca0.25FeO3-δ (LCF7525) was synthesized via a citric acid – ethylenediaminetetraacetate sol-gel route. Crystal structure, phase purity, and lattice constants were determined by powder X-ray diffraction and Rietveld refinement. The oxygen exchange kinetics (chemical surface exchange coefficients and chemical diffusion coefficients of oxygen) and the electronic conductivity were studied by in-situ dc-conductivity (relaxation) measurements at 600–800 °C and 1 × 10−3 ≤ pO2/bar ≤ 0.1. The thermal expansion coefficient was determined by dilatometry at 30–1000 °C and 1 × 10−3 ≤ pO2/bar ≤1. The oxygen nonstoichiometry was measured as a function of temperature and oxygen partial pressure by thermogravimetry and could be described by a point defect model. Experimental data of the chemical diffusion coefficient of oxygen and results from defect chemical modelling were used to estimate self-diffusion coefficients of oxygen and oxygen vacancies, as well as the ionic conductivity. Based on the results obtained for the mass and charge transport properties and the thermal expansion behaviour, it can be concluded that LCF7525 may be an attractive Sr- and Co-free material for air electrodes in intermediate temperature solid oxide fuel cells and solid oxide electrolyser cells.

Chemical diffusion coefficient of oxygen in thoria based fuel with 3 and 8 weight % urania

The chemical diffusion coefficients D˜ of oxygen in (Th1-yUy)O2+x cuboid specimens with 3 and 8 wt% UO2 were measured using the thermogravimetry method in the temperature range of 1316 K ≤ T ≤ 1428 K. The magnitude of the measured diffusion coefficients D˜ in these two specimens was as low as 5.3 × 10−11 ±2 × 10−11 m2 s−1 and as high as 4.5 × 10−10 ±2.2 × 10−12 m2 s−1 that generally increased with increasing uranium content. The activation energies for interstitial oxygen diffusion in the specimen thoria-urania matrix were found to range from 150.4 to 174.2 kJ mol−1, which was slightly higher than previously reported. An analytical solution approach with weight measurements was the technique used to assess the chemical diffusion coefficient in the specimens.

Strontium-free rare earth perovskite ferrites with fast oxygen exchange kinetics: Experiment and theory

The Sr-free mixed ionic electronic conducting perovskites La0.8Ca0.2FeO3-δ (LCF82) and Pr0.8Ca0.2FeO3-δ (PCF82) were synthesized via a glycine-nitrate process. Crystal structure, phase purity, and lattice constants were determined by XRD and Rietveld analysis. The oxygen exchange kinetics and the electronic conductivity were obtained from in-situ dc-conductivity relaxation experiments at 600–800°C and 1×10−3≤pO2/bar≤0.1. Both LCF82 and PCF82 show exceptionally fast chemical surface exchange coefficients and chemical diffusion coefficients of oxygen. The oxygen nonstochiometry of LCF82 and PCF82 was determined by precision thermogravimetry. A point defect model was used to calculate the thermodynamic factors of oxygen and to estimate self-diffusion coefficients and ionic conductivities. Density Functional Theory (DFT) calculations on the crystal structure, oxygen vacancy formation as well as oxygen migration energies are in excellent agreement with the experimental values. Due to their favourable properties both LCF82 and PCF82 are of interest for applications in solid oxide fuel cell cathodes, solid oxide electrolyser cell anodes, oxygen separation membranes, catalysts, or electrochemical sensors.