Oxygen Exchange Coefficient Research Articles

Equivalent Circuit Modelling for Electrode-Supported Solid Oxide Cell Based on Analysis of Gas and Temperature Dependence

Electrochemical impedance spectroscopy (EIS) measurement is used to determine the performance of solid oxide cells (SOCs). In the commercial-type of SOC such as an electrode-supported cell (ESC), all reaction resistance is sometimes combined into one spectrum, making it difficult to deconvolute the reaction resistance. The distributed-relaxation times (DRT) is often used to deconvolute the impedance spectra into a several reaction resistances. However, the DRT analysis cannot give any physical meaning to each reaction resistance of SOC’s components. Thus, it is necessary to build an equivalent circuit model to describe the physical meaning of each resistance.In this study, the ESC (Nexceris, USA) with a composition of Ni-YSZ (yttria stabilized zirconia) as a fuel electrode, 8YSZ as an electrolyte, Ce0.9Gd0.1O1.95 as an interlayer, and La0.6Sr0.4CoO3- δ (LSC) as an electrode was measured by EIS as a function of oxygen partial pressure, hydrogen partial pressure, water vapor partial pressure, temperature, and carbon dioxide partial pressure under open circuit voltage (OCV). At first, the impedance spectra were analyzed by the DRT method using software “FTIKREG” [1] based on Tikhonov regularization. After that, the complex nonlinear least square (CNLS) equivalent circuit was developed and fitted to the obtained impedance spectra.The gas conversion and diffusion at a very low-frequency region were analyzed by the Warburg impedance, meanwhile, the electrode resistance was perfectly fitted by the Gerischer impedance. The analysis result on the air electrode resistance of LSC showed the transport properties such as oxygen exchange coefficient and oxide ion diffusivity of the LSC air electrode close to the reported data on the bulk electrode [2]. Meanwhile, the analysis result on the Ni/YSZ fuel electrode resistance showed a good agreement on the reaction resistance and chemical capacitance from the reported data [3].Despite the developed model is “well-fitted” with the obtained impedance spectra, the analysis only done at OCV. In the real operating condition such as fuel cell or electrolysis mode the applied bias is used. Thus, the impedance spectra will slightly be different to those obtained under OCV. Therefore, the developed model will be also fitted to the impedance spectra under applied bias. The analysis result will be used to discuss the application of the developed equivalent circuit model. Acknowledgement This study was supported by the New Energy and Industrial Technology Development Organization (NEDO), grant number JPNP16002. Reference [1] J. Weese, Computer Physics Communications, 69 (1992) 99-111.[2] D. K. Hohnke et al., Solid State Ionics,5 (1981) 531-534.[3] M. Takeda, K. Yashiro, R. A. Budiman, S. Hashimoto, T. Kawada, J. Electrochem. Soc., 170 (2023) 034505.

Recovery and Delay of Impurity-Induced Poisoning of Oxygen Exchange Kinetics in Mixed Conducting Oxides Via Controlled Surface Acidity

The oxygen exchange kinetics in mixed conducting oxide surfaces are critical to a variety of energy-related technologies, including solid oxide fuel/electrolyser cells (SOFCs/SOECs), permeation membranes, batteries, and sensors. Such devices, however, commonly suffer from performance degradation from impurity-poisoning during long-term operation, for example, chromia poisoning in SOFCs from metal interconnects or silica from furnace refractories. The acidity/basicity of binary additives has recently been found to be a sensitive descriptor for determining the oxygen exchange coefficient (kchem ) of mixed ionic/electronic conducting oxides (MIEC), systematically varing by over 6 orders of magnitude via controlled surface infiltration with binary oxides. These huge variations in kchem are attributed to changes in surface electron concentration induced by the additives.Inspired by these ideas, in this work we first demonstrate the ability to recover the oxygen exchange kinetics of MIEC oxides (e.g. Pr0.1Ce0.9O2-δ (PCO) and Sr(Ti,Fe)O3 (STF)) by counteracting acidic infiltrant’s impact on degradation, e.g. by chromia or silica, by compensating with a basic infiltrant like lithia or calcia. Further, we further demonstrate the ability to enhance impurity tolerance of MIEC oxides by pre-coating basic additives onto the electrodes. Such pre-treatments not only markedly improve the initial kchem of the pristine MIEC oxides, but markedly lower the rate of degradation induced by subsequent coatings with acidic poisons. Electrical conductivity relaxation measurements on porous MIEC specimens were used to examine the impact of infiltrants on kchem . In parallel, the area-specific resistance of screen-printed symmetric cells (cathode|electrolyte|cathode) were examined by electrochemical impedance spectroscopy.

Surface oxygen exchange kinetics of mixed conducting oxides: Dilatometric vs electrical conductivity relaxation study

The surface oxygen exchange kinetics of mixed ionic-electronic conducting (MIEC) oxides play a crucial role in a variety of applications, including solid oxide fuel/electrolysis cells (SOFCs/SOECs), permeation membranes and sensors. To date, a variety of methods, including isotope exchange, electrical conductivity, optical absorptivity and thermogravimetry relaxation, and impedance spectroscopy have been used to measure the oxygen exchange coefficient of MIEC oxides. Each of these methods have their advantages and limitations, depending on the physical and chemical properties of the investigated materials and their sample dimensions and densities. Here, we demonstrate the ability to precisely measure the surface oxygen exchange coefficient (kchem) of MIEC model material Pr0.1Ce0.9O2-δ (PCO) by use of dilatometric relaxation measurements, particularly of interest for materials that exhibit larger chemical expansion coefficients. This is achieved by use of porous bulk specimens to ensure that the contribution of oxygen exchange to the overall kinetics is dominant. We demonstrate that kchem values extracted from chemical expansion relaxation measurements on PCO are nearly identical to those derived from electrical conductivity relaxation measurements. This provides the opportunity to precisely investigate the oxygen exchange and chemical expansion kinetics of a wide range of materials used in high-temperature applications, particularly where more conventional methods are difficult or inappropriate to apply.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and weattribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

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.

Oxygen permeability and surface kinetics of composite oxygen transport membranes based on stabilized δ-Bi2O3

Composite ceramic membranes based on the ionic conducting Tm-stabilized δ-Bi2O3 (BTM) and the electronic conducting (La0.8Sr0.2)0.99MnO3-δ (LSM) exhibit among the highest oxygen flux values reported for Bi2O3-based membranes. Here, we use pulse-response isotope exchange (PIE) and oxygen flux measurements to elaborate on limiting factors for the oxygen permeation in BTM - 40-70 vol% LSM composites. Once both phases percolate, between 30 and 50 vol% BTM, the flux is essentially independent of the BTM/LSM volume ratio. The oxygen permeability is under mixed diffusion- and surface control, gradually becoming more bulk-limited with increasing temperature. The oxygen exchange coefficients of BTM-LSM are significantly higher than its constituent phases, revealing that a cooperative surface exchange mechanism enhances the kinetics. Some of the Tm was substituted with Pr to introduce electronic conductivity in BTM. (Bi0.8Tm0.15Pr0.05)2O3-δ (BTP) exhibits higher surface exchange coefficients compared to BTM, but the oxygen flux remains one order of magnitude lower than that of percolating BTM-LSM composites.

Triple Perovskite Nd1.5Ba1.5CoFeMnO9-δ-Sm0.2Ce0.8O1.9 Composite as Cathodes for the Intermediate Temperature Solid Oxide Fuel Cells.

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd1.5Ba1.5CoFeMnO9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (RP) of NBCFM is 1.1273 Ω cm2~0.1587 Ω cm2 in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10−6 K−1 at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce0.8Sm0.2O2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10−6 K−1 to 15.25 × 10−6 K−1. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10−4 to 10−3 cm s−1 over the range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm2 at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm2). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.

Thermally Controlled Activation and Passivation of Surface Chemistry and Oxygen-Exchange Kinetics on a Perovskite Oxide

The state-of-the-art oxygen electrode materials used in solid oxide cells at 600–800 °C can deteriorate by gradual passivation of their surfaces related to changes in the surface chemistry. We have recently observed the unexpected recovery of the oxygen-exchange activity on La0.6Sr0.4FeO3 after short-term exposure to high temperatures; a 10 h-long thermal treatment at above 800 °C resulted in a 40-fold increase in the oxygen-exchange coefficient, kchem, from 1 × 10–5 to 4 × 10–4 cm/s. The present work addresses the underlying mechanism of this improvement by investigating the chemical and morphological changes at the surface with respect to thermal history. Using repeated sequences of 10 h-long thermal treatments at 1000 °C over weeks-long aging at around 650 °C, we have probed the oxygen-exchange kinetics and surface chemistry concurrently by electrical conductivity relaxation and X-ray photoelectron spectroscopy (XPS). The oxygen-exchange coefficient decreases with aging at 650 °C following Avrami-type kinetics but sharply increases after each high-temperature treatment. An increased amount of strontium [40% cation fraction; XSr/(XLa + XSr + XFe)] is found on the surface of all thermally treated samples (compared to the bulk nominal 20%). Hence, the passivation/reactivation is not due to the total Sr enrichment as such. By XPS, two different states of Sr are observed on the surface (labeled “lattice-bound” and “secondary phase”). The relative amounts in these two phases vary with time, differently at 650 °C versus 1000 °C. The amount of “lattice-bound Sr” decreases and the amount of “secondary phase Sr” increases with aging at 650 °C and the opposite at 1000 °C. We show that it is the Sr in the secondary phases, rather than the total amount, which is the main culprit for performance degradation. The findings are further supported by the observations of morphological changes on the surface in the form of SrO, SrCO3, and SrSO4 precipitates and layers. Decomposition of secondary Sr phases at 1000 °C and simultaneous dissolution of Sr back in a perovskite lattice rejuvenate the surface and lead to a significant increase in exchange kinetics.

Investigation of structural, electrical and ionic properties of La0.6Ca0.4Fe0.8Ni0.2O3 as an IT-SOFC cathode synthesized through microwave-assisted method

In this paper, structural, electrical and ionic properties of La0.6Ca0.4Ni0.2Fe0.8O3-δ synthesized with microwave-assisted heating is investigated and compared with a sample synthesized by conventional method. Samples are prepared by sol-gel method. X-ray diffraction indicates that both samples have orthorhombic symmetry with Pnma space group. The electrical conductivity is measured by four-probe method and the maximum electrical conductivity of the microwave-assisted sample (LCFN-MWC) is about 29.6 Scm−1. Ionic properties of samples are studied using electrical relaxation technique. Oxygen diffusion coefficient and oxygen exchange coefficient of LCFN-MWC are found to be 1.3 × 10−4 cm2S−1 and 6.4 × 10−3 cmS−1 at 600°C, respectively. Moreover, the associated activation energies are about 0.70 eV and 0.30 eV for LCFN-MWC.

Understanding the oxygen reduction kinetics on Sr2-xFe1.5Mo0.5O6-δ: Influence of strontium deficiency and correlation with the oxygen isotopic exchange data

This paper presents, for the first time, the results of studies of the electrochemical reaction of oxygen reduction on Sr2Fe1.5Mo0.5O6-δ and Sr-deficient Sr1.95Fe1.5Mo0.5O6-δ, as promising electrodes for solid state electrochemical devices, by the electrochemical impedance method with the subsequent interpretation of the data using the concepts outlined in the Adler et al. model. It was established that the oxygen reduction reaction for both electrodes is determined by two relaxation processes associated with oxygen diffusion, oxygen surface exchange, and Knudsen diffusion in the pores of the electrode. Strontium deficiency was found to have a positive effect on the electrochemical activity of the electrodes, enhancing the stage related to oxygen diffusion and surface exchange. Also, for the first time, for Sr2Fe1.5Mo0.5O6-δ, a quantitative correlation for the surface oxygen exchange and diffusion coefficients obtained from the impedance data and by the isotopic exchange method is demonstrated.

Silicon Contamination of the Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient

Chemical oxygen surface exchange coefficients are used to quantify and rank the performance of oxygen exchange catalysts used in solid oxide fuel cell, solid oxide electrolysis cell, oxygen separation membrane, catalytic converter, and other oxygen-exchange-enabled devices. Unfortunately, during the manufacture and operation of these devices it is easy to introduce siliceous contaminants that degrade their oxygen exchange performance. Surprisingly, despite the well-known negative impact siliceous impurities have on the oxygen transport properties of ceria-based materials, quantitative measurements of how siliceous impurities impact the chemical oxygen surface exchange coefficient of praseodymium doped ceria are largely absent from the literature. In this work, the crystal structure, film thickness, surface composition, bulk composition, and chemical oxygen exchange coefficient of (100)-oriented PCO thin films with, and without, surface silica contaminants were evaluated via X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS), and Curvature Relaxation (κR) experiments, respectively. The results show that siliceous surface phases only a few nanometers thick are capable of (1) reducing the PCO chemical oxygen surface exchange coefficient by approximately 3 orders of magnitude and (2) approximately doubling the activation energy for oxygen exchange.

Determination of relevant factors affecting the surface oxygen exchange coefficient of solid oxide fuel cell cathode with ionic conducting oxide coating

In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.

AMPK Phosphorylation Effect of Genistein Is Independent of GPR30

Abstract Objectives Genistein promotes fatty acid oxidation, improves glucose tolerance and increases energy expenditure. These effects are associated with AMPK phosphorylation in skeletal muscle. However, the molecular mechanism by which genistein activates the signaling pathway to activate AMPK phosphorylation is not yet known. Several target proteins have been suggested, one o them is GPR30, but it has not been studied. Thus, the aim of this study was to determine if AMPK phosphorylation by genistein is activated by GPR30. Methods To determine if GPR30 is involved in the mechanism that promotes AMPK phosphorylation we used C2C12 GPR30 KD (knock down) myotubes obtained by transduction with a shRNA, and also GPR30 −/− primary myotubes. AMPK phosphorylation was determined via western blot in the presence of genistein after 1 hour in both cultures. Furthermore, in order to establish in vivo if the beneficial effects of genistein were associated to GPR30, C57BL/6 mice (wild type WT) and GPR30 −/− mice were fed with both control and high fat sucrose diets (HFSD) containing genistein for 2 months. During two months of treatment, weight gain, food consumption, and body composition were measured as well as a glucose tolerance test (GTT), indirect calorimetry and AMPK phosphorylation by western blot in skeletal muscle of all of groups. Results Genistein at concentrations of 0.3,1,3 and 10µM increased AMPK phosphorylation in GPR30 KD, and GPR30 −/− primary myotubes, indicating that GPR30 is not directly involved in AMPK phosphorylation by genistein in an in vitro model. The in vivo study showed that the effect of genistein in GPR30 −/− mice on weight gain, body composition, GTT and indirect calorimetry was similar compared to wild type mice. The mice that consumed a HFSD with genistein gained less weight and less body fat, regardless of genotype (WT or GPR30 −/−). Similar effects were observed in the glucose tolerance test, oxygen exchange coefficient and oxygen consumption. Wild type and GPR30 −/− mice that consumed genistein had a better glucose tolerance than those that did not consume genistein. Conclusions This study showed that GPR30 its not directly involved in promoting AMPK phosphorylation by genistein. In addition, genistein reduced weight gain, increased glucose tolerance and increased oxygen consumption even in the absence of GPR30. Funding Sources INCMNSZ.

Influence of Sr and Co deficiency on the transport properties and oxygen reduction reaction of La0.6Sr0.4Co0.2Fe0.8O3-δ

The surface oxygen exchange coefficient (k*) and the oxide ion diffusivity (D*) in the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) related perovskite materials with various strontium (Sr) and cobalt (Co) compositions were determined by a combination of isotope exchange (IE) technique and secondary ion mass spectrometry (SIMS). Results show that the derived values of k* and D* decrease with decreasing Sr and Co content in those LSCF samples. To corroborate the results, electrochemical impedance spectroscopy (EIS) was performed on porous LSCF samples with the same Sr and Co deficiency; obtained impedance spectra were analyzed by the transmission line model (TLM) to derive k* and D*. It showed that those k* and D* values obtained from the isotope exchange experiments are in good agreement with the electrochemically determined values. For further analysis, thermogravimetry was performed to evaluate the oxygen vacancy concentration (δ) for respective samples. It has been found that changes in log D* can be well correlated with the log (δ/(3-δ)) as expected from the vacancy mechanism for the oxide ion diffusion. On the other hand, changes in log k* are not correlated only with the ionic properties like log (δ/(3-δ)) but also with the electron properties; this can be seen in the h plane coordinated with log D* and log k*.

Rate-Determining Steps of Oxygen Surface Exchange Kinetics on Sr2Fe1.5Mo0.5O6−δ

The oxygen surface kinetics of Sr2Fe1.5Mo0.5O6−δ was determined using the 16O2/18O2 isotope exchange method with gas phase analysis at 600–800 °C. The heterogeneous exchange rates (rH) and the oxygen diffusion coefficients (D) were calculated by processing the concentration dependences of the 18O fraction using Ezin’s model. The rates of oxygen dissociative adsorption (ra) and incorporation (ri) were calculated based on a model using the three exchange type rates. It has been established that the rates ra and ri were comparable in this temperature range. Assumptions were made about the effect of the chemical composition of the surface on the rate of oxygen adsorption. It was found that the oxygen exchange coefficient (k) of Sr2Fe1.5Mo0.5O6−δ is comparable to that of La0.6Sr0.4MnO3±δ oxide. High values of the oxygen diffusion coefficient were found for Sr2Fe1.5Mo0.5O6−δ. The values were comparable to those of the double cobaltite praseodymium-barium and exceed by more than an order those of lanthanum-strontium manganite.

Determination of Factors Governing Surface Composition and Degradation of La0.6Sr0.4Co0.2Fe0.8O3-δ Electrode under Sulfur-Contained Air

Factors governing the change of the surface composition and the degradation of transport properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) under low concentration sulfur-contained air (0.01 ppm) were examined as functions of water vapor pressure (p(H2O)) and oxygen partial pressure (p(O2)). Bulk LSCF samples were heat-treated at 973 K in dry or wet air containing SO2 and more enhanced SrSO4 formation/growth was observed under wet compared to dry air. When the p(O2) was changed from 0.21 to 0.01 bar, the SrSO4 precipitates became smaller in size and higher in number, indicating that the p(H2O) and the p(O2) have different effects on secondary phase formation. Detailed analysis of the surface composition indicates the presence of CoFe2O4 in the vicinity of the SrSO4 particle, suggesting the Sr and Co depletion in the subsurface region. Based on the analysis, the water vapor affects the secondary phase growth due to the possibility of additional reaction pathways that may enhance the growth rate. This behavior is correlated with the logarithmic surface oxygen exchange coefficient, log k*, that was observed to decrease monotonically within 20 h of annealing time but its degradation was found to be faster under wet air, in accordance with Sr and Co depletion rate.

Influence of oxygen partial pressure on the oxygen diffusion and surface exchange coefficients in mixed conductors

The performances of mixed ionic and electronic conductors is used in many applications, such as oxygen transport membranes or electrodes in solid oxide fuel cells. The performances of these systems depend mainly on two fundamental parameters including oxygen diffusion (DO) and the oxygen exchange coefficient (k). This work focuses on the impact of the oxygen partial pressure on oxygen diffusion and surface exchange coefficients of mixed conducting materials, as reported studies are scarce in the literature. In this way, two different mixed conducting materials are studied, namely, La0.6Sr0.4Fe0.6Ga0.4O3-δ, a perovskite-type material, and the nickelate La2NiO4+δ. The DO and k coefficients are determined by a specific oxygen permeation measurement and by the isotopic exchange depth profile method.

Fine-tuning of oxygen vacancy and interstitial concentrations in La1.85Ce0.15CuO4+δ by electrical bias

Defects present in oxide materials are well known to strongly impact transport properties, as well as interfacial reaction kinetics, such as oxygen exchange with the atmosphere. Most commonly, oxygen defect concentrations, or equivalently oxygen nonstoichiometry, are set in a given material by controlling the external oxygen partial pressure and temperature. This approach, however, is limited by the range of oxygen partial pressures readily experimentally achievable and the time it takes to reach equilibrium. In this study, we instead rapidly fine-tune oxygen defect concentrations in promising rare earth cuprate (RE2CuO4: RE = rare earth) SOFC cathode materials by controlling the electrical potential applied across a YSZ supporting electrolyte. We then monitor the induced oxygen nonstoichiometry variations by in-situ measurement of chemical capacitance. These layered perovskites, that incorporate both oxygen interstitials and vacancies, show promise as SOFC cathodes. In this study, we attempt to correlate the changing defect and transport properties with the measured area-specific-resistance, or equivalently the surface oxygen exchange coefficient. Success is also demonstrated in measuring in-plane film conductivity over a wide range of effective oxygen partial pressure, with the results agreeing with predictions from appropriate defect models.

Ceria: Recent Results on Dopant-Induced Surface Phenomena

Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO2 whose vapour pressure is considerably higher than that of ZrO2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O2−δ pellets leading to a porous surface zone of Ce2Zr2O7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO2 or H2O since re-oxidation temperatures of (Ce,Zr)O2−δ are known to be shifted towards lower temperatures with increasing ZrO2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10−4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10−7 mol·O/(cm3·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.

Oxygen reduction reaction process of LaNi0.6Fe0.4O3 − δ film – porous Ce0.9Gd0.1O1.95 heterostructure electrode

Relevant factors affecting an oxygen reduction reaction (ORR) in a heterostructure electrode comprising a Ce0.9Gd0.1O1.95 porous layer on LaNi0.6Fe0.4O3−δ film electrode were evaluated using electrochemical impedance spectroscopy and isotope depth profile analyses. Despite the LaNi0.6Fe0.4O3−δ film – porous Ce0.9Gd0.1O1.95 electrode having higher interfacial conductivity,σE, compared to a bare LaNi0.6Fe0.4O3−δ film electrode, σEbehavior as a function of p(O2) was similar to each measured temperature. This exhibits a good correlation with the isotopic oxygen depth profile analysis where the evaluated surface oxygen exchange coefficient, k*, showed enhancement compared to bare LaNi0.6Fe0.4O3−δ film and exhibited the same behavior on p(O2). Based on the analyses, there are two plausible reasons for this enhancement: (a) the formation of triple phase boundaries at the junction of LaNi0.6Fe0.4O3−δ film, porous Ce0.9Gd0.1O1.95, and gas phase, where the incorporation process during an ORR process takes place via the porous Ce0.9Gd0.1O1.95; and (b) a modification of the subsurface layer just below the film surface where the presence of diffused Ce and Gd appears to accelerate the ORR process.