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The surface exchange and bulk transport of oxygen are highly relevant to ceria-based redox materials, which are envisaged for the solar thermochemical splitting of carbon dioxide in the future. Experimental investigations of oxygen isotope exchange on CeO2-δ, Ce0.9M3+0.1O1.95-δ (with M3+ = Y, Sm) and Ce0.9M4+0.1O2-δ (with M4+ = Zr) samples were carried out for the first time utilizing oxygen-isotope-enriched C18O2 gas atmospheres as the tracer source, followed by Secondary Ion Mass Spectrometry (SIMS), at the temperature range 300 ≤ T ≤ 800 °C. The experimental K ˜ O and D ˜ O data reveal promising results in terms of CO2 splitting when trivalent (especially Sm)-doped ceria is employed. The reaction temperatures are lower than previously proposed/reported due to the weak temperature dependency of the parameters K ˜ O and D ˜ O . The majority of isotope exchange experiments show higher values of K ˜ O and D ˜ O for Sm-doped cerium dioxide in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. The apparent activation energies for both K ˜ O and D ˜ O are lowest for Sm-doped ceria. Using Zr-doped cerium oxide exhibits various negative aspects. The Zr-doping of ceria enhances the reducibility, but the possible Zr-based surface alteration effects and dopant-induced migration barrier enhancement in Zr-doped ceria are detrimental to surface exchange and oxygen diffusion at lower temperatures of T ≤ 800 °C. [ABSTRACT FROM AUTHOR]

Oxygen chemical surface exchange coefficient kδ\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${k}^{\updelta }$$\end{document} for Ba0.5Sr0.5(Co0.8Fe0.2)1−xMexO3−ẟ (Me = Ta, W) has been measured by oxygen pressure relaxation method in the temperature range 600–800℃ and oxygen pressure 1.3–34.7 mbar. The comparison of the values of the tracer k∗\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${k}^{*}$$\end{document} and chemical kδ\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${k}^{\updelta }$$\end{document} oxygen surface exchange rate constants allowed to evaluate the additional oxygen capacity of the surface layer, which is different from the bulk oxygen capacity, characterized by the thermodynamic factor wO=12∂lnpO2∂ln3-δ\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${\text{w}}_{\text{O}}=\frac{1}{2}\frac{\partial \text{ln}\left({\text{pO}}_{2}\right)}{\partial \text{ln}\left(3-\updelta \right)}$$\end{document} calculated from the T-pO2-3-δ\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\text{T}-{\text{pO}}_{2}-\left(3-\updelta \right)$$\end{document}–diagram. The possible reasons were related to the specific phase composition of the surface layers responsible for the oxygen exchange process. The pO2\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${\text{pO}}_{2}$$\end{document} dependence of the chemical oxygen exchange coefficient was discussed in terms of surface coverage with adsorbed oxygen anionic forms. The relationship between the mechanism of surface oxygen exchange, determined either during equilibration of oxygen pressure or gas phase composition (oxygen isotope exchange), was explained in terms of Fleig’s theory (https://doi.org/10.1039/b618765j). The relationship between the chemical composition of the surface and the mechanism of the surface oxygen exchange is discussed. [ABSTRACT FROM AUTHOR]

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.

[Display omitted] • A cubic fluorite/perovskite Pr 6 O 11 -Pr 1.2 Sr 0.8 NiO 4+δ (PO-PSNO) heterostructure cathode is prepared by impregnation method. • The ORR reactive sites at the heterointerface are increased, thus enhancing the electrochemical performance of the heterostructure cathode. • About 10% PO impregnation can make the heterointerface have the best ORR reactive site. Pr 2 NiO 4+δ -based (Ruddlesden-Popper) perovskite-structured oxide materials are promising candidates for low and intermediate temperature solid oxide fuel cells (SOFCs) systems. In this study, we prepared a high-performance cubic-fluorite/perovskite heterostructure Pr 6 O 11 -Pr 1.2 Sr 0.8 NiO 4+δ (PO-PSNO) cathode for intermediate temperature SOFCs by solution impregnation method. By changing the impregnation amount of Pr 6 O 11 , the micro-morphology of the heterostructure was regulated, so that the heterostructure interface had the best reactive site. The oxygen reduction reaction (ORR) performance of Pr 1.2 Sr 0.8 NiO 4+δ is improved, and the content of surface Sr in the heterostructure electrode material is reduced. Our results indicate that the synergistic effect of PO particles and PSNO skeleton makes the heterostructure exhibit a high oxygen surface exchange properties and catalytic performance. When the impregnation amount of PO particles reaches about 10% of the PSNO skeleton, the heterostructure has the best ORR activity. The oxygen surface exchange coefficient (K chem) of 2-PO-PSNO is 5.8 × 10-4 cm·s−1, being 1.45 times as that of PSNO at 750 °C. Meanwhile, the polarization impedance (R p) of 2-PO-PSNO is reduced by 66% to only 0.114 Ω cm2. Our study provides a new approach for developing high-performance RP structured perovskite cathodes and demonstrates practical applications in the field of SOFCs. [ABSTRACT FROM AUTHOR]

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La 1– x Sr x ScO 3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO 3 oxide. However, for the strontium-doped oxides La 1– x Sr x ScO 3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La 0.91 Sr 0.09 ScO 3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La 1– x Sr x ScO 3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (p H 2 O = 2.35 kPa, p O 2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С. • The oxygen surface exchange rates are measured in La 1− x Sr x ScO 3−δ oxides. • The oxygen diffusion coefficients are measured in La 1− x Sr x ScO 3−δ oxides. • The proton transference numbers are estimated in La 1− x Sr x ScO 3−δ oxides. [ABSTRACT FROM AUTHOR]

Abstract The interaction between oxygen in the gas phase and La 2– x Ca x Zr 2 O 7– α (x = 0; 0.05; 0.1) with a pyrochlore structure was studied by the isotope exchange method with the gas phase equilibration in the temperature range of 600–900 °C and the oxygen pressure of 1.01 kPa. The oxygen diffusion and oxygen surface exchange coefficients were calculated for the investigated oxides. The rate-determining stage of the oxygen surface exchange kinetics for La 1.9 Ca 0.1 Zr 2 O 7– α was found to be different from that for La 1.95 Ca 0.05 Zr 2 O 7– α and La 2 Zr 2 O 7– α. The influence of the calcium content in La 2– x Ca x Zr 2 O 7– α (x = 0; 0.05; 0.1) oxides on the heterogeneous exchange rates and oxygen tracer diffusion coefficients were discussed. Graphical abstract Electron back-scattered diffraction results. fx1 Highlights • The oxygen surface exchange rates are measured in doped lanthanum zirconates. • The oxygen diffusion coefficients are measured in doped lanthanum zirconates. • The calcium segregation process is found in doped lanthanum zirconates. [ABSTRACT FROM AUTHOR]

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films. [ABSTRACT FROM AUTHOR]

Abstract: A novel double perovskite Sr2FeMo2/3Mg1/3O6−δ is prepared and characterized as an anode material for solid oxide fuel cells (SOFCs). X‐ray diffraction refinement reveals that Mg and Mo cations locate separately in two different B sites (B and B′ in A2BB′O6) while Fe occupies both B and B′ sites, forming the lattice structure with the form of Sr2(Mg1/3Fe2/3)(Mo2/3Fe1/3)O6−δ. The inactive element Mg doping not only endows the material with excellent redox structural stability but also triggers the creation of antisite defects in the crystal lattice, which provide the material with excellent electrochemical activity. The anode performance of Sr2FeMo2/3Mg1/3O6−δ is characterized in an La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte supported cell with La0.58Sr0.4Fe0.8Co0.2O3−δ cathode. A peak power density of 531, 803, 1038, and 1316 mW cm−2 at 750, 800, 850, and 900 °C, respectively, is achieved in humidified H2. The Sr2FeMo2/3Mg1/3O6−δ shows suitable thermal expansion coefficient (16.9(2) × 10−6 K−1), high electrical conductivity, and good tolerance to carbon deposition and sulfur poisoning. First‐principle computations demonstrate that the presence of FeBOFeB′ bonds can promote the easy formation and fast migration of oxygen vacancies in the lattice, which are the key to affecting the anode reaction kinetics. The excellent overall performance of Sr2FeMo2/3Mg1/3O6−δ compound makes it a promising anode material for SOFCs. [ABSTRACT FROM AUTHOR]

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-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. [ABSTRACT FROM AUTHOR]

The interaction of oxygen in the gas phase with double perovskites LnBaCo 2 O 6 − δ (Ln = Pr, Sm, Gd) was studied by the isotope exchange method with gas phase equilibration in the temperature range of 600–850 °C and the oxygen pressure range of 1.3 · 10 − 3 –6.6 · 10 − 2 atm. The oxygen nonstoichiometry of the oxides was determined by means of the thermogravimetric analysis as a function of oxygen partial pressure and temperature in the ranges − 3 ≤ log( Po 2 /atm) ≤ − 0.68 and 600 ≤ T, °C ≤ 850, respectively. The influence of the oxygen nonstoichiometry on the heterogeneous exchange rate and the oxygen tracer diffusion coefficient was discussed. The rate-determining stage of the oxygen surface exchange kinetics for PrBaCo 2 O 6 − δ was found to be different in comparison with SmBaCo 2 O 6 − δ and GdBaCo 2 O 6 − δ . The possible reasons of this observation were considered. [ABSTRACT FROM AUTHOR]

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.

This work presents the effect of dispersed copper oxide (CuO) nanoparticles on the oxygen reduction reaction (ORR) on a typical solid oxide fuel cell (SOFC) electrocatalyst, La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF). The ORR kinetics were enhanced by a factor up to 4 at 750 °C as demonstrated by electrical conductivity relaxation measurements used to determine the chemical surface exchange coefficient, k chem . The value of k chem increased from 2.6 × 10 −5 cm s −1 to 9.3 × 10 −5 cm s −1 at 750 °C when the LSCF surface was coated with submicron CuO particles. The enhanced k chem was attributed to additional reactions that occur on the CuO surface and at the LSCF-CuO-gas three-phase boundaries (3PBs) as suggested by the k chem dependence on CuO coverage and 3PB length. This enhancement was further demonstrated by the introduction of CuO nanoparticles into LSCF electrodes. CuO infiltrated electrodes reduced the interfacial polarization resistance from 2.27 Ω cm 2 to 1.5 Ω cm 2 at 600 °C and increased the peak power density from 0.54 W cm −2 to 0.72 W cm −2 at 650 °C. Electrochemical impedance spectroscopy indicated that the reduced resistance was due to the shrinkage of the low frequency arc, which is associated with the electrochemical surface exchange reaction. [ABSTRACT FROM AUTHOR]

The influence of dense membrane microstructures on semi permeation performance is still not well understood, and no consensus or explanation can be established from the literature. The apparent discrepancy is likely due to a poor understanding of the oxygen transport mechanisms through the membrane and, specifically, to the impact of the microstructure on the oxygen surface exchange kinetics. The aim of this paper is to provide a better understanding of the impact of microstructures on oxygen transport mechanisms through the membrane. Two reference materials, Ba0.5Sr0.5Fe0.7Co0.3O3 (BSFCo) and La0.5Sr0.5Fe0.7Ga0.3O3 (LSFG) perovskites, are considered to explain the discrepancies observed in the literature. [ABSTRACT FROM AUTHOR]

The effect of scandium content in CaZrScO on electrical conductivity and oxygen exchange kinetics was investigated. The electrical conductivity was studied by means of impedance spectroscopy depending on temperature (700-850 °С) and oxygen partial pressure (2.1 · 10 > pO > 10 Pa). The oxygen exchange kinetics was studied using the isotope exchange method with gas phase equilibration in the temperature range of 700-850 °С and oxygen pressures of 0.13-6.7 kPa. The oxygen tracer diffusion coefficient, the oxygen heterogeneous exchange rate, and the electrical conductivity of oxides with the growing scandium content were found to increase. It was assumed that the electronic defects play an important role in the oxygen surface exchange. [ABSTRACT FROM AUTHOR]

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is a mixed ionic-electronic conducting perovskite (ABO 3 ) material which has attracted significant research interest in recent years as a cathode for solid oxide fuel cell (SOFC) applications at intermediate temperatures (500 °C–750 °C). Fundamental understanding of the oxygen surface exchange kinetics and diffusion in LSCF cathodes is necessary to optimize the cell performance at the required operating temperatures. In this study, we controlled the surface microstructure and crystalline orientation of LSCF thin films on (100) Gd 2 O 3 -doped CeO 2 thin films (GDC) prepared by pulsed laser deposition on yttria-stabilized zirconia (YSZ) single crystal substrates. Tailoring of the LSCF microstructure was achieved by modifying the underlying GDC thin film surface morphology and microstructure. To clarify the effect of the LSCF microstructure on its oxygen surface exchange kinetics, we employed the 18 O/ 16 O oxygen isotope exchange technique in conjunction with secondary ion mass spectroscopy (SIMS) depth profile analysis. Comparison of the qualitative features of the measured concentration of the 18 O profile and evaluation of the oxygen surface exchange coefficient k * were performed and the results correlated to the LSCF microstructure. [ABSTRACT FROM AUTHOR]

Oxygen surface exchange properties of lanthanum nickelates La 2 NiO 4+δ , La 3 Ni 2 O 7-δ and La 4 Ni 3 O 10-δ (LNOs) have been investigated using the Pulsed Isotope 18O/16O Exchange (PIE) technique in the temperature range 300–700 ​°C. The evaluation of the oxygen exchange rate from these lanthanum nickelates was conducted by measurement of isotope fractions 18O 2 , 16O18O, and 16O 2 in the effluent pulse at the exhaust from the reactor with a packed bed. The rates of oxygen heterogenous surface exchange (r H), dissociative adsorption (r a) and incorporation (r i) were calculated. The oxygen surface exchange was found to vary between the three nickelates, with r H decreasing between samples La 3 Ni 2 O 7-δ ​> ​La 4 Ni 3 O 10-δ ​> ​La 2 NiO 4+δ , which was attributed to a decreasing presence of Ni cations on the particle surfaces. Despite possessing the lowest oxygen surface exchange, La 2 NiO 4+δ had the best electrode electrochemical performance, which was attributed to highly mobile interstitial oxygen atoms leading to the largest r i (surface oxygen incorporation rate into the bulk) of the nickelates. This study reveals the key limiting factors determining lanthanum nickelate cathode performance in SOFCs, and we suggest avenues to improve these materials towards functional cathodes in devices. [Display omitted] • La 2 NiO 4 oxygen surface exchange is limited by dissociative adsorption of oxygen. • For La 3 Ni 2 O 7 , La 4 Ni 3 O 10 oxygen surface exchange is limited by oxygen incorporation. • Low presence of Ni on the particle surfaces decreases oxygen surface exchange rate. [ABSTRACT FROM AUTHOR]

The mixed ionic and electronic conductor Sr(Ti,Fe)O 3 - α (STF) exhibits fast oxygen surface exchange kinetics, with electron concentration potentially playing a key role. The effect of La donor doping on electron concentration, Fermi level, and overall defect chemistry of STF is investigated on Sr 1 - y La y Ti 0.65 Fe 0.35 O 3 - α (LSTF, 0 ≤ y ≤ 0.5) thin films. Defect chemical modeling, optical absorption, and electrical conductivity measurements in oxidizing conditions indicate compensation of donors by an increase in oxygen and electron concentrations, increase in Fermi level, decrease in oxygen vacancy and hole concentrations, and formation of cation vacancies (for [La] > [Fe]). The surface exchange coefficient, measured by impedance spectroscopy, decreased with increasing donor concentration, suggesting that oxygen exchange kinetics in LSTF are limited by low oxygen vacancy and/or hole concentrations, rather than electron transfer. [ABSTRACT FROM AUTHOR]

The surface oxygen exchange kinetics on the double perovskite PrBaCo2O5+δ has been studied using pulse 18O–16O isotope exchange measurements in the temperature range from 300 to 500°C. The exchange rate, at oxygen partial pressure of 0.21atm, shows an activation energy of 97±3kJmol−1. A two-step exchange mechanism is applied to account for the observed distribution of 16O2, 16O18O, and 18O2 in the effluent pulses. The results indicate that the oxygen exchange kinetics on PrBaCo2O5+δ is determined by the rates of homolytic oxygen dissociative adsorption on the PrBaCo2O5+δ surface and incorporation of oxygen adatoms in the oxide lattice. A changeover of the rate-determining step occurs upon increasing pO2 from 0.1 to 0.95atm. [ABSTRACT FROM AUTHOR]

Abstract: Numerous mixed ionic electronic conducting materials with a perovskite structure have been investigated in the literature as potential membrane materials for oxygen separation applications. However, the effect of cation substitution in the A site of a perovskite structure on the oxygen semi-permeation flux is not clearly understood. The goal of this paper is to provide insight into the effect of cation substitution in the A-site on oxygen semi-permeation flux in La0.5A0.5Fe0.7Ga0.3O3−δ and La0.5A0.5Fe0.7Co0.3O3−δ perovskite membranes with A = Ca, Sr, and Ba. In addition, the rate-determining step in the trans-membrane oxygen transport was clearly identified based on oxygen diffusion and oxygen surface exchange coefficients for these two perovskite membrane series. [Copyright &y& Elsevier]

Highlights: [•] The ratio of Sm3+ concentration to Nd3+ had significant effect on the electrical properties of SNDC. [•] The SNDC (x =0.6) had the highest ionic conductivity with slowest oxygen surface exchange. [•] The SNDC (x =0.6) showed the largest transference number of oxygen ions. [•] Co-doping can enhance beneficial properties of SNDC. [Copyright &y& Elsevier]

Abstract: To clearly identify the rate determining step (r.d.s.) in the oxygen semi-permeation of dense Ln2NiO4+δ membranes (with Ln=La, Pr and Nd), a specific apparatus was designed to measure the oxygen semi-permeation flux and activity gradient at both membrane surfaces. From these measurements, the coefficients for the oxygen diffusion and surface exchange at both membrane surfaces were determined. Their values were compared to the values reported in the literature for similar membrane materials. We concluded that the oxygen flux through these membranes was limited by the oxygen surface exchange at the oxygen-lean face and not oxygen volume diffusion, as previously claimed. [Copyright &y& Elsevier]

Abstract: Here, an in situ curvature relaxation (κR) method was used to measure chemical oxygen surface exchange coefficients ( ) under well-characterized stress, temperature, and oxygen partial pressure conditions. These were measured by analyzing the transient curvature of yttria stabilized zirconia supported La 0.6 Sr 0.4 FeO 3−δ thin films reacting to oxygen partial pressure step changes. The sputtered thin film measured here were consistent with extrapolated bulk sample , but larger than those reported for pulsed laser deposited thin films. This is the first time that the curvature response of a system has been used to characterize thin film oxygen surface exchange kinetics. The simultaneous measurement of film stress and provided by the curvature relaxation method may help explain the large discrepancies observed in the literature. [Copyright &y& Elsevier]

Solid-state heterointerfaces are of interest for emergent local behavior that is distinct from either bulk parent compound. One technologically relevant example is the case of mixed ionic/electronic conductor (MIEC)-metal interfaces, which play an important role in electrochemistry. Metal-MIEC composite electrodes can demonstrate improved catalytic activity vs single-phase MIECs, improving fuel cell efficiency. Similarly, MIEC surface reaction kinetics are often evaluated using techniques that place metal current collectors in contact with the surface under evaluation, potentially altering the response vs the native surface. Techniques enabling direct and local in situ observation of the behavior at and around such heterointerfaces are needed. Here, we develop a spatially resolved optical transmission relaxation (2D-OTR) method providing continuous evaluation of local, high-temperature, controlled atmosphere defect kinetics across a ∼1 cm 2 sample area simultaneously in a contact-free manner. We apply it to observe the spatial variance of oxygen incorporation and evolution rates at ∼525-620 °C, in response to step changes in oxygen partial pressure, on MIEC SrTi 0.65 Fe 0.35 O 3- x films as a function of distance from porous Pt and Au layers. Using this model geometry, we find significant enhancements in kinetics adjacent to the metals that decay over a few millimeter distance. To extract kinetic parameters, we fit the short-term optical data (initial portion of relaxations) with an exponential decay function appropriate for surface-exchange-limited kinetics, yielding apparent surface exchange coefficients ( k chem ) with spatial resolution, decreasing with distance from the metal. To understand the kinetic processes governing the complete (long-term) optical relaxations, we performed COMSOL simulations, which demonstrated that a combination of laterally varying k comes from demonstrations of changing surface and bulk chemistry vs distance from the metal-MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.chem and in-plane diffusion controls the observed kinetics over the full time range. Further support for spatially varying k chem comes from demonstrations of changing surface and bulk chemistry vs distance from the metal-MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.

Abstract: The catalytic kinetics of oxygen surface exchange and fuel oxidation for a perovskite membrane is investigated in terms of the thermodynamic state in the immediate vicinity of or on the membrane surface. Perovskite membranes have been shown to exhibit both oxygen perm-selectivity and catalytic activity for hydrocarbon conversion. A fundamental description of their catalytic surface reactions is needed. In this study, we infer the kinetic parameters for heterogeneous oxygen surface exchange and catalytic fuel conversion reactions, based on permeation rate measurements and a spatially resolved physical model that incorporates detailed chemical kinetics and transport in the gas-phase. The conservation equations for surface and bulk species are coupled with those of the gas-phase species through the species production rates from surface reactions. It is shown that oxygen surface exchange is limited by dissociative/associative adsorption/desorption of oxygen molecules onto/from the membrane surface. On the sweep side, while the catalytic conversion of methane to methyl radical governs the overall surface reactions at high temperature, carbon monoxide oxidation on the membrane surface is dominant at low temperature. Given the sweep side conditions considered in ITM reactor experiments, gas-phase reactions also play an important role, indicating the significance of investigating both homogeneous and heterogeneous chemistry and their coupling when examining the results. We show that the local thermodynamic state at the membrane surface should be considered when constructing and examining models of oxygen permeation and heterogeneous chemistry. [Copyright &y& Elsevier]

Abstract: The effects of water on oxygen surface exchange kinetics and its degradation of dense La0.6Sr0.4CoO3−δ (LSC) thin films on yttria-stabilized zirconia (YSZ) are investigated by impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). Two distinguishable effects of water on oxygen surface exchange are identified. While humidification of the oxidant accelerates the oxygen incorporation in the short-term range, the long-term stability of the cathode decreases substantially. XPS analysis indicates that the accelerated LSC degradation rate under wet conditions originated from an increased change of the surface cation composition. [Copyright &y& Elsevier]

The electronic charge carrier concentration in LaSrFeO was shown to depend on the partial pressure of O ( pO). Chemical diffusion coefficient and surface exchange coefficient, k, were determined by conductivity relaxation in O/N and CO/CO mixtures. k was proportional to pO in O/N, while in CO/CO k was controlled by a reaction mechanism involving both CO and CO. [ABSTRACT FROM AUTHOR]

Abstract: Mixed ionic and electronic conductors (MIEC) have presented great economical and environmental interests for these last years because of their potential applications for electrode materials in solid oxide fuel cell and for oxygen separation form air such as in catalytic membrane reactors for methane conversion into syngas (H2–CO mixture) (A.F. Sammells, M. Schwartz, R.A. Mackay, T.F. Barton, D.R. Peterson , U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch , H.J.M. Bouwmeester, B.A. Boukamp ). A good compromise between oxygen permeability, chemical stability and physical properties is required to optimize the process. La(1−x)Sr x Fe(1−y)Ga y O3−δ materials fulfill this requirement and were retained as membrane for catalytic membrane reactor (CMR) (Y. Teraoka , G. Etchegoyen, T. Chartier ). Oxygen semi-permeations through La(1−x)Sr x Fe(1−y)Ga y O3−δ membranes have been measured under various oxygen partial pressure gradients from 973K to 1273K, and compared with the values obtained by isotopic oxygen exchange depth experiments (S. Kim, S. Wang, X. Chen, Y.L. Yang, N. Wu, A. Ignatiev, A.J. Jacobson, and B. Abeles ). Those results lead to a better understanding of the oxygen transport through the membrane and the influence of Sr and Ga amounts on oxygen semi-permeation through the membrane. The influence of Ga amount is not limited to the improvement of dimensional stabilities but it also increases oxygen diffusion and surface exchange kinetic. This paper suggests that La0.6Sr0.4Fe0.6Ga0.4O3−δ perovskite is a very good compromise for membrane reactor materials and opens new perspectives on membrane architecture development. [Copyright &y& Elsevier]

Abstract: Catalytic membrane reactors (CMR) have been an economically attractive process for natural gas reforming to syngas (H2 +CO) since more than twenty years. The CMR studied in this paper consists of a mixed ionic and electronic conductor dense layer (La(1−X)Sr X Fe(1−Y)Ga Y O3−δ ). High temperature X-ray diffraction analysis, from room temperature to 900°C under air and nitrogen atmosphere, show a reversible monoclinic to rhombohedral phase transition around 300°C, and good chemical and dimensional stabilities of La0.8Sr0.2Fe0.7Ga0.3O3−δ material. The La0.8Sr0.2Fe0.7Ga0.3O3−δ dense layer elaborated by tape casting has been respectively coated with La0.8Sr0.2Fe0.7Ga0.3O3−δ on the air side and La0.8Sr0.2Fe0.7Ni0.3O3−δ on the inert side using screen printing. The influences of the dense membrane microstructure and of the surface exchange kinetics on the oxygen semi-permeation performances are evaluated. Small grain size, mainly below 1μm in the dense membrane significantly increases the oxygen flux. A porous layer of La0.8Sr0.2Fe0.7Ni0.3O3−δ or La0.8Sr0.2Fe0.7Ga0.3O3−δ on the air or inert side of the membrane increased strongly the specific oxygen semi-permeation. The impact of the porous layer is much more important than the reduction of the grain size. In this case, surface exchange kinetics are the limiting steps of oxygen permeation, and Ni-containing formulation leads to the highest flux. [Copyright &y& Elsevier]

Abstract: Dense CeO2 layers of 350 nm thickness, with columnar grains of size 25 nm, were RF-sputtered on silicon. 18O isotope exchange in the temperature range from 200 to 575 °C showed surface exchange at CeO2 as the rate-determining step, while diffusion through the polycrystalline thin layer was fast. The surface exchange coefficients controlled by surface or grain boundary processes were found to be or . Comparison with literature data on doped ceria revealed that surface exchange could be dominated by grain boundary processes. A lower limit for the diffusion coefficient was determined as 10−15 cm2 s−1 at 575 °C. [Copyright &y& Elsevier]

Abstract: 18O/16O isotope exchange in combination with SIMS depth profiling was used to investigate oxygen transport in Li2O-deficient single crystalline LiNbO3 in the temperature range 983≤ T/K≤1188 at 200mbar oxygen. Within the limit of experimental error and for the investigated range of temperatures no significant differences between transport parallel and transport perpendicular to the c-axis were found. The following temperature dependencies were determined: for oxygen tracer diffusion D =6.4×10−3exp[−333kJ/mol/(RT)]m2/s; and for oxygen surface exchange k =7.8×102exp[−288kJmol−1/(RT)]m/s. The activation enthalpy obtained for tracer diffusion can be interpreted as the enthalpy of migration of extrinsic oxygen vacancies induced by impurities with lower valency on niobium sites. [Copyright &y& Elsevier]

Abstract: Dense YSZ–LSM composites were fabricated for application as an oxygen separation membrane. A density about 95% was achieved for a sintering temperature of 1350 °C; however, an insulating La2Zr2O7 phase was formed during this high-temperature sintering. The percolation threshold was identified at about 30 wt.% (or 28 vol.%) of LSM. Isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method was used to investigate the oxygen diffusion and surface exchange properties of the composites. The two composites, YSZ–30 wt.%LSM and YSZ–40 wt.%LSM (or 38 vol.% LSM), with a 3-D interconnected network, had higher oxygen diffusivity than the pure LSM, but slightly lower than the parent YSZ material. The oxygen surface exchange coefficient of the YSZ was significantly enhanced by the introduction of the electronic conductor LSM, which confirmed the supposition that the availability of electrons limits the oxygen surface exchange reaction on the pure YSZ materials. [Copyright &y& Elsevier]

Isotope Exchange/Depth Profiling (IEDP) using Secondary Ion Mass Spectrometry (SIMS) has been used to determine the oxygen tracer diffusion and surface exchange coefficients of (100) oriented 9.5 mol% yttria stabilised zirconia single crystals. Exchange experiments performed with molecular oxygen are compared with the exchange using an oxygen plasma. The surface exchange coefficient for specimens in a plasma is up to 100 times higher compared to measurements with normal molecular oxygen. For the exchange experiments we used an inductively coupled radio frequency (rf) oxygen plasma with a maximum radio frequency power of 250 W. Double probe measurements and optical emission spectrometry are used for the characterisation of the plasma. The measured electron temperatures are within the range of 5–12 eV. [Copyright &y& Elsevier]

The chemical surface exchange of oxygen at single-crystalline Y2O3-stabilized ZrO2 surfaces is studied using the in situ optical absorption relaxation technique. The surface exchange coefficients depend on temperature, oxygen partial pressure, and redox-active dopant concentration in a dilute concentration range; they depend only slightly on surface orientation, and become independent of annealing time after a long time treatment (typically at 800 °C). Using the thermodynamic factor of oxygen (which sensitively depends on redox-active impurities) derived from the known defect chemical parameters of this material, the effective macroscopic chemical rate constants are analyzed. The correlation with effective tracer rate coefficients is discussed for this model electron-poor ionic conductor. [Copyright &y& Elsevier]

This study intends to illustrate the application of transient kinetics in combinatorial research. Thus, it is shown through various case studies how high-throughput transient kinetics may both accelerate the search for new catalytic materials and bring fundamental insights in reaction mechanisms. As a first case study, the cracking reactions of the C6 isomers (n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,3-dimethylbutane) over a fluidised catalytic cracking (FCC) catalyst at temperatures between 300 and 650 °C have been studied in a temporal-analysis-of-product (TAP) reactor. A mathematical model taking into account the extra- and intracrystalline transport phenomena was used to determine the diffusion, sorption, and kinetic parameters for the hydrocarbons studied. Sorption and diffusion decrease with increasing branching in agreement with literature data. Except product distribution, the activation energies for the overall cracking of the C6 isomers are similar for all isomers. As a second case study, a “paralleled” TAP reactor with a multisample holder is shown to be an efficient tool for acquiring adsorption/desorption parameters for a series of zeolites, tested to evaluate their potential as catalyst supports in the oxidative dehydrogenation of ethane. Finally, the applied combinatorial study was found also suitable for discovering new materials precursors of dense oxygen-conducting membranes. From high-throughput transient TAP and TPD experiments, it was possible to derive a strategy directing the rapid combinatorial evolution within the available parameter space. It was shown that high-quality information is accessible from these data, providing all parameters necessary for a well fitting description with a kinetic model. [Copyright &y& Elsevier]

BaCe 0.2 Fe 0.8-x Pr x O 3-δ (x = 0–0.3) is studied as a cobalt-free cathode material for proton-conducting solid oxide fuel cells. The cathode is composed of a cubic BaFeO 3-δ phase and an orthorhombic BaCeO 3-δ phase, and Pr is doped in both phases. The partial substitution of Pr for Fe decreases the content of the BaFeO 3-δ phase, leading to a lower electrical conductivity. BaCe 0.2 Fe 0.6 Pr 0.2 O 3-δ has the most adsorbed oxygen and Fe3+ on the surface, resulting in the fastest oxygen surface exchange kinetics and the highest activity. The partial pressure of H 2 O shows a negligible effect on the polarization resistance of the cathode. In contrast, the polarization resistance increases remarkably with the decrease of oxygen partial pressure, indicating that the rate of the cathode process is controlled by the surface exchange of oxygen. At 700 °C, BaCe 0.2 Fe 0.6 Pr 0.2 O 3-δ shows the lowest polarization resistance of 0.057 Ω cm2, and a single cell with that cathode exhibits the highest maximum power density of 562 mW cm−2. The results demonstrate that Pr doped BaCe 0.2 Fe 0.8 O 3-δ is a promising cobalt-free cathode material for proton-conducting solid oxide fuel cells. • Pr doped BaCe 0.2 Fe 0.8 O 3 is studied as a cathode material for proton-conducting SOFC. • The surface exchange of oxygen is accelerated with the doping of Pr. • The polarization resistance of the cathode is 0.057 Ω cm2 at 700 °C. [ABSTRACT FROM AUTHOR]

Abstract: Double-perovskite Gd1−x Pr x BaCo2O5+ δ membranes showed appreciable oxygen permeability at moderate temperatures. The overall oxygen permeation process of GdBaCo2O5+ δ was found to be controlled mainly by the bulk diffusion step with the membrane thickness larger than 0.8 mm, and the limitation by oxygen surface exchange came into play at reduced thickness of 0.8 mm. The electrical conductivity measurement showed Gd1– x Pr x BaCo2O5+ δ samples possessed a semiconducting behavior at a wide temperature range below 300 °C and a metallic behavior at 300–850 °C with a high conductivity of nearly 103 S cm−1. [Copyright &y& Elsevier]

Mixed ionic-electronic conductor LSCF is a popular cathode material for intermediate to low temperature solid oxide fuel cells; nevertheless, there are concerns about its surface-limited oxygen reduction reaction. In this work, Ruddlesden-Popper-structured La 2 NiO 4+δ (LNO) nanoparticles are introduced onto the surface of (La 0.6 Sr 0.4) 0.995 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathode and the electrode performance is examined by means of electrochemical impedance spectroscopy (EIS) in the temperature range of 550–750 °C. Impedance spectra analysis shows that an 81% reduction in polarization resistance can be achieved by infiltration of LNO nanoparticles. The distribution of relaxation times (DRT) technique is used to gain insights on the physicochemical properties of the system under study. Obtained results reveal that by correlating oxygen surface exchange, bulk diffusivity, ionic conduction, and particle size of nano-structured LNO with the electronic and ionic conductivity of LSCF, the electrochemical behavior of the cathodes is improved. [ABSTRACT FROM AUTHOR]

The third-order Ruddlesden-Popper phase Pr 4 Ni 2.7 Co 0.3 O 10-δ (PNCO43) was synthesized by a freeze drying process. Phase purity and crystal structure were determined by X-ray diffraction and Rietveld analysis. The electronic conductivity of a bulk sample obtained by a two-step sintering process was measured by the four-point dc van der Pauw method as a function of temperature (50 ≤ T/°C ≤ 800) and oxygen partial pressure (1 × 10−3 ≤ pO 2 /bar ≤1). Dense thin-film PNCO43 microelectrodes were prepared by pulsed laser deposition and photolithography on yttria-stabilised zirconia substrates. The thin-films were characterized by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and inductively coupled plasma optical emission spectroscopy. Individual resistive and capacitive processes were investigated with electrochemical impedance spectroscopy as a function of the oxygen partial pressure (1 × 10−3 ≤ pO 2 /bar ≤1) and temperature (600 ≤ T/°C ≤ 850). Oxygen surface exchange coefficients kq, calculated from the resistance of the electrode, show relatively high values (e.g. kq = 1.5 × 10−6 cm s−1 at 800 °C and 2 × 10−1 bar pO 2). Chemical surface exchange coefficients k chem of oxygen were obtained from the peak frequency or the chemical capacitance as determined by impedance spectroscopy. • The third-order Ruddlesden-Popper phase Pr 4 Ni 2.7 Co 0.3 O 10-δ was synthesized. • Study on the sintering behaviour of PNCO43 was performed. • High electronic conductivity with low pO 2 dependence of the PNCO43 bulk sample. • PNCO43 thin-film on YSZ (100) deposited with pulsed laser deposition. • EIS on PNCO43 microelectrodes shows fast oxygen exchange kinetics. [ABSTRACT FROM AUTHOR]