Isotope Exchange Depth Profiling Research Articles

Oxygen diffusion and surface exchange coefficients measurements under high pressure: Comparative behavior of oxygen deficient versus over‐stoichiometric air electrode materials

AbstractMixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 and 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900°C has been developed. The main goal of this study was to compare the behavior of two types of reference materials: the oxygen deficient La‐Sr‐Fe‐Co perovskites, and the overstoichiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure.

Quantifying Diffusion Across Solid-Solid Interfaces in Electrochemical Cells

Enabling an increased rollout of hydrogen-based technologies requires decarbonisation of both hydrogen production and conversion. Solid Oxide Cells (SOCs), in the form of electrolysers and fuel cells, could play a significant role in achieving this, however lifetime and degradation of SOCs remain major obstacles to commercialisation. Understanding the mechanisms governing ionic transport in ceramics is important to improving the performance and durability of SOCs. Properties such as the oxygen tracer diffusivity, , can be measured by Isotopic Exchange Depth Profiling (IEDP) as developed by Kilner et al. [1].This method has, so far, primarily been used to characterise the properties of single materials; however, SOCs are multilayer devices with solid-solid interfaces that may also affect transport. Some investigations into the diffusion behaviour across multiple layers using the IEDP technique have been done in thin film samples [2][3]. Profiles obtained from these studies appear to show an abrupt concentration drop at the interface between certain materials, indicating an interface which significantly impacts the diffusion behaviour (and by extension the overall cell performance). However, no attempt was made to quantify this interface effect with a theoretical modelling approach.A finite-difference model for diffusion in a system containing multiple layers with interfaces is developed. It numerically solves Fick’s second law of diffusion with various boundary conditions. This model can be used to fit experimental data obtained from tracer diffusion SIMS data, yielding new way of quantifying interfacial resistance. A new interfacial resistance parameter, , has been defined, which quantifies the magnitude of concentration drop across any given interface and thus offers a universal way of numerically characterising resistance to diffusion.The validity of the developed method has been experimentally tested in samples containing layers of lanthanum strontium cobalt ferrite (LSCF) and gadolinium-doped ceria (GDC), a commonly used system in SOCs. Initial data from tracer diffusion experiments as seen in Figure 1 have shown the presence of a significant concentration drop at the interface of a LSCF-GDC stack which can be fitted to the numerical model developed by the authors. This approach is used to measure the changing interface properties under various ageing conditions, as well as the influence of interlayers and material selection on the interfacial resistances both in SOC and other diffusion systems.[1] J. A. Kilner, B. C. H. Steele, and L. Ilkov. Oxygen self-dffusion studies using negative-ion secondary ion mass-spectrometry (sims). Solid State Ionics, 12(MAR):89-97, 1984.[2] K. Develos-Bagarinao, H. Yokokawa, H. Kishimoto, T. Ishiyama, K. Yamaji, and T. Horita. Elucidating the origin of oxide ion blocking effects at gdc/srzr(y)o-3/ysz interfaces. Journal of Materials Chemistry A, 5(18):8733-8743, 2017.[3] Katherine Develos-Bagarinao, Haruo Kishimoto, Jeffrey De Vero, Katsuhiko Yamaji, and Teruhisa Horita. Effect of la0.6sr0.4co0.2fe0.8o3-delta microstructure on oxygen surface exchange kinetics. Solid State Ionics, 288:6-9, 2016.Fig. 1. Oxygen isotopic fraction data of an LSCF layer deposited on a GDC substrate and annealed in Oxygen-18 atmosphere at 650 C for 1h obtained using cross-sectional SIMS (left) and fit of the data using the multi-layer diffusion model developed by the authors (right). Figure 1

Hydrogen Exchange and Diffusion Kinetics at Elevated Temperatures in Proton-Conducting Solid Oxide Materials

Storage of purified hydrogen is one of the central challenges in addressing climate change and reducing our reliance on fossil fuels for energy conversion and storage, and therefore there is a global surge in research and development concerning hydrogen purification and storage. In this regard, we are studying proton conduction in solid oxide materials at elevated temperatures for applications in hydrogen separation and compression membranes. Hydrogen compression is the most recommended method to store hydrogen for automotive applications as it allows an increase in the hydrogen volumetric energy density.Traditionally the protonic conductivity in these materials is measured by indirect methods. For example, conductivity measurements in mixed gas atmospheres, comparing for example dry N2 with humidified N2, thereby allowing the contribution of protons to be evaluated. In this study, we for the first time report the evaluation of protonic conductivity in BaZr1-xCexY0.2O3−δ, BaZr0.1Ce0.7Y0.2–xYbxO3–δ and Ba7Nb4MoO20 by direct measurements afforded by the Isotope Exchange Depth Profiling technique with deuterium labelling. We also report the kinetics of H/D transport through the bulk materials and across metal-ceramic interfaces with a particular interest in the behaviour of the interface between the key Pd/Pd alloy catalyst component and the hydrogen-transporting oxide ceramic material. The transport and interface behaviour information will be of significance in designing hydrogen separation and compression membranes. Figure 1

Densification Study of Pr6O11 as a Limiting Step for the Determination of Its Oxygen Transport Properties

This study describes the successful sintering of Pr6O11, which is recently developed as a promising oxygen electrode for Solid Oxide Cells (SOCs). Using dense pellets or thin films is mandatory prior to an accurate measurement of the ionic transport properties via the isotopic oxyen exchange method here involved. Dilatometry experiments performed under different atmospheres (argon, air, oxygen) and sintering tests with different thermal cycles allowed overcoming the detrimental phase changes of Pr6O11 and densifying this oxide with a sintered relative density of 95%. Dense pellets of large sizes were prepared. In a second part, the ionic transport properties of Pr6O11 have been determined in the range 400 < T °C < 800 thanks to Isotopic Exchange Depth Profiling (IEDP) measurements.

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn y O3±δ epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties.

Direct Measurements of Hydrogen Exchange and Diffusion Kinetics at Elevated Temperatures in Proton-Conducting Solid Oxide Materials

Storage of purified hydrogen is one of the central challenges in addressing climate change and reducing our reliance on fossil fuels for energy conversion and storage, and therefore there is a global surge in research and development concerning hydrogen purification and storage. In this regard, we are studying proton conduction in solid oxide materials at elevated temperatures for applications in hydrogen separation and compression membranes. Hydrogen compression is the most recommended method to store hydrogen for automotive applications as it allows an increase in the hydrogen volumetric energy density.Traditionally the protonic conductivity in these materials is measured by indirect methods. For example, conductivity measurements in mixed gas atmospheres, comparing for example dry N2 with humidified N2, thereby allowing the contribution of protons to be evaluated. In this study, we for the first time report the evaluation of protonic conductivity in BaZr1-xCexY0.2O3−δ (BZCY) and BaZr0.1Ce0.7Y0.2–xYbxO3–δ (BZCYYb) by direct measurements afforded by the Isotope Exchange Depth Profiling (IEDP) technique with deuterium labelling. We also report the kinetics of H/D transport through the bulk materials and across metal-ceramic interfaces with particular interest in the behaviour of the interface between the key Pd/Pd alloy catalyst component and the hydrogen transporting oxide ceramic material. The transport and interface behaviour information will be of significance in designing hydrogen separation and compression membranes. Figure 1

A‐site acceptor‐doping strategy to enhance oxygen transport in sodium–bismuth–titanate perovskite

AbstractSodium–bismuth–titanate (NBT) has recently been shown to contain high levels of oxide ion conductivity. Here we report the effect of A‐site monovalent ions, M+ = K+ and Li+, on the electrical conductivity of NBT. The partial replacement of Bi3+ with monovalent ions improved the ionic conductivity by over one order of magnitude without an apparent change of the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on an acceptor‐doping approach. The 18O tracer‐diffusion coefficient (D*) determined by the isotope exchange depth profile method in combination with secondary ion mass spectrometry confirmed that oxygen ions are the main charge carriers in the system. Among these acceptor‐doped samples, 4% Li doping provides the highest total conductivity, leading to a further discussion of doping strategies for NBT‐based materials to enhance the electrical behavior, is discussed. Comparisons with other oxide‐ion conductors and an oxygen‐vacancy diffusivity limit model in perovskite lattice suggested that the doped NBT‐based materials might already have achieved the optimization of the ionic conductivity.

Characterising Ionic Transport Resistance at Solid-Solid Interfaces in SOFC Using Isotopic Exchange and Numerical Modelling

Understanding the mechanisms governing ionic transport in ceramics is important for improving the performance and durability of Solid Oxide Fuel Cells (SOFCs). Properties such as the tracer diffusivity, D* , can be measured by Isotopic Exchange Depth Profiling (IEDP) as developed by Kilner et al. [1]. This method has, so far, primarily been used to characterise the material properties of single materials; however, SOFCs are multilayer devices with solid-solid interface that may also affect transport. Some investigations into the diffusion behaviour across multiple layers using the IEDP technique have been done in thin film samples [2][3]. Profiles obtained from these studies appear to show an abrupt concentration drop at the interface between certain materials, indicating an interface which significantly impacts the diffusion behaviour (and by extension the overall cell performance). However, no attempt was made to quantify this interface effect with a theoretical modelling approach.A finite-difference model for diffusion in a system containing multiple layers with interfaces has been developed. It numerically solves Fick’s second law of diffusion with various boundary conditions. This model can be used to fit experimental data obtained from tracer diffusion SIMS data, yielding a new way of quantifying interfacial resistance. A new interfacial resistance parameter, r int , has been defined, which quantifies the resistance to diffusion across an interface.The validity of the developed method has been experimentally demonstrated in a sample consisting of a layer of lanthanum strontium cobalt ferrite (LSCF) on a gadolinium-doped ceria (GDC) substrate, both common materials in SOFCs. Initial data from tracer diffusion experiments have shown the presence of a significant concentration drop at the interface of the LSCF-GDC stack, which can be fitted to the numerical model developed by the authors. This approach could be used to measure the changing interface properties under various ageing conditions, as well as the influence of interlayers and material selection on the interfacial resistances both in SOFC and other diffusion systems with interfaces, such as solid state batteries.[1] J. A. Kilner, B. C. H. Steele, and L. Ilkov. Oxygen self-dffusion studies using negative-ion secondary ion mass-spectrometry (sims). Solid State Ionics, 12(MAR):89-97, 1984.[2] K. Develos-Bagarinao, H. Yokokawa, H. Kishimoto, T. Ishiyama, K. Yamaji, and T. Horita. Elucidating the origin of oxide ion blocking effects at gdc/srzr(y)o-3/ysz interfaces. Journal of Materials Chemistry A, 5(18):8733-8743, 2017.[3] Katherine Develos-Bagarinao, Haruo Kishimoto, Jeffrey De Vero, Katsuhiko Yamaji, and Teruhisa Horita. Effect of la0.6sr0.4co0.2fe0.8o3-delta microstructure on oxygen surface exchange kinetics. Solid State Ionics, 288:6-9, 2016.Figure: Isotope Exchange Depth Profile Data taken by linescan from an LSCF-GDC multi-layer sample. Original image data and isotopic fraction profile, as well as the corresponding fit obtained using the numerical model is shown. Figure 1

Development of an Isotopic Oxygen Exchange Setup Operating Under High Pressure: Preliminary Measurements Performed on Ln2NiO4+d (Ln=La, Pr, Nd, MIEC) Oxides

MIEC (Mixed Ionic Electronic Conductors) are commonly used as oxygen electrodes in SOC (Solid Oxide Cells) either for fuel cell or high-temperature steam electrolysis applications. In addition to a high electronic conductivity, the electrodes require both high oxygen diffusivity (D) and surface exchange (k) coefficient in the operating temperature range to be considered as candidates.The two parameters D and k can be directly determined thanks to the IEDP (Isotopic Exchange Depth Profiling) method: a dense material is annealed during controlled time under enriched 18O atmosphere, prior to the recording of the 18O/(18O+16O) profile inside the ceramic using a TOF-SIMS analyser. The tracer coefficients for the oxygen diffusion (D*) and surface exchange (k*) can be determined by fitting this experimental 18O penetration profile: it is achieved by using the solution for Fick’s second law proposed by Crank [1], when applied to the diffusion into a semi-infinite medium.To date, the reported measurements in the literature have been performed at ambient pressure and even below [2]. However, in order to reach a higher efficiency at the system level or for a ‘Power-to-Gas’ application, it would be helpful to operate the Solid Oxide Electrolysers at high pressures, between 10 to 60 bars [3,4]. A preliminary study is therefore needed, which consists in assessing the oxygen reduction/oxidation and the transport properties of the electrodes materials when they are submitted to such high pressure. In order to address these questions, an innovative setup able to operate up to a total pressure of 50 bars has been developed in collaboration between the ICMCB-CNRS and the CEA-LITEN. Its technical characteristics will be described in the first part of the article.This setup has been used to characterize the properties of nickelate dense pellets with the K2NiF4-type structure. These materials, whose main defects and charge carriers are interstitial oxygens, have been developed for several years as an alternative to the more conventional oxygen deficient perovskites [5]. Oxygen exchanges have been performed in the range 500 < T°C < 700, under P (air) = 30 bars (equivalent to p(O2) ~ 6.5 bars. Compared to atmospheric condition, the oxygen over-stoichiometry is as expected significantly increased after the oxygen exchanges, whatever the nickelate. Preliminary results (D* and k*) obtained on Ln2NiO4+ d (Ln=La, Pr, Nd) MIEC oxides will be presented and discussed.

Kinetics of competing exchange of oxygen and water at the surface of functional oxides.

The presence of water vapour in the input gas stream influences the performance of air electrodes of solid oxide cells. In this work, the oxygen transport kinetics were determined by isotopic exchange depth profiling at 350 °C on polycrystalline La0.6Sr0.4Co0.2Fe0.8O3-δ samples in humidified oxygen, comparing the differences in tracer diffusion profile using either 18O2 or H218O as the labelling medium. The apparent surface exchange coefficients of oxygen were determined in each case and used together to estimate the oxygen surface exchange coefficients of molecular oxygen and water. It was found that, in humid conditions, the surface exchange coefficient of molecular oxygen is significantly decreased in comparison to a reference in dry conditions. In addition, the surface exchange coefficient of water is higher than that for molecular oxygen. This is in good agreement with the hypothesis that, water monopolises the active exchange sites at the material surface and thus oxygen from water exchanges faster than the one of molecular oxygen.

Operando Isotopic Exchange in Solid Oxide Fuel Cells: Oxygen-Transport Dependency on Applied Potential.

The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell (YSZ: yttria-stabilized zirconia). Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established.

Strain-induced structure and oxygen transport interactions in epitaxial La0.6Sr0.4CoO3\u2212\u03b4 thin films

The possibility to control oxygen transport in one of the most promising solid oxide fuel cell cathode materials, La0.6Sr0.4CoO3−δ, by controlling lattice strain raises questions regarding the contribution of atomic scale effects. Here, high-resolution transmission electron microscopy revealed the different atomic structures in La0.6Sr0.4CoO3−δ thin films grown under tensile and compressive strain conditions. The atomic structure of the tensile-strained film indicated significant local concentration of the oxygen vacancies, with the average value of the oxygen non-stoichiometry being much larger than for the compressive-strained film. In addition to the vacancy concentration differences that are measured by isotope exchange depth profiling, significant vacancy ordering was found in tensile-strained films. This understanding might be useful for tuning the atomic structure of La0.6Sr0.4CoO3−δ thin films to optimize cathode performance.

Effect of Interfacial Properties on the Electrochemical Performance of Solid Oxide Fuel Cells

In this study, we examined the properties of model heterointerfaces created with yttrium-doped SrZrO3 (SZY) thin films, a type of secondary phase that most frequently occurs at interfaces of solid oxide fuel cells utilizing yttria-stabilized zirconia (YSZ) electrolytes. Electrochemical impedance results of symmetrical cells prepared with LSCF cathodes were evaluated using DRT (distribution of relaxation times) analysis, and 18O isotope exchange depth profiling with SIMS (secondary ion mass spectrometry) analysis was performed on post-tested samples to examine the ionic transport across heterointerfaces. Results showed high polarization resistance contributed by interfacial resistance associated with interfaces with SZY, i.e., LSCF/SZY, SZY/YSZ or SZY/GDC (gadolinia-doped ceria). The careful selection of heterostructure configurations lead to the conclusion that the interfacial contact of SZY with LSCF, YSZ and GDC results into high interfacial resistances, possibly due to an ion blocking effect.

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.

Investigation of rate-determining step of LaNi0.6Co0.4O3-δ film electrode

The rate-determining step (RDS) in the oxygen reduction process on a LaNi0.6Co0.4O3-δ film electrode was analyzed using electrochemical impedance spectroscopy in varied temperatures (873–1073 K), oxygen partial pressures (1–10−3 bar), and DC bias. From the evaluation of the area-specific interfacial conductivity, σE, and chemical capacitance, CE, it is suggested that the RDS is surface reaction under low p(O2) (10−2–10−3 bar) but there appears a contribution of bulk diffusion under high p(O2) in the film electrode at any given temperature. The change of the RDS was also confirmed using isotope exchange depth profile method where a significant change in the oxygen profile in the film electrode was observed. This is attributed to the rate of the bulk diffusion of O2− which is almost constant when the oxygen nonstoichiometry of LaNi0.6Co0.4O3-δ is small, whereas the surface reaction rate strongly depends on p(O2). At low p(O2), the surface reaction is slower than the bulk diffusion, so that the potential drop on the surface of the electrode film is large. In contrast, at high p(O2), the surface reaction is faster than the bulk diffusion where the potential decay is sluggish inside the film electrode.

Improvement of ionic conductivity in A-site lithium doped sodium bismuth titanate

Oxide-ion conductors play a significant role in various applications such as solid oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Recently, high ionic conductivity (~1×10−4Scm−1 at 600°C) was found in sodium bismuth titanate (NBT), which originates from oxygen vacancies compensating the introduced Bi-deficiency. By providing pathways with low diffusion barriers, the highly polarizable Bi3+ ions with 6s2 lone pair electrons and weak BiO bonds are also beneficial for the migration of oxygen ions. Here we report the influence of lithium doping on the electrical properties of NBT. The optimal doping level of 4at% Li on the Bi-site improves the ionic conductivity by one order of magnitude to ~7×10−3Scm−1 at 600°C without changing the conduction mechanism, which could be attributed to an increase in the oxygen vacancy concentration based on an acceptor doping mechanism. A further increase in Li content does not improve the total conductivity. Oxygen diffusion data were acquired by the Isotope Exchange Depth Profile (IEDP) method in combination with Secondary Ion-Mass Spectrometry (SIMS). The oxygen self-diffusion coefficients (e.g. 7.04×10−9cm2s−1 at 600°C) are in excellent agreement with the values derived from impedance spectroscopy data, suggesting that the oxygen ions are the main charge carriers in the system. Furthermore, a degradation test was performed for 100h under a variety of atmospheres, showing only a slight decrease in conductivity in both air and oxygen atmospheres attributed to the loss of material from the A-site. Comparison with other oxide-ion conductors indicates that Li-doped NBT materials are promising candidates for intermediate temperature SOFC applications.

Oxygen Transport of Lscrf-Scsz Based Mixed Conducting Ceramic Composites

Materials for membrane reactors must possess both high ionic and electronic conductivity as well as good stability over a wide range of oxygen partial pressure. In order to improve the oxygen diffusion, substituting elements is an approach commonly adopted to increase vacancy concentration, but it may also reduce the chemical stability of the membrane [1]. Single phase materials, e.g. La1-x Sr x Co1-y Fe y O3-δ (LSCF), have been shown to possess excellent oxygen permeation properties but suffer from dimensional and chemical instabilities under operation. To improve the long-term stability manganese, chromium etc. can substitute for cobalt but this will sacrifice the permeability of a membrane [2]. Alternatively, focus has been shifted onto dual phase materials which consist of one phase as a good ionic conductor and the other phase as a good electronic conductor. Among all the essential criteria, transport properties of the dense composite layer can profoundly affect the membrane performance. Previous transport measurements work have been carried out on La0.8Sr0.2MnO3-δ (LSM)-Yttria-Stabilised Zirconia (YSZ) [3], LSCF-Ce0.9Gd0.1O2-δ [4] etc. dual phase systems, whilst in this work the La1-x Sr x Cr1-y Fe y O3-δ (LSCrF)-Scandia-Stabilised Zirconia (ScSZ) based dual phase composite system has been investigated. Isotopic Exchange Depth Profiling (IEDP)[5] combined with Secondary Ion Mass Spectrometry (SIMS) was performed in this present work to investigate the mass transport in the composite materials. In order to achieve improved performance optimisation strategies have been applied, which include changing doping level and doping elements in the component LSCrF and ScSZ, and using different volume fractions (LSCrF:ScSZ) in the dual phase composites and optimising the microstructures of the dual phase composites. Additionally, the effects of a humidified environment on the diffusion behaviours of both the single phase and dual phase samples have been studied by using pure labelled H2 18O to reflect operating atmospheres. The diffusion coefficient (D*) at 800 ̊C of LSCrF was found to increase dramatically by 3 orders of magnitude compared to a similar experiment under dry conditions indicating an increase of oxygen vacancy concentration whilst ScSZ remains a good ionic conductor in a pure water atmosphere, and moreover the surface exchange coefficient (k) of ScSZ has been improved to 1.54x10-6 cm/s at 800 ̊C while in dry O2 conditions almost no 18O has been exchanged into the sample. An explanation for this behaviour is that electronic species are not required for the surface exchange process in pure water atmosphere [6] so the rate-limiting steps of surface exchange in ScSZ shifts from dissociation and oxygen reduction in the case of pure oxygen to adsorption steps for water. To obtain further understanding of how the surface has changed during the annealing, X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) have been performed in order to observe the surface and sub-surface chemistry and compositional information.

Influence of Crystal Orientation and Annealing on the Oxygen Diffusion and Surface Exchange of La2NiO4+δ

La2NiO4+δ is a mixed ionic–electronic conducting material with the 2D K2NiF4-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k*) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La2NiO4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450–600 °C. The differences in k* have been correlated w...

Water-Induced Decoupling of Tracer and Electrochemical Oxygen Exchange Kinetics on Mixed Conducting Electrodes

Isotope exchange depth profiling and electrochemical impedance spectroscopy are usually regarded as complementary tools for measuring the surface oxygen exchange activity of mixed conducting oxides, for example used in solid oxide fuel cell (SOFC) electrodes. Only very few studies compared electrical (kq) and tracer (k*) exchange coefficients of solid–gas interfaces measured under identical conditions. The 1:1 correlation between kq and k* often made is thus more an assumption than experimentally verified. In this study it is shown that the measured rates of electrical and tracer exchange of oxygen may strongly differ. Simultaneous acquisition of kq and k* on La0.6Sr0.4FeO3-δ and SrTi0.3Fe0.7O3-δ thin film electrodes revealed that k* > 100 kq in humid oxidizing (16O2 + H218O) and humid reducing (H2 + H218O) atmospheres. These results are explained by fast water adsorption and dissociation on surface oxygen vacancies, forming two surface hydroxyl groups. Hence, interpreting experimentally determined k* values in terms of electrochemically relevant oxygen exchange is not straightforward.

Evidence for enhanced oxygen surface exchange reaction in nanostructured Gd2O3-doped CeO2 films

The effect of microstructure of Gd2O3-doped CeO2 (GDC) films on oxygen surface exchange and diffusion is reported. Epitaxial GDC (10 mol% Gd) films up to 1 μm in thickness are prepared using pulsed laser deposition on (100) yttria-stabilized zirconia single-crystal substrates and subjected to high-temperature annealing at 1300 °C in air to induce microstructural modifications. Characterization using atomic force microscopy and transmission electron microscopy reveals granular morphologies comprised of densely packed columnar nanostructures for the as-grown GDC films; however, significant microstructural reconstruction of the entire GDC layer occurs after high-temperature annealing. 18O isotope exchange depth profiling with dynamic secondary ion mass spectroscopy is employed to evaluate the oxygen surface exchange coefficient k* and the diffusion coefficient D* at T = 600 °C. The as-grown GDC exhibits up to 10 times higher k* than the annealed film. The strong differences in oxygen surface reaction are correlated to the observed film properties including surface microstructure and cerium oxidation state as evaluated using electron energy loss spectroscopy in scanning transmission electron microscopy.