Oxygen Exchange Kinetics Research Articles

(Invited) Enhancement of Surface Oxygen Exchange Kinetics for Pr0.1Ce0.9O2- δ with Deposition of La or Sm Oxide

Fast oxygen exchange kinetics, a key figure of merit in solid oxide fuel cell (SOFC) electrodes, is often dramatically hindered by the presence of even small concentrations of impurities at the surface. Si, for example, is common in ceramics processing. As discussed in this presentation, it was found that rapid degradation of oxygen exchange kinetics occurred on dense thin films of Pr0.1Ce0.9O2-d (PCO), a mixed ionic electronic conducting electrode for SOFCs. A new optical transmission relaxation (OTR) technique was developed to aid in measuring kinetics on the bare film surfaces. A key advantage of this technique is that it does not require current collectors typical of conventional measurements. Si was identified by TEM and XPS as a significant surface impurity, which forms a blocking layer on the electrode surface. Deposition of a La oxide film was found to result in full recovery of oxygen exchange kinetics on the measured sample. La and Si oxides were found to interact with each other as identified by a shift in the O1s XPS peak and a systematic OTR study of films that likely react with Si (e.g. La, Sm oxides) and those that do not (e.g. Nb, Ti, Zn oxides). These results indicate that La may "clean" the surface of the SOFC electrode, thereby dramatically improving oxygen transport kinetics. Preliminary measurements of OTR on a La-PCO solid solution indicates improved long-term electrode kinetics stability, demonstrating the commercial potential of this method to achieve Si impurity tolerance.

Surface Modification of LaNi0.6Fe0.4O3-δ Film Electrode by Ce0.9Gd0.1O1.95 Porous Layer

1. Introduction Our previous study on the composite electrode between mixed ionic and electronic conductor (MIEC) of LaNi0.6Fe0.4O3-δ (LNF) and ion-conducting oxide of Ce0.9Gd0.1O1.95(GDC) showed high electrochemical performance [1]. The lack of understanding on reaction mechanism makes the enhancement of composite electrode always connected to the enhancement of active region length of the electrode. However, for electrode which has quite high ionic conductivity such as LNF, another factor may enhance as well [2]. The surface oxygen exchange rate may enhance due to the hetero junction effect with GDC. To prove our hypothesis, LNF film electrode was prepared and GDC porous layer added on top of the surface of the film. In this study, the effect of the GDC was analyzed and discussed. 2. Experimental A LNF film electrode was prepared by pulse laser deposition method by Nd-YAG laser on the GDC electrolyte. Porous LNF as a counter electrode was deposited on the opposite side to the LNF film on the GDC electrolyte. The GDC porous layer was deposited into top of the film surface by screen printing method. The diameter of GDC porous layer was smaller than LNF film electrode. The samples were calcined at 1273 K for 6 hours. Electrochemical measurement was taken to compare two samples between LNF film and LNF film-GDC porous layer as a function of Temperature (973 – 1073 K) and oxygen partial pressure (10-3-1 bar). For the impedance set up, the alumina ring which was covered with Pt mesh was used as a current collector. The diameter of the alumina ring was larger compared to the GDC layer that the alumina ring didn’t touch GDC. In the analyses of the impedance responses, tentative equivalent circuits with two or three R-CPE elements (a resistance, R, and a constant phase element, CPE, connected in parallel) were used to identify their physical meanings. Secondary ion mass spectrometry measurement (SIMS) was also performed to determine the surface oxygen exchange kinetics of the samples. The samples with LNF film and LNF film – GDC porous layer were cut into pieces of 3 mm x 3 mm. The samples were pre-annealed at 973 K under 10-1 bar for 3 hours and then quenched to room temperature. The isotope exchange was performed by flowing 16O2 at 973 K under 10-1 bar for 30 minutes before the gas abruptly changed into 18O2for 10 minutes. The quenched samples after the isotope exchange were used for SIMS measurement. For LNF film – GDC porous sample, the GDC porous layer was removed from LNF film prior to SIMS measurement. The isotope depth profile technique was used to determine the surface exchange kinetics for both of the samples. 3. Results and Discussion From electrochemical impedance spectroscopy (EIS) result, the depressed semicircle of impedance spectra was observed for the LNF film and LNF film – GDC porous electrode. The low frequency arc was related to the electrode reaction on both of the samples for which the area specific conductivity was calculated. Fig. 1 shows the comparison of area specific conductivity (ASC) of LNF film and LNF film - GDC porous. The LNF film - GDC porous layer has higher ASC compared to the LNF film electrode. It shows that the activity of oxygen reduction process enhances due to the GDC existence on the top of LNF film surface. The ASC which was estimated from SIMS were close compared to the EIS result as shown in Fig. 1. The area specific conductivity which was estimated from k*by the following equation [3]: σe= (k*.2.n.F 2.Co )/(R.T) Where the F, and Co are faraday number and oxygen concentration in the bulk sample. The n (between 2 to 4) is dependent on the reaction kinetics. Two measurements by EIS and SIMS proved that GDC on the surface of LNF film enhances the surface oxygen kinetics. Within the surface modification by porous GDC, the ASC enhances significantly. It may be due to the GDC on the LNF gives the oxygen vacancy at the interface between GDC and LNF film surface. The oxygen vacancy from GDC increases the rate of oxygen reduction reaction on the electrode surface. At the current result, the solid conclusion on the mechanism remains unclear. Reference [1] R. A. Budiman et al., ECS Trams., 57(2013) 1873. [2] M. Nishi et al., ECS Trans., 45, 171 (2012). [3] T.Kawada et al., Solid State Ionic, 121 (1999) 271. Figure 1

(Invited) Molecular Understanding of Oxygen Exchange in Solid Oxide Fuel Cell Cathodes

Materials with the ability to rapidly exchange oxygen with the environment play a critical role in many existing and developing technologies, e.g., solid oxide fuel cells (SOFCs), gas separation membranes, oxygen sensors, chemical looping devices, and memristors. In particular, reducing the working temperature of SOFCs, which is critical to their increased commercialization, is significantly inhibited by the slow oxygen exchange kinetics at the cathode. The oxygen exchange is an essential step in the overall oxygen reduction reaction (ORR), where O2 (gas) combines with 4e- to incorporate as 2O2- into the electrolyte, facilitated by the cathode. Oxygen exchange rates are typically quantified by the oxygen exchange coefficient, K*, which represents the response of the rate of oxygen exchange to a driving force. In our work we are developing techniques to predict K* and provide a molecular scale understanding of processes involved in oxygen surface exchange, with the long-term goal of designing more active oxygen exchange materials. Our work has focused on perovskite materials. A few years ago we identified the distance of the bulk oxygen 2p-band center to the Fermi level as an easily calculated descriptor for oxygen exchange in electronic conducing perovskites.[1] These results suggested that overall oxyphilicity plays a critical role in determining K* and provides a tool for high-throughput ab initio screening of promising materials. We will report on recent predictions of active oxygen exchange systems using these screening approaches. In more recent work we have initiated an effort to model oxygen exchange at the molecular level for the (La,Sr)CoO3-d (LSC) system. Using ab inito energetics and the elementary kinetics formalism developed by Adler, et al.[2] We have modeled the possible reaction steps on the LSC (001) surface. This modeling has yielded valuable insights in the rate limiting steps of oxygen exchange through a perovskite surface, the role of surface termination and Sr segregation on K*,[3, 4] and the absolute limits of surface exchange rates. Acknowledgement We gratefully acknowledge financial support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0001284, and computing support from NSF National Center for Supercomputing Applications (NCSA - DMR060007).

Improving the Si Impurity Tolerance of Pr0.1Ce0.9O2−δ SOFC Electrodes with Reactive Surface Additives

Fast oxygen exchange kinetics, a key figure of merit in solid oxide fuel cell (SOFC) electrodes, is often dramatically hindered by the presence of even small concentrations of impurities, for example, Si which is common in ceramics processing. In this work, rapid degradation of oxygen exchange kinetics was found on dense thin films of Pr0.1Ce0.9O2−δ (PCO), a mixed ionic electronic conducting electrode for SOFCs, using a new optical transmission relaxation (OTR) technique which facilitates kinetics measurements of bare film surfaces (i.e., in the absence of current collectors typical of conventional measurements). Si was identified by TEM and XPS as a significant impurity, forming a blocking layer on the electrode surface. Full recovery of oxygen exchange kinetics on the measured samples was achieved after deposition of a La oxide film. Subsequent TEM studies indicate the formation of a porous La- and Si-containing film. Interaction between La and Si oxides was identified by a shift in the O 1s XPS peak an...

Modification of Oxygen/(Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ Interfaces Derived by Metal-Organic Deposition

In this study, nanoscaled (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) thin films (with thicknesses below 100 nm) have been successfully deposited onto NdGaO3 single crystal substrates using a metal-organic deposition (MOD) method. In contrast to physical vapor deposition techniques, e.g. pulsed-laser deposition or magnetron sputtering, BSCF thin films formed by an MOD method provide the possibility to custom-tailor the surface morphology by subsequent annealing treatments. Changes of surface area or phase composition (cubic, hexagonal BSCF, others) are expected to affect the catalytic activity of oxygen-transport membranes. A study of surface morphologies achieved by a variation of the post-deposition thermal treatment is presented. The electrical properties of selected MOD thin films were determined and compared to data for epitaxial BSCF thin films and bulk samples. Lastly, oxygen exchange kinetics were investigated by electrical conductivity relaxation measurements analyzed in the frequency domain, yielding oxygen surface exchange coefficients k δ(T).

(Invited) Enhancement of Surface Oxygen Exchange Kinetics for Pr0.1Ce0.9O2- δ with Deposition of La or Sm Oxide

Rapid oxygen transport across the gas/solid interface (oxygen exchange) is critical for solid oxide fuel cell electrode performance. In this work, La and Sm are shown to dramatically improve the oxygen exchange rate on Pr0.1Ce0.9O2-δ thin films (>50 x at 450 oC) as compared to a freshly deposited film without La or Sm oxide. A surface cleaning effect from the La and Sm is suspected as the source of the enhancement, though catalytic activity from La and Sm may also play a role.

Nature and Functionality of Oxygen/Cathode/Electrolyte-Interfaces in SOFCs

The functionality of oxygen electrodes for solid oxide fuel cells (SOFC) is mainly determined by intrinsic material properties and fabrication controlled microstructure parameters for an enhanced oxygen reduction reaction at the solid/gas interface. La1-xSrx Co1-yFeyO3-δ (LSCF) delivers high oxygen-exchange kinetics and is widely used as a cathode material for high performing anode-supported fuel cells. LSCF is however chemically instable against Zr-based electrolytes (f.e. Y stabilized ZrO2 - YSZ) and is therefore separated by a 5-8 µm thick diffusion barrier interlayer, consisting of rare earth doped Cerium oxide (f.e. doped with Gd - GDC). This study shows that variations in GDC manufacturing conditions tremendously influence the cathode performance. The LSCF-GDC-YSZ layer sequence reveals a complex interacting nature during fabrication that needs to be understood for further optimization and reproducible performance of the oxygen/cathode /electrolyte interface in SOFCs.

Low temperature, non-stoichiometric oxygen-isotope exchange coupled to Fe(II)–goethite interactions

The oxygen isotope composition of natural iron oxide minerals has been widely used as a paleoclimate proxy. Interpretation of their stable isotope compositions, however, requires accurate knowledge of isotopic fractionation factors and an understanding of their isotopic exchange kinetics, the latter of which informs us how diagenetic processes may alter their isotopic compositions. Prior work has demonstrated that crystalline iron oxides do not significantly exchange oxygen isotopes with pure water at low temperature, which has restricted studies of isotopic fractionation factors to precipitation experiments or theoretical calculations. Using a double three-isotope method (18O–17O–16O and 57Fe–56Fe–54Fe) we compare oxygen and iron isotope exchange kinetics, and demonstrate, for the first time, that oxygen-isotope exchange between structural oxygen in crystalline goethite and water occurs in the presence of aqueous Fe(II) (Fe(II)aq) at ambient temperature (i.e., 22–50°C). The three-isotope method was used to extrapolate partial exchange results to infer the equilibrium, mass-dependent isotope fractionations between goethite and water. In addition, this was combined with a reversal approach to equilibrium by reacting goethite in two unique waters that vary in composition by about 16‰ in 18O/16O ratios. Our results show that interactions between Fe(II)aq and goethite catalyzes oxygen-isotope exchange between the mineral and bulk fluid; no exchange (within error) is observed when goethite is suspended in 17O-enriched water in the absence of Fe(II)aq. In contrast, Fe(II)-catalyzed oxygen-isotope exchange is accompanied by significant changes in 18O/16O ratios. Despite significant oxygen exchange, however, we observed disproportionate amounts of iron versus oxygen exchange, where iron-isotope exchange in goethite was roughly three times that of oxygen. This disparity provides novel insight into the reactivity of oxide minerals in aqueous solutions, but presents a challenge for utilizing such an approach to determine equilibrium isotope fractionation factors. Despite the uncertainty from extrapolation, there is consistency in goethite–water fractionation factors for our reversal approach to equilibrium, with final weighted average fractionation factors of Δ18OGth-water=3.0 (±2.5‰) and 0.2 (±0.9‰) at 22°C and 1.9 (±1.5‰) and −1.6 (±0.8‰) at 50°C for nano-particulate and micron-sized goethite, respectively (errors at 2σ level). This variability of Δ18OGth-water with particle size may reflect differences in the grain boundaries of goethite and ratios of surface area to volume. Reaction of ferrihydrite with Fe(II)aq in two distinct waters resulted in a quantitative conversion to goethite and complete oxygen-isotope exchange in each case, and similar fractionation factors were observed for experiments using the two waters. Comparison of our results with previous studies of oxygen isotope fractionation between goethite and water suggests that particle size may be a contributing factor to the disparity among experimental studies.

Strontium-doped samarium manganite as cathode materials for oxygen reduction reaction in solid oxide fuel cells

SmxSr1−xMnO3 with x = 0.3, 0.5 and 0.8, denoted as SSM37, SSM55 and SSM82, respectively, have been prepared via a sol–gel route as materials for cathodes in solid oxide fuel cells. Their activities in the oxygen reduction reaction (ORR) have been evaluated in comparison with the state-of-the-art cathode material La0.8Sr0.2MnO3 (LSM82) by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and thermogravimetry (TG). Among all the prepared cathodes, the SSM55 exhibits the lowest values, while the LSM82 exhibits the highest polarization resistance, at open circuit voltage (OCV) and temperatures from 650 to 800 °C. This result indicates that the prepared SmxSr1−xMnO3 is a promising replacement for LSM82 as cathode material for SOFCs, and the SSM55 represents the optimal concentration in SmxSr1−xMnO3 series. The remarkably high ORR activity of the SSM55 is ascribed to its high surface Mn4+/Mn3+ and Oad/Olattice ratios and fast surface oxygen exchange kinetics.

Accelerated oxygen exchange kinetics on Nd2NiO(4+δ) thin films with tensile strain along c-axis.

The influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was investigated at the surface of Nd2NiO4+δ (NNO). Nanoscale dense NNO thin films with tensile, compressive and no strain along the c-axis were fabricated by pulsed laser deposition on single-crystalline Y0.08Zr0.92O2 substrates. The ORR kinetics on the NNO thin film cathodes was investigated by electrochemical impedance spectroscopy at 360-420 °C in air. The oxygen exchange kinetics on the NNO films with tensile strain along the c-axis was found to be 2-10 times faster than that on the films with compressive strain along the c-axis. A larger concentration of oxygen interstitials (δ) is found in the tensile NNO films compared to the films with no strain or compressive strain, deduced from the measured chemical capacitance. This is consistent with the increase in the distance between the NdO rock-salt layers observed by transmission electron microscopy. The surface structure of the nonstrained and tensile strained films remained stable upon annealing in air at 500 °C, while a significant morphology change accompanied by the enrichment of Nd was found at the surface of the films with compressive strain. The faster ORR kinetics on the tensile strained NNO films was attributed to the ability of these films to incorporate oxygen interstitials more easily, and to the better stability of the surface chemistry in comparison to the nonstrained or compressively strained films.

Surface chemistry of La0.6Sr0.4CoO3−δ thin films and its impact on the oxygen surface exchange resistance

Correlation between the surface chemistry of LSC thin films analyzed by LEIS and ICP-MS and the oxygen exchange kinetics.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

Surface activation of asymmetric CaTi1−xFexO3−δ tubular membranes for oxygen separation

Tubular asymmetric mixed ionic-electronic conducting CaTi0.9Fe0.1O3−δ (CTF) membranes can be used for oxygen separation from air. However, the oxygen permeation flux through thin CTF supported membranes may be limited by surface oxygen exchange kinetics and gas diffusion limitation in the porous support. To overcome surface exchange limitation, the membranes can be coated with an activation layer, which can either be a porous layer of the membrane material itself to increase the active surface area, or a porous layer of a material exhibiting superior oxygen exchange properties. In this study, asymmetric tubular CTF membranes were prepared by plastic extrusion and dip-coating. The membranes were further coated by spray-coating with both types of activation layers. La0.8Sr0.2CoO3−δ (LSC) or La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) were selected based on similar thermal expansion and high mixed ion and electron conductivity. We report here on the effects of spray-coating parameters and annealing treatment on both the adhesion and the porosity of the spray-coated layers. In addition, we discuss results of oxygen flux and permeability in the temperature range 750–1000°C in relation with potential limitations from surface exchanges and gas diffusion in the porous tubular supports.

Defect chemistry and surface oxygen exchange kinetics of La-doped Sr(Ti,Fe)O3 − α in oxygen-rich atmospheres

The mixed ionic and electronic conductor Sr(Ti,Fe)O3-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 Sr1-yLayTi0.65Fe0.35O3-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.

Invited Presentation: Effect of Elastic Strain and Dislocations on Oxide Ion Diffusion and Oxygen Exchange Kinetics on Oxides

How can the oxygen non-stoichiometry, oxygen diffusion and oxygen reduction kinetics be altered in functional oxides by lattice strain? This question has recently attracted wide interest for enabling a new route for attaining fast oxygen exchange and transport kinetics, and obtaining fundamental answers to this question is important for a range of technologies including high performance solid oxide fuel cells, oxide based sensors, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of the reactions involved on the material can be altered. For example, in theory one can turn an endothermic reaction into an exothermic one, and reduce the energy barriers by applying lattice strain. This is the so-called mechano-chemical coupling, and when applied on electrochemical systems we can introduce the term mechano-electro-chemistry.In this talk, we will discuss recent work that probes quantitatively the mechanisms by which elastic strain impacts surface reactivity and diffusion kinetics in fluorite and perovskite oxides. Computationally, using ab initio and atomistic simulations, we discovered that lattice strain can accelerate the ionic transport in 8%Y2O3-ZrO2 by reducing the ion migration energy barriers. Combining surface chemical characterization on model thin films with ab initio modeling, we demonstrated for the first time that tensile lattice strain favors oxygen exchange kinetics on both the oxygen-deficient (La,Sr)CoO3-d and (La,Sr)MnO3-d and the oxygen-excess Nd2NiO4+d by impacting the energies of oxygen adsorption and dissociation, oxygen vacancy formation, oxygen diffusion, cation segregation and the electron transfer process. The favorable impact of tensile lattice strain on these elementary reactions was validated at the collective level by quantifying the oxygen surface exchange and diffusion kinetics with isotope exchange measurements on (La,Sr)CoO3-d. Lastly, we will discuss the impact of dislocations in ionic crystals, as doped CeO2 and ZrO2 on the distribution of charged defects and mobility of oxygen vacancies. These results together put forth both elastic strain and the dislocations to be important factors that impressively changes the kinetics of charge transport and reactivity in functional oxides.

Effect of Lattice Strain on Surface Chemistry, Oxygen Non-Stoichiometry and Oxygen Reduction Reactivity of Nd2NiO4+ δ Thin Films

Lattice strain effects have received significant interest recently for altering the kinetics of oxygen ion diffusion and surface reactions in functional oxides used as electrodes and electrolytes of solid oxide fuel cells (SOFCs). For example, tensile strained perovskite oxide thin films, as exemplified by (La,Sr)CoO3, were shown to clearly exhibit enhanced charge transfer properties and oxygen reduction reactivity as cathode material. Ruddlesden-Popper (RP) phases, with the chemical formula An+1MnO3n+1, have drawn broad interest as a competitive candidate for new cathode materials. The oxygen diffusion in these materials is highly anisotropic: diffusivity along the a-b plane is much higher than those along c-axis. While the oxygen exchange and diffusion in perovskite oxides is mediated by oxygen vacancies and is isotropic, RP phase oxides preferentially incorporate and transport oxygen interstitials along the a-b plane. Because of this structural anisotropy, the strain response of RP oxides in oxygen exchange and diffusion is expected also to be also anisotropic, and likely stronger along the c-axis compared to the strain response of perovskite materials.We systematically investigated the effect of lattice strain on surface chemistry, non-stoichiometry and oxygen surface exchange kinetics of Nd2NiO4 (NNO) epitaxial thin films as a model RP structure, using both experimental and computational methods. NNO thin films with both (100) orientation (fast diffusion path exposed) and (001) orientation (fast diffusion path buried), and both tensile and compressive strain states were fabricated. In situ high-temperature X-ray diffraction (HTXRD) and in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) data is combined to assess the variation of oxygen non-stoichiometry and surface cation chemistry during annealing at high temperature. The strain along the c-axis, rather than the crystal orientation, was found to primarily determine the oxygen non-stoichiometry and Ni cation oxidation state of the NNO oxide thin films. Moreover, the NNO films with compressive strain along the c-axis are more reducible in low oxygen partial pressure compared to the tensile strained films. Nd segregation and surface morphology changes during annealing are more evident for the films on the NNO films with compressive strain along the c-axis. We attribute this to the elastic energy minimization along the rock-salt layers that is compressively stressed. Based on these observations, faster oxygen exchange kinetics is expected on the (100) oriented films with tensile strain along the c-axis. Investigations to determine and compare the oxygen exchange kinetics directly and quantitatively have been done using both electrical conductivity relaxation (ECR) and electrochemical impedance spectroscopy (EIS). ECR and EIS results proved faster surface exchange for (100) samples compared to (001) samples, also that tensile strain along c axis accelerated the oxygen surface exchange. On the other hand, we also utilized ab initio density functional theory (DFT) to study the effect of strain along c axis on oxygen interstitial formation and oxygen diffusivity in NNO, which also showed that tensile strain along c axis facilitates interstitial formation and oxygen transport along the a-b plane. In this work, the strain response of RP phase represented by the model Nd2NiO4+δ composition was found to be anisotropic, and the strain along the c-axis was found to be a key parameter for altering the non-stoichiometry, surface chemistry and oxygen reduction reactivity. These results have important implications to surface oxygen exchange kinetics on SOFC cathodes, and enable a comparison of the strain response of different classes of oxides.

INVITED TALK - In Situ Electronic Structure Measurements and Correlations to Reactivity on Hetero-Structures for Solid Oxide Fuel Cells

Strontium-doped lanthanum cobaltite is one of the most promising cathode candidates for intermediate temperature solid oxide fuel cells. Recently, the hetero-interfaces between the perovskite (La,Sr)CoO3 (LSC113) and the Ruddlesden-Popper (La,Sr)2CoO4 (LSC214) phases have shown highly enhanced oxygen exchange kinetics. This observation offers the potential for a new class of cathode structure design, either via multilayering or by forming vertical heteroepitaxial nanocomposites, with high densities of these special interfaces. In this work, nanoscale multilayers and vertically aligned nanocomposites of LSC113/ 214 have been prepared by pulsed laser deposition. Their surface chemistry was characterized by Auger Electron Spectroscopy with high spatial resolution, and their surface electronic structure was probed by scanning tunneling microscopy at elevated temperatures and in oxygen gas. The intimate contact of LSC214 to the LSC113 phase, rendered the charge transfer on the LSC214 surface to be more facile, contributing to enhanced oxygen reduction reactivity near the LSC113/ 214 interfacial regions intersecting the surface. The enhancement of the ORR kinetics is also confirmed by EIS measurement at relatively low temperatures below 400 oC. The surface chemistry at higher temperatures ages by time and hides the improvements that arise from the coupling of these two phases through their interface. These observations lead to a novel means for SOFC cathode fabrication with enhanced performance and provide guidelines for identifying other promising cathode candidates.

“Imaging” LEIS of micro-patterned solid oxide fuel cell electrodes

Understanding the kinetics of oxygen exchange between the gas phase and a ceramic electrode is key to optimising the performance of electrochemical energy conversion devices such as Solid Oxide Fuel Cells. Clearly the surface chemistry of these materials is important, and surface sensitive techniques such as Low Energy Ion Scattering (LEIS) can provide important compositional information key to unravelling electrode kinetics.In this work, we use high lateral resolution LEIS to perform local analyses of a micropatterned electrode structure, of the type often used for studies of the geometrical dependences of electrode performance. We find that the results are comparable to those for bulk materials, but detect evidence of cation interdiffusion from the electrode to the electrolyte. Finally, we note that this preliminary study could open the prospect of in situ measurements of cells near operating conditions.

Oxygen transport kinetics of the misfit layered oxide Ca3Co4O9+δ

Ca3Co4O9+δ exhibits fast oxygen exchange kinetics rendering it a promising electrode for application as an air electrode in solid oxide cells.

Revealing the atomic structure and strontium distribution in nanometer-thick La0.8Sr0.2CoO3−δ grown on (001)-oriented SrTiO3

Surface segregation in metal oxides can greatly influence the oxygen transport and surface oxygen exchange kinetics critical to the performance of solid-state devices such as oxygen permeation membranes and solid oxide fuel/electrolytic cell electrodes. Unfortunately detecting elemental distributions at the atomic scale near the surface remains challenging, which hampers the understanding of underpinning mechanisms and control of surface segregation for the design of high-performance materials. Using the coherent Bragg rod analysis (COBRA) method, we report the first direct 3D atomic imaging of a 4 nm-thick “La0.8Sr0.2CoO3–δ”/SrTiO3 epitaxial film. Of significance, energy differential COBRA revealed pronounced Sr segregation (La1−xSrxCoO3−δ, x ∼ 0.4) in the four unit cells from the top surface while complete Sr depletion was detected in the five unit cells from the “La0.8Sr0.2CoO3−δ”/SrTiO3 interface. The drastic strontium compositional changes in the film were associated with large changes in the atomic positions of apical oxygen sites in the perovskite structure. Such Sr segregation tendencies toward the surface were also found in nominal “La0.6Sr0.4CoO3−δ” thin films, which can greatly enhance the surface oxygen exchange properties of oxides. The results presented here show that COBRA and the differential COBRA methods can be used to investigate a variety of electrochemically active systems providing atomic scale structural and chemical information that can help understand the physical and chemical properties of these systems and serve as a basis for comparison with DFT calculations.