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Cobaltite oxides, like Ba(La)CoO3, are very promising candidates as cathode materials for solid oxide fuel cells and electrolysers (SOFCs and SOECs). Lattice distortions have been shown as a promising approach to improve cathodic properties. To investigate this possible approach, biaxial strain is induced on model systems, i.e. through the epitaxial growth of barium lanthanum cobaltite on single crystalline substrates. The influence of strain on the oxidation state of cobalt is probed through energy electron loss spectroscopy. XRD measurements indicate that films with biaxial compressive strain exhibit a larger unit cell volume. Electron energy-loss spectroscopy (EELS) shows that biaxial compressive strain induces a higher Co2+/Co3+ ratio, probably enabled through the larger ionic radius of Co2+. Electrical conductivity relaxation measurements reveal that the activation energy for the surface exchange is decreased with the increasing amount of Co2+. This strongly indicates the connection between the unit cell volume, the oxidation state of cobalt, and the surface exchange kinetics of Ba(La)CoO3. An improved understanding of the fundamental kinetics in cathode materials for SOFCs is important for further materials development.

Mixed ionic-electronic conductors (MIECs) that display high oxide ion conductivity (σo ) and electronic conductivity (σe ) constitute an important family of electrocatalysts for a variety of applications including fuel cells and oxygen separation membranes. Often MIECs exhibit sufficient σe but inadequate σo . It has been a long-standing challenge to develop MIECs with both high σo and stability under device operation conditions. For example, the well-known perovskite oxide Ba0.5 Sr0.5 Co0.8 Fe0.2 O3- δ (BSCF) exhibits exceptional σo and electrocatalytic activity. The reactivity of BSCF with CO2 , however, limits its use in practical applications. Here, the perovskite oxide Bi0.15 Sr0.85 Co0.8 Fe0.2 O3- δ (BiSCF) is shown to exhibit not only exceptional bulk transport properties, with a σo among the highest for known MIECs, but also high CO2 tolerance. When used as an oxygen separation membrane, BiSCF displays high oxygen permeability comparable to that of BSCF and much higher stability under CO2 . The combination of high oxide transport properties and CO2 tolerance in a single-phase MIEC gives BiSCF a significant advantage over existing MIECs for practical applications.

Mixed ionic-electronic conductors (MIECs) that display high oxide ion conductivity (σ

In this work, an innovative approach for determining the surface stoichiometry of complex metal oxide (CMO) thin films is presented. The procedure is based on treatment of the sample surface with different etching solutions, followed by on-line analysis of the derived eluates using inductively coupled plasma – mass spectrometry (ICP-MS). Via consecutive treatment of the sample surface with water and diluted HCl, a differentiation between water soluble and acid soluble parts of near surface regions in thin films is enabled. The developed procedure was applied for the analysis of dense La0.6Sr0.4CoO3−δ (LSC) thin films indicating the presence of a water soluble Sr-rich phase with sub nm-thickness on top of the LSC films. The step-wise optimization of this technique is reported and the results are compared to measurements performed by low-energy ion scattering (LEIS). The detrimental effect of the water soluble Sr rich phase on the oxygen exchange activity of LSC thin film electrodes is verified by electrochemical impedance spectroscopy (EIS).

Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage.

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO3) and iron-doped strontium titanate (SrTi1–xFexO3−δ). Our study reveals that while O2 dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O2 molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O2 molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O2 molecules. Our focus on the SrO-terminated surface, rather than the TiO2 layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spect...

The segregation of constituent elements and impurities affects the surface composition of solid oxide electrode materials, and hence, the rate of the surface exchange reaction. In this work, we compare segregation occurring at 400 °C in Pr0.5Sr0.5CoO3-δ (PSCO) and PrBaCo2O5-δ (PBCO). The surface composition changes rapidly on annealing at low temperature, with segregation of the divalent cation evident after just 15 minutes. The PBCO contained a Na impurity which undergoes similar segregation behavior to the Ba, becoming the dominant surface cation after 2 hours of annealing. Although cation mobility in these materials is slow compared to oxygen, it is sufficient to cause significant changes in surface composition on timescales of the order of hours.

Layered Ruddlesden-Popper (RP) type oxides, formulated Lan+1NinO3n+1 (n = 1, 2 and 3), have recently been suggested as candidates for IT-SOFC cathodes. The oxygen surface exchange activity is strongly influenced by the surface composition and atomic surface structure where the adsorption and dissociation of molecular oxygen take place. It is therefore necessary to understand the surface and near-surface chemical compositions of the cathode materials. In this work, the surface and near-surface composition of epitaxial films in the RP series was systematically investigated using low-energy ion scattering (LEIS).

In this work, we investigate and compare the performance and cell polarization resistance of Ba 0.5 La 0.5 CoO 3−δ (BLC) and double perovskite oxide BaGd 0.8 La 0.2 Co 2 O 6−δ (BGLC) anode on cathode supported protonic steam electrolysis cells using a 20 μm SrZr 0.5 Ce 0.4 Y 0.1 O 3−δ electrolyte with Ni-SZCY541 composite as the cathode. The kinetics of protons through the bulk and across the gas electrolyte interfaces of both anode materials were also studied by direct measurement of their tracer diffusions using time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS). Cell terminal voltages of 1.74 and 1.93 V, were observed at a current density of 0.5 A cm −2 for both BLC and BGLC whereas a hydrogen evolution rate of 121.85 and 111.15 μmol cm −2 every minute was also obtained at the same current density, translating to a current efficiency of 78 and 72% respectively. Hydrogen tracer diffusion studies confirm BGLC can incorporate protons into the bulk relative to BLC even though the present steam electrolysis results show a better performance for BLC at 600 °C.

The accuracy and precision of isotopic analysis in Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) relies on the appropriate reduction of the dead-time and detector saturation effects, especially when analyzing species with high ion yields or present in high concentrations. Conventional approaches to avoid these problems are based on Poisson dead-time correction and/or an overall decrease of the total secondary ion intensity by reducing the target current. This ultimately leads to poor detection limits for the minor isotopes and high uncertainties of the measured isotopic ratios. An alternative strategy consists of the attenuation of those specific secondary ions that saturate the detector, providing an effective extension of the linear dynamic range. In this work, the selective attenuation of secondary ion signals (SASI) approach is applied to the study of oxygen transport properties in electroceramic materials by isotopic labeling with stable (18)O tracer and ToF-SIMS depth profiling. The better analytical performance in terms of accuracy and precision allowed a more reliable determination of the oxygen surface exchange and diffusion coefficients while maintaining good mass resolution and limits of detection for other minor secondary ion species. This improvement is especially relevant to understand the ionic transport mechanisms and properties of solid materials, such as the parallel diffusion pathways (e.g., oxygen diffusion through bulk, grain boundary, or dislocations) in electroceramic materials with relevant applications in energy storage and conversion devices.

Composites consisting of a perovskite-based electronic or mixed conductor with a fluorite-structured ionic conductor are often used as electrodes in solid oxide electrochemical energy conversion devices. After sintering the materials, there is often evidence for inter-reaction between the two phases, or inter-diffusion of cations or impurities between the two phases. We studied the18O exchange properties of a composite consisting of CGO and LSCF in a 50 : 50 ratio. High resolution ToF-SIMS mapping reveals that the18O fraction at the very outer surface of grains of the CGO phase is much higher than expected fromD* andk* values for the single-phase parent material. Surface compositional analysis by ToF-SIMS and low energy ion scattering (LEIS) spectroscopy suggests that the surfaces of the CGO grains in the composite do not show the impurities which typically segregate to the surface in single-phase CGO. Thus, the “cleaning” of impurities from the CGO surface by dissolution into the perovskite phase may be one explanation for the apparent enhanced surface exchange for CGO in these composites.

The surface and near-surface chemical composition of electroceramic materials often shows significant deviations from that of the bulk. In particular, layered materials, such as cation-ordered LnBaCo2O5+δ perovskites (Ln = lanthanide), undergo surface and sub-surface restructuring due to the segregation of the divalent alkaline-earth cation. These processes can take place during synthesis and processing steps (e.g. deposition, sintering or annealing), as well as at temperatures relevant for the operation of these materials as air electrodes in solid oxide fuel cells and electrolysers. Furthermore, the surface segregation in these double perovskites shows fast kinetics, starting at temperatures as low as 400 °C over short periods of time and leading to a decrease in the transition metal surface coverage exposed to the gas phase. In this work, we use a combination of stable isotope tracer labeling and surface-sensitive ion beam techniques to study the oxygen transport properties and their relationship with the surface chemistry in ordered LnBaCo2O5+δ perovskites. Time-of-Flight Secondary-Ion Mass Spectrometry (ToF-SIMS) combined with 18O isotope exchange was used to determine the oxygen tracer diffusion (D*) and surface exchange (k*) coefficients. Furthermore, Low Energy Ion Scattering (LEIS) was used for the analysis of the surface and near surface chemistry as it provides information from the first mono-atomic layer of the materials. In this way, we could relate the compositional modifications (e.g. cation segregation) taking place at the electrochemically-active surface during the exchange at high temperatures and the oxygen transport properties in double perovskite electrode materials to further our understanding of the mechanism of the surface exchange process.

Understanding the surface chemistry and oxygen surface exchange activity in mixed conducting perovskite and related perovskite oxides is of great relevance in developing electrochemical devices. Mixed conducting Ruddlesden–Popper Lan+1NinO3n+1 phases (n = 1, 2 and 3) have been considered as promising electrodes for electrochemical energy conversion cells due to their layered structure allowing non-stoichiometric defect structures. This study focuses on a systematic investigation of the chemical composition of the outermost atomic surfaces of as-deposited and annealed epitaxial films of Lan+1NinO3n+1 (n = 1, 2 and 3). For both as-deposited and annealed films, the analysis of the outermost surface using low energy ion scattering shows preferential LaO-termination. The results also provide evidence of an associated Ni-enrichment below the outermost surface. These findings suggest significant atomic rearrangement occurs during deposition and subsequent annealing. To investigate the thermal stability of these films during deposition, further microstructural analysis was carried out by means of high-resolution scanning transmission electron microscopy, showing significant re-orientation of LaO layers after a post-annealing heat treatment. In thin films of n = 2, 3 phases, surface restructuring reduces the epitaxy of the films and hence any potential beneficial anisotropy in transport properties will be lost. Care must therefore be exercised in processing these materials for electrode applications.

Low Energy Ion Scattering (LEIS) is a powerful tool to analyse the composition of the very outermost atomic monolayer of a material. However, most studies so far have been conducted on ceramic pellets of the materials of interest. In this work, we apply laterally resolved LEIS analysis to electrochemical cells consisting of micro-patterned arrays of LSCF electrodes on YSZ electrolytes. The results suggest the migration of constituent elements and impurities between the two phases which has the potential to influence device performance.

The electro-catalytically active surfaces of mixed ionic electronic conductors (MIECs) are critically important for the operation of electrochemical devices such as solid oxide electrolyser cells (SOECs) and fuel cells (SOFCs). However, the composition and morphology of surfaces of real samples are often poorly characterized, and thus our understanding of the influence of these factors on the surface exchange reactions is rudimentary. In this work, we apply low energy ion scattering (LEIS) spectrometry and atomic force microscopy (AFM) to investigate the evolution of the surface composition and morphology of two layered perovskite type materials, showing the dynamic nature of these surfaces even at room temperature.

We study the outer atomic surfaces of a series of perovskite-based ceramics using low energy ion scattering spectroscopy. After high temperature treatment, segregated A-site (or acceptor substituent) cations dominate the outer surfaces with no B-site cations detected. We also find evidence of an associated B-cation enriched region below the surface.

The analysis of the surface chemistry of electrode materials for solid oxide electrochemical conversion devices is of great importance to understand the fundamental mechanism of the oxygen exchange reaction, and the influence of changes in surface chemistry on device degradation. Low Energy Ion Scattering (LEIS) is an extremely surface sensitive analysis technique, capable of probing this chemistry in the very outer atomic layer of a material under conditions relevant to its operational state. In this contribution, we use LEIS to study the Sr segregation process in the perovskite La0.6Sr0.4Co0.2Fe0.8O3 − δ (LSCF), and show that, conversely, there is no segregation of either lanthanide cation in Ruddlesden–Popper structured Pr1La1NiO4 + δ (PLNO).

Surfaces and interfaces determine the performance and long-term durability of solid-oxide fuel cells (SOFCs). In most cases, the surface chemical composition of the materials used in these electrochemical energy conversion devices shows significant deviations from the bulk chemistry. This might be as a result of surface cation segregation processes, as well as long-term surface poisoning due to external impurities. Both processes have implications for the electrochemical performance of the devices, leading to the degradation of the cell components. In order to suppress this degradation, an effort to lower the operation temperature to 500–800°C has been made. This article provides an overview of present research progress related to surface segregation and poisoning for low-temperature SOFCs.

Cu(In,Ga)Se2 (CIGS) single-crystal epitaxial films have been analyzed by low energy ion scattering, which is sensitive to exactly the outermost surface atomic layer, to determine the surface chemistry as a function of preparation conditions. The samples were grown by a hybrid sputtering and evaporation method on cation (A) or anion (B) terminated (111) GaAs substrates and had smooth surfaces. The samples were exposed to excited atomic oxygen or hydrogen beams or were sputtered with 500 eV Ar+ ions. Atomic O* treatment resulted in an otherwise clean, oxidized surface including all film constituents. Atomic H* resulted in strong enhancement of the surface Ga population, probably due to a preexisting Ga native oxide in the outermost atomic layer. Sputtering produced a clean surface that was closest to the bulk composition of the film as measured by energy-dispersive spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.

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.

The anode in a solid oxide electrolysis cell (SOEC) is often the rate-determining component, in terms of both device performance and degradation rates. In order to improve device characteristics, better electrode materials must be developed. Interstitial oxygen conducting electrodes, such as those in the A2BO4 Ruddlesden-Popper family, have been shown to perform well as anodes in SOECs. In this work, we characterize the electrode properties of Ruddlesden-Popper structured (Pr,La)2NiO4+δ electrode materials, and relate these electrochemical properties to the surface composition characterized by Low Energy Ion Scattering (LEIS) spectroscopy.

Impurities on the surface of SOFC electrodes can dramatically reduce their interfacial transport. In this work, we use a novel in situ optical absorption relaxation technique to derive the oxygen exchange rate in the absence of electrodes, thereby studying the “innate” material. Si is found to be an impurity in high concentration, with its presence associated with an orders of magnitude decrease in oxygen exchange rate. Addition of La to the electrode surface is found to substantially improve oxygen exchange rate.

The measurement of the mass transport and oxygen exchange in materials for SOFCs has been greatly aided by the use of ion beam analysis techniques. In particular this has focused on the use of Secondary Ion Mass Spectrometry (SIMS) to measure diffusion profiles of oxygen 18 in ceramics to determine the kinetics of the bulk transport of oxygen. A second ion beam technique, Low Energy Ion Scattering (LEIS) can provide important information about the composition of the outermost atomic layers of the same materials where the gas/solid oxygen exchange process is taking place. The application of the newer technique of LEIS to materials for air electrodes is discussed and the behaviour of some common perovskite materials is explored.

The authors use SIMS measurements to examine the composition of a Pr0.1Ce0.9O2−δ thin film following an oxygen surface exchange measurement via optical absorption relaxation. A significant concentration of impurities (Si, K, Na, and Ca) were observed to accumulate on the film surface, with a concentration profile extending approximately 60 nm into the film, following the high temperature measurements. This resulted in a significant decrease in the oxygen exchange coefficient, kex . Potential sources for the impurities are discussed.

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. The intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention. One is the critical issue of the compositional/structural surface modifications occurring during operation and how these affect the photoelectrochemical performance. The second concerns the relation between the electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance since it is exactly at the surface where the visible light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art of oxynitride thin film fabrication and characterization before focusing on LaTiO2N selected as representative photocatalyst. We report the investigation of the initial physicochemical evolution of the surface. Then we show that, after stabilization, the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations and be up to 5 times larger than for polycrystalline samples.

It is well known that alkaline earth additives in conventional perovskite-based solid oxide electrode materials tend to segregate rapidly to the outer surfaces, which is associated with degradation in device performance. It may be possible to avoid these problems by developing materials which do not contain alkaline earth dopants; however, without the oxygen vacancies introduced by alkaline earth doping, the oxygen transport and electro-catalytic properties may suffer. Here, we investigate oxygen transport in the nickel doped lanthanum cobaltite system, and find that the relatively low oxygen diffusivity is dominated by grain boundary transport. Despite this, at higher temperatures, the surface exchange coefficient is comparable to state-of-the-art Sr doped cobaltites. The surface composition of the annealed samples measured by Low Energy Ion Scattering spectroscopy does not appear different from the as-polished samples, suggesting that the surface composition is indeed more stable for this alkaline earth free material.

The effects of microstructure on the kinetics of strontium segregation in La0.6Sr0.4CoO3-δ ceramic samples are investigated. A faster segregation of the Sr cations towards the surface was observed after annealing for 1 h at 600 °C in dry oxygen (pO2= 200 mbar) as the density of grain boundaries at the surface increases (i.e. with a decrease of the grain size). The presence of surface impurities, such as Na, partially inhibits the Sr segregation towards the surface. Optical micrographs and ToF-SIMS chemical imaging showed that the segregation and precipitation of Sr starts at the grain boundaries, with an apparent enrichment of the surface around the grain boundary via Sr surface diffusion.

CGO-LSCF composite membranes have been characterised with a view to applications in oxygen separators for oxyfueled boilers in Carbon Capture and Storage (CCS) systems. Although commonly believed to be chemically stable together, evidence is presented that an interdiffusion reaction occurs on sintering at 1250ºC for 4 hours. Isotopic oxygen exchange measurements show that this does not appear to have any detrimental effect on the oxygen exchange properties. Similarly, electrical conductivity was also measured, and no detrimental effect was observed for these properties.

TiO2 foam-like scaffolds with pore size ~300 μm and >95% porosity were fabricated by the foam replication method. A new approach to improve the structural integrity of the as-sintered foams, which exhibit extremely low compression strength, was explored by coating them with poly-(d,l-lactic acid) (PDLLA) or PDLLA/Bioglass® layers. The PDLLA coating was shown to improve the mechanical properties of the scaffold: the compressive strength was increased by a factor of ~7. The composite coating involving Bioglass® particles was shown to impart the rutile TiO2 scaffold with the necessary bioactivity for the intended applications in bone tissue engineering. A dense hydroxyapatite layer formed on the surface of the foams upon immersion in simulated body fluid for 1 week.

Proton ceramic fuel cells (PCFC) for electrochemical energy conversion target operation in a temperature range between 300-600 °C, reducing the degradation rates and the materials cost in comparison with high-temperature oxide ion conducting fuel cells (SOFCs). In both cases (SOFC and PCFC), one of the main limitations for cell performance is the kinetics of the cathode reaction which takes place at the cathode-electrolyte interface when the cathode materials shows only electronic conductivity (i.e. at the triple phase boundary (TPB) where cathode surface, electrolyte and gas phase are in contact). Conversely, in mixed ionic-electronic conducting cathodes (MIECs), the oxygen reduction reaction sites are extended to the whole effective surface of the air electrode, providing high electrocatalytic activity even at the intermediate and low temperature range. Recently, MIEC cathode materials used for SOFC have been proposed as promising candidates for air electrodes on proton conducting electrolytes for PCFC. Among these oxygen-conducting materials (O-MIECs), single perovskite and double perovskites have been suggested (e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ and NdBaCo2O5+δ, respectively) (1, 2). The assumption for the use of these O-MIECs in PCFCs is that sufficient proton conductivity is present, i.e. there are three mobile carriers through the bulk of the cathode (electrons, protons and oxygen ions). Although these materials work effectively on proton conducting electrolytes, there are few direct studies on the specific ionic carriers involved in the bulk transport. On the other hand, the chemical stability of the cathode material can be compromised, especially in steam or CO2-containing atmospheres (3). Many of the suggested cathode materials are doped with aliovalent cations in order to increase their MIEC character. Nevertheless, even at the low working temperatures of PCFC devices, the segregation of the substitutional cations lead to a specific surface termination. For instance, Ba-containing double perovskites, such as GdBaCo2O5+δ, shows fast segregation of the Ba cations at 400°C, leading to a Ba-rich outer surface after annealing in an oxygen atmosphere (e.g. 15 minutes) (4, 5). The dynamics of the segregation of the divalent cations are likely to be affected by the annealing atmosphere, as well as the surface reactivity of the segregated species in the presence of CO2 and H2O. In this work, the surface composition and the ionic transport properties of MIEC air electrodes in wet atmospheres will be discussed. The relative importance of the oxygen and proton conductivities in MIEC materials is directly measured by a combination of stable isotope exchange experiments (18O and 2D2O tracers) and Time-of-Flight Secondary Ion Mass Spectrometry depth profiling (ToF-SIMS), while the surface composition of the material is studied by Low-Energy Ion Scattering (LEIS). 1. A. Grimaud, F. Mauvy, J. M. Bassat, S. Fourcade, L. Rocheron, M. Marrony and J. C. Grenier, J Electrochem Soc, 159, B683 (2012). 2. G. Goupil, T. Delahaye, G. Gauthier, B. Sala and F. L. Joud, Solid State Ionics, 209, 36 (2012). 3. S. Upasen, P. Batocchi, F. Mauvy, A. Slodczyk and P. Colomban, Ceramics International, 41, 14137 (2015). 4. H. Tellez, J. Druce, J. A. Kilner and T. Ishihara, Faraday Discuss, 182, 145 (2015). 5. H. Téllez, J. Druce, Y.-W. Ju, J. Kilner and T. Ishihara, International Journal of Hydrogen Energy, 39, 20856 (2014).

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO3) and iron-doped strontium titanate (SrTi1–x FexO3−δ). Our study reveals that while O2 dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O2 molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O2 molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O2 molecules. Our focus on the SrO-terminated surface, rather than the TiO2 layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO3 after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO2-perovskite layer is not exposed at the gas–solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction. Further calculations were performed using the non-equilibrium Green's function method combined with density functional theory (NEGF-DFT) to study the ballistic electron transport through a few layers of iron-doped strontium titanate. This computational method can provide critical information for the surface reactivity and surface active sites which is comparable with the STM experiments. Our study shows that the electron transport is enhanced if oxygen vacancies are present in the surface layed and the electron transport is significantly suppressed by overlayers of SrO. We believe that this study provides useful information for the control over the surface activity of SrTiO3 and explains the mechanisms of the material degradation with SrO surface segregation. Finally, we investigate the properties of the SrTiO3 / SrO interface to elucidate the mechanism of oxygen ion transport between the perovskite and the oxide phase. Figure 1

The drive to reduce the operating temperatures of solid oxide fuel cells (SOFCs) and electrolysers (SOECs) requires the discovery of new materials showing enhanced oxygen transport and electrocatalytic properties. As well as modifying the composition of materials with the simple perovskite structure, lately there has also been interest in materials with other, often, layered structures including the Ruddlesden-Popper type materials, such as La2NiO4, and double perovskites such as GdBaCo2O5 (GBCO) and PrBaCo2O5 (PBCO) [1]. Whilst Sr-substituted lanthanum cobaltite (La0.6Sr0.4CoO3-d, LSC64) is widely used, the Ba-substituted analogue La1-xBaxCoO3-d (BLC) is attracting recent interest showing competitive electrode performances [2,3]. On the other hand, the double perovskite PrBaCo2O5+d (PBCO) has high diffusivity and surface exchange activity, which is often attributed to the anisotropic diffusion of oxygen due to the ordering of Pr and Ba cations on the A-site. However, experimental measurements suggest that the degree of anisotropy between diffusion in along the c axis and in the ab plane is only around half an order of magnitude [4]. Therefore, the family of compositions between single perovskite BLC and double perovskite PBCO offers an interesting system to study the effects of cation ordering on transport behaviour. In this work, we have studied the structural and oxygen transport properties of materials in the La0.5-x/2Prx/2Ba0.5CoO3-d compositional space. The materials were synthesised by a conventional solid state reaction route, and X-ray diffraction revealed that Pr-rich compositions (0.7 < x < 1) retain the same double perovskite structure as PBCO, with cation ordering on the A-site, and hence more properly written as La1-xPrxBaCo2O5+d. The La-rich compositions (0 < x Oxygen isotope exchange studies on the Ba0.5La0.5CoO3 end member show that the oxygen tracer diffusion (D*) and exchange (k*) coefficients are similar to those of PBCO, and of Sm0.5Sr0.5CoO3, which are some of the highest reported [5], explaining the high electrochemical performance of the single perovskite material [2,3]. Preliminary results indicate these values change only slightly across the composition field, with no significant increase as the structure changes from single to double perovskite. These findings seem to suggest that it is not the A-cation ordering in the crystal structure of PBCO that provides per se its high performance, but the defect chemistry, namely the high concentration of oxygen vacancies in the material. References [1] A. Tarancon et al., J. Mater. Chem. 20, pp 3799 – 3813 (2010) [2] T. Ishihara et al., Solid State Ionics 152-153, pp 609 - 613 (2002) [3] G.M. Rupp et al., J. Electrochem. Soc. 163(6), pp F564-F573, (2016) [4] M. Burriel et al., Chem. Mater. 24, pp 613 – 621 (2012) [5] I. Fullarton et al., Ionics 1, pp 51 – 58 (1995)

Oxygen electrodes in solid oxide electrochemical cells such as Solid Oxide Fuel Cells (SOFCs) or electrolysers (SOECs) are typically based on LaBO3 perovskites, where B is a transition metal such as Co, Mn or Fe. In order to impart oxide ion conductivity and electro-catalytic activity towards oxygen exchange, the La is often partially substituted with a divalent alkaline earth cation such as strontium. Although this Sr substitution is necessary to improve the electrode performance, the Sr tends to segregate to the outer surface of the electrode, where it is implicated in the decrease of surface exchange activity, and hence device performance [1]. Our group has shown that this segregation occurs rapidly, with significant changes in the surface composition of polished dense ceramics of these materials observed in times as short as 15 minutes at temperatures as low as 400 °C [2,3]. Clearly the driving forces are sufficiently high, and the kinetics (thought to be mediated by grain boundary diffusion [4]) so fast that suppressing this segregation is a challenge. One strategy to altogether avoid the problems associated with Sr segregation would be to identify new candidate materials which do not require Sr doping in the first place. In this way, the surface composition, and hence oxygen exchange activity, is expected to be more stable. However, the absence of Sr to introduce oxygen vacancies may severely hamper the oxygen transport and exchange properties. Therefore, it would be necessary to investigate alterative doping strategies to tailor the oxygen transport properties. In this work, we study Sr-free perovskite materials, such as LaCo1-xNixO3-d (LCN), in terms of their surface composition and oxygen exchange properties. The measurements combine Low Energy Ion Scattering (LEIS) spectroscopy studies of the surface composition, combined with 18O exchange-Secondary Ion Mass Spectrometry (SIMS) measurements of the oxygen transport properties. Although preliminary results indicate that the oxygen diffusivity in these materials is not as high as would be desired for a mixed conducting oxide electrode, the transport behavior is superior to that of the Sr-containing perovskite La0.8Sr0.2MnO3+d, which does find commercial application. At low to intermediate temperatures, the oxygen diffusion profiles (figure 1) show a significant contribution from grain boundary diffusion (Harrison type B kinetics [5]), whereas around 700 ºC, the diffusion kinetics transitions towards the Harrison “A” regime, where no grain boundary tailing is evident. More interestingly, the obtained surface exchange coefficients (k*) are relatively high. Around 700 ºC, the Sr-free materials show k* values comparable to La0.6Sr0.4CoO3-d. This may be related to the surface composition; LEIS spectroscopy studies of the composition of the very outer atomic surfaces of the samples (fig. 2) show that the transition metal cations persist at the surface, whereas their coverage is very low in Sr-containing materials such as La0.6Sr0.4CoO3-dafter comparable treatments. However, although no divalent cation segregation is present, sodium impurities are observed at the outer surfaces after annealing at higher temperatures. It is not clear as of writing whether this is due to the segregation of bulk impurities, or cross contamination in the annealing apparatus. These changes in surface composition may have implications for the longer-term stability of these materials. REFERENCES: [1] G.M. Rupp et al., J. Mater. Chem. A 3(45), pp. 22759 – 22769 (2015) [2] H. Tellez et al., Int. J. Hydrogen Energy 39(35), pp. 20856-20863, (2014) [3] J. Druce et al., ECS Trans. 66(2), pp 61-68, (2015) [4] M. Kubicek et al., Phys. Chem. Chem. Phys. 16(6), pp. 2715-2726 (2014) [5] L.G. Harrison, Trans. Faraday Soc. 57, p. 1191 (1961) Figure 1

Cation segregation is a common phenomenon ocurring at high and intermediate temperatures in the perovskite materials often used as oxygen electrodes in solid oxide fuel cells and electrolyzers (SOFC/SOEC). Most of these perovskite oxides, with the general formula ABO3, are doped either in the A-site or B-site lattice positions in order to tailor their stability, electronic conductivity and ionic transport properties. For instance, LaCoO3perovskites are typically doped with Sr cations at the A-site in order to increase the oxygen vacancy concentration, and hence improve the catalytic activity of the material for oxygen reduction/evolution reactions, as well as the ionic conductivity. Nevertheless, at the typical processing and operation temperatures for SOFC/SOEC applications, aliovalent doping of (3,3) perovskites results in surface segregation due to elastic and electrostatic interactions within the host lattice. This leads to a compositional gradient at the near- and outer surface of the oxide electrode, showing a full SrO-perovskite surface termination and, in the longer term, to the precipitation of secondary phases (Figure 1) [1]. Recent studies based on low-energy ion scattering (LEIS) spectroscopy have shown that this segregation occurs in a short time-scale and at relatively low temperatures. For instance, Sr segregation occurs even during the fabrication of dense polycrystalline thin films of La0.6Sr0.4CoO3- d(LSC) by pulsed laser deposition of [2]. The Sr cations which segregate during the deposition of the film and its subsequent cooling can be removed by rinsing in deionized water - indicating that some water-soluble Sr secondary phases can be formed. However, when the LSC thin films are subjected to further annealing in oxygen at temperatures higher than 500°C, segregation of the divalent dopant takes place again, indicating this is the equilibrium surface termination at the high temperature. The kinetics of the surface segregation is, however, highly dependent on the microstructure of the oxide material. As observed in Figure 2, the size of the grains (i.e. the grain boundary density) will have a strong impact effect on the kinetics of the Sr segregation. In the case of the LSC thin films (with an average grain size of about 200 nm), annealing at 600°C for 1 h results in a significant Sr surface coverage, whereas the Sr segregation is less evident for a polycrystalline ceramic sample annealed in the same conditions (grain size of around 40 mm). These results are in agreement with a previous study by Kubicek et al. [3], in which the diffusion of Sr in LSC thin films at the temperatures of interest was estimated to be three orders of magnitude faster through the grain boundaries compared to the bulk. Given the detrimental effect of Sr segregation on the electrochemical performance of the electrode (e.g. by increasing the electrode surface resistance), the impact of faster cation diffusion through the grain boundaries should be considered when optimizing the microstructure of the electrode material. Although decreasing grain size may improve performance, it could lead to faster segregation. On the other hand, decreasing the grain boundary density at the surface while reducing the working temperature (T < 500°C) might sufficiently impede the segregation kinetics and enhance durability. On the other hand, nanostructured thin film electrodes with high grain boundary density would be expected to show very rapid degradation. References: 1. J. Druce, H. Téllez, M. Burriel, M. D. Sharp, L. J. Fawcett, S. N. Cook, D. S. McPhail, T. Ishihara, H. H. Brongersma and J. A. Kilner, Energy & Environmental Science 2014, 7, 3593. 2. G. M. Rupp, H. Téllez, J. Druce, A. Limbeck, T. Ishihara, J. Kilner and J. Fleig, J. Mater. Chem. A 2015, 3, 22759. 3. M. Kubicek, G. M. Rupp, S. Huber, A. Penn, A. K. Opitz, J. Bernardi, M. Stoger-Pollach, H. Hutter and J. Fleig, Physical Chemistry Chemical Physics 2014, 16, 2715. Figure 1

La0.8Sr0.2Fe0.6Ni0.4O3-x - Ce0.9Gd0.1O2-x and La0.8Sr0.2Fe0.8Co0.2O3-x - Ce0.9Gd0.1O2-x nanocomposites were synthesized via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in acetone with addition of a surfactant, followed by drying and sintering at temperatures up to 1200°C. The evolution of the structure of samples with sintering temperature was studied by XRD and TEM with EDX and compared with the data on conductivity, oxygen isotope exchange, O2 TPD, H2 and CH4 TPR. Preliminary testing of button-size cells with cathodes supported on thin YSZ layer covering Ni/YSZ cermet demonstrated a high and stable performance of LSNF–GDC composite promising for the practical application.

The exchange of oxygen between the gas phase and a mixed conducting ceramic, and its subsequent solid state diffusion, is key to the operation of electrochemical energy conversion devices such as Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysers (SOECs), as well as dense membranes for oxygen separation. Indeed, it is often the surface reaction that determines the overall performance in such applications. However, whilst the mechanisms of oxygen diffusion in the bulk of these materials are generally well understood in terms of the defect chemistry and crystal structure, such an atomistic description of the surface exchange reaction still eludes us. Whilst 18O isotopic tracer diffusion experiments coupled with SIMS analysis have been used for more than 30 years to study the kinetics of oxygen exchange and transport in candidate mixed conductors, relatively little attention has been paid to the characterization of the realistic surface composition of such materials; despite the great attention which is paid to the bulk chemistry, it is often assumed that the surface is a simple termination of the bulk. More recently, it is becoming apparent that the surface and near-surface composition after high temperature treatment is often radically different from the bulk. Some of this recent work has been facilitated by the availability of new tools to probe the surface composition. Our group in particular has been applying Low Energy Ion Scattering (LEIS) spectroscopy, which is uniquely capable of determining the elemental composition of the single outer atomic layer of a material (i.e. the very same atomic surface that interacts with oxygen in the gas phase). In this contribution we will use the commonly used perovksite La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) to illustrate how LEIS and SIMS can contribute to our understanding of the relevant surface composition, complementing the existing understanding of the bulk properties of this commonly used electrode material. Starting from studies of surface segregation and restructuring in dense ceramic pellets after heat treatments similar to sintering conditions, we will then show how compositional changes can also occur at targeted device operating temperatures (around 500 ºC). Finally, we will show how laterally resolved LEIS analysis can be applied to understand compositional changes and interdiffusion occurring in the very outer atomic surfaces of LSCF microelectrode – YSZ electrolyte electrochemical half cells.

Many of our predictions of the functional properties of layered perovskites and related oxide materials are based on the assumption that their chemical composition and structure is homogeneous. From these predictions and further experimental studies performed on model systems (i.e. epitaxial thin films, single crystals or dense ceramic materials), the mechanisms of the oxygen surface exchange and diffusion processes are investigated to understand their behaviour and improve their efficiency and durability as oxygen electrodes for solid oxide fuel cell (SOFCs) and solid oxide electrolysers (SOECs). Although the bulk diffusion properties in these functional oxides are, in general, well known in terms of the bulk defect chemistry, the oxygen exchange between the surface and the gas phase is still not fully understood. For instance, the surface exchange process is likely to be strongly dependent on the chemical composition and microstructure of the outermost surface, where the adsorption and incorporation of oxygen takes place (or oxygen evolution in the case of the SOEC). Many of the fundamental studies on the ionic transport properties of mixed-ionic electronic conductors (MIECs) with application as air electrodes in SOFCs/SOECs are performed on polycrystalline ceramic materials. Nevertheless, recent studies have shown that the surface and near-surface (e.g. 2-3 nm) chemistry of perovskite and perovskite-related polycrystalline materials can significantly differ from the bulk composition. These surface deviations are related to the segregation of matrix species or impurities present in the materials after being subjected to temperatures relevant for materials processing and cell operation (e.g. 400 to 1000°C).1 , 2 Therefore, the local (defect) chemistry at the immediate and near-surface is likely to have a significant impact on oxygen exchange and diffusion mechanisms as a consequence of these atomic re-arrangements. On the other hand, high quality crystal epitaxial thin films are very often used in order to deconvolute the influence of the crystal orientation (anisotropy) and the influence of strain on the ionic transport properties. Typical thin film growth techniques, such as vapour deposition techniques or pulsed laser deposition, also involve annealing steps at relatively high temperature (e.g. 400-800°C), and hence, these materials are susceptible to similar segregation processes as occurring for ceramic materials3. For instance, PLD deposition at 850°C of a 24 nm-thick GdBaCo2O5+δ epitaxial thin film grown on (001)-SrTiO3 led to an outermost surface which is predominantly BaO-terminated (Figure 1, solid black line) and a Co-enriched sub-surface (Figure 1, red dotted line) when compared with the bulk cation composition (Figure 1, blue dashed line), as analysed by Low-Energy Ion Scattering (LEIS). In this work, we use high sensitivity LEIS analysis to understand the compositional deviation at the atomic level of the surface and near-surface in polycrystalline and epitaxial thin films of double perovskites (GdBaCo2O5+ d) and how these systems are modified under the thermal treatments relevant for material processing or cells operation. These studies are essential in order to characterize the realistic surfaces that will play a role for the oxygen exchange or evolution at the electrode/gas phase interface. 1. J. Druce, H. Téllez, M. Burriel, M. Sharp, L. Fawcett, S. N. Cook, D. McPhail, T. Ishihara, H. H. Brongersma and J. A. Kilner, Energ Environ Sci, 2014, 7, 3593-3599. 2. H. Téllez, J. Druce, Y.-W. Ju, J. Kilner and T. Ishihara, Int J Hydrogen Energ, 2014, 39, 20856-20863. 3. J. H. Lee, G. Luo, I. C. Tung, S. H. Chang, Z. Luo, M. Malshe, M. Gadre, A. Bhattacharya, S. M. Nakhmanson, J. A. Eastman, H. Hong, J. Jellinek, D. Morgan, D. D. Fong and J. W. Freeland, Nat Mater, 2014, 13, 879-883. Figure 1

The surface segregation and inter-diffusion of cations and impurities in La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) micro-patterned electrode structures on a single crystal Zr0.84Y0.16O1.92(YSZ) electrolyte was investigated by high-resolution ion beam analysis (Time-of Flight Secondary Ion Mass Spectrometry and Low-Energy Ion Scattering). The outermost and near surface of these microelectrodes showed significant changes in the distribution of the cations and impurities after micro-patterning, annealing and electrochemical testing of the cells. As occurring for bulk LSCF ceramics, strong segregation of Sr towards the electrode surface is observed during annealing of the microelectrode, as well as due to the thermal treatments required for electrochemical measurements. This segregation might have a drastic effect on the surface oxygen exchange reaction as the electrochemically active transition metals (Fe, Co) are not directly exposed to the gas phase. Impurities, such as Si and Ca, are also observed at the annealed electrolyte surface. Furthermore, the microstructured electrode showed that impurities can also migrate between the different components (i.e. Na and Si impurities migrate from the electrolyte to the LSCF electrode, showing a “cleaner” surface after the thermal treatments). This study has provided a further understanding of the migration and segregation of cations and impurities taking place in real devices, building upon previous studies performed on ceramics materials. Figure 1

Layered Ruddlesden-Popper (RP) type oxides, formulated Lan+1NinO3n+1(n = 1, 2 and 3), have recently been suggested as candidates for intermediate temperature solid oxide fuel cell (IT-SOFC) cathodes. Compared to traditional cathodes (e.g. LSM), these mixed ionic-electronic conductors (MIECs) possess highly promising oxygen surface exchange kinetics, which is not limited to the triple phase boundary (TPB), but the entire surface exposed to the gas phase. The oxygen surface exchange activity is strongly influenced by the surface composition and atomic surface structure where the adsorption and dissociation of molecular oxygen take place. These effects may influence the rate determining step [1], and consequently the ultimate efficiency of the SOFC device. More importantly, compared to bulk samples, the oxygen transport kinetics for thin-film samples is significantly dominated by the surface process [2]. A series of Lan+1NinO3n+1(n = 1, 2 and 3) epitaxial films have been deposited using pulsed laser deposition (PLD) technique for the fundamentally surface analysis [3]. It is therefore of great importance to understand the surface and near-surface chemical compositions and the structure (or termination) of the obtained films. These investigations will aid in the understanding of the electrochemical properties of lanthanum nickelate cathode materials in surface regions [4]. In this work, the kinetic nature of Lan+1NinO3n+1(n = 1, 2 and 3) epitaxial films deposited by PLD was systematically investigated using high-sensitivity low-energy ion scattering (HS-LEIS). The compositional analysis for the outermost and near-surface of these obtained films was also studied. The results reveal that the phenomena of surface reconstruction may decrease the active surface area during the deposition and post-treatment process of the thin-film materials. References [1] S. B. Adler, Chem. Rev. 2004, 104, 4791. [2] G. Kim, S. Wang, A. J. Jacobson, C. L. Chen, Solid State Ionics 2006, 177, 1461. [3] K.-T. Wu, Y.-A. Soh, S. J. Skinner, Mater. Res. Bull. 2013, 48, 3783. [4] J. Kilner, S. Skinner, H. Brongersma, J. Solid State Electr. 2011, 15, 861.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and low-energy ion scattering (LEIS) are recently attracting great interest in energy materials research due to their capabilities in terms of surface sensitivity and specificity, spatial resolution and their ability to analyse the isotopic chemical composition. This work shows the synergy provided by this powerful combination to further our understanding of the surface chemistry and structure that ultimately determines the electrochemical performance in advanced electro-ceramic materials for energy storage and energy conversion applications. In particular, this novel approach has been applied to the analysis of (Li3xLa2/3−x□1/3−2x)TiO3 perovskite materials used as the electrolyte in lithium batteries and (La, Sr)2CoO4+δ epitaxial thin films used as oxygen electrodes in solid oxide fuel cells and solid oxide electrolysers. The analysis of these two promising materials requires the development and optimisation of new analytical approaches that take advantage of the recent instrumental developments in order to characterise the outermost and near-surfaces at the atomic scale.

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