Ionically conducting thin films are of important for integration into miniaturized Si-based devices in the areas of energy processing. For single phase solid solutions such as doped-ceria electrolytes in fuel cells, the lattice constant is generally linearly dependent on the ion radius of the dopant with a classic “order parameter” describing cation-cation and oxygen vacancy long range order at these high dopant levels. Characteristics such as oxygen vacancy formation, their concentration and ion mobility rely on anion-cation spacing and coordination, and the nature of the dopant in these solid solution systems. State-of-the-art solid solutions are processed through sintering at temperatures far higher than those where transport properties and structural characteristics are investigated; stable structures and constant diffusion lengths persist. Despite the growing interest unequivocal elucidation of near and long range structural order changes and their implication for ionic diffusivities and mobilities have been unclear for films processed at low temperature far from classical sintering conditions. This work presents a case study experiment in which Ce0.8M0.2O1.9-x (M=Sc3+, Lu3+, Gd3+ and La3+) thin films are compared to sintered pellets with respect to their structural, chemical and electrical properties. In general, an increase in dopant radii resulted in a systematic lattice expansion for the films similar to that of the pellets. However, exceptionally large decreases in lattice constant and (micro-)strain were observed for the fully crystalline films with increasing temperature anneals, contrary to that observed for the pellets. Raman spectroscopy on the films confirms the compaction of the O-Ce-O bonds upon annealing. There is also a dependency of strain on solute-host radii mismatch. Scandia, prone to clustering to oxygen vacancies, reveals the largest microstrain and lattice strain changes. Gd-doped films exhibit a comparable activation energy (Ea) to bulk pellets of 0.71 eV for T>450°C. However, for T<450°C, the Ea decreases from 1.1 to 0.7 eV in the films annealed at 1000 oC compared to those annealed at 600 oC with frozen-in structural states. Interestingly, this coincides with the strain decrease from 0.3% to almost zero, lattice constant compression by -1.5%, and a shift in Raman. These results point to a decrease in the space available for migration and potentially a reduction in defect concentration upon high-temperature annealing of the films, recovering them from their metastable nature. This is thin film processing and doping dependent. This is especially relevant for T<450°C and Gd doped ceria films as electrolyte used in micro-fuel cells. Fundamentally, one may also remark that the observed structural changes are larger for a given dopant in a ceria film then to the effect of doping with different cations in bulk pellets.

There is increasing interest in enhancing the ionic conductivity of fluorite-based oxides, such as zirconia and ceria, through nanostructuring. Experimental investigations, however, give conflicting reports, with the nanoscale effect varying from slight decreases, through modest increases, to a “colossal” enhancement. In addition, the reasons for these nanoscale effects are seldom clear. Two effects are often claimed, without quantitative treatment, to be responsible: altered point-defect distributions in interfacial space-charge layers or modified migration barriers in the vicinity of the interface due to lattice strain. In this work the effect of lattice strain on oxygen vacancy migration in the fluorite-structured oxide CeO₂ is investigated at the atomistic level. Static lattice simulation techniques, employing density functional theory (DFT) calculations, are used to determine the migration energies for oxygen vacancies, ΔE_mig, in ceria lattices subjected to isotropic, uniaxial or biaxial strain. Analysis of the data yields the activation volumes, ΔV_mig, and activation enthalpies, ΔH_mig. Enhancement of the oxygen-ion conductivity of an oxide heterostructure through space-charge effects is also discussed.

Solid oxide fuel cells (SOFCs) normally need to operate at >800 °C due in part to low oxygen ion conductivity of the traditional yttria stablized zirconia (YSZ) electrolyte. Finding ways to improve the low temperature oxygen ion conduction of the electrolyte, thereby decreasing the operating temperature, will play a decisive role in utilization of SOFCs in stationary or vehicular power applications. Recently, nanostructured multilayers that consist of two oxygen ion conductors (doped zirconia and ceria) or one oxygen ion conductor and one insulating oxide (e.g.,YSZ/Y2O3) have been reported to exhibit higher oxygen ion conductivity by orders of magnitude compared to that of either of their constituent layer materials. Particular interest is focused on the presence of lattice strains and interfacial regions with a high concentration of planar defects, as these are supposed to be the causes for the higher oxygen ion conductivity in these multilayers. These results remain controversial and difficult to repeat, especially in heteroepitaxial growth in which a large lattice mismatch exists at the interface. Here we present the structural and electrical properties of multilayers composed of ceria-zirconia solid solutions deposited via sputtering onto single crystal Al2O3 substrates. A unique thin film deposition route has been used, allowing the fabrication of multilayer films with composition controlled at the nanometer level. The strain induced at the interface between two layers was adjusted by altering the Ce/Zr ratios. The aim of this study is to quantify the influence of strain on oxygen ion vacancy mobility and concentration.

One of the most important goals in Solid Oxide Fuel Cell (SOFC) research is to decrease the operating temperature. To realize this objective, it is imperative to identify electrolyte materials that show enhanced conductivity in the intermediate temperature range (773 K to 1073 K). Previously, we developed a Kinetic Lattice Monte Carlo (KLMC) model that takes into account repulsive interactions between vacancies to investigate oxygen diffusion in doped ceria and to compute ionic conductivity as a function of temperature and dopant concentration. One effect not included in this KLMC model was the effect of dopants on lattice parameter expansion that is experimentally observed. The effect of lattice parameter expansion for 20 % mole fraction of dopant content in Gd-doped ceria (GDC) was calculated using density functional theory (DFT+U) for an expanded 96-atom lattice, and resulted in slightly lower activation energies for oxygen vacancy migration. Inclusion of these revised energies in the KLMC model showed roughly an order of magnitude increase in ionic conductivity for GDC in the intermediate temperature range. Reasons for this remarkable increase in ionic conductivity as a function of lattice parameter expansion in GDC will be discussed. The large effect of lattice parameter expansion on ionic conductivity suggests that the addition of other dopants and co-dopants that expand the lattice further may be beneficial, and this effect may be an important guide in searching for new dopants for ceria. Overall, the ionic conductivities computed using the revised KLMC model are in reasonable agreement with experiment.

It is clear that fundamental materials research is a crucial component in the discovery and characterisation of ionic and mixed conducting materials for solid oxide fuel cells (SOFCs). In this context, advanced computational techniques are now well-established tools for probing the properties of energy materials on the atomic- and nano-scale. This presentation will highlight recent studies on oxide-ion, proton and mixed conductors [1] focusing on perovskite-type materials (such as doped BaZrO3 and La2CoO4) and novel structures containing tetrahedral units (such as Si/Ge-based apatites). Key properties investigated include ion transport mechanisms, defect-dopant nano-cluster formation and surface structures. Our studies are closely correlated with related experimental work on these materials (e.g. diffraction, conductivity, solid-state NMR). [1] C. Tealdi et al., J. Mater. Chem., 22, 8969 (2012); P. Panchmatia et al., Angewandte Chemie, 50, 9328 (2011); L. Malavasi et al., Chem. Soc. Rev. 39, 4370 (2010).

Energy generation and storage is an increasingly important area of materials development, with several competing technologies vying for commercialisation, each of which present technological challenges. One leading technology is the solid oxide fuel cell which typically is based on a ceramic oxide electrolyte with fast oxide ion conduction. Currently there are only a limited number of electrolyte materials available to developers due to the stingent conditions imposed on the materials (stable in a wide pO2 range, fast ion conduction, no reactivity with other components etc) and hence there is a need for new electrolyte materials. Electrolytes are typically based on a simple cubic structure type with oxygen vacancies introduced through substituion with aliovalent cations. In this work we report on an alternative strategy: that of introducing structurally complex materials with excess oxygen content leading to fast ion conduction. Here we report on the preparation of a substituted LaNbO4 based oxide in which additional oxygen content is accommodated through the adoption of a superstructure leading to interstital ion conducting pathways. The clearly oxygen excess materials have then been processed as dense ceramics and formed into single fuel cells with conventional anode and cathode layers, demonstrating no reactivity under operating conditions and generating a reasonable power ouput of > 100mW cm2 at 900oC with an OCV of 1V on a thick electrolyte. These are promising data that offer considerable scope for optimisation.

The apatite-type lanthanum silicate is an ionic conductor showing the oxygen migration, and is regarded as one of the candidates of the electrolyte for the mid-temperature solid oxide fuel cell. We have recently indicated theoretically that the interstitialcy diffusion is the main conduction mechanism of La9.33(SiO4)6O2, and that it is strongly perturbed by the intrinsic lanthanum vacancy. From this theory, it will be expected that Al-doped silicate, La10(Si(2/3)Al(1/3)O4)6O2, would exhibit a lower activation energy by the elimination of the lanthanum vacancy. However, experiments have shown that the activation energy is not affected much by the doping. Here we propose the model on the effects of the Al doping to the oxygen conduction on the basis of the density-functional calculations. Presented are the calculation results of the stable configuration, the formation enthalpies, and the diffusion path of the oxygen related defects. The result has shown that the interstitial oxygen ion in the Al-doped apatite resides at the site forming the pyramid structure with the AlO4 cluster. This is contrast to that in the starting material, La9.33(SiO4)6O2, the split interstitial. It is predicted by the calculated formation enthalpies that this configuration stabilizes the oxygen interstitial to the oxygen vacancy even at rather oxygen poor condition. Moreover, similar to La9.33(SiO4)6O2, the oxygen interstitial in the double negatively-charged state will exhibits a lower activation energy in the migration than the positively charged vacancy. In spite of the preference of the oxygen interstitial, due to the significant change of the stable structure and the diffusion path, the substitution by Al will not lower oxygen a migration barrier of the oxygen interstitial. This research was partly supported by Grant-in-Aid for Scientific Research (C) (No. 22560663) from Japan Society for the Promotion of Science.

Rear-earth silicate/germanate materials having an apatite-type crystal structure are known to exhibit high oxide-ion conductivities, and are expected as electrolytes for solid oxide fuel cells. However, the oxide-ion conductivity has to be further improved for the practical application, and thus the atomic-level ionic conduction mechanism is of scientific and industrial importance and should be clarified in more detail. In this study, first-principles calculations were performed to investigate the oxide-ion conduction mechanism in lanthanum silicate and germanate and discuss a physical origin of the fast oxide-ion conduction. A first-principles plane-wave based PAW method was used for the present calculations and minimum energy diffusion pathways and their potential barriers for oxide-ion conduction in different crystallographic directions were analyzed. In the case of lanthanum silicate, it was found that the energetically most stable interstitial O5 site is located close to the O4 column along the c axis. During the diffusion process, the interstitialcy mechanism with O4 is likely to occur for O5 diffusion along the c axis. In contrast, during O5 diffusion normal to the c axis, the interstitial O5 ion can favorably interact with SiO4 groups and form SiO5-like units as intermediate low-energy structure. After the SiO5 formation, an oxide ion in the SiO5 unit was readily released into the interstitial region in the crystal lattice. This result shows a novel oxide-ion diffusion mechanism that SiO4 groups can act as relay points of interstitial oxide ions to diffuse in lanthanum silicate. Such a relay mechanism was also confirmed for oxide-ion conduction in lanthanum germanate, which should be a key to explain the large oxide-ion conductivity observed experimentally.

A major goal of ongoing research in Solid Oxide Fuel Cells (SOFCs) is the reduction of the operating temperature from the high temperature range (800-1000°C) to intermediate temperatures (500-750°C). Such a reduction could solve numerous reliability problems in SOFC systems and reduce the overall cost of cell stacks. This goal can be achieved by the development of new electrolyte materials with high ionic conductivity and/or by the reduction of the thickness of the electrolyte. In this work, we explore the use of pulsed laser deposition (PLD) in the synthesis of thin-film electrolytes based on gadolinium-doped barium zirconate (BZG). BZG has recently been predicted to have high protonic conductivity on par with or perhaps even superior to that of yttrium-doped barium zirconate, which currently holds the record for the highest measured thin-film protonic conductivity. BZG deposition was carried out using ablation targets prepared from commercially available barium zirconate (BaZrO₃) and gadolinium oxide (Gd₂O₃) powders. The masses of the constituent powders were adjusted in suitable mixtures to yield targets containing 5, 10, and 15 mol. % of Gd in BaZrO₃. Targets of pure BaZrO₃ were also prepared for comparison. Targets where pressed at 2800 psi and annealed at 1200°C for 12 hours in air. By ablating these targets in a controlled environment, PLD is achieved on various substrates including silicon, platinum, and MgO. PLD was performed using a KrF excimer laser (248 nm) with energy density between 1 and 2 J/cm² and pulse repetition rate of 30 Hz. Substrates were maintained at temperatures ranging from 500°C to 700°C. Deposition took place in O₂ atmosphere at various pressures in the 40-200 mTorr range in a vacuum system with base pressure below 5.0×10^minus;7 Torr. Based on deposition rate calibration data, PLD was carried out for sufficient time to achieve film thicknesses in the 1-2 mu;m range. X-ray diffraction (XRD) measurements on as-deposited films show reflections consistent with polycrystalline BaZrO₃ in the cubic structure when films are deposited at temperatures above 650°C and O₂ pressure of 200 mTorr. Scanning electron microscopy (SEM) studies on these crystalline films reveal smooth, high-quality surfaces with average grain size of approximately 200 nm. As-deposited films synthesized at O₂ pressures in the 40-150 mTorr tended to be amorphous, as indicated by XRD measurements and by the absence of grain structure in the SEM analysis. All films crystallized upon post-deposition annealing at 850°C in O₂-rich environment. Energy dispersive X-ray measurements indicate the successful incorporation of Gd in the films in concentrations that scale with the original Gd content in the ablation targets. Targets prepared with 5, 10, and 15 mol. % of Gd yielded films with Gd concentration of approximately 0.5, 1.0, and 1.9 at. %, respectively. Results of electrochemical impedance spectroscopy measurements on obtained films will be discussed.

The typical electrode in solid oxide fuel cells is complex both compositionally and microstructurally leading to difficulties in achieving reproducible performance and isolating the key rate limiting steps controlling performance. As a result, there has been great interest, in recent years, in studying dense thin films offering simple geometries and microstructures and good compositional reproducibility. Furthermore, thin films lend themselves well to high resolution analysis by scanning probe techniques including scanning tunneling microscopy (STM) capable of interrogating the surface electronic structure of the films. In this presentation, we begin by reviewing recent progress made in correlating the oxygen exchange kinetics exhibited by Sr(Ti,Fe)O_3-δ mixed conductors with their electronic structure and the impact that segregated surface Sr species has on these kinetics. We next examine means for measuring oxygen nonstoichiometry in (Ce,Pr)O_2-δ thin films and correlating that with their cathodic performance. We also demonstrate that these films exhibit significant chemical expansion under similar operating conditions. We finish by reporting recent studies on (La,Sr)CoO_3-δ/(La,Sr)₂CoO_4+δ multilayers by STM which serve to explain the exceptionally high surface exchange rates exhibited by this materials couple.

Transition metal (TM= Cr, Mn, Fe, Co, and Ni) perovskites are current, and potentially future cost-effective catalysts for electrochemical energy conversion devices such as fuel cells, electrolytic water splitting devices, metal air batteries, etc. The materials have low cost as well as good ability to catalyze the oxygen reduction reaction (ORR) and oxygen evolution reactions (OER). While an ab initio Density Functional Theory (DFT) based descriptor approach has been very valuable in understanding and optimizing catalytic activity in ORR/OER metal catalysts, prediction of ORR/OER activity for TM perovskites using first-principles based descriptors is less well developed. Such descriptors are potentially more challenging to establish due to intrinsic errors of DFT for strongly correlated electronic systems and the significant complexity of TM perovskite surfaces. For example, while basic DFT methods predict clear trends in d-electron filling and oxygen binding energetics, it is not established that these trends correlate as expected with relevant experimental data on oxygen surface coverage, thermogravimetry, and catalytic activity. In this talk, a DFT based database, including both DFT-GGA and GGA+U approaches, are used to assess various electronic structure and binding energy based descriptors that have been proposed to effectively correlate with experimentally measured ORR/OER activities of TM perovskites. We assess these descriptors against measured low temperature ORR/OER and high temperature ORR activity data, and highlight issues in their application.

The slow rate of oxygen reduction reaction (ORR) at the cathode is one of the main barriers for implementation of high-performance solid oxide fuel cells (SOFCs) at intermediate temperatures (500-700 °C). The structural, chemical and electron transfer properties of the cathode surface are of importance to improve the ORR reactivity. However, the dynamic nature of surface structure and chemistry, driven by the harsh conditions of high temperatures and oxygen partial pressure in SOFCs, has resulted in the difficulty to probe and fundamentally understand the origin of the activation and deactivation of ORR kinetics on the surfaces. Cation segregation and phase separation on the surface of perovskite oxides has been commonly observed, but the exact mechanism for the segregation and its impact on the surface activity remains controversial. In this work, we systematically assessed the mechanisms of the cation segregation and phase separation on perovskite thin films by consideration of the elastic and electrostatic interactions. The effects of the elastic energy on the cation segregation were investigated with two control parameters, A-site stoichiometry and the lattice mismatch between the dopant (Ca, Sr, Ba) and the host cation (La) in doped lanthanum manganite as model system. The effects of the electrostatic energy were investigated by controlling the oxygen chemical potential during annealing, which influences the distribution of charged oxygen defects in the films. The surface cation stoichiometry, bonding states, and the surface structure were investigated at the film surface. The higher chemical stability upon annealing was observed on A-site deficient Sr-doped lanthanum manganite (LSM), and on the lanthanum manganite films with smaller size mismatch between the dopant and the host cation. The relatively higher stability is considered to be due to the excess space available to dopant in the A-site sub-lattice in the bulk. The oxygen vacancy distribution near the film surface affected the cation segregation because of the electrostatic interactions between the dopants and the oxide ion vacancies. Observed difference in extent of surface segregation and phase separation with the control of the elastic and electrostatic interactions suggests the different electronic and electrochemical properties on the surface. Density functional theory (DFT) calculations are performed to develop a physical model for the mechanisms of the cation segregation on perovskite surfaces. Electronic and electrochemical investigations of the film surface will be discussed to elucidate the impact of cation segregation and phase separation on the ORR kinetics at the cathode surface.

La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskites are promising mixed-conducting cathodes for solid oxide fuel cells (SOFCs) due to their catalytic activity. Previous studies have shown that the oxygen reduction process at the cathode surface is a rate-limiting step in the performance of SOFCs [1]. Using in-situ synchrotron x-ray diffraction, the oxygen exchange reaction mechanism under applied electrochemical potential was studied using pseudo half-cells. The cells consisted of an epitaxial LSCF thin film (60 nm) grown on an yttria stabilized zirconia (YSZ) substrate with a gadolinium doped ceria (GDC) interlayer. The dependence of oxygen vacancy concentration on electrochemical potential was spatially resolved by measuring the c-lattice parameter with a 50 mu;m x-ray beam as a function of applied cathodic and anodic overpotential at different oxygen partial pressures (1.5, 15 and 150 Torr) in a temperature range from 350°C-600°C. Typically, the out-of-plane c-lattice parameter of LSCF increases with increasing oxygen vacancy concentration and temperature. We measured both the absolute change as well as the time dependence of the change in lattice parameters upon application of an electric potential with a time resolution of a tenth of a second. The transition from an initial state to steady state upon potential application is described in terms of a characteristic time constant tau;. Activation energies were extracted from the slopes of lines fitted to Arrhenius plots for both the lattice parameter and electrical current data. The time constants, surface exchange rates and activation energies of the current and of lattice parameter shifts are compared to determine average versus local (from the x-ray measurements) behavior. Time dependent correlations between oxygen vacancy formations, oxygen transfer rates and oxygen ion diffusion processes are revealed. 1. S. B. Adler, Chemical Reviews, 104, 4791 (2004).

The mixed conducting perovskites investigated as SOFC cathode materials cover a variety of compositions ranging from (La,Sr)MnO3-d (LSM) to (La,Sr)(Co,Fe)O3-d (LSCF) and (Ba,Sr)(Co,Fe)O3-d (BSCF). Measurements on pore-free samples (18O exchange for dense ceramics, e.g. [1]; impedance spectroscopy for dense PLD films on YSZ substrates, e.g. [2,3]) allows us to obtain effective rate constants k for the oxygen surface exchange reaction (unaffected by morphology changes as it would be the case for porous films). For the mentioned perovskites, the k values span a range of 5 orders of magnitude at 750°C. The analysis of possible correlations of k with (bulk) materials properties emphasizes the importance of ionic conductivity - i.e. high oxygen vacancy concentration as well as vacancy mobility - as a key factor for the surface oxygen exchange rate [4]. This interpretation is corroborated by ab initio calculations for (La,Sr)MnO3-d [5], which indicate that the approach of an oxygen vacancy to oxygen intermediates adsorbed on the surface is the rate-determining step for a number of perovskites. Measurements of surface rate constant as well as oxygen diffusivity for BSCF perovskite films indicate that for this family of materials k has a linear correlation with the ionic conductivity [3]. Having identified the importance of a high oxygen vacancy concentration as well as mobility for fast oxygen exchange, materials such as (Bi,Sr)(Co,Fe)O3-d are currently explored [6]. [1] R.A. De Souza, J.A. Kilner, Solid State Ionics 126 (1999) 153 [2] F. S. Baumann, J. Fleig, G. Cristiani, B. Stuhlhofer, H.-U. Habermeier, J. Maier, J. Electrochem. Soc. 154 (2007) B931 [3] L. Wang, R. Merkle, J. Maier, J. Electrochem. Soc. 157 (2010) B1802 [4] L. Wang, R. Merkle, Y.A. Mastrikov, E.A. Kotomin, J. Maier, J. Mat. Res. (2012) doi: 10.1557/jmr.2012.186 [5] Y.A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, J. Maier, J. Phys. Chem. C 114 (2010) 3017 [6] A. Wedig, R. Merkle, B. Stuhlhofer, H.-U. Habermeier, J. Maier, E. Heifets, Phys. Chem. Chem. Phys. 13 (2011) 16530

Small scale cells (including button cells, and one to five cells printed on an inert substrate) with segmented-in-series cell design were fabricated using a LSM composite as cathode, a Ni cermet as anode, and ScSZ as the electrolyte. These cells were tested under simulated fuel cell system operating conditions with moisture on the cathode side and reformate fuel (through the reversed water shift reaction of H2 and CO2) on the anode side. After 2000 to 8000 hours of testing, the LSM composite cathodes, especially the cathode/electrolyte interface, were characterized using transmission electron microscope (TEM) and electron energy-loss spectroscopy (EELS) for second phase formation, microstructural evolution, phase stability, and the presence of impurities. The detailed analysis revealed changes in the LSM and ionic phase during long-term operation under fuel cell system conditions. AC impedance was measured before and after durability tests of small-scale cells to understand electrochemical process changes associated with cathode chemistry and microstructural changes, and to deconvolute contributions to area specific resistance (ASR) from different components. The effect of cathode stability during fuel cell operation on long-term degradation will be discussed. A mitigation approach to improve LSM cathode stability during fuel cell operation was investigated. Small-scale cells with a new cathode formulations were tested under the same conditions. Cathode long-term durability and lower degradation rate were demonstrated. Post-test analysis confirmed that the modified LSM composite shows better phase stability after long term fuel cell operation.

Complex perovskite materials with the ABO3 lattice structure promise to improve efficiency of energy conversion devices due to a high concentration of oxygen vacancies and enhanced oxygen reactions. (Bax/LaxSr1minus;x)(Co1minus;yFeyO3minus;δ) (BSCF/LCSF) perovskites are considered among perspective materials for cathodes in SOFC and oxygen permeation membranes because a good oxygen exchange performance, mixed ionic and electronic conductivity, and low oxygen vacancy diffusion activation barrier. However, understanding of the interplay between the chemical composition, structural order/disorder, and crystalline stability in most perovskites is extraordinarily complex and essentially unexplored. Our first principles DFT and thermodynamics calculations of an ideal BSCF crystal, the crystal containing defects, a set of solid solutions, and phase diagrams suggest that while the hexagonal phase of BSCF is favored over the cubic phase for small oxygen deficiency, at low temperatures, the material decomposes into a mixture of cubic and hexagonal phases, which are likely to form grain boundaries and surface interfaces. This instability is considered a disadvantage for fast oxygen transport chemistry and impedes the applicability of BSCF-based SOFC. We discuss possible mechanisms of defect-induced (in)stability in the context of available experiments, explain the observed SOFC performance reduction, and provide insights on enhancing energy conversion in devices. This work is supported partly by GIF (project 1-1025-5-10/2009) and NSF.

Mixed ionic-electronic conducting La0.6Sr0.4CoO3-δ (LSC) is one of the state-of-the-art cathode materials for thin film and miniaturized SOFC [1] operated at intermediate temperatures. The electrochemical kinetics is believed to be limited by oxygen incorporation at the perovskite/air interface. Hence, for porous electrodes, increasing the number of sites for oxygen exchange either by making the electrode thicker or by producing nanosized LSC grains can improve the cathode performance. In this work, nanoporous LSC cathodes are deposited by spray pyrolysis onto gadolinium-doped ceria (GDC) electrolyte substrates. The as-deposited films are dense an amorphous. Different porous microstructures are produced by changing the heat-treatment, 600°C and 800°C for 4 hours in air in this study. XRD and DSC analyses reveal that the layers are crystalline after heat treatment. The electrode microstructure is analyzed using focus-ion beam nanotomography, continuous phase size distribution (c-PSD) and mercury intrusion porosimetry PSD (MIP-PSD)[2]. The latter methods, based on the analysis of 2D-micrographs and 3D-reconstructions allow for the determination of the phase sizes, volume fractions, surface areas and percolation levels of the various phases present in the annealed films. The area specific resistance (ASR) of symmetrical LSC/GDC/LSC cell is measured in air between 400 and 600°C by impedance spectroscopy. The 800°C annealed LSC layers, with an average grain size (r₅₀) of ca. 30 nm show higher electrochemical performance than those annealed at 600°C (r₅₀ = 12 nm), ca. 0.12 Omega;cm² and 0.35 Omega;cm², respectively. This is attributed to an improved percolation level of pores (ca. 99% vs. 60%) making more LSC surface available for oxygen exchange. The principle of c-PSD and MIP-PSD will be described. More results on the relationships between microstructural features (such as percolation factor, constrictivity, and tortuosity) and oxygen reduction activity will be reported. [1] A. Evans, S. Karalicacute;, J. Martynczuk, M. Prestat, R. Tölke, Z. Yáng, and L.J. Gauckler, ECS Transactions, 45 (2012) 333. [2] B. Muench and L. Holzer, Journal of the American Ceramic Society, 91 (2008) 4059.

Lanthanum strontium cobaltites LaxSr1-xCoO3-δ (LSCO) are promising materials for solid oxide fuel cells, electrochemical sensors, and other energy storage and conversion technologies, with properties exhibiting a complex interplay of magnetic, ionic, and electronic phenomena. Spatial distribution and mobility of oxygen vacancies (influenced by vacancy ordering) are key to determining their functionality. Here, we determine local vacancy concentration using lattice parameter mapping by aberration-corrected Scanning Transmission Electron Microscopy (STEM), extending the concept of chemical expansivity to sub-unit cell scale. We use the collected data to develop a theoretical description of the vacancy ordering behavior. For the study we examine several compositions of LSCO grown by Pulsed Laser Deposition on Yttria-stabilized Zirconia (YSZ), La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) and NdGaO3 (NGO) substrates. The studied films show oxygen vacancy ordering, where alternating cobalt oxide planes are oxygen-depleted or stoichiometric. Vacancy ordering is not uniform across the samples and has multiple topological defects. Intriguingly, types and behaviour of these defects show similarities to classical ferroic systems including ferroelectrics and ferroelastics, such as “domain walls” separating vacancy ordered regions with different ordering direction, anti-phase boundaries with oxygen sublattice displaced by a fraction of the ordered unit cell (cation sublattice stays intact), and relaxor-like microstructures with small domains of ordered phase incorporated into disordered matrix. We can therefore treat vacancy ordering as a ferroic transition and develop a Ginzburg-Landau-Devonshire (GLD) description for this phenomenon. Using atomically resolved data on defect configurations in LSCO, we demonstrate that the gradient and interfacial terms for GLD description can be quantitatively determined. These results suggest that a predictive theory can be developed for vacancy ordering transitions, facilitating optimization of material performance. * This research is sponsored by the Materials Sciences and Engineering Division (YMK, SJP, SVK, DNL, AYB), Office of BES of the U.S. DOE, and by appointment (YMK) to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE. Research partially conducted at Center for Nanophase Materials Sciences (MDB), with is supported at Oak Ridge National Laboratory by Office of Basic Energy Sciences of the US DOE.

In the past years, the layered rare earth nickelate Ruddlesden-Popper (R-P) type phases, Ln_n+1Ni_nO_3n+1, have attracted attention mostly as potential alternate cathode materials for solid-oxide fuel cells (SOFC) as well as for oxygen separation membranes or nature gas separation. The lanthanum nickelate R-P structure is generated by stacking up n perovskite-type layers (LaNiO₃) separated by rock salt layers (LaO) along the c-axis. The n = 1 member of this series, La₂NiO_4+x, adopts the K₂NiF₄ structure and consists of alternating perovskite and rock salt layers, whereas the n = infin; member corresponds to the three dimensional perovskite LaNiO₃. La₂NiO_4+x can accommodate an extraordinarily large oxygen excess (0 < x le; 0.3), see e.g. Refs. [1,2]. In contrast the R-P compounds with n = 2 and 3 are mostly oxygen deficient. Herein we present a Mössbauer investigation of the La_n+1Ni_n-y⁵⁷Fe_yO_3n+1 series for n = 1, 2, and 3 with y = 0.02, 0.05, 0.1, and 0.9 in the temperature range from room temperature up to 1000°C and at oxygen activities, a_O2, ranging between loga_O2 = 0 and loga_O2 = minus;4. The temperature dependent measurements provide information on the evolution of local structure around the nuclear probes. Especially in the case of the oxygen excess material La₂Ni_1-yFe_yO_4+x, the spectra show a clear dependence on oxygen activity and it is concluded that quadrupolar interactions increase with decreasing stoichiometry parameter x. Possible origins of these uncommon results and their connection with the oxygen disorder of the material are discussed. References [1] S. J. Skinner, Solid State Sci. 5 (2003) 419 [2] A. Aguadero, J.A. Alonso, M.J. Martinez-Lope, M.T. Fernandez-Diaz, M.J. Escudero, L. Daza, J. Mater. Chem 16 (2006) 3402

In the last years, a rapidly growing attention is being directed towards the investigation of the ionic conducting properties of oxide thin films and hetero-structures, with the main aim of the development of micro-solid oxide fuel cells. Experimental evidence has been reported showing that interfacial phenomena at hetero-phase interfaces, including film-substrate interfaces, give rise to faster ion conduction pathways than the bulk or homo-phase interfaces. This has been ascribed either to the building up of space charge regions at the interfaces or to interfacial strains derived from the lattice mismatch between the two adjacent materials. However, controversial results have been reported: in some cases the nature of the ionic carriers has been questioned, or the presence of strains did not enhance the ionic conductivity. These findings show the need of a deeper understanding of the interface transport properties to unravel the interfacial conduction mechanisms. This talk will present the recent efforts performed in our lab towards the fabrication by pulsed laser deposition (PLD) of ionic (both proton or oxygen-ion) conducting oxide thin films and superlattices, and their electrochemical characterization to clarify the causes for enhanced ionic conductivity at oxide hetero-interfaces. One example will be the PLD fabrication of superlattices based on ceria and zirconia, doped and undoped, on (001)-oriented MgO single-crystal wafers. To allow an epitaxial growth of the hetero-structures, a thin buffer layer of SrTiO₃ (STO) deposited by PLD was used. The growth of each layer was monitored in-situ by reflection high energy electron diffraction (RHEED). Structural analysis showed the high quality of the deposited interfaces: very well ordered super-lattices were obtained even for samples with a thickness of each layer as small as 5 unit cells. A computational density functional theory study of the structural and electronic properties of the (100) and (111) ZrO₂-CeO₂ interfaces was performed to support experimental findings.

The growing demand for miniaturized systems in energy conversion and storage has prompted extensive research aimed at fabricating solid-state ionic devices in thin-film form such as micro-solid oxide fuel cells (mu;-SOFCs). These devices can readily be miniaturized using thin film deposition technologies such as Pulsed Laser Deposition (PLD) [1]. Oxide materials with fluorite structure, such as gadolinium-doped-ceria (20%) (CGO) or yttria stabilized zirconia (ZrO2:8 mol% Y2O3) are particularly promising as electrolytes for mu;-SOFCs [1-6]. High quality, dense epitaxial thin film of fluorite is crucial to obtain physical stability in reducing/oxidizing atmospheres in the temperature range 400-800C [3]. In this work we have studied the growth of thin epitaxial fluorite electrolyte films as candidate materials for low temperature solid oxide fuel cell (LT-SOFC) applications. The grown films were characterized using various ex-situ techniques to understand their structure, crystalline quality, chemical composition and the electrochemical properties. In order to improve the quality of the films, a thin buffer layer (a few nm thick) is introduced to facilitate the epitaxial growth of the fluorite film [4-6]. [1] U. P. Muecke, D. Beckel, A. Bernard, A. Bieberle-Hütter, S. Graf, A. Infortuna, P. Müller, J. L. M. Rupp, J. Schneider, L. J. Gauckler, Adv. Funct. Mater., 2008, 18, 3158. [2] V. Esposito, E. Traversa, J. Am. Ceram. Soc., 2008, 91, 1037. [3] S. Sanna , V. Esposito , D. Pergolesi , A. Orsini , A. Tebano , S. Licoccia , G. Balestrino , E. Traversa , Adv. Funct. Mater., 2009 , 19, 1713. [4] S. Sanna, V. Esposito, A. Tebano, S. Licoccia, E. Traversa, and G. Balestrino, Small, 2010, 6, No. 17, 1863. [5] K. Mohan Kant, V. Esposito, and N. Pryds, App. Phys. Lett., 2010, 97, 143110. [6] E. Wachsman, K.T. Lee, Science, 2011, 334, 935.

A topic of great interest in recent years is the study of the ionic conduction in oxide thin film heterostructures. Evidence has been reported of significant enhancement in electrical conduction in these materials (1-8 orders of magnitude, eg. refs [1-3]) while the cause of this has remained ambiguous. In order to investigate the true nature of the conduction mechanism in systems such as these, we have developed a novel experimental technique for directly studying the oxygen diffusion behaviour. An exchange anneal in an atmosphere of high oxygen-18 concentration is first performed on a sample with a surface oxygen-blocking layer with a single region exposed. Time of flight SIMS is then performed in an area containing both exposed and covered heterostructure material. The 3-dimensional nature of the analyses performed by ToF-SIMS allows the extraction of individual diffusion coefficients for individual layers down to approximately 10 nm thickness. Systems consisting of repeated layer units of alternating doped and undoped ceria (reported in [4]), yttrium-stabilised zirconia (YSZ) and undoped ceria, and samarium-doped ceria and praseodymium nickel copper gallate (PNCG) all grown on MgO by PLD by several collaborators were studied by this method. The diffusion coefficients obtained can be used to calculate ionic contributions to the total electrical conductivity however these are never seen to be greater than that expected for the appropriate single crystal lattice component. This work has been made possible through collaborations with ORNL, USA, NIMS, Japan and Kyushu University, Japan. [1] J. Garcia-Barriocanal et al., “Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures,” Science, vol. 321, no. 5889, p. 676, Aug. 2008. [2] M. Sillassen et al., “Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized Zirconia,” Advanced Functional Materials, vol. 20, no. 13, pp. 2071-2076, 2010. [3] I. Kosacki et al., “Nanoscale effects on the ionic conductivity in highly textured YSZ thin films,” Solid State Ionics, vol. 176, no. 13-14, pp. 1319-1326, 2005. [4] J. M. Perkins et al., “Anomalous Oxidation States in Multilayers for Fuel Cell Applications,” Advanced Functional Materials, vol. 20, no. 16, pp. 2664-2674, Aug. 2010.

The recently reported fast oxygen reduction (OR) kinetics at the interface of (La,Sr)CoO3-δ (LSC113) and (La,Sr)2CoO4+δ (LSC214) phases opened up new questions for the potential role of dissimilar interfaces in advanced cathodes for solid oxide fuel cells (SOFCs). Obtaining a microscopic level understanding for such behavior is important for designing novel interfaces for very high-performance SOFC cathodes. In this work, in order to probe the governing mechanism of such high OR kinetics, we implemented a novel combination of in-situ scanning tunneling spectroscopy and focused ion beam milling and performed density functional theory calculations. In particular, we investigated the OR activity on the basis of local electronic structure and quantitatively assessed the oxygen incorporation mechanism near the interfaces of model (La0.8Sr0.2)CoO3-δ/(La0.5Sr0.5)2CoO4+δ superlattices. The strongly anisotropic oxygen adsorption and dissociation on LSC214 and the lattice strain in the vicinity of the LSC113/214 interface are computationally found to contribute to the OR activity enhancement significantly, providing 4×10^2 times faster oxygen incorporation kinetics at 500 °C. Furthermore, in situ tunneling spectroscopy measurements demonstrated that, starting at 200-300 °C, the LSC214 layers are electronically activated through interface coupling with LSC113. Such electronic activation is expected to facilitate charge transfer from the surface to the oxygen adsorbates in the reduction process, providing about 10^3 times faster OR kinetics. These results show that the electronic activation of LSC214 in concert with its anisotropically fast oxygen incorporation kinetics are likely the key mechanisms governing the very fast oxygen reduction kinetics observed near the LSC113/214 interfaces. Insights gained from our results provide a new understanding of oxide hetero-interfaces at high temperatures and point towards electronically coupled oxide structures as novel cathodes.

The interface between the perovskite La_0.8Sr_0.2CoO_3-δ (LSC-113) and the K₂NiF₄-type (La_0.5Sr_0.5)₂CoO_4-δ (LSC-214) heterostructure was recently reported to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest for developing better functioning cathodes for electrochemical devices such as solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ~0.9 eV energy gain for exchanging a Sr from LSC-113(25%Sr) with a La from LSC-214(50%Sr). We report an even larger (~1.3 eV) energy gain for exchanging a Sr from LSC-113(40%Sr) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that, in a typical experimental temperature range of 500-600 °C, the Sr concentrations in the LSC-214 phase in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr) may be enriched to about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase such Sr enrichment is expected to enhance the oxygen vacancy concentration by 2-2.5 orders of magnitude under typical experimental conditions. Such an increased vacancy concentration in LSC-214 near the interface can potentially explain most of the enhanced oxygen kinetics observed up until now in these heterostructures. The results of this work have been published in M. J. Gadre, Y.-L. Lee, D. Morgan, Physical Chemistry Chemical Physics, 14, 2606-2616 (2012).

The kinetics of electrochemical electrode reactions strongly affects the performance of electrochemical cells such as solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs). The corresponding polarization resistances may depend on many parameters including electronic and ionic conductivity of the electrode, grain size and porosity, surface chemistry of electrode and electrolyte, etc. A deconvolution into the effects of each individual parameter is highly sophisticated, particularly for porous electrodes. Therefore thin film electrodes became very popular in mechanistic studies. Still, it is far from trivial to relate a measured overall polarization resistance of, for example, a mixed conducting thin film electrode to the atomistic processes, i.e. to the electrochemical reaction at the surface, ion transport through the electrode, ion transfer across the electrode/electrolyte interface, or electron conduction. The latter may be particularly relevant in H2/H2O atmosphere since most of the commonly used acceptor-doped mixed conductors exhibit low conductivity under reducing conditions. Accordingly, there is urgent need for experiments and tools revealing additional information on the mechanistic origin of polarization resistances. In this contribution, several approaches are discussed that help identifying and separating the processes determining the polarization resistance of electrode reactions in SOFC/SOEC related materials. Even though impedance spectroscopic studies may allow separation of different resistive contributions, their interpretation is often ambiguous. Combination with oxygen tracer experiments as a second tool for probing the electrode kinetics yields much less ambiguous results. This is exemplified for (La,Sr)CoO3-x electrodes on YSZ. It is also shown that film preparation strongly affects the importance of surface exchange and ion diffusion and that a clear correlation of different properties with local structure (analyzed by TEM) and cation mobility exists. As a second tool we introduce a novel method of impedance studies on mixed conducting electrodes: Two interdigitally arranged Pt electrodes were embedded into a Sr(Ti,Fe)O3 thin film microelectrode and two different types of impedance measurements were performed in H2/H2O atmosphere: one between the two Pt electrodes (in-electrode), and one in a conventional arrangement versus a counter electrode on the solid electrolyte. By fitting both spectra simultaneously it is possible to determine up to six materials parameters including ionic and electronic film conductivity and surface reaction rate. As a third approach leading to additional information we employed switching between two electrode reaction for a given material, i.e. instead of “only” varying the oxygen partial pressure we analyzed (La,Sr)FeO3 microelectrodes in H2/H2O as well as in oxygen containing atmosphere. This revealed further details on the parameters affecting the polarization resistance.

A solid oxide fuel cell (SOFC) is a type of fuel cell which is comprised of oxide-based electrolyte and electrodes. SOFCs have been actively studied as promising next-generation energy conversion devices because these have high energy efficiency and can utilize any hydrocarbon fuel currently available, other than pure hydrogen. Traditional SOFCs operate at high temperatures (ge; 800 oC) to secure oxygen ion conductivity and catalytic activity of oxide components. This high temperature operation induces problems, such as chemical reactions between the cell components and microstructural degradation. When minimizing the problems associated with high temperature operation, one of the key issues in the SOFC research is the reduction of the operating temperature without compromising performance. Low temperature operation is of significant technical importance to both conventional large-capacity and portable miniaturized SOFCs. It ensures reliability and cost-effectiveness, providing a pathway to commercialization of the former system, and facilitates thermal management, resulting in reduction of size of the latter system. Therefore, the attempt of improving the low temperature performance of SOFCs by introducing thin electrolytes and nano-structure electrodes surged during the past decade. As a result, thin-film and nano-technologies, which have been relatively unfamiliar techniques to the SOFC in the past, are actively studied nowadays. However, as a consequence of overlooking the fatal issues related to the usage of the thin films and nano-structures at elevated operating temperatures, tremendous research efforts turned into vain in attaining targeted performances and thermomechanical reliabilities for thin-film and nano-structure based SOFCs (TF-SOFC). Among many issues, the thermomechanical vulnerability of the ultra-thin electrolyte membrane and the fast degradation of nano-porous metal electrodes, even in the low operation temperature regime of SOFCs (le; 500 °C), have been the main limitations. During the last several years, we have reported our achievements of both high-performance at low operating temperatures and improved thermomechanical reliability in the TF-SOFC. In our approach, the structural stability of thin-film electrolytes was secured by realizing nano-porous supporting structure. In terms of the nano-structure electrode, fabricating nano-composite electrodes and employing structurally supporting templates significantly mitigated the degradation in long-term operation. In the current presentation, the overview of the KIST TF-SOFC research and recent advances will be introduced. This presentation will provide insights into how to break the limitation of the usage of thin-film and nano-structure materials for high-temperature operating devices including SOFCs, leading to the critical performance possibly achievable.

Cathode materials have been receiving a great deal of attention because oxygen incorporation at the cathode is often considered as the key factor limiting development of intermediate temperature SOFCs (IT-SOFCs). In order to identify the important factors governing oxygen reduction kinetics at the cathode, our group has introduced the Sr(Ti,Fe)O3-δ (STF) system as a model mixed conducting cathode material. The STF cathode was observed to exhibit typical mixed ionic-electronic conducting (MIEC) behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple phase boundary. The surface oxygen exchange reaction was confirmed to be the rate determining step (RDS), and values for the surface exchange coefficient, k, were found to be comparable in magnitude to those exhibited by other popular MIEC oxides such as (La,Sr)(Co,Fe)O3. Interestingly, the surface oxygen exchange rate of STF was found to be only weakly correlated with the transport properties of the majority electronic and ionic carriers (σ_el and σ_ion), in contradiction to common expectations. Instead, charge transfer from the electrode surface to oxygen molecules, mediated by the availability of minority electrons (STF is a p-type conductor), was found to control the overall reaction kinetics at high pO2. In this work, we extend our studies to further examine this correlation between k and minority charge carrier density (n for STF) by controlling the band gap energy (Eg) and the reduction enthalpy (ΔHred). We do so by introducing Ba on the A site of STF i.e., (Ba,Sr)(Ti,Fe)O3-δ (BSTF). We have shown in preliminary work that Ba2+ substitution for smaller Sr2+ leads to an increase in lattice constant of BSTF, with a corresponding decrease in reduction enthalpy, band gap, and activation energy for oxygen surface exchange, presumably due to weaker bonds. To further understand this behavior, we have extended our studies of oxygen nonstoichiometry of bulk BSTF by thermogravimetric analysis (TGA). In addition, BSTF thin film cathodes, deposited onto single crystal yttria stabilized zirconia by pulsed laser deposition, with well defined area and thickness, are examined by electrochemical impedance spectroscopy as a function of electrode geometry, temperature and pO2. Special care is taken to understand the role of surface reaction products (e.g. carbonates) and surface segregation of the alkaline earth elements. Preliminary defect, transport and oxygen exchange models are presented.

To improve the efficiency of solid oxide fuel cells, a more fundamental understanding of the oxygen reduction reaction at the cathode is needed. The surface exchange coefficient (k_chem) for oxygen incorporation into a cathode can be measured by changes in the oxygen vacancy concentration in response to the electrochemical potential. Since k_chem could vary due to the surface crystallographic orientation or grain boundaries, it is advantageous to control the cathode microstructure by using thin films. Our approach is to use in situ X-ray diffraction on pulsed laser deposited thin films to indirectly measure the oxygen vacancy concentration. The advantage of this method is that it can be used to study the effects of applied potential on the thin films, which is not possible for electrical conductivity measurements. Our samples were La_0.6Sr_0.4Fe_1-xCo_xO₃ (x=0,0.2,0.95) on yttria stabilized zirconia (YSZ) single crystal substrates, running in a half cell configuration between 680-890K in air. Although the 3 different types of PLD films had different degrees of epitaxy with the YSZ(111) substrate, we found that chemical expansion occurred under cathodic potentials in all of the films. We also monitored for Sr segregation during our experiment and found that it starts to occur at 890K. Changes in the chemical expansion in response to the applied potential as the Sr segregation develops will be discussed.

Ni / (Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 (Ni / SCSZ) cermet is one of anode materials with good catalytic activity towards hydrogen oxidation, superior ionic conductivity and low carbon deposition for use in Solid Oxide Fuel Cell (SOFC). The mechanical properties of anode materials play an important role in the reliability and durability of solid oxide fuel cells operating at high temperatures in a reducing environment. Mechanical properties of porous Ni / SCSZ cermet bars reduced from 65wt%NiO-35wt%SCSZ ceramics were tested and characterized. Young&’s modulus as well as strength and fracture toughness of non-reduced and reduced anodes has been measured, both at room and at high temperatures. High temperature experiments were performed in the reducing environment of forming gas. It is shown that while at 700 °C and 800 °C, the anode specimens exhibited purely brittle deformation, a brittle-to-ductile transition occurred for heating above 800 °C and the anode deformed plastically at 900 °C. Fractographies of the anode specimens were performed to identify the fracture modes of anodes tested at different temperatures.

Ceria co-doped with samaria and neodymia has been proposed to be a better solid electrolyte than singly doped ceria. In the present work, structural properties and energetics of samaria-doped ceria (Ce1-xSmxO2-x/2), neodymia-doped ceria (Ce1-xNdxO2-x/2 and samaria-neodymia co-doped ceria (Ce1-xSmx/2Ndx/2O2-x/2 ( all with 0 < x < 0.3) have been studied. The enthalpies of formation at 25 °C relative to constituent oxides, enthalpies of mixing, and enthalpies of oxygen vacancy-dopant association were determined by high temperature oxide melt drop solution calorimetry in molten sodium molybdate at 702 °C. The energetics of the solid solutions were analyzed considering cation size mismatch and defect association. At concentrations below x = 0.05, endothermic destabilization is attributed to regular solution behavior due to size mismatch and limited defect association. There is considerable stabilization at concentrations x > 0.05 for singly-doped ceria systems. This behavior is explained by dominant dopant cation-oxygen vacancy defect associates. However, there is less destabilization and shift towards higher dopant concentration in the enthalpy of formation maximum in co-doped Ce1-xSmx/2Ndx/2O2-x/2 compared to singly-doped ceria systems. Enthalpies of defect associates in co-doped Ce1-xSmx/2Ndx/2O2-x/2 were found to be less exothermic than those of singly-doped materials.

A main factor limiting the conversion efficiency of solid oxide fuel cells (SOFCs) is the oxygen reduction reaction (ORR) at the cathode. Therefore, many studies have been performed for enhancing ORR activity.(1-3) As the ABO3 perovskite oxide compositions show their intrinsic ORR limitations, there is a need to search for alternative types of mixed ionic and electronic conductors to enhance SOFC efficiencies. Recently, heterostructured interfaces (such as ABO3/A2BO4)(4-6) of oxides have shown remarkably high transport or oxygen surface exchange properties and dramatically enhanced ORR activity. However, the mechanism of enhanced ORR activity is not well understood. In this study, we examine the ORR activity on interfaces between the same perovskite phase with different B-site cations (Mn and Co). La0.8Sr0.2MnO3-δ (LSM113) is decorated on epitaxial La0.8Sr0.2CoO3-δ (LSC113) thin films prepared by pulsed laser deposition (PLD) with three different amounts of coverage (100, 500, and 1500 pulses, using a linear extrapolation is equivalent to ~ 0.5 nm, ~ 3 nm, ~ 9 nm). Crystallinity, and crystallographic relationships of the films were analyzed using 4-circle X-ray diffraction. The ORR activity of epitaxial LSC113 films decorated with LSM113 was examined using electrochemical impedance spectroscopy (EIS) measurements conducted on patterned micro-electrodes. X-ray photoelectron spectroscopy (XPS) was performed to study how the interface chemistry changes as a function of the amount of LSM113 surface coverage. Interestingly, LSM113 fully decorated LSC113 film shows comparable surface exchange rate (kq) and activation energy (~1.83eV) with bare LSM113 thin films, which is an order of magnitude lower than that of bare LSC113 thin film. In contrast, LSC113 film partially decorated with LSM113 show an order of magnitude higher kq compared to a bare LSC113 thin film and comparable activation energy (~0.85eV) for kq with A2BO4 (La2NiO4+δ) decorated LSC113 film. The mechanism of ORR activity on LSM113/LSC113 interfaces of oxides will be discussed. References 1. E. J. Crumlin, E. Mutoro, S. J. Ahn, G. J. la O, D. N. Leonard, A. Borisevich, M. D. Biegalski, H. M. Christen and Y. Shao-Horn, Journal of Physical Chemistry Letters, 1, 3149 (2010). 2. G. J. la O, S. J. Ahn, E. Crumlin, Y. Orikasa, M. D. Biegalski, H. M. Christen and Y. Shao-Horn, Angew. Chem.-Int. Edit., 49, 5344. 3. E. Mutoro, E. J. Crumlin, M. D. Biegalski, H. M. Christen and Y. Shao-Horn, Energy Environ. Sci., 4, 3689. 4. M. Sase, F. Hermes, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai and H. Yokokawa, Journal of the Electrochemical Society, 155, B793 (2008). 5. M. Sase, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, K. Yamaji, T. Horita and H. Yokokawa, Solid State Ionics, 178, 1843 (2008). 6. K. Yashiro, T. Nakamura, M. Sase, F. Hermes, K. Sato, T. Kawada and J. Mizusaki, Electrochemical and Solid State Letters, 12, B135 (2009).

The development of electromechanically active materials is of considerable practical and scientific importance. The large majority of miniaturized motors in use today are based on piezoelectrics or electrostrictors and despite decades of intense research, the highest piezoelectric coefficients achieved have not increased significantly and currently do not exceed 60 C/m2. The performance of the best electrostrictive materials (<1 µF/m) is either similar to or inferior to that of piezoelectrics. In materials with a large concentration of vacancies, such as ionic conductors with low electron conductivity, application of an electric field (tens of kV/cm) at low temperatures does not generate a flux of vacancies. Nevertheless, such a field may in fact be sufficiently strong to produce local rearrangement of atoms in the vicinity of the point defects. Proceeding on this premise, we have prepared cantilevers consisting of a [metal\ 500 nm thick Gd-doped ceria film\metal] multilayer structure on a 40×8×0.15 mm glass substrate. An electric field up to 80kV/cm was applied perpendicular to the plane of the cantilever to films both with and without in-plane compressive strain. All films show electrostriction-type behavior (stress proportional to the square of the electric field). The stress generated in strain-free films reaches 60 MPa, which is higher than the saturation stress of commercial electrostriction materials. Films with compressive strain (>0.1%) produce stress of asymp;0.5 GPa. This corresponds to an “equivalent piezoelectric constant” of >0.4 GPa/80 kV/cm, i.e. >50 C/m² and comparable with the best piezoelectric materials currently in use. Application of an electric field perpendicular to the film plane, in either direction, for an extended period of time (a few hours) significantly increases the magnitude of the electromechanical response. In situ X-ray absorption spectroscopy measurements showed that during this process, the first coordination shell (O) of the Ce ions changes but that of the Gd ions does not. This supports our contention that the originally distorted environment of the Ce_Ce-V_o complexes is modified by the field, and thereby gives rise to electrostriction. This finding is also able to explain the dependence of the electrostriction on the in-plane strain and strongly suggests that a similar effect should be observable in other ionic conductors. Therefore, one may expect that the next generation of electromechanically active materials may indeed come from the family of oxygen ion conductors.

To separate the effects of ionic, electronic and proton diffusion, we used null-point ellipsometry (lambda;=632.8 nm) with lock-in detection to monitor electric field-induced changes in a thin film of Ce_0.8 Gd_0.2O_1.9 at 75-160 °C. The 250-400 nm films were sputtered and annealed in air for 2 hrs at 450°C ; the bottom electrode was 1 mm glass covered with 100-150nm sputtered Cr ; and the top electrode was semitransparent e-beam evaporated Au (15±3 nm). Application of an external bias > 0.05 V between the Cr and Au layers results in a readily detectable change in the ellipsometer signal, which is due to an increase in the refractive index, i.e. sample density, of the Ce_0.8 Gd_0.2O_1.9 film at one of its interfaces. Since the concentration of free electrons in the film (<10⁶ cm^-3) is too low to affect its refractive index, we interpret the observed change as arising from the accumulation of oxygen ions. From the dependence of the signal amplitude on the applied DC voltage, we deduced the interface potential to be 0.4 V at 110 °C. From the measurement of the system relaxation time, we estimated that the oxygen diffusion coefficient is (7±3.5)×10^-20 cm²/sec at 120 °C and the activation energy is 1.6±0.1 eV. These results demonstrate the usefulness of the technique for studying diffusion mechanisms in ceramics near room temperature. Combined with impedance spectroscopy, null-point ellipsometry could also effectively contribute to the identification of the dominant conduction mechanism.

Ethanol is an attractive fuel for electricity production in solid oxide fuel cells (SOFCs) because it is renewable and widely available. In the present study, hydrogen production from ethanol gradual internal reforming (GIR) was investigated in SOFCs with a catalytic layer deposited over the anode. The GIR mechanism allows for the operation of direct (dry) ethanol according to the theoretical reactions: H₂ + O^2- → H₂O + 2e^- (1) ; C₂H₅OH + 3H₂O → 2CO₂ + 6H₂ (2). A ceria-based catalytic layer promotes the ethanol reforming producing hydrogen, which is oxidized in the anode/electrolyte interface generating steam. Therefore, as long as electric current is drained from the fuel cell reactions (1) and (2) continuously sustain each other. Single SOFCs were fabricated using electrolyte supports of yttria-stabilized zirconia (YSZ) and conventional anode (Ni-YSZ) and cathode (LSM) electrode materials. A mixture of 10 mol% gadolinia doped-ceria with 0.1 wt.% of Ir was deposited onto the anode as the catalytic layer (~100 µm thick). Electrochemical characterization of the single cells was carried out at 850°C using hydrogen and anhydrous ethanol balanced with argon as fuels. Polarization curves measured for both ethanol and hydrogen are similar, adding further evidence for the ethanol GIR. More importantly, fuel cells were able to operate continously over 100 h with dry ethanol without significant degradation, strongly suggesting that no carbon is formed. Scanning electron microscopy analyses after fuel cell operation with ethanol confirmed the absence of carbon deposits in the anode.

The global performance of the cathode in solid oxide fuel cells (SOFC) is microstructure sensitive, and it is well known that the topological connectivity of two solid electrode phases and total length of triple phase boundaries are key parameters affecting the cathode performance. In order to improve the performance of SOFC composite cathode via microstructure engineering and to predict the transport processes in complex porous cathodes, a better understanding of the microstructural characteristics, such as the geometrical and topological properties, is required. Newly developed methods are applied here to extract the topological parameters such as the tortuosity, dihedral angle, and TPB length for 3D phase-field simulation microstructures. These methods are further applied to samples that have been experimentally reconstructed using FIB-SEM techniques to determine the tortuosity value for the pore phase, the dihedral angle distributions, and the total TPB length in a LSM/YSZ/pore three-phase electrode microstructure. The methods described in this work enable microstructurally focused optimization of SOFC performance, facilitate examination of the transport properties of cathode at the macroscopic scale, and quantitatively describe microstructural evolution.

Cobalt containing perovskites of the LnCoO3 (Ln= lanthanide) type and its substituted derivatives, e.g. Ln_1-xA_xCo_1-yB_yO₃ (Ln= lanthanide, A= alkaline earth metal and B=transition metal), have been subject of many studies during the last twenty years and Sr and Fe substituted compounds i.e. the La_1-xSr_xCo_1-yFe_yO₃ type, have attracted a lot of attention due to their high ionic and electronic conductivity. The materials have been considered as potential cathode materials for solid oxide fuel cells (SOFCs). The material suffers however, from high thermal expansion due to spin state transitions in Co³⁺ and in some cases also from an increase of oxygen vacancy formation. The spin state transition in Co³⁺ seem to govern a large part of the thermal expansion as observed for LaCoO₃ with average thermal expansion coefficients around ~21-22 ppm/K between 25- 900°C. Although, many studies have been made on the composition-structure-properties relations for this intriguing perovskite cobaltate, few have been concerned with investigating how the properties change in a systematic way with changes in the amount and oxidation state of cobalt, without any influence from oxygen vacancy formation. We here present the results of our investigation of the high-temperature thermal expansion and electronic transport properties of the B-site substituted La₂Co_1+z(Ti_1-xMg_x)_1-zO₆ with both variable oxidation state of cobalt between +2 and +3 and variable Co³⁺-content relative to the other B-cations, by changing the relative ratios of Co: (Ti⁴⁺:Mg²⁺). Due to an insignificant number of oxygen vacancies of La₂Co_1+z(Ti_1-xMg_x)_1-zO₆ samples prepared in air at elevated temperatures, the investigated system is proposed as a good model system for the investigation of influences of Co oxidation state and stoichiometry on different properties in perovskite cobalt oxides. The structural characterization, thermal expansion, Seebeck coefficients, high temperature magnetic properties and electronic conductivity of the system will be presented.

The deactivation resulting from carbon dissolution includes the breakage of the Ni-Ni conducting network as well as the formation of microscopic cracks within the Ni-YSZ material. The microstructure of the Ni-YSZ cermet exposed to methane at 750 °C was investigated using a dual beam focused ion beam scanning electron microscopy (FIB-SEM) system and the deactivation process within the cermet was confirmed in the reconstructed microstructure prepared by FIB-SEM. Furthermore, promotion of precious metal nanoparticles (NPs) on the Ni-YSZ cermet was studied to improve the carbon tolerance of Ni. We prepared M-NPs (M = Pd, Pt, Au) using a novel method, so-called a physical vapor deposition on powder (PVDP) and introduced them into Ni-YSZ cermets. We demonstrated that promotion of precious metal NPs effectively alleviated carbon deposition in Ni-YSZ anodes and these results were consistent with molecular insights obtained in the DFT calculations.

Among perovskite-structured proton conductors, yttrium-doped barium zirconate is one of the most promising high-temperature proton conductors due to its high proton conductivity and sound chemical stability. Since the solid state proton conductors contain grain boundaries, protons should migrate across them. Frequently observed sum;m (m=3, 5, and 9) tilt grain boundaries in perovskite structure were constructed and evaluated for proton migration across them. The grain boundary energy was high in order of sum;5, 9, and 3 tilt boundaries. The yttrium dopant was segregated at sum;5 and 9 tilt boundaries, while it was not at sum;3 tilt boundary. The energy barriers for proton migration across the tilt grain boundaries were all higher than that in bulk, showing that there was no easy path for proton across the grain boundaries.

Solid Oxide Fuel Cells (SOFC) are of great interest in many energy conversion systems. To obtain suitable efficiency by co-generation, high operating temperatures (800-1000°C) are required, leading to degradation issues and an increase of material cost. Thus, many current works are devoted to the reduction of the operating temperature down to 600°C. Solid Oxide Fuel Cells based on protonic conducting electrolyte, so-called H+-SOFC, operating at temperatures between 400 and 600 °C, may represent an interesting alternative. However, several drawbacks remain when working at such low temperatures; for instance the oxygen reduction reaction occurring at the cathode side is characterized by a high activation energy and becomes the major cause of performance loss for T < 800°C, even though this can be partially compensated by the low activation energy of the thermal dependence of the electrolyte protonic conduction. To overcome this problem, new H+-SOFC cathode materials are required. In this context, our researches aim to elaborate new cathode materials for H+-SOFC, with high mixed conductivites and good electrocatalytic properties toward oxygen reduction, between 400 and 600°C. On the basis of the best SOFC cathode materials, a selection of H+-SOFC cathodes has been made, Ruddlesden-Popper phases and double perovskites showing the best electrochemical activities. To provide the delocalisation of electrode reaction (2H+ + 2e- + ½ O2 → H2O) all over the gas / electrode interface, mixed protonic and electronic conductors should be used. While ionic O2- conduction in rare earth nickelates and related phases is well known, protonic conduction has never been evidenced in these compounds. Our work is devoted to the comprehension of cathode electrochemical behaviour. In this way, structural characterizations as well as electrochemical properties of Ruddlesden-Popper oxides and layered perovskites associated with barium cerate (BCY10) as electrolyte, have been studied under zero dc and current conditions versus temperature, oxygen and water partial pressures. The results are discussed in terms of triple mixed conductivity (e-/H+/O2-) of these materials.

Solid oxide fuel cells (SOFCs) have an advantage on the use of various fuels, not only H2 but hydrocarbon-based fuels such as town gas, LP gas, kerosene, and biogas after simple reforming processes at the anodes, because of high operational temperature. However, un-reformed hydrocarbons and any impurities in actual fuels flown into the SOFC actual systems can trigger in-plane distributions of voltage characteristic, carbon deposition, structural change and others. Therefore, toward the practical use of SOFCs, we must precisely measure in-plane distributions of performance and durability in a comparatively large cell like a real SOFC. In this study, SOFCs with in-plane segmented electrodes were prepared, and the performance at each segment was evaluated separately in various conditions, in order to explain in-plane distributions of performance and durability in the SOFC systems. Typical electrolyte-supported single cells were used in this study. Scandia-stabilized zirconia (ScSZ: 10mol% Sc2O3-1mol% CeO2-89mol% ZrO2) plates with a thickness of 300~350 µm and an area of 25cm2(5cm×5cm) were used as electrolytes. NiO-ScSZ cermet and LSM ((La0.8Sr0.2)0.98MnO3)-ScSZ composite material were used for the anode and the cathode, respectively. Electrode layers were prepared via screen printing technique. Screen printed green electrode layers were heat-treated at 1300 oC for the anode and 1200 oC for the cathode. Ni mesh for the anode and Pt mesh for the cathode were used as current collectors. An electrode 16cm2(4cm×4cm) was separated into 3 parts, in order to evaluate various in-plane distributions from fuels inlet to outlet. For typical experimental conditions, a fuel, operation temperature, current density, and fuel utilization rate were 50% pre-reforming CH4(S/C=2.5), 800oC, 0.2A/cm2 25% respectively. By changing pre-reforming rates or fuel utilization rates, or by mixing various impurities such as sulfur, cell voltage and various overvoltages were measured. Then, cells after tests were observed with SEM and EDX. Furthermore, results were compared to the computational analysis done by commercially available CFD software such as FLUENT, and in-plane distributions of performance and durability in the SOFC systems were investigated. When the 50% pre-reforming CH4(S/C=2.5) fuels with (a) neither H2S nor C3H8, (b) 3ppm H2S, (c) 3% C3H8, (d) both 3ppm H2S and 3% C3H8 were provided, similar to the button cells, cell performance for (a) and (b) was stable for 40 hours. Cell voltage for (b) and (d) dropped after adding H2S as a result of increasing overvoltage, and started to recover toward the initial cell voltage after stopping H2S. When the fuel was (d), carbon was deposited at the fuel inlet. Carbon deposition was occurred on the anode surface, according to the SEM image of the inlet cross-section surface. As the co-poisoning effect of H2S and C3H8, it has been revealed that carbon deposition is likely to occur at the fuel inlet was obtained.

Silver electrodes have been deposited deep inside a porous structure of the proton conducting perovskite BaCe0.5Zr0.3Y0.16Zn0.04O3-d using Tollens&’ reaction. The nanostructured silver layer serves as a base for the electrodeposition of copper metal. The silver and copper electrode microstructures were characterized by electron microscopy and the composition distribution by EDX. The silver electrodes were used to determine the bulk and grain boundary conductivity of the proton conductor. A cell with electrodeposited copper as the cathode and impregnated nickel as the anode was studied in fuel cell and in electrolysis modes at 617 C. It is shown that the electrodeposited copper can be used for the co-electrolysis of CO2 and H2/H2O with a performance similar to that of impregnated copper. The authors would like to thank Sasol for funding.

The oxygen conductor yttria-stabilized-zirconia (YSZ) is widely used in miniaturized solid oxide fuel cells (mu;SOFC) and may be suitable for solid state ion emitter applications e.g. as miniaturized ion engines for electric propulsion. Since the growth of the YSZ films is not completely free of strain, cracks in fabricated free standing membranes are often observed. YSZ thin films were deposited on Si substrates by radio frequeny sputtering. Free standing YSZ membranes were fabricated by anisotropic wet chemical etching of a Silicon substrate using different masking patterns defined by electron beam lithography. The pattern optimization allows one to reduce stress induced cracking. We show how different patterns, sizes and etching conditions influence the strain in the fabricated membranes. To characterize these membranes we used light and laser microscopy as well as scanning electron microscopy and atomic force microscopy. Additionally we investigated the strain profiles in the membranes with Raman mapping.

Complex ABO3-type perovskite solid solutions with oxygen deficiency exhibit a perceptible ionic conductivity, leading to their promising use as electrolytes ((La,Sr)(Ga,Mg)O3-δ), oxygen permeation membranes, and solid oxide fuel cell (SOFC) cathodes ((La,Sr,Ba)(Mn,Fe,Co)O3-δ). Oxygen stoichiometry strongly affects transport properties of the materials, which, in turn, defines suitability of the material for targeted applications. The oxygen migration in those perovskites occurs via the vacancy mechanism in which the vacancy moves through a bottleneck formed by the "critical triangle" of one B site cation and two A site cations. In addition to their influence on bulk transport properties, the concentration and mobility of oxygen vacancies are two major factors determining the surface oxygen incorporation rate. Although oxygen vacancies are being extensively studied, our understanding of their effects on behavior of the materials and the corresponding performance of practical devices is far from complete. By using first principles massively parallel DFT calculations combined with large 40-320 atom supercells, we analyze and compare the formation and migration of oxygen vacancies in (La,Sr)(Co,Fe)O3-δ (LSCF) and (Ba,Sr)(Co,Fe)O3-δ (BSCF) perovskites. The atomic relaxation, charge redistribution, migration barrier, and the structure of transition states for oxygen ion migration are obtained. We explore differences between BSCF perovskites [1,2], which exhibit considerably lower migration barriers than other perovskites, and LSCF. We discuss relevant implications for the oxygen surface and bulk reaction and hence for energy conversion in practical devices. [1] E.A. Kotomin et al, Solid State Ionics, 188, 1 (2011) [2] R.Merkle et al, J Electrochem. Soc. 159, B 219 (2012).

8 mol% yttria stabilized zirconia, Y0.16Zr0.84O1.92 (8YSZ), is a high-temperature oxygen ion conductor commonly used in the electrodes and electrolytes of solid oxide fuel cells, electrolyzers, and oxygen sensors. When 8YSZ is used as an electrolyte, dopants which enhance the 8YSZ densification kinetics are desirable (as long as they do not deteriorate the material&’s electrical and/or mechanical properties) because they lower manufacturing costs by reducing processing times and/or temperatures. When 8YSZ is used in an electrode, however, dopants which enhance the 8YSZ densification kinetics are undesirable because they lower the 8YSZ-pore and/or 8YSZ-electrocatalyst interfacial areas; thereby degrading electrode electrochemical performance. In this study, the 25-1450C sintering kinetics of a single batch of Tosoh 8YSZ powder surface- doped with barium, calcium, cobalt, copper, iron, lithium, magnesium, manganese, nickel, strontium or zinc at the 1, 3 and 5 mol% impurity level were investigated via constant heating rate dilatometry in air. Post-dilatometry x-ray diffraction, scanning electron microscopy, and AC impedance spectroscopy were used to observe the effect of surface-doping on the 8YSZ phase purity, grain size, and electrical conductivity, respectively. The sintering aid effectiveness comparison in this this study is important for optimizing 8YSZ high-temperature processing and operation conditions.

Yttria stabilized zirconia (YSZ) is the conventional electrolyte material in SOFCs. Conductivity degradation of YSZ electrolyte directly leads to degradation of electrical efficiency of the SOFC systems. The phase equilibrium for the ZrO₂-Y₂O₃ system [1] and conductivity degradation mechanism of YSZ [2] have not been completely clarified. In our previous report, conductivity degradation behaviour of NiO doped YSZ was examined focusing on the phase transformation behaviour of YSZ electrolyte [3]. Conductivity degradation behaviour of 1mol% NiO doped 8mol% Y₂O₃ stabilized zirconia (1Ni-8YSZ) electrolyte was measured at open circuit voltage (OCV) under a SOFC environment with in-situ Raman spectroscopy. We demonstrate that phase transformation of YSZ gradually proceeded under OCV conditions with gradual conductivity degradation. In this study, phase transformation behaviour on the surface of the NiO doped YSZ electrolyte is analyzed by in-situ micro Raman spectroscopy with simultaneous conductivity measurement. The effect of applied voltage, redox cycle and thermal cycle were discussed. [1] Ex, M. Yashima et al., Solid State Ionics, 86-88 (1996) 1131- 1149 [2] Ex, K. Nomura et al., Solid State Ionics, 132 (2000) 235; B. Butz et al., Solid State Ionics, 177 (2006) 3275 [3] H. Kishimoto et al., MRS Online Proceedings Library (2012) 1385; H. Kishimoto et al., Electrochimica Acta, in press (2012), DOI : 10.1016/j.electacta.2012.04.148

Reactivity in the Sr ions incorporated LaGaO3 with Ni system, which is the main constituent of the anodes for intermediate temperature solid oxide fuel cells (IT-SOFC), has been investigated both computationally and experimentally. For the first time, an isobarothermal section of the La-Sr-Ga-Ni-O system was constructed. It has been found that NiO reacts with LaGaO3 and there exist extended solid solutions of La(Ga,Ni)O3, La(Ga,Ni)2O4 and La4(Ni,Ga)3O10. It has also been found that, in the presence of Sr ions and direct contact conditions, NiO diffusion from the anode and cathode materials to LaGaO3 is inevitable.

The electrical conductivity degradation of yttria-stabilized zirconia (YSZ) is a factor of performance degradation in real SOFCs stacks. We have shown that the electrical conductivity degradation of Ni-doped YSZ is accompanied by tetragonal phase transformation [1]. We have proposed that the tetragonal phase transformation in Ni-doped YSZ occurs via cation vacancy diffusion judging from the agreement between the tetragonal transformed and the low oxygen potential regions [2]. Furthermore, we have calculated the oxygen potential profile based on electronic conductivity measurements [3]. In this study, the visualization of the phase transformed region in Ni-doped YSZ was successfully achieved using a Raman spectra mapping, when the electrolyte was placed under several oxygen potential gradients. Total correspondence was found for tetragonal phase transformation and the calculated low oxygen potential regions. [1] H. Kishimoto, K. Yashiro, T. Shimonosono, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, J. Mizusaki, Journal of Materials Research, accepted in 2011. [2] T. Shimonosono, H. Kishimoto, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics, “Phase transformation related electrical conductivity degradation of NiO doped YSZ” in press (2012). [3] T. Shimonosono, H. Kishimoto, K. Yamaji, M.E. Brito, T. Horita, H. Yokokawa, Solid State Ionics, “Electronic conductivity of Ni-doped yttria-stabilized zirconia” in press (2012).

Oxygen removal from non-stoichiometric oxides produces volume changes that can lead to substantial stresses when these materials are constrained in thin film form. This effect is particularly relevant in ceria electrolytes for solid oxide fuel cells. To measure these stresses, in situ wafer curvature measurements were conducted on ceria films during oxidation-reduction cycling, over a range of temperatures and oxygen partial pressures. Undoped and Gd doped ceria films were grown by metal organic chemical vapor deposition (MOCVD), and Pr doped ceria films were grown by pulsed laser deposition (PLD). In undoped nanocrystalline films, reversible stresses increased with decreasing grain size, and were larger than expected based on literature values for the chemical expansion and elastic modulus. These results demonstrate that grain boundary effects dominate compositional stresses in nanocrystalline material. This provides strong experimental support for the idea that space charge effects near surfaces and interfaces can substantially enhance stresses due to chemical expansion. These undoped films also exhibit an unexpected stress reversal at temperatures above 600 C, which suggests that there are multiple stress-inducing mechanisms associated with grain boundaries. In comparison, the doped films exhibit stresses that are closer to the bulk material properties and the stress reversals at high temperatures were not observed. These observations are analyzed in terms of the relevant defect incorporation mechanisms for the undoped and doped materials.

Due to their well-defined geometry and dense nature, thin film Mixed Ionic Electronic Conductor (MIEC) electrodes have recently been favored as a means of measuring intrinsic MIEC electrochemical properties such as the oxygen surface exchange coefficient. In this study, a novel bi-layer curvature relaxation technique has been used to measure the chemical oxygen surface exchange coefficients of sputtered La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ thin films atop yttria stabilized zirconia single crystal substrates. This bi-layer curvature relaxation technique allows accurate oxygen surface exchange coefficients to be determined as a function of easily modulated and measured film stress, atmosphere, and temperature for materials exhibiting significant mechano-chemical coupling. The optical nature of this technique makes it one of the few techniques suitable for in-situ thin-film oxygen surface exchange coefficient measurements. In addition, the electrode-free nature of this technique makes it especially compelling for surface exchange measurements of low conductivity MIEC films, which previously required the use of inter-digitated electrodes which unfortunately could 1) chemically react with the underlying film, 2) induce complicated film stress variations and 3) induce preferential oxygen incorporation at the electrode-film-air triple phase boundary.

Electrode-electrolyte interactions at elevated temperatures are major contributors to long-term degradation in solid oxide cells (SOC). The use of silver-based electrodes for SOC is promising due to their good performance at reduced temperatures and chemical stability with other SOC materials. A symmetric cell configuration with applied voltage bias has been used to investigate interactions between yttria-stabilized zirconia (YSZ) electrolytes and silver-based electrodes. Composition changes and microstructural evolution at the electrode-electrolyte interfaces have been studied using electron microscopy, energy dispersive x-ray spectroscopy, X-ray diffraction, and secondary ion mass spectroscopy techniques. No additional phase formation was detectable at the silver-YSZ interfaces. However, bulk porosity formation, severe surface undulations, and trace impurities were observed at the interfaces under a wide variety of operating conditions. The interaction is explained in terms of voltage-enhanced diffusion of impurities to the silver-YSZ interface.

The high temperature electrolysis of water, driven by off peak surplus energy from renewable sources or nuclear power, allows the efficient and economical production of hydrogen using solid oxide electrolyser cells (SOECs). In these devices, water is dissociated into hydrogen and oxygen at the cathode (or steam electrode), and the oxygen is removed by electrochemically pumping it through an electrolyte with an external applied potential, to be evolved at the anode (or oxygen electrode), leaving humidified hydrogen as the product at the steam electrode. One attractive aspect of these devices is that if the cell is run in the opposite direction, the device operates as a Solid Oxide Fuel Cell (SOFC), generating electricity from the stored hydrogen. A system involving such a reversible cell offers great potential to store and subsequently utilise surplus energy. Typical state-of-the-art SOEC&’s based on the high temperature SOFC materials “set” (especially those using (La,Sr)MnO_3-δ-based oxygen electrodes) suffer from problems of stability and degradation, particularly by delamination at the oxygen electrode / electrolyte interface at high current densities, due to the build-up of high pO₂&’s at the interface. Part of the reason for this is that the kinetics of oxygen surface exchange at the oxygen electrode are not fast enough to cope with the large fluxes of oxygen generated. To overcome this problem, we need to develop new materials for the oxygen electrode that show both enhanced surface exchange kinetics and improved ionic conductivity, ideally in both electrolyser and fuel cell modes. Interstitial oxygen ionic conductors such as the first order Ruddlesden-Popper type phases with the general formula A₂MO_4+δ (A = La,Pr,Nd, B = Ni,Co) are particularly interesting candidate materials. The Nd₂NiO_4+δ and Pr₂NiO_4+δ materials have already been seen^1,2 to show superior performance in electrolysis cells than typical LSM electrode materials. In this contribution, a series of compositions in the pseudo-binary system defined by the La₂NiO_4+δ and Pr₂NiO_4+δ end members are synthesised and characterised in terms of their defect chemistry and their performance as oxygen electrodes for reversible SOEC/SOFC devices. References: 1. T. Ogier et al., Proc. SOFC-XII: Electrochem. Soc. 219th meeting, May 2011 Montréal Canada 2. F. Chauveau et al., Journal of Power Sources 195 (2010) 744-749

Mixed ionic and electronic conductors (MIECs) have attracted considerable attention because of their promising application as a cathode material for solid oxide fuel cells and oxygen permeable membranes. Among a number of MIECs, Sr-doped lanthanum cobaltites, La_1-xSr_xCoO_3-δ exhibits excellent mixed conductivity as well as high catalytic activity for oxygen reduction. For the Sr-doped lanthanum cobaltites, it is well known that Sr segregation takes place during preparation and also use as a cathode. In this study, we have studied Sr-segregation behavior for La_1-xSr_xCoO_3-δ (LSC) as functions of Sr and carbon dioxide concentration. All of the samples were prepared by using a solid-state reaction process. Heat treatment was carried out in various CO₂ concentration at 1173 K for 24 h. For 10%Sr (LSC91) and 20%Sr-doped (LSC82) samples, no secondary phase was observed after the CO₂ heat treatment. As Sr concentration increases higher than 30%, a carbonate compound covers in partial the surface of LSC. For 50%Sr-doped (LSC55) sample, the precipitation phase seems to comprise of two layers; in addition, Sr-less and porous region along grain boundary appears. In the case of higher Sr concentration (ge;70%), the whole surface was covered with the secondary phase. This implies that Sr segregation driven by the presence of CO₂ can be controlled by adjusting Sr and CO₂ concentration. To clarify the stable regime of LSCs, CO₂ adsorption isotherms were measured. Based on the isotherms, their thermodynamic properties will be discussed.

Gadolinium doped CeO₂ (GDC) is a component material for some intermediate temperature (500 °C to 700 °C) solid oxide fuel cell (IT-SOFC) electrolytes and anodes due its high oxygen ion conductivity. The anode generally consists of an intimate mixture of Ni metal and GDC called a cermet. The electrochemical oxidation of the fuel takes place at the three-phase boundaries (TPBs) where the gas phase, metal particle and the ceramic component meet. An investigation of nanoscale changes taking place at the TPBs is essential to understand the functionality of the anode. In situ environmental transmission electron microscopy (ETEM) is a powerful technique that allows us to observe these changes under near-reaction conditions. In this research, the nanostructural changes observed in a Ni-GDC cermet anode during in situ reduction in the ETEM are explored. An intimate mixture of NiO and GDC powders in a 3:2 ratio by weight was used to synthesize compacted circular disks of 0.8 mm thickness and 18 mm in diameter. The dc electrical conductivity of the reduced anode disks were found to be dependent on the reduction procedure and the pre-treatment given to the starting powders of NiO and GDC. The anodes with highest conductivity (0.3 S/cm²) were used for the in situ investigation in the ETEM. In addition to pure NiO and GDC grains, some grains appeared to consist of a composite of NiO and GDC, which was confirmed with electron energy loss spectroscopy (EELS). After in situ reduction in a flowing H₂/N₂ mixture at 700 °C, a change in the structure of these grains was observed wherein Ni nanoparticles nucleated on the GDC grain under reducing conditions. Corresponding EELS spectra revealed a lowering in the Ni L_2,3 peak intensity ratio confirming the reduction of NiO to Ni. This indicated that upon reduction of the embedded NiO, Ni diffused out of the GDC grain, possibly due to the poor solubility of Ni in GDC at higher temperatures under reducing conditions. A reversal in Ce M_4,5 peaks was also observed in these grains and indicated the reduction of Ce to an oxidation state less than +4, contrary to the pure GDC grains which did not show a change in Ce oxidation state under the same conditions. This phenomenon was attributed to an H spillover effect, previously observed by us in Niminus;Pr-doped CeO₂ SOFC anode materials.

The morphology and structural stability of Yttria stabilized zirconia-strontium doped lanthanum manganite (YSZ-LSM) composite has been investigated under a wide range of sintering conditions. Besides the zirconate formation, the YSZ and LSM morphology changed under different reaction conditions. A porous YSZ morphology in contact with LSM was observed. LSM and YSZ powders with and without pore formers were mixed uniformly, and then uniaxially pressed into pellets. The weight ratio of YSZ to LSM was kept at 1%, 10% to 50%. The pellets were sintered at 1400 °C for 2 to 10 hours in air. The morphology change and microstructural evolution were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. 1%YSZ-LSM samples showed porous structure, 10%YSZ-LSM showed partial porous and partial dense YSZ structure, and 50%YSZ-LSM showed only dense YSZ structure. A mechanism for the interactions is proposed. It is postulated that the morphology and crystal structure change is primarily due to Mn diffusion, dissolution, precipitation and evaporation. To prove this hypothesis, experiments were conducted using a simplified binary system containing 1wt% MnO2 and YSZ mixture. The quenched MnO2-YSZ samples showed dense YSZ structure at 1400 °C, 1300 °C and 1250 °C, porous YSZ structure at 1200 °C and lower temperature. The different Mn solubility in YSZ at different temperature is believed to be responsible for the porous YSZ formation.

Ceria (CeO2), especially at ultra-small size range has unique catalytic and redox properties based on its enhanced oxygen ion conductivity and high mobility of oxygen vacancies in the nanosize system. The smaller the particle size the larger the Ce3+ concentration and also the greater the number of oxygen vacancies to maintain charge neutrality. This in fact leads to the increased ability of ultra-small ceria to either absorb or release oxygen effortlessly in the surface layers of ceria nanocrystals. This study demonstrates this process using high resolution x-ray photoelectron spectroscopy (XPS) by monitoring the redox cycle of ceria overtime after external exposure to oxygen sources. Specifically, high resolution Ce3d XPS spectra are measured at different stages of the redox cycle and the Ce3+/Ce4+ ratio is quantified using spectrum deconvolution method.

The wide spread application of the current solid oxide fuel cells devices is limited by the high operation temperatures. An strong research effort is being directed to increase the conductivity of the electrolyte to minimize ohmic losses at lower temperatures. Interface effects in epitaxial ionic conducting heterostructures appear as a promising pathway towards novel artificial electrolytes for cooler fuel cells or other electrochemical devices. Heterostructures combining transition metal oxides, as compared to other materials, are able to accommodate very large amounts of epitaxial strain without breaking into islands or structural domains. Coherently strained interfaces are an interesting playground for the search of materials with enhanced ion diffusivities, of interest in devices for energy generation and storage. In this regard, it has been proposed that the coherent growth of strained interfaces in heterostructures combining materials with different degrees of lattice mismatch may promote ion diffusivity and thus, these heterostructures may play an important role in the optimization of materials for energy generation and storage. This is the case of the Y2O3- ZrO2 / SrTiO3 (YSZ/STO) superlattices where different structures (fluorite vs. perovskite) are combined with a large lattice mismatch of 7%. The interface between highly dissimilar structures stabilizes a disordered oxygen sublattice with an increased number of oxygen vacancies which promote oxygen diffusion. In this talk we will highlight the importance of the interface structure of highly strained YSZ/PMO superlattices alternating YSZ and transition metal oxides with a perovskite related structure (PMO), in determining changes of their ionic conductivity. We will show results of different PMO systems including SrTiO3, LaAlO3, YAlO3, where epitaxial strain can be consistently modified. We will discuss the role of growth orientation in controlling the structure and morphology of the interface. Results of density functional theory calculations are discussed, showing that the incompatibility of the oxygen positions at the interface planes plays a key role in stabilizing the high values of ionic conductivities.

In the last decade some studies in heteroepitaxial growth of oxide ionic conducting YSZ films on different substrates along with multilayered structures combining YSZ with other materials (mostly non ionic conducting compounds) have revealed a significant dependence of ionic conductivity with film microstructure and epitaxial strain. Particularly, for the controversial case of epitaxial SrTiO3-YSZ heterostructures, some experimental studies on SrTiO3-YSZ (1nm thick) multilayers have claimed up to 7-8 order of magnitude enhancement in ionic conductivity [1]. In this direction it has been reported through theoretical calculations that coherent films up to nominal +7% tensile strain may show several orders of magnitude enhanced ionic conductivity provided the fluorite structure is preserved up to such high strain [2]. However, some other studies in YSZ structure at the interface with SrTiO3 under tensile strain have revealed a certain instability of fluorite phase [3]. In this study we confirm that the structure of YSZ nanolayers (of a nominal thickness of a few nanometres) sandwiched between SrTiO3 slabs may be substantially modified to form strained tetragonal and monoclinic zirconia interfaces, and in some cases to form a modified structure, not compatible with the fluorite, which resembles that of a perovskite-type Zr-O containing layer. Depending on film orientation, either deposited on 100 or 110 SrTiO3 substrates, as well as on the substrate chemical termination, either SrO or TiO2, the YSZ film growth can be controlled to form continuous YSZ slabs with fluorite structure. However, in all cases the YSZ grows almost fully relaxed, as proven by XRD measurements. We also provide evidence that, despite the disturbed microstructure and the unstrained state of the YSZ, the electrical conductivity of the films might be enhanced by about 6-7 orders of magnitude (if one attributes the conductivity changes only to the YSZ material). However, the pO2 power dependence of the conductance with exponent +1/4 measured for these films is an indication of the p-type electronic conductivity in the system. This was attributed to the SrTiO3 conductance enhancement at the interface [4]. Isotopic 18O tracer diffusion experiments did not show any evidence of ionic diffusion along the multilayer structure parallel to the interfaces. Heating-cooling cycles in air at different rates reveal a strong dependence of the electrical properties of the layers. This is explained in terms of interface phenomena associated with oxygen diffusion between YSZ and SrTiO3 and changes in SrTiO3 oxygen stoichiometry. This results in a large span of metastable states of the samples depending on their history. [1] J. Garcia-Barriocanal et al, Science 321 (2008) 676 [2] A. Kushima and B. Yildiz, J. Mater. Chem. 20 (2010) 4809-4819 [3] W. L. Cheah and M. W. Finnis J Mater Sci (2012) 47:1631-1640 [4] A. Cavallaro et al. Solid State Ionics 181(2010) 592-601

In the recent years, there has been an emerging interest towards the use of thin-film technology for miniaturization of solid oxide fuel cells (SOFCs) with thinner components (particularly the electrolyte) that can potentially reduce the operating temperature of the cell while delivering enhanced performance. Moreover, development of novel nano-architectures not only has led to enhancing the properties of individual components at the nano-scale, but also has made the down-scaling of the cells to smaller dimensions possible for development of micro-solid oxide fuel cells (µSOFCs) as well as for other applications. Toward these goals, we present a novel design and fabrication process of a two-phase one-dimensional hetero-nanostructured electrolyte architecture grown on silicon substrate. The developed hetero-nanostructure is composed of a backbone vertically-oriented nanotubes array of strontium titanate SrTiO3 (STO) that has been synthesized for the first time on silicon substrate using the electrochemical anodization and hydrothermal processes. Then the nanotubes are coated with yttria-stabilized zirconia (Y2O3)x(ZrO2)1-x (YSZ) using metal-organic chemical vapor deposition (MOCVD) to form a core-shell nanotubular structure. Analysis of the crystal lattice strain and the defect chemistry of the YSZ/STO hetero-interface is performed using a field emission scanning electron microscopy (FESEM), high-temperature glazing angle X-ray diffraction (HT-GAXRD), and high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). Local atomic/electronic structure of the YSZ coating is also obtained around Y and Zr centers using the X-Ray Absorption Fine Structure (XAFS) measurements. Results obtained from the preliminary cell testing shows promise toward the use of the developed YSZ/STO nanotubes structure as an electrolyte membrane of µSOFCs. Correlations between the charge transfer characteristics and the microstructure of the developed nano-composite at the interface are also described. The developed approach proposes a cost-effective method for design and fabrication of hetero-nanostructured electrolyte membranes for µSOFCs and a potential route to develop silicon-based energy devices for portable applications.

Oxide ionic conductivity in the laminated film with nm thickness is attracting much interest. In this study, layered thin film using Cu- and Ga-doped Pr2NiO4 and Sm-doped CeO2 (PNCG/SDC) was fabricated by pulsed laser deposition (PLD) method, and its change in oxide ion conductivity was studied in details. It was confirmed that prepared film was successfully deposited by PLD method, and its electrical conductivity was improved with decreasing the thickness of SDC layer. In order to identify the charge carrier, ion blocking method was applied. From the results on PO2 dependency, laminated film shows p-type dependence in high oxygen partial pressure and slope for dependency is close to that of PNCG bulk material, suggesting that hole conductivity in laminated film is assigned to the hole in PNCG layer. On the other hand, in wide PO2 range, conductivity is independent of oxygen partial pressure suggesting dominated oxide ion conductivity. This result might suggest that SDC layer is reduced state and formation of electron is suppressed in a laminated film. As a result, improved conductivity observed in laminated film could be assigned to oxide ionic conductivity. The estimated transport number by ion blocking method is also close to unity and the ionic transport number increased with decreasing SDC film thickness and increasing number of interface.

Innovation in advanced materials and devices for embeddable power sources is crucial to the realization of autonomous systems operable in complex and dynamic environment. In this general framework, I will discuss our on-going efforts to realize ultra-thin solid oxide fuel cells with electrolytes at the electron tunneling limit. Such studies allow one to probe rigorously electronic-ionic transport in two-dimensional oxide membranes that are self-supported and proximal to free surfaces. Special instrumentation designed to probe transport and thermo-mechanical stability real-time in such structures at extreme chemical potential gradient and elevated temperatures will be described. Experimental techniques to synthesize atomically tailored designer materials for electrolyte versus electrode functionality in thin film form in suspended structures will be considered. Experimental realization of high performance two-dimensional solid oxide fuel cells operable in a variety of fuels will be presented. The ability to operate such SOFCs at low temperatures (< 500 deg.C) creates an intriguing opportunity: the exploration of charge ordered complex oxides as fuel cell components while retaining memory of the electronic complexity. I will consider this briefly as well.

The power output of a solid oxide fuel cell (SOFC) quickly decreases to zero when the fuel supply is interrupted. Materials that could store energy during the fuel cell operation and restitute this energy when the fuel supply is interrupted would enable new modalities in SOFCs. This could also be assisted by the on-going research trend to reduce the operating temperature of solid oxide fuel cells without significant loss of power density. One such strategy we have explored is to use compositionally complex oxide anodes that can rapidly change valence states but also possess sufficiently high electronic conductivity to enable mixed conduction pathways. We synthesized vanadium oxide thin films by RF magnetron sputtering for thin film solid oxide fuel cell anodes. The electrolyte was nanostructured yttria stabilized zirconia (YSZ) deposited by RF magnetron sputtering and the cathode was porous platinum deposited by DC sputtering at high Ar pressure. It is well known that the oxidation state of vanadium oxide can vary depending on the ambient conditions, thus providing the ability to store energy electrochemically. Furthermore, hydrogen insertion has been reported in the various vanadium oxides, providing an additional pathway for energy storage. Compared to reference porous platinum anode thin film solid oxide fuel cells (Pt/YSZ/Pt SOFCs), the vanadium oxide anode SOFCs (VOx/YSZ/Pt) provide energy much longer once the fuel supply is interrupted. The time during which the Pt/YSZ/Pt fuel cells delivered energy was always about 15 seconds once the fuel supply was interrupted. Our vanadium oxide anode SOFCs delivered energy up to 210 seconds after the fuel supply was interrupted, depending on the current density and the vanadium oxide thickness. Regarding their performance during regular fuel cell operation, the VOx/YSZ/Pt SOFCs had lower open circuit potentials and maximum power densities compared to the reference Pt/YSZ/Pt SOFCs. This is thought to be due to the poorer catalytic properties of vanadium oxide for hydrogen oxidation compared to platinum and clearly points out new research vectors. We will discuss these results in detail along with efforts to explore the concept of multi-functional electrode materials for emerging fuel cell applications. The authors thank Mr. Kian Kerman for assistance in manufacturing the thin film solid oxide fuel cells.

Layered cathode materials with mixed ionic and electronic transport properties, such as those with double perovskite and K2NiF4-type structures, are widely studied as promising materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The intrinsic anisotropic structure of these families of compounds enables high oxygen diffusivity in a preferential direction due to the presence of fast diffusion pathways or interstitial sites. Moreover, the oxygen surface exchange could also be highly enhanced for a particular orientation due to differences in the outermost surface structure and composition, which would result in different pathways for the oxygen incorporation process. With this in mind we have selected two compounds within these families: GdBaCo2O5+δ (GBCO) double perovskite and (La0.5Sr0.5)2CoO4 (LSC214) with K2NiF4-type structure. Both GBCO and LSC214 have been deposited as epitaxial thin films on two different substrates, giving us the opportunity of studying them as model materials with defined orientations. The oxygen diffusion and exchange properties were analysed by isotopic 18O exchange depth profiling (IEDP) and Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) along longitudinal and transverse directions using a particular methodology which was developed for thin films. In the first study we deposited GBCO films with different orientations, either pure c-axis or a-axis oriented, on SrTiO3 (001) and NdGaO3 (110) single crystals, respectively, and exchanged the samples using different temperatures and exposure times. As a result we found that both longitudinal diffusion coefficients (Dc* and Da*) increase following a thermally activated dependence, but with different apparent activation energies for c- and a-axis oriented films. In this case the corresponding oxygen surface exchange rates (k*) do not show any evidence of anisotropy with very similar values within 20-30% differences for films with different orientations. The second study is focused on the study of LSC214 thin films grown on single-crystal SrTiO3 (001) and LaSrAlO4 (100),respectively. We have studied and compared the k* and D* values along the (001) and (100) directions of LSC214, with a particular emphasis in the different surface activities between the two orientations. Finally, we have also studied the effect of surface cation chemistries on the exchange coefficient of these films by gentle HCl etching. We show how this work involving the combination of two layered model materials, each of them grown with two different orientations, has helped us acquire a deeper and better understanding of the effect of crystallographic orientation on the oxygen exchange and diffusion kinetics of layered epitaxial thin film cathodes.

As the ABO3 perovskite oxide compositions show their intrinsic oxygen reduction reaction (ORR) limitations, there is a need to search for alternative types of mixed ionic and electronic conductors to enhance solid oxide fuel cells efficiencies. The Ruddlesden-Popper La2NiO4+δ (LNO214) is of particular interest(1, 2) due to their ability to incorporate excess oxygen and their anisotropic oxygen transport properties. Recently, Bassat et al.(3), have reported that the activation energy of the surface exchange reaction measured in the (a,b) plane is lower than that in the c plane. The molecular dynamics simulation of oxygen interstitial migration mechanism of LNO214(4) showed that the apical ion takes up residence in an adjacent interstitial site. Despite these efforts suggesting oxygen surface exchange dependence on orientation, this relationship is not well understood. We here examine the ORR activity of a-axis normal epitaxial growth of LNO214 thin films possessing varying amounts of strain on (001)-oriented single-crystal yttria-stabilized zirconia (YSZ), which can provide insight into ORR mechanisms of Ruddlesden-Popper type materials. Pulsed laser deposition (PLD) was utilized to deposit a-axis normal epitaxial thin LNO214 films (50 nm, 120 nm, 210 nm, corresponding to -3.4 %, -5 %, and -5.8% c-lattice mismatch and 1%, 0.32%, 0.3% a-lattice mismatch) on YSZ with gadolinium-doped ceria (GDC) as the buffer layer. Crystallinity, crystallographic relationships, and strains of the films were analyzed using 4-circle X-ray diffraction. Using electrochemical impedance spectroscopy (EIS) measurements conducted on patterned micro-electrodes ORR activity of epitaxial LNO214 films of different amounts of strain was examined. Interestingly, the surface exchange rate (kq) of a-axis normal epitaxial growth of LNO214 thin films strongly depends on c-lattice mismatch which may be caused by the change in chemical composition of LNO. These findings suggest that c-lattice mismatch may play an important role in controlling the formation of oxygen interstitial which can be contributed to ORR activity. The mechanism of ORR activity on Ruddlesden-Popper La2NiO4+δ thin films will be discussed. References 1. E. Boehm, J. M. Bassat, P. Dordor, F. Mauvy, J. C. Grenier and P. Stevens, Solid State Ion., 176, 2717 (2005). 2. V. V. Kharton, A. A. Yaremchenko, A. L. Shaula, M. V. Patrakeev, E. N. Naumovich, D. I. Loginovich, J. R. Frade and F. M. B. Marques, J. Solid State Chem., 177, 26 (2004). 3. J. M. Bassat, P. Odier, A. Villesuzanne, C. Marin and M. Pouchard, Solid State Ion., 167, 341 (2004). 4. A. Chroneos, D. Parfitt, J. A. Kilner and R. W. Grimes, J. Mater. Chem., 20, 266.

Thin-film solid oxide fuel cells (SOFCs), or micro-SOFCs, are under intense study due to their high energy conversion efficiency and relatively low operation temperature (<500 ^oC). Electrolyte materials with high ionic conductivity and long-term stability are a key component for successful operation. It is of great interest to develop a high-throughput methodology to screen candidate materials with high reliability and resolution. In this paper, we developed a high-throughput methodology to investigate ionic conductivity in oxygen-ion conductors. Yttria stabilized zirconia (YSZ) composition-spread thin films with nanometer-size grains were prepared by 90^o off-axis reactive RF co-sputtering. The morphology and microstructure were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The ionic conductivity of the YSZ composition spreads was evaluated using electrochemical impedance spectroscopy (EIS) measurements. We compare results for two electrode configurations, namely, out-of-plane (parallel plate) and in-plane (planar interdigitated electrode) and find that the contribution from the intragrain conductivity in YSZ thin films (150 nm) is more explicit in the latter configuration because it greatly diminishes electrode effects. The intragrain oxygen ion conductivity of thin film YSZ was systematically measured as a function of yttria concentration over the range 2 mol% to 12 mol%. The results show that the measured conductivity of the YSZ thin films is close to that of corresponding bulk materials with a peak value around 3 x 10^-4 S cm^-1 at 440 °C at the optimum Y₂O₃ concentration of 8 mol%. Validation of this technique means that it can be applied to novel chemical systems for which systematic bulk measurements have not been attempted.

The strain in a material has a profound influence over its properties including: ionic and electronic conduction, chemical (re)activity, electronic band structure, magnetic and superconducting. Moreover, it offers an environmentally benign solution to engineering desirable properties compared to traditional methods, such as doping. However, it is difficult to capture and fine-tune strain within a ‘bulk&’ material. Here we show how ‘nanotechnology&’ enables strain engineering and its effect upon ionic conductivity and chemical activity. In particular, we explore, using atomistic simulation and experiment, supported thin-films, nanoparticles, nanorods and three-dimensional (meso)porous architectures.

The search for lower temperature solid oxide fuel cells (SOFCs) has led to a strong interest in enhancing the oxygen reduction activity of the perovskites widely used for cathodes in these systems. Recent experimental and theoretical work on interfaces and thin-films suggests dramatic enhancements compared to bulk in both defect concentrations and their mobility. Here we consider in detail the (La,Sr)CoO3-d system, whose films have shown oxygen defect concentrations, oxygen exchange rates, and cation ordering that are not seen in the bulk material. We demonstrate that for significant Sr doping the vacancy concentrations are enhanced for either tensile or compressive strain, but that the strain effects are quite different in the undoped LaCoO3-d material. These results are compared to recent experimental studies of defect chemistry changes in films to assess where strain might be playing a major role. We will also discuss results of using high-throughput tools to explore the oxygen vacancy formation and migration energetics of a range of B-site cations to determine more general trends of defect coupling with strain and transition metal character.

The kinetics of the oxygen reduction (OR) reaction on mixed ionic and electronic conductors is a key factor determining the performance of solid oxide fuel cell (SOFC) systems at intermediate temperatures. In addition to temperature and pressure as the thermodynamic and kinetic drivers for OR, lattice strain in the material can also impact the OR activity of the SOFC cathode-related materials by influencing the surface cation chemistry, oxygen stoichiometry and electronic structure.[1] However, owing to the complexity of the electrochemical processes, a detailed understanding of how the OR activity is related to the strain state on a molecular level is still missing. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to assess the ¹⁸O tracer diffusion in strained LSC films in order to clarify the role of strain on the oxygen exchange and diffusion kinetics. For this purpose, epitaxial thin LSC films (~20 nm) with a 1.4% tensile strain on single-crystal SrTiO₃ (100) and with a -1.5% compressive strain on LaAlO₃ (100) were grown by pulsed laser deposition. Ex-situ X-ray diffraction reciprocal space mapping was used to confirm the film strain state. The OR reaction kinetics was investigated by oxygen isotope exchange experiments in ¹⁸O-rich atmosphere from 300-450°C and the subsequent in-depth analysis using ToF-SIMS. Oxygen isotope concentration profiles were measurable despite the very small film thickness (i.e. 20 nm) and thus the surface exchange coefficients k and diffusion coefficients D could be calculated. It is shown that both k and D of LSC films are strongly influenced by their strain states. The tensile strained LSC exhibits much faster oxygen exchange kinetics in the investigated temperature range, with k being about a factor 4 higher and D being almost an order of magnitude higher compared to compressively strained LSC. The different ¹⁸O concentration profile of differently strained LSC films directly proved our hypothesis that the lattice strain in oxides can alter the OR kinetics significantly, consistent with the changes in oxygen stoichiometry and electronic structure determined previously.[1, 2] These results represent an initial set of direct correlations of oxygen incorporation kinetics to the strain state on epitaxial thin film perovskite cathodes. In addition, this method can be applied to other strained oxide cathode material systems, where a fundamental understanding of the oxygen incorporation behavior at the interfaces and surfaces is required in order to enable low temperature oxygen reactivity and mobility. [1] J. Am. Chem. Soc., 2011, 133 (44), 17696 [2] J. Mater. Chem., 2011, 21, 18983

Yttria-stabilized zirconia (YSZ) and gadolinia doped ceria (GDC) are very interesting materials for industrial applications such as fuel cells or electrochemical sensors, due to their stability and moderately high ionic conductivity. Recently, nanoscale thin films are reported to have highly improved conductivity. The presence of interfaces and lattice strain are suggested as possible causes, but the results remain controversial and difficult to repeat. In this work, the fabrication process and structural properties of RF sputtered strained zirconia and ceria based thin films are analyzed. Under the deposition conditions used in this study, cubic zirconia and ceria based thin films have been obtained with a thickness from ~100 nm to ~6nm. The ionic conductivity of the strained nanoscale thin films was determined in the temperature range from 673 K to 1073 K, from which the effect of interfaces and lattice strain on ionic conductivity are analyzed.

The local environment of Ce and Gd ions in CeO₂-Gd₂O₃ solid solutions, among the most important of oxygen ion conductors, was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). From X-ray diffraction (XRD) measurements of the powdered samples, specially prepared to maximize homogeneity and crystallinity, we found that the unit cell volume reaches a maximum between 20 and 25 mol% Gd, which is the composition range in which fluorite (Fm-3m - CeO₂) structure is observed to transform into double fluorite (Ia-3) structure. The average near neighbor Gd-O distance, as deduced by EXAFS, decreases gradually with increasing Gd content, whereas the average Ce-O distance drops sharply at the phase transition. An abrupt change in the local environment of Ce, which is more likely to contain oxygen vacancies than that of Gd, is also supported by the XANES spectra. Efforts at extending or modifying the oxygen conductive Fm-3m phase should therefore concentrate on tailoring the local environment of the Ce ion. Extension of these findings to other similar solid solutions would be based on the concept that the structural element controlling phase stability is the cation with the larger number of near neighbor oxygen vacancies.

Solid oxide fuel cells (SOFCs) provide means for converting chemical directly to electrical energy with high efficiency. A key limitation in low temperature operation of SOFCs, desirable for portable applications and use of alternative sealing materials, is sluggish electrode kinetics. The use of non-stoichiometric oxides has been shown to improve electrode performance due to the presence of both electronic conductivity, typically through electron hopping along multivalent elements, and oxide ion conductivity. At the same time, the dependence of electronic and ionic carrier concentration on temperature and oxygen partial pressure, typically exhibited by these non-stoichiometric oxides also results in a defect induced, chemical expansion. This can lead to large stresses which, in turn, has been shown to contribute to mechanical failure in SOFC and oxygen permeation membrane devices. Recently, we demonstrated that the origin of chemical expansion, in the two key fluorite oxides for SOFC technology, ceria and zirconia, is the result of two competing processes, the formation of an oxygen vacancy (leading to a lattice contraction) and a cation radius change (leading to a lattice expansion). A key finding was that lattice contraction from oxygen vacancy formation was larger in zirconia than in ceria and drawing on this, we show that a solid solution of ceria-zirconia displays a chemical expansion coefficient approximately half that of ceria. Experimental and computational investigations, undertaken to determine the chemical expansion coefficient in ceria-zirconia, will be discussed along with interpretations of the origin of the decreased chemical expansion coefficient.

Gadolinia-doped ceria, Ce_0.9Gd_0.1O_1.95, (GDC) is commonly used as as a solid oxide fuel cell (SOFC) electrolyte and electrode component. . Typically, GDC electrolytes have relative densities between 92 and 100%, while GDC electrode scaffolds have relative densities that range from ~45 to 80%. A knowledge of the mechanical behavior of porous GDC is therefore essential for the design and manufacture of mechanically robust SOFCs. Unfortunately, no systematic studies of the mechanical properties of GDC as a function of relative density currently exist in the literature. In this study, the room-temperature mechanical properties (Young&’s modulus, shear modulus, Poisson&’s ratio, hardness and fracture toughness) of cold die pressed and sintered GDC with relative densities ranging from roughly 70 to 95% were investigated. A non-destructive resonant ultrasound spectroscopy (RUS) technique was used to measure the elastic moduli, while hardness and fracture toughness were evaluated via Vickers indentation.

One of the major challenges of Solid Oxide Fuel Cell (SOFC) devices is to improve their long-term durability and decrease their costs to achieve market penetration. One way to achieve this is by lowering their operating temperature down to 500-600 °C, so that cheap metallic interconnects can be used. However, this requires materials with higher activities. For example, at 500-600 °C, the conductivity of yttria stabilized zirconia, the current choice for the electrolyte, is not enough to efficiently run a SOFC and, for this reason, more conducting materials have to be used in this temperature range [1]. Several such materials have been suggested, such as doped CeO2, Bi2O3, apatite materials and doped LaGaO3. Here we focus on the latter. In this study, we use a combination of Density Functional Theory (DFT) and Molecular Dynamics (MD) calculations to study the factors affecting the conduction mechanism of this material. First the DFT calculations are used to parameterize a polarizable interionic potential [2]. These potentials are then tested by reproducing a series of static and dynamic experimental properties (lattice parameters, thermal expansion coefficients, ionic conductivities and activation energies). These potentials are then used to perform long (~ 10 ns) MD simulations to study in depth the conduction mechanism of this material under realistic SOFC conditions (high temperature, large concentrations of defects). We find that the vacancies in doped LaGaO3 tend to order with respect to the dopant cation and, also, with respect to each other. We show how these ordering tendencies affect the conductivity of this material and draw a comparison with previous studies on fluorite-structured materials [3-4]. [1] E Wachsman et al., Science 334, 935 (2011) [2] Burbano et al., Journal of Physics: Condensed Matter 23, 255402 (2011) [3] Marrocchelli et al., Chemistry of Materials 23, 1356 (2011) [4] Burbano et al., Chemistry of Materials 24, 222 (2012)

With ionic conductivities superior to conventional doped zirconias and cerias at intermediate temperatures (IT, 700-800 °C), bismuth oxide (BiO_1.5) materials based on the defect fluorite structure have the potential to reduce the operating temperatures of solid oxide fuel cells (SOFCs). In order to investigate the energetics of stabilized BiO_1.5 in the fluorite structure, DyO_1.5-stabilized BiO_1.5 (DSB) over a range of compositions was synthesized by solid state reaction and quenched. Using high temperature oxide melt solution calorimetry in molten 3Na₂O-4MoO₃ at 702 °C, enthalpies of formation were determined. Relative to the room temperature phases of the oxide end-members, the formation of Bi_1-xDy_xO_1.5 is endothermic at x < 0.30, and becomes slightly exothermic near the upper phase boundary (x = 0.50). A negative interaction parameter for mixing in the solid solution suggests a tendency for short range ordering, and the increasingly exothermic ΔH_mix with increasing x parallels the trend that conductivity decreases with increasing dopant content. Results will be discussed in light of low temperature transformations, structure-conductivity relationships, and component volatility.

In recent years there has been an increasing interest in the perovskite (ABO₃) phase Na_0.5Bi_0.5TiO₃ (NBT) as a promising lead-free piezoelectric material to replace Pb(Zr_1-xTi_x)O₃ -based system. One of the main drawbacks of NBT for piezoelectric applications is the high leakage current, the origin of which is, however, not clear. Impedance Spectroscopy (IS) studies on NBT polycrystalline samples show that low levels of A-site nonstoichiometry significantly changes the bulk electrical conductivity and conduction mechanisms. In particular, nominal compositions with Bi-deficiency exhibit bulk conductivity ~ 3-4 orders of magnitude higher than that in compositions with Bi-excess. Impedance data suggest the high conductivity in Bi-deficient compositions is related to oxygen ion conduction, whereas electronic conduction dominates insulating Bi-excess compositions. Electromotive Force (EMF) measurements using air/nitrogen gas confirm the presence of oxygen ionic conduction with high ionic transport number > 0.9 at 600-700 ^oC. In addition, acceptor (Mg) doping on the B-site can further increase the ionic conductivity to ~ 0.01 S cm^-1 at 600 ^oC as well the electrolyte stability in 5%H₂/95%N₂ up to 550 ^oC. In summary, low level of nonstoichiometry in NBT can induce a switch between electronic and ionic conduction accompanied with a significant change in electrical conductivity. This work not only points out a possibility of NBT being used as an ionic conductor apart from piezoelectric applications, but also raises a general question about the influence of A-site deficiency on the conduction mechanisms in titanate-based perovskites.

An SOFC in operando is a complex entity with many different processes occurring throughout the different materials involved. The majority of analytical techniques available to the scientist are ex situ techniques and relating data obtained in this manner to an operating SOFC involves many assumptions. It is much more desirable to study SOFC materials in situ under conditions as close to operation as possible. In this study we present in situ X-ray absorption near edge spectroscopy (XANES) and AC impedance spectroscopy (ACIS) data collected simultaneously on symmetrical cells at 600 °C. This required the development of a custom furnace that utilised infra-red heating to achieve a compact design with short heating/cooling times. The materials studied were the layered Ruddlesden-Popper phases La2NiO4+δ, La4Ni3O10-δ, and composites of the two. These are known to show promise as potential SOFC cathode materials. XANES allows for the redox properties of nickel to be investigated; this is known to be key in the oxygen reduction and incorporation reaction as well as governing the number of oxygen vacancies or interstitials. ACIS allows for the properties of the electrode as a whole to be studied. For the first time it is possible to correlate these data to gain further understanding of the materials and processes involved. It was found that composite electrodes showed improved performance over single-phase electrodes, that the redox chemistry is dominated by thermal effects, and that DC bias causes improvement in the electrode properties but no change in the nickel oxidation state.

Perovskite-type oxides of general formula LnBaM2O6-δ (Ln=lanthanide M= transition metal) can show layered-type ordering of the Ba and Rare Earth atoms. The ordered compounds have different properties than the oxides with a random distribution of the cations. In this sense, higher oxygen diffusion has been reported in ordered GdBaMn2O6-δ than in the corresponding disordered phase (1). This fact has originated an increasing interest in materials based on layered perovskites as possible cathodes for intermediate temperature-operating solid oxide fuel cells. Interesting electrical properties and electrochemical performance of BaGdCo2O5+δ have been reported (2). Studies on double perovskites such as oxygen exchange in polycrystalline samples of PrBaCo2O5+δ have demonstrated that the oxygen kinetics in this material are significantly faster than in disordered perovskites (3). Recent studies of SmBaCo2O5+δ have also demonstrated a high performance of this oxide, at intermediate temperature ranges, as a cathode on a Sm0.2Ce0.8O2 electrolyte (4). We are studying the influence of the cation ordering and the microstructure in the electronic properties of Gd0.5Ba0.5B1-xB&’xO3-δ (B, B&’= Mn, Co, Fe). Preparation of these materials under different reducing or oxidizing conditions have a significant effect on Charge Ordering and in the ordering of both the Gd/Ba cations and anion vacancies. We have studied these materials by Selected Area Electron Diffraction and High Resolution Transmission Electron Microscopy. The ordered phases GdBaB2-xB&’xO6-δ show different superstructures of the perovskite-type, associated to CO effects and ordering of cations and/or vacancies. Superstructure formation also affects the electronic properties of these materials. We present in here a study of the the behaviour of some of these oxides as cathodes for SOFCs as well as the composition-crystal structure- microstructure properties relationships. References: 1. Taskin A. et al. Progress in Solid State Chemistry 35, (2007), 481; Kim, G. et al. J. Mater. Chem. 17, (2007), 2500. 2. Chang A. et al., Solid State Ionics 177, (2006), 2009; Tarancoacute;n A. et al. J. Mater. Chem. 17, (2007), 3175. Tarancoacute;n A. et al. Solid State Ionics 179, (2008), 2372. 3. 5. Kim, G. et al.J. Mater. Chem. 17, (2007), 2500. M. Burriel et al. Chem. Mater. 24(3), (2012), 613. 4. Zhou et al. J. Power Sources 185, (2008), 754.

Recently, Co-free Bi-Sr-Fe-based perovskite-type oxides have been highly expected as a cathode material of IT-SOFC [1], even though bismuth containing oxides generally have been studied as electrolyte materials and gas sensors because of their high ionic conductivity. Our group has reported high oxygen flux and mixed ionic and electronic conductivity for Bi_1-xSr_xFeO_3-δ system in the wide range of Sr concentration [2]. In addition, Wedig et al have reported that a partial occupation of A-site by the highly polarizable Bi³⁺ improves the surface oxygen exchange kinetics for SrFeO_3-δ [3]. Nonetheless, their poor electronic conductivity limits the application as a cathode of IT-SOFC. In this study, we have selected lanthanum as a further dopant on Bi-site of (Bi, Sr)FeO_3-δ, which might cause an increase in electronic conductivity and thus cathode property. The purpose of this study is to prepare Bi_0.7-xLa_xSr_0.3FeO_3-δ (BLSF) perovskite-type oxides, and to clarify their crystal structures and electrochemical properties including defect equilibrium. BLSF with dopant concentration of 0, 10, 30, 50 and 70 mol % La were prepared by using a solid-state reaction technique. Single phase with perovskite-type structure was prepared over the whole La-doped samples. BSF is a cubic perovskite-type (221, Pm-3m) while LSF has orthorhombic symmetry (62, Pbnm). The structural change from cubic to orthorhombic was found between x = 0.1 and 0.3. Substitution for Bi by La also resulted in a decrease in lattice volume, which suggests change in defect equilibrium; La-Sr-Fe-based perovskite-type oxide favors charge compensation by electron holes than Bi-Sr-Fe-based oxides that energetically prefer the formation of oxygen vacancies. Electrical conductivity in the temperature range of 300 to 850 °C in air was enhanced by La-doping as expected. This implies that La-doping is clearly advantageous for raising the electron hole concentration. Meanwhile, oxygen permeability tests were conducted to clarify their minor oxide-ion conductivity and surface exchange kinetics. Electrochemical properties of BLSF will be also discussed. [1] Y. Niu, J. Sunarso, W. Zhou, F. Liang, L. Ge, Z. Zhu and Z. Shao, Int. J. Hydrogen Energy, 36 (2011) 3179. [2] K. Brinkman, T. Iijima, and H. Takamura, Solid State Ionics, 181 (2010) 53. [3] A. Wedig, R. Merkle, B. Stuhlhofer, H. Habermeier, J. Maier and E. Heifets, Phys. Chem. Chem. Phys., 13 (2011) 16532-16533

Mixed oxide-ion and electronic conductors (MIECs) have been attracting much attention for use as a cathode of SOFCs and as an oxygen permeable membranes. Among a number of MIECs with a perovskite-type structure, a Ba-Sr-Co-Fe-based oxide, especially Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), exhibits excellent mixed conductivity and the resultant oxygen transport property. To further extend the application of BSCF, its redox and oxygen transport properties at relatively low temperatures, i.e. 350 to 700°C were investigated in this study. BSCF powders were prepared by a solid-state reaction technique. The sample sintered at 1100 to 1200°C was a single phase with a perovskite-type structure. For oxygen permeation tests, 100 mu;m-thick BSCF membranes with or without surface modification were prepared by using tape-casting and screen-printing techniques. Oxygen nonstoichiometry was measured by a Sievert-type apparatus and thermogravimetry. Table 1 summarizes the redox behavior of BSCF powders at 350 and 500°C measured by changing air and hydrogen atmospheres at five-minutes intervals. Oxygen storage and release capacity exceeding 200 mu;mol-O₂/g was stably and reversibly observed. This suggests that BSCF can be utilized as a support of automobile catalysts. Oxygen permeation tests were also performed at low temperatures under air-helium atmospheres. The 100mm-thick BSCF membranes with a bare surface exhibits an oxygen flux of 1.2 mu;mol/cm²s at 700°C and an activation energy of 182 kJ/mol. This large activation energy implies that the oxygen permeation of 100 mu;m-thick BSCF membrane is limited by surface exchange kinetics. By applying a catalyst layer, the activation energy was reduced down to 55 kJ/mol, which shows a good agreement with that of bulk diffusion coefficient of oxygen vacancy in BSCF reported by Wang et al. The oxygen transport and redox behavior of BSCF at low temperatures will be further discussed in the context of surface exchange kinetics.

Novel anodes for SOFC component layers have been prepared using three scalable techniques: Continuous hydrothermal flow synthesis (CHFS), screen printing and electroless and electrolytic deposition of metals. 1.- CHFS is feasible for producing industrial-scale quantities of YSZ and CGO nanoparticles (>kg/h at the university level) . We have synthesised a range of phase pure electrolyte nanomaterials Ce0.9Gd0.1O2, Ce0.8Sm0.2O2, (Y2O3)0.08(ZrO2)0.92. Characterised by XRD, BET and FEG-SEM. These materials show particle sizes of around 5 nm, surface areas up to 200 m2g-1 and are phase pure. 2.- Screen printing is currently used in fuel cell technology and widely used in other industries. Inks containing these materials with pore formers have been produced and screen printed onto YSZ electrolytes (300 microns thick), producing a range of porous scaffolds (ca. 10 microns) well bonded to the electrolyte and with open and controlled porosity. These have been structurally characterised by XRD and SEM. 3.- Electroless and electrolytic deposition techniques are used for the first time for the fabrication of anodes in porous scaffolds. The scaffolds prepared by screen printing have then been impregnated with nickel and copper using conventional impregnation and/or electroless deposition and electrodeposition techniques. The electrochemical properties of these anodes have been preliminarily assessed and operational cells Ni/YSZ/LSM have been produced to show the potential of these three technologies. Selected samples have been analysed by FIB-Tomography techniques to obtain a 3-D analysis of the microstructure.

We will review synchrotron x-ray techniques for in situ investigation of model thin film cathode systems for solid oxide fuel cells. The advanced x-ray techniques to be covered in the talk include x-ray reflectivity, total external reflection fluorescence spectroscopy, depth sensitive absorption spectroscopy (XANES and EXAFS), and small angle x-ray scattering (SAXS). In situ sample cells examined in this study were made of LSM, LSC, and LSCF thin films epitaxially grown on YSZ single crystal substrates by the pulse laser deposition technique. We will present and discuss recent results obtained from these model cathodes. In collaboration with K.-C. Chang, B. Ingram, E. Perret, J. Eastman, P. Fuoss, P. Salvador, B. Yildiz. This work and use of the Advanced Photon Source were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Oxygen transport kinetics in ceramic materials play a key role in determining the efficiency of solid oxide fuel cells (SOFC). Among the different methodologies to study oxygen transport, the isotope exchange depth profiling (IEDP) method, which combines ¹⁸O-tracer exchange experiments and secondary ion mass spectrometry (SIMS) depth profiling, has proved to be a useful tool to estimate oxygen tracer diffusion coefficients (D^*) and surface exchange coefficients (k^*). Nevertheless, the accurate estimation of the surface exchange coefficient (k^*) is challenging due to the transient signals occurring during the initial stages of SIMS depth profiling. In this sense, low-energy ion scattering (LEIS) represents an exceptional tool for the study of oxygen exchange process in the gas-solid interface due to its single-atomic-layer sensitivity, which would allow the reliable determination of the surface ¹⁸O concentration. Furthermore, additional information on the exact cation composition of the outermost surface atomic layer can give an invaluable and unique insight of the steps occurring in the oxygen-incorporation process and about the relationship between surface chemistry and oxygen reduction reaction (ORR) kinetics, which are still not well understood. Moreover, surface compositional modifications during the exchange at high temperatures (e.g. cation segregation) can be obtained by post-anneal analysis using LEIS. In this work, we show the LEIS analysis of the topmost surface composition of (La_0.5Sr_0.5)₂CoO₄ thin films (LSC214) grown on single-crystal SrTiO₃ (001) and LaSrAlO₄ (100). These results provide an accurate estimation of the oxygen surface concentration and a full insight into the compositional surface modifications taking place during the exchange process. By studying films with different orientations (i.e. (001) and (100)) and different surface treatments (e.g. HCl etching) we have been able to investigate the oxygen incorporation and the factors limiting the oxygen exchange process, including cation segregation and secondary phase formation occurring at high temperatures.

A major challenge in solid oxide fuel cell (SOFC) cathode performance is the slow kinetics of oxygen reduction on perovskite oxide surfaces, particularly at the lower temperatures (T<700 oC) desired for material durability. For development of cathodes with enhanced electrocatalytic activity, a fundamental understanding of the relation of surface chemistry to oxygen reduction kinetics on the perovskite-type oxides is important. This open question is further complicated by segregation of cations onto the surface of the cathode. However, a unified theory that explains the origin of the cation segregation on the complex oxides does not yet exist. Here we quantitatively assess the electrostatic and elastic interactions of the dopant with the surrounding lattice as the key driving forces to cation segregation on perovskites, using density functional theory (DFT) calculations. The manganite based perovskite oxide (ABO3 with B=Mn), a widely studied SOFC cathode material, is chosen as the model material system. To systematically induce elastic energy differences in the system, the radius of the selected dopant cations is varied with respect to the host cation on the A-site - La3+ and Sm3+ as the host and Ca2+, Sr2+, and Ba2+ as the dopant cations. Electrostatic energy differences are induced by controlling the distribution of charged oxygen- and cation-vacancies. Our results show that the larger dopant cation, which introduces larger elastic energy into the system, tends segregate more strongly toward the surface. Electrostatic attraction of these dopants to surface oxygen vacancies also favors stronger surface segregation. A physical model based on the cation radius and charge state, using the database generated by these DFT calculations, is constructed to quantitatively assess and predict the enhancement or depletion behavior of dopants on the perovskite surface. Our results demonstrated that both the elastic and electrostatic interactions of the dopant with the surrounding lattice are major driving forces to describe the surface segregation phenomena. The relative importance of these two driving forces in various perovskite compounds is discussed.

Continued advancement of fuel cell technologies requires a greater understanding of the complex governing electrochemical processes through in-situ studies within their actual operating regime. Scanning probe microscopy provides an array of methodologies capable of examining a vast range of phenomena on the exact scale which they occur, critical to the fundamental understanding of these complex processes and discovering the origin of performance detriments. Here the design and development of a miniature reaction chamber for use with a standard commercial atomic force microscope while isolating the required fuel cell operating conditions (temperature, gas environment) from the surrounding ambient environment is presented. Variable temperature scanning surface potential microscopy (VT-SSPM) studies were performed of perovskite La_0.8Sr_0.2FeO₃ (LSF) infiltrated-scandia stabilized zirconia (ScSz) electrode - ScSZ electrolyte symmetrical cells to distinguish functional property variations within the active zone region. The response of the electrode/electrolyte interface manifests itself as sharp, localized potential gradients within a < 10 mu;m region of the electrode adjacent to the electrolyte. Comparisons between LSF and La_0.8Sr_0.2MnO₃ (LSM) - infiltrated electrodes on both ScSZ and yttria-stabilized zirconia (YSZ) electrolytes will also be presented and discussed.

Much global research activity is being directed towards methods that will result in enhancement of the oxide ionic conductivity in oxide materials. Such materials are needed for application in a new generation of efficient and economic solid-state electrochemical devices such as reduced-temperature Solid Oxide Fuel Cells (SOFCs). There have been numerous reports in the literature of significantly enhanced ionic conductivity in multilayer heterostructures formed from ionic conductors and insulators. [1-3] There have also been a number of reports suggesting that these enhancements are due to electronic rather than ionic conductivity. [4,5] Many of these reports have focused on heterostructures between SrTiO3 (STO) and yttria-stabilised zirconia (YSZ). [3,5] It would be highly informative to understand how the lattice mismatch (>7%) between STO and YSZ is accommodated. It has been suggested that a highly disordered region is present at each interface in a trilayer structure, which results in high local concentration of oxygen vacancies. Electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has been used to address this question but close proximity of the atomic numbers (Sr:38, Y:39, Zr:40) make overlap of ionization edges problematic. [5,6] In this contribution we will report on the new insights obtained using high resolution energy-dispersive X-ray (EDX) analysis in the STEM. The new generation of detectors allows for near-atomic scale spatial resolution chemical mapping with high signal-to-noise ratio. Using this method we have obtained quantitative information on the atomic scale fluctuations in cation distributions, which give unforeseen insights into the strain-accommodation within the STO/YSZ heterostructures. We will discuss the difference between STO/YSZ and YSZ/STO interfaces and the relationship between the composition, structure and properties in these materials. [1] Kosacki, I., Rouleau, C. M., Becher, P. F., Bentley, J., & Lowndes, D. H. (2005). Solid State Ionics, 176(13-14), 1319-1326. [2] Sata, N., Jin-Phillipp, N. Y., Eberl, K., & Maier, J. (2002). Solid State Ionics, 154, 497-502. [3] Garcia-Barriocanal, J., Rivera-Calzada, A., Varela, M., Sefrioui, Z., Iborra, E., Leon, C., Pennycook, S.J. & Santamaria, J. (2008). Science, vol. 321, no. 5889, pp. 676-680. [4] Guo, X. (2011). Scripta Materialia, 65(2), 96-101. [5] Cavallaro, A., Burriel, M., Roqueta, J., Apostolidis, A., Bernardi, A., Tarancon, A., Srinivasan, R., Cook, S.N., Fraser, H.L., Kilner, J.A., McComb, D.W. & Santiso, J. (2010). Solid State Ionics, vol. 181, no. 13-14, pp. 592-601 [6] Pennycook, T.J., Oxley, M.P., Garcia-Barriocanal, J., Bruno, F.Y., Leon, C., Santamaria, J., Pantelides, S.T., Varela, M. & Pennycook, S.J. (2011). European Physical Journal-Applied Physics, vol. 54, no. 3, pp. 33507

Multi-layered (Y2O3)0.08(ZrO2)0.92 / (Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 (YSZ/SCSZ) electrolytes have been designed, so that the inner SCSZ layers provided superior ionic conductivity and the outer YSZ skin layers maintained good chemical and phase stability. Those YSZ/SCSZ electrolytes with 3, 4 and 6 layers design were manufactured via tape-casting, laminating and sintering. After sintering, the thickness of YSZ outer layers were kept constant at ~30 mu;m, the thickness of SCSZ inner layer varies from ~30 mu;m for a Y-SC-Y three layered electrolyte, ~60 mu;m for a Y-2SC-Y four layered electrolyte, and ~120 mu;m for a Y-4SC-Y six layered electrolyte. Due to the mismatch of coefficients of thermal expansion (CTEs) from each layer of different composition, the thermal residual stresses were calculated, and it was found the compressive residual stress in outer layers and tensile stress in inner layers appear at 800 °C operation temperature. The values of the residual stresses can be controlled by adjusting the layers thickness ratios. The biaxial flexure strength of layered electrolytes was tested via a ring-on-ring method at RT and 800 °C. In the primary analysis, the applied load and the normalized applied load at the fracture showed an improved strength in Y-xSC-Y type electrolytes at 800 °C in comparison of electrolytes with single YSZ or SCSZ composition; whereas at RT, the Y-xSC-Y type electrolytes showed lower strength. It was considered that the residual stresses due to the mismatch of thermal expansion might play an important role in determine the flexure strength of laminated electrolytes.

In Solid Oxide Fuel Cells (SOFCs) stacks, the formation of insulating phases along the cathode/electrolyte interfaces is detrimental to the individual cell performance. A classical example is the formation of SrZrO₃ along the interface of lanthanum strontium cobalt iron oxide (LSCF) cathodes and the yttria stabilized zirconia (YSZ) electrolyte. An “interlayer” consisting gadolinium doped ceria (GDC) is used to avoid reactions at the interface. Nevertheless, the SrZrO₃ formation seems to be still a problem depending upon the dense nature of the GDC interlayer. To understand the strontium (Sr) transport behavior through the GDC layer is, indeed, an important step towards effective design of diffusion barriers. We have investigated the cation diffusion from a dense film LSCF towards dense sintered GDC in diffusion couples. Both bulk and grain boundary diffusion of La into dense sintered GDC was noticeable, whereas the Sr, Fe, and Co diffusion was thought to be negligible. Similar results were found when a porous LSCF was used in the diffusion couple. In principle, our results are in agreement with recently published data pointing out the positive role of dense GDC as an effective diffusion barrier. In this work, LSCF (porous)/GDC(dense)/YSZ diffusion triplets were used to further investigate the diffusion behavior of Sr towards GDC when YSZ could act as a diffusion driving force and/or as a reaction sink for Sr. The PLD deposited GDC film was approximately 5-6 mu;m in thickness. The LSCF/GDC/YSZ triplets were annealed at 1100 ^oC and 1200 ^oC for 168 h. Crystalline phases were identified by a standard X-ray powder diffraction (XRD) procedure. The cross-sectional studies were performed using scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) analysis. A typical result can be described as follows: Distinctively, a new phase in form of spots grew along both the GDC/YSZ and the LSCF/GDC interfaces in all samples. XRD and EDX allow the identification of the new phase as SrZrO₃. In brief, the SrZrO₃ formation at the interfaces, in the temperature range investigated, suggests a dominant and fast grain boundary diffusion of Sr across the dense GDC interlayer. Acknowledgement: Part of this study was financially supported by NEDO, Japan under the Project “SOFC system and elemental technology development”.

Using single crystal epitaxial thin films grown at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL), hard x-ray photoelectron spectroscopy (HAXPES) studies were conducted at National Institute of Standards and Technology beamline X24A of the National Synchrotron Light Source at Brookhaven National Laboratory. Samples studied were 20 percent strontium A-site doped lanthanum manganite (LSM-20) and 40 percent strontium A-site doped lanthanum strontium cobalt ferrite (LSCF-40). Oxygen reduction at the cathode of a working device is dependent on both the cathode composition and the transition metal oxidation states. To understand the metal oxidation states, HAXPES measurements were performed. HAXPES offers some advantages over lower energy XPS techniques. Notably, higher incident photon energies permit more bulk-sensitive analysis, minimizing the contribution of surface contamination thus allowing material analysis without the need to treat the sample in vacuum. Also, HAXPES allows for a tunable penetration depth, so correlations can be made between the depth-dependent cation oxidation states determined with this method and the depth-dependent composition as measured by Total Reflection X-Ray Fluorescence (TXRF). Systematic changes in the binding energies of strontium were observed in LSCF-40 samples annealed at 800°C in air and quenched after 0.5, 1, 2, 5 and 20 hours. For LSM-20, an as-deposited sample was compared to samples quenched from 700°C and 800°C in oxygen partial pressures of 0.30 atm and 0.030 atm. The results of these studies will be discussed.

In today&’s world affected by energy shortage, cleaner and more efficient fuel cells offer promising alternatives. Solid oxide fuel cells (SOFC) process traditional fuels to produce electricity and syngas, which can further be processed via the Water-Gas Shift (WGS) reaction to yield additional hydrogen fuel and to concentrate the CO2 for potential capture. Traditionally, for WGS reactions, metals have been treated as the primary reactive substrates. Recent work, however, divulges that the support, in addition to providing structural integrity, also participates in the overall reaction. A variety of support materials, e.g. CeO2, Al2O3, Si and MgO have been studied for the WGS reaction of which ceria exhibits the highest activity [Wheeler et al., (2004), J. of Catal., 223]. The goal of this work is to determine the surface phase diagram of ceria under various relevant chemical environments, at ambient and extreme temperatures and pressures using first principles thermodynamics (FPT) and density functional theory (DFT) calculations. Developing a strategy for the high-fidelity prediction of such surface phase diagrams will be of broad importance, as it will allow us to rapidly identify regions of catalytic interest of a given system under a given set of chemical conditions [Reuter & Scheffler, (2003), PRL, 90(4)]. Our DFT work has been carried out at two levels of sophistication to critically examine methods to capture energetics accurately. Specifically, using our FPT strategy, we explore the non-stoichiometry of the ceria (111) surface as past studies indicate its high stability (and likelihood of being prevalent under experimental conditions). Oxygen desorption/adsorption on the (111) surface could occur from the surface or the sub-surface layer, and our work indicates that desorption of sub-surface oxygen is thermodynamically favored similar to the results obtained earlier. Our surface phase diagrams are determined by considering a pure oxygen environment, as well as redox environments such as CO2/CO, NO2/NO and H2O/H2. The computed phase diagrams portray a trend wherein high temperatures and low pressures are required to observe any appreciable reduction for an undoped ceria surface. A comparison of several features of our predicted phase diagrams with available experimental data (e.g., stoichiometry at various temperatures, pressures and redox environments, and the predominance of sub-surface vacancies) shows remarkable agreement. The present contribution exemplifies the power of FPT techniques in quantitative predictions of surface phase equilibria and provides new opportunities for catalyst design.

The presentation addresses the identification of new cathode materials based on the synthesis of new complex oxides in bulk^1,2 and thin film³ form. In addition to conventional synthesis strategies we will discuss the extent to which computation can assist in the identification of suitable structures and compositions. This leads to modelling studies of potential structures at the interfaces within the topical YSZ-STO heterostructures⁴, where crystal chemical considerations lead to the suggestion of novel structural motifs at the interfaces between the constituent structural units. 1 Demont, A., Dyer, M. S., Sayers, R., Thomas, M. F., Tsiamtsouri, M., Niu, H. N., Darling, G. R., Daoud-Aladine, A., Claridge, J. B. & Rosseinsky, M. J. Chemistry of Materials 22, 6598-6615 (2010). 2 Li, M.-R., Kuang, X., Chong, S. Y., Xu, Z., Thomas, C. I., Niu, H., Claridge, J. B. & Rosseinsky, M. J. Angewandte Chemie-International Edition 49, 2362-2366 (2010). 3 Grygiel, C., McMitchell, S. R. C., Xu, Z., Yan, L., Niu, H. J., Giap, D., Bacsa, J., Chalker, P. R. & Rosseinsky, M. J. Chemistry of Materials 22, 1955-1957 (2010). 4 Dyer, M. S., Darling, G. R., Claridge, J. B. & Rosseinsky, M. J. Angewandte Chemie-International Edition 51, 3418-3422 (2012).

Global energy demand is projected to rise as high as 60% over the next 30 years. It is a challenging trend that may be met only by revolutionary breakthroughs in energy science and technology. Nanotechnology has the potential for making impact on this effort by developing the advanced materials, tools and devices that are more efficient. A unique aspect of nanomaterials is the significantly enhanced their properties related to increased relative surface or grain boundary area, and the dominance of quantum effects. These effects enhance chemical reactivity, making some nanomaterials useful as catalysts, sensors, and components of fuel cells and batteries to improve their efficiency. These devices play also important role in the development of novel energy conversion processes. Increased understanding of the structure and behavior of materials at the nanoscale is advancing our ability to design novel devices. In this lecture, the ability to use nanomaterials in energy conversion will be discussed. The ability to enhance the physical properties of oxygen - CeO2, ZrO2:Y, Sc and proton - SrCeO3:Yb conductors will be presented. Scaling factors, such as grain size, thin film thickness, and porosity will be discussed in the context of a lattice defect model, and will be illustrated by recent results obtained for nanoceramic thin films, nanoscale superlattices, and mesoporous materials. To demonstrate the advantage of nanomaterials, they were evaluated in solid oxide fuel cells and hydrogen gas sensors. The results showed an enhancement in power and current densities, sensitivity and response time for nanomaterial-based devices as compared to the commonly used materials.

This presentation reports on a facile, cost effective and versatile method to fabricate nanostructured, mesoporous NiO-Gd-CeO2 (GDC) Gd-CeO2, La1-xSrxFe1-yCoyO3+delta and GDC/LSCF films. These films were synthesized through the “sol-gel method” combined with the dip-coating approach. Compared to spray-pyrolysis, APCVD or ALD methods, the proposed method avoids the use of expensive equipment and metal-organic precursors. In addition, we focussed on relatively low temperature processes that allow retaining a well-organized nanostructure within the material. Different techniques were used to characterize the films: FE-SEM, XRD, XPS, HR-TEM. Additionally, thermal or porosimetry Ellipsometry were also performed to define the heat treatment that allows synthesizing films with controlled porosity and particle size and to characterize the porous network, respectively. Using X-ray diffraction techniques coupled with a modelling approach, we have studied the formation of particle into these mesoporous films and we have shown that particle growth is governed by the surface curvature within the films. Based on these results, a discussion on the stability of the film has been proposed. Electrochemical characterization has been performed to characterize our films under various conditions of temperature and atmosphere (air and 5%H2/N2). For NiO/GDC mesoporous films, we have developed an “in-situ” technique based on impedance spectroscopy to understand the mechanism of reduction of NiO to Ni into the mesoporous Gd-CeO2 films. These studies allow us designing the films with the adequate microstructure and composition. In parallel, we have extended this impedance based approach to the characterization of the cathode, GDC/LSCF. This study permits to propose a heat-treatment that conduces to the formation of highly conductive GDC/LSCF. Finally, these films were used to synthesize a micro-SOFC and performances will be discussed as function of the architecture of the micro-SOFC device.

There has been a substantial effort to increase the conductivity of electrolyte materials at lower temperatures for use in solid oxide fuel cells (SOFCs). This has led to interest in the enhancement of the ionic transport properties at oxide interfaces, which can be investigated through the fabrication of nanoscale membranes in the form of thin films and multilayers. Single thin films of yttria-stabilised zirconia (YSZ) grown on magnesium oxide (MgO) have been a popular system of study due to the high ionic conductivity and stability of YSZ coupled with the insulating properties of MgO. Results in the literature have been inconsistent with reported conductivity ranging from an enhancement of up to three orders of magnitude [1, 2] to a reduction by a factor of around four [3]. Lattice strain, dislocation density and dopant segregation are possible causes of these effects, however conclusive evidence still remains elusive and the interfaces have not yet been comprehensively characterised. In this work thin films of YSZ have been grown on (100) oriented MgO substrates by pulsed laser deposition (PLD). The objective was to fully investigate the influence of substrate roughness and surface chemistry on the nature of the YSZ films as a function of film thickness and PLD growth parameters. The structural and chemical properties of the substrates and thin films will be reported as characterised by x-ray diffraction (XRD), low energy ion scattering (LEIS), secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM) and electron microscopy (EM). Finally the resultant effect on the ionic transport properties based on electrochemical impedance spectroscopy (EIS) will be presented. [1] Sillassen, M., et al., Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized Zirconia. Advanced Functional Materials, 20(13):2071-2076, 2010. [2] Kosacki, I., et al., Nanoscale effects on the ionic conductivity in highly textured YSZ thin films. Solid State Ionics, 176(13-14):1319-1326, 2005. [3] Guo, X., et al., Ionic conduction in zirconia films of nanometer thickness. Acta Materialia, 53(19):5161-5166, 2005

In the course of the NEDO project on reliability and durability of solid oxide fuel cells (SOFCs), in Japan, and among the performance degradation factors identified for early stages of operation in several types of stacks, a noticeable increase in ohmic resistance has been reported. We advocated ourselves to understand this phenomenon by analysis of disassembled stacks and by reproducing the phenomena at a laboratory scale. We have initially focused on the electrical conductivity degradation of yttria-stabilized zirconia (YSZ) electrolyte considering the influence of processing conditions at rather high temperatures and ubiquitous dissolution of Ni in YSZ. As a main achievement we have shown the electrical conductivity degradation of Ni-doped YSZ to be accompanied by its transformation from cubic to tetragonal phases [1-2]. Furthermore, ex-situ micro-Raman Spectroscopy in mapping mode was used to determine the regions in which phase transformation takes place[3]. In this study, the influence of SOFC operating condition on the electrical conductivity degradation of Ni-doped YSZ electrolyte was investigated. Evidently, a decrease in operating temperature significantly suppressed the electrical conductivity degradation, however, unexpectedly a similar effect was found for electrical loading. Based on calculations of the oxygen potential distribution under SOFC operating condition, a low oxygen potential region in the Ni-doped YSZ was shown to be decreased by electrical loading. This result is in excellent agreement with the tetragonal transformed regions. It is concluded that, without doubt, the electrical conductivity degradation of Ni-doped YSZ is directly related with the tetragonal phase transformation. As we have proposed early [3], this transformation seems to be dominated by cation vacancy diffusion, which in turn, is induced by the reduction of the nickel dissolved in the YSZ lattice. [1] H. Kishimoto, K. Yashiro, T. Shimonosono, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, J. Mizusaki, MRS Online Proceedings Library (2012) 1385. [2] H. Kishimoto, K. Yashiro, T. Shimonosono, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, J. Mizusaki, Electrochimica Acta, in press (2012). [3] T. Shimonosono, H. Kishimoto, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics, in press (2012).

Ceria (CeO₂) is a material which has found applications in several technologically relevant areas, such as, catalysis and energy production. As a result of this, the defect structure and the processes controlling the ionic diffusion in doped and anion deficient ceria have been widely studied over the years.¹ Rare earth (RE) doped ceria (Ce_1-xRE_xO_2-x/2) plays a key role as the electrolyte in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs), which operate between 873 K and 1073 K. The aliovalent doping of ceria with RE metals (RE₂O₃) leads to the formation of charge-balancing vacancies which act as charge carriers. In particular, yttria doped ceria (YDC) combines good ionic conductivity (σ_i asymp; 5.0x10^-3Scm^-1 at 873 K) and relatively low production costs. There remains, however, a lack of clarity surrounding the nature of the defect interactions that lead to the rise and fall in conductivity that is observed as the dopant concentration is varied in these systems. Modern simulation techniques have become a mainstay within the materials science community, not only because they afford researchers information which is complementary to their experiments, but also because they can serve as predictive tools. The work presented here rationalizes the ionic conductivity process in YDC under typical operating temperatures using a combination of Density Functional Theory (DFT) and Interatomic Potential (IP) simulations along with neutron diffraction and impedance spectroscopy data.^{2, 3} These results are also compared to simulations for Ce_1-xRE_xO_2-x/2, where RE = Sc, La, Nd, Sm and Gd, which use newly developed IPs parameterized from hybrid-DFT calculations. This series of experiments and calculations served to identify how cation-vacancy and vacancy-vacancy interactions determine the observed maximum in conductivity. It was found that even in the case where the effects of cation-vacancy association are minimized, it is the interaction between vacancies which ultimately limits the conductivity in these fluorite-structured electrolytes. 1. Mogensen, M.; Sammes, N. M.; Tompsett, G. A., Solid State Ionics 2000, 129 (1-4), 63-94. 2. Burbano, M.; Marrocchelli, D.; Yildiz, B.; Tuller, H. L.; Norberg, S. T.; Hull, S.; Madden, P. A.; Watson, G. W., J. Phys.: Condens. Matter 2011, 23, 255402. 3. Burbano, M.; Norberg, S. T.; Hull, S.; Eriksson, S. G.; Marrocchelli, D.; Madden, P. A.; Watson, G. W., Chem. Mater. 2012, 24 (1), 222-229.

Direct ethanol fuel cells (DEFCs) allow easy handling and storage of high energy density liquid ethanol; therefore provides a promising alternative to hydrogen fuel cells in portable applications. It was demonstrated that up to 96% of 1M ethanol can be directly oxidized with Pt/Ru catalysts at temperature of 145°C. It is expected that further oxidation at higher temperatures is achievable to attain maximized fuel efficiency. Currently, most DEFCs are demonstrated with polymer-based electrolyte membranes with cell operating temperatures limited below 150°C. Increasing the operating temperature of DEFC is advantageous, not only to fully oxidize the ethanol fuel on anode, but also to enhance the oxygen reduction kinetics on cathode. However, the conductivity of polymer electrolyte membranes decreases drastically with further increased temperature due to the loss of water content in the electrolyte that is strongly associated with effective ion conduction. Another common inconvenience of polymer electrolyte is the fuel crossover from anode to cathode that promotes the adsorption of the fuel and its intermediates onto the cathode surface, which blocks the oxygen reduction reaction (ORR). On the other hand, a proton-conducting ceramic electrolyte can be an alternative for DEFCs operating at temperature above 150°C. Ceramic electrolyte has no fuel crossover due to a dense structure and doesn&’t need water management for proton-conduction. Furthermore, ceramic electrolyte needs higher operating temperatures than current high temperature polymer electrolyte DEFC, which helps to accelerate the electrode reactions kinetics, enhance anode CO tolerance, and virtually eliminate fuel crossover through electrolyte membrane and water. In this work, the polymer electrolyte membrane is substituted with yttrium-doped barium zirconate(BYZ) to overcome the above-mentioned problems. BYZ is a proton conductor that has high ion conductivity over other conventional oxide ion conductors. The proton conductivity of BYZ at targeted temperature is 10-3 Scm-1. For a operating temperature range of 200°C and 450°C with a targeted area specific resistance (ASR) value of 0.15 #8486;cm2, it requires the thickness of BYZ to be in the range of 750 nm to 20 µm. A nanoscale thin film micro fuel cell was fabricated on micromachined Si substrate with the help of lithography and etching techniques. We deposited BYZ electrolyte by pulsed laser deposition (PLD) on silicon substrate with help of photolithography techniques to fabricate a micro fuel cell structure. Both anode and cathode of porous Pt alloy were deposited by DC magnetron sputtering. Electrochemical impedance spectroscopy (EIS) and I-V characterization were performed to verify the cell performance at temperature range of 200°C and 450°C.

In this work we use density functional theory calculations to address the proton incorporation process of a ceramic membrane in proton conductor-based solid oxide fuel cells (SOFCs). We consider the simple case of a model SOFC based on yttrium-doped barium zirconate as the proton-conductor perovskite oxide. Further, to study the proton incorporation process at the triple phase boundary (TPB) regions in the anode of SOFCs, we consider both palladium and nickel metal particles in contact with the (100) surface of the cubic perovskite oxide. To achieve a full understanding of the proton incorporation process, we compute (i) the energy and migration energy barriers of protons in the bulk and at the surface of Pd and Ni, (ii) the energy of protons in the bulk and at the surface of Y-doped barium zirconate, and (iii) the energy of protons in proximity of the interface of a Pd/Ni metal particle with the gas phase and the ceramic membrane. Our calculations show that protons resulting from the dissociation of gaseous H-containing species on the surface of metallic particles diffuse readily on the surface and need to overcame energy barriers of about 0.4 eV to penetrate into the bulk of the metallic particles. With respect to (half) the energy of gaseous molecular hydrogen, protons exhibit an energy stability of about -0.3 eV and -0.5 eV on the (111) surface of Pd and Ni, respectively. In the bulk of these same metals, the energy of a proton increases to about 0 eV and 0.1 eV, respectively. In the bulk of doped barium zirconate, protons attain an energy of about -1 eV, and when bonded to oxygen ions in the first coordination shell of Y dopands the energy drops to -1.3 eV. At the BaO and ZrO2 terminations of the (100) surface, the energy of protons drop to values of about -2.1 and -2.6 eV, respectively. These results show that protons form very stable hydroxyl species on the surface of the perovskite oxide and that to excape these surface traps, protons need to overcome energy barriers larger than 1.1 eV. At the interface between the ceramic membrane and a Pd/Ni metal particle, the energy of protons assume values intermediate to those obtained in the bulk of the two separate materials. In particular, we obtained values ranging between -1 eV and -0.1 eV. Overall, our study shows that the kinetics of the proton incorporation process of the ceramic membrane is limited by the occurrence of trap states at both the surface of the oxide and interface with the matallic catalyst. To incorporate the ceramic membrane, the energetically most favoured path for protons on the surface of the metal is diffusing across the bulk of the metal particle toward the metal/oxide interface. This process has an energy cost of about 0.4-0.5 eV and is more likely to occur near the TPB regions.

Introduction Fuel cells operating around intermediate temperature (300-500 oC) have attracted worldwide attention owing to their high efficiency, and a new electrolyte which can operate that temperature region has been studied so far. Recently, we succeeded in preparing silicate glasses with proton transport number (tH)=1 around intermediate temperature region by considering “mixed alkali effect”, and fuel cell operation at that temperature have been confirmed [1]. This glass shows good chemical and thermal durability with very low fabrication cost. Here, we report the state of the non-bridging oxygen (P=Onb), proton infiltration and fuel cell performances around intermediate temperature. Experiments Borosilicate and phosphosilicate glasses were prepared by conventional melting method using an alumina crucible (SSA-H). The starting materials were analytical grade reagents Na2CO3 (Kishida chemical Co.), K2CO3 (Kishida chemical Co.), SiO2 (Kishida chemical Co.), H3BO3 (Kishida chemical Co.), P2O5 (Nacalai Tesque, Inc.). Appropriate amounts of the materials were melted at 1600 oC for 2 h, and then cast on a carbon plate. The glasses were then annealed from 600 °C down to room temperature. A hydrogen concentration cell was constructed with a 1.5 mm thick glass. The proton transport number TH was calculated as follow: E = tH(RT/2F)ln(P1/P2) where E is the electromotive force (emf) [mV], R is the universal gas constant [J K-1 mol-1], T is the absolute temperature [K], and P1 and P2 are the partial pressure of H2. In the fuel cell test, H2 and O2 gases were supplied to the anode and cathode, respectively. Results and discussion The borosilicate glass had TH asymp; 0, which indicates that few protons migrate in this glass. By contrast, when P2O5 (amount-of-substance fraction 3 mol%) was added to the borosilicate glass, the TH was 0.23 at 500 oC. Furthermore, the tH of the phosphosilicate glass increased up to 0.58 at the same temperature. However, in these glasses, Na+ ions migrated in addition to the protons, and these glasses were not applied as fuel cell electrolytes. The pronounced changes in properties resulting from the addition of a second alkali oxide to a glass have been called the mixed-alkali effect. To decrease the mobility of Na+ ions, we prepared a mixed-alkali glass with both sodium and potassium ions. It was apparent that the tH of the mixed-alkali glass increased to approximately 1.0. These results could be applied to solid state ionics to develop new proton conducting electrolytes. Chemical/thermal durabilities, conductivity as well as fuel cell test at 500°C for more than 1000 hours are also reported. Reference [1] Y. Daiko et al., Electrochem. Solid-State Lett., 14 (2011) B63-B65.

As the working electrolyte in solid oxide fuel cells (SOFCs), the ionic conduction in ZrO2 is of prime importance. Owing to the relatively low mobility of oxygen vacancies, the ionic conductivity of ZrO2 is small at low to intermediate temperatures; therefore, there is always an interest in enhancing the ionic conductivity. Decreasing the feature size (grain size or film thickness) from the micrometer to the nanometer scale usually results in a remarkable change in the transport properties of materials. Owing to the high interfacial density, the materials properties of nanostructured materials can be tuned by varying the spacing of interfaces. In CaF2/BaF2 heterostructures (e.g. Maier et al., Nature 2000, Nature Mater. 2005), a two-orders-of-magnitude enhancement of ionic conductivity was achieved by decreasing the feature size from 500 to 16 nm. In this context a fascinating question emerges: Is it possible to achieve significantly higher ionic conductivity in nanostructured ZrO2? In 2008 a colossal ionic conductivity was reported for SrTiO3/ZrO2/SrTiO3 epitaxial heterostructures (Garcia-Barriocanal et al., Science 2008). But the claimed eight-orders-of-magnitude-conductivity enhancement is ionic or just electronic (Guo, Science 2009)? In this contribution, up-to-date works on the ionic conductivity of nanostructured ZrO2 bulk ceramics, thin films and heterostructures are evaluated, with a purpose of finding out an answer to this question: Can we really achieve significantly higher ionic conductivity in nanostructured ZrO2?

Thermomechanical challenges place restrictions on the choice of electrolyte materials for self-supported thin film solid oxide fuel cells (TF-SOFCs) operating at low temperatures. Utilizing thin films opens up the possibility of a bottom-up fabrication approach to material synthesis for fuel cell applications. We have explored this capability using a co-sputtering technique. Homogeneous solid solutions and compositionally graded electrolytes comprised of doped CeO2 and ZrO2 components have been synthesized. The latter of which is made by simultaneously varying the deposition rate of individual components via respective target power. This approach demonstrates the synthesis of functionally graded, self-supported electrolytes having varying volume fraction of each constituent as a function of film thickness. Further, we fabricated TF-SOFCs to investigate the characteristics of such electrolytes. TF-SOFCs integrating these structures exhibit power densities that exceed 1000 W/cm2 and 650 mW/cm2 at 520 °C, using H2 and CH4 fuel respectively. The kinetic barrier to high power density, thermodynamic voltage, and evolution of metallic electrode morphology were found to be strongly affected by compositional differences. Our results indicate that deviation from ideal Nernst potential in TF-SOFCs with high volume fraction of doped CeO2 hinder voltage efficiencies and should be addressed accordingly. In this presentation, we will discuss these results in detail along with a simple physical model to understand percolative electronic pathways in nanometric electrolytes with mixed phases.

Recently, there are strongly interests on nano structure control of cathode for SOFC in order to improve the catalytic activity. In particular, application of composite oxide consisting of oxide ion conductor and mixed conductor for cathode shows high activity to oxygen activation, however, because of limited interface, activity could be further improved by control of interface structure. In our previous work, we studied the Ni-Fe anode supported cell exhibited the excellent power generation property at intermediated temperature. However, by decreasing temperature, the power generating property became insufficient with increasing cathodic internal resistance. Therefore, in this study, we fabricated columnar type cathode by pulse laser deposition and also applied for high power density SOFC at low temperature. Double columnar structure of Ce_0.8Sr_0.2O₂-Sm_0.5Sr_0.5CoO₃ (SDC-SSC) was successfully deposited by pulse laser deposition method. Much higher power density is achieved by using double columnar structure electrode for cathode. In particular, at higher temperature, improvement in power density is much higher than the conventional cell and the maximum power density is as high as 2.15 W/cm² at 973K. In addition, at 673 K, the maximum power density was slightly increased 0.154 W/cm² to 0.164 W/cm² by inserting double columnar interlayer. These increments could be explained by the expanded reaction area by using composite interlayer. In addition, in this study, anisotropy in electrical conductivity and composition of SDC-SSC composite were also studied. We observed that the power generation property was strongly dependent on the composition and the deposition temperature.

Conventional solid oxide fuel cells (SOFCs) use yttria-stabilized zirconia (YSZ) as the electrolyte. However, its conductivity characteristics require an operating temperature of over 800 °C even with 10-mm-thick YSZ membranes. Reduction of the operating temperature of SOFCs to below 800 °C is one of the main goals of current SOFC research. However, the electrochemical reaction at the SOFC cathodes becomes considerably slower as temperature is reduced. Hence, it is essential to optimize cathode, electrolyte and anode in order to achieve good electrochemical performance at temperature below 800 °C. Improvement of cathode performance can be achieved by development of new materials and/or optimization of electrode architecture/microstructure. YSZ and lanthanum strontium cobalt ferrite (LSCF) nanofibers have been fabricated by the electrospinning method. The LSCF nanofibers and the YSZ nanofibers decorated with lanthanum strontium manganite (LSM) have been used for the cathode of intermediate-temperature (SOFC) with the YSZ electrolyte. The nanofibers have formed a three-dimensional network cathode that has high porosity, which is good for gas transport. The nanofibers network provides continuous pathway for charge transport. As a result, such a nanostructured cathode exhibits reduced polarization resistance as compared to the conventional micron-sized powder cathode. The nanofiber cathode also shows good thermal stability at the intermediate-temperature. Since the porous nanofiber network structure is a good scaffold for infiltration, gadolinia doped ceria (GDC) has been infiltrated into the LSCF nanofiber cathode in order to enhance the transport of oxygen ions in the cathode. The fuel cell with the LSCF-20%GDC composite cathode shows a power density of 1.07 W/cm2 at 1.9 A/cm2 at 750 oC. Our work has demonstrated that the performance of intermediate-temperature SOFCs can be enhanced by engineering the electrode architecture on the nanoscale.

Solid oxide fuel cells (SOFCs) with thin (~15 mu;m) La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) electrolytes and nano-structured electrodes have recently been shown to yield power density > 1 W/cm² down to 550°C. In order to avoid Ni-LSGM interactions during co-firing, the LSGM electrolytes were co-fired with LSGM supports that were subsequently infiltrated with Ni to form the anode. However, a thick LSGM support is not ideal given its lack of electronic conductivity, poor mechanical strength, and the high cost of Ga. The present paper describes thin-LSGM-electrolyte SOFCs fabricated by co-firing on Sr_0.8La_0.2TiO₃ (SLT) supports. SLT was chosen because of its good electronic conductivity, relatively low materials cost, reasonable mechanical strength, and good thermal expansion coefficient match with LSGM. Also, the La dopant is desirable to avoid La out-diffusion from LSGM. Button cells were formed by dry pressing SLT, then either drop coating or screen printing an LSGM functional layer (in some cases) and an LSGM electrolyte, followed by co-firing and application of a LSCF-GDC cathode. Dense electrolyte and suitable functional-layer microstructure, without interdiffusion as measured by SEM-EDS, were achieved. Anode Ni infiltration (15-20 vol%) followed by ~700°C calcination produced Ni nanoparticles within the SLT support and, when used, the porous LSGM functional layer. EDS mapping and SEM imaging showed that the Ni infiltrate penetrated fully into the functional layer and showed Ni nanoparticles ranging in size from 10-100 nm. Cells without the LSGM functional layer produced relatively low open-circuit voltage; the improvement to near theoretical with the functional layer suggests it may act as a barrier that reduces Ti diffusion into the electrolyte. Maximum power densities in air and humidified hydrogen were 330 W/cm² at 600°C and 510 W/cm² at 650°C. Work is underway to improve upon these results by using an infiltrated cathode, thinner electrolyte, and optimized Ni infiltration.

Metal nanoparticles are of significant importance for chemical and electrochemical catalysis due to their high surface-to-volume ratio and possible unique catalytic properties. However, the poor thermal stability and the resulting sintering behavior of nano-sized particles limit their use only to low temperature conditions. Accordingly, the application of nanoscale metal catalysts (diameter < 10 nm) to solid oxide fuel cell (SOFC) electrodes at elevated temperature (> 600C) has received virtually no attention in the literature. Here we report a novel strategy for stabilizing metal nanoparticles by using Sm_0.2Ce_0.8O_2-δ (SDC) films with highly porous and vertically oriented morphology. Films of SDC are grown on single crystal YSZ substrates by means of pulsed laser deposition. The SDC serves as a supporting structure for metal nanoparticles as well as a mixed conducting transport layer for hydrogen electro-oxidation reactions. A small amount of Pt (10 mu;g/cm²) is subsequently applied onto the SDC film by sputtering. Resulting structures are examined by TEM, SIMS, and electron diffraction, confirming that Pt nanoparticles are stabilized on the porous SDC structure even after annealing at 650C for 100h. Electrochemical properties are investigated by impedance spectroscopy under H₂-H₂O-Ar atmospheres in the temperature regime 450 - 650C. The Pt-decorated porous SDC structure demonstrates exceptionally reduced electrode impedance as well as activation energy associated with hydrogen-oxidation reaction, when compared with the porous or dense SDC structure without Pt. Effects of the Pt nanoparticles are systematically investigated, and the implication of these results for thin-film-based SOFCs at reduced operating temperature is discussed.

In solid oxide fuel cell (SOFC), electricity is electrochemically generated at high temperature using oxygen ion mobility from oxidizing fuel. The materials used in various components of SOFC; anode, cathode, electrolyte and interconnects/sealants need to satisfy unique electronic properties such as for SOFC anode, appropriate balance of mixed electronic/ionic conductivity and thermal expansion coefficient matching with the electrolyte material is necessary. In typical SOFC electrolytes, the oxygen vacancies in group 4 transition metal oxides in fluorite structure can be increased by replacing them with a transition metal (matching ionic radii) either from group 3 or lanthanoids with +3 valence state. Additional oxygen vacancies at SOFC cathodes with perovskite ABO3 structure can be created by doping group 2 elements with +2 valence state at A or B site. Despite of favorable material choices, the SOFC performance at the intermediate operating temperatures can be limited due to poor oxygen ion conductivity in the bulk. Therefore, modified material design principles need to be applied to enhance the fuel cell efficiency. Better understanding of grain boundaries and interfaces can help to solve efficiency challenges faced by SOFCs at intermediate temperatures. Cerium oxide is known to promote oxygen conduction at intermediate operating temperatures compared to its YSZ counterpart. Complex oxygen mobility studies in nanocrystalline CeO2 will be discussed to correlate the oxygen desorption, diffusivity with oxygen transport properties. Even though exact quantification of simultaneously occurring complex oxygen mobility effects like exchange interaction, diffusion, adsorption and desorption in the bulk remains a challenge, a shift in the 18O concentration maxima during nuclear reaction analysis experiments indicated oxygen desorption from the surface region. Due to the lack of long range lattice ordering, high defect density in nanocrystalline cerium oxide did not necessarily convert into high conductivity. To promote the oxygen transport across material interfaces, modified material design principles were applied to create systematic high density material interfaces using molecular beam epitaxy and glancing angle sputter depositions. Apart from some preferential segregation effects, improvement in the conductivity is observed at the intermediate temperatures. Reduction studies on Ni-YSZ as an anode for the direct hydrocarbon fuel feed and anode supported SOFC applications show mobile nature of nanocrystalline Ni at the YSZ interface. In this context, future SOFC material design challenges will also be discussed.

Optimization of the microstructure of porous electrodes plays a crucial role in enhancing the efficiency of solid oxide fuel cells (SOFCs). Using a variety of numerical modeling approaches, we investigate the correlation between the morphological properties of SOFC electrode microstructures and their resulting effective material properties. Based on this analysis, we present strategies for increasing the electrochemical performance of SOFC electrodes by controlling certain key morphological parameters. The porous anode and cathode of SOFCs can be considered binary mixtures of electronic and ionic conducting spherical particles. Two techniques for numerically describing an electrode&’s microstructure are employed: In the first approach, three-dimensional packings of randomly distributed, overlapping spheres are generated, while in the second approach the microstructure is described based on an assumed average particle coordination number and percolation theory. The influence of the particle overlap as well as advantages and shortcomings of both approaches will be discussed. In a parameter study, we systematically investigate the impact of key morphological parameters on effective material properties that are of special interest for predicting the electrochemical performance of SOFC electrodes. For this purpose, the disposable active three-phase boundary, the electronic and ionic conductivities, the mean pore radii and the tortuosity of the pore space are computed as functions of the electron and ion conducting particle sizes, their volume fraction and their size ratio. For calculating the effective size of the three-phase boundary we propose an extension of prior approaches, which we judge to be physically more sound and, in particular, well suited for characterizing mixtures of particles of different sizes. The computed effective material properties are subsequently employed as input parameters in a one-dimensional finite-element cell level model encompassing the entire set of processes of gas transport, electronic and ionic conduction as well as the electrochemical reactions. This approach enables us to demonstrate the influence of electron and ion conducting particle sizes and volume fractions on the maximum achievable power densities in anode and cathode. Under certain conditions, cathode microstructures having electronic conducting particles of size different from that of the ionic conducting particles become preferable and yield a higher maximum power density when compared to the best possible configuration of monodisperse particles.

The microstructure of the Solid Oxide Fuel Cells electrodes governs a major part of their efficiency, durability and reliability. The 3D characterization of this microstructure is mandatory to determine the morphological properties of the electrodes that are essential in modeling and analyzing the SOFC electrochemical response. Several studies have been undertaken, especially in the anode part, which has nickel and yttria-stabilized zirconia (YSZ) constituents. Previous reports using destructive FIB-SEM and non-destructive tomography techniques give promising results. However, the volume studied (~10x10x10µm3) is too small to be representative of the microstructure and to obtain good statistics. In this work, 3D imaging of 50x50x50µm3 volumes in the anode electrode with 25 nm spatial resolution was performed using region of interest X-ray holotomography at the nano-imaging station ID22NI of the ESRF. Local acquisitions at high energy and specific sample preparation protocols were developed to improve the quality of the images and allow the identification of the three different phases (nickel, YSZ and pores) within this large volume. Both the thick current collector substrate and the thin functional layer have been studied. Algorithms for image analysis and numerical procedures have been developed to compute from the reconstructions the microstructure properties such as volume fraction of each phase, specific surface area, mean pore radius, phase percolation and triple phase boundary length. Finite element simulations based on the 3D reconstructions have been used to determine directly the tortuosity factor for a typical highly porous SOFC support. Special attention has been paid to estimate the Representative Volume Elements (RVEs) of the analysed media. Because of its coarse microstructure, it has been shown that a volume as large as 35x35x35 µm3 is required to be statistically representative for the current collector substrate (for the fluid properties). Finally, the impact of boundary conditions used for the simulations has been estimated.

High Temperature Steam Electrolysis (HTSE) is one of the most promising ways for hydrogen mass production. To make this technology economically suitable, each component of the system has to be optimized to reach high energetic efficiency, especially the single solid oxide electrolysis cell (SOEC). Improving the oxygen electrode performances is of particular interest as this electrode contributes to a large extent to the cell polarization resistance. The present study is focused on alternative structured oxygen electrodes. The Ln2NiO4+δ (Ln = La, Pr or Nd) rare-earth nickelate oxides (with K2NiF4-type structure) were selected as oxygen electrode material with respect to their aptitude to accommodate oxygen overstoichiometry, leading to a mixed electronic and ionic conductivity. A thin ceria-based interfacial layer was added between the electrode and the dense zirconia-based electrolyte to improve mechanical and electrochemical properties and to limit the reactivity with this electrolyte. The selected interfacial materials were yttria-doped ceria Ce0.8Y0.2O2-δ (YDC) and gadolinium-doped ceria Ce0.8Gd0.2O2-δ (CGO). These structured electrodes were screen-printed, then characterized by electrochemical impedance spectroscopy measurements performed on symmetric electrolyte supported cells, under zero dc conditions and anodic polarization. The lowest polarization resistance RP and the best anodic overpotential eta;A vs. current density curves were obtained for the Pr2NiO4+δ / YDC structured electrode: RP is lessened down to 0.06 Omega;.cm2 at 800°C under air and zero dc conditions. The oxygen reaction rate determining step was determined by varying the oxygen partial pressure P(O2) in the range 10-3 - 1 atm. At 800°C, for the Pr2NiO4+δ / YDC electrode, molecular oxygen absorption/desorption is limiting. These results will be discussed in terms of oxygen evolution processes in the temperature range 600 - 800°C. Then, hydrogen electrode supported cells including the Pr2NiO4+δ / YDC structured oxygen electrode were characterized in terms of electrochemical performances. At 800°C, when the inlet gas composition is 90% H2O - 10% H2 at the hydrogen electrode, and flowing air at the oxygen electrode, the current density determined at 1.3 V reaches - 1 A.cm-2, the steam conversion rate being 64 %. These results will be compared to those obtained with a commercial cell including the oxygen deficient perovskite-type La0.6Sr0.4Fe0.8Co0.2O3-δ as oxygen electrode.

In order to improve the tolerance of solid oxide fuel cell for coke deposition and reoxidation, application of oxide for anode is highly requested. In our previous work, we found that mixed oxide of Ce(Mn,Fe)O₂-La(Sr)Fe(Mn)O₃ is highly active for anode of direct hydrocarbon type solid oxide fuel cells. However, because of insufficient electrical conductivity, the power density of the cell at decreased temperature is not high. In this study, effects of small amount of additives like RuO₂, Pd, or Pt were investigated for improving the power density in intermediate temperature range. In case of Pt or Ag which is known as active catalyst for oxidation, anodic overpotential was much enlarged by addition, resulting in the decreased power density. Among the examined additives, fairly high maximum power densities of 0.3 W cm^-2 (in H₂) and 1.5 W cm^-2 (in C₃H₈) at 800 ^oC were achieved on the anode modified with 10 wt% RuO₂. The cells showed stable power density, and negligible carbon formation, even after operation for more than 50 hours at 1A cm^-2. The improved power density was due to decreased anodic overpotential and IR losses, in turn because of the improved electrical conductivity contributed by the RuO₂ particle additives.

Direct supply of hydrocarbons especially methane and natural gas as fuel for solid oxide fuel cells is of strong interest, since this contributes to simplification of the total fuel cell system and potentially impacts cost per unit power produced. In this study, thin film micro-solid oxide fuel cells (mu;SOFCs) utilizing nanoporous ruthenium (Ru) and Ru - gadolinia doped ceria (CGO) nano-composite anodes were fabricated and investigated for operation with various fuels. Thin film of 8 mol% yttria-stabilized zirconia was fabricated as self-supported electrolytes on a silicon substrate, with porous platinum deposited as cathode and porous Ru and nano-composite Ru-CGO deposited as anode electrodes. mu;SOFCs were tested up to 530 degC with hydrogen, methane and natural gas as the fuel and air as the oxidant. At 500 degC mu;SOFC with Ru anode exhibited 450 mW/cm2 with room temperature humidified methane as fuel, where mu;SOFC with Ru-CGO anode exhibited 275 mW/cm2 with the same fuel. Studies on various fuels including natural gas will be presented. Anode electrodes were investigated before and after the fuel cell tests by SEM for structural changes, and by x-ray photoelectron spectroscopy for chemical composition evolution. The crucial role of materials synthesis in fabricating a percolating mixed conducting ionic-electronic nanocomposite that is thermomechanically stable in a self-supported membrane configuration under extreme reducing conditions will be considered in detail.

Mechanical endurance of the ceramic component layers at high operating temperatures is extremely important to ensure the reliability of solid oxide fuel cells (SOFCs) and similar devices based on dense ion-conducting ceramic membranes. A key issue, requiring acute attention is the effects due to thermal cycling on the mechanical stability of the SOFC, particularly over long term periodic operation. The effects of the repeated thermal shocks on the mechanical stability of NiO-8YSZ anode/8YSZ electrolyte bi-layer SOFC membrane structures were evaluated in details in this study. The SOFC membrane structures were subjected to rapid thermal cycling between 100 and 800 °C in ambient air atmosphere. The bi-layer membranes show a strong resistance to degradation in mechanical properties even after 500 repeated cycles of rapid thermal shocks. This resistance to the degrading effects of thermal cycling is attributed to the structural stability and compatibility of the NiO-8YSZ anode with the 8YSZ electrolyte layer, primarily due to the close matching in their coefficient of thermal expansion.

The effect of different fuels on the nanostructure and chemical evolution of the interfaces from the anode of commercial Solid Oxide Fuel Cells was analyzed using the Transmission Electron Microscopy. The fuels evaluated in this present study include H2, synthesis gas, and fuels with trace (ppm) phosphine. Previous study shows the existence of interface ribbon phase of NiO in the Ni/YSZ interfaces, but not along the Ni/Ni grain boundaries. Previous study also indicates that the thickness of the NiO ribbon layer is dependent upon cell operation time, with thicker NiO ribbon phases developing at longer operation times. The present study further demonstrates that evolution of the NiO ribbon phase is also fuel dependent. For the cells operated for an equivalent duration of 196 hours, the thickness of the NiO layer is about 10 nm under 97.5% H2 (with balance of N2), and the thickness of the NiO layer is about 50 nm under 25% H2 (balance N2). The interaction of trace (ppm) phosphine with Ni-YSZ anode of commercial solid oxide fuel cells has also been investigated and evaluated for both synthesis gas and hydrogen fuels. Experiments indicate that degradation rates and mechanisms are fuel dependent. Degradation of cells operated in synthesis gas (syngas) with phosphine is more severe than that from cells operated in hydrogen with phosphine. In the cell operated in syngas containing 10 ppm phosphine, significant microstructure degradation was observed within both the Ni phase and the YSZ phase. In addition to the formation of Ni-P phases on the outer layer of the anode, significant pitting corrosion was observed in the Ni grains. At the YSZ side, a previously undetected YPO4 phase is observed at the YSZ/YSZ/Ni triple phase boundaries, and tetragonal YSZ and cubic YSZ domains with sizes of several tens nano meters are observed along the Ni/YSZ interface. These observations contrast with data obtained for the cell operated in hydrogen with phosphine, where no YPO4 phase is observed and the alternating tetragonal YSZ and cubic YSZ domains at the Ni/YSZ interface are smaller with typical sizes of 5-10 nm.

While solid oxide fuel cells (SOFCs) can use hydrocarbon-based fuels such as town gas, LP gas, and kerosene after simple reforming processes, un-reformed hydrocarbons and sulfur could flow into the SOFC systems simultaneously as minor fuel impurities. In this study, co-poisoning effects in the simultaneous presence of H2S and hydrocarbons to electrolyte- supported SOFCs single cell in supplying 3ppm H2S and 3% hydrocarbons are experimentally investigated. After H2S poisoning experiments, carbon deposition is analyzed by field-emission scanning electron microscope (FESEM), scanning transmission electron microscope (STEM) and gas chromatograph (GC) in order to discuss the co-poisoning mechanisms. Typical electrolyte-supported single cells were used in this study. Scandia-stabilized zirconia (ScSZ: 10mol% Sc2O3-1mol% CeO2-89mol% ZrO2) plates with a thickness of 200 µm were used as electrolytes. Ni-ScSZ cermet and LSM-ScSZ composite material were used for the anode and the cathode, respectively. Mixture of H2, CO, CH4 and CO2, simulating 50 % pre-reformed CH4-based gas with S/C = 2.5, was supplied, and the cell voltage was then measured at a constant current density of 0.2 A cm-2 at 800 oC. H2S and hydrocarbons was added into or removed from the fuel gas by just changing between pure N2 gas and H2S and hydrocarbons-containing N2 gas. After poisoning test in the case of the fuel gas containing 3% C2H6 and 3ppm H2S, the fuel gas containing 3% C3H8 and 3ppm H2S, the fuel gas containing i-C4H10, and the fuel gas containing i-C4H10 and 3ppm H2S, carbon fibers were formed on the anode layer. The structure, diameter, and amount of carbon fibers depended on the carbon number of the fuels. The carbon formation rate was derived by considering the carbon balance between the amount of carbons of hydrocarbons, CO, and CO2 supplied and the amount of carbon in the exhaust fuel gases. Based on the quantitative evaluation of H2S effects on the carbon deposition by changing the kind of the hydrocarbons on the fuel, it was found that the carbon formation tends to be accelerated in the presence of H2S as the co-poisoning effect.

Recently, solid oxide fuel cells (SOFCs) have been actively developed as a type of high-efficiency electrical power generation system to utilize efficiently the world&’s remaining limited fossil fuel. Energy devices such as SOFC need no destruction of materials during the operation period and the understandings of mechanical properties under SOFC operation conditions are necessary to ensure long-term durability of the devices. In the practical uses of SOFC systems for residential applications, natural disaster such as unanticipated earthquakes and lightning strikes, emergency stops caused by blackouts and local fuel shortages can happen. These situations expose anode materials to oxidation conditions, and cause destruction of the cells. Therefore, re-oxidation is very significant problem and it is considered that the tolerance for the reduction and oxidation (redox) is one of important issues for SOFC cells. It has been already reported that a segmented-in-series(SIS)-type SOFC has high tolerance against redox conditions. In this study, we examined the basic mechanical properties such as Yong&’s modulus of substrate materials for SIS-type SOFCs under the operation conditions in order to get a guiding principle of material design. Three types of composition, such as Al2O3-NiO-YSZ (ANY), TiO2-NiO-YSZ (TNY), and MgO-NiO-YSZ (MNY), were prepared. In the preparation, each powder was physically mixed and pressed uniaxially into rectangular pellets and they were sintered for 2hours at 1500°C. Some of them were reduced in 4% H2 balanced N2 reduction atmosphere for 100hours at 800°C. Their Young&’s modulus of both as sintered and reduced samples were measured at the temperature in the range from room temperature to 1000°C. In ANY and TNY samples, compared to as sintered samples, reduced samples showed lower Yong&’s modulus around high temperature ranges. On the other hand, in MNY samples, both as sintered and reduced samples showed almost same behaviors at measured temperature range. After Young&’s modulus measurement, the cross sections of samples were analyzed using a scanning electron microscope. In ANY and TNY samples, the micro structures after reduction were drastically different from those of as sintered samples. In MNY samples, the macroscopic structures of both as sintered and reduced samples were almost same although nano-size Ni particles were observed in reduced samples. These results indicate that micro-structural changes caused the behavior changes of Young&’s modulus at high temperature ranges between as sintered and reduced samples. In the future work, we are planning to examine the mechanical behaviors of samples which are reduced for longer time, and conducted redox treatments.