Publications
547 results found
Leonard K, Druce J, Thoreton V, et al., 2018, Exploring mixed proton/electron conducting air electrode materials in protonic electrolysis cell, Solid State Ionics, Vol: 319, Pages: 218-222, ISSN: 0167-2738
In this work, we investigate and compare the performance and cell polarization resistance of Ba 0.5 La 0.5 CoO 3−δ (BLC) and double perovskite oxide BaGd 0.8 La 0.2 Co 2 O 6−δ (BGLC) anode on cathode supported protonic steam electrolysis cells using a 20 μm SrZr 0.5 Ce 0.4 Y 0.1 O 3−δ electrolyte with Ni-SZCY541 composite as the cathode. The kinetics of protons through the bulk and across the gas electrolyte interfaces of both anode materials were also studied by direct measurement of their tracer diffusions using time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS). Cell terminal voltages of 1.74 and 1.93 V, were observed at a current density of 0.5 A cm −2 for both BLC and BGLC whereas a hydrogen evolution rate of 121.85 and 111.15 μmol cm −2 every minute was also obtained at the same current density, translating to a current efficiency of 78 and 72% respectively. Hydrogen tracer diffusion studies confirm BGLC can incorporate protons into the bulk relative to BLC even though the present steam electrolysis results show a better performance for BLC at 600 °C.
Pergolesi D, Gilardi E, Fabbri E, et al., 2018, Interface effects on the ionic conductivity of doped ceria-yttria-stabilized zirconia heterostructures, ACS Appl Mater Interfaces, Vol: 10, Pages: 14160-14169, ISSN: 1944-8244
Multilayered heterostructures of Ce0.85Sm0.15O2-δ and Y0.16Zr0.92O2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.
Cavallaro A, Pramana S, Ruiz Trejo E, et al., 2018, Amorphous-cathode-route towards low temperature SOFC, Sustainable Energy & Fuels, Vol: 2, Pages: 862-875, ISSN: 2398-4902
Lowering the operating temperature of solid oxide fuel cell (SOFC) devices is one of the major challenges limiting the industrial breakthrough of this technology. In this study we explore a novel approach to electrode preparation employing amorphous cathode materials. La0.8Sr0.2CoO3−δ dense films have been deposited at different temperatures using pulsed laser deposition on silicon substrates. Depending on the deposition temperature, textured polycrystalline or amorphous films have been obtained. Isotope exchange depth profiling experiments reveal that the oxygen diffusion coefficient of the amorphous film increased more than four times with respect to the crystalline materials and was accompanied by an increase of the surface exchange coefficient. No differences in the surface chemical composition between amorphous and crystalline samples were observed. Remarkably, even if the electronic conductivities measured by the Van Der Pauw method indicate that the conductivity of the amorphous material was reduced, the overall catalytic properties of the cathode itself were not affected. This finding suggests that the rate limiting step is the oxygen mobility and that the local electronic conductivity in the amorphous cathode surface is enough to preserve its catalytic properties. Different cathode materials have also been tested to prove the more general applicability of the amorphous-cathode route.
Staykov A, Tellez H, Druce J, et al., 2018, Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3, Science and Technology of Advanced Materials, Vol: 19, Pages: 221-230, ISSN: 1468-6996
Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO3 and iron doped SrTiO3 were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO3 and compared it to DOS of iron-doped SrTiO3 with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO3 and iron-doped SrTiO3. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO3, are accessible only on TiO2 terminated SrTiO3 surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.
Skinner SJ, ryan MP, pramana S, et al., 2017, Crystal structure and surface characteristics of Sr-doped GdBaCo2O6-δ double perovskites: oxygen evolution reaction and conductivity, Journal of Materials Chemistry A, Vol: 6, Pages: 5335-5345, ISSN: 2050-7496
A cheap and direct solution towards engineering better catalysts through identification of novel materials is required for a sustainable energy system. Perovskite oxides have emerged as potential candidates to replace the less economically attractive Pt and IrO2 water splitting catalysts. In this work, excellent electrical conductivity (980 S cm−1) was found for the double perovskite of composition GdBa0.6Sr0.4Co2O6−δ which is consistent with a better oxygen evolution reaction activity with the onset polarisation of 1.51 V with respect to a reversible hydrogen electrode (RHE). GdBa1−xSrxCo2O6−δ with increasing Sr content was found to crystallise in the higher symmetry tetragonal P4/mmm space group in comparison with the undoped GdBaCo2O6−δ which is orthorhombic (Pmmm), and yields higher oxygen uptake, accompanied by higher Co oxidation states. This outstanding electrochemical performance is explained by the wider carrier bandwidth, which is a function of Co–O–Co buckling angles and Co–O bond lengths. Furthermore the higher oxygen evolution activity was observed despite the formation of non-lattice oxides (mainly hydroxide species) and enrichment of alkaline earth ions on the surface.
Téllez Lozano H, Druce J, Cooper SJ, et al., 2017, Double perovskite cathodes for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic conductors?, Science and Technology of Advanced Materials, Vol: 18, Pages: 977-986, ISSN: 1468-6996
Published by National Institute for Materials Science in partnership with Taylor & Francis. 18 O and 2 H diffusion has been investigated at a temperature of 300 °C in the double perovskite material PrBaCo 2 O 5+δ (PBCO) in flowing air containing 200 mbar of 2 H 2 16 O. Secondary ion mass spectrometry (SIMS) depth profiling of exchanged ceramics has shown PBCO still retains significant oxygen diffusivity (~1.3 × 10 −11 cm 2 s −1 ) at this temperature and that the presence of water ( 2 H 2 16 O), gives rise to an enhancement of the surface exchange rate over that in pure oxygen by a factor of ~3. The 2 H distribution, as inferred from the 2 H 2 16 O − SIMS signal, shows an apparent depth profile which could be interpreted as 2 H diffusion. However, examination of the 3-D distribution of the signal shows it to be nonhomogeneous and probably related to the presence of hydrated layers in the interior walls of pores and is not due to proton diffusion. This suggests that PBCO acts mainly as an oxygen ion mixed conductor when used in PCFC devices, although the presence of a small amount of protonic conductivity cannot be discounted in these materials.
Harrington GF, Skinner SJ, Kilner JA, 2017, Can solute segregation in ceramic materials be reduced by lattice strain?, Journal of the American Ceramic Society, Vol: 101, Pages: 1310-1322, ISSN: 0002-7820
Lattice strain is a relatively unexplored route to modify the degradation effects in functional oxides for high-temperature electrochemical devices. In this paper, we present results on the segregation of Gd to the surface of strained Gd0.1Ce0.9O2-δ films using low-energy ion scattering to assess the surface composition. The potential for strain-modified segregation is discussed as well as the challenges in studying and implementing it.
Skinner SJ, li C, ni N, et al., 2017, Surface chemistry of La<sub>0.99</sub>Sr<sub>0.01</sub>NbO<sub>4-d</sub> and its implication for proton conduction, ACS Applied Materials and Interfaces, Vol: 9, Pages: 29633-29642, ISSN: 1944-8244
Acceptor-doped LaNbO4 is a promising electrolyte material for proton-conducting fuel cell (PCFC) applications. As charge transfer processes govern device performance, the outermost surface of acceptor-doped LaNbO4 will play an important role in determining the overall cell performance. However, the surface composition is poorly characterized, and the understanding of its impact on the proton exchange process is rudimentary. In this work, the surface chemistry of 1 atom % Sr-doped LaNbO4 (La0.99Sr0.01NbO4-d, denoted as LSNO) proton conductor is characterized using LEIS and SIMS. The implication of a surface layer on proton transport is studied using the isotopic exchange technique. It has shown that a Sr-enriched but La-deficient surface layer of about 6–7 nm thick forms after annealing the sample under static air at 1000 °C for 10 h. The onset of segregation is found to be between 600 and 800 °C, and an equilibrium surface layer forms after 10 h annealing. A phase separation mechanism, due to the low solubility of Sr in LaNbO4, has been proposed to explain the observed segregation behavior. The surface layer was concluded to impede the water incorporation process, leading to a reduced isotopic fraction after the D216O wet exchange process, highlighting the impact of surface chemistry on the proton exchange process.
Skinner SJ, McComb DW, Harrington GF, et al., 2017, The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films, Physical Chemistry Chemical Physics, Vol: 19, Pages: 14319-14336, ISSN: 1463-9084
Enhanced conductivity in YSZ films has been of substantial interest over the last decade. In this paper we examine the effects of substrate lattice mismatch and film thickness on the strain in YSZ films and the resultant effect on the conductivity. 8 mol% YSZ films have been grown on MgO, Al2O3, LAO and NGO substrates, thereby controlling the lattice mismatch at the film/substrate interface. The thickness of the films was varied to probe the interfacial contribution to the transport properties, as measured by impedance spectroscopy and tracer diffusion. No enhancement in the transport properties of any of the films was found over single crystal values, and instead the effects of lattice strain were found to be minimal. The interfaces of all films were more resistive due to a heterogeneous distribution of grain boundaries, and no evidence for enhanced transport down dislocations was found.
Cooper SJ, Niania M, Hoffmann, et al., 2017, Back-exchange: a novel approach to quantifying oxygen diffusion and surface exchange in ambient atmospheres, Physical Chemistry Chemical Physics, Vol: 19, Pages: 12199-12205, ISSN: 1463-9084
A novel two-step Isotopic Exchange (IE) technique has been developed to investigate the influence of oxygen containing components of ambient air (such as H₂O and CO₂) on the effective surface exchange coefficient (k*) of a common mixed ionic electronic conductor material. The two step 'back-exchange' technique was used to introduce a tracer diffusion profile, which was subsequently measured using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The isotopic fraction of oxygen in a dense sample as a function of distance from the surface, before and after the second exchange step, could then be used to determine the surface exchange coefficient in each atmosphere. A new analytical solution was found to the diffusion equation in a semi-infinite domain with a variable surface exchange boundary, for the special case where D* and k* are constant for all exchange steps. This solution validated the results of a numerical, Crank-Nicolson type finite-difference simulation, which was used to extract the parameters from the experimental data. When modelling electrodes, D* and k* are important input parameters, which significantly impact performance. In this study La₀.₆Sr₀.₄Co₀.₂Fe₀.₈O₃-δ (LSCF6428) was investigated and it was found that the rate of exchange was increased by around 250% in ambient air compared to high purity oxygen at the same pO₂. The three experiments performed in this study were used to validate the back-exchange approach and show its utility.
Wu K-T, Tellez H, Druce J, et al., 2017, Surface chemistry and restructuring in thin-film Lan+1NinO3n+1 (n=1, 2 and 3) Ruddlesden-Popper oxides, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 5, Pages: 9003-9013, ISSN: 2050-7488
Understanding the surface chemistry and oxygen surface exchange activity in mixed conducting perovskite and related perovskite oxides is of great relevance in developing electrochemical devices. Mixed conducting Ruddlesden–Popper Lan+1NinO3n+1 phases (n = 1, 2 and 3) have been considered as promising electrodes for electrochemical energy conversion cells due to their layered structure allowing non-stoichiometric defect structures. This study focuses on a systematic investigation of the chemical composition of the outermost atomic surfaces of as-deposited and annealed epitaxial films of Lan+1NinO3n+1 (n = 1, 2 and 3). For both as-deposited and annealed films, the analysis of the outermost surface using low energy ion scattering shows preferential LaO-termination. The results also provide evidence of an associated Ni-enrichment below the outermost surface. These findings suggest significant atomic rearrangement occurs during deposition and subsequent annealing. To investigate the thermal stability of these films during deposition, further microstructural analysis was carried out by means of high-resolution scanning transmission electron microscopy, showing significant re-orientation of LaO layers after a post-annealing heat treatment. In thin films of n = 2, 3 phases, surface restructuring reduces the epitaxy of the films and hence any potential beneficial anisotropy in transport properties will be lost. Care must therefore be exercised in processing these materials for electrode applications.
Buannic L, Orayech B, Lopez Del Amo J-M, et al., 2017, Dual Substitution Strategy to Enhance Li+ Ionic Conductivity in Li7La3Zr2O12 Solid Electrolyte, CHEMISTRY OF MATERIALS, Vol: 29, Pages: 1769-1778, ISSN: 0897-4756
Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li7La3Zr2O12 appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga3+, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc3+, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li7La3Zr2O12 according to the resulting stoichiometry, Li7–3x+yGaxLa3Zr2–yScyO12. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li6.65Ga0.15La3Zr1.90Sc0.10O12, with preferential Ga3+ occupation of tetrahedral Li24d sites over the distorted octahedral Li96h sites. 7Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10–3 S cm–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively.
Aguesse F, Manalastas W, Buannic L, et al., 2017, Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal., ACS Applied Materials and Interfaces, Vol: 9, Pages: 3808-3816, ISSN: 1944-8244
All-solid-state batteries including a garnet ceramic as electrolyte are potential candidates to replace the currently used Li-ion technology, as they offer safer operation and higher energy storage performances. However, the development of ceramic electrolyte batteries faces several challenges at the electrode/electrolyte interfaces, which need to withstand high current densities to enable competing C-rates. In this work, we investigate the limits of the anode/electrolyte interface in a full cell that includes a Li-metal anode, LiFePO4 cathode, and garnet ceramic electrolyte. The addition of a liquid interfacial layer between the cathode and the ceramic electrolyte is found to be a prerequisite to achieve low interfacial resistance and to enable full use of the active material contained in the porous electrode. Reproducible and constant discharge capacities are extracted from the cathode active material during the first 20 cycles, revealing high efficiency of the garnet as electrolyte and the interfaces, but prolonged cycling leads to abrupt cell failure. By using a combination of structural and chemical characterization techniques, such as SEM and solid-state NMR, as well as electrochemical and impedance spectroscopy, it is demonstrated that a sudden impedance drop occurs in the cell due to the formation of metallic Li and its propagation within the ceramic electrolyte. This degradation process is originated at the interface between the Li-metal anode and the ceramic electrolyte layer and leads to electromechanical failure and cell short-circuit. Improvement of the performances is observed when cycling the full cell at 55 °C, as the Li-metal softening favors the interfacial contact. Various degradation mechanisms are proposed to explain this behavior.
Cooper SJ, Bertei A, Shearing PR, et al., 2016, TauFactor: An open-source application for calculating tortuosity factors from tomographic data, SoftwareX, Vol: 5, Pages: 203-210, ISSN: 2352-7110
TauFactor is a MatLab application for efficiently calculating the tortuosity factor, as well as volume fractions, surface areas and triple phase boundary densities, from image based microstructural data. The tortuosity factor quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media. TauFactor was originally developed to improve the understanding of electrode microstructures for batteries and fuel cells; however, the tortuosity factor has been of interest to a wide range of disciplines for over a century, including geoscience, biology and optics. It is still common practice to use correlations, such as that developed by Bruggeman, to approximate the tortuosity factor, but in recent years the increasing availability of 3D imaging techniques has spurred interest in calculating this quantity more directly. This tool provides a fast and accurate computational platform applicable to the big datasets (>10^8 voxels) typical of modern tomography, without requiring high computational power.
Akbay T, Staykov A, Druce J, et al., 2016, The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4, Journal of Materials Chemistry A, Vol: 4, Pages: 13113-13124, ISSN: 2050-7496
Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage.
Burriel M, Tellez H, Chater RJ, et al., 2016, Influence of Crystal Orientation and Annealing on the Oxygen Diffusion and Surface Exchange of La2NiO4+delta, Journal of Physical Chemistry C, Vol: 120, Pages: 17927-17938, ISSN: 1932-7455
La2NiO4+δ is a mixed ionic–electronic conducting material with the 2D K2NiF4-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k*) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La2NiO4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450–600 °C. The differences in k* have been correlated with differences in surface chemistry: surface termination, near-surface rearrangement, and the presence of extrinsic impurities. Finally, the predominant La termination at the immediate outer surface is evidenced, confirming recent results for other Ruddlesden–Popper phases with mixed ionic–electronic conducting properties.
Chater RJ, Cavallaro A, Bayliss RD, et al., 2016, Fast grain boundary oxygen ion diffusion in the α-phase of Bi2O3, Solid State Ionics, Vol: 299, Pages: 89-92, ISSN: 0167-2738
The low temperature stable α-phase of pure Bi2O3 is known to have an oxygen ion diffusivity that is over 7 orders of magnitude lower than the high temperature fluorite-structured δ-phase. Tracer oxygen-18 diffusion studies of polycrystalline α-Bi2O3 at 600 °C using an ion microscope with lateral resolutions of ~ 50 nm for the surface distributions of the oxygen isotopes have resolved the secondary fast migration pathways as well as the normal bulk diffusion profile in the grains. A grain boundary pathway for the oxygen migration is distinguished from other extended defects, some of which are also active in the overall oxygen diffusivity. This experimental study highlights the potential manipulation of the micro-structure of this material for enhanced oxygen ion conduction in intermediate temperature solid oxide fuel cells as has been shown for perovskite MIEC electrode materials.
Limbeck A, Rupp GM, Kubicek M, et al., 2016, Dynamic etching of soluble surface layers with on-line inductively coupled plasma mass spectrometry detection - a novel approach for determination of complex metal oxide surface cation stoichiometry, Journal of Analytical Atomic Spectrometry, Vol: 31, Pages: 1638-1646, ISSN: 1364-5544
In this work, an innovative approach for determining the surface stoichiometry of complex metal oxide (CMO) thin films is presented. The procedure is based on treatment of the sample surface with different etching solutions, followed by on-line analysis of the derived eluates using inductively coupled plasma – mass spectrometry (ICP-MS). Via consecutive treatment of the sample surface with water and diluted HCl, a differentiation between water soluble and acid soluble parts of near surface regions in thin films is enabled. The developed procedure was applied for the analysis of dense La0.6Sr0.4CoO3−δ (LSC) thin films indicating the presence of a water soluble Sr-rich phase with sub nm-thickness on top of the LSC films. The step-wise optimization of this technique is reported and the results are compared to measurements performed by low-energy ion scattering (LEIS). The detrimental effect of the water soluble Sr rich phase on the oxygen exchange activity of LSC thin film electrodes is verified by electrochemical impedance spectroscopy (EIS).
Tellez H, Druce J, Ishihara T, et al., 2016, Effects of microstructure on surface segregation: role of grain boundaries, 229th Meeting of The-Electrochemical-Society / Symposium on Ionic and Mixed Conducting Ceramics, Publisher: Electrochemical Society, Pages: 57-69, ISSN: 1938-5862
The effects of microstructure on the kinetics of strontium segregation in La0.6Sr0.4CoO3-δ ceramic samples are investigated. A faster segregation of the Sr cations towards the surface was observed after annealing for 1 h at 600 °C in dry oxygen (pO2= 200 mbar) as the density of grain boundaries at the surface increases (i.e. with a decrease of the grain size). The presence of surface impurities, such as Na, partially inhibits the Sr segregation towards the surface. Optical micrographs and ToF-SIMS chemical imaging showed that the segregation and precipitation of Sr starts at the grain boundaries, with an apparent enrichment of the surface around the grain boundary via Sr surface diffusion.
Druce J, Tellez H, Ishihara T, et al., 2016, Surface composition and oxygen exchange properties of alkaline earth-free perovskites; LaCo0.6Ni0.4O3, 229th Meeting of The-Electrochemical-Society / Symposium on Ionic and Mixed Conducting Ceramics, Publisher: Electrochemical Society, Pages: 71-80, ISSN: 1938-5862
It is well known that alkaline earth additives in conventional perovskite-based solid oxide electrode materials tend to segregate rapidly to the outer surfaces, which is associated with degradation in device performance. It may be possible to avoid these problems by developing materials which do not contain alkaline earth dopants; however, without the oxygen vacancies introduced by alkaline earth doping, the oxygen transport and electro-catalytic properties may suffer. Here, we investigate oxygen transport in the nickel doped lanthanum cobaltite system, and find that the relatively low oxygen diffusivity is dominated by grain boundary transport. Despite this, at higher temperatures, the surface exchange coefficient is comparable to state-of-the-art Sr doped cobaltites. The surface composition of the annealed samples measured by Low Energy Ion Scattering spectroscopy does not appear different from the as-polished samples, suggesting that the surface composition is indeed more stable for this alkaline earth free material.
Kilner JA, Druce J, Ishihara T, 2016, Electrolytes, High-Temperature Solid Oxide Fuel Cells for the 21st Century: Fundamentals, Design and Applications: Second Edition, Pages: 85-132, ISBN: 9780124104532
This chapter discusses fundamental and practical aspects of electrolyte materials for solid oxide fuel cells (SOFCs). We focus on two main families of ceramic electrolytes; those with the fluorite crystal structure (ZrO2- and CeO2-based) and those with perovskite-related structures (La1-xSrxGa1-yMgyO3-(x+y)/2, CaTiO3 and Brownmillerite). Fundamental factors influencing the oxide ionic conductivity are discussed, including the crystal structure, along with strategies to improve the conductivity by substitution with aliovalent cations to introduce extrinsic point defects (oxygen vacancies). Protonic conductivity in perovskite-structured ceramics is also discussed. Finally, we consider some of the alternative materials, including apatite-structured silicates and LAMOX. Although the latter two families are interesting, we highlight some of the challenges to be overcome for their implementation in practical SOFC systems.
Staykov A, Tellez H, Akbay T, et al., 2015, Oxygen activation and dissociation on transition metal free perovskite surfaces, Chemistry of Materials, Vol: 27, Pages: 8273-8281, ISSN: 0897-4756
Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO3) and iron-doped strontium titanate (SrTi1–xFexO3−δ). Our study reveals that while O2 dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O2 molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O2 molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O2 molecules. Our focus on the SrO-terminated surface, rather than the TiO2 layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO3 after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO2-perovskite layer is not exposed at the gas–solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction.
Horsfield AP, Wu J, skinner S, et al., 2015, Why Ni is absent from the surface of La2NiO4+delta?, Journal of Materials Chemistry A, Vol: 3, Pages: 23760-23767, ISSN: 2050-7488
La2NiO4+δ (LNO214) is a potential intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material which belongs to the Ruddlesden–Popper (RP) structure series An+1BnO3n+1. There is interest in this material as it offers a way to avoid Sr segregation and associated degradation, as LNO214 can take in oxygen interstitials and become catalytically active without A-site doping. While the bulk ionic conduction mechanisms are well studied, its surface structure and chemistry are still a matter of debate. Recent experimental studies (both with and without dopants) reveal that it has a La-terminated surface and a highly Ni deficient surface layer. These results disagree with previous computer simulations, and undermine the conventional explanation for the oxygen reduction process at the surface. In this work we evaluate the thermodynamic stability of La2NiO4+δ at IT-SOFC operation temperatures. We find that the decomposition of La2NiO4+δ to produce La2O3 and higher order RP phases is indeed thermodynamically favourable. A hypothesis for the formation mechanism of the La-terminated and Ni deficient surface based on partial decomposition and surface passivation is proposed and evaluated.
Bayliss RD, Cook SN, Kotsantonis S, et al., 2015, Oxygen Ion Diffusion and Surface Exchange Properties of the alpha- and delta-phases of Bi2O3 (vol 4, 1301575, 2014), Advanced Energy Materials, Vol: 5, ISSN: 1614-6832
Taub S, Neuhaus K, Wiemhoefer H-D, et al., 2015, The effects of Co and Cr on the electrical conductivity of cerium gadolinium oxide, Solid State Ionics, Vol: 282, Pages: 54-62, ISSN: 1872-7689
Tellez H, Druce J, Hall A, et al., 2015, Low energy ion scattering: surface preparation and analysis of Cu(In,Ga)Se<sub>2</sub> for photovoltaic applications, PROGRESS IN PHOTOVOLTAICS, Vol: 23, Pages: 1219-1227, ISSN: 1062-7995
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Jaem R, Nakayama M, Manalastas W, et al., 2015, Insights into the Lithium-Ion Conduction Mechanism of Garnet-Type Cubic Li5La3Ta2O12 by ab-Initio Calculations, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 20783-20791, ISSN: 1932-7447
Jalem R, Nakayama M, Manalastas W, et al., 2015, Insights into the lithium-ion conduction mechanism of garnet-type cubic Li5La3Ta2O12 by ab-initio calculations, Journal of Physical Chemistry C, Vol: 119, Pages: 20783-20791, ISSN: 1932-7455
Chen Y, Tellez H, Burriel M, et al., 2015, Segregated Chemistry and Structure on (001) and (100) Surfaces of (La<sub>1-<i>x</i></sub>Sr<i><sub>x</sub></i>)<sub>2</sub>CoO<sub>4</sub> Override the Crystal Anisotropy in Oxygen Exchange Kinetics, CHEMISTRY OF MATERIALS, Vol: 27, Pages: 5436-5450, ISSN: 0897-4756
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Colomer MT, Kilner JA, 2015, Effect of sintering time on structural, microstructural and chemical composition of Ni-doped lanthanum gallate perovskites, JOURNAL OF SOLID STATE CHEMISTRY, Vol: 228, Pages: 167-173, ISSN: 0022-4596
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