Publications
250 results found
Yatoo MA, Kawale SS, Skinner SJ, 2019, Perovskite and layered oxide materials for intermediate temperature solid oxide fuel cells, Intermediate Temperature Solid Oxide Fuel Cells: Electrolytes, Electrodes and Interconnects, Pages: 315-346, ISBN: 9780128174456
Intermediate temperature solid oxide fuel cells (IT-SOFCs) offer an attractive route to low carbon power generation that is both scalable and fuel flexible. In order to produce these devices it is essential that effective electrode materials that act as oxygen reduction catalysts are developed, and ABO3 perovskite-based oxides have been leading contenders amongst cell developers. More recently a range of double perovskite (A2B2O6-δ) and Ruddlesden-Popper (An+1BnO3n+1; n=1,2,3) phases have been considered as potential electrodes, as single materials or as part of a composite system. Here the requirements for effective IT-SOFC cathodes are discussed, and key aspects of their design and synthesis considered. An extensive discussion of the electrochemical properties of the major classes of cathode materials is provided, focusing mainly on the key parameters governing cell performance such as area-specific resistance and electrode durability.
Celikbilek O, Thieu C-A, Agnese F, et al., 2019, Enhanced catalytic activity of nanostructured, A-site deficient(La0.7Sr0.3)0.95(Co0.2Fe0.8)O3−δ for SOFC cathodes, Journal of Materials Chemistry A, Vol: 7, Pages: 25102-25111, ISSN: 2050-7488
Lower operating temperatures (≤650 °C) of solid oxide fuel cells (SOFCs) are sought in order to decrease the system costs and improve material compatibility and durability issues. Here, we report A-site deficient (La0.7Sr0.3)0.95(Co0.2Fe0.8)O3−δ (LSCF) perovskite film as a potential high-performance cathode with microstructural details at the nanometre length scale. This cathode exhibits area specific resistance values of as low as 0.037 and 0.1 Ω cm2 in a symmetrical cell and peak power densities of 1.4 and 1.0 W cm−2 in a Ni/YSZ anode-supported cell at 650 and 600 °C, respectively. These values are among the highest reported data for LSCF-type cathodes. X-ray diffraction and electron microscopy analyses revealed a closely related two-phase perovskite structure for LSCF and a well-dispersed, nanoscale B-site spinel phase (CoFeOx) decorating the LSCF surfaces. Detailed investigations were carried out to correlate the surface to bulk elemental composition changes on the film, the catalytic activity of the spinel phase and the crystal structures of the constituents with the oxygen reduction reaction (ORR) kinetics. The oxygen transport parameters calculated from the electrochemical impedance spectra indicate an increase by one-to-two-orders of magnitude in the oxygen surface-exchange coefficient in comparison to nominally stoichiometric, state-of-the-art La0.6Sr0.4Co0.2Fe0.8O3−δ. Such substantial improvements in the electrode performance were attributed to the catalytically active B-site spinel phase precipitated as a result of the A-site deficiency and to the very high active surface area of the film.
Skinner S, 2019, Recent advances in the understanding of the evolution of surfaces and interfaces in solid oxide cells, Advanced Materials Interfaces, Vol: 6, ISSN: 2196-7350
In solid oxide electrochemical cells the critical processes of fuel oxidation and oxygen reduction occur at surfaces, and ultimately define the performance of the devices. Understanding how these process occur and the role of defects such as dislocations and grain boundaries in these charge transfer processes, and the effect of cation segregation on surface exchange are all topics of current significance. To address these issues, a suite of advanced analytical techniques have been developed to allow in-situ analysis of key processes, complementing established ex-situ techniques. In addition the application of surface sensitive techniques such as surface diffraction and ion scattering spectroscopy, offer unprecedented levels of information on the surface chemistry and structure of functional materials.
Shen Z, Skinner SJ, Kilner JA, 2019, Oxygen transport and surface exchange mechanisms in LSCrF-ScCeSZ dual-phase ceramics, Physical Chemistry Chemical Physics, Vol: 21, Pages: 13194-13206, ISSN: 1463-9076
For the mechanisms by which the oxygen gets incorporated in a dual-phase composite system, three hypotheses, i.e. cation inter-diffusion, spillover type and self-cleaning of the perovskite-structured phase, have been provided in the literature. However, experimentally a consensus on the most likely mechanism is yet to be reached. In this work, a specially fused sample of the lanthanum strontium chromium ferrite (LSCrF)-scandia/ceria-stabilised zirconia (ScCeSZ) dual-phase material was investigated. Among the three potential mechanisms, no obvious cation inter-diffusion was firstly observed. A cleaner surface of the ScCeSZ phase was confirmed in the fused sample than in the isolated ScCeSZ single-phase sample while impurity layers were clearly observed on the LSCrF surface, suggesting the cleaning effect from the perovskite. However, more evidence implies that the cleaning effect is not the only reason for the synergistic effects between these two phases. Observations via SIMS analysis lend strong support to the 'spillover-type' mechanism as the oxygen isotopic fraction on the surface of the ScCeSZ increased compared to the isolated single-phase and as the distance to the heterojunction increases, the oxygen isotopic fraction decreases. Moreover, oxygen depleted layers were clearly seen on the top layers of the LSCrF surface which may be associated with the higher oxygen diffusivity in the surface/sub-surface layers, oxygen grain boundary fast diffusion and the impurities on the perovskite phase. For this sample, a combination of 'spillover' and 'self-cleaning' type mechanisms is suggested to be the potential possibility while the contribution from the cation inter-diffusion for this specific sample is proven to be low.
Skinner S, 2019, Energy Storage and Conversion Materials, Publisher: Inorganic Materials Series, ISBN: 9781788010900
Showcasing recent developments in inorganic materials in an area of societal interest and importance, this book provides an up-to-date introduction to the contemporary use of functional solids in emerging technologies.
Ni N, Wang CC, Jiang SP, et al., 2019, Synergistic effects of temperature and polarization on Cr poisoning of La <inf>0.6</inf> Sr <inf>0.4</inf> Co <inf>0.2</inf> Fe <inf>0.8</inf> O <inf>3-: δ</inf> solid oxide fuel cell cathodes, Journal of Materials Chemistry A, Vol: 7, Pages: 9253-9262, ISSN: 2050-7496
La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) solid oxide fuel cell cathodes were poisoned by Cr at different temperatures and polarization conditions with a Cr-Fe alloy as the interconnect. Cr induced degradation was analysed by electrochemical impedance spectroscopy (EIS) focusing on the electrochemical resistance (R chem ) that reflects the cathode electrochemical properties. It was found that R chem increased more with increasing temperatures. However cathodic polarization exhibited a synergistic effect with the temperature, which accelerated the LSCF cathode degradation at 800 °C while lowering the degree of degradation at 900 °C. By correlating complementary micro- and nano-scale microstructure characterization with the impedance analysis, the degradation mechanisms were investigated. A new Cr incorporation mechanism involving preferential formation of nanometre size Fe-Co-Cr-O spinel particles within the cathode up to the cathode/electrolyte interface was found to be responsible for the reduced degradation at 900 °C combined with cathodic polarization. The new mechanism reveals that the activity of B site elements in LSCF and possibly other perovskite cathodes plays an important role under certain combined temperature and polarization conditions, therefore future research in designing Cr resistant perovskite cathode materials may consider strategies that utilize the exsolution of B site elements for the formation of beneficial spinel phases.
Law J, Franco V, Conde A, et al., 2019, Modification of the order of the magnetic phase transition in cobaltites without changing their crystal space group, Journal of Alloys and Compounds, Vol: 777, Pages: 1080-1086, ISSN: 0925-8388
It has been found that GdBa1-xSrxCo2O6-δ can exhibit consecutive magnetic transitions: antiferromagnetic-ferromagnetic (AFM-FM) transition followed by ferromagnetic-paramagnetic transition (FM-PM), which give rise to a coexistence of inverse and conventional magnetocaloric effect (MCE), respectively. In the pristine compound (x = 0), its AFM-FM transition is shown to belong to a first-order phase transition and the FM-PM to a second-order type. Despite it is widely known that the properties of cobaltites are highly influenced by their oxygen content and type of doping carriers, in this work, further evaluation using magnetocaloric analysis (universal curve method and a quantitative criterion using magnetic field dependence of the magnetic entropy change) reveals that the first-order AFM-FM phase transition converts into a second-order character with just Sr doping of x = 0.1 (despite of having the same space group at room temperature and type of dopant carrier as x = 0), severely affecting its thermomagnetic properties. Moreover, the peaks of these two MCE span over a temperature range that is larger than those reported for cobaltite-type materials, making it closer to room temperature applications.
Harris CM, Skinner SJ, 2019, Redox behaviour and solid solubility of cerium ortho-niobates, Journal of Solid State Chemistry, Vol: 271, Pages: 135-143, ISSN: 0022-4596
Using Ce L III edge X-ray absorption near edge spectroscopy (XANES), it is shown that acceptor dopants introduced to cerium members of the rare earth ortho-niobate series, Ce 1-x Sr x NbO 4±δ and Ce 1-x Ca x NbO 4±δ , are charge compensated by the formation of holes on the cerium sublattice. These spectroscopic studies are complemented by structural studies, using X-ray and neutron powder diffraction, determining that the solubility limit of the strontium and calcium dopants within the CeNbO 4+δ structure is ~10% and ~30% respectively. Under oxidising conditions, the Ce 3+ /Ce 4+ redox couple facilitates reversible redox processes, and it is observed that the Ce 1-x Sr x NbO 4±δ and Ce 1-x Ca x NbO 4±δ materials form commensurate and incommensurately modulated oxygen hyperstoichiometric phases as a function of temperature. Under reducing atmospheres, this redox activity is suppressed, and charge-compensating Ce 4+ holes are annihilated.
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Skinner S, Li C, Pramana S, 2019, Room temperature structure and transport properties of the incommensurate modulated LaNb0.88W0.12O4.06, Dalton Transactions, Vol: 48, Pages: 1633-1646, ISSN: 1477-9234
The crystal structure of a (3 + 2)D incommensurate modulated LaNb0.88W0.12O4.06 phase, a novel oxygen ionic conductor, is refined using a combination of synchrotron X-ray diffraction and electron diffraction data. The superspace group I2/c(α10γ1)00(α20γ2)00 (a = 5.4131(1) Å, b = 11.6432(2) Å, c = 5.2963(1) Å, β = 91.540(1)°, q1 = 0.2847(5)a* + 0.1098(9)c* and q2= −0.1266(9)a* + 0.2953(1)c*) was chosen for the refinement. Similar to other scheelite type modulated structures, the modulation of LaNb0.88W0.12O4.06 stems from the cation occupancy ordering in the xz plane. To facilitate the modulated cation sub-lattice, and to compensate for the difference in their size and charge, the B site polyhedra are distorted by stretching the B–O bond lengths. Consequently, an extension in the B site coordination number from 6 to 8 is observed in the modulated phase. An interconnected 3D network of BOx polyhedra, similar to that of modulated CeNbO4.25, is obtained as a result of the structure modulation, which is not available in the unmodulated parent structure. Tracer diffusivity measurements indicate that the composition is an oxygen ion conductor, which relies on an intersticalcy conduction mechanism. Oxygen tracer diffusivity of 2.46 × 10−9 cm2 s−1, at 750 °C is reported.
Dutton SE, Skinner S, Snyder GJ, 2019, Interfaces in energy materials, APL Materials, Vol: 7
Yatoo M, Aguadero A, Skinner S, 2019, LaPr3Ni3O9.76 as a candidate solid oxide fuel cell cathode: Role of microstructure and interface structure on electrochemical performance, APL Materials, Vol: 7, ISSN: 2166-532X
A new higher-order Ruddlesden-Popper phase composition LaPr3Ni3O9.76 was synthesised by a sol-gel route and studied forpotential intermediate-temperature solid oxide fuel cell cathode properties by electrochemical impedance spectroscopy. The focus of the work was optimisation of the microstructure and interface structure to realise the best performance, and thereforesymmetrical cells after impedance testing were subsequently studied by scanning electron microscopy for post-microstructuralanalysis. It was observed that the cathode ink prepared after ball milling the material and then triple roll milling the prepared inkgave the lowest area specific resistance (ASR) of 0.17Ωcm2 at 700◦C when a La0.8Sr0.2Ga0.8Mn0.2O3-δ (LSGM) electrolyte thathad been previously polished was used. The post-microstructural studies, as expected, showed an improved interface structureand relatively good particle interconnectivity and much less sintering when compared to the symmetrical half-cells constructedusing the ink prepared from the as-synthesised material. The interface structure was further improved significantly by adding a∼10μm thick LSGM ink interlayer, which was reflected in the electrochemical performance, reducing the ASR of the material from 0.17Ωcm2 to 0.08Ωcm2 at 700◦C. This is to date the best performance reported for an n = 3 Ruddlesden-Popper phase material with LSGM as the electrolyte.
Paik H, Berenov A, Skinner SJ, et al., 2019, Hydrogen oxidation kinetics on platinum-palladium bimetallic thin films for solid acid fuel cells, APL Materials, Vol: 7, ISSN: 2166-532X
Solid acid fuel cells (SAFCs) based on the proton-conductive electrolyte CsH2PO4 have shown promising power densities at an intermediate operating temperature of ~ 250 C. However, Pt loadings in SAFCs remain higher than desirable, and the electrocatalysis mechanisms in these devices are still unknown. Here, hydrogen oxidation kinetics on Pt and Pt-Pd bimetallic thin film electrodes on CsH2PO4 have been evaluated to establish the potential for a beneficial role of Pd in SAFC anodes. Symmetric cells fabricated by depositing metal film on both sides of electrolyte discs are characterized for studying hydrogen electro-oxidation across the gas | metal | CsH2PO4 structure. It was found that Pd reacts with CsH2PO4, forming palladium phosphide at the metalelectrolyte interface. Accordingly, the activity of Pd was examined in a bilayer geometry of Pd | Pt | CsH2PO4 | Pt | Pd. The bilayer Pt | Pd films showed much higher activity for hydrogen electro-oxidation than films of Pt alone, as measured by AC impedance spectroscopy. Ex-situ low energy ion scattering and scanning transmission electron microscopy revealed that Pd diffused into the Pt layer under operating conditions. The dramatic impact of Pd along with its presence throughout the film suggests it catalyzes reactions at both the metal-gas and metalelectrolyteinterfaces, as well as increasing hydrogen diffusion rates through the films.
Peral D, Castillo D, Araguas-Rodriguez S, et al., 2019, RELIABLE TEMPERATURE MEASUREMENT WITH THERMAL HISTORY PAINTS: AN UNCERTAINTY ESTIMATION MODEL, ASME Turbo Expo: Turbomachinery Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS
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Shen Z, Kilner J, Skinner S, 2018, Mass Transport in (La0.8Sr0.2)0.95CrxFe1–xO3−δ–Scandia-stabilised zirconia dualphase composite as a dense layer in oxygen transport membranes, The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces, Vol: 122, Pages: 27135-27147, ISSN: 1932-7447
Electrical and oxygen-ion transport in the dual-phase composite systems (La0.8Sr0.2)0.95CrxFe1–xO3−δ (LSCrF) (x = 0.3, 0.5, 0.7)–10 mol % Sc2O3–1 mol % CeO2–89 mol % ZrO2 (10Sc1CeSZ) have been investigated. In these three (x = 0.3, 0.5, 0.7) dual-phase systems, the pure ionic conductor 10Sc1CeSZ dominates the oxygen bulk diffusion whereas the mixed electronic and ionic conductor LSCrF is the predominant phase for oxygen surface exchange and provides pathways for a counter flow of electrons to maintain electrical neutrality. Hence, the electrical conductivity of the dual-phase composite materials increases whereas the diffusion coefficient decreases with increase of the LSCrF content, as expected. However, the surface exchange coefficients as a function of the LSCrF composition show significant scatter. For both phases, once the volume fraction is lower than 30%, the continuous network starts to disconnect and percolation thresholds were observed for both electrical conductivity and oxygen diffusion coefficients in the composites. For the composites with three-dimensional networks of both phases, no obvious difference was observed for the electrical conductivity and oxygen tracer diffusion behavior and it was also confirmed that the microstructures may have a minor effect on the oxygen diffusion behavior of the dual-phase materials. Furthermore, the microscale studies of oxygen diffusion in each phase of the dual-phase composite reveal a synergistic effect between these two phases: the surface exchange coefficient, k, of LSCrF decreases while that for the 10Sc1CeSZ phase k increases when compared with their corresponding isolated single-phase materials.
Niania M, Podor R, Britton TB, et al., 2018, In situ study of strontium segregation in La<inf>0.6</inf>Sr<inf>0.4</inf>Co<inf>0.2</inf>Fe<inf>0.8</inf>O<inf>3- δ</inf>in ambient atmospheres using high-temperature environmental scanning electron microscopy, Journal of Materials Chemistry A, Vol: 6, Pages: 14120-14135, ISSN: 2050-7496
Samples of the solid oxide fuel cell cathode material La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) were annealed using High-Temperature Environmental Scanning Electron Microscopy (HT-ESEM) from room temperature to 1000 °C in atmospheres of pure oxygen, pure water and ambient lab air. Image series of each heat treatment were taken where microstructural changes were observed and compared between samples. Strontium segregation rate was found to be significantly increased in the presence of pure water as compared to pure O2and ambient air. Electron backscattered diffraction (EBSD) was performed in order to assess the effect of crystal orientation on particle formation and surface sensitive chemical analysis techniques were used to determine the chemical changes at the grain surface as a result of the different heat treatments. It was shown that crystal orientation affected the nature and growth rate of strontium-based particles, however, due to the pseudo-symmetry of La0.6Sr0.4Co0.2Fe0.8O3-δ, precise crystal orientation relationships could not be determined. The chemical composition of the grain surface was found to be approximately equal under each atmosphere.
Kilner J, Shen Z, Skinner SJ, 2018, Electrical conductivity and oxygen diffusion behaviour of the (La0.8Sr0.2)0.95CrxFe1-xO3-δ (x=0.3, 0.5 and 0.7) A-site deficient perovskites, Physical Chemistry Chemical Physics, Vol: 20, Pages: 18279-18290, ISSN: 1463-9076
Lanthanum strontium chromite ferrite ((La0.8Sr0.2)0.95CrxFe1−xO3−δ, LSCrF) pellets with 5% A-site deficiency were fabricated and the electrical conductivity and oxygen diffusion behaviour with different Cr substitution levels (x = 0.3, 0.5 and 0.7) were investigated. As the Cr content increased, the electrical conductivity increased and then a maximum value was achieved at x = 0.7. In the oxygen diffusion studies, all the measured materials present good surface exchange rates (>9 × 10−8 cm s−1 at 900 °C) while the bulk diffusivity of the investigated materials decreased as the Cr substitution level increased: at 900 °C the oxygen diffusion coefficients of the LSCrF materials (x = 0.3, 0.5 and 0.7) are 1.1 × 10−10 cm2 s−1, 3.7 × 10−12 cm2 s−1 and 8.6 × 10−13 cm2 s−1, respectively. Oxygen diffusion in the perovskite materials (LSCrF) is shown to be bulk diffusion limited and it was found that analysis on this type of material using the line scan mode in Time-of-Flight Secondary Ion Mass Spectrometry may result in significant underestimation of the surface exchange coefficient due to the oxygen saturation, while the depth profile mode provides more reliable results but the obtained surface exchange coefficients may also only reach a lower limit. Moreover, fast grain boundary diffusion behaviour was observed in the LSCrF (x = 0.7) material and the Le Claire, and Chung and Wuensch approximations were applied to analyse the oxygen diffusion profiles. For this material, the two approximations provided similar results for the grain boundary product (Dgbδ) and under the assumption that the width of a grain boundary is on the nanometre scale, the oxygen diffusion coefficient of the grain boundaries was about 3–4 orders of magnitude higher than that of the bulk at temperatures ≤900 °C.
Yatoo MA, Du Z, Zhao H, et al., 2018, La2Pr2Ni3O10±δ Ruddlesden-Popper phase as potential intermediate temperature-solid oxide fuel cell cathodes, Solid State Ionics, Vol: 320, Pages: 148-151, ISSN: 0167-2738
Ruddlesden-Popper phases are layered oxides composed of nABO 3 perovskite layers sandwiched between two AO rock-salt layers. Herein a new composition of n = 3 Ruddlesden-Popper phases, La 2 Pr 2 Ni 3 O 10±δ , synthesised by the citrate sol-gel method is reported. A preliminary microstructure investigation combined with studies of the electrochemical performance of this new composition, La 2 Pr 2 Ni 3 O 10±δ as a potential cathode material in both symmetrical and single cell configurations is reported. The area specific resistance of the La 2 Pr 2 Ni 3 O 10±δ cathode was found to be 0.34 Ω cm 2 at 800 °C, which is significantly better than previous reports for the La 4 Ni 3 O 10±δ analogue under similar conditions. A modest peak power density of 0.19 W cm −2 at 800 °C was found, whilst electrode adhesion was identified as contributing to the modest performance.
Niania M, Podor R, Britton TB, et al., 2018, Correction: In situ study of strontium segregation in La<inf>0.6</inf>Sr<inf>0.4</inf>Co<inf>0.2</inf>Fe<inf>0.8</inf>O<inf>3- δ</inf>in ambient atmospheres using high-temperature environmental scanning electron microscopy (J. Mater. Chem. A (2018) DOI: 10.1039/c8ta01341a), Journal of Materials Chemistry A, Vol: 6, Pages: 14464-14464, ISSN: 2050-7488
© 2018 The Royal Society of Chemistry. Correction for 'In situ study of strontium segregation in La0.6Sr0.4Co0.2Fe0.8O3-δin ambient atmospheres using high-temperature environmental scanning electron microscopy' by Mathew Niania et al., J. Mater. Chem. A, 2018, DOI: 10.1039/c8ta01341a. The authors regret that the name of the first author of ref. 37 was displayed as "F. Pisigkin". The correct name should be "F. Piskin". The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.
Araguás Rodríguez S, Jelínek T, Michálek J, et al., 2018, Accelerated thermal profiling of gas turbine components using luminescent thermal history paints, Journal of the Global Power and Propulsion Society, Vol: 2, Pages: 344-361, ISSN: 2515-3080
Environmental requirements to reduce CO2 emissions and the drive towards higher efficiencies have resulted in increased operating temperatures in gas turbines. Subsequently, Original Equipment Manufacturer (OEMs) require improved component design and material selection to withstand the harsher conditions. This demands rapid evaluation of new components and their surface temperature to accelerate their market entry. Accurate temperature information proves key in the design of more efficient, longer-lasting machinery and in monitoring thermal damage. A number of traditional temperature measurement techniques are available, but can incur a number of limitations. Online temperature measurements, such as pyrometry or phosphor thermography, often require optical access to the component during operation and are therefore not suitable for inaccessible components. Other options including thermocouples can only provide point measurements and cannot deliver profiles across the surface. Offline techniques store temperature information that can be measured and analysed following operation. Several of these, however, are of destructive nature, can affect local thermal gradients and only provide point measurements. This article discusses an innovative offline measurement technique: luminescent Thermal History Paints (THPs). THPs are comprised of ceramic pigments in a binder matrix that can be applied to any hot component as a thin coating. These pigments are doped with optically active ions, which will phosphoresce when excited with a light source. The coating material experiences irreversible structural changes depending on the temperature it is exposed to. Changes in the material structure are reflected in its phosphorescent properties, which are measured with standard optical instrumentation at any surface location. Since the changes are permanent, the temperature information is stored in the coating and can be extracted after operation. Following calibration, it is therefore p
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.
Shih D, Aguadero A, Skinner SJ, 2018, Improvement of Ionic Conductivity in A-site Lithium Doped Sodium Bismuth Titanate, Solid State Ionics, Vol: 317, Pages: 32-38, ISSN: 0167-2738
Oxide-ion conductors play a significant role in various applications such as solid oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Recently, high ionic conductivity (~ 1 × 10− 4 S cm− 1 at 600 °C) was found in sodium bismuth titanate (NBT), which originates from oxygen vacancies compensating the introduced Bi-deficiency. By providing pathways with low diffusion barriers, the highly polarizable Bi3 + ions with 6s2 lone pair electrons and weak Bisingle bondO bonds are also beneficial for the migration of oxygen ions. Here we report the influence of lithium doping on the electrical properties of NBT. The optimal doping level of 4 at% Li on the Bi-site improves the ionic conductivity by one order of magnitude to ~ 7 × 10− 3 S cm− 1 at 600 °C without changing the conduction mechanism, which could be attributed to an increase in the oxygen vacancy concentration based on an acceptor doping mechanism. A further increase in Li content does not improve the total conductivity. Oxygen diffusion data were acquired by the Isotope Exchange Depth Profile (IEDP) method in combination with Secondary Ion-Mass Spectrometry (SIMS). The oxygen self-diffusion coefficients (e.g. 7.04 × 10− 9 cm2 s− 1 at 600 °C) are in excellent agreement with the values derived from impedance spectroscopy data, suggesting that the oxygen ions are the main charge carriers in the system. Furthermore, a degradation test was performed for 100 h under a variety of atmospheres, showing only a slight decrease in conductivity in both air and oxygen atmospheres attributed to the loss of material from the A-site. Comparison with other oxide-ion conductors indicates that Li-doped NBT materials are promising candidates for intermediate temperature SOFC applications.
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.
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, Ravella UK, liu J, et al., 2017, Cationic Interdiffusion at the SOFC Electrolyte/Cathode Interface in La2Mo2O9/La0.8Sr0.2MnO3-δ, Chemistry Select, Vol: 2, Pages: 5616-5623, ISSN: 2365-6549
In this work cation diffusion between a La2Mo2O9 (LM) ionic conductor and the conventional Solid Oxide Fuel Cell (SOFC) cathode material La0.8Sr0.2MnO3-δ (LSM), was probed using secondary ion mass spectrometry (SIMS), and diffusion coefficients of Sr, Mo and Mn cations within both materials evaluated. Diffusion coefficients extracted from samples with a Sr solution deposited on the LM pellets and from a Mo solution deposited on LSM pellets were found to be orders of magnitude higher than the cross-diffusion through the interface between two dense pellets in direct contact. These differences may be due to uncertainty in determining the interface position, or to a real dependence on the source of the diffusing cation. In the most favorable case, that of pellets in direct contact, extrapolation of diffusion coefficients down to a typical SOFC operating temperature, 800 °C, show that Mo diffusion in LSM (diffusion coefficient ∼ 10−14 cm2.s−1) is much higher than Sr or Mn diffusion in LM, and incompatible with use in a SOFC device, unless an efficient buffer layer is used.
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.
Tonus F, Bahout M, Dorcet V, et al., 2017, A-site order–disorder in the NdBaMn2O5+d SOFC electrode material monitored in situ by neutron diffraction under hydrogen flow, Journal of Materials Chemistry A, Vol: 5, Pages: 11078-11085, ISSN: 2050-7496
The A-site disordered perovskite manganite SOFC electrode material, Nd0.5Ba0.5MnO3, has been obtained by heating the A-site-ordered and vacancy ordered layered double perovskite, NdBaMn2O5, in air at 1300 °C for 5 h. Combined Transmission electron microscopy (TEM) images and Neutron powder diffraction (NPD) analysis at 25 °C revealed that Nd0.5Ba0.5MnO3 has a pseudotetragonal unit cell with orthorhombic symmetry (space group Imma, √2 ap × 2 ap × √2 ap) at 20 °C with the cell dimensions a = 5.503(1) Å, b = 7.7962(4) Å, c = 5.502(1) Å, in contrast to Pm-3m or Cmcm that have been previously stated from X-ay diffraction studies. The in situ neutron diffraction study carried out on Nd0.5Ba0.5MnO3 in hydrogen flow up to T~ 900 °C, allows monitoring the A-site cation disorder-order structural phase transition of this representative member of potential SOFC anode materials between air sintering conditions and hydrogen working conditions. Oxygen loss form Nd0.5Ba0.5MnO3 proceeds with retention of A-site disorder until the oxygen content reaches the Nd0.5Ba0.5MnO2.5 composition at 600 °C. The phase transition to layered NdBaMn2O5 with localization of the oxygen vacancies in the Nd layer proceeds at 800 °C with retention of the oxygen content. Impedance spectroscopy measurements for the A-site ordered electrode material, NdBaMn2O6, screen printed on a Ce0.9Gd0.1O2-δ (CGO) electrolyte showed promising electrochemical performance with polarization resistance of 1.09 Ω cm2 at 700 °C in air without any optimization.
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.
Pramana SS, Cavallaro A, Qi J, et al., 2017, Understanding surface structure and chemistry of single crystal lanthanum aluminate, Scientific Reports, Vol: 7, ISSN: 2045-2322
The surface crystallography and chemistry of a LaAlO3 single crystal, a material mainly used as a substrate to deposit technologically important thin films (e.g. for superconducting and magnetic devices), was analysed using surface X-ray diffraction and low energy ion scattering spectroscopy. The surface was determined to be terminated by Al-O species, and was significantly different from the idealised bulk structure. Termination reversal was not observed at higher temperature (600°C) and chamber pressure of 10-10 Torr, but rather an increased Al-O occupancy occurred, which was accompanied by a larger outwards relaxation of Al from the bulk positions. Changing the oxygen pressure to 10-6 Torr enriched the Al site occupancy fraction at the outermost surface from 0.245(10) to 0.325(9). In contrast the LaO, which is located at the next sub-surface atomic layer, showed no chemical enrichment and the structural relaxation was lower than for the top AlO2 layer. Knowledge of the surface structure will aid the understanding of how and which type of interface will be formed when LaAlO3 is used as a substrate as a function of temperature and pressure, and so lead to improved design of device structures.
Skinner SJ, Yanez-Gonzalez A, Ruiz-Trejo E, et al., 2016, Development of an optical thermal history coating sensor based on the oxidation of a divalent rare earth ion phosphor, Measurement Science & Technology, Vol: 27, ISSN: 1361-6501
The measurement of temperatures in gas turbines, boilers, heat exchangers and othercomponents exposed to hot gases is essential to design energy efficient systems and improvemaintenance procedures. When on-line measurements, such as those performed withthermocouples and pyrometers, are not possible or inconvenient, the maximum temperaturesof operation can be recorded and measured off-line after operation. Although thermal paintshave been used for many years for this purpose, a novel technique based on irreversiblechanges in the optical properties of thermographic phosphors, can overcome some of thedisadvantages of previous methods.In particular, oxidation of the divalent rare earth ion phosphor BaMgAl10O17:Eu(BAM:Eu) has shown great potential for temperature sensing between 700 °C and 1200 °C.The emission spectra of this phosphor change with temperature, which permits to define anintensity ratio between different lines in the spectra that can be used as a measurand of thetemperature. In this paper, the study of the sensing capabilities of a sensor coating based onBAM:Eu phosphor material is addressed for the first time. The sensitivity of the intensityratio is investigated in the temperature range from 800 °C to 1100 °C, and is proved to beaffected by ionic diffusion of transition metals from the substrate. The use of an interlayermade of zirconia proves efficient in reducing ionic diffusion and coatings with this diffusionbarrier present sensitivity comparable to that of the powder material.
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