Publications
249 results found
Aguadero A, Wilson G, Seymour I, et al., 2022, Fast redox kinetics in SrCo1-xSbxO3-δ perovskites for thermochemical energy storage and oxygen separation, Journal of The Electrochemical Society, Vol: 169, ISSN: 0013-4651
The use of perovskite materials for thermochemical energy storage and oxygen separation has been gaining momentum in recent years due to their ability to topotactically exchange large volumes of oxygen, and their chemical and structural flexibility. B-site substituted SrCoO3-δ derivatives have previously been investigated as promising materials for intermediate temperature solid oxide fuel cell cathodes due to the stabilization of a 3 C perovskite structure with high electronic and ionic conductivity that allows large oxygen storage capabilities. Here, antimony-substituted strontium cobalt oxides are investigated and identified as new candidate materials for thermochemical oxygen separation applications. In this work we shed light on the exceptional redox kinetics and cyclability of antimony-substituted variants undergoing oxygen exchange at intermediate temperatures (500 to 800 °C). Through the use of density functional theory and isothermal gas atmosphere switching, we demonstrate how the inductive effect of the more electronegative antimony dopants in the Co position, facilitates the kinetics of metal oxide oxidation, whilst hindering reduction reactions. SrCo0.95Sb0.05O3−δ was identified to isothermally evolve 3.76 cm3 g−1 of oxygen at 500 °C and calculated to produce up to 10.44 cm3 g−1 under temperature-swing reaction configurations aligning with previously reported materials.
Skinner S, Sha Z, Basbus J, et al., 2022, In situ neutron diffraction study of BaCe0.4Zr0.4Y0.2O3-δ proton conducting perovskite: insight on phase transition and proton transport mechanism., Journal of Materials Chemistry A, Vol: 10, Pages: 9037-9047, ISSN: 2050-7488
The understanding of protonic defect transport mechanism in Ba(Ce,Zr)O3 perovskites oxides and its proper temperature range of conductivity are fundamentals for materials design and their technological applications as electrolyte for solid oxide fuel and electrolyzer cells, isotopic separation membranes and hydrogen sensors. Structural features of the material, as well as the lattice distortions and proton diffusion, are key factors defining the protonic conduction. The crystal structure of protonated and deuterated BaCe0.4Zr0.4Y0.2O3−δ (BCZY) perovskite and its correlation with protonic transport was studied by in situ Neutron Powder Diffraction (NPD) and complementary Thermogravimetry (TG), Quasi-Elastic Neutron Scattering (QENS) and Isotope Exchange Depth Profiling (IEDP) techniques. A 2nd order phase transition from rhombohedral to cubic symmetry takes place at intermediate temperatures (400 - 600 ºC). Dynamic measurements of NPD allowed the detection of the temperature of the phase transition for BCZY at around 520 ºC. Crystallographic and microstructural parameters, including deuterium occupancy and anisotropic thermal parameters, were determined from high resolution NPD data. The deuterium (and oxygen) occupancy for pre-hydrated BCZY is maximum at low temperature and decreases above 400 ºC, even through the phase transition and beyond 600 ºC. By contrast, proton diffusion increases with temperature above the phase transition. The combination of both effects, deuterium content and diffusion coefficient, explains previous results showing that the proton conductivity dominates the ionic conductivity over the oxygen vacancy mechanism until 600 ºC. The phase transition is mainly related to oxygen sublattice relaxation and does not impact on the protonic transport mechanism.
Skinner SJ, Birss V, Rupp J, et al., 2022, Introduction to the special issue in honour of Prof. John Kilner's 75th birthday, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 10, Pages: 2149-2151, ISSN: 2050-7488
Heiba HF, Bullen JC, Kafizas A, et al., 2022, The determination of oxidation rates and quantum yields during the photocatalytic oxidation of As(III) over TiO2, Journal of Photochemistry and Photobiology A: Chemistry, Vol: 424, Pages: 113628-113628, ISSN: 1010-6030
The determination of reaction rates for the photocatalytic oxidation (PCO) of arsenite (As(III)) using TiO2 under UV radiation is challenging due to the numerous experimental processes. This includes chemical processes running simultaneously with PCO (e.g. adsorption of arsenic species, direct UV photolysis of As(III)) and the analytical approach used (e.g. whether As(III) or As(V) are measured and used in the calculation of the PCO rate). The various experimental approaches used to date have led to oxidation rates and rate constants which vary by orders of magnitude and contradicting information on rate laws. Here we present the results of a critical examination of possible controls affecting the experimental determination of PCO rates. First, we demonstrate that the choice of analytical technique is not critical, provided that the rate constants are calculated based on the depletion of As(III) after correction of the directly adsorbed As(III). Second, we show the correction of the directly adsorbed As(III) at each time interval is best done by running two parallel experiments (one under UV and the other in dark) instead of running sequential experiment (i.e. running the experiment in the dark then turning on the UV lamp). These findings are supported by XPS analysis of the oxidation state of TiO2-sorbed As. Third, we demonstrate that photolysis by the light source itself, as well as the chemical composition of the solution (i.e. the effect of HEPES and the ionic strength), can significantly increase As(III) oxidation rates and need to be corrected. Finally, to determine the quantum yield of As(III) oxidation, we measured the photon absorption by the TiO2 photocatalyst. Our results showed that the quantum yield (Ø) for this oxidation reaction was low, and in the region of 0.1 to 0.2 %.
Yatoo MA, Skinner SJ, 2022, Ruddlesden-Popper phase materials for solid oxide fuel cell cathodes: A short review, Materials Today: Proceedings, Vol: 56, Pages: 3747-3754, ISSN: 2214-7853
In the last couple of decades, researchers have been working on Ruddlesden-Popper phases to realise them as components of solid oxide cells. Ruddlesden-Popper phase materials have been particularly proposed as materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). As such a sizeable literature was produced on Ruddlesden-Popper phases and in this short review we look at these studies with a particular focus on the structural chemistry, oxygen transport and electrical conductivity, electrochemical performance, and stability of these materials under operating conditions. More specifically, the materials have been studied for cathodes and, therefore, we believe a review dedicated to cathode applications of these materials will be beneficial for the community. A brief outlook on the future directions in the field will also be provided.
Jayaseelan DD, Pramana S, Grasso S, et al., 2021, Fabrication and characterisation of single-phase Hf2Al4C5 ceramics, Journal of the European Ceramic Society, Vol: 42, Pages: 1292-1301, ISSN: 0955-2219
Single-phase Hf2Al4C5 ternary carbide was fabricated from Hf/Al/C powder mixtures by pressure assisted sintering techniques such as hot pressing and spark plasma sintering at 1900 °C for 3 h and 10 min, respectively. XRD confirmed that the ternary carbide started to form at temperatures as low as 1500 °C and with total formation of Hf2Al4C5 after reactive sintering for 1 h at 1900 °C. It is evident from HRTEM that two Hf-C layers were sandwiched with 4 Al-C layers (Al4C3) in the Hf2Al4C5 ternary carbide. Tight interlocking of grains, faceted grains and stacking faults were occasionally observed. Thermal conductivity of Hf2Al4C5 is measured to be 14 w m−1k−1 from room temperature to 1300 °C. The oxidation studies carried out at 1300 °C for 3 h reveal that the oxidation layer thickness is around 220 μm and it contains microcracks closer to sample surface whereas the interface looks seamless without any cracking or spallation of the oxide layer.
Skinner S, Develos-Bagarinao K, Celikbilek O, et al., 2021, On the role of surfaces and interfaces on electrochemical performance and long-term stability of nanostructured LSC thin film electrodes, Journal of Materials Chemistry A, Vol: 10, Pages: 2445-2459, ISSN: 2050-7488
Innovative concepts for novel electrode structures have been actively pursued over recent years to achieve both superior electrochemical performance as well as long-term stability for the development of solid oxide cells (SOCs) with efficient energy conversion. In this study, towards understanding the role of surfaces and interfaces on electrochemical performance and long-term stability of nanostructured La0.6Sr0.4CoO3−δ (LSC) thin film electrodes, a systematic investigation of the effect of deposition temperature and long-term annealing was conducted. The surface conditions of the LSC thin films, in terms of the proportion of surface-bound Sr and valence state of Co, are highly influenced by the deposition temperature; however, prolonged annealing at 700 °C in air of LSC thin films deposited at various temperatures essentially transforms their surfaces to a final state having similar chemical environment and crystalline properties. On the other hand, Sr diffusion across the LSC/GDC interfaces is promoted at higher deposition temperatures and is further accelerated with prolonged heating at 700 °C. Significant improvement in the electrochemical performance for symmetrical cells using LSC thin films is attributed to two main factors: enhancement of the surface exchange property as mediated by a distinctive nanostructure that allows the retention of a high porosity, and better stability of the electrode–electrolyte interfaces owing to the suppressed cation diffusion. This work paves the way to obtaining highly active and durable electrodes through tuning surfaces and interfaces and provides guidance for designing novel electrode materials with excellent performance for SOC applications.
Skinner S, Cali E, Shen Z, et al., 2021, Significantly enhanced oxygen transport properties in mixed conducting perovskite oxides under humid reducing environments, Chemistry of Materials, Vol: 33, Pages: 8469-8476, ISSN: 0897-4756
Mixed ionic and electronic conducting (MIEC) perovskite oxides (ABO3) have a substantial role in carbon-neutral clean energy conversion and storage technologies. Owing to their favorable catalytic properties, high ionic and electronic conductivity, and chemical and redox stability, MIEC perovskite oxides are promising electrode materials in multiple applications, such as solid oxide fuel/electrolysis cells, oxygen transport membranes, metal–air batteries, electrochemical sensors, and electrocatalysts for water splitting. Here, taking (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ (LSCrF8255) as a model MIEC perovskite oxide, we demonstrate that the oxygen mass transport properties are significantly enhanced under a humid reducing water vapor environment (pO2 < 1 mbar, pH2O = 30 mbar) by up to 4 orders of magnitude compared to those measured under dry (pO2 = 200 mbar) and wet (pO2 = 200 mbar, pH2O = 30 mbar) oxygen atmospheres. A 0.8 eV decrease in the activation energy for oxygen bulk diffusion was also found under water vapor, and a decrease in activation energy of 0.7 eV for water surface exchange compared to oxygen surface exchange was found. The mechanisms underpinning these enhancements were explored. Furthermore, LSCrF8255 has also exhibited a consistent surface composition evolution regarding Sr segregation and phase separation and an excellent bulk stability under both oxidizing and reducing environments at elevated temperatures.
Celikbilek O, Wells M, Driscoll JL, et al., 2021, Ag/Ce<sub>0.9</sub>Gd<sub>0.1</sub>O<sub>2-</sub> <sub>δ</sub> Based Vertically Aligned Nanocomposite Thin Film Electrodes for Low-Temperature Solid Oxide Cells, ECS Meeting Abstracts, Vol: MA2021-02, Pages: 1377-1377
Skinner S, Cavallaro A, Cali E, et al., 2021, Analysis of H2O-induced surface degradation in SrCoO3-derivatives and its impact on redox kinetics, Journal of Materials Chemistry A, Vol: 9, Pages: 24528-24538, ISSN: 2050-7488
Substituted SrCoO3 perovskites have been proposed as promising mixed ionic electronic conductors for a range of applications including intermediate temperature solid oxide fuel cells (IT-SOFCs), electrolysers and thermochemical water splitting reactors for H2 production. In this work we investigate the effect of sample exposure to water in substituted SrCoO3 powders and thin films and correlate it with the degradation of oxygen mobility and kinetics. SrCo0.95Sb0.05O3−δ (SCS) thin films have been deposited on different single crystal substrates by pulsed laser deposition (PLD). After water cleaning and post annealing at 300 °C, the sample surface presented an increase of the SrO-surface species as observed by ex situ X-ray Photoemission Spectroscopy (XPS) analysis. This increase in SrO at the sample surface has also been confirmed by the Low Energy Ion Scattering (LEIS) technique on both SCS thin film and powder. Thermochemical water splitting experiments on SCS and SrCo0.95Mo0.05O3−δ (SCM) powder revealed a phase degradation under water oxidising conditions at high temperature with the formation of the trigonal phase Sr6Co5O15. Transmission Electron Microscopy (TEM) analysis of SCS powder treated with water suggests that this phase degradation could already superficially start at Room Temperature (RT). By isotope exchange depth profile experiments on SCS thin films, we were able to quantify the oxygen diffusivity in this SCS surface decomposed layer (D* = 5.1 × 10−17 cm2 s−1 at 400 °C). In the specific case of bulk powder, the effect of water superficial decomposition translates into a lower oxidation and reduction kinetics as demonstrated by comparative thermogravimetric analysis (TGA) studies.
Skinner SJ, Li C, Pramana S, 2021, Investigating the modulated structures in the La(Nb, W)O<sub>4+d</sub> family of oxide ion conductors, Publisher: INT UNION CRYSTALLOGRAPHY, Pages: C396-C396, ISSN: 2053-2733
Williams N, Seymour I, Leah R, et al., 2021, Intrinsic Dipole Moments and Electron Transfer at the MIEC-Gas Interfaces, ECS Meeting Abstracts, Vol: MA2021-03, Pages: 171-171
<jats:p> The catalytic and transport properties of mixed ionic-electronic conducting (MIEC) electrodes for solid oxide fuel cells (SOCs) are well documented and utilised, yet poorly understood. In the current study, a novel kinetic framework for the electrochemical behaviour of hydrogen at the MIEC-gas interface will be discussed for three parallel treatments: as an ideal gas, as an adsorbate on the electrode surface and as a charge carrier dissolved in the oxide lattice. This model gives a physically meaningful reason for the enhancement in electrochemical activity of a MIEC electrode as the steam pressure is increased in both fuel cell and electrolysis modes, and is ubiquitous for any electrochemical system where the electric double layer is described as a Heaviside step function.</jats:p> <jats:p>The process of charge transfer at the MIEC electrode/gas interface comprises of ambipolar exchange of ions and electronic species. The result of such a process causes charge separation and an associated dipole moment at the electrode surface. By applying an overpotential η to the MIEC electrode, an electrostatic surface potential shift away from equilibrium may be established where an effective double layer is formed between the electrode surface and the adsorbed species. Although no net charge transfer occurs, this surface potential shift modifies the surface chemistry and is the driving force for the ambipolar exchange of ions and electronic species.</jats:p> <jats:p>Density Functional Theory (DFT) calculations were used to study the electrostatic potential at the ceria [111] surface, where we were able to calculate the dipole moment and adsorption energy as a function of hydroxyl coverage. The theory of the electrostatic potential at the MIEC-gas interface was then applied to predict the surface electrochemical properties as a function of local overpotential. The mechanistic understanding gained from this
Skinner S, Williams N, Seymour I, et al., 2021, Theory of the electrostatic surface potential and intrinsic dipole moments at the mixed ionic electronic conductor (MIEC)–gas interface, Physical Chemistry Chemical Physics, Vol: 23, Pages: 14569-14579, ISSN: 1463-9076
The local activation overpotential describes the electrostatic potential shift away from equilibrium at an electrode/electrolyte interface. This electrostatic potential is not entirely satisfactory for describing the reaction kinetics of a mixed ionic–electronic conducting (MIEC) solid-oxide cell (SOC) electrode where charge transfer occurs at the electrode–gas interface. Using the theory of the electrostatic potential at the MIEC–gas interface as an electrochemical driving force, charge transfer at the ceria–gas interface has been modelled based on the intrinsic dipole potential of the adsorbate. This model gives a physically meaningful reason for the enhancement in electrochemical activity of a MIEC electrode as the steam and hydrogen pressure is increased in both fuel cell and electrolysis modes. This model was validated against operando XPS data from previous literature to accurately predict the outer work function shift of thin film Sm0.2Ce0.8O1.9 in a H2/H2O atmosphere as a function of overpotential.
Irvine J, Rupp JLM, Liu G, et al., 2021, Roadmap on inorganic perovskites for energy applications, Journal of Physics: Energy, Vol: 3, Pages: 031502-031502
<jats:title>Abstract</jats:title> <jats:p>Inorganic perovskites exhibit many important physical properties such as ferroelectricity, magnetoresistance and superconductivity as well their importance as energy materials. Many of the most important energy materials are inorganic perovskites and find application in batteries, fuel cells, photocatalysts, catalysis, thermoelectrics and solar thermal. In all these applications, perovskite oxides, or their derivatives offer highly competitive performance, often state of the art and so tend to dominate research into energy material. In the following sections, we review these functionalities in turn seeking to facilitate the interchange of ideas between domains. The potential for improvement is explored and we highlight the importance of both detailed modelling and <jats:italic>in situ</jats:italic> and operando studies in taking these materials forward.</jats:p>
Shen Z, Wu J, Shorvon MW, et al., 2021, Partially Anion-Ordered Cerium Niobium Oxynitride Perovskite Phase with a Small Band Gap, Chemistry of Materials, Vol: 33, Pages: 4045-4056, ISSN: 0897-4756
Skinner S, Zhou Y, Shiraiwa M, et al., 2021, Protonic conduction in BaNdInO4 structure achieved by acceptor doping, Chemistry of Materials, Vol: 33, Pages: 2139-2146, ISSN: 0897-4756
The potential of calcium-doped layered perovskite compounds, BaNd1–xCaxInO4–x/2 (where x is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1–2 orders of magnitude higher than those of the pristine material, BaNdInO4. The highest total conductivity of 2.6 × 10–3 S cm–1 was obtained in the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250–750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H2O) was 1.3 × 10–4 S cm–1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the x = 0.2 sample, we conclude that experimental evidence indicates that BaNd1–xCaxInO4–x/2 is a fast proton conductor.
Skinner S, Yashima M, Zhou Y, et al., 2021, High Oxide-Ion Conductivity through the Interstitial Oxygen Site in Ba7Nb4MoO20-Based Hexagonal Perovskite Related Oxides, Nature Communications, Vol: 12, ISSN: 2041-1723
Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10-26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10-4 S cm-1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1-O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.
Williams NJ, Seymour ID, Leah RT, et al., 2021, Intrinsic dipole moments and electron transfer at MIEC-Gas Interfaces, ECS Transactions, Vol: 103, Pages: 973-979, ISSN: 1938-6737
The magnitude of the electrostatic potential at the surface of a mixed ionicelectronic conducting (MIEC) solid-oxide cell (SOC) electrode is dependent on the intrinsic dipole moment of adsorbed gas species. Using density functional theory, we have investigated the electrostatic nature of hydroxyl adsorbates and the kinetics of electron transfer in the water electrolysis reaction at the pristine ceria-gas interface. First principles data ware used in a generalised kinetic model to predict reaction kinetics near equilibrium and under an applied overpotential. The mechanistic understanding gained from this model is widely applicable to a range of MIEC systems and provides a basis upon which the operating conditions can be tailored.
Celikbilek O, Cavallaro A, Kerherve G, et al., 2020, Thermal History Induced Surface Restructuring of La<sub>0.6</sub>Sr<sub>0.4</sub>CoO<sub>3-δ </sub>thin-Film Solid Oxide Electrodes and Its Significance for the Catalytic Activity and Stability, ECS Meeting Abstracts, Vol: MA2020-02, Pages: 3763-3763
<jats:p> Thin-films deposited by Pulsed Laser Deposition (PLD) have enabled the fabrication of cathodes for solid oxide cells (SOC) to possess various microstructures, crystalline states and surface chemistry. These films are often subjected to various processing and thermal treatments which influence the chemical activity and stability of the films. Here, we investigate how the catalytic activity and stability is affected by the thermally induced surface restructuring in complex transition metal oxides, in particular, La<jats:sub>0.6</jats:sub>Sr<jats:sub>0.4</jats:sub>CoO<jats:sub>3-δ</jats:sub> (LSC) thin films grown by PLD.(1) To this end, the influence of substrate temperature during PLD as well as post-annealing treatments on the film microstructure and surface chemistry were studied. In comparison to high-temperature grown films, the post-annealed low-temperature grown films showed 2-fold lower degradation rate in the long-term electrochemical tests at 500 °C. The differences in the electrochemical stability were found to be related to the initial microstructure and morphology of the films (Fig 1). The post-annealed low-temperature grown film showed localised Sr-segregation into protruding particles, leaving the remaining surface with catalytically active stoichiometry. On the other hand, the high-temperature grown films showed a fully covered surface with Sr-rich particles, which continued to thicken with time. Here we emphasise the importance of processing and thermal history in the elemental surface distribution, especially for the stability of LSC64 electrodes and propose that it should be considered as among the main pillars in the design of the active surface.</jats:p> <jats:p>Fig. 1 Top-view (a,d,g) and cross-section view (b,e,h) SEM micrographs and corresponding AFM images (c,f,i) showing the topography of LSC64 films deposited by the PLD: (a−c) LT-grown film, (d&min
Bullen J, Kenney J, Fearn S, et al., 2020, Improved accuracy in multicomponent surface complexation models using surface-sensitive analytical techniques: adsorption of arsenic onto a TiO2/Fe2O3 multifunctional sorbent, Journal of Colloid and Interface Science, Vol: 580, Pages: 834-849, ISSN: 0021-9797
Many novel composite materials have been recently developed for water treatment applications, with the aim of achieving multifunctional behaviour, e.g. combining adsorption with light-driven remediation. The application of surface complexation models (SCM) is important to understand how adsorption changes as a function of pH, ionic strength and the presence of competitor ions. Component additive (CA) models describe composite sorbents using a combination of single-phase reference materials. However, predictive adsorption modelling using the CA-SCM approach remains unreliable, due to challenges in the quantitative determination of surface composition. In this study, we test the hypothesis that characterisation of the outermost surface using low energy ion scattering (LEIS) improves CA-SCM accuracy. We consider the TiO2/Fe2O3 photocatalyst-sorbents that are increasingly investigated for arsenic remediation. Due to an iron oxide surface coating that was not captured by bulk analysis, LEIS significantly improves the accuracy of our component additive predictions for monolayer surface processes: adsorption of arsenic(V) and surface acidity. We also demonstrate non-component additivity in multilayer arsenic(III) adsorption, due to changes in surface morphology/porosity. Our results demonstrate how surface-sensitive analytical techniques will improve adsorption modelling for the next generation of composite sorbents.
Sha Z, Cali E, Kerherve G, et al., 2020, Oxygen diffusion behaviour of A-site deficient (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ perovskites in humid conditions, Journal of Materials Chemistry A, Vol: 8, Pages: 21273-21288, ISSN: 2050-7488
In the development of high temperature electrochemical devices such as oxygen transport membranes (OTMs) and solid oxide fuel cells (SOFCs), solid-state (ceramic) technologies have proven to be particularly promising. For example, doped lanthanum chromite perovskites, which display high thermo-chemical stability in aggressive environments as mixed ionic and electronic conducting (MIEC) perovskite electrodes, show potential for use as OTMs. Previous studies on the range of these MIEC perovskites have focussed on material behaviour under pure oxygen conditions. Recently, however, it has been suggested that components of air such as humid vapour may modify the materials' chemistry under device operating conditions, affecting device performance and durability. We have designed and carried out fundamental research into the effect of humidity on the oxygen surface exchange and diffusion kinetics of a commercialized (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ (LSCrF8255) perovskite material under elevated OTM and SOFC operating conditions. The water surface exchange and oxygen ion diffusion behaviour of LSCrF8255 perovskites were measured through Isotopic Exchange Depth Profiling-Secondary Ion Mass Spectrometry (IEDP-SIMS) in a designed humid condition with an oxygen partial pressure of 200 mbar and a constant water vapour pressure of 30 mbar, from intermediate to high temperatures (600 °C to 900 °C). Our study demonstrates consistency between oxygen ion bulk diffusion kinetics in wet (pO2 = 200 mbar, pH2O = 30 mbar) and dry (pO2 = 200 mbar, pH2O = 0 mbar) oxygen atmospheres. However, limited surface exchange between water and the LSCrF material was observed above 800 °C. To study the limited water surface exchange behaviour, angle-resolved X-ray photoelectron spectroscopy (ARXPS), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) have been applied to correlate the changes in water surface exchange with chemical or topograp
Sadia Y, Gelbstein Y, Skinner SJ, 2020, Structure, electrical conductivity and electrochemical behavior of (La1-xSrx)2(Ni0.9Mn0.1)O4+δ based compounds, Journal of Solid State Chemistry, Vol: 290, Pages: 1-4, ISSN: 0022-4596
Solid oxide fuel cells typically operate at temperatures near 800 °C. One obstacle to reducing this temperature is finding a high-performance cathode for lower temperatures. La2NiO4+δ (LNO) has shown promise as a good material as a cathode being a well-known mixed electronic and ionic conductor. However, increasing the surface exchange in LNO is important for high performance applications. Mn substitution has been shown to increase the surface exchange of LNO with Sr required to stabilize the structure. Changing the ratio of Mn:Sr will change the oxidation state of Mn and Ni and the fundamental properties of the material. Changing the Sr content in (La1-xSrx)2(Ni0.9Mn0.1)O4+δ was investigated with x = 0.1-0.5. The XRD patterns shows a single phase K2NiF4 structures. Above 35% Sr the ‘a’ parameter of the unit cell starts increasing and the ‘c’ parameter starts to decrease. Increasing Sr content showed increased electrical conductivity from 40 S cm−1 to 261 S cm−1. Impedance data of the 10% Sr sample showed behavior similar to LNO sample with slightly increased area specific resistance (ASR); any further increase in Sr content further increases ASR of the electrode.
Skinner S, Aguadero A, Tsai C-Y, 2020, High electrical conductivity and crystal structure of the solid oxide cell electrode Pr4Ni3O10-δ, Journal of Solid State Chemistry, Vol: 289, Pages: 1-9, ISSN: 0022-4596
Pr4Ni3O10-δ is a promising air electrode for solid oxide fuel cells and electrolysers with comparable performance to Pr2NiO4+δ but with improved thermodynamic stability at a cell operating temperature of 600 °C–800 °C. To fully understand and integrate Pr4Ni3O10-δ into commercial devices there are several aspects that remain to be addressed. This study provides a systematic analysis of the synthesis kinetics, crystal structure, oxygen content and electrical conductivity providing clear paths for the development of Pr4Ni3O10-δ-based low temperature solid oxide electrochemical devices with enhanced performance and stability. We prove that the material can reach a remarkable electrical conductivity of 235 Scm-1 at 700 °C in air, much higher than the previously reported values of 86 Scm-1 and 56 Scm-1, rising to 278 Scm-1 at 450 °C. High resolution neutron powder and in-situ X-ray diffraction confirmed that Pr4Ni3O10-δ crystallises in the monoclinic P21/a space group, and that no phase transitions were observed on heating to 1000 °C in air. An electrical conductivity anomaly was found at ~300 °C and is attributed to a subtle change in the local structure of the material, potentially associated with changes in the oxygen content.
Celikbilek O, Cavallaro A, Kerherve G, et al., 2020, Surface restructuring of thin-film electrodes based on thermal history and its significance for the catalytic activity and stability at the gas/solid and solid/solid interfaces, ACS Applied Materials & Interfaces, Vol: 12, Pages: 34388-34401, ISSN: 1944-8244
Electrodes in solid-state energy devices are subjected to a variety of thermal treatments, from film processing to device operation at high temperatures. All these treatments influence the chemical activity and stability of the films, as the thermally induced chemical restructuring shapes the microstructure and the morphology. Here, we investigate the correlation between the oxygen reduction reaction (ORR) activity and thermal history in complex transition metal oxides, in particular, La0.6Sr0.4CoO3−δ (LSC64) thin films deposited by pulsed laser deposition. To this end, three ∼200 nm thick LSC64 films with different processing and thermal histories were studied. A variety of surface-sensitive elemental characterization techniques (i.e., low-energy ion scattering, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry) were employed to thoroughly investigate the cationic distribution from the outermost surface to the film/substrate interface. Moreover, electrochemical impedance spectroscopy was used to study the activity and the stability of the films. Our investigations revealed that, despite the initial comparable ORR activity at 600 °C, the degradation rates of the films differed by twofold in the long-term stability tests at 500 °C. Here, we emphasize the importance of processing and thermal history in the elemental surface distribution, especially for the stability of LSC64 electrodes and propose that they should be considered as among the main pillars in the design of active surfaces.
Skinner S, Yatoo M, Du Z, et al., 2020, LaxPr4-xNi3O10-d: Mixed A-site cation higher-order Ruddlesden- Popper phase materials as intermediate-temperature solid oxide fuel cell cathodes, Crystals, Vol: 10, ISSN: 2073-4352
Systematic studies of the air electrode and full solid oxide fuel cell performance of La3PrNi3O9.76, and La2Pr2Ni3O9.65 n = 3 Ruddlesden–Popper phases are reported. These phases were found to adopt orthorhombic symmetry with a decrease in lattice parameters on increasing Pr content, consistent with the solid solution series end members. From electrochemical impedance spectroscopy measurements of symmetrical cells, the electrodes were found to possess area specific resistances of 0.07 Ω cm2 for the La2Pr2Ni3O9.65 cathode and 0.10 Ω cm2 for the La3PrNi3O9.76 cathode at 750 °C, representing a significant improvement on previously reported compositions. This significant improvement in performance is attributed to the optimisation of the electrode microstructure, introduction of an electrolyte interlayer and the resulting improved adhesion of the electrode layer. Following this development, the new electrode materials were tested for their single-cell performance, with the maximum power densities obtained for La2Pr2Ni3O9.65 and La3PrNi3O9.76 being 390 mW cm−2 and 400 mW cm−2 at 800 °C, respectively. As these single-cell measurements were based on thick electrolytes, there is considerable scope to enhance over cell performance in future developments.
Goel P, Gupta MK, Mittal R, et al., 2020, Phonons and oxygen diffusion in Bi2O3 and (Bi0.7Y0.3)2O3, Journal of Physics: Condensed Matter, Vol: 32, ISSN: 0953-8984
We report investigation of phonons and oxygen diffusion in Bi2O3 and (Bi0.7Y0.3)2O3. The phonon spectra have been measured in Bi2O3 at high temperatures up to 1083 K using inelastic neutron scattering. Ab-initio calculations have been used to compute the individual contributions of the constituent atoms in Bi2O3 and (Bi0.7Y0.3)2O3 to the total phonon density of states. Our computed results indicate that as temperature is increased, there is a complete loss of sharp peak structure in the vibrational density of states. Ab-initio molecular dynamics simulations show that even at 1000 K in δ-phase Bi2O3, Bi-Bi correlations remain ordered in the crystalline lattice while the correlations between O-O show liquid like disordered behavior. In the case of (Bi0.7Y0.3)2O3, the O-O correlations broadened at around 500 K indicating that oxygen conductivity is possible at such low temperatures in (Bi0.7Y0.3)2O3 although the conductivity is much less than that observed in the undoped high temperature δ-phase of Bi2O3. This result is consistent with the calculated diffusion coefficients of oxygen and observation by QENS experiments. Our ab-initio molecular dynamics calculations predict that macroscopic diffusion is attainable in (Bi0.7Y0.3)2O3 at much lower temperatures, which is more suited for technological applications. Our studies elucidate the easy directions of diffusion in δ-Bi2O3 and (Bi0.7Y0.3)2O3.
Li C, Pramana S, Bayliss R, et al., 2020, Evolution of structure in the incommensurate modulated LaNb1–xWxO4+x/2 (x = 0.04–0.16) oxide ion conductors, Chemistry of Materials, Vol: 32, Pages: 2292-2303, ISSN: 0897-4756
Hyper-stoichiometric CeNbO4+d phases demonstrate remarkable oxygen diffusivity and provide an interesting structural template for oxygen ion conductors. Previously, we have reported the room temperature structure of the incommensurate modulated LaNb0.88W0.12O4.06, a structural analogue of CeNbO4+d. We have confirmed that it is a pure oxygen ion conductor, with anions diffusing via an interstitialcy mechanism. However, the high temperature structural information for the LaNb1–xWxO4+d (x = 0.04–0.16) family, which is key to understanding the structure–property relationship in oxygen ionic conductors with complex structures at operating conditions, is unreported. In this contribution, we address this question by investigating the high temperature structural evolution of the LaNb1–xWxO4+2/x phases using a combination of thermal analysis, scattering techniques, and 17O and 93Nb nuclear magnetic resonance spectroscopy. We reveal a series of phase transitions between a modulated monoclinic phase, a high temperature modulated tetragonal phase, and a high temperature unmodulated tetragonal phase. These findings are correlated with the ion transport and offer insights into the design of new materials for solid state electrochemical devices.
van den Bosch C, Cavallaro A, Moreno R, et al., 2020, Revealing strain effects on the chemical compositionof Perovskite oxide thin films surface, bulk, and interfaces, Advanced Materials Interfaces, Vol: 7, ISSN: 2196-7350
Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high-quality epitaxial thin films of the model perovskite La0.5Sr0.5Mn0.5Co0.5O3-δ (LSMC), under compressive or tensile strain, were characterized with a combination of in situ and ex situ bulk and surface-sensitive techniques. The results demonstrate a non-linear correlation of mechanical and chemical properties as a function of the operation conditions. It was observed that the effect of strain on reducibility is dependent on the “effective strain” induced on the chemical bonds. In plain strain, and in particular the relative B-O length bond, are the key factor controlling which of the B-site cation would be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically-induced strain effects was proven. With this, it was confirmed that tensile strain favors the stabilization of a more reduced lattice, accompanied by greater segregation of strontium secondary phases and a decrease of oxygen exchange kinetics on LSMC thin films.
Tsai C-Y, McGilvery CM, Aguadero A, et al., 2019, Phase evolution and reactivity of Pr2NiO4+δ and Ce0.9Gd0.1O2-δ composites under solid oxide cell sintering and operation temperatures, International Journal of Hydrogen Energy, Vol: 44, Pages: 31458-31465, ISSN: 0360-3199
In developing a new compositae air electrode for Solid Oxide Cells (SOCs) it is essential to fully understand the phase chemistry of all components. Ruddlesden-Popper type electrodes such as Pr2NiO4+δ have previously been proposed as attractive alternatives to conventional La0·6Sr0·4Fe0·8Co0·2O3-δ/Ce1-xGdxO2-δ compositae air electrodes for both fuel cell and electrolyser modes of operation. However, Pr2NiO4+δ have been shown to have limited stability, reacting with a Ce1-xGdxO2-δ interlayer to form a Ce1-x-yGdxPryO2-δ (CGPO) phase of unknown stoichiometry. Additionally, Pr2NiO4+δ are known to decompose to Pr4Ni3O10 ± δ under certain conditions.In this work detailed understanding of the chemical reaction between Pr2NiO4+δ and Ce0.9Gd0.1O2-δ (CGO10) under normal solid oxide cell fabrication and operating temperatures was obtained, identifying the composition of the resulting CGPO phase reaction products. It is shown that, in addition to the unreacted CGO10 present after sintering the compositae at 1100 °C for up to 12 h, a series of CGPO chemical compositions were formed with various Ce, Gd and Pr ratios depending on the relative distance of the doped ceria phases from the Pr2NiO4+δ phases. The extent of the chemical reaction was found to depend on the sintering time and the contact area of the two phases. Further thermal treatment of the resulting products under SOC air electrode operating temperature (800 °C) resulted in the initiation of Pr2NiO4+δ decomposition, forming Pr4Ni3O10 ± δ and Pr6O11 with no detectable change in the composition of previously formed Pr-substituted ceria phases. It is apparent that the Pr2NiO4+δ/CGO10 compositae is unsuitable as an air electrode, but there is evidence that the decomposition products, Pr4Ni3O10 ± δ and Ce1-x-yGdxPryO2-δ are stable and suitable candidates for SOC electrodes.
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.
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