Stephen J Skinner | Imperial College London (original) (raw)
Papers by Stephen J Skinner
Acta Crystallographica Section C-crystal Structure Communications, Mar 20, 2004
The structure of the high-temperature scheelite-type polymorph of cerium niobium tetraoxide, CeNb... more The structure of the high-temperature scheelite-type polymorph of cerium niobium tetraoxide, CeNbO 4 , has been determined using time-of-¯ight neutron powder diffraction data collected both in situ at 1073 K in air and in vacuo. In both cases, the structure was found to be tetragonal, with I4 1 /a symmetry and without any signi®cant deviation from the stoichiometric composition.
Intermediate Temperature Solid Oxide Fuel Cells, 2020
Abstract Intermediate temperature solid oxide fuel cells (IT-SOFCs) offer an attractive route to ... more Abstract 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.
International Journal of Hydrogen Energy, 2019
Ce 1-x-y Gd x Pr y O 2-d phase formed under solid oxide cell sintering conditions. Stable coexist... more Ce 1-x-y Gd x Pr y O 2-d phase formed under solid oxide cell sintering conditions. Stable coexistence of Ce 1-x Gd x Pr y O 2-d and Ce 1-x Gd x O 2-d under operating conditions. Gradation of composition away from the Pr 2 NiO 4 /Ce 1-x Gd x O 2-d interface. Pr 2 NiO 4þd decomposition at operating temperatures but not at sintering conditions.
The Journal of Physical Chemistry C, 2018
Electrical and oxygen ion transport in the dual-phase composite systems (La 0.8 Sr 0.2) 0.95 Cr x... more Electrical and oxygen ion transport in the dual-phase composite systems (La 0.8 Sr 0.2) 0.95 Cr x Fe 1-x O 3-δ (LSCrF) (x = 0.3, 0.5, 0.7)-10mol% Sc 2 O 3-1mol% CeO 2-89mol% ZrO 2 (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 while 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 while 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 3-dimensional networks of both phases, no obvious difference was observed for the electrical conductivity and oxygen tracer diffusion behaviour and it was also confirmed that the microstructures may have a minor effect on the oxygen diffusion behaviour of the dual-phase materials. Furthermore, the micro-scale 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 for the 10Sc1CeSZ phase k increases when compared with their corresponding isolated single phase materials.
European Microscopy Congress 2016: Proceedings, 2016
Solid State Ionics, 2013
Atomic scale simulations have been used to investigate the impact of co-doping (yttrium and gadol... more Atomic scale simulations have been used to investigate the impact of co-doping (yttrium and gadolinium) and strain on oxygen diffusion and binding of dopant-vacancy clusters in ceria. Doped ceria in its relaxed or strained form is used as an electrolyte for solid oxide fuel cell applications. For unstrained co-doped ceria we calculate an activation energy for migration of 0.70-0.75 eV in the temperature range of 973-1873 K. Co-doping with yttrium and gadolinium only affected oxide ion diffusion to a small degree when compared to single doping. The diffusion coefficient was substantially increased by tensile strain while compressive strain caused a decrease. To gain further insight why tensile strain leads to higher diffusivity static simulations were employed. It is calculated that tensile strain reduces the binding energies of clusters between oxygen vacancies and trivalent dopant atoms while compressive strain leads to higher binding energies.
Solid State Ionics, 2006
The tracer diffusion coefficient (D ⁎) and the surface exchange coefficient (k ⁎) provide vital i... more The tracer diffusion coefficient (D ⁎) and the surface exchange coefficient (k ⁎) provide vital information for materials used in high temperature electrochemical devices (e.g. solid oxide fuel cells or oxygen permeation membranes). These values were established for the high temperature tetragonal scheelite structured CeNbO 4+δ (monoclinic fergusonite at room temperature), which is of interest due to its wide range of oxygen stoichiometries varying from stoichiometric CeNbO 4 to CeNbO 4.33. Measurements of D ⁎ and k ⁎ were performed by the isotopic exchange/line scan technique with SIMS (secondary ion mass spectrometry) used to determine 18 O stable isotope depth distribution. This process was carried out between temperatures of 1073 K and 1173 K at 500 mbar of 16 O/ 18 O. These measurements were then correlated with oxide ion conductivity data previously determined from four probe d.c. and e.m.f. measurements using the Nernst-Einstein relation.
Sensors and Actuators A: Physical, 2011
The operating temperatures of surfaces in the hot sections of gas turbines are of great practical... more The operating temperatures of surfaces in the hot sections of gas turbines are of great practical importance, but are often very hard to measure. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for the utilisation of phosphorescent coatings as thermal history sensors was proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their photoluminescent properties and are a function of both the temperature and duration of exposure. If care is taken to ensure steady state conditions during exposure, subsequent off-line analysis of emission in the laboratory can reveal the temperature experienced by the coating. In this paper, an investigation of the amorphous-to-crystalline change of Y 2 SiO 5 :Tb is reported and used to provide a proof of concept for a phosphorescent thermal history sensor. Phosphor powder was calcined at different temperatures and for different periods, and characterised using photoluminescence spectroscopy. A calibration curve was generated and shows that this phosphor is suitable for temperature measurements over a temperature range from 600 • C to at least 1000 • C. With more advanced signal processing routines it is anticipated that the dynamic range might be extended to 1400 • C. Such routines and other materials/physical processes are the subject of ongoing research in the area at Imperial College and Southside Thermal Sciences.
J. Mater. Chem., 2011
ABSTRACT
Electrodes in solid-state energy devices are subjected to a variety of thermal treatments, from f... more 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, La 0.6 Sr 0.4 CoO 3−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., lowenergy 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.
ECS transactions, May 19, 2023
A series of Ruddlesden-Popper phase materials – La3PrNi3O10- δ, La2Pr2Ni3O10-δ and LaPr3Ni3O10- δ... more A series of Ruddlesden-Popper phase materials – La3PrNi3O10- δ, La2Pr2Ni3O10-δ and LaPr3Ni3O10- δ – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure. The thermal expansion coefficient was calculated for all compositions and was found to be in the range of 13.0 - 13.4 × 10-6 ˚C-1, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells. A weak preference for Pr occupying the M(2) site in the rocksalt layer was also observed. The majority of the oxygen vacancies were confined to the perovskite layers with a particular preference for O(1) and O(5) sites at high temperatures observed by neutron diffraction measurements. Further, a preference for a curved oxygen transport pathway around the NiO6 octahedra was observed which agrees with the published literature for these materials.
International Journal of Inorganic Materials, Oct 1, 2000
The crystal structures of chromium-doped spinel-type lithium manganese oxides, LiMn 2؊y Cr y O 4 ... more The crystal structures of chromium-doped spinel-type lithium manganese oxides, LiMn 2؊y Cr y O 4 (y ؍ 0, 1 9 , 1 6 , and 1 3), have been studied by neutron and X-ray powder di4raction. Rietveld re5nements of their neutron di4raction data revealed that Cr is substituted for Mn selectively and that Li occupies only the 8a site. Thus, the above solid solutions can be expressed as Li(Mn 2؊y Cr y)O 4. The fractional coordinates of oxygen were almost the same in all the samples while the lattice parameter a decreased linearly with increasing Cr content. The lengths of (Mn, Cr)+O bonds in (Mn, Cr)O 6 octahedra changed in a manner expected from average ionic radii for Mn 2؊y Cr y. Rietveld analyses of the X-ray di4raction data showed the crystallite size and anisotropic strain to decrease with increasing Cr content.
Journal of materials chemistry. A, Materials for energy and sustainability, 2019
Lower operating temperatures (≤ 650 °C) of solid oxide fuel cells (SOFCs) are sought in order to ... more 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 Asite deficient (La 0.7 Sr 0.3) 0.95 (Co 0.2 Fe 0.8)O 3−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 cm 2 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 (CoFeO x) 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 surfaceexchange coefficient in comparison to nominally stoichiometric, state-of-the-art La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−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.
Nature Communications, Aug 30, 2022
Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold gr... more Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold great promise for a variety of energy applications, such as CO 2 capture and conversion, gas separation, energy storage, and redox catalytic processes. Copper-based mixed oxides are one of the most promising candidate materials with a high oxygen storage capacity. However, the structural deterioration and sintering at high temperatures is one key scientific challenge. Herein, we report a precursor engineering approach to prepare durable copper-based redox sorbents for use in thermochemical looping processes for combustion and gas purification. Calcination of the CuMgAl hydrotalcite precursors formed mixed metal oxides consisting of CuO nanoparticles dispersed in the Mg-Al oxide support which inhibited the formation of copper aluminates during redox cycling. The copper-based redox sorbents demonstrated enhanced reaction rates, stable O 2 storage capacity over 500 redox cycles at 900°C, and efficient gas purification over a broad temperature range. We expect that our materials design strategy has broad implications on synthesis and engineering of mixed metal oxides for a range of thermochemical processes and redox catalytic applications. Anthropogenic emissions of CO 2 generated from the burning of fossil fuels are a primary driver for global warming and climate change. Besides the large-scale deployment of renewable energy and increased energy end-use efficiencies, CO 2 capture, utilization, and storage (CCUS) technologies are likely to be essential for mitigating industrial CO 2 emissions to an extent that allows the ambitious global Net-Zero emissions to be fulfilled. Amongst many emerging CCUS technologies, chemical looping processes have attracted significant attention for energy-efficient conversion of fuels with inherent CO 2 capture 1-3. Chemical looping processes involve the use of oxygen storage materials, commonly derived from metal oxides, which serve to transport oxygen through cyclic reduction and oxidation reactions (Fig. 1a) 2. This concept has been extended to a variety of chemical processes, including chemical looping combustion (CLC) 4,5 , methane reforming and partial oxidation 6-8 , CO 2 conversion 9-11 , hydrogen production 12,13 , air separation 14-17 , and redox catalytic processes for chemical
A series of higher-order Ruddlesden-Popper phase materials – La3PrNi3O10-d, La2Pr2Ni3O10-d and La... more A series of higher-order Ruddlesden-Popper phase materials – La3PrNi3O10-d, La2Pr2Ni3O10-d and LaPr3Ni3O10-d – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis. All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO6 octahedra was observed which agrees with the published literature for Rud...
Journal of Materials Chemistry A
Significantly different surface chemical compositions in SOCs are correlated with the dynamic mas... more Significantly different surface chemical compositions in SOCs are correlated with the dynamic mass transport phenomena such as Sr segregation, Cr evaporation and redeposition and linked with the material's oxygen transport properties.
Advanced Materials Interfaces, 2019
Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is... more 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, are characterized with a combination of in situ and ex situ bulk and surface‐sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is 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 BO length bond, is the key factor controlling which of the B‐site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it...
ECS Transactions, 2015
In this study the surface properties of LSCF have been studied in in-situ from room temperature t... more In this study the surface properties of LSCF have been studied in in-situ from room temperature to 1000oC using a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) under a number of different atmospheres. Dense, polished La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF) was observed during thermal annealing under a vacuum, pure oxygen, pure water and humid air atmospheres. The effect each atmosphere had on the surface morphology has been characterized in real time and analysed chemically using EDX. Significant differences in the surface behaviour were seen under each atmosphere. The results observed in the HT-ESEM were complimentary to in-operando surface exchange measurements and have enabled a more detailed understanding of SOFC cathode decomposition.
Acta Crystallographica Section C-crystal Structure Communications, Mar 20, 2004
The structure of the high-temperature scheelite-type polymorph of cerium niobium tetraoxide, CeNb... more The structure of the high-temperature scheelite-type polymorph of cerium niobium tetraoxide, CeNbO 4 , has been determined using time-of-¯ight neutron powder diffraction data collected both in situ at 1073 K in air and in vacuo. In both cases, the structure was found to be tetragonal, with I4 1 /a symmetry and without any signi®cant deviation from the stoichiometric composition.
Intermediate Temperature Solid Oxide Fuel Cells, 2020
Abstract Intermediate temperature solid oxide fuel cells (IT-SOFCs) offer an attractive route to ... more Abstract 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.
International Journal of Hydrogen Energy, 2019
Ce 1-x-y Gd x Pr y O 2-d phase formed under solid oxide cell sintering conditions. Stable coexist... more Ce 1-x-y Gd x Pr y O 2-d phase formed under solid oxide cell sintering conditions. Stable coexistence of Ce 1-x Gd x Pr y O 2-d and Ce 1-x Gd x O 2-d under operating conditions. Gradation of composition away from the Pr 2 NiO 4 /Ce 1-x Gd x O 2-d interface. Pr 2 NiO 4þd decomposition at operating temperatures but not at sintering conditions.
The Journal of Physical Chemistry C, 2018
Electrical and oxygen ion transport in the dual-phase composite systems (La 0.8 Sr 0.2) 0.95 Cr x... more Electrical and oxygen ion transport in the dual-phase composite systems (La 0.8 Sr 0.2) 0.95 Cr x Fe 1-x O 3-δ (LSCrF) (x = 0.3, 0.5, 0.7)-10mol% Sc 2 O 3-1mol% CeO 2-89mol% ZrO 2 (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 while 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 while 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 3-dimensional networks of both phases, no obvious difference was observed for the electrical conductivity and oxygen tracer diffusion behaviour and it was also confirmed that the microstructures may have a minor effect on the oxygen diffusion behaviour of the dual-phase materials. Furthermore, the micro-scale 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 for the 10Sc1CeSZ phase k increases when compared with their corresponding isolated single phase materials.
European Microscopy Congress 2016: Proceedings, 2016
Solid State Ionics, 2013
Atomic scale simulations have been used to investigate the impact of co-doping (yttrium and gadol... more Atomic scale simulations have been used to investigate the impact of co-doping (yttrium and gadolinium) and strain on oxygen diffusion and binding of dopant-vacancy clusters in ceria. Doped ceria in its relaxed or strained form is used as an electrolyte for solid oxide fuel cell applications. For unstrained co-doped ceria we calculate an activation energy for migration of 0.70-0.75 eV in the temperature range of 973-1873 K. Co-doping with yttrium and gadolinium only affected oxide ion diffusion to a small degree when compared to single doping. The diffusion coefficient was substantially increased by tensile strain while compressive strain caused a decrease. To gain further insight why tensile strain leads to higher diffusivity static simulations were employed. It is calculated that tensile strain reduces the binding energies of clusters between oxygen vacancies and trivalent dopant atoms while compressive strain leads to higher binding energies.
Solid State Ionics, 2006
The tracer diffusion coefficient (D ⁎) and the surface exchange coefficient (k ⁎) provide vital i... more The tracer diffusion coefficient (D ⁎) and the surface exchange coefficient (k ⁎) provide vital information for materials used in high temperature electrochemical devices (e.g. solid oxide fuel cells or oxygen permeation membranes). These values were established for the high temperature tetragonal scheelite structured CeNbO 4+δ (monoclinic fergusonite at room temperature), which is of interest due to its wide range of oxygen stoichiometries varying from stoichiometric CeNbO 4 to CeNbO 4.33. Measurements of D ⁎ and k ⁎ were performed by the isotopic exchange/line scan technique with SIMS (secondary ion mass spectrometry) used to determine 18 O stable isotope depth distribution. This process was carried out between temperatures of 1073 K and 1173 K at 500 mbar of 16 O/ 18 O. These measurements were then correlated with oxide ion conductivity data previously determined from four probe d.c. and e.m.f. measurements using the Nernst-Einstein relation.
Sensors and Actuators A: Physical, 2011
The operating temperatures of surfaces in the hot sections of gas turbines are of great practical... more The operating temperatures of surfaces in the hot sections of gas turbines are of great practical importance, but are often very hard to measure. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for the utilisation of phosphorescent coatings as thermal history sensors was proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their photoluminescent properties and are a function of both the temperature and duration of exposure. If care is taken to ensure steady state conditions during exposure, subsequent off-line analysis of emission in the laboratory can reveal the temperature experienced by the coating. In this paper, an investigation of the amorphous-to-crystalline change of Y 2 SiO 5 :Tb is reported and used to provide a proof of concept for a phosphorescent thermal history sensor. Phosphor powder was calcined at different temperatures and for different periods, and characterised using photoluminescence spectroscopy. A calibration curve was generated and shows that this phosphor is suitable for temperature measurements over a temperature range from 600 • C to at least 1000 • C. With more advanced signal processing routines it is anticipated that the dynamic range might be extended to 1400 • C. Such routines and other materials/physical processes are the subject of ongoing research in the area at Imperial College and Southside Thermal Sciences.
J. Mater. Chem., 2011
ABSTRACT
Electrodes in solid-state energy devices are subjected to a variety of thermal treatments, from f... more 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, La 0.6 Sr 0.4 CoO 3−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., lowenergy 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.
ECS transactions, May 19, 2023
A series of Ruddlesden-Popper phase materials – La3PrNi3O10- δ, La2Pr2Ni3O10-δ and LaPr3Ni3O10- δ... more A series of Ruddlesden-Popper phase materials – La3PrNi3O10- δ, La2Pr2Ni3O10-δ and LaPr3Ni3O10- δ – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure. The thermal expansion coefficient was calculated for all compositions and was found to be in the range of 13.0 - 13.4 × 10-6 ˚C-1, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells. A weak preference for Pr occupying the M(2) site in the rocksalt layer was also observed. The majority of the oxygen vacancies were confined to the perovskite layers with a particular preference for O(1) and O(5) sites at high temperatures observed by neutron diffraction measurements. Further, a preference for a curved oxygen transport pathway around the NiO6 octahedra was observed which agrees with the published literature for these materials.
International Journal of Inorganic Materials, Oct 1, 2000
The crystal structures of chromium-doped spinel-type lithium manganese oxides, LiMn 2؊y Cr y O 4 ... more The crystal structures of chromium-doped spinel-type lithium manganese oxides, LiMn 2؊y Cr y O 4 (y ؍ 0, 1 9 , 1 6 , and 1 3), have been studied by neutron and X-ray powder di4raction. Rietveld re5nements of their neutron di4raction data revealed that Cr is substituted for Mn selectively and that Li occupies only the 8a site. Thus, the above solid solutions can be expressed as Li(Mn 2؊y Cr y)O 4. The fractional coordinates of oxygen were almost the same in all the samples while the lattice parameter a decreased linearly with increasing Cr content. The lengths of (Mn, Cr)+O bonds in (Mn, Cr)O 6 octahedra changed in a manner expected from average ionic radii for Mn 2؊y Cr y. Rietveld analyses of the X-ray di4raction data showed the crystallite size and anisotropic strain to decrease with increasing Cr content.
Journal of materials chemistry. A, Materials for energy and sustainability, 2019
Lower operating temperatures (≤ 650 °C) of solid oxide fuel cells (SOFCs) are sought in order to ... more 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 Asite deficient (La 0.7 Sr 0.3) 0.95 (Co 0.2 Fe 0.8)O 3−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 cm 2 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 (CoFeO x) 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 surfaceexchange coefficient in comparison to nominally stoichiometric, state-of-the-art La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−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.
Nature Communications, Aug 30, 2022
Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold gr... more Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold great promise for a variety of energy applications, such as CO 2 capture and conversion, gas separation, energy storage, and redox catalytic processes. Copper-based mixed oxides are one of the most promising candidate materials with a high oxygen storage capacity. However, the structural deterioration and sintering at high temperatures is one key scientific challenge. Herein, we report a precursor engineering approach to prepare durable copper-based redox sorbents for use in thermochemical looping processes for combustion and gas purification. Calcination of the CuMgAl hydrotalcite precursors formed mixed metal oxides consisting of CuO nanoparticles dispersed in the Mg-Al oxide support which inhibited the formation of copper aluminates during redox cycling. The copper-based redox sorbents demonstrated enhanced reaction rates, stable O 2 storage capacity over 500 redox cycles at 900°C, and efficient gas purification over a broad temperature range. We expect that our materials design strategy has broad implications on synthesis and engineering of mixed metal oxides for a range of thermochemical processes and redox catalytic applications. Anthropogenic emissions of CO 2 generated from the burning of fossil fuels are a primary driver for global warming and climate change. Besides the large-scale deployment of renewable energy and increased energy end-use efficiencies, CO 2 capture, utilization, and storage (CCUS) technologies are likely to be essential for mitigating industrial CO 2 emissions to an extent that allows the ambitious global Net-Zero emissions to be fulfilled. Amongst many emerging CCUS technologies, chemical looping processes have attracted significant attention for energy-efficient conversion of fuels with inherent CO 2 capture 1-3. Chemical looping processes involve the use of oxygen storage materials, commonly derived from metal oxides, which serve to transport oxygen through cyclic reduction and oxidation reactions (Fig. 1a) 2. This concept has been extended to a variety of chemical processes, including chemical looping combustion (CLC) 4,5 , methane reforming and partial oxidation 6-8 , CO 2 conversion 9-11 , hydrogen production 12,13 , air separation 14-17 , and redox catalytic processes for chemical
A series of higher-order Ruddlesden-Popper phase materials – La3PrNi3O10-d, La2Pr2Ni3O10-d and La... more A series of higher-order Ruddlesden-Popper phase materials – La3PrNi3O10-d, La2Pr2Ni3O10-d and LaPr3Ni3O10-d – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis. All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO6 octahedra was observed which agrees with the published literature for Rud...
Journal of Materials Chemistry A
Significantly different surface chemical compositions in SOCs are correlated with the dynamic mas... more Significantly different surface chemical compositions in SOCs are correlated with the dynamic mass transport phenomena such as Sr segregation, Cr evaporation and redeposition and linked with the material's oxygen transport properties.
Advanced Materials Interfaces, 2019
Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is... more 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, are characterized with a combination of in situ and ex situ bulk and surface‐sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is 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 BO length bond, is the key factor controlling which of the B‐site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it...
ECS Transactions, 2015
In this study the surface properties of LSCF have been studied in in-situ from room temperature t... more In this study the surface properties of LSCF have been studied in in-situ from room temperature to 1000oC using a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) under a number of different atmospheres. Dense, polished La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF) was observed during thermal annealing under a vacuum, pure oxygen, pure water and humid air atmospheres. The effect each atmosphere had on the surface morphology has been characterized in real time and analysed chemically using EDX. Significant differences in the surface behaviour were seen under each atmosphere. The results observed in the HT-ESEM were complimentary to in-operando surface exchange measurements and have enabled a more detailed understanding of SOFC cathode decomposition.