Imperial College London

ProfessorJohnKilner

Faculty of EngineeringDepartment of Materials

Senior Research Investigator
 
 
 
//

Contact

 

+44 (0)20 7594 6745j.kilner Website

 
 
//

Location

 

214Royal School of MinesSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

547 results found

Kilner JA, 2023, (Invited) Mixed Ionic Electronic Conductors (MIECs): The Importance of Fast and Slow Processes, ECS Meeting Abstracts, Vol: MA2023-02, Pages: 2220-2220

<jats:p> MIECs are ideal materials for use as electrodes in solid state ionic devices. They have found application in SOFCs, SOECs, and solid state battery devices. In this contribution we will focus on MIEC used in the SOFC/SOEC where oxygen is the mobile ion. Most of these materials are complex perovskite materials, for example La<jats:sub>1-x</jats:sub>Sr<jats:sub>x</jats:sub>Co<jats:sub>y</jats:sub>Fe<jats:sub>1-y</jats:sub>O<jats:sub>3-δ</jats:sub> (LSCF). The focus of experimental observation in these materials has been on the fast process of transport of the highly mobile oxygen ion and much is known about the effect of non-stoichiometry in determining the level of oxygen ion transport. Most of these materials are very good mixed conductors at temperatures above 600°C but the lattice oxygen transport drops off markedly at lower temperatures. Recently there have been attempts to improve the lower temperature behaviour by producing nanomaterials, usually in thin film form [1-3], with a high density of grain boundaries. These have been shown to introduce fast diffusion paths along the grain boundaries and have been shown to improve the electrochemical performance.</jats:p> <jats:p> What has been neglected in the study of these materials is the evolution of the cation distribution in these complex oxides. The cation distribution has a marked effect upon the performance of an MIEC electrode, for example, the segregation of Sr to the surface which is detrimental to the oxygen exchange [4, 5], the development of defect clusters which can trap the oxygen vacancies, and the local chemistry of the grain boundaries [6], affecting transport in polycrystalline materials. Cation transport is a slow process but has importance during the short times and high temperatures during processing, and even more relevantly for the low temperatures but very long times associated wi

Journal article

Nguyen T, Ishihara T, Kilner J, Staykov Aet al., 2023, Effect of Electronic Interactions on Oxide-Ion Mobility in Solid Electrolytes with a Fluorite Structure, Journal of Physical Chemistry C, Vol: 127, Pages: 22437-22446, ISSN: 1932-7447

The high oxide-ion transport in three fluorite-structured oxide electrolytes is examined by using density functional theory. Our study elucidates the oxide-ion diffusion mechanism in yttrium (Y)-doped ZrO2, Y-doped CeO2, δ-Bi2O3, and α-Bi2O3, including the major change in oxygen mobility that occurs in Bi2O3 at the δ-α phase transition. This research focuses on the partial covalent interactions, often neglected in atomistic simulations, and their effect on the migrating oxygen ion. We found a direct relationship between the degree of hybridization (i.e., partial covalent interactions) in the resting state, transition state, and the activation barrier for oxygen migration. From this, we can understand the oxygen-ion migration energetics by analyzing the resulting electron density. The oxide ions migrate nonlinearly between two adjacent lattice sites to maximize partial covalent interactions with the neighboring cations. We also provide an explanation for the enhanced oxide-ion conductivity of δ-Bi2O3 compared to α-Bi2O3, related to the electron density due to different 6s lone-pair orientations. The disordered δ-phase has a complex lone-pair orientation, which also changes as the oxygen ion is migrating, resulting in a very low barrier to oxygen-ion migration. In contrast, α-Bi2O3, a material with exactly the same stoichiometry, has a well-ordered lattice, and the lone-pair orientation is always arranged to hinder the oxygen diffusion pathway.

Journal article

Skinner S, Sha Z, Kerherve G, Wilson G, Kilner J, Held G, Van Spronsen Met al., 2023, Studying surface chemistry of mixed conducting perovskite oxide electrodes with synchrotron-based soft X-rays, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 127, Pages: 20325-20336, ISSN: 1932-7447

A fundamental understanding of the electrochemical reactions and surface chemistry at the solid–gas interface in situ and operando is critical for electrode materials applied in electrochemical and catalytic applications. Here, the surface reactions and surface composition of a model of mixed ionic and electronic conducting (MIEC) perovskite oxide, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255), were investigated in situ using synchrotron-based near-ambient pressure (AP) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The measurements were conducted with a surface temperature of 500 °C under 1 mbar of dry oxygen and water vapor, to reflect the implementation of the materials for oxygen reduction/evolution and H2O electrolysis in the applications such as solid oxide fuel cell (SOFC) and electrolyzers. Our direct experimental results demonstrate that, rather than the transition metal (TM) cations, the surface lattice oxygen is the significant redox active species under both dry oxygen and water vapor environments. It was proven that the electron holes formed in dry oxygen have a strong oxygen character. Meanwhile, a relatively higher concentration of surface oxygen vacancies was observed on the sample measured in water vapor. We further showed that in water vapor, the adsorption and dissociation of H2O onto the perovskite surface were through forming hydroxyl groups. In addition, the concentration of Sr surface species was found to increase over time in dry oxygen due to Sr surface segregation, with the presence of oxygen holes on the surface serving as an additional driving force. Comparatively, less Sr contents were observed on the sample in water vapor, which could be due to the volatility of Sr(OH)2. A secondary phase was also observed, which exhibited an enrichment in B-site cations, particularly in Fe and relatively in Cr, and a deficiency in A-site cation, notably in La and relatively in Sr

Journal article

Riedl C, Siebenhofer M, Nenning A, Wilson GE, Kilner J, Rameshan C, Limbeck A, Opitz AK, Kubicek M, Fleig Jet al., 2023, Surface decorations on mixed ionic and electronic conductors: effects on surface potential, defects, and the oxygen exchange kinetics, ACS Applied Materials and Interfaces, Vol: 15, Pages: 26787-26798, ISSN: 1944-8244

The oxygen exchange kinetics of epitaxial Pr0.1Ce0.9O2−δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO2, TiO2) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO2 dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

Journal article

Siebenhofer M, Nenning A, Wilson GE, Kilner JA, Rameshan C, Kubicek M, Fleig J, Blaha Pet al., 2023, Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 11, Pages: 7213-7226, ISSN: 2050-7488

Journal article

Skinner S, Sha Z, Shen Z, Kilner J, Cali Eet al., 2023, Understanding surface chemical processes in perovskite oxide electrodes, Journal of Materials Chemistry A, Vol: 11, Pages: 5645-5659, ISSN: 2050-7488

The effect of operating conditions on the surface composition and evolution of (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ (LSCrF8255) as a model perovskite oxide was investigated. LSCrF8255 pellets were annealed under dry oxygen (pO2 = 200 mbar), wet oxygen (pO2 = 200 mbar, pH2O = 30 mbar), and water vapour (pO2 < 1 mbar, pH2O = 30 mbar) environments to reflect the applications of perovskite materials as electrodes for oxygen reduction/evolution and H2O electrolysis in electrochemical energy conversion devices such as solid oxide fuel/electrolysis cells (SOFCs/SOECs) and oxygen transport membranes (OTMs). A series of comprehensive surface characterization techniques were applied, including low energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDX). Our comprehensive study showed that after annealing at 900 °C for 27 hours, a severe level of Sr surface segregation occurred on the sample annealed in both dry oxygen and water vapour but in different manners, whereas on the sample annealed in wet oxygen, Sr segregation was likely suppressed. In addition, the Sr segregation behaviour can be correlated to other mass transport phenomena, such as Cr evaporation and redeposition and Si deposition, as well as to crystal orientation and defects such as grain boundaries and dislocations. Apart from the Sr-enriched surface precipitates, phase separation was consistently observed on the samples annealed in all three conditions. The secondary phase was found to be B-site cation enriched (significantly Fe enriched, relatively Cr enriched) and A-site cation (La and Sr) deficient. Moreover, in contrast to the Sr enriched surface, a La enriched surface was observed on samples annealed in dry oxygen at 600 and 700 °C, which was found to be potentially caused by the Sr and Cr su

Journal article

Thoreton V, Niania M, Druce J, Tellez H, Kilner JAet al., 2022, Oxygen Diffusion in Ceramic Mixed Conducting La<sub>0.6</sub>Sr<sub>0.4</sub>Co<sub>0.2</sub>Fe<sub>0.8</sub>O<sub>3-δ</sub>: The Role of Grain and Twin Boundaries, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 169, ISSN: 0013-4651

Journal article

Zhao Y, Kilner J, Ishihara T, Yoshizawa K, Staykov Aet al., 2022, Effect of electronic interactions and coordination spheres on ionic diffusion in La<sub>x</sub>Sr<sub>1-x</sub>GayMg<sub>1-y</sub>O<sub>3-δ</sub>, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, Vol: 161, ISSN: 0022-3697

Journal article

Brugge RH, Chater RJ, Kilner JA, Aguadero Aet al., 2021, Experimental determination of Li diffusivity in LLZO using isotopic exchange and FIB-SIMS, JOURNAL OF PHYSICS-ENERGY, Vol: 3, ISSN: 2515-7655

Journal article

Gao R, Fernandez A, Chakraborty T, Luo A, Pesquera D, Das S, Velarde G, Thoreton V, Kilner J, Ishihara T, Nemsak S, Crumlin EJ, Ertekin E, Martin LWet al., 2021, Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes, ADVANCED MATERIALS, Vol: 33, ISSN: 0935-9648

Journal article

Thoreton V, Niania M, Kilner J, 2021, Kinetics of competing exchange of oxygen and water at the surface of functional oxides, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 23, Pages: 2805-2811, ISSN: 1463-9076

Journal article

Kuganathan N, Grimes R, Rushton M, Kilner J, Gkanas Eet al., 2021, Self-diffusion in garnet-type Li7La3Zr2O12 solid electrolytes, Scientific Reports, Vol: 11, Pages: 1-10, ISSN: 2045-2322

Tetragonal garnet-type Li7La3Zr2O12 is an important candidate solid electrolyte for all-solid-state lithium ion batteries because of its high ionic conductivity and large electrochemical potential window. Here we employ atomistic simulation methods to show that the most favourable disorder process in Li7La3Zr2O12 involves loss of Li2O resulting in lithium and oxygen vacancies, which promote vacancy mediated self-diffusion. The activation energy for lithium migration (0.45 eV) is much lower than that for oxygen (1.65 eV). Furthermore, the oxygen migration activation energy reveals that the oxygen diffusion in this material can be facilitated at higher temperatures once oxygen vacancies form.

Journal article

Niania MAR, Rossall AK, Van den Berg JA, Kilner JAet al., 2020, The effect of sub-surface strontium depletion on oxygen diffusion in La<sub>0.6</sub>Sr<sub>0.4</sub>Co<sub>0.2</sub>Fe<sub>0.8</sub>O<sub>3-<i>δ</i></sub>, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 19414-19424, ISSN: 2050-7488

Journal article

Niania M, Sharpe M, Webb R, Kilner JAet al., 2020, The surface of complex oxides; ion beam based analysis of energy materials, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Vol: 480, Pages: 27-32, ISSN: 0168-583X

LEIS depth profiles, obtained by low energy (0.5 keV) Ar+ sputtering, have been analysed for the mixed conducting oxide material La0.6Sr0.4Co0.2Fe0.8O3-δ. Samples have been examined after differing thermal treatments to examine the sub-surface reorganisation of the cation species. The profiles have shown considerable changes, but these are not strongly correlated with the thermal treatments. The similarity between the profiles suggests that preferential sputtering effects can dominate the sub-surface region (~1–3 nm) where sputtering has not reached equilibrium. Preferential sputtering of oxygen in oxide materials is well known, but here we provide evidence of the preferential sputtering of the cationic species in a complex multicomponent oxide. Of note is strong enrichment (~30%) of the sputtered surface with the heaviest of the elements, La. Simulations using the code TRIDYN have confirmed the observations, in particular, La surface enrichment and the fluence needed to achieve steady state sputtering of > 3 × 1016 cm−2.

Journal article

Celikbilek O, Cavallaro A, Kerherve G, Fearn S, Chaix-Pluchery O, Aguadero A, Kilner JA, Skinner SJet 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.

Journal article

Ghuman KK, Gilardi E, Pergolesi D, Kilner J, Lippert Tet al., 2020, Microstructural and Electronic Properties of the YSZ/CeO<sub>2</sub> Interface via Multiscale Modeling, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 124, Pages: 15680-15687, ISSN: 1932-7447

Journal article

Hess F, Staykov AT, Yildiz B, Kilner Jet al., 2020, Solid Oxide Fuel Cell Materials and Interfaces, Handbook of Materials Modeling: Applications: Current and Emerging Materials, Second Edition, Pages: 1275-1305, ISBN: 9783319446790

Perovskite materials are widely used as catalysts and electrocatalysts in energy conversion devices. Their bulk transport properties can be tuned flexibly for many applications. While the ion and electron transport properties in their bulk are generally well-understood, the surfaces remain much less explored and challenging. Surface instabilities give rise to compositional deviations in the near-surface region or even the precipitation of secondary phases. These deteriorate the surface catalytic properties under operating conditions, thus reducing efficiency and lifetime of devices. In this contribution, the bulk and surface chemistry of perovskite oxides for solid oxide fuel cell applications are explored for selected compounds relevant for application. Starting with a short description of the bulk defect chemistry of perovskite oxides, the challenges for both theory and experiment in treating these materials are highlighted. Surface instabilities and a selection of mechanisms for surface stabilization are explored, such as electronic reconstruction, ionic segregation, and the formation of stable AO surface terminations. Finally, the O2 dissociation is examined at two different stable AO terminations, where it is revealed that the nature of the active sites differs for the SrO and LaO terminations on SrTiO3 and La2NiO4+δ, respectively, so that the O2 dissociation follows fundamentally different mechanisms on these surfaces.

Book chapter

Li M, Niu H, Druce J, Tellez H, Ishihara T, Kilner JA, Gasparyan H, Pitcher MJ, Xu W, Shin JF, Daniels LM, Jones LAH, Dhanak VR, Hu D, Zanella M, Claridge JB, Rosseinsky MJet al., 2019, A CO2-tolerant perovskite oxide with high oxide ion and electronic conductivity, Advanced Materials, Vol: 32, Pages: 1-8, ISSN: 0935-9648

Mixed ionic–electronic conductors (MIECs) that display high oxide ion conductivity (σo) and electronic conductivity (σe) constitute an important family of electrocatalysts for a variety of applications including fuel cells and oxygen separation membranes. Often MIECs exhibit sufficient σe but inadequate σo. It has been a long‐standing challenge to develop MIECs with both high σo and stability under device operation conditions. For example, the well‐known perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) exhibits exceptional σo and electrocatalytic activity. The reactivity of BSCF with CO2, however, limits its use in practical applications. Here, the perovskite oxide Bi0.15Sr0.85Co0.8Fe0.2O3−δ (BiSCF) is shown to exhibit not only exceptional bulk transport properties, with a σo among the highest for known MIECs, but also high CO2 tolerance. When used as an oxygen separation membrane, BiSCF displays high oxygen permeability comparable to that of BSCF and much higher stability under CO2. The combination of high oxide transport properties and CO2 tolerance in a single‐phase MIEC gives BiSCF a significant advantage over existing MIECs for practical applications.

Journal article

Gao R, Jain ACP, Pandya S, Dong Y, Yuan Y, Zhou H, Dedon LR, Thoreton V, Saremi S, Xu R, Luo A, Chen T, Gopalan V, Ertekin E, Kilner J, Ishihara T, Perry NH, Trinkle DR, Martin LWet al., 2019, Designing optimal perovskite structure for high ionic conduction, Advanced Materials, Vol: 32, Pages: 1-9, ISSN: 0935-9648

Solid‐oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure–property relationships that would enable the rational design of better materials. Here, using epitaxial thin‐film growth, synchrotron radiation, impedance spectroscopy, and density‐functional theory, the impact of structural parameters (i.e., unit‐cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9Sr0.1Ga0.95Mg0.05O3–δ . As compared to the zero‐strain state, compressive strain reduces the unit‐cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit‐cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit‐cell volumes and octahedral rotations decrease migration barriers and create low‐energy migration pathways, respectively. The desired combination of large unit‐cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit‐cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion‐conducting perovskite electrolytes.

Journal article

Brugge RH, Kilner JA, Aguadero A, 2019, Germanium as a donor dopant in garnet electrolytes, Solid State Ionics, Vol: 337, Pages: 154-160, ISSN: 0167-2738

Cubic Li 7 La 3 Zr 2 O 12 (LLZO) garnet electrolytes continue to be viewed as an enabler of all-solid-state lithium battery technologies, or as protective membranes for next-generation lithium battery systems. Supervalent dopants at the lithium sublattice are commonly used to stabilise the conductive cubic phase, through the creation of lithium vacancies. The use of germanium (Ge 4+ ) as a higher valent dopant substituting for Li + was studied here and shown to stabilise the cubic LLZO phase through substitution of x = 0.10 mol of Ge (at the tetrahedral 24d Li sites of the space group Ia-3d, based on the neutron powder diffraction result). This substitution preference follows that of Al 3+ (having a similar ionic radius and reported to reside at 24d sites), but with a lower critical concentration for cubic phase stabilisation, in agreement with charge neutrality arguments to obtain the required Li content (ca. 6.4–6.6 per formula unit) for optimum conductivity. The x = 0.10 composition gave the highest bulk Li ion conductivity of 2.8 × 10 −4 S cm −1 at 25 °C, on the order of reported values for Al-doped LLZO. Surface chemical analysis using time of flight secondary ion mass spectrometry showed Ge homogeneously distributed within the grains as well as some Li-, O- and Ge- enrichment along the grain boundaries. Cyclic voltammetry of the cells containing Ge-doped LLZO showed a redox stability up to +5 V.

Journal article

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.

Journal article

Manalastas W, Rikarte J, Chater RJ, Brugge R, Aguadero A, Buannic L, Llordés A, Aguesse F, Kilner Jet al., 2019, Mechanical failure of garnet electrolytes during Li electrodeposition observed by in-operando microscopy, Journal of Power Sources, Vol: 412, Pages: 287-293, ISSN: 0378-7753

Metallic Li anodes are key to reaching high energy densities in next-generation solid-state batteries, however, major problems are the non-uniform deposition of Li at the interface and the penetrative power of Li metal during operation, which cause failure of the ceramic electrolyte, internal short-circuits and a premature end of battery life. In this work, we explore the anode-electrolyte interface instability of a Li metal-garnet electrolyte system during Li electrodeposition, and its implications for mechanical fracture, Li metal propagation, and electrolyte failure. The degradation mechanism was followed step-by-step during in-operando electrochemical cycling using optical and scanning electron microscopy. High amounts of Li electrodeposition in a localized zone of the interface lead to ceramic fracture followed by an electrode-to-electrode electrical connection via a conductor Li metal filament. This work enables deeper understanding of battery failure modes in all-solid-state batteries containing a ceramic electrolyte membrane.

Journal article

Akbay T, Kilner JA, Ishihara T, Atkinson Cet al., 2019, Explicit solution to extract self-diffusion and surface exchange coefficients from isotope back-exchange experiments, The Journal of Physical Chemistry C, Vol: 123, Pages: 258-264, ISSN: 1932-7447

Multistep 18O isotope exchange procedures and subsequent analytical techniques can be used to elucidate the effect of ambient gas atmospheres on the transport properties of oxide ion-conducting materials utilized in high-temperature solid oxide devices for electrochemical energy conversion. In this contribution, we provide an explicit solution to the one-dimensional transient diffusion equation to estimate oxygen self-diffusion and surface exchange coefficients of oxide ion conducting materials exposed to multistep 18O exchange procedures. Although an analytical solution exists for representing the diffusion profiles of labeled species obtained from a single-step isotope exchange procedure, it is not applicable to the diffusion profiles resulted from consecutive procedures with dynamically altered initial and surface boundary conditions. Hence, a new analytical solution is found for the diffusion problem representing the isotope back-exchange procedure in a semi-infinite spatial domain. The explicit solution is then used to determine the self-diffusion and surface exchange coefficients as fitting parameters for tracer gas diffusion profiles obtained from multistep isotope exchange experiments conducted in different oxidizing gas atmospheres. It is demonstrated that the explicit solution provides a great flexibility in analyzing the effects of oxidizing gas atmospheres on transport properties of oxide ion conducting materials.

Journal article

Staykov A, Fukumori S, Yoshizawa K, Sato K, Ishihara T, Kilner Jet al., 2018, Interaction of SrO-terminated SrTiO3 surface with oxygen, carbon dioxide, and water, Journal of Materials Chemistry A, Vol: 6, Pages: 22662-22672, ISSN: 2050-7496

The interaction of SrO terminated SrTiO3 surface with molecular carbon dioxide and water has been investigated using first-principle theoretical methods and surface analysis techniques. We have studied the formation of a surface SrCO3 layer and various possible products of H2O interaction with the SrO surface, such as, surface chemisorbed water and the formation of a surface hydroxide layer. The co-adsorption of CO2 and H2O was explained both theoretically and experimentally showing that its products follow a complex temperature dependence and as a result, the surface composition may vary between carbonate and surface chemisorbed water. Our theoretical simulations have shown that the presence of water molecules in the gas phase might assist the molecular oxygen/lattice oxygen exchange reaction by stabilization of the surface oxo species in the transition state with a hydrogen bond mechanism. As a result, the activation barrier for molecular oxygen dissociation is decreased leading to an increase in the surface exchange rate constant. Our study demonstrates that the SrO terminated SrTiO3 surface is not static but instead, dynamically responds to external factors such as gas composition, humidity, and temperature. As a result, the surface phases can show different trends for the surface exchange reaction with molecular oxygen by either an increase or decrease in the exchange rate.

Journal article

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.

Journal article

Saranya AM, Morata A, Pla D, Burriel M, Chiabrera F, Garbayo I, Hornes A, Kilner JA, Tarancon Aet al., 2018, Unveiling the outstanding oxygen mass transport properties of Mn-rich perovskites in grain boundary-dominated La0.8Sr0.2(Mn1-xCox)(0.85)O-3 +/-delta nanostructures, Chemistry of Materials, Vol: 30, Pages: 5621-5629, ISSN: 0897-4756

Ion transport in solid-state devices is of great interest for current and future energy and information technologies. A superior enhancement of several orders of magnitude of the oxygen diffusivity has been recently reported for grain boundaries in lanthanum–strontium manganites. However, the significance and extent of this unique phenomenon are not yet established. Here, we fabricate a thin film continuous composition map of the La0.8Sr0.2(Mn1–xCox)0.85O3±δ family revealing a substantial enhancement of the grain boundary oxygen mass transport properties for the entire range of compositions. Through isotope-exchange depth profiling coupled with secondary ion mass spectroscopy, we show that this excellent performance is not directly linked to the bulk of the material but to the intrinsic nature of the grain boundary. In particular, the great increase of the oxygen diffusion in Mn-rich compositions unveils an unprecedented catalytic performance in the field of mixed ionic–electronic conductors. These results present grain boundaries engineering as a novel strategy for designing highly performing materials for solid-state ionics-based devices.

Journal article

Niania M, Podor R, Britton TB, Li C, Cooper SJ, Svetkov N, Skinner S, Kilner Jet 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.

Journal article

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.

Journal article

Wu J, Fuji K, Yashima M, Staykov A, Akbay T, Ishihara T, Kilner JAet al., 2018, A systematic evaluation of the role of lanthanide elements in functional complex oxides; implications for energy conversion devices, Journal of Materials Chemistry A, Vol: 6, Pages: 11819-11829, ISSN: 2050-7488

Lanthanide containing complex oxides, especially the ABO3 perovskite and A(n+1)BnO(3n+1) Ruddlesden–Popper series, attract much interest as promising catalytic materials in many renewable energy applications such as electro-chemical energy conversion and hydrogen production. Recent experimental and theoretical studies on some members of these materials, e.g. La2NiO4, revealed that the La–O terminated surfaces are catalytically active under operational conditions. These findings suggested that the conventional understanding of such oxides being fully ionized, and composed of catalytically inert La3+ ions needs to be revised. In this study, generalized gradient approximation and hybrid density functional theory methods were used to study and compare the electronic structures of La and Sr in related oxides. Density functional theory approaches based on both Gaussian and plane wave basis sets were employed to ensure robustness of this study. Consistent results were obtained across different ab initio methods and approaches used. Density of states plots and charge analysis results showed that La exhibits a partially occupied d-orbital and an atomic charge of +2 instead of its nominal valence number (+3) in the oxides, while Sr does not show similar characteristics. Electron density maps obtained from synchrotron X-ray diffraction experiments confirmed the simulation findings as well. The presence of the available d-orbital electron on La and associated partial covalency were postulated as being responsible for the catalytic behaviour observed in experiments. In addition, Pr and Ba electronic structures in related oxides were also calculated. A similar trend to the La and Sr charges was observed. Based on these findings, the traditional concept of atomic “ionicity” was briefly reviewed and adapted as a catalysis descriptor for possible performance evaluation.

Journal article

Brugge R, Hekselman A, Cavallaro A, Pesci F, Chater R, Kilner J, Aguadero Aet al., 2018, Garnet electrolytes for solid state batteries: visualization of moisture-induced chemical degradation and revealing its impact on the Li-ion dynamics, Chemistry of Materials, Vol: 30, Pages: 3704-3713, ISSN: 0897-4756

In this work, we reveal the impact of moisture-induced chemical degradation and proton–lithium exchange on the Li-ion dynamics in the bulk and the grain boundaries and at the interface with lithium metal in highly Li-conducting garnet electrolytes. A direct correlation between chemical changes as measured by depth-resolved secondary ion mass spectrometry and the change in transport properties of the electrolyte is provided. In order to probe the intrinsic effect of the exchange on the lithium kinetics within the garnet structure, isolated from secondary corrosion product contributions, controlled-atmosphere processing was first used to produce proton-free Li6.55Ga0.15La3Zr2O12 (Ga0.15-LLZO), followed by degradation steps in a H2O bath at 100 °C, leading to the removal of LiOH secondary phases at the surface. The proton-exchanged region was analyzed by focused ion beam secondary ion mass spectrometry (FIB-SIMS) and found to extend as far as 1.35 μm into the Ga0.15-LLZO garnet pellet after 30 min in H2O. Impedance analysis in symmetrical cells with Li metal electrodes indicated a greater reactivity in grain boundaries than in grains and a significantly detrimental effect on the Li transfer kinetics in the Li metal/garnet interface correlated to a 3-fold decrease in the Li mobility in the protonated garnet. This result indicates that the deterioration of Li charge transfer and diffusion kinetics in proton-containing garnet electrolytes have fundamental implications for the optimization and integration of these systems in commercial battery devices.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00003269&limit=30&person=true