529 results found
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.
Li M, Niu H, Druce J, et 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.
Gao R, Jain ACP, Pandya S, et 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.
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.
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.
Manalastas W, Rikarte J, Chater RJ, et 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.
Akbay T, Kilner JA, Ishihara T, et 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.
Staykov A, Fukumori S, Yoshizawa K, et 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.
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.
Saranya AM, Morata A, Pla D, et 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.
Niania M, Podor R, Britton TB, et al., 2018, In situ study of strontium segregation in La<inf>0.6</inf>Sr<inf>0.4</inf>Co<inf>0.2</inf>Fe<inf>0.8</inf>O<inf>3- δ</inf>in ambient atmospheres using high-temperature environmental scanning electron microscopy, Journal of Materials Chemistry A, Vol: 6, Pages: 14120-14135, ISSN: 2050-7496
Samples of the solid oxide fuel cell cathode material La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) were annealed using High-Temperature Environmental Scanning Electron Microscopy (HT-ESEM) from room temperature to 1000 °C in atmospheres of pure oxygen, pure water and ambient lab air. Image series of each heat treatment were taken where microstructural changes were observed and compared between samples. Strontium segregation rate was found to be significantly increased in the presence of pure water as compared to pure O2and ambient air. Electron backscattered diffraction (EBSD) was performed in order to assess the effect of crystal orientation on particle formation and surface sensitive chemical analysis techniques were used to determine the chemical changes at the grain surface as a result of the different heat treatments. It was shown that crystal orientation affected the nature and growth rate of strontium-based particles, however, due to the pseudo-symmetry of La0.6Sr0.4Co0.2Fe0.8O3-δ, precise crystal orientation relationships could not be determined. The chemical composition of the grain surface was found to be approximately equal under each atmosphere.
Kilner J, Shen Z, Skinner SJ, 2018, Electrical conductivity and oxygen diffusion behaviour of the (La0.8Sr0.2)0.95CrxFe1-xO3-δ (x=0.3, 0.5 and 0.7) A-site deficient perovskites, Physical Chemistry Chemical Physics, Vol: 20, Pages: 18279-18290, ISSN: 1463-9076
Lanthanum strontium chromite ferrite ((La0.8Sr0.2)0.95CrxFe1−xO3−δ, LSCrF) pellets with 5% A-site deficiency were fabricated and the electrical conductivity and oxygen diffusion behaviour with different Cr substitution levels (x = 0.3, 0.5 and 0.7) were investigated. As the Cr content increased, the electrical conductivity increased and then a maximum value was achieved at x = 0.7. In the oxygen diffusion studies, all the measured materials present good surface exchange rates (>9 × 10−8 cm s−1 at 900 °C) while the bulk diffusivity of the investigated materials decreased as the Cr substitution level increased: at 900 °C the oxygen diffusion coefficients of the LSCrF materials (x = 0.3, 0.5 and 0.7) are 1.1 × 10−10 cm2 s−1, 3.7 × 10−12 cm2 s−1 and 8.6 × 10−13 cm2 s−1, respectively. Oxygen diffusion in the perovskite materials (LSCrF) is shown to be bulk diffusion limited and it was found that analysis on this type of material using the line scan mode in Time-of-Flight Secondary Ion Mass Spectrometry may result in significant underestimation of the surface exchange coefficient due to the oxygen saturation, while the depth profile mode provides more reliable results but the obtained surface exchange coefficients may also only reach a lower limit. Moreover, fast grain boundary diffusion behaviour was observed in the LSCrF (x = 0.7) material and the Le Claire, and Chung and Wuensch approximations were applied to analyse the oxygen diffusion profiles. For this material, the two approximations provided similar results for the grain boundary product (Dgbδ) and under the assumption that the width of a grain boundary is on the nanometre scale, the oxygen diffusion coefficient of the grain boundaries was about 3–4 orders of magnitude higher than that of the bulk at temperatures ≤900 °C.
Wu J, Fuji K, Yashima M, et al., 2018, A systematic evaluation of the role of the Lanthanide elements in complex functional oxides; implications for energy conversion devices, Journal of Materials Chemistry A, Vol: 6, Pages: 11819-11829, ISSN: 2050-7496
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.
Leonard K, Druce J, Thoreton V, et al., 2018, Exploring mixed proton/electron conducting air electrode materials in protonic electrolysis cell, Solid State Ionics, Vol: 319, Pages: 218-222, ISSN: 0167-2738
In this work, we investigate and compare the performance and cell polarization resistance of Ba 0.5 La 0.5 CoO 3−δ (BLC) and double perovskite oxide BaGd 0.8 La 0.2 Co 2 O 6−δ (BGLC) anode on cathode supported protonic steam electrolysis cells using a 20 μm SrZr 0.5 Ce 0.4 Y 0.1 O 3−δ electrolyte with Ni-SZCY541 composite as the cathode. The kinetics of protons through the bulk and across the gas electrolyte interfaces of both anode materials were also studied by direct measurement of their tracer diffusions using time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS). Cell terminal voltages of 1.74 and 1.93 V, were observed at a current density of 0.5 A cm −2 for both BLC and BGLC whereas a hydrogen evolution rate of 121.85 and 111.15 μmol cm −2 every minute was also obtained at the same current density, translating to a current efficiency of 78 and 72% respectively. Hydrogen tracer diffusion studies confirm BGLC can incorporate protons into the bulk relative to BLC even though the present steam electrolysis results show a better performance for BLC at 600 °C.
Brugge R, Hekselman A, Cavallaro A, et 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.
Pergolesi D, Gilardi E, Fabbri E, et al., 2018, Interface effects on the ionic conductivity of doped ceria-yttria-stabilized zirconia heterostructures, ACS Appl Mater Interfaces, Vol: 10, Pages: 14160-14169, ISSN: 1944-8244
Multilayered heterostructures of Ce0.85Sm0.15O2-δ and Y0.16Zr0.92O2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.
Cavallaro A, Pramana S, Ruiz Trejo E, et al., 2018, Amorphous-cathode-route towards low temperature SOFC, Sustainable Energy & Fuels, Vol: 2, Pages: 862-875, ISSN: 2398-4902
Lowering the operating temperature of solid oxide fuel cell (SOFC) devices is one of the major challenges limiting the industrial breakthrough of this technology. In this study we explore a novel approach to electrode preparation employing amorphous cathode materials. La0.8Sr0.2CoO3−δ dense films have been deposited at different temperatures using pulsed laser deposition on silicon substrates. Depending on the deposition temperature, textured polycrystalline or amorphous films have been obtained. Isotope exchange depth profiling experiments reveal that the oxygen diffusion coefficient of the amorphous film increased more than four times with respect to the crystalline materials and was accompanied by an increase of the surface exchange coefficient. No differences in the surface chemical composition between amorphous and crystalline samples were observed. Remarkably, even if the electronic conductivities measured by the Van Der Pauw method indicate that the conductivity of the amorphous material was reduced, the overall catalytic properties of the cathode itself were not affected. This finding suggests that the rate limiting step is the oxygen mobility and that the local electronic conductivity in the amorphous cathode surface is enough to preserve its catalytic properties. Different cathode materials have also been tested to prove the more general applicability of the amorphous-cathode route.
Staykov A, Tellez H, Druce J, et al., 2018, Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3, Science and Technology of Advanced Materials, Vol: 19, Pages: 221-230, ISSN: 1468-6996
Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO3 and iron doped SrTiO3 were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO3 and compared it to DOS of iron-doped SrTiO3 with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO3 and iron-doped SrTiO3. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO3, are accessible only on TiO2 terminated SrTiO3 surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.
Skinner SJ, ryan MP, pramana S, et al., 2017, Crystal structure and surface characteristics of Sr-doped GdBaCo2O6-δ double perovskites: oxygen evolution reaction and conductivity, Journal of Materials Chemistry A, Vol: 6, Pages: 5335-5345, ISSN: 2050-7496
A cheap and direct solution towards engineering better catalysts through identification of novel materials is required for a sustainable energy system. Perovskite oxides have emerged as potential candidates to replace the less economically attractive Pt and IrO2 water splitting catalysts. In this work, excellent electrical conductivity (980 S cm−1) was found for the double perovskite of composition GdBa0.6Sr0.4Co2O6−δ which is consistent with a better oxygen evolution reaction activity with the onset polarisation of 1.51 V with respect to a reversible hydrogen electrode (RHE). GdBa1−xSrxCo2O6−δ with increasing Sr content was found to crystallise in the higher symmetry tetragonal P4/mmm space group in comparison with the undoped GdBaCo2O6−δ which is orthorhombic (Pmmm), and yields higher oxygen uptake, accompanied by higher Co oxidation states. This outstanding electrochemical performance is explained by the wider carrier bandwidth, which is a function of Co–O–Co buckling angles and Co–O bond lengths. Furthermore the higher oxygen evolution activity was observed despite the formation of non-lattice oxides (mainly hydroxide species) and enrichment of alkaline earth ions on the surface.
Téllez Lozano H, Druce J, Cooper SJ, et al., 2017, Double perovskite cathodes for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic conductors?, Science and Technology of Advanced Materials, Vol: 18, Pages: 977-986, ISSN: 1468-6996
Published by National Institute for Materials Science in partnership with Taylor & Francis. 18 O and 2 H diffusion has been investigated at a temperature of 300 °C in the double perovskite material PrBaCo 2 O 5+δ (PBCO) in flowing air containing 200 mbar of 2 H 2 16 O. Secondary ion mass spectrometry (SIMS) depth profiling of exchanged ceramics has shown PBCO still retains significant oxygen diffusivity (~1.3 × 10 −11 cm 2 s −1 ) at this temperature and that the presence of water ( 2 H 2 16 O), gives rise to an enhancement of the surface exchange rate over that in pure oxygen by a factor of ~3. The 2 H distribution, as inferred from the 2 H 2 16 O − SIMS signal, shows an apparent depth profile which could be interpreted as 2 H diffusion. However, examination of the 3-D distribution of the signal shows it to be nonhomogeneous and probably related to the presence of hydrated layers in the interior walls of pores and is not due to proton diffusion. This suggests that PBCO acts mainly as an oxygen ion mixed conductor when used in PCFC devices, although the presence of a small amount of protonic conductivity cannot be discounted in these materials.
Harrington GF, Skinner SJ, Kilner JA, 2017, Can solute segregation in ceramic materials be reduced by lattice strain?, Journal of the American Ceramic Society, Vol: 101, Pages: 1310-1322, ISSN: 0002-7820
Lattice strain is a relatively unexplored route to modify the degradation effects in functional oxides for high-temperature electrochemical devices. In this paper, we present results on the segregation of Gd to the surface of strained Gd0.1Ce0.9O2-δ films using low-energy ion scattering to assess the surface composition. The potential for strain-modified segregation is discussed as well as the challenges in studying and implementing it.
Skinner SJ, li C, ni N, et al., 2017, Surface chemistry of La<sub>0.99</sub>Sr<sub>0.01</sub>NbO<sub>4-d</sub> and its implication for proton conduction, ACS Applied Materials and Interfaces, Vol: 9, Pages: 29633-29642, ISSN: 1944-8244
Acceptor-doped LaNbO4 is a promising electrolyte material for proton-conducting fuel cell (PCFC) applications. As charge transfer processes govern device performance, the outermost surface of acceptor-doped LaNbO4 will play an important role in determining the overall cell performance. However, the surface composition is poorly characterized, and the understanding of its impact on the proton exchange process is rudimentary. In this work, the surface chemistry of 1 atom % Sr-doped LaNbO4 (La0.99Sr0.01NbO4-d, denoted as LSNO) proton conductor is characterized using LEIS and SIMS. The implication of a surface layer on proton transport is studied using the isotopic exchange technique. It has shown that a Sr-enriched but La-deficient surface layer of about 6–7 nm thick forms after annealing the sample under static air at 1000 °C for 10 h. The onset of segregation is found to be between 600 and 800 °C, and an equilibrium surface layer forms after 10 h annealing. A phase separation mechanism, due to the low solubility of Sr in LaNbO4, has been proposed to explain the observed segregation behavior. The surface layer was concluded to impede the water incorporation process, leading to a reduced isotopic fraction after the D216O wet exchange process, highlighting the impact of surface chemistry on the proton exchange process.
Skinner SJ, McComb DW, Harrington GF, et al., 2017, The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films, Physical Chemistry Chemical Physics, Vol: 19, Pages: 14319-14336, ISSN: 1463-9084
Enhanced conductivity in YSZ films has been of substantial interest over the last decade. In this paper we examine the effects of substrate lattice mismatch and film thickness on the strain in YSZ films and the resultant effect on the conductivity. 8 mol% YSZ films have been grown on MgO, Al2O3, LAO and NGO substrates, thereby controlling the lattice mismatch at the film/substrate interface. The thickness of the films was varied to probe the interfacial contribution to the transport properties, as measured by impedance spectroscopy and tracer diffusion. No enhancement in the transport properties of any of the films was found over single crystal values, and instead the effects of lattice strain were found to be minimal. The interfaces of all films were more resistive due to a heterogeneous distribution of grain boundaries, and no evidence for enhanced transport down dislocations was found.
Cooper SJ, Niania M, Hoffmann, et al., 2017, Back-exchange: a novel approach to quantifying oxygen diffusion and surface exchange in ambient atmospheres, Physical Chemistry Chemical Physics, Vol: 19, Pages: 12199-12205, ISSN: 1463-9084
A novel two-step Isotopic Exchange (IE) technique has been developed to investigate the influence of oxygen containing components of ambient air (such as H₂O and CO₂) on the effective surface exchange coefficient (k*) of a common mixed ionic electronic conductor material. The two step 'back-exchange' technique was used to introduce a tracer diffusion profile, which was subsequently measured using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The isotopic fraction of oxygen in a dense sample as a function of distance from the surface, before and after the second exchange step, could then be used to determine the surface exchange coefficient in each atmosphere. A new analytical solution was found to the diffusion equation in a semi-infinite domain with a variable surface exchange boundary, for the special case where D* and k* are constant for all exchange steps. This solution validated the results of a numerical, Crank-Nicolson type finite-difference simulation, which was used to extract the parameters from the experimental data. When modelling electrodes, D* and k* are important input parameters, which significantly impact performance. In this study La₀.₆Sr₀.₄Co₀.₂Fe₀.₈O₃-δ (LSCF6428) was investigated and it was found that the rate of exchange was increased by around 250% in ambient air compared to high purity oxygen at the same pO₂. The three experiments performed in this study were used to validate the back-exchange approach and show its utility.
Wu K-T, Tellez H, Druce J, et al., 2017, Surface chemistry and restructuring in thin-film Lan+1NinO3n+1 (n=1, 2 and 3) Ruddlesden-Popper oxides, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 5, Pages: 9003-9013, ISSN: 2050-7488
Understanding the surface chemistry and oxygen surface exchange activity in mixed conducting perovskite and related perovskite oxides is of great relevance in developing electrochemical devices. Mixed conducting Ruddlesden–Popper Lan+1NinO3n+1 phases (n = 1, 2 and 3) have been considered as promising electrodes for electrochemical energy conversion cells due to their layered structure allowing non-stoichiometric defect structures. This study focuses on a systematic investigation of the chemical composition of the outermost atomic surfaces of as-deposited and annealed epitaxial films of Lan+1NinO3n+1 (n = 1, 2 and 3). For both as-deposited and annealed films, the analysis of the outermost surface using low energy ion scattering shows preferential LaO-termination. The results also provide evidence of an associated Ni-enrichment below the outermost surface. These findings suggest significant atomic rearrangement occurs during deposition and subsequent annealing. To investigate the thermal stability of these films during deposition, further microstructural analysis was carried out by means of high-resolution scanning transmission electron microscopy, showing significant re-orientation of LaO layers after a post-annealing heat treatment. In thin films of n = 2, 3 phases, surface restructuring reduces the epitaxy of the films and hence any potential beneficial anisotropy in transport properties will be lost. Care must therefore be exercised in processing these materials for electrode applications.
Buannic L, Orayech B, Lopez Del Amo J-M, et al., 2017, Dual Substitution Strategy to Enhance Li+ Ionic Conductivity in Li7La3Zr2O12 Solid Electrolyte, CHEMISTRY OF MATERIALS, Vol: 29, Pages: 1769-1778, ISSN: 0897-4756
Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li7La3Zr2O12 appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga3+, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc3+, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li7La3Zr2O12 according to the resulting stoichiometry, Li7–3x+yGaxLa3Zr2–yScyO12. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li6.65Ga0.15La3Zr1.90Sc0.10O12, with preferential Ga3+ occupation of tetrahedral Li24d sites over the distorted octahedral Li96h sites. 7Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10–3 S cm–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively.
Aguesse F, Manalastas W, Buannic L, et al., 2017, Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal., ACS Applied Materials and Interfaces, Vol: 9, Pages: 3808-3816, ISSN: 1944-8244
All-solid-state batteries including a garnet ceramic as electrolyte are potential candidates to replace the currently used Li-ion technology, as they offer safer operation and higher energy storage performances. However, the development of ceramic electrolyte batteries faces several challenges at the electrode/electrolyte interfaces, which need to withstand high current densities to enable competing C-rates. In this work, we investigate the limits of the anode/electrolyte interface in a full cell that includes a Li-metal anode, LiFePO4 cathode, and garnet ceramic electrolyte. The addition of a liquid interfacial layer between the cathode and the ceramic electrolyte is found to be a prerequisite to achieve low interfacial resistance and to enable full use of the active material contained in the porous electrode. Reproducible and constant discharge capacities are extracted from the cathode active material during the first 20 cycles, revealing high efficiency of the garnet as electrolyte and the interfaces, but prolonged cycling leads to abrupt cell failure. By using a combination of structural and chemical characterization techniques, such as SEM and solid-state NMR, as well as electrochemical and impedance spectroscopy, it is demonstrated that a sudden impedance drop occurs in the cell due to the formation of metallic Li and its propagation within the ceramic electrolyte. This degradation process is originated at the interface between the Li-metal anode and the ceramic electrolyte layer and leads to electromechanical failure and cell short-circuit. Improvement of the performances is observed when cycling the full cell at 55 °C, as the Li-metal softening favors the interfacial contact. Various degradation mechanisms are proposed to explain this behavior.
Cooper SJ, Bertei A, Shearing PR, et al., 2016, TauFactor: An open-source application for calculating tortuosity factors from tomographic data, SoftwareX, Vol: 5, Pages: 203-210, ISSN: 2352-7110
TauFactor is a MatLab application for efficiently calculating the tortuosity factor, as well as volume fractions, surface areas and triple phase boundary densities, from image based microstructural data. The tortuosity factor quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media. TauFactor was originally developed to improve the understanding of electrode microstructures for batteries and fuel cells; however, the tortuosity factor has been of interest to a wide range of disciplines for over a century, including geoscience, biology and optics. It is still common practice to use correlations, such as that developed by Bruggeman, to approximate the tortuosity factor, but in recent years the increasing availability of 3D imaging techniques has spurred interest in calculating this quantity more directly. This tool provides a fast and accurate computational platform applicable to the big datasets (>10^8 voxels) typical of modern tomography, without requiring high computational power.
Akbay T, Staykov A, Druce J, et al., 2016, The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4, Journal of Materials Chemistry A, Vol: 4, Pages: 13113-13124, ISSN: 2050-7496
Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage.
Burriel M, Tellez H, Chater RJ, et al., 2016, Influence of Crystal Orientation and Annealing on the Oxygen Diffusion and Surface Exchange of La2NiO4+delta, Journal of Physical Chemistry C, Vol: 120, Pages: 17927-17938, ISSN: 1932-7455
La2NiO4+δ is a mixed ionic–electronic conducting material with the 2D K2NiF4-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k*) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La2NiO4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450–600 °C. The differences in k* have been correlated with differences in surface chemistry: surface termination, near-surface rearrangement, and the presence of extrinsic impurities. Finally, the predominant La termination at the immediate outer surface is evidenced, confirming recent results for other Ruddlesden–Popper phases with mixed ionic–electronic conducting properties.
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