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Ion intercalation of perovskite oxides in liquid electrolytes is a very promising method for controlling their functional properties while storing charge, which opens the potential application in different energy and information technologies. Although the role of defect chemistry in the oxygen intercalation in a gaseous environment is well established, the mechanism of ion intercalation in liquid electrolytes at room temperature is poorly understood. In this study, the defect chemistry during ion intercalation of La0.5Sr0.5FeO3-{\delta} thin films in alkaline electrolytes is studied. Oxygen and proton intercalation into the LSF perovskite structure is observed at moderate electrochemical potentials (0.5 V to -0.4 V), giving rise to a change in the oxidation state of Fe (as a charge compensation mechanism). The variation of the concentration of holes as a function of the intercalation potential was characterized by in-situ ellipsometry and the concentration of electron holes was indirectly quantified for different electrochemical potentials. Finally, a dilute defect chemistry model that describes the variation of defect species during ionic intercalation was developed.

Electrochemical devices such as solid oxide fuel cells (SOFC) may greatly benefit from the implementation of nanoengineered thin-film multifunctional layers providing, alongside enhanced electrochemical activity, improved mechanical, and long-term stability. In this study, an ultrathin (400 nm) bilayer of samarium-doped ceria and a self-assembled nanocomposite made of Sm0.2Ce0.8O1.9-La0.8Sr0.2MnO3-$\delta$ was fabricated by pulsed laser deposition and is employed as a functional oxygen electrode in an anode-supported solid oxide fuel cell. Introducing the functional bilayer in the cell architecture results in a simple processing technique for the fabrication of high-performance fuel cells (power density 1.0 W.cm-2 at 0.7 V and 750 $^\circ$C). Durability tests were carried out for up to 1500 h, showing a small degradation under extreme operating conditions of 1 A.cm-2, while a stable behaviour at 0.5 A.cm-2. Post-test analyses, including scanning and transmission electron microscopy and electrochemical impedance spectroscopy, demonstrate that the nanoengineered thin film layers remain mostly morphologically stable after the operation., Comment: 24 pages, 12 figures

The call for material systems with enhanced mass transport properties is central in the development of next-generation fuel cells, batteries and solid state energy devices in general. While two-dimensional doping by artificial heterostructuring or nanoscaling has shown great potential for overcoming kinetic limitations of ion diffusion, the length scale of interface effects requires the development of advanced tools for capturing and quantifying local phenomena in greater detail. In the present paper, an in-depth study of grain boundary oxygen conduction in Sr-doped lanthanum manganite films is presented by means of novel isotope-exchange atom probe tomography. Local pathways for fast mass transport are directly mapped by two-dimensional reconstructions and line profiles of the oxygen isotope concentration. Accurate finite element modelling is employed to retrieve the local kinetic parameters, highlighting an enhancement of two orders of magnitude for both the diffusivity and surface exchange rate with respect to the bulk (D*gb=2.1×1014 cm2 s1 and k*gb=4.3×1010 cm s1, respectively, for grain boundaries at 550 °C). Co-acquired reconstruction of the cationic distribution reveals strong inhomogeneities (dopant de-mixing) across the grain boundaries and in the sub-surface region, leading to local Sr accumulation. The findings provide unequivocal quantitative assessment of fast grain boundary oxygen diffusion in lanthanum strontium manganite, giving further insights into local stoichiometry deviations and promoting isotope exchange-atom probe tomography as a powerful tool for the study of local interface effects with high resolution. Different models for the explanation of the phenomena are critically discussed on the basis of the experimental findings.

La2NiO4 is a very promising cathode material for intermediate and low temperature solid oxide cell applications, due to its good electronic and ionic conductivity, together with its high oxygen exchange activity with a low activation energy. Oxygen incorporation and transport in La2NiO4 (L2NO4) thin films are limited by surface reactions. Hence, tailoring the morphology is expected to lead to an overall improvement of the electrode performance. We report the growth of nano-architectured La2NiO4 thin film electrodes by Pulsed Injection Metal Organic Chemical Vapour Deposition (PI-MOCVD), achieving vertically gapped columns with a multi-fold active surface area, leading to much faster oxygen exchange. This nano-columnar structure is rooted in a dense bottom layer serving as a good electronic and ionic conduction pathway. The microstructure is tuned by modification of the growth temperature and characterised by SEM, TEM and XRD. We studied the effect of surface activity by electrical conductivity relaxation measurements in fully dense and nano-columnar La2NiO4 thin films of various thicknesses grown on several different single crystal substrates. Our results demonstrate that the increased surface area, in combination with the opening of different surface terminations, leads to a significant enhancement of the total exchange activity in our films with an optimized nano-architectured microstructure. This journal is, This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 824072 (Harvestore) and under the Marie Skłodowska-Curie grant agreement no. 840787 (Thin-CATALYzER).

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient LaSrMn O epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of La Mn × antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties., This research was supported by the funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 101017709 (EPISTORE) and the 'Generalitat de Catalunya' (2017 SGR 1421, NANOEN). LY acknowledges support from the MINECO (Spain) through the IJC2018-037698-I grant. MICIIN projects PID2019-106165GB-C21 and RED2018-102609-T are also acknowledged. PN acknowledges the support from the AGAUR through the 2021 FI_B 00157 Grant. RADS acknowledges funding from German Research Foundation (DFG) from project DE 2854/12-1 and from the collaborative research center SFB917 'Nanoswitches'. J S acknowledges the financial support of the Spanish Ministry of Economy, Industry and Competitiveness (Project: PID2019-108573GB-C21).

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry., The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition.

The implementation of nano-engineered composite oxides opens up the way towards the development of a novel class of functional materials with enhanced electrochemical properties. Here we report on the realization of vertically aligned nanocomposites of lanthanum strontium manganite and doped ceria with straight applicability as functional layers in high-temperature energy conversion devices. By a detailed analysis using complementary state-of-the-art techniques, which include atom-probe tomography combined with oxygen isotopic exchange, we assess the local structural and electrochemical functionalities and we allow direct observation of local fast oxygen diffusion pathways. The resulting ordered mesostructure, which is characterized by a coherent, dense array of vertical interfaces, shows high electrochemically activity and suppressed dopant segregation. The latter is ascribed to spontaneous cationic intermixing enabling lattice stabilization, according to density functional theory calculations. This work highlights the relevance of local disorder and long-range arrangements for functional oxides nano-engineering and introduces an advanced method for the local analysis of mass transport phenomena., J.S. acknowledges the support of ICN2 (funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme SEV-2017-0706) for the XRD measurements. M.A. acknowledges the support from the Feodor Lynen Research Fellowship Program of the Alexander von Humboldt Foundation and the Isaac Newton Trust, 17.25(a). M.A. and J.D. acknowledge the support from the EPSRC Centre of Advanced Materials for Integrated Energy Systems (CAM-IES) under EP/P007767/1. J.D. also acknowledge support from EPSRC grants EP/N004272/1, EP/T012218/1, the Royal Academy of Engineering- CIET1819_24, ERC POC grant 779444, Portapower. X.W. and H.W. acknowledge the funding support from the U.S. National Science Foundation for the TEM effort at Purdue University (DMR-1565822 and DMR-2016453). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 824072 (HARVESTORE), No 681146 (ULTRASOFC) and No 101017709 (EPISTORE) and was supported by an STSM Grant from the COST Action MP1308: Towards Oxide-Based Electronics (TO-BE), supported by COST (European Cooperation in Science and Technology).

Oxide interfaces have attracted a huge amount of interest in recent years due to their novel functionalities that are not possible in bulk. Here we employ atomistic simulation techniques to study the defect properties and oxygen ion migration energy in bulk SrTiO3, LaCrO3 and SrTiO3–LaCrO3 interface. The SrO and La2O3 “Schottky-like” defects are found to be the most favourable intrinsic defects in their respective bulk structures and also at interface layers. While defect energies calculated at interface layers are not significantly changed compared to that calculated in the bulk LaCrO3, significant changes are noted in Schottky defects and oxygen Frenkel between bulk SrTiO3 and the interface. The activation energies for the oxygen ion diffusion in bulk SrTiO3 and LaCrO3 are calculated to be 0.64eV and 0.53eV, respectively, suggesting that both materials will present favourable diffusion pathways. In general, there is a reduction in the activation energies of oxygen ion migration at the interface compared to bulk structures. In particular, in the Cr layer, activation energy is calculated to be 0.36eV implying that interface plays an important role in the oxygen ion migration and therefore such an interface is of interest for use in advanced electrochemical devices.

The implementation of nano-engineered composite oxides opens up the way towards the development ofa novel class of superior energy materials. Vertically aligned nanocomposites are characterized by acoherent, dense array of vertical interfaces, which allows for the extension of local effects to the wholevolume of the material. Here, we use such a unique architecture to fabricate highly electrochemicallyactive nanocomposites of lanthanum strontium manganite and doped ceria with unprecedented stabilityand straight applicability as functional layers in solid state energy devices. Direct evidence of synergisticlocal effects for enhancing the electrochemical performance, stemming from the highly ordered phasealternation, is given here for the first time using atom-probe tomography combined with oxygen isotopicexchange. Interface-induced cationic substitution, enabling lattice stabilization, is presented as the originof the observed long-term stability. These findings reveal a novel route for materials nano-engineeringbased on the coexistence between local disorder and long-range arrangement.

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO3 were examined by electrochemical impedance spectroscopy (EIS) at low (25-160°C) and intermediate (500-700°C) temperatures. At intermediate temperatures, the electronic and ionic conductivity as well as the chemical capacitance of SrTiO3 were determined as a function of T and p(O2) by employing a modified transmission line equivalent circuit to accurately describe the measured system. Defect modelling based on chemical capacitance measurements is established as a new method to determine the concentrations and the thermodynamic properties of ionic and electronic defects in SrTiO3. This method has potential for a wide application for mixed ionic and electronic conducting materials. Impedance spectroscopy at low temperatures was used to further quantify the electronic trapping energies of the main ionic defects of SrTiO3. Utilization of the chemical capacitance allows the establishment of a defect model based solely on electrochemical measurements, which correctly predicts the conductivity and the chemical capacitance, unveiling the concentrations of internal defects. This analysis yields a concentration of 6 ppm for acceptor-type titanium vacancies in the investigated SrTiO3 single crystals, which was experimentally confirmed by complementary Positron Annihilation Lifetime Spectroscopy measurements. The employed method is sensitive for electronically relevant defects in concentrations even below 1 ppm.

Tuning oxygen mass transport properties at the nanoscale offers a promising approach for developing high performing energy materials. A number of strategies for engineering interfaces with enhanced oxygen diffusivity and surface exchange has recently been proposed. However, the origin and the local magnitude of such local effects remain largely undisclosed to date. This is ascribed to the lack of direct measurement tools with sufficient resolution. In this work, we use atom probe tomography with sub-nanometric resolution to study oxygen mass transport on oxygen-isotope exchanged thin films of lanthanum chromite. We present a direct visualization of nanoscaled highly conducting oxygen incorporation pathways along grain boundaries, with reliable quantification of fast oxygen diffusion at grain boundaries and correlative link to local chemistries. Combined with finite element simulations of the precise 3D nanostructure, we quantify an enhancement in the grain boundary oxygen diffusivity and in the surface exchange coefficient of lanthanum chromite of about 4 and 3 orders of magnitude, respectively, compared to the bulk. This remarkable increase of the oxygen diffusivity in an interface-dominated material is unambiguously attributed to grain boundary conduction highways thanks to the use of a powerful technique that can be straightforwardly extended to the study of currently inaccessible multiple nanoscale mass transport phenomena.

High temperature solid oxide solar cells based on SrTiO3 (STO) are investigated. Schottky contacts between STO and different materials, including La1-xSrxCrO3, La0.8Sr0.2MnO3, La0.6Sr0.4CoO3 and metals like Au and Pt are illuminated by UV light with a wavelength of 365 nm at 350 °C in air and the resulting voltages and currents are measured. With certain material combinations, e.g. LaCrO3/STO or Au/STO heterojunctions, high photovoltages of more than 1.0 V are obtained. Current measurements reveal changes in the bulk STO due to stoichiometry polarization, leading to a self-enhancement effect of the photovoltaic cell. Mechanistic insight in the processes under UV light is gained by electrochemical impedance spectroscopy, especially regarding the space charge region at the heterojunction and bulk changes due to stoichiometry polarization. In addition to the electronic effects upon illumination, also photo-ionic effects of oxygen transport caused by chemical potential changes were observed.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K2NiF4-type insulating La2CuO4 (LCO) and perovskite-type conductive metallic LaNiO3 (LNO) layers with different thicknesses to assess the heterostructure—thermoelectric property—relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contr...

The utilization of interface effects in epitaxial systems at the nanoscale has emerged as a very powerful approach for engineering functional properties of oxides. Here we present a novel structure fabricated by a state-of-the-art oxide molecular beam epitaxy method and consisting of lanthanum cuprate and strontium (Sr)-doped lanthanum nickelate, in which interfacial high-temperature superconductivity (Tc up to 40 K) occurs at the contact between the two phases. In such a system, we are able to tune the superconducting properties simply by changing the structural parameters. By employing electron spectroscopy and microscopy combined with dedicated conductivity measurements, we show that decoupling occurs between the electronic charge carrier and the cation (Sr) concentration profiles at the interface and that a hole accumulation layer forms, which dictates the resulting superconducting properties. Such effects are rationalized in the light of a generalized space-charge theory for oxide systems that takes account of both ionic and electronic redistribution effects.

We present a methodology based on \textit{ex-situ} (post-growth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La$_2$CuO$_{4+y}$ grown epitaxially on substrates of isostructural LaSrAlO$_4$. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and describe the resulting methodology as a platform to be explored in thin films of other transition metal oxides., Comment: 10 pages, 4 figures

Mixed ionic electronic conductors (MIEC) are pivotal materials in a number of electrochemical devices that are relevant for clean energy technologies and industrial chemical processes. In this report progress, an overview of the recent strategies to tune surfaces and interfaces of MIEC fluorite and perovskite oxides for solid oxide fuel cells and microsolid oxide fuel cells electrodes is provided. Most of the works presented focus on salient strategies to improve the oxygen reduction reaction (ORR) and the fuel oxidation reaction kinetics. Herein insights on the current understanding of heterointerfaces in epitaxial thin films and vertically aligned nanocomposites for ORR kinetics is provided. A selection of oxide materials having potential for thin-film anode application is also presented. Further, recent salient results in grain boundary engineering and ex-solution as well as strain engineering and photoactivation for the development of advanced electrodes is discussed.

Oxide interfaces have attracted a huge amount of interest in recent years due to their novel functionalities that are not possible in bulk. Here we employ atomistic simulation techniques to study the defect properties and oxygen ion migration energy in bulk SrTiO3, LaCrO3 and SrTiO3–LaCrO3 interface. The SrO and La2O3 “Schottky-like” defects are found to be the most favourable intrinsic defects in their respective bulk structures and also at interface layers. While defect energies calculated at interface layers are not significantly changed compared to that calculated in the bulk LaCrO3, significant changes are noted in Schottky defects and oxygen Frenkel between bulk SrTiO3 and the interface. The activation energies for the oxygen ion diffusion in bulk SrTiO3 and LaCrO3 are calculated to be 0.64 eV and 0.53 eV, respectively, suggesting that both materials will present favourable diffusion pathways. In general, there is a reduction in the activation energies of oxygen ion migration at the interface compared to bulk structures. In particular, in the Cr layer, activation energy is calculated to be 0.36 eV implying that interface plays an important role in the oxygen ion migration and therefore such an interface is of interest for use in advanced electrochemical devices.

Using spherical aberration corrected high-resolution and analytical scanning transmission electron microscopy, we have quantitatively studied the lattice distortion and the redistribution of charges in two-dimensionally strontium (Sr)-doped La2CuO4 superlattices, in which single LaO planes are periodically replaced by SrO planes. As shown previously, such structures show Tc up to 35 K as a consequence of local charge accumulation on both sides of the nominal SrO planes position. This is caused by two distinct mechanisms of doping: heterogeneous doping at the downward side of the interface (space-charge effect) and "classical" homogeneous doping at the upward side. The comparative chemical and atomic-structural analyses reveal an interrelation between local CuO6 octahedron distortions, hole spatial distribution, and chemical composition. In particular we observe an anomalous expansion of the apical oxygen-oxygen distance in the heterogeneously doped (space-charge) region, and a substantial shrinkage of the apical oxygen-oxygen distance in the homogeneously doped region. Such findings are interpreted in terms of different Jahn-Teller effects occurring at the two interface sides (downward and upward).

Infiltration of nano-catalysts in ionic-conductive backbones is receiving increasing attention to fabricate highly performing electrodes for Solid Oxide Cells application. In particular, nanostructured, high surface area scaffolds based on ceria and infiltrated with functional perovskites have already proved their excellent catalytic activity as oxygen electrodes. A major challenge for this type of nanocomposites is keeping the enhanced performance when up-scaling to large area cells and during long term operation. In this work, Ce0.8Gd0.2O1.9-La0.6Sr0.4Co0.2Fe0.8O3-δ infiltrated mesoporous oxygen electrodes were fabricated and tested in state-of-the-art 25 cm2 area fuel electrode supported solid oxide electrolysis cells. Injected currents as high as 11.2 A (0.7 A cm−2) at 1.3 V were measured in co-electrolysis mode at 750 °C showing improved performances with respect to button cell counterparts. Stability tests at injected currents of 8 A (0.5 A cm−2) for more than 600 h yielded a degradation rate of 126 mV kh−1 mainly related to the metallic nickel depletion approaching the fuel electrode-electrolyte interface, proving the stability of the oxygen electrode under highly demanding operating conditions. The excellent results presented here anticipate the relevance of nanostructured infiltrated electrodes for the next generation of enhanced Solid Oxide Cells.

New emerging disciplines such as Nanoionics and Iontronics are dealing with the exploitation of mesoscopic size effects in materials, which become visible (if not predominant) when downsizing the system to the nanoscale. Driven by the worldwide standardisation of thin film deposition techniques, the access to radically different properties than those found in the bulk macroscopic systems can be accomplished. This opens up promising approaches for the development of advanced micro-devices, by taking advantage of the nanostructural deviations found in nanometre-sized, interface-dominated materials compared to the “ideal” relaxed structure of the bulk. A completely new set of functionalities can be explored, with implications in many different fields such as energy conversion and storage, or information technologies. This manuscript reviews the strategies, employed and foreseen, for engineering mass transport properties in thin film ceramics, with the focus in oxide ionic and mixed ionic-electronic conductors and their application in micro power sources.

The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices. By means of molecular beam epitaxy whole LaO-layers were periodically replaced through SrO-layers providing a charge reservoir, yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muSR. We find a quasi-2D superconducting state which couples to the antiferromagnetic order in a non-trivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions.

In the last few years, high temperature solid oxide electrolysis cells (SOECs) have emerged as a promising solution for energy conversion and storage. However, state-of-the-art systems suffer from technological limitations, which prevent their widespread use and market penetration. Particularly, the electrode–electrolyte interface represents a critical element due to the high oxygen potential located in this area, which in turn determines interface delamination. In this context, the use of mesoporous materials, whose architecture is characterized by concatenated nanometric-size pores and high specific surface area, represents a powerful strategy toward the achievement of long-term stability of the electrode. Such structures have been recently proposed as ionically conducting electrode scaffolds for solid oxide fuel cells (SOFCs), exhibiting good performances and low degradation rates. In particular, it has been shown that a highly increased triple phase boundary (TPB)-active points distribution along the electrode may be achieved upon infiltration with a catalytically active material. In this study, infiltrated mesoporous cerium was used as a functional oxygen electrode layer in a fuel electrode supported SOEC system. The results indicate that an enhancement in both long-term stability and electrolysis cell performance are achieved. This is attributed to the decrease in high current density paths and areas of high oxygen potential and to the superior thermal stability of such a nanostructured composite, which allows better current distribution. First, the morphological characterization of the as-synthesized mesoporous Ce0.8Gd0.2O1.9 (CGO), which was carried out by TEM microscopy and low-angle X-ray diffraction (LA-XRD), is presented. A structural and functional investigation of the fuel electrode supported cells, in which the oxygen electrode is composed of catalytically active La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) infiltrating a CGO scaffold, was studied by means of XRD, scanning electron microscopy and spectroscopy, and electrochemical measurements. Characterization under co-electrolysis mode (45% H2O, 45% CO2 and 10% H2) shows an injected current density higher than 1.2 A cm−2 at 1.4 V at 750 °C. Electrochemical impedance spectroscopy was carried out at regular time intervals during galvanostatic (0.5 and 0.75 A cm−2) long-term operation for 1400 h, exhibiting a decrease in polarization resistance and only slight increase in serial resistance during operation. Total degradation rates lower than 2% kh−1 at 0.5 A cm−2 and 1% kh−1 at 0.75 A cm−2 were obtained. Finally, a mechanism for cell degradation based on evolution of the fuel electrode is proposed.

Among the range of complex interactions, especially at the interfaces of epitaxial oxide systems, contributing to the occurrence of intriguing effects, a predominant role is played by the local structural parameters. In this study, oxide molecular beam epitaxy grown lanthanum cuprate-based bilayers (consisting of a metallic (M) and an insulating phase (I)), in which high-temperature superconductivity arises as a consequence of interface effects, are considered. With the aim of assessing the role of the dopant size on local crystal structure and chemistry, and on the interface functionalities, different dopants (Ca2+, Sr2+ and, Ba2+) are employed in the M-phase, and the M–I bilayers are investigated by complementary techniques, including spherical-aberration-corrected scanning transmission electron microscopy. A series of exciting outcomes are found: (i) the average out-of-plane lattice parameter of the bilayers is linearly dependent on the dopant ion size, (ii) each dopant redistributes at the interface with a characteristic diffusion length, and (iii) the superconductivity properties are highly dependent on the dopant of choice. Hence, this study highlights the profound impact of the dopant size and related interface chemistry on the functionalities of superconducting oxide systems.

In this paper we present the atomic-layer-by-layer oxide molecular beam epitaxy (ALL-oxide MBE) which has been recently installed in the Max-Planck Institute for Solid State Research and we report on its present status, providing some examples that demonstrate its successful application in the synthesis of different layered oxides, with particular reference to superconducting La2CuO4 and insulator-to-metal La2−xSrxNiO4. We briefly review the ALL-oxide MBE technique and its unique capabilities in the deposition of atomically smooth single-crystal thin films of various complex oxides, artificial compounds and heterostructures, introducing our goal of pursuing a deep investigation of such systems with particular emphasis on structural defects, with the aim of tailoring their functional properties by precise defects control.

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry.

Strontium titanate (SrTiO3 ) has received much attention due to its wide range of potential applications, including in electrochemical devices such as solid oxide fuel cells and capacitors. The stability and safety features of SrTiO3 led to the development of promising electrodes for Li-ion batteries. Here, we use density functional theory simulations to examine the incorporation of lithium from its gas-phase and bulk forms. The results show that a single Li atom is thermodynamically stable in bulk SrTiO3 with respect to its gas-phase and slightly unfavorable compared to its bulk. Multiple Li incorporation up to six is also considered, and the incorporation is exoergic with respect to both gas-phase and bulk forms. Charge analysis confirmed the presence of Li+ ions in the lattice. Li incorporation turns the insulating nature of SrTiO3 into metallic and non-magnetic into magnetic. Lithium incorporation facilitates the formation of Sr, Ti, and O vacancies. The loss of Li2 O is exoergic, suggesting that oxygen vacancy mediated self-diffusion will be promoted