Solid state ionics: advanced concepts and devices

Resume : Solid-state batteries offer great potential for large improvements in safety and lifetime, as well as higher energy and power densities. However, the interfacial composition and structure between solid electrolytes and electrode materials often present major deviations from those of the bulk materials. Elucidating the nature of the involved interfaces is required to establish a rational approach towards the successful combination of materials in a new generation of solid-state cells. Controlled interfaces between a solid-state electrolyte (Li0.33La0.5TiO3), cathode (LiMn2O4) and anode (LI4Ti5O12) have been realized in 2D-planar and 3D-vertical thin film geometries by applying pulsed laser deposition. The influence of the temperature and deposition rate on the morphology evolution of lithium-based vertically aligned nanocomposites is modelled by applying Kinetic Monte Carlo Simulations with activation energies for hopping obtained experimentally and with minimum restrictions for hopping directions. Epitaxial engineering is used to control the crystal orientation within the 2D and 3D geometries, which enables a unique insight into the relation between electrochemistry and crystal directionality of such chemically complex inorganic interfaces, not obtainable in single crystals or polycrystalline samples. D.M. Cunha et al., ‘Morphology Evolution during Lithium-Based Vertically Aligned Nanocomposite Growth’, ACS Appl. Mater. Interfaces 2019, 11, 44444−44450.

Resume : Thin film solid state batteries are called to play a prominent role as a power supply for future micro-devices. Despite few commercial solutions exist, many efforts are still invested in the development of such devices with improved capabilities. The challenge is to develop appropriate electrodes with high capacity, stability and fast performance, and electrolytes with a low ionic resistance at room temperature. Furthermore, it is crucial to pay attention to the electrochemical compatibility between the components and to provide good quality interfaces. Here we present the development of LiMn2O4 (LMO) and Li4Ti5O12 spinel electrodes, and Li1 xAlxTi2-x(PO4)3 electrolyte thin films. The materials have been deposited by means of Large Area Pulsed Laser Deposition (LA-PLD), using multi-layering strategies to balance lithium content of the films. Exhaustive structural and electrochemical characterization of the layers has been carried out. In particular, recently developed operando spectroscopic ellipsometry and Raman spectroscopy techniques have been used for the study of ion-transport phenomena and the track of Lithium content and volume expansion during cycling.

Resume : Thin film deposition techniques can be useful for the fabrication of better solid-state batteries. Reducing the thickness of the electrolyte material to a few hundred nanometers facilitates ionic conductance for faster charge-discharge and reduces the total volume of inactive material, therefore increasing energy density. Lithium garnet Li7La3Zr2O12 (LLZO) electrolyte is a promising ionic superconductor for solid-state lithium batteries. In bulk, this material has demonstrated high ionic conductivities (0.1 - 1 mS/cm), as well as a wide electrochemical stability window (against metallic lithium anode and high potential cathode materials). However, processing this ceramic material in the form of films presents still some challenges. We present a method for fabricating crystalline LLZO thin films at about 700°C (significantly below the standard processing temperatures of about 1100°C) with densities and ionic conductivities comparable to the values observed in bulk ceramic pellets. By engineering the interface between a thin film LiCoO2 cathode and the solid-state electrolyte, we demonstrate a low interfacial resistance between cathode and electrolyte. For the first time a functional thin film lithium battery with sputtered ceramic LLZO as solid-state electrolyte is demonstrated.

Resume : Energy storage is essential for many applications such as portable electronic devices and electric vehicles. Lithium ion batteries (LIBs) are the most commonly used energy storage devices because of their high energy density and stability over repeated charge-discharge cycling. High rate performance of LIBs is nowadays one of the most important requirements for electric vehicles. However, due to the limitations of conventional graphite anodes, the charge-discharge process of the battery is limited to 1 C (or even lower). Titanium-based oxides are promising candidates to fulfill those requirements. However, in order to reach high power density, researchers usually use strategies like incorporation of carbon within the composites and/or a mesoporous structure design, which reduce the volumetric capacity. Niobium tungsten oxides have recently been shown to exhibit very fast (dis)charging capacity owing to their stable host structure suitable for lithium diffusion. It was originally suggested that dimensional reduction of the material would have a negligible effect on its electrochemical performance, and recent studies on 300 nm thick nanofibers seemingly confirmed that hypothesis by failing to demonstrate enhanced energy storage property in comparison to bulk materials. However, in this contribution we will provide conclusive evidence for the dependence of the lithiation process of Nb18W16O93 anodes on the grain size. We will show that the lithiation dynamics of niobium tungsten oxide are significantly enhanced when the secondary grain size is below 100 nm. This study provides a new perspective on the importance of nanoscaling this material to further improving the electrochemical performance of Nb18W16O93 anodes for realizing fast (dis)charging for future energy storage devices.

Resume : Proton-conducting ceramics, such as yttrium doped barium zirconates/cerates referred to as BZCY, are studied for intermediate temperature applications, including protonic-ceramic fuel/electrolysis cells, electrochemical compressors, or catalytic membrane reactors. In the presence of steam, the material hydrates with the consumption of oxygen vacancies and the formation of protonic defects. Hydration-induced chemical expansion (lattice expansion upon protonic defect formation) can be critical during the sample preparation and testing. Techniques for thermal and chemical expansion measurements will be presented together with challenges associated with analysis and interpretation. A short overview of the literature data on the expansion in proton-conducting ceramics will be provided. While all these measurements are performed in a single atmosphere, it is important to predict the expansion with different gas compositions on both sides of the membrane, thus representing real device applications. To do so, we developed a computational model based on a Nernst-Planck-Poisson formulation and included a chemo-thermo-mechanical component. Examples of crack formation due to uncontrolled hydration on symmetrical cells (BZCY-NiO//BZCY//BZCY-NiO) prepared by tape-casting will be shown.

Resume : BaFeO3-δ perovskites with mobile oxygen vacancies, holes, and protons, can be used as cathode material for Protonic Ceramic Fuel Cells (PCFC). Their mixed conductivity including protons is important to make the whole cathode surface active for oxygen reduction. The proton concentration in such cathode perovskites was determined by thermogravimetry. [1,2] Crystal structure, symmetry and local lattice distortions have an important impact on the proton uptake. Partial substitution of Fe by redox-inactive and oversized Zn2+ or Y3+ is beneficial for proton uptake. [2] This can be assigned to local lattice distortions decreasing the covalency of Fe-O bonds. This enhances the basicity of the oxygen ions, which is important for dissociative water incorporation via an acid-base reaction. The local environment and bonding of the cations and of O2- is probed using Extended X-Ray Absorption Fine Structure (Fe,Zn,Y K-edges) and X-Ray Raman Scattering (O K-edge) for oxidized ("Fe4+"), reduced ("Fe3+") samples. [3] The rather small Fe edge shift between reduced and oxidized samples and strong pre-edge features at the O K-edge indicate that electron holes are largely delocalized to oxygen states. The variation in the pre-edge peak and edge position at the Fe K-edge demonstrates that the degree of electron hole transfer from the transition metal to oxygen depends sensitively on cation composition and formal Fe oxidation state. Zn and Y doped samples show the smallest energy shifts upon oxidation (i.e., even oxidized samples exhibit largely Fe3+ character). This may be related to the fact that the partial Y3+, Zn2+ substitution on the perovskite's B site increases the overall basicity of the perovskite, which allows for larger electron transfer from oxygen to iron. For oxidized samples without Y,Zn the EXAFS analysis shows very small deviation from an undistorted cubic structure. Reduction of the samples as well as partial Fe substitution by Y3+, Zn2+ strongly suppresses the third shell peak, indicating significant static disorder and buckling of B–O–B connections. These local distortions decrease the Fe–O covalency and disfavor hole transfer from iron to O. This is further confirmed by the decreased pre-peak in the O-K edge XRS spectra of Y,Zn-doped samples. The corresponding increased oxide ion basicity then leads to increased proton uptake. This consistent picture of the interplay of chemical, geometrical, and electronic structure features, and their impact on proton uptake can serve as guideline for further PCFC cathode material optimization. [1] D.Poetzsch et al. Phys.Chem.Chem.Phys.16,16446 (2014). [2] R.Zohourian et al. Adv.Funct Mater.28,1801241 (2018). [3] G. Raimondi et al., Chem. Mater. 32, 8502-8511 (2020).

Resume : Mixed proton-, oxygen ion- and electron-conducting ceramics such as self-generated nanocomposites from the BaCeO3-δ-BaFeO3-δ system [1] offer attractive options for application in protonic ceramic fuel and electrolyser cells or hydrogen separation membranes. In the present work, partial substitution of Ce and Fe by Y in the BaCeO3-δ BaFeO3-δ system is investigated to further increase the proton uptake and gain a deeper understanding of the interrelation between chemical composition and oxygen-/proton-exchange processes. The precursor BaFe0.4Ce0.4Y0.2O3-δ was synthesised via a sol-gel process. After thermal treatment, a composite of a cubic (Pm-3m) Fe-rich and a trigonal (R-3c) Ce-rich perovskite was obtained. With increased annealing temperature, the fraction of the cubic phase increases (reaching 98 wt-% at 1370°C) and lattice parameters change systematically. Analytical scanning transmission electron microscopy is used to determine the distribution of both phases and their local cation stoichiometry. The water uptake measured by thermogravimetry shows characteristic differences depending on cation composition between the homogeneous BaFe0.4Ce0.4Y0.2O3-δ perovskite and 2-phase composites. These results will be related to water incorporation trends and deviations from ideally dilute defect chemistry observed for (Ba,Sr,La)FeO3-δ perovskites [2]. [1] S.Cheng et al., Angew. Chem. Int. Ed. 2016, 55, 10895. [2] R.Zohourian et al., Adv. Funct. Mat. 2018, 28, 1801241.

Resume : In recent years manganites with tailored functional properties have attracted special attention for their use in solid oxide fuel cells, electrolyzers, oxygen permeation membranes and memristive devices. The presence of oxygen vacancies and, more importantly, their migration under a particular driving force, such as temperature, oxygen partial pressure, voltage or electrochemical potential, defines their performance when used as functional components. In this talk I will give new insights on the oxygen transport properties of manganite thin films by presenting new tuning strategies and a novel methodology to characterize the exchange kinetics. First, I will show experimental evidence on how the combination of extended defects and strain relaxation accelerates the oxygen transport across LaMnO3±δ (LMO) thin films. Epitaxial LMO films were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on two different substrates. On SrTiO3 the films are perfectly epitaxial and defect free, while on LaAlO3, with a higher film-substrate mismatch, the strain is released by the formation of a high density of extended structural defects. By combining the oxygen transport measurements at the temperature range of interest (500°C- 600°C) with a detailed structural characterization using several complementary techniques, we showed that the combination of structural defects and strain relaxation accelerates the oxygen transport across the LMO/LAO films. Additionally, we demonstrated that the oxygen diffusion at 500 °C is extraordinarily high due to the prevalence of the orthorhombic structure in the film, together with a high concentration of oxygen vacancies. Next, I will present a very innovative methodology to measure the oxygen exchange kinetics, which has been proved for Sr- and Co- substituted lanthanum manganite (La0.8Sr0.2(Mn1−xCox)0.85O3-δ) thin films. The widely used isotope exchange depth profile methodology combined with secondary ion mass spectrometry (IEDP-SIMS) allows to directly obtain the oxygen transport parameters, i.e. surface exchange (k*) and tracer diffusion (D*) coefficients, from isotope depth profiles. However, it has a number of drawbacks as it is destructive, rather expensive and time consuming. The new technique presented here is based on the combination of 18O isotopic exchange and Raman spectroscopy, and allows to follow the evolution of the 18O concentration changes with time in situ, and to easily evaluate the effect of changes in the gas atmosphere (e.g. humidity, contaminants). Furthermore, it also has the advantages of being new non-destructive, simple and cheap, showing a great promise to be extended to many other functional materials in the near future.

Resume : The comprehension of the complex processes taking place at Solid Oxide Fuel Cells (SOFC) materials requires using complementary characterization techniques. In these devices, the ionic/electronic charge transport and the solid-gas electrode reactions depend on the atomic and electronic structures of the electrode surface and electrode/electrolyte's bulk. However, SOFC works at non-ambient conditions, which could induce changes in material structures affecting their performances. Therefore, simultaneous electrical or electrochemical experiments with in-situ studies using neutron or synchrotron radiation techniques are essential to understand the whole phenomena. In this work, we discuss results for both, proton conductor electrolyte and perovskite-based symmetrical electrode materials. In oxides with proton conductivity, O-ion and/or electronic conduction usually co-exist. Thus, in-situ X-ray diffraction combined with Impedance Spectroscopy (IS) and neutron-based techniques were used to elucidate the crystalline structure and validate the temperature range where BaCe0.4Zr0.4Y0.2O3-δ can operate as proton electrolyte. On the other hand, we evaluate the chance of using Ni-doped SrTi0.3Fe0.7O3-d perovskite as anode/cathode by studying the reversibility of reduction-oxidation cycles with the aim to explore electrode regeneration process. We used near ambient synchrotron spectroscopies, electron microscopy and IS aiming to understand the mechanism of surface decoration by nanoparticles exsolution and how it affects the electrode reactions.

Resume : Mixed ionic and electronic conductor (MIECs) thin films offer an ideal platform for the study of oxygen mass transport mechanisms, which play a crucial role in many energy and information technologies, such as solid oxide fuel cells (SOFCs) and non-volatile resistive memory devices. For these applications, the defect chemistry is known to play a major role in determining the overall performances and the knowledge of the point defect concentration is needed to tailor the oxides’ functional properties. Nevertheless, traditional methods used in bulk materials presents many challenges in thin films’ form, especially at low temperature, since the small masses and volumes lower the sensitivity of the measurements. The objective of present work is to investigate the defect chemistry in La1-xSrxFeO3-δ (LSF) thin films as a function of equivalent oxygen partial pressure, temperature (450-350 ºC) and Sr concentration by in-situ ellipsometry measurements. LSF thin films were deposited on YSZ (001) substrate by Pulsed Laser Deposition (PLD). The oxygen chemical potential of the LSF was progressively varied by applying a voltage bias between the thin film and the counter-back electrode, while recording the ellipsometry spectra. The results show that it is possible to quantify the defect concentration in the LSF thin films at intermediate-to-low temperatures using ellipsometry, this method offers new insights into the effect of the point defect concentration on the material’s electronic structure and on the phenomena involved in the reduction and oxidation of LSF thin films.

Resume : Mixed ionic and electronic conducting (MIEC) perovskites are often used as electrodes in high temperature electrochemical devices such as solid oxide fuel/electrolysis cells (SOFC/SOEC) and oxygen transport membranes (OTMs) due to their excellent catalytic activity for oxygen reduction and ionic and electronic conductivity. Previous studies on the use of MIEC perovskites for these applications have focussed on material behaviour and properties under pure oxygen conditions. Recently, however, it has been suggested that humid vapour may modify the materials’ chemistry under device operating conditions and impact device performance and durability [1-2]. Study of water-induced oxygen transport mechanisms on MIEC perovskites under different humid atmospheres is vital as the aforementioned electrochemical devices approach market viability. In this study, the oxygen surface exchange and bulk diffusion kinetics of MIEC perovskite (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255) are probed through Isotopic Exchange Depth Profiling – Secondary Ion Mass Spectrometry (IEDP-SIMS). Isotopic exchange annealing was carried out from 600 to 900 °C under pure water (pO2 < 1 mbar, pH2O = 30 mbar), humidified oxygen (pO2 = 200 mbar, pH2O = 30 mbar), and dry oxygen (pO2 = 200 mbar, pH2O = 0 mbar) conditions. We have observed a significant enhancement in oxygen mass transport properties under the pure water vapour condition compared to the dry and the wet oxygen conditions with high pO2. This is primarily due to the higher concentration of oxygen vacancies generated in the materials during exchange annealing. To probe the defect chemistry of the samples, in particular the oxygen non-stoichiometry changes under the different annealing conditions, thermogravimetric analysis and neutron diffraction were used. Our study also demonstrates limited surface exchange between water and the LSCrF phase under the humidified oxygen conditions. This is primarily due to the dominance of homo-exchange between the humid vapour and gaseous oxygen molecules at high temperatures, as observed through in-situ residual gas analysis carried out during wet oxygen isotopic exchange. In addition to the study of transport properties, we have investigated the effect of the atmosphere on the stability of the LSCrF8255 surface using angle-resolved X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy, correlating changes in oxygen transport kinetics with cation segregation processes to provide a detailed understanding of potential degradation processes in the solid oxide cell or OTM devices under humid conditions. 1. Sha, Z., Cali, E., Kerherve, G. and Skinner, S.J., 2020. Oxygen diffusion behaviour of A-site deficient (La0.8Sr0.2)0.95Cr0.5Fe 0.5O3−δ perovskites in humid conditions. Journal of Materials Chemistry A, 8(40), pp.21273-21288. 2. Staykov, A., Fukumori, S., Yoshizawa, K., Sato, K., Ishihara, T. and Kilner, J., 2018. Interaction of SrO-terminated SrTiO3 surface with oxygen, carbon dioxide, and water. Journal of Materials Chemistry A, 6(45), pp.22662-22672.

Resume : Solid oxide fuel cells (SOFC) stand out among other technologies for stationary power and quality heat co-generation due to their high energy conversion efficiency that can reach 80% [1]. SOFC generate electricity from chemical fuels such as H2, CO, CH4 at high temperatures (> 800 °C). The same device could become a solid oxide electrolyzer cell (SOEC), to generate reversely fuels from electricity. Large commercialization of this technology requires lower operation temperatures (< 600°C) to reduce costs and mitigate degradation but then typical electrode materials, i.e. electronic conductors and Ni-cermets, suffer from poisoning, C deposits, and increased activation overpotentials. Tackling these issues requires engineering new electrode materials for intermediate temperature SOFC [2]. Ni-doped Sr(Ti,Fe)O3-d (STFN) is a mixed ionic electronic-conducting perovskite that exsolves Fe Ni nanoparticles (NPs) in a reducing atmosphere. These Fe-Ni NPs boost the performance of the supporting STFN electrodes but the underlying mechanisms at the solid/gas interface are not well understood [3-5]. In this study, we combine ambient pressure X-ray photoelectron and absorption spectroscopies with electrochemical impedance spectroscopy and polarization in SOFC and SOEC modes on model cells while redox cycling the atmosphere. We characterize the surface chemistry of STFN electrodes under operando conditions to investigate the tunability of the exsolution process, which could impact the life cycle. [1] “Technology Report Hydrogen and fuel cells”, International Energy Agency, Springer, 2015. [2] “A perspective on low-temperature solid oxide fuel cells”, Z. Gao, L.V. Mogni, E.C. Miller, J.G. Railsback, S.A. Barnett, Energy and Environmental Science, 9 (2016) 1602-1644. [3] “Ni-Substituted Sr(Ti,Fe)O3 SOFC Anodes: Achieving High Performance via Metal Alloy Nanoparticle Exsolution”, T. Zhu, H. Troiani, L.V. Mogni, M. Han, S.A. Barnett, Joule, 2 (2018) 478-496. [4] “Exsolution and chemistry in perovskite SOFC anodes Role of Stoichiometry in Sr(Ti,Fe,Ni)O3”, T. Zhu, H. Troiani, L.V. Mogni, M. Santaya, M. Han, S.A. Barnett, Journal of Power Sources, 439 (2019) 227077. [5] “Study of phase stability of SrTi0.3Fe0.7O3−d perovskite in reducing atmosphere: Effect of microstructure”, Solid State Ionics, 342(2019) 3-9.

Resume : Mixed conductors find application in sensors, fuel/electrolysis cell electrodes, reactors for chemical fuel production, and gas separation membranes. In some cases, low-to-intermediate temperature processing and operation may be advantageous for limiting initial energy expenditure and long-term degradation, respectively. In such conditions, amorphous or poorly crystalline structures may prevail, though their performance has not been widely studied. Therefore, in recent work, we have been applying X-ray absorption spectroscopy and in situ impedance and optical measurements during crystallization, in order to explore how mixed conductor structure-property relationships are impacted by the degree of crystallinity. Our initial studies on thin films of the perovskite SrTi0.65Fe0.35O3-x (STF) demonstrate that the amorphous material, prepared by pulsed laser deposition at room temperature, contains relatively under-coordinated cations with a lower average oxidation state for Fe than in the crystalline counterpart, prepared via higher temperature deposition or annealing. These results are consistent with an observed increase in sub-gap optical absorption and electrical conductivity during crystallization, which is attributed to an increase in hole concentration. These local structural and defect chemical changes can help to explain the evolution of other functional properties of STF during crystallization, such as the dramatic increase in oxygen exchange kinetics.

Resume : There is tremendous interest in making the next super battery, but state-of-the-art Li-ion technology works well and has inertia in several commercial markets. Supplanting Li-ion will be difficult. Recent material breakthroughs in Li and Na metal solid-state electrolytes could enable a new class of non-combustible solid-state batteries (SSB) delivering twice the energy density (1,200 Wh/L) compared to Li-ion. However, technological and manufacturing challenges remain. The discussion will consist of recent milestones and attempts to bridge knowledge gaps to include: • The physical and mechano-electrochemical phenomena that affect the stability and kinetics of the Li and Na metal-solid electrolyte interface • Thin film processing and Li integration with LLZO • Plating and stripping dynamics of Li and Na metal Despite the challenges, SSB technology is rapidly progressing. Multi-disciplinary research in the fields of materials science, solid-state electrochemistry, and solid-state mechanics will play an important role in determining if SSB will make the lab-to-market transition. Key words: energy storage, Li, Na

Resume : The use of metallic lithium as anode material is crucial for achieving maximal energy densities in all solid state Li ion batteries (ASSLIB). Therefore, understanding the nature of ion transfer at the interface between the lithium metal and solid electrolyte is essential for further optimization of ASSLIB. In this contribution, the lithium transfer across the SE | lithium metal interface is investigated by means of ab-initio calculations based on density functional theory (DFT). The aluminum doped garnet Li6.25Al0.25La3Zr2O12 (LLZO) is considered as a model SE due to its practical stability against lithium metal. By combining the information of both ab-initio molecular dynamics (AIMD) simulations and calculations of energy barriers using the Nudged Elastic Band (NEB) method, a schematic energy landscape of the LLZO | lithium metal interface is constructed, implying that the charge transfer process across this interface is not the rate limiting step, neither during stripping, nor during plating conditions. This charge transfer reaction is additionally analyzed by tracking the transformation of the charge density profile during the interfacial jump. We observe, that the ionization of lithium atoms occurs after the energy barrier has been overcome. This confirms that the transfer of the electron from the ionized lithium to the delocalized metallic electron cloud does not impose any intrinsic energy barrier to the interfacial transport.

Resume : Na metal All-Solid-State batteries (ASSBs) could provide a single solution to simultaneously meet several notorious objectives for new generations of cells: outstanding safety, high energy density, fast charging rates and reduced environmental impact. Among the best Na+ conducting ceramic solid electrolytes, the family of Na+ SuperIonic CONductors (NaSICON) of composition Na1+xZr2SixP3-xO12 (0 ? x ? 3) has long been recognized for its remarkably high ionic conductivity. Thanks to perpetuated research over several decades, the ionic conductivity of NaSICON ceramics was improved and now reaches values of up to 5.0 mS cm-1 at room temperature for the composition Na3.4Zr2Si2.4P0.6O12 [1]. Interfaces in NaSICON-based ASSBs ? and in particular the Na metal|NaSICON interface - have however yet to be optimized. In this work, the experimental conditions influencing the Na|NaSICON interface resistance are assessed. The impact of various polishing and annealing conditions on the surface composition of NaSICON ceramics are studied by a combination of X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) and correlations with the electrochemical performance of Na|NaSICON interfaces are established. This study reveals the existence of an in-situ formed sodium phosphate film on the surface of thermally treated NaSICON ceramics and demonstrates that its presence systematically leads to Na|NaSICON interface resistances below 1 ? cm2 at room temperature. DFT simulations demonstrated that the segregation of this sodium phosphate layer on the surface of NaSICON ceramics was driven by a minimization of surface energies. NaSICON ceramics possessing this surface film were able to withstand high current densities of up to 8 mA.cm-2 which paves the way for fast-charging Na metal ASSBs. References [1] Ma, Qianli, et al. "Room temperature demonstration of a sodium superionic conductor with grain conductivity in excess of 0.01 S cm? 1 and its primary applications in symmetric battery cells." Journal of Materials Chemistry A 7.13 (2019): 7766-7776.

Resume : Na3PS4 is a promising electrolyte for future sodium all-solid state batteries. Its readily available components make it a compelling and more sustainable alternative to recent Li-technologies. At ambient temperature, the ionic conductivity in its cubic crystal structure is in the order of 10−4 cm−1 [1]. Even though several studies focused on explaining the dynamic properties of cubic Na3PS4, the driving forces that lead to fast Na+ exchange are not yet completely clear. Here, we synthesized nanocrystalline, defect-rich cubic Na3PS4 via a solid-state synthesis with subsequent annealing at 250 °C for 12 h. Additionally, we synthesized doped Na3PS4 to investigate the influence of foreign atoms in the crystal structure on the ionic transport properties. Ion dynamics of the powder samples were analysed using high-precision broadband impedance spectroscopy and variable-temperature, time-domain 23Na NMR spin-lattice relaxation rate measurements. We were able to separate bulk ion dynamics from electrical relaxation associated with grain boundary regions. While macroscopic transport is characterized by an activation energy of 0.36 eV, 23Na NMR indicates a much lower value of 0.18 eV, see also [2]. This discrepancy points to length-scale dependent dynamic parameters. Indeed, electric modulus spectroscopy, i.e., the analysis of resistivity peaks ρν (= M''/ω)(1/T), revealed a low-temperature activation energy of 0.13 eV, which is consistent with our result from NMR. We attribute this barrier to extremely fast local Na hopping processes constituting the basis for long-range ionic transport in Na3PS4. [1] A. Hayashi et al., Nat. Commun., vol. 3, pp. 856-860, 2012. [2] C. Yu et al., J. Mater. Chem. A, vol. 4, pp. 15095-15105, 2016.

Resume : Different charge compensation mechanisms are known for ionic solids. Among them are the formation of compensating defects such as electronic or ionic defects, the valence changes of atoms and the segregation of dopants. In principle, the introduction of positive charges by donor doping or reduction results either in the compensation by electrons, negatively charged intrinsic acceptors as metal vacancies, the reduction of one of the species in the compound, or in the segregation of the dopant species. The situation is reversed for the addition of negative charges. While the different mechanisms are well-documented for different materials, predicting the prevailing compensation mechanism in a material is hardly possible. It is a common perception that the Fermi energy is determined by the defect concentrations. However, it is also possible to describe the concentration of defects as a function of the Fermi energy. This reveals Brouwer diagrams, which are identical to those obtained using standard defect chemistry calculations. In addition, it enables a direct comparison of the different compensation mechanisms.

Resume : The development of fast-ion-conducting solid electrolytes for use in all-solid-state batteries requires a detailed understanding of the key structural principles that give high ionic conductivities in specific materials. Lithium argyrodites based on Li6PS5X (X= Cl, Br, or I) are promising lithium ion solid electrolytes with room temperature ionic conductivities of up to 10-2 S cm-1. Recent experimental studies of these materials have shown that introducing subvalent cations into the host framework to give lithium stoichimetries x(Li)>6 can give significant increases in lithium ion conductivities. The microscopic cause for this behaviour is uncertain, however, and has been attributed in different studies to either a change in the anion substructure, or to purely the increase in lithium stoichiometry. To better understand the relationship between lithium stoichiometry, host-framework composition, and lithium conductivity in the lithium argyrodites we have performed a series of molecular dynamics studies on the series Li6 xPS5 xI1-x. To help deconvolve the coupled effects of changing the framework composition and changing the lithium stoichiometry in these stoichiometric systems, we have performed further simulations of non-stoichiometric systems, allowing us to vary each parameter independently. From these simulations we find a much greater effect from varying the host-framework stoichiometry than from varying lithium stoichiometry, and conclude that it is the change in framework chemistry, rather than lithium stoichiometry, that underpins the trends reported in previous experimental studies. A mechanistic analysis shows that lithium diffusion is effected by concerted motion of lithium ions at all stoichiometries. The quantitative mechanistic behaviour, however, does vary with host-framework composition (and to a lesser extent, with lithium stoichiometry) which we attribute to differing degrees of disorder in the lithium substructure.

Resume : Protonic ceramic fuel cells (PCFC) attract growing interest, since BaZr(1-x)Y(x)O(3-x/2) electrolytes offer a higher ionic conductivity compared to oxide ion conductors < 600°C. Optimized cathode materials with mixed protonic and electronic conductivity (e.g. (La,Ba,Sr)(Co,Fe,Zn,Y)O3-delta [1]) are crucial for PCFC performance. However, they generally show lower degrees of hydration compared to BaZr(1-x)Y(x)O(3-x/2). We perform DFT calculations based on the Hubbard-type (PBE+U and SCAN+U) functionals for Ba(1-x)Sr(x)FeO3-delta [2], BaCoO3-delta and SrCoO3-delta in order to look deeper in the materials properties changes induced by oxygen vacancies and protons. We, therefore, present a detailed analysis of the electronic DOS, volume and local geometry changes, oxidation and hydration energies as a function of oxygen deficiency (nominal Fe and Co oxidation state, hole concentration). An important comparison with La(1-x)Sr(x)FeO3-delta [3] on the role of A-cation is also given. The hydration energy is more negative for Ba(1-x)SrxFeO3-delta than for La(1-x)Sr(x)FeO3-delta, which can be assigned to the larger oxide ion basicity. The hydration energy becomes less negative with increasing hole concentration, which is related to hole delocalization to the oxide ions. Thus, we achieve a comprehensive understanding of water incorporation in mixed protonic-electronic conductors. [1] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Adv. Funct. Mater. 28 (2018), 1801241 [2] M. Hoedl, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Phys. Chem. C 124 (2020), 11780 [3] D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Mater. Chem. A 4 (2016), 13093

Resume : It is becoming ever more apparent that, in complex compounds such as pyrochlores, the detailed arrangement of the cations drives functionality. For example, both radiation tolerance and ionic conductivity have been linked to how easily cations can mix across sublattices. This is due to the fact that mass transport in these materials is a strong function of the cation distributions. However, the actual relationship between the cation, or chemical, structure of these compounds and the rates of transport are still not well established. While some reports find enhanced ionic conductivity in disordered materials, others find higher conductivity in ordered phases. Many of these studies use chemistry to influence the disorder, essentially changing multiple variables at once. However, the degree to which cations mix can be finely controlled using radiation damage without changing chemistry. Here, we use radiation damage as a tool to induce changes in the cation structure of thin-film model pyrochlores. We then characterize the extent to which those changes impact mass transport. We combine these experimental efforts with state-of-the-art simulation methodologies to understand how atomic scale mechanisms dictating mass transport change when the cation structure is modified. We have found that even small changes in cation structure can lead to large changes in the transport characteristics of the material. Our results provide new insight into mass transport in materials that exhibit chemical complexity well beyond the model systems studied where chemical disorder dictates the fundamental behavior of the material.

Resume : Cathode materials for protonic ceramic fuel cells (PCFC) require sufficient proton conductivity to extend the reaction zone beyond the three phase boundary. The hydration thermodynamics of BaFeO3-d-related perovskites was studied by thermogravimetry. [1] Despite a high oxygen vacancy concentration, the degree of hydration is lower for cathode materials compared to Ba(Zr,Y)O3-x electrolytes. A partial substitution of iron by redox-inactive, oversized Zn2+ or Y3+ drastically increases the proton uptake. Measurements of oxygen nonstoichiometry and proton uptake indicate pronounced deviations from ideally dilute defect chemistry (hole-hole and hole-proton defect interactions [1]). Based on DFT calculations [2] and EXAFS/XRS measurements [3], these interactions are related to the partial transfer of holes from iron to adjacent oxygen ions, which in turn disfavors protonation. The obtained detailed defect-chemical understanding serves as the basis for PCFC cathode optimization, in particular since proton uptake, catalytic activity, electronic conductivity, and stability show conflicting trends. [1] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Adv. Funct. Mater. 28, 1801241 (2018) [2] M. F. Hoedl, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Phys. Chem. C 124, 11780 (2020) [3] G. Raimondi, A. Chiara, F. Giannici, R. Merkle, J. Maier, Chem. Mater. 32, 8502 (2020) We thank GIF (I-1342-302.5/2016) for financial support

Resume : Mixed Proton and Electron conducting Ceramics (MPEC’s) are central in the development of efficient positrode electrodes for Proton Ceramic Electrochemical Cells (PCEC’s). As for most high temperature oxygen electrodes, the mass transfer overpotential is limiting the electrode performance, and the interplay between catalytic activity for the surface red-ox reaction and the inherent partial proton conductivity determines the electroactive surface area during operation. Furthermore, the electronic conductivity of the electrode is important for good current collection and low ohmic overpotential, but mobile electronic defects also affects proton stability. The stability of proton defects in a ceramic structure strongly correlates with anion basicity.[1] It has previously been shown that the basicity of the oxide ions is governed by the cation-anion charge transfer characteristics,[2] and electron structure and mobility is therefore essential with respect to both red-ox activity and partial proton conductivity. In a perovskite structure, the B-site cation facilitates electron transfer and the A-site cation influences the structure and overall electronegativity. In this work, we investigate the influence of A- and B-site substitutions on proton concentration and electrochemical performance in Ba- and La-based cobaltites with perovskite- or perovskite-related structures. Thermogravimetric Analysis (TGA) has been used to investigate proton concentrations, and Electrochemical Impedance Spectroscopy (EIS) was used to investigate polarisation resistance of the positrode reactions for model electrodes and porous electrodes. In order to investigate the positrode in both anodic and cathodic operation, the EIS studies has been performed under positive and negative DC bias, and the results reveal that B-site substitutions affect water oxidation and oxygen reduction differently. Substituting Ti4+ for Co3+ in a BaLa0.8Gd0.2Co2O6-δ electrode promotes the anodic water oxidation reaction, while substituting Zn2+ for Co3+ impedes the anodic reaction and promotes the cathodic reduction of oxygen. Polarization resistances of several electrode compositions are obtained, and pO2, pH2O and T-dependencies are analysed for electrode materials with different hydration levels. The influence of B-site cation substitution on hydration is also shown for the BaLaCoO3-BaLaFeO3 system. Acknowledgements: The Research Council of Norway (Grant nᵒ 272797 “GoPHy MiCO” and nᵒ 299736 “FunKey Cat”) [1] T.S. Bjørheim, M.F. Hoedl, R. Merkle, E.A. Kotomin, J. Maier, The Journal of Physical Chemistry C 124 (2020) (2) 1277. [2] M.F. Hoedl, D. Gryaznov, R. Merkle, E.A. Kotomin, J. Maier, The Journal of Physical Chemistry C 124 (2020) (22) 11780.

Resume : The local activation overpotential describes the electrostatic potential shift away from equilibrium at an electrode/electrolyte interface. This electrostatic potential is not entirely satisfactory for describing the reaction kinetics of a mixed ionic-electronic conducting (MIEC) solid-oxide cell (SOC) electrode where charge transfer occurs at the electrode-gas interface. Using the theory of the electrostatic potential at the MIEC-gas interface as an electrochemical driving force, charge transfer at the ceria-gas interface has been modelled based on the intrinsic dipole potential of adsorbate.1 This model gives a physically meaningful reason for the enhancement in electrochemical activity of a MIEC electrode as the steam pressure is increased in both fuel cell and electrolysis modes. We validated this model against operando XPS data to accurately predict the outer work function shift of thin film Sm¬0.2Ce0.8O1.9 in a H2/H2O atmosphere as a function of overpotential.2 1 J. Fleig, Phys. Chem. Chem. Phys., 2005, 7, 2027–2037. 2 Z. A. Feng, C. Balaji Gopal, X. Ye, Z. Guan, B. Jeong, E. Crumlin and W. C. Chueh, Chem. Mater., 2016, 28, 6233–6242.

Resume : This presentation will survey the continuing controversy surrounding the modeling of surfaces and interfaces in ionic conductors, and will move beyond it to a discussion of the feasibility of device-scale models incorporating microscopically accurate, nanoscale depictions of solid state electrochemical interfaces. Microscopic evidence makes it clear that dilute-solution theories cannot be trusted in concentrated ionic solutions, where 'concentrated' refers to anything greater than approximately one mole percent. The results of a quantitative analysis of a microscopic dataset and corresponding statistical comparison of dilute-case theories vs. those incorporating concentrated solution thermodynamics will be presented. Devices based on ionic materials -- such as high temperature fuel cells and electrolyzers -- require accurate models of interfaces that control ionic conductivity and where rate-limiting reactions take place. Device-scale models of ceria-based electrolyzers for carbon dioxide with nanoscale interface models are now in development at West Virginia and the latest results will be presented.

Resume : Solid oxide cells which convert chemical energy to electrical energy offer an opportunity toward the deployment of miniaturized energy generation systems for battery-free portable devices (1, 2). In particular, mixed ionic-electronic conducting oxide (MIEC)-based ceramic layers, which combine electronic and ionic conductivity, exhibit a promising combination of catalytic and transport behavior (3). In recent times, engineering the electrochemical properties of MIEC materials at the grain boundary level has shown great potential (4, 5). In order to improve upon the design of such materials, a better understanding of the nanoscale structure and especially the direct analysis of the grain boundary behavior is therefore key. Atom probe tomography (APT) has the unique ability to probe the nanoscale 3-D distribution of species down to tens of parts-per-million concentrations. This has provided fundamental insights into the compositions and composition profiles in and around precipitates, grain boundaries, phase boundaries, and dislocations in metal alloys, electronic materials, and ceramic materials (6). Additionally, since this time-of-flight technique has sufficient mass resolution to detect particular isotopes of species, the introduction of tracer isotopic species can be used to distinguish sources of an element. For example, this technique has been used to investigate hydrogen embrittlement of steels using a deuterium tracer (7). Here, annealing of lanthanum manganite and chromite films at controlled temperatures and times was carried out in an atmosphere containing an 18O isotopic tracer followed by APT analysis of these films. From the measured distribution of the 18O species, the diffusion of reactant oxygen is distinguished from that in the original oxide film structures allowing fundamental investigations on oxygen kinetics on the nanoscale. The experimental results display the local distributions of 18O and other species in and around the grain boundaries and phase boundaries in the MIEC films allowing for direct comparisons with simulation profiles. This work presents a novel powerful tool for the fundamental understanding of grain boundary electrochemical properties in MIEC materials. 1. E. D. Wachsman, K. T. Lee, Lowering the Temperature of Solid Oxide Fuel Cells. Science 334, 935-939 (2011). 2. S. S. Shin et al., Multiscale structured low-temperature solid oxide fuel cells with 13 W power at 500 °C. Energy & Environmental Science 13, 3459-3468 (2020). 3. Z. Gao, L. V. Mogni, E. C. Miller, J. G. Railsback, S. A. Barnett, A perspective on low-temperature solid oxide fuel cells. Energy & Environmental Science 9, 1602-1644 (2016). 4. E. Navickas et al., Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films. Physical Chemistry Chemical Physics 17, 7659-7669 (2015). 5. F. Chiabrera et al., Engineering Transport in Manganites by Tuning Local Nonstoichiometry in Grain Boundaries. Advanced Materials 31, 1805360 (2019). 6. A. Devaraj et al., Three-dimensional nanoscale characterisation of materials by atom probe tomography. International Materials Reviews, 1-34 (2017). 7. Y. S. Chen et al., Direct observation of individual hydrogen atoms at trapping sites in a ferritic steel. Science 355, 1196 (2017).

Resume : Nickel/gadolinia-doped ceria (Ni/GDC) is the currently most promising alternative fuel electrode material for solid oxide fuel and electrolysis cells. For a targeted optimisation of real porous Ni/GDC cermet electrodes, a detailed insight into the role of the material properties for the electrochemical polarisation resistance is crucial. Here, model-composite GDC thin film electrodes with embedded current collectors were used in the first step to characterize the electrochemical elementary parameters of this material. In the second step, the results from model experiments are transferred to the interpretation of the impedance of porous cermet electrodes. Analytic fits of the electrode impedance are done by using a transmission line circuit, which reflects the physically correct relationship of the relevant elementary processes on Ni/GDC cermet electrodes. With this approach, it is possible to separate and quantify the individual contributions to the electrode polarisation resistance, such as oxygen ion transport across the electrolyte/GDC interface, ionic conductivity within the porous Ni/GDC electrode, and oxygen exchange at the GDC surface. Comparison with our model studies yields very good quantitative agreement. With these detailed insights, we can explain the excellent performance of real porous Ni/GDC fuel electrodes, which is enabled by the mixed ion/electron conduction of GDC and a microstructure with small GDC and large Ni grains. Moreover, we show that anode functional layers consisting entirely of GDC can even surpass the performance of cermet anodes (at 800 °C an ASR of 0.012 Ωcm² can be achieved), if the functional layer is sufficiently thin and good contact to a current collecting layer is established.

Resume : Perovskites from the La1-xSrxCo1-yFeyO3-δ series are promising materials for solid oxide cell air electrodes. However, Sr-segregation and reaction with acidic impurities lead to significant long-term degradation. Thus, research efforts are directed towards the development of alternative materials with a weak driving force for cation-segregation, low basicity, and excellent mass- and charge transport properties. Recently, promising results were obtained for La0.8Ca0.2FeO3-δ, which shows fast oxygen exchange kinetics and good long-term stability. In the present study, we explore the effects of Nd-substitution on structure-property relations in the La0.8-xNdxCa0.2FeO3-δ series in order to further optimize the mass- and charge transport properties and long-term stability. Single-phase orthorhombic perovskites (space group Pnma) were obtained in the range 0≤x≤0.6. Atomically resolved STEM-EDX maps confirm that La, Nd, and Ca are distributed homogeneously on the A-site. As expected from considerations of the ionic radii, increasing Nd-substitution leads to a decrease in unit cell volume. The orthorhombic/trigonal phase transition, which is observed for x=0 at approx. 750°C, is suppressed for x≥0.1. A maximum in the electronic conductivity was observed for x=0.1-0.2, whereas the thermal expansion coefficient and the oxygen nonstoichiometry are nearly independent of x at 0.1≤x≤0.6. First results on the oxygen exchange kinetics of LNCF (x=0.6) show high activity towards oxygen reduction.

Resume : The performance of mixed conducting oxides, such as cathode materials for solid oxide fuel cells, is strongly correlated with oxygen exchange processes between the gas phase and the ceramic oxide which can be described by oxygen diffusion as well as the surface exchange reaction. The long-term stability of cathode materials is highly affected by the occurrence of inert surface particles blocking the surface exchange reaction. Amongst others, conductivity relaxation experiments are a powerful tool for the investigation of the oxygen exchange properties. It is the aim of this contribution to present finite element modeling (FEM) of relaxation curves for ceramic samples as a function of surface coverage of inert particles. In particular, the effect of the particle shape as well as size distribution is studied in detail. Basically, the FEM simulations have been carried out on thick (0.05 cm) as well as thin samples (0.5 – 5 µm). A bimodal distribution of the surface particles is accomplished by a combination of large particles (100 µm) with significantly smaller particles (1 – 17 µm). In addition, large particles (100 µm) combined with particles of comparable size (20 – 40 µm) have been investigated. Moreover, the effect of surface particles with a square shaped cross-section as well as a rectangular cross section has been taken into account. Interestingly, the oxygen exchange kinetics is affected by flux constriction especially in the case of fairly large particle sizes.

Resume : Power-to-gas technologies are on the way to become economically viable in order to make more use of fluctuating renewable energy, but the rational design of hydrogen evolution reaction (HER) electrocatalysts that are competitive with platinum remains to be an outstanding challenge. We present the delafossites PdCrO2, PdCoO2 and PtCoO2 as a new family of highly efficient electrocatalysts for the HER in acidic media and show that the reductive operation can modify their surface and hence, the catalytic performance in different ways. For PdCoO2, the inherently strained Pd metal sublattice acts as a pseudomorphic template for the growth of a tensile strained Pd rich capping layer that ranges up to 400 nm, far beyond epitaxial methods. Its formation continuously improves the electrocatalytic activity by simultaneously increasing the exchange current density j0 and by reducing the Tafel slope down to 38 mV/decade, leading to overpotentials η_10< 15 mV for 10 mA/cm², superior to bulk platinum. We attribute these effects to the strain facilitated operando formation of a β-palladium hydride phase with drastically enhanced surface catalytic properties with respect to bulk or nanostructured palladium. These findings illustrate how operando induced electrochemical modifications can be used as a long ranging top-down design concept for rational surface and property engineering through the strain-stabilized formation of catalytically active phases. F. Podjaski et al. ”Rational strain engineering in delafossite oxides for highly efficient hydrogen evolution catalysis in acidic media.” Nat Catal 3, 55–63 (2020) doi:10.1038/s41929-019-0400-x

Resume : Proton ceramic fuel cells (PCFCs) are a electrochemical devices based a ceramic electrolyte that exhibits high protonic conductivity at elevated temperatures. PCFCs have received big attentions as alternatives of oxygen-ion conducting solid oxide fuel cells (SOFCs), because they have high ionic conductivity with low activation of protons, compared to that of SOFCs at intermediate temperatures [1, 2]. Recently, Lee et al. reported that the PCFC with cell size of 5×5 cm2 achieved a power density of 1.3 W‧cm-2 at 600 ℃ by using a Ni-cermet anode-supported cell design with the BaCe0.7Zr0.1Y0.1Yb0.1O3-δ(BCZYYb) electrolyte and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) cathode [3]. However, the BSCF cathode material has limited durability in terms of Sr segregation phenomena and high thermal expansion coefficient (TEC) under PCFC condition. Thus, many researchers try to develop the highly active and stable cathode materials with low activation energy at intermediate temperatures [4]. Usually, cobaltites have good catalytic properties for oxygen reduction reaction. However, TEC of cobaltite-materials is almost twice larger than that of state-of-art BCZYYb electrolyte for PCFCs [5]. Exceptionally, layered cobaltites have low TEC of ~ 10×10-6 K-1 [6]. Hence, in this study, we investigate layered cobaltites as a cathode material to improve thermal stability of PCFCs. In addition, to improve the catalytic property of cathode materials, various metal oxides are doped into layered cobaltites. In addition, doped layered cobaltites are prepared using various synthesis methods such as combustion, citrate-hydrothermal, and acetate methods. To confirm phase purity of synthesized powders, we use Rigaku X-ray diffraction spectroscopy. The symmetric cells are fabricated with the BCZYYb electrolyte and measured by electrochemical impedance spectroscopy at 450-800℃ under wet and dry condition to investigate electrochemical property of materials. References [1] Kim, J., S. Sengodan, S. Kim, O. Kwon, Y. Bu, and G. Kim, Renewable & Sustainable Energy Reviews 109, 606-618 (2019). [2] Duan, C. C., J. H. Tong, M. Shang, S. Nikodemski, M. Sanders, S. Ricote, Science 349 (6254), 1321-1326 (2015). [3] An, H., H. W. Lee, B. K. Kim, J. W. Son, K. J. Yoon, H. Kim, D. Shin, H. I. Ji, and J. H. Lee, Nature Energy 3 (10), 870-875 (2018) [4] Rioja-Monllor, L., C. Bernuy-Lopez, M. L. Fontaine, T. Grande, and M. A. Einarsrud, Journal of Materials Chemistry A 7 (14), 8609-8619 (2019) [5] Danilov, N. A., A. P. Tarutin, J. G. Lyagaeva, E. Y. Pikalova, A. A. Murashkina, D. A. Medvedev, M. V. Patrakeev, and A. K. Demin, Ceramics International 43 (17), 15418-15423 (2017) [6] Rolle, A., S. Boulfrad, K. Nagasawa, H. Nakatsugawa, O. Mentre, J. Irvine, and S. Daviero-Minaud, Journal of Power Sources 196 (17), 7328-7332 (2011). Keywords: Protonic ceramic fuel cells, Thermal expansion coefficient, Electrochemical spectroscopy, Cobaltite cathode * Corresponding author: jyoung@sejong.ac.kr(J. Y. Park)

Resume : Proton-conducting oxides are promising materials for electrochemical devices like fuel cell, hydrogen pump, hydrogen sensor, etc., and also for tritium purification and recovery system in nuclear fusion reactors. The hydrogen concentration in such oxide materials is very fundamental and important, but its precise measurement is not easy. Tritium tracer method such as tritium imaging plate (TIP) technique is a powerful tool not only for measuring hydrogen concentration but also for clarifying hydrogen behavior in oxide materials. In the present study, hydrogen solubility and diffusivity behavior in three different proton-conducting oxides, BaZr0.9Y0.1O2.95 (BZY), BaZr0.955Y0.03Co0.015O2.97 (BZYC), and CaZr0.9In0.1O2.95 (CZI), were studied by the TIP method using a hydrogen-tritium gas mixture (HT) (T = 0.0001%, ~1.3kPa) and also partially-tritiated heavy water vapor (DTO) (T = 0.1%, ~2kPa) in the temperature range of 623 to 1273 K. The hydrogen solubility and diffusivity behavior in BZY, BZYC, and CZI were also studied by thermal desorption spectroscopy (TDS) using deuterium (D2), heavy water (D2O), and O-18 enriched water (or H218O). The oxide specimens were prepared with conventional powder metallurgy using BZY, BZYC, and CZI powders separately by being die-pressed, cold-isostatically-pressed (200MPa), and sintered in air at 1913 K for 20 h. The specimens obtained were having a disc shape (~7.5 mm in diameter, ~2.3 mm in thickness) and more than 95% TD. From IP images for the surface of all tritium (T) exposed specimens, uniform T distribution was found. Observing cross-sectional T concentration profiles of cut specimens allowed us to determine hydrogen solubility and diffusivity in the specimens. In all cases of tritium exposure (HT and DTO), BZYC always shows the highest hydrogen solubility, while CZI always shows the lowest one. In DTO exposure, the BZY shows the highest hydrogen diffusivity, while in HT the BZYC shows the highest one. In all cases, CZI shows almost one order lower solubility and diffusivity than the BZY and BZYC. These results mean that BaZrO3 shows better electrochemical performance than the CaZrO3 and that a small amount of Y and Co doping in BaZrO3 may play a vital role in the enhancement of its electrochemical activity. Detail comparison of TDS results with TIP data for all three samples will be discussed in the paper in detail. Keywords: BaZrO3; CaZrO3; tritium, deuterium, heavy water, and H218O exposure, hydrogen solubility and diffusivity, fusion reactor materials.

Resume : The thin film vanadium pentoxide (V2O5) as ion-storage layer for electrochromic device is deposited on ITO glass by an RF magnetron sputtering. The electrochromic properties of the film are discussed after various thermal annealing temperature with/without oxygen flow ambient. It is found that the crystal structure of V2O5 thin film transfer from amorphous to crystalline phases (110) and (021) as the annealed temperature is 400 C. And the grain size of the sample without oxygen flow is larger than that of with oxygen flow after annealing. A degradation of electrochromic property is observed at sample both after the treatment of oxygen flow and thermal annealing. This is due to the oxygen occupies the ion-sites of bleach in/out and results in the reduction of charge capacity. While, an improvement of electrochromic property is obtained at the sample after thermal annealing without oxygen flow. The charge capacity of 69.68 mC/cm2 with a transparent difference, △T between colored/bleached processes of 28.7 % was obtained at the sample after 300 C thermal annealing

Resume : Oxide materials can dissociatively incorporate water into oxygen vacancies, but the thermodynamic feasibility of this reaction varies greatly. The individual contributions from proton affinity of lattice oxide ions and hydroxide affinity of oxygen vacancies to the hydration enthalpy are experimentally not accessible. This impedes an in-depth understanding of the hydration trends for different materials. We calculate proton and hydroxide affinities applying a thermochemical cycle based on reaction energies from first-principles DFT calculations; band alignment with respect to vacuum ensures the comparability of the calculated affinities [1]. This scheme is applied to a large variety of oxides ranging from binary oxides (Cs2O to SiO2) to ternary oxides, e.g. BaZrO3. Although the proton and hydroxide affinities describe purely ionic reactions, they strongly correlate with the oxide’s electronic structure, in particular with the ionization potential (position of O2p states relative to vacuum level). The slope is steeper for the proton affinity, naturally explaining the often found phenomenological correlation between basicity and more favorable hydration of the oxide.[1] This analysis will be extended to open-shell proton and electron mixed conducting perovskites (e.g. cathode materials). [1] T.S. Bjorheim, M.F. Hoedl, R. Merkle, E.A. Kotomin, J. Maier, J. Phys. Chem. C 124 (2020) 1277

Resume : ABSTRACT: Biosensors based on field-effect devices (bioFETs) have gained immense research over the past few decades because of their numerous advantages over existing technologies. Yet, their commercialization remains very limited. The biggest challenge for bioFET realization is the extremely short Debye screening length at high ionic strengths. This problem becomes significantly more severe at the solution-oxide interface due to high ion concentration induced due to the charged oxide surface groups which cripples any attempt to use field-effect mechanism to ‘sense’ the target analytes. In this work, we propose an electrostatic approach to remove the double layer (DL) excess ion concentration, thereby forcing the DL ion concentration to match the bulk concentration[1]. This consequently forces bulk screening length at the DL, thus ‘exposing’ target biomolecules to the underlying FET. To this end, local tunable surface electric fields are introduced to the DL using surface passivated-metal electrodes. The effect of these electric fields on the DL ion distribution are examined numerically and analytically. Also, the feasibility of the proposed approach is demonstrated numerically for a fully-depleted silicon-on-insulator based bioFET. We show how a significant twofold increase in the threshold voltage shift is achieved due to the presence of target molecules upon the removal of the surface excess ion population. Reference [1] I. M. Bhattacharyya, G. Shalev, Electrostatically-governed Debye screening length at the solution-solid interface for biosensing applications, ACS Sensors, 2019. https://doi.org/10.1021/acssensors.9b01939

Resume : Pulsed Laser Deposition (PLD) is a widely used technique to grow complex oxide films of a given stoichiometry. Due to the complex nature of the process itself, many parameters are known to influence thin film properties, structure or defect concentrations of the deposited thin films. However, the effect of ultraviolet radiation emitted by the plasma plume on thin film and substrate was so far a widely uncharted territory. Recent advances in the understanding of the photoconductivity and the effect of UV radiation on SrTiO3 now raise the question, if and how the UV radiation of the PLD plasma plume affects the electrical properties of an STO substrate during pulsed laser deposition. For this purpose STO single crystals with Pt current collectors were investigated by the means of in situ impedance spectroscopy during pulsed laser deposition (IPLD). By shielding the sample with a quartz disc the effect of the UV light could be isolated from potential effects of impinging species and real film growth. Our measurements revealed an increase of the STO conductivity as a response to the UV light, which persists after the illumination. When a thin STO layer is deposited on top, the conductivity decreases and the aforementioned effect disappears or is cloaked by another effect. This indicates that on the one hand the oxygen exchange on the STO surface is strongly affected by the UV light and that, on the other hand, thin layers of material deposited on the surface dramatically change the behavior of the whole system. The results of in- and across-plane impedance measurements are presented in this contribution.

Resume : The material class of perovskites is well known for over a century and one of their most prominent and most researched representatives is strontium titanate (SrTiO3, short STO). STO is used as a model system in many applications such as solid oxide fuel cells or resistive switching. [1,2] In contrast, the material class of hybrid perovskites is a rather young research field. It has attracted huge attention in the last decade as absorber material in photovoltaic applications. [3] The main difference between those two material classes is that one component of the usually inorganic perovskite is replaced by an organic molecule, hence the name hybrid perovskite, and one promising candidate of this group is methylammonium lead iodide (CH3NH3PbI3, short MAPI). Both materials – STO and MAPI – have in common that their applications require external electric fields which can influence the material properties in general and particularly the ion transport. In this study we conducted molecular dynamics simulations using the LAMMPS code [4] and empirical pair potentials derived by Pedone et al. [5] for STO and Mattoni et al. [6] for MAPI. These potential sets have already shown by our group to be capable of describing oxygen and iodine diffusion in the respective system. We are able to describe the field-dependent oxide-ion mobility in STO very well with an analytical model from Genreith-Schriever and De Souza [7]; in contrast the field-dependent mobility of iodide ions in the hybrid perovskite MAPI showed some unusual features. This behavior will be discussed. Literature: [1] R. Merkle, Angew. Chem. Int. Ed. 2011, 47, 3874. [2] R. Waser, Adv. Mater. 2009, 21, 2632. [3] P. Gao, Energy Environ. Sci. 2014, 7, 2448. [4] S. Plimpton, J. Comp. Phys. 1995, 117, 1. [5] A. Pedone, J. Phys. Chem. B 2006, 110, 11780. [6] A. Mattoni, J. Phys. Condens. Mat. 2017, 29, 043001. [7] A. R. Genreith-Schriever, Phys. Rev. B 2016, 94.

Resume : Solid oxide electrolysis cells (SOECs) have received growing attention in the last few years as they offer a way to highly efficient hydrogen production. However, degradation effects occurring at the air electrode are a major problem, thus efforts are made to characterise and optimise its performance. The perovskite-type oxide La0.6Sr0.4CoO3-δ (LSC) is a promising material for the air electrode due to its mixed ionic-electronic conductivity and high catalytic activity. Although LSC has been extensively tested in solid oxide fuel cells (SOFCs), comparatively few studies have been made on LSC in the SOEC mode. In this work, the defect chemistry of LSC thin film microelectrodes at varying anodic DC voltages was investigated by analysing the chemical capacitance. Depending on the electrode morphology, unexpected peaks of the chemical capacitance up to 1000 F/cm^3 at overpotentials higher than 100 mV were observed after several hours of annealing at around 600 °C or after applying high bias voltage up to 1 V. Correlations of these capacitive effects with variations of the oxygen exchange kinetics were also investigated. Supposedly, strontium segregation to the surface and a consequential formation of A-site vacancies in the bulk of the electrodes is responsible for these peaks of the chemical capacitance. Hence, a novel defect chemical mechanism connecting the oxygen exchange reaction and A-site vacancies is suggested to contribute to the chemical capacitance of oxides.

Resume : Mathematical models are ubiquitous in science and engineering and are used to interpret and predict the outcome of experiments and the behavior of devices or systems. Models with varying degrees of complexity are found in the study of electroactive materials, and they have been used to explain the nature of electrochemical reactions. Electrochemical impedance spectroscopy (EIS) is one of the most powerful electrochemical characterization techniques used in electrochemistry research. One of the most challenging steps associated with EIS is interpreting the data, which typically uses non-unique equivalent circuit models. The distribution of relaxation time (DRT) method is an alternative approach to equivalent circuits [1]. However, obtaining DRT from EIS data involves solving an ill-posed inverse problem [2]. We tackle this issue by formulating the DRT problem in a Bayesian statistical framework [3]. The inherent flexibility of the Bayesian framework allows adding our prior knowledge about the DRT. We propose two prior models. First, the regularization penalty term and the regularization parameter itself are treated as multivariate random vectors. Such formulation leads to a timescale dependent regularization giving better DRT recovery than the conventional timescale independent approach. The second prior is proposed for the weights of the fitting residuals. We show by treating them as random vectors that anomalies present in the EIS data can be detected, leading to a better-quality DRT recovery. Also, as the DRT problem is a Bayesian setting, we show that, for a given prior hypothesis, we can sample many possible DRTs from the data [4]. We extend this approach to the context of Gaussian processes (Figure 1). Nevertheless, the DRT obtained from the conventional ridge regularization represents the most probable DRT. Consequently, the Bayesian formulation generalizes the DRT problem. We wish to emphasize the power of the DRT method and the need for using principles of data sciences in the analysis of the measured electrochemical data. Such applications may enable a more profound understanding beyond the ones obtained from conventional analysis methods. Further, they can be used to assess data quality [6-8]. References [1] M. Saccoccio, T.H. Wan, C. Chen, and F. Ciucci. Electrochimica Acta, 147, 470-482 (2014) [2] T.H. Wan, M. Saccoccio, C. Chen, and F. Ciucci. Electrochimica Acta, 184, 483-499 (2015) [3] F. Ciucci and C. Chen. Electrochimica Acta 167, 439-454 (2015) [4] M.B. Effat and F. Ciucci. Electrochimica Acta, 247, 1117-1129 (2017) [5] F. Ciucci http://sites.google/drttools & https://github.com/ciuccislab [6] E. Quattrocchi, T.H. Wan, A. Curcio, S. Pepe, M.B. Effat, and F. Ciucci. 324, 20 November 2019, 134853 (2019) [7] J. Liu and F. Ciucci. Electrochimica Acta, 331, 135316 (2020) [8] J. Liu and F. Ciucci. Journal of The Electrochemical Society, 167, 026506 (2020)

Resume : Surface point defects represent centers of enhanced energy and thus enhanced reactivity.[1] For example, the oxygen exchange rate of Gd- and Pr-doped ceria was found to increase strongly with dopant concentration, which increases the concentration of oxygen vacancies (VO..) in case of Gd, and VO.. as well as Pr3 /4 redox active centers for Pr doping.[2] Here, oxidation kinetics of CO and CH4 as test reaction is studied on systematically Pr, Gd, Nb–doped ceria, and Y- and Pr–doped zirconia.[3] CO and CH4 oxidation proceed via the Mars–Van–Krevelen mechanism; the rate–determining step involves the reaction of adsorbed CO with surface layer oxygen, followed by fast CO2 desorption and VO.. formation. Under certain conditions, the competition of oxygen consumption by CO and catalyst re-oxidation by O2 leads to a kinetically determined decreased effective oxygen partial pressure (pO2,eff) inside the catalyst particles, as evidenced by the increased steady state oxygen deficiency. This accelerates the oxygen incorporation until CO oxidation and O incorporation rates are balanced. Samples with lower Pr content exhibit lower pO2,eff values. The presence of pO2,eff affects also the apparent reaction orders. No decreased pO2,eff appears during methane oxidation, because the CH4 oxidation branch is slower than the oxygen incorporation. [1] J. Maier, Chem. Eur. J. 2001, 7, 4762 [2] M. Schaube et al., J. Mat. Chem. A 2019, 7, 21854 [3] M. Schaube et al., J. Phys. Chem. C 2020, 124, 18544

Resume : Solid oxide fuel cells (SOFCs) represent a clean, efficient, and universal chemical energy-electric energy conversion technology. Reducing its operating temperature to intermediate range (650-850℃) or even lower temperature range (400-650℃) is an important practical requirement. However, the main obstacle to lowering the operating temperature is due to the slow kinetics of the oxygen reduction reaction (ORR) on the cathode side at lower temperatures, and therefore it is important to explore new cathode materials with good ORR activity. Recently, Mixed ion and electron conductor (MIEC) cathodes are widely studied for their good ionic and electronic conductivities. Among them, Sr-doped Lanthanum Cobaltite based cathode is one of the current research hotspots as a MIEC material. The La1-xSrxCoO3-δ (from x = 0 to x = 0.8) thin films were investigated as the electron conductivity and oxygen reduction activity of a medium and low temperature solid oxide fuel cell (SOFC) cathodes. Thin film materials can avoid external interference caused by microstructure and crystal orientation to better explain the complex relationship between chemical composition, electronic conductivity and ORR activity. It was observed that the electronic conductivity and the polarization resistance varied together, and had the best electronic conductivity and polarization resistance at the Sr doping concentration of x = 0.4. The Co3 /Co4 change explains that the electronic conductivity and oxygen reduction activity improve as the Sr doping concentration increases from x = 0 to x < 0.4. As the Sr doping content is further increased, a doping condition exceeding 0.4 favors the oxygen vacancy formation process and can be demonstrated by forming Co2 to balance the system charge. Therefore, Co3 /Co4 is no longer the best choice for surface oxygen reduction. The increased oxygen vacancy concentration provides more oxygen ion exchange sites for the surface oxygen reduction reaction process, and at the same time, the electron conductivity is deteriorated, thereby lowering the charge transfer efficiency of the oxygen reduction process. Although the concentration of oxygen vacancies is enhanced, the highly doped LSCO film, the non-optimal Co3 /Co4 valence state and poor electronic conductivity synergistically cause a decrease in oxygen reduction activity. Finally, it is shown that the electronic conductivity and oxygen reduction activity are related to the Co valence state and the surface composition. Our results demonstrate that tuning the mixed valence state with the electronic band structure can be a valid root in designing cathode materials. In particular, heavily doped LSCO film electrodes could have interesting potentials for SOFCs applications at lower temperature. 1. Zhaoxin Zhu, Yanuo Shi, Carmela Aruta, and Nan Yang, ACS Appl. Energy Mater. 2018, 1, 5308−5317.

Resume : The strategy most commonly employed for the valorization of the non-profitable by-products in the industry is the burning of these, generating heat and power. The EU project iCAREPLAST considers this approach for the valorisation of a by-product gas mixture composed by pyrolysis gases, oxygenates and other hydrocarbons resulting from the up‐cycling of non-recycled plastic waste into alkyl-aromatics. iCAREPLAST project considers the burning of these undesired gas streams by using pure O2 (oxyfuel combustion) in specially-conceived catalytic membrane reactors (Oxygen Transport Membrane modules) to produce heat and power (integrated in a combined cycle) while applying efficient CO2 capture. This work presents the advances conducted within iCAREPLAST project activities in the application of oxyfuel combustion with OTM modules. Several material compositions have been considered for burning gas hydrocarbon streams (CH4/Ar mixtures) at temperatures in the range of 1000-850 ºC. The CH4 conversion and CO2 selectivity for different membrane composition have been studied. The membranes have been surface activated by adding a porous catalytic layer by screen-printing for the promotion of permeation and combustion. The best results are obtained for BSCF membranes, nearly achieving a total combustion of the HC stream (10% CH4 in Ar), nevertheless, the limited stability of BSCF membranes under reducing and CO2-containing environments makes necessary the selection of more stable alternatives. Other tested compositions show lower results with respect to CH4 combustion towards CO2 generation, despite their high stability.

Resume : Current commercially available rechargeable Li-ion batteries, for example LiCoO2, are working mostly in the 4 V regime. One often suggested possibility to improve the effectivity of Li ion batteries are the creation of the 5 Volt cathode materials. We performed quantum mechanical calculations on the average battery voltage for the Li2CoxMn4-xO8 (x=0,1,2,3 and 4) cathode materials by means of the WIEN2k computer program package. The calculated average battery voltages for x = 0,1,2,3 and 4 are equal to 3.95, 5, 4.47, 4.19 and 3.99 V [1-3]. Our ab initio calculation results are compared with the available experimental data for x = 0, 1, 2 and 4 which are equal to 4, 5, 5 and 4 Volt. Thereby, for the Li2Co1Mn3O8 battery cathode material, our calculated average battery voltage around 5 Volt is in perfect agreement with the experimentally available battery voltage values of 5 Volt. Nevertheless, our calculated average battery voltage is underestimated (4.47 V) for the Li2Co2Mn2O8 cathode material, which also experimentally exhibits the 5 Volt voltage. References: [1] Roberts Eglitis, Int. J. Mod. Phys. B 33, 1950151 (2019) [2] R. I. Eglitis, Phys. Scr. 90, 094012 (2015) [3] R. I. Eglitis and G. Borstel, Phys. Stat. Sol. A 202, R13 (2005)

Resume : Zirconia based materials are known for having high values of ionic conductivity at high temperatures, therefore, they are used as a solid electrolyte in the construction of solid oxide fuel cells (SOFCs). In this work, we studied the effect of the chemical composition of zirconia based crystals on their phase composition, local structure and transport characteristics. A comparative analysis of two systems was carried out: zirconia stabilized only with ytterbium oxide and zirconia stabilized together with ytterbium and scandium oxides. Crystals of (ZrO2)1-x-y(Sc2O3)x(Tb2O3)y and (ZrO2)1-x-y(Sc2O3)x(Yb2O3)y were grown by directional crystallization of a melt in a cold container using direct high-frequency heating. Investigations of the phase composition were carried out by Raman scattering and x-ray phase analysis. The conductivity of the crystals was measured by impedance spectroscopy. The local structure of the grown crystals was studied by optical spectroscopy using Eu3+ ions as a spectroscopic probe. This work was performed with the financial support of the Russian Science Foundation (grant № 19-72-10113).

Resume : Acceptor doped barium zirconate is a promising proton conductor suitable as electrolyte in protonic-ceramic fuel cells. The doping with e.g. yttria leads to the formation of oxygen vacancies and exposing to a hydrating atmosphere at elevated temperatures introduces mobile protons into the system leading to high proton conductivity. The degree of hydration is a function of the partial pressure water and the temperature. In most studies, the classical mass action law for non-interacting defects is applied to model the defect chemistry of this system and connect the proton concentration to the thermodynamic parameters. However, at typical defect concentrations the interactions of defects are no longer negligible. The present study investigates influence of defect interactions on the free energy of hydration in yttrium-doped barium zirconate. Combining a DFT derived interaction model with an MMC multistage sampling approach [1,2], we obtain the free energy of interaction, which can be separated into contributions to the internal energy and the configurational entropy. Neglecting volume changes, the interaction dependent part of the equilibrium constant is deductible from the free energy of interaction. This enables an ab-initio calculation of the relation between water partial pressure and degree of hydration for a concentrated, interacting system. First results indicate intermediate degrees of hydration as the energetically favorable state, as the attractive interactions between dopant and protons cannot compensate rising proton-proton repulsion and entropy loss indefinitely. [1] J. P. Valleau and D. N. Card, J. Chem. Phys., 57, 5457, 1972 [2] S. Grieshammer and M. Martin, J. Mater. Chem. A, 5, 9241, 2017

Resume : Oxides which change their defect concentration with the oxygen partial pressure p(O2) are important materials for solid oxide fuel and electrolysis cells (SOFCs, SOECs) and sensors. Exact characterization of these materials over a large p(O2) range is necessary to understand their defect related properties such as ionic and electronic conductivity. For such measurements two different oxygen pump systems were designed and constructed: one was integrated in the hull of the experimental chamber while the other one is insertable into an experimental chamber. The capability of these setups are exemplified with measurements on LaSrFeO3 (LSF) thin film electrodes and SrTi O3 (STO) single crystals. The first setup shows an outstanding pumping performance (from 10-5 bar to 10-17 bar oxygen partial pressure in less than 5 minutes). The second setup can be equipped with an additional oxygen sensor directly next to sample in the electrochemical measurement cell. This design has two advantage: i) the oxygen pump is kept at one temperature while the electrochemical cell operates at various measurement temperatures and ii) the setup can be operated statically with different chamber base pressures or with continuous gas stream. The oxygen partial pressure is controlled with a PID controller to automatically change the desired set point. Additionally, the setup can be connected to mass flow controllers and a furnace controller to automatically change all experimental parameters. Different cells can be implemented, for example for Van-der-Pauw, current voltage and impedance measurements on thin films, pellets and microelectrodes.

Resume : Recent experimental work has demonstrated that catalytically active nanoparticles with enhanced stability can be readily dispersed on a perovskite oxide support through a process of ex-solution. This process produces socketed nanoparticles, which show high activity and are more resistant to catalytic poisoning. One such example is exsolved nickel on lanthanum doped strontium titanate support that can be used for syngas production by methane steam reforming. However, the transport of the dopant and the formation of the metal nanoparticle are still not well understood. We have begun to address this by using a range of atomistic simulation techniques. Potential based molecular dynamics were used to investigate the dynamics of nickel diffusion in lanthanum-doped strontium titanate. While nickel favourably resides on B-sites, A-site deficiency is found to be a key factor in determining the migration barrier and the diffusion rate. We also found evidence of enhanced lanthanum and nickel diffusion, as seen in experiments. However, when larger concentrations of lanthanum dopants are present, lanthanum rich layers form and are predicted to inhibit diffusion. We also investigated the transport at surfaces and found low energy barriers and trapping sites on surface layers. One of the advantages of atomistic simulation is that compositional changes can be easily made, and we illustrate this by exploring the effect of different dopants such as Fe and Mn and the effect of different host lattices by replacing strontium titanate with calcium titanate.

Resume : Today, within the framework of materials science topics, the leading positions are consistently occupied by the studies on the perovskite-like structures with general formula ABO3. Despite saving sustained interest in these compounds over the years and a wide range of studied compositions and their properties, continued development in this direction does not lose its relevance up to this day. In particular, layered compounds with alterating rock-salt and perovskite blocks - so called Ruddlesden-Popper phases, for example manganites, till now were not studied in much detail compared to other perovskite structure modifications. However, with a more thorough research, these objects show curious features, thereby potentially representing scientific interest. In the scope of current work, layered perovskite-like oxides corresponding to the chemical formula Sr4-yCayMn3-xFexO10-δ, where y = 0; 3, x = 0; 1; 2, were synthesized using the glycine-nitrate method. The X-ray diffraction confirmed the obtained compounds to be single-phase. This work includes the studying of oxygen non-stoichiometry dependences on temperature and oxygen pressure, calculation and measurement of specific heat effects, as well as thermal cycling test of degradation resistance. Accordingly, these compounds appear to be promising not only for a further detailed research, but also for practical application as oxygen carrier materials in the developing technology of thermochemical energy storage.

Resume : Complex oxides of d-metals with perovskite-like structure are acquiring great attention in modern science. Apparently, the number of fields these compounds can be applied for is constantly growing due to the enormous amount of their functional properties discovered. For instance, the so-called “double perovskites” possess high electron/ion mobility, good catalytic and magneto-transport properties in wide range of external conditions (temperature, pressure, magnetic field etc.). In this regard, the practical aspects of their usage are undoubtedly better studied experimentally rather than theoretically. Accordingly, the lack of theoretical knowledge should be somehow filled with. This work is aimed at ab initio investigation of various typical representatives – strontium molybdates, layered cobaltites and recently discovered Co-based double perovskites with high Ta/Nb concentrations. All these are perspective candidates for practical use as solid oxide fuel cell electrodes. In this study we try to analyze using combined theoretical and experimental approaches how the electron band structure features of the initial phases are changing upon doping and what are the reasons underlying the respective alterations in functional properties of materials considered. The results of the research done provide solid fundamental for recent experimental findings and also propose some new advantageous doping strategies. This work was supported by the RFBR under grant №19-33-90173

Resume : Many state-of-the-art energy materials can be regarded as solid oxides with a certain amount of dopant atoms which act to enhance their properties. It is well known that the dopant species can exhibit segregation at interfaces in the material (e.g. surfaces, grain boundaries), and that this can have a significant effect on key properties of the material such as its conductivity. However, a detailed understanding of this phenomenon is lacking. There is considerable interest in being able to determine: 1) for a given dopant element and concentration, to what extent segregation will occur at different types of interface in the material; 2) what effect the segregation will have on key properties of the material. Computer simulation is in principle able to answer these questions. However, due to the large activation energies associated with dopant diffusion, often equilibrium cannot be reached in reasonable timescales using standard simulation methods. We have explored alternative simulation methods for studying segregation, and have found that Monte-Carlo-based methods allow the atomic-scale equilibrium segregation profiles at grain boundaries and surfaces to be determined efficiently, even when used in conjunction with state-of-the-art models commonly employed in molecular dynamics simulations of oxides. Here we present results for ceria-zirconia, a material of interest due to its catalytic properties. We use Monte-Carlo methods to determine the miscibility gap and cation ordering tendencies in bulk ceria-zirconia, as well as the segregation profiles at various surfaces and grain boundaries. Our simulations provide fundamental insights into ordering and segregation phenomena in ceria-zirconia and similar systems.

Resume : In the field of Solid-State Ionics, it is often necessary to determine the absolute oxygen stoichiometry of materials. It is commonly measured in a given temperature range and at a given oxygen partial pressure by coupling iodometric titration to thermogravimetric measurement. Iodometry titration provides the absolute oxygen stoichiometry at room temperature and thermogravimetric measurement provides the relative change of oxygen stoichiometry. It is possible as well to determine an absolute oxygen stoichiometry at high temperature by reducing the material to a definite mixture of components, commonly metal oxides (rock-salt structure) or metal. In certain conditions, these approaches may fail to provide valid results. We report here an alternative approach by isotopic titration. The method consists in equilibrating a sample with a definite amount of labelled diatomic oxygen. The isotopic fraction of labelled oxygen is measured in the gas phase before and after annealing the system at high temperature until complete homogenisation of the isotope fraction in the sample and in the atmosphere is achieved. Benefits and limitations of the approach are discussed.

Resume : Free-radical copolymerization of a reactive solution consisting of a difunctional poly(ethylene glycol) diallyl ether oligomer (PEGDAE), a monofunctional reactive diluent 4-vinyl-1,3-dioxolan-2-one (VEC), and a stock solution containing lithium salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in a carbonate-free non-volatile plasticizer, poly(ethylene glycol) dimethyl ether (PEGDME) is achieved by means of ultraviolet (UV)-light irradiation. Hence obtained cross-linked polymer electrolytes (XPEs) possessed a unique structure with cycling carbonate moieties attached to linear polyethylene chains which are cross-linked by ethylene oxide (EO) units. By changing [O]/[Li ] ratio from 24 to 3, a series of XPEs were prepared and physicochemical properties are characterized by thermogravimetric analysis–mass spectrometry, differential scanning calorimetry, NMR, etc., and electrochemical techniques. Quantum chemical calculations provided insights into the coordination of Li -ions and EO units. The XPEs exhibited RT ionic conductivity in the order of 10-4 and 10-5 S/cm. The addition of lithium bis(fluorosulfonyl)imide (LiFSI) salt along with LiTFSI resulted in dual-salt XPEs exhibiting improved physical and electrochemical properties. All the single- and dual-salt XPEs exhibited electrochemical stability between 4.2 and 5V vs. Li|Li facilitating them to be employed against high-voltage cathodes in lithium metal batteries (LMBs). Remarkably, NCA-based lithium metal cells displayed excellent cycling stability (capacity retention >50%) even after 1000 cycles when operated at 20oC.

Resume : Solid-state diffusion in lithium intercalation compounds is one of the most important processes determining the overall kinetics in lithium-ion cells and placing practical constraints on the composition and morphology of composite electrode coatings. Thin films of lithium intercalation compounds offer a suitable platform for the extraction of the underlying intrinsic material properties from electrochemical impedance spectra. However, although lithium intercalation compounds generally constitute a class of mixed ionic and electronic conductors, the relevant literature currently relies on the traditional finite-length diffusion model that neglects electronic resistances and treats the resulting Warburg impedance as a simple two-terminal element in equivalent circuits. Most commonly, the reflective-boundary Warburg element is incorporated into the intuitively derived Randles circuit. In this work, we apply the generalized transmission line model proposed by Jamnik and Maier [Phys. Chem. Chem. Phys., 2001, 3, 1668-1678] to the specific case of lithium intercalation materials and thus also go beyond the assumptions underlying the traditional Randles model by considering both electronic and ionic resistances in the material as well as differentiated pathways for capacitive and ohmic charge transfer across the contact interfaces. This model is applied to study ambipolar diffusion in oriented LiCoO2 thin films. An aqueous three-electrode cell is used to control the state of charge (SOC) and to measure cross-plane impedance spectra. This revealed information on SOC-dependent conductivities, interfacial resistances, and chemical capacitances. Furthermore, we extend our analysis to in-plane impedance measurements at low SOC, where cross-plane impedance spectra typically do not allow a separation of solid-state diffusion from interfacial charge transfer kinetics. Finally, we discuss the relationships between the ambipolar diffusion coefficient, chemical capacitance, and electronic and ionic conductivities.

Resume : Cubic Li7La3Zr2O12 (LLZO) based garnet electrolytes are promising for solid state batteries, due to their high conductivity and stability towards Li-metal anodes. The stability of LLZO towards high voltage cathodes, however, is less clear. According to DFT calculations, the thermodynamic stability window of LLZO ranges only up to 3-3.3 V vs. Li0, whereas experimental studies with ion blocking electrodes report a stability window up to 5-6 V vs Li0. The practically usable voltage range of LLZO electrolytes is therefore an important unresolved question. LiCoO2 (LCO) is a well understood and widely used cathode material ranging up to about 4.2 V. Fortunately, the electrolyte degradation is slow at room temperature, so operation of LCO|LLZO cells is possible. For cell fabrication, however, higher temperatures are required at which undesired solid-state reactions occur between LLZO and LCO. Various phases, consisting of Li, La, Zr, Co and O, are formed at the LLZO-LCO interface. These undesired phases have low Li-ion conductivity, hence reducing the overall Li-ion conductivity of the system. Furthermore, diffusion of Co into the garnet may be an issue. A method to minimize these undesired reactions is therefore critical for the use of LLZO electrolytes with high voltage cathodes. In this work, we aim at the fabrication of kinetically fast LLZO-LCO interfaces by sputter deposition of LCO thin film electrodes on LLZO electrolytes. Among others, we tested the effect of a Li-Nb-O interlayer, which partly prevents the undesired reactions. The kinetics of Co diffusion in LLZO at various temperatures was determined via secondary ion mass spectroscopy (SIMS). The effects of cobalt impurities on the bandgap and conductivity of LLZO were investigated by means of UV-VIS spectroscopy. Changes of ionic and electronic conductivities were determined using DC and AC methods. The electrochemical Li (de)intercalation properties of symmetrical LCO-LLZO-LCO cells and LCO-LLZO-Li cells were measured by means of DC-cycling and electrochemical impedance spectroscopy (EIS) in order to find optimal processing parameters for efficient lithium transfer. The interfacial impedance was measured as function of the state of charge by means of microelectrodes. We found that the charge transfer kinetics of the LLZO-LCO interface can differ drastically from liquid electrolytes and strongly depend on the state of charge and fabrication parameters. We expect that our studies will help to better understand and optimize the kinetics of Li transfer at the LLZO-LCO interface.

Resume : Gadolinium-doped ceria (CGO) barrier layer placed between yttrium-stabilized zirconia (YSZ) electrolyte and strontium-based electrode aims to avoid the formation of insulating phase SrZrO3 during solid oxide fuel cell operation. However, this phase was observed in as-fabricated state-of-the-art cells, which evidences cation inter-diffusion during the sintering process [1]. The use of vacuum techniques such as sputtering and Large Area Pulsed Laser Deposition (LA-PLD) allows the implementation of dense layers at lower temperature compared with traditional deposition techniques (i. e. screen printing). CGO barrier layer deposition and annealing parameters were optimized in a previous work from the group and led to an increase of 70% in power density at 750ºC and 0.7 V compared to a state-of-the-art button cell. Remarkable results were also obtained when the PLD barrier layer was up-scaled to large area cells of 80 cm2 and tested in short-stack configuration for long-term operation [2]. This short-stack, that includes state-of-the-art cells as well as the large-area PLD ones, was aged for a total of 14,000 h at 750ºC for the first 8,000 h and 700ºC until the end of the experiment. Cells with PLD barrier layer showed enhanced initial performances with a low degradation rate. This work presents a post-mortem characterization of the barrier layers and interfaces in both types of cells. Scanning-electronic microscope, Electron Probe Micro Analysis with Wavelength Dispersive X-Ray (EPMA-WDX), micro-Raman spectroscopy and TEM were used to observe any microstructural changes as well as any changes in composition due to cation inter-diffusion. [1] Morales et al., Journal of Power Sources 344 (2017) 141-151. [2] Morales et al. ACS Applied Energy Materials 1 (2018) 1955-1964.

Resume : State of the art SOEC are complex structures requiring many fabrication steps and are still presenting durability issues. On one side, fuel electrodes, usually made with Ni YSZ (yttria stabilized zirconia Y2O3-ZrO2) cermets, suffer re-oxidation, Ni volatility and agglomeration and coking issues during pure CO2 electrolysis and co-electrolysis. On the other side, oxygen electrodes, usually made with La1 xSrxMO3 δ (LSM) YSZ composites or by mixed ionic-electronic conducting (MIEC) perovskite like La1 xSrxCo1 yFeyO3 δ (LSCF), are subject to delamination problem or inter-diffusion of ions. This work presents the study of symmetrical solid oxide cells made with Sr2Fe1.5Mo0.5O6−δ (SFM) electrodes. The symmetrical configuration allow a reduction of sintering steps and an improvement of the electrodes-electrolyte thermomechanical compatibility meanwhile the use of MIEC perovskite/metallic electrodes enables operations under steam electrolysis (SOEC) or co-electrolysis (co-SOEC) without the use of reducing safe gas at the fuel electrode. YbScSZ tapes previously coated with a Ce1 xGdxO1.9 (GDC) barrier layer grown by pulsed laser deposition were used as electrolyte supports. Electrode sintering temperature was optimized by means of electrochemical impedance spectroscopy (EIS) measurements in both air and H2 symmetrical atmosphere. The cell was then characterized at 900ºC in SOEC and co-SOEC modes without the use of any safe gas obtaining high current densities of 1.4 and 1.1 A·cm-2 at 1.3 V respectively. Short-term reversibility was finally proven by switching the gas atmosphere between the cathode and anode sides while keeping the electrolysis conditions. Similar performances are obtained in both configurations.

Resume : The functional properties of ceramics are classically tailored by designing point defects and interfaces. Dislocations as heavily charged line defects have so far been underappreciated as a means to tune functionality but are finding increasing attention today. To modify electrical properties of rutile (TiO2), (a semiconductor with wide bandgap prevalent due to its important applications in gas sensors and solar cells), defect engineering via chemical doping has an important role. However, often the solubility limit of the dopants restricts this method for tailoring material properties significantly while it increases material complexity [1]. Here, we demonstrate the possibility to induce equivalent conductivity enhancements akin to conventional chemical doping by mechanically introduced dislocations. We combine the understanding of mesoscopic dislocation structure and its behavior at elevated temperatures, which results in highly arranged dislocation bundles in single crystals. By controlling the mesoscopic structure of dislocations, we are able to both enhance and reduce the electrical conductivity. These changes are documented by temperature and oxygen partial pressure dependent conductivity measurements. Furthermore, local impact of dislocations is afforded by micro-contact impedance spectroscopy. In this way, the prospect of dislocations as “self-dopant” is presented, where the additional design parameter of the dislocation arrangement renders them potentially superior to conventional chemical doping strategies. [1] M. K. Nowotny, L. R. Sheppard, T. Bak, J. Nowotny, Phys. Chem. C 2008, 112, 5275-5300

Resume : Owing to the unique properties of solid electrolytes, rechargeable solid-state sodium-ion batteries (SSSBs) hold great promise for safe and sustainable energy storage applications. In addition to safety, more energy-dense batteries can be built by using higher voltage cathodes together with dense stacking architectures. Moreover, SSSBs are a more cost-effective alternative due to the higher abundance of sodium compared to conventional lithium. All of these advantages make SSSBs particularly attractive for large-scale grid storage applications. However, the poor electrochemical stability of current sulfide-based solid electrolytes, especially when used in conjunction with higher-voltage oxide cathodes, has largely limited their long-term cycling performance and practicality. Here, we report the computationally-assisted discovery of a chloride Na-ion conductor, Na3-xY1-xZrxCl6 (NYZC), that is both chemically compatible with oxide cathodes and electrochemically stable (up to 3.8 V vs. Na/Na+). Its exhibits a relatively good ionic conductivity of 6.6 x 10-5 S cm-1 at ambient temperature, several orders of magnitude higher than commonly used oxide coatings. This performance is attributed to abundant Na vacancies and facilitated by cooperative MCl6 polyanionic rotation, resulting in an extremely low interfacial impedance. This makes NYZC a promising candidate to be adopted in a cathode composite with an oxide cathode (such as NaCrO2) in order to mitigate the detrimental interfacial reactions commonly observed with sulfide electrolytes. As such, a SSSB comprising a NaCrO2 + NYZC composite cathode, Na3PS4 electrolyte, and Na-Sn anode exhibits an exceptional first-cycle Coulombic efficiency of 97.1% at room temperature and demonstrates impressive cycling capabilities: over 1000 cycles with 89.3% capacity retention at 40°C. These findings validate the coupled computational/experimental approach to find new electrolyte materials and highlight the immense potential of halide ion conductors for SSSB applications. Reference: Wu, E.; Banerjee, S.; Tang, H.; Richardson, P. M.; Doux, J.-M.; Qi, J.; Zhu, Z.; Grenier, A.; Li, Y.; Zhao, E.; Deysher, G.; Nguyen, H.; Stephens, R.; Verbist, G.; Chapman, K. W.; Clément, R. J.; Banerjee, A.; Meng, Y. S.; Ong, S. P. A Stable Cathode-Solid Electrolyte Composite for Long-Cycle-Life, High Voltage Solid-State Sodium-Ion Batteries. 2020. https://doi.org/10.26434/chemrxiv.12730964.v1.

Resume : Ammonia (NH3) is an important chemical source of fertilizers and possible candidates for hydrogen storage materials [1]. To facilitate these applications, fast conversion between NH3 and H2-N2 is required. Recently, calcium compounds combined with nitrogen and hydrogen attracts attention as promising catalysts for the fast conversion. Among them, Ca2NH and CaNH exhibit high catalytic performance in NH3 synthesis and decomposition, respectively [1,2,3]. To quantitatively understand the mechanism of the catalytic reactions, epitaxial thin film surfaces can provide an ideal platform due to well-defined size and crystal orientation. However, neither Ca2NH nor CaNH thin films have been reported to date. In addition, due to the structural similarity between Ca2NH and CaNH, it is difficult to selectively stabilize each phase using epitaxial effects. In this study, under various gas conditions, we study a route to selectively fabricate Ca2NH and CaNH epitaxial thin films using reactive magnetron sputtering. Ca-N-H thin films were deposited on MgO(110) substrates using reactive magnetron sputtering with a mixture of Ar, N2, and H2 gases. Structural properties were characterized using X-ray diffraction and Raman spectroscopy. At lower H2 partial pressure (PH2) of 0.020 Pa, we observed diffraction peaks at 2-theta= 50.9°. As PH2 increased to 0.074 Pa, the 2theta position was shifted to 49.9°. This suggests the phase transition from Ca2NH (440, 2-theta = 51.0°) to CaNH (220, 2theta = 50.1°). This phase transition scenario was confirmed by evaluating the chemical composition using Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), and nuclear reaction analysis (NRA). As a result, the chemical compositions of thin films fabricated under PH2 = 0.016 and 0.074 Pa were CaN0.55H0.37 (~Ca2N1.1H0.73) and CaN0.85H0.82, respectively. These values are consistent with the phase transition scenario, indicating PH2 plays an important role in selective control of the Ca2NH/CaNH phases. To the best of our knowledge, this study is the first report of metal-nitrogen-hydrogen epitaxial thin films. [1] M. Kitano et al., Chem. Sci., 2016, 7, 4036. [2] P. Yu et al., J. Energy Chem., 2020, 46, 16-21. [3] K. Ogasawara et al., Proceeding of Annual/Fall meetings of the Japan Petroleum Institute, 2019, 26, (in Japanese).

Resume : Energy conversion and storage systems based on solid oxide fuel cells (SOFCs) are promising candidates for future power solutions due to their high efficiency, low cost, hazard-free and sustainable material components[1]. Miniaturisation and portabilisation require low operation temperatures (≤500°C). However, this leads to decreased electrochemical performance and increased polarization resistance, with sluggish cathode surface activity being the major source. Engineering structural characteristics on a nanoscale is key for improved ionic properties and advanced performance. In this study we investigate the Ruddlesden-Popper phase La2NiO4, a highly interesting electrode material for micro-SOFCs, due to its low activation energy of the surface exchange activity[2]. By growing La2NiO4 thin films by pulsed injection Metalorganic Chemical Vapor Deposition (PI-MOCVD) on various substrates and with different thicknesses we were able to tune the film orientation and strain and analyze its effect on the oxygen exchange activity. Microstructure, morphology and chemical composition were characterized by XRD, SEM, AFM, TEM, Raman spectroscopy and XPS. The oxygen activity was studied by electrical conductivity relaxation and electrochemical impedance spectroscopy. We found very high surface exchange coefficients and low area specific resistance (ASR) values at low temperatures (400-500°C) as compared to other cathode materials. The oxygen incorporation in these films was identified to be surface limited, in which the specific step depends on the type of crystal growth (e.g. epitaxial or polycrystalline). TEM analysis of the material revealed a largely increased surface area through the combination of vertically spaced grain growth on top of a dense layer. Hence, the remarkable high oxygen activity found in the studied thin films is likely caused or enhanced by these morphological features, pointing towards a possible pathway to further foster the electrochemical performance of this promising cathode material. [1] Connor, P. A. et al. Tailoring SOFC Electrode Microstructures for Improved Performance. Adv. Energy Mater. 8, 1–20 (2018). [2] Kilner, J. A. & Burriel, M. Materials for Intermediate-Temperature Solid-Oxide Fuel Cells. Annu. Rev. Mater. Res. 44, 365–393 (2014). Acknowledgment: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 824072 (Harvestore).

Resume : In recent times, an alternative method to conventional catalyst preparation has gained significant interest: the exsolution of catalytically active metal species from perovskite-type host oxides, thus obtaining oxide-supported catalysts with superior properties. In a recent study we showed that this type of catalysts shows the interesting feature of being electrochemically switchable between a very active metallic state and a less active oxidic state [1]. However, a better understanding of the electrochemical switching behavior of the exsolved catalyst particles is of major interest. In the present study, the switching point between high and low activity states was investigated on different perovskite electrode materials. Electrodes were subjected to different electrochemical polarization and atmospheres (mixtures of H₂/H₂O in argon) and the electrochemical behavior was studied via impedance measurements and current-voltage characteristics. Furthermore, changes in the surface species were detected by near ambient pressure photoelectron spectroscopy (NAP-XPS). These measurements were carried out on La_0.6Sr_0.4FeO_3-δ (LSF) and Nd_0.6Ca_0.4FeO_3-δ(NCF) thin film electrodes, which were grown by pulsed laser deposition on yttria stabilized zirconia crystals. In addition, further experiments were conducted to reveal how doping with elements easier reducible than iron (e.g. Co or Ni on the B site of the perovskite) influences the current voltage characteristics and how they alter the electrochemical switchability of the studied exsolution catalysts. [1] A. K. Opitz et al. Understanding electrochemical switchability of perovskite-type exsolution catalysts. Nat. Communications. 2020, 11, 1-10.

Resume : In solid oxide fuel cells (SOFCs) oxygen reduction is one of the major factors limiting cell performance. Improving the mechanistic understanding of the oxygen exchange reaction is thus of high importance. Even though platinum is not a typical SOFC cathode material, platinum microelectrodes on yttria-stabilised zirconia (YSZ) electrolyte offer an excellent model system for investigation of triple phase boundary (TPB) dominated cathode kinetics and have thus been extensively investigated in the past . Despite this long and extensive history of inquiry, the exact reaction mechanism of oxygen exchange on Pt/YSZ is not completely understood until now. The goal of this study was to achieve a more detailed mechanistic understanding of the oxygen exchange on the free YSZ surface close to the TPB. Especially the possibility of an electron transfer via the electrolyte and thus a significant role of the YSZ defect chemistry is in the focus of this work. Dense and geometrically well-defined microelectrodes were prepared by magnetron sputter deposition of Pt on YSZ single crystals followed by micro-structuring via photolithography and ion-beam etching. The electrodes were electrochemically characterised by impedance and current voltage curve measurements at different temperatures and oxygen partial pressures. In addition, buried hetero-layers with different electronic conductivity are applied to alter the lateral electronic conductivity of the electrolyte, thus being able to modify the decay length of electrochemical activity at the TPB.

Resume : Resistive switching (RS) is the change in resistance of a Metal-Insulator (or Semiconductor)-Metal (MIM) capacitor-like structure under an external electric field. This topic has attracted researchers’ attention for its application in Non-Volatile Memories (Resistive Random Access Memories, or ReRAMs) and neuromorphic computing systems due to their simple structure, high operation speed and low power consumption [1]. This work uses La2NiO4+δ (L2NO4), a mixed ionic electronic oxide well known for its highly mobile oxygen interstitial ions, as a memristive layer. It has also been reported that the oxygen content in the L2NO4 structure, which influences the memory characteristics, can be tuned by thermal treatments in different gas atmospheres [2]. Some recent studies reported very promising results on nickelate memristive devices grown as epitaxial thin films [2,3]. However, they still present shortcomings, such as the impossibility of building L2NO4-based memristors in cross-bar array architectures and their integration into Complementary Metal Oxide Semiconductor (CMOS) technology. Thus, we focused on using Pt/TiO2/SiO2/Si, a CMOS-compatible substrate, to build Ti/L2NO4/Pt heterostructures as memristive devices in a “top-bottom” vertical configuration. The L2NO4 films were grown using Pulsed Injection Metal-Organic Chemical Vapour Deposition (PI-MOCVD). The deposition temperature and precursor ratio were optimized to obtain highly dense and thin films suitable for memristive devices. The successful growth of polycrystalline L2NO4 films on Pt was confirmed by the Grazing Incidence X-ray Diffraction (GI-XRD). Moreover, to study the interaction between the electrodes, L2NO4 film and substrate layers, Scanning Transmission Electron Microscopy (STEM) cross-section analyses were carried out after the electrodes’ evaporation on top of the film, showing sharp interfaces and the absence of interdiffusion between the layers. An analogue-type bipolar resistive switching behavior has been demonstrated for the first time in the Ti/L2NO4/Pt devices in top-bottom configuration. Besides, highly reproducible RS with well-defined resistance states is observed when cycling the devices multiple times. These results open the door to the use of L2NO4-memristors in cross-bar architectures in CMOS integrated circuits and to their testing as novel elements for neuromorphic computing. [1] Z. B. Yan and J. M. Liu, “Resistance switching memory in perovskite oxides,” Ann. Phys. (N. Y)., vol. 358, pp. 206–224, 2015. [2] K. Maas et al., “Tuning Memristivity by Varying the Oxygen Content in a Mixed Ionic – Electronic Conductor,” vol. 1909942, pp. 1–10, 2020. [3] K. Maas et al., “Using a mixed ionic electronic conductor to build an analog memristive device with neuromorphic programming capabilities,” J. Mater. Chem. C, 2019.

Resume : In situ techniques are advanced and important experimental methods to study lithium battery materials and full cell batteries [1-2]. Among various approaches, in situ Raman spectroscopy was recently used to investigate such promising cathode materials as LixNi0.8Co0.15Al0.05O2 [3], LixNi0.8Mn0.1Co0.1O2 [4], LiNi0.5Mn0.5O2 [5], xLi2MnO3·(1–x)LiMO2 [6], a commercial composite of NMC811&NMC111 [7]. Thanks to this method we can obtain information about their local structural changes at the surface due to lithium deintercalation/intercalation and study the phase transitions due to electrochemical reaction. In this work, we performed in situ Raman spectroscopy on a commercial NMC111 material to study the structural changes upon first cycles. Additionally, we completed ex situ XRD and Raman experiments to further explore the structural transformations. The first cycle shows the most significant changes once lithiated to high potentials vs. Li+/Li (Fig. 1). The spectral evolution during the delithiation and lithiation exhibits four major changes (new modes development and intensity variations). Based on data collected from in situ Raman and ex situ methods, a further theoretical approach will be employed to study Raman mode positions to improve a recent interpretation of peak evolution. This work helps to extend the knowledge on the structural changes and reversibility at the high voltage charging and deep discharging of NMC-type materials especially atractive for a solid-state battery designs. D.A.Z. and M.B. thank the support through the Homing program of the Foundation for Polish Science (POIR.04.04.00-00-5EC3/18-00) co-financed by the European Union under the European Regional Development Fund. References: [1] S. Hwang et al. J. Phys. D: Appl. Phys. 2020, 53, 113002 [2] P.P.R.M.L.Harks et al. J. Power Sources 288 (2015) 92-105 [3] E. Flores et al. Chem. Mater. 2018, 30, 14, 4694-4703 [4] C. Ghanty et al. ChemElectroChem 2015, 2, 1479 – 1486 [5] S. Hy et al. J. Am. Chem. Soc. 2014, 136, 3, 999-1007 [6] P. Lanz et al. Electrochimica Acta 2014, 130, 206-212 [7] C.-Y. Li et al. J. Phys. Chem. C 2020, 124, 7, 4024-4031

Resume : In comparison to bulk properties of Gd-doped ceria (GDC), little is known about the electrical grain boundary properties in thin films. This lack of knowledge arises from the small cross-sectional area of thin films, which makes separation of impedance features rather challenging. Here, we use embedded interdigitating Pt electrodes with spacing of few micrometres to separate grain and grain boundary conductivity of GDC thin films and characterise the effect doping and atmosphere [1]. Since the used embedded Pt thin film electrodes are blocking for oxygen ions and reversible for electrons, they allow determination of partial electronic and ionic conductivities by comparison of the high frequency and low frequency intercepts of the impedance arcs. Surprisingly, the ionic conductivity of the films increases by up to one order of magnitude when going from oxidising to reducing atmosphere, although the concentration of oxygen vacancies is dominated by the p(O2) independent extrinsic vacancies. We find the largest effect for films with low doping concentration and low temperature, where the grain boundary blocking factor is highest. Preliminary results also indicate a strong p(O2) dependence of the ion conduction in porous GDC-based SOFC anodes, which can only be explained by grain boundary effects. After defect chemical modelling, we find that the p(O2) dependence of the grain boundary resistance is in line with the widely accepted grain boundary space charge model. The results are of high relevance for optimising the properties of GDC in anodes and electrolytes for solid oxide fuel cells, as well as for understanding electrostrictive and memristive devices, for which oxygen partial pressure dependent ionic conductivity is an important new aspect. [1] Nenning, A.; Opitz, A. Low Oxygen Partial Pressure Increases Grain Boundary Ion Conductivity in Gd-Doped Ceria Thin Films. J. Phys. Energy 2019, 2, 014002, doi:10.1088/2515-7655/ab3f10.

Resume : Layered oxide cathode materials are used in modern Li-ion batteries. However, during prolonged device operation they suffer from surface degradation, transition metal dissolution and destructive phase transitions at the cathode-electrolyte interface. One way to suppress the chemical instability and reactivity of the cathode with the electrolyte is to coat the cathode surface with inert oxides such as niobium oxide (Nb2O5). To investigate the effect of oxide coatings, all functional battery layers can be deposited as thin films, which offers a flexible platform for investigating interfacial phenomena. The anode, electrolyte and cathode layers are clearly distinct over a large area, isolating interfacial phenomena and simplifying their investigation. An ALD process for Nb2O5 is presented, which acts as a coating for the thin-film LiCoO2 cathode fabricated by magnetron sputtering. Impedance measurements showed that it was necessary to lithiate the niobium oxide coating for an adequate ion transport, which was achieved by annealing the coated cathode at 700 °C for 1 hour. To investigate the electrochemical performance, liquid electrolyte half-cells were assembled with Li foil as anode. In this first step, the focus lies on the power density of the modified cathodes. By investigating cathode and coating thickness, increased rate performance could be demonstrated. Such Nb2O5-coated LiCoO2 cathodes showed at 20 C charge rate, 80% capacity retention relative to 1 C, which for uncoated cathodes, no capacity remained at that rate. Detailed elemental analysis with TOF-SIMS and XPS revealed both bulk and surface effects of the niobium oxide coating. These results suggest that ALD as a coating method can stabilize the cathode-electrolyte interface and improve the long-term performance of layered transition-metal oxide cathodes.

Resume : Amorphous oxide semiconductors (AOS) based on mixtures of transition metal cations (zinc, indium, gallium, tin) are widely used in thin film optoelectronics. Memristors with AOS are typically reported based on ion migration. The charge-trapping type switching in thin film diodes is less explored. However, it offers certain advantages for passive crossbar applications, such as area-scaling, low-power programming, and self-rectification without requiring electroforming. The common switching characteristic is a set in the diode’s forward polarity. In this poster, an overview is presented of charge-trapping type resistive switching in diodes of the most common AOS (indium-gallium-zinc oxide and zinc-tin oxide). All devices were crosspoint structures, patterned by photolithography. The state-dependent conduction mechanisms are discussed and compared to pure barrier height switching observed in crystalline ZnO thin film diodes.

Resume : In a context where the Internet of Things technologies will be needing a great deal of self-powered devices, the development of sustainable and efficient energy microdevices becomes critical in order to meet the energy demand of wireless electronic systems and low-power grid nodes. Reversible, full-ceramic micro solid oxide cells (-SOCs) represent a compelling approach for harvesting ambient energy in chemical form (electrolysis mode) and to release it as electric power (fuel cell mode). In the present study, we present an investigation on the structural and electrochemical properties of full-ceramic hydrogen and air electrodes. One of the limiting areas that -SOCs face is given by the selection of the hydrogen electrode, as it must present mixed ionic-electronic conductivity and high thermal stability in reducing conditions, while keeping the catalytic activity for the hydrogen oxidation reaction. In this work, results on the synergistic behavior of different hydrogen electrode materials (Ce- and Cr/Mn-based) are presented. The architectures studied consist in both dense and partially porous films, fabricated either as a nanocomposite or following a bilayer configuration. The films were fabricated by pulsed laser deposition (PLD) and characterized by complementary structural and electrochemical techniques (X-ray diffraction, electron microscopy, atomic force microscopy, and electrochemical impedance spectroscopy). We propose heterostructures based on (La,Sr)(Cr,Mn)O3 and samarium-doped-ceria, which present high electrochemical performance and overcome the limitations in terms of sheet conductivity of state-of-art doped-ceria. To shed light on the properties of oxygen ceramic electrodes, a high throughput screening methodology based on combinatorial PLD was designed for the analysis of the La0.8Sr0.2CoxFeyMnzO3 (x=1-y-z) perovskite family. This method allows to fabricate, in a single deposition process, a wafer containing the composition of the parent compounds together with a gradient distribution of the intermediate stoichiometries. The multicomponent sample was characterized by X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry, leading to a mapping of the structural and optical properties. The electrochemical properties were measured sequentially along the wafer by electrochemical impedance spectroscopy. The study provides a general method for the full characterization of a library of materials fabricated in a single run and measured under the exact same conditions.

Resume : Solid Oxides Cells (SOC) has been playing a fundamental role in the green and sustainable energy businesses due to the possibility of integrating the direct usage of carbon-based fuels and renewable energies to convert by-products into valuable fuels. Through all the components of these devices, electrodes are a key part in terms of the efficiency and durability. Studies have been carried on for decades in order to improve the matrix of chemical composition, microstructure, and performance. Moreover, research on efficient synthesis and exsolution of nanostructured perovskite oxides are attracting intensive attention nowadays. Surfaces decorated with consistently catalytically active nanoparticles assume a crucial role in numerous fields, such as sustainable power source, photocatalysis, conversion, and in high-temperature catalysis applications such as in solid oxide cells as electrodes. Hence, to implement this strategy in practical electrodes for solid oxide fuel cells, understanding the exsolution process in terms of synthesis temperature and the atmosphere is a prerequisite. Herein, we demonstrate the study of synthesis and characterization of different perovskite electrode materials based on the exsolution of catalytically active dopants by the solid precipitation of the functional active elements, such as: Ni on the Ni doped Gd0.1Ce0.9O2-δ (GDC) perovskite (Ni-GDC) as anode materials and Ag doped La0.8Sr0.2MnO3 (Ag-LSM) to lower the cathode operation temperatures. Both electrode materials can - under determined conditions of temperature and reducing atmosphere - exsolve the dopant element that remains exposed on the surface, increasing the catalytic activity of the former material. Insights into the exsolution mechanism are also derived from both experiments. Microstructural and morphological characterization of the electrode materials with exsolved active phases was carried out by X-ray diffraction, differential scanning calorimetry (DSC), temperature-programmed reduction (TPR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). Electrochemical impedance spectroscopy (EIS) was carried out at operating temperature to evaluate the performance of the exsolved materials as electrodes for SOFC.

Resume : Many efforts in the field of Li-ion batteries are focusing on the development and implementation of solid electrolytes in order to overcome the drawback related to their liquid counterparts such as: flammability, complex encapsulation requirements, high cost and complexity manufacturing processes. Li-conducting glass-based materials gained attention in the last years as solid state electrolytes for lithium batteries, thanks to their good ionic conductivity at room temperature (10-3-10-4 S cm-1) and their wide electrochemical stability windows. In the present work, Li1.5Al0.5Ge1.5P3O12 (LAGP) glass was 3D-printed by robocasting and stereolitography (SLA) in free-form robust self-standing structures with the main target to obtain 3D batteries with high active area (allowing high specific energy and power per unit volume). The use of 3D printing techniques allowed the fabrication of simple as well as complex architectures, with enhanced contact area with the electrodes. The inks and the printing processes were both optimized in order to reach an accuracy up to ≈100 µm. The printed structures demonstrated an excellent densification after the firing/sintering treatment, as a result of the optimization of the inks formulation, printing process and sintering conditions. Furthermore, ionic conductivity of 3D-printed LAGP electrolyte resulted to be 1.8 10-4 S cm-1 at room temperature, in accordance with the values measured on the same material processed with conventional methods. No detrimental reaction products or degradation phenomena were detected by chemical analyses after the sintering. The successful implementation of 3D printing techniques in LAGP processing represents an innovative approach that will push further the development of all solid state Li-ion batteries with enhanced energy density, thanks to the easy fabrication of 3D structured solid electrolytes.

Resume : Metal exsolution has evolved as an efficient synthesis route for supported nanoparticles with great potential for diverse applications in the field of catalysis and electrochemistry. Catalytically active dopants are released to the perovskite oxide surface in the form of metallic nanoparticles via a thermal treatment in reducing atmosphere. This allows for the fabrication of nanostructured composite materials, which is particularly interesting for the in-situ activation of fuel electrodes for solid oxide cells (SOCs). In the present study, we employ SrTi0.9Nb0.05Ni0.05O3-ẟ (STNNi) thin films fabricated by pulsed laser deposition (PLD) as model systems for the systematic study of the mechanistic processes governing nickel exsolution. Here, the growth of nanoparticles at the (001) crystal facet is observed and the release of nickel to the perovskite surface is monitored by chemical and structural analysis of the thin films. Microscopic investigations reveal spontaneous phase separation and the presence of Ni-rich nanostructures embedded within the perovskite host lattice, which serve as centers for the formation of metallic nanoparticles in the perovskite bulk. Based on the specific structural characteristics of the material system, a modified exsolution pathway needs to be considered which is shown to result in a considerably different exsolution behaviour compared to exsolution from solid solutions. Using a defect engineering approach, the interplay of exsolution behaviour and lattice defects is studied, where the defect structure of the perovksite host lattice is controlled and systematically varied based on the laser fluence applied for thin film growth. Here, a limitation of the exsolution dynamics for non-stoichiometric thin films is found to be correlated to a distortion of the perovskite host lattice.

Resume : Grain boundaries are known to impact ionic conductivities in solid electrolytes, with mobile ions often possessing reduced ion mobilities in grain boundary regions. The resistive properties of grain boundaries can generally be separated into two contributions: 1) an increased potential energy barrier caused by the structural rearrangement of bulk crystals at the grain boundary core, and 2) the formation of space-charge regions, areas of bulk electrolyte adjacent to the core where charged defect concentrations are altered due to an accumulation or depletion of charged defects at the core. Conventional approaches to investigating space-charge formation use one-dimensional, continuum mean-field models, such as the Mott-Schottky and Gouy-Chapman models. These mean-field, dilute limit models predict an exponential decay of charged defects away from the interface. However, these models have been shown to perform less well outside of the dilute limit, where defect interactions are unlikely to be well described by the simple mean-field approach. To test the performance and validity of the Mott-Schottky model beyond the dilute limit, we use kinetic Monte Carlo simulations on a simple, model grain boundary system, and extract resultant space-charge profiles for a number of concentrations and relative permittivities. We find the Mott-Schottky model performs well in a dilute regime but deteriorates quickly for concentrated systems. We propose an analytical form for the deviated charged defect profiles at high concentrations. This study improves our understanding of the behaviour of charged defects at interfaces in concentrated systems. This improved understanding could help facilitate the generation of methods to reduce the interfacial resistances in solid electrolytes.

Resume : The most common resistive switching memory systems show filamentary switching, which stochastic filament formation process causes a high variability. In this work, we studied in detail heterostructures of the mixed valence managnite PrxCa1-xMnO3 and insulating interface tunnel barrier layers. This type of memory devices show area-dependentn currents, a gradual SET and RESET and suffer less from device variability. This provides analog-switching and the possibility to adept the device current to a given circuit requirement. We will present the impact of different interface layers and their processing conditions on the switching characteristics of polycrystaline PrxCa1-xMnO3 thin films grown on Pt substrates. The difference between an oxide (TaOx, WOx and AlOx) and a metal (Ta, W, Al and Mo) as interface layer will be presented and the underlying mechanisms will be explained. The gradual switching type enables us to demonstrate learning algorithms, such as Spike Timing Dependent Plasticity or Long Term Potentiation/Depression. Furthermore, we investigated the influence of pulse height and length variations during the LTD/LTP measurement and perceptron simulations with the measured variations will be presented.

Resume : Single crystalline aluminum nitride (AlN) shows attractive properties if used as piezoelectric resonator at temperatures above 500 °C. The covalent bonds present in the material are favorable as they enable low acoustic losses. Application of AlN resonators as robust pressure or temperature sensor in harsh environments appears to be feasible. However, high quality bulk crystals and understanding of the defect mechanisms are required to take advantage of these properties. AlN crystals are initially grown by physical vapor transport under varying conditions to achieve samples that exhibit different doping concentrations for carbon, oxygen and silicon. They exhibit, for example, a wide range of electrical conductivity. The determination of electrical and acoustic properties and their correlations are the focus of this work. Beside fundamental crystal properties, the temperature dependences of electrical and resonant properties are determined from room temperature up to about 900 °C. The resonance spectra are acquired by resonant piezoelectric spectroscopy and used to extract the acoustic loss. The latter is of particular interest and in general caused by several mechanisms that dominate commonly in certain temperature ranges and that depend on the type and/or concentration of the defects. The investigations include intrinsic phonon-phonon interaction (minor contribution at high temperatures), anelastic relaxation of point defects and losses related to electrical conductivity. The temperature dependences of the latter two mechanisms can be described by Debye functions using materials parameters such as relaxation times and electrical conductivity. Superposition of several Debye functions is applied to model the overall loss and to obtain indications about the dominant mechanisms. At low and medium temperatures, high contributions of anelastic relaxation of point defects are observed in oxygen-dominated AlN samples. The origin is assumed to be point defect complexes (Al vacancies and oxygen defects) and possibly electrically inactive defects. The lowest overall losses are found for lowest concentration of oxygen impurities. At higher oxygen concentration ([O]/[C] = 4.0 and 1.7), a relatively high background governs the total losses up to about 250 °C. For high C or O concentrations the relaxation of charge carriers dominates at temperatures of from about 650 to 900 °C. Further, the long-term stability of AlN crystals at 900 °C and low oxygen partial pressure of 10-17 bar is investigated over 22 days. Within this period of time, the resonance frequency decreases by only 0.36%. The losses are low and remain virtually unchanged. Finally, the losses in AlN are compared with those of other high temperature stable piezo-electric crystals. Above about 500 °C and at a given resonance frequency, AlN exhibits lower losses than the high-temperature stable piezoelectric oxide crystal langasite (La3Ga5SiO14).

Resume : Hexagonal rare earth manganites, h-RMnO3 δ (R=Sc, Y, Lu-Ho), are known to accommodate large amounts of interstitial oxygen [1] at intermediate temperatures of 200-400 °C [2], making them applicable as catalysts and oxygen carriers in chemical looping combustion (CLC) [3]. As these materials are highly refractory, nanocrystalline particles can withstand harsh operating conditions and are stable against grain growth at higher temperatures [4]. The magnitude of δ, as well as the thermal stability of these interstitial defects, can be fine-tuned by aliovalent doping and particle size, but these effects are not well understood. Here, we study the effects of R-cation, crystallite size and donor doping on oxygen absorption in h-RMnO3 δ (R=Y, Ho, Dy). Thermogravimetry and high temperature X-ray diffraction show that oxygen absorption increases from R=Y through R=Ho, Dy, and with decreasing crystallite size. Donor doping with Ti further enhances oxygen absorption for all R and stabilize excess oxygen to significantly higher temperatures, extending the operation temperature window for catalysis and CLC. Density functional theory (DFT) calculations reveal trends in the effects of R-cation and donor doping on the enthalpy of oxidation and oxygen migration through the lattice. [1] Skjærvø et al. Nature Commun. 7 (2016) 13745. [2] Remsen et al. Chem. Mater. 23 (2012) 3818. [3] Fossdal et al. Int. J. Greenhouse Gas Control 5 (2011) 483. [4] Bergum et al. Dalton Trans. 40 (2011) 7583.

Resume : In the last decades, ceramic based energy devices, such as Solid Oxide Fuel and Electrolysis Cells (SOFC and SOEC) increased significantly their role on the new energy scenario based on renewable sources. The current state of SOC devices production is characterized by expensive and time-consuming processes, since it involved numerous steps and procedures, such as tape-casting, screen-printing, annealing, manual stacking, joining, sealing, etc. As an innovative and potential solution, the application of Additive Manufacturing (AM) to the SOC manufacturing could bring a simplification and optimization of the process due to the freedom to design, which is provided by AM and allows the enhancement of the SOC-based devices performance, as well as an increase of the efficiency, together with the reduction of waste material. For this purpose, the combination of different AM techniques in a single printing process is needed, and makes possible the production of monolithic SOFC stack during one-step printing. For this final goal, hybrid 3D printing technology based on stereolithography (SLA) and robocasting (RC) has been developed. The technique allows to combine different functional ceramic materials in the one fabricated piece. Thus, 8YSZ is used for the electrolyte fabrication by SLA and state-of-art SOFC electrode materials are deposited by RC. The elaborated hybrid multimaterial 3D printing technology renders possible complete SOC stack fabrication as a single-step process. Fully printed SOFC cells, their co-sintering process and their characterization are here presented while their advantages, limitations and perspectives of the fabrication process are discussed.

Resume : Redox-based based memristive devices are one of the most attractive candidates for future non-volatile memory applications and neuromorphic circuits. For the most common memristive band insulators, resistive switching is induced by the formation of oxygen vacancies and the resulting valence change on the cation sublattice. However, manganites, cobaltites and ferrates undergo a topotatic phase transition between the perovskite and the brownmillerite structure upon reduction that might also be induced during resistive switching. Due to the spatially confined redox-process, experimental proofs of topotactic phase transitions in memristive devices are very rare. By employing X-ray absorption spectromicroscopy, we demonstrate that the reversible topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5, and the conductive perovskite phase, SrFeO3, gives rise to resistive switching of SrFeOx memristive devices. Interestingly, we found that the electric field induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111) grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from bottom to the top electrode, show a localized phase transition. We attribute this difference in the extension of the topotactic phase transition to the anisotropic oxygen ionic conduction in the brownmillerite structure.

Resume : Memristive devices are attracting a great deal of attention for memory, logic and sensing applications due to their simple structure, high density integration, low-power consumption, and fast operation. In particular, multi-terminal structures controlled by active gates would certainly provide novel concepts for reconfigurable electronic systems with engineered functionality. In this work we will show the potential of reversible field-induced metal-insulator transition (MIT) in strongly-correlated metallic oxides for the design and obtention of multi-terminal memristive transistor-like devices [1, 2]. We have studied the highly correlated cuprate YBa2Cu3O7-δ as a model system, which is able to display non-volatile volume MIT, driven through local oxygen migration. In this way, vertical and lateral oxygen mobility may be modulated, at the micro- and nano-scale, by tuning the applied bias voltage and operating temperature. The key advantage of this material is the possibility to homogeneously modulate the oxygen vacancy diffusion not only in a confined filament or interface, as observed in widely explored insulating strongly correlated oxides, but also toward the whole film thickness. We will show different device configurations in which the lateral conduction of a bridge is controlled by active gate-tunable volume resistance changes. Large design flexibility can be obtained by changing the switching performance of different gates, thus offering the possibility to locally adjust the conductance response as required for non volatile memories and neuromorphic functionalities. References [1] A. Palau et al. ACS Applied Materials & Interfaces. 10, 30522, 2018 [2] J.C. Gonzalez-Rosillo et al. Adv. Elect. Mater. 1800629, 2019

Resume : Mixed-valence manganites are very promising candidate materials for resistive-switching devices due to the possibility to generate multilevel resistance states as well as area-dependent switching. This enables their use in future non-volatile memories or novel neuromorphic circuits. The aim of our work is to gain a deeper understanding of the microscopic mechanisms of resistive switching in mixed valence manganites with the focus on Sr-doped LaMnO3 (LSMO). It is widely accepted that ionic transport, especially oxide-ion transport, plays an important role in the field of resistive switching. Nevertheless, the role played by dislocations (one dimensional lattice defects) in the switching process is unknown. LSMO is unusual amongst the perovskite oxides, since it is the only system to show fast diffusion of oxygen along dislocations. The reasons for this behaviour are however not understood. To this end, we studied the ionic transport in bulk LSMO as well as along low-tilt grain boundaries (that consist of a periodic array of dislocation) by Molecular Dynamics simulations, employing empirical pair potentials. Additionally, we studied the ionic transport experimentally in comparable thin films, grown by Pulsed Laser Deposition (PLD), using Secondary-Ion Mass Spectrometry (SIMS).

Resume : Manganite thin films exhibit interesting properties for applications such as resistive switching (RS) memories [1], Micro Solid Oxide Fuel Cell cathodes, spintronic sensors and micro solar cells [2]. In thin films, the functional properties depend on the microstructure and composition. In particular, for valence change memories (VCM) the resistance change relies on the oxygen vacancies drift, so RS phenomena are expected to be tunable by varying the film nanostructure. Strontium substituted LaMnO3 (La1-xSrxMnO3±δ, LSM) was selected as a potential candidate to improve the performance and reliability of VCMs. 20% Sr-substituted LSM thin films were grown by Pulsed Injection Metal-Organic Chemical Vapour Deposition under optimized conditions. Different nanostructures were obtained by growing epitaxial LSM thin films on LaAlO3 (LAO) and SrTiO3 (STO) single crystals, which results in compressive and tensile in-plane strain, respectively. Devices of Ti/LSM\Pt in top-top configuration were microfabricated on both types of films to assess the role of the nanostructure on the RS phenomena. Whereas the devices on both types of films show reproducible, counter-8-wise (C8W) and analog response, HRS/LRS ratios one order of magnitude higher as well as much larger retention times were measured for the LAO substrate-devices. The RS response was related to the nanostructure characteristics of the manganite films. These results allow further comprehension of the LSM nanoionic properties and can lead to the optimization of manganite memristive devices with high storage capacity and fast operation time. [1] R. Rodriguez-Lamas, D. Pla, O. Chaix-Pluchery, B. Meunier, F. Wilhelm, A. Rogalev, L. Rapenne, X. Mescot, Q. Rafhay, H. Roussel, M. Boudard, C. Jiménez, M. Burriel, Beilstein J. Nanotechnol. 2019, 10, 389. [2] D. Pla, C. Jimenez, M. Burriel, Adv. Mater. Interfaces 2017, 4, 1600974.  

Resume : Perturbing the crystal lattice away from the equilibrium structure via an applied lattice strain has been investigated from some time as a method to realise considerable improvements in oxygen-ion conductivity. Interest in this approach has, however, waned over recent years for two primary reasons: (i) a lack of consistency and reproducibility in the reported experimental findings and (ii) typically only modest changes in ionic transport are observed. In this presentation, we will address both reasons and make the case for lattice strain still being a promising route to enhanced oxygen transport. By using an unconventional method of thermally annealing out strain which occurs during deposition of epitaxial rare earth-substituted films grown by PLD, we were able to tailor the strain with no influence from grain boundaries or interfaces. Through careful analysis of the literature, we managed, for the first time, to develop a quantitative consensus on the variation of the transport properties of ceria as a function of lattice strain. We also experimentally demonstrate the effects of migration direction with respect to the biaxially strained plane. Surprisingly, we find that the change in activation energy with strain is dependent on the size of the dopant cation. Combined with computational calculations of the same system, we show that the strain-modified conductivity is dependent both on the migration edge barrier and the defect-association. These findings give unique insights into the atomistic interaction of strain on the ionic transport of oxygen in substituted CeO2, and suggest that substantially improved conductivity in strained oxides may yet still be achieved if migration direction and defect association are optimised.

Resume : The performance of mixed conducting porous SOC electrodes depends on the concentration and mobility of electronic and ionic defects in the bulk, as well as the surface structure and chemistry, which determines the reactivity for oxygen exchange reactions. The simultaneous characterization of SOC electrodes with electrochemical impedance spectroscopy (EIS) and ambient pressure XPS (APXPS) delivers detailed insight into the reaction mechanism. Since the bulk and surface defect chemistry vary with atmosphere and overpotential of the investigated electrode, precise knowledge of these parameters is an important, but highly non-trivial experimental requirement, especially when the investigated electrode is porous and thus has a low polarization resistance. Therefore, combined EIS and APXPS studies were so far mostly performed on model cells with thin film electrodes. In the presented study we use a novel three electrode solid oxide cell design with three porous electrodes. This allows precise control of half-cell overpotentials and measurement of virtually artefact free half-cell impedance spectra in an APXPS chamber. We exemplify the strength of this design on porous La0.6Sr0.4FeO3 (LSF) electrodes. Equivalent circuit fitting of the half-cell impedance spectra reveals defect chemistry, surface reactivity and ion conductivity of the LSF phase, also when a DC bias is applied. Another important aspect of the cell design is the current collecting layer of the LSF electrode, which is realized in form a of a thin film Pt grid between electrode and electrolyte. Therefore, the LSF phase is accessible to ambient pressure XPS (APXPS) measurements at various bias and atmosphere conditions. The combination of these methods reveals a consistent picture about the relation of bulk and surface defect chemistry, mixed ion/electron conduction and redox kinetics. Specifically, we can show that in oxidizing atmosphere, overpotential drastically changes concentration of oxygen vacancies in LSF. By impedance fitting with a transmission line type equivalent circuit we find an according bias dependence of the ionic conductivity and chemical capacitance. Since oxygen vacancies are not directly detectable by APXPS, the surface chemistry depends only moderately on the overpotential. In reducing (H2+H2O) atmosphere, the vacancy concentration is high, irrespective of the applied overpotential. Still, the I-V curve is strongly asymmetric and much steeper in electrolysis direction. There, the overpotential has a strong effect on the oxidation states of Fe. We find Fe3+, Fe2+ and Fe0 states, depending on the overpotential. When cathodic bias is applied, metallic iron is present in form of exsolved nanoparticles, which explain the excellent H2O electrolysis kinetics. Therefore we deliver direct proof that Fe exsolutions strongly accelerate the water splitting kinetics also on porous electrodes.

Resume : Ion transport across grain boundaries in diverse polycrystalline ionic conductors is often found to be hindered. Such behaviour is commonly attributed to the presence of a highly resistive second phase or to the presence of space‐charge zones, in which mobile charge carriers are strongly depleted. One other possible cause – the severe perturbation of the crystal structure within the grain‐boundary core – is widely ignored. Employing molecular dynamics (MD) simulations of the model Σ5(310)[001] grain boundary in fluorite‐structured ceria, we demonstrate an approach to extract the intrinsic structural resistance of a grain boundary (to ionic transport across it), and we determine this excess resistance as a function of temperature. Compared with space‐charge resistances predicted for a dilute solution of charge carriers the structural resistance of this interface is orders of magnitude smaller at temperatures below T≈1000 K but at T>1200 K it is no longer negligible.

Resume : Highly oxygen defective ionic metal oxides fluorites such, as ceria and bismuth oxides, are sustainable, non-classical electrostrictors with properties that are superior to lead-based piezoelectric metal oxides. Here, we report recent findings underlying the exceptional electromechanical performances of such materials, both as bulk ceramics and thin films. We especially highlight the effect of dopants, microstructure, and crystallography on low-temperature actuation. We also show how the materials perform in the nanoscale and how these impact micro/nano-electromechanical system´s design and some related technologies.

Resume : The chemo-mechanical effect in solids refers to dimensional change due to change in stoichiometry. Dimensional change due to electrochemically-induced compositional change has been termed the electro-chemo-mechanical (ECM) effect. The mechanical instability inherent in this effect is clearly deleterious for batteries or fuel cells, but, as recently suggested, has potential for use in actuation[1]. The structure of an actuator device that operates on the ECM principle comprises a micrometer thick solid electrolyte (SE) sandwiched between two ECM-active, working body (WB) layers. An electrochemical reaction must occur in these layers, causing them to alternately expand or contract. In order to facilitate the ECM response, the WB layers should have mixed ionic and electronic conductivity and a large chemical expansion coefficient. We have constructed a 2mm diameter thin film membrane ECM actuator device comprising 20mol% Gd doped CeO2 (20GDC) as the SE and [TiO(2-δ)\20GDC] or [V2O(5-δ)\20GDC] composites as the WBs[2]. Selected area electron diffraction measurements showed the composite to be nanocrystalline, a morphology that promotes interfacial oxygen ion diffusion. Synchrotron X-ray absorption (XAS) measurements detected a mixture of Ce3+/Ce4+ (~[0.4]/[0.6]) and Ti3+/Ti4+ (~[0.1]/[0.9]) oxidation states in the WB. XAS measurements under bias showed changes in the short-range order of Ti and V oxides supporting the presence of a redox reaction. The deformation of the ECM actuator was observed to be in the bending regime producing large vertical displacements (~3 μm) and ~4 MPa stress. The stress/voltage ratio yields a pseudo piezoelectric stress coefficient of e31=1.26 C/m2, comparable to common lead-free piezoelectrics such as lithium niobate, lithium tantalate and alkali niobates. [1] J. G. Swallow et al., Nat. Mater. (2017) 16, 749 [2] E. Makagon et al. Adv. Funct. Mater. (2020), 2006712.

Resume : A recently discovered family of superionic conductors exhibits colossal equivalent permittivity when the material is placed between two metallic electrodes. This is remarkable because ionic conductors with comparable ionic conductivity usually do not exhibit such huge permittivity. As a matter of fact, these titanium-based lamellar perovskites of general formula M2Ti2O5 where M=Rb,K… are also found to exhibit memristive properties. Systematic measurements have allowed us to track the behavior of the permittivity as function of frequency and temperature in both the Rb and K compounds. We evidenced a maximum in the real equivalent permittivity at around 270 K for the Rb-compound and at 300K for the K-compound, together with Warburg diffusion at low frequency. The variation of the permittivity is correlated to the variation of the ionic conduction, thus pointing to a common origin for both phenomena. We will present here different investigations on the nature of the conducting ions. In particular by measuring the charge distribution inside the sample, we were able to demonstrate that the ions accumulating at the anode are of negative sign and that the material becomes locally conducting on the cathode side, creating a virtuel cathode, which accounts for all observed features. We will then present our latest results and conclusions on the nature of the migrating ions and we will discuss possible applications for this material.

Resume : Cerium oxide is a versatilely applicable ceramic, e.g. as high temperature coating material. Doped with trivalent oxides, such as gadolinium oxide, the material shows high oxygen ion conductivity exceeding that of commonly applied yttria-stabilized zirconia. In this study, we estimate thermal conductivity of pure and doped cerium oxide by equilibrium molecular dynamics simulations based on widely applied empirical pair potentials and making use of the Green–Kubo formalism. The simulations show the limitations of most of the present potentials to correctly describe the thermal expansion and thermal conductivity. Based on our results we apply the Green–Kubo formalism for Gd-doped ceria to obtain thermal and ionic conductivity. The simulations do not only yield the diagonal Onsager phenomenological coefficients but also the off-diagonal coefficients. In this way it is possible to describe the coupling between mass and heat transport, which also known as thermodiffusion or the Soret-effect.

Resume : Sodium bismuth titanate (NBT) based ceramics are excellent lead-free ferroelectrics and relaxor materials. The defect chemistry of these materials is, however, very complex. NBT can actually be modified from highly ionically conductive to highly resistive. For example, acceptor doping does not lead to the hardening of ferroelectric properties as it was initially expected. Instead, mobile oxygen vacancies are induced making the material an excellent oxygen ion conductor [1]. This behavior is very interesting from a research perspective but it may be very detrimental for the transfer of NBT-ferroelectrics into application. Aging and fatigue models from other well-known ferroelectrics might not be applicable. Thus, a detailed understanding of the defect chemistry of NBT and its solid solutions is of high importance. We developed a model to elucidate the defect chemistry of NBT-ceramics [2]. Furthermore, methods to control the ionic conductivity, ferroelectric properties and the microstructure will be discussed in this work [3]. This will illustrate the extraordinary opportunities to alter properties of NBT-based material for multiple applications. 1. Li, M., et al., A family of oxide ion conductors based on the ferroelectric perovskite NBT. Nature Materials, 2014. 13(1): p. 31-35. 2. Koch, L., et al., Ionic conductivity of acceptor doped sodium bismuth titanate: influence of dopants, phase transitions and defect associates. Journal of Materials Chemistry C, 2017. 5(35): p. 8958-8965. 3. Steiner, S., et al., The effect of Fe-acceptor doping on the electrical properties of NBT and 0.94 NBT–0.06 BT. Journal of the American Ceramic Society, 2019. 102(9): p. 5295-5304.

Resume : Since the discovery of anomalous polarization phenomena in solar cells based on organic-inorganic hybrid perovskites, much attention has been devoted to the mixed ionic-electronic conducting properties of these materials.1, 2 Strikingly, investigation of mixed conduction in methylammonium lead iodide under light showed – along with the expected increase in electronic conductivity – huge enhancement of ionic conductivity.3 As mixed anion perovskites show greatest potential for achieving high-efficiency solar cells, understanding the role of anion composition on this “photo-ionic effect” is of utmost importance. In this contribution, we quantify the ionic and electronic conductivities for thin films of iodide, bromide as well as of mixed halide perovskites in devices with ion blocking electrodes. Compared with the iodide-based counterparts, bromide-based perovskites experience less enhancement of ionic conductivity. 3, 4 This corroborates the self-trapped hole model for the photo-induced ion transport owing to the fact that Br is less polarizable than the counterpart of I.5 On the basis of these observations, we propose a model explaining the photo-demixing in mixed halide perovskites where the formation of neutral iodine interstitial defects stabilizes the coexistence of I-rich and Br-rich domains. The connection between the photo-effect on iodide-based perovskites and the driving force for demixing3 represents a novel contribution to the thermodynamics of phase segregation and emphasizes the importance of the coupling between electronic and ionic defects in the phase behavior of halide perovskites. 1. T. Y. Yang, G. Gregori, N. Pellet, M. Gratzel and J. Maier, Angew Chem Int Ed Engl, 2015, 54, 7905-7910. 2. A. Senocrate, I. Moudrakovski, G. Y. Kim, T. Y. Yang, G. Gregori, M. Gratzel and J. Maier, Angew Chem Int Ed 2017, 56, 7755-7759. 3. G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel and J. Maier, Nature materials, 2018, 17, 445-449. 4. G. Y. Kim, A. Senocrate, Y.-R. Wang, D. Moia and J. Maier, Angewandte Chemie International Edition, 2021, 60, 820-826. 5. R. A. Evarestov, E. A. Kotomin, A. Senocrate, R. K. Kremer and J. Maier, Phys Chem Chem Phys, 2020, 22, 3914-3920.

Resume : Solid-state Li-ion conductors have received a great deal of attention in recent years due to their potential advantages compared to commercial liquid electrolytes, namely the improved safety and higher energy densities achievable through the implementation of Li-metal anodes. Unfortunately, despite their superior mechanical properties, solid state batteries are afflicted by the same short-circuiting challenges that affect their liquid electrolyte counterparts, a phenomenon broadly described as lithium “dendrites”. A fundamental, concerted understanding of the effects of mechanical, ionic and electronic transport and interfacial properties of solid electrolytes on dendrite formation and penetration is necessary to guide the development of the solid electrolytes of the future. Anti-perovskites of the Li2OHX family (X= Cl, Br) are an interesting class of Li-ion conductors with tunable properties and a low melting point that could represent an ideal model solid electrolyte system for this thorough investigation. In my talk, I will discuss the progress we have made and future steps.

Resume : Solid-state electrolytes for battery systems is a fast developing field.[1] Here, we focus on a new class of solid-state electrolytes based on metal organic frameworks (MOFs). An endless number of linkers and metal centres can be used to build a tremendous variety of different MOFs with tunable properties. MOFs are interesting materials to investigate as solid-state electrolytes as they are thermally stable and highly porous. In this study MIL-121 (Al centres linked by pyromellitic acid) was synthesized by a hydrothermal route [2] and post synthetically modified with lithium acetate and sodium acetate. After this lithiation and sodiation step samples were soaked with LiClO4 or NaClO4 in propylene carbonate in order to increase the ion content even further. X-ray powder diffraction (XRD) revealed that the structure of pristine MIL-121 could be largely maintained after lithiation or sodiation together with a small loss in crystallinity. At 303 K a conductivity of 4.6 ∙ 10−6 S/cm−1 for Li ions and 1.2 ∙ 10−4 S/cm−1 for Na ions was measured. This is in line with previous studies on MOFs as solid-state electrolytes.[3] Interestingly, activation energies were different at higher and lower temperatures. The kink in the Arrhenius curve could not be assigned to structural changes or phase transitions. Hence, the observed non-Arrhenius behaviour was attributed to a change from correlated to uncorrelated motion as suggested in the model of Ngai.[4] Low activation energies of 0.28 eV (above 323 K) and 0.36 eV (above 283 K) for Li and Na, respectively, were found in the higher temperature region. 7Li NMR spectroscopy confirmed the mobility of both ions and showed the vital role of the soaking electrolyte. Line shapes and spin-lattice relaxation (SLR) NMR suggested two different conduction processes: one can likely be ascribed to ions forming stronger bonds with carboxylic groups whereas the other one corresponds to weaker-bonded solvated mobile ions in MOF pores. 1H SLR NMR measurements revealed that indeed the alkali metal ion is the moving species in the material. A correlation between the alkali metal ion and hydrogen might, however, play a major role in the diffusion process at lower temperatures. The successful modifications (lithiation, sodiation) of MIL-121 lead to encouraging conductivities and proved the potential suitability in batteries of this young class of solid-state ion conductors. [1] C. W. Sun, J. Liu, Y. D. Gong, D. P. Wilkinson, J. J. Zhang, Nano Energy 2017, 33, 363-386. [2] C. Volkringer, T. Loiseau, N. Guillou, G. Ferey, M. Haouas, F. Taulelle, E. Elkaim, N. Stock, Inorg Chem 2010, 49, 9852-9862. [3] a) R. Ameloot, M. Aubrey, B. M. Wiers, A. P. Gomora-Figueroa, S. N. Patel, N. P. Balsara, J. R. Long, Chem-Eur J 2013, 19, 5533-5536; b) B. M. Wiers, M. L. Foo, N. P. Balsara, J. R. Long, J Am Chem Soc 2011, 133, 14522-14525. [4] a) K. L. Ngai, A. K. Jonscher, C. T. White, Nature 1979, 277, 185-189; b) K. L. Ngai, C. T. White, Phys Rev B 1979, 20, 2475-2486.

Resume : Layered transition-metal (TM) oxides have played a central role as cathodes for Li-ion and Na-ion rechargeable batteries. Alleviating the deleterious structural transitions that occur in these materials during electrochemical cycling is still a critical challenge which requires a fundamental understanding of the mechanisms taking place at the atomistic level. In this work we investigated how Te6 doping into the AxNiO2 (A=Li and Na) family of materials leads to changes in the local structure and electrochemical performance. The Li-Ni-Te-O system provides a rich phase space in which two layered polymorphs and one disordered rock salt phase can be synthesised. Significant Ni2 migration occurs in both layered polymorphs which leads to rapid capacity fading. The disordered rock salt structure displayed the highest reversible capacity out of the three polymorphs, which could be rationalised based on the presence of fast Li-transport on the partially ordered Li/Ni sublattice.[1] In the Na-Ni-Te-O system, the inclusion of Te6 results in the suppression of TM layer shearing during desodiation. Using a combination of X-ray diffraction, solid-state NMR and DFT calculations it can be shown that the improvement of the electrochemical cycling is related to the inclusion of Na ions into the TM layer which suppresses Na-ordering. The conclusions from this work have wider implications for the design of new Li-ion and Na-ion cathode materials. [1] Grundish et al. Chem. Mater. 31 (2019).