In high-temperature settings, amorphous alumina thin film insulators offer distinct advantages over crystalline materials, especially including a reduction in pinholes and leakage currents. Nonetheless, the phase stability of amorphous structures is a significant factor in determining their performance as insulators over a broad temperature range. With increasing temperatures, amorphous alumina undergoes a series of crystallization processes, resulting in irreversible alterations such as cracking and delamination due to volume changes between the different Al2O3 polymorphs. In this study, amorphous alumina doped with yttrium and zirconium has been deposited via physical vapor deposition, demonstrating the stability of the amorphous structure up to 1200 °C in ambient air and vacuum. Above 1000 °C, the amorphous films transform into a two-phase system consisting of tetragonal yttria-stabilized zirconia (YSZ) embedded in an amorphous alumina matrix. Compared to undoped alumina, films alloyed with 4.3, 12.2, and 20.7 at. % ZrY exhibited enhanced thermal stability. Samples alloyed with 4-12 at. % ZrY additionally showed stable and reversible resistivity behavior under thermal cycling. Notably, films containing only 4.3 at. % ZrY achieved uncharted resistivity values of (3.33 ± 0.06) × 105 and (2.83 ± 0.07) × 105 Ω·m at 750 and 850 °C, respectively, representing a substantial enhancement relative to undoped alumina, which exhibited a value of (8.33 ± 0.04) × 104 Ω·m at 750 °C.

Mixed ionic electronic transfer (MIET) reactions, such as the oxygen reduction reaction (ORR) at oxide surfaces, are of paramount importance to manifold technologically highly relevant processes and fundamental understanding must be developed to improve performance and tailor highly efficient electrodes and catalysts. Understanding such complex multi-step reactions, requires the study of kinetic processes, underlying thermodynamic properties, i.e. ionic and electronic defect concentrations and electrostatic surface effects. However conventional techniques struggle to uncover the complete picture within the same sample/measurement. Here, we overcome this limitation by introducing bias-triggered conductivity relaxation (BCR) as a novel tool to investigate MIET reactions on oxides. It is based on alternating out-of-plane coulometric titration/polarization and in-plane electrical conductivity relaxation measurements, providing simultaneous electronic, ionic and extraordinarily rich surface kinetics information. This innovative combination of electrical and chemical driving forces synergizes information depth, with enhanced time resolution, versatility and speed, yet it lifts the weaknesses of the individual approaches, while remaining cost-effective and surprisingly simple. Furthermore, BCR allows to disentangle overpotential induced electrostatic modifications of the surface kinetics in a unique manner. We showcase the advantages of BCR in this work by studying the ORR in model (La,Sr)FeO_3-δ thin film electrodes and reporting on their thermodynamic and kinetic properties.

Solid-state oxygen ion batteries (OIBs) are a novel technology for electrochemical energy storage, based on the exchange of oxygen between two mixed conducting oxide electrodes via an oxide ion-conducting electrolyte. Suitable electrode materials not only require good ionic and electronic conductivity, but also a highly variable oxygen non-stoichiometry δ to chemically store large amounts of charge. Another desirable characteristic for anodes is good material stability down to very reducing oxygen chemical potentials. This work focuses on the exploration of La0.5Sr0.5Cr0.2Mn0.8O3-δ and its electrochemical and defect chemical properties, with particular focus on its applicability in anodes of oxygen ion batteries. Thin film model cells were prepared by pulsed laser deposition (PLD) of electrodes on 100-oriented Y:ZrO2 single crystals. These planar half-cells were sealed with ZrO2 and glass to inhibit oxygen exchange with the atmosphere. Electrode capacities of up to 930 mAh cm-3 were achieved and confirmed to be stable over more than 70 cycles at 400 °C between -0.07 V and -2.07 V vs. 1 bar O2. Charge/discharge curves revealed the existence of two plateaus at -0.8 V and -1.4 V. Further, electrochemical impedance measurements on samples with microelectrodes were employed to study the chemical capacitance C chem, oxygen diffusion coefficient, and ionic resistivity of La0.5Sr0.5Cr0.2Mn0.8O3-δ over the same range of potentials. High resolution C chem vs. oxygen chemical potential measurements revealed two clearly separated peaks, indicating two separate redox processes, which correspond to the two distinct plateaus found in the charge/discharge curve. A defect chemical model (Brouwer diagram) was developed, based on a two stage transition: Mn4+ → Mn3+ → Mn2+. The model can quantitatively explain the location of both peaks in the chemical capacitance curve and the corresponding plateaus of the charge/discharge curve. Furthermore, X-ray photoelectron spectroscopic measurements of the Mn3+ → Mn2+ transition fully confirmed this model. Altogether, this study showed that La0.5Sr0.5Cr0.2Mn0.8O3-δ is a highly promising anode material for oxygen ion batteries operating at high voltages.

Reducing catalyst material usage while maintaining high performance is critical for scalable green hydrogen production via electrolysis. In this work, low-loading alkaline hydrogen-evolution reaction (HER) cathodes are fabricated by direct current magnetron sputtering of Ni and Ni₄Mo, providing a one-step, solvent-free route to catalyst-coated substrates. By tuning sputter power and Ar pressure, the deposited layers are steered toward rough nanostructures with increased accessible surface area, which correlates with improved HER activity compared with denser reference films, as shown by three-electrode measurements. The sputtered Ni-Mo has the targeted Ni₄Mo composition and exhibits a nanocrystalline Ni-fcc-type structure rather than the equilibrium intermetallic phase. Process transfer from planar substrates to porous carbon paper yields catalyst loadings of 0.046-0.186 mg cm⁻² (Ni) and 0.053-0.318 mg cm⁻² (Ni4Mo). Three-electrode electrochemical benchmarking showed decreasing HER overpotential with increasing loading, reaching ~104 mV at 10 mA cm⁻² for a NiMo cathode with 0.21 mg/cm². Implemented in a PGM-free AEM electrolyzer, along with an uncatalyzed stainless steel anode, Ni4Mo outperforms Ni at high current densities, delivering 1.95 V at 1 A cm⁻² and 60 °C (1.88 V at 80 °C) and enabling a mass activity of ~4.7 A mg⁻¹ at 1.95 V. A 500 h steady-state durability experiment at 1 A cm⁻² demonstrates stable performance with a reduced degradation rate toward the end of the test.

Mixed ionic and electronic conducting (MIEC) oxides are central to energy devices, but quantifying their intrinsic ionic transport and defect thermodynamics remains challenging in representative systems such as La0.6Sr0.4Co0.2Fe0.8O3−δ, where electronic conduction typically dominates and masks the much smaller ionic contribution. In this work, a method based on stoichiometry polarization and transmission line modeling of impedance data is proposed to extract the ionic conductivity and chemical capacitance of MIEC oxide thin films as functions of temperature and oxygen chemical potential. This framework enables the simultaneous determination of transport properties and defect concentrations from a single measurement. Moreover, it unlocks measurements at high oxygen chemical potentials (oxygen partial pressure » 1 bar) that are otherwise hardly accessible. Quantification of defect concentrations allows for the extraction of the partial molar enthalpy and entropy of lattice oxygen. The method provides a novel experimental route to investigate oxygen thermodynamics and transport in mixed conductors, and opens new possibilities for an in-depth understanding of functional oxide materials.

At heterojunctions between mixed ionic and electronic conductors (MIECs), band alignment takes place in order to equilibrate electronic and ionic charge carriers. The bulk properties of such MIECs can be used to describe and understand the corresponding interfacial space charges. For ultrathin films in MIEC heterolayers, however, these interfacial effects may differ from those between bulk materials. In this work, the interfacial regions between SrTiO3 (STO) and the two MIECs (La, Sr)FeO3-δ (LSF) and (La, Sr)MnO3-δ (LSM) are considered. Stacks of LSF|LSM and LSM|LSF were deposited on STO single crystals, and the resulting space-charge regions in STO are characterized by means of impedance spectroscopy at 500 °C in the p(O2) range between 1 and 5 × 10-4 bar. By extracting the STO bulk and space-charge resistances from the impedance data, space-charge potentials are derived. Interestingly, even extremely thin LSM interlayers (0.5 nm) cause bulk-like LSM band bending beneath LSF top layers. STO space charges at LSF interlayers of the same thickness, however, are strongly affected by LSM top layers. A model is introduced to interpret the measured difference in critical thicknesses of LSF and LSM in terms of different accumulation- and depletion-layer thicknesses in MIECs.

The electrical and electrochemical properties of mixed conducting oxides often depend on the oxygen partial pressure p(O2) and numerous studies have dealt with the p(O2) dependencies found in bulk materials. However, measurements regarding the properties of interfaces between two different mixed conducting oxides are much less common. This work investigates the interfacial space charge region in SrTiO3 (STO) caused by the contact with another mixed ionic and electronic conductor (MIEC), specifically La0.6Sr0.4FeO3-δ (LSF), La0.65Sr0.35MnO3-δ (LSM) and La0.9Sr0.1CrO3-δ (LSCr). The space charge regions were investigated by means of electrochemical impedance spectroscopy at 500 °C in the broad p(O2) range of 1 bar to 10-32 bar. All measurable space charge potentials Δϕ show a decrease with decreasing p(O2). A model correlates the change in Δϕ with the strongly p(O2) dependent defect concentrations of the MIECs. The measured partial pressure dependencies of Δϕ can be fully attributed to the different p(O2) dependencies of the respective bulk Fermi levels. The suggested model is broadly applicable to MIEC|MIEC interfaces in general and can also be used to predict transitions of space charges from hole depleted to electron depleted layers.

The oxygen stoichiometry is key to tune functional properties of advanced oxides and has motivated numerous studies of the oxygen off-stoichiometry diagram, aiming to determine and control structural and functional (electronic/ionic, electrochemical, optical) properties, as well as the oxygen storage capacity. Here, a novel approach is developed, allowing to project selected oxygen chemical potential regions onto a single thin film sample with unprecedented control. Therefore, a specifically designed electrochemical cell geometry is deployed, resulting in a well-defined, in-plane oxygen concentration gradient, whose endpoints can be flexibly controlled via the pO2 and applied overpotentials, and which is independent of variations in the materials electrical resistivity. This allows for an unparalleled study of materials properties as continuous function of the oxygen content using spatially resolved tools (spectroscopic, diffraction, microscopy, etc.) and thereby greatly reduces experimental efforts while avoiding sample-to-sample variability, multi-step treatments, degradation effects, etc. This work presents the proof-of-concept of in-plane oxygen gradients, based on spatially resolved ex/in situ and novel fixed-energy X-ray absorption near edge spectroscopy (XANES), X-ray diffraction, ellipsometry, and electrical measurements in hyper-stoichiometric La2NiO4+δ and sub-stoichiometric (La, Sr)FeO3-δ thin films. It demonstrates the readiness and wide applicability of this innovative approach, highly relevant for fundamental as well as applied research.

The catalytic and electrochemical properties of many non-stoichiometric oxides are governed by their defect chemistry. Therefore, detailed knowledge of their oxygen non-stoichiometry under operating conditions is desired. For this, coulometric titration can offer a valuable tool that can have advantages in terms of required sample mass, accuracy and reachable p(O2) range over other established techniques, such as thermogravimetric analysis (TGA). Here, we present a new design for an easy to fabricate miniature coulometric titration setup using materials selected for optimal electrode kinetics. The small chamber volume (0.03-0.05 ml), small sample mass (about 30 mg) and kinetically fast electrodes allow for a precise variation of the p(O2) from 1 bar down to 10-32 bar at 625 degrees C. This is a much wider range than typically achievable under gas flow in TGA or with other titration setups described in the literature. A characterisation of the titration setup showed that residual errors in the defect chemistry of the investigated materials are in the range of 10-4 to 10-3 p.f.u. Exemplary measurements on CeO2-delta and Sr1-xTi0.6Fe0.4O3-delta (STF) showcase how this wide p(O2) range can not only be used to study oxygen non-stoichiometry at very reducing conditions and the p(O2) at which vacancy ordering phenomena occur (for CeO2-delta), but also detect and quantify small amounts of redox-active secondary phases (for STF).

Mixed ionic and electronic conductors (MIECs) are a highly relevant material class in the field of solid-oxide cells and are, for example, promising candidates for electrodes with fast interfacial reaction kinetics. While there are many studies dealing with the bulk conductivities of such MIECs, models describing the interfaces between two mixed-conducting oxides have been far less developed. This study focuses on the investigation of space charges at the interfaces of the model perovskite SrTiO3 with different MIECs. Impedance spectroscopic measurements at 500 °C revealed that the MIECs under investigation can be divided into materials leading to negligible (YBa2Cu3O7-δ), moderate [(La,Sr)FeO3-δ, (La,Sr)CoO3-δ], and large [(La,Sr)MnO3-δ, (La,Sr)CrO3-δ] space charge resistances in SrTiO3 single crystals. The fundamental cause for these different space charge resistances is different space charge potentials, and we show that these can be determined by various methods with excellent agreement, ranging from X-ray photoelectron spectroscopy to impedance spectroscopy and photovoltage measurements. A model is introduced to correlate the ionic and electronic driving forces determining the space charges and to predict the space charge potentials from the electronic and ionic bulk properties of the corresponding mixed-conducting oxides. This model is also used to relate space charge potentials with reducibilities of MIECs, i.e., transition points from hole to vacancy compensation of an acceptor dopant in defect chemical Brouwer diagrams. The predicted trends are in good agreement with thermodynamic data on defect formation energies from the literature. Accordingly, the given model provides a widely applicable framework to predict and describe the space charge properties of a variety of MIEC heterojunctions.

Green synthesis and defect engineering of LaCoO3 model nanocatalysts by femtosecond pulsed laser ablation in liquid (fs-PLAL) led to the formation of two types of nanoperovskites: stoichiometric LaCoO3 and nonstoichiometric cobalt-rich nanoparticles. Micro-Raman analysis revealed pronounced second-order phonon scattering, suggesting a high defect density. The defect spatial distribution was evaluated by high-resolution electron microscopy, employing Fourier filtering and image reconstruction. Increasing the laser fluence increases the surface defect density due to the fast cooling of primary nanoparticles, a process intensified by the inherently ultrashort pulses. Laser-produced nanoparticles exhibited internal defects, a characteristic absent in those produced by a chemical method. Chemically derived nanoparticles, originally perfectly crystalline, formed grain/twin boundaries during calcination when their irregular shapes coalesced. Compared to a chemically synthesized reference catalyst, nanoparticles laser-synthesized at 5.8 J cm-2 showed the highest CO conversion during PROX in excess H2 at 400 °C. Perovskite produced at 5.8 J cm-2 and 5.1 J cm-2 also showed higher CO2 selectivity (89% and 83%, respectively, versus 28% of the reference), as well as excellent stability at 350-400 °C.

The increasing demand for energy storage solutions has spurred intensive research into next-generation battery technologies. Oxygen-ion batteries (OIBs), which leverage mixed ionic-electronic conducting (MIEC) oxides, have emerged as promising candidates due to their solid, non-flammable nature and potential for high power densities. This study investigates the use of over-stoichiometric La2NiO4+delta (L2NO4) as a cathode material for OIBs, exploring its capacity for electrochemical energy storage. Half-cell measurements reveal that L2NO4 with a closed-pore microstructure can store oxygen, achieving a volumetric charge of 63 mA h cm- 3 at 400 degrees C with a current density of 3.6 mu A cm- 2 and potentials up to 0.75 V vs. 1 bar O2. Additionally, a functional full cell combining over-stoichiometric L2NO4 and under-stoichiometric La0.5Sr0.5Cr0.2Mn0.8O3-delta (LSCrMn) has been successfully developed, demonstrating excellent cyclability and coulomb efficiency. The full cell reaches a maximum volumetric charge of 90 mA h cm- 3 at 400 degrees C, 17.8 mu A cm- 2, and a cut-off voltage of 1.8 V. This proof of concept underscores the viability of combining over- and under-stoichiometric MIEC materials in OIBs and provides critical insights into optimizing electrode materials and tuning oxygen content for improved performance. This research lays the groundwork for future advancements in OIB technology, aiming to develop materials with lower resistance and higher efficiency.

Oxygen exchange on mixed conducting oxide surfaces and how to modulate its kinetics has been in the focus of research for decades. Recent studies have shown that surface modifications can be used to tune the high temperature oxygen exchange kinetics of a single material systematically over several orders of magnitude, shifting the focus of research from bulk descriptors to a material's outermost surface. Herein, we aim to unify bulk and surface perspectives and derive general design principles for fast oxygen exchange based on three fundamental material properties: oxide reducibility, adsorption energetics, and surface acidity. We explain in detail how these properties relate to a material's electronic structure to facilitate guided materials discovery and design. We first introduce the connection of a material's electronic structure with its equilibrium defect chemistry and doping compensation mechanisms, and consequently to experimental observables, such as the oxidation enthalpy. We then present a molecular orbital model for oxygen adsorption on mixed conducting oxide surfaces, rationalizing trends of adsorption energies with a material's chemistry and electronic structure. Using this model we explore the effect of surface modifications on adsorption energetics, partially clarifying the effect of surface acidity on oxygen exchange kinetics. Building on this discussion, we show why the bulk O 2p band center and the work function are the two fundamental quantities that need to be tuned to achieve fast oxygen exchange kinetics on pristine surfaces and we discuss corresponding material design strategies. Lastly, we discuss potential implications for stability under operating conditions.

Interface engineering reduces interfacial resistance in yttria-stabilized zirconia electrolyte thin films, enabling efficient low-temperature oxygen ion battery operation.

The (electro)chemical properties of electrode materials in solid oxide cells or oxide-based redox catalysts are determined by the surface chemistry of these materials under operation conditions. Surface point defect concentrations strongly depend on the oxygen stoichiometry in the bulk and the gas phase's chemical composition (e.g., oxygen activity). However, many chemically sensitive surface analysis techniques rely on UHV conditions, leading to a two-fold deviation from surfaces under operational conditions. On the one hand, adsorbed gas phase species are missing in UHV. On the other hand, transition metal oxidation states and the oxygen vacancy concentration at surfaces are connected to the oxygen stoichiometry in the bulk of the material, which is inevitably altered during cell transfer from electrochemical measurement to UHV-based analytics. To reduce this two-fold gap between analytical studies and typical operation conditions, we present a novel solid oxide cell design for electrochemical oxygen activity control of surfaces in UHV-based analytic tools. Its key feature is an oxygen-ion buffering counter electrode containing a Fe|FeO phase equilibrium with known oxygen activity. A defined voltage between this counter electrode and the oxide under investigation (used as working electrode) defines the oxygen activity of the relevant oxide surface. Moreover, simultaneous thin film coulometry allows the determination of the bulk oxygen deficiency in the respective oxides. As a proof of concept, we use UHV-based XPS to compare the bulk and surface reducibility of fluorite-type Gd-doped ceria and perovskite-type Fe-doped SrTiO3 under electrochemical oxygen activity control. We show that the cell voltage can tune the transition metal oxidation states and oxygen vacancy concentration at the surface. These relate well to the actual solid oxide cell operation at the same temperature and p(O2).

Improving materials for energy conversion and storage devices is deeply connected with an optimization of their surfaces and surface modification is a promising strategy on the way to enhance modern energy technologies. This study shows that surface modification with ultra-thin oxide layers allows for a systematic tailoring of the surface dipole and the work function of mixed ionic and electronic conducting oxides, and it introduces the ionic potential of surface cations as a readily accessible descriptor for these effects. The combination of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) illustrates that basic oxides with a lower ionic potential than the host material induce a positive surface charge and reduce the work function of the host material and vice versa. As a proof of concept that this strategy is widely applicable to tailor surface properties, we examined the effect of ultra-thin decoration layers on the oxygen exchange kinetics of pristine mixed conducting oxide thin films in very clean conditions by means of in-situ impedance spectroscopy during pulsed laser deposition (i-PLD). The study shows that basic decorations with a reduced surface work function lead to a substantial acceleration of the oxygen exchange on the surfaces of diverse materials.

In this study, a detailed structural characterization of epitaxial La0.6Sr0.4CoO3−δ (LSC) films grown in (100), (110), and (111) orientations was conducted. LSC is a model air electrode material in solid oxide fuel and electrolysis cells and understanding the correlation of bulk structure and catalytic activity is essential for the design of future electrode materials. Thin films were grown on single crystals of the perovskite material La0.95Sr0.05Ga0.95Mg0.05O3−δ cut in three different directions. This enabled an examination of structural details at the atomic scale for a realistic material combination in solid oxide cells. The investigation involved the application of atomic force microscopy, X-ray diffraction, and high-resolution transmission electron microscopy to explore the distinct properties of these thin films. Interestingly, ordering phenomena in both cationic as well as anionic sublattices were found, despite the fact that the thin films were never at higher temperatures than 600 °C. Cationic ordering was found in spherical precipitates, whereas the ordering of oxygen vacancies led to the partial transition to brownmillerite in all three orientations. Our results indicate a very high oxygen vacancy concentration in all three thin films. Lattice strains in-plane and out-of-plane was measured, and its implications for the structural modifications are discussed.