Abstract A strong societal and political drive is motivating the development and optimization of novel energy conversion and storage systems for decarbonization. The successful implementation of solid state devices such as fuel cells and secondary batteries depends, however, on achieving ambitious targets in terms of performance, reliability and cost competitiveness. Research and technology are addressing these needs through a holistic approach including exploration of new materials and nanoarchitectures, as well as system engineering. These significant efforts require the support of appropriate characterization tools capable of assessing nanometer-scale phenomena such as concentration profiles of ionic and electronic charges, local chemical compositions and their evolution over time across interfaces. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real-time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2025 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2024 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications.

Modifying mixed ionic and electronic conductor (MIEC) surfaces has gained attention as a strategy to enhance oxygen exchange reaction kinetics and attenuate surface degradation. This study investigates the high-temperature stability and cation segregation behavior of La0.6Sr0.4CoO3-delta (LSC) thin films modified with similar to 0.5 nm CaO and SnO2 overlayers after annealing at 800 degrees C. Combining Time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Secondary electron microscopy (SEM)/Energy dispersive X-ray analysis (EDX), and Auger-Meitner electron spectroscopy (AMES), we provide a comprehensive picture of surface and sub-surface changes, segregation, interdiffusion, and secondary phase formation. Our results show that Sr enrichment during high-temperature annealing occurs at the surface regardless of the overlayer. However, significant differences in surface morphology emerge depending on the overlayer. Our results indicate that surface acidity, modulated by the oxide overlayer, is of fundamental importance for the formation of secondary phases and determines the interaction with acidic gas-phase impurities. These findings suggest that surface modifications are not a viable strategy to prevent Sr segregation at high temperatures. However, they can lead to complicated surface dynamics and significantly change the secondary phase formation processes induced by Sr segregation.

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.

In this work, we systematically investigate the structure of Sr and Mg doped lanthanum gallate single crystals using x-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis revealed the presence of multiple domains with distinct lattice parameters. High-resolution TEM and integrated differential phase contrast imaging captured two regions, one with 90° symmetry and the other with non-90° symmetry. In addition, ordered oxygen vacancies in the non-90° region were revealed, while vacancies in the 90° region were randomly distributed. Energy-dispersive x-ray spectroscopy confirmed that the difference in the doping level of the two regions is within the detectability limits. However, the electron energy loss near-edge structure revealed detectable differences in their bonding character.

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.

In this work, we investigated La0.6Sr0.4CoO3−δ / SrTi0.3Fe0.7O3−δ multilayer systems with different layer thicknesses. Reciprocal space mapping showed splitting of the reflections in the sample with 5 nm layers while for 15 nm thick layers reflections have been elongated. Using transmission electron microscopy, we investigated this phenomenon at the atomic scale and showed that the alternating 15 nm layers gradually increase their tensile out-of-plane strain, whereas the sample with 5 nm layers maintains a relatively stable strain state. In-plane strain relaxation is similar in both samples. Still, it differs in the strain relaxation mechanism, which involves the formation of amorphous regions in the 5 nm sample and the formation of edge dislocations in the 15 nm sample. Electron energy loss spectroscopy was employed to probe the oxidation states of Co, Fe, and Ti. In both samples, the oxygen vacancy concentration increases toward the surface.

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.

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.

Recent studies showed that binary oxide modifications can induce substantial changes in the oxygen exchange kinetics at the surface of Pr0.1Ce0.9O2-delta electrodes. The effect of these oxides was linked to their Smith acidity. In this work, these observations of oxides at the surface of mixed conducting oxides are extended to a metal-like conducting oxide La1-xSrxCoO3-delta. The samples of La1-xSrxCoO3-delta with various Sr contents deposited by PLD were investigated in situ during the deposition process by impedance spectroscopy, before and after decoration with SrO, CaO and SnO2 at different temperatures and pO(2). Considering experimental observations from the literature, the kinetics of decorated surfaces follow the Smith acidity of the binary oxide, which confirms that this scale is a good descriptor for sorting impurities. No notable effects of the Sr content on the decoration impact were measured and the effect of impurities was compared between La1-xSrxCoO3-delta and Pr0.1Ce0.9O2-delta from a previous study. Basic impurities added at the surface of La1-xSrxCoO3-delta electrodes showed less influence on the reaction kinetics than on Pr0.1Ce0.9O2-delta, but a stronger impact was found with acidic oxides. This effect is supposed to originate from the acidity difference that controls a major part of the oxygen exchange kinetics. These results underline that the outermost surface is decisive for solid oxide cell electrode materials and that the relative insensitivity of the kinetics to bulk properties might lead to a high flexibility in terms of electrode design and material combination.

Summary The epiphytic orchid Caularthron bilamellatum sacrifices its water storage tissue for nutrients from the waste of ants lodging inside its hollow pseudobulb. Here, we investigate whether fungi are involved in the rapid translocation of nutrients. Uptake was analysed with a 15N labelling experiment, subsequent isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (ToF‐SIMS and NanoSIMS). We encountered two hyphae types: a thick melanized type assigned to ‘black fungi’ (Chaetothyriales, Cladosporiales, and Mycosphaerellales) in ant waste, and a thin endophytic type belonging to Hypocreales. In few cell layers, both hyphae types co‐occurred. 15N accumulation in both hyphae types was conspicuous, while for translocation to the vessels only Hypocreales were involved. There is evidence that the occurrence of the two hyphae types results in a synergism in terms of nutrient uptake. Our study provides the first evidence that a pseudobulb (=stem)‐born endophytic network of Hypocreales is involved in the rapid translocation of nitrogen from insect‐derived waste to the vegetative and reproductive tissue of the host orchid. For C. bilamellatum that has no contact with the soil, ant waste in the hollow pseudobulbs serves as equivalent to soil in terms of nutrient sources.

Sub-nanometer modifications of mixed ionic electronic conducting (MIEC) materials, like the perovskite La0.6Sr0.4CoO3-delta (LSC), represent a promising approach to improving their oxygen exchange kinetics and degradation stability. The complex interactions between decoration layers and the host material are still not fully understood and are a subject of current research. Under these circumstances, a novel approach using time-offlight secondary ion mass spectrometry (ToF-SIMS) was developed to gain deeper insight into the electronic and chemical interactions of LSC with different oxidic decorations. The investigated samples were prepared by pulsed laser deposition (PLD) on YSZ single crystals, starting with a 100 nm thin LSC layer, which was then modified by sub-nm decorations of CaO, TiO2, and SnO2 (nominally 0.05-0.5 nm) on top. By using ToF-SIMS with a sampling depth of 1-2 nm, it was possible to extract information simultaneously on the decoration layer and the host oxide. Significant differences in secondary ion (SI) intensities of the host material LSC were found that can be attributed to the formation of surface-near dipoles as a consequence of the acidic or basic nature of the decoration oxide. Further, relative stoichiometric variations of the La+, Sr+, and Co+ signals depending on the decoration oxide were observed, suggesting different preferential decoration sites on the LSC surface.

The oxygen exchange kinetics and the surface chemistry of epitaxially grown, dense La0.6Sr0.4CoO3-δ (LSC) thin films in three different orientations, (001), (110), and (111), were investigated by means of in situ impedance spectroscopy during pulsed laser deposition (i-PLD) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). i-PLD measurements showed that pristine LSC surfaces exhibit very fast surface exchange kinetics but revealed no significant differences between the specific orientations. However, as soon as the surfaces were in contact with acidic, gaseous impurities, such as S-containing compounds in nominally pure measurement atmospheres, NAP-XPS measurements revealed that the (001) orientation is substantially more susceptible to the formation of sulfate adsorbates and a concomitant performance decrease. This result is further substantiated by a stronger increase of the work function on (001)-oriented LSC surfaces upon sulfate adsorbate formation and by a faster performance degradation of these surfaces in ex situ measurement setups. This phenomenon has potentially gone unnoticed in the discussion of the interplay between the crystal orientation and the oxygen exchange kinetics and might have far-reaching implications for real solid oxide cell electrodes, where porous materials exhibit a wide variety of differently oriented and reconstructed surfaces.

The potentiality of mixed conducting oxides as electrodes in rechargeable oxygen ion batteries is exemplified by impedance measurements and galvanostatic cycling. Porous La0.6Sr0.4CoO3−δ (LSC) thin film electrodes were prepared on yttria-stabilized zirconia electrolytes with a dense ZrO2 blocking layer on top to prevent oxygen exchange with the measurement atmosphere. Half cell measurements performed between 350 and 460 °C revealed electrode capacities up to 135 mAh/cm3. Measured charge/voltage curves are compared with those reconstructed from chemical capacitance values and with model calculations. Two different storage mechanisms were identified: At low potentials (<0.05 V vs. 1 bar O2), mainly oxygen vacancies become filled during charging. At higher potentials, however, O2 gas formation (>500 bar at potentials >0.1 V) in closed pores dominates charge/voltage characteristics, which can be described by calculations based on a real gas model. Moreover, full oxygen ion batteries were investigated, where such a porous LSC electrode served as cathode and a dense La0.9Sr0.1CrO3−δ (LSCr) film was used as anode. Owing to the much lower reducibility of LSCr, a cell voltage of 1.2 V was obtained at 460 °C with electrode related capacities and energy densities up to 100 mAh/cm3 and 53 mWh/cm3, respectively.

The electron beam, during high-resolution transmission electron microscopy, was employed to induce a phase transition in La0.6Sr0.4CoO2.5 (LSC) from a brownmillerite ordering to an oxygen deficient perovskite structure. Prior to irradiation, a strongly alternating out-of-plane lattice parameter was observed, reflecting electrostatic interactions between AO and BO/BO2 planes in the brownmillerite ordering. During electron beam irradiation for one hour, the oxygen vacancy ordering vanished gradually, and a uniform cubic perovskite structure prevailed. To exclude beam-induced heating effects, in situ heating experiments were performed, revealing a stable brownmillerite ordering in the relevant temperature range (up to at least 500 °C). Thus, we conclude that the phase transition is caused by knock-on processes that affect oxygen vacancies in terms of a transition from structural vacancies toward extremely high concentrations of randomly distributed point defects in the ABO3 structure.

Spinels of the general formula Li2-δM2O4 are an essential class of cathode materials for Li-ion batteries, and their optimization in terms of electrode potential, accessible capacity, and charge/discharge kinetics relies on an accurate understanding of the underlying solid-state mass and charge transport processes. In this work, we report a comprehensive impedance study of sputter-deposited epitaxial Li2-δMn2O4 thin films as a function of state-of-charge for almost the entire tetrahedral-site regime (1 ≤ δ ≤ 1.9) and provide a complete set of electrochemical properties, consisting of the charge-transfer resistance, ionic conductivity, volume-specific chemical capacitance, and chemical diffusivity. The obtained properties vary by up to three orders of magnitude and provide essential insights into the point defect concentration dependences of the overall electrode potential. We introduce a defect chemical model based on simple concentration dependences of the Li chemical potential, considering the tetrahedral and octahedral lattice site restrictions defined by the spinel crystal structure. The proposed model is in excellent qualitative and quantitative agreement with the experimental data, excluding the two-phase regime around 4.15 V. It can easily be adapted for other transition metal stoichiometries and doping states and is thus applicable to the defect chemical analysis of all spinel-type cathode materials.

Minimizing the overpotential at the air electrode of solid oxide fuel cells (SOFC) is one of the key challenges regarding a broad applicability of this technology. Next to novel materials and geometry optimization, surface modification is a promising and flexible method to alter the oxygen exchange kinetics at SOFC cathode surfaces. Despite extensive research, the mechanism behind the effect of surface decorations is still under debate. Moreover, for Sr decoration, previous studies yielded conflicting results, reporting either a beneficial or a detrimental impact on the oxygen exchange kinetics. In this contribution, in situ impedance spectroscopy during pulsed laser deposition was used to investigate the effect of Sr containing decorations under different deposition conditions. Depending on deposition temperature and interactions with the gas phase, opposing effects of Sr decoration were found. In combination with near-ambient pressure X-ray photoelectron spectroscopy and non-ambient X-ray diffractometry, it was possible to trace this phenomenon back to different chemical environments of the surface Sr. At high temperatures, Sr is deposited as SrO, which can have a beneficial effect on the oxygen exchange kinetics. At low temperatures, SrCO3 adsorbates are formed from trace amounts of CO2 in the measurement atmosphere, causing a decrease of the oxygen exchange rate. These results are in excellent agreement with the concept of surface acidity as a descriptor for the effect of surface decorations, providing further insight into the oxygen exchange kinetics on SOFC cathode surfaces and its degradation. In addition, this study shows that Sr segregation itself initially does not lead to performance degradation but that segregated SrO readily reacts with acidic compounds, reducing the catalytic capability of mixed conducting oxides.

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