Impact of point defects on the elastic properties of BaZrO3: Comprehensive insight from experiments and ab initio calculations

Abstract Acceptor doped BaZrO3 is the prototype of proton conducting perovskites which are of strong interest as electrolytes for intermediate temperature fuel cells. Elastic properties of both dry and hydrated Y-doped BaZrO3 (1.5–17 mol% Y) were determined using ultrasound time of flight (TOF) measurements, and complemented by ab initio calculations which allow for an analysis of the different contributions. The experimental and theoretical findings are consistent and reveal a strong decrease of the Young's, shear and bulk moduli upon increasing dopant concentration. This decrease is attributed to a combined effect of (i) macroscopic lattice chemical expansion mainly caused by differing io…

First principles calculations of oxygen vacancy formation and migration in mixed conducting Ba0.5Sr0.5Co1−yFeyO3−δ perovskites

Abstract First-principles supercell calculations of oxygen vacancies in the Ba 0.5 Sr 0.5 Co 1− y Fe y O 3− δ (BSCF) perovskites are presented. The density of states is determined for different iron content and oxygen vacancy concentrations, and the characteristic differences for Co and Fe are discussed. We analyze the dependences of the defect (oxygen vacancy) formation and migration energies on the Fe content and compare the calculated properties with those of related LaCoO 3 and LaFeO 3 perovskites.

X-ray Spectroscopy of (Ba,Sr,La)(Fe,Zn,Y)O3-δIdentifies Structural and Electronic Features Favoring Proton Uptake

Mixed protonic–electronic conducting oxides are key functional materials for protonic ceramic fuel cells. Here, (Ba,Sr,La)(Fe,Zn,Y)O3−δ perovskites are comprehensively investigated by X-ray spectroscopy (in oxidized and reduced states). Extended X-ray absorption fine structure shows that Zn,Y doping strongly increases the tendency for Fe–O–Fe buckling. X-ray absorption near-edge spectroscopy at the Fe K-edge and X-ray Raman scattering at the O K edge demonstrate that both iron and oxygen states are involved when the samples are oxidized, and for the Zn,Y doped materials, the hole transfer from iron to oxygen is less pronounced. This can be correlated with the observation that these material…

Interdependence of Oxygenation and Hydration in Mixed-Conducting (Ba,Sr)FeO3-δPerovskites Studied by Density Functional Theory

Financial support by the German–Israeli Foundation for Scientific Research and Development (grant I-1342-302.5/2016) and the Latvian Council of Science (grant lzp-2018/1-0147 (D.G., E.A.K.)) is gratefully acknowledged. The authors further thank Guntars Zvejnieks for help with CRYSTAL code calculations.

Surface termination effects on the oxygen reduction reaction rate at fuel cell cathodes

This research was partly funded by the Latvian project IMIS2 with the computer resources provided by the High Performance Computing Centre Stuttgart (HLRS) (Project DEFTD 12939). The authors thank D. Gryaznov for fruitful discussions and M. Sokolov for technical assistance. MMK is grateful to the Office of the Director of National Science Foundation for support under the Independent Research and Development program. The ndings, conclusions, and recommendations expressed in this material are those of the authors and do not necessarily reect the views of NSF and other funding agencies.

Jahn-Teller distortion aroundFe4+inSr(FexTi1−x)O3−δfrom x-ray absorption spectroscopy, x-ray diffraction, and vibrational spectroscopy

$\mathrm{Sr}({\mathrm{Fe}}_{x}{\mathrm{Ti}}_{1\ensuremath{-}x}){\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ perovskites (strontium titanate ferrite solid solution) with well-defined oxygen stoichiometry have been studied as a function of iron concentration by x-ray diffraction, Fe and Ti $K$-edge x-ray absorption spectroscopy (XAS), and vibrational (Raman and infrared) spectroscopy. In reduced $\mathrm{Sr}({\mathrm{Fe}}_{x}{\mathrm{Ti}}_{1\ensuremath{-}x}){\mathrm{O}}_{3\ensuremath{-}x∕2}$ samples, the analysis of the Fe $K$-edge extended x-ray absorption fine structure indicates the expected presence of oxygen vacancies ${\mathrm{V}}_{\mathrm{O}}^{∙∙}$ in the first coordination shell…

Nanosession: Ionics - Redox Kinetics, Ion Transport, and Interfaces

Oxygen Incorporation Reaction into Mixed Conducting Perovskites: a Mechanistic Analysis for (La,Sr)MnO3 Based on DFT Calculations

Based on DFT calculations of intermediates and transition states, several hypothetical mechanisms for oxygen incorporation into mixed conducting La1-xSrxMnO3{plus minus}d perovskites are discussed. In the most probable mechanism, the rate-determining step comprises the encounter of a highly mobile surface oxygen vacancy and a molecular oxygen adsorbate. Starting from these results, the variation of reaction rates for different materials is explored.

Long-Range and Short-Range Structure of Proton-Conducting Y:BaZrO3

Yttrium-doped barium zirconate (BZY) is the most promising candidate for proton-conducting ceramics and has been extensively studied in recent years. The detailed features of the crystal structure, both short-range and long-range, as well as the crystal chemistry driving the doping process, are largely unknown. We use very high resolution X-ray diffraction (HR-XRD) to resolve the crystal structure, which is very slightly tetragonally distorted in BZY, while the local environment around Zr4+ and Y3+ is probed with extended X-ray absorption fine structure (EXAFS), and the symmetry and vibrations are investigated by using Raman spectroscopy. It is found that barium zirconate shows some degree …

Theoretical and Experimental Studies of Charge Ordering in CaFeO 3 and SrFeO 3 Crystals

(Invited) The Effect of (La,Sr)MnO 3 Cathode Surface Termination on Its Electronic Structure

La1-xSrxMnO3 (LSM) was one of the first perovskites used as SOFC cathode material. Its (001) surface has two possible terminations, LaSrO and MnO2, with quite different properties and oxygen reduction efficiencies. To avoid effects of surface polarity and the dipole moment across the material, symmetric non-stoichiometric slabs are commonly used in theoretical calculations with identical terminating planes on its both sides. We analyzed the dependence of the electronic structure (density of states) and charge distribution (effective atomic charges and chemical bond covalency) on the slab termination and Mn ion oxidation state (controlled by the Sr content and slab nonstoichiometry).

Jahn-Teller effect in the phonon properties of defective SrTiO3from first principles

the Jahn–Teller effect occurs, thus reducing the cubic symmetry of a perfect crystal and leading to the appearance of both Raman- and infrared-active vibrational modes. The calculated phonon densities of states and group-theoretical analysis of defect-induced phonon frequencies were used for the interpretation of the relevant experimental data, once defect-induced local modes are identified. The temperature dependence of the Vo formation energy based on the calculated Gibbs free energy was also compared with experiments, and the phonon contribution therein estimated.

Ab initio modelling of oxygen vacancies and protonic defects in La1−xSrxFeO3−δ perovskite solid solutions

Using hybrid density functionals, detailed ab initio calculations were performed for oxygen vacancies and protons in La1−xSrxFeO3−δ perovskite solid solutions which may serve as a cathode material in protonic ceramic fuel cells. The atomic and electronic structures of different configurations of defects and the role of Fe oxidation state are analyzed in detail. The energetics of the reduction and hydration reactions are investigated. The hydration energy is found to be significantly smaller than for Ba(Zr1−xYx)O3−x/2 electrolyte materials, and the role of basicity as one decisive factor is discussed.

Formation and migration of oxygen vacancies in La1−xSrxCo1−yFeyO3−δperovskites: insight from ab initio calculations and comparison with Ba1−xSrxCo1−yFeyO3−δ

The formation and migration of oxygen vacancies in the series of (La,Sr)(Co,Fe)O3−δ perovskites, which can be used as mixed conducting SOFC cathode materials and oxygen permeation membranes, are explored in detail by means of first principles density functional calculations. Structure distortions, charge redistributions and transition state energies during the oxygen ion migration are obtained and analyzed. Both the overall chemical composition and vacancy formation energy are found to have only a small impact on the migration barrier; it is rather the local cation configuration which affects the barrier. The electron charge transfer from the migrating O ion towards the transition metal ion…

Energy Conversion: Solid Oxide Fuel Cells: First-Principles Modeling of Elementary Processes

Fuel cells are electrochemical devices that directly transform the chemical free energy of combustion (e.g., H2 + O2 and CHx + O2) into electrical energy. The avoidance of a thermal detour guarantees high theoretical efficiency. As far as the temperature regimes are concerned, we distinguish between high temperature ceramic fuel cells, intermediate-temperature fuel cells, and low temperature (i.e., only slightly above room temperature) fuel cells. The high temperature fuel cells are usually based on oxide components (ternary transition metal oxides as cathodes, Ni or Cu cermets as anodes, and acceptor-doped zirconia or ceria as electrolytes). The high temperature necessary for ion conductio…

First Principles Calculations of Oxygen Vacancy Formation and Migration in Ba1−xSrxCo1−yFeyO3−δPerovskites

Based on first principles DFT calculations, we analyze oxygen vacancy formation and migration energies as a function of chemical composition in complex multicomponent (Ba,Sr)(Co,Fe)O3−δ perovskites which are candidate materials for SOFC cathodes and permeation membranes. The atomic relaxation, electronic charge redistribution and energies of the transition states of oxygen migration are compared for several perovskites to elucidate the atomistic reason for the exceptionally low migration barrier in Ba0.5Sr0.5Co0.8Fe0.2O3−δ that was previously determined experimentally. The critical comparison of Ba1−xSrxCo1−yFeyO3−δ perovskites with different cation compositions and arrangements shows that …

Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells

Solid oxide fuel cells (SOFC) are under intensive investigation since the 1980's as these devices open the way for ecologically clean direct conversion of the chemical energy into electricity, avoiding the efficiency limitation by Carnot's cycle for thermochemical conversion. However, the practical development of SOFC faces a number of unresolved fundamental problems, in particular concerning the kinetics of the electrode reactions, especially oxygen reduction reaction. We review recent experimental and theoretical achievements in the current understanding of the cathode performance by exploring and comparing mostly three materials: (La,Sr)MnO3 (LSM), (La,Sr)(Co,Fe)O3 (LSCF) and (Ba,Sr)(Co,…

Comparison of Permeation Measurements and Hybrid Density Functional Calculations on Oxygen Vacancy Transport in Complex Perovskite Oxides

The oxygen vacancy (VO••) formation enthalpy (ΔHF) was determined from the effective activation energies obtained in the permeation measurements on a series of solid solutions La0.98–xSrxCo0.2Fe0.8O3−δ (x = 0.125–0.8) and compared with the results of ab initio calculations at finite temperatures based on the hybrid (PBE0) and GGA+U calculations, as well as previous thermogravimetric experiments. The Mulliken atomic charges, magnetic moments, and the Fe-VO•• and Sr-VO•• distances are analyzed. The strong dependence of formation enthalpy of VO•• on the Fe oxidation state due to variation in Sr and VO•• concentrations is discussed.

Proton, Hydroxide Ion, and Oxide Ion Affinities of Closed-Shell Oxides: Importance for the Hydration Reaction and Correlation to Electronic Structure

Phenomenologically, the enthalpy of the dissociative water incorporation (hydration) of oxides is often found to be more favorable for more basic oxides. In the present work, we investigate proton,...

First principles calculations of oxygen reduction reaction at fuel cell cathodes

This study was partly supported by M-ERA-NET project SunToChem (EK, YM). The computer resources were provided by Stuttgart Super-computing Center (Project DEFTD 12939). Authors thank E. Heifets, M. M. Kuklja, M. Arrigoni, D. Morgan, R. Evarestov, and D. Gryaznov for fruitful discussions.