Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3 and SrTiO3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.Keywords: (La,Sr)MnO3; dislocation; epitaxial thin film; oxygen diffusion; oxygen surface exchange; strain; ToF-SIMS;

Water splitting and chemical fuel production as a promising carbon-neutral energy solution relies critically on an efficient electrochemical process over catalyst surfaces. The fundamentals within the surface redox pathways, including the complex interactions of mobile ions and electrons between the bulk and the surface, along with the role of adsorbates and electrostatic fields remain yet to be understood quantitatively. This work presents a detailed kinetics study and nonstoichiometry characterization of Ce0.5Zr0.5O2−δ (CZO), one of the most recognized catalysts for water splitting. The use of CZO leads to >60% improvement in the kinetic rates as compared with undoped ceria with twice the total yield at 700 °C, resulting from the improved reducibility. The peak H2 production rate is 95 μmol g–1 s–1 at 700 °C, and the total production is 750 μmol g–1. A threshold temperature of 650 °C is required to achieve significant H2 production at fast rates. The redox kinetics is modeled using two-step surface chemistry with bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted, and the model predictions show good agreement with the measurements. The enthalpy of bulk defect formation for CZO is found to be 262 kJ/mol, >40% lower than that of undoped ceria. As oxygen vacancy is gradually filled up, the surface H2O splitting chemistry undergoes a transition from exothermic to endothermic, with the crossover around δ = 0.04 to 0.05, which constrains the further ion incorporation process. Our kinetics study reveals that the H2O splitting process with CZO is kinetics limited at low temperature and transitions to partial-equilibrium with significantly enhanced backward reaction at high temperature. The charge-transfer step is found to be the rate-limiting step for H2O splitting. The detailed kinetics and nonstoichiometric equilibria should be helpful in guiding the design and optimization of CZO as a catalyst, oxygen storage material, as well as oxygen carrier for water-splitting applications.

CO2 splitting via thermo-chemical or reactive redox has emerged as a novel and promising carbon-neutral energy solution. Its performance depends critically on the properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface ion-incorporation pathways, along with the role of surface defects and the adsorbates remain largely unknown. This study presents a detailed kinetics study of CO2 splitting using CeO2 and Ce0.5Zr0.5O2 (CZO) in the temperature range 600–900 °C. Given our interest in fuel-assisted reduction, we limit our study to relatively lower temperatures to avoid excessive sintering and the need for high temperature heat. Compared to what has been reported previously, we observe higher splitting kinetics, resulting from the utilization of fine particles and well-controlled experiments which ensure a surface-limited-process. The peak rates with CZO are 85.9 μmole g−1 s−1 at 900 °C and 61.2 μmole g−1 s−1 at 700 °C, and those of CeO2 are 70.6 μmole g−1 s−1 and 28.9 μmole g−1 s−1. Kinetic models are developed to describe the ion incorporation dynamics, with consideration of CO2 activation and the charge transfer reactions. CO2 activation energy is found to be −120 kJ mole−1 for CZO, half of that for CeO2, while CO desorption energetics is analogous between the two samples with a value of ∼160 kJ mole−1. The charge-transfer process is found to be the rate-limiting step for CO2 splitting. The evolution of CO32− with surface Ce3+ is examined based on the modeled kinetics. We show that the concentration of CO32− varies with Ce3+ in a linear-flattened-decay pattern, resulting from a mismatch between the kinetics of the two reactions. Our study provides new insights into the significant role of surface defects and adsorbates in determining the splitting kinetics.

The presence of interlaminar interstitial defects like hydrogen affects the mechanical properties of van der Waals-bonded layered materials such as transition metal chalcogenides. While the embrittling effect of hydrogen is well understood in metals, the impact of hydrogen defects on the mechanical behavior of layered chalcogenides remained unexplored. In this article, we use density functional calculations to reveal the influence of different hydrogen point defects on important mechanical metrics, including binding energies, elastic moduli and tensile and shear strengths of a prototypical ionic layered material, mackinawite, Fe1+xS. We find that one of the low-energy hydrogen defect structures, interlaminar molecular H2 interstitials, severely degrades the strength of inter-layer van der Waals interactions in the mackinawite crystal. This leads to a significant (over 80%) degradation in the mechanical properties of the mackinawite crystal and enables facile interlayer sliding and exfoliation. This finding suggests the mechanisms for cathodic exfoliation of transition metal chalcogenides like Fe1+xS, and presents a plausible mechanism for the poor protectiveness of layered passive films like mackinawite that undergo failure by spalling or delamination.

The sluggish oxygen reduction reaction (ORR) greatly reduces the energy efficiency of solid oxide fuel cells (SOFCs). Here we report our findings in dramatically enhancing the ORR kinetics and durability of the state-of-the-art La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode using a hybrid catalyst coating composed of a conformal PrNi0.5Mn0.5O3 (PNM) thin film with exsoluted PrOx nanoparticles. At 750 °C, the hybrid catalyst-coated LSCF cathode shows a polarization resistance of ∼0.022 Ω cm2, about 1/6 of that for a bare LSCF cathode (∼0.134 Ω cm2). Further, anode-supported cells with the hybrid catalyst-coated LSCF cathode demonstrate remarkable peak power densities (∼1.21 W cm−2) while maintaining excellent durability (0.7 V for ∼500 h). Near Ambient X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) analyses, together with density functional theory (DFT) calculations, indicate that the oxygen-vacancy-rich surfaces of the PrOx nanoparticles greatly accelerate the rate of electron transfer in the ORR whereas the thin PNM film facilitates rapid oxide-ion transport while drastically enhancing the surface stability of the LSCF electrode.

We present a multi-scale approach to predict equilibrium defect concentrations across oxide/oxide hetero-interfaces. There are three factors that need to be taken into account simultaneously for computing defect redistribution around the hetero-interfaces: the variation of local bonding environment at the interface as epitomized in defect segregation energies, the band offset at the interface, and the equilibration of the chemical potentials of species and electrons via ionic and electronic drift-diffusion fluxes. By including these three factors from the level of first principles calculation, we build a continuum model for defect redistribution by concurrent solution of Poisson's equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. This model solves for and preserves the continuity of the electric displacement field throughout the interfacial core zone and the extended space charge zones. We implement this computational framework to a model hetero-interface between the monoclinic zirconium oxide, m-ZrO2, and the chromium oxide Cr2O3. This interface forms upon the oxidation of zirconium alloys containing chromium secondary phase particles. The model explains the beneficial effect of the oxidized Cr particles on the corrosion and hydrogen resistance of Zr alloys. Under oxygen rich conditions, the ZrO2/Cr2O3 heterojunction depletes the oxygen vacancies and the sum of electrons and holes in the extended space charge zone in ZrO2. This reduces the transport of oxygen and electrons thorough ZrO2 and slows down the metal oxidation rate. The enrichment of free electrons in the space charge zone is expected to decrease the hydrogen uptake through ZrO2. Moreover, our analysis provides a clear anatomy of the components of interfacial electric properties; a zero-Kelvin defect-free contribution and a finite temperature defect contribution. The thorough analytical and numerical treatment presented here quantifies the rich coupling between defect chemistry, thermodynamics and electrostatics which can be used to design and control oxide hetero-interfaces.

Plastic strain engineering was applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO3 as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n-type conductivity by 50 times at oxygen pressures below 10−5 atm at 650 °C. At higher oxygen pressures the p-type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO3 arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro-solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects.

Topotactic phase transition of functional oxides induced by changes in oxygen nonstoichiometry can largely alter multiple physical and chemical properties, including electrical conductivity, magnetic state, oxygen diffusivity, and electrocatalytic reactivity. For tuning these properties reversibly, feasible means to control oxygen nonstoichiometry-dependent phase transitions in functional oxides are needed. This paper describes the use of electrochemical potential to induce phase transition in strontium cobaltites, SrCoOx (SCO) between the brownmillerite (BM) phase, SrCoO2.5, and the perovskite (P) phase, SrCoO3−δ. To monitor the structural evolution of SCO, in situ X-ray diffraction (XRD) was performed on an electrochemical cell having (001) oriented thin-film SrCoOx as the working electrode on a single crystal (001) yttria-stabilized zirconia electrolyte in air. In order to change the effective pO2 in SCO and trigger the phase transition from BM to P, external electrical biases of up to 200 mV were applied across the SCO film. The phase transition from BM to P phase could be triggered at a bias as low as 30 mV, corresponding to an effective pO2 of 1 atm at 500 °C. The phase transition was fully reversible and the epitaxial film quality was maintained after reversible phase transitions. These results demonstrate the use of electrical bias to obtain fast and easily accessible switching between different phases as well as distinct physical and chemical properties of functional oxides as exemplified here for SCO.

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.Keywords: electric field effect; resistive switching; scanning tunneling microscopy; strontium ruthenate; tantalum oxide;

Topotactic phase transition in SrCoOx (x = 2.5–3, denoted as SCO) has become a focal point for the study of this unique functional oxide system, sparked by the large alteration in the physical and chemical properties from brownmillerite (BM) to perovskite (P) phases. Recently, we showed that applying electrochemical bias could be a convenient way to control the oxygen stoichiometry in SCO and trigger its topotactic phase transition. In this paper, we utilized in situ ambient pressure X-ray spectroscopic tools to reveal the electronic structure and oxygen nonstoichiometry evolution during the BM → P phase transition of SCO. During the BM → P transition via intercalation of oxygen anions into the structure, we found a lowering of the Fermi level due to creation of Co 3d–O 2p hybridized unoccupied states. X-ray absorption spectra showed that the formed unoccupied states have largely O 2p characteristics. Finally, we utilized the time-dependent relaxation of the X-ray absorption intensity as a new approach to study the phase transformation kinetics and rate-limiting mechanisms. The results deepen the understanding of the electronic structure of SCO as a function of its oxygen stoichiometry and phase and may guide the design of SCO properties for electrocatalyst and memristor functionality.

Chemical-looping water splitting is a novel and promising technology for hydrogen production with CO2 separation. Its efficiency and performance depend critically on the reduction and oxidation (redox) properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface redox pathways, including the complex interactions of mobile ions and electrons between the bulk and the surface, along with the adsorbates and electrostatic fields, remain yet unresolved. This work presents a detailed redox kinetics study with emphasis on the surface ion-incorporation kinetics pathway, using time-resolved and systematic measurements in the temperature range 600–1000 °C. By using fine ceria nanopowder, we observe an order-of-magnitude higher hydrogen production rate compared to the state-of-the-art thermochemical or reactive chemical-looping water splitting studies. We show that the reduction is the rate-limiting step, and it determines the total amount of hydrogen produced in the following oxidation step. The redox kinetics is modeled using a two-step surface chemistry (an H2O adsorption/dissociation step and a charge-transfer step), coupled with the bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted with excellent agreement with measurements. The model reveals that the surface defects are abundant during redox conditions, and charge transfer is the rate-determining step for H2 production. The results establish a baseline for developing new materials and provide guidance for the design and the practical application of water splitting technology (e.g., the design of OC characteristics, the choice of the operating temperatures, and periods for redox steps, etc.). The method, combining well-controlled experiment and detailed kinetics modeling, enables a new and thorough approach for examining the defect thermodynamics in the bulk and at the surface, as well as redox reaction kinetics for alternative materials for water splitting.

The effect of dislocations on the chemical, electrical and transport properties in oxide materials is important for electrochemical devices, such as fuel cells and resistive switches, but these effects have remained largely unexplored at the atomic level. In this work, by using large-scale atomistic simulations, we uncover how a ⟨100⟩{011} edge dislocation in SrTiO3, a prototypical perovskite oxide, impacts the local defect chemistry and oxide ion transport. We find that, in the dilute limit, oxygen vacancy formation energy in SrTiO3 is lower at sites close to the dislocation core, by as much as 2 eV compared to that in the bulk. We show that the formation of a space-charge zone based on the redistribution of charged oxygen vacancies can be captured quantitatively at atomistic level by mapping the vacancy formation energies around the dislocation. Oxide-ion diffusion was studied for a low vacancy concentration regime (ppm level) and a high vacancy concentration regime (up to 2.5%). In both cases, no evidence of pipe-diffusion, i.e., significantly enhanced mobility of oxide ions, was found as determined from the calculated migration barriers, contrary to the case in metals. However, in the low vacancy concentration regime, the vacancy accumulation at the dislocation core gives rise to a higher diffusion coefficient, even though the oxide-ion mobility itself is lower than that in the bulk. Our findings have important implications for applications of perovskite oxides for information and energy technologies. The observed lower oxygen vacancy formation energy at the dislocation core provides a quantitative and direct explanation for the electronic conductivity of dislocations in SrTiO3 and related oxides studied for red–ox based resistive switching. Reducibility and electronic transport at dislocations can also be quantitatively engineered into active materials for fuel cells, catalysis, and electronics.

Attaining fast oxygen exchange kinetics on perovskite and related mixed ionic and electronic conducting oxides is critical for enabling their applications in electrochemical energy conversion systems. This study focuses on understanding the relationship between surface chemistry and the surface oxygen exchange kinetics on epitaxial films made of (La1–xSrx)2CoO4, a prototypical Ruddlesden–Popper structure that is considered as a promising cathode material for fuel cells. The effects of crystal orientation on the surface composition, morphology, oxygen diffusion, and surface exchange kinetics were assessed by combining complementary surface-sensitive analytical techniques, specifically low energy ion scattering, X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and secondary ion mass spectroscopy. The films were grown in two different crystallographic orientations, (001) and (100), and with two different Sr compositions, at x = 0.25 (LSC25) and 0.50 (LSC50), by using pulsed laser deposition. In the as-prepared state, a Sr enriched layer at the top surface and a Co enriched subsurface layer were found on films with both orientations. After annealing at elevated temperatures in oxygen, the Sr enrichment increased, followed by clustering into Sr-rich secondary phase particles. Both the LSC25 and LSC50 films showed anisotropic oxygen diffusion kinetics, with up to 20 times higher oxygen diffusion coefficient along the ab-plane compared that along the c-axis at 400–500 °C. However, no dependence of surface oxygen exchange coefficient was found on the crystal orientation. This result indicates that the strong Sr segregation at the surface overrides the effect of the structural anisotropy that was also expected for the surface exchange kinetics. The larger presence of Co cations exposed at the LSC25 surface compared to that at the LSC50 surface is likely the reason for the faster oxygen surface exchange kinetics on LSC25 compared to LSC50. This work demonstrated the critical role of surface chemistry on the oxygen exchange kinetics on perovskite related oxides, which are thus far underexplored at elevated temperatures, and provides a generalizable approach to probe the surface chemistry on other catalytic complex oxides.

The hetero-interfaces between the perovskite (La1−xSrx)CoO3 (LSC113) and the Ruddlesden-Popper (La1−xSrx)2CoO4 (LSC214) phases have recently been reported to exhibit fast oxygen exchange kinetics. Vertically aligned nanocomposite (VAN) structures offer the potential for embedding a high density of such special interfaces in the cathode of a solid oxide fuel cell in a controllable and optimized manner. In this work, VAN thin films with hetero-epitaxial interfaces between LSC113 and LSC214 were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC214 domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC113 domains above 250 °C in 10−3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC214 with LSC113 also provides for a more stable cation chemistry at the surface of LSC214 within the VAN structure, as compared to single phase LSC214 films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC113 and LSC214 in the temperature range of 320–400 °C. The higher reactivity of the VAN surface to the oxygen reduction reaction is attributed to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation. The instability of the LSC113/214 hetero-structure surface chemistry at temperatures above 400 °C, however, was found to lead to degraded ORR kinetics. Thus, while VAN structures hold great promise for offering highly ORR reactive electrodes, efforts towards the identification of more stable hetero-structure compositions for high temperature functionality are warranted.

The adsorption energy of reactant molecules and reaction intermediates is one of the key descriptors of catalytic activity of surfaces and is commonly used as a metric in screening materials for design of heterogeneous catalysts. The efficacy of such screening schemes depends on the accuracy of calculated adsorption energies under reaction conditions. These adsorption energies can depend strongly on interactions between adsorbed molecules in the adlayer. However, these interactions are typically not accounted for in screening procedures that use DFT-based zero-coverage adsorption energies. Identifying the physical mechanisms behind these interactions is essential to model realistic catalyst surfaces under reaction conditions and to understand the dependence of adsorption energies on reaction parameters like surface strain and composition. This article describes a method to quantitatively resolve the observed inter-adsorbate interactions into various direct adsorbate–adsorbate interactions (i.e. Coulombic and steric) and surface-mediated interactions (i.e. adsorbate-induced surface relaxation and change in electronic structure) by combining density functional theory and cluster-expansion calculations of coverage-dependent adsorption energies. The approach is implemented on a model catalyst surface of FeS2(100) reacting with H2S molecules. We find that the adsorption energy of H2S molecules can be affected by over 0.55 eV by the repulsive inter-adsorbate interactions caused primarily by the adsorbate-induced changes to the electronic structure of the FeS2 surface. These interactions also show a strong monotonic dependence on surface strain, being three times stronger on compressively strained surfaces than on surfaces under tensile strain. The large magnitude of inter-adsorbate interactions as well as their strong dependence on lattice strain demonstrate the need for using coverage-dependent adsorption energies for more accurate screening, for example for strained catalytic systems like core–shell and overlayer structures.

Cation diffusion is an important rate-limiting process in the growth of pyrrhotite (Fe1−xS) in passivating films on steels exposed to sulfidic environments, and for proposed synthetic applications of Fe1−xS, for example single-phase magnetic switching devices. Above the Néel temperature TN of 315 °C, where Fe1−xS is paramagnetic and structurally disordered, iron self-diffusivity *DFe predictably follows a standard, established Arrhenius law with temperature. However, we report 57Fe tracer diffusion measurements below TN, obtained using secondary ion mass spectrometry (SIMS), that demonstrate a 100-fold reduction in diffusion coefficient as compared to the extrapolated, paramagnetic Arrhenius trend at 150 °C. The results can be described by a magnetic diffusion anomaly, where the vacancy migration energy for the spontaneously-magnetized cation sublattice is increased by approximately 40% over the paramagnetic state. These constitute the first set of consistent diffusivity data obtained in magnetic pyrrhotite, allowing more accurate prediction of pyrrhotite growth rates and determination of magnetic properties for synthetic devices.

The influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was investigated at the surface of Nd2NiO4+δ (NNO). Nanoscale dense NNO thin films with tensile, compressive and no strain along the c-axis were fabricated by pulsed laser deposition on single-crystalline Y0.08Zr0.92O2 substrates. The ORR kinetics on the NNO thin film cathodes was investigated by electrochemical impedance spectroscopy at 360–420 °C in air. The oxygen exchange kinetics on the NNO films with tensile strain along the c-axis was found to be 2–10 times faster than that on the films with compressive strain along the c-axis. A larger concentration of oxygen interstitials (δ) is found in the tensile NNO films compared to the films with no strain or compressive strain, deduced from the measured chemical capacitance. This is consistent with the increase in the distance between the NdO rock-salt layers observed by transmission electron microscopy. The surface structure of the nonstrained and tensile strained films remained stable upon annealing in air at 500 °C, while a significant morphology change accompanied by the enrichment of Nd was found at the surface of the films with compressive strain. The faster ORR kinetics on the tensile strained NNO films was attributed to the ability of these films to incorporate oxygen interstitials more easily, and to the better stability of the surface chemistry in comparison to the nonstrained or compressively strained films.Keywords: electrochemistry; fuel cells; lattice strain; nanoscale thin film; oxygen reduction reaction; Ruddlesden−Popper oxide;

The fast kinetics of oxygen reduction reaction (ORR) at oxide hetero-structures made of La0.8Sr0.2CoO3 and (La0.5Sr0.5)2CoO4 (LSC113/214) attracted great interest to enable high performance cathodes for solid oxide fuel cells. The aim of this work is to uncover the underlying mechanism of fast ORR kinetics at the LSC113/214 system from a defect chemistry and electronic structure perspective. X-ray photoelectron spectroscopy with depth profiling was used to compare the reducibility of the Co cation and the valence band offset in the LSC113/214 multilayer (ML) and in single phase LSC113 and LSC214 films. At 250 °C, the Co 2p core-level photoelectron spectra showed the presence of Co2+ across the ML interfaces in both the LSC113 and LSC214 layers. While this is similar to the Co valence state of LSC113 single phase films, it is contrary to the single phase LSC214 films which had only the higher oxidation state of cobalt, Co3+. The greater reducibility of Co in LSC214 in the ML structure compared to that of Co in the LSC214 single phase film was attributed to electron donation and transfer of oxygen vacancies from LSC113 across the interfaces, and it is one mechanism to enhance the oxygen reduction activity at the LSC113/214 hetero-structure.

The collective behavior of point defects formed on the free surfaces of ionic crystals under redox conditions can lead to initiation of local breakdown by pitting. Here, we controllably generated sulfur vacancies on single crystal FeS2(100) through in vacuo annealing, and investigated the resulting evolution of surface chemistry using synchrotron x-ray photoelectron spectroscopy (XPS). By measuring the S 2p photoemission signal intensity arising from sulfur defects as a function of temperature, the enthalpy of formation of sulfur vacancies was found to be 0.1 ± 0.03 eV, significantly lower than the reduction enthalpy of bulk FeS2. Above 200 °C, the created sulfur vacancies together with preexisting iron vacancies condensed into nm-scale defect clusters, or pits, at the surface, as evidenced by scanning tunneling microscopy (STM). We provide a mechanistic description for the initiation of pits that requires the concerted behavior of both the sulfur and iron vacancies, and validate this model with kinetic Monte Carlo (kMC) simulations. The model probes realistic length and time scales, providing good agreement with the experimental results from XPS and STM measurements. Our results mechanistically and quantitatively describe the atomic scale processes occurring at pyrite surfaces under chemically reducing environments, important in many natural and technological settings, ranging from its role as a passivating film in corrosion to its potential use as a photovoltaic absorber in solar energy conversion.

In the energy-structure paradigm, we analyzed the defects that can arise in tetragonal zirconium oxide (T-ZrO2) involving the hydrogen atom or the hydrogen molecule using density functional theory. Our results indicate that the dominant hydrogen defect under reducing conditions is , a complex formed between the hydride ion and a doubly charged oxygen vacancy. This result is consistent with the experimental observation that under reducing conditions, the solubility of hydrogen is proportional to the degree of hypostoichiometry of T-ZrO2. Under oxidizing conditions we found three different hydrogen defects, each predominating in a specific range of the chemical potential of electrons. Starting from the valence band top toward the conduction band bottom, these defects are the interstitial proton, , a complex formed between two hydrogen species and a zirconium vacancy with a net effective charge of (2−), , and finally a complex similar to the latter but with a net effective charge of (4−), . In the two hydrogens exist in the form of hydroxyl groups, while in they exist in the form of a hydrogen molecule. In addition, we found that up to three hydrogen species can favorably accumulate in a zirconium vacancy under oxidizing conditions. The clustering of hydrogen in cation vacancies can be a precursor for the deleterious effects of hydrogen on the mechanical properties and stability of metal oxides, in analogy with hydrogen embrittlement in metals. Finally we observed a red-shift and a blue-shift for the vibrational frequencies of all the hydroxyl groups and all the hydrogen molecules, respectively, in T-ZrO2 when compared to the gas phase frequencies. This is an important characteristic for guiding future experimental efforts to detect and identify hydrogen defects in T-ZrO2. The insights presented in this work advance our predictive understanding of the degradation behavior of T-ZrO2 as a corrosion resistant passive layer, as a gate dielectric and in biomedical applications.

Zirconium–niobium alloys are currently proposed for applications in water-cooled nuclear reactors. However, the mechanisms by which Nb impacts the corrosion resistance of these alloys are yet to be clarified. In this work, we utilize a thermodynamic framework informed by density functional theory calculations to predict the effect of Nb on the equilibria of charged defects in tetragonal ZrO2, and discuss how the changes in the defect concentrations affect the protectiveness of this oxide that grows natively on Zr alloys during oxidation. Our analysis shows that Nb dissolves predominantly in the oxidation state 5+ as a substitutional defect on the Zr sublattice, with charge compensation achieved by the negatively charged Zr vacancies. Moreover, Nb dissolution is limited to the oxygen-rich conditions, i.e., in the oxide surface facing oxidizing environment. We validate this finding by performing X-ray photoelectron spectroscopy on oxidized Zr–Nb alloys. The introduction of Nb in tetragonal ZrO2 is found to enhance the concentration of Zr vacancies and of free electrons and to decrease the concentration of oxygen vacancies. We conclude that the net effect of Nb on the corrosion kinetics of Zr alloys is favorable if the rate limiting process is oxygen transport, while Nb would be detrimental if electron transport limits the oxidation kinetics.

Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

•Methodology to quantify the surface band gap from FeS2 scanning tunneling spectra.•Intrinsic and defect-related electronic surface states are considered.•The measured surface band gap is 0.4 ± 0.1 eV, as compared with 0.95 eV in the bulk.The interfacial electronic properties and charge transfer characteristics of pyrite, FeS2, are greatly influenced by the presence of electronic states at the crystal free surface. We investigate the surface electronic structure of FeS2 (100) using scanning tunneling spectroscopy (STS) and interpret the results using tunneling current simulations informed by density functional theory. Intrinsic, dangling bond surface states located at the band edges reduce the fundamental band gap Eg from 0.95 eV in bulk FeS2 to 0.4 ± 0.1 eV at the surface. Extrinsic surface states from sulfur and iron defects contribute to Fermi level pinning but, due to their relatively low density of states, no detectable tunneling current was measured at energies within the intrinsic surface Eg. These findings help elucidate the nature of energy alignment for electron transfer processes at pyrite surfaces, which are relevant to evaluation of electrochemical processes including corrosion and solar energy conversion.

Probing the mechanisms of defect–defect interactions at strain rates lower than 106 s−1 is an unresolved challenge to date to molecular dynamics (MD) techniques. Here we propose an original atomistic approach based on transition state theory and the concept of a strain-dependent effective activation barrier that is capable of simulating the kinetics of dislocation–defect interactions at virtually any strain rate, exemplified within 10−7 to 107 s−1. We apply this approach to the problem of an edge dislocation colliding with a cluster of self-interstitial atoms (SIAs) under shear deformation. Using an activation–relaxation algorithm [Kushima A, et al. (2009) J Chem Phys 130:224504], we uncover a unique strain-rate–dependent trigger mechanism that allows the SIA cluster to be absorbed during the process, leading to dislocation climb. Guided by this finding, we determine the activation barrier of the trigger mechanism as a function of shear strain, and use that in a coarse-graining rate equation formulation for constructing a mechanism map in the phase space of strain rate and temperature. Our predictions of a crossover from a defect recovery at the low strain-rate regime to defect absorption behavior in the high strain-rate regime are validated against our own independent, direct MD simulations at 105 to 107 s−1. Implications of the present approach for probing molecular-level mechanisms in strain-rate regimes previously considered inaccessible to atomistic simulations are discussed.

Transition metal-doped ferrites are attractive candidates for a wide range of applications including catalysis and electronic and magnetic devices. Although their bulk characteristics are well-understood, very little is known about their surface properties at the molecular level. Here, we demonstrate high reactivity of NiFe2O4 (111) surfaces, a Ni-doped ferrite, by elucidating the surface structure and water adsorption mechanism using density functional theory with on-site correction for Couloumb interaction (DFT + U). The surface reactivity of NiFe2O4 (111) surfaces (with 0.25 ML Fetet1 and 0.5 ML Feoct2–tet1 terminations) is shown to be significantly higher in comparison with the undoped Fe3O4 (111) surfaces. Dissociation of water is found to be highly favorable with an adsorption energy of −1.11 eV on the 0.25 ML Fetet1 terminated surface and −2.30 eV on the 0.5 ML Feoct2–tet1 terminated surface. In addition, we computed a low activation barrier of 0.18 eV for single water molecule dissociation on the 0.25 ML Fetet1 termination, while the corresponding dissociation reaction on the 0.5 ML Feoct2–tet1 termination proceeded without a barrier. The reactivity of NiFe2O4 surfaces toward water is understood based on strong interactions between the adsorbing OH radical molecular orbitals and the d orbitals of the surface Fe atom. In particular, the new bonding orbitals created due to the interaction of the OH 3σ orbital and the Fe d states are pushed deeper down the energy axis resulting in a greater energy gain and higher water adsorption strength in the case of 0.5 ML Feoct2–tet1 termination. Furthermore, transition-metal surface resonances (TMSR) are found to be good descriptors of the surface reactivity in the two ferrites investigated and is a useful measure to design ferrite-based catalytic systems. These findings have strong implications toward the use of NiFe2O4 as an effective metal-doped ferrite catalyst in a typical industrial process such as the water-gas shift (WGS) reaction and are of significance in fuel materials durability in nuclear reactors where ferrites are known to trap boron resulting in failure of the reactors.

The correlation between the surface chemistry and electronic structure is studied for SrTi1−xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi0.95Fe0.05O3 (STF5), SrTi0.65Fe0.35O3 (STF35) and SrFeO3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 °C and 10−3 mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrOx on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.

The recently reported fast oxygen reduction kinetics at the interface of (La,Sr)CoO3−δ (LSC113) and (La,Sr)2CoO4+δ (LSC214) phases opened up new questions for the potential role of dissimilar interfaces in advanced cathodes for solid oxide fuel cells (SOFCs). Using first-principles based calculations in the framework of density functional theory, we quantitatively probed the possible mechanisms that govern the oxygen reduction activity enhancement at this hetero-interface as a model system. Our findings show that both the strongly anisotropic oxygen incorporation kinetics on the LSC214 and the lattice strain in the vicinity of the interface are important contributors to such enhancement. The LSC214(100) surface exposed to the ambient at the LSC113/LSC214 interface facilitates oxygen incorporation because the oxygen molecules very favorably adsorb onto it compared to the LSC214(001) and LSC113(001) surfaces, providing a large source term for oxygen incorporation. Lattice strain field present near the hetero-interface accelerates oxygen incorporation kinetics especially on the LSC113(001) surface. At 500 °C, 4 × 102 times faster oxygen incorporation kinetics are predicted in the vicinity of the LSC113/LSC214 hetero-interface with 50% Sr-doped LSC214 compared to that on the single phase LSC113(001) surface. Contributions from both the anisotropy and the local strain effects are of comparable magnitude. The insights obtained in this work suggest that hetero-structures, which have a large area of (100) surfaces and smaller thickness in the [001] direction of the Ruddlesden–Popper phases, and larger tensile strain near the interface would be promising for high-performance cathodes.

La0.6Sr0.4CoO3−δ(LSC) thin film cathodes synthesized by pulsed laser deposition at 450°C (LSC_450°C) and 650°C (LSC_650°C) exhibit different electrochemical performance. The origin of the differences in the oxygen reduction activity and stability of these cathodes is investigated on the basis of their surface chemistry and their surface atomic and electronic structures. Angle resolved X-ray photoelectron spectroscopy and nanoprobe Auger electron spectroscopy are used to identify the surface cation content, chemical bonding environment, and the spatial heterogeneities with nanoscale resolution. The higher electrochemical activity of LSC_450°C is attributed to the more stoichiometric cation content on the surface and the more uniform lateral and depth distribution of constituent cations. The poorly crystalline atomic structure of the LSC_450°C was found to prohibit the extensive segregation and phase separation on the surface because of the more favorable elastic and electrostatic interactions of Sr in the bulk. Upon annealing in air at 600 °C, the surface of the LSC_650°C undergoes a structural change from a Sr-rich LSC state to a SrO/Sr(OH)2-rich phase-separated state. The partial blockage of the surface with the heterogeneously distributed SrO/Sr(OH)2-rich phases, the decrease in oxygen vacancy content, and the deterioration of the electron transfer properties as evidenced from the Co oxidation state near the surface are found responsible for the severe electrochemical deactivation of the LSC_650°C. These results are important for advancing our ability to tailor the electrochemical performance of solid oxide fuel cell cathodes by understanding the relation of surface chemistry and structure to the oxygen reduction activity and stability, and the dependence of cation segregation on its driving forces including material microstructure.Keywords: cathode activity; La0.6Sr0.4CoO3−δ; phase separation; surface segregation;

In this work, we demonstrate the mechanism by which electronic charge localization increases the chemical expansion coefficient in two model systems, CeO2−δ and BaCeO3−δ. Using Density Functional Theory calculations, we predict that this coefficient is increased by more than 70% when charge is fully localized, consistent with the observation that materials with a smaller degree of charge localization have smaller chemical expansion coefficients. This finding has important consequences for devising materials with smaller chemical expansion coefficients and for the reliability of the widely-used Shannon's ionic radii.

The main goal of this study is to assess the resistance of ceria against hydrogen penetration into its bulk, in the context of its application as a protective surface coating against hydrogen embrittlement in metals. We evaluate the reaction mechanisms between the H2S and H2O molecules and the CeO2(111) surface and their kinetic descriptors, using first principles based calculations in the density functional theory framework. Our approach is validated by performing an extensive comparison with the available experimental data. We predict that hydrogen penetration into CeO2(111) is a surface-absorption-limited process with a high-energy barrier (1.67 eV) and endothermicity (1.50 eV), followed by a significantly lower bulk dissolution energy and diffusion barrier (0.67 and 0.52 eV, respectively). We find that the presence of surface vacancies and higher coverages affects significantly the energetics of H2S/H2O adsorption, dissociation, and hydrogen subsurface absorption, facilitating most of these processes and degrading the protectiveness of ceria against hydrogen penetration. The reasons behind these effects are discussed. Overall we expect ceria to hinder the hydrogen incorporation significantly due to the effectively large energy barrier against subsurface absorption, provided vacancy formation is suppressed.

The present study investigates the adsorption and dissociation reaction pathways of boric acid, B(OH)3, and the reaction kinetic descriptors on NiO(001) and ZrO2(1̅11) surfaces. Density functional theory is employed for ground-state calculations, while the nudged elastic band method is used for obtaining reaction barriers. Strong electron correlations in the case of NiO are included using the DFT + U approach. Adsorption of boric acid on clean ZrO2(1̅11) is found to be more favorable compared with that on NiO(001), in agreement with prior experiments. Dissociative adsorption is observed to dominate over molecular adsorption in the case of ZrO2(1̅11), whereas NiO(001) favors molecular adsorption. The most stable configuration for B(OH)3 on NiO(001) is a hydrogen-bonded molecular structure, Nis-(OH)B(OH)(OH)···Os (s = surface atom), with an adsorption energy of −0.74 eV. On ZrO2(1̅11), a single O–H dissociated structure, Zrs-(O)B(OH)(HO)-Zrs + Os-H, with an adsorption energy of −1.61 eV, is the most stable. Our results reveal lower activation barriers for B(OH)3 dissociation on NiO(001) than on ZrO2(1̅11). We demonstrate the importance of both the surface transition-metal atom and oxygen states and discuss bonding mechanisms leading to different adsorption configurations on such metal oxides. The analysis of surface reactivity presented here is useful in designing metal oxides for catalytic applications and is of significant importance in fuel materials durability in nuclear energy systems.

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La0.8Sr0.2CoO3 (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10–3 mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8–1.5 eV at room temperature to a metallic-like state with no energy gap at 200–300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.

We present the structural and dynamic nature of water ultraconfined in the quasi-two-dimensional nanopores of the highly disordered calcium−silicate−hydrate (C-S-H), the major binding phase in cement. Our approach is based on classical molecular simulations. We demonstrate that the C-S-H nanopore space is hydrophilic, particularly because of the nonbridging oxygen atoms on the disordered silicate chains which serve as hydrogen-bond acceptor sites, directionally orienting the hydrogen atoms of the interfacial water molecules toward the calcium−silicate layers. The water in this interlayer space adopts a unique multirange structure: a distorted tetrahedral coordination at short range up to 2.7 Å, a disordered structure similar to that of dense fluids and supercooled phases at intermediate range up to 4.2 Å, and persisting spatial correlations through dipole−dipole interactions up to 10 Å. A three-stage dynamics governs the mean square displacement (MSD) of water molecules, with a clear cage stage characteristic of the dynamics in supercooled liquids and glasses, consistent with its intermediate-range structure identified here. At the intermediate time scales corresponding to the β-relaxation of glassy materials, coincident with the cage stage in MSD, the non-Gaussian parameter indicates a significant heterogeneity in the translational dynamics. This dynamic heterogeneity is induced primarily because of the heterogeneity in the distribution of hydrogen bond strengths. The strongly attractive interactions of water molecules with the calcium silicate walls serve to constrain their motion. Our findings have important implications on describing the cohesion and mechanical behavior of cement from its setting to its aging.

Mechanisms by which lattice strain alters the oxygen reduction reaction (ORR) kinetics are important to understand in order to increase the ORR activity of solid oxide fuel cell cathodes. Here we assess the mechanistic and quantitative effects of strain on oxygen diffusion on the LaCoO3(LCO)(001) surface using density functional theory calculations. Planar tensile strain is found to reduce the migration barrier of oxygen vacancy anisotropically on the LCO(001) surface, inducing an enhanced mobility along the [10] direction and a suppressed mobility along the [110] direction. The increase of space around Co that the oxygen (vacancy) traverses with a curved path is the cause of the enhanced mobility along the [10]. The increasing octahedral distortions with planar tensile strain inhibit the migration of oxygen vacancy along the [110] direction. Furthermore, the mobility of the adsorbed oxygen atom is suppressed with increasing strain due to its stronger adsorption on the surface. On the basis of rate theory estimates, the significantly lower energy barrier for oxygen vacancy diffusion is expected to dominate the other degrading factors and actually accelerate the ORR kinetics on LCO(001) up to 3% strain. The insights obtained here are useful for designing strategies to control the desired anisotropic and uni-directional oxygen transport along strained hetero-interfaces.

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La2CoO4+δ. The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La2CoO4+δ. Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principles-based calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling–untilting of the CoO6 octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800–1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT- and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31–0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden–Popper structures.

Effects of strain on the surface cation chemistry and the electronic structure are important to understand and control for attaining fast oxygen reduction kinetics on transition-metal oxides. Here we demonstrate and mechanistically interpret the strain coupling to Sr segregation, oxygen vacancy formation, and electronic structure on the surface of La0.7Sr0.3MnO3 (LSM) thin films as a model system. Our experimental results from X-ray photoelectron spectroscopy and scanning tunneling spectroscopy are discussed in light of our first principles-based simulations. A stronger Sr enrichment tendency and a more facile oxygen vacancy formation prevail for the tensile-strained LSM surface. At 500 °C in 10−3 mbar oxygen, both LSM film surfaces exhibit a metallic-like tunneling conductance, with a higher density of electronic states near the Fermi level on the tensile-strained LSM surface, contrary to the behavior at room temperature. Our findings illustrate the potential role and mechanism of lattice strain in tuning the reactivity of perovskite transition-metal oxides with oxygen in solid oxide fuel cell cathodes.Keywords: density functional theory; electronic structure; La0.7Sr0.3MnO3; LSM; perovskite oxide; scanning tunneling microscopy; segregation; strain; surface;

We present the mechanism and the extent of increase in the oxygen anion diffusivity in Y2O3 stabilized ZrO2 (YSZ) under biaxial lattice strain. The oxygen vacancy migration paths and barriers in YSZ as a function of lattice strain was assessed computationally using density functional theory (DFT) and nudged elastic band (NEB) method. Two competing and non-linear processes acting in parallel were identified to alter the migration barrier upon applied strain: (1) the change in the space, or electronic density, along the migration path, and (2) the change in the strength of the interatomic bonds between the migrating oxygen and the nearest neighbor cations that keep the oxygen from migrating. The increase of the migration space and the weakening of the local oxygen–cation bonds correspond to a decrease of the migration barrier, and vice versa. The contribution of the bond strength to the changes in the migration barrier is more significant than that of the opening of migration space in strained YSZ. A database of migration barrier energies as a function of lattice strain for a set of representative defect distributions in the vicinity of the migration path in YSZ was constructed. This database was used in kinetic Monte Carlo (KMC) simulations to estimate the effective oxygen diffusivity in strained YSZ. The oxygen diffusivity exhibits an exponential increase up to a critical value of tensile strain, or the fastest strain. This increase is more significant at the lower temperatures. At the strain states higher than the critical strain, the diffusivity decreases. This is attributed to the local relaxations at large strain states beyond a limit of elastic bond strain, resulting in the strengthening of the local oxygen–cation bonds that increases the migration barrier. The highest enhancement of diffusivity in 9%-YSZ compared to its unstrained state is 6.8 × 103 times at 4% strain and at 400 K. The results indicate that inducing an optimal strain state by direct mechanical load or by creating a coherent hetero-interface with lattice mismatch can enable desirably high ionic conductivity in YSZ at reduced temperatures. The insights gained here particularly on the nonlinear and competing consequences of lattice strain on the local bonding structure and charge transport process are of importance for tuning the ionic transport properties in a variety of solid-state conducting material applications, including but not limited to fuel cells.

Nuclear energy can be used as the primary energy source in centralized hydrogen production through high-temperature thermochemical processes, water electrolysis, or high-temperature steam electrolysis. Energy efficiency is important in providing hydrogen economically and in a climate friendly manner. High operating temperatures are needed for more efficient thermochemical and electrochemical hydrogen production using nuclear energy. Therefore, high-temperature reactors, such as the gas-cooled, molten-salt-cooled and liquid-metal-cooled reactor technologies, are the candidates for use in hydrogen production. Several candidate technologies that span the range from well developed to conceptual are compared in our analysis. Among these alternatives, high-temperature steam electrolysis (HTSE) coupled to an advanced gas reactor cooled by supercritical CO2 (S-CO2) and equipped with a supercritical CO2 power conversion cycle has the potential to provide higher energy efficiency at a lower temperature range than the other alternatives.

Researchers at the Massachusetts Institute of Technology in the US are developing techniques for applying strain to materials, in order to accelerate oxygen-reduction reactions for applications in solid oxide fuel cells and electrolysis cells.

•We investigated the encapsulation of Sr-90 in CSH and in 9 Å-tobermorite.•Strontium can be substituted for calcium in C–S–H and 9 Å-tobermorite.•The integrity of the silicate chains in both phases is maintained upon Sr-90 substitution.•Limited degradation of the mechanical properties is observed.•These results suggest that CSH is a good candidate for immobilizing Sr-90.Cementitious materials are considered to be a waste form for the ultimate disposal of radioactive materials in geological repositories. We investigated by means of atomistic simulations the encapsulation of strontium-90, an important radionuclide, in calcium–silicate–hydrate (C–S–H) and its crystalline analog, the 9 Å-tobermorite. C–S–H is the major binding phase of cement. Strontium was shown to energetically favor substituting calcium in the interlayer sites in C–S–H and 9 Å-tobermorite with the trend more pronounced in the latter. The integrity of the silicate chains in both cementitious waste forms were not affected by strontium substitution within the time span of molecular dynamics simulation. Finally, we observed a limited degradation of the mechanical properties in the strontium-containing cementitious waste form with the increasing strontium concentration. These results suggest the cement hydrate as a good candidate for immobilizing radioactive strontium.Graphical abstractDownload full-size image

The presence of interlaminar interstitial defects like hydrogen affects the mechanical properties of van der Waals-bonded layered materials such as transition metal chalcogenides. While the embrittling effect of hydrogen is well understood in metals, the impact of hydrogen defects on the mechanical behavior of layered chalcogenides remained unexplored. In this article, we use density functional calculations to reveal the influence of different hydrogen point defects on important mechanical metrics, including binding energies, elastic moduli and tensile and shear strengths of a prototypical ionic layered material, mackinawite, Fe1+xS. We find that one of the low-energy hydrogen defect structures, interlaminar molecular H2 interstitials, severely degrades the strength of inter-layer van der Waals interactions in the mackinawite crystal. This leads to a significant (over 80%) degradation in the mechanical properties of the mackinawite crystal and enables facile interlayer sliding and exfoliation. This finding suggests the mechanisms for cathodic exfoliation of transition metal chalcogenides like Fe1+xS, and presents a plausible mechanism for the poor protectiveness of layered passive films like mackinawite that undergo failure by spalling or delamination.

In this work, we demonstrate the mechanism by which electronic charge localization increases the chemical expansion coefficient in two model systems, CeO2−δ and BaCeO3−δ. Using Density Functional Theory calculations, we predict that this coefficient is increased by more than 70% when charge is fully localized, consistent with the observation that materials with a smaller degree of charge localization have smaller chemical expansion coefficients. This finding has important consequences for devising materials with smaller chemical expansion coefficients and for the reliability of the widely-used Shannon's ionic radii.

In the energy-structure paradigm, we analyzed the defects that can arise in tetragonal zirconium oxide (T-ZrO2) involving the hydrogen atom or the hydrogen molecule using density functional theory. Our results indicate that the dominant hydrogen defect under reducing conditions is , a complex formed between the hydride ion and a doubly charged oxygen vacancy. This result is consistent with the experimental observation that under reducing conditions, the solubility of hydrogen is proportional to the degree of hypostoichiometry of T-ZrO2. Under oxidizing conditions we found three different hydrogen defects, each predominating in a specific range of the chemical potential of electrons. Starting from the valence band top toward the conduction band bottom, these defects are the interstitial proton, , a complex formed between two hydrogen species and a zirconium vacancy with a net effective charge of (2−), , and finally a complex similar to the latter but with a net effective charge of (4−), . In the two hydrogens exist in the form of hydroxyl groups, while in they exist in the form of a hydrogen molecule. In addition, we found that up to three hydrogen species can favorably accumulate in a zirconium vacancy under oxidizing conditions. The clustering of hydrogen in cation vacancies can be a precursor for the deleterious effects of hydrogen on the mechanical properties and stability of metal oxides, in analogy with hydrogen embrittlement in metals. Finally we observed a red-shift and a blue-shift for the vibrational frequencies of all the hydroxyl groups and all the hydrogen molecules, respectively, in T-ZrO2 when compared to the gas phase frequencies. This is an important characteristic for guiding future experimental efforts to detect and identify hydrogen defects in T-ZrO2. The insights presented in this work advance our predictive understanding of the degradation behavior of T-ZrO2 as a corrosion resistant passive layer, as a gate dielectric and in biomedical applications.

We present a multi-scale approach to predict equilibrium defect concentrations across oxide/oxide hetero-interfaces. There are three factors that need to be taken into account simultaneously for computing defect redistribution around the hetero-interfaces: the variation of local bonding environment at the interface as epitomized in defect segregation energies, the band offset at the interface, and the equilibration of the chemical potentials of species and electrons via ionic and electronic drift-diffusion fluxes. By including these three factors from the level of first principles calculation, we build a continuum model for defect redistribution by concurrent solution of Poisson's equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. This model solves for and preserves the continuity of the electric displacement field throughout the interfacial core zone and the extended space charge zones. We implement this computational framework to a model hetero-interface between the monoclinic zirconium oxide, m-ZrO2, and the chromium oxide Cr2O3. This interface forms upon the oxidation of zirconium alloys containing chromium secondary phase particles. The model explains the beneficial effect of the oxidized Cr particles on the corrosion and hydrogen resistance of Zr alloys. Under oxygen rich conditions, the ZrO2/Cr2O3 heterojunction depletes the oxygen vacancies and the sum of electrons and holes in the extended space charge zone in ZrO2. This reduces the transport of oxygen and electrons thorough ZrO2 and slows down the metal oxidation rate. The enrichment of free electrons in the space charge zone is expected to decrease the hydrogen uptake through ZrO2. Moreover, our analysis provides a clear anatomy of the components of interfacial electric properties; a zero-Kelvin defect-free contribution and a finite temperature defect contribution. The thorough analytical and numerical treatment presented here quantifies the rich coupling between defect chemistry, thermodynamics and electrostatics which can be used to design and control oxide hetero-interfaces.

The hetero-interfaces between the perovskite (La1−xSrx)CoO3 (LSC113) and the Ruddlesden-Popper (La1−xSrx)2CoO4 (LSC214) phases have recently been reported to exhibit fast oxygen exchange kinetics. Vertically aligned nanocomposite (VAN) structures offer the potential for embedding a high density of such special interfaces in the cathode of a solid oxide fuel cell in a controllable and optimized manner. In this work, VAN thin films with hetero-epitaxial interfaces between LSC113 and LSC214 were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC214 domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC113 domains above 250 °C in 10−3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC214 with LSC113 also provides for a more stable cation chemistry at the surface of LSC214 within the VAN structure, as compared to single phase LSC214 films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC113 and LSC214 in the temperature range of 320–400 °C. The higher reactivity of the VAN surface to the oxygen reduction reaction is attributed to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation. The instability of the LSC113/214 hetero-structure surface chemistry at temperatures above 400 °C, however, was found to lead to degraded ORR kinetics. Thus, while VAN structures hold great promise for offering highly ORR reactive electrodes, efforts towards the identification of more stable hetero-structure compositions for high temperature functionality are warranted.

The fast kinetics of oxygen reduction reaction (ORR) at oxide hetero-structures made of La0.8Sr0.2CoO3 and (La0.5Sr0.5)2CoO4 (LSC113/214) attracted great interest to enable high performance cathodes for solid oxide fuel cells. The aim of this work is to uncover the underlying mechanism of fast ORR kinetics at the LSC113/214 system from a defect chemistry and electronic structure perspective. X-ray photoelectron spectroscopy with depth profiling was used to compare the reducibility of the Co cation and the valence band offset in the LSC113/214 multilayer (ML) and in single phase LSC113 and LSC214 films. At 250 °C, the Co 2p core-level photoelectron spectra showed the presence of Co2+ across the ML interfaces in both the LSC113 and LSC214 layers. While this is similar to the Co valence state of LSC113 single phase films, it is contrary to the single phase LSC214 films which had only the higher oxidation state of cobalt, Co3+. The greater reducibility of Co in LSC214 in the ML structure compared to that of Co in the LSC214 single phase film was attributed to electron donation and transfer of oxygen vacancies from LSC113 across the interfaces, and it is one mechanism to enhance the oxygen reduction activity at the LSC113/214 hetero-structure.

The adsorption energy of reactant molecules and reaction intermediates is one of the key descriptors of catalytic activity of surfaces and is commonly used as a metric in screening materials for design of heterogeneous catalysts. The efficacy of such screening schemes depends on the accuracy of calculated adsorption energies under reaction conditions. These adsorption energies can depend strongly on interactions between adsorbed molecules in the adlayer. However, these interactions are typically not accounted for in screening procedures that use DFT-based zero-coverage adsorption energies. Identifying the physical mechanisms behind these interactions is essential to model realistic catalyst surfaces under reaction conditions and to understand the dependence of adsorption energies on reaction parameters like surface strain and composition. This article describes a method to quantitatively resolve the observed inter-adsorbate interactions into various direct adsorbate–adsorbate interactions (i.e. Coulombic and steric) and surface-mediated interactions (i.e. adsorbate-induced surface relaxation and change in electronic structure) by combining density functional theory and cluster-expansion calculations of coverage-dependent adsorption energies. The approach is implemented on a model catalyst surface of FeS2(100) reacting with H2S molecules. We find that the adsorption energy of H2S molecules can be affected by over 0.55 eV by the repulsive inter-adsorbate interactions caused primarily by the adsorbate-induced changes to the electronic structure of the FeS2 surface. These interactions also show a strong monotonic dependence on surface strain, being three times stronger on compressively strained surfaces than on surfaces under tensile strain. The large magnitude of inter-adsorbate interactions as well as their strong dependence on lattice strain demonstrate the need for using coverage-dependent adsorption energies for more accurate screening, for example for strained catalytic systems like core–shell and overlayer structures.

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La2CoO4+δ. The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La2CoO4+δ. Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principles-based calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling–untilting of the CoO6 octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800–1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT- and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31–0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden–Popper structures.

Cation diffusion is an important rate-limiting process in the growth of pyrrhotite (Fe1−xS) in passivating films on steels exposed to sulfidic environments, and for proposed synthetic applications of Fe1−xS, for example single-phase magnetic switching devices. Above the Néel temperature TN of 315 °C, where Fe1−xS is paramagnetic and structurally disordered, iron self-diffusivity *DFe predictably follows a standard, established Arrhenius law with temperature. However, we report 57Fe tracer diffusion measurements below TN, obtained using secondary ion mass spectrometry (SIMS), that demonstrate a 100-fold reduction in diffusion coefficient as compared to the extrapolated, paramagnetic Arrhenius trend at 150 °C. The results can be described by a magnetic diffusion anomaly, where the vacancy migration energy for the spontaneously-magnetized cation sublattice is increased by approximately 40% over the paramagnetic state. These constitute the first set of consistent diffusivity data obtained in magnetic pyrrhotite, allowing more accurate prediction of pyrrhotite growth rates and determination of magnetic properties for synthetic devices.

Mechanisms by which lattice strain alters the oxygen reduction reaction (ORR) kinetics are important to understand in order to increase the ORR activity of solid oxide fuel cell cathodes. Here we assess the mechanistic and quantitative effects of strain on oxygen diffusion on the LaCoO3(LCO)(001) surface using density functional theory calculations. Planar tensile strain is found to reduce the migration barrier of oxygen vacancy anisotropically on the LCO(001) surface, inducing an enhanced mobility along the [10] direction and a suppressed mobility along the [110] direction. The increase of space around Co that the oxygen (vacancy) traverses with a curved path is the cause of the enhanced mobility along the [10]. The increasing octahedral distortions with planar tensile strain inhibit the migration of oxygen vacancy along the [110] direction. Furthermore, the mobility of the adsorbed oxygen atom is suppressed with increasing strain due to its stronger adsorption on the surface. On the basis of rate theory estimates, the significantly lower energy barrier for oxygen vacancy diffusion is expected to dominate the other degrading factors and actually accelerate the ORR kinetics on LCO(001) up to 3% strain. The insights obtained here are useful for designing strategies to control the desired anisotropic and uni-directional oxygen transport along strained hetero-interfaces.

We present the mechanism and the extent of increase in the oxygen anion diffusivity in Y2O3 stabilized ZrO2 (YSZ) under biaxial lattice strain. The oxygen vacancy migration paths and barriers in YSZ as a function of lattice strain was assessed computationally using density functional theory (DFT) and nudged elastic band (NEB) method. Two competing and non-linear processes acting in parallel were identified to alter the migration barrier upon applied strain: (1) the change in the space, or electronic density, along the migration path, and (2) the change in the strength of the interatomic bonds between the migrating oxygen and the nearest neighbor cations that keep the oxygen from migrating. The increase of the migration space and the weakening of the local oxygen–cation bonds correspond to a decrease of the migration barrier, and vice versa. The contribution of the bond strength to the changes in the migration barrier is more significant than that of the opening of migration space in strained YSZ. A database of migration barrier energies as a function of lattice strain for a set of representative defect distributions in the vicinity of the migration path in YSZ was constructed. This database was used in kinetic Monte Carlo (KMC) simulations to estimate the effective oxygen diffusivity in strained YSZ. The oxygen diffusivity exhibits an exponential increase up to a critical value of tensile strain, or the fastest strain. This increase is more significant at the lower temperatures. At the strain states higher than the critical strain, the diffusivity decreases. This is attributed to the local relaxations at large strain states beyond a limit of elastic bond strain, resulting in the strengthening of the local oxygen–cation bonds that increases the migration barrier. The highest enhancement of diffusivity in 9%-YSZ compared to its unstrained state is 6.8 × 103 times at 4% strain and at 400 K. The results indicate that inducing an optimal strain state by direct mechanical load or by creating a coherent hetero-interface with lattice mismatch can enable desirably high ionic conductivity in YSZ at reduced temperatures. The insights gained here particularly on the nonlinear and competing consequences of lattice strain on the local bonding structure and charge transport process are of importance for tuning the ionic transport properties in a variety of solid-state conducting material applications, including but not limited to fuel cells.