“Stretching” the energy landscape of oxides—Effects on electrocatalysis and diffusion

Elastic strain engineering offers a new route to enable high-performance catalysts, electrochemical energy conversion devices, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of reactions involved in the material can be altered. This is the so-called mechano-chemical coupling. Here we discuss how elastic strain activates reactions on metals and oxides. We also present analogies to strained polymer reactions. A rich set of investigations have been performed on strained metal surfaces over the last 15 years, and the mechanistic reasons behind strain-induced reactivity are explained by an electronic structure model. On the other hand, the potential of strain engineering of oxides for catalytic and energy applications has been largely underexplored. In oxides, mechanical stress couples to reaction and diffusion kinetics by altering the oxygen defect formation enthalpy, migration energy barrier, adsorption energy, dissociation barrier, and charge transfer barrier. A generalization of the principles for stress activated reactions from polymers to metals to oxides is offered, and the prospect of using elastic strain to tune reaction and diffusion kinetics in functional oxides is discussed.National Science Foundation (U.S.) (NSF CAREER award, Division of Materials Research, Ceramics Program, Grant No.1055583))United States. Dept. of Energy (Office of Science, Basic Energy Sciences, Grant No. DE-SC0002633

Role of lattice strain and defect chemistry on the oxygen vacancy migration at the (8.3%Y2O3-ZRO2)/SrTiO3 hetero-interface: A first principles study

We report on the mechanism and the upper limits in the increase of oxygen ion conductivity at oxide hetero-interfaces, particularly the 8.3%Y2O3-ZrO2/SrTiO3 (YSZ/STO) as a model interface. We consider two factors contributing to the increase in ionic conductivity at or near the interface: 1) a favorable strain state to shift and/or change the symmetry of electron energy levels to provide improved charge transfer and mobility. 2) the alteration of the defect chemistry to enhance the density and distribution of oxygen vacancies. First principles and Kinetic Monte-Carlo simulations were performed to identify the atomic-scale nature of the hetero-interface and the oxygen vacancy migration barriers and diffusivity. Our results suggest that the modulation in both the lattice strain and the defect chemistry due to the YSZ/STO interface can enhance the ionic conductivity in YSZ up to six orders of magnitude by reducing the migration barrier and increasing the oxygen vacancy concentration, respectively

Factors that Influence Cation Segregation at the Surfaces of Perovskite Oxides

As the oxygen reduction reaction (ORR) becomes more critical for development of solid oxide fuel cells (SOFCs) that operate at 500-700 °C, the correlation between the surface chemistry and electrochemical performance is important to understand and enable design of cathode materials with optimal surface chemistry. Recently we demonstrated that elastic and electrostatic interactions of the dopant with the host lattice drive dopant segregation, a detrimental process on the surface of perovskite cathodes [1]. Motivated by those results, here we investigated the effects of A-site stoichiometry in La0.8Sr0.2MnO3 (LSM) thin films on the surface chemistry and electrochemical activity. Angle-resolved X-ray photoelectron spectroscopy was employed to identify the surface cation content and chemical bonding states. A-site deficient LSM films showed higher chemical stability against Sr segregation and secondary phase formation upon annealing. This was correlated with a higher electrochemical activity measured by AC impedance spectroscopy. Given the insulating nature of secondary phases created on the surface upon annealing, observed higher electrochemical stability in A-site deficient LSM films can be ascribed to the suppressed surface segregation and phase separation.United States. Dept. of Energy (Office of Fossil Energy, Grant No. DE-NT0004117

Stabilizing single atoms and a lower oxidation state of Cu by a ½[110]{100} edge dislocation in Cu-CeO₂

Stabilizing atomically dispersed catalytic metal species at surfaces is a significant challenge for obtaining high-performance single atom catalysts. This is because of the strong tendency for the dispersed metal atoms to agglomerate. We propose that dislocations can provide a strong anchor for stabilizing single atoms. A ½[110]{100} edge dislocation in Cu doped ceria, Cu-CeO₂, is investigated as a model system with density functional theory. The defect formation energies are found to be lower at the dislocation core, with a large segregation energy ranging within 0.8–2.5 eV depending on the site and species at the dislocation core. The high segregation energy indicates that the edge dislocations can enrich Cu defects in an atomically sized area and, thus, have a potential to strongly anchor single atom species at surfaces. Moreover, the edge dislocation also stabilizes reduced cation species, Cu (1+) and Ce (3+). The more reduced dislocation core can offer high concentration of oxygen vacancy as well as in-gap electronic states which provide more reactivity for surface reactions.United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0002633

The Effect of Group Counseling Program on Dealing with School Exhaustion of 7th Grade Students

In this research, the effect of Group Counseling Program on coping with school exhaustion was investigated. The study was conducted with 7th grade students attending Private Bogazici Educational Center in Bakirkoy, Istanbul during 2016-2017 academic year. “the School Burnout Scale for the Second Level of Primary Education”, which was developed by Aypay (2011) was used to assess school exhaustion levels. Sixteen (8 males and 8 females) out of 96 students with high school exhaustion levels and willing to participate in the study was selected randomly. 8 of them were randomly assigned as experimental group (4 males and 4 females) and the other 8 students were randomly assigned as control group. In this research, pre-test post-test control group design was used. In the study, group counseling was applied to the experimental group for 8 weeks and no treatment was given to the control group. For the analysis of the data, Mann-Whitney-U test was used for intra-group comparison; and Wilcoxon test was used for inter-group comparison. As a result, it was found that group counseling program was effective in decreasing school exhaustion; however, it was not effective in family originated exhaustion and school inadequacy, which are sub-dimensions of school exhaustion. The results showed that there was a significant difference between pre-test and post-test scores of total exhaustion scores. Obtained data was interpreted and recommendations were given in the scope of the literature. Keywords: school exhaustion, group counseling, 7th grade student

Edge dislocation slows down oxide ion diffusion in doped CeO2 by segregation of charged defects

Strained oxide thin films are of interest for accelerating oxide ion conduction in electrochemical devices. Although the effect of elastic strain has been uncovered theoretically, the effect of dislocations on the diffusion kinetics in such strained oxides is yet unclear. Here we investigate a 1/2{100} edge dislocation by performing atomistic simulations in 4–12% doped CeO₂ as a model fast ion conductor. At equilibrium, depending on the size of the dopant, trivalent cations and oxygen vacancies are found to simultaneously enrich or deplete either in the compressive or in the tensile strain fields around the dislocation. The associative interactions among the point defects in the enrichment zone and the lack of oxygen vacancies in the depletion zone slow down oxide ion transport. This finding is contrary to the fast diffusion of atoms along the dislocations in metals and should be considered when assessing the effects of strain on oxide ion conductivity.United States. Department of Energy (DE-SC0002633)National Science Foundation (U.S.) (TG-DMR110004)National Science Foundation (U.S.) (TG-DMR120025

Interstitialcy diffusion of oxygen in tetragonal La₂CoO_4+δ

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

Surface Chemistry and Non-Stoichiometry of Nd2NiO4+ Epitaxial Thin Films with Different Orientation and Strain

The influence of lattice strain on non-stoichiometry and surface chemical composition was investigated for epitaxial Nd2NiO4+ä (NNO) films during annealing in ultra high vacuum (below 10[superscript -8] mbar) and temperatures of up to 700oC. (100)- and (001)-oriented films with tensile and compressive lattice strain along c-axis were fabricated using pulsed laser deposition method. A significant decrease in the c-lattice parameter during annealing was found by x-ray diffraction (XRD) for the tensile strained films. X-ray photoelectron spectroscopy (XPS) showed that Ni reduction during annealing takes place only in compressively strained films, indicating the lower content of oxygen interstitials. A lower interstitial content in the compressively strained NNO films is consistent with the smaller c-lattice parameter measured by XRD and the easier reducibility of Ni measured by XPS. Cation segregation and morphological changes were found only for the compressively strained film surfaces. These results show that lattice strain along the c-axis is an important parameter that can alter the surface chemistry, and thus the oxygen exchange kinetics, on Nd2NiO4+ä at elevated temperatures.National Science Foundation (U.S.) (Division of Materials Research, Ceramics Program, CAREER award

Electro-chemo-mechanical effects of lithium incorporation in zirconium oxide

Understanding the response of functional oxides to extrinsic ion insertion is important for technological applications including electrochemical energy storage and conversion, corrosion, and electronic materials in neuromorphic computing devices. Decoupling the complicated chemical and mechanical effects of ion insertion is difficult experimentally. In this work, we assessed the effect of lithium incorporation in zirconium oxide as a model system, by performing first-principles based calculations. The chemical effect of lithium is to change the equilibria of charged defects. Lithium exists in ZrO_{2} as a positively charged interstitial defect, and raises the concentration of free electrons, negatively charged oxygen interstitials, and zirconium vacancies. As a result, oxygen diffusion becomes faster by five orders of magnitude, and the total electronic conduction increases by up to five orders of magnitude in the low oxygen partial pressure regime. In the context of Zr metal oxidation, this effect accelerates oxide growth kinetics. In the context of electronic materials, it has implications for resistance modulations via ion incorporation. The mechanical effect of lithium is in changing the volume and equilibrium phase of the oxide. Lithium interstitials together with zirconium vacancies shrink the volume of the oxide matrix, release the compressive stress that is needed for stabilizing the tetragonal phase ZrO_{2} at low temperature, and promote tetragonal-to-monoclinic phase transformation. By identifying these factors, we are able to mechanistically interpret experimental results in the literature for zirconium alloy corrosion in different alkali-metal hydroxide solutions. These results provide a mechanistic and quantitative understanding of lithium-accelerated corrosion of zirconium alloy, as well as, and more broadly, show the importance of considering coupled electro-chemo-mechanical effects of cation insertion in functional oxides