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Ab initio DFT+U analysis of oxygen transport in LaCoO3: the effect of Co3+ magnetic states
No full textAlthough solid oxide fuel cells (SOFCs) provide clean and efficient electricity generation, high operating temperatures (T > 800 °C) limit their widespread use. Lowering operating temperatures (600 °C < T < 800 °C) requires developing next-generation mixed ion-electron conducting (MIEC) cathodes that permit facile oxygen transport. One promising MIEC material, La1-xSrxCo1-yFeyO 3 (LSCF), can operate at intermediate temperatures, has a longer cell lifetime, and permits less expensive interconnect materials. However, the road to optimization of LSCF compositions for SOFC applications would benefit from fundamental, atomic-scale insight into how local chemical changes affect its oxygen ion conductivity. We provide this insight using ab initio density functional theory plus U (DFT+U) calculations to analyze the factors governing oxygen transport in the LSCF parent material LaCoO3. We show that oxygen diffusion in LaCoO3 depends strongly on the spin state of the Co3+ ions: in particular, low spin Co3+ promotes higher oxygen vacancy concentrations than other spin states. We also predict that different spin states of Co3+ significantly affect the oxygen ion migration barrier. Through electronic structure analysis, we uncover the fundamental details which govern oxygen diffusivity in LaCoO3
Ab initio evaluation of oxygen diffusivity in LaFeO3: the role of lanthanum vacancies
No full textSolid oxide fuel cells (SOFCs) are attractive for clean and efficient electricity generation, but high operating temperatures (T > 800 °C) limit their widespread usage. Oxygen ion conducting cathode materials (mixed ion-electron conductors, MIECs), such as La1−x Sr x Co1−y Fe y O3 (LSCF), enable lower T by reducing cathode polarization losses. Understanding how composition affects oxygen diffusion in LaFeO3 is vitally important for designing high-performance LSCF cathodes. To do this, we employ first-principles density functional theory plus U (DFT+U) calculations to show how lanthanum vacancies in LaFeO3 dramatically change the oxygen diffusion coefficient. Our ab initio results show that A-site substoichiometry is a viable route to increased oxygen diffusion and higher SOFC performance
Oxygen Transport in Perovskite-Type Solid Oxide Fuel Cell Materials: Insights from Quantum Mechanics
No full textGlobal advances in industrialization are precipitating increasingly rapid consumption of fossil fuel resources and heightened levels of atmospheric CO2. World sustainability requires viable sources of renewable energy and its efficient use. First-principles quantum mechanics (QM) studies can help guide developments in energy technologies by characterizing complex material properties and predicting reaction mechanisms at the atomic scale. QM can provide unbiased, qualitative guidelines for experimentally tailoring materials for energy applications.
This Account primarily reviews our recent QM studies of electrode materials for solid oxide fuel cells (SOFCs), a promising technology for clean, efficient power generation. SOFCs presently must operate at very high temperatures to allow transport of oxygen ions and electrons through solid-state electrolytes and electrodes. High temperatures, however, engender slow startup times and accelerate material degradation. SOFC technologies need cathode and anode materials that function well at lower temperatures, which have been realized with mixed ion–electron conductor (MIEC) materials.
Unfortunately, the complexity of MIECs has inhibited the rational tailoring of improved SOFC materials. Here, we gather theoretically obtained insights into oxygen ion conductivity in two classes of perovskite-type materials for SOFC applications: the conventional La1–xSrxMO3 family (M = Cr, Mn, Fe, Co) and the new, promising class of Sr2Fe2–xMoxO6 materials. Using density functional theory + U (DFT+U) with U–J values obtained from ab initio theory, we have characterized the accompanying electronic structures for the two processes that govern ionic diffusion in these materials: (i) oxygen vacancy formation and (ii) vacancy-mediated oxygen migration. We show how the corresponding macroscopic oxygen diffusion coefficient can be accurately obtained in terms of microscopic quantities calculated with first-principles QM.
We find that the oxygen vacancy formation energy is a robust descriptor for evaluating oxide ion transport properties. We also find it has a direct relationship with (i) the transition metal–oxygen bond strength and (ii) the extent to which electrons left behind by the departing oxygen delocalize onto the oxygen sublattice. Design principles from our QM results may guide further development of perovskite-based MIEC materials for SOFC applications
Oxide ion transport in Sr2Fe1.5Mo0.5O6-delta, a mixed ion-electron conductor: new insights from first principles modeling
No full textWe use ab initio density functional theory + U calculations to characterize the oxide ion diffusion process in bulk Sr2Fe1.5Mo0.5O6-d (SFMO) by analyzing the formation and migration of oxygen vacancies. We show that SFMO's remarkable ionic conductivity arises from its intrinsic content of oxygen vacancies and a predicted very low migration barrier of such vacancies. Theoretical analysis of the electronic structure reveals a crucial role played by strongly hybridized Fe 3d/O 2p states to achieve the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. We predict a next-nearest-neighbor-type migration pathway for the O2- ion should dominate. The low energy barrier of this pathway is mainly related to electrostatic interactions with homogeneously distributed Mo in the SFMO sublattice. We identify the reasons why Fe-rich perovskites, with the key addition of a certain concentration of Mo, produce excellent electronic and ionic transport properties so crucial for efficient operation of intermediate temperature solid oxide fuel cells
First-Principles Study of Lanthanum Strontium Manganite: Insights into Electronic Structure and Oxygen Vacancy Formation
No full textWe characterize the structural, electronic, and defect behavior of La1–xSrxMnO3 (LSM) (xSr = 0.0, 0.25, and 0.5) by means of density functional theory + U (DFT+U) and hybrid DFT methods. Aliovalent substitution of Sr2+ for La3+ induces formation of holes in the LSM electronic structure. These holes affect electron and oxide ion transport, two key processes occurring within LSM when used as a solid oxide fuel cell (SOFC) cathode. To improve fundamental understanding of these processes, we investigated the atomic-scale effects of increasing Sr content and two different Mn magnetic moment alignments. In agreement with low-temperature experiments, we find a metallic, ferromagnetic (FM) electronic ground state with holes delocalized across the Mn and O sublattices. We also employ an antiferromagnetic (AFM) arrangement of Mn ions to model LSM’s high-temperature paramagnetic state. In contrast to FM LSM, the holes in AFM LSM localize to form Mn4+ ions, consistent with the observed high-temperature polaronic transport in LSM. The formation of oxygen vacancies governs oxide ion transport in bulk LSM. We find that the ease with which oxygen vacancies form is strongly influenced by the Sr content and the overall magnetic arrangement of Mn ions. These atomic-scale insights enable us to propose new guidelines for enhanced nanoscale LSM-based SOFC cathodes
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Ab Initio DFT plus U Analysis of Oxygen Vacancy Formation and Migration in La1-xSrxFeO3-delta (x=0, 0.25, 0.50)
No full textIncorporating mixed oxygen-ion-electron conducting (MIEC) cathode materials is a promising strategy to make intermediate-temperature solid oxide fuel cells (IT-SOFCs) viable; however, a lack of fundamental understanding of oxygen transport in these materials limits their development. Density functional theory plus U (DFT+U) calculations are used to investigate how the Sr concentration affects the processes that govern oxygen ion transport in La1-xSrxFeO3-delta (LSF, x = 0, 0.25, and 0.50). Specifically, we show that oxygen vacancies compensate holes introduced by Sr and that this compensation facilitates oxygen vacancy formation in LSF. We also find that oxygen migration in LaFeO3 is accompanied by electron transfer in the opposite direction. Our results explicitly identify and clarify the role of electron-deficient substitutions in promoting oxygen diffusion in LSF. This atomic level insight is important for enabling rational design of iron-based SOFC cathode materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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