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Abstract

The winner of the 2023 TPS Best Paper Award has been selected (please refer to our TPS home page for details about the award at https://ieee-npss.org/publications/transactions-on-plasma-science/). This year is the fifth year that the award is being given, and I am pleased to announce that this year’s winner is the paper, “Effect of Nonuniform Emission on Miram Curves” Published in IEEE TRANSACTIONS ON PLASMA SCIENCE, Volume: 48, Issue: 1, January 2020, Page(s): 146–155. The nine coauthors of this paper are David Chernin, John J. Petillo, and Serguei Ovtchinnikov from Leidos, Inc., Reston, VA, USA; Y. Y. Lau and Abhijit Jassem from the University of Michigan, Ann Arbor, MI, USA; and Dongzheng Chen, Ryan Jacobs, Dane Morgan, and John H. Booske from the University of Wisconsin–Madison, Madison, WI, USA. The abstract of the paper and the photos and biosketches of the coauthors are given as follows. The award plaques, certificates, and award checks have been sent to the coauthors by IEEE. This paper is an open access paper, freely available to all our readers. Congratulations to the team of coauthors David Chernin, John J. Petillo, Serguei Ovtchinnikov, Y. Y. Lau, Abhijit Jassem, Dongzheng Chen, Ryan Jacobs, Dane Morgan, and John H. Booske on this accomplishment.

Recent experimental thermionic emission characterization measurements of the perovskite oxide SrVO3 demonstrated that low work functions can be achieved with intrinsic surface dipoles on monolithic polar perovskites, while recent density functional theory (DFT) calculations suggest that many perovskites in addition to SrVO3 may also show low work function. In this investigation, we studied the thermionic emission behavior of another perovskite suggested from DFT, BaMoO3, together with its related physical properties. The temperature limited emission current density increases and then saturates with increasing voltage, consistent with patch field theory. The overall effective work functions are 2.6–2.7 eV, comparable to other thermionic emitters like LaB6, but much higher than the 1.0 eV DFT-predicted lowest work function. We attribute this discrepancy to patch field effects caused by nanoscale features decorating individual surface facets. The resulting heterogeneity yields thermionic emission characteristic of an effective work function higher than the lowest local facet work functions. Additionally, the material shows some instability when operating at high temperatures over 1200°C. Nevertheless, BaMoO3 shows emission behavior comparable to LaB6 when operated at temperatures of $< 1200~^{\circ }\text{C}$ , and may find use as a vacuum electron source in vacuum electronic applications such as electron microscopes and electron beam writers.

In a current density versus temperature ( ${J}$ – ${T}{)}$ (Miram) curve in thermionic electron emission, experimental measurements often yield a smooth transition between the exponential region and the saturated emission regions, which is sometimes referred to as the “roll-off” or “Miram curve knee.” The shape of the Miram curve knee is an important figure of merit for thermionic vacuum cathodes. Specifically, cathodes with a sharp Miram curve knee at low temperature with a flat saturated emission current are typically preferred. Our previous work on modeling nonuniform thermionic emission revealed that the space charge effect and the patch field effect are key pieces of physics, which set the Miram curve shape. This work provides a more complete understanding of the physical factors connecting these physical effects and their relative impact on the shape of the knee. For our analyses, we use a periodic, equal-width striped (“zebra crossing”) work function distribution as a model system and illustrate how the space charge and patch field effects restrict the emission current density near the Miram curve knee. Our results indicate that there are three main physical parameters that significantly impact the shape of the Miram curve: 1) the normalized work function difference; 2) the patch-diode size ratio; and 3) the patch-diode field ratio. These parameters relate the patch size, work function values, anode–cathode voltage, and anode–cathode gap distance to the shape of the Miram curve, providing new understanding and a guide to the design of thermionic cathodes used as electron sources in vacuum electronic devices.

The transition from fossil fuels to renewable energy requires the development of efficient and cost-effective energy storage technologies. A promising way forward is to harness the energy of intermittent renewable sources, such as solar and wind, to perform (electro)catalytic reactions to generate fuels, thus storing energy in the form of chemical bonds. However, current catalysts rely on the use of expensive, rare, or geographically localized elements, such as platinum. Widespread adoption of new (electro)catalytic technologies hinges on the discovery and development of materials containing earth-abundant elements, which can efficiently catalyze an array of (electro)chemical reactions.

LaB6has been used as a commercial electron emitter for decades. Despite the large number of studies on the work function of LaB6, there is no comprehensive understanding of work function trends in the hexaboride material family. In this study, we use density functional theory calculations to calculate trends of rare-earth hexaboride work function and rationalize these trends based on the electronegativity of the metal element. We predict that alloying LaB6with Ba can further lower the work function by ∼0.2 eV. Interestingly, we find that alloyed (La, Ba)B6can have lower work functions than either LaB6or BaB6, benefiting from an enhanced surface dipole due to metal element size mismatch. In addition to hexaborides, we also investigate work function trends of similar material families, namely, tetraborides and transition metal nitrides, which, like hexaborides, are electrically conductive and refractory and thus may also be promising materials for electron emission applications. We find that tetraborides consistently have higher work functions than their hexaboride analogues as the tetraborides have less ionic bonding and smaller positive surface dipoles. Finally, we find that HfN has a low work function of about 2.2 eV, making HfN a potentially promising new electron emitter material.

Abstract

Radiation-induced segregation is well known in metals, but has been rarely studied in ceramics. We discover that radiation can induce notable segregation of one of the constituent elements to grain boundaries in a ceramic, despite the fact that the ceramic forms a line compound and therefore has a strong thermodynamic driving force to resist off-stoichiometry. Specifically, irradiation of silicon carbide at 300 °C leads to carbon enrichment near grain boundaries, whereas the enrichment diminishes for irradiation at 600 °C. The temperature dependence of this radiation-induced segregation is different from that shown in metallic systems. Using an ab initio informed rate theory model, we demonstrate that this difference is introduced by the unique defect energy landscapes present in the covalent system. Additionally, we discover that grain boundaries in unirradiated silicon carbide grown by chemical vapour deposition are intrinsically carbon-depleted. The inherent grain boundary chemistry and its evolution under radiation are both critical for understanding the many properties of ceramics associated with grain boundaries.

Advances in machine learning have impacted myriad areas of materials science, such as the discovery of novel materials and the improvement of molecular simulations, with likely many more important developments to come. Given the rapid changes in this field, it is challenging to understand both the breadth of opportunities and the best practices for their use. In this review, we address aspects of both problems by providing an overview of the areas in which machine learning has recently had significant impact in materials science, and then we provide a more detailed discussion on determining the accuracy and domain of applicability of some common types of machine learning models. Finally, we discuss some opportunities and challenges for the materials community to fully utilize the capabilities of machine learning.

During aqueous corrosion, atoms in the solid react chemically with oxygen, leading either to the formation of an oxide film or to the dissolution of the host material. Commonly, the first step in corrosion involves an oxygen atom from the dissociated water that reacts with the surface atoms and breaks near-surface bonds. In contrast, hydrogen on the surface often functions as a passivating species. Here, we discovered that the roles of O and H are reversed in the early corrosion stages on a Si-terminated SiC surface. O forms stable species on the surface, and chemical attack occurs by H that breaks the Si–C bonds. This so-called hydrogen scission reaction is enabled by a newly discovered metastable bridging hydroxyl group that can form during water dissociation. The Si atom that is displaced from the surface during water attack subsequently forms H2SiO3, which is a known precursor to the formation of silica and silicic acid. This study suggests that the roles of H and O in oxidation need to be reconsidered.

Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in situ synchrotron wide-angle X-ray scattering and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape: rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveal that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding the phase–structure–property relationship, guiding future design and synthesis of nanoparticles for promising applications.

The kinetics of chemical reactions occurring during the metal–organic vapor phase epitaxy (MOVPE) of GaAs1–yBiyhave been studied using density functional theory (DFT). GaAs1–yBiyis a metastable semiconductor alloy that has potential applications in high-performance long-wavelength emitters. Its growth is complicated by the low solubility of Bi within the GaAs lattice, which leads to phase segregation under conventional III–V semiconductor growth conditions. In this study, the thermochemical and kinetic parameters of the gas-phase pyrolysis and surface reactions occurring in the MOVPE growth of GaAs1–yBiyfrom trimethyl bismuth, tertiary butyl arsine, and triethyl gallium are calculated from first-principles electronic structure and vibrational mode calculations. These calculations indicate that the pyrolysis products AsH2and Bi(CH3)2are the principle sources for the deposition of their respective metallic elements. The surface-adsorbed methyl species and their interaction with the gas-phase pyrolysis products lead to the self-limiting growth described within this model. The calculated thermochemical and kinetic values provide initial parameters for the development of a microkinetic model of GaAs1–yBiydeposition.

We have developed a machine learning model for critical cooling rates for metallic glasses based on computational properties, supporting in-silico screening for desired Rcvalues and significantly reducing reliance on time-consuming laboratory work. We compare results for features derived from easy-to-compute functions of elemental properties to more complex physically motivated properties using ab initio, machine-learning potentials, and empirical potential molecular dynamics methods. The established approach enables property acquisition across a diverse range of alloys. Analysis of various features for 34 alloys from 20 chemical systems shows that the best model for critical cooling rates was learned from one elemental property-based feature and three simulated features. The elemental property based feature is an ideal entropy value based on alloy stoichiometry. The simulated features were acquired from estimates of energies above the convex hull, changes in heat capacity, and the fraction of icosahedra-like Voronoi polyhedra. Models were assessed through a demanding cross validation test based on repeatedly leaving out full chemical systems as test sets and had an R2of 0.78 and a mean average error of 0.76 in units of log10(K/s). We demonstrate with Shapley additive explanation analysis that the most impactful features have physically reasonable influence on model predictions. The established methodology can be applied to other high-throughput studies of material properties of diverse compositions.

Two-dimensional (2D) ZnO nanosheets with highly concentrated Zn vacancies (VZn) of up to approximately 33% were synthesized by ionic layer epitaxy at the water–toluene interface. This high cation vacancy concentration is unprecedented for ZnO and may provide unique opportunities to realize exotic properties not attainable in the conventional bulk form. After annealing, the nanosheets showed characteristic magnetic hysteresis with saturation magnetization of 57.2 emu/g at 5 K and 50.9 emu/g at room temperature. This value is 1 order of magnitude higher than other ZnO nanostructures and comparable to the conventional ferrimagnetic Fe3O4. Density functional theory calculations, with the support of experimental results, suggest that a high concentration of VZn(approximately one-third of the Zn sites) can form spontaneously during synthesis when stabilized by H ions, and the formation of VZncould be further facilitated by the presence of grain boundaries. It is essential to remove the H for the nanosheets to show ferromagnetism. The mechanisms identified for the origin of the high magnetism in ZnO nanosheets presents an intriguing example of a kinetically stabilized, non-equilibrium, highly defective 2D nanomaterial with a significantly enhanced physical property.

Valleyite, Ca4(Fe,Al)6O13, is a new sodalite-type mineral discovered in late Pleistocene basaltic scoria from the Menan Volcanic Complex near Rexburg, Idaho, U.S.A. It is an oxidation product of basaltic glass during the early stage of the scoria formation and is associated with hematite (α-Fe2O3), maghemite (γ-Fe2O3), luogufengite (ε-Fe2O3), and quartz on the surface of vesicles. The measured crystal size of valleyite ranges from ~250 to ~500 nm. The empirical chemical formula of valleyite is (Ca3.61Mg0.39)(Fe3.97Al1.91Ti0.09)O13. The mineral has a space group of I4̅3m. The (Fe,Al)-O bond distance and unit-cell edge are slightly larger than those reported for synthetic Ca4Al6O13, presumably due to the presence of the larger Fe3+cations, compared with Al3+, in the structure. Density functional theory calculations predict that valleyite may be a metastable phase at low temperatures. Measured Curie temperatures for valleyite and luogufengite are 645 and 519 K, respectively. Their magnetization hysteresis loop indicates the magnetic exchange coupling between valleyite (soft magnet) and luogufengite (hard magnet) that aids in the understanding of magnetic properties and paleo-magnetism of basaltic rocks. This new mineral, valleyite, with the sodalite-type cage structure is potentially a functional magnetic material.

We have synthesized pyrite-type PtO2(py-PtO2) at 50–60 GPa and successfully recovered it at 1 bar. The observed O–O stretching vibration in Raman spectra provides direct evidence for inter-oxygen bonding in the structure. We also identified the O–H vibrations in py-PtO2synthesized from the low-temperature areas, indicating hydrogenation, py-PtO2Hx(x≤ 1). Diffraction patterns are consistent with a range of degrees of hydrogenation controlled by temperature. We found that py-PtO2has a high bulk modulus, 314 ± 4 GPa. The chemical behaviors found in py-PtO2have implications for the hydrogen storage in materials with anion–anion bonding, and the geochemistry of oxygen, hydrogen, and transition metals in the deep planetary interiors.

Abstract

Electronic structure descriptors are computationally efficient quantities used to construct qualitative correlations for a variety of properties. In particular, the oxygen p-band center has been used to guide material discovery and fundamental understanding of an array of perovskite compounds for use in catalyzing the oxygen reduction and evolution reactions. However, an assessment of the effectiveness of the oxygen p-band center at predicting key measures of perovskite catalytic activity has not been made and would be highly beneficial to guide future predictions and codify best practices. Here, we have used density functional theory at the Perdew–Burke–Ernzerhof (PBE), PBEsol, PBE + U, strongly constrained and appropriately normed functional, and Heyd–Scuseria–Ernzerhof (HSE06) levels to assess the correlations of numerous measures of catalytic performance for a series of technologically relevant perovskite oxides, using the bulk oxygen p-band center as an electronic structure descriptor. We have analyzed correlations of the calculated oxygen p-band center for all considered functionals with the experimentally measured X-ray emission spectroscopy oxygen p-band center and multiple measures of catalytic activity, including high-temperature oxygen reduction surface exchange rates, aqueous oxygen evolution current densities, and binding energies of oxygen evolution intermediate species. Our results show that the best correlations for all measures of catalytic activity considered here are made with PBE-level calculations, with strong observed linear correlations with the bulk oxygen p-band center (R2= 0.81–0.87). This study shows that strong linear correlations between numerous important measures of catalytic activity and the oxygen p-band bulk descriptor can be obtained under a consistent computational framework, and these correlations can serve as a guide for future experiments and simulations for development of perovskite and related oxide catalysts.

The development of Ruddlesden–Popper oxides as oxygen exchange and transport materials for applications such as solid oxide fuel cells, oxygen separation membranes, and chemical looping will benefit from a detailed mechanistic understanding of how oxygen is transported through these materials. Using density functional theory, we found that there are two distinct oxygen interstitial diffusion mechanisms involving two different oxygen interstitial species that can be active in La2–xSrxNiO4+δand, we believe, in hyperstoichiometric Ruddlesden–Popper oxides in general. The first mechanism is the previously proposed interstitialcy-mediated mechanism, which consists of diffusing oxide interstitials. The second mechanism is newly discovered in this work and consists of both oxide and peroxide interstitial diffusing species. This mechanism exhibits a similar or possibly lower migration barrier than the interstitialcy mechanism for high oxidation states. Which mechanism contributes to the oxygen interstitial diffusion is the result of the change in relative stability between the oxide interstitial (2–charge) and peroxide interstitial (1–charge), which directly affects the migration barriers for these two different mechanisms. The stability of the oxide and peroxide and therefore the competition between the two oxygen diffusion mechanisms is highly sensitive to the overall oxidation state of the system. Therefore, the oxygen diffusion mechanism is a function of the material composition, oxygen off-stoichiometry, operating temperature, and oxygen partial pressure. We also examined the effect of epitaxial strain on both oxygen diffusion mechanisms and found that tensile and compressive epitaxial strain of up to 2% had less than 100 meV/(% strain) effects on oxygen interstitial formation, migration, and activation energies and that total achievable activation energy reductions are likely less than 100 meV for up to ±2% epitaxial strain. The presented understanding of factors governing interstitial oxygen diffusion potentially has significant implications for the engineering of Ruddlesden–Popper oxides in numerous alternative energy technologies.

Activation barriers associated with ion diffusion and chemical reactions are vital to understand and predict a wide range of phenomena, such as material growth, ion transport, and catalysis. In the calculation of activation barriers for non-redox processes in semiconductors and insulators, it has been widely assumed that the charge state remains fixed to that of the initial electronic ground state throughout a dynamical process. In this work, we demonstrate that this assumption is generally inaccurate and that a rate-limiting transition state can have a different charge state from the initial ground state. This phenomenon can significantly lower the activation barrier of a dynamical process that depends strongly on charge state, such as carbon vacancy diffusion in 4H-SiC. With inclusion of such transition state redox, the activation barrier varies continuously with Fermi level, in contrast to the step-line feature predicted by the traditional fixed-charge assumption. In this study, a straightforward approach to include the transition state redox effect is provided, the typical situations where the effect plays a significant role are identified, and the relevant electron dynamics are discussed. Modeling of reaction dynamics can be improved by rethinking a traditional belief about the charge of a transition state. Chemists usually assume that for a non-redox reaction, the state with the highest energy along a reaction path, or the transition state, has the same charge as the initial state. But Guangfu Luo, Thomas Kuech and Dane Morgan from the University of Wisconsin-Madison in the USA have now demonstrated that a transition state with unfixed charge is more realistic for dynamical processes involving semiconductors or insulators. Through atomistic simulations of dynamics, such as vacancy diffusion in gallium arsenide, the team reports that allowing a change of charge during a dynamical process can reduce its energy barrier relative to that predicted by the traditional approach and predict a more realistic continuous variation of energy barrier with Fermi level. Ab initiocalculations reveal that the rate-limiting transition state during a dynamical process in a semiconductor or an insulator can have a different charge state from that of the initial ground state, rather than the same charge state as generally assumed. Such “transition state redox” effect may significantly lower the activation barrier and can be realized through the hopping of an initial excited state and possibly an electron exchange with the bulk during the dynamical process.

The slow kinetics of the oxygen reduction reaction (ORR) is one of the key challenges in developing high performance energy devices, such as solid oxide fuel cells. Straining a film by growing on a lattice-mismatched substrate has been a conventional approach to enhance the ORR activity. However, due to the limited choice of electrolyte substrates to alter the degree of strain, a systematic study in various materials has been a challenge. Here, we explore the strain modulation of the ORR kinetics by growing epitaxial La0.6Sr0.4CoO3−δ(LSCO) films on yttria-stabilized zirconia substrates with the film thickness below and above the critical thickness for strain relaxation. Two orders of magnitude higher ORR kinetics is achieved in an ultrathin film with ∼0.8% tensile strain as compared to unstrained films. Time-of-flight secondary ion mass spectrometry depth profiling confirms that the Sr surface segregation is not responsible for the enhanced ORR in strained films. We attribute this enhancement of ORR kinetics to the increase in oxygen vacancy concentration in the tensile-strained LSCO film owing to the reduced activation barrier for oxygen surface exchange kinetics. Density functional theory calculations reveal an upshift of the oxygen 2p-band center relative to the Fermi level by tensile strain, indicating the origin of the enhanced ORR kinetics.

Electrolyte decomposition reactions on Li-ion battery electrodes contribute to the formation of solid electrolyte interphase (SEI) layers. These SEI layers are one of the known causes for the loss in battery voltage and capacity over repeated charge/discharge cycles. In this work, density functional theory (DFT)-based ab initio calculations are applied to study the initial steps of the decomposition of the organic electrolyte component ethylene carbonate (EC) on the (101̅4) surface of a layered Li(Nix,Mny,Co1-x-y)O2(NMC) cathode crystal, which is commonly used in commercial Li-ion batteries. The effects on the EC reaction pathway due to dissolved Li+ions in the electrolyte solution and different NMC cathode surface terminations containing adsorbed hydroxyl −OH or fluorine −F species are explicitly considered. We predict a very fast chemical reaction consisting of an EC ring-opening process on the bare cathode surface, the rate of which is independent of the battery operation voltage. This EC ring-opening reaction is unavoidable once the cathode material contacts with the electrolyte because this process is purely chemical rather than electrochemical in nature. The −OH and −F adsorbed species display a passivation effect on the surface against the reaction with EC, but the extent is limited except for the case of −OH bonded to a surface transition metal atom. Our work implies that the possible rate-limiting steps of the electrolyte molecule decomposition are the reactions on the decomposed organic products on the cathode surface rather than on the bare cathode surface.

In this work, the diffusion barriers of A-site cation and dopants in bulk LaMnO3+-d (LMO) are determined based on density functional theory (DFT) simulations. These properties are highly relevant for solid oxide fuel cell (SOFC) cathode applications. Due to the weak repulsive interactions (0.2~0.3 eV) between the nearest neighbor A-site and B-site cation vacancies, the A-site and B-site cation vacancy clusters can readily form in LMO. The DFT modeling unveils a facile A-site cation diffusion pathway via a cation vacancy cluster mechanism with a migration barrier reduction of about 1.3 eV versus the direct A-site vacancy migration. By combining these results with the bulk LMO defect modeling, the temperature and oxygen partial pressure dependences of the A-site cation tracer diffusion coefficients are assessed based on the concentration of transport carriers and the migration barriers associated with the A-site cation transport in bulk LMO. The trends in the migration barriers vs. various types of metal cations relevant to SOFC applications i.e. La3+, Sr2+, Zr4+, Y3+, are also investigated, and the results obtained suggest that both the ionic charge and the ionic radius correlate with the calculated cation migration barriers

An ultrathin MgO coating was synthesized via atomic layer deposition (ALD) to improve the surface properties of the Li[Ni0.5Mn0.3Co0.2]O2(NMC) cathode. An in-situ quartz crystal sensor was used to monitor the “self-limiting” surface reactions during ALD process and estimate the density of the deposited film. The electrochemical performance of the MgO-coated NMC cathode was evaluated in a half-cell assembly and compared to other ALD-based coatings, such as Al2O3and ZrO2. Cyclic voltammetry studies suggested that ALD MgO has a higher Li-diffusion coefficient which resulted in lower overpotential on the NMC cathode surface and improved Li-ion battery rate performance. MgO-coated NMC also yielded improved capacity retention over uncoated NMC in a long-range cycling test.

Both the experimental and first-principles modeling results revealed the dependence of defect energetics on oxygen non-stoichiometry and magnetic coupling of Fe in the Fe-based perovskite oxides. A generalized defect thermodynamic model of the proton-conducting (La,Ba)Fe1-xMxO3-δperovskite oxide is developed to allow inclusion of nonlinear δ dependent terms in three key defect reaction energies, namely, the oxygen vacancy formation, hydration, and charge disproportionation reactions. A transition from a large polaron description at lower δ values to a small polaron expression at higher δ is also considered in our analysis. Based on the functional forms of defect energetics on δ as guided by first principles modeling and literature data, the Brouwer diagrams of BaFe0.9Y0.1O3-δare assessed to provide information on electronic and ionic defect concentration (including the proton species) as a function of O2and H2O pressure at different temperatures for solid-oxide cell applications.

Two-dimensional (2D) ferromagnetic (FM) materials with nanoscale thickness and spontaneous net magnetization have emerged as a promising class of functional materials for applications in next-generation spintronics, quantum processing, and data storage devices. However, most 2D materials exhibit weak FM even at low temperatures, limiting their potential applications in many technological fields. The fabrication of strong room-temperature FM 2D materials is highly desirable for the development of practical applications. Here, we demonstrate an ionic layer epitaxy strategy to synthesize few-layered NiOOH nanosheets with strong room-temperature FM and a saturation magnetization up to 409.86 emu cm–3at 300 K. The results are consistent with the ab initio predictions of a stable FM NiOOH nanolayer structure with an FM configuration. The FM strength of the NiOOH nanosheets can be tuned by controlling the surfactant monolayer density and annealing. This work offers a promising strategy for achieving strong high-temperature FM in 2D materials for spintronic applications.

Metal oxide coatings can improve the electrochemical stability of cathodes and hence, their cycle-life in rechargeable batteries. However, such coatings often impose an additional electrical and ionic transport resistance to cathode surfaces leading to poor charge–discharge capacity at high C-rates. Here, a mixed oxide (Al2O3)1–x(Ga2O3)xalloy coating, prepared via atomic layer deposition (ALD), on Li[Ni0.5Mn0.3Co0.2]O2(NMC) cathodes is developed that has increased electron conductivity and demonstrated an improved rate performance in comparison to uncoated NMC. A “co-pulsing” ALD technique was used which allows intimate and controlled ternary mixing of deposited film to obtain nanometer-thick mixed oxide coatings. Co-pulsing allows for independent control over film composition and thickness in contrast to separate sequential pulsing of the metal sources. (Al2O3)1–x(Ga2O3)xalloy coatings were demonstrated to improve the cycle life of the battery. Cycle tests show that increasing Al-content in alloy coatings increases capacity retention; whereas a mixture of compositions near (Al2O3)0.5(Ga2O3)0.5was found to produce the optimal rate performance.

Determining the three-dimensional (3D) atomic structure of nanoparticles is critical to identifying the structures controlling their properties. Here, we demonstrate an integrated genetic algorithm (GA) optimization tool that refines the 3D structure of a nanoparticle by matching forward modeling to experimental scanning transmission electron microscopy (STEM) data and simultaneously minimizing the particle energy. We use the tool to create a refined 3D structural model of an experimentally observed ∼6000 atom Au nanoparticle.

Abstract

Abstract

Platinum-cobalt alloy nanoparticles are of great interest as cathode catalysts for PEMFCs as they have been shown to have enhanced activity versus platinum. However, their relative stability against loss of electrochemically-active surface area in relation to Pt catalysts is still debated. In this study, the evolutions of Pt3Co particle size distributions (PSDs) in fuel cell and aqueous environments were followed during accelerated stress tests (ASTs) using in-operando ASAXS. The measured evolutions showed a degradation mechanism dominated by loss of particles smaller than the critical particle diameter (<5.2 to 6.1 nm, CPD), which depended on environment and the AST. These evolutions were compared to that of a Pt catalyst with a similar initial PSD, which was found to have a lower degradation rate than Pt3Co. The ASAXS data, as well as data from aqueous dissolution, X-ray absorption spectroscopy, individual particle energy-dispersive spectroscopy, X-ray fluorescence spectrometry, and kinetic Monte Carlo calculations support a loss mechanism of increased Pt dissolution from Pt3Co versus Pt due to destabilization caused by extensive dealloying of particles <[?]5 nm during ASTs.

In this study we employ density functional theory (DFT) methods to investigate the surface energy barrier for electron emission (surface barrier) and thermodynamic stability of Ba and Ba–O species adsorption (relative to formation of bulk BaO) under conditions of high temperature (approximately 1200 K) and low pressure (approximately 10–10Torr) on the low-index surfaces of bixbyite Sc2O3. We employ both the standard generalized gradient approximation (GGA) and the hybrid HSE functional to calculate accurate surface barriers from relaxed GGA structures. The role of Ba in lowering the cathode surface barrier is investigated via adsorption of atomic Ba and Ba–O dimers, where the highest simulated dimer coverage corresponds to a single-monolayer film of rocksalt BaO. The change of the surface barrier of a semiconductor due to adsorption of surface species is decomposed into two parts: a surface dipole component and a doping component. The dipole component is the result of charge rearrangement at the surface and described by the electrostatic Helmholtz equation. The doping component is due to charge transfer from the surface species, which changes the Fermi level and thereby changes the surface barrier. Different initial geometries, adsorption sites, and coverages were tested for the most stable low-index Sc2O3surfaces ((011) and (111)) for both atomic Ba and Ba–O dimers. The lowest surface barrier with atomic Ba on Sc2O3was found to be 2.12 and 2.04 eV for the (011) and (111) surfaces at 3 and 1 Ba atoms per surface unit cell (0.250 and 0.083 Ba per surface O), respectively. The lowest surface barrier for Ba–O on Sc2O3was found to be 1.21 eV on (011) for a 7 Ba–O dimer-per-unit-cell coverage (0.583 dimers per surface O). Generally, we found that Ba in its atomic form on Sc2O3surfaces is not stable relative to bulk BaO, while Ba–O dimer coverages between 3 and 7 Ba–O dimers per (011) surface unit cell (0.250 and 0.583 dimers per surface O) produce stable structures relative to bulk BaO. Ba–O dimer adsorption on Sc2O3(111) surfaces was found to be unstable versus BaO over the full range of coverages studied. Investigation of combined n-type doping and surface dipole modification showed that their effects interact to yield a reduction less than the two contributions would yield separately.

Surface modification of perovskites is a new approach to develop highly active and stable cathodes for solid oxide fuel cells. Here, we report that La0.8Sr0.2MnO3−δ(LSM82) surface decoration led to markedly enhanced activity and stability for surface exchange kinetics of the (001)pseudocubic-oriented epitaxial La0.8Sr0.2CoO3−δ(LSC82) thin films on (001)-oriented yttria-stabilized zirconia (YSZ). In-plane and out-of-plane strains of the LSC82 films at elevated temperatures determined from in situ high resolution X-ray diffraction were not influenced by LSM82 decoration. Atomic force microscopy and scanning electron microscopy analysis showed that the formation of secondary particles observed upon annealing on the undecorated LSC82 surface was eliminated by the LSM82 decoration, which was accompanied by increased strontium (Sr) on the surface as revealed by auger electron spectroscopy. The enhanced stability of LSM82-decorated LSC82 against surface decomposition is in good agreement with density functional theory (DFT) calculations that showed a considerable energy gain for Mn substitution in LSC82. The surface exchange coefficients (kq) of LSC82, determined from electrochemical impedance spectroscopy, increased significantly with LSM82 coverage with average thicknesses up to ∼1 nm, while LSM82 thicknesses of ∼3.5 nm and greater reduced kq. Moreover, LSM82 decoration increased the stability of LSC82 for oxygen surface exchange. Remarkably, the kqof the LSC82 film with 0.9 nm-thick LSM82 coverage was not changed significantly over 70 h at 550 °C, after which time it exhibited activities 2 orders of magnitude higher than that of the undecorated LSC82. DFT calculations support the hypothesis that the enhanced oxygen surface exchange kinetics and stability of LSM82-decorated LSC82 can be attributed primarily to manganese (Mn) substitution in the LSC82 enabling the stabilization of higher Sr concentration in the perovskite structure near the surface as compared to the undecorated LSC82.

Complex oxides such as transition metal perovskites, particularly ABO3 perovskites of the type ABO3 (A=La, B= transition metal, O=oxygen) are promising catalysts for the aqueous oxygen reduction reaction (ORR), with potential application in in Alkaline Fuel Cell cathodes. There is a significant interest in using atomistic simulations to help design cathode materials through the study of the nature of surface binding. However, modeling of ORR on perovskite oxides has been previously restricted only to BO2-terminated surfaces, although in practice a mixture of both AO and BO2 terminations may exist, as the exact nature of surface termination for thin-film catalyst electrodes is difficult to synthesize as well as characterize. In this study we use Density Functional Theory based thermodynamic modeling to probe the differences in the binding of oxygen (O) and hydroxyl species (OH and OOH,) on the (001)-LaO and (001)-BO2 terminations, with the focus on the material LaCrO3. We show that in the relevant aqueous conditions for alkaline fuel cell cathodes, surface species *OH, *OOH and *O bind ~2eV more strongly to the LaO-termination compared to the CrO2-terminated surfaces, and therefore, when the LaO termination is stable, can potentially passivate the oxygen reduction and evolution reaction activity of the thin-films of LaCrO3.

Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.

We have investigated the migration of La3+ and Sr2+ by an A-site vacancy (VA) mechanism in La1-xSrxMnO3 (LSMO). Ab initio calculations determine migration barriers of 2.96 eV and 2.42 eV, for La3+ and Sr2+, respectively, and that repulsion between Sr2+ and VA is well-described by a screened electrostatic potential of the form E = 2.8 exp(-0.2r)/r eV, (r being separation in A). Using these results to parameterize kinetic Monte Carlo (KMC) and analytical calculations for diffusion coefficients we find good agreement with experiment observations of A-site diffusivity in other perovskites. We use these results to consider the tendency for Sr2+ to kinetically demix.

This work demonstrates how magnetic free energy terms can be incorporated into defects models, with a focus on those used to study solid oxide fuel cell materials. We apply the approach to extending a LaMnO3 defect model recently published in Physical Chemistry Chemical Physics 14 (1), 290-302 (2012). The magnetic contributions to the defect reactions energies are found to be much smaller than the total defect reaction energetics, and are close to the accuracy range of the adopted modeling approaches. These results show that the magnetic contributions are relatively minor and can often be ignored without significant loss of accuracy.

Oxygen adsorption on Pt(111) has many implications for a wide range of technologies including PEM fuel cells. Using DFT, we have calculated the stable phases for oxygen on Pt(111) surfaces up to one monolayer of oxygen coverage. Our predicted stable phases are consistent with electrochemical measurements and are in agreement with Temkin/Frumkin isotherm conditions. We predict a new phase at one monolayer that suggests a simple mechanism for oxygen place-exchange to subsurface positions. Analysis of the phase diagram provides a possible explanation for the hysteresis observed in Pt cyclic voltammograms in aqueous environments.

A single-pulse X-ray diffraction (XRD) diagnostic has been developed for the investigation of shocked material properties on a very short time scale. The diagnostic, which consists of a 37-stage Marx bank high-voltage pulse generator coupled to a needle-and-washer electron beam diode via coaxial cable, produces line-and-bremsstrahlung X-ray emission in a 40 ns pulse. The molybdenum anode produces 0.71 Å characteristic Kαlines used for diffraction. The X-ray beam passes through a pinhole collimator and is incident on the sample with an approximately 2 mm×5 mm spot and 1° full width at half maximum angular divergence. Coherent scattering from the sample produces a Debye-Scherrer diffraction pattern on an image plate located at 75 mm from the polycrystalline sample surface. An experimental study of the polycrystalline structures of zirconium and tin under high-pressure shock loading has been conducted with single-pulse XRD. The experimental targets were 0.1-mm-thick foils of zirconium and tin using 0.4-mm-thick vitreous carbon back windows for shock loading, and the shocks were produced by either Detasheet or PBX-9501 high explosives buffered by 1-mm-thick 6061-T6 aluminum. The diffraction patterns from both shocked zirconium and tin indicated a phase transition into a polymorphic mix of amorphous and new solid phases.

The loss of Pt nanoparticle surface area is of great importance when considering the durability of cathodes in polymer electrolyte membrane fuel cells. We here present a model for the loss of Pt surface area that includes both the Pt dissolution/precipitation reaction and the oxide formation/removal reaction. We discuss implications of this model when applied to a distribution of Pt particles under electrochemical potential in both fuel cell and aqueous electrolyte experiments. The importance of particle size distribution shape and distance of the hydrogen crossover Pt sink are explored in detail. We find that particle size distribution shape alters the surface area vs. time curve's functional form and contributes variability to long-term surface area stability. Our model shows that moving the hydrogen crossover Pt sink away from the Pt surface decreases surface area loss and shifts the dominant loss mechanism from mass loss to coarsening. We predict a shift in dominant surface area loss mechanism for in-situ vs. ex-situ experiments (from mass loss to coarsening, respectively) and suggest a critical role for oxide roughening in cycling experiments. The activation energy for Pt dissolution for potential cycling conditions (0.6-1.0 V) is estimated to be 160 meV/atom lower than potentiostatic conditions (0.95 V).

The efficiency of Solid Oxide Fuel Cells depends strongly on the oxygen reduction reaction (ORR) rate on the cathode. Recent studies have suggested that large enhancements in defect thermokinetics and catalytic rates may be possible through use of epitaxially strained materials. Using ab-initio methods to simulate defect energetics under epitaxial strain, we quantify the effects of strain on oxygen stoichiometry in La{1-x}Sr{x}CoO{3-d} (x=0.125) thin-films. We find at most a 2-3 fold increase in the vacancy concentration (at near 1000K) compared to bulk LSC over the range of strains from -2% (compressive) to about 4% (tensile). The results of this study suggest that epitaxial strain alone has a relatively weak effect of LSC oxygen vacancy concentration under SOFC conditions.

Loss of nanoparticle surface area under polymer electrolyte membrane fuel cell (PEMFC) operating conditions is an important barrier to the practical application of PEMFCs. We have developed an electrochemical rate theory model to better understand a number of aspects of Pt surface area loss, including the effects of particle size distribution, crossover molecular hydrogen, and Pt oxidation. As a specific example we here discuss some initial efforts to understand the effects of temperature on Pt nanoparticle stability. We compare our results to a previously developed first-order kinetic model and use this comparison to evaluate the role of the kinetic activation enthalpy for Pt dissolution and precipitation on overall surface area loss.

Perovskite materials of the form ABO3 are a promising family of compounds for use in solid oxide fuel cell (SOFC) cathodes. Study of the physics of these compounds under SOFC conditions with ab initio methods is particularly challenging due to high temperatures, exchange of oxygen with O2 gas, and correlated electron effects. This paper discusses an approach to performing ab initio studies on these materials for SOFC applications and applies the approach to calculate vacancy formation energies in LaBO3 (B = Mn, Fe, Co, Ni) compounds.

We use a random forest (RF) model to predict the critical cooling rate (RC) for glass formation of various alloys from features of their constituent elements. The RF model was trained on a database that integrates multiple sources of direct and indirect RCdata for metallic glasses to expand the directly measured RCdatabase of less than 100 values to a training set of over 2000 values. The model error on 5-fold cross-validation (CV) is 0.66 orders of magnitude in K/s. The error on leave-out-one-group CV on alloy system groups is 0.59 log units in K/s when the target alloy constituents appear more than 500 times in training data. Using this model, we make predictions for the set of compositions with melt-spun glasses in the database and for the full set of quaternary alloys that have constituents which appear more than 500 times in training data. These predictions identify a number of potential new bulk metallic glass systems for future study, but the model is most useful for the identification of alloy systems likely to contain good glass formers rather than detailed discovery of bulk glass composition regions within known glassy systems.

Two different Pt/C samples (Pt/Vulcan and Pt/C-high-surface- area) were subject to potential cycling between 0.6V and 1.0V vs. reversible hydrogen electrode, where significant platinum area loss was found. Both cycled MEA cathode samples were examined and compared in detail by glancing angle X-ray powder Diffraction and transmission electron microscopy (TEM) to reveal processes responsible for observed platinum loss. TEM data and analyses of pristine and cycled Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes as reported recently1: i) Ostwald ripening on carbon at the nanometer- scale; and ii) diffusion of soluble platinum species in the ionomer phase at the micrometer-scale, chemical reduction of these species by cross-over H2 molecules and precipitation of platinum particles in the cathode ionomer phase.