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The inevitable formation of passivating oxide films on the surface of the TiFe intermetallic compound limits its performance as a stationary hydrogen storage material. Extensive experimental efforts have been dedicated to the activation of TiFe, i.e. oxide layer removal prior to utilization for hydrogen storage. However, development of an efficient activation protocol necessitates a fundamental understanding of the composition and structure of the air-exposed surface and its interaction with hydrogen, which currently is absent. Therefore, in this study we explored the growth and nature of the oxide films on the most exposed TiFe surface (110) in depth using static and dynamic first-principles methods. We identified the lowest energy structures for six oxygen coverages up to approximately 1.12 nm of thickness with a global optimization method and studied the temperature effects and structural evolution of the oxide phases in detail via ab-initio molecular dynamics (AIMD). Based on structural similarity and coordination analysis, motifs for TiO2, TiFeO3 as well as Ti(FeO2)x (x = 2, 3 or 5) phases were identified. On evaluating the interaction of the oxidized surface with hydrogen, a minimal energy barrier of 0.172 eV was predicted for H2 dissociation while the H migration from the top of the oxidized surface to the bulk TiFe was limited by several high-lying energy barriers above 1.4 eV. Our mechanistic insights will prove themselves valuable for informed designs towards new activation methods of TiFe and related systems as hydrogen storage materials.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage

MXene transition-metal carbides and nitrides are of growing interest for energy storage applications. These compounds are especially promising for use as pseudocapacitive electrodes due to their ab...

MXenes are a novel class of two dimensional materials, discovered by Barsoum and Gogotsi [M. Naguib, J. Come, B. Dyatkin, V. Presser, P. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochemistry Communications 16, 61-64 (2012); B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nature Reviews Materials vol. 2, 16098 (2017)]. Their large specific surface area and the tunability of their physicochemical properties as a function of the transition metal and surface terminal group make them a unique design platform for various applications, a primary example of which is pseudocapacitive energy storage. However, there is still incomplete understanding of how the transition metal chemistry and stoichiometry, and the surface termination affect charge storage mechanisms in MXene. In this study, we have performed systematic first-principles calculations for bulk MXene and found that the atomic charge of the metal cations, which is related to their valence, decreases across the d-electron metal series. Electronic-structure indicators of performance are examined to understand the energy storage behavior, whereby charges are stored between the terminal groups and adsorbing cations. Importantly, we found that the differential Bader charges show good agreement with theoretical capacitances and are useful in predicting charge storage trends in MXene-based pseudocapacitors. Furthermore, we have performed first-principles and grand canonical Monte Carlo calculations for the slab systems, finding that the solvent plays a critical role in enhancing the pseudocapacitive response.

Band alignment effects of anatase and rutile nanocrystals in TiO$_2$ powders lead to an electron hole separation, increasing the photo catalytic efficiency of these powders. While size effects and types of possible alignments have been extensively studied, the effect of interface geometries of bonded nanocrystal structures on the alignment is poorly understood. In order to allow conclusive studies of a vast variety of bonded systems in different orientations, we have developed a new density functional tight binding parameter set to properly describe quantum confinement in nanocrystals. Applying this set we found a quantitative influence of the interface structure on the band alignment., Comment: 28 pages, 11 figures (18 graphics), 2 tables

Reactive Ti-5Al-5 V-5Cr-3Mo (Ti5553) alloy feedstock powder is used in laser powder-bed fusion (LPBF). Due to the large quantity of powder necessary for LPBF, powder is reused numerous times under oxidizing environments. Transformations to the powder’s native surface oxide may impact the LPBF process and lead to deviations from expected behavior of printed parts. Here, we present a multimodal characterization of new and reused powder by combining X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of individual powder particles to understand how the surface oxide composition changes, grows, and becomes hydroxylated and hydrated due to reuse. We show that the native oxide film of Ti5553 powder is a hydroxylated mixed oxide composed of TiO2 and Al2O3 that grows from 5.6 +/- 0.7 nm to 8.3 +/- 1.1 nm due to reuse. Al oxide regions of the surface oxide become more hydroxylated during reuse in comparison to Ti oxide regions. This indicates that an outer oxide of pure TiO2 may enhance recyclability of Ti5553 powder while the presence of Al oxides may decrease recyclability. Further characterization of surface transformations due to recyclability may eliminate unnecessary variation in LPBF.

The energy density of ruthenia (${\mathrm{RuO}}_{2}$) pseudocapacitor electrodes is critically dependent on their surface structure. To understand this dependence, we simulate the electrochemical response of ${\mathrm{RuO}}_{2}$(110), ${\mathrm{RuO}}_{2}$(100), and ${\mathrm{RuO}}_{2}$(101) in aqueous environments using a self-consistent continuum solvation (SCCS) model of the solid-liquid interface. The insertion of protons into the ${\mathrm{RuO}}_{2}$(110) sublayer is found to profoundly affect the voltage-dependent characteristics of the system, leading to a sharp transition from a battery-type to capacitor-type response. The calculated charge-voltage properties for ${\mathrm{RuO}}_{2}$(101) are in qualitative agreement with experiment, albeit with a pseudocapacitance that is significantly underestimated. In contrast, the ${\mathrm{RuO}}_{2}$(100) facet is correctly predicted to be pseudocapacitive over a wide voltage window, with a calculated pseudocapacitance in close agreement with experimental voltammetry. These results establish the SCCS model as a reliable approach to predict and optimize the facet-dependent pseudocapacitance of polycrystalline systems.

Glass surfaces are of considerable interest due to their disproportionately large influence on the performance of glass articles in many applications. However, the behavior of glass surfaces has proven difficult to model and predict due to their complex structure and interactions with the environment. Here, the effects of glass network topology on the surface reactivity of glasses have been investigated using reactive and nonreactive force field-based molecular dynamics simulations as well as density functional theory. A topological constraint-based description for surface reactivity is developed, allowing for improved understanding of the physical and chemical origins of surface reactivity. Results show evidence for the existence of a chemically stable intermediate phase on the surface of the glass where the glass network is mechanically isostatic.

Germanium telluride is a well-known phase change material (PCM) used in non-volatile memory cells and radio frequency switches. Controlling the properties of GeTe for improved PCM device performance has sometimes been achieved by doping and/or alloying with metals, often at concentrations greater than 10 at. % and using non-equilibrium methods. Since switching PCMs between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of metal-incorporated GeTe ( Ge 0.5 − x M x Te 0.5) films is also critical to practical implementation of these materials in electronic and optoelectronic devices. In this work, we use both density-functional theory and experimental characterization methods to probe the solubility and critical properties of Ge 0.5 − x M x Te 0.5 films. Using first-principles calculations, we determine the enthalpy of formation for GeTe with 2.08, 4.17, and 6.25 at. % of Cu, Fe, Mn, Mo, and Ti and show trends between the stability of the Ge 0.5 − x M x Te 0.5 systems and the atomic position, composition, and distribution of the metal atoms in the GeTe matrix. Out of all the studied systems, Mo was the only metal to cluster within GeTe. Analysis of the Ge–Te bond lengths and volumes of the Ge 0.5 − x M x Te 0.5 supercells shows that increasing the atomic concentration (2.08, 4.17, 6.25 at. %) of the different metals causes varied distortions of the crystal structure of GeTe that are accompanied by significant changes in the projected density of states. Computational predictions concerning metal solubility and the effect of metal incorporation on critical properties of GeTe are compared to experimental results in the literature (Cu, Mn, Mo, and Ti) and to transmission electron microscopy and transport data from newly characterized co-sputtered Ge 0.5 − x Fe x Te 0.5 films. The computational predictions of decreasing solubility (Mn > Cu, Fe > Ti, Mo) shows good agreement with experimental observations (Mn, Cu > Fe > Ti, Mo), and Ge 0.5 − x Fe x Te 0.5 films exhibited increased crystallization temperatures from pure GeTe.

Aluminum alloys are widely used in many applications due to their inherent corrosion resistance, which is attributed to the formation of a protective oxide layer. However, the strength and durability of many Al-based materials is reduced in humid environments. Consequently, detailed mechanistic understanding of corrosion reactions over time is necessary to predict the cost, safety, and performance. Most of the current knowledge of atmospheric aluminum corrosion is built on response relationships upon extensive corrosion and typically involve a first-order correlation of corrosion data with environmental conditions. However, mechanistic understanding of corrosion processes is still limited, particularly at the early stages of exposure to humid environments. In addition, the effects of electrolytes such as sodium chloride present in the environment are still not fully understood. This presentation will focus on our recent efforts to reveal the mechanistic dependence of electrochemical aluminum corrosion in humid air in the presence of NaCl. The rate of corrosion is explored through complimentary experimental and theoretical analyses relating corrosion rate to electrolyte chemistry, mass uptake and corrosion products. As an experimental platform, we use quartz crystal microbalance (QCM) electrodes coated with aluminum and sodium chloride using sputtering techniques. Structure, composition, and surface morphology of the samples were characterized by grazing-incidence X-ray diffraction, FTIR spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning transmission electron microscopy, and electron energy loss spectroscopy. The QCM provides operando data with nanogram resolution on the formation of the corrosion product layer, from which we can derive rate constants for aluminum surface reactions. We find that the reaction rate in dry air is negligible, but a dramatic acceleration is observed at 50% RH and above. In addition, this study provides additional insight into: 1) the influence of the sodium chloride film thickness on the rate and reaction products; 2) the role of surface termination and roughness on aluminum oxide/hydroxide formation; and 3) the electrochemical corrosion reaction rate acceleration as a function of temperature. Finally, based on the interplay between theory and experiment we achieved preliminary understanding of the rate-limiting steps, which we will use to develop validated computational models of early-stage (hours to several days) aluminum corrosion. Figure 1

Pseudocapacitors are energy-storage devices characterized by fast and reversible redox reactions that enable them to store large amounts of electrical energy at high rates. We simulate the response of pseudocapacitive electrodes under realistic conditions to identify the microscopic factors that determine their performance, focusing on ruthenia (RuO2) as a prototypical electrode material. Electronic-structure methods are used together with a self-consistent continuum solvation (SCCS) model to build a complete dataset of free energies as the surface of the charged electrode is gradually covered with protons under applied voltage. The resulting dataset is exploited to compute hydrogen-adsorption isotherms and charge-voltage responses by means of grand-canonical sampling, finding close agreement with experimental voltammetry. These simulations reveal that small changes on the order of 5 {\mu}F/cm2 in the intrinsic double-layer capacitance of the electrode-electrolyte interface can induce variations of up to 40 {\mu}F/cm2 in the overall pseudocapacitance., Comment: 8 pages with 6 figures and 1 table

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50–100 h) and one slower-decaying species (lifetimes on the order of 1000 h or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO2(1 1 0) to phenoxyl radicals on the same substrate indicate distinct changes in the π–π∗ transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(1 1 0), indicating that this shift in the π–π∗ transition may be taken as a general hallmark of phenoxyl radical formation.

Self-interaction is a central problem for the accuracy of density-functional approximations in describing the electronic structure of atoms and molecules. In this work, we discuss the different types of self-interaction errors commonly encountered in density-functional calculations, providing precise definitions for each of them. Based upon these definitions, we derive an orbital-dependent density-functional method, called the Koopmans-compliant approach, which simultaneously corrects the different self-interaction errors, by enforcing piecewise linearity with respect to fractional particle counts and by imposing the correct asymptotic behavior of the one-electron potential in approximate energy functionals. We illustrate the very good performance of this new method in predicting the electronic properties of atoms and molecules, while preserving or improving the prediction of total energies and equilibrium geometries. These results highlight the accuracy and efficiency of Koopmans-compliant functionals as an attractive solution to the self-interaction problem.