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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Exceptional activity of sub-nm Pt clusters on CdS for photocatalytic hydrogen production: A combined of experimental and first-principles study

In this work we have explored a new concept of substantially increasing photocatalytic activity for H₂ production of conventional semiconductors by modifying them with sub-nm Pt particles. By combining both experimental and theoretical approaches, we have also developed new mechanistic insights into the 17 times increase in photocatalytic activity of Pt modified CdS catalysts.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2015

Microhydrated dihydrogen phosphate clusters probed by gas phase vibrational spectroscopy and first principles calculations

We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H 2 PO 4 - (H 2 O) n anions (n = 2–12) in the spectral range of the stretching and bending modes of the solute anion (600–1800 cm-1). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogen bonding to the two terminal P=O groups, these are hydrated before the two P–OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H 2 PO 4 - (aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H 2 PO 4 - , provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

First-principles assessment of hole transport in pure and Li-doped NiO

Alloying nickel oxide (NiO) with lithium oxide (Li 2 O) at high Li concentrations may reduce NiO's band gap and expand its use as a light absorber in photocatalytic and tandem dye-sensitized solar cell technologies. In this work, we evaluate the viability of this alloy as a p-type hole transport material. We use embedded cluster models, along with unrestricted Hartree–Fock and complete active space self-consistent field theories, to study the impact of alloying on polaronic transport of holes. Our calculated energy barrier for hole transfer in undoped NiO is in excellent agreement with the experimental value of ~0.1 eV. We predict that hole transport in NiO is anisotropic and mostly confined parallel to the (111) ferromagnetic planes. Applying the same model to Li-doped NiO indicates that isolated Li ions do not introduce free holes into NiO samples. However, free holes can be created in the homogeneous Li 0.125 Ni 0.875 O alloy, in which the Li concentration is very high. Our kinetic Monte Carlo calculations show that hole mobility in this alloy is lower than in undoped NiO. However, the additional free holes and the predicted lower band gap of Li 0.125 Ni 0.875 O should increase hole conductivity compared to NiO upon alloy formation. Therefore, Li 0.125 Ni 0.875 O alloys have potential for use as a hole transporter, as well as a sunlight absorber, in a variety of solar energy applications.

25 ENERGY STORAGE↗
DOE OSTI2015

Solvation effects on the band edge positions of photocatalysts from first principles

The band edge positions of photocatalysts relative to the redox potentials of water play an important role in determining the efficiency of photoelectrochemical cells. These band positions depend on the structure of the solid–liquid interface, but direct ab initio molecular dynamics calculations of these interfaces, while expected to be accurate, are too computationally demanding for high-throughput materials screening. Thus rapid theoretical screening of new photocatalyst materials requires simplified continuum solvation models that are suitable for treating solid–liquid interfaces. In this paper, we evaluate the accuracy of the recently developed CANDLE and SaLSA continuum solvation models for predicting solvation effects on the band positions of several well-studied surfaces [Si(111), TiO 2 (110), IrO 2 (110) and WO 3 (001)] in water. We find that the solvation effects vary considerably, ranging from <0.5 eV for hydrophobic surfaces, 0.5–1 eV for many hydrophilic oxide surfaces, to ~2 eV for oxygen-deficient surfaces. The solvation model predictions are in excellent agreement (within ~0.1 eV) with ab initio molecular dynamics results where available, and in good agreement (within ~0.2–0.3 eV) with experimental measurements. We also predict the energetics for surface oxygen vacancies and their effect on the band positions of the hydrated WO 3 (001) surface, leading to an explanation for why the solvation shift observed experimentally is substantially larger than predicted for the ideal surface. Finally, based on the correlation between solvation shift and the type of surface and solvent, we suggest approaches to engineer the band positions of surfaces in aqueous and non-aqueous solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Investigation of sodium insertion–extraction in olivine Na x FePO 4 (0 ≤ x ≤ 1) using first-principles calculations

Olivine NaFePO4 has recently attracted the attention of the scientific community as a promising cathode material for Na-ion batteries. In this work we combine density functional theory (DFT) calculations and high resolution synchrotron X-ray diffraction (HRXRD) experiments to study the phase stability of NaxFePO4 along the whole range of sodium compositions (0 ≤ x ≤ 1). DFT calculations reveal the existence of two intermediate structures governing the phase stability at x = 2/3 and x = 5/6. This is in contrast to isostructural LiFePO4, which is a broadly used cathode in Li-ion batteries. Na2/3FePO4 and Na5/6FePO4 ground states both align vacancies diagonally within the ab plane, coupled to a Fe2+/Fe3+ alignment. HRXRD data for NaxFePO4 (2/3 < x < 1) materials show common superstructure reflections up to x = 5/6 within the studied compositions. The computed intercalation voltage profile shows a voltage difference of 0.16 V between NaFePO4 and Na2/3FePO4 in agreement with the voltage discontinuity observed experimentally during electrochemical insertion.

Saracibar, A.↗
DOE OSTI2016

Energetics of Sn 2+ isomorphic substitution into hydroxylapatite: First-principles predictions

In this study, the energetics of Sn 2+ substitution into the Ca 2+ sublattice of hydroxylapatite (HA), Ca 10 (PO 4 ) 6 (OH) 2 , has been investigated within the framework of density functional theory. Calculations reveal that Sn 2+ incorporation via coupled substitutions at Ca(II) sites is energetically favourable up to a composition of Sn 6 Ca 4 (PO 4 ) 6 (OH) 2 , and further substitutions at Ca(I) sites proceed once full occupancy of Ca(II) sites by Sn 2+ is achieved. Compositions of Sn x Ca 10–x (PO 4 ) 6 (OH) 2 (x = 4–9) are predominant, with an optimal stoichiometry of Sn 8 Ca 2 (PO 4 ) 6 (OH) 2 , and Sn-substituted HA follows approximately Vegard's law across the entire composition range.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Li-ion site disorder driven superionic conductivity in solid electrolytes: a first-principles investigation of β-Li 3 PS 4

The attractive safety and long-term stability of all solid-state batteries has added a new impetus to the discovery and development of solid electrolytes for lithium batteries. Recently several superionic lithium conducting solid electrolytes have been discovered. All the superionic lithium containing compounds (β-Li 3 PS 4 and Li 10 GeP 2 S 12 and oxides, predominantly in the garnet phase) have partially occupied sites. This naturally begs the question of understanding the role of partial site occupancies (or site disorder) in optimizing ionic conductivity in these family of solids. In this paper, we find that for a given topology of the host lattice, maximizing the number of sites with similar Li-ion adsorption energies, which gives partial site occupancy, is a natural way to increase the configurational entropy of the system and optimize the conductivity. For a given topology and density of Li-ion adsorption sites, the ionic conductivity is maximal when the number of mobile Li-ions are equal to the number of mobile vacancies, also the very condition for achieving maximal configurational entropy. We demonstrate applicability of this principle by elucidating the role of Li-ion site disorder and the local chemical environment in the high ionic conductivity of β-Li 3 PS 4 . In addition, for β-Li 3 PS 4 we find that a significant density of vacancies in the Li-ion sub-lattice (~25%) leads to sub-lattice melting at (~600 K) leading to a molten form for the Li-ions in an otherwise solid anionic host. This gives a lithium site occupancy that is similar to what is measured experimentally. We further show that quenching this disorder can improve conductivity at lower temperatures. As a consequence, we discover that (a) one can optimize ionic conductivity in a given topology by choosing a chemistry/composition that maximizes the number of mobile-carriers i.e. maximizing both mobile Li-ions and vacancies, and (b) when the concentration of vacancies becomes significant in the Li-ion sub-lattice, it becomes energetically as well as entropically favorable for it to remain molten well below the bulk decomposition temperature of the solid. Finally, this principle may already apply to several known superionic conducting solids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

First-principles study of adsorption–desorption kinetics of aqueous V 2+ /V 3+ redox species on graphite in a vanadium redox flow battery

Vanadium redox flow batteries (VRFBs) represent a promising solution to grid-scale energy storage, and understanding the reactivity of electrode materials is crucial for improving the power density of VRFBs. However, atomistic details about the interactions between vanadium ions and electrode surfaces in aqueous electrolytes are still lacking. Here, we examine the reactivity of the basal (0001) and edge (11[2 with combining macron]0) graphite facets with water and aqueous V2+/V3+ redox species at 300 K employing Car–Parrinello molecular dynamics (CPMD) coupled with metadynamics simulations. The results suggest that the edge surface is characterized by the formation of ketonic C[double bond, length as m-dash]O functional groups due to complete water dissociation into the H/O/H configuration with surface O atoms serving as active sites for adsorption of V2+/V3+ species. The formation of V–O bonds at the surface should significantly improve the kinetics of electron transfer at the edge sites, which is not the case for the basal surface, in agreement with the experimentally hypothesized mechanism.

Jiang, Zhen↗
DOE OSTI2017

Transmutation effects on long-term Cs retention in phyllosilicate minerals from first principles

The accidental release and incorporation of radiocesium into soil minerals represents a massive environmental, technical and social challenge. Accurately forecasting the evolving distribution and fate of long- and medium-lived isotopes such as 137Cs and 134Cs over decadal time scales is essential. The cesium cation has long been modeled as a strongly and selectively sorbed species into clay mineral interlayers; however, because of the time scales involved by the radioisotopes half-lives, the effects of radioactive decay on Cs retention have been unknown. We report density functional theory (DFT) simulations of transmutation effects of radiocesium on long-term Cs retention in phlogopite. The calculations show that the progressive appearance of daughter product Ba2+ is accompanied by a proportional increase in thermodynamic driving force to preferentially discharge remaining Cs, both radioactive and stable, back into aqueous solution. Based on thermodynamic analysis, the findings indicate that radiocesium transmutation provides a mean to weaken the binding of Cs in phyllosilicate minerals, therefore potentially involving a premature re-release of Cs back into the environment. In the case where radiogenic Ba2+ ions accumulate in the mineral, collateral effects would ultimately be an increase in the overall interlayer binding energy and a lower resorption capacity.

Sassi, Michel JPC↗
DOE OSTI2017

A first principles study of spinel ZnFe 2 O 4 for electrode materials in lithium-ion batteries

Zinc ferrite (ZnFe 2 O 4 ) is a perspective lithium ion battery material, owing to its large theoretical capacity (1000 mAh/g). Here, we report a density functional study of the discharge process at early stage from ZnFe 2 O 4 up to LixZnFe2O4 (x = 2), aiming to provide fundamental understanding of the mechanism. According to our calculations, with x increasing up to 1 the intercalation of Li+ ions prefer octahedral 16c sites, which is accompanied by Zn 2+ ion displacement from tetrahedral 8a sites to 16c sites starting at x = 0.25 and the gain in stability; while the stability decreases for 1 < x ≤ 2 due to the occupation of Li+ ions at the less active tetrahedral 8a/48f/8b sites. The open-circuit voltages estimated based on the structures of stable intermediates identified by the DFT calculations are in good agreement with the experimental values. Our results highlight the importance of the interplay among Li, O 2- , Fe 3+ and Zn 2+ in enabling the high performance as LIB materials.

25 ENERGY STORAGE↗
DOE OSTI2018

Anharmonic contribution to the stabilization of Mg(OH) 2 from first principles

Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P$\bar{3}$ (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P$\bar{3}$m1 (No. 164, point group -3m) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH) 2 , in particular for the PP$\bar{3}$m1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P$\bar{3}$m1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P$\bar{3}$m1 structure towards P$\bar{3}$. Here, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Magnetic structure of UO 2 and NpO 2 by first-principle methods

Here, the magnetic structure of the actinide dioxides (AnO 2 ) remains a field of intense research. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic ground-state, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO 2 , whereas a ferromagnetic (111) ground-state with R¯3 with combining macron]m (No. 166) has been established for NpO 2 . Band structure calculations have been performed to analyse these results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗