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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

A computational study of Al n - and Al n -1 1Pt - clusters: the effects of doping and a uniform tuning gauge for single-atom nanocatalysts

Results of density functional theory calculations on Al n - and Al n -1 1Pt - , n = 2-8, clusters are presented and analyzed. The analysis includes different structural forms of the clusters characterized in terms of binding energy, spin and symmetry, and a comparative evaluation of various properties of the two systems viewed as connected through a single-Pt substitutional doping and examined in terms of their respective most stable structures. Furthermore, the Al n -1 1Pt - clusters are then used as a paradigmatic (model) case of single-atom nanocatalysts, with Pt as the catalytic center and Al n-1 as its support, to implement a uniform descriptor for gauging the tuning effects of all the parameters ( "knobs ") of a nanocatalyst that include the identity of the active center and the material and size of its support.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

Nitrogen and sulfur co-doped porous carbon sheets for energy storage and pH-universal oxygen reduction reaction

Developing efficient electrocatalysts for energy storage and oxygen reduction reaction (ORR) is of great sig-nificance for the utilization of renewable energy. In particular, designing catalysts with both promising activityand long stability for ORR in pH-universal electrolytes still remain as a tremendous challenge. To tackle such aproblem, metal-free nitrogen and sulfur co-doped porous carbon sheet (NSPCS) was rationally designed in thiswork in order to integrate the two reported routes of enhancing the electrocatalytic activity of graphene. The as-prepared NSPCS has an onset potential of 0.89 V vs. RHE, and half-wave potential E1/2 ≈ 0.75 V during ORR inacidic solution, making it as the most active ORR catalyst. Moreover, the resulting NSPCS also shows a 0.03 Vpositive shift of half-wave potential than commercial Pt/C for ORR and excellent charge capacitive performancein alkaline media. Electron microscopy revealed high degree of defects on NSPCS surface. This, coupled withsynergistic doping effects of nitrogen and sulfur, optimized the active sites and charge transfer, rationalized theoutstanding performance in both oxygen reduction reactions and supercapacitors.Developing efficient electrocatalysts for energy storage and oxygen reduction reaction (ORR) is of great significance for the utilization of renewable energy. In particular, designing catalysts with both promising activity and long stability for ORR in pH-universal electrolytes still remain as a tremendous challenge. To tackle such a problem, metal-free nitrogen and sulfur co-doped porous carbon sheet (NSPCS) was rationally designed in this work in order to integrate the two reported routes of enhancing the electrocatalytic activity of graphene. The as-prepared NSPCS has an onset potential of 0.89 V vs. RHE, and half-wave potential E-1/2 approximate to 0.75 V during ORR in acidic solution, making it as the most active ORR catalyst. Moreover, the resulting NSPCS also shows a 0.03 V positive shift of half-wave potential than commercial Pt/C for ORR and excellent charge capacitive performance in alkaline media. Electron microscopy revealed high degree of defects on NSPCS surface. This, coupled with synergistic doping effects of nitrogen and sulfur, optimized the active sites and charge transfer, rationalized the outstanding performance in both oxygen reduction reactions and supercapacitors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Quantitative Identification of Dopant Occupation in Li‐Rich Cathodes

Elemental doping is widely used to improve the performance of cathode materials in lithium‐ion batteries. However, macroscopic/statistical investigation on how doping sites are distributed in the material lattice, despite being a key prerequisite for understanding and manipulating the doping effect, has not been effectively established. Herein, to solve this predicament, a universal strategy is proposed to quantitatively identify the locations of Al and Mg dopants in lithium‐rich layered oxides (LLOs). Solid evidence confirms that Al prefers to occupy the transition metal (TM) layer, while Mg evenly occupies both TM and Li layers. As a result, Mg significantly reduces the thickness of LiO 2 slabs at room temperature, which will increase the energy barrier of oxygen activation and enhance the structure stability of LLOs. The suppressed oxygen activity in Mg‐doped LLO can be kinetically unlocked at 55 °C. The different characteristics of Al and Mg enlighten an Al/Mg co‐doping strategy to optimize LLOs, which significantly improves the cycle performance while lifting the capacity. In conclusion, these insights from the quantitative identification of doping sites shed light on the manipulation of doping effects toward better cathodes.

25 ENERGY STORAGE↗
DOE OSTI2015

One-pot hydrothermal synthesis of zeolite/sodium tantalate composite and its photodegradation of methyl orange

Highlights: • Sodalite/NaTaO{sub 3} composite is prepared by a one-pot hydrothermal synthesis. • Enhanced photodegradation is achieved due to the heterogeneous doping effect. • Structure distortion is found for NaTaO{sub 3} after removing sodalite by acid washing. - Abstract: Sodalite/NaTaO{sub 3} composite was prepared by a one-pot hydrothermal synthesis method. Sodalite and NaTaO{sub 3} grow interpenetrated, and the resulting composites have similar morphology as the pure sodalite. The sodalite/NaTaO{sub 3} composite has a lower band gap of 3.35 eV due to the heterogeneous doping effect, and exhibits an enhanced photodegradation of methyl orange under UV irradiation as compared to the pure NaTaO{sub 3}. A slight structure distortion is found for NaTaO{sub 3} after removing sodalite by acid washing the sodalite/NaTaO{sub 3} composite, and such result further confirms the co-growth of the two crystals. This one-pot hydrothermal method opens up new avenues for the preparation of photocatalytic composites.

36 MATERIALS SCIENCE↗
NASA NTRS1985

High-efficiency silicon solar-cell design and practical barriers

A numerical evaluation technique is used to study the impact of practical barriers, such as heavy doping effects (Auger recombination, band gap narrowing), surface recombination, shadowing losses and minority-carrier lifetime (Tau), on a high efficiency silicon solar cell performance. Considering a high Tau of 1 ms, efficiency of a silicon solar cell of the hypothetical case is estimated to be around 29%. This is comparable with (detailed balance limit) maximum efficiency of a p-n junction solar cell of 30%. Value of Tau is varied from 1 second to 20 micro. Heavy doping effects, and realizable values of surface recombination velocities and shadowing, are then considered in succession and their influence on cell efficiency is evaluated and quantified. These practical barriers cause the cell efficiency to reduce from the maximum value of 29% to the experimentally achieved value of about 19%. Improvement in open circuit voltage V sub oc is required to achieve cell efficiency greater than 20%. Increased value of Tau reduces reverse saturation current and, hence, improves V sub oc. Control of surface recombination losses becomes critical at higher V sub oc. Substantial improvement in Tau and considerable reduction in surface recombination velocities is essential to achieve cell efficiencies greater than 20%.

Mokashi, A.↗
DOE OSTI2021

Mesoporous Textured Fe-N-C Electrocatalysts as Highly Efficient Cathodes for Proton Exchange Membrane Fuel Cells

A new platinum group metal (PGM)-free proton exchange membrane fuel cell (PEMFC) cathode catalyst materials, synthesized using the VariPore™ method by Pajarito Powder, LLC, are characterized for their structure and activity. The physico-chemical analysis of the iron-nitrogen-carbon (Fe-N-C) electrocatalysts show mesoporous carbon material effectively doped with iron and nitrogen. The materials have an average pore size of 7-8 nm and high specific surface area. The Fe-N-C catalysts exhibit good oxygen reduction reaction (ORR) activity in 0.5 M H2SO4 electrolyte with high half-wave potential and sustainable electrochemical stability over 10,000 repeated potential cycles with insignificant losses in their activities. As cathode catalysts in a PEMFC, the Fe-N-C materials deliver remarkably good fuel cell performance at low overpotential approaching that of the commercial Pt catalyst. The high ORR electrocatalytic activity of these Fe-N-C catalysts is credited to the synergy between nitrogen-moieties, specifically pyrrolic-N, pyridinic-N, and graphitic-N, and iron in addition to the high mesoporosity that facilitate an effective reaction path in boosting the electrocatalytic activity and stability.

DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Effect of doping and chemical ordering on the optoelectronic properties of complex oxides: Fe 2 O 3 –V 2 O 3 solid solutions and hetero-structures

The electronic and optical properties of a-(Fe1xVx)2O3 at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time dependent density functional theory. At low V concentrations the onset of the optical absorption is B0.5 eV (i.e., nearly 1.6 eV lower than that in pure a-Fe2O3) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe–V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe2O3/V2O3 hetero-structures is as low as B0.3 eV and the corresponding peak is at B0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (B1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe2O3/V2O3 hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in a-(Fe1xVx)2O3 thin films synthesized with well-controlled spatial distribution of Fe and V species.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2019

Effect of doping some lanthanide oxides on optical and radiation shielding properties of cadmium borate glasses

Ce{sub 2}O{sub 3}, La{sub 2}O{sub 3} and Nd{sub 2}O{sub 3} and Sm{sub 2}O{sub 3} doped in the cadmium borate host glass were prepared. Some optical and physical properties such as density, molar volume, optical band gap energy, the width of the band tails (Urbach energy) and radiation shielding properties were measured. In addition, the attenuation parameters, are calculated at different photon energies using the XCOM program and practical before and after irradiation. A correlation between the results from density, UV–Vis and FTIR measurements indicates that CdO plays a role as a network modifier and alter within the atomic structure is attributable to the formation of BO{sub 4} units. The attenuation parameters are calculated at different photon energies using the XCOM program and measured in experiments. These values show that there is a correlation between theoretical and experimental values. Hence, these results indicate that the samples have gamma-ray shielding in this energy range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effects of Doping on the Surface Energies of Nanocrystals and Evidence from Studies at High Pressure

Zn{sub 1-X}Mn{sub X}S (X = 0.85% and 1.26%) nanoparticles have been synthesized using a specially designed equipment and we have studied the influence of doping Mn{sup 2+} on the surface energy of ZnS. The high pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that doping with Mn{sup 2+} increases the surface energy of the nanocrystals. The theoretical result has been further corroborated by our experimental observation of an increase in the phase transition pressure of Mn{sup 2+} doped ZnS nanocrystals in diamond-anvil-cell studies.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Effect of Doping on Surface Reactivity and Conduction Mechanism in Sm-doped CeO2 Thin Films

Scanning probe microscopy measurements show irreversible surface electrochemistry in Sm-doped CeO2 thin films, which depends on humidity, temperature and doping concentration. A systematic study by electrochemical strain microscopy (ESM) in samples with two different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in water adsorption and splitting with subsequent proton liberation. We measure the behavior of the hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first order reversal curve (FORC) method. Complementing our study with spectroscopic measurements by hard x-ray photoemission spectroscopy we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity and conduction mechanism clearly emerges from all of our experimental results. We find that at lower Sm concentration proton conduction is prevalent, featured by lower activation energy and higher mobility. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A study of various geometries of gold nanowires and effect of doping

We present here the simulations, using Gupta Potential, of gold cylinders under stress, showing the changes leading to the formation of chains at nano level. When one of these short chains is stretched, it transforms from a zigzag configuration to a straight one, passing through various qualitatively different phases and ultimately disintegrating via an interesting ‘dimer’ phase. Strain vs stress for all the cases are presented and young’s modulus calculated. Using Density functional theory (DFT) study we have shown that doping of gold strand with Silicon or Carbon atoms increases its thermodynamic stability as suggested in a recent study.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Hydrogen passivation of titanium impurities in silicon: Effect of doping conditions

While the contamination of solar silicon by fast diffusing transition metals can be now limited through gettering, much attention has been drawn to the slow diffusing species, especially the early 3d and 4d elements. To some extent, hydrogen passivation has been successful in healing many deep centers, including transition metals in Si. Recent deep-level transient spectroscopy (DLTS) measurements concerning hydrogen passivation of Ti revealed the existence of at least four electrical levels related to Ti{sub i}H{sub n} in the upper-half of the gap. These findings challenge the existing models regarding both the current level assignment as well as the structure/species involved in the defects. We revisit this problem by means of density functional calculations and find that progressive hydrogenation of interstitial Ti is thermodynamically stable in intrinsic and n-doped Si. Full passivation may not be possible to attain in p-type Si as Ti{sub i}H{sub 3} and Ti{sub i}H{sub 4} are metastable against dissociation and release of bond-centered protons. All DLTS electron traps are assigned, namely, E40′ to Ti{sub i}H(-/0), E170′ to Ti{sub i}H{sub 3}(0/+), E(270) to Ti{sub i}H{sub 2}(0/+), and E170 to Ti{sub i}H(0/+) transitions. Ti{sub i}H{sub 4} is confirmed to be electrically inert.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electronic properties of graphene and effect of doping on the same

The electronic structure of pure and doped two dimensional crystalline material graphene have been computed and analyzed. Density functional theory has been employed to perform calculations. The electronic exchange and correlations are considered using local density approximation (LDA). The doped material is studied within virtual crystal approximation (VCA) upto 0.15e excess as well as deficient charge per unit cell. Full Potential Linear Augmented Plane Wave basis as implemented in ELK code has been used to perform the calculations. To ensures the monolayer of graphene, distance after which energy is almost constant when interlayer seperation is varied, is taken as separating distance between the layers. The obtained density of states and band structure is analyzed. Results show that there is zero band gap in undoped graphene and conduction and valence band meets at fermi level at symmetry point K. PDOS graph shows that near the fermi level the main contribution is due to 2p{sub z} electrons. By using VCA, calculations for doped graphene are done and the results for doped graphene are compared with undoped graphene. We found that by electron or hole doping, the point where conduction and valence bands meet can shift below or above the fermi level. The shift in bands seems almost as per rigid band model upto doping concentration studied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗