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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 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Low temperature and limited water activity reveal a pathway to magnesite via amorphous magnesium carbonate

Forsterite carbonated in thin H 2 O films to magnesite via amorphous magnesium carbonate during reaction with H 2 O-bearing liquid CO 2 at 25 °C. This novel reaction pathway contrasts with previous studies that were carried out at higher H 2 O activity and temperature, where more highly hydrated nesquehonite was the metastable intermediate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Molecular growth upon ionization of van der Waals clusters containing HCCH and HCN is a pathway to prebiotic molecules

The growth mechanisms of organic molecules in an ionizing environment such as the interstellar medium are not completely understood. Here we examine by means of ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) computations the possibility of bond formation and molecular growth upon ionization of van der Waals clusters of pure HCN clusters, and mixed clusters of HCN and HCCH, both of which are widespread in the interstellar medium. Ionization of van der Waals clusters can potentially lead to growth in low temperature and low-density environments. Our results show, that upon ionization of the pure HCN clusters, strongly bound stable structures are formed that contain N∙∙∙H bonds, and growth beyond pairwise HCN molecules is seen only in a small percentage of cases. In contrast, mixed clusters, where HCCH is preferentially ionized over HCN, can grow up to 3 or 4 units long with new carbon–carbon and carbon–nitrogen covalent bonds. Moreover, cyclic molecules formed, such as the radical cation of pyridine, which is a prebiotic molecule. The results presented here are significant as they provide a feasible pathway for molecular growth of small organic molecules containing both carbon and nitrogen in cold and relatively denser environments such as in dense molecular clouds but closer to the photo-dissociation regions, and protoplanetary disks. In the mechanism we propose, first, a neutral van der Waals cluster is formed. Once the cluster is formed it can undergo photoionization which leads to chemical reactivity without any reaction barrier.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Anaerobic respiration pathways and response to increased substrate availability of Arctic wetland soils

The availability of labile carbon (C) compounds in Arctic wetland soils is expected to increase due to thawing permafrost and increased fermentation as a result of decomposition of organic matter with warming. How microbial communities respond to this change will affect the balance of CO 2 and CH 4 emitted during anaerobic organic matter decomposition, and ultimately the net radiative forcing of greenhouse gas emissions from these soils. While soil water content limits aerobic respiration, the factors controlling methanogenesis and anaerobic respiration are poorly defined in suboxic Arctic soils. In this study, we conducted incubation experiments on two tundra soils from field sites on the Seward Peninsula, Alaska, with contrasting pH and geochemistry to determine the pathways of anaerobic microbial respiration and changes with increasing substrate availability upon warming. In incubation of soils from the circumneutral Teller site, the ratio of CO 2 to CH 4 dropped from 10 to <2 after 60 days, indicating rapid depletion of alternative terminal electron acceptors (TEAs). Addition of acetate stimulated production of CO 2 and CH 4 in a nearly 1:1 ratio, consistent with methanogenesis, and the composition of the microbial community shifted to favor clades capable of utilizing the added acetate such as the Fe(III)-reducing Geobacter and the methanogenic archaea Methanosarcina. In contrast, both CO 2 and CH 4 production declined with acetate addition during incubation of soils from the more acidic Council site, and fermentative microorganisms increased in abundance despite the high availability of fermentation products. These results demonstrate that the degree to which increasing substrate availability stimulates greenhouse gas production in tundra wetlands will vary widely depending on soil pH and geochemistry.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Directly measuring the structural transition pathways of strain-engineered VO 2 thin films

Epitaxial films of vanadium dioxide (VO 2 ) on rutile TiO 2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO 2 thin films deposited on isostructural MgF 2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF 2 substrate clearly reveals how elongation of V–V dimers accompanied by the shortening of V–O bonds triggers the intermediate M2 phase in the temperature range between 70–80 °C upon the heating–cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO 2 thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Pathways to fluorescence via restriction of intramolecular motion in substituted tetraphenylethylenes

The design of materials with enhanced luminescence properties is a fast-developing field due to the potential applicability of these materials as light-emitting diodes or for bioimaging. A transparent way to enhance the emission properties of interesting molecular candidates is blocking competing and unproductive non-radiative relaxation pathways by the restriction of intramolecular motions. Rationalized functionalization is an important possibility to achieve such restrictions. Using time-dependent density functional theory (TD-DFT) based on the ωB97XD functional and the semiempirical tight-binding method including long-range corrections (TD-LC-DFTB), this work investigates the effect of functionalization of the paradigmatic tetraphenylethylene (TPE) on achieving restricted access to conical intersections (RACI). Photodynamical surface hopping simulations have been performed on a larger set of compounds including TPE and ten functionalized TPE compounds. Functionalization has been achieved by means of electron-withdrawing groups, bulky groups which block the relaxation channels via steric hindrance and groups capable of forming strong hydrogen bonds, which restrict the motion via the formation of hydrogen bond channels. Most of the investigated functionalized TPE candidates show ultrafast deactivation to the ground state due to their still existing structural flexibility, but two examples, one containing –CN and –CF 3 groups and a second characterized by a network of hydrogen bonds, have been identified as interesting candidates for creating efficient luminescence properties in solution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Theoretical assessments of Pd–PdO phase transformation and its impacts on H 2 O 2 synthesis and decomposition pathways

The direct synthesis of H 2 O 2 from O 2 and H 2 provides a green pathway to produce H 2 O 2 , a popular industrial oxidant. Here, in this study, we theoretically investigate the effects of Pd oxidation states, coordination environments, and particle sizes on primary H 2 O 2 selectivities, assessed by calculating the ratio of rate constants for the formation of H 2 O 2 (via OOH* reduction; k O–H ) and the decomposition of OOH* (via O–O cleavage; k O–O ). For Pd metals, the k O–H /k O–O ratio decreased from 10 -4 for Pd(111) to 10 -10 for the Pd 13 cluster at 300 K, indicating poorer H 2 O 2 selectivity as Pd particle size decreases and low primary selectivities for H 2 O 2 overall. As the oxygen chemical potential increases and metals form surface and bulk oxides, the perturbation of Pd–Pd ensemble sites by lattice O atoms results in selectivities that become dramatically higher than unity. For instance, at 300 K, the k O–H /k O–O ratio increases significantly from 10 -4 to 10 9 to 10 16 as Pd(111) oxidizes to Pd 5 O 4 /Pd(111) and to PdO(100), respectively. In contrast, such selectivity enhancements are not observed for surface and bulk oxides that persistently contain rows of more metallic, undercoordinated Pd–Pd ensemble sites, such as PdO(101)/Pd(100) and PdO(101). These Pd–Pd ensembles are also absent when smaller Pd nanoparticles fully oxidize, indicating that smaller PdO clusters can be more selective for H 2 O 2 synthesis. These trends for primary H 2 O 2 selectivities were found to inversely correlate with trends for H 2 O 2 decomposition rates via O–O bond cleavage, demonstrating that catalysts with high primary H 2 O 2 selectivity can also hinder H 2 O 2 decomposition. Ab initio thermodynamic calculations are used to estimate the thermodynamically favored phase among Pd, PdO/Pd and PdO in O 2 , H 2 O 2 /H 2 O, and O 2 /H 2 environments. These results are combined to show that smaller Pd nanoparticles are more prone to be oxidized at lower oxygen chemical potentials, upon which they become more selective than larger Pd particles for H 2 O 2 synthesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Kinetic pathways of fast lithium transport in solid electrolyte interphases with discrete inorganic components

The transport of lithium ions in the solid electrolyte interphase (SEI) has been previously accepted to proceed in two steps: a fast pore diffusion through the outer, porous organic layer followed by a slow knock-off or vacancy diffusion in the inner, dense inorganic layer. The second step is believed to be the rate-limiting step during fast-charging. In this study, we have intentionally constructed a thicker SEI (SEI-rich) structure on the surface of monoclinic Nb 2 O 5 (H-Nb 2 O 5 ) by adding LiNO 3 into a conventional ethylene carbonate based electrolyte. The electrochemical performance of two electrodes, one SEI-rich and one with few SEI (SEI-lean), was found to be almost the same, including their fast-charging capability and cycling stability, despite the significant difference in their SEI structure. Importantly, analysis using cryogenic scanning/transmission electron microscopy showed the discrete decoration of individual inorganic particles (e.g., Li 2 O) and amorphous species (LiN x O y /organic components) over the surface of H-Nb 2 O 5 . These discrete inorganic particles are in contradiction to the formation of dense inner inorganic layer, which has been commonly postulated. Based on these findings, we propose a new mechanism for Li ion transport through the SEI: one-step pore diffusion, without the second step slow diffusion. This one-step pore diffusion process provides an extremely fast Li ion transport, and effectively removes the kinetic limitation of Li ion transport in the SEI for fast charging. Finally, these results strongly suggest that the influence of SEI structure on the transport kinetics of lithium ions is much less significant than previously accepted. These results offer a new understanding of possible lithium ion transport pathway within SEI and may have implications for the future designs of fast-charging battery materials.

25 ENERGY STORAGE↗
DOE OSTI2023

Local microenvironment tuning induces switching between electrochemical CO 2 reduction pathways

Gas diffusion layers (GDL) have become a critical component in electrochemical CO 2 reduction (CO 2 R) systems because they can enable high current densities needed for industrially relevant productivity. Besides this function, it is often assumed that the choice of catalyst and electrolyte play much more important roles than the GDL in influencing the observed product selectivity. Here, we show that tuning of the GDL pore size can be used to control the local microenvironment of the catalyst and hence, effect significant changes in catalytic outcomes. This concept is demonstrated using sputtered Ag films on hydrophobic PTFE substrates with 6 different pore sizes. Although Ag is known to be a predominantly CO generating catalyst, we find that smaller pore sizes favor the generation of formate up to a faradaic efficiency of 43%. Combined experimental and simulation results show that this is due to the influence of the pore size on CO 2 mass transport, which alters the local pH at the electrode, resulting in reaction pathway switching between CO and formate. Importantly, our results highlight the importance of the local microenvironment as an experimental knob that can be rationally tuned for controlling product selectivity: a key consideration in the design of CO 2 R systems.

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

The transformation of lepidocrocite (γ-FeOOH) with Fe($\tiny{II}$) (aq) in slightly acidic media: intermediate pathways and biomimetic behavior

Lepidocrocite (LP) is commonly found in natural or anthropogenic environments and oxidized alloy steel waste storage containers. Despite its importance, the end products formed and its mineral transformation pathways, including intermediate steps and underlying mechanisms under Fe(II) (aq) catalysis still need to be clarified due to decades of dispersed research. Here, in this work, we investigated LP's catalytic transformation with 10 mM and 0.2 mM Fe(II) (aq) at their natural solution pH's via bulk (X-ray Diffraction/XRD, Raman and Attenuated Total Reflectance Fourier Transform Infrared/ATR-FTIR) and micro/nano-scale (semi in situ Transmission Electron Microscopy/TEM) analysis. In general, we observed that goethite (GT) and LP were the main end products. However, a series of two major distinct intermediate events that were initiated by a dissolution type of reaction along with an “induction period” (lack of dissolution) on LP occurred. Fascinatingly, two of the intermediate steps along its mineral transformation presented novel types of non-classical mechanisms of crystallization via some type of guided oriented particle attachment. Furthermore, one of these intermediate steps is biomimetic in appearance, similar to what is observed during bacterial particle attachment. However, it uses inorganic nano-wire antennas that have a sensory-like function as observed with bacterial fimbriae and/or flagellum through an electron transparent film (similar to a bio-film matrix). Finally, this work leads us to comprehend the evolution of some well documented crystal morphologies for GT commonly observed in natural and anthropogenic settings.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Advanced pathways for hydrogen production: a collective view from a technical experts meeting

Hydrogen is an essential fuel and feedstock that can be produced in multiple ways to meet requirements for technological sectors that include energy storage, transportation, petroleum refining, and ammonia synthesis. To consider the future state of hydrogen manufacturing, a team of experts has assembled and examined three emerging hydrogen production technologies – photoelectrochemical, biological, and thermochemical. Each of these emerging technologies holds significant long-term potential for cost reduction while lowering industrial emissions associated with conventional methods of hydrogen manufacture (e.g., steam methane reforming) by using sunlight and renewable resources as primary sources of energy and feedstock, respectively. All three are currently at low technology readiness levels, however their applications, cost reduction opportunities and performance improvement pathways are under active development. In this work, opportunities and outlook for research that can directly advance the technologies are discussed.

08 HYDROGEN↗