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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Enhancing photoluminescence of graphene quantum dots by thermal annealing of the graphite precursor

Highlights: • The enhancement of GQD photoluminescence by precursor modification is proposed. • GQD photoluminescence 4 times increased with doubled quantum yield. • Higher content of peroxyl, aldehyde and epoxy groups results in better capacitance. - Abstract: Different approaches for enhancement of the graphene quantum dots (GQDs) photoluminescence have been developed lately. We report modified electrochemical procedure for GQDs synthesis. Graphitic rods were thermal annealing at 1700 °C in vacuum and used for electrochemical quantum dots synthesis. We found that the modified GQDs (GQDs-1700) consist of two while in the structure of pristine (p-GQDs) three graphene layers are measured. The main structural difference between GQDs is the increased structural disorder, caused by a higher amount of the oxygen containing functional groups in the structure of GQDs-1700. The photoluminescence of GQDs-1700 is 4 times more intense, red shifted and with doubled Quantum Yield compared to p-GQDs. In addition, the changes in electrochemical behavior of GQDs are observed: the higher capacitance was noticed if GQDs posses more reducible functional groups, such as peroxyl, aldehyde and epoxy groups while increase in keto functional groups leads to the lower capacitance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Au nanoparticles and graphene oxide co-loaded graphitic carbon nitride: Synthesis and photocatalytic application

Highlights: • The 0.5%Au/GO/g-C3N4 composite exhibites excellent photocatalytic activity. • GO nanoslice acts as an e- transporter and acceptor Au/GO/g-C3N4 composite. • Au particles in Au/GO/g-C3N4 composite act as electron trap to separate e- and h{sup +}. • Adsorbed OH and h{sup +} are involved in reducing and oxidizing process, respectively. - Abstract: Inhibition of charge-carrier recombination in semiconductors is one of the most efficient methods to enhance the photocatalytic performance in visible light irradiation. Noble metals and graphene oxide (GO) are currently regarded as fine channels to enhance the charge-carrier transfer efficiency. Herein, novel Au nanoparticles and GO co-loaded graphitic carbon nitride (Au/GO/g-C{sub 3}N{sub 4}) composite was obtained by a single step ultrasonic chemical process, and the photocatalytic activity was tested in Rhodamine B (Rh B) photodegradation in visible light region. The Au/GO/g-C{sub 3}N{sub 4} composite containing 0.5%Au (molar Au per unit molar of g-C{sub 3}N{sub 4}) exhibited excellent photocatalytic activity and might be attributed to the important part of GO nanoslices and Au nanoparticles. An electron transporter and acceptor have been acted by the GO nanoslices to facilitate the charge transfer and against the recombination of photoexcited electron-hole pairs. In the meantime, an electron trap was formed on the Au nanoparticles to separate the charges. Furthermore, it was found that the reactive species of adsorbed OH and holes took part in the reduction and oxidization actions in the photocatalytic process, respectively. The design of the Au/GO/g-C{sub 3}N{sub 4} composite provides a novel route to synthesis of highly efficient photocatalysts in environmental application.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Expanded graphite for thermal conductivity and reliability enhancement and supercooling decrease of MgCl2⋅6H2O phase change material

Highlights: • MgCl{sub 2}·6H{sub 2}O/EG composite PCMs show the superior thermal transfer ability. • 83.33 wt.% of MgCl{sub 2}·6H{sub 2}O encapsulated in composite PCM was obtained. • EG and nucleating agents decreased the supercooling degree. - Abstract: Expanded graphite (EG) is considered as a promising supporter for phase change material (PCM) due to its unique porous structure and excellent heat transfer ability. In the study, EG with different mass fractions (9 wt.%, 13 wt.%, 16.67 wt.%, and 20 wt.%) was respectively blended with MgCl{sub 2}·6H{sub 2}O. The thermal conductivity data of composite PCMs with 9 wt.%, 13 wt.%, 16.67 wt.%, and 20 wt.% of EG was respectively measured to be 0.942 W/m K, 1.053 W/m K, 1.354 W/m K and 1.658 W/m K. DSC analysis showed that 16.67 wt.% of EG decreased the degree of supercooling by 29.4 °C and that the addition of 3.0 wt.% SrCO{sub 3} further decreased the degree of supercooling by 17.9 °C. The maximum encapsulation weight percentage of MgCl{sub 2}·6H{sub 2}O reached 83.33 wt.% after 30 phase change cycles without significantly reducing its latent heat value, exhibiting the relatively stable thermal reliability.

36 MATERIALS SCIENCE↗
DOE OSTI2018

ZnCr-LDH/N-doped graphitic carbon-incorporated g-C3N4 2D/2D nanosheet heterojunction with enhanced charge transfer for photocatalysis

Highlights: • Synthesize a novel ZnCr-LDH/modified-g-C{sub 3}N{sub 4} photocatalyst by a facile template free in situ coprecipitation method approach. • The synergistic effect of adsorption and catalytic improve the ability of photodegradation. • Composites with 2D-2D nanosheets structure present superior catalytic activity than pure phases. • Calcinated photocatalyst has high visible-light respond capacity and low recombination rate of electron-hole pairs due to the formation of heterojunction. - Abstract: In this work, we successfully synthesized a novel ZnCr-LDH/N-doped graphitic carbon-incorporated g-C{sub 3}N{sub 4} (g-C{sub 3}N{sub 4}-C(N)) hybrid with unique two-dimensional/two-dimensional (2D-2D) structure for the efficient degradation of Congo red under visible light irradiation. It was demonstrated that ZnCr-LDH/g-C{sub 3}N{sub 4}-C(N) exhibited lower photogenerated electron-hole recombination rate and enhanced light harvesting. Furthermore, its derived calcined product also showed outstanding performance due to the unique heterogeneous structure and the synergistic effect between photocatalysis and adsorption. In addition, the mixed metal oxide ZnO and ZnCr{sub 2}O{sub 4} in ZnCr-CLDH/g-C{sub 3}N{sub 4}-C(N) dispersed uniformly on the g-C{sub 3}N{sub 4}-C(N) exposed more active sites and facilitated the photodegradation of pollutants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Decoration of Fe3O4 and CoWO4 nanoparticles over graphitic carbon nitride: Novel visible-light-responsive photocatalysts with exceptional photocatalytic performances

Highlights: • g-C{sub 3}N{sub 4}/Fe{sub 3}O{sub 4}/CoWO{sub 4} nanocomposites as efficient photocatalysts are reported. • The g-C{sub 3}N{sub 4}/Fe{sub 3}O{sub 4}/NiWO{sub 4} (10%) nanocomposite exhibited the highest activity. • Activity was 12.5-folds greater than g-C{sub 3}N{sub 4} in RhB degradation under visible light. • The nanocomposite is magnetically separated from the treated solution. - Abstract: In this work, nanoparticles of Fe{sub 3}O{sub 4} and CoWO{sub 4} were decorated over graphitic carbon nitride (g-C{sub 3}N{sub 4}) to fabricate very efficient visible-light-responsive photocatalysts through a facile refluxing-calcination method. Among the fabricated photocatalysts, the g-C{sub 3}N{sub 4}/Fe{sub 3}O{sub 4}/CoWO{sub 4} (10%) nanocomposite displayed superior activity in degradation of RhB, which is 12.5 and 11.5 times superior than the pristine g-C{sub 3}N{sub 4} and g-C{sub 3}N{sub 4}/Fe{sub 3}O{sub 4} photocatalysts, respectively. Additionally, exceptional performance of the ternary photocatalyst was confirmed in degradations of three organic pollutants under visible light. Higher harvesting ability of visible light and effective separation of electron/hole pairs from recombination are two main factors responsible for the observed exceptional photocatalytic activity of the ternary nanocomposite. Due to enough saturation magnetization of the ternary nanocomposite, the photocatalyst was entirely separated from the degradation system by a conventional magnet. Finally, photostability of the ternary photocatalyst during four successive runs was confirmed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced charge separation ability and visible light photocatalytic performance of graphitic carbon nitride by binary S, B co-doping

Highlights: • S, B hertroatoms were successfully doped into g-C{sub 3}N{sub 4} by thermal treatment. • S, B heteroatoms effectively change the intrinsic electronic structure of g-C{sub 3}N{sub 4}. • The SBCN shows much higher photocatalytic activity than CN, SCN and BCN. • The SBCN has high photocatalytic stability and reusability. - Abstract: As a metal-free visible light photocatalyst, the photocatalytic performance of graphitic carbon nitride (g-C{sub 3}N{sub 4}) is limited. To improve the activity of g-C{sub 3}N{sub 4}, we herein report a binary S, B co-doped g-C{sub 3}N{sub 4} (SBCN) by thermally treating the precursor mixture of thiourea, melamine and boron oxide. Characterization results show the obtained SBCN has ultrathin sheet-like morphology and these doped heteroatoms are uniform distributed in the sample. Compared to the pristine g-C{sub 3}N{sub 4}, the SBCN has a small band gap with additional absorption in visible light region, decreased photoluminescence emission and electric resistance and enhanced photocurrent response. These advantages endue SBCN with excellent photocatalytic activity for degradation of organic dyes, and its reaction rate is 6.5 times higher than that of the g-C{sub 3}N{sub 4}. Moreover, the successive cycles demonstrate that this SBCN possesses a superior stability and reusability. Finally, we propose a possible photocatalytic mechanism based on the band structure and mainly active species.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Superconductivity in graphite intercalation compounds

This study examines the field of superconductivity in the class of materials known as graphite intercalation compounds which has a history dating back to the 1960s. This paper recontextualizes the field in light of the discovery of superconductivity in CaC₆ and YbC₆ in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how this relates to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Interplay of Mechanochemistry and Material Processes in the Graphite to Diamond Phase Transformation

The manifestation of intramolecular strains in covalent systems is widely known to accelerate chemical reactions and open alternative reaction paths. This process is moderately well understood for isolated molecules and unimolecular processes. However, in condensed matter processes such as phase transformations, material properties and structure may influence typical mechanochemical effects. Therefore, we utilize steered molecular dynamics to induce out of plane strains in graphite and compress the system under a constant strain rate to induce phase transformation. We show that the out of plane strain allows phase transformations to initiate at small amounts of compressive strain. Yet, in contrast to typical mechanochemical results, the sum of compressive and out of plane work needed to form a diamond has a local minimum due to altered defect formation processes during phase transformation. Additionally, these altered processes slow the kinetics of the phase transformation, taking longer from initiation to total material transformation.

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

Graphitized Mesoporous Engineered Carbon Support for Fuel Cell Applications

As proton exchange membrane fuel cells mature into commercial devices capable of powering a wide range of stationary and automotive applications, they need materials with tunable properties to improve their performance and durability. Carbon supports used for platinum (Pt) nanoparticle dispersion is typically based on a furnace black-type material with a random structure, thereby hindering progress in catalyst development. To address this challenge, engineered carbons with a tunable mesoporous structure offer an opportunity to maximize catalyst performance and durability. In this article, we report on the development of a graphitized, mesoporous carbon support with a high degree of ordering, labeled here as ECS4005, for the dispersion of Pt nanoparticles. Pt/ECS4005 shows significantly improved kinetic activity due to its mesoporous structure that mitigates Pt poisoning by sulfonate functional groups in the ionomer while simultaneously enabling favorable accessibility to reactants.

25 ENERGY STORAGE↗
DOE OSTI2020

Pyrazine-Fused Porous Graphitic Framework-Based Mixed Matrix Membranes for Enhanced Gas Separations

Membrane-based separations can mitigate the capital- and energy-intensive challenges associated with traditional thermally driven processes. To further push the boundary of gas separations, mixed matrix membranes (MMMs) have been extensively exploited; however, identifying an optimal nanofiller to boost the separation performance of MMMs beyond Robeson permeability-selectivity upper bounds remains an ongoing challenge. In this study, a new class of MMMs based on pyrazine-fused crystalline porous graphitic frameworks (PGFs) is reported. At a loading of 6 wt % PGFs, the MMMs surpass the current H 2 /CH 4 Robeson upper bound, ideally suited for applications such as H 2 regeneration. In addition, the fabricated MMMs exhibit appealing CO 2 separation performance, closely approaching the current Robeson upper bounds for CO 2 separation. Compared with the pristine polymeric membranes, the PGF-based MMMs display a record-high enhancement of gas permeability over 120% while maintaining intrinsic gas selectivities. Highlighting the crucial role of the crystallinity of nanofillers, this study demonstrates a facile and effective approach in formulating high-performance MMMs, complementing state-of-the-art membrane formation processes. The design principles open the door to energy-efficient separations of gas mixtures with enhanced productivity compatible with the current membrane manufacturing.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chemical Interplay of Silicon and Graphite in a Composite Electrode in SEI Formation

In this study, we investigated the effect of the Si/graphite weight ratio in half-cells on the solid electrolyte interphase (SEI) layer's chemistry. The nominal concentrations of active materials were (wt % Si/wt % Gr) 15/73, 30/58, 60/28, and 80/0. The electrolyte in the cells consisted of either 1.2 M LiPF 6 in ethylene carbonate/ethyl methyl carbonate (3:7 by wt) or 1.2 M LiPF 6 in ethylene carbonate:ethyl methyl carbonate (3:7 by wt) + 10 wt % fluoroethylene carbonate. These coin cells were cycled five times at the C/10 rate. As expected, the addition of silicon to the electrode significantly increased the measured capacity. Examination of the aged composite material showed that the electrolyte influenced the concentration of chemical environments on the surface. Depth profiling revealed that these concentrations of surface environments changed with sputtering time. A statistics-of-mixtures model was used to deconvolute how silicon and graphite interacted during the formation of these species and how the interaction changed with depth.

25 ENERGY STORAGE↗
DOE OSTI2022

Probing the Role of Multi-scale Heterogeneity in Graphite Electrodes for Extreme Fast Charging

Electrode-scale heterogeneity can combine with complex electrochemical interactions to impede lithium-ion battery performance, particularly during fast charging. This research investigates the influence of electrode heterogeneity at different scales on the lithium-ion battery electrochemical performance under operational extremes. We employ image-based mesoscale simulation in conjunction with a three-dimensional electrochemical model to predict performance variability in 14 graphite electrode X-ray computed tomography data sets. Our analysis reveals that the tortuous anisotropy stemming from the variable particle morphology has a dominating influence on the overall cell performance. Cells with platelet morphology achieve lower capacity, higher heat generation rates, and severe plating under extreme fast charge conditions. On the contrary, the heterogeneity due to the active material clustering alone has minimal impact. Our work suggests that manufacturing electrodes with more homogeneous and isotropic particle morphology will improve electrochemical performance and improve safety, enabling electromobility.

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

Seeding Iron Trifluoride Nanoparticles on Reduced Graphite Oxide for Lithium-Ion Batteries with Enhanced Loading and Stability

Development of electric vehicles and portable electronic devices during the past decade calls for lithium-ion batteries (LIBs) with enhanced energy density and higher stability. Integration of FeF 3 phases and carbon structures leads to promising cathode materials for LIBs with high voltage, capacity, and power. In this study, FeF 3 ·0.33H 2 O nanoparticles were synthesized on reduced graphite oxide (rGO) nanosheets using an in situ approach. By chemically tuning the interfacial bonding between FeF 3 ·0.33H 2 O and rGO, we successfully achieved high particle loading and enhanced cycling stability. Specifically, a discharge capacity of ~208.3 mAh g –1 was observed at a current density of 0.5 C. The FeF 3 ·0.33H 2 O/rGO nanocomposites also demonstrate great cycle capability with a reversible discharge capacity of 133.1 mAh g –1 after 100 cycles at 100 mA g –1 ; the capacity retention is about 97%. Furthermore, this study provides an alternative strategy to further improve the stability and performance of iron fluoride/carbon nanocomposite materials for LIB applications.

25 ENERGY STORAGE↗
DOE OSTI2022

Quantifying Graphite Solid-Electrolyte Interphase Chemistry and its Impact on Fast Charging

The solid-electrolyte interphase (SEI) enables the remarkable capacity retention of lithium-ion batteries, yet a comprehensive quantitative description of the SEI composition remains elusive. Using a combination of differential electrochemical mass spectrometry and mass spectrometry titration, we quantify graphite SEI components formed under electrolytes of varying salt concentrations. We find that, regardless of salt concentration, a conversion of initially deposited lithium ethylene dicarbonate to monocarbonates (likely lithium ethylene monocarbonate) and noncarbonate species occurs, and the extent of this conversion increases with electrolyte aging. Here, we additionally demonstrate that as the concentration increases (up to 2.0 M LiPF6), the SEI becomes progressively thinner with more LiF and less solid carbonates deposited. Finally, we reveal that less dead lithium formation and less solid carbonate deposition occur during prolonged fast charging for higher-concentration electrolytes. Because of the advantages imparted by a thinner SEI, the onset state of charge for lithium plating for the 2.0 M electrolyte is later than that predicted by a standard electrochemical model, underscoring the importance of explicit SEI effects in future electrochemical models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Development of Nanocrystalline Graphite from Lignin Sources

Carbon composites are attractive to a variety of high-impact applications, such as carbon fibers, batteries, and vehicle parts, due to their multifunctional properties. The properties of carbon are highly dependent on the allotrope the carbon takes and the functionality, impurities, and defects contained within the structure. The increase in demand for sustainable carbon sources in energy storage devices motivates interest in understanding synthesis parameters of lignin value-added products. Also, as the dependence on oil for fuel decreases, alternative sources for carbon in many applications will be needed. In this work, the thermochemical conversion of lignin powders from different feedstocks was evaluated via small and wide-angle X-ray scattering techniques to resolve the amorphous, disordered, and crystalline domains present in the lignin carbons. Scattering analyses indicated an evolution of hierarchical structures along with an increase in ordered domains as a function of carbonization temperature. Qualitative and quantitative methods were used to describe isotropic scattering intensity profiles at multiple length scales. The use of power law models in the mesoscopic region served as the basis to describe morphological changes related to structural features, for example, graphene stacking, degree of roughness, and surface fractals. Kraft softwood and switchgrass produced carbon powder with the most crystalline domains and the least surface roughness. Softwoods reached the highest degree of crystallinity followed by switchgrass samples and had less variability in particle sizes. These results suggest lignin carbons extracted from softwoods and switchgrass are viable substitutes for graphite. Interpretation of X-ray scattering data from lignin carbon powders elucidates feedstock- and processing-dependent morphological features across multiple length scales providing a straightforward framework to evaluate the feasibility of leveraging lignin carbons for producing tunable application-specific materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Crystal structure of graphite under room-temperature compression and decompression

Recently, sophisticated theoretical computational studies have proposed several new crystal structures of carbon (e.g., bct-C{sub 4}, H-, M-, R-, S-, W-, and Z-carbon). However, until now, there lacked experimental evidence to verify the predicted high-pressure structures for cold-compressed elemental carbon at least up to 50 GPa. Here we present direct experimental evidence that this enigmatic high-pressure structure is currently only consistent with M-carbon, one of the proposed carbon structures. Furthermore, we show that this phase transition is extremely sluggish, which led to the observed broad x-ray diffraction peaks in previous studies and hindered the proper identification of the post-graphite phase in cold-compressed carbon.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Benchmarking of HEU Mental Annuli Critical Assemblies with Internally Reflected Graphite Cylinder

Three experimental configurations of critical assemblies, performed in 1963 at the Oak Ridge Critical Experiment Facility, which are assembled using three different diameter HEU annuli (15-9 inches, 15-7 inches and 13-7 inches) metal annuli with internally reflected graphite cylinder are evaluated and benchmarked. The experimental uncertainties which are 0.00055, 0.00055 and 0.00055 respectively, and biases to the detailed benchmark models which are -0.00179, -0.00189 and -0.00114 respectively, were determined, and the experimental benchmark keff results were obtained for both detailed and simplified model. The calculation results for both detailed and simplified models using MCNP6-1.0 and ENDF VII.1 agree well to the benchmark experimental results with a difference of less than 0.2%. These are acceptable benchmark experiments for inclusion in the ICSBEP Handbook.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗