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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Ion-acoustic solitons, double layers and supersolitons in a plasma with two ion- and two electron species

The polarity of ion-acoustic solitons that arise in a plasma with two (same mass, different temperature) ion species and two (different temperature) electron species is investigated. Two different fluid models are compared. The first model treats all species as adiabatic fluids, while the second model treats the ion species as adiabatic, and the electron species as isothermal. Nonlinear structures are analysed via the reductive perturbation analysis and pseudo-potential analysis. Each model supports both slow and fast ion-acoustic solitons, associated with the two (slow and fast) ion-acoustic speeds. The models support both positive and negative polarity solitons associated with the slow ion-acoustic speed. Moreover, results are in good agreement, and both models support positive and negative polarity double layers. For the fast ion-acoustic speed, the first model supports only positive polarity solitons, while the second model supports solitons of both polarity, coexistence of positive and negative polarity solitons, double layers and supersolitons. A novel feature of our analysis is the evaluation of nonlinear structures at critical number densities where polarity changes occur. This analysis shows that solitons that occur at the acoustic speed are neither a necessary nor a sufficient condition for the phenomenon of coexistence. The relationship between the existence regions of supersolitons and soliton polarity is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2002

Double Layers in Expanding Plasmas and Their Relevance to the Auroral Plasma Processes

When a dense plasma consisting of a cold and a sufficiently warm electron population expands, a rarefaction shock forms. In the expansion of the polar wind in the magnetosphere, it has been previously shown that when a sufficiently warm electron population also exists, in addition to the usual cold ionospheric one, a discontinuity forms in the electrostatic potential distribution along the magnetic field lines. Despite the lack of spatial resolution and the assumption of quasi-neutrality in the polar wind models, such discontinuities have been called double layers (DLs). Recently similar discontinuities have been invoked to partly explain the auroral acceleration of electrons and ions in the upward current region. By means of one-dimensional Vlasov simulations of expanding plasmas, for the first time we make here the connection between (i) the rarefaction shocks, (ii) the discontinuities in the potential distributions, and (iii) DLs. We show that when plasmas expand from opposite directions into a deep density cavity with a potential drop across it and when the plasma on the high-potential side contains two electron populations, the temporal evolution of the potential and the plasma. distribution generates evolving multiple double layers with an extended density cavity between them. One of the DLs is the rarefaction-shock (RFS) and it forms by the reflections of the cold electrons coming from the high-potential side; it supports a part of the potential drop approximately determined by the hot electron temperature.

Singh, Nagendra↗
NASA NTRS1987

Effect of double layers on magnetosphere-ionosphere coupling

The Earth's auroral zone contains dynamic processes occurring on scales from the length of an auroral zone field line which characterizes Alfven wave propagation to the scale of microscopic processes which occur over a few Debye lengths. These processes interact in a time-dependent fashion since the current carried by the Alfven waves can excite microscopic turbulence which can in turn provide dissipation of the Alfven wave energy. This review will first describe the dynamic aspects of auroral current structures with emphasis on consequences for models of microscopic turbulence. A number of models of microscopic turbulence will be introduced into a large-scale model of Alfven wave propagation to determine the effect of various models on the overall structure of auroral currents. In particular, the effects of a double layer electric field which scales with the plasma temperature and Debye length is compared with the effect of anomalous resistivity due to electrostatic ion cyclotron turbulence in which the electric field scales with the magnetic field strength. It is found that the double layer model is less diffusive than in the resistive model leading to the possibility of narrow, intense current structures.

Lysak, Robert L.↗
DOE OSTI2023

Modeling the Environment-Dependent Kinetics of Oxygen Reduction Reaction – a Continuum Model for Electric Double Layer

Here, for proton-exchange-membrane fuel cells (PEMFCs) to achieve broad commercialization, improved energy-conversion efficiency with minimal Pt-based electrocatalyst is required. Because the sluggish rate of oxygen reduction reaction (ORR) limits the efficiency of PEMFCs, the efficiency improvement requires a better understanding of ORR kinetics and mechanism to design better catalyst. To understand the ORR mechanism, theoretical and experimental analyses have been conducted. While previous studies reasonably explained the catalyst-dependent activity on single crystal catalysts in 0.1 M perchloric acid solution, the explicit effect of electrolyte and related microenvironments is not thoroughly understood. The change in the electrolyte alters the electric-double-layer (EDL) structure and thus the local microenvironment at the electrode/electrolyte interface. Thus, the structure of the EDL should be carefully analyzed to uncover the electrolyte-dependent reaction kinetics. In this talk, we propose a multiscale continuum model to predict the EDL structure and examine the effect of perchloric acid concentration on ORR activity on Pt (111). The model includes Density Potential Functional Theory (DPFT) for electron density and Modified Poisson Boltzmann equation for species’ density and electric potential. Also, the interaction between adsorbents and electric field is taken into account by minimizing the grand potential. After model validation with experimentally measured double-layer capacity data as a function of applied potential and concentration, the effect of the perchloric acid concentration (0.02 M – 0.2 M) on ORR activity is analyzed and discussed. It is shown that the model reproduces the specific activity obtained in the experiments when assuming the oxygen adsorption is limiting the rate, which can be attributed to the large energetic barrier for solvent reorganization. Then, extension of the model to PEMFC ionomer electrolytes will be introduced. Overall, the model framework and findings provide insights into the ORR mechanism and guidance on how to tailor catalyst materials for increased PEMFC performance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Analysis of thermocapacitive effects in electric double layers under a size modified mean field theory

Thermodynamics of the electric double layer has received renewed interest for its potential applications in low-grade waste heat harvesting and reversible heating/cooling in supercapacitors. We apply a size-modified mean field theory to analytically capture the influence on the pseudo-Seebeck coefficient S = ∂φ 0 /∂T) σ of different factors, including the electrode potential φ 0 , asymmetry in ion sizes, and ion concentration, under a fixed electrode surface charge σ. The pseudo-Seebeck coefficient is predicted to scale as φ 0 /T at low electrode potentials, but it reaches limiting values when the electrode potential exceeds crossover values due to the steric effect. The qualitative behavior changes substantially, however, when the temperature dependence of the permittivity is taken into account. The pseudo-Seebeck coefficient S is then predicted to scale linearly with φ 0 even at high electrode potentials, significantly over-predicting the experimental values. Lastly, this suggests a strong influence of phenomena not captured in the mean field theory, such as deviation of local effective permittivity from the bulk value, thermally facilitated adsorption or desorption of ions on electrode surfaces, and weakening of ionic associations with temperature.

42 ENGINEERING↗
NASA NTRS2014

Low Temperature Double-Layer Capacitors Using Asymmetric and Spiro-Type Quaternary Ammonium Salts

Double-layer capacitors capable of operating at extremely low temperatures (e.g., as low as -80.degree. C.) are disclosed. Electrolyte solutions combining a base solvent (e.g., acetonitrile) and a cosolvent are employed to lower the melting point of the base electrolyte. Example cosolvents include methyl formate, ethyl acetate, methyl acetate, propionitrile, butyronitrile, and 1,3-dioxolane. A quaternary ammonium salt including at least one of triethylmethylammonium tetrafluoroborate (TEMATFB) and spiro-(1,1')-bipyrrolidium tetrafluoroborate (SBPBF.sub.4), is used in an optimized concentration (e.g., 0.10 M to 0.75 M), dissolved into the electrolyte solution. Conventional device form factors and structural elements (e.g., porous carbon electrodes and a polyethylene separator) may be employed.

Brandon, Erik J.↗
DOE OSTI2016

Stable electrolyte for high voltage electrochemical double-layer capacitors

A simple electrolyte consisting of NaPF 6 salt in 1,2-dimethoxyethane (DME) can extend the voltage window of electric double-layer capacitors (EDLCs) to >3.5 V. DME does not passivate carbon electrodes at very negative potentials (near Na/Na + ), extending the practical voltage window by about 1.0 V compared to standard, non-aqueous electrolytes based on acetonitrile. The voltage window is demonstrated in two- and three-electrode cells using a combination of electrochemical impedance spectroscopy (EIS), charge-discharge cycling, and measurements of leakage current. DME-based electrolytes cannot match the high conductivity of acetonitrile solutions, but they can satisfy applications that demand high energy density at moderate power. The conductivity of NaPF 6 in DME is comparable to commercial lithium-ion battery electrolytes and superior to most ionic liquids. Lastly, factors that limit the voltage window and EDLC energy density are discussed, and strategies to further boost energy density are proposed.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

Effect of Solution pH on Layered Double Hydroxide Formation on Electrogalvanized Steel Sheets

The effect of solution pH on layered double hydroxide (LDH) formation on electrogalvanized steel sheets (EG steel) was investigated in this study. EG steel with a Zn coating weight of 64 g/m{sup 2} was used as a substrate and was immersed in a 0.25 M Na{sub 2}Al{sub 2}O{sub 4}-based solution having several pH values to form a Zn-Al-based LDH conversion coating. After rinsing and drying, corrosion resistance was evaluated by EIS. It was found that corrosion resistance increased with increasing pH up to pH 12.6. However, above this pH, corrosion resistance tended to display a sharp decrease. To understand this trend, the conversion coating was characterized by FT-IR, XRD and GDOES. The conversion coating was found to consist mainly of Zn{sub 2}Al(OH){sub 6}(CO{sub 3}){sub 0.5}·xH{sub 2}O crystals (an LDH). The thickness of the conversion coating layer increased with increasing pH up to pH 12.4, but above this pH, its thickness tended to decrease due to the formation of ZnO. The reason for the dependence of corrosion resistance on solution pH is discussed from the viewpoints of the thickness and changes in the microstructure of the conversion coating layer, including its crystal size, density and uniformity, which could vary during the initial formation of ZnO at pH values above 12.6.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A facile mechanochemical approach to synthesize Zn-Al layered double hydroxide

In this study, a mechanochemical route to synthesize Zn-Al layered double hydroxide (LDH) was introduced, in which Zn basic carbonate and Al hydroxide were first dry milled into an activated state and then agitated in water to obtain the final products. The as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Thermogravimetry (TG) and Scanning electron microscopy (SEM). The products possessed a high crystallinity of Zn–Al LDH phase without any other impurities, proving a facile and effective preparation of Zn–Al LDH by using non-heating mechanochemical approach. - Highlights: • A non-heating mechanochemical route to synthesize Zn-Al LDH. • The products possessed high crystalline Zn-Al LDH phase. • No emission of other impurities or wastewater.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1978

Experimental observations of strong double layers

A computer simulation is applied to the production of strong electric potential double layers (DL) in a triple plasma device. The simulation is intended to represent DL in the low magnetosphere above the auroral zones. The DL are described as standing electrostatic shocks with different energy coefficients in their strong and weak forms. The strong DL was generally found to be unstable, but stability could be imparted if a population of trapped electrons was presented. Stability increased with the length of the system. A schematic for the system is presented, and a phase-space plot of electrons (indicating system stability) is graphed.

Coakley, P.↗
DOE OSTI2021

Colloidal synthesis and charge carrier dynamics of Cs 4 Cd 1-x Cu x Sb 2 Cl 12 (0 ≤ x ≤ 1) layered double perovskite nanocrystals

The toxicity and instability of lead-based metal halide perovskites are the two main obstacles that prevent perovskite materials from implementation in applications. Recently, layered double perovskites (LDPs) emerge as a new family of perovskite materials which provide a new route to solve these problems by lead-component replacement and reduction of crystal structure dimensionality. However, LDP nanocrystals (NCs) have been rarely studied, limiting the further property exploration and application realization. In this work, we report the colloidal synthesis of a series of Cs 4 Cd 1-x Cu x Sb 2 Cl 12 (0 ≤ x ≤ 1) LDP NCs by tuning the stoichiometry of metal precursors. The composition-structure-property relationships of the resulting LDP NCs are studied through materials characterizations, density functional theory calculations, and transient-absorption spectroscopy. In addition, we demonstrate that high-performance high-speed photodetectors can be fabricated using the colloidal LDP NCs through solution-processing. This work premises further expansion of such LDP-based materials for both fundamental studies and application integrations.

25 ENERGY STORAGE↗
DOE OSTI2020

Insights from machine learning of carbon electrodes for electric double layer capacitors

Recent years have witnessed the broad use of carbon electrodes for electric double layer capacitors (EDLCs) because of large surface area, high porosity and low cost. Whereas experimental investigations are mostly focused on the device performance, computational studies have been rarely concerned with electrochemical properties at conditions remote from equilibrium, limiting their direct applications to materials design. Through a comprehensive analysis of extensive experimental data with various machine-learning methods, we report herein quantitative correlations between the structural features of carbon electrodes and the in-operando behavior of EDLCs including energy and power density. Machine learning allows us to identify important characteristics of activated carbons useful to optimize their efficiency in energy storage.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Highly Efficient Iodine Capture by Layered Double Hydroxides Intercalated with Polysulfides

We demonstrate strong iodine (I-2) vapor adsorption using Mg/Al layered double hydroxide (MgAl-LDH) nanocomposites intercalated with polysulfide (S-x(2-)) groups (S-x-LDH, x = 2, 4, 6). The as-prepared LDH/polysulfide hybrid materials display highly efficient iodine capture resulting from the reducing property of the intercalated polysulfides. During adsorption, the I-2 molecules are reduced to I-3(-) anions by the intercalated [S-x](2-) groups that simultaneously are oxidized to form S8. In addition to the chemical adsorption, additional molecular I-2 is physically captured by the LDH composites. As a result of these parallel processes, and despite their very low BET surface areas, the iodine capture capacities of S-2-LDH, S-4-LDH, and S-6-LDH are similar to 1.32, 1.52, and 1.43 g/g, respectively, with a maximum adsorption of 152% (wt %). Thermogravimetric and differential thermal analysis (TG-DTA), energy dispersive X-ray spectroscopy (EDS), and temperature-variable powder X-ray diffraction (XRD) measurements show the resulting I-3(-) ions that intercalated into the LDH gallery have high thermal stability (>= 350 degrees C). The excellent iodine adsorption performance combined with the facile preparation points to the S-x-LDH systems as potential superior materials for adsorption of radioactive iodine, a waste product of the nuclear power industry.

Ma, Shulan↗
DOE OSTI2025

Boosting the Oxygen Evolution Reaction by Tuning the Interfacial Iron Adsorption on Layered Double Hydroxide

Understanding the interaction between ions in the electrolyte and electrode materials plays an important role in optimizing the water electrolysis performance for hydrogen production. Herein, the synergistic effect of iron (Fe) in the electrolyte and interlayer anions within the layered structure on the oxygen evolution reaction (OER) has been investigated by combining material synthesis with controlled structure, multiple characterization techniques, and first-principles calculations. Nickel aluminum layered double hydroxides (NiAl-LDHs) with different interlayer anions (CO 3 2– , Cl – , and Br – ) show similar oxygen evolution activity in the absence of Fe species in the electrolyte. The addition of Fe into the electrolyte results in improved performance for all of the NiAl-LDHs, following the rank LDH-Br > LDH-Cl > LDH-CO 3 , under all of the conditions with varied concentration of Fe. X-ray absorption spectroscopy and identical location electron microscopy analyses show that the LDH structure remains unchanged after the OER activity test, while in situ stationary probe rotating disk electrode inductively coupled plasma mass spectrometry (SPRDE-ICP-MS) measurements show partial dissolution of the intercalating halide ions during cycling, with less dissolution for Br-intercalated materials. Insights from theoretical calculations demonstrate the thermodynamic preference of Br – to remain intercalated in the presence of Fe, while the stronger adsorption of Fe(OH) 3 species on the LDH-Br sample promotes the OER activity. In conclusion, these results provide mechanistic insights into the rational design of active layered materials with an enhanced OER performance for efficient water electrolysis.

58 GEOSCIENCES↗
NASA NTRS1984

Numerical simulations of double layers and auroral electric fields

Recent one-dimensional and two-dimensional numerical simulations of double layers (DLs) in the electric fields of the auroral plasma are reviewed, with reference to observational data. It is found that two-dimensional DLs driven by current sheets of finite thickness have different characteristics, depending on whether the layer thickness is less than or much greater than the ion gyroradius: When thickness is less than ion gyroradius, V-shaped DLs form with nearly equal parallel and perpendicular potential drops; when layer thickness is much greater than ion gyroradius the major parallel potential drop occurs outside the current sheet and the perpendicular electric fields are localized at the edges of the current sheet. It is shown that some features of the simulated fields, such as the amplitudes and scale lengths, are qualitatively similar to those observed in space.

Singh, N.↗
DOE OSTI2021

Realization of AlSb in the Double-Layer Honeycomb Structure: A Robust Class of Two-Dimensional Material

Exploring new two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a bandgap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. Here, the successful growth of a DLHC material here opens the door for the realization of a large family of novel 2D DLHC traditional semiconductors with unique excitonic, topological, and electronic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Layered Double Hydroxide Derivatives for Polyolefin Upcycling

While Ru-catalyzed hydrogenolysis holds significant promise in converting waste polyolefins into value-added alkane fuels, a major constraint is the high cost of noble metal catalysts. Here, in this work, we propose, for the first time, that Co-based catalysts derived from CoAl-layered double hydroxide (LDH) are alternatives for efficient polyolefin hydrogenolysis. Leveraging the chemical flexibility of the LDH platform, we reveal that metallic Co species serve as highly efficient active sites for polyolefin hydrogenolysis. Furthermore, we introduced Ni into the Co framework to tackle the issue of restricted hydrogenation ability associated with contiguous Co-Co sites. In-situ analysis indicates that the integration of Ni induces electron transfer and facilitates hydrogen spillover. This dual effect synergistically enhances the hydrogenation/desorption of olefin intermediates, resulting in a significant reduction in the yield of low-value CH 4 from 27.1 to 12.6%. Through leveraging the unique properties of LDH, we have developed efficient and cost-effective catalysts for the sustainable recycling and valorization of waste polyolefin materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗