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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Dielectric Properties of Organic Solvents in an Electric Field

The effect of an external electric field ( E ) on the dielectric constant (ε) of several pure solvents [propylene carbonate (PC), ethylene carbonate (EC), acetonitrile (MeCN), and dimethyl carbonate (DMC)] and for several EC/DMC mixtures is explored using classical molecular dynamics simulation. Force fields were chosen that accurately predict the density and zero-field dielectric constant with respect to experiment. The simulation results for ε( E ) for field strengths up to 0.4 V/Å are calculated and fit to the Booth model, which is a standard functional form for the dependence of the dielectric constant on electric field. For PC and DMC, the Booth model gives an excellent representation of the data at all electric fields studied. For EC and MeCN, the Booth model works well at lower field strengths (up to 0.15 and 0.2 V/Å, respectively), but at the higher electric fields, these systems are observed to crystallize, a phenomenon referred to as electrofreezing . In conclusion, this work will provide useful input data for the continuum modeling of devices, such as electric double-layer capacitors, that utilize organic electrolyte solvents.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Impurity Effects on Charging Mechanism and Energy Storage of Nanoporous Supercapacitors

Room-temperature ionic liquids (RTILs) have been widely used as electrolytes to enhance the capacitive performance of electrochemical capacitors also known as supercapacitors. Whereas impurities are ubiquitous in RTILs (e.g., water, alkali salts, and organic solvents), little is known about their influences on the electrochemical behavior of electrochemical devices. In this work, we investigate different impurities in RTILs within the micropores of carbon electrodes via the classical density functional theory (CDFT). We find that under certain conditions impurities can significantly change the charging behavior of electric double layers and the shape of differential capacitance curves even at very low concentrations. More interestingly, an impurity with a strong affinity to the nanopore can increase the energy density beyond a critical charging potential. As a result, our theoretical predictions provide further understanding of how impurity in RTILs affects the performance of supercapacitors.

25 ENERGY STORAGE↗
DOE OSTI2017

Interfacial Structure and Electric Field Probed by in Situ Electrochemical Vibrational Stark Effect Spectroscopy and Computational Modeling

Interfacial electric fields play crucial roles in electrochemistry, catalysis, and solar energy conversion. Understanding of the interfacial electric field effects has been hindered by the lack of a direct spectroscopic method to probe of the interfacial field at the molecular level. Here, we report the characterization of the field and interfacial structure at Au/diisocyanide/aqueous electrolyte interfaces, using a combination of in situ electrochemical vibrational sum frequency generation (SFG) spectroscopy, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. For 1,4-phenylene diisocyanide (PDI), 4,4'-biphenyl diisocyanide (BPDI), and 4,4"-p-terphenyl diisocyanide (TPDI), our results reveal that the frequency of the gold-bound NC stretch mode of the diisocyanide self-assembled monolayer (SAM) increases linearly with the applied potential, suggesting that SFG can be an in situ probe of the strength of the electric field at electrode/electrolyte interfaces. Using DFT-computed Stark tuning rates of model complexes, the electric field strength at the metal/SAM/electrolyte interfaces is estimated to be 108–109 V/m. The linear dependence of the vibrational frequency (and field) with applied potential is consistent with an electrochemical double-layer structure that consists of a Helmholtz layer in contact with a diffused layer. The Helmholtz layer thickness is approximately the same as the molecular length for PDI, suggesting a well-ordered SAM with negligible electrolyte penetration. For BPDI and TPDI, we found that the Helmholtz layer is thinner than the monolayer of molecular adsorbates, indicating that the electrolyte percolates into the SAM, as shown by molecular dynamics simulations of the Au/PDI/electrolyte interface. The reported analysis demonstrates that a combination of in situ SFG probes and computational modeling provides a powerful approach to elucidate the structure of electrochemical interfaces at the detailed molecular level.

Ge, Aimin↗
DOE OSTI2018

Interplay between Ion Transport, Applied Bias, and Degradation under Illumination in Hybrid Perovskite p-i-n Devices

In this work, we studied ion transport in hybrid organic–inorganic perovskite p-i-n devices as a function of applied bias under device operating conditions. Using electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, we elucidated various resistive and capacitive elements in the device. We show that ion migration is predictably influenced by a low applied forward bias, characterized by an increased capacitance at the hole-transporting (HTM) and electron-transporting material (ETM) interfaces, as well as in bulk. However, unlike observations in n-i-p devices, we found that there is a capacitive discharge leading to ion redistribution in the bulk at high forward biases. Furthermore, we show that a chemical double-layer capacitance buildup as a result of ion accumulation impacts the electronic properties of the device, likely by inducing either charge pinning or charge screening, depending on the direction of the ion-induced field. Finally, we extrapolate ion diffusion coefficients (~10 –7 cm 2 s –1 ) and ionic conductivities (~10 –7 S cm –1 ) from the Warburg mass (ion) diffusion response and show that, as the device degrades, there is an overall depletion of capacitive effects coupled with increased ion mobility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors

Well-tailored mixtures of distinct ionic liquids can act as optimal electrolytes that extend the operating electrochemical window and improve charge storage density in supercapacitors. Here, we explore two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF 4 ). We study their electric double-layer behavior in the neat state and as binary mixtures on the external surfaces of onion-like carbon electrodes using quasielastic neutron scattering (QENS) and classical density functional theory techniques. Computational results reveal that a mixture with 4:1 EmimTFSI/EmimBF 4 volume ratio displaces the larger [TFSI – ] anions with smaller [BF 4 – ] ions, leading to an excess adsorption of [Emim + ] cations near the electrode surface. These findings are corroborated by the manifestation of nonuniform ion diffusivity change, complementing the description of structural modifications with changing composition, from QENS measurements. In conclusion, molecular-level understanding of ion packing near electrodes provides insight for design of ionic liquid formulations that enhance the performance of electrochemical energy storage devices.

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

Electrochemical Behavior of Nanoporous Supercapacitors with Oligomeric Ionic Liquids

According to a recent study [Matsumoto, M. ; J. Am. Chem. Soc. 2017, 139, 16072-16075], an oligomeric ionic liquid (OIL) may exhibit a wide electrochemical window and an exceptionally high capacitance in electric double layer (EDL) transistors. However, little is known on applicability of OILs in EDL capacitors with porous electrodes. To understand the capacitive performance of OILs, here we investigate the charging behavior, EDL structure, and capacitance for different types of OILs using the classical density functional theory. In contrast to their application to field-effect transistors, the capacitive performance of OILs in nanoporous electrodes is sensitive to the charging potential. We report that although adoption of oligomeric cations shows little advantage for the electrical charging of a positive electrode, they enhance the energy storage density for negative electrodes.

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

Ionic Liquid Mixture Expands the Potential Window and Capacitance of a Supercapacitor in Tandem

Mixing room temperature ionic liquids (RTIL) may optimize electrolyte properties for the better performance of electrical double layer capacitors (EDLCs). To understand the composition effects, we theoretically investigate the potential window, interfacial structure, capacitance, and energy density of RTIL mixtures consisting of one common cation and two anions of different sizes. A combination of quantum density functional theory (QDFT) and classical density functional theory (CDFT) calculations indicates that the electrolyte composition has significant effects on the EDL structure and potential of zero charge, thereby altering the integral capacitance and electrochemical potential window. The two quantities can be optimized together to achieve the best EDLC performance.

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

Acid–Base Chemistry of a Model IrO 2 Catalytic Interface

Iridium oxide (IrO 2 ) is one of the most efficient catalytic materials for the oxygen evolution reaction (OER), yet the atomic scale structure of its aqueous interface is largely unknown. Herein, the hydration structure, proton transfer mechanisms and acid-base properties of the rutile IrO 2 (110)-water interface are investigated using ab-initio based deep neural-network potentials and enhanced sampling simulations. The proton affinities of the different surface sites are characterized by calculating their acid dissociation constants, which yield a point of zero-charge in good agreement with experiments. A large fraction (≈ 80%) of adsorbed water dissociation is observed, together with a short lifetime (≈ 0.5 ns) of the resulting terminal hydroxy groups, due to rapid proton exchanges between adsorbed H 2 O and adjacent OH species. As a result, this rapid surface proton transfer supports the suggestion that the rate-determining step in the OER may not involve proton transfer across the double layer into solution, as indicated by recent experiments.

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

Vibrational Probe at the Electrochemical Interface: Dependence on Plasmon Coupling and Potential of the Lineshape in Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy of vibrational probes at an electrode surface shows promise for studying the structural dynamics at an active electrochemical interface. This interface is a complex environment where the solution structures in response to the applied potential. A strategy for achieving the necessary monolayer sensitivity is to use a plasmonically active electrode, which enhances the electromagnetic fields that produce the spectroscopic response. Here, in this study, we show how the coupling between the plasmon and the vibrations of the molecular monolayer impacts the FTIR and 2D IR spectroscopy, with an emphasis on the electrochemical potential difference spectra. We show how mixing between the vibrational and plasmonic states gives rise to the distortions that are observed in these measurements. This provides an important step toward 2D IR measurements of vibrational probes at the electrochemical interface as a tool for probing the structural dynamics in the double layer.

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

Understanding the MXene Pseudocapacitance

MXenes have attracted great attention as next-generation capacitive energy-storage materials, but the mechanisms underlying their pseudocapacitive behavior are not well understood. Here we provide a theoretical description of the surface redox process of Ti3C2Tx (T = O, OH), a prototypical MXene, in 1 M H2SO4 electrolyte, based on joint density functional theory with an implicit solvation model and the analysis of Gibbs free energy under a constant-electrode potential. From the dependence of the O/OH ratio (or the surface H coverage) and the surface charge on the applied potential, we obtain a clear picture of the capacitive energy-storage mechanism of Ti3C2Tx that shows good agreement with previous experimental findings in terms of the integral capacitance and Ti valence change. We find a voltage-dependent redox/double-layer co-charging behavior: the capacitive mechanism is dominated by the redox process, but the electric double-layer charge works against the redox process. This new insight may be useful in improving the capacitance of MXenes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Aqueous Carbon Quantum Dot-Embedded PC60-PC61 BM Nanospheres for Ecological Fluorescent Printing: Contrasting Fluorescence Resonance Energy-Transfer Signals between Watermelon-like and Random Morphologies

To go beyond the PC60 surfactant structure, the double-layer micelle morphology in water motivates exploration of altered protocols to produce new morphologies. Furthermore, the low photoluminescence quantum yield of aqueous fullerene-based particles encourages high fluorescence to create a light-emitting display. With this in mind, we established new hybrid n-type nanospheres with carbon quantum dot (CQD)-embedded PC60-PC61BM particles, processed using two different protocols. The homogenizer-assisted PC60-CQD-PC61BM resulted in a watermelon-shaped spherical particle, whereas a circular morphology with randomly embedded CQDs was observed in the microwave-treated hybrids. More surprisingly, the watermelon-shaped colloid induced efficient fluorescence resonance energy transfer (FRET) between the CQD and C-60 molecules of PC61BM, and the FRET-mediated emission signature diminished gradually as the stripe patterns collapsed. This phenomenon allowed different fluorescent colors in the colloidal printing film. We thereby provided the new carrier dynamics of the particle photonic activities of the developed aqueous PC60-based colloids with the possibility of ecological utilization.

carbon dot particles↗
DOE OSTI2024

Key Experimental Considerations When Evaluating Functional Ionic Liquids for Combined Capture and Electrochemical Conversion of CO 2

Ionic liquids (ILs) are considered functional electrolytes for the electrocatalytic reduction of CO 2 (ECO 2 R) due to their role in the double-layer structure formation and increased CO 2 availability at the electrode surface, which reduces the voltage requirement. However, not all ILs are the same, considering the purity and degree of the functionality of the IL. Further, there are critical experimental factors that impact the evaluation of ILs for ECO 2 R including the reference electrode, working electrode construction, cosolvent selection, cell geometry, and whether the electrochemical cell is a single compartment or a divided cell. Here, we describe improved synthesis methods of imidazolium cyanopyrrolide IL for electrochemical studies in consideration of precursor composition and reaction time. We explored how IL with cosolvents (i.e. acetonitrile, dimethylformamide, dimethyl sulfoxide, propylene carbonate, and n-methyl-2-pyrrolidone) affects conductivity, CO 2 mass transport, and ECO 2 R activation overpotential together with the effects of electrode materials (Sn, Ag, Au, and glassy carbon). Acetonitrile was found to be the best solvent for lowering the onset potential and increasing the catalytic current density for the production of CO owing to the enhanced ion mobility in combination with the silver electrode. Further, the ECO 2 R activity of molecular catalysts Ni(cyclam)Cl 2 and iron tetraphenylsulfonato porphyrin (FeTPPS) on the carbon cloth electrode maintained high Faradaic efficiencies for CO in the presence of the IL. This study presents best practices for examining nontraditional multifunctional electrolytes amenable to integrated CO 2 capture and conversion technologies for homogeneous and heterogeneous ECO 2 R.

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

Ion Transport in Charged Membranes: Linking Electric-Field-Driven Mechanisms to Pore Size via Perturbation Analysis

Ion-exchange membranes are a critical component in electrochemical systems. Nevertheless, the understanding and modeling of ion transport within these porous structures have been limited by particular complexity reductions, either ignoring the dimensionality of their porous network architecture or imposing geometric assumptions (i.e., overlapping double layers). Before addressing this morphology-transport gap, a framework that relates the driving forces of transport to the geometry of a single pore is required. In this work, our modeling domain consists of a two-dimensional single pore with charged walls, connecting two identical electrolyte reservoirs. Using the Poisson-Nernst-Planck equations and regular perturbation theory, we decouple the electric fields and analyze the driving forces of ion transport, specifically electromigration and induced electroosmosis within the pore. These processes are described as analytical functions of the interaction aspect ratio, ?, defined as the ratio of the pore radius to the Debye length. Using this parameter, our study (i) describes the interplay between electromigrative and electroosmotic mechanisms that set ionic conductivity, (ii) identifies a dimensionless group of intrinsic electrolyte properties that indicates the predominant driving force, and (iii) provides a qualitative, confinement-dependent perspective on selectivity in ion-conducting membranes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Coexistence and Coupling of Multiple Charge Orderings and Spin States in Hexagonal Ferrite

The coupling between charge and spin orderings in strongly correlated systems plays a crucial role in fundamental physics and device applications. As a candidate of multiferroic materials, LuFe 2 O 4 with a nominal Fe 2.5+ valence state has the potential for strong charge–spin interactions; however, these interactions have not been fully understood until now. Here, combining complementary characterization methods with theoretical calculations, two types of charge orderings with distinct magnetic properties are revealed. The ground states of LuFe 2 O 4 are decided by the parallel/antiparallel coupling of both charge and spin orderings in the adjacent FeO double layers. Whereas the ferroelectric charge ordering remains ferrimagnetic below 230 K, the antiferroelectric ordering undergoes antiferromagnetic–ferrimagnetic–paramagnetic transitions from 2 K to room temperature. This study demonstrates the unique aspects of strong spin-charge coupling within LuFe 2 O 4 . Our results shed light on the coexistence and competing nature of orderings in quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Mapping the Transmission Functions of Single-Molecule Junctions

Charge transport characteristics of single-molecule junctions are often governed by a transmission function that dictates the probability of electrons or holes tunneling across the junction. Here, we present a new and simple technique for measuring the transmission function of molecular junctions in the coherent tunneling limit, over an energy range of 2 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different areas exposed, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction’s transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed ab initio. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Nanoporous Polymer Films with a High Cation Transference Number Stabilize Lithium Metal Anodes in Light-Weight Batteries for Electrified Transportation

To suppress dendrite formation in lithium metal batteries, high cation transference number electrolytes that reduce electrode polarization are highly desirable, but rarely available using conventional liquid electrolytes. Here, we show that liquid electrolytes increase their cation transference numbers (e.g., ~0.2 to >0.70) when confined to a structurally rigid polymer host whose pores are on a similar length scale (0.5–2 nm) as the Debye screening length in the electrolyte, which results in a diffuse electrolyte double layer at the polymer–electrolyte interface that retains counterions and reject co-ions from the electrolyte due to their larger size. Lithium anodes coated with ~1 μm thick overlayers of the polymer host exhibit both a low area-specific resistance and clear dendrite-suppressing character, as evident from their performance in Li–Li and Li–Cu cells as well as in post-mortem analysis of the anode’s morphology after cycling. High areal capacity Li–S cells (4.9 mg cm –2 ; 8.2 mAh cm –2 ) implementing these high transference number polymer-hosted liquid electrolytes were remarkably stable, considering ~24 μm of lithium was electroreversibly deposited in each cycle at a C-rate of 0.2. In conclusion, we further identified a scalable manufacturing path for these polymer-coated lithium electrodes, which are drop-in components for lithium metal battery manufacturing.

25 ENERGY STORAGE↗
DOE OSTI2021

Electrolyte-Guided Design of Electroreductive CO Coupling on Copper Surfaces

Engineering the interfacial distribution of electrolytic ions can aid in modulating the electrocatalyst performance and efficiency. Using a hybrid quantum-classical modeling approach, we describe how predictive tuning of the solution microenvironment on copper can enhance the efficiency of CO 2 reduction (CO 2 R) to C 2 products. We elucidate how competing electrolyte constituents in mixed electrolyte solutions stimulate restructuring of the electrochemical double layer (EDL) and stabilize the OCCO* dimer (* denotes surface adsorbed), with predictions validated in flow reactors using copper gas diffusion electrodes (Cu-GDEs). Furthermore, our findings highlight how molecular-scale electrolyte engineering with informed models of the EDL can be leveraged to tailor CO 2 R activity and selectivity toward C 2 products.

36 MATERIALS SCIENCE↗