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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Insights into the Enhanced Reversibility of Graphite Anode Upon Fast Charging Through Li Reservoir

Increasing the charging rate and reducing the charging time for Li-ion batteries are crucial to realize the mainstream of electric vehicles. However, it is formidable to avoid the Li plating on graphite anode upon fast charging. Despite the tremendous progress in Li detection techniques, the fundamental mechanism of Li plating and its chemical/electrochemical responses upon cycling still remains elusive. Herein, we present a comprehensive electrochemical method to investigate the fast charging behavior of graphite electrode. A detailed analysis is directed toward understanding the changes in phase, composition, and morphology of the fast-charged graphite. By applying a resting process, we scrutinize the further reactions of the plated Li, which readily transforms into irreversible (dead) Li. We further develop a modified graphite electrode with a thin Ag coating as the Li reservoir. The plated Li can be "absorbed" by the Ag layer to form the Li-Ag solid solution that suppresses the formation of dead Li and provides structural stability, thus promoting the further lithiation of graphite and enhancing the reversibility. Here this work not only provides additional insights into the fast charging behavior of graphite electrode but also demonstrates a potential strategy to improve the fast charging performance of graphite anode.

25 ENERGY STORAGE↗
DOE OSTI2020

Reversible Electrochemical Phase Change in Monolayer to Bulk-like MoTe 2 by Ionic Liquid Gating

Transition-metal dichalcogenides (TMDs) exist in various crystal structures with semiconducting, semi-metallic, and metallic properties. The dynamic control of these phases is of immediate interest for next-generation electronics such as phase change memories. Of the binary Mo and W-based TMDs, MoTe 2 is attractive for electronic applications because it has the lowest energy difference (40 meV) between the semiconducting (2H) and semi-metallic (1T') phases, allowing for MoTe 2 phase change by electrostatic doping. Here, we report phase change between the 2H and 1T' polymorphs of MoTe 2 in thicknesses ranging from the monolayer to bulk-like case (73 nm) using an ionic liquid electrolyte at room temperature and in air. We find consistent evidence of a partially reversible 2H-1T' transition using in situ Raman spectroscopy where the phase change occurs in the topmost layers of the MoTe 2 flake. We find a thickness-dependent transition voltage where higher voltages are necessary to drive the phase change for thicker flakes. We also show evidence of electrochemical activity during the gating process by observation of Te metal formation. This finding suggests the formation of Te vacancies which have been reported to lower the energy difference between the 2H and 1T' phases, potentially aiding the phase change process. Furthermore, our discovery that the phase change can be achieved on the surface layer of bulk-like materials reveals that this electrochemical mechanism does not require isolation of a single layer and the effect may be more broadly applicable than previously thought.

2d materials↗
DOE OSTI2023

Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.

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

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water–Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously rising. However, the lack of understanding of the dynamic behaviors of SACs during applications hinder catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO 2 -anatase SAC (Pd 1 /TiO 2 ) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in-situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO 2 by H 2 alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H 2 is accompanied by the partial sintering of single Pd atoms (Pd 1 ) into disordered, flat, ~1 nm diameter clusters (Pd n ). The highly active Pd sites in the new coordination environment under H 2 are eliminated by oxidation, which, when performed at high temperature, also re-disperses Pd n and facilitates the reduction of TiO 2 . In contrast, Pd 1 sinters into crystalline, ~5 nm particles (Pd NP ) during CO treatment, deactivating Pd 1 /TiO 2 . During the rWGS reaction, the two Pd evolution pathways co-exist. The activation by H 2 dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar with the ones formed under H 2 . Finally, this work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pre-treatments, and how their activity is modulated by these behaviors. These insights on SAC dynamics and structure-function relationship are valuable to mechanistic understanding and catalyst design.

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

Unveiling the Multielectron Acceptor Properties of π-Expanded Pyracylene: Reversible Boat to Chair Conversion

In this work, the chemical reduction of a hybrid pyracylene-hexa-peri-hexabenzocoronene (HPH) nanographene was investigated with different alkali metals (Na, K, Rb) to reveal its remarkable multielectron acceptor abilities. The UV–vis and 1 H NMR spectroscopy monitoring of the stepwise reduction reactions supports the existence of all intermediate reduction states up to the hexaanion for HPH. Tuning the experimental conditions enabled the synthesis of the HPH anions with gradually increasing reduction states (up to –5) isolated with different alkali metal ions as crystalline materials. The single-crystal X-ray diffraction structure analysis demonstrates that the highly negatively charged HPH anions (–4 and –5) exhibit a drastic geometry change from boat-shaped (observed in the neutral parent, mono- and dianions) to a chair conformation, which was proved to be fully reversible by NMR spectroscopy. DFT calculations show that this geometry change is induced by an enhanced interaction between the coordinated metal ions and negatively charged HPH core in the chair conformation.

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

Concerted Formation of Reversibly Precipitated Sulfur Species and Its Importance for Lean Electrolyte Lithium–Sulfur Batteries

Achieving high energy densities for lithium-sulfur batteries remain elusive. Largely limited by the volume of electrolyte used, lean electrolyte conditions (electrolyte/sulfur mass ratio <3) present enormous challenges that have led to very poor specific capacity and rate performance. Previous studies have identified that the high concentration of polysulfide is responsible for the poor discharge voltage. However, there still lacks sufficient understanding of the processes occurring at lean electrolyte conditions. Here, in this work we uncovered a polysulfide concentration regulating mechanism that operates through the precipitation and redissolution of solid sulfur-based species (reversibly precipitated sulfur species, RPSS). This occurs in a concerted manner in a global sense through the cathode and can be measured using impedance spectroscopy. It was found that the more RPSS that is formed, the higher the energy density of discharge. We propose that high concentration of polysulfide tends to supersaturate, which impeded the formation of RPSS. Employing an electrolyte with low Li ion concentration along with using poorly dissociating lithium salts allowed for more RPSS formation and ultimately enabled discharge at >2.0 V at 0.05 C, at E/S = 2.5, and at room temperature without the use of an engineered cathode.

25 ENERGY STORAGE↗
DOE OSTI2024

Synthetically Reversible, Proton-Mediated Nitrite N–O Bond Cleavage at a Dicopper Site

A monocationic dicopper(I,I) nitrite complex [Cu 2 (μ-κ 1 :κ 1 -O 2 N)DPFN][NTf 2 ] (2) (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine, NTf 2 – = N(SO 2 CF 3 ) 2 – ), was synthesized by treatment of a dicopper acetonitrile complex, [Cu 2 (μ-MeCN)DPFN][NTf 2 ] 2 (1), with tetrabutylammonium nitrite ([nBu 4 N][NO 2 ]). DFT calculations indicate that 2 is one of three linkage isomers that are close in energy and presumably accessible in solution. Reaction of the μ-κ 1 :κ 1 -O 2 N complex with p-TolSH produces nitrous acid (HONO) and the corresponding dicopper thiolate species via an acid–base exchange reaction. Notably, treatment of 2 with HNTf 2 results in N–O bond cleavage in the putative, HONO-ligated complex to form the more thermodynamically favorable nitrosyl-bridged dicopper complex [Cu 2 (μ-NO)(μ-OH)DPFN][NTf 2 ] 2 (4). This scission can be reversed via deprotonation of the hydroxy ligand with KO t Bu. X-ray diffraction studies confirmed the solid-state molecular structures of 2 and 4. DFT calculations were used to construct a reaction coordinate diagram detailing formation of the μ-NO complex and to describe its electronic structure. The nitrosyl ligand in 4 is chemically labile, as demonstrated by its ready displacement in reactions with CO or NO 2 – .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Reversible Interfacial Hydride Transfer as a Complementary Tool To Measure Molecular Hydricity

Hydride transfer is an essential elementary reaction across the chemical value chain, but there are limited methods available for quantifying thermodynamic hydricity (ΔG H- ), particularly among main group reagents. Herein, we exploit facile H 2 activation and reversible hydride transfer from a metal surface to a molecular reagent, the net hydrogen reduction reaction (HRR), to develop a potentiometric method for quantifying ΔG H- of main group reagents recalcitrant to conventional methods. HRR potentiometry is first validated with a benzimidazole-based hydride donor and then applied to uncover the impact of the reaction environment on hydricity. For a benzimidazole-based hydride donor, HRR equilibrium potentials are roughly invariant across solvents, indicating that the solvent dependence of its hydricity largely reflects the differential solvation of H - across media. For formate, HRR potentials and corresponding hydricities depend strongly on water content. Additionally, for borohydrides, HRR potentiometry reveals that effective hydricity values are strongly influenced by Lewis acid-base adduct formation with the hydride acceptor but are minimally influenced by the countercation. Together with these studies, the advantages, limitations, and practical considerations of the HRR potentiometry method are discussed, highlighting the power of this methodology as a complementary tool to measure molecular hydricity.

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

Atomic Alignment in PbS Nanocrystal Superlattices with Compact Inorganic Ligands via Reversible Oriented Attachment of Nanocrystals

Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials with NC self-assembly offering opportunities to enable novel material properties. Here, in this work, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as Sn 2 S 6 4– and AsS 4 3– , can self-assemble into all-inorganic superlattices with both long-range superlattice translational and atomic-lattice orientational order. Structural characterizations reveal that the NCs adopt an unexpected edge-to-edge alignment, and numerical simulation clarifies that orientational order is thermodynamically stabilized by many-body ion correlations originating from the dense electrolyte. Furthermore, we show that the superlattices of Sn 2 S 6 4– -functionalized PbS NCs can be fully disassembled back into the colloidal state, which is highly unusual for orientationally attached superlattices with atomic-lattice alignment. The reversible oriented attachment of NCs, enabling their dynamic assembly and disassembly into effectively single-crystalline superstructures, offers a pathway toward designing reconfigurable materials with adaptive and controllable electronic and optoelectronic properties.

Landau-Ginzburg theory↗
DOE OSTI2019

Record-Setting Sorbents for Reversible Water Uptake by Systematic Anion Exchanges in Metal–Organic Frameworks

The reversible capture of water vapor at low humidity can enable transformative applications such as atmospheric water harvesting and heat transfer that uses water as a refrigerant, replacing environmentally detrimental hydro- and chloro-fluorocarbons. The driving force for these applications is governed by the relative humidity at which the pores of a porous material fill with water. Here, we demonstrate modulation of the onset of pore-filling in a family of metal-organic frameworks with record water sorption capacities by employing anion exchange. Unexpectedly, the replacement of the structural bridging Cl - with the more hydrophilic anions F - and OH - does not induce porefilling at lower relative humidity, whereas the introduction of the larger Br - results in a substantial shift toward lower relative humidity. We rationalize these results in terms of pore size modifications as well as the water hydrogen bonding structure based on detailed infrared spectroscopic measurements. Fundamentally, our data suggest that, in the presence of strong nucleation sites, the thermodynamic favorability of water pore-filling depends more strongly on the pore diameter and the interface between water in the center of the pore and water bound to the pore walls than the hydrophilicity of the pore wall itself. On the basis of these results, we report two materials that exhibit record water uptake capacities in their respective humidity regions and extended stability over 400 water adsorption-desorption cycles.

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

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal–Ligand Cooperation

The reversible activation of H 2 via a pathway involving metal–ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. In this paper, we present an in-depth kinetic study of the 1,2-addition of H 2 via an MLC pathway to the widely used iron catalyst [( i Pr PNP)FeH(CO)] ( 1 ) ( i Pr PNP = N(CH 2 CH 2 P i Pr 2 ) 2 – ). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H 2 using protic additives, which operate as “proton shuttles”. Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H 2 activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H 2 activation. The results of this study provide guidance about the design of catalysts and additives to promote H 2 activation via an MLC pathway.

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

A Reversible Structural Phase Transition by Electrochemically-Driven Ion Injection into a Conjugated Polymer

We nd that conjugated polymers can undergo reversible structural phase transitions during electrochemical oxidation and ion injection. We study poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2- methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated side chains. Using grazing incidence wide angle X-ray scattering (GIWAXS), we show that, in contrast to previously known polymers, this polymer switches between two structurally distinct crystalline phases associated with electrochemical oxidation/reduction in an aqueous electrolyte. Importantly, we show that this unique phase change behavior has important physical consequences for ion transport. Notably, using moving front experi- ments visualized by both optical microscopy and super-resolution photoinduced force microscopy (PiFM), we show that a propagating ion front in PB2T-TEG exhibits non-Fickian transport, retaining a sharp step-edge prole, in stark contrast to the Fickian diusion more commonly observed. This structural phase transition is reminiscent of those accompanying ion uptake in inorganic materials like LiFePO4. We propose that engineering similar properties in future conjugated polymers may enable the realization of new materials with superior performance in electrochemical energy storage or neuromorphic memory applications.

Bischak, Connor G.↗
DOE OSTI2022

Climate Leads to Reversed Latitudinal Changes in Chinese Flood Peak Timing

Changes in river floods under a changing climate are of worldwide concerns, but relevant knowledge is limited especially over the East Asia monsoon region. Here we examine changes in flood peak timing based on the most comprehensive flood dataset to date across China during 1961-2017. The temporal shifts of Chinese flood peak timing range from -19.0 days earlier to +14.0 days later per decade. Earlier flood occurrences are mainly observed in northern China, with delayed flood occurrences in the south. The spatial pattern of reversed latitudinal changes in flood timing leads to growing spatial extents of concurrent river floods over China. Soil moisture plays an important role in modulating such changes, despite precipitation determines the climatological mean dates of flood occurrences. Our results highlight a strong climate signal on the changes in flood timing over China, and call for coordinated efforts in alleviating expanding flood hazards related to the changes in flood timing.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Atmospheric Feedbacks Reverse the Sensitivity of Modeled Photosynthesis to Stomatal Function

Stomata mediate fluxes of carbon and water between terrestrial plants and the atmosphere. These fluxes are governed by stomatal function and can be modulated in many Earth system models by an empirical parameter within the calculation of stomatal conductance, the stomatal slope (𝑔 1⁢𝑀 ). Intuitively, 𝑔 1⁢𝑀 represents the marginal water cost of carbon, relating it to the emergent plant property of water use efficiency. Observations show that 𝑔 1⁢𝑀 can range widely across and within plant types in varying environments, and this distribution of 𝑔 1⁢𝑀 is not captured within Earth system models which represent each plant type with a single 𝑔 1⁢𝑀 value. Here we examine how 𝑔 1⁢𝑀 influences photosynthesis using coupled Earth system model simulations by perturbing 𝑔 1⁢𝑀 to observed 5⁢t⁢h and 95⁢t⁢h percentiles for each plant type. We find that high 𝑔 1⁢𝑀 reduces photosynthesis nearly everywhere, while low 𝑔 1⁢𝑀 has regionally dependent responses. Under fixed atmospheric conditions, low 𝑔 1⁢𝑀 increases photosynthesis in the Amazon and central North America but decreases photosynthesis in boreal Canada. These responses reverse when the atmosphere responds interactively due to spatially differing sensitivity to increases in temperature and vapor pressure deficit. Choice of 𝑔 1⁢𝑀 also influences photosynthetic response to changes in atmospheric carbon dioxide (CO 2 ), with lower and higher 𝑔1⁢𝑀 modifying total global response to elevated 2x preindustrial CO 2 by +6.4% and −9.6%, respectively. Our work demonstrates that atmospheric feedbacks are critical for determining the photosynthetic response to 𝑔 1⁢𝑀 assumptions and some regions are particularly sensitive to choice of 𝑔 1⁢𝑀 .

Liu, Amy X. [University of Washington, Seattle, WA↗
DOE OSTI2019

Reversible intercalation of methyl viologen as a dicationic charge carrier in aqueous batteries

The interactions among charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. In this work we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic solid electrode, 3,4,9,10-perylenetetracarboxylic dianhydride. This is the largest insertion charge carrier when non-solvated ever reported for batteries; surprisingly, the kinetic properties of the (de) insertion of methyl viologen are excellent with 60% of capacity retained when the current rate is increased from 100 mA g -1 to 2000 mA g -1 . Characterization reveals that the insertion of methyl viologen causes phase transformation of the organic host, and embodies guest-host chemical bonding. First-principles density functional theory calculations suggest strong guest-host interaction beyond the pure ionic bonding, where a large extent of covalency may exist. This study extends the boundary of battery chemistry to large molecular ions as charge carriers and also highlights the electrochemical assembly of a supramolecular system.

25 ENERGY STORAGE↗
DOE OSTI2020

Piezoelectricity and topological quantum phase transitions in two-dimensional spin-orbit coupled crystals with time-reversal symmetry

Abstract Finding new physical responses that signal topological quantum phase transitions is of both theoretical and experimental importance. Here, we demonstrate that the piezoelectric response can change discontinuously across a topological quantum phase transition in two-dimensional time-reversal invariant systems with spin-orbit coupling, thus serving as a direct probe of the transition. We study all gap closing cases for all 7 plane groups that allow non-vanishing piezoelectricity, and find that any gap closing with 1 fine-tuning parameter between two gapped states changes either the Z 2 invariant or the locally stable valley Chern number. The jump of the piezoelectric response is found to exist for all these transitions, and we propose the HgTe/CdTe quantum well and BaMnSb 2 as two potential experimental platforms. Our work provides a general theoretical framework to classify topological quantum phase transitions, and reveals their ubiquitous relation to the piezoelectric response.

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

Structural coordination between active sites of a CRISPR reverse transcriptase-integrase complex

CRISPR-Cas systems provide adaptive immunity in bacteria and archaea, beginning with integration of foreign sequences into the host CRISPR genomic locus and followed by transcription and maturation of CRISPR RNAs (crRNAs). In some CRISPR systems, a reverse transcriptase (RT) fusion to the Cas1 integrase and Cas6 maturase creates a single protein that enables concerted sequence integration and crRNA production. To elucidate how the RT-integrase organizes distinct enzymatic activities, we present the cryo-EM structure of a Cas6-RT-Cas1—Cas2 CRISPR integrase complex. The structure reveals a heterohexamer in which the RT directly contacts the integrase and maturase domains, suggesting functional coordination between all three active sites. Together with biochemical experiments, our data support a model of sequential enzymatic activities that enable CRISPR sequence acquisition from RNA and DNA substrates. These findings highlight an expanded capacity of some CRISPR systems to acquire diverse sequences that direct CRISPR-mediated interference.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Cell specific photoswitchable agonist for reversible control of endogenous dopamine receptors

Dopamine controls diverse behaviors and their dysregulation contributes to many disorders. Our ability to understand and manipulate the function of dopamine is limited by the heterogenous nature of dopaminergic projections, the diversity of neurons that are regulated by dopamine, the varying distribution of the five dopamine receptors (DARs), and the complex dynamics of dopamine release. In order to improve our ability to specifically modulate distinct DARs, here we develop a photo-pharmacological strategy using a Membrane anchored Photoswitchable orthogonal remotely tethered agonist for the Dopamine receptor (MP-D). Our design selectively targets D1R/D5R receptor subtypes, most potently D1R (MP-D1 ago ), as shown in HEK293T cells. In vivo, we targeted dorsal striatal medium spiny neurons where the photo-activation of MP-D1 ago increased movement initiation, although further work is required to assess the effects of MP-D1 ago on neuronal function. Our method combines ligand and cell type-specificity with temporally precise and reversible activation of D1R to control specific aspects of movement. Our results provide a template for analyzing dopamine receptors.

59 BASIC BIOLOGICAL SCIENCES↗