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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Kinetic Control over Disproportionation Stabilizes Wurster’s Blue Catholyte for Nonaqueous Redox Flow Batteries

Redoxmers are organic molecules that serve as charge carriers in redox flow batteries. While these materials are affordable and easy to source, insufficient stability of their charged states (radical ions) remains a challenge. A common reaction of these species is their disproportionation. Furthermore, this reversible reaction yields unstable multiply charged states, shifting the overall charge transfer equilibrium toward the decomposition products. Here we show how kinetic controls can be engineered into a redoxmer molecule to suppress these unwanted charge transfer reactions. This approach is used to transform Wurster's blue, which is historically the first example of a stable radical ion in organic chemistry, into an exceptionally durable redoxmer molecule that persists over thousands of electrochemical cycles.

Cations↗
DOE OSTI2020

Scaling Laws of Exciton Recombination Kinetics in Low Dimensional Halide Perovskite Nanostructures

Carrier recombination is a crucial process governing the optical properties of a semiconductor. Although various theoretical approaches have been utilized to describe carrier behaviors, a quantitative understanding of the impact of defects and interfaces in low dimensional semiconductor systems is still elusive. Here, we develop a model system consisting of chemically tunable, highly luminescent halide perovskite nanocrystals to illustrate the role of carrier diffusion and material dimensionality on the carrier recombination kinetics and luminescence efficiency. Our advanced synthetic methods provide a well-controlled colloidal system consisting of nanocrystals with different aspect ratios, halide compositions, and surface conditions. Using this system, we reveal the scaling laws of photoluminescence quantum yield and radiative lifetime with respect to the aspect ratio of nanocrystals. The scaling laws derived herein are not only a phenomenological observation but proved a powerful tool disentangling the carrier dynamics of microscopic systems in a quantitative and interpretable manner. The investigation of our model system and theoretical formulation bring to light the dimensionality, as a hidden constraint on carrier dynamics, and identify the diffusion length as an important parameter that distinguishes nanoscale and macroscale carrier behaviors. Furthermore, the conceptual distinction in carrier dynamics in different dimensionality regimes informs new design rules for optical devices where complex microstructures are involved.

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

Catalytic Dynamic Kinetic Resolutions in Tandem to Construct Two-Axis Terphenyl Atropisomers

The defined structure of molecules bearing multiple stereogenic axes is of increasing relevance to materials science, pharmaceuticals, and catalysis. However, catalytic enantioselective approaches to control multiple stereogenic axes remain synthetically challenging. Here, we report the catalytic synthesis of two-axis terphenyl atropisomers, with complementary strategies to both chlorinated and brominated variants, formed with high diastereo- and enantioselectivity. The chemistry proceeds through a sequence of two distinct dynamic kinetic resolutions: first, an atroposelective ring opening of Bringmann-type lactones produces a product with one established axis of chirality, and second, a stereoselective arene halogenation delivers the product with the second axis of chirality established. In order to achieve these results, a class of Brønsted basic guanidinylated peptides, which catalyze an efficient atroposelective chlorination, is reported for the first time. In addition, a complementary bromination is reported, which also establishes the second stereogenic axis. These bromo-terphenyls are accessible following the discovery that chiral anion phase transfer catalysis by C 2 -symmetric phosphoric acids allows catalyst control in the second stereochemistry-determining event. Accordingly, we established the fully catalyst-controlled stereodivergent synthesis of all possible chlorinated stereoisomers while also demonstrating diastereodivergence in the brominated variants, with significant levels of enantioselectivity in all cases.

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

Redox-Induced Structural Reorganization Dictates Kinetics of Cobalt(III) Hydride Formation via Proton-Coupled Electron Transfer

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH 3 )] 2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)] + through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH 3 )] 2+ complexes (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)), peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pK a units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (k pl ) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (k plateau,depe = 3.5 x 10 7 M -1 s -1 , k plateau,dppe = 1.7 x 10 7 M -1 s -1 , k plateau,dcpe = 7.1 x 10 4 M -1 s -1 ). Mechanistic studies were conducted to identify the cause of the aberrant $k_PT^{app}$–ΔpK a trends. When the deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)] + . Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

14 SOLAR ENERGY↗
DOE OSTI2023

A Telomerase-Activated Magnetic Resonance Imaging Probe for Consecutively Monitoring Tumor Growth Kinetics and In Situ Screening Inhibitors

Telomerase has long been considered as a biomarker for cancer diagnosis and a therapeutic target for drug discovery. Detecting telomerase activity in vivo could provide more direct information of tumor progression and response to drug treatment, which, however, is hampered by the lack of an effective probe that can generate an output signal without a tissue penetration depth limit. Here in this study, using the principle of distance-dependent magnetic resonance tuning, we constructed a telomerase-activated magnetic resonance imaging probe (TAMP) by connecting superparamagnetic ferroferric oxide nanoparticles (SPFONs) and paramagnetic Gd-DOTA (Gd(III) 1,4,7,10- tetraazacyclododecane-1,4,7,10-tetraazacyclododecane-1,4,7,10-tet-raacetic acid) complexes via telomerase-responsive DNA motifs. Upon telomerase-catalyzed extension of the primer in TAMP, Gd-DOTA-conjugated oligonucleotides can be liberated from the surface of SPFONs through a DNA strand displacement reaction, restoring the T 1 signal of the Gd-DOTA for a direct readout of the telomerase activity. Here we show that, by tracking telomerase activity, this probe provides consistent monitoring of tumor growth kinetics during progression and in response to drug treatment and enables in situ screening of telomerase inhibitors in whole-animal models. This study provides an alternative toolkit for cancer diagnosis, treatment response assessment, and anticancer drug screening.

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

Mechanism-Guided Kinetic Analysis of Electrocatalytic Proton Reduction Mediated by a Cobalt Catalyst Bearing a Pendant Basic Site

Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pKa units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co(III) hydride intermediate is formed through reduction of the Co(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co(II) catalyst can be protonated, and the Co(III) hydride can react directly with acid to release H 2 . With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. Furthermore, these findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.

08 HYDROGEN↗
DOE OSTI2025

Interfacial Cation Arrangement Controls Electrocatalytic Kinetics in CO 2 Reduction

The identity of electrolyte cations is known to strongly influence electrocatalytic activity, but the relationship between their interfacial arrangement and observed performance remains poorly understood. Organic cations, with their molecular tunability, provide a powerful platform for systematically probing these effects. Here, we leverage phosphonium-based geminal dications to control interfacial cation arrangement and identify the variables that most strongly influence catalytic rates. As a case study, we examine CO 2 reduction to CO over polycrystalline silver electrodes in dry aprotic acetonitrile. Through a combination of rotating disk electrode measurements, electrochemical impedance spectroscopy, and molecular dynamics simulations, we decouple the effects of cation–electrode distance and interfacial cation density on catalytic rates. We find that smaller, more densely packed cations induce stronger interfacial electric fields, which lower the activation barrier for CO 2 adsorption and increase reaction rates. Using geminal phosphonium dications [C n (P mmm ) 2 ][ClO 4 ] 2 , we demonstrate that both the vertical and lateral positioning of organic cations within the electrical double layer independently affect reactivity. These results demonstrate that electrolyte cation identity primarily influences catalytic kinetics by determining how efficiently charge can be arranged at electrochemical interfaces. Altogether, our findings support an electrostatic view of cation effects in catalysis and provide design principles for next-generation electrolytes.

Cations↗
DOE OSTI2026

Microcanonical Kinetics of Water-Mediated Proton Transfer in 4ABAH + ·(H 2 O) n = 4–6 Clusters (ABA = Aminobenzoic Acid): A Model System for Size-Dependent Relaxation to Ergodic Behavior

Here, we leverage the unique properties of the 4ABAH + · (H 2 O) n clusters (ABA = 4-aminobenzoic acid, n = 4−6) to quantitatively address how a finite, isolated system evolves into an ergodic condition starting from localized arrangements in configuration space. This system adopts two distinct structural isomers in which water molecules cluster around the cationic centers of its two protomers with widely separated positive charge centers. These isomers arise from excess proton attachment to either the acid (O) or amino (N) group on opposite sides of the benzene ring. Both forms are captured and kinetically trapped using cryogenic ion methods and then selectively vibrationally excited through their mutually exclusive IR bands involving NH and OH stretching fundamentals. Because the IR excitation lies below the water binding energy, the system can evolve to explore slow, rare events that lead to the interconversion between the two isomers. The rates of these intracluster reactions are determined by using a pump−probe scheme involving ∼5 ns IR pump and UV probe lasers. The rates occur on the microsecond time scale, leading to steady state populations of the isomers, thus revealing the cluster size-dependent fractionation between the two species at microcanonical equilibrium. The steady state distributions are correlated with the expected trend in the cluster size-dependent reaction energetics, which are in turn consistent with changes in the relative densities of states of the two species. These results thus provide an unusually clear example in which complex, protic-solvent-mediated chemical transformations are captured within a finite system at a precisely determined internal energy.

Rana, Abhijit [Yale Univ., New Haven, CT (United S↗
DOE OSTI2026

Kinetics vs Thermodynamics: Engineering Photoredox Reactivity from an Upper Excited State of Fe II

Ultrafast deactivation of metal-to-ligand charge-transfer (MLCT) states into low-lying metal-centered states has long limited the utility of first-row transition-metal complexes in a broad range of applications, including photoredox catalysis. Here we bypass such limitations using a Fe II -pyridinium carbene complex to enable electron transfer reactivity directly from a higher-lying MLCT state. Time-resolved spectroscopic data revealed that the higher-energy MLCT manifold relaxes to lower-lying metal-centered states with a time constant of ca. 3 ps. The nature of the metal-centered (MC) excited state as a 3 MC (i.e., S = 1) was inferred from variable-temperature transient absorption studies. These experiments also revealed that the ground-state recovery process occurs at or very near the “Marcus barrierless regime”, thereby allowing for an estimate of its excited-state redox potential. The detailed picture of the energetics of this compound that emerged enabled fine-tuning of competing thermodynamic and kinetic pathways to effect electron transfer from the higher-energy MLCT excited state prior to relaxation to the ligand-field manifold. We believe these results open an unexplored landscape for the use of earth-abundant first-row transition-metal-based chromophores for applications in light-to-chemical energy conversion.

charge transfer↗
DOE OSTI2020

Tearing Instability in Alfvén and Kinetic-Alfvén Turbulence

Recently, it has been realized that magnetic plasma turbulence and magnetic field reconnection are inherently related phenomena. Turbulent fluctuations generate regions of sheared magnetic field that become unstable to the tearing instability and reconnection, thus modifying turbulence at the corresponding scales. In this contribution, we give a brief review of some recent results on tearing-mediated magnetic turbulence. We illustrate the main ideas of this rapidly developing field of study by concentrating on two important examples—magnetohydrodynamic Alfvén turbulence and small-scale kinetic-Alfvén turbulence. Due to various potential applications of these phenomena in space physics and astrophysics, we specifically try not to overload the text by heavy analytical derivations but rather present a qualitative discussion accessible to a non-expert in the theories of turbulence and reconnection.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Assessing Latent and Kinetic Energy Trend Changes in Extratropical Cyclones From 1940 to 2020: Results From ERA‐5 Reanalysis

Abstract Baroclinic or extratropical cyclones (ETCs) transport heat and moisture to higher latitudes, making it fundamentally important to understand how their influence changes as Earth's climate evolves. A 2–8‐day Lanzcos bandpass filter is applied to European Center for Medium Range Weather Forecasting 5th Generation Reanalysis latent energy (LE) and kinetic energy (KE) data to assess how ETCs have changed from 1940 to 2020 relative to full‐scale changes in LE and KE. Full‐scale KE trends are more positive at high latitudes relative to mid‐latitudes, confirming several previous studies that ETCs have shifted poleward. LE increases have occurred globally, and trends in both full‐scale LE and KE are statistically significant in the southern high latitudes. The high relative fractional contribution of 2–8‐day LE wave power and trend clearly suggest that ETCs have an increasingly important role in poleward moisture transport but are not solely responsible for the observed statistically significant increases.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

The Vertical Structure of Turbulence Kinetic Energy Near the Arctic Sea‐Ice Surface

Atmospheric turbulence over the Arctic sea-ice surface has been understudied due to the lack of observational data. In this study, we focus on the turbulence kinetic energy (TKE) over sea ice and distinguish its two different vertical structures, the “Surface” type and the “Elevated” type, using observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition (MOSAiC). The “Surface” type has the maximum TKE near the surface (at 2 m), while the “Elevated” type has the maximum TKE at a higher level (6 m). The TKE budget analysis indicates that the “Elevated” type is caused by the increased shear production of TKE at 6 m. In addition, spectral analysis reveals that the contribution to TKE by horizontal large eddies is enhanced in the “Elevated” type. Finally, how the vertical structure of TKE affects the parameterization of turbulent momentum flux is discussed.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Convective and Orographic Origins of the Mesoscale Kinetic Energy Spectrum

The mesoscale spectrum describes the distribution of kinetic energy in the Earth's atmosphere between length scales of 10 and 400 km. Since the first observations, the origins of this spectrum have been controversial. At synoptic scales, the spectrum follows a –3 spectral slope, consistent with two-dimensional turbulence theory, but a shallower –5/3 slope was observed at the shorter mesoscales. The cause of the shallower slope remains obscure, illustrating our lack of understanding. Through a novel coarse-graining methodology, we are able to present a spatio-temporal climatology of the spectral slope. We find convection and orography have a shallowing effect and can quantify this using “conditioned spectra.” These are typical spectra for a meteorological condition, obtained by aggregating spectra where the condition holds. This allows the investigation of new relationships, such as that between energy flux and spectral slope. Potential future applications of our methodology include predictability research and model validation.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Complex Dependence of Calcite Crack Kinetics on Salinity: The Role of DLVO and Hydration Forces

Abstract Subcritical crack growth (SCG) plays an important role in many geological processes such as delayed earth rupture and rock weathering. The complex dependency of SCG on the in‐crack fluid chemistry, however, is still poorly understood. In this study, we utilize the newly developed surface force‐based fracture theory (SFFT) to elucidate the relative contributions of surface forces and solute transport to the crack growth kinetics of calcite in NaCl solutions. Expanding on Barenblatt's cohesive crack model, SFFT introduces an effective stress intensity at the crack tip that encompasses all the relevant intermolecular forces across the crack in addition to the external far‐field stresses. The nonlinear system of equations portraying the crack opening profile, the solute distribution in a propagating crack, and the crack growth velocity are numerically solved via an implicit scheme. After carefully calibrating the model for calcite‐water systems, the SFFT is used to predict the SCG response of calcite at different NaCl concentrations, based on various hypotheses. These predictions are then compared to existing SCG data from the literature. We demonstrate that the experimentally observed variation of SCG rate with NaCl concentration cannot be explained solely by DLVO forces (electrostatic and Van der Waals interactions). This can be remediated by introducing an exponentially decaying hydration force with a nonlinear, nonmonotonic dependence on NaCl concentration. Furthermore, we demonstrate that accounting for both diffusive and advective transport of ions is important in explaining the absence of a stage‐II SCG response for calcite in electrolyte solutions. Plain Language Summary Subcritical crack growth (SCG) refers to the slow propagation of cracks in materials under a stress below the threshold for catastrophic failure. SCG is a key process in many geological events, for example, delayed earth ruptures and rock weathering. New initiatives such as underground CO 2 and H 2 storage in carbonate reservoirs further call for better understanding of SCG in carbonate minerals subjected to varying fluid chemistry. This study examines the SCG of calcite, a key mineral found in carbonate rocks, intergranular cement in sandstones, and filling material in mineral veins and faults, determining their deformation and strength. A mathematical model is developed to describe how the crack opens and propagates, how solutes (like salts) distribute within the crack, and how the crack surfaces interact with each other. We used the model to predict calcite SCG in water at different salt concentrations and compared it with experimental data. Our results revealed that the hydration force is the dominating factor in determining the complex, non‐linear dependency of SCG on salinity. We also found that both the movement of ions by diffusion and by bulk water flow are crucial for explaining the SCG rates, especially when the cracks grow quickly. Key Points Surface Force‐Based Fracture Theory predicts the complex subcritical crack growth patterns of calcite crystals immersed in NaCl solutions Results highlight the dominant role of hydration forces in altering the fracture behavior of calcite compared to VdW and electric double‐layer forces Advective solute transport explains the absence of stages‐II and ‐III subcritical crack growth responses in solid‐liquid systems

DLVO↗
DOE OSTI2022

Ultrafast kinetics of the antiferromagnetic-ferromagnetic phase transition in FeRh

Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temporal detection window of THz emission probes of transient states without sacrificing any loss of resolution or sensitivity. It allows us to study the kinetics of emergent ferromagnetism from the femtosecond up to the nanosecond timescales in FeRh/Pt bilayers. Our results strongly suggest a latency period between the initial pump-excitation and the emission of THz radiation by ferromagnetic nuclei.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Neural network kinetics for exploring diffusion multiplicity and chemical ordering in compositionally complex materials

Diffusion involving atom transport from one location to another governs many important processes and behaviors such as precipitation and phase nucleation. The inherent chemical complexity in compositionally complex materials poses challenges for modeling atomic diffusion and the resulting formation of chemically ordered structures. Here, we introduce a neural network kinetics (NNK) scheme that predicts and simulates diffusion-induced chemical and structural evolution in complex concentrated chemical environments. The framework is grounded on efficient on-lattice structure and chemistry representation combined with artificial neural networks, enabling precise prediction of all path-dependent migration barriers and individual atom jumps. To demonstrate the method, we study the temperature-dependent local chemical ordering in a refractory NbMoTa alloy and reveal a critical temperature at which the B2 order reaches a maximum. The atomic jump randomness map exhibits the highest diffusion heterogeneity (multiplicity) in the vicinity of this characteristic temperature, which is closely related to chemical ordering and B2 structure formation. The scalable NNK framework provides a promising new avenue to exploring diffusion-related properties in the vast compositional space within which extraordinary properties are hidden.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The kinetics of static recovery by dislocation climb

Abstract The initial microstructure of a wide range of structural materials is conditioned by thermo-mechanical treatments such as hot-working, tempering, or solution annealing. At the elevated temperatures associated with these treatments the dislocation microstructure evolves, usually decreasing in density through a process known as static recovery. Despite its technological relevance, static recovery is not fully characterized from a theoretical standpoint, with even the controlling mechanisms subject to debate. In this study, a climb-enabled discrete dislocation dynamics (DDD) capability is leveraged to explore the kinetics of static recovery in pure Fe when controlled by dislocation climb. Quantitative data from these simulations is used to develop a revised static recovery law, and provides the parameters appropriate for predictive microstructure models in Fe. This law differs from previous analytical derivations invoking climb of dislocations, following the logarithmic trends typical of experimental observations where prior work did not. Direct comparison between the recovery law derived from DDD to experimental recovery data in alpha Fe shows strong agreement across a range of temperatures, and suggests that climb is the controlling mechanism for static recovery in pure metals.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Isotope-dependent Tafel analysis probes proton transfer kinetics during electrocatalytic water splitting

Proton transfer plays an important role in both hydrogen and oxygen evolution reactions during electrocatalytic water splitting to produce green hydrogen. However, directly adapting the conventional proton/deuterium kinetic isotope effect to study proton transfer in heterogeneous electrocatalytic processes is challenging. Here we propose using the shift in the Tafel slope between protic and deuteric electrolytes, or the Tafel slope isotope effect, as an effective probe of proton transfer characteristics. Comparison of the Tafel slope isotope effect for diverse hydrogen and oxygen evolution reaction electrocatalysts in different pH environments reveals that proton transfer is both pH and structure dependent. Using ruthenium oxide as an example, we show that local structure modification can change the rate-determining step from an electrochemical, concerted proton–electron transfer step to a chemical step and improve the oxygen evolution activity in acid. The isotope-dependent Tafel analysis will facilitate a better understanding of the proton transfer behaviours during electrocatalytic processes and provide guidance for designing efficient electrocatalysts.

Chemistry↗