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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 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Structural investigations of La 0.6 Sr 0.4 FeO 3-δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La 0.6 Sr 0.4 FeO 3-δ , LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (R$\overline{3}$c to Pm$\overline{3}$m) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Revisiting complexation thermodynamics of transplutonium elements up to einsteinium

Literature casts einsteinium as a departure from earlier transplutonium actinides, with a decrease in stability constants with aminopolycarboxylate ligands. This work studies transplutonium chemistry – including Am, Bk, Cf, and Es – with aminopolycarboxylate ligands. Es complexation follows similar thermodynamic and structural trends established by the earlier actinides, consistent with first-principle calculations.

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

Density functional theory study of thermodynamic and kinetic isotope effects of H 2 /D 2 dissociative adsorption on transition metals

We studied here the thermodynamic isotope effects (TIEs) and kinetic isotope effects (KIEs) for H 2 /D 2 dissociative adsorption using periodic, density functional theory (DFT)-based calculations. We examined the TIEs on the close-packed, open, and stepped surfaces, of twelve transition metals (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Re, Ir, Pt, and Au), and the KIEs on the surfaces of three noble metals (Cu, Ag, and Au). Both TIEs and KIEs were evaluated at 1/9 ML coverage. We find distinct TIEs on different adsorption sites, indicating that TIEs could be used in conjunction with binding energies to determine the dominant adsorption sites for hydrogen. Additionally, we find that while H 2 dissociative adsorption may traditionally be considered structure insensitive in terms of reaction rates, it can exhibit structure sensitivity in terms of its KIEs. Complementarily to TIEs, KIEs might therefore be useful for identifying active sites for H 2 dissociative adsorption on the three noble metal transition metal catalysts studied.

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

Thermodynamics of the S 2 -to-S 3 state transition of the oxygen-evolving complex of photosystem II

The room temperature pump–probe X-ray free electron laser (XFEL) measurements used for serial femtosecond crystallography provide remarkable information about the structures of the catalytic (S-state) intermediates of the oxygen-evolution reaction of photosystem II. However, mixed populations of these intermediates and moderate resolution limit the interpretation of the data from current experiments. The S 3 XFEL structures show extra density near the OEC that may correspond to a water/hydroxide molecule. However, in the latest structure, this additional oxygen is 2.08 Å from the Oε2 of D1–E189, which is closer than the sum of the van der Waals radii of the two oxygens. In this work, we use Boltzmann statistics and Monte Carlo sampling to provide a model for the S 2 -to-S 3 state transition, allowing structural changes and the insertion of an additional water/hydroxide. Based on our model, water/hydroxide addition to the oxygen-evolving complex (OEC) is not thermodynamically favorable in the S 2 g = 2 state, but it is in the S 2 g = 4.1 redox isomer. Thus, formation of the S 3 state starts by a transition from the S 2 g = 2 to the S 2 g = 4.1 structure. Then, electrostatic interactions support protonation of D1–H190 and deprotonation of the Ca 2+ -ligated water (W3) with proton loss to the lumen. The W3 hydroxide moves toward Mn4, completing the coordination shell of Mn4 and favoring its oxidation to Mn( IV ) in the S 3 state. In addition, binding an additional hydroxide to Mn1 leads to a conformational change of D1–E189 in the S 2 g = 4.1 and S 3 structures. In the S 3 state a fraction of D1–E189 release from Mn1 and bind a proton.

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

Structure–thermodynamics relationship of schoepite from first-principles

The relationship between the structure and thermodynamic properties of schoepite, an important uranyl phase with formula [(UO 2 ) 8 O 2 (OH) 12 ]·12H 2 O formed upon corrosion of UO 2 , has been investigated within the framework of density functional perturbation theory (DFPT). Experimental crystallographic lattice parameters are well reproduced in this study using standard DFT. Phonon calculations within the quasi-harmonic approximation predict standard molar entropy and isobaric heat capacity of S 0 = 179.60 J mol -1 K -1 and C 0 P = 157.4 J mol -1 K -1 at 298.15 K, i.e. , ~6% and ~4% larger than existing DFPT-D2 calculations. The computed variation of the standard molar isobaric heat capacity with water content from schoepite (UO 3 · x H 2 O, x = 2.25) to dehydrated schoepite ( x = 1) is predicted to be essentially linear along isotherms ranging from 100 to 500 K. Finally, these findings have important implications for the dehydration of layered uranyl corrosion phases and hygroscopic materials.

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

Unique interfacial thermodynamics of few-layer 2D MoS 2 for (photo)electrochemical catalysis

The electronic structure of few-layer MoS2 is studied by in-situ and operando spectroelectrochemistry in conditions relevant to its use as an electrocatalyst. We show that electron injection into the conduction band is coupled with a redshift of the exciton resonance, the magnitude of which depends on the number of vertical MoS 2 layers. In addition, the applied electric field/electronic doping imparts uniaxial tensile strain evidenced by broadening Raman signals, indicating that under conditions of electrocatalysis, the system is structurally different from equilibrium. We demonstrate that field/carrier induced changes to the electronic structure of MoS 2 alter the band edge positions which changes the fundamental thermodynamic driving force for charge transfer. This property is a function of the applied potential, an effect unique to 2D semiconductors. The dynamic band edge potentials change the relevant interfacial energetics for charge transfer and has strong implications for the mechanistic understanding of (photo)catalytic fuel-forming reactions using two-dimensional systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interpreting Tafel behavior of consecutive electrochemical reactions through combined thermodynamic and steady state microkinetic approaches

Assessing the reaction pathway of multi-electron-transfer reactions is an essential yet difficult task for the rational design of electrocatalysts. In this work, we develop a heuristic approach that combines thermodynamic adsorption energetics calculated through density functional theory with microkinetic modeling using the steady state approximation to interpret the potential-dependent Tafel behavior of consecutive electrochemical reactions. In doing so, we introduce a kinetic framework for ab initio calculations that ensures self-consistent adsorption energetics based on kinetically limited adsorbate coverages. The approach is applied to experimental results on CoO x (OH) 2-x single crystal electrocatalyst particles yielding coverage dependent mechanistic information and identification of the rate-limiting step with standard rate constants for the oxygen evolution reaction on the ($11\bar{2}0$) surfaces of the β-Co(OH) 2 , β-CoOOH, and CoO 2 bulk phases. Finally, this generalizable method enables catalyst benchmarking based on determining the active species involved and associated intrinsic reaction rate constants in consecutive multi-electron-transfer reactions.

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

Chemical short-range order in derivative Cr–Ta–Ti–V–W high entropy alloys from the first-principles thermodynamic study

The development of high-entropy alloys (HEAs) focuses on exploring compositional regions in multi-component systems with all alloy elements in equal or near-equal atomic concentrations. Initially it was based on the main idea that high mixing configurational entropy contributions to the alloy free energy could promote the formation of a single solid solution phase. By using the ab-initio based Cluster Expansion (CE) Hamiltonian model constructed for the quinary bcc Cr–Ta–Ti–V–W system in combination with Monte Carlo (MC) simulations, we show that the phase stability and chemical short-range order (SRO) of the equiatomic quinary and five sub-quaternary systems, as well as their derivative alloys, can dramatically change the order–disorder transition temperatures (ODTT) as a function of alloy compositions. In particular, it has been found, that the equiatomic quaternary Ta–Ti–V–W and Cr–Ta–Ti–W alloys had the lowest order–disorder transition temperature (500 K) among all the analysed equiatomic compositions. In all investigated alloy systems, the strongest chemical ordering has been observed between Cr and V, which led to the conclusion that decreasing the concentration of either Cr or V might be beneficial in terms of decreasing the ODTT. It also predicts that increasing concentration of Ti significantly decreases the ODTT. Our analysis of chemical SRO as a function of alloy composition allows to understand the microstructure evolution of HEAs as a function of temperature in excellent agreement with available experimental observations. Importantly, our free energy of mixing and SRO calculations predict that the origin of precipitates formed by Cr- and V-rich in the sub-quaternary Cr–Ta–V–W system is driven by the thermodynamics. The modelling results are in an excellent agreement with experimental observation of Cr and V segregation in the W 0.38 Ta 0.36 Cr 0.15 V 0.11 alloy which in turns shows an exceptional radiation resistance.

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

Bridging the thermodynamics and kinetics of temperature-induced morphology evolution in polymer/fullerene organic solar cell bulk heterojunction

The performance of organic solar cells (OSC) critically depends on the morphology of the active layer. After deposition, the active layer is in a metastable state and prone to changes that lead to cell degradation. Here, a high efficiency fullerene:polymer blend is used as a model system to follow the temperature-induced morphology evolution through a series of thermal annealing treatments. Electron microscopy analysis of the nano-scale phase evolution during the early stages of thermal annealing revealed that spinodal decomposition, i.e. spontaneous phase separation with no nucleation stage, is possibly responsible for the formation of a fine scale bicontinuous structure. In the later evolution stages, large polycrystalline fullerene aggregates are formed. Optical microscopy and scattering revealed that aggregate-growth follows the Johnson–Mehl–Avrami–Kolmogorov equation indicating a heterogeneous transformation process, i.e., through nucleation and growth. These two mechanisms, spinodal decomposition vs. nucleation and growth, are mutually exclusive and their co-existence is surprising. This unexpected observation is resolved by introducing a metastable monotectic phase diagram and showing that the morphology evolution goes through two distinct and consecutive transformation processes where spinodal decomposition of the amorphous donor:acceptor blend is followed by nucleation and growth of crystalline acceptor aggregates. Lastly, this unified thermodynamic and kinetic mechanism allows us to correlate the morphology evolution with OSC degradation during thermal annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Unexpected electrochemical behavior of an anolyte redoxmer in flow battery electrolytes: solvating cations help to fight against the thermodynamic–kinetic dilemma

Redoxmers are redox-active molecules that can store energy in electrolytes for redox flow batteries (RFBs), and their electrochemical properties are significantly affected by the choice of supporting electrolytes. Herein, we use 2,1,3-benzothiadiazole (BzNSN) as a model system to scrutinize the supporting electrolyte impact. By systemically varying the components of supporting salts, BzNSN not only shows substantial redox potential shifts but also exhibits varying electrochemical stabilities. Specifically, changing the size of cations can effectively alter the coordination between the supporting salt and BzNSN species. From Li + , Na + , K + , to NEt 4 + , the redox potential of BzNSN shifts negatively, from -1.63 V to -1.82 V vs. Ag/Ag + . Molecular dynamics and density functional theory simulations revealed that smaller cations, like Li + , are closer to the charged BzNSN when coordinated, implying stronger coordination, while larger cations, like K + and NEt 4 + , are farther away. Interestingly, the large cation electrolytes also lead to much improved electrochemical stability, evidenced by the extraordinarily enhanced kinetic lifetime from electron paramagnetic resonance measurement. This study demonstrates the first example of tuning an anolyte redoxmer toward a concurrent improvement of lowered redox potentials AND enhanced calendar lives via solvation means, which is usually constrained by the thermodynamic–kinetic relation.

2,1,3-benzothiadiazole↗
DOE OSTI2020

Thermodynamic and kinetic properties of layered-CaCo2O4 for the Ca-ion batteries: a systematic first-principles study

One of the more promising directions in multivalent energy storage is systems based on Ca ion intercalation due to the potential for high voltage and capacity. A major challenge for enabling such a battery is to find cathode materials capable of fast ionic diffusion and reversible insertion of Ca ions. Here, on the basis of first-principles calculations, we have demonstrated that layered CaCo 2 O 4 exhibits favorable thermodynamic and kinetic properties that should enable topotactic Ca ion intercalation reactions. The P3-type layered Ca x Co 2 O 4 (0 < x < 1) with either of space groups of P 1 or P 2 1 / m are stable at multiple Ca concentrations and show a smooth voltage plateau higher than 3 V up to X = 0.5. The energy barriers of the single Ca ion migration are as low as 0.36 eV and 0.27 eV at the dilute and high vacancy concentration limits, respectively. Therefore, although varying the vacancy environments of the diffusing atom influences the migration barriers, they do not exceed 0.6 eV. Stochastic analysis of Ca hopping events performed by ab initio molecular dynamics (AIMD) simulation has shown that the migration barriers are lower than 0.32 eV. Therefore, the Ca diffusivity at room temperature extrapolated from the AIMD results is comparable to Li diffusivity (>10 -10 cm 2 s -1 ) in conventional Li cathode materials, suggesting the feasibility of layered Ca x Co 2 O 4 as multivalent cathode materials. Finally, the structural factors that enable fast diffusion are discussed.

25 ENERGY STORAGE↗
DOE OSTI2021

Efficient separation of xylene isomers by using a robust calcium-based metal–organic framework through a synergetic thermodynamically and kinetically controlled mechanism

Adsorptive separation of physically and chemically similar molecules and understanding the underlying host–guest interactions at the molecular level are of significant scientific and practical importance. Here we report the development of a novel calcium-based metal–organic framework, formulated as Ca 3 (Htcpp) 2 (H 4 tcpp = 2,3,5,6-tetrakis(4-carboxyphenyl)-pyrazine) featuring microporosity and high stability. This compound shows distinct adsorption behavior toward xylene isomers and is capable of separating them efficiently at an industrially relevant temperature. The selective adsorption is attributed to a synergetic thermodynamic and kinetic effect. Here, the host–guest interactions were probed directly by single-crystal X-ray diffraction analysis and the adsorption affinity was evaluated through computational molecular simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗