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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Redox-active dinuclear oxorhenium(V) pyrazolate complexes

Four new structurally similar dinuclear oxorhenium(V) complexes, [{Re(O)X(PPh 3 )} 2 (μ-O)(μ-4-x'-pz) 2 ], where pz = pyrazolate anion, X = X' = Cl (1) and Br (4), X = Cl, X' = Br (2), and X = Br, X' = Cl (3), have been synthesized and characterized. Little variation in spectroscopic features – 1 H NMR, IR, UV–Vis – exists among the four complexes. All complexes possess a bent Re-O-Re core as well as distorted octahedral coordination geometry around the rhenium centers. Finally, a reversible one-electron electrochemical process is observed at approximately 0.84 V vs. Fc + /Fc in all four complexes; however, changing the terminal halide from chloride to bromide slightly destabilizes the oxidized Re(VI) center.

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

Structural and Electronic Complexities of a Sulfur‐Bridged Di‐Iron Complex Composed of Mono‐ and Di‐Nitrosyl Units

The delocalized, thermodynamically stable cation, [(N 2 S 2 )Fe(NO)•Fe(NO) 2 ] + , an adduct of mono-nitrosyl and dinitrosyl iron units, is analyzed to address the unusual stability of the sulfur-bridged diiron complex in its three overall redox levels, +, 0, and −. X-ray diffraction and myriad spectroscopic techniques probe products of sequential electron uptake in the corresponding neutral and anionic species. Conundrums include unified blueshifts of the overall 3-band, ν(NO), pattern with added electrons. One-electron reduction changes the anti-ferromagnetically coupled, S = 0, cationic diiron species to the neutral analog, S = ½, with unpaired spin mainly localized on the MNIU, which decreases its ∠Fe–N–O angle by 10 degrees in response to the extra electron density. Subsequent reduction to the anionic species, S = 1, involves a major geometric change at the MNIU, which moves the Fe in {Fe(NO)} 8 out of the N 2 S 2 plane. Site-specific 15 N labeling of nitrosyl in the MNIU confirms the IR analysis and shows rapid NO exchange between the MNIU/DNIU (mono-nitrosyl iron unit/dinitrosyl iron unit) pairs during its synthesis at RT. Mössbauer spectroscopy, S K-edge XAS, and molecular orbital calculations confirm the ability of NO and the versatility of sulfur bridges to buffer and distribute electrons, a key to their major importance in metalloenzymes.

DNIC↗
DOE OSTI2020

Identification of a Na + -Binding Site near the Oxygen-Evolving Complex of Spinach Photosystem II

The oxygen-evolving complex (OEC) of photosystem II (PSII) is an oxomanganese cluster composed of four redox-active Mn ions, and one redox-inactive Ca 2+ ion, with two nearby bound Cl – ions. Sodium is a common counter-ion of both chloride and hydroxide anions, and a sodium-specific binding site has not been identified near the OEC. Furthermore, we find that the oxygen-evolution activity of spinach PSII increases with [Na + ], particularly at high pH. A Na + -specific binding site next to the OEC, upon deprotonation of the D1-H337 amino-acid residue, is suggested by the analysis of two recently published PSII cryo-EM maps in combination with quantum mechanical calculations and molecular dynamics simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2021

Tunable Pseudocapacitive Intercalation of Chloroaluminate Anions into Graphite Electrodes for Rechargeable Aluminum Batteries

Rechargeable aluminum-graphite batteries using chloroaluminate-containing electrolytes have been the focus of significant research, particularly due to their high-rate capabilities. Engineered graphite electrodes have been shown to exhibit supercapacitor-like rate performance, despite the fact they store charge via the electrochemical intercalation of polyatomic AlCl4− anions. However, the origins of such rate capabilities are not well understood. Here, using electrochemical techniques, we disentangle quantitatively the diffusion-limited Faradaic, pseudocapacitive, and capacitive contributions to charge storage, revealing that AlCl4− anions intercalate into graphite with significant pseudocapacitive characteristics due to low ion diffusion limitations. Pristine and mildly exfoliated graphites are compared, where exfoliation resulted in significantly higher pseudocapacitive AlCl4− intercalation at the highest potential redox pair as well as higher galvanostatic capacity retention at faster discharge rates. The relationships between graphite structure, ion mass transport, and the overall rate of electrochemical AlCl4− intercalation are discussed. Ion diffusion within the electrolyte phase of the porous electrode is shown to play a key role in controlling the rate of intercalation at higher potentials and faster rates, which can be enhanced by reducing electrode tortuosity. The results establish that chloroaluminate anion intercalation into graphite exhibits non-diffusion-limited pseudocapacitive contributions that are tunable by modifying the graphite structure.

Jeffrey H. Xu↗
DOE OSTI2023

Electrolytes with Solvating Inner Sheath Engineering for Practical Na–S Batteries

Sodium–sulfur (Na–S) batteries with durable Na-metal stability, shuttle-free cyclability, and long lifespan are promising to large-scale energy storages. However, meeting these stringent requirements poses huge challenges with the existing electrolytes. Herein, a localized saturated electrolyte (LSE) is proposed with 2-methyltetrahydrofuran (MeTHF) as an inner sheath solvent, which represents a new category of electrolyte for Na–S system. Unlike the traditional high concentration electrolytes, the LSE is realized with a low salt-to-solvent ratio and low diluent-to-solvent ratio, which pushes the limit of localized high concentration electrolyte (LHCE). The appropriate molecular structure and solvation ability of MeTHF regulate a saturated inner sheath, which features a reinforced coordination of Na + to anions, enlarged Na + -solvent distance, and weakened anion-diluent interaction. Such electrolyte configuration is found to be the key to build a sustainable interphase and a quasi-solid–solid sulfur redox process, making a dendrite-inhibited and shuttle-free Na–S battery possible. With this electrolyte, pouch cells with decent cycling performance under rather demanding conditions are demonstrated.

25 ENERGY STORAGE↗
DOE OSTI2022

Access to Ru(IV)–Ru(V) and Ru(V)–Ru(VI) Redox in Layered Li 7 RuO 6 via Intercalation Reactions

The push to increase the capacity of oxides to store charge as Li-ion battery cathodes has challenged our knowledge of the boundaries of redox chemistry and its relationship to the atomic structure. Layered phases with Ru are now classical models in the quest for refined theories. Current understanding dictates that the highest formal state that can be reached in them is Ru(V), after which activity centered at the oxide anions is triggered. Yet, this picture is challenged by this investigation of the intercalation chemistry of Li 7 RuO 6 . Coupling structural and spectroscopic analysis with computational simulations, we show that Li 7 RuO 6 is capable of highly reversible lithiation to Li 8 RuO 6 through the Ru(V)/Ru(IV) redox couple. Further, Li 7 RuO 6 can also undergo anodic Li deintercalation, showing clear evidence of the reversible formation of Ru(VI) with octahedral coordination, an unusual state that is not accessible in other layered oxides. The results highlight the versatility of the Ru-O bond to undergo distinct redox transitions depending on the specific layered arrangements. They enrich our understanding of redox chemistry in solids while underscoring the need for descriptions of charge compensation that reflect the nuance of covalent interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Copper Single Atom-Triggered Niobia–Ceria Catalyst for Efficient Low-Temperature Reduction of Nitrogen Oxides

To reduce nitrogen oxide (NO x ) emission from diesel engines in the cold-start process benefitting the atmospheric environment, catalysts with superior low-temperature NO x removal efficiency are highly demanded. Herein, we report an efficient Nb 2 O 5 /CuO/CeO 2 (NbCuCe) oxide catalyst for the selective catalytic reduction (SCR) of NO x , showing much higher DeNO x activity below 200 °C, superior sulfur resistance, faster response, and much less NH 3 slip than the state-of-the-art Cu-CHA zeolite catalyst. Additionally, atomically dispersed Cu species facilitate the strong interaction between Cu and the Nb/Ce base catalyst, which significantly improves the low-temperature redox properties at Cu–O–Ce sites and NH 3 adsorption/activation at Nb–O–Cu sites, thus contributing to the superior SCR performance of NbCuCe at low temperatures. The developed NbCuCe catalyst is highly promising for efficient DeNO x from cold-start diesel engines and can be coupled with Cu-CHA to achieve a broad operation temperature window.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrochemical Behavior of Cerium at an Indium Tin-Doped Oxide Electrode in Acidic Media

The redox behavior and speciation of cerium at mesoporous thin films composed of nanoparticles of indium tin-doped oxide (nITO) electrodes were characterized in pH 4.8, 0.1 M acetate buffer and both 0.1 and 1 M HNO 3 using electrochemical techniques and X-ray photoelectron spectroscopy. Anodic deposition of ceria species from Ce(III) to the nITO electrode was achieved under all solvent conditions via spontaneous condensation of electrochemically generated ceric hydroxide species. In 1 M nitric acid, the rate of CeO 2 dissolution is on the same order as CeO 2 deposition, resulting in negligible amounts of CeO 2 electrodeposited at the nITO surface. The cathodic stripping of CeO 2 from the nITO substrate deposited in 0.1 M nitric acid or pH 4.8 acetate buffer follows a 2-step process where Ce(IV)-oxide is initially reduced to an unstable Ce(III)-oxide species that rapidly undergoes acid catalyzed dissolution to yield soluble Ce(III) (aq) . These findings provide a foundation for the pH and anodic potential controlled deposition of CeO 2 thin films to ITO substrates, which can aid in the development of materials composed of ceria. As a result, they can also be used to infer likely analogous actinide redox behavior and speciation at these electrodes.

Cerium↗
DOE OSTI2022

Theoretical and Experimental Investigation of Functionalized Cyanopyridines Yield an Anolyte with an Extremely Low Reduction Potential for Nonaqueous Redox Flow Batteries

Abstract Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of −2.2 V or lower vs. ferrocene +/0 (Fc +/0 ), but the 3‐CNPy⋅ − radical anion forms a sigma‐dimer that is re‐oxidized at E ≈−1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge‐discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu 4 ][PF 6 ] shows that 2‐CNPy and 4‐CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density‐functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E 1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3‐CN‐6‐PhPy and 3‐CN‐4‐PhPy are the most promising as anolytes. They exhibit reversible reductions at E 1/2 =−2.19 and −2.22 V vs. ferrocene +/0 , respectively, and retain about half of their capacity after 30 bulk charge‐discharge cycles. An improved version of 3‐CN‐6‐PhPy with three methyl groups (3‐cyano‐4‐methyl‐6‐(3,5‐dimethylphenyl)pyridine) has an extremely low reduction potential of −2.50 V vs. Fc +/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge‐discharge cycling in acetonitrile.

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

I – /I 3 – Redox-Assisted Synthesis and Properties of Low Dimensional, Mixed-Valent Gold Iodide Perovskite Derivatives

Here, we report a set of three new mixed-valent Au I Au III iodides: (ClPy) 3 [AuI 2 ] 2 [AuI 4 ] [1], (BrPy) 3 [AuI 2 ] 2 [AuI 4 ] [2], and (ClPy) 2 [AuI 2 ][AuI 4 ] [3], as well as three new monovalent Au III iodides: (XPy) 2 [AuI 4 ][I 3 ] (Py = 4-X-pyridinium X = Cl, Br, and I) [4–6]. Two of these mixed-valent compounds (1 and 2) incorporate both monovalent Au I ···Au I (aurophilic bonding) and mixed-valent Au I I···Au III I couples (Au–I halogen bonding), to the best of our knowledge an unprecedented structural feature. These same two mixed-valent compounds also exhibit a rare low-dimensional molecular architecture with respect to second sphere Au···I interactions, namely, 1D chains of Au···I interactions, extending along a single crystallographic axis. All compounds were synthesized with the assistance of the I – /I 3 – redox couple and tacit manipulation of the polyiodide content during synthesis. Air-free synthesis was used to influence the redox process of I – /I 3 – , resulting in better selection for mixed-valent products. Compounds 1–2 and 4–5 exhibit a characteristically narrow bandgap (1.04–1.25 eV), as measured via diffuse reflectance spectroscopy (DRS). Computational analyses were used to rationalize the specific assembly modes of [AuI 2 ] – and [AuI 4 ] – species, and they show that the Au I ···I interaction type is favored over the Au III ···I.

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

Photoelectrochemical Nitrate and Nitrite Reduction Using Cu 2 O Photocathodes

Nitrate in wastewater streams causes eutrophication, and nitrate removal is of great importance for environmental protection. Electrochemical nitrate reduction has the advantage of directly converting nitrate to benign or useful chemicals, but it typically requires a considerable overpotential. Here, in this study, photoelectrochemical nitrate reduction is investigated using a Cu 2 O photocathode, where photoexcited electrons in the conduction band inherently have an overpotential of >1.6 V for nitrate reduction. The Cu 2 O photocathode is found to reduce nitrate to nitrite selectively with a high Faradaic efficiency (>85%). More importantly, as the surface of Cu 2 O is particularly catalytic for nitrate reduction, nitrate reduction on Cu 2 O kinetically suppresses photocorrosion of Cu 2 O without the need for additional catalyst or protection layers. In addition to nitrate reduction, nitrite reduction on Cu 2 O is examined to compare the effects of nitrate and nitrite reduction kinetics on the photocurrent generation and photocorrosion of Cu 2 O photocathodes.

25 ENERGY STORAGE↗
DOE OSTI2025

Electrochemical Activation of Ni–Fe Oxides for the Oxygen Evolution Reaction in Alkaline Media

The oxygen evolution reaction (OER) is essential to many key electrochemical devices, including H 2 O electrolyzers, CO 2 electrolyzers, and metal−air batteries. NiFe oxides have been historically identified as active for the OER, though they have been less studied in their more commercially relevant bulk oxide forms, such as NiFe 2 O 4 . Past works have demonstrated that the initial starting phase of Ni(Fe) precatalysts can influence their activation to the Ni(Fe)OOH active phase, including the rate and degree of conversion, pointing to the necessity of understanding activation protocols and in situ characteristics of catalyst materials at the device level. In this work, we investigate the characteristics of commercially relevant NiFe bulk oxides (NiFe 2 O 4 and a physical mixture of NiO and γ-Fe 2 O 3 ) during multiple activation procedures. Our results demonstrate that significant performance enhancement is observed for these bulk oxides regardless of the Fe incorporation in the initial form (i.e., atomically or macroscopically integrated), leading to significant performance enhancement (up to 30×) over time on stream. We hypothesize that this activation is due to the formation of NiFeOOH active sites on the surface, supported by in situ cyclic voltammetry and Raman spectroscopy results. We further show that not only the starting material but also the method of activation influences the number of Ni(Fe)OOH active sites formed and suggest that these sites can be quantified from the Ni 2+ to Ni 3+ redox transition using cyclic voltammetry. Broadly, this work demonstrates the necessity of in situ characterization of catalyst materials for cell-level design and testing.

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

Systematic Study of Different Anion Doping on the Electrochemical Performance of Cobalt-Free Lithium–Manganese-Rich Layered Cathode

Regardless of the appealingly high energy density (1000 Wh kg –1 ) of the lithium–manganese-rich layered oxide cathode (LMR-NMO), this material still suffers from rapid capacity and voltage decay after continuous cycling. LMR-NMO involves the redox reaction of both transition metals and oxygen to gain additional capacity in comparison with a conventional NMC cathode. Due to the use of a high voltage range (beyond 4.4 V), the oxygen release from the structure initiates and in turn generates intergranular cracks and spinel formation, consequently, into rock-salt structure. Here, LMR-NMO with different doping ranges (1–5 mol %) of F, S, and Cl are being examined to summarize the benefits and drawbacks of each anion. This study shows that F is the best candidate as it increases average voltage (voltage retention 96–97% after 200 cycles at 0.5 C), improves ionic conductivity (nearly two times higher than pristine), reduces cation mixing, minimizes oxygen release, and offers high stability during high temperature cycle. However, for S and Cl, the results are conflicted. An optimal amount of anion dopants should be considered as the side effects might overcome the benefits when the doping amount is excessive.

25 ENERGY STORAGE↗
DOE OSTI2026

Leveraging the redox activities of cerium and dibenzotetrathiafulvalene to discover a photo-responsive magnetic material

Stimuli-responsive changes in lanthanide-based materials are a promising research direction. In this study, [DBTTF] 4 [Ce 2 Cl 10 ] DBTTF = dibenzotetrathiafulvalene (1) was synthesized by a light-induced crystallization, where photo-oxidation of DBTTF enables formation of the cerium dimer [Ce 2 Cl 10 ] 4− . Intermolecular interactions between the stacked organic units of the crystal result in charge transfer bands in the visible-NIR (near-infrared) region, evident in the solid-state absorption spectrum upon comparison with the solution spectrum. The assignments of the sublattice oxidation states were made with single-crystal X-ray diffraction (SC-XRD) structural characterization, Raman spectroscopy, X-ray absorption spectroscopy, and magnetometry. Continuous 532 nm laser irradiation of the microcrystalline solid modulates the redox states in 1, leading to ∼40% reduction in the observed magnetization at 2 K. Density functional theory PBE+U/HSE06 band structure calculations predict Mott insulating behavior in 1, with a bandgap of 0.54/0.81 eV, and further support the conjecture that the observed photo-induced change in magnetization results from electron transfer from the [Ce 2 Cl 10 ] 4− anions to the π-stacked [DBTTF] 2 2+ organic dimer subunits. An enhancement in conductivity is similarly observed upon 532 nm irradiation, determined by single-crystal transport measurements. The findings reveal that photo-responsive lanthanide-based materials can be achieved by integration of redox-active organic moieties with redox-active lanthanide cations for the realization of switchable, photo-magnetic materials.

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

Iodine effective diffusion coefficients through volcanic rock: Influence of iodine speciation and rock geochemistry

Accurate prediction of the subsurface transport of iodine species is important for the assessment of long-term nuclear waste repository performance, as well as monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty, given that radioiodine decays into radioxenon. However, the transport of iodine through intact geologic media is not well understood, compromising our ability to assess risk associated with radioiodine migration. The current study's goal is to quantify the matrix diffusion of iodine species through saturated volcanic rock, with particular attention paid to the redox environment and potential speciation changes. Here, diffusion experiments were run for iodide through lithophysae-rich lava, lithophysae-poor lava, and welded tuff, whereas iodate diffusion was studied through welded tuff. Iodine transport was compared with a conservative tracer, HDO, and effective diffusion coefficients were calculated. Likely due to a combination of size and anion exclusion effects, iodine species diffused more slowly than the conservative tracer through all rock types tested. Furthermore, oxidation of iodide to iodate was observed in the lithophysae-poor lava, affecting transport. Results provide much needed data for subsurface transport models that predict radioiodine migration from underground sources, and indicate the pressing need for geochemical and redox interactions to be incorporated into these models.

58 GEOSCIENCES↗
DOE OSTI2025

Structural Evolution of Mixed-Addenda Keggin Polyoxometalate Anions with Atom-by-Atom Substitution

Polyoxometalates (POMs) are molecular metal oxides with distinctive electronic properties that make them promising materials for applications in energy, sensors, and memory devices. One of the most promising methods of tuning the stability, photochromic, redox, and electron-spin properties of POMs is through the substitution of the metal “addenda” atoms that, along with oxygen, constitute their cage-like structures. Because traditional synthesis methods typically produce a distribution of POMs, the isolation and characterization of multimetallic POMs with predetermined stoichiometry remains challenging. The presence of multiple energetically accessible isomers further complicates the experimental characterization and theoretical modeling of multimetallic POMs. Herein, we leverage the distinguishing mass-selection capabilities of ion soft landing to prepare stoichiometrically selected Keggin PMo x W 12-x O 40 3- (x = 0 – 6, 8, 10, and 12) POMs on self-assembled monolayer surfaces free of the solvent molecules and counterions that often confound characterization of complex species at interfaces. The structures of the supported POMs are characterized with atom-by-atom precision using in situ infrared (IR) reflection absorption spectroscopy complemented by detailed density functional theory calculations. Our joint experimental and theoretical results reveal an almost linear shift in the positions of the IR bands towards lower wavenumbers with an increase in the number of lighter molybdenum atoms compared to heavier W atoms in PMoxW 12-x O 40 3- . The theoretical calculations also indicate that numerous isomeric structures may be populated at the experimental conditions and, consequently, contribute to the overall IR spectra. In conclusion, our findings indicate that in addition to the number of substituted addenda atoms and the presence of multiple isomeric structures, interactions with the surface play an important role in determining the IR spectra and structure of supported bimetallic POMs.

Prabhakaran, Venkateshkumar [Pacific Northwest Nat↗