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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

An organic bifunctional redox active material for symmetric aqueous redox flow battery

Symmetric redox flow batteries utilizing a single redox active material as catholyte and anolyte, offer a plausible and unique strategy to alleviate cross-contamination issues during cycling, making them an ideal energy storage device for large scale applications. In this work, a low-cost riboflavin organic molecule is coupled with a 2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) radical molecule to form a single Riboflavin-TEMPO (RIBOTEMPO) bifunctional redox active material, which is used in both aqueous anolyte and catholyte. Cyclic voltammetry analysis of the combined molecule displays electrochemically reversible reactions for the redox couples at 0.72 V and 0.51 V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.23 V. A symmetric cell testing demonstrated over 100 consecutive charge/discharge cycles with nearly 80% coulombic efficiency and capacity retention of 44.7% at a current density of 2.5 mA cm(-2). By utilizing a pH neutral supporting electrolyte, a lowcost, safe, and environmental-friendly redox flow battery is achieved. This work provides a pathway for the design and development of bifunctional redox active molecules for symmetric battery applications.

25 ENERGY STORAGE↗
DOE OSTI2022

Elucidating facet dependent electronic and electrochemical properties of Cu 2 O nanocrystals using AFM/SCEM and DFT

Cuprous oxide (Cu 2 O) has extensively been studied owing to its excellent optical, magnetic, and catalytic properties. Many of these properties are facet-dependent and have not been well elucidated. This work synthesized cubic, cuboctahedral, octahedral, and rhombic dodecahedral shaped Cu 2 O nanocrystals of ~300 nm in size to evaluate the facet-dependent electrochemical activities. Here, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were firstly used to reveal the average electrochemical activities at the ensemble level. Atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) was further used to assess the electrochemical activities of different Cu 2 O nanocrystals at the facet level. Hexaammineruthenium (III) chloride ({Ru(NH 3 ) 6 }Cl 3 ) was employed as the probe molecules that reacted with four different Cu 2 O nanocrystals under —400mV and yielded ~300 pA current between the probing tip and the nanocrystal surface. The tip-current mapping results indicate that rhombic dodecahedral Cu 2 O exhibits higher electrocatalytic activity than other shaped Cu 2 O, due to the presence of dominant exposed facet of {110} as indicated by the relatively high tip current. Density-functional theory (DFT) calculations confirmed the facet dependence of local surface energy and electronic structure of Cu 2 O nanocrystals. Besides electrochemical activity, the surface work function and adsorptive properties were both observed to vary with the shape and dominant exposed facets of Cu 2 O. This study presented a unique experimental and computational chemistry approach to analyze surface electrochemical properties of Cu 2 O crystals at a crystalline facet level.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A series of four-coordinate heteroleptic copper(I) complexes with diimine and β-diketiminate ligands

In this paper, a series of heteroleptic bis-chelate copper(I) complexes of the general formula Cu(N^N)(NacNac R ), where N^N is the diimine ligand and NacNac R is the β-diketiminate ligand are introduced. The complexes are easily prepared by reacting NacNac ligands with CuO t Bu in the presence of diimine ligands. The molecular structures of all complexes are characterized by NMR spectroscopy and X-ray crystallography. The studies of cyclic voltammetry and UV–vis absorption spectra show that the HOMO and the LUMO energy level can be tailored by changing the substitution pattern of each ligand. The oxidation potential is dependent on the NacNac ligand, and the reduction potential shows the dependence on the diimine ligand. In conclusion, tunable redox potentials and a wide range of visible absorption can make these copper(I) complexes promising candidates for applications in solar light harvesting.

14 SOLAR ENERGY↗
DOE OSTI2024

Dynamics of precatalyst conversion and iron incorporation in nickel-based alkaline oxygen evolution reaction catalysts

The efficiency of alkaline water electrolyzers is limited by the oxygen evolution reaction (OER). The design of improved OER catalysts requires understanding of material changes induced by the electrolyte under oxidizing potentials. We compare four Ni-based thin-film precatalysts—Ni, NiO, Ni(OH) 2 , and NiS x —in 0.1 M KOH with and without Fe impurities. Precatalyst conversion to the active oxyhydroxide catalysts and their OER performance are induced and followed using cyclic voltammetry. Without Fe electrolyte impurities, the precatalysts convert at different rates to a similar, modestly active NiOOH catalyst. Added Fe impurities are incorporated concurrently with the oxyhydroxide formation leading to active Ni 1-x Fe x OOH catalysts. The NiS x and Ni(OH) 2 precatalysts rapidly convert to oxyhydroxides both with and without Fe, while conversion of Ni and especially NiO is slowed down by Fe impurities. Choice of the precatalyst and presence of Fe impurities are key factors in designing active Ni 1-x Fe x OOH OER catalysts for electrolyzers.

alkaline electrolysis↗
DOE OSTI2024

Monitoring SEIRAS on a Graphitic Electrode for Surface-Sensitive Electrochemistry: Real-Time Electrografting

The ubiquity of graphitic materials in electrochemistry makes it highly desirable to probe their interfacial behavior under electrochemical control. Probing the dynamics of molecules at the electrode/electrolyte interface is possible through spectroelectrochemical approaches involving surface-enhanced infrared absorption spectroscopy (SEIRAS). Usually, this technique can only be done on plasmonic metals such as gold or carbon nanoribbons, but a more convenient substrate for carbon electrochemical studies is needed. Here, we expanded the scope of SEIRAS by introducing a robust hybrid graphene-on-gold substrate, where we monitored electrografting processes occurring at the graphene/electrolyte interface. These electrodes consist of graphene deposited onto a roughened gold-sputtered internal reflection element (IRE) for attenuated total reflectance (ATR) SEIRAS. The capabilities of the graphene-gold IRE were demonstrated by successfully monitoring the electrografting of 4-amino-2,2,6,6-tetramethyl-1-piperidine N -oxyl (4-amino-TEMPO) and 4-nitrobenzene diazonium (4-NBD) in real time. These grafts were characterized using cyclic voltammetry and ATR-SEIRAS, clearly showing the 1520 and 1350 cm –1 NO 2 stretches for 4-NBD and the 1240 cm –1 C–C, C–C–H, and N–Ȯ stretch for 4-amino-TEMPO. Successful grafts on graphene did not show the SEIRAS effect, while grafting on gold was not stable for TEMPO and had poorer resolution than on graphene-gold for 4-NBD, highlighting the uniqueness of our approach. The graphene-gold IRE is proficient at resolving the spectral responses of redox transformations, unambiguously demonstrating the real-time detection of surface processes on a graphitic electrode. This work provides ample future directions for real-time spectroelectrochemical investigations of carbon electrodes used for sensing, energy storage, electrocatalysis, and environmental applications.

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

A Free-Standing Boron-Doped Diamond Grid Electrode for Fundamental Spectroelectrochemistry

Spectroelectrochemistry (SEC) is a powerful technique that enables a variety of redox properties to be studied, including formal potential (E o ), thermodynamic values (ΔG, ΔH, ΔS), diffusion coefficient (D), electron transfer stoichiometry (n), and others. SEC requires an electrode which light can pass through while maintaining sufficient electrical conductivity. This has been traditionally composed of metal or metal oxide films atop transparent substrates like glass, quartz, or metallic mesh. Robust electrode materials like boron-doped diamond (BDD) could help expand the environments in which SEC can be performed, but most designs are limited to thin films (~100–200 nm) on transparent substrates less resilient than free-standing BDD. Here, this work presents a free-standing BDD grid electrode (G-BDD) for fundamental SEC measurements, using the well-characterized Fe(CN) 6 3–/4– redox couple as proof-of-concept. With a combination of cyclic voltammetry (CV), thin-layer SEC, and chronoabsorptometry, several of the redox properties mentioned above were calculated and compared. For E o' , n, and D, similar results were obtained when comparing the CV [E o' = +0.279 (±0.002) V vs Ag/AgCl; n = 0.97; D = 4.1 × 10 –6 cm 2 ·s –1 ] and SEC [E o' = +0.278 (±0.001) V vs Ag/AgCl; n = 0.91; D = 5.2 × 10 –6 cm 2 ·s –1 ] techniques. Both values align with what has been previously reported. To calculate D from the SEC data, modification of the classical equation used in chronoabsorptometry was required to accommodate the G-BDD electrode geometry. Overall, this work expands on the applicability of SEC techniques and BDD as a versatile electrode material.

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

Functionalization of Electrodes with Tunable [EMIM] x [Cl] x +1 – Ionic Liquid Clusters for Electrochemical Separations

Functionalization of electrodes with clusters of hydrophobic molecules may improve the energy efficiency and selectivity of electrochemical separations by modulating the desolvation process occurring at the interface. Ionic liquids (IL), which have a broad range of compositions and properties, are potential candidates for controlling the transport, desolvation, and adsorption of target ions at electrochemical interfaces. We report a joint experimental and theoretical investigation of the structure, stability, and selective adsorption properties of the IL clusters 1-ethyl-3-methylimidazolium chloride [EMIM] x [Cl] x+1 - (x = 1 – 10) and demonstrate their ability to adsorb and separate ions from solution. The structure and stability of the IL clusters are determined experimentally using high-mass-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and negative ion photoelectron spectroscopy. Global optimization theory and ab initio molecular dynamics simulations provide molecular-level insight into the bonding and structural fluxionality of these species. Ion soft landing is used to selectively functionalize the surface of highly oriented pyrolytic graphite (HOPG) working electrodes with [EMIM] 1 [Cl] 2 - , [EMIM] 3 [Cl] 4 - , and [EMIM] 5 [Cl] 6 - clusters. Kelvin probe microscopy provides insight into the relative stability of the clusters on HOPG and their effect on the work function of IL-functionalized electrodes. Cyclic voltammetry measurements reveal irreversible adsorption of Fe(CN) 6 4-/3- anions during redox cycling, while electrochemical impedance spectroscopy indicates a substantial decrease in the electron transfer resistance of the IL-functionalized electrodes due to adsorption of Fe(CN) 6 4-/3- . Overall, our findings demonstrate that IL clusters with different size and stoichiometry may be used to increase the efficiency of electrochemical separations, opening new horizons in selective electrode functionalization.

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

Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core–Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of non-precious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiP x -FeO x core-shell nanocatalysts with an amorphous NiP x core designed for enhanced OER activity. Using ex-situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeO x shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH - /H 2 O. Thus, the FeO x shell preserves the amorphous NiP x core from rapid oxidation to Ni 3 (PO 4 ) 2 and Ni(OH) 2 . On the other hand, the incorporation of Ni from the core into the FeO x shell facilitates absorption of hydroxide ions for OER. As a result, the Ni/Fe(OH) x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such synthetic route involving the FeO x shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core-shell morphology can be tuned to leverage the strength of each metallic component for improved electrochemical activities.

25 ENERGY STORAGE↗
DOE OSTI2025

Microelectrode Arrays for Electrochemical Cycling of Individual Battery Particles

The electrochemical characterization of battery materials is usually performed on porous electrodes containing conductive additives, binders, and ensembles of redox-active particles at densities as high as 10 15 cm –3 . These complex structures often obscure individual battery particles’ intrinsic properties, hindering insights into the fundamentals of battery electrochemistry. To address this challenge, we developed microelectrode arrays to enable the charge and discharge of individual battery particles in liquid electrolytes. In this Methods and Protocols paper, we describe the fabrication of the microelectrode array chips, the assembly of particles onto microelectrodes, and the different types of electrochemical techniques that can be used, including galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration, and electrochemical impedance spectroscopy. We use micron-sized lithium cobalt oxide as a model system to showcase the reliability and versatility of single-particle electrochemical experiments enabled by our microelectrode array. In conclusion, we aim to provide a detailed description to promote future uses of microelectrode arrays in fundamental research of batteries and other electrochemical systems.

25 ENERGY STORAGE↗
DOE OSTI2025

Surface Manipulation on Pt 2.2 Ni(111) Nanocatalysts for Boosting Their ORR Performance in Alkaline Media

We have previously shown that Pt–Ni alloy nano-octahedra with {111} facets exhibit outstanding electrochemical performance in the oxygen reduction reaction (ORR) in acidic media when their surfaces are finely tailored at the atomic level. Here, in this investigation, we further refine the surface structure of Pt 2.2 Ni octahedral nanocatalysts to improve ORR performance in a 0.1 M KOH solution using diverse surface manipulation techniques. Through systematic analysis using electrochemical CO stripping, cyclic voltammetry, and X-ray photoelectron spectroscopy, we examined the surfaces of Pt 2.2 Ni octahedral nanocatalysts pretreated with various methods, including etching in acetic acid or perchloric acid, and subsequent electrochemical activation in an alkaline solution or an acidic solution. Among these treatments, those involving acidic media, particularly electrochemical cycling in acidic electrolytes, demonstrated significantly enhanced ORR activity in 0.1 M KOH. The latter exhibited a mass activity of 2.95 A/mg pt and a specific activity of 8.71 mA/cm 2 at 0.90 V, surpassing state-of-the-art Pt/C by 12-fold and 34-fold, respectively. Furthermore, this identified nanocatalyst displayed robust stability, with negligible activity decay observed after 10,000 cycles. This study suggests that the improved ORR activity can be attributed to the Pt-rich surfaces with well-preserved {111} lattices on the surface-modified Pt–Ni nano-octahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spectroscopic Investigation into P(NDI2OD-T2) Charge Localization

P(NDI2OD-T2), commonly referred to as N2200, stands out as a promising electron-transporting (n-type) polymer for low-cost, flexible (photo)electrochemical applications due to its reversible two-electron reduction and high electron mobility. UV–vis spectroelectrochemistry in the tetrabutyl ammonium hexafluorophosphate/acetonitrile electrolyte shows two sets of chemically reversible redox signals in the cyclic voltammetry corresponding to the reduction of the neutral polymer film to polaronic and bipolaronic species. These electrochemical signatures suggest a distinct electronic reorganization upon reduction (polaron/bipolaron formation), highlighting the need for molecular-level insights into how charges are accommodated within the polymer backbone. While it has been previously hypothesized that charge predominantly localizes on the naphthalene diimide (NDI) unit during reductive charging, specific changes in atomic environments that confirm this localization have not been characterized in n-type polymers. Herein, we use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe electronic transitions in an electrochemically charged polymer to deduce charge localization. The O K-edge (1s) spectra exhibit two distinct π* peaks; the intensity of the lower-energy π* a peak that corresponds to an excitation to a largely localized carbonyl state decreases with reductive potentials relative to the higher-energy π* b peak. We corroborate this with Raman spectroscopy at different potentials, which shows a decrease in intensity on the C–C/C═C and C═O stretching bands of NDI as well as a red shift of the carbonyl band due to the formation of a polaron on the NDI. Additionally, new Raman active NDI signals associated with elongated C═O and C═C bonds are observed at lower energy during the formation of charged states. Together with theoretical calculations, these findings show that the injected charge spatially localizes on the NDI units and is dominantly distributed on the carbonyl groups. In conclusion, the combination of NEXAFS, optical and vibrational spectroscopies, and theoretical calculations is generalizable to other pi-conjugated polymers and can identify charge localization for the further development of organic semiconductors.

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

Synthesis and Characterization of [Ni(H 2 O)(7-P Ph 2 N ArSO3 ) 2 ](NaBF 4 ) for Light-Driven Quantum Dot-Catalyst Hydrogen Evolution

Light-driven hydrogen generation from water is a route to carbon-neutral fuels. However, the integrated light absorbers and catalysts must be compatible and functional under the same conditions. A sulfate-functionalized complex, [Ni(H 2 O)(7-P Ph 2 N ArSO3 ) 2 ](NaBF 4 ), where P Ph 2 N ArSO3 = 4-(3,6-diphenyl-1,3,6-azadiphosphepan-1-yl)benzenesulfonate), was synthesized and characterized. A solid-state structure from single-crystal X-ray crystallography is also reported. The electrocatalytic activity for hydrogen evolution of this complex in 7:3 H 2 O:CH 3 CN was characterized by cyclic voltammetry and controlled potential electrolysis, with a maximum observed rate of 83 s –1 . Density functional theory was used to characterize the favorable binding of the sulfate functionalities to cadmium sulfide quantum dots. Experimental assembly of these catalysts onto cadmium sulfide quantum dots was successful. In conclusion, both fast electron transfer and light-driven pH-dependent hydrogen evolution were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Graphitized Biocarbon Derived from Hydrothermally Liquefied Low-Ash Corn Stover

Graphitized biocarbon can be utilized for energy storage applications such as supercapacitors. The scientific community have geared their attention to obtain such value-added product from abundantly available and low-cost biomass feedstock agricultural residues such as corn stover. Lignocellulosic components embedded within the cell wall of biomass substrates can provide a fine template for enhanced ion storage, transport, and rate capabilities, desirable for electrochemical storage. Presented is the utilization of homogenized low ash content corn stover milled and sieved to desired specifications, which underwent hydrothermal liquefaction in the presence of Ni-based catalytic salts at 275 o C. The hydrochar obtained by solid residue extracted from the reaction slurry was washed to acid neutral and subjected to catalytic activation using ZnCl 2 , followed by thermal annealing at 400 o C for morphological and pore size enhancement of the resulting biochar. Carbonization was performed on acid neutralized hydrochar at 850 °C to further enhance pore structures and increase graphitization for improved conductivity. Catalytic materials exhibited a specific capacitance of 316 F g -1 and held a 100% retention beyond 10,000 cycles. BET, Raman, XRD, cyclic voltammetry, chronopotentiometry, and EIS are discussed herein.

Biomass↗
DOE OSTI2025

Alizarin Multilayers Adsorbed onto Glassy Carbon Electrodes for Electrochemical Sequestration of Manganese, Sodium, and Lithium Cations

The development of efficient methods for metal ion recovery and water remediation is critical for addressing the nation’s urgent need for secure domestic supply chains, overcoming critical materials challenges, and ensuring resilient manufacturing. Here, we present a novel approach for synthesizing stable Alizarin (Alz, 1,2-dihydroxy anthraquinone) multilayers on glassy carbon electrodes (GCEs) for electrochemical sequestration of manganese, sodium, and lithium cations. Alz-GCEs exhibit negatively shifted reduction potentials, indicating strong interactions between Alz and metal cations through metal-coupled electron transfer (MCET) mechanisms. The cation binding interactions and redox behavior of these electrodes were investigated using cyclic voltammetry (CV) and density functional theory (DFT) calculations. Our results demonstrate that Alz forms multilayer structures on GCEs with redox properties that are modulated by the presence of metal cations in the electrolyte. DFT calculations provide insights into the electrochemical mechanism, indicating both stepwise and concerted pathways for metal binding. The findings highlight the potential of Alz-GCEs for efficient and selective metal ion capture, which could be useful for developing sustainable materials for critical metal recovery and water remediation. This work suggests that redox-mediated organic adlayers are promising candidates for advancing electrochemical separation technologies of metal ions.

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

Cobalt-Doped Tungsten Oxide Nanoparticles for Electrochemical Sensing and Detection of a Penicillin Antibiotic: Amoxicillin

A sensitive electrochemical sensor for the determination of antibacterial amoxicillin (AMXL) in pharmaceutical dosage and environmental samples was developed, since its residues were found in water, food, and biological samples to pose adverse health risks such as immune–allergic reactions and antimicrobial resistance. WO 3 -based sensors are reported to provide higher sensitivity, selectivity, and stability during the electrochemical reaction. Considering this, Co-doped WO 3 nanostructures were synthesized by the hydrothermal method yielding a mixture of nanoparticle and nanorod structures and applied to the assay of AMXL. Extensive characterizations were carried out by SEM, XRD, TEM, and XPS analysis that confirm the monoclinic crystal structure with a uniform Co distribution in the WO 3 lattice. The optimum doping amount of Co in the lattice is found to be 1.80 at %. The electrochemical performance of the Co-WO 3 /carbon paste electrode (CPE) was evaluated for the determination of AMXL under optimum conditions using cyclic voltammetry, electrochemical impedance spectroscopy, and square wave voltammetry. The Co-WO 3 /CPE showed two linear detection ranges at 0.01–0.1 μM with the linear regression I pa = 3.73 [AMXL] + 0.77 with R 2 = 0.96 and at 0.1 to 30.0 μM with the linear regression I pa = 3.76 [AMXL] + 0.69 with R 2 = 0.95. The limit of detection was 9.1 nM (SNR = 3), and the limit of quantification was 30.6 nM (SNR = 10). The sensitivities for lower and higher concentration ranges were 63.22 and 63.73 μA μM –1 cm –2 , respectively. Furthermore, Co-WO 3 /CPE showed high selectivity and anti-interference capabilities. In conclusion, the sensing and disposable Co-WO 3 /CPE sensor showed a great potential for AMXL analysis in spiked soil, water, and Amoxil tablet samples with desirable percentage recovery.

Shanbhag, Mahesh M. [KLE Technological University,↗
DOE OSTI2020

[(MeCN)Ni(CF 3 ) 3 ] - and [Ni(CF 3 ) 4 ] 2– : Foundations toward the Development of Trifluoromethylations at Unsupported Nickel

Nickel anions [(MeCN)Ni(CF 3 ) 3 ] - and [Ni(CF 3 ) 4 ] 2– were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF 3 to [(dme)NiBr 2 ] in the presence of the [PPh 4 ] + counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF 3 ) 3 ] - and [Ni(CF 3 ) 4 ] 2- , these complexes are better physically described as d 9 metal complexes. [(MeCN)Ni(CF 3 ) 3 ] - is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF 3 ) 3 ] 2– salt, revealing the broader potential of [(MeCN)Ni(CF 3 ) 3 ] - in the development of “ligandless” trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF 3 ) 3 ] - with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh 4 ] 2 [Ni(CF 3 ) 4 (SO 4 )], which was derived through the two-electron oxidation of [Ni(CF 3 ) 4 ] 2– .

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

Controlling the Reduction of Chelated Uranyl to Stable Tetravalent Uranium Coordination Complexes in Aqueous Solution

The solution-state interactions between octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) chelating ligands and uranium were investigated and characterized by UV-visible spectrophotometry and X-ray absorption spectroscopy (XAS), as well as electrochemically via spectroelectrochemistry (SEC) and cyclic voltammetry (CV) measurements. Depending on the selected chelator, we demonstrate the controlled ability to bind and stabilize U IV , generating with 3,4,3-LI(1,2-HOPO), a tetravalent uranium complex that is practically inert toward oxidation or hydrolysis in acidic, aqueous solution. At physiological pH values, we are also able to bind and stabilize U IV to a lesser extent, as evidenced by the mix of U IV and U VI complexes observed via XAS. Finally, CV and SEC measurements confirmed that the U IV complex formed with 3,4,3-LI(1,2-HOPO) is redox inert in acidic media, and U VI ions can be reduced, likely proceeding via a two-electron reduction process.

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

Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes

The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, ( Mes PDP Ph )UO 2 (THF) and ( Cl2Ph PDP Ph )UO 2 (THF), are reported as the U VI uranyl adducts of the bulky aryl substituted pincers ( Mes PDP Ph ) 2– and ( Cl2Ph PDP Ph ) 2– (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H 2 Mes PDP Ph , Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H 2 Cl2Ph PDP Ph , Cl 2 Ph = 2,6-dichlorophenyl), respectively). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li 2 Ar PDP Ph , Ar = Mes of Cl 2 Ph), salt metathesis with [UO 2 Cl 2 (THF) 2 ] 2 afforded both compounds in moderate yields. The characterization of each species has been undertaken by a combination of solid- and solution-state methods, including combustion analysis, infrared, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal X-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional ( Ar PDP Ph ) 2– pincer ligand. The electrochemical analysis of both compounds by cyclic voltammetry in tetrahydrofuran (THF) reveals rich redox profiles, including events assigned as U VI /U V redox couples. Here, a time-dependent density functional theory study has been performed on ( Mes PDP Ph )UO 2 (THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗