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At least 451 records · Page 25

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DOE OSTI2025

Conformational Control as a Design Strategy to Tune the Redox Behavior of Benzotriazole Negolytes for Nonaqueous Flow Batteries

Here, we present a molecular engineering strategy to tune the reduction potentials of benzotriazole derivatives as high-energy-density negolytes in nonaqueous redox flow batteries. Within nonaqueous electrolytes, these derivatives, notably 2-(o-tolyl)-2H-benzo[d][1,2,3]triazole (1), demonstrate a theoretical capacity of up to 93.8 Ah L⁻¹ and a reduction potential of –2.35 V vs ferrocene/ferrocenium (Fc/Fc⁺). Introducing dimethyl substitution (i.e., 2-(2,6-dimethylphenyl)-2H-benzo[d][1,2,3]triazole (4)) shifts the reduction potential even more negatively to –2.55 V vs Fc/Fc⁺. We ascribe the nonlinear effect of dimethyl substitution on reduction potential to ground-state conformational effects. Flow battery tests with negolyte 1 and ferrocene posolyte demonstrated >90% Coulombic efficiency at 6.7 mA cm⁻² with improved cyclability in the presence of lithium bis(trifluoromethylsuylfonyl)imide supporting salt.

25 ENERGY STORAGE↗

DOE OSTI2025

A formal Fe III/V redox couple in an intercalation electrode

Iron redox cycling between low-valent oxidation states of Fe II and Fe III drives crucial processes in nature. The Fe II/III redox couple charge compensates the cycling of lithium iron phosphate, a positive electrode (cathode) for lithium-ion batteries. High-valent iron redox couples, involving formal oxidation higher than Fe III , could deliver higher electrochemical potentials and energy densities. However, because of the instability of high-valent Fe electrodes, they have proven difficult to probe and exploit in intercalation systems. Here, in this study, we report and characterize a formal Fe III/V redox couple by revisiting the charge compensation mechanism of (de)lithiation in Li 4 FeSbO 6 . Valence-sensitive experimental and computational core-level spectroscopy reveal a direct transition from Fe III (3d 5 ) to a negative-charge-transfer FeV (3d 5 L 2 ) ground state on delithiation, without forming Fe IV , or oxygen dimers. We identify that the cation ordering in Li 4 FeSbO 6 drives a templated phase transition to stabilize the unique Fe V species and demonstrate that disrupting cation ordering suppresses the Fe III/V redox couple. Exhibiting resistance to calendar aging, high operating potential and low voltage hysteresis, the Fe III/V redox couple in Li 4 FeSbO 6 provides a framework for developing sustainable, Fe-based intercalation cathodes for high-voltage applications.

Ramachandran, Hari [Stanford Univ., CA (United Sta↗

DOE OSTI2025

Computationally efficient models for aqueous organic redox flow batteries

The rising usage of intermittent energy has garnered the need for large scale energy storage systems. Redox flow batteries (RFB) based energy storage system shows promising potential. Numerical simulations and machine learning approaches have been widely used to study RFB performance. The development of autonomous material discovery framework and digital twin of energy storage system usually needs to query cell performance through fast response models. In this study, two computationally efficient models are introduced: a physics-based analytical flow battery model (EZBattery), and a machine learning operator model (Deep Operator Network, denoted by DeepONet). Both models can provide cell performance near instantly, and prediction accuracy was systematically examined on an application of evaluating the performances of a 780 cm 2 aqueous organic redox flow battery (AORFB), using potential anolyte candidates in dihydroxyphenazine (DHP)-based family of organic materials. A validated computationally expansive 3-dimensional multi-physics finite element model by COMSOL was used as the ground truth and provided the training data set for the DeepONet. 1280 samples were generated with 10 properties to mimic the different possible anolyte candidates, and the cell performances were evaluated under 10 different combined operating conditions. The accuracy comparisons for the two computationally efficient models show that both models can provide comparable accuracy in predicting cell charging/discharging voltage curves. DeepONet can provide slightly higher overall accuracy than EZBattery with faster calculation speed, but highly relies on the training dataset. EZBattery does not need a training dataset and can provide interpretable physics-based explanations of the results, while being more flexible to adjust to adapt any different cell designs, flow battery architectures, and electrolyte materials.

Analytical model↗

DOE OSTI2024

A Class of Sodium Transition-Metal Sulfide Cathodes With Anion Redox

Sodium-ion batteries (SIBs) are entering commercial relevance as a sustainable and low-cost alternative to lithium-ion batteries. Improving the energy density of SIBs is critical to enable their widespread adoption. Here, in this work, a new class of cathode materials Na 6 M S 4 ( M = Co, Mn, Fe, and Zn) that exhibit high charge-storage capacity is reported. Using Na 6 CoS 4 as a prototypical example, a six-electron conversion reaction dominated by anion redox is observed, confirmed through various electrochemical and spectroscopic techniques. After the initial cycle, Na 6 CoS 4 delivers a high capacity of 392 mA h g -1 with a long lifespan of over 500 cycles. The reaction involves, initially, the transformation of crystalline Na 6 CoS 4 to a nearly amorphous structure consisting of mainly CoS and sulfur nanoparticles, which then reversibly cycles between nearly amorphous a-CoS/S and a-Na 6 CoS 4 . Such anion-redox-driven conversion-type cathodes hold the potential to enable energy-dense, stable SIBs.

25 ENERGY STORAGE↗

DOE OSTI2020

Phyllosilicate mineral dissolution upon alkaline treatment under aerobic and anaerobic conditions

The dissolution of phyllosilicate minerals exposed to high-pH environments has been studied to determine the influence of alkaline treatments and variable redox conditions on clay dissolution and the potential formation of new phases via incongruent dissolution phenomena. The present study focused on the dissolution of phyllosilicate minerals (illite, muscovite, and montmorillonite) under anaerobic and aerobic conditions using comparative solutions (sodium hydroxide and ammonium hydroxide) at similar ionic strength in the presence and absence of oxygen. Our batch data show that there is a rapid decrease in aluminum dissolution (< 240 hrs) and slow increase in silica over time (up to 1440 hrs). This trend was particularly evident for montmorillonite which shows the greatest dissolution for ammonium hydroxide, likely due to intercalation of the polyatomic cation ammonium into the mineral’s expandable layers. When comparing alkaline treatments, the strong base, sodium hydroxide, dissolved more of the mica minerals, illite and muscovite, likely due to ion-pairing between the silicate tetrahedra [SiO4]n- and Na+ cations in solution. In addition, redox conditions affect the systems similarly with treatment, although the sodium hydroxide treatment results in greater variability in solution redox conditions. For all investigated phyllosilicates, the calculated aqueous aluminum and silicon ratios over time are significantly different from the minerals’ stoichiometric ratios. As a result, we conclude that incongruent dissolution occurred and suggest formation of secondary precipitates. Understanding the potential for clay mineral alterations from interaction with alkaline solutions has implications for in situ remediation, mining operations, and waste interactions with the subsurface. This research shows that incongruent dissolution is likely and will lead to secondary precipitation which may have long term physical and chemical impacts in the subsurface.

mineral dissolution, phyllosilicate, alkaline, amm↗

DOE OSTI2023

Redox Chemistry of the Subphases of α-CsPbI2Br and β-CsPbI2Br: Theory Reveals New Potential for Photostability

The logic in the design of a halide-mixed APb(I 1-x Br x ) 3 perovskite is quite straightforward: to combine the superior photovoltaic qualities of iodine-based perovskites with the increased stability of bromine-based perovskites. However, even small amounts of Br doped into the iodine-based materials leads to some instability. In the present report, using first-principles computations, we analyzed a wide variety of α-CsPbI 2 Br and β-CsPbI 2 Br phases, compared their mixing enthalpies, explored their oxidative properties, and calculated their hole-coupled and hole-free charged Frenkel defect (CFD) formations by considering all possible channels of oxidation. Nanoinclusions of bromine-rich phases in α-CsPbI 2 Br were shown to destabilize the material by inducing lattice strain, making it more susceptible to oxidation. The uniformly mixed phase of α-CsPbI 2 Br was shown to be highly susceptible towards a phase transformation into β-CsPbI 2 Br when halide interstitial or halide vacancy defects were introduced into the lattice. The rotation of PbI 4 Br 2 octahedra in α-CsPbI2Br allows it either to transform into a highly unstable apical β-CsPbI 2 Br, which may phase-segregate and is susceptible to CFD, or to phase-transform into equatorial β-CsPbI 2 Br, which is resilient against the deleterious effects of hole oxidation (energies of oxidation >0 eV) and demixing (energy of mixing <0 eV). Thus, the selective preparation of equatorial β-CsPbI 2 Br offers an opportunity to obtain a mixed perovskite material with enhanced photostability and an intermediate bandgap between its constituent perovskites.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Examination of actinide chemistry at solid-water interfaces to support advanced actinide separations (Final Report)

In this work we have examined the thermodynamics of actinide interactions with solid interfaces in the presence and absence of multi-functional complexing ligands. The term “interactions” is a global term to capture many potential reactions including aqueous complexation, sorption/surface complexation, surface-mediated redox reactions, electrostatic attraction and repulsion, surface passivation, surface precipitation, and counter-ion competition. Each of these potential reactions may control actinide interactions with a solid surface. This work is helping to develop a more fundamental understanding of actinide bonding at solid:water interfaces and examine unique surface mediated oxidation/reduction reactions which are not expected based on the reduction potentials of aqueous species. We place a large emphasis on understanding both sorption and desorption processes as a means of understanding the sorption mechanisms. This work is directly aligned with the DOE BES Heavy Element Chemistry programs interests in understanding “the chemical and physical properties of these elements to determine solution, interfacial and solid-state bonding and reactivity”.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗

NASA NTRS1984

An electromotive force series in a borosilicate glass-forming melt

An electromotive force series for redox couples was defined as a function of oxygen fugacity in a borosilicate melt at 1150 C. The resulting order of relative reduction potentials can be used to estimate the amounts of redox species in glass during processing. The electromotive force series in this melt is comparable to those in other silicate glass-forming melts and in aqueous systems but differs in detail because of interaction of the solvents with individual redox couples.

Schreiber, H. D.↗

DOE OSTI2023

Activity of Metal-Fluorine States upon Delithiation of Disordered Rocksalt Oxyfluorides

The capacity of transition-metal oxides as Li-ion battery cathodes is limited by instabilities that arise when high states of charge are achieved. Oxyfluorides with a disordered rocksalt structure have emerged as attractive alternatives, but the role of F in their electrochemical function, particularly when cationic redox produces high formal oxidation states, remains to be ascertained. Here, using X-ray absorption spectroscopy, we confirm the existence of Mn-F covalent interactions in Li 2 MnO 2 F and Li 2 Mn 2/3 Nb 1/3 O 2 F. New unoccupied states evolve from hybrid 3d-2p states of both Mn-F and Mn-O bonds when the phases are delithiated, particularly in the presence of Mn(IV). The results challenge the assumption of F as anion whose covalent states with the metal are tapped at very high potentials, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggest the existence of unique knobs of design of battery cathodes by manipulating the covalent interactions between transition metals and two different anions.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Reactive iron, not fungal community, drives organic carbon oxidation potential in floodplain soils

Wetlands host ∼20% of terrestrial organic carbon and serve as a major sink for atmospheric carbon. Anoxic soils and sediments accrue soil organic carbon (SOC) partly by hampering the activity of extracellular oxidative enzymes that break down phenolic polymers. Upon aeration, fungal-driven oxidative enzymatic depolymerization and microbial respiration of released monomers ensue. Redox-active metals can simultaneously catalyze abiotic nonspecific oxidation of SOC, notable examples including Mn(III) or Fe(II) through Fenton-like, hydrogen peroxide-catalyzed oxidative radical production. However, the extent of reactive metal contributions to biotic and abiotic SOC degradation is not understood in the context of natural environments with diverse redox chemistry. We tested the relative contributions of fungi, Mn(III) and Fe(II) to phenolic substrate (L-DOPA) oxidation in floodplain soils representing a range of transient redox conditions driven by permanent vs. periodic flooding. Phenol oxidative potential was highest in permanently flooded soils with fewer fungal taxa known for observed (per)oxidase activity and instead correlated with HCl-extractable Fe(II), Fe(total) and Fe(II)/Fe(total), suggesting a specific role of Fe(II). Fe(II) additions enhanced phenol oxidative potential in sterilized and non-sterilized soils in the presence of hydrogen peroxide, indicating abiotic Fe-mediated radical chemistry could significantly enhance wetland SOC oxidative depolymerization throughout redox-active floodplain soils. Fungal community composition did not correlate to phenol oxidative potential overall and only more oxic soils adjacent to the river with diverse fungal communities showed declining oxidative potential after sterilization. Mn(III) addition did not significantly enhance phenol oxidative potential across all soils, although it appeared to drive fungal-mediated oxidative potential in the most aerated floodplain soils. Understanding how metals mediate SOC depolymerization as abiotic oxidants or microbially-harnessed enzyme cofactors and substrates in soils under variable hydrologic controls will improve our ability to represent depolymerization in terrestrial carbon models in wetland and other frequently saturated soils.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2023

Proton-regulated alcohol oxidation for high-capacity ketone-based flow battery anolyte

Redox flow batteries have a unique architecture that potentially enables cost-effective long-duration energy storage to address the intermittency introduced by increased renewable integration for the decarbonization of the electric power sector. Targeted molecular engineering has demonstrated electrochemical reversibility in natively redox-inactive ketone molecules in aqueous electrolytes. Yet, the kinetics of fluorenone-based flow batteries continue to be limited by slow alcohol oxidation. We show how strategically designed proton regulators can accelerate alcohol oxidation and thus enhance battery kinetics. Fluorenone-based flow batteries with the organic additive ß-cyclodextrin demonstrate enhanced rate capability, high capacity, and long cycling. This study opens a new avenue to improve the kinetics of aqueous organic flow batteries by modulating the reaction pathway with a homogeneous catalyst.

25 ENERGY STORAGE↗

DOE OSTI2024

Influence of crossover on capacity fade of symmetric redox flow cells

Volumetrically unbalanced compositionally symmetric cell cycling with potentiostatic (CV) or galvanostatic-with-potential-hold (CCCV) protocols is a rigorous technique for evaluating the calendar lifetime of reactants for redox flow batteries. Here, we evaluate the influence of reactant crossover through the membrane on symmetric cell cycling behavior. We tested symmetric cells of anthraquinone disulfonic acid (AQDS) with Nafion membranes of varied thickness and manufacture (NR211, NR212, N115, and N117, ranging 25–183 μm). Membranes were tested both as-received and pretreated with a common procedure of soaking in water at elevated temperature and then in dilute hydrogen peroxide. We found no significant difference in capacity fade rates of symmetric cells with any of the membranes as-received, indicating a negligible influence of crossover. However, we observed increased capacity fade with increased permeability through pretreated membranes. Supported by zero-dimensional modeling and operando UV-vis spectrophotometry, we propose a mechanism for net crossover in AQDS symmetric cells based on a higher time-averaged concentration of quinhydrone dimers in the non-capacity limiting side (NCLS) compared to the capacity limiting side (CLS), driving net crossover of AQDS reactants out of the CLS. Further, we illustrate other hypothetical scenarios of net crossover using the zero-dimensional model. Overall, many membrane–electrolyte systems used in symmetric cell studies have sufficiently low crossover flux as to avoid the influence of crossover on capacity fade, but under conditions of higher crossover flux, complex interactions of crossover and chemical reactions may result in diverse capacity fade trajectories, the mechanisms of which may be untangled with operando characterization and modeling.

25 ENERGY STORAGE↗

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 OSTI2020

Deciphering the Oxygen Absorption Pre-edge: A Caveat on its Application for Probing Oxygen Redox Reactions in Batteries

The pre-edges of oxygen-K X-ray absorption spectra have been ubiquitous in transition metal (TM) oxide studies in various fields, especially on the fervent topic of oxygen redox states in battery electrodes. However, critical debates remain on the use of the O-K pre-edge variations upon electrochemical cycling as evidences of oxygen redox reactions, which has been a popular practice in the battery field. This study presents an investigation of the O-K pre-edge of 55 oxides covering all 3d TMs with different elements, structures, and electrochemical states through combined experimental and theoretical analyses. It is shown unambiguously that the O-K pre-edge variation in battery cathodes is dominated by changing TM-d states. Furthermore, the pre-edge enables a unique opportunity to project the lowest unoccupied TM-d states onto one common energy window, leading to a summary map of the relative energy positions of the low-lying TM states, with higher TM oxidation states at lower energies, corresponding to higher electrochemical potentials. The results naturally clarify some unusual redox reactions, such as Cr 3+/6+ . This work provides a critical clarification on O-K pre-edge interpretation and more importantly a benchmark database of O-K pre-edge for characterizing redox reactions in batteries and other energy materials.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Promotion and Tuning of the Electrochemical Reduction of Hetero‐ and Homobimetallic Zinc Complexes**

Abstract Compounds containing multiple metals attract significant interest due to the useful redox and reactivity properties of such species. Here, the electrochemical properties of a family of macrocyclic complexes that feature a zinc(II) center paired with a secondary redox‐inactive metal cation in heterobimetallic (Na + , Ca 2+ , Nd 3+ , Y 3+ ) motifs or homobimetallic (Zn 2+ ) motifs have been investigated. The new complexes were prepared via a divergent strategy, isolated, and structurally characterized with single‐crystal X‐ray diffraction (XRD) analysis. XRD results show that the structures of the complexes are modulated by the identity of the incorporated secondary metal ions. Cyclic voltammetry data reveal that ligand‐centered reduction is promoted in the bimetallic complexes and that the paired metal ions synergistically influence the redox properties of the complexes. Similar to prior work from our group and others, the bimetallic complexes containing stronger Lewis acids undergo more significant reduction potential shifts; contrasting with prior work on complexes containing redox‐active metals, however, the zinc(II) complexes studied here display faster electron transfer (as judged by lower reorganization energies, λ) when incorporating di‐ or tri‐valent Lewis acids in contrast to monovalent (and more weakly acidic) sodium. The quantified trends in these data offer insights that could help distinguish metal‐ versus ligand‐centered reduction of bimetallic complexes.

Kelsey, Shaun R.↗

DOE OSTI2024

Thermodynamic Limits of Redox-Based Thermochemical Processes (REDOTHERM)

Solar thermochemical fuel production is a potential pathway for the production of sustain liquid drop-in fuels, which can help decarbonize the aviation and maritime sectors. In an attempt to analyze the commercial viability of this technology, several studies have been conducted, including system and technoeconomic analysis (TEA) modeling. However, most studies to date simply assume a given redox reactor efficiency, which is significantly higher than demonstrated values to date. While it is widely recognized that utilizing a counter-current flow (CF) configuration could increase the redox reactor efficiency, an over-simplification in the thermodynamic modeling may lead to unphysical results which has been included in multiple publications. The fact that the solar redox reactor is the least developed component in the process chain makes it hard to identify technology gaps and evaluate pathways to deployment at scale using this approach. In this work, a thermodynamic model for a moving oxide system has been developed, in a general form that allows to analyze the system for different redox-active materials, under a wide range of operating conditions, for both parallel and countercurrent flows. The model capabilites are demonstrated, and the model's code will be shared as an open-source on GitHub in the next few months.

chemical looping↗

DOE OSTI2021

The impact of alkali‐ion intercalation on redox chemistry and mechanical deformations: Case study on intercalation of Li, Na, and K ions into FePO 4 cathode

Abstract Batteries made of charge carriers from Earth‐crust abundant materials (e.g., Na, K, and Mg) have received extensive attention as an alternative to Li‐ion batteries for grid storage. However, a lack of understanding of the behavior of these larger ions in the electrode materials hinders the development of electrode structures suitable for these large ions. In this study, we investigate the impact of alkali ions (Li, Na, and K) on the redox chemistry and mechanical deformations of iron phosphate composite cathodes by using electrochemical techniques and in situ digital image correlation. Na‐ion and Li‐ion intercalation demonstrate a nearly linear correlation between electrochemical strains and the state of charge and discharge. The strain development shows nonlinear dependance on the state of charge and discharge for K ions. Strain rate calculations show that K ion intercalation results in a progressive increase in the strain rate for all cycles. Li and Na intercalation induce nearly constant strain rates with the exception of the first discharge cycle of Na intercalation. When the same amount of ions are inserted into the electrode, the electrode shows the lowest strain generation upon Li intercalation compared to larger alkali ions. Na and K ions induce similar volumetric changes in the electrode when the state of charge and discharge is around 30%. Although the electrode experiences larger absolute strain generation at the end of the discharge cycles upon Na intercalation, strain rates were found to be greater for K ions. Potential‐dependent behaviors also demonstrate more sluggish redox reactions during K intercalation, compared to Li and Na. Our quantitative analysis suggests that the strain rate, rather than the absolute value of strain, is the critical factor in amorphization of the crystalline electrode.

25 ENERGY STORAGE↗

DOE OSTI2021

Photocatalytic Oxidation of Dissolved Mn 2+ by TiO 2 and the Formation of Tunnel Structured Manganese Oxides

The redox reaction of manganese (Mn) is of great environmental, geological, and public health significance, as Mn oxides control the distribution and electron flow of numerous nutrients and contaminants in natural and engineered environments. Current understanding on the oxidation pathways of Mn(II) to Mn(III/IV) mainly focuses on biotic processes due to their much higher oxidation rates than those of abiotic processes. This study demonstrates rapid photocatalytic oxidation of Mn 2+ (aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO 2 minerals. Notably, the photocatalytic oxidation rates are comparable to or even higher than those of reported biotic/abiotic processes. In addition, the rapid photocatalytic oxidation leads to the formation of large tunnel structured Mn oxides (todorokite and romanechite) on the surface of TiO 2 . These findings suggest that photocatalytic oxidation of Mn 2+ (aq) by natural semiconducting minerals is likely an important yet previously overlooked pathway for understanding the occurrence of natural Mn oxide coatings on rock surfaces. In addition, considering the increasing input of photoreactive engineered nanoparticles into environmental systems, this work shows the potential impacts of nanoparticles on influencing natural redox cycles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗