Fluoride Salts Research Articles

Molecular Design of Imino Anion Acceptors Enables Long‐Life Fluoride Ion Batteries

AbstractAnion acceptors (AAs) enable to dissolve metal fluoride salts and achieve reversible fluorination and defluorination in fluoride ion batteries (FIBs). However, most reported strategies only focus on boron‐ and alcohol‐based AAs with strong Lewis acidity and excessive hydrogen bond (HB) strength, which often leads to the uncontrollable mass loss of active materials and the inferior reduction stability of electrolyte. Although amino and imine groups possess preferable anti‐reductive property, their HB strengths are apparently too weak to dissociate fluoride salts. Here a novel strategy is proposed for molecular structure design toward imino AAs by introducing double bonds and pyridine‐N into the five‐membered‐ring of pyrrolidine. Therein the conjugation effect, inductive effect, and α effect are synergistically utilized to enhance the Lewis acidity of imino group. Theoretical calculations and experiments prove that 1,2,4‐triazole AA retains the reduction stability in the maximum extent while increasing the HB strength of imino group. Based on this imino AA, the electrolyte achieves an unprecedented wide electrochemical stability window (5.5 V), enabling highly reversible cycling of fluorination and defluorination for CuF2||Pb full cells (>300 cycles) with a Cu+‐mediated two‐step redox mechanism, for PbF2‐Pb||PbF2‐Pb symmetric cells (1600 h) with low overpotential, and for PbF2||Pb asymmetric cells with high coulombic efficiency.

Oleylammonium fluoride passivated blue-emitting 2D CsPbBr3 nanoplates with near-unity photoluminescence quantum yield: safeguarding against threats from external perturbations.

Quantum-confined, two-dimensional (2D) CsPbBr3 (CPB) nanoplates (NPLs) have emerged as exceptional candidates for next-generation blue LEDs and display technology applications. However, their large surface-to-volume ratio and detrimental bromide vacancies adversely affect their photoluminescence quantum yield (PLQY). Additionally, external perturbations such as heat, light exposure, moisture, oxygen, and solvent polarity accelerate their transformation into three-dimensional (3D), green-emitting CPB nanocrystals (NCs), thereby resulting in the loss of their quantum confinement. Until now, no reported strategies have successfully addressed all these issues simultaneously. In this study, for the first time, we prepared oleylammonium fluoride (OAmF) salt and applied it post-synthetically to CPB NPLs with thicknesses of n = 3 and n = 4. Steady state and time-resolved photoluminescence (TRPL) measurements like fluorescence upconversion and TCSPC confirmed the elimination of detrimental deep trap states by fluoride ions, resulting in an unprecedented improvement in PLQY to 85% for n = 3 and 98% for n = 4. Furthermore, the formation of robust Pb-F bonds, coupled with strong electrostatic and hydrogen-bonding interactions, resulted in a highly stable NPL surface-ligand interaction. This concrete surface architecture restricts the undesired phase transition of 2D NPLs into 3D NCs under various external perturbations, including heat up to 363 K, strong UV irradiation, water, atmospheric conditions, and solvent polarity. Also, the temperature dependent TRPL measurements provide an insight into the charge carrier dynamics under thermal stress conditions and reveal the location of shallow trap states, which lie below 7 meV from the conduction band edge. In brief, our innovative OAmF salt has effectively addressed all the critical issues of 2D CPB NPLs, paving the way for next-generation LED applications. This breakthrough not only enhances the stability and PLQY of CPB NPLs but also offers a scalable solution for the advancement of perovskite-based technologies.

Remineralisation of enamel erosive lesions by daily-use fluoride treatments: network meta-analysis of an in situ study set

ObjectivesDaily-use fluoride products are first-line protection against enamel wear from dietary-acid exposure (DAE). This study aimed to understand effects of fluoride concentration, fluoride salt, product form and ingredients in daily-use products on remineralisation and demineralisation, via network meta-analysis (NMA) of 14 studies using one well-established in-situ model. Remineralisation (surface-microhardness recovery, SHMR) after treatment, and protection against subsequent demineralisation (acid-resistance ratio, ARR) were measured.Materials and methodsHealthy participants, wearing intra-oral palatal appliances holding enamel specimens eroded with standardised DAE, used test products once. Enamel hardness was assessed (Knoop microhardness probe) pre-DAE; post-DAE; after 4 h intra-oral remineralisation; and after post-remineralisation DAE. NMA was performed using a mixed-models approach on subject-level data to estimate and compare means.ResultsThere was a dose-response for fluoride ion in toothpastes (0-1426ppm F; p < 0.001 for SMHR and ARR). One toothpaste (silica-based, 1150ppm F as NaF) showed a benefit for SMHR versus placebo [mean(standard error)]: 8.8%(0.6%) (33.0% vs. 24.2%; p < 0.001); for ARR: 0.27(0.03) (0.43 vs. 0.15; p < 0.001; 9 mutual studies). Use of fluoride mouthwash after fluoride toothpaste increased SMHR [2.4%(1.1%); p = 0.043; 3 studies]; the effect on ARR [0.08(0.05)] was not significant (p = 0.164). Negative effects of polyvalent metal ions and polyphosphates on SMHR (p < 0.05) were observed.ConclusionsNMA proved effective in discriminating between fluoride-based treatments in this in-situ study, highlighting the importance of fluoride ion to enamel protection and showing formulation ingredients can affect its performance.Clinical RelevanceDaily-use fluoride products can protect enamel against dietary acids, but careful formulation is required for optimal performance.

Role of liquid immiscibility in the formation of the rare metal granites of the Katugin massif, Aldan shield

The paper discusses possible immiscibility between fluoride salt (“cryolite”) and silicate liquids into which the parental melt of the Katugin massif exsolves, and the petrological implications of this phenomenon. Results of a detailed study of the cryolite and zircon are presented. Liquid immiscibility is demonstrated to have triggered the massive crystallization of zircon and, together with the processes of subsequent evolution of the cryolite melt, contributed to the formation of the large cryolite bodies. Data on mineralhosted inclusions were used to estimate the crystallization temperatures of fluoride salt and silicate melts and outline the pathways of their evolution during the formation of the massif. It is shown that the granites of the Katugin and West Katugin massifs were most likely derived from distinct sources, that differed mainly in fluorine content. Data on the chemical composition of three zircon generations identified in the granites of the Katugin massif are presented.

Shielding Fluoride Ion Basicity through Diurea Coordination for Nonaqueous Fluoride Shuttle Batteries

AbstractThe strong basicity of fluoride ions leads to detrimental nucleophilic attack on organic components in the electrolytes, such as β‐hydrogen elimination reactions with organic cations and solvents, converting “naked” F− into corrosive and unstable bifluoride (HF2−) ions. These reactions significantly constrain the choice of suitable solvents and salts to develop electro(chemical) stable fluoride ion electrolytes. In this work, we replaced the triple water ligands typically present in industrial organic fluoride salts with dual 1,3‐diphenylurea (DPU) coordination via hydrogen bonding interaction. This modification successfully suppressed the Lewis basicity of fluoride ions, providing long‐term chemical stability (over 1000 hours) across a wide range of aprotic solvents, a broadened electrochemical stability window (−2.5–0.9 V vs. Ag+/Ag) and high ionic conductivity (1.7 mS cm−1) at room temperature. Additionally, the weaker hydrogen bonding in F−‐DPU coordination, compared to the conventional boron‐based anion acceptor (AA) strategy that relies on intensive Lewis acid‐base interactions, facilitates faster (de)fluorination kinetics at the electrode. The proposed room temperature fluoride ion batteries sustain improved electrochemical performance by pairing with the Pb‐PbF2 anode and BiF3 or Ag cathode.

Shielding Fluoride Ion Basicity through Diurea Coordination for Nonaqueous Fluoride Shuttle Batteries.

The strong basicity of fluoride ions leads to detrimental nucleophilic attack on organic components in the electrolytes, such as β-hydrogen elimination reactions with organic cations and solvents, converting "naked" F- into corrosive and unstable bifluoride (HF2 -) ions. These reactions significantly constrain the choice of suitable solvents and salts to develop electro(chemical) stable fluoride ion electrolytes. In this work, we replaced the triple water ligands typically present in industrial organic fluoride salts with dual 1,3-diphenylurea (DPU) coordination via hydrogen bonding interaction. This modification successfully suppressed the Lewis basicity of fluoride ions, providing long-term chemical stability (over 1000 hours) across a wide range of aprotic solvents, a broadened electrochemical stability window (-2.5-0.9 V vs. Ag+/Ag) and high ionic conductivity (1.7 mS cm-1) at room temperature. Additionally, the weaker hydrogen bonding in F--DPU coordination, compared to the conventional boron-based anion acceptor (AA) strategy that relies on intensive Lewis acid-base interactions, facilitates faster (de)fluorination kinetics at the electrode. The proposed room temperature fluoride ion batteries sustain improved electrochemical performance by pairing with the Pb-PbF2 anode and BiF3 or Ag cathode.

Imidazolium-Based Ionic Liquid Electrolytes for Fluoride Ion Batteries.

The fluoride-ion battery (FIB) is a post-lithium anionic battery that utilizes the fluoride-ion shuttle, achieving high theoretical energy densities of up to 1393 Wh L-1 without relying on critical minerals. However, developing liquid electrolytes for FIBs has proven arduous due to the low solubility of fluoride salts and the chemical reactivity of the fluoride ion. By introducing a chemically stable electrolyte based on 1,3-dimethylimidazolium [MMIm] bis(trifluoromethanesulfonyl)imide [TFSI] and tetramethylammonium fluoride (TMAF), we achieve an electrochemical stability window (ESW) of 4.65 V, ionic conductivity of 9.53 mS cm -1, and a solubility of 0.67 m. The origin of this high solubility and the solvation structure were investigated using NMR spectroscopy and neutron total scattering, showing a fluoride solvation driven by strong electrostatic interactions and weak hydrogen bonding without covalent H-F character. This indicates the chemical stability of 1,3-dimethylimidazolium toward the fluoride ion and its potential as an electrolyte for high-voltage FIBs.

SCALE inventory and reactivity analysis as part of the Hermes 2021 PSAR review

The readiness of SCALE for comprehensive studies of pebble-bed reactors has been demonstrated through detailed analysis of a fluoride salt–cooled, high-temperature pebble-bed reactor (PB-FHR). The methods developed for pebble-bed reactor modeling in SCALE, particularly for inventory generation, have proven effective in gaining insights into the reactor physics of this advanced reactor. Excellent agreement with another code package has been observed, further highlighting SCALE’s strong performance.The SCALE results supported the US Nuclear Regulatory Commission’s construction permit application review of the Hermes low-power PB-FHR demonstration reactor. A SCALE model of the Hermes reactor was developed at Oak Ridge National Laboratory using information from the Preliminary Safety Analysis Report (PSAR) and supplemented with publicly available data. SCALE reactivity coefficient simulations reproduced PSAR results within 1σ statistical uncertainties. Sensitivity studies emphasized the importance of graphite specifications for accurate keff predictions.

Improved Thermal Stability of Ionic Liquid through Hydrogen Bond Donor As an Electrolyte for Fluoride-Ion Battery

Fluoride ion batteries (FIBs) have recently gained much attention as a next generation battery system because of their potential to surpass lithium-ion batteries (LIBs) in many aspects such as high energy density and low cost. FIBs with solid or liquid electrolyte have been studied intensively, amongst liquid-based FIBs has an advantage of good interfacial contact between the electrolyte and electrode, and low-temperature operation over all-solid-state FIBs. However, stability has been a major concern for liquid electrolytes. The instability caused by the strong basicity of F− ion attacking most of the commonly used organic solvents leading to side reactions.1 So far bis(2,2,2-trifluoroethyl) ether (BTFE), which has good base resistance, has been proposed as an effective organic solvent for electrolyte.2 However, the boiling point of BTFE is relatively low (64 °C). While looking beyond organic solvents, ionic liquids (ILs) can be a choice as an electrolyte for FIBs owing to their good properties such as nonvolatility and wide electrochemical window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion. One way to weaken the basicity of F- ion is to solvate them by a complexing agent. Hagiwara et al. reported that imidazolium-based ILs containing ethylene glycol are stable despite the presence of F− ion.6 Bond formation between F− ion and hydrogen in the hydroxy groups of ethylene glycol and imidazolium-based cation prevents the F− ion from attacking the β-hydrogen of the cation's alkyl chain. However, there have been no reports of FIBs using ILs containing solvated F− ion as electrolytes. In this study, we investigated the thermal stability of choline bis(trifluoromethanesulfonyl)amide (N111(2OH) TFSA), an IL with hydrogen bond donor functional group, against F− ions. Furthermore, we performed charge-discharge tests on liquid-based FIBs using N111(2OH) TFSA containing fluoride salt as an electrolyte.N111(2OH) TFSA (Fig. 1 (a)) was synthesized by ion exchange of N111(2OH) Cl with K TFSA. The melting point of the synthesized N111(2OH) TFSA was estimated to be 38 °C from DSC measurement and it was solid at room temperature. 0.4 mol kg−1 tetramethylammonium fluoride (TMAF) in N111(2OH) TFSA was found to be stable up to about 150 °C (Fig. 1(c)). TGA measurements of N,N,N-trimethyl-N-propylammonium (N1113 TFSA) (Fig. 1(b)) without hydroxy groups were also performed under the same conditions for comparison. A solution of 0.4 mol kg−1 TMAF in N1113 TFSA showed a gradual weight loss right after the measurement was started, with a significant weight loss at about 120 °C (Fig. 1(c)). These results suggest that N111(2OH) TFSA with the hydroxy group exhibits higher thermal stability toward F− ion. This was attributed to the hydrogen bonding of the hydroxy group to F− ion, which weakened the basicity of F− ion. Finally, charge-discharge tests were performed in a three-electrode cell using BiF3 working electrode, Pb counter electrode, and Ag/Ag+ reference electrode. Figure 1 (d) shows charge-discharge curves of BiF3 electrode with 0.4 mol kg−1 TMAF in N111(2OH) TFSA as an electrolyte at 60 °C. The first discharge capacity of the BiF3 electrode was 300 mAh g−1, which represents 99 % of the theoretical capacity of BiF3 (302 mAh g−1) with a columbic efficiency of 85%. This result suggests that the BiF3 was reversibly defluorinated/fluorinated with 0.4 mol kg−1 TMAF in N111(2OH) TFSA.AcknowledgmentThis presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).References(1) V. K. Davis, et al., Mater. Chem. Front., 2019, 3, 2721-2727.(2) V. K. Davis, et al., Science, 2018, 362, 1144–1148.(3) K. Okazaki, et al., ACS Energy Lett., 2017, 2, 1460–1464(4) S. Raiguel, et al., Green Chem., 2020, 22, 5225-5252.(5) H. Sun, and S. G. Dimagno, J. Am. Chem. Soc., 2005, 127, 2050-2051.(6) Z. Chen, et al., J. Phys. Chem. Lett., 2018, 9, 6662−6667 Figure 1

Dual Fluoride Salt-Based Hydrate-Melt Electrolyte: Advancing Aqueous Fluoride Shuttle Battery

Introduction Fluoride shuttle battery (FSB) is an attractive post lithium-ion battery (LIB) candidate owing to its high theoretical energy density due to the use of multi-electron conversion reactions on transition metal fluorides as high as 5000 Wh L−1. However, its utilization of charge-discharge reactions at room temperature (RT) has been hindered by the scarcity to adopt adequately conductive electrolytes which enable reversible fluorination, and most of the reported FSBs employ solid-state electrolytes at operating high-temperatures1. Few reports on the liquid electrolytes suggested the employment of a saturated organic solution to control the metal fluoride dissolution for improving its cycling performance2. However, inorganic fluoride salts do not readily dissolve into organic solvents, and concentrated electrolytes were not obtained despite the addition of anion receptors, which hinders the fluoride conductivity3. Therefore, the use of organic solvents carries many demerits such as poor ionic conductivity, flammability, toxicity and so on. In this context, concentrated aqueous electrolytes could be a great substitute to address these issues owing to a) the toxic-free nature of water, b) the high solubility of inorganic fluoride salts in water, c) electrochemical stability enhancement at the electrodes due to the minimal free solvent molecules, d) facile room temperature transport properties4. Recently, a concentrated 35 mol kg-1 CsF electrolyte with a potential window of 3.1 V and ionic conductivity of around 50 mS cm–1 was reported5. However, there are still some challenges in terms of reversibility of metal fluoride electrodes.Here we report a fluoride hydrate melt with the highest F- concentration of 45.2 mol kg-1, which corresponds to a F-:water molar ratio of 1:1.2. We used a eutectic mixture of CsF-RbF, inspired by our previous report on eutectic dual salt-based hydrate-melts for aqueous LIBs4. We studied the electrochemical properties of the CsF-RbF hydrate melt and applied it to an aqueous electrolyte for FSBs. Experiment CsF and RbF were mixed with water in several molar ratios to find the highest solubility for synthesizing hydrate-melts. Linear sweep voltammetry (LSV) tests were conducted using Pt metal, Pt mesh, and Ag/AgCl as the working, counter and reference electrodes, respectively. The ionic conductivity was evaluated by electrochemical impedance spectroscopy using symmetric cells with Pt electrodes. To explore the RT cycling performance of Cu electrodes in the hydrate-melt electrolytes, cyclic voltammetry (CV) of the Cu (negative electrode) – CuF2 (positive electrode,) with Ag/AgCl as the reference electrode was conducted. Both the Cu electrode and CuF2 electrode were prepared by mixing Cu or CuF2, acetylene black, and PTFE with a weight ratio of 85:10:5. Results and Discussion We discovered a novel RT hydrate-melts of fluoride salts with minimal water, which was denoted as CsF0.6RbF0.4. 1.2H20 with the concentration of 45.2 mol kg-1 (Figure 1a). In the LSV study, the oxidation and reduction potential of the hydrate-melt were observed to be 2.19 V and -1.4 V vs. Ag/AgCl, respectively (Figure 1b), which gave a wide potential window of 3.49 V on the Pt electrode. In addition, the ionic conductivity of the CsF-RbF hydrate-melt was found to be 20.5 mS cm-1 at 25 0C. As compared to lithium-based hydrate meltat similar high concentrations (potential window: 2.7 V, ionic conductivity: 3 mS cm-1)4, the CsF-RbF hydrate melt shows both wide potential window and high ionic conductivity.The CV profile exhibited reversible fluorination of Cu at -0.25 V and defluorination at around -0.9 V. This could also be a hint where the dissolution of the active material is inhibited despite many cycles, as the concentration is high enough and amount of free water is insufficient for further dissolution. Thus, the application of the novel hydrate-melt electrolyte elucidates the intersection of enhanced solubility, non-flammability, low toxicity, wider potential window, high ionic conductivity and active material dissolution prevention for the FSBs. References M. A. Reddy, M. Fichtner, J. Mater. Chem., 2011, 21 (43), 17059-17062.H. Konishi, T. Minato, T. Abe, Z. Ogumi, Z, J. Appl. Electrochem. 2018, 48, 1205-1211.K. I. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett., 2017, 2 (6), 1460-1464.Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy,2016, 1 (10), 1-9.O. Alshangiti, G. Galatolo, G.J. Rees, H. Guo, H, J.A. Quirk, J.A. Dawson, M. Pasta, ACS Energy Lett., 2023, 8 (6), 2668-2673. Figure 1

(General Student Poster Award Winner, 3rd Place) Designing and Evaluating Microelectrodes for Molten Salt Corrosion Electrochemistry

Due to the coupling of high temperature operation (>500°C) and propensity to contain oxidizing impurities (e.g., moisture, metallic cations), corrosion of structural alloys in molten chloride and fluoride salts is a major concern among developers of Generation IV nuclear reactors. To mitigate corrosion, it is essential to understand the corrosion mechanism and accurately determine its rate. Prior studies employ a forensic approach for corrosion testing, which does not provide insights on instantaneous rate of corrosion [1]. The use of electrochemical impedance spectroscopy (EIS) can provide in-operando method to interrogate dissolution processes. However, due to the high electrical conductivity and double-layer capacitance of metal salt interface [2], low frequency regimes normally dominated by polarization resistance (Rp) in aqueous systems require data acquisition at low frequencies. (<10-3 Hz). This renders access to charge-transfer resistance (Rct) and diffusional impedance effects difficult. This difficulty is compounded by issues such as signal noise associated with the use of long cables in conventional molten salt corrosion cells [2].To overcome this challenge, a working electrode may be designed as a 2D microdisk electrode to was used to interrogate the low-frequency impedance regimes. This disk has a non-uniform current distribution at such low Wagner number but it is easier to access the charge transfer and/or diffusional impedance regime, otherwise unattainable with the conventionally used partially immersed 3D macro-electrodes utilized in most studies [3]. In this work, a flush mounted microdisk electrode, compatible for use in molten LiF-NaF-KF (i.e., FLiNaK) salts at 600°C was designed and evaluated for its effectiveness towards accurately determining Rp through the EIS method. Pure Cr was selected as the model working electrode since its corrosion behavior in FLiNaK at 600°C has been thoroughly investigated [4,5,6],. Cr microdisks, with areas ranging from 100 to 10-2 cm2, were fabricated and subsequently exposed in FLiNaK containing 1 wt.% of EuF3 at 600°C for 24 hrs. EIS measurements were carried out over the commonly utilized frequency range from 105 to 10-2 Hz. Moreover, the error introduced by implementing various low “cut off” frequencies was examined. Acknowledgment Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). This work was performed in the Department of Materials Science and Engineering (DMSE) in the Center for Electrochemical Science and Engineering (CESE) at the University of Virginia. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility (NMCF) with National Science Foundation (NSF) under award CHE-2102156. H.C. acknowledges the National Science Foundation Graduate Research Fellowship (GRFP), GRANT#: 1842490.

Cathodic Decomposition Reference Electrodes As a Generalized Approach for Thermodynamic Calculations in Complex Molten Salts

The rise of molten salt use in green energy applications has led to increased efforts to model and understand the underlying thermochemical properties of active species within molten salts. Molten salts can become increasingly complex during the operational lifetime through corrosion or generation of new species, resulting in changes to the thermophysical and thermodynamic properties. For example, next-generation molten salt nuclear reactors start with a binary or ternary fuel salt, but generation of fission products during operation can significantly change the behavior of active species in the salt and interactions with container materials. While processes in established molten salt electrolytes such as LiCl-KCl eutectic can use well-characterized reference electrodes such as Ag/AgCl to quantify thermochemical properties1, emerging salt systems do not have well-established reference electrode systems. Using standard reference electrodes based upon anodic electrolyte decomposition (e.g., chlorine or fluorine gas-based electrodes) introduces additional engineering and safety challenges during construction and measurement of thermochemical properties. These experimental barriers have previously limited investigation of the thermodynamics of novel molten salt systems.Here, we use cathodic decomposition electrodes (CDEs) to measure the thermochemical properties of molten salt systems. The CDE defines 0 V as the cathodic decomposition of the pure solvent electrolyte at all temperatures. A CDE was used to make activity measurements in chloride and fluoride salts. Both steady-state emf measurements in concentration cells and transient measurements were made in an electrolyte containing active species such as GdCl3 or NiF2. The universal nature of a CDE reference can improve accuracy and repeatability of activity measurements in molten salt systems and fill the knowledge gaps that prevent prediction of thermochemical and thermophysical properties in molten salt systems. L. YANG and R. G. HUDSON, “Some Investigations of the Ag∕AgCl in LiCl-KCl Eutectic Reference Electrode,” J. Electrochem. Soc. 106 11, 986 (1959); https://doi.org/10.1149/1.2427195.

Oxide Speciation and Oxy-Fluoro Complexes in Molten Fluoride Salts

The presence and speciation of oxides are relevant to various engineering applications of molten fluoride salts. Here, our studies are directed toward two primary applications: 1) as coolants, fuel salts, or breeding blankets in advanced fission and fusion reactors, and 2) as the reaction medium for the novel direct synthesis of Li-ion battery cathodes from Li-ores.In an advanced nuclear reactor, oxide might be introduced to the molten fluoride coolant/blanket via ingress of oxygen or moisture from air. Oxygen and moisture act as oxidants, reacting with the metal fluoride to produce metal oxides or hydroxides, along with corrosive HF. By these reactions, the presence of oxide is an indicator of leaks as well as of the progression of corrosion. Quantification of oxides by square wave voltammetry (SWV) has been proposed as a technique for electrochemical oxide sensors [1]. The intensity of the peak current for the oxidation of oxide to oxygen gas is proportional to the quantity of oxide present, so comparison to a calibration curve can indicate how much oxide is present in the melt. True engineering systems, however, would likely contain multiple solutes including corrosion products, fission products, and activation products. Multi-component chemistries are further complicated by differences in solvent-solute interactions; different fluoride solvents have varying degrees of polymerization, so they incorporate solutes into their structure differently. Practically, this means that oxides may exist in different complexes depending on the solvent and other solutes, therefore oxide quantification via SWV is complicated by solvent and solute chemistry.The synthesis of Li-ion battery cathodes via reaction in molten fluoride salts is currently being studied within a DOE Office of Science BES project as a lower-carbon emissions alternative to traditional cathode manufacturing. This concept involves the dissolution of spodumene (LiAlSi2O6) in molten fluorides and the electrochemical deposition of disordered transition metal (M) oxyfluoride anion rocksalt (Li1+xTM1-xO2-zFz) for use as a battery cathode. The anion disorder in the rocksalt is relevant due to its relation to cathode performance. Energy storage capacity from the transition metal redox states increases as F is substituted for O. Cathode synthesis, therefore, requires tuning of the chemical potentials and transport properties in melts containing fluoride and oxide anions. In this context, a deeper understanding of the structure and speciation of oxide in molten fluorides is critical to formulating the reaction pathway and required composition and configuration of the rocksalt product.Given these applications, we investigate the speciation of oxide and its existence within oxo-fluoro complexes in molten fluoride salts using electrochemistry. We present results of electroanalytical studies of oxides in molten 2LiF-BeF2 and 46.5LiF-11.5NaF-42KF, using cyclic voltammetry and SWV. We make connections between solute speciation and solvent structure, allowing for mutually enhanced understanding of solubility and fluoroacidity. Broadly, electroanalytical studies provide a deeper understanding of oxide’s speciation in molten fluoride systems which, in turn, informs electrochemical sensor development and electrochemical synthesis processes. By investigating the fundamental science of oxide and oxide complexes, we hope to advance technologies which enable the transition to clean energy.[1] L. Massot, L. Cassayre, P. Chamelot, and P. Taxil, “On the use of electrochemical techniques to monitor free oxide content in molten fluoride media,” J. Electroanal. Chem., vol. 606, no. 1, pp. 17–23, Aug. 2007, doi: 10.1016/j.jelechem.2007.04.005.

The Effect of Cold Work and Irradiation on the Dealloying Behavior of Ni-Cr Alloys in Molten Fluoride Salts

Molten salt nuclear reactor (MSR) environments present several challenges to structural materials, such as corrosion coupled with radiation damage occurring at high homologous temperatures. When a corrosion driving force exists, such as the presence of an oxidizing impurity with a half-cell redox potential higher than those of the alloying elements, selective dealloying of the less noble element (LN) in a multi-component structural alloy can occur [1]. This process can result in interesting corrosion morphologies such as the formation of a relatively uniform penetrating bicontinuous porous structures in grain interiors and penetrating dealloying along grain boundaries. Irradiation can induce solute segregation and/or saturation of non-equilibrium defects that interact with the corrosion front [2]. Currently, it remains uncertain how irradiation damage within an alloy impacts the process of molten salt dealloying, rendering the morphological instability.In this work, the corrosion dealloying behavior of Ni20Cr alloy (wt.%) was studied in molten LiF-NaF-KF (or FLiNaK) salts at 600°C. Ni20Cr alloys were subjected to two different conditions of Ni ion radiation, one carried out at 25°C and the other at 600°C, creating a peak damage depth nearing ~3µm at approximately 60 dpa. As a surrogate for microstructural damage by radiation, a separate set of Ni20Cr alloys were cold-worked to achieve reductions of thickness of 10%, 30%, and 50% which induce lattice defects in the form of dislocations. Additional samples were subjected to the identical potentiostatic holds attempted previously in molten FLiNaK in carefully selected electrochemical potential regimes where only Cr dealloying occurs in FLiNaK [2]. A porous Ni-rich ligament was formed that undergoes coarsening and densification. Electrochemical impedance spectroscopy (EIS) was used to quantify the porous structures. The pre-irradiated then post-corrosion samples were examined by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques to characterize micro and nanoscale changes in structure and composition. Irradiation produced deeper dealloying depths than observed on annealed samples. Furthermore, the roles of dislocation array and grain boundaries dislocation pile ups, which serve as short-circuit paths for outward Cr transport, during the dealloying process, were closely examined. Acknowledgment Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). This work was performed in the Department of Materials Science and Engineering (DMSE) in the Center for Electrochemical Science and Engineering (CESE) at the University of Virginia. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility (NMCF) with National Science Foundation (NSF) under award CHE-2102156. H.C. acknowledges the National Science Foundation Graduate Research Fellowship (GRFP), GRANT#: 1842490.

Evaluation of an Ag/AgF Reference Electrode for Electrochemical Measurements of Molten 2LiF-BeF2

Despite the wide application of molten fluoride salts, such as 2LiF-BeF2 (FLiBe), as heat transfer fluids, fuel solvents, and fusion blanket materials in advanced nuclear power concepts, the mechanisms of ion transport, speciation, and reaction kinetics in these systems have not been comprehensively characterized. Though electrochemical methods are powerful tools for probing these properties of molten fluorides, current studies are limited by the lack of a reliable thermodynamic reference electrode (TRE) compatible with the fluoride environment. The reference electrode designs used in fluorides can be classified into three main categories: quasi-reference electrodes (qREs), dynamic reference electrodes (DREs), and thermodynamic reference electrodes. qREs and DREs are not well suited to in-line measurements of molten fluorides since the reference redox couple of the noble qRE material cannot be assumed to be constant and the time scale of the DRE’s relaxation of the transient redox system is too short.Therefore, explorations of the FLiBe system have pursued the development of a TRE which can hold a thermodynamically stable reference potential for an extended time. Current designs using the H2/HF or Ni/Ni2+ couples suffer from either difficult handling of gas electrodes at high temperatures [1] or long equilibriation times. Since pure NiF2 does not melt at reactor operating temperatures (~500-1000°C), the NiF2 used must dissolve into a reservoir of supporting FLiBe, a process which takes over 30 hours. In this study, the performance of a reference electrode using the Ag/Ag+ redox couple is assessed for long-term electrochemical stability. The Ag/AgF system was chosen to address the limitations of the Ni/Ni2+ TRE, as the low melting point of AgF (Tmelt = 435°C) allows for the direct use of the pure salt in the electrode, eliminating the time-consuming dissolution step required for NiF2.The Ag/AgF TRE was constructed by melting AgF salt into a boron-nitride crucible fitted with a LaF3 fluoride membrane and inserting a silver wire into the salt reservoir. The stability and reproducibility of the electrode potential were evaluated through long-term open circuit potential measurements against a platinum qRE in a FLiBe melt at 600°C. The reversibility of the Ag/Ag+ reaction was assessed using a linear polarization study. Finally, the location of the redox potential was found by comparing the open circuit potential of the Ag/AgF TRE against a beryllium rod electrode. The development of a reliable and easy-to-operate thermodynamic reference electrode will facilitate future studies on the thermochemical properties of molten FLiBe while laying the foundation for advancing in-line monitoring and redox control in nuclear technologies.[1] Jenkins, Howard W., "Electrochemical Measurements in Molten Fluorides. " PhD diss., University of Tennessee, 1969. https://trace.tennessee.edu/utk_graddiss/3072

Photocatalytic low-temperature defluorination of PFASs.

Polyfluoroalkyl and perfluoroalkyl substances (PFASs) are found in many everyday consumer products, often because of their high thermal and chemical stabilities, as well as theirhydrophobic and oleophobic properties1. However, the inert carbon-fluorine (C-F) bonds that give PFASs their properties also provide resistance to decomposition through defluorination, leading to long-term persistence in the environment, as well as in the human body, raising substantial safety and health concerns1-5. Despite recent advances in non-incineration approaches for the destruction of functionalized PFASs, processes for the recycling of perfluorocarbons (PFCs) as well as polymeric PFASs such as polytetrafluoroethylene (PTFE) are limited to methods that use either elevated temperatures or strong reducing reagents. Here we report the defluorination of PFASs with a highly twisted carbazole-cored super-photoreductant KQGZ. A series of PFASs could be defluorinated photocatalytically at 40-60 °C. PTFE gave amorphous carbon and fluoride salts as the major products. Oligomeric PFASs such as PFCs, perfluorooctane sulfonic acid (PFOS), polyfluorooctanoic acid (PFOA) and derivatives give carbonate, formate, oxalate and trifluoroacetate as the defluorinated products. This allows for the recycling of fluorine in PFASs as inorganic fluoride salt. The mechanistic investigation reveals the difference in reaction behaviour and product components for PTFE and oligomeric PFASs. This work opens a window for the low-temperature photoreductive defluorination of the 'forever chemicals' PFASs, especially for PTFE, as well as the discovery of new super-photoreductants.

Carbon black densified matrix graphite to enhance its anti-infiltration capability against molten salt

A novel graphite matrix was prepared by adding carbon black (CB) as a densification agent to traditional matrix A3-3 graphite preparation. The particle size of CB densifier we chose was approximately 50–150 nm, which was smaller than the pore diameter of densified matrix graphite (500-1000 nm). Five different mass fractions of CB were mixed and compared with as-received A3-3 by various characterization methods, which indicated that CB can effectively fill the pores of A3-3 and improve its anti-infiltration capability against molten salt (fluoride salt for molten salt reactor and heat storage salt). Meanwhile, considering the requirements for A3-3 graphite in scenarios of solid fuel molten salt reactor and thermal energy storage system, all the samples were compared as well before and after purification. The results showed that purification treatment resulted in slight increase of the pore size and open porosity of graphite and thus weakened its anti-infiltration capability against molten salt, but the thermal properties of graphite improved inversely. However, the addition of excessive CB could cause the significantly decline of thermal properties of A3-3. Comprehensively considering the anti-infiltration effect and requirements of thermal properties, adding 5 % CB was the most preferrable choice for the matrix graphite before and after purification.

In-line detection of salt formation during vitrification using millimeter wave radiometry and interferometry

Vitrification is an internationally significant industrial process used for the treatment and immobilization of hazardous and radioactive waste. For successful silicate melt vitrification, molten salt formation during melter operation must be avoided. As such, proper process controls and continuous monitoring are critical to minimizing melting problems. Several in-situ process technologies for glass melters are well-developed; however, the lack of in-situ surface salt formation detection methods presents a risk to vitrification at the Waste Treatment & Immobilization Plant (WTP) on the US Hanford Site. While proposed previously, millimeter wave (MMW) radiometry and interferometry are demonstrated for the first time for in-line detection of salt formation in simulated nuclear waste glass melts. The experimental radiometer and interferometer setup uses the optical properties of the melt and a dual receiver operating at ∼ 137 GHz to elucidate melting behavior. A series of previously characterized glasses supersaturated with sulfate (Na2SO4), chloride (NaCl), or fluoride (NaF) salts are analyzed using the MMW system. This provides insight into volatile losses, fining, salt formation, salt identity, crystallization, and optical properties of a heterogeneous melt. Relevant terahertz (MMW/THz) optical properties are also compiled. Millimeter wave measurements are evaluated here for the ability to detect phase changes in salt-forming glass melt compositions without opaque body radiometry assumptions. This contribution demonstrates MMW radiometry with interferometry as a useful method for in-situ salt detection, enabling risk reduction in nuclear waste vitrification melters.

Highly enhanced fluoride removal from aqueous systems via solvent extraction utilizing trialkyl phosphine oxide as an efficient extractant

Solvent extraction (SX) has been recognized as an efficient approach for the large-scale removal of impurities, demonstrating considerable potential for fluoride extraction in industrial applications. However, existing fluoride extraction methods typically require the incorporation of additives to form complexes with fluoride for its removal. Due to the excessive additive requirements for efficient fluoride removal, these methods inevitably result in secondary contamination of feed solution, necessitating additional purification efforts to remove them. In this study, four extractants (trioctyl amine, tributyl phosphate, trioctyl phosphate, and trialkyl phosphine oxide (TRPO)) were investigated for fluoride removal via hydrogen bonding with hydrofluoric acid (HF). Various parameters, including the concentrations of extractants and stripping agents, pH values, extraction time, and temperature were initially optimized. It was found that TRPO exhibits the highest extraction efficiency for HF, with a single-stage extraction rate exceeding 70 %. Complete fluoride removal (> 99.9 %) was achieved through a consecutive four-stage extraction process. Further experiments, including maximum loading and slope analysis, combined with FT-IR and 19F NMR characterization, and DFT calculations elucidate the mechanism of the extraction. The transfer of HF into the organic phase is accomplished by the formation of a 1:1 hydrogen-bonded complex with TRPO. The loaded fluoride in the organic phase can be easily stripped by a base to get a fluoride salt. This study paves the way for the construction of novel SX systems to remove fluoride as HF.

Faster and Safer "In situ" Synthesis of Germanane and Silicane.

Germanane (GeH) and silicane (SiH), members of the Xanes family, have garnered significant attention as 2D materials due to their diverse properties, which hold promise for applications in electronics, optoelectronics, energy storage, and sensing. Typically, highly concentrated hydrochloric acid (HCl) or hydrofluoric acid (HF) is employed in the synthesis of these Xanes, but both routes are problematic due to slow kinetics and safety concerns, respectively. Here for the first time, a faster and safer method is demonstrated for Xanes synthesis that harnesses the generation of HF "in situ" using a solution of HCl and lithium fluoride (LiF) salt, overcoming the key challenges of the conventional methods. A variety of characterization techniques to establish a baseline is utilized for both Xanes and to provide a holistic knowledge regarding this method, the possible consequences of this approach, and the possibility of applying it to other layered Zintl phases. The novel synthesis protocol results in high-quality GeH and SiH with bandgaps (Eg) of 1.75 and 2.47eV respectively, highlighting their potential suitability for integration into semiconductor applications.