Critical materials, such as rare-earth metals, are essential to numerous applications, including clean energy; however, the present industrial practices for producing rare-earth metals involve environmentally damaging and thus unsustainable chemical and electrochemical processes. An alternative moderate-temperature chloride-based molten salt electrolysis process can address these issues, providing energy efficient and sustainable metal production. While it is being developed presently for rare-earth electrowinning, one can easily envision its broader application to rare-earth electrorefining and the electrolytic production of high-volume metals like Fe and Al. Presently, these high-volume metals industries account for nearly 10% of global greenhouse gas emissions. Thus, the chloride MSE process presents a huge opportunity for truly achieving sustainability if it is developed further for producing Fe, Al, Ti, Mg, and other commodity metals.

Redox flow batteries (RFBs) are membrane-separated rechargeable flow cells with redox electrolytes, offering the potential for large-scale energy storage and supporting renewable energy grids. Yet, creating a cost-effective, high-performance RFB system is challenging. In this work, we investigate an Fe/Mn RFB alkaline system based on the [(TEA)Fe-O-Fe(TEA)](3-/4-) and MnO4-/2- redox couples with a theoretical cell voltage of similar to 1.43 V. This combination has not been systematically studied previously, but it can lead to a very low-cost and sustainable materials for high energy storage. Constant current cycling tests were performed at +/- 41 mA cm(-2)(2) between 20% and 80% SOC over 800 h (400 cycles) with an apparent Coulombic efficiency (CE) approaching 100%, while the voltage efficiency (VE) gradually decreased from similar to 75.3% to similar to 61.4% due to increasing internal resistances. The voltage efficiency loss can be mitigated through a periodic acid treatment to remove MnO2 deposits from the separator.

Neodymium metal is a critical component of rare earth magnets, essential for electric vehicles and the green energy transition, but its production has severe environmental impacts across its mining, separation, purification, and metal electrowinning steps. Specifically, conventional neodymium electrowinning in oxyfluoride molten salts using a consumable graphite anode generates greenhouse gases, e.g., carbon dioxide and perfluorocarbon (PFC). Here, we propose an alternative chloride-based molten salt electrolysis process utilizing a novel dimensionally stable anode (DSA). Our process lowers the specific electrical energy consumption compared to the state of the art, while producing reusable chlorine gas and eliminating direct CO2 and PFC emissions. Chloride-based molten salt electrolysis of NdCl3 (1.65 M) added to a LiCl-KCl eutectic (45:55 wt %), while using a RuO2-coated DSA enables high Coulombic efficiency (>80%), low specific energy consumption (2.3 kWh/kg-Nd), and excellent electrowon Nd product purity (>97 wt %). Life cycle analysis, excluding the common input feedstock (Nd2O3), shows that the global warming potential for the proposed chloride-based electrolysis approach is 5 kg CO2 equivalent, compared to 9-16 kg CO2 equivalent for the conventional process, representing a 44-69% reduction in CO2 emissions.

Neodymium production has become increasingly important due to use of neodymium–iron–boron (NdFeB) magnets in green energy, consumer technology and defense applications. The state-of-the-art practice features a neodymium fluoride and lithium fluoride molten salt with a consumable carbon anode converting dissolved neodymium oxide and carbon to neodymium metal and CO 2 . 1-2 The anode effect results in PFCs and CO emissions, which make the current neodymium electrowinning process unsustainable. 3-4 Some attempts have been made to electrolyze the chloride salt of neodymium to produce chlorine at the anode. Chlorine is a value-added product which neither consumes the anode nor produces other deleterious GHGs. Additionally, the chloride of neodymium can be non-carbothermically produced through a spontaneous reaction with hydrochloric acid, producing only water as a side product. However, a common problem in chloride rare earth electrowinning is the low Coulombic efficiency (10-50%) obtained in lab-scale and pilot-scale efforts. 5 The neodymium chloride electrolysis process notoriously encounters a two-step reduction producing an intermediate divalent (Nd 2+ ) oxidation state which is stable in the chloride media. 6-7 The intermediate neodymium species diffuses away from the cathode leading to Coulombic efficiency loss. Additionally, the kinetics of the chlorine evolution reaction (CER) in chloride molten salts are known to be relatively sluggish, causing significant anodic overpotentials which elevate the specific energy consumption during electrowinning. Furthermore, electrowinning from molten salts often produces spongy or dendritic deposits of metal requiring energy-intensive purification. In this talk, we will address all aforementioned technical hurdles and report on our efforts achieving high Coulombic efficiency (~85%), and compact deposits with superior as-electrowon neodymium metal purity (>99 wt.%). Furthermore, we will report the development of a new anode which catalyzes chlorine evolution, lowering the anodic overpotential considerably during electrolysis at high current densities of practical interest for electrowinning. Specific energy consumption is calculated for some example cases and found to be between 2.1 and 3.5 kWh/kg representing a significant improvement over the conventional oxyfluoride process. Figure 1

The significance of easily detecting rare earth elements (REEs) has increased due to the growing demand for REEs. Addressing this need, we present an innovative electrochemical biosensor, focusing on cerium as a model REE. This biosensor utilizes a modified EF-hand loop peptide sequence, incorporating cysteine for covalent attachment to a gold working electrode and tyrosine as an electrochemically active amino acid. The sensor was designed such that binding to cerium induces a conformational change in the peptide, affecting tyrosine's proximity to the electrode surface, modulating the current. A calibration curve was generated from cyclic voltammetry current peaks at similar to 0.55-0.65 V versus a silver pseudo-reference electrode, with cerium concentrations ranging from 0 to 67 mu M in artificial urine. The sensor exhibited a biologically relevant limit of detection of 35 mu M and a sensitivity of -0.0024 +/- 0.002 (mu A mu M)(-1). These findings offer insights into designing peptide sequences for electrochemical biosensing.

Utility-scale zinc (Zn) batteries are a promising solution to address the problem of intermittency of renewable energy sources; however, Zn-metal anodes in these batteries suffer from capacity loss due to spontaneous corrosion of the Zn especially when high-surface area anode configurations are employed. Additionally, Zn dendrites are known to form during battery charging limiting the cycle-life of these batteries. Electrolyte additives have been explored that prevent aforementioned issues, but these too come at a cost, i.e., surface-blocking additives polarize the electrode surface leading to loss in the voltaic and energy efficiencies of the battery. In this contribution, a novel electrolyte additive, benzyldimethylhexadecylammonium chloride (BDAC), is investigated for its ability to suppresses corrosion of Zn in an acidic (pH = 3) electrolyte. An attribute of BDAC distinct from previously studied additives is that it selectively suppresses electrochemical activity when the Zn electrode is at its corrosion potential; however, during high-rate Zn deposition (charging) or stripping (discharging), BDAC is essentially deactivated and thus it does not appreciably polarize the electrode surface, thus minimizing voltaic efficiency losses. This selective corrosion suppression behavior is explored using slow-scan voltammetry, which reveals hysteresis implying a potential- or current-dependent BDAC adsorption mechanism in which BDAC reaches higher surface coverages when the partial currents at the Zn surface are low (e.g., at or near the corrosion potential), but BDAC coverage is reduced considerably when the Zn deposition or stripping rates are increased. Numerical simulations of the BDAC diffusion-adsorption process corroborate this mechanism. Ramifications of our approach to the selective suppression of Zn dendrites are discussed.

Redox organic deep eutectic solvents (DESs) based on redox organic electrolytes have great potential as a versatile and energy dense electrochemical energy storage system but have been held back by lack of understanding of their irregular behavior when organic redox molecules are transitioned from other solvent systems. In this work, the hydrogen bonding characteristics of a series of redox organic molecules are investigated through infrared spectroscopy and molecular modeling. New understanding of these interactions is then used to explain their various electrochemical behaviors in a DES electrolyte. A model is used to predict the behavior of new derivatives towards the design of an optimized redox organic DES system. Hydrogen bonding between the redox molecules and the solvent was found to hinder transport to an electrode surface when the surface contains adsorbed hydrogen bond donors and to significantly shift the potential of a redox reaction more positive when a hydrogen bond forms at the redox active site. It is predicted that functionalizing a molecule with electron withdrawing groups to lower the electron density of a redox active function group lowers the strength of the hydrogen bond and alleviates the undesirable potential shift. This hypothesis is demonstrated by the addition of nitro groups to fluorenones.

Large-scale energy storage is required to meet a multitude of current energy challenges. These challenges include modernizing the grid, incorporating intermittent renewable energy sources (so as to dispatch continuous electrical energy), improving the efficiency of electricity transmission and distribution, and providing flexibility of storage independent of geographical and geological location. Through efforts supported by ARPA-E and the Department of Energy Office of Electricity, one technology we are developing utilizes an approach based on sustainable, low cost iron electrolytes in an iron flow battery (IFB). Advantages of the IFB include abundant, non-toxic, and non-corrosive materials that are used to provide an energy storage solution that has inherently safe operation and is environmentally friendly. In our approach to the all iron RFB, we introduced a flowing carbon slurry as the negative electrode as a substrate for plating iron during charging allowing storage of the plated iron in a tank external to the stack. This configuration decouples energy and power capacities. The work supported by DOE allowed us to address challenges encountered in scaling up from small laboratory cells to a large commercial size slurry IFB single cell. 1 In this presentation, we will address some of the challenges we encountered for demonstrating the technology within an academic environment in order to position it for commercial development. Even with funding to overcome the valley-of-death and technology-to-market assistance given by ARPA-E, we faced many challenges including hiring expertise to address scale-up, design and testing. We will discuss challenges faced with identifying partners and licensees for moving discovery to product. Also, we will describe the many types of interactions we had with the university Technology Transfer Office including patent disclosure, IP protection, and negotiating licenses. Acknowledgements: This research was supported by ARPA-E contract DE-AR0000352. Partial support also was received from the DOE Office of Electricity Award #1111358,0 under Dr. Imre Gyuk. 1 N.S. Sinclair, R.F. Savinell and J.S. Wainright, MRS Energy & Sustainability, 2022; doi: 10.1557/s43581-022-00046-8

The all-iron flow battery is currently being developed for grid scale energy storage. As with all flow batteries, the membrane in these systems must meet stringent demands for ionic conductivity while limiting unwanted reactant (Fe3+) crossover. In addition, for the all-iron chemistry proton transport across the membrane is highly desirable to maintain the pH levels in the negative and positive electrolytes. Two membranes are considered, Nafion and an in-house developed composite based on Daramic (a commercial microporous separator) and poly(vinyl alcohol). Their performance is compared for various metrics specific to the all-iron flow battery chemistry.

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO 2 ) from aluminum oxide (Al 2 O 3 ) and carbon. 1–3 Similar molten salt techniques have been developed for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium is of particular interest recently with significant increases in demand being driven by the increased production of technologies such as wind turbines, electric vehicles, and hard disk drives that require neodymium in the form of neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The current procedure for neodymium processing uses a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd 2 O 3 ) and carbon to neodymium metal and CO 2 . 4,6 As an unfortunate byproduct of the fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously. The formation of PFCs combined with the emission of a significant amount of other greenhouse gases (CO,CO 2 ) make the current process for neodymium electrowinning is dangerous and undesirable from an environmental standpoint. However rare earth metal production remains profitable enough that illegal processing operations make up a significant portion of the world’s supply of rare earth elements like neodymium. 7,8 Due to this, few places currently produce neodymium metal and virtually none is produced in the United States, creating supply chain risks with dire consequences. 5 An alternative molten salt process has been proposed in which the fluoride salts have been replaced with chloride salts consisting of lithium chloride (LiCl) and potassium chloride (KCl). Rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into the LiCl-KCl molten salt and neodymium is electroplated via the below set of reactions. 9,10 Cathode: 2NdCl 3 + 6e - → 2Nd (solid) + 6Cl - Anode: 6Cl - → 3Cl 2 + 6e - Overall: 2NdCl 3 → 2Nd (solid) + 3Cl 2 This process has several distinct advantages. Utilizing the chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine produced could then be recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride. This talk focuses on recent advances to improve purity and lower energy consumption (kWh/kg). Our work evaluates anode and cathode behavior during this neodymium chloride molten salt process in order to determine its viability. Overpotential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Additional purification techniques such as vapor distillation procedures are developed to ensure a product that is viable for industrial use. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Portions of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. T. R. Beck, Electrochem. Soc. Interface , 23 , 36–37 (2014). G. G. Botte, Electrochem. Soc. Interface , 23 , 49–55 (2014). W. E. Haupin, J. Chem. Educ. , 60 , 279–282 (1983). M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte . B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol. , 48 , 9506–9513 (2014). V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576 , 10 , 576 (2020). H. Vogel, B. Friedrich, H. Vogel, and B. Friedrich, Int. J. Nonferrous Metall. , 6 , 27–46 (2017). J. C. K. Lee and Z. Wen, Nat. Sustain. 2018 110 , 1 , 598–605 (2018). R. Akolkar, J. Electrochem. Soc. , 169 , 043501 (2022). D. Shen and R. Akolkar, J. Electrochem. Soc. , 164 , H5292–H5298 (2017).

Eutectic solvents (ESs) have been widely studied due to their tunable solvation and physical properties. The properties of eutectic solvents that are formed from mixtures of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) are governed by a complex hydrogen (H) bonded network among their constituents. However, an understanding of the key factors effecting the H-bonding structure and dynamics, and the resulting physical and electrochemical properties at interfaces is still in its infancy. In this study, the influence of the cation's alkyl chain length and the anion chemical identity on the characteristics of the H-bonding network in eutectic solvents were examined. In particular, ESs formed from mixtures of tetraalkylammonium salts as HBAs and ethylene glycol as the HBD were studied. Complementary spectroscopic studies revealed the changes in the solvation environment as a function of the ammonium chain length (C1, C2, C3, and C4) and the anion type (Cl-, Br-, I-). With an increase in the alkyl chain length or a decrease in the electronegativity of halide anions (X-), it was found that the O-H bond length shortens and the H center dot center dot center dot X- bond elongates in the H-bond (O-H center dot center dot center dot X- ) structure, thus resulting in an increase of the solvated ion size and a decrease in the bulk ionic conductivity. Further, double layer capacitance measurements showed that increased capacitance was correlated with shorter H-bonds in anion solvates.

Deep Eutectic Solvents (DESs) have recently gained interest as flow battery electrolytes. Their advantages include a wider electrochemical stability window compared to aqueous electrolytes, higher solubility for redox-active species, and negligible volatility. However, DESs are often highly viscous, and suffer from low ionic conductivities. This can make assessing redox kinetics difficult when attempting to determine their viability for energy storage. In classical voltammetric measurements, low ionic conductivity manifests as high solution resistance, thereby requiring "live" compensation of the electrolyte ohmic drop when performing fast-scan voltammetry. An uncompensated or inadequately-compensated ohmic drop leads to misinterpretation of the voltammetric behavior, e.g., assessing reversibility vs. irreversibility of a redox reaction. Here, we present micro-fabricated electrodes as facile "meso-scale" electrodes, which overcome these issues by nearly eliminating the ohmic drop while retaining uniformity of the current distribution over the electrode surface. Their use in precise transport-kinetics measurements is demonstrated using a redox-active organic, i.e., 4-Hydroxy-TEMPO in an aqueous medium and in ethaline, which is a viscous DES. This study provides a methodical approach to design and to implement voltammetry experiments using meso-scale electrodes leading to reliable measurements of diffusion-reaction properties of 4-Hydroxy-TEMPO.

Chapter 35 Fe / Fe Flow Battery Robert F. Savinell, Robert F. Savinell Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorNicholas Sinclair, Nicholas Sinclair Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorXiaochen Shen, Xiaochen Shen Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorJulia Song, Julia Song ESS Inc., 26440 SW Parkway, Wilsonville, OR, 97070 USASearch for more papers by this authorJesse S. Wainright, Jesse S. Wainright Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this author Robert F. Savinell, Robert F. Savinell Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorNicholas Sinclair, Nicholas Sinclair Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorXiaochen Shen, Xiaochen Shen Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this authorJulia Song, Julia Song ESS Inc., 26440 SW Parkway, Wilsonville, OR, 97070 USASearch for more papers by this authorJesse S. Wainright, Jesse S. Wainright Case Western Reserve University, Department of Chemical and Biomolecular Engineering, 10700 Euclid Avenue, Cleveland, Ohio, 44106 USASearch for more papers by this author Book Editor(s):Christina Roth, Christina RothSearch for more papers by this authorJens Noack, Jens NoackSearch for more papers by this authorMaria Skyllas-Kazacos, Maria Skyllas-KazacosSearch for more papers by this author First published: 06 January 2023 https://doi.org/10.1002/9783527832767.ch35 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Summary This chapter describes the operating principles and key features of the all-iron flow battery (IFB). This energy storage approach uses low-cost iron metal (Fe) ions for both the positive and negative electrode reactions thereby requiring less stringent membrane properties. 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Nonflammable eutectic solvents show great potential to enhance the concentrations of the redox-active materials and the cell voltages for redox flow batteries (RFBs). Herein, we report a promising redox-active eutectic electrolyte (1.5 M total redox species) with viologen and ferrocene derivatives where both of the redox reactions are reversible with a maximum open-circuit voltage of 1.35 V and an energy density of 15.1 Wh L-1, which is relevant to large-scale energy storage. The charge-discharge (from 75 to 25% state of charge) characteristics in a flow cell (0.15 M negolyte and 0.3 M posolyte) showed that it can be cycled with consistent discharge capacity for 12 h (19 cycles), beyond which pressure-driven crossover between the posolyte and negolyte reservoirs leads to capacity decay. This study points to promising new directions toward eutectic electrolyte development for RFBs where we demonstrate increasing the polarity, functionalizing the redox molecules, and separating redox intermediates to prevent undesired side reactions can make improvements in operating cell voltage, energy density, and cyclability.

Deep eutectic solvent (DES) systems offer unique properties as electrolytes for redox flow batteries. They consist of hydrogen bond donor and acceptor pairs and often offer larger electrochemical windows than traditional aqueous electrolytes with lower vapor pressure than other non-aqueous systems like acetonitrile. They can contain a large concentration of charge carries and have the potential to be high energy density electrolytes1. When paired with redox organic species it is possible to tune both the electrolyte and redox actives to obtain desirable electrochemical properties such as the solubility of active species, kinetic and transport behaviors and even thermodynamic properties such as redox potential2. When attempting to understand these behaviors in DES it becomes important to understand the redox organic molecule’s interaction with the hydrogen bond network. For example, it has been show that altering the ratio of hydrogen bond donor to acceptor sites in a molecule can have dramatic effects on its solubility3. Hydrogen bonding between the DES and the redox organic in its varying charge states can dramatically impact many aspects of the electrochemical behavior. This work focuses on interactions that can have apparent effects on the kinetic and thermodynamic behavior of redox organics. We explore several molecules such as viologen derivatives, PTIO and N-methyl-phthalimide, some of which exhibit reversible behavior in both acetonitrile and a choline chloride and ethylene glycol based DES while other do not. Some see a dramatic shift in redox potential while other do not. We correlate these changes in behavior to influences of hydrogen bonding through the use of IR spectroscopy paired with voltammetry. Red shift in the stretching frequencies of redox active functional groups indicates the presence or lack of hydrogen bonding and this is related to changes in voltammetry as ethylene glycol, a hydrogen bond donor, is added to the redox organic in acetonitrile electrolyte system. It was observed that radical/cation reactions such as PTIO/PTIO+ or viologen were unaffected both kinetically and thermodynamically while reactions involving anionic species such as PTIO-/PTIO or N-methyl-phthalimide would either become irreversible or shift potential. This can be observed for PTIO in the accompanying figure where the cation reaction (more positive couple) does not change while the anion reaction (more negative couple) changes potential dramatically. We propose that some anionic species hydrogen bond more strongly at the redox active sites due to their higher electron density at a hydrogen bond critical point. Electron density has been strongly correlated to hydrogen bond strength4 and molecular modeling is being done to determine the comparative hydrogen bond strength of the anion and cation species as well as reversible and irreversible cationic species. These insights can lead to the intelligent design of highly soluble redox species that remain reversible and make use of the full electrochemical window of a given DES. E. L. Smith, A. P. Abbott and K. S. Ryder, Chem. Rev., 114, 11060–11082 (2014). N. S. Sinclair, D. Poe, R. F. Savinell, E. J. Maginn and J. S. Wainright, J. Electrochem. Soc., 168, 020527 (2021). B. Chen, S. Mitchell, N. Sinclair, J. Wainright, E. Pentzer and B. Gurkan, Mol. Syst. Des. Eng., 5, 1147–1157 (2020). G. Gilli and P. Gilli, J. Mol. Struct., 552, 1–15 (2000). Figure 1

Government funding is critical for testing concepts and ideas of technical approaches to demonstrate their value to attract attention for commercial development. In the US for energy projects, this funding often comes from ARPA-E, but similar types of funding agencies exist in other countries as well. However, independent of the funding sources, government or private, often unanticipated challenges arise that require pivots and flexibility, and leap-frogging scale-up levels can hinder achieving the knowledge needed for technology development. By incorporating a conducting carbon slurry in the negative electrolyte of an all iron flow battery, the decoupling of power from energy design becomes possible for this normally hybrid flow battery system. This approach offers the potential for very low cost large-scale energy storage with safe and sustainable materials. Government funding of this project allowed the demonstration of the concept during the seedling stage, but with the use of carbon nanotubes that would not meet cost targets. The second phase of the project demonstrated that low cost carbons with certain properties could also be used effectively. The third phase of the project then sought to scale-up the lab cells to a full-size stack. This paper summarizes some of the technical challenges encountered and pivots in approach that were taken. This project was sponsored by a commercialization-focused government agency (US ARPA-E in this case) and we point out some constraints and expectations of attracting commercial funding sources that hindered the development, or complicated solving the necessary design and materials issues to make the technology interesting for further investment. The lessons learned here will be applicable to other commercialization driven projects sponsored by government agencies in the US and elsewhere.

We developed concentrated hydrogen bonded electrolytes (CoHBEs) derived from a mixture of choline chloride (ChCl) and ethylene glycol (EG) containing ferrocene and viologen redox species for redox flow batteries. CoHBEs are similar to deep eutectic solvents (DESs) in terms of having distinct physical properties including wide electrochemical window, and low volatility. However, CoHBEs do not necessarily meet the requirement of “deep eutectic temperature” at a specific composition of the parent compounds that form the DES. CoHBEs formed with viologen and ferrocene species in ChCl:EG demonstrate reversible redox reactions. More importantly, 0.5 M of a viologen derivative coupled with 1 M of a ferrocene derivative was achieved owing to the good solvent strength of ChCl:EG at 1:4 and 1:6 compositions. The resulting electrolyte presents about 2M equivalent concentration of the redox couple since the viologen derivative is able to undergo two successive electron transfer. A theoretical cell voltage of 1.35V is possible with this electrolyte. This presentation will discuss the electrochemical and transport properties of this electrolyte system, and their applicability in redox flow batteries as studied by spectro-electrochemical and flow cell experiments.

Deep eutectic solvent (DES), formed by the intermolecular hydrogen bonding between its components of an H–bond acceptor (HBA) and an H–bond donor (HBD), has been extensively studied in the area of material synthesis, catalysis, electrochemistry, etc. The properties of DES including melting point, color, density, viscosity, conductivity, etc. are closely related to the H-bond network. Establishing an understanding of the H-bond structure offers an opportunity to fine-tune these properties. Hence, in this study, we systematically investigated the effects of halide anion type and alkyl chain length on the hydrogen bond network of a series of eutectic solvent systems and attempted to establish an unambiguous H-bond correlation with the conductivity. We varied the halide anion from Cl − to I − (Cl − , Br − , I − ) and the alkyl chain length from methyl to butyl ( n c = 1, 2, 4), and performed FTIR and conductivity measurements. Results show that the OH stretching peak exhibits a blue shift with the halide anion varies in the order of Cl − , Br − , I − when compared with the neat ethylene glycol (Figure 1a), indicating an increase in H-bond strength. A similar trend can be found with the increasing alkyl chain length as well. A stronger H-bond network inhibits the mobility of these halide anions in the eutectic system, as a result, the conductivity will decrease as shown in Figure 1b. Further probing the H-bond network with ultrafast spectroscopy is currently underway. Figure 1. (a) FTIR spectrum of neat EG and TBAX:EG(1:10) sample, X= Cl − , Br − ,I − . (b) Trend between the OH stretching peak shift (vs. neat EG) and conductivity of these investigated samples. Acknowledgememts This work was supported as part of the Breakthrough Electrolytes for Energy Storage (BEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409 Figure 1

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO2) from aluminum oxide (Al2O3) and carbon.1–3 Similar molten salt techniques have been developed for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium is of particular interest recently with significant increases in demand being driven by the increased production of new technologies such as electrified vehicles and magnetic data storage that require neodymium in the form of neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The state of the art procedure for neodymium processing, similar to the aluminum process, utilizes a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd2O3) and carbon to neodymium metal and carbon dioxide.4,6 As an unfortunate byproduct of this process performed in a fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously alongside carbon dioxide from the sacrificial anode. The formation of PFCs combined with the emission of a significant amount of greenhouse gas (carbon dioxide) make the current process for neodymium electrowinning undesirable from an environmental standpoint.7 An alternative molten salt process has been proposed in which the fluoride salts have been replaced with chloride salts consisting of lithium chloride (LiCl) and potassium chloride (KCl). Rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into the LiCl-KCl molten salt and neodymium is electroplated via the below set of reactions.8,9 Cathode: 2NdCl3 + 6e- → 2Nd(solid) + 6Cl- Anode: 6Cl- → 3Cl2 + 6e- Overall: 2NdCl3 → 2Nd(solid) + 3Cl2 This process has several distinct advantages. Utilizing the chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine produced could then be recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride. Our work evaluates anode and cathode behavior during this neodymium chloride molten salt process in order to determine its viability. Overpotential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Additional purification techniques such as vapor distillation procedures are developed to ensure a product that is viable for industrial use. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through a subcontract from the Ames Laboratory with funding from the Department of Energy - Energy Efficiency and Renewable Energy under contract No. DE-AC02-07CH11358; Agreement No. 26110-AMES-CMI. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. T. R. Beck, Electrochem. Soc. Interface, 23, 36–37 (2014). G. G. Botte, Electrochem. Soc. Interface, 23, 49–55 (2014). W. E. Haupin, J. Chem. Educ., 60, 279–282 (1983). M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte. B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol., 48, 9506–9513 (2014). V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576, 10, 576 (2020). H. Vogel, B. Friedrich, H. Vogel, and B. Friedrich, Int. J. Nonferrous Metall., 6, 27–46 (2017). R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022). D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017).

The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxidation states, and can provide both positive and negative redox couples for a flow battery. A flow battery using the PTIO/Ch1EG4 electrolyte demonstrated nearly 50% round trip efficiency with an approximately 1 V open circuit potential. Inefficiencies were primarily due to membrane resistance which can be significantly lowered with increased temperature. While PTIO appears stable over short periods (hours), the oxidized form is not stable in the DES-like electrolyte over longer times. Molecular modeling was performed to investigate the relative stability of PTIO in DES as compared to the previously studied 4-hydroxy-TEMPO (4HT). It was found that the oxoammonium cation 4HT + exhibits a noticeably larger nucleophilic reactive cloud as compared to PTIO + , indicating a higher reactivity. This method to predict stability of the oxoammonium cation shows promise to inform the design and synthesis of promising redox systems based on nitroxide radicals in DES electrolytes to identify new chemistries for large scale energy storage.