Energy Frontier Research Center Research Articles

Corrosion Behavior of Binary Ni-Cr Alloys in Molten FLiNaK Salts at Different Electrochemical Potentials

The development of molten salt reactors (MSRs) is a collaborative effort among academia, government laboratories, and industry, with the aim of realizing the potential of clean nuclear energy. However, the corrosion of structural alloys presents a significant engineering challenge in this application. Addressing this issue requires investment not only in testing and characterizing candidate alloys, but also in developing a scientific framework to understand the fundamental mechanisms governing the corrosion of metallic materials in molten fluorides which can then guide alloy design [1].In this work, the corrosion behavior of binary Ni-Cr alloys (5-20 wt.% Cr) in molten LiF-NaF-KF (or FLiNaK) salts between 550oC and 750oC was interrogated using electrochemical methods coupled with thermo-kinetic analysis. Electrochemical techniques, including linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry, were utilized to elucidate the rate-limiting kinetic factors and to identify the dependence of corrosion morphology on applied potential. Additionally, Ni-Cr alloys were also immersed 99.9% pure FLiNaK salts with a known oxidizer concentration (e.g., 0.1-5 wt.% EuF3) coupled with open-circuit potential measurement to provide a time-based description of morphological evolution in the case of a dynamically changing electrode potential. These results are corroborated by examination of the post-corrosion morphology using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques as well as salt composition analysis using inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD) and in situ cyclic voltammetry.The systematic choice of potential revealed multiple potential regimes in which Ni-Cr alloys exhibit distinctly different corrosion morphologies. At intermediate potentials, a porous bicontinuous structure is formed by dealloying of the less noble element (Cr) and percolation into the Ni-rich alloy within grains and along grain boundaries. At other potentials, charge transfer control dissolution of both Cr and Ni leads to the formation of a crystallographic faceted structure. The morphological differences suggest the presence of multiple corrosion processes in Ni-Cr in FLiNaK controlled by either transport or interfacial reaction. These findings can be explained through electrochemical processes at the alloy-salt interface, as well as the roles of surface, grain boundary and bulk diffusion kinetics involving the alloying constituents. Furthermore, the effect of radiation damage on Ni20Cr alloys on these rate-limiting corrosion regimes will be investigated. 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).[1] Chan, H.L., et al. Npj Materials Degradation 6(1), 46 (2022).

(General Student Poster Award Winner, 3rd Place) Corrosion Electrochemistry in Molten Flinak Salts

An existing knowledge gap in the field of molten salt (MS) corrosion is the lack of in situ, diagnostical and instantaneous measurements of the underlying corrosion system. Numerous investigations comparing the corrosion behavior of commercial alloys with complex microstructures and compositions in molten fluoride salts have been conducted during the past five years. A few of these have been expanded from exposure studies to those using electrochemical methods[1,2]. In essence, electrochemical techniques offer an in situ and diagnostical way to analyze the corrosion behavior of potential materials in real-time, either in open-circuit (static immersion) or polarized conditions. Investigations on MS corrosion have found few mechanistic insights that elucidate the fundamentals of electrode reactions and characteristics of the electrode-salt interface. However, fate of elements oxidized are not tracked. Nonetheless can be difficult to understand the electrochemical corrosion behavior of structural alloys in non-aqueous, aggressive media, such as molten LiF-NaF-KF (FLiNaK) salts, as they may be influenced by a wide range of parameters (such as microstructure, impurity, and potential).Understanding the mechanism of an alloy's most susceptible alloying constituent's dissolution and describing the rate-determining steps (RDS) of this process using the principles of thermodynamics, kinetics, and electrochemistry are fundamental initial steps in understanding the complexities of alloy corrosion. Chromium (Cr) is frequently referred to in molten fluorides research as an essential yet harmful alloying element in candidate molten salt reactor structural alloys (i.e.Ni-based superalloys and stainless steels) due to its high thermodynamic driving force to dissolve relative to other alloying elements. Because of that Cr is a crucial material for research because it serves as the basis for work on Fe-Cr, Ni-Cr, and Fe-Ni-Cr alloys as well as other alloys. However, published research does not consistently describe the mechanism relating to Cr corrosion in FLiNaK.The objective of this work was to explore the electrochemical corrosion behavior of pure Cr in molten FLiNaK salts at 600oC using electrochemical methods. In this study, we describe and clarify the regimes of Cr corrosion in FLiNaK at 600 oC that are potential-dependent, rate-limiting charge-transfer and mass-transport regulated.A range of measurements based on electrochemistry and material characterization, such as linear, cyclic, and potentiostatic polarization techniques, is presented. A key finding in this work was the attempt to establish a connection between the reactivity of Cr in an aggressive, non-aqueous electrolyte such as molten FLiNaK to electrochemical potentials and corrosion morphology predicted thermodynamically. It was found, that the corrosion mechanism of Cr in FLiNaK was dependent upon mixed potentials established by the anodic Cr/Cr(II) and/or Cr(II) /Cr(III) oxidations coupled with cathodic reactions by multiple oxidizer candidates whose predominance is a function of salt impurity level and Cr exposure times. Results suggest that the early stage of Cr dissolution is regulated by charge-transfer (or activation-controlled) in FLiNaK at 600oC. This type of mechanism can occur at low overpotential (i.e. lower driving force) and can be interrupted by the oversaturation of Cr at the surface. Its presence may be eliminated if the salt film is present. The electrochemical potential in which mass transport regime is rate-limiting falls within the phase stability field where CrF6 3- species dominates, and leading to the formation of a K3CrF6 salt film. The post-exposure SEM analysis revealed that Cr dissolution in this regime involves volume transport within each grain, resulting in the revelation of surface facets containing crystallographic features of cubic crystals.The application of electrochemistry enables a more straightforward way to identify potential RDS that control the rate of Cr corrosion and offers a temporal description of the corrosion mechanism. Such fundamental understanding, especially on the nature of cathodic and anodic reactions of Cr, can provide a baseline to understand the corrosion mechanism of complex alloys. The post-corrosion microstructure investigation, the XRD of the solidified salt combination, and the thermodynamic analysis of the stable phases present in each corrosion regime all corroborated these conclusions. Acknowledgments Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Wang, Y. L., et al. Corrosion Science 109, 43–49 (2016).Chan, H.L., et al. NPJ Materials Degradation 6(1), 46 (2022).

Proton Radiation Effect on Iron Thermal Oxides: Point Defects, Microstructure & Corrosion Electrochemistry

Iron(Fe)-based alloys, such as austenitic stainless steels, form protective surface oxide layers when exposed to oxygen, aqueous electrolyte, and water vapor, which effectively inhibit corrosion [1]. One attribute that underpins the electrochemical reactivity of surface oxide layers is the presence and properties of charged point defects (e.g. vacancies and interstitials charge compensated by electrons or holes), which govern the rate and mechanism of ionic transport when exposed to an oxidizing environment such as an aqueous electrolyte [2,3,4]. However, the effect of radiation on the relationship between point defects, oxide microstructure, and oxidation behavior is poorly understood. Structural materials used in nuclear applications are particularly susceptible to radiation damage, which can degrade the underlying surface oxides. To investigate this phenomenon, thermally oxidized pure Fe was selected as a model material system to study radiation-induced defects in iron oxide scales [3]. Iron oxides were fabricated by thermal oxidation of pure Fe metal in air at 400°C, 600oC, and 800°C for 1 hour. Each oxides layer exhibits an outer hematite (α-Fe2O3) and inner magnetite (Fe3O4)/wüstite (FeO) layers with varying thickness depending on the oxidation conditions. These oxides were then irradiated with 200 keV, 0.03 dpa protons at room temperature. The oxide microstructure and phase compositions were characterized with transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). These results were corroborated with X-Ray diffraction (XRD) and Raman spectroscopy analysis. The relative vacancy concentrations and size distributions in the oxide layers were investigated using positron annihilation spectroscopy (PAS) and positron annihilation lifetime spectroscopy (PALS) techniques. Lastly, these oxides were exposed to a 0.1 M borate buffer solution (pH 9.2) at applied potential where corrosion reactivity was governed by interfacial reactions and ionic defects to regulate the growth processes observed. The potentials were cycled over the ranges where oxides growth and reduction occurred involving Fe/Fe(II)/Fe(III) reactions at a pH which suppressed chemical reactions at the oxide/electrolyte interface and would lead to solubilization of Fe species. The subsequent oxidation, reduction, and semiconductor properties of the oxides were evaluated using AC and DC electrochemical methods, both with and without prior irradiation. Current results demonstrate that radiation damage can manifest itself in the form of vacancy clustering and possible maghemite (γ-Fe2O3) formation. These phenomena were corroborated with donor density and diffusivity values calculated using Mott-Schottky analysis and electrochemical impedance spectroscopy (EIS) analysis and equivalent circuit model (ECM) interpretation, respectively. Radiation damages to the outer rate-regulating α-Fe2O3 layer correlates with enhanced anodic current density corresponding to Fe/Fe(II)/Fe(III) reactions. However, proton radiation may also interact with the metal/oxide and oxide/oxide interfaces depending on the oxide structure and H+ beam penetration depth, leading to either degradation or improvement in the Fe-oxide corrosion responses. Nevertheless, the damage persists and can continue to impact corrosion mechanisms long after the irradiation source is removed. The multiple techniques adopted, each characterizing a material property of a different length scale (from meso-, micro-, to macroscopic) enable us to hypothesize the roles that point defects played after irradiation and during corrosion and how the radiation-induced changes in the defect characteristics influence corrosion electrochemistry. Acknowledgment Research primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer were supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156. The TEM sample preparation and characterization was performed at the Analytical Instrumentation Facility at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. DOE (Contract No. 89233218CNA000001).

(Invited) The Effect of Surfactants on Vanadium Electrochemistry in Porous Carbon Electrodes

We present a study of vanadium electrochemistry in porous electrodes with and without sodium dodecyl sulfate (SDS) additives present. Carbon paper and felt electrodes are compared. Carbon felt electrodes are the de facto standard in flow batteries. There are significant differences in the electronic conductivity of these media. The carbon paper has a much higher conductivity than the felt, which has an electronic conductivity similar to the ionic conductivity of the electrolytes used. The latter fact motivates an extension of available theory for polarization curve fitting to explicitly include both electronic and ionic contributions in the porous electrode. We demonstrate agreement between our model and an existing model evaluated at the limit where electrode conductivity is significantly greater than electrolyte conductivity. Our model uses the product of the electrode surface area per volume ratio and the heterogeneous electron transfer rate constant, and the Damköhler number as fitting parameters for carbon paper and felt electrodes. We discuss the effect of SDS additives on kinetics in the context of the cation hydration shell for the vanadium ions. To this point, we apply Marcus-Hush-Chidsey formalism to our porous electrode model to quantify reorganization energies obtained through polarization curve fitting. This work was supported as part of the Breakthrough Electrolytes for Energy Storage (BEES2), an Energy Frontier Research Center funded by the United States. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.

Electrode Imbibition and Surfactant Interactions with Vanadium Flow Battery Electrolytes

Redox flow batteries (RFB) are a critical technology for stationary grid-scale energy storage, with potential usage with intermittent power sources, such as wind and solar. RFBs are attractive for long-duration energy storage because the anolyte and catholyte are stored separately external to the electrochemical reactor. Separating the electrolytes and the electrochemical cell decouples energy and power densities, unlike batteries that store charge in the solid-state, such as Li-ion, Pb-acid, or Ni-Cd. A standard RFB chemistry is the all-vanadium redox flow battery (VRFB), which utilizes the four oxidation states of vanadium, with V2+, V3+, V4+, and V5+. Recently our group found that adding small amounts of sodium dodecyl sulfate (SDS) to a VFRB increased the kinetics of the V2+ to V3+oxidation.Furthermore, surfactants play essential roles in battery manufacturing and operation. Surfactants can change metal oxide crystal structure, inhibit dendrite growth, limit corrosion, and prevent nanoparticle agglomeration. Surfactants are never wholly removed during manufacturing and can be present during battery operation. Therefore, it is crucial to understand how surfactants impact battery performance and their effect on the materials.This talk describes SDS's equilibrium surface properties and adsorption characteristics at the liquid−air and liquid-solid interfaces, thus providing insights into the interactions of SDS with a VFRB electrolyte. Experiments focusing on the effects of sulfuric acid and vanadium ions on SDS's equilibrium surface tension and critical micelle concentration (CMC) were completed. The Szyszkowski equation is used to calculate SDS's adsorption parameters, such as interfacial surface excess, Γsz [mol/m2], the surface excess at the saturated interface, Γ∞ [mol/m2], the minimum molecular area in the adsorption layer at the saturated interface, Amin [nm2], and the Gibbs free energy of adsorption, ΔGads [kJ/mol]. Furthermore, an examination of the spontaneous imbibition of the VFRB electrolyte with SDS into a porous carbon electrode is presented. The microstructure of porous systems, interfacial tension, and fluid−solid interactions controls the capillary-driven flow and is vital in multiphase flow in porous media. Therefore, understanding SDS's effects on the VFRB electrolyte in an electrode is critical to understanding the electrochemical reactions and molecular interactions occurring.This work was supported as part of the Breakthrough Electrolytes for Energy Storage and Systems (BEES2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.

Effect of Anionic, Cationic, and Non-Ionic Surfactants on the Structural Stability and Morphology of Perfluorinated Sulfonic Acid Membranes

Redox flow batteries (RFB) are a critical technology for stationary grid-scale energy storage, with potential usage with intermittent power sources, such as wind and solar. RFBs are attractive for long-duration energy storage because the anolyte and catholyte are stored separately external to the electrochemical reactor, which allows the RFBs to be modular and stationary. Separating the electrolytes and the electrochemical cell decouples energy and power densities, unlike batteries that store charge in the solid-state, such as Li-ion, Pb-acid, or Ni-Cd. A standard RFB chemistry is the all-vanadium redox flow battery (VRFB), which utilizes the four oxidation states of vanadium, with V2+, V3+, V4+, and V5+. Recently, new electrolytes are being studied in energy storage devices. Such as the first redox flow battery to utilize microemulsions as the electrolyte.Microemulsions are a breakthrough electrolyte for energy storage (BEES) that is emerging as an alternative to traditional RFB electrolytes because they produce thermodynamically stable, homogeneous solutions of polar and non-polar molecules. The stabilization of oil and water solutions allows microemulsion electrolytes to accommodate organic redox species with fast electron transfer rates that are not soluble in the aqueous phase while still offering the high conductivity of an aqueous salt electrolyte. Microemulsions consist of two immiscible phases stabilized by a surfactant, a non-polar "oil" phase, solubilizing the redox material, and the polar phase, which typically consists of an aqueous brine that provides charge balance and conductivity. Additionally, if the redox molecule was a non-polar liquid, it could serve as the entire oil phase, maximizing the energy density. An attractive property of a microemulsion electrolyte is that each component can be environmentally and biologically safe. For example, the aqueous phase only contains salt and water, and surfactants are found in many household, personal, and food products. Furthermore, some redox materials investigated in our lab are vitamins such as menadione (vitamin K3) and tocopherol (vitamin E).This talk describes the structural, mechanical, and morphological studies of surfactants' effects on perfluorinated sulfonic acid membranes. Small-angle X-ray scattering monitored the morphological changes as a function of surfactant concentration and soaking time, revealing how the surfactant penetrated and altered the polymer membrane. FTIR and NMR spectroscopy are also employed, providing insight into any polymer membrane degradation or dissolution. We demonstrate that surfactant choice is not only crucial to the stability of the microemulsion used as an electrolyte but can ultimately destroy the separator in a redox flow battery.This work was supported as part of the Breakthrough Electrolytes for Energy Storage and Systems (BEES2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.

Revisiting Open Circuit Corrosion of Cr in Unpurified Molten FLiNaK Salts with a Multi-Modal Approach

Corrosion and mitigation of environmental degradation of structural materials in high-temperature Gen IV molten salt reactors, are critical issues. However, any corrosion studies have historically been forensic post-test examinations and materials development has occurred mainly by trial and error until recently. Meanwhile, a fundamental understanding can only be achieved by recognizing how the corrosion system of model materials in molten fluoride salts evolves as a function of time. A number of in-situ spectroscopy techniques to identify corrosion product compositions in molten salts have been suggested in recent years. Raman, high-temperature UV-Vis absorption spectroscopy, and X-ray absorption spectroscopy are a few of them [1,2]. These techniques' major limitations include the fact that they are mainly qualitative, and complex hardware requiring a beam focusing inside the glove box is needed. At the same time, electrochemical techniques have been presented which can provide the information on behavior of both metal and its corrosion product in the continuously evolving corrosion system in molten salts. However, use of single isolated methods renders it difficult to obtain mechanistic information on instantaneous, time-dependent processes and phenomena during corrosion. These are needs gaps opportunities. In this work we present a detailed electrochemical analysis of the corrosion of polycrystalline chromium at 600°C in FLiNaK over 50 h including an electrochemical analysis of in-situ near-instantaneous corrosion rates corroborated with other methods.A variety of corroborating methods including (1) in-situ electrochemical diagnostic techniques, (2) static immersion exposure with gravimetric and SEM analysis, (3) electrochemical impedance spectroscopy, (4) XRD, (5) ICP-OES, was utilized to study the corrosion behavior in unpurified FLiNaK at 600°C. The focus was to evaluate the viability of using an external non-reactive electrode to measure the concentration of corrosion products via redox process analysis in order to determine the corrosion rate during the process of OCP corrosion of Cr in FLiNaK. This work reports the possibility of using of electrochemical techniques such as CV for the instantaneous determination of Cr corrosion rates when Cr is exposed to a 600oC FLiNaK salts for 50 h. It was found that the concentration of dissolved corrosion products, in the more likely possible forms of CrF3 - and CrF6 3- , might be calculated by established analytical approaches to CV analysis such as Berzins-Delahay (soluble/insoluble) and Randles-Ševcik equations (soluble/soluble). The calculated values showed the definite concentration of Cr(II)/Cr(III) corrosion products at the distance of the external electrode (due to the static corrosion conditions). These finding supported by XRD and ICP-OES. A semi-infinite diffusion model was employed in this study to adjust the calculated concentrations assuming a stagnant solution and diffusion based one-dimensional concentration diffusion profile in order to calculated the Cr(III) concentration at a fixed position in the Cr(III) concentration gradient. It was shown that the semi-infinite diffusion model helps correctly describe this profile between 10 and 50 h, based on the geometry of the working electrode and corrosion cell. Based on these results the OCP corrosion rate was interpreted to switch over 50 h from initially charge transfer control Cr(II) oxidation at early times with likely HF and Cr(III) reduction, followed by mass transport control regulated by a salt film and then possible stifling due to depletion of HF and Cr(III) solubility limits.The results of half-cell redox potentials and peak currents pertaining to oxidation/reduction of Cr corrosion products were correlated to the concentrations of particular Cr species and compared with the results of mass loss experiment and ex-situ ICP-OES of post-exposed FLiNaK. The mass loss of the Cr(II)/Cr(III) metal that was approximated by spatially integrating the concentration profiles at a selected time, and the result of Cr mass loss separately obtained from gravimetric measurements showed a reasonable agreement. Furthermore, the rate of accumulation of Cr ions in the residual FLiNaK salt after 50 h of exposure determined by ICP-OES, correlated with the electrochemical CV and EIS analysis. In addition to this, the oxidation and reduction peak potentials pertaining to Cr/CrF3 - and CrF3 -/CrF6 3- redox reactions (measured on Pt wire) were compared with the OCP (measured on Cr) to provide insights on the predominant anodic and cathodic reactions. Acknowledgments This research was supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156. References Y. Liu et al., (2020) Corrosion Science. 169 108636. https://doi.org/10.1016/j.corsci.2020.108636.S. Fayfar, et al., (2022). https://doi.org/10.26434/chemrxiv-2022-3nspm.

(Invited) Understanding Fluid Structure and Properties of Eutectic and Microemulsion Electrolytes for Energy Storage

The increase of adding intermittent renewable energy sources like solar and wind to the electricity grid requires a commensurate need for large scale energy storage. Flow batteries are the most likely storage technology to meet this need because of its ability to decouple power load from energy storage capacity, and its promise for longer-term storage capacity. Flow battery electrolytes are mostly based on aqueous systems with limited voltage window for energy storage redox couples, with some systems based on organic solvents that are volatile and often toxic. The BEES (Breakthrough Electrolytes for Energy Storage) EFRC (a DOE Emerging Frontier Research Center), in its first four years (2018 to 2022), studied two classes of electrolytes, more specifically reported here is work on deep eutectic solvents (DESs) and microemul sions (µEs). While each class of liquid presents a unique aspect that can be leveraged for energy storage, collectively they represent novel electrolytes that are structured at the molecular (DESs) to meso-scale (µEs) level. The motivation to explore DES for electrolytes is the potential for high energy density, safety, and sustainability.1 The motivation to explore µEs for electrolytes is the ability to decouple power and energy density through composition.2 A fundamental understanding of the physical, transport, and electrochemical properties of model systems representative of each class of electrolytes in relation to their bulk and interfacial structure was achieved through collaborative experimental and theoretical studies within BEES. This presentation will describe our motivation and approach, and give examples of a few research highlight from BEES. Detailed results of BEES in developing standards and identifying the key structure-property relations and mechanisms, as well as intrinsic limitations and prospects are reported in BEES affiliated peer-reviewed publications.3 Acknowledgment: This work was funded by 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. 1 Ghahremani, Savinell, Gurkan, J Electrochem Soc, 2022, 169, 3 2 Barth, Imel, Nelms, Goenaga, Zawodzinski, Front Chem, 2022, 10, 831200 3 In Google Scholar search the DOE award number: DE-SC0019409

Ion Transport in Polymer/Inorganic Composite Electrolytes – a Comparison between Broadband Dielectric Spectroscopy and Impedance Spectroscopy

Significant efforts have been made to develop composite electrolytes combining polymer matrix with Li-ion conducting inorganic solids for feasible construction of solid-state batteries. However, the Li-ion transfer kinetics at the interface between polymer/inorganic electrolyte is unfavorable for Li-ion conduction in the composite electrolyte because of the interfacial resistance and the activation energy barrier for interfacial Li-ion transfer. The activation energy barrier for the Li-ion transfer reaction at the interface is correlated with the interaction between Li-ion and polymer matrix in the polymer electrolyte, interaction between Li-ion and anions in inorganic electrolytes and the interaction between polymer and inorganic electrolytes, therefore, the electrolyte composition significantly affects the interfacial charge transfer kinetics in composites.In this work, Li-ion transport properties in the composite electrolytes are investigated by using broadband dielectric spectroscopy (BDS) and impedance spectroscopy (IS). We particularly focus on Li-ion charge transfer at the polymer/inorganic interface from the dual ion conducting polymer to the single ion conducting polymer electrolytes with different Li-ion conducting salts such as LiN(SO2CF3)2 (LiTFSI) and (LiMTFSI) in vinyl ethylene carbonate (VEC) or polymerized vinyl ethylene carbonate (PVEC). The interfacial resistances of inorganic electrolytes Li0.34La0.56TiO3 (LLTO) in polymer electrolytes are elucidated. To further evaluate the polymer/inorganic electrolyte interfaces, various types of cell are assembled in a glove box filled with argon gas and characterized by a series of techniques such as BDS and IS. The Li-ion transfer kinetics at the interface significantly affects the Li-ion flux through the composite electrolytes and will be presented. In addition, we will present a systematic comparison between BDS and IS results in order to build connections between the two communities. Acknowledgements This work was supported as part of the Fast and Cooperative Ion Transport in Polymer-Based Materials (FaCT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences at Oak Ridge National Laboratory under contract DE-AC05-00OR22725.

Investigation of Surface Adsorption and Electrode Passivation during Electrochemical Oxidation of Redox-Active Organics

Redox-active organic molecules (ROMs) have gained interest as electroactive materials for redox flow batteries (RFBs). Electrochemical properties of ROMs, i.e., their solubility and redox potential, can be tuned through molecular engineering of the base molecule. Previously, we reported values of the apparent anodic charge transfer coefficient α (values approaching 1) associated with the electrochemical oxidation of a model ROM, 4-hydroxy-TEMPO.1 Also, we have proposed an adsorption-mediated mechanism to explain this unusual value of α, and have presented evidence to support the occurrence of adsorbed intermediates. Here, we extend this investigation to gain further insights using techniques of slow-scan voltammetry and electrode cycling, and apply them to other ROMs. We will discuss results of spectroscopic and micro-gravimetric studies, which provide further insights into the physicochemical properties of the adsorbed surface-passivating films. Ramifications of these to implementation of ROMs in flow batteries will be discussed too.[1] N. A. Shaheen, M. Ijjada, M. B. Vukmirovic, R. Akolkar. J. Electrochem. Soc., 167 (2020) 143505. Acknowledgements: Investigation of the model ROM was supported by 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. Work completed at Argonne was supported by the Joint Center for Energy Storage Research (JCESR), a U.S. Department of Energy, Energy Innovation Hub, and by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract #DE‐SC0014664. All opinions expressed in this paper are the author’s and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

Revealing 3D Morphological Evolution and Reaction Kinetics of Metals and Alloys in Molten Salts Via Synchrotron X-Ray Nano-Tomography and Multimodal Studies

The use of molten salts for large-scale solar concentrated power plants and molten salt reactors has been driving the research to better understand how metals and alloys interact with the molten salt. As the metals may undergo morphological, chemical, and structural change in molten salt environments, it is critical to understand the fundamental mechanisms in these changes. In this work, we will present how we utilized synchrotron X-ray nano-tomography to better understand the 3D morphological evolution of Ni, Cr, and their alloys in molten salt.The effects of temperature and additives in the salt on the morphological evolution will be discussed. At the higher temperature, a characteristic bicontinuous structure can form from molten salt dealloying a binary alloy. [1] This contrasts to the intergranular corrosion found in the same system reacted at a lower temperature. [2] Different additives in the salt were also found to alter the morphological changes of the alloys and can create planar corrosion, percolation dealloying, or redeposition.To complement the morphological studies by X-ray nano-tomography, a suite of X-ray and electron microscopy analyses were also carried out to better understand the chemical and structural (both short-and long-range ordering) evolution. Taking it as a multimodal approach, we will discuss how we couple the analysis from synchrotron operando X-ray absorption spectroscopy, diffraction, and imaging, as well as the multiscale imaging studies from both X-ray and electron microscopy. This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy, Office of Science.

Reactivity of Metal Ions with Excess Electrons in Molten MgCl2-KCl Mixtures

Molten salts are proposed as liquid fuels or coolants in a new fleet of molten salt nuclear reactors that would have operational and safety advantages over present reactor systems. Under those conditions, the salt will be exposed to high radiation levels, and understanding the chemical effects of radiolysis on the molten salt fuel or coolant is essential to reliable, efficient and sustainable reactor operation. Building this understanding begins with identifying primary salt radiolysis products and characterizing their reactivities, for which we perform high-temperature pulse radiolysis transient absorption spectroscopy at the BNL Laser-Electron Accelerator Facility. Salt mixtures containing monovalent and divalent cations are of particular interest because of their tunable Lewis acidity-basicity that can be used to control the solubility of dissolved metal ions in the reactor. Changing the MgCl2:KCl mixing ratio alters the absorption spectra of radiolytically-produced excess electrons, with increasing blue shifts related to the probable number of Mg2+ ions close to the cavity electron. These strong blue shifts imply significant changes in the electron’s energetics and reactivity that we quantify by measuring reaction kinetics with metal ion electron acceptors. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Electrochemical Corrosion Study on Nickel and Chromium with Different Surface Orientations in Molten MgCl2-KCl

In the present work, the electrochemical corrosion tests were performed on single-crystal nickel and chromium with different surface orientations having (100), (110), and (111) facets, in a MgCl2-KCl eutectic molten salt at 500oC under an argon atmosphere. The corrosion rate of Ni is lower than that of Cr, indicating that Ni is electrochemically more stable than Cr in the molten salt. The anodic corrosion rates of different single crystals increase in the order of (111) < (100) < (110). This observation shows that the degree of electrochemical stability is related to the coordination of surface atoms and lower density surfaces yield higher corrosion rates. In addition, the corrosion rate of a polycrystalline sample is similar to that of the single-crystal (110) sample, which suggests that the corrosion rate of the polycrystalline surface is dominated by the orientation with the lowest surface density.This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science. Keywords: Tafel, Nickel, Chromium, MgCl2-KCl, Single crystal.

Exposure Aging of Passive Films on Multi-Principal Element Alloys and Ramifications Towards Local Corrosion Resistance

Both highly engineered legacy and emerging alloys lack information on the precise attributes of protective passive films especially after long-time exposures when it really matters. Relevant field exposure periods are nearly infinite in time but both accelerated testing and high-fidelity experiments are often very short term in duration. Recent studies have explored passivation and protection of a number of different MPEAs connecting oxide chemistry to corrosion electrochemistry but most reports are after reality brief exposure periods.The goals of this work are to (a) review the nature of passive films (i.e., solid solution oxides, phase separated single element oxides, and/or complex oxides) and (b) determine the effect of exposure aging on the evolution of oxide features such as composition, structure, phase separation and physical attributes such as thickness in MPEAs; and to correlate these features with both protectiveness in the passive range and local corrosion resistance. Furthermore, we aim to understand what factors limit beneficial alloying element enrichment in the oxide, and further insights on potential third element effects.The focus of the current work extends from the solid solution Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W family to a variety of emerging MPEAs containing “d-block” elements. AC and DC electrochemistry during potential sweeps and potential step passivation studies were conducted in 0.1 M NaCl pH 4 (HCl) as well as other solutions in the passive range. Exposure and high-fidelity characterizations were investigated over 10 s to 10 days at select potentials.Improvements in passive films protectiveness were correlated with the enrichment of certain elements and depletion of others. Mn oxides were found to be detrimental when formed over the long term in the outer layer of oxides on MPEAs. The most corrosion-resistant alloys possess passivating films that are efficacious not only just after formation, but that self-heal as well as improve over long exposure times as evident from impedance spectroscopy and metastable pitting resistance. Such alloys were observed to be the most resistant when interrogated for local corrosion compared to inferior alloys whose impedance response degraded over time as the passive film evolved. The latter alloys eventually arrived at conditions susceptible to local corrosion. Time-potential-transformation diagrams based on long-term exposures in Cl- followed by high fidelity characterization is suggested to be an approach to improve scientific understanding of long-term passive-protection features relevant to chloride exposures.This work was supported as part of the Center of Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584; the Office of Naval Research under MURI ONR N00014-16-1-2280 and N00014-19-1-2420.

(Invited) Effects of Halide Anion Type and Alkyl Chain Length on Hydrogen Bonding in Eutectic Solvent System

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.AcknowledgememtsThis 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

Repassivation Temperature of One-Dimensional Artificial Pits in Stainless Steel

A new framework has been developed in recent works to describe pit growth stability 1-8, based on which many critical scientific issues in localized corrosion were addressed, such as the rate-controlling step in pit growth, the salt film growth and self-regulating mechanism, and the kinetics and the activation energy of metal dissolution in the local pit environment. Although many new insights were generated, there are still some phenomena that are not well understood, such as the pit repassivation temperature (T rp). According to the new framework, T rp is predicted to decrease with the increase of applied potential (E app). However, T rp sometimes exhibits an unexpected dependence on E app that contradicts with the prediction, and the underlying mechanism is not clear.In this study, T rp of one-dimensional artificial pits made from 316L stainless steel was investigated in 3.5 wt.% NaCl by downward temperature scanning. For a given pit depth, T rp exhibits an unexpected “S” dependence on applied potential, which has never been reported previously. Further analysis shows that this relationship is derived from repassivation underneath the salt film, which is different from repassivation that occurs when the pit surface concentration decreases below a critical value. This indicates that there is a reversal of the effect of the salt film on pit growth stability. The systematic study shows that such a reversal depends on temperature, E app, and pit depth. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.References G. S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164 (4), C180-C181 (2017).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (9), C484-C491 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (11), C762-C770 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (6), C115-C124 (2019).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (11), C3341-C3354 (2019).T. Li, J. Wu, and G. S. Frankel, Corrosion Science, 182 109277 (2021).T. Li, J. Wu, X. Guo, A. M. Panindre, and G. S. Frankel, Corrosion Science, 193 109901 (2021).T. Li, D. E. Perea, D. K. Schreiber, M. G. Wirth, G. J. Orren, and G. S. Frankel, Corrosion Science, 174 108812 (2020).

Probing Local Ion Insertion through Strain-Current Correlation

Among different electrochemical energy storage devices, the ones that store a higher amount of energy (e.g., batteries) often undergo larger ion insertion-induced structural transformation and volume change, which results in lower rate performance. The ones that store a lower amount of energy (e.g., supercapacitors) usually show gradual structural change and smaller volume change. There is a strong correlation between electrochemistry and mechanical properties. Understanding the electro-chemo-mechanical coupling and controlling the ion insertion-induced strain is crucial for achieving high power and high energy storage devices. In-situ atomic force microscopy (AFM) is well suited to tackle this task as it allows tracking local volume changes and other mechanical responses sub-nanometer spatial resolution under the conditions close to the device operationIn this work, we monitored electrode volume change via in-situ AFM and demonstrated the electro-chemo-mechanical coupling behaviors during proton insertion into WO3 materials. The concept of mechanical cyclic voltammetry (mCV) curves was developed, and the relationship between electrochemical current and strain was investigated with simplified models. The results revealed multiple ion-intercalation processes with different mechanical responses are involved during electrode cycling. Local heterogeneity was investigated via mCV mapping, confirming that the charging mechanisms varied across the electrode. These local variations could be further correlated to local morphology, crystal orientations, or chemical compositions. We further demonstrate that the mCV approach is applicable to a variety of energy storage materials (e.g., birnessite and MXene) with the increasing complexity of current-deformation relationships.The work was supported by the Fluid Interface Reactions, Structures, and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

Bioinspired Supercharging of Photoredox Catalysis for Applications in Energy and Chemical Manufacturing.

For more than a decade, photoredox catalysis has been demonstrating that when photoactive catalysts are irradiated with visible light, reactions occur under milder, cheaper, and environmentally friendlier conditions. Furthermore, this methodology allows for the activation of abundant chemicals into valuable products through novel mechanisms that are otherwise inaccessible. The photoredox approach, however, has been primarily used for pharmaceutical applications, where its implementation has been highly effective, but typically with a more rudimentary understanding of the mechanisms involved in these transformations. From a global perspective, the manufacture of everyday chemicals by the chemical industry as a whole currently accounts for 10% of total global energy consumption and generates 7% of the world's greenhouse gases annually. In this context, the Bio-Inspired Light-Escalated Chemistry (BioLEC) Energy Frontier Research Center (EFRC) was founded to supercharge the photoredox approach for applications in chemical manufacturing aimed at reducing its energy consumption and emissions burden, by using bioinspired schemes to harvest multiple electrons to drive endothermically uphill chemical reactions. The Center comprises a diverse group of researchers with expertise that includes synthetic chemistry, biophysics, physical chemistry, and engineering. The team works together to gain a deeper understanding of the mechanistic details of photoredox reactions while amplifying the applications of these light-driven methodologies.In this Account, we review some of the major advances in understanding, approach, and applicability made possible by this collaborative Center. Combining sophisticated spectroscopic tools and photophysics tactics with enhanced photoredox reactions has led to the development of novel techniques and reactivities that greatly expand the field and its capabilities. The Account is intended to highlight how the interplay between disciplines can have a major impact and facilitate the advance of the field. For example, techniques such as time-resolved dielectric loss (TRDL) and pulse radiolysis are providing mechanistic insights not previously available. Hypothesis-driven photocatalyst design thus led to broadening of the scope of several existing transformations. Moreover, bioconjugation approaches and the implementation of triplet-triplet annihilation mechanisms created new avenues for the exploration of reactivities. Lastly, our multidisciplinary approach to tackling real-world problems has inspired the development of efficient methods for the depolymerization of lignin and artificial polymers.

Probing Local Ion Insertion via Electro-Chemo-Mechanical Coupling Responses

Electrochemical energy storage devices (e.g., Li-ion batteries and supercapacitors) store energy through electrolytic ion insertion in the electrode. During the ion insertion process, the electrode needs to open up or reorganize its crystallographic structure in order to host the ions, which results in macroscopic and microscopic deformation. The amount of energy barrier required for electrode structural change during the ion insertion process dictates the energy and power density and cycle life of the device. Therefore, understanding the electrode deformation and changes in mechanical responses during electrochemical events and their impact on devices’ performance is crucial for achieving high power and high energy storage devices. In-situ atomic force microscopy (AFM) is well suited to tackle this task as it allows tracking local volume changes and other mechanical responses sub-nanometer spatial resolution under the conditions close to the device operationIn this work, we demonstrate the electro-chemo-mechanical coupling behaviors of proton insertion into WO3 materials via in-situ AFM. The concept of mechanical cyclic voltammetry (mCV) curves was developed, and the relationship between electrochemical current and strain were investigated with simplified models. The results revealed multiple ion-intercalation process with different mechanical responses are involved during electrode cycling. Local heterogeneity was investigated via mCV mapping, confirming that the charging mechanisms varied across the electrode. These local variations could be further corelated to local morphology, crystal orientations or chemical compositions. Besides providing information of redox heterogeneity, the mechanical CV methodology allows us to circumvent the common issues that are often encountered in the global electrochemical characterizations (ex: cell resistance and unwanted parasitic reaction). We further demonstrate that the mCV approach is applicable to a variety of energy storage materials with increasing complexity of current-deformation relationships.The work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

How has external knowledge contributed to lithium-ion batteries for the energy transition?

SummaryInnovation in clean-energy technologies is central toward a net-zero energy system. One key determinant of technological innovation is the integration of external knowledge, i.e., knowledge spillovers. However, extant work does not explain how individual spillovers come about: the mechanisms and enablers of these spillovers. We ask how knowledge from other technologies, sectors, or scientific disciplines is integrated into the innovation process in an important technology for a net-zero future: lithium-ion batteries (LIBs), based on a qualitative case study using extant literature and an elite interview campaign with key inventors in the LIB field and R&D/industry experts. We identify the breakthrough innovations in LIBs, discuss the extent to which breakthrough innovations—plus a few others—have resulted from spillovers, and identify different mechanisms and enablers underlying these spillovers, which can be leveraged by policymakers and R&D managers who are interested in facilitating spillovers in LIBs and other clean-energy technologies.