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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
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DOE OSTI2022

Solvent selection criteria for temperature-resilient lithium–sulfur batteries

All-climate temperature operation capability and increased energy density have been recognized as two crucial targets, but they are rarely achieved together in rechargeable lithium (Li) batteries. Herein, we demonstrate an electrolyte system by using monodentate dibutyl ether with both low melting and high boiling points as the sole solvent. Its weak solvation endows an aggregate solvation structure and low solubility toward polysulfide species in a relatively low electrolyte concentration (2 mol L −1 ). These features were found to be vital in avoiding dendrite growth and enabling Li metal Coulombic efficiencies of 99.0%, 98.2%, and 98.7% at 23 °C, −40 °C, and 50 °C, respectively. Pouch cells employing thin Li metal (50 μm) and high-loading sulfurized polyacrylonitrile (3.3 mAh cm −2 ) cathodes (negative-to-positive capacity ratio = 2) output 87.5% and 115.9% of their room temperature capacity at −40 °C and 50 °C, respectively. This work provides solvent-based design criteria for a wide temperature range Li-sulfur pouch cells.

25 ENERGY STORAGE↗
DOE OSTI2020

Asymmetric Composition of Ionic Aggregates and the Origin of High Correlated Transference Number in Water-in-Salt Electrolytes

"Water-in-salt" electrolytes open up exciting new avenues for expanding the electrochemical window of aqueous electrolytes. We investigated the solvation structure and dynamics of highly concentrated lithium bis(trifluoromethane)sulfonimide aqueous electrolyte using experimentally corroborated molecular dynamics simulations. The simulations revealed that the heterogeneous structure of the electrolyte comprises percolating networks of ion and water domains/aggregates. Interestingly, the ionic regions are composed of more TFSI - ions than Li + ions. The Li + -ion transport mechanism was further explored. Li + ions can hop along the coordinated TFSI- ions in the ionic aggregates. The calculated correlated transference number of the 20 m electrolyte is similar to 0.32, which is reasonably high for the high concentration due to a weak negative correlation between the motion of cations and anions within the heterogeneous microscopic domains. These molecular dynamics results connect the heterogeneous structure of the electrolyte to the correlated dynamics of the Li + ion and provide a new understanding of the Li + -ion transport mechanism in this novel electrolyte.

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

Synergistic Anion and Solvent-Derived Interphases Enable Lithium-Ion Batteries under Extreme Conditions

Lithium-ion batteries (LIBs) face increasingly stringent demands as their application expands into new areas, including extreme temperatures and fast charging. To meet these demands, the electrolyte should enable fast lithium-ion transport and form stable interphases on electrodes simultaneously. In practice, however, improving one aspect often compromises another. For instance, the trend toward electrolytes forming anion-derived interphases typically reduces transport efficiency due to weak-solvating solvents. Here, we propose that instead of relying on anions to form the interphase, leveraging both solvents and anions to form interphases can potentially lead to a balancing point between robust interphase formation and effective ion transport. Guided by this design principle, 2,2-difluoroethyl ethyl carbonate (DFDEC) was identified as the promising solvent. With the new electrolyte using DFDEC as the major solvent and lithium bis(fluorosulfonyl) imide (LiFSI) as the salt, graphite||LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) full cells are capable of fast charging and demonstrate long-term cycling stability with a cutoff voltage of 4.5 V. Notably, the battery shows a capacity retention of 84.3% after 500 cycles with an average Coulombic efficiency (CE) as high as 99.93%. This new electrolyte also enables stable battery cycling across a wide temperature range (-20 to 60 °C), with excellent capacity retention.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2021

Do Deep Eutectic Solvents Behave Like Ionic Liquid Electrolytes? A Perspective from the Electrode-Electrolyte Interface

The differential capacitance as a function of potential and composition for choline chloride (ChCl) and ethylene glycol (EG) at 1:2,1:4, 1:6, and 1:20 molar ratios were studied using electrochemical impedance spectroscopy (EIS) on glassy carbon. In addition, ChCl:EG at the known ethaline DES composition (1:2) and 1:20 molar ratio were studied on Au and Pt electrodes, and compared to glyceline (ChCl:glycerol, 1:2). The capacitance-potential curves on glassy carbon were best interpreted by the modified Gouy-Chapman model. A dampened U-shape was observed similar to dilute electrolytes, however, the non-negligible ionic and H-bonding interactions in these electrolytes resulted in ambiguity about the point of zero charge where capacitance was weakly dependent on potential. Additionally, the differential capacitance was independent of composition on glassy carbon due to limited availability of mobile free ions and constraints by solvated ion sizes within the interface. Increases in capacitance are observed with Au as a result of desolvation and specific adsorption of Cl, and with Pt upon decreased Cl concentrations. These results indicate DESs and H-bonded electrolytes with high salt concentrations are distinct from ionic liquids and present no pronounced over screening or crowding with applied potential.

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

Influence of Water Sorption on Ionic Conductivity in Polyether Electrolytes at Low Hydration

Ion-containing polymers are subject to a wide range of hydration conditions across electrochemical and water treatment applications. Significant work on dry polymer electrolytes for batteries and highly swollen membranes for water purification has informed our understanding of ion transport under extreme conditions. However, knowledge of intermediate conditions (i.e., low hydration) is essential to emerging applications (e.g., electrolyzers, fuel cells, and lithium extraction). Ion transport under low levels of hydration is distinct from the extreme conditions typically investigated, and the relevant physics cannot be extrapolated from existing knowledge, stifling materials design. In this study, we conducted ion transport measurements in LiTFSI-doped polyethers that were systematically hydrated from dry conditions. A semiautomated apparatus that performs parallel measurements of water uptake and ionic conductivity in thin-film polymers under controlled humidity was developed. For the materials and swelling range considered in this study (i.e., <0.07 g water/g dry polymer electrolyte), ionic conductivity depends nonlinearly on water uptake, with the initial sorbed water weakly affecting conductivity. With additional increases in swelling, more significant increases in conductivity were observed. Remarkably, changes in conductivity induced by water sorption were correlated with the number of water molecules per lithium ion, with the normalized molar conductivity of different samples effectively collapsing onto one another until this unit of hydration exceeded the solvation number of lithium ions under aqueous conditions. Furthermore, these results provide important knowledge regarding the effects of trace water contamination on conductivity measurements in polymer electrolytes and demonstrate that the lithium-ion solvation number marks a key transition point regarding the influence of water on ion transport in ion-containing polymers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A new cyclic carbonate enables high power/ low temperature lithium-ion batteries

The modern lithium-ion battery (LIB) configuration was enabled by the "magic chemistry" between ethylene carbonate (EC) and graphitic carbon anode. Despite the constant changes of cathode chemistries with improved energy densities, EC-graphite combination remained static during the last three decades. While the interphase generated by EC protects the fragile graphitic structure, the intrinsic disadvantages of EC (high viscosity, high melting point, excessive interphase growth) lead to mediocre power density and poor performances of LIB at sub-zero temperatures, where lithium depositions form upon charging. Such performance compromises arise from the fundamental dilemma between requiring effective interphase protection and high impedance from excessive growth of interphase. In this work, we designed and synthesized a "double EC" molecule as electrolyte additive to resolve the above dilemma. Erythritol bis(carbonate) (EBC) possesses lower LUMO energy level than EC and hence tends to decompose prior to EC reduction, but its weak solvation toward Li + restricts the extent of its reduction, thus minimizing the interphase thickness and the corresponding impedances. Electrolytes containing EBC enables both the charging and discharging of ampere-size LIB pouch cells at sub-zero temperatures from 0 to -20°C, demonstrating that the key approach to improve low temperature performances lies in how to tailor interphasial chemistry rather than the bulk electrolyte composition.

25 ENERGY STORAGE↗
DOE OSTI2022

A Carboranyl Electrolyte Enabling Highly Reversible Sodium Metal Anodes via a “Fluorine‐Free” SEI

Abstract Realization of practical sodium metal batteries (SMBs) is hindered due to lack of compatible electrolyte components, dendrite propagation, and poor understanding of anodic interphasial chemistries. Chemically robust liquid electrolytes that facilitate both favorable sodium metal deposition and a stable solid‐electrolyte interphase (SEI) are ideal to enable sodium metal and anode‐free cells. Herein we present advanced characterization of a novel fluorine‐free electrolyte utilizing the [HCB 11 H 11 ] 1− anion. Symmetrical Na cells operated with this electrolyte exhibit a remarkably low overpotential of 0.032 V at a current density of 2.0 mA cm −2 and a high coulombic efficiency of 99.5 % in half‐cell configurations. Surface characterization of electrodes post‐operation reveals the absence of dendritic sodium nucleation and a surprisingly stable fluorine‐free SEI. Furthermore, weak ion‐pairing is identified as key towards the successful development of fluorine‐free sodium electrolytes.

Tomich, Anton W.↗
DOE OSTI2025

Optimizing semi-hydrogenation of unsaturated hydrocarbons by electrolyte engineering approach

Electrochemical hydrogenation of unsaturated hydrocarbons, when powered by renewables, represents a unique opportunity to substitute current energy-intensive synthetic routes. Modulation of adsorption energies of the organic substrate and key intermediates of the reaction is critical for fine tuning of the yield, selectivity and kinetics of the reaction. Interestingly, mounting evidence exists regarding the role of electrolyte composition in the outcome of semi-hydrogenation reactions. Nevertheless, electrolyte optimization is a complex task, owing to its hybrid nature. Indeed, it is composed of water serving as a proton source, an organic solvent necessary to dissolve the organic substrate and a conducting salt. Herein, we demonstrate that varying conducting salt and organic solvent has a dramatic impact on the outcomes of semi-hydrogenation of alkynes. By varying salt and water concentrations, we demonstrate that water does not serve as a proton source, and instead addition of an acid is necessary. While increasing the acid concentration increases the yield of the reaction, at too large concentrations the hydrogen evolution reaction becomes predominant. Furthermore, by combining electrochemical measurements with spectroscopic techniques including Fourier transform infrared (FTIR) spectroscopy and small angle X-ray spectroscopy (SAXS), we demonstrate that the electrolyte solvation structure dramatically impacts the yield of the reaction. Organic solvents weakly interacting with water, including acetonitrile, form aqueous nanoheterogeneities that prevent the organic substrate from accessing the catalyst interface and thus lead to limited yields. Instead, solvents such as dimethylformamide form homogeneous mixtures with which all reactants can access the interface, leading to yields greater than 80% for optimized compositions.

Zhang, Rongyu↗
DOE OSTI2026

Hydration and transport properties of cesium hydroxide and mixed cesium hydroxide–sodium nitrite aqueous solutions

Here, this study explores the hydration and transport properties of aqueous cesium hydroxide (CsOH) solution, with or without 1 molar (M) sodium nitrite (NaNO 2 ). Historic studies of electrolyte solutions indicate that Cs + ions decrease viscosity and increase diffusion rates, whereas OH − ions have the opposite effect. Here, the influence of OH − was dominant in CsOH solutions, leading to increased viscosity and reduced diffusion rates. There was a linear relationship between diffusion coefficients and water activity, emphasizing the significant role of ion–water interactions in determining transport properties. This may be because the interaction between Cs + and the anions is weak even when they are in direct contact with each other. The weakness of the ion-pairing was established through thermodynamic analysis. The findings suggest that ion-pairing is not the only important interaction controlling transport properties when ion-pairing is weak. Nonetheless, ion-pairing or obstructions did result in more sluggish transport properties as electrolyte concentrations increased. Overall, the research enhances the understanding of the complexities underlying ion interactions in multicomponent solutions.

Reynolds, Jacob G. [Hanford Site (HNF), Richland, ↗
DOE OSTI2023

Electrolyte design for Li-ion batteries under extreme operating conditions

The ideal electrolyte for the widely used LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811)||graphite lithium-ion batteries is expected to have the capability of supporting higher voltages (≥4.5 volts), fast charging (≤15 minutes), charging/discharging over a wide temperature range (±60 degrees Celsius) without lithium plating, and non-flammability. No existing electrolyte simultaneously meets all these requirements and electrolyte design is hindered by the absence of an effective guiding principle that addresses the relationships between battery performance, solvation structure and solid-electrolyte-interphase chemistry. Here we report and validate an electrolyte design strategy based on a group of soft solvents that strikes a balance between weak Li+-solvent interactions, sufficient salt dissociation and desired electrochemistry to fulfil all the aforementioned requirements. Remarkably, the 4.5-volt NMC811||graphite coin cells with areal capacities of more than 2.5 milliampere hours per square centimetre retain 75 per cent (54 per cent) of their room-temperature capacity when these cells are charged and discharged at -50 degrees Celsius (-60 degrees Celsius) at a C rate of 0.1C, and the NMC811||graphite pouch cells with lean electrolyte (2.5 grams per ampere hour) achieve stable cycling with an average Coulombic efficiency of more than 99.9 per cent at -30 degrees Celsius. The comprehensive analysis further reveals an impedance matching between the NMC811 cathode and the graphite anode owing to the formation of similar lithium-fluoride-rich interphases, thus effectively avoiding lithium plating at low temperatures. Additionally, this electrolyte design principle can be extended to other alkali-metal-ion batteries operating under extreme conditions.

25 ENERGY STORAGE↗
DOE OSTI2022

A Carboranyl Electrolyte Enabling Highly Reversible Sodium Metal Anodes via a “Fluorine-Free” SEI

Realization of practical sodium metal batteries (SMBs) is hindered due to lack of compatible electrolyte components, dendrite propagation, and poor understanding of anodic interphasial chemistries. Chemically robust liquid electrolytes that facilitate both favorable sodium metal deposition and a stable solid-electrolyte interphase (SEI) are ideal to enable sodium metal and anode-free cells. Herein we present advanced characterization of a novel fluorine-free electrolyte utilizing the [HCB 11 H 11 ] 1- anion. Symmetrical Na cells operated with this electrolyte exhibit a remarkably low overpotential of 0.032 V at a current density of 2.0 mA cm -2 and a high coulombic efficiency of 99.5 % in half-cell configurations. Additionally, surface characterization of electrodes post-operation reveals the absence of dendritic sodium nucleation and a surprisingly stable fluorine-free SEI. Furthermore, weak ion-pairing is identified as key towards the successful development of fluorine-free sodium electrolytes.

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

A quantitative figure of merit for battery SEI films and their use as functional solid-state electrolytes

As a key passivation film that governs battery operation, the solid electrolyte interphase (SEI) has long been credited for enabling high-performance batteries or blamed for their eventual death. However, qualitative descriptions of the SEI often found in the literature (e.g., “conductive,” “passivating”) highlight our incomplete understanding of this layer, where even the most basic properties foundational to SEI function remain difficult to measure. Here, we quantify SEI conductivities and SEI transference numbers using a separator-free Cu|SEI|Li architecture that treats the SEI as a functional solid-state electrolyte (SSE). We find that while any SEI property alone (e.g., electronic conductivity) is weakly correlated (R 2 < 0.67) with battery performance (e.g., Coulombic efficiency), a strong correlation (R 2 > 0.99) can be achieved by defining the “SEI cT number” as a product between the SEI transference number (T) and the ratio of SEI conductivities (c). Analogous to the thermoelectric figure of merit (i.e., zT ), SEI cT quantitatively benchmarks the holistic impact of SEI properties on battery performance and underscores the pitfalls of citing such properties in isolation. Perhaps most strikingly, we demonstrate that Li metal deposition and stripping at room temperature is possible in our separator-free Cu|SEI|Li cell, confirming that the SEI can function precisely as an SSE. Together, these results enrich our understanding of the SEI, not just as a passivation layer but as a functional structure that can potentially have important implications for solid-state batteries.

Science & Technology - Other Topics↗
DOE OSTI2020

Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes

The emergence of magnesium and calcium batteries as potential beyond Li ion energy storage technologies has generated significant interest into the fundamental aspects of alkaline earth metal cation coordination in multivalent electrolytes and the impact of coordination on application-critical electrolyte properties such as solubility, transport, and electrochemical stability. Understanding these details in calcium electrolytes is of immediate importance due to recent, unprecedented demonstrations of reversible calcium metal electrodeposition in a limited number of ethereal solvent-based systems. In this work, we provide insight connecting Ca 2+ coordination tendencies to important calcium battery electrolyte properties. Our results demonstrate a clear solvent:Ca 2+ coordination strength trend across a series of cyclic ether and linear glyme solvents that controls the extent of ion association in solutions of “weakly” coordinating salts. We apply understanding gained from these results to rationalize relative anion:Ca 2+ coordination tendencies and attendant Ca 2+ coordination structures using two oxidatively stable anions of particular interest for current battery electrolytes. Armed with this understanding of solvent and anion interactions with Ca 2+ , we demonstrate and interpret differences in electrochemical calcium deposition behavior across several electrolyte exemplars with varying solvent and anion coordination strengths. Our findings demonstrate that solvents exhibiting especially strong coordination to Ca 2+ , such as triglyme, can inhibit reversible calcium deposition despite effective elimination of anion:Ca 2+ coordination while solvents exhibiting more modest coordination strength, such as 1,2-dimethoxyethane, may enable deposition provided anion:Ca 2+ coordination is substantially limited. These results reveal that the strength of coordination of both anion and solvent should be considered in the design of electrolytes for calcium batteries.

25 ENERGY STORAGE↗
DOE OSTI2025

What is the Role of Relative Humidity on Conductivity in Polymer Electrolytes?

Ion transport properties in polymer electrolytes have been widely studied under rigorously dry and highly water-swollen conditions. However, the transition between these extremes, i.e., the low hydration regime, is still poorly understood at the molecular level and relevant to applications ranging from battery electrolytes to electrophoretic separations. Here, in this study, we apply atomistic molecular dynamics simulations to probe the role of hydration on ion mobilities and conductivities in LiTFSI-doped polyethers at low water content (less than 10% water by volume), which corresponds to 0−80% relative humidity conditions. With increasing water content, Li + ions exhibit two distinct regimes in their mobilities. At low water content, the majority of the Li + ions are weakly hydrated (two or fewer water molecules per Li + ion), maintaining strong interactions with their polymer solvation cage, and therefore exhibit only a weak increase in mobility relative to dry conditions. As water content increases, Li + ions form complete hydration shells, residing within water-rich domains that promote faster mobilities. Further, Li + mobilities increase more significantly in polymer electrolytes composed of more hydrophilic polymers and higher salt concentrations due to the formation of larger water-rich domains. In contrast, TFSI − has comparatively weak interactions with both the polymer and water, exhibiting a monotonic increase in mobility as a function of water content. Overall, these results help clarify the molecular mechanisms underlying ionic mobilities and conductivities in polymer electrolytes at low water content.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Stable, Efficient Iron Electrodeposition via Anion-Directed Control of Fe(II) Coordination

Traditional steelmaking processes consume about 7% of the world’s energy supply, with reduction of iron oxides into iron via blast furnaces representing the most energy-demanding and capital-intensive step. To economize and modularize iron reduction processes, we aim to develop an electrodeposition technique to reduce aqueous iron ions to metallic iron. However, the hydrogen reduction reaction (HER) occurs at a more positive standard reduction potential than the iron reduction reaction. In addition, aqueous Fe(II) cations easily precipitate at mildly acidic conditions (pH ≥ 3), which limits the deposition efficiency and degrades deposit quality. To address these challenges, we first search for anions that have intermediate coordination strength with Fe(II) based on the hard-soft acid-base theory, trading a slightly more negative Fe(II) reduction potential for a considerably broader pH stability range. We select citrate with predicted intermediate coordination strength, in combination with more weakly coordinating anions (e.g., SO 4 2- , Cl - ) to control the coordination structure of Fe 2+ for improved electrolyte stability and electrodeposition behavior. We find that citrate coordination stabilizes Fe 2+ -based electrolytes at higher pH conditions (4.8–5.5), significantly extending their shelf life while also suppressing HER during Fe electrodeposition by orders of magnitude. To measure Faradaic efficiencies (FE), we developed a straightforward, titration-based methodology to quantify the amount of deposited iron regardless of the rate of concurrent HER. Although coordination between citrate and Fe 2+ decreases the reduction potential of Fe(II), high FE (≥98%) was achieved at 10 mA cm -2 . FE and achievable deposition rates are tunable by both concentration and the ratio of Fe 2+ to citrate. In all cases, Raman spectroscopy and X-ray diffraction (XRD) reveal that iron deposition in citrate-containing electrolytes suppresses iron oxide/hydroxide precipitation, in contrast to deposits generated in citrate-free electrolytes. Altogether, this work demonstrates that citrate-mediated anion coordination enables high-purity iron electrodeposition with increased FE, high current density, and improved electrolyte stability. This multi-anion coordination strategy provides a versatile framework for designing stable electrolyte and efficient metal electrodeposition.

coordination↗
DOE OSTI2023

Revealing the Anion–Solvent Interaction for Ultralow Temperature Lithium Metal Batteries

Anion solvation in electrolytes can largely change the electrochemical performance of the electrolytes, yet has been rarely investigated. Herein, three anions of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and derived asymmetric (fluorosulfonyl)(trifluoro-methanesulfonyl)imide (FTFSI) are systematically examined in a weakly Li + cation solvating solvent of bis(3-fluoropropyl)ether (BFPE). In-situ liquid secondary ion mass spectrometry demonstrates that FTFSI - and FSI - anions are associated with BFPE solvent, while weak TFSI - /BFPE cluster signals are detected. Molecular modeling further reveals that the anion–solvent interaction is accompanied by the formation of H-bonding-like interactions. Further, anion solvation enhances the Li + cation transfer number and reduces the organic component in solid electrolyte interphase, which enhances the Li plating/stripping Coulombic efficiency at a low temperature of -30 °C from 42.4% in TFSI-based electrolytes to 98.7% in 1.5 m LiFTFSI and 97.9% in LiFSI-BFPE electrolytes. The anion–solvent interactions, especially asymmetric anion solvation also accelerate the Li + desolvation kinetics. The 1.5 m LiFTFSI-BFPE electrolyte with strong anion–solvent interaction enables LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811)||Li (20 µm) full cell with stable cyclability even under -40 °C, retaining over 92% of initial capacity (115 mAh g -1 , after 100 cycles). The anion–solvent interactions insights allow to rational design the electrolyte for lithium metal batteries and beyond to achieve high performance.

25 ENERGY STORAGE↗
DOE OSTI2020

An Inorganic-Rich Solid Electrolyte Interphase for Advanced Lithium-Metal Batteries in Carbonate Electrolytes

In carbonate electrolytes, the organic-inorganic solid electrolyte interphase (SEI) formed on the lithium (Li) metal anode surface is strongly bonded to Li and experiences the same volume change as Li, thus it undergoes continuous cracking/reformation during plating/stripping cycles. Here we report, an inorganic-rich SEI is designed on a Li metal surface to reduce its bonding energy with Li metal by dissolving 4 M concentrated LiNO 3 in dimethyl sulfoxide (DMSO) as an additive for a fluoroethylene carbonate (FEC) based electrolyte. Due to the aggregate structure of NO 3 - ions and its participation in the primary Li + solvation sheath, abundant Li 2 O, Li 3 N, and LiN x O y grains are formed in the resulting SEI, in addition to the uniform LiF distribution from the reduction of PF 6 - ions. The inorganic-rich SEI’s weak bonding (high interface energy) to Li can effectively promote Li diffusion along the SEI/Li interface and prevent Li dendrite penetration into the SEI. As a result, our designed carbonate electrolyte enables a Li anode to achieve a high Li plating/stripping CE of 99.55% (1 mA cm -2 , 1.0 mAh cm -2 ) and the electrolyte also enables a Li||LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NMC811) full cell (2.5 mAh cm -2 ) to retain 75%of its initial capacity after 200 cycles with an outstanding CE of 99.83%.

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

Non-thermal atmospheric pressure plasma–liquid synthesis of organic acids in aqueous solution from carbon monoxide

This work aims at understanding the conversion of CO to organic acids, namely oxalic acid and formic acid, using non-thermal atmospheric pressure plasma over aqueous solutions. CO exhibited significantly higher conversion to organic acids (more than 15×) compared to CO 2 under the same reaction conditions. The result bolsters a proposed two-step process for CO 2 fixation, whereby CO 2 is first converted to CO, and then CO is converted to organic acids. The organic acids produced from CO are intermediates in the water–gas shift (WGS) reaction of CO in the presence of an aqueous solution to dissolved CO 2 and hydrogen gas. Based on a simple thermodynamic analysis, the organic acid yield was increased by lowering the plasma–liquid reaction temperature using an ice bath to cool the reaction flask. The composition of the organic acids could be varied by changing the pH of the solution. Oxalate was formed in higher concentrations with increasing solution pH above the pK a of the radical species (CO 2 )˙ − . Below the pK a value, formate was the exclusive organic acid formed. The production of formate has a rather weak pH dependence but is enhanced slightly at a basic pH above 10. Furthermore, at basic pH, the effect of electrolyte concentration comes into play. Higher electrolyte concentrations, leading to shorter electrolyte Debye lengths, resulted in lowered organic acid yields. The highest yields of organic acids obtained in our system were 122 mg L −1 for oxalate and 77 mg L −1 for formate at an optimum 1 mM NaOH concentration in the starting solution. This work is a successful pioneering example of CO to organic acids conversion using non-thermal plasmas, which opens the pathway for a promising two-step conversion process of CO 2 to organic acids.

Sudagar, Alcina Johnson [Washington University in ↗