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Breakthrough Electrolytes for Energy Storage Energy Frontier Research Center
Show moreSilicon is one of the most earth abundant elements, and thus, the fate and reactivity of silicate materials are often important for various energy and environmental technologies including carbon sequestration, where CO2 is captured and stored as a thermodynamically stable solid carbonate phase. Thus, understanding the structures and chemistries of different silicate phases has become an important research aim. In this study, the changes in the silicate structures (Q0–Q4) of heat-treated Mg-bearing mineral (serpentine) exposed to a CO2-water system (carbonic acid) was investigated using 29Si MAS NMR, XRPD and ICP-OES and the identified structures were employed to explain complex leaching behaviors of silicate materials. The 29Si MAS NMR and XRPD analysis indicated that the heat-treated serpentine is a mixture of amorphous (Q1: dehydroxylate I, Q2: enstatite, Q4: silica) and crystalline (Q0: forsterite, Q3: dehydroxylate II and serpentine) phase, while natural serpentine mineral has single crystalline Q3 silicate structure. The leaching experiments showed that both Mg and Si in the amorphous silicate structures (Q1: dehydroxylate I, Q2: enstatite) are more soluble than those in crystalline phase (Q0: forsterite, Q3: dehydroxylate II and serpentine). Therefore, tuning the silicate structure towards Q1 and Q2 would significantly improve carbon sequestration potential of silicate minerals, whereas silicate materials with Q3 structure would provide great chemical stabilities in acidic conditions. The solubilities of silicate structures were in the order of Q1 (dehydroxylate I) > Q2 (enstatite) ≫ Q0(forsterite) > Q3 (dehydroxylate II) > Q3 (serpentine) and this finding can be used to better design a wide range of energy and environmental materials and reaction systems.
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Show moreDeep eutectic solvents (DES), on account of their low-cost, non-flammability and electrochemical stability, are attracting attention for their potential use in applications such as electrodeposition and energy storage. In these applications, knowledge of the transport and electrochemical kinetics properties of DES is critically important. To date, attempts to measure the kinetics parameters of the Cu2+ + e Cu1+ reaction in ethaline DES have yielded a cathodic charge transfer coefficient (α) in the range of 0.2–0.3 suggesting an unexpected asymmetric polarization behavior. In the present work, we pursued a comprehensive study of the kinetics and transport properties of the aforementioned reaction. Using steady-state and transient polarization measurements on RDE and microelectrodes combined with diffusion-reaction modeling, we demonstrate that the Cu2+/Cu1+ transition exhibits a charge transfer coefficient in the range of 0.49–0.54 and an exchange current density in the range 1.72–1.88 mA/cm2. Recommendations are provided for avoiding pitfalls in kinetics analysis of highly resistive DES electrolytes. These recommendations include the application of accurate correction and the use of electrode configurations where the secondary current distribution is relatively uniform.
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Show moreWe report on the effect of the substrate on electrochemical deposition of Cu from deep eutectic solvent ethaline. We investigated the polarization behavior during electrodeposition of Cu on Pt and glassy carbon (GC) from both Cu2+ and Cu+ containing ethaline using cyclic voltammetry (CV). Formation of bulk Cu deposits on both substrates underwent nucleation and growth processes; however, the nucleation was considerably sluggish on GC compared to Pt. While experiments in Cu+ solutions indicated that coalescence of Cu islands on Pt is a slow process and that its surface may not be fully covered by Cu, such determination of Cu coverage could not be made on GC. Cu dissolution is also slower from GC than from Pt. It was observed that CV of Cu deposition on GC is influenced by the surface preparation method. Since ethaline has high chloride concentration, a parallel study in aqueous 3 M NaCl solution was conducted in order to examine the influence of the chloride medium on the electrodeposition process. This revealed that electrodeposition in both media occurred in the same manner but with different charge and mass transfer rates caused by the differences in viscosity and chloride concentrations of the two solutions.
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Show moreThe influence of ionic associations and potential-dependent interactions on the electrode–electrolyte interfacial structure of ionic liquids (ILs) is studied by electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) for a variety of asymmetric quaternary ammonium ILs. Specifically, the impact of cation alkyl chain length (C = 4, 8 and 16) and ether functionality on the interfacial structuring of ILs at the glassy carbon electrode surface is examined. Ammonium cations with alkyl chain length of 8 and 16 carbons are found to stabilize the formation of the bis(trifluorosulfonyl)imide, [TFSI], anion dense Stern layer at positive electrode potentials leading to larger capacitances. The longer alkyl chain of the cation is believed to screen the ion–ion repulsion among the anions by intruding into the interfacial anion layer. SERS suggests the presence of carbon-containing rings at the interface at both positive and negative electrode potentials, which can be explained by the buckling of the long alkyl chains. Inclusion of an ether functionality allowed for more symmetry in the camel-shaped potential-dependent differential capacitance curves, suggesting similar excess ion density at both positive and negative potentials. This work contributes to understanding and predicting the interfacial electrode capacitance in ILs by understanding the balance of ionic interactions and the associated repulsions at electrode–electrolyte interfaces that are pertinent to electrochemical energy storage, electrocatalysis, and electrochemical sensors.
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Show moreWe report an electrochemical study of bromine reduction to tribromide in a single nitrobenzene (NB) droplet during a single collision event. The feasibility of this study is based on the favorable distribution of Br2 in NB rather than in water, which was demonstrated by voltammetry at the NB/water interface. The NB-in-water emulsions containing the ionic liquids trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide (ILPA) as the supporting electrolyte and sodium dodecyl sulfate (SDS) as the surfactant were prepared by a high-power ultrasonication. Its single droplet as an attoliter reactor, colliding on a 25 μm Pt UME, was monitored by chronoamperometry. The spike-type current transients of Br2 reduction can elucidate the size and size distribution of NB, and measure the diffusion in the droplet as analyzed by a bulk electrolysis model. The frequency of collisions can provide diffusion coefficient of the droplets to the UME. Moreover, the i-t decay behavior can be simulated, which fits well with the experimental one.
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Show moreThe Ag/AgCl reference electrode is commonly used in choline chloride based deep eutectic solvents. However, we found it undergoes significant potential shifts in electrochemical tests which previous reports largely ignored. In this work, we studied the degradation mechanism leading to its instability. Results show that due to the high Cl− concentration in ethaline, the AgCl film easily dissolves and forms AgCl2 − species causing a potential shift. Therefore, we suggest a [Fe(CN)6]3−/[Fe(CN)6]4− reference electrode based on the reversibility and low diffusivity of [Fe(CN)6]3−/[Fe(CN)6]4− redox couple in ethaline, which was demonstrated to be reliable and stable over weeks of operation.
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Show moreThe discovery of electrolytes that have low vapor pressures and high solubility towards redox active species with an ability to undergo multiple electron transfer reactions is a challenge to realize in large-scale energy storage. Herein, we investigate the feasibility of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) derived halide salts as redox active species in ethaline (1 : 2 ethylene glycol : choline chloride), a deep eutectic solvent (DES). Hydrogen bonding and electrostatic interactions achieved by the functionalization of TEMPO are shown to improve solubility in ethaline. This is the first study evaluating the physical properties and electrochemical behavior of the newly synthesized TEMPO-salts in ethaline providing insight into the impact of chemical functionality on the utility of these redox active species in DESs.
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Show moreSolvation and transport properties of methly viologen dichloride (MVCl2) in 1:2, 1:4, and 1:6 molar mixtures of choline chloride (ChCl) and ethylene glycol (EG), including the deep eutectic solvent (DES) ethaline (1:2 mixture), were studied through the application of the hole theory to measured physical properties, cyclic voltammetry, and Raman spectroscopy. The ChCl:EG mixtures were compared to the ionic liquid (IL) 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([PYR13][TFSI]) and choline bis(trifluoromethylsulfonyl)imide (ChTFSI) EG mixtures with the same molar ratios in order to understand the impact of the anion and hydrogen bond donor on solvation. Exchanging the chloride anion with TFSI is found to increase the fluidity of the solvent and promote stronger solute–solvent interactions. Raman spectroscopy suggests MVCl2 is strongly solvated by EG in ChTFSI:EG solutions and interstitially accommodated in holes in ChCl:EG mixtures and [PYR13][TFSI]. Complex solvents such as ILs and DESs are regarded as “designer solvents”, and it is demonstrated here that the physical properties and solvation characteristics of these fluids strongly depend on the choice of the anion.
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Show moreDeep eutectic solvents (DES) are environmentally-friendly electrolytes that are gaining interest for electrodeposition and energy storage applications. In these applications, metal electrodeposits with smooth, non-dendritic morphology are desired and thus effective strategies for suppressing roughness evolution are critically needed. A commonly employed and rather effective strategy for suppressing roughness evolution in metal electrodeposition is the use of electrolyte additives; however, the availability of such additives in DES electrolytes is limited and so is the understanding of the mechanisms through which additives suppress roughness amplification in DES media. In the present contribution, we demonstrate that polyethylenimine (PEI) is an effective electrolyte additive that suppresses roughness evolution during Cu electrodeposition in ethaline DES. PEI, due to its adsorption–deactivation properties, exhibits a unique hysteresis response during voltammetric studies of Cu electrodeposition – this response is analyzed using a mathematical model incorporating the relevant PEI transport, surface adsorption and deactivation processes. The model provides guidelines for selection of optimal conditions (e.g., PEI concentration) for effective suppression of roughness amplification in Cu electrodeposition.
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Show moreA range of techniques including physical property measurements, neutron scattering experiments, ab initio molecular dynamics, and classical molecular dynamics simulations are used to probe the structural, thermodynamic, and transport properties of a deep eutectic solvent comprised of a 1:2 molar ratio of choline chloride and ethylene glycol. This mixture, known as Ethaline, has many desirable properties for use in a range of applications, and therefore, understanding its liquid structure and transport properties is of interest. Simulation results are able to capture experimental densities, diffusivities, viscosities, and structure factors extremely well. The solvation environment is dynamic and dominated by different hydrogen bonding interactions. Dynamic heterogeneities resulting from hydrogen bonding interactions are quantified. Rotational dynamics of molecular dipole moments of choline and ethylene glycol are computed and found to exhibit a fast and slow mode.
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Show moreIn the past decade, reports detailing the preparation, characterization, and application of deep eutectic solvents (DESs) have grown in number significantly, showing signs of increased interest and attention. Indeed, these systems provide tunable polar environments attractive for their ease of synthesis and lack of need for purification. DESs are homogeneous systems composed of two or more components having a significantly depressed melting point compared to either constituent material. As interest and application of DESs grow, the need for a common understanding of their preparation and characterization is required. In this Perspective, we discuss metal-free DESs, focusing on their preparation, characterization of physical properties, and considerations for their application. We highlight inconsistencies or omissions in literature reports as well as factors for researchers to consider when investigating these systems.
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Show moreWe have investigated the charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM] TFSI) and various concentrations of lithium salt bis(trifluoromethylsulfonyl)imide (LiTFSI) confined within a SiO2 matrix, prepared via a sol–gel method. The translational diffusion coefficients of BMIM+, TFSI–, and Li+ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures, covering the 20–100 °C range, using nuclear magnetic resonance (NMR) pulsed field gradient diffusion spectroscopy. The mobility of BMIM+, TFSI–, and Li+ was found to increase with the [BMIM] TFSI/LiTFSI ratio, exhibiting an almost liquid-like mobility in IGs. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring 1H, 19F, and 7Li spin–lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles) and using two different models based on translational and rotational diffusion and reorientation mediated by translational displacements. The existence and weighting of these two fractions of ions were obtained from the direct diffusion measurements. The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li+ batteries.
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Show moreDeep eutectic solvent (DES) based electrolytes are gaining attention for electrochemical applications. As such, knowledge of the charge transfer kinetics in DES and its dependence on electrolyte composition and temperature is important. Using Cu2+ + e ↔ Cu1+ as a model system, we demonstrate that metal redox reactions in chloride-containing DES media suffer from sluggish charge transfer kinetics. The exchange current density, i0, displayed a peculiar inverse relationship with the bulk chloride concentration, [Cl−]b. The i0 decreased from 7.9 to 3.0 mA/cm2 when [Cl−]b increased from 2.8 to 4.6 M at 30°C. Such dependence is shown to originate from Cl− complexation with reactant and product species. Furthermore, i0 increased with an increase in Cun+ bulk concentration in the 50–200 mM range. Also, increase in temperature elevated i0. The charge transfer coefficient αc, however, remained constant (~0.5) and was unaffected by Cun+ or Cl− concentrations or by temperature. To explain how charge transfer kinetics depend on the various system parameters, a model incorporating complexation phenomena was developed and its predictions were compared to experiments.
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