Understanding the Electrode-Electrolyte Interfaces of Ionic Liquids and Deep Eutectic Solvents.

Developing unconventional electrolytes such as ionic liquids (ILs) and deep eutectic solvents (DESs) has led to remarkable advances in electrochemical energy storage and conversion devices. However, the understanding of the electrode-electrolyte interfaces of these electrolytes, specifically the liquid structure and the charge/electron transfer mechanism and rates, is lacking due to the complexity of molecular interactions, the difficulty in studying the buried interfaces with nanometer-scale resolution, and the distribution of the time scales for the various interfacial events. This Feature Article outlines the standing questions in the field, summarizes some of the exciting approaches and results, and discusses our contributions to probing the electrified interfaces by electrochemical impedance spectroscopy (EIS), surface-enhanced Raman spectroscopy (SERS), and neutron reflectivity (NR). The related findings are analyzed within electrical double-layer models to provide a framework for studying ILs, DESs, and, more broadly, the concentrated hydrogen-bonded electrolytes.

Solvation Structure, Dynamics, and Charge Transfer Kinetics of Cu2+ and Cu+ in Choline Chloride Ethylene Glycol Electrolytes.

Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study electrolytes containing CuCl2 and CuCl salts in mixtures of choline chloride (ChCl) and ethylene glycol (EG). The study focused on the concentration of 100 mM of both CuCl2 and CuCl with the ratio of ChCl/EG varied from 1:2, 1:3, 1:4, to 1:5. It was found that the Cu2+ and Cu+ have different solvation environments in their first solvation shell. Cu2+ is coordinated by both Cl- anions and EG molecules, whereas Cu+ is only solvated by EG. However, both Cu2+ and Cu+ show strong interactions with their second solvation shells, which include both Cl- anions and EG molecules. Considering both the first and second solvation shells, the concentrations of Cu2+ and Cu+ that have various coordination numbers in each solution were calculated and were found to correlate qualitatively with the exchange current density trends reported in previous experiments of Cu2+ reduction to Cu+. This finding makes a connection between atomic solvation structure observed in MD simulations and redox reaction kinetics measured in electrochemical experiments, thus revealing the significance of the solvation environment of reduced and oxidized species for electrokinetics in deep eutectic solvents.

From Salt in Solution to Solely Ions: Solvation of Methyl Viologen in Deep Eutectic Solvents and Ionic Liquids.

Solvation 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.

CO2-responsive microemulsions based on reactive ionic liquids.

We demonstrate that the nanodomains within a ternary system consisting of oil, surfactant, and a new reactive ionic liquid can be tuned reversibly upon exposure to and removal of CO2 under mild conditions of temperature and pressure. The equilibrium microstructures of these domains have been characterized by small-angle neutron scattering and demonstrate that control over emulsion morphology (and therefore physicochemical properties such as viscosity) and the breaking of emulsions can be achieved without the need for irreversible changes in system composition or significant energy input.

Reaction kinetics of CO2 absorption in to phosphonium based anion-functionalized ionic liquids.

The reaction kinetics between CO2 and trihexyl(tetradecyl)phosphonium ([P66614])-based ionic liquids (ILs) with prolinate ([Pro]), 2-cyanopyrrolide ([2-CNpyr]), and 3-(trifluoromethyl)pyrazolide ([3-CF3pyra]) anions are studied at temperatures from 22-60 °C. The absorption of CO2 is carried out in a stirred reactor under pseudo first order conditions. ILs are diluted to concentrations of 0.05, 0.1 and 0.15 M with tetraglyme--a nonreactive, low volatility solvent with much lower viscosity than the ILs. Physical solubility of CO2 in the mixtures is calculated using correlations developed from CO2 solubility measurements in tetraglyme and the N2O-analogy for ILs and dilute IL solutions. The diffusivity of CO2 is estimated from viscosity-dependent correlations chosen after a thorough literature review. The results indicate partial first order reaction kinetics with respect to IL with values ranging from 19,500 L mol(-1) s(-1) ([P66614][Pro]) to 3200 L mol(-1) s(-1) ([P66614][3-CF3pyra]) at 22 °C. The second order reaction rate constants follow Arrhenius behavior with the highest activation energy of 43 kJ mol(-1) measured for [P66614][Pro]. ILs with aprotic heterocylic anions (AHA), on the other hand, show small activation energies of 18 and 11 kJ mol(-1) for [P66614][3-CF3pyra] and [P66614][2-CNpyr], respectively. The ILs studied in this work exhibit reactivity comparable to or higher than common aqueous amines. High reaction rates and tunable capacity make ILs, and AHA ILs in particular, attractive solvents for CO2 separations.

Effect of water and temperature on absorption of CO2 by amine-functionalized anion-tethered ionic liquids.

Amine-functionalized anion-tethered ionic liquids (ILs) trihexyl(tetradecyl)phosphonium asparaginate [P(66614)][Asn], glutaminate [P(66614)][Gln], lysinate [P(66614)][Lys], methioninate [P(66614)][Met], prolinate [P(66614)][Pro], taurinate [P(66614)][Tau], and threoninate [P(66614)][Thr] were synthesized and investigated as potential absorbents for CO(2) capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature were determined. Furthermore, the CO(2) absorption isotherms of [P(66614)][Lys], [P(66614)][Tau], [P(66614)][Pro], and [P(66614)][Met] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. The most important result of this study is that the viscosity of [P(66614)][Pro] only increased by a factor of 2 when fully complexed with 1 bar of CO(2) at room temperature. This is in stark contrast to the other chemically reacted ILs investigated here and all other amino acid-based ILs reported in the literature, which dramatically increase in viscosity, typically by 2 orders of magnitude, when complexed with CO(2). The unique behavior of [P(66614)][Pro] is likely due to its ring structure, which limits the number and availability of hydrogen atoms that can participate in a hydrogen bonding network. We found that water can be used to further reduce the viscosity of the CO(2)-complexed IL, while only slightly decreasing the CO(2) capacity. Finally, from temperature-dependent isotherms, we estimate a heat of absorption of -63 kJ/mol of CO(2) for the 1:1 reaction of CO(2) with [P(66614)][Pro], when we use the two-reaction model.

Equimolar CO(2) absorption by anion-functionalized ionic liquids.

Amino acid ionic liquid trihexyl(tetradecyl)phosphonium methioninate [P(66614)][Met] and prolinate [P(66614)][Pro] absorb CO(2) in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO(2) capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO(2). Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.