Structure Of Deep Eutectic Solvents Research Articles

Deciphering the structure of deep eutectic solvents: A computational study from the solute's viewpoint

In the last few years, Deep Eutectic Solvents (DESs) have emerged as sustainable alternatives to traditional organic solvents. The high degree of freedom when tailoring DES structures represents, at the same time, their most praised and most complicated feature. Indeed, given the enormous number of possible combinations, the selection of the most suitable DES for a given application cannot be based on a trial-and-error approach; therefore, reliable computational tools are needed to fully exploit DESs potentialities. In this work, we propose the first computational protocol to investigate absorption properties of solutes dissolved in DES. The protocol combines an accurate sampling of the solute-solvent phase-space by means of Molecular Dynamics (MD) and a fully atomistic Quantum Mechanics/Molecular Mechanics (QM/MM) approach to describe intermolecular interactions and spectral properties. In this way, specific interactions such as hydrogen bonding, which characterize DES complex interaction patterns, are properly modelled. The robustness and reliability of the method are proved by comparing computed data with experimental spectra of 2-hydroxymethylfurfural and syringic, vanillic, p-coumaric, gallic, and caffeic acids dissolved in DES and water. These molecules have been chosen because of their relevance in the frame of biomass valorization. Remarkably, the protocol allows getting insights into DES/water mixtures and opens up to the prediction of aqueous DES behaviour of the water threshold which causes the switch from water-in-DES to aqueous electrolyte-like environments.

Green and low-cost deep eutectic solvents for efficient extraction of basic and non-basic nitrides in simulated oils

The removal of nitrogenous compounds from fuel oil is essential to reduce NOx emissions and improve the quality of the oil. In the present work, a new method of nitrogen removal using deep eutectic solvents (DESs) as extractants is reported. Seven green and low-cost quaternary ammonium acidic DESs were prepared and used for the extraction of basic N-compounds (quinoline) and non-basic N-compounds (indole) in model oil. The results indicated that the structure of DESs has a profound effect on denitrification performance. Under the same conditions, DES composed of tetrabutylammonium bromide and formic acid ([TBAB:2FA]) showed the best denitrification performance and could effectively remove both basic and non-basic nitrogen compounds. The main experimental parameters such as the mass ratio of DESs to model oil, extraction temperature, time, initial concentration of N-compounds and water content when [TBAB:2FA] were used as extractants were investigated. Under the optimal conditions (initial N-compounds concentration of 200 ppm, DESs-oil mass ratio of 1:5, 30 °C, and 60 min of extraction), the extraction efficiency of quinoline and indole were 99.60% and 98.42%, respectively. In addition, the DESs were regenerated with deionized water. When reused four times, the denitrification capacity remained high. Finally, the mechanism of the extraction process was investigated by FT-IR spectroscopy. This research is expected to provide a theoretical reference for the removal of nitrogen compounds from fuel oil.

Deep Eutectic Solvents with Excellent Catalytic Ability for Extractive and Oxidative Desulfurization under Room Temperature

Deep eutectic solvents (DESs) have been intensively investigated as promising green solvents for extraction combined with oxidative desulfurization (EODS) of fuel oil. However, it is still a challenge to explore the effect of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) structure on the formation of DESs and the performance of desulfurization. In this study, exploratory density functional theory (DFT) calculation was first carried out to reveal the influence of the electron density of HBA on the preparation of DES. Then, the structures of DESs were optimized, and their interaction energy was calculated. The structure of HBA and HBD has an important effect on the formation of DES and the capacity of EODS. Under optimal conditions (30 °C, O/S = 2), the desulfurization efficiency of phenol/formic acid (Ph/FA) DES can reach as high as 99.8%. Further, the desulfurization mechanism was also investigated by experiments, characterizations, and verified DFT calculations. This study not only provides a new aspect for designing and understanding DESs but also provides a novel catalyst for efficient EODS.

Understanding ion-ion and ion-urea interactions in mixtures of urea and choline oxyanions salts

Rotational and oscillatory rheology, Raman, and infrared spectroscopy were used to examine the anion effect on the structure of deep eutectic solvents (DES) by contrasting reline (the 1:2 mixture of choline chloride and urea, [Chol]Cl:U) with other mixtures at the same composition based on oxyanion salts of different acceptor and donor characteristics for hydrogen bonding (choline acetate, [Chol][Ac], and choline dihydrogen phosphate, [Chol][DHP]). Oscillatory rheological measurements of [Chol][DHP] suggest the presence of hydrogen-bonded [DHP] chains that can be broken and recombined. Far-infrared spectra also show that [Chol][DHP] has stiffer dynamics when compared to the other two mixtures under investigation. The high-frequency range of the vibrational spectra provides further proof of differences in the hydrogen bond strengths across the systems under study. The fingerprint region of the vibrational spectra provides information about [Chol] and urea conformations in the presence of different anions in the liquid phase.

Efficient and eco-friendly isolation and purification of lignin from black liquor with choline chloride-based deep eutectic solvents

Abstract This study developed an efficient and green method for isolating and purifying lignin from black liquor (BL) using deep eutectic solvents (DESs). After the short and mild process (700 W microwave-assisted, 100 °C, DES: BL 1:5 (v: v), 30 min) using optimized DES (lactic acid (LA): choline chloride (ChCl) 10:1, molar ratio), the yield and purity of lignin obtained from 100 mL BL was 1.58 g and 88.12%, respectively, which was more efficient than the results of 1.18 g and 78.54% of the conventional process. Furthermore, more than 95% of initial results were obtained after three cycles. The composition and structure of DESs and conventionally purified lignin were slightly different. The solvent costs for both processes were also evaluated, showing that the DES process has great potential to replace traditional bulky and environmentally unfriendly procedures for lignin isolation and purification and to help develop new strategies for the potential added value of lignin.

The role of HBA structure of deep eutectic solvents consisted of ethylene glycol and chlorides of a choline family for improving the ammonia capture performance

The current study deals with four novel deep eutectic solvents (DES) formed by choline-like chlorides (ChCl), namely ammonium salts with 2-hydroxyethyl and 2-(2-hydroxyethoxy)ethyl substituents, and ethylene glycol (EG) in mole ratio 1 : 2. Their thermophysical properties such as density (ρ), viscosity (η), refractive index (nD) were carefully investigated in a broad range of temperatures. The ammonia solubilities in considered DESs at T = (303.2–333.2) K and p = (100–636) kPa were measured and considering the slight deviation from ideality Krichevsky − Kasarnovsky equation were used for fitting the experimental data. Based on this, the Henry's law constants (H), partial molar volumes at infinite dilution of NH3 in DESs (VNH3∞), and then the changes of enthalpy (ΔsolH°), Gibbs free energy (ΔsolG°) and entropy (ΔsolS°) during the NH3 absorption process were obtained. For the investigated DESs an improvement of ammonia absorption capacity at 313.2 K and 101 kPa was found to be by 33 % (3.640 mol · kg−1) if changing one methyl to 2-hydroxyethyl group, by 57 % (4.295 mol · kg−1) if changing two methyl to two 2-hydroxyethyl groups, by 5 % (2.861 mol · kg−1) if changing one methyl to 2-(2-hydroxyethoxy)ethyl group, by 13 % (3.081 mol · kg−1) if changing two methyl to 2-hydroxyethyl and 2-(2-hydroxyethoxy)ethyl groups in HBA structure compared to the ethaline (ChCl : EG = 1 : 2; 2.729 mol · kg−1). This assumes occurring the gas capture via – most likely – hydrogen bonds interactions between the ammonia molecule(s) and the hydroxyalkyl fragments, while the ether’s groups in HBA structure depress the positive effect of latter and hence decrease the general ammonia solubility in the considered DESs. Indeed, the strong linking emerging between hydroxyls of HBA and NH3 leads to the deformation of solvent intrinsic structure, which was proven by 1H NMR and FTIR spectroscopy.

Few-layered Ti3C2Tx MXene synthesized via water-free etching toward high-performance supercapacitors

MXene has drawn considerable attention in energy storage due to particular physicochemical properties. At present, among most near-ambient temperature preparation methods, water is usually served as the main solvent. However, MXene is usually subjected to fast structural degradation on account of water molecules attacking in aqueous solution. Herein, we report a novel water-free etching strategy for synthesizing few-layered Ti3C2Tx MXenes in deep eutectic solvents at near-ambient temperature. Benefitting from the absence of water and macromolecular structure of deep eutectic solvents, the as-synthesized few-layered Ti3C2Tx (DES-Ti3C2Tx) MXene presents abundant −O terminations and low oxidation degree. As a consequence, the DES-Ti3C2Tx MXene displays excellent specific capacitance of 320 F/g at 2 mV/s. Impressively, the DES-Ti3C2Tx MXene exhibits splendid long-term stability that 97% specific capacitance retention can be acquired over 50 000 cycles at high current density of 50 A/g. Therefore, this study offers a new thought for preparing high performance MXene-based materials by water-free etching method.

Structure of Deep Eutectic Solvents (DESs): What We Know, What We Want to Know, and Why We Need to Know It.

Deep eutectic solvents (DESs) are a tunable class of solvents with many advantageous properties including good thermal stability, facile synthesis, low vapor pressure, and low-to-negligible toxicity. DESs are composed of hydrogen bond donors and acceptors that, when combined, significantly decrease the freezing point of the resulting solvent. DESs have distinct interfacial and bulk structural heterogeneity compared to traditional solvents, in part due to various intramolecular and intermolecular interactions. Many of the physiochemical properties observed for DESs are influenced by structure. However, our understanding of the interfacial and bulk structure of DESs is incomplete. To fully exploit these solvents in a range of applications including catalysis, separations, and electrochemistry, a better understanding of DES structure must be obtained. In this Perspective, we provide an overview of the current knowledge of the interfacial and bulk structure of DESs and suggest future research directions to improve our understanding of this important information.

Deep Eutectic Solvents as Nonflammable Electrolytes for Durable Sodium‐Ion Batteries

Radiative Cooling In article number 2100159, Dries De Sloovere, An Hardy, and co-workers, show that careful investigation and optimization of the coordination structure of deep eutectic solvents allows the preparation of a viable electrolyte alternative for sodium-ion batteries. The optimized electrolyte is durable and nonflammable, considerably improving the safety of battery operation. Furthermore, it can offer a more durable electrochemical performance compared to conventional electrolytes.

One-step extraction of oat protein by choline chloride-alcohol deep eutectic solvents: Role of chain length of dihydric alcohol

The aim of this study is to investigate the effect of dihydric alcohol chain length (1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol) on the structure of deep eutectic solvents (DESs) and the properties of the extracted oat proteins. Herein, five anhydrous and nine hydrated DESs were successfully prepared by mixing choline chloride, dihydric alcohol, and/or water in a heating method. The structures of DESs were confirmed by FTIR and 1H NMR. Among them, only four anhydrous and six hydrated DESs were able to extract oat proteins from flours by one-step extraction. SDS-PAGE and FTIR analyses indicated that the structural properties of the oat proteins were highly reliant on the composition of the DESs; while physicochemical properties were primarily ruled by the environmental pH. Overall, the hydrated DES composing of all food-grade compounds, including choline chloride, 1,3-propanediol, and water at a molar ratio of 1:3:1, demonstrated its great potential for one-step biorefinery of oat proteins.

Tuning the solvation of indigo in aqueous deep eutectics.

The solubility of synthetic indigo dye was measured at room temperature in three deep eutectic solvents (DESs)-1:3 choline chloride:1,4-butanediol, 1:3 tetrabutylammonium bromide:1,4-butanediol, and 1:2 choline chloride:p-cresol-to test the hypothesis that the structure of DESs can be systematically altered, to induce specific DES-solute interactions, and, thus, tune solubility. DESs were designed starting from the well-known cholinium chloride salt mixed with the partially amphiphilic 1,4-butanediol hydrogen bond donor (HBD), and then, the effect of increasing salt hydrophobicity (tetrabutylammonium bromide) and HBD hydrophobicity (p-cresol) was explored. Measurements were made between 2.5 and 25 wt. % H2O, as a reasonable range representing atmospherically absorbed water, and molecular dynamics simulations were used for structural analysis. The choline chloride:1,4-butanediol DES had the lowest indigo solubility, with only the hydrophobic character of the alcohol alkyl spacers. Solubility was highest for indigo in the tetrabutylammonium bromide:1,4-butanediol DES with 2.5 wt. % H2O due to interactions of indigo with the hydrophobic cation, but further addition of water caused this to reduce in line with the added water mole fraction, as water solvated the cation and reduced the extent of the hydrophobic region. The ChCl:p-cresol DES did not have the highest solubility at 2.5 wt. % H2O, but did at 25 wt. % H2O. Radial distribution functions, coordination numbers, and spatial distribution functions demonstrate that this is due to strong indigo-HBD interactions, which allow this system to resist the higher mole fraction of water molecules and retain its solubility. The DES is, therefore, a host to local-composition effects in solvation, where its hydrophobic moieties concentrate around the hydrophobic solute, illustrating the versatility of DES as solvents.

Intermolecular interactions in tetrabutylammonium chloride based deep eutectic solvents: Classical molecular dynamics studies

Deep eutectic solvents (DESs) based on tetrabutylammonium chloride have been studied using atomistic molecular dynamics simulations. Two different hydrogen bond donors (HBDs) glycerol and ethylene glycol were used to explore the effect of HBDs on the structure of DESs. Interaction between the anion and the HBD was found to be the most dominant interaction from the structural analysis using radial and spatial distribution functions. Anions and HBDs interact via hydrogen bonds formed between them. The mean hydrogen bond lifetime for these bonds was found to be relatively shorter in case of ethylene glycol compared to that of glycerol, which agrees with the binding energy of the complexes in the gas phase determined using quantum chemical calculations. Diffusivity of components in DES having ethylene glycol as HBD is higher than that of the DES having glycerol due to the greater extent of hydrogen bonding in the latter. The presence of peaks and anti-peaks in the lower wave vector region in both the DESs signifies the existence of long-range structural ordering. The vapor – liquid interface of both the systems showed enrichment of ions at the interface compared to the bulk region.

Extractive desulfurization of dibenzothiophene from normal octane using deep eutectic solvents as extracting agent

Among the most important causes of air pollution we may refer to hydrocarbon fuels with a large content of organosulfur compounds. These compounds must be removed to the level of standard concentration for which different techniques are commonly in use. Since, desulfurization of aromatic sulfur compounds, such as thiophenes, is not possible by common methods such as hydrodesulfurization, other methods should be considered. In recent years, scientists have focused on extractive desulfurization using Deep Eutectic Solvents (DES) as a new method for removing organosulfurs from hydrocarbon fuels. In this research, two new DESs are synthesized. As the model fuel, we used n-octane, and dibenzothiophene (DBT) was chosen as the organosulfur content with a 1000 ppm concentration. DESs were synthesized by mixing quaternary ammonium salts and trifluoroacetic acid in a heating method. The structures of DESs were confirmed by FTIR and H-NMR analyses. The extraction process was first carried out at temperatures ranging from 25 to 40 °C. The results showed that the best temperature was 35 °C. Then, the effect of fuel to DES mass ratio (i.e. different compositions of feed), on extraction efficiency was examined. For thermodynamic modeling of liquid–liquid equilibrium the Non Random Two Liquid (NRTL) activity coefficient model was implemented. The parameters of the NRTL model were correlated using the binary data obtained from solubility experiments. Finally, the adjusted parameters were used for predicting ternary phase equilibrium data obtained in this work. The root mean square deviation from experimental data was less than 0.11% for each solvent.

The construction of a new deep eutectic solvents system based on choline chloride and butanediol: The influence of the hydroxyl position of butanediol on the structure of deep eutectic solvent and supercapacitor performance

The precise regulation of the deep eutectic solvents (DESs) system has an important influence on its structure, performance and application. In this work, choline chloride (ChCl) and three kinds of butanediols are used as hydrogen bond acceptors and donors, respectively. The research focuses on the influence of two hydroxyl positions in butanediols on the structure and properties of DES systems with various characterization techniques. The results show that the DES system composed of 1, 2-butanediol and ChCl (the distance of two hydroxyl group is closest) possesses the largest conductivity (1.26 mS cm−1) and the lowest viscosity (56.99 cP). Moreover, using ChCl and 1, 2-butanediol as the electrolyte, the corresponding voltage of activated carbon (AC) based supercapacitor can reach 2 V, the specific capacitance can reach 116 F g−1 and the highest energy density is 16.14 Wh kg−1. More importantly, this work uses the DFT model to design and verify the DES systems in detail, and then explain the mechanism of the influence of the hydroxyl position on the structure and electrochemical performance of the DES system from a theoretical perspective. Obviously, the DES systems having difference structures and DFT calculation proposed in this article provide a significant way and reference for in-depth understanding of DES electrolytes and applications.

Deep Eutectic Solvent Choline Chloride/p-toluenesulfonic Acid and Water Favor the Enthalpy-Driven Binding of Arylamines to Maleimide in Aza-Michael Addition.

Deep eutectic solvents (DESs) have been considered "the organic reaction medium of the century" because they can be used as solvents and active catalysts in chemical reactions. However, experimental and theoretical studies are still needed to provide information on the structures of DESs, the kinetics and thermodynamics properties, the interactions between the DESs and the substrates, the effect of water on the DES supramolecular network and its physicochemical properties, and so forth. This information is very useful to understand the essence of the processes that take place in the catalysis of chemical reactions and, therefore, to help in the design of a DES for a specific reaction and sample. This article shows a systematic study of the impact of DES choline chloride/p-toluenesulfonic acid and DES choline chloride/p-toluenesulfonic acid-water in the aza-Michael addition of arylamines to maleimide to obtain aminopyrrolidine-2,5-dione derivatives. The derivatives are obtained under very mild reaction conditions with good yield. The global reaction is exothermic, spontaneous, permitted by enthalpy, and prohibited for entropy. The calculated potential energy surface shows a reaction mechanism of six steps controlled by enthalpy (except the last step that is controlled by entropy). The water incorporated in the supramolecular DES complex stabilizes the transition states and favors the enthalpy-driven binding. A set of H/D exchange NMR experiments validates the transition state existing in the fourth stage of the mechanism.

Can Water be an Additive? - Towards the Understanding of the Water Content Effect on Ni and Cr Electrodeposition from Choline Chloride-Based Deep Eutectic Solvent

Thin Ni and Cr coatings are commonly electrodeposited from aqueous electrolytes [1,2]. These often contain series of additives, whose role is to improve the leveling and adhesion of deposited films to the substrate, decrease internal stress, and limit the hydrogen evolution reaction (HER). However, certain additives, such as thiourea as well as Cr(VI) compounds are highly hazardous [1,2]. Therefore, due to the increasing environmental awareness of the society, the electrolytes containing these constituents require replacement by green alternatives, such as Deep Eutectic Solvents (DESs).DESs, usually composed of quaternary ammonium salt and hydrogen bond donor, i.e. ethylene glycol, have wider electrochemical potential window than H2O, which in principle enables obtaining metallic coatings without the HER [3]. However, due to the low conductivity and high viscosity of DESs, it is necessary to use additives, i.e. nicotinic acid, improving these physico-chemical properties and enhancing the electrodeposition process. However, due to the unavoidable presence of water (DESs are hygroscopic), H2O can be considered an additive to DESs. The addition of H2O can be beneficial since it improves the ion mobility, and therefore increases the reduction rate [4]. The current studies focus on understanding the effect of water content on the physico-chemical properties, and the structure of DESs. However, a little is known on the influence of H2O on the electrodeposition of Ni and Cr, especially on the nucleation and growth, the effect of the morphology and chemical composition of deposited films. Therefore, an in-depth study on this topic is required.The electrolytic reduction of Ni and Cr cations in choline chloride – ethylene glycol (1ChCl:2EG) on glassy carbon and steel substrates was investigated in function of water content. This study was performed by means of electrochemical techniques, such as linear sweep voltammetry and chronoamperometry. The obtained deposits were analyzed with surface sensitive methods, i.e. scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In order to bring the understanding on the influence of H2O as an additive on the early stage electrodeposition of Ni and Cr from DES, a combination of computational modelling with UV-vis spectroscopy was involved. This approach allowed correlating the observed electrochemical behavior of Ni and Cr cations with certain water concentrations. Establishing such relationship is of high relevance, as it may enable optimizing the process parameters for Ni and Cr electroreduction from DESs. [1] N. V. Mandich and D. L. Snyder. Modern Electroplating, chapter Electrodeposition of Chromium, pages 205–248. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010.[2] G. DiBari. Modern Electroplating, chapter Electrodeposition of Nickel, pages 79-114. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010.[3] E.L. Smith, A.P. Abbot, K.S. Ryder, Deep Eutectic Solvents (DESs) and Their Application, American Chemical Society Publications, 2014, pp. 11060 – 11082.[4] M. Lukaczynska, E.A. Mernissi Cherigui, A. Ceglia, K. Van den Bergh, J. De Strycker, H. Terryn, and J. Ustarroz, Influence of water content and applied potential on the electrodeposition of Ni coatings from deep eutectic solvents, Electrochimica Acta, 319: 690-704, 2019.

Toward an Atomistic Understanding of Deep Eutectics Solvents Electrochemical Interfacial Structure

Green, stable, and wide electrochemical window deep eutectics solvents (DESs) are ideal candidates for electrochemical systems, such as batteries, supercapacitors, catalysis, electrodeposition, and many more. However, since these mixtures are highly dense and composed of only large asymmetric ionic and molecular components, short-range ion-ion, ion-molecule interactions play a dominant role both in the bulk as well as at the interface properties. This results in breakdown of the standard mean-field approximation, which is the basis of dilute solution theory. In addition, due to the hygroscopic nature of DESs, the presence of latent water is unavoidable. Therefore, understanding the DESs-electrode interface (electrical double layer structure, EDLS) together with the role of water at a molecular scale is of great importance for the widespread use of these solvents in electrochemical systems [1-3]. In this presentation, we will first review the benchmark study on conventional dilute solution electrical double layer structure and secondly present our recent findings and theoretical developments on the electrochemical interfacial structure of deep eutectic solvents using a combined experimental and a novel atomistic molecular dynamics approach. In the presentation, we focused on the computational approach. The use of such an atomistic-molecular approach allows investigation of all the possible bulk and interfacial interactions and, consequently, helps to shed light on the nanoscale electrochemical interfacial structure of DESs. Unlike the interfacial structures observed and proposed for other electrolyte-electrode interfaces (compact- diffuse double layer for dilute solution [4]; overscreening or crowding for concentrated solutions [5]), the simulations of DESs-electrode interfacial structure show an unexpected and previously unrecognized phenomenon: the electrochemical interface is composed of a mixed layer structure followed by a mixed charged clustered layer regardless of the surface polarization [1,2]. Some of the findings will be discussed in this presentation.

Monoethanolamine based DESs for CO2 absorption: Insights from molecular dynamics simulations

In order to mitigate issues related to greenhouse gases, several ionic liquids have been designed to efficiently absorb CO2. Among them, novel deep eutectic solvents (DESs), with their commercially attractive properties, have recently been studied for CO2 capture process. Earlier, we have analyzed molecular structure of methyltriphenylphosphonium bromide (MTPPBr) and mono ethanol amine (MEA) based DESs at molar ratio 1:6, 1:7 and 1:8, which are known to have high CO2 absorption capacity. Herein, we further explore the interactions between the above mentioned DESs and CO2 using molecular dynamics simulations to systematically explore the mechanism(s) of CO2 absorption. Results based on interaction energy and radial distribution functions (rdfs) reveal strong interactions between MEA/CO2 and Br/CO2, suggesting the role of these key components of the DESs in CO2 capture. The observed absorption capacity is significantly higher than the traditionally used MEA solvent. Furthermore, the DESs have high viscosity, or low diffusion of the components, which is an impediment to its commercial development. Hence, we examined the effect of temperature and addition of water onto diffusion of the DESs’ components and CO2 absorption capacity. Results show that structure of DESs and its CO2 absorption capacity remain unaffected up to 70 mol% water in the DESs; however, the increase in diffusion is also insufficient. On the other hand, a 30 K rise in the temperature led to a significant increase in the diffusion while maintaining a high absorption capacity. Taken together, work assists in understanding interactions between CO2 and DESs’ components and the absorption mechanism of these DESs and additionally suggests methods to decrease DESs’ viscosity, along with optimized process conditions, for further industrial implementation.

Hydrated-Metal-Halide-Based Deep-Eutectic-Solvent-Mediated NiFe Layered Double Hydroxide: An Excellent Electrocatalyst for Urea Electrolysis and Water Splitting.

The liquid structures of deep eutectic solvents (DESs) based on hydrated metal halides and their application as electrolytes have been widely studied. However, little attention has been paid to the direct use of this type of DES in the preparation of micro-/nanomaterials. Herein, an FeCl3 ⋅6 H2 O/urea DES was used in the one-step synthesis of NiFe-LDH_D with a nanoflower morphology. In alkaline media, this catalyst promoted excellent electrocatalytic activity for the oxidation of urea at potential of 1.32 V (vs. RHE) and for the oxygen-evolution reaction at a potential of 1.39 V to achieve a current density of 10 mA cm-2 . These results were superior to the results with NiFe-LDH/NF that was obtained from an aqueous solution of FeCl3 , as well as most of the previously reported transition-metal catalysts. Furthermore, NiFe-LDH_D/NF could be readily implemented as both a cathode and an anode for the electrolysis of urea and water splitting. The use of hydrated-metal-halide-based DESs for the preparation of LDH catalysts through a dipping-redox strategy should both enrich the research of DESs and offer guidance for the rational surface engineering of catalysts for the electrolysis of urea and overall water splitting with high performance.

Pressure-induced amorphization and crystallization of Choline chloride/Ethylene glycol deep eutectic solvent

The pressure-induced phase transitions of Choline chloride/Ethylene glycol (ChCl/EG) deep eutectic solvent have been studied at different rates of 0.2 and 1.0GPa/s up to about 5.0GPa. Two crystalline phases formed at 2.6 and 3.9GPa with increasing pressure at lower compression rate, however, the amorphous phase solidified with superpressurized glass above 2.8GPa at higher compression rate. The unusual results may be explained by the unique and characteristic structure of deep eutectic solvent and the supercooling induced by compression. The P versus T phase diagram of ChCl/EG deep eutectic solvent was constructed, and the solidification mechanism of ChCl-based deep eutectic solvent was discussed through the molecules packing of ChCl.