Electrochemical behavior of several iron complexes in hydrophobic room-temperature ionic liquids

Ionic Liquids Effect on the Stability of 17-Electron Cation Product of the Electrochemical Oxidation of Cymantrene

The oxidative electrochemistry of cymantrene, CpMn(CO)3 (1; Cp = [η5-C5H5]–), was examined in ionic liquids (ILs) composed of anions of varying Lewis base properties. It was observed that the cyclic voltammetric responses strongly depended on the nucleophilic properties of the IL anion. Still, all observations are consistent with the initial formation of 1+ followed by an attack from the IL anion. In bis(trifluoromethylsulfonyl)amide [NTf2]-based ILs, the process shows close to ideal electrochemical reversibility as the reaction between 1+ and [NTf2] anion is very slow. On the other hand, in tetrafluoroborate and trifluoromethanesulfonate-based IL, the oxidation of 1 shows different levels of electrochemical reversibility with a marked sign of anion attack to 1+. In contrast, 1 exhibits an irreversible oxidation process in hexafluorophosphate-based IL. The reaction rate constants for the interaction of 1+ with the different IL anions were estimated by fitting the experimental data to digital simulations of the proposed mechanism. Besides, the use of [NTf2]-based ILs as a supporting electrolyte in CH2Cl2 was also examined. The oxidation process of 1 shows a close to ideal electrochemical reversibility but low to non-chemical reversibility. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their limited utility for investigating the redox properties of metal carbonyl compounds. It also intends to warn the reader on how the IL media may influence an electrochemical study if care is not exercised.

A Porphyrin-Based Organic Network Comprising Sustainable Carbon Dots for Photopolymerization

Sustainable carbon dots (CDs) based on furfuraldehyde (F-CD) resulted in a photosensitive material after pursuing the Alder-Longo reaction. The porphyrin moiety formed connects the F-CDs in a covalent organic network. This heterogeneous material (P-CD) was characterized by XPS indicating incorporation of the respective C, N and O moieties. Time resolved fluorescence including global analysis showed contribution of three linked components to the overall dynamics of the excited state. Electrochemical and photonic properties of this heterogeneous material facilitated photopolymerization in a photo-ATRP setup where either CuBr₂ /TPMA, FeBr₃ /Br^- or a metal free reaction setup activated controlled polymerization. Chain extension experiments worked in all three cases showing end group fidelity for activation of controlled block copolymerization using MMA and styrene as monomers. Traditional radical polymerization using a diaryl iodonium salt as co-initiator failed.

Electrochemical cell recharging by solvent separation and transfer processes

Electrochemical conversion and storage of unutilized renewable energy will contribute to decarbonization. Here, we create the concept of a liquid electrochemical cell that discharges between the anodic and cathodic sides by reverse reactions of the same redox couple in different solvation states, which are created by differences in the mixture ratios of two solvents called the main solvent (MS) and the transferred solvent (TS). The cell can be charged by a transfer of the TS between the discharged anolyte and catholyte. As an example, we demonstrate a cell utilizing a ferro-/ferricyanide redox couple. Stable discharging and charging via the proposed method is achieved by utilizing water (MS) and acetone (TS). Additionally, dominating factors in the design of a high-performance system are discussed, focusing on the electron acceptability of the MS and the TS. The cell voltages are successfully tuned, and a cell voltage of 0.63 V is achieved by the combination of dimethyl sulfoxide (MS) and water (TS). Moreover, the cell can be customized by various electrochemical reaction systems, which can allow multiple options for the charging processes. This concept provides new approaches for the utilization of diverse energy sources as an input for the charging of electrochemical cells.

Tannic acid-Fe complex derivative-modified electrode with pH regulating function for environmental remediation by electro-Fenton process

A heterogeneous electro-Fenton (hetero-EF) system can effectively broaden the applicable pH range, although the decreased electrogeneration efficiency of H₂O₂ at elevated pH (especially neutral conditions) is unfavorable for the efficient removal of organic pollutants. Herein, a tannic acid-Fe complex derivative-modified carbon felt (TFD@CF) cathode was prepared for hetero-EF treatment of organic pollutants over a wide pH range. Interestingly, the as-prepared hetero-EF cathode could act as a pH regulator that acidified the solution over a wide pH range. As expected, the TFD@CF cathode exhibited excellent hetero-EF activity for the removal of diverse organic pollutants (such as methyl orange, methylene blue, sulfamerazine, bisphenol A and 2,4-dichlorophenoxyacetic acid) at neutral and even alkaline pH (removal efficiency >90 %). A total of 2.98 kWh kg^-1 COD^-1 with 83.2 % COD removal could be achieved by the TFD@CF cathode for the treatment of actual textile dyeing secondary wastewater. Electrochemical characterizations proved that the TFD@CF cathode had excellent electrochemical properties with improved electron transfer ability and a well-pronounced Fe(III) electroreductive response. Meanwhile, more acidic groups were newly generated during the electrochemical reaction (an increase of 30.1 %), thus dissociating more H⁺ into solution. The identification of reactive oxygen species suggested that OH and ¹O₂ could be responsible for the removal of organic pollutants in the TFD@CF EF system. These interesting findings may provide new insights into the design of multifunctional hetero-EF cathodes for the removal of refractory organic pollutants.

Screening of metal complexes and organic solvents using the COSMOSAC-LANL model to enhance the energy density in a non-aqueous redox flow cell: an insight into the solubility

In this paper, we have proposed a first-principles methodology to screen transition metal complexes against a particular organic solvent and organic solvents against a particular transition metal complex based on their solubility information without the knowledge of heat of fusion and melting temperature. The energy density of a non-aqueous redox flow cell directly depends on the solubility of the redox active species in the non-aqueous medium. We have used the "COSMOSAC-LANL" activity coefficient model (A. Karmakar, R. Mukundan, P. Yang and E. R. Batista, RSC Adv., 2019, 18506-18526; A. Karmakar and R. Mukundan, Phys. Chem. Chem. Phys., 2019, 19667-19685) which is based on first-principles COSMO calculations where the microscopic information is passed to the macroscopic world via a dielectric continuum solvation model, followed by a post-statistical thermodynamic treatment of the self-consistent properties of the solute particle to calculate the solubility. To model the activity coefficient at infinite dilution for the binary mixtures, a 3-suffix Margules (3sM) function is introduced for the quantitative estimation of the asymmetric interactions and, for the combinatorial term, the Staverman-Guggenheim (SG) form is used. The new activity coefficient model is separately called the "LANL" activity coefficient model. The metal complex and the organic solvent have been treated as a simple binary mixture. The present model has been applied to a set of 14 different organic solvents and 16 different transition metal complexes. Using the new LANL activity coefficient model in combination with the ADF-COSMOSAC-2013 model, we have shown how one can improve the solubility of a transition metal complex in an organic solvent. We applied our model to screen 84 binary mixtures to predict the compatible pair of redox active species and organic solvent to increase the energy density. The solvation mechanism of the transition metal complexes in the organic solvents was obtained using the new model. The results have been compared with the experimental and theoretical results where they are available.

Metal-Based Ionic Liquids in Oxidative Desulfurization: A Critical Review

Ionic liquids (ILs) as novel functional desulfurization materials have attracted increasing attentions. Metal-based ionic liquids (MILs) are classified into three types of metal chloride ILs, metal oxide ILs, and metal complex ILs based on the definition and basic structure of MILs in this critical review. On the basis of the properties of ILs such as structure designability, super dissolution performance, good thermal and chemical stability, nonflammability, and wide electrochemical window, MILs exhibit unique advantages on hydrophobicity, oxidation performance, and Brönsted-Lewis acidity. Therefore, MILs possess both the absorption and oxidation centers for the intramolecular adsorption and oxidation to improve the oxidative desulfurization (ODS) process. During the novel nonaqueous wet oxidative desulfurization process (Nasil), H₂S can be oxidized into elemental sulfur with hydrophobic MILs, which can be regenerated by oxygen for recycle, to solve the problems of low sulfur capacity, low sulfur quality, and severe secondary pollution in the aqueous Lo-Cat wet oxidative desulfurization process. Another outstanding feature of MILs in ODS is biomimetic catalysis, which has the function of activating molecular oxygen and improving the oxidation performance. Metal oxide ILs and metal complex ILs are used in combination with hydrogen peroxide or oxygen with the existing water to generate a Fenton-like reaction to convert hydrophobic organic sulfur or SO₂ into hydrophilic sulfoxide/sulfone or sulfur acid, respectively. However, the corrosion of Cl^- to the equipment and emulsification phenomenon in the extraction process of sulfoxide/sulfone separation still need further study. Furthermore, the promising strategies to construct highly efficient and green desulfurization processes for large-scale applications are provided.

Direct dissolution of UO2 in carboxyl-functionalized ionic liquids in the presence or absence of Fe-containing ionic liquids

Dissolution of UO₂ is a prerequisite for the reprocessing of spent nuclear fuel. This study showed that UO₂ could be directly dissolved in a single carboxyl-functionalized ionic liquid (IL), [HOOCMmim][Tf₂N] 1-carboxymethyl-3-methylimidazolium bistriflimide, or [HOOCEtmim][Tf₂N] 1-carboxyethyl-3-methylimidazolium bistriflimide. The addition of an extra Fe-containing IL, [Emim][FeCl₄] (Emim, 1-ethyl-3-methylimidazolium) or [Bmim][FeCl₄] (Bmim, 1-butyl-3-methylimidazolium) could significantly improve the dissolution kinetics. Results demonstrated that the dissolution process in the early stage could be described by using the pseudo first-order rate law. The apparent activation energy for UO₂ dissolution in the mixture of the Fe-containing IL and carboxyl-functionalized IL was calculated to be ∼67 kJ mol^-1, implying that the reaction was mainly controlled by a chemical process. Nevertheless, the influence of the diffusion process is non-negligible since the IL has a relatively high viscosity that can retard the diffusion of the formed uranyl species from the UO₂ surface. Spectroscopic studies and density functional theory calculations indicated that the uranyl ion coordinated with carboxylate groups is the predominant product for UO₂ dissolution in the single carboxyl-functionalized IL, while uranyl chloride complexes would also form in the mixed ILs. The dissolved uranyl species can be successfully recovered from the ILs by extraction. The success of UO₂ dissolution in the carboxyl-functionalized IL with or without the Fe-containing IL indicates that the Fe-containing IL and oxygen can serve as an effective catalyst and oxidant for the dissolution of UO₂, respectively.

Redox-active glyme-Li tetrahalogenoferrate(iii) solvate ionic liquids for semi-liquid lithium secondary batteries

Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries. In this work, we synthesized a series of novel redox-active glyme(oligoether)–Li salt-based SILs, consisting of a symmetric ([Li(G3)]⁺) or asymmetric ([Li(G3Bu)]⁺) triglyme–Li salt complex and redox-active tetrahalogenoferrate ([FeX]⁻ (X = Br₄, Cl₃Br, Cl₄)), for use as the catholyte in semi-liquid lithium secondary batteries. The successful formation of stable molten complexes of [Li(G3/G3Bu)][FeX] was confirmed by Raman spectroscopy and thermogravimetry. The melting point (T_m) depended on both the molecular weights of the complex anions and the structures of the complex cations. [Li(G3)][FeCl₄] comprised complex cations with a symmetric structure, and the smallest complex anions showed the lowest T_m of 28.2 °C. The redox properties of the [FeX]⁻/[FeX]²⁻ couple strongly suggested the suitability of [Li(G3/G3Bu)][FeX] as a catholyte. The discharge capacities of semi-liquid lithium secondary batteries utilizing the [Li(G3/G3Bu)][FeX] catholyte depended on the structure of the SILs, and the cell with [Li(G3)][FeCl₄] showed the highest capacity with relatively good capacity retention. This study confirmed the feasibility of the glyme-based redox-active SILs as catholytes for scalable redox-flow type batteries.

Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries.

Electrochemical characteristics of the decolorization of three dyes by laccase mediator system (LMS) with synthetic and natural mediators

Laccase mediator system (LMS), a very attractive candidate for refractory organics biodegradation, harbors tremendous potential on industry application. However, the performance of LMS usually varies with the discrepancy of mediators and substrates in their chemical structures. Here, we adopt electrochemical analysis that is able to assess the degradation performance of various LMS on three different dyes by quantitative analysis of reaction outcome. Two mechanisms were suggested to explain the grafting of three mediators (1-Hydroxybenzotriazole, Violuric Acid and Acetosyringone), involving the transformation of proton or electron to produce active moieties, which subsequently react with target substrates. A thorough electrochemical insight into the redox features of mediators and its change in the presence of laccase and substrates were carried out using electrochemical analysis. The effectiveness of each kind of LMS on substrates was preliminarily evaluated by analyzing the change of the peak current and potential of mediators. The actual conversion rate of dyes was used to verify the analysis results, which confirms the important role of the stability of the oxidized form as well as their redox potential of the mediators in determining the mechanism of substrate oxidation. The application of electrochemical analysis in efficiency evaluation of LMS shed new light on effective selection of suitable mediators for degradation of refractory organics. It was therefore possible to prejudge the efficacy of LMS by analyzing the electrochemical parameters of target substances and mediators, which undoubtedly has broad further application prospects of LMS.

A Stretchable and Self-Healing Energy Storage Device Based on Mechanically and Electrically Restorative Liquid-Metal Particles and Carboxylated Polyurethane Composites

Stretchable and self-healing (SH) energy storage devices are indispensable elements in energy-autonomous electronic skin. However, the current collectors are not self-healable nor intrinsically stretchable, they mostly rely on strain-accommodating structures that require complex processing, are often limited in stretchability, and suffer from low device packing density and fragility. Here, an SH conductor comprising nickel flakes, eutectic gallium indium particles (EGaInPs), and carboxylated polyurethane (CPU) is presented. An energy storage device is constructed by the two SH electrodes assembled with graphene nanoplatelets sandwiching an ionic-liquid electrolyte. An excellent electrochemical healability (94% capacity retention upon restretching at 100% after healing from bifurcation) is unveiled, stemming from the complexation modulated redox behavior of EGaIn in the presence of the ligand bis(trifluoromethanesulfonyl)imide, which enhances the reversible Faradaic reaction of Ga. Self-healing can be achieved where the damaged regions are electrically restored by the flow of liquid metal and mechanically healing activated by the interfacial hydrogen bonding of CPU with an efficiency of 97.5% can be achieved. The SH conductor has an initial conductivity of 2479 S cm^-1 that attains a high stretchability with 700% strain, it restores 100% stretchability even after breaking/healing with the electrical healing efficiency of 75%.

Solubility model of metal complex in ionic liquids from first principle calculations

A predictive model based on first principles calculations has been proposed to study the solid–liquid equilibria comprising of metal complexes and ionic liquids. The model is based on first principle COSMO calculation followed by post statistical thermodynamical treatment of self-consistent properties of solute and solvent molecules. The metal complex and ionic liquid have been treated as a simple binary mixture. The ionic liquid has been treated here as a single intact molecule. The experimentally observed dual-solute relationship between the ionic liquid and redox active species in presence of a third organic solvent has been established using our model in this work. Within the model, the dual-solute relationship appeared as a simple Gibbs–Duhem relationship between these two species at ambient condition. The dual-solute relationship between the metal complex (V(acac)₃, Cr(acac)₃ and Mn(acac)₃) and ionic liquid ([Tea][BF₄]) has been validated by calculating the Gibbs–Duhem relationship, x_solutevs. x_solvent(IL) and 1/γ_solutevs. x_solvent(IL) plots. The present model has been applied to a set of ionic liquids, metal complexes and organic solvent (acetonitrile) for which experimental study has been done. The solvation mechanism of the metal complexes in those ionic liquids was obtained using the model. According to our findings, the ionic liquid containing imidazolium cation and [NTf₂]⁻ anion is appeared as a suitable solvent for the non-aqueous redox flow cell. We have compared our results with the already reported experimental results where they were available for the non-aqueous solvents.

A predictive model based on first principles calculations has been proposed to study the solid–liquid equilibria comprising of metal complexes and ionic liquids.

Voltammetric and spectroscopic study of ferrocene and hexacyanoferrate and the suitability of their redox couples as internal standards in ionic liquids

The hexacyanoferrate(iii/ii) redox couple is useful as a complementary redox standard with ferrocenium/ferrocene because in many instances only one of these couples exhibits reversible behaviour.

The Chemistry of Redox-Flow Batteries

The development of various redox-flow batteries for the storage of fluctuating renewable energy has intensified in recent years because of their peculiar ability to be scaled separately in terms of energy and power, and therefore potentially to reduce the costs of energy storage. This has resulted in a considerable increase in the number of publications on redox-flow batteries. This was a motivation to present a comprehensive and critical overview of the features of this type of batteries, focusing mainly on the chemistry of electrolytes and introducing a thorough systematic classification to reveal their potential for future development.

Solute diffusion in ionic liquids, NMR measurements and comparisons to conventional solvents

Diffusion coefficients of a variety of dilute solutes in the series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides ([Prn1][Tf2N], n = 3, 4, 6, 8, and 10), trihexyltetracedecylphosphonium bis(trifluoromethanesulfonyl)imide [P14,666][Tf2N], and assorted imidazolium ionic liquids are measured using pulsed field gradient (1)H NMR. These data, combined with available literature data, are used to try to uncover the solute and solvent characteristics most important in determining tracer diffusion rates. Discussion is framed in terms of departures from simple hydrodynamic predictions for translational friction using the ratio ζobs/ζSE, where ζobs is the observed friction, determined from the measured diffusion coefficient D via ζobs = kBT/D, and ζSE = 6πηR is the Stokes friction on a sphere of radius R (determined from the solute van der Waals volume) in a solvent with viscosity η. In the case of neutral solutes, the primary determinant of whether hydrodynamic predictions are accurate is the relative size of solute versus solvent molecules. A single correlation, albeit with considerable scatter, is found between ζobs/ζSE and the ratio of solute-to-solvent van der Waals volumes, ζobs/ζSE = {1 + a(VU/VV)(-p)}, with constants a = 1.93 and p = 1.88. In the case of small solutes, the observed friction is over 100-fold smaller than predictions of hydrodynamic models. The dipole moment of the solute has little effect on the friction, whereas solute charge has a marked effect. For monovalent solutes of size comparable to or smaller than the solvent ions, the observed friction is comparable to or even greater than what is predicted by hydrodynamics. These general trends are shown to be quite similar to what is observed for tracer diffusion in conventional solvents.

Ionic liquids as oxidic media for electron transfer studies

We review the basic ideas underlying the electron free energy level diagrams that have been found useful in considering the thermodynamics of redox processes in molten silicates and related high temperature ionic liquid (IL) solvents, and then show how closely they link to behavior observable in ambient temperature ionic liquids. Much of the information available on redox levels in molten oxides has been gleaned from chemical analysis and spectroscopic species distribution studies, but it is simpler to obtain the data electrochemically. Here, we report some cyclic voltammetry measurements of the Fe(II)∕Fe(III) redox equilibrium in aprotic ionic liquids whose anions provide oxide environments for the redox species that are of different electronic polarizability character from the high temperature solvents, and relate the observations to those of the earlier studies. Quasi-reversible behavior is found in each of the cases studied. As might be expected, the Fe(II)∕Fe(III) equilibrium experiences a more basic environment in an acetate IL than it experiences in any of the common glassforming oxide media, while triflate anions contrast by providing a more acid environment than does the most acid of the molten oxide glassformers studied (an alkali phosphate). The difference can amount to well over 1 V, suggesting the possibility of a "basicity cell" where the same redox couple locates in anode and cathode compartments of the cell, and only the anion environment is different.

X-ray photoelectron spectroscopy of ferrocenyl- and ferrocenium-based ionic liquids

X-ray photoelectron spectroscopy (XPS) is used to study a series of five 1-(ferrocenylmethyl)-3-methylimidazolium- and 1-(ferroceniummethyl)-3-methylimidazolium-based salts. All samples emit good photoelectron fluxes with sharp, well-resolved photoelectron peaks. Due to the low volatility of imidazolium-salts at ambient temperature, no modification of the XP spectrometer was required. Two of the salts exhibit supercooling behaviour, allowing XPS to be recorded at room temperature on liquid samples without the need for charge neutralisation. The photoelectron peaks can be assigned to the component atoms of the salts, based on previous studies on ferrocene, ferrocenium-compounds and ionic liquids. Oxidation of the ferrocenyl moiety to ferrocenium shiftsthe Fe 2p and cyclopentadienyl C 1s photoelectron peaks to higher binding energy but does not affect the imidazolium and anion peaks. Under charge-neutralisation conditions, in which the sample is flooded with low-energy electrons, the ferrocenium moiety of the salt 1-(ferroceniummethyl)-3-methylimidazolium di(hexafluorophosphate) is partially reduced.

Role of H+ in Polypyrrole and Poly(3,4-ethylenedioxythiophene) Formation Using FeCl3·6H2O in the Room Temperature Ionic Liquid, C4mpyrTFSI

The polymerisation reaction of pyrrole and 3,4-ethylenedioxythiophene using the chemical oxidant FeCl3·6H2O in the room temperature ionic liquid butyl-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C4mpyrTFSI) has been investigated using cyclic voltammetry, UV/vis and IR spectroscopy. The voltammetric data for the Fe2+/3+ reaction is complicated by the presence of H+ introduced upon dissolution of the iron salt by deprotonation of the coordinated waters. The voltammetric and chemical reaction studies show that H+ itself, introduced to solution as trifluoromethanesulfonic acid (HTFSI), can act as the chemical oxidant for the polymerisation reaction. Voltammetric data also implies that in this system the Fe2+/3+ redox couple may not actually be involved in the polymerisation reaction and that the H+ introduced upon dissolution of the FeCl3·6H2O may be the sole cause of the oxidation reaction.