Towards an all-copper redox flow battery based on a copper-containing ionic liquid.

The first redox flow battery (RFB), based on the all-copper liquid metal salt [Cu(MeCN)4][Tf2N], is presented. Liquid metal salts (LMS) are a new type of ionic liquid that functions both as solvent and electrolyte. Non-aqueous electrolytes have advantages over water-based solutions, such as a larger electrochemical window and large thermal stability. The proof-of-concept is given that LMSs can be used as the electrolyte in RFBs. The main advantage of [Cu(MeCN)4][Tf2N] is the high copper concentration, and thus high charge and energy densities of 300 kC l(-1) and 75 W h l(-1) respectively, since the copper(i) ions form an integral part of the electrolyte. A Coulombic efficiency up to 85% could be reached.

Liquid Nickel Salts: Synthesis, Crystal Structure Determination, and Electrochemical Synthesis of Nickel Nanoparticles.

New nickel-containing ionic liquids were synthesized, characterized and their electrochemistry was investigated. In addition, a mechanism for the electrochemical synthesis of nanoparticles from these compounds is proposed. In these so-called liquid metal salts, the nickel(II) cation is octahedrally coordinated by six N-alkylimidazole ligands. The different counter anions that were used are bis(trifluoromethanesulfonyl)imide (Tf2 N(-) ), trifluoromethanesulfonate (OTf(-) ) and methanesulfonate (OMs(-) ). Several different N-alkylimidazoles were considered, with the alkyl sidechain ranging in length from methyl to dodecyl. The newly synthesized liquid metal salts were characterized by CHN analysis, FTIR, DSC, TGA and viscosity measurements. An odd-even effect was observed for the melting temperatures and viscosities of the ionic liquids, with the complexes with an even number of carbon atoms in the alkyl chain of the imidazole having a higher melting temperature and a lower viscosity than the complexes with an odd number of carbons. The crystal structures of several of the nickel(II) complexes that are not liquid at room temperature were determined. The electrochemistry of the compounds with the lowest viscosities was investigated. The nickel(II) cation could be reduced but surprisingly no nickel deposits were obtained on the electrode. Instead, nickel nanoparticles were formed at 100 % selectivity, as confirmed by TEM. The magnetic properties of these nanoparticles were investigated by SQUID measurements.

A QCM study of ORR-OER and an in situ study of a redox mediator in DMSO for Li-O2 batteries.

The oxygen reduction reaction and oxygen evolution reaction (ORR-OER) in DMSO were investigated by cyclic voltammetry and potentiostatic methods. A quartz crystal microbalance (QCM) was used to detect which products are formed during reduction and to evaluate the reversibility of the reactions. The studied parameters include the scan rate and the applied cathodic potential. We confirm by the QCM that LiO2 is soluble: this conclusion comes from the time delay we observed between the deposition of the expected mass (based on Faraday's law) and the measured mass. Ambiguity in reported literature values for the slope of the deposited mass per electron M/z is due to the negligence in considering this time delay. The average M/z value versus cathodic charge indicates that soluble LiO2 is the first product of the ORR which reacts further to form Li2O2, either via a disproportionation reaction or via further electrochemical reduction of LiO2. For strong negative potentials and thus large depths of discharge, Li2O is the main discharge product. The reaction pathways hence strongly depend on the experimental conditions applied; especially the reduction potential. The redox mediator tetrathiafulvalene (TTF) was investigated and its influence on reversibility was confirmed by cycling at moderate depths of discharge, where Li2O2 is the main discharge product.

Electrodeposition of thick palladium coatings from a palladium(II)-containing ionic liquid.

The first palladium-containing Liquid Metal Salts (LMS) are presented and shown to be suitable electrolytes for the electrodeposition of palladium. The homoleptic LMS of formula [Pd(MeIm)4][Tf2N]2 or [Pd(EtIm)4][Tf2N]2 (MeIm = N-methylimidazole, EtIm = N-ethylimidazole) have higher melting points than the heteroleptic [Pd(MeIm)2(EtIm)2][Tf2N]2, which is proved to be the most promising electrolyte. The deposition reaction in these LMS was found to be irreversible but smooth and dense palladium layers can be deposited that are crack-free up to a thickness of 10 microns.

High current density electrodeposition of silver from silver-containing liquid metal salts with pyridine-N-oxide ligands.

New cationic silver-containing ionic liquids were synthesized and used as non-aqueous electrolytes for the electrodeposition of silver layers. In the liquid state of these ionic liquids, a silver (i) cation is coordinated by pyridine-N-oxide (py-O) ligands in a 1 : 3 metal-to-ligand ratio, although in some cases a different stoichiometry of the silver center crystallized out. As anions, bis(trifluoromethanesulfonyl)imide (Tf2N), trifluoromethanesulfonate (OTf), methanesulfonate (OMs) and nitrate were used, yielding compounds with the formulae [Ag(py-O)3][Tf2N], [Ag(py-O)3][OTf], [Ag(py-O)3][OMs] and [Ag(py-O)3][NO3], respectively. The compounds were characterized by CHN analysis, FTIR, NMR, DSC, TGA and the electrodeposition of silver was investigated by cyclic voltammetry, linear potential scans, scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). With the exception of [Ag(py-O)3][Tf2N], which melts at 108 °C, all the silver(i) compounds have a melting point below 80 °C and were tested as electrolytes for silver electrodeposition. Interestingly, very high current densities were observed at a potential of -0.5 V vs. Ag/Ag(+) for the compounds with fluorine-free anions, i.e. [Ag(py-O)3][NO3] (current density of -10 A dm(-2)) and [Ag(py-O)3][OMs] (-6.5 A dm(-2)). The maximum current density of the compound with the fluorinated anion trifluoromethanesulfonate, [Ag(py-O)3][OTf], was much lower: -2.5 A dm(-2) at -0.5 V vs. Ag/Ag(+). Addition of an excess of ligand to [Ag(py-O)3][OTf] resulted in the formation of the room-temperature ionic liquid [Ag(py-O)6][OTf]. A current density of -5 A dm(-2) was observed at -0.5 V vs. Ag/Ag(+) for this low viscous silver salt. The crystal structures of several silver complexes could be determined by X-ray diffraction, and it was found that several of them had a stoichiometry different from the 1 : 3 metal-to-ligand ratio used in their synthesis. This indicates that the compounds form crystals with a composition different from that of the molten state. The electrochemical properties were measured in the liquid state, where the metal-to-ligand ratio was 1 : 3. Single crystal X-ray diffraction measurements showed that silver(i) is six coordinate in [Ag(py-O)3][Tf2N] and [Ag(py-O)3][OTf], while it is five coordinate in the other complexes. In [Ag3(py-O)8][OTf]3, there are two different coordination environments for silver ions: six coordinate central silver ions and five coordinate for the outer silver ions. In some of the silver(i) complexes, silver-silver interactions were observed in the solid state.

Synthesis and properties of alkoxy- and alkenyl-substituted peralkylated imidazolium ionic liquids.

Novel peralkylated imidazolium ionic liquids bearing alkoxy and/or alkenyl side chains have been synthesized and studied. Different synthetic routes towards the imidazoles and the ionic liquids comprising bromide, iodide, methanesulfonate, bis(trifluoromethylsulfonyl)imide ([NTf2](-)), and dicyanamide {[N(CN)2](-)} as the anion were evaluated, and this led to a library of analogues, for which the melting points, viscosities, and electrochemical windows were determined. Incorporation of alkenyl moieties hindered solidification, except for cations with high symmetry. The alkoxy-derivatized ionic liquids are often crystalline; however, room-temperature ionic liquids (RTILs) were obtained with the weakly coordinating anions [NTf2](-) and [N(CN)2](-). For the viscosities of the peralkylated RTILs, an opposite trend was found, that is, the alkoxy derivatives are less viscous than their alkenyl-substituted analogues. Of the crystalline compounds, X-ray diffraction data were recorded and related to their molecular properties. Upon alkoxy substitution, the electrochemical cathodic limit potential was found to be more positive, whereas the complete electrochemical window of the alkenyl-substituted imidazolium salts was shifted to somewhat more positive potentials.

Room-temperature silver-containing liquid metal salts with nitrate anions.

The synthesis, structural, thermal and electrochemical properties of fluorine-free silver-containing ionic liquids are presented. The ionic liquid cations are formed by a silver(i) ion surrounded by two 1-alkylimidazole ligands, with the counter anions being nitrate ions. Depending on the alkyl chain length, the complexes were found to be liquids at room temperature or melting slightly above this. For the solid compounds it was possible to elucidate the structure by single crystal X-ray analysis. The ionic liquids are electroactive, have good mass transport properties and can be used for the electrodeposition of silver at high current densities. The thermal properties and stability of these compounds were tested by differential scanning calorimetry and thermogravimetric analysis. The viscosity of the ionic liquids follows a Vogel-Tamman-Fulcher relationship as a function of temperature. The electrochemical properties of the complexes were tested by cyclic voltammetry and the resulting electrodeposits were examined using scanning electron microscopy and atomic force microscopy.

Electrodeposition of germanium from the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide.

The electrodeposition of germanium from the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide ([BMP][DCA]) and a mixture of [BMP][DCA] and 1-butyl-1-methylpyrrolidinium chloride ([BMP]Cl) was studied using cyclic voltammetry and using an electrochemical quartz crystal microbalance (EQCM). [GeCl4(BuIm)2] (BuIm = N-butylimidazole) was used as germanium source as it has a solubility of 0.47 M, up to 13 times the solubility of GeCl4 in [BMP][DCA]. Cyclic voltammograms show an irreversible electrochemical behavior and two reduction waves were observed. The wave at the more positive potential was assigned to the reduction of Ge(4+) to Ge(2+). The wave at the more negative potential was attributed to the formation of Ge(0). The diffusion coefficient of Ge(4+) in [BMP][DCA] containing 0.1 M [GeCl4(BuIm)2] is 1.1 × 10(-12) m(2) s(-1), and the exchange current density is 2 × 10(-4) A m(-2) at 50 °C. Polymerization of dicyanamide anions took place at the anode in the solution of [BMP][DCA]. The polymerization reaction could be avoided by using an equimolar [BMP]Cl-[BMP][DCA] mixture as electrolyte. Smooth, porous germanium films were electrodeposited on both copper and silicon substrates.

Direct electroplating of copper on tantalum from ionic liquids in high vacuum: origin of the tantalum oxide layer.

In this paper, it is shown that high vacuum conditions are not sufficient to completely remove water and oxygen from the ionic liquid 1-ethyl-3-methylimidazolium chloride. Complete removal of water demands heating above 150 °C under reduced pressure, as proven by Nuclear Reaction Analysis (NRA). Dissolved oxygen gas can only be removed by the use of an oxygen scavenger such as hydroquinone, despite the fact that calculations show that oxygen should be removed completely by the applied vacuum conditions. After applying a strict drying procedure and scavenging of molecular oxygen, it was possible to deposit copper directly on tantalum without the presence of an intervening oxide layer.

Continuous synthesis of peralkylated imidazoles and their transformation into ionic liquids with improved (electro)chemical stabilities.

A versatile and efficient method to synthesize tetrasubstituted imidazoles via a one-pot modified Debus-Radziszewski reaction and their subsequent transformation into the corresponding imidazolium ionic liquids is reported. The tetrasubstituted imidazoles were also synthesized by means of a continuous flow process. This straightforward synthetic procedure allows for a fast and selective synthesis of tetrasubstituted imidazoles on a large scale. The completely substituted imidazolium dicyanamide and bis(trifluoromethylsulfonyl)imide salts were obtained via a metathesis reaction of the imidazolium iodide salts. The melting points and viscosities are of the same order of magnitude as for their non-substituted analogues. In addition to the superior chemical stability of these novel ionic liquids, which allows them to be applied in strong alkaline media, the improved thermal and electrochemical stabilities of these compounds compared with conventional imidazolium ionic liquids is also demonstrated by thermogravimetrical analysis (TGA) and cyclic voltammetry (CV). Although increased substitution of the ionic liquids does not further increase thermal stability, a definite increase in cathodic stability is observable.

Heteroleptic silver-containing ionic liquids.

The first examples of structurally characterised mixed-ligand metal-containing ionic liquids (ILs) are presented, synthesised by the use of different N-alkylimidazoles. The cations consist of two-coordinate silver(i) centres ligated by two different N-alkylimidazole ligands. It is shown that the resulting ionic liquids have lower melting points than the single ligand ILs.

High current density electrodeposition from silver complex ionic liquids.

Liquid metal salts are electrolytes with the highest possible metal concentration for electrodeposition, because the metal ion is an integral part of the solvent. This paper introduces the new ionic silver complexes [Ag(MeCN)(4)](2)[Ag(Tf(2)N)(3)], [Ag(MeCN)][Tf(2)N] and [Ag(EtIm)(2)][Tf(2)N], where MeCN stands for acetonitrile, EtIm for 1-ethylimidazole and Tf(2)N is bis(trifluoromethylsulfonyl)imide. These complexes have been characterized by differential scanning calorimetry, single crystal X-ray crystallography, thermogravimetrical analysis, Raman spectroscopy and cyclic voltammetry. [Ag(MeCN)(4)](2)[Ag(Tf(2)N)(3)] is a room temperature ionic liquid. Smooth silver layers of good quality could be deposited from it, at current densities of up to 25 A dm(-2) in unstirred solutions. [Ag(EtIm)(2)][Tf(2)N] melts at 65 °C and can be used as an electrolyte for silver deposition above this temperature. [Ag(MeCN)][Tf(2)N] has a melting point that is too high to be useful in electrodeposition. Addition of thiourea or 1H-benzotriazole to the electrolyte decreased the surface roughness of the silver coatings. The morphology of the metal layers was investigated by atomic force microscopy (AFM). Adsorption of 1H-benzotriazole on the silver metal surface has been proven by Raman spectroscopy. This work shows the usefulness of additives in improving the quality of metal films electrodeposited from ionic liquids.

Oscillating electrochemical reaction in copper-containing imidazolium ionic liquids.

An example of an electrochemical oscillator in ionic liquids is presented. Solutions of the ionic liquid 1-ethyl-3-methylimidazolium chloride, [C(2)mim]Cl, which contain both Cu(+) and Cu(2+) ions, show current oscillations during potentiostatic polarization. The oscillations were analyzed by the Quartz Crystal Microbalance (QCM) technique and by Electrochemical Impedance Spectroscopy (EIS). The electrochemical oscillations are of the N-NDR-type, because the low frequency end of the impedance spectrum has negative real impedances. The oscillating current leads to an oscillating growth speed of a metallic copper layer. Besides the presence of both Cu(+) and Cu(2+), the presence of chloride is a necessary, yet not a sufficient, condition for the occurrence of current oscillations. Oscillating currents were also observed for the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-butyl-2,3-dimethylimidazolium chloride, but not for tributyltetradecylphosphonium chloride and N-butylpyridinium chloride.

Copper(I)-containing ionic liquids for high-rate electrodeposition.

New metal-containing ionic liquids [Cu(CH(3)CN)(n)][Tf(2)N] (n=2, 4; Tf(2)N=bis(trifluoromethylsulfonyl)- amide) have been synthesised and used as a non-aqueous electrolyte for the electrodeposition of copper at current densities greater than 25 A dm(-2). The tetrahedral copper(I)-containing cation in [Cu(CH(3)CN)(4)][Tf(2)N] is structurally analogous to quaternary ammonium and phosphonium ionic liquids and overcomes problems of metal solubility and mass transport. Two CH(3)CN ligands are removed at elevated temperatures to give [Cu(CH(3)CN)(2)][Tf(2)N], which can be used as a concentrated non-aqueous electrolyte. The structural and electrochemical characterisation of these compounds is described herein.

Cobalt(II) complexes of nitrile-functionalized ionic liquids.

A series of nitrile-functionalized ionic liquids were found to exhibit temperature-dependent miscibility (thermomorphism) with the lower alcohols. Their coordinating abilities toward cobalt(II) ions were investigated through the dissolution process of cobalt(II) bis(trifluoromethylsulfonyl)imide and were found to depend on the donor abilities of the nitrile group. The crystal structures of the cobalt(II) solvates [Co(C(1)C(1CN)Pyr)(2)(Tf(2)N)(4)] and [Co(C(1)C(2CN)Pyr)(6)][Tf(2)N](8), which were isolated from ionic-liquid solutions, gave an insight into the coordination chemistry of functionalized ionic liquids. Smooth layers of cobalt metal could be obtained by electrodeposition of the cobalt-containing ionic liquids.