Electrode kinetics and mechanism of iodine reduction in the room-temperature ionic liquid [C(4)mim][NTf2]

The fast electrochemical reduction of iodine in the RTIL 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide, [C(4)mim][NTf2], is reported and the kinetics and mechanism of the process elucidated. Two reduction peaks were observed. The first reduction peak is assigned to the process

Electrooxidation of the iodides [C(4)mim]I, LiI, NaI, KI, RbI, and CsI in the room temperature ionic liquid [C(4)mim][NTf2]

The electrochemical oxidation of 1-butyl-3-methylimidazolium iodide, [C(4)mim]I, has been investigated by cyclic voltammetry at a platinum microelectrode at varying concentrations in the RTIL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(4)mim][NTf2]. Two oxidation peaks were observed. The first peak is assigned to the oxidation of iodide to triiodide, in an overall two-electron process: 3I(-)- 2e(-) -> I-3(-). At higher potentials, the electrogenerated triiodide oxidizes to iodine, in an overall one-electron process: I-3(-) - e(-) -> 3/2I(2). An average diffusion coefficient, D, for I- of 1.55 x 10(-11) m(2) s(-1) was obtained. A digital simulation program was used to simulate the voltammetric response, and kinetic parameters were successfully extracted. The parameters deduced from the simulation include D for I-, I-3(-), and I-2 and K-eq,K-2, the equilibrium constant for the reaction of iodide and iodine to form triiodide. Values for these parameters are of the same order as those previously published for the oxidation of Br- in the same RTIL [Allen et al. J. Electroanal. Chem. 2005, 575, 311]. Next, the cyclic voltammetry of five different inorganic iodide salts was studied by dissolving small amounts of the solid in [C(4)mim][NTf2]. Similar oxidation peaks were observed, revealing diffusion coefficients of ca. 0.55, 1.14, 1.23, 1.44, and 1.33 x 10(-11) m(2) s(-1) and solubilities of 714, 246, 54, 83, and 36 mM for LiI, NaI, KI, RbI, and CsI, respectively. The slightly smaller diffusion coefficients for the XI salts (compared to [C(4)mim]I) may indicate that I- is ion-paired with Li+, Na+, K+, Rb+, and Cs+ in the RTIL medium.

Electroreduction of sulfur dioxide in some room-temperature ionic liquids

The mechanism of sulfur dioxide reduction at a platinum microelectrode was investigated by cyclic voltammetry in several room-temperature ionic liquids (RTILs)-[C(2)mim][NTf2], [C(4)mim][BF4], [C(4)mim][NO3], [C(4)mim][PF6], and [C(6)mim][Cl] where [C(2)mim] is 1-ethyl-3-methylimidazolium, [C(4)mim] is 1-butyl-3-methylimidazolium, [C(6)mim] is 1-hexyl-3-methylimidazolium, and [NTf2] is bis(trifluoromethylsufonyl)imide-with special attention paid to [C(4)mim][NO3] because of the well-defined voltammetry, high solubility, and relatively low diffusion coefficient of SO2 obtained in that ionic liquid. A cathodic peak is observed in all RTILs between -2.0 and -1.0 V versus a silver quasi-reference electrode. In [C(4)mim][NO3], the peak appears at -1.0 V, and potential step chronoamperometry was used to determine that SO2 has a very high solubility of 3100 (+/-450) mM and a diffusion coefficient of 5.0 (+/-0.8) x 10(-10) m(2) s(-1) in that ionic liquid. On the reverse wave, up to four anodic peaks are observed at ca. -0.4, -0.3, -0.2, and 0.2 V in [C(4)mim][NO3]. The cathodic wave is assigned to the reduction of SO2 to its radical anion, SO2-center dot. The peaks at -0.4 and -0.2 V are assigned to the oxidation of unsolvated and solvated SO2-center dot, respectively. The peak appearing at 0.2 V is assigned to the oxidation of either S2O42- or S2O4-center dot. The activation energy for the reduction of SO2 in [C(4)mim][NO3] was measured to be 10 (+/-2) kJ mol(-1) using chronoamperometric data at different temperatures. The stabilizing interaction of the solvent with the reduced species SO2-center dot leads to a different mechanism than that observed in conventional aprotic solvents. The high sensitivity of the system to SO2 also suggests that [C(4)mim][NO3] may be a viable solvent in gas sensing applications.

Voltammetric characterization of the ferrocene vertical bar ferrocenium and cobaltocenium vertical bar cobaltocene redox couples in RTILs

Ferrocene, Fc, and cobaltocenium hexafluorophosphate, CcPF(6), have been recommended for use as internal reference redox couples in room-temperature ionic liquids (RTILs), as well as in more conventional aprotic solvents. In this study, the electrochemical behavior of Fc and CcPF(6) is reported in eight commonly used RTILs; [C(2)mim][NTf2], [C(4)mim][NTf2], [C(4)mim][BF4], [C(4)mim][PF6], [C(4)mim][OTf], [C(4)mim][NO3], [C(4)mpyrr][NTf2], and [P-14,P-6.6,P-6][FAP], where [C(n)mim](+) = 1-butyl-3-methylimidazolium, [NTf2](-) = bis(trifluoromethylsulfonyl)imide, [BF4](-) = tetrafluoroborate, [PF6](-) = hexafluorophosphate, [OTf](-) = trifluoromethylsulfonate, [NO3](-) = nitrate, [C(4)mpyrr](+) = N-butyl-N-methylpyrrolidinium, [P-14,P-6,P-6,P-6](+) = tris(ri-hexyl)-tetradecylphosphonium and [FAP](-) = trifluorotris(pentafluoroethyl)phosphate, over a range of concentrations and temperatures. Solubilities and diffusion coefficients, D, of both the charged and neutral species were determined using double potential-step chronoamperometry, and CcPF(6) (36.5-450.0 mM) was found to be Much more Soluble than Fc (27.5-101.8 mM). It was observed that classical Stokes-Einstein diffusional behavior applies for Fc and CcPF(6) in all eight RTILs. Diffusion coefficients of Fc and CcPF(6) were calculated at a range of temperatures, and activation energies calculated. It was also determined that D for Fc and CcPF(6) does not change significantly with concentration. This supports the use of both Fe and CcPF(6) to provide a well-characterized and model redox couple for use as a voltammetric internal potential reference in RTILs contrary to previous literature reports in the former case.

Electrochemical ammonia gas sensing in nonaqueous systems: A comparison of propylene carbonate with room temperature ffonc liquids

First, the direct and indirect electrochemical oxidation of ammonia has been studied by cyclic voltammetry at glassy carbon electrodes in propylene carbonate. In the case of the indirect oxidation of ammonia, its analytical utility of indirect for ammonia sensing was examined in the range from 10 and 100 ppm by measuring the peak current of new wave resulting from reaction between ammonia and hydroquinone, as function of ammonia concentration, giving a sensitivity 1.29 x 10(-7) A ppm(-1) (r(2)=0.999) and limit-of-detection 5 ppm ammonia. Further, the direct oxidation of ammonia has been investigated in several room temperature ionic liquids (RTILs), namely 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim] [BF4]), 1-butyl-3-methylimiclazolium trifluoromethylsulfonate ([C4mim] [OTf]), 1-Ethyl -3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(2)mim] [NTf2]), 1-butyl-3-methylimidazolium bis(tritluoromethylsulfonyl)imide ([C4mim] [NTf2]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim] [PF6]) on a 10 put diameter Pt microdisk electrode. In four of the RTILs studied, the cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen, N-2, and protons, which are transferred to an ammonia molecule, forming NH4+ via the protonation of the anion(s) (A(-)). However, in [C4mim] [PF6], the protonated anion was formed first, followed by NH4+. In all five RTILs, both HA and NH4+ are reduced at the electrode surface, forming hydrogen gas, which is then oxidized. The analytical ability of this work has also been explored further, giving a limit-of-detection close to 50 ppm in [C(2)mim] [NTf2], [C(4)mim] [OTf], [C(4)mim] [BF4], with a sensitivity of ca. 6 x 10(-7) A ppm(-1) (r(2) = 0.999) for all three ionic liquids, showing that the limit of detection was ca. ten times larger than that in propylene carbonate since ammonia in propylene carbonate might be more soluble in comparison with RTILs when considering the higher viscosity of RTILs.

The electrochemical oxidation and reduction of nitrate ions in the room temperature ionic liquid [C(2)mim][NTf2]; the latter behaves as a 'melt' rather than an 'organic solvent'

The electrochemical oxidation of 1-butyl-3-methylimidazolium nitrate [C(4)mim][NO3] was studied by cyclic voltammetry in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [C(2)mim][NTf2]. A sharp peak was observed on a Pt microelectrode (d = 10 mu m), and a diffusion coefficient at infinite dilution of ca. 2.0 x 10(-11) m(2) s(-1) was obtained. Next, the cyclic voltammetry of sodium nitrate (NaNO3) and potassium nitrate (KNO3) was studied, by dissolving small amounts of solid into the RTIL [ C2mim][ NTf2]. Similar oxidation peaks were observed, revealing diffusion coefficients of ca. 8.8 and 9.0 x 10(-12) m(2) s(-1) and solubilities of 11.9 and 10.8 mM for NaNO3 and KNO3, respectively. The smaller diffusion coefficients for NaNO3 and KNO3 (compared to [C(4)mim][NO3]) may indicate that NO3- is ion-paired with Na+ or K+. This work may have applications in the electroanalytical determination of nitrate in RTIL solutions. Furthermore, a reduction feature was observed for both NaNO3 and KNO3, with additional anodic peaks indicating the formation of oxides, peroxides, superoxides and nitrites. This behaviour is surprisingly similar to that obtained from melts of NaNO3 and KNO3 at high temperatures ( ca. 350 - 500 degrees C), and this observation could significantly simplify experimental conditions required to investigate these compounds. We then used X-ray photoelectron spectroscopy (XPS) to suggest that disodium( I) oxide (Na2O), which has found use as a storage compound for hydrogen, was deposited on a Pt electrode surface following the reduction of NaNO3.

Electrochemical studies of gold and chloride in ionic liquids

For the first time, the electrochemistry of gold has been studied in detail in a 'second-generation', non-haloaluminate, ionic liquid. In particular, the electrochemical behaviour of Na[AuCl4] has been investigated in 1-butyl-3-methylimidazolium bis{(tifluoromethyl)sulfonyl} imide, [C(4)mim][NTf2], over gold, platinum and glassy carbon working electrodes. The reduction of [AuCl4](-) initially forms [AuCl2](-) before deposition on the electrode as Au(0). To enable stripping of deposited gold or electrodissolution of bulk gold, the presence of chloride, trichloride or chlorine is required. Specifically trichloride and chlorine have been identified as the active species which preferentially form Au(I) and Au(III), respectively.

Liquid-liquid miscibility and volumetric properties of aqueous solutions of ionic liquids as a function of temperature

The volumetric properties of seven {water + ionic liquid} binary mixtures have been studied as a function of temperature from (293 to 343) K. The phase behaviour of the systems was first investigated using a nephelometric method and excess molar volumes were calculated from densities measured using an Anton Paar densimeter and fitted using a Redlich-Kister type equation. Two ionic liquids fully miscible with water (1-butyl-3-methylimidazolium tetrafluoroborate ([CCIm][BF]) and 1-ethyl-3-methylimidazolium ethylsulfate ([CCIm][EtSO])) and five ionic liquids only partially miscible with water (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CCIm][NTf]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CCIm][NTf]), 1-butyl-3-methylimidazolium hexafluorophosphate ([CCIm][PF]), 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([CCPyrro][NTf]), and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N][NTf])) were chosen. Small excess volumes (less than 0.5 cm · mol at 298 K) are obtained compared with the molar volumes of the pure components (less than 0.3% of the molar volume of the pure ionic liquid). For all the considered systems, except for {[CCIm][EtSO] + water}, positive excess molar volumes were calculated. Finally, an increase of the non-ideality character is observed for all the systems as temperature increases. © 2009 Elsevier Ltd. All rights reserved.

The Reduction of Oxygen in Various Room Temperature Ionic Liquids in the Temperature Range 293-318 K: Exploring the Applicability of the Stokes-Einstein Relationship in Room Temperature Ionic Liquids

The voltammetry for the reduction of oxygen at a microdisk electrode is reported in six commonly used RTILs: [C(4)mim][NTf2], [C(4)mpyrr][NTf2], [C(4)dmim][NTf2], [C(4)mim][BF4], [C(4)mim][PF6], and [N-6.2.2.2][NTf2], where [C(4)mim](+) is 1-butyl-3-methylimidazolium, [NTf2](-) is bis(trifluoromethanesulfonyl)imide, [C(4)mpyrr](+) is N-butyl-N-methylpyrrolidinium, [C(4)dmim](+) is 1-butyl-2,3-methylimidazolium, [BF4](-) is tetrafluoroborate, [PF6](-) is hexafluorophosphate, and [N-6.2.2.2](+) is n-hexyltriethylammonium at varying scan rates (50-4000 mV s(-1)) and temperatures (293-318 K). Diffusion coefficients, D, of oxygen are deduced at each temperature from potential-step chronoamperometry, and diffusional activation energies are calculated. Oxygen solubilities are also reported as a function of temperature. In the six ionic liquids, the Stokes-Einstein relationship (D proportional to eta(-1)) was found to apply only very approximately for oxygen. This is considered in relationship to the behavior of other diverse solutes in RTILs.

Effect of Water on the Electrochemical Window and Potential Limits of Room-Temperature Ionic Liquids

The effect of water content on room-temperature ionic liquids (RTILs) was studied by Karl Fischer titration and cyclic voltammetry in the following ionic liquids: tris(P-hexyl)tetradecylphosphonium trifluorotris(pentafluoroethyl)phosphate [P-14,P-6,P-6,P-6][NTf2], N-butyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [C(4)mpyrr][NTf2], 1-hexyl-3-methylimidazolium tris(perfluoroethyl)trifluorophosphate [C(6)mim][FAP], 1-butyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(4)mim][NTf2], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(4)dmim][NTf2], N-hexyltriethylammonium bis(trifluoromethylsolfonyl)imide [N-6,N-2,N-2,N-2][NTf2], 1-butyl-3-methylirnidazolium hexafluorophosphate [C(4)mim][PF6], F6], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(2)mim][NTf2], 1-butyl-3-methylimidazolium tetrafluoroborate [C(4)mim][BF4], 1-hexyl-3-methylimidazolium iodide [C(4)mim][I], 1-butyl-3-methylimidazolium trifluoromethylsulfonate [C(4)mim][OTf], and 1-hexyl-3-methylimidazolium chloride [C(6)mim][Cl]. In addition, electrochemically relevant properties such as viscosity, conductivity, density, and melting point of RTILs are summarized from previous literature and are discussed. Karl Fisher titrations were carried out to determine the water content of RTILs for vacuum-dried, atmospheric, and wet samples. The anion in particular was found to affect the level of water uptake. The hydrophobicity of the anions adhered to the following trend: [FAP](-) > [NTf2](-) > [PF6](-) > [BF4](-) > halides. Cyclic voltammetry shows that an increase in water content significantly narrows the electrochemical window of each ionic liquid. The electrochemical window decreases in the following order: vacuum-dried > atmospheric > wet at 298 K > 318 K > 338 K. The anodic and cathodic potentials vs ferrocene internal reference are also listed under vacuum-dried and atmospheric conditions. The data obtained may aid the selection of a RTIL for use as a solvent in electrochemical applications.

Speciation of Rare-Earth Metal Complexes in Ionic Liquids: A Multiple-Technique Approach

The dissolution process of metal complexes in ionic liquids was investigated by a multiple-technique approach to reveal the solvate species of the metal in solution. The task-specific ionic liquid betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) is able to dissolve stoichiometric amounts of the oxides of the rare-earth elements. The crystal structures of the compounds [Eu-2(bet)(8)(H2O)(4)][Tf2N](6), [Eu-2(bet)(8)(H2O)(2)][Tf2N](6)center dot 2H(2)O, and [Y-2(bet)(6)(H2O)(4)][Tf2N](6) were found to consist of dimers. These rare-earth complexes are well soluble in the ionic liquids [Hbet][Tf2N] and [C(4)mim]- [Tf2N] (C(4)mim = 1-butyl-3-methylimidazolium). The speciation of the metal complexes after dissolution in these ionic liquids was investigated by luminescence spectroscopy, H-1, C-13, and Y-89 NMR spectroscopy, and by the synchrotron techniques EXAFS (extended X-ray absorption fine structure) and HEXS (high-energy X-ray scattering). The combination of these complementary analytical techniques reveals that the cationic dimers decompose into monomers after dissolution of the complexes in the ionic liquids. Deeper insight into the solution processes of metal compounds is desirable for applications of ionic liquids in the field of electrochemistry, catalysis, and materials chemistry.

Templated electrodeposition of silver nanowires in a nanoporous polycarbonate membrane from a nonaqueous ionic liquid electrolyte

Template electrodeposition has been used to prepare a wide range of nanostructures but has generally been restricted to aqueous electrolytes. We report the deposition of silver nanowires in a commercial nuclear track-etched polycarbonate template from the nonaqueous ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) using silver electrochemically dissolved from the anode. Transmission electron microscopy (TEM) shows that the nanowires have a very high aspect ratio with an average diameter of 80 nm and length of 5 mu m. Ionic liquid electrolytes should greatly extend the range of metals that can be electrodeposited as nanowires using templates.

Ionic liquids as solvent and solvent additives for the synthesis of sol-gel materials

The ionic liquid (IL) 1-butyl-3-methylimidazolium chloride was used as a drying control chemical additive in the synthesis of silica sol-gel materials with and without methanol as a co-solvent. The resulting gels were characterized by using thermogravimetric analysis, differential scanning calorimetry, infrared spectroscopy and water sorption kinetics. Calcined gels were analyzed using scanning electron microscopy and nitrogen adsorption isotherms for surface area and pore volume determination. Non-calcined gels were monolithic and showed general cloudiness with lesser degrees observed at higher IL volumes. Calcinations resulted in the formation of powders with increased available surface area as the amount of IL volume was increased. This is consistent with an increase in respective pore volume but a general decrease in average pore size. The resulting materials exhibited conventional structural microdomains, in contrast to periodicity reported when other ionic liquids were used as templates.

Sensor technologies based on a cellulose supported platform

A simple approach to sensor development based on encapsulating a probe molecule in a cellulose support followed by regeneration from an ionic liquid solution is demonstrated here by the codissolution of cellulose and 1-(2-pyridylazo)-2-naphthol in 1-butyl-3-methylimidazolium chloride followed by regeneration with water to form strips which exhibit a proportionate (1 : 1) response to Hg(II) in aqueous solution.

In search of ionic liquids incorporating azolate anions

Twenty-eight novel salts with tetramethyl-, tetraethyl-, and tetrabutylammonium and 1-butyl-3-methylimidazolium cations paired with 3,5-dinitro-1,2,4-triazolate, 4-nitro-1,2,3-triazolate, 2,4-dinitroimidazolate, 4,5-dinitroimidazolate, 4,5-dicyanoimidazolate, 4-nitroimidazolate, and tetrazolate anions have been prepared and characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and single-crystal Xray crystallography. The effects of cation and anion type and structure on the physicochemical properties of the resulting salts, including several ionic liquids, have been examined and discussed. Ionic liquids (defined as having m.p. <100 degrees C) were obtained with all combinations of the 1-butyl-3-methylimidazolium cation ([C(4)mim](+)) and the heterocyclic azolate anions studied, and with several combinations of tetraethyl or tetrabutylammonium cations and the azolate anions. The [C(4)mim](+) azolates were liquid at room temperature exhibiting large liquid ranges and forming glasses on cooling with glasstransition temperatures in the range of -53 to -82 degrees C (except for the 3,5-dinitro-1,2,4-triazolate salt with m.p. 33 degrees C). Six crystal structures of the corresponding tetraalkylammonium salts were determined and the effects of changes to the cations and anions on the packing of the structure have been investigated.