Showing papers on "Supporting electrolyte published in 1997"

Electrochemical reduction of carbon dioxide on flat metallic cathodes

Abstract: The most important methods for the electrochemical reduction of carbon dioxide on flat metallic cathodes have been systematically summarized using a novel classification approach. In contrast to the usual classification systems that were based solely on the products of electrolysis, the electroreduction procedures have been grouped according to both the nature of the cathode (sp or d group metal electrodes) and the solvent used for the supporting electrolyte (aqueous or nonaqueous solutions). The new classification system allows the identity of the electroreduction product to be better related to the nature of the metallic electrode and the supporting electrolyte. Similar reduction products are formed by each of the four possible combinations of electrodes and supporting electrolytes (sp group metals in aqueous and nonaqueous electrolytes, and d group metals in aqueous and nonaqueous electrolytes, respectively). The discussion has included both synthetic aspects and mechanistic considerations. Of special interest in this review is the discussion of procedures for the selective preparation of formic acid and for the manufacturing of hydrocarbons and/or alcohols using carbon dioxide as the carbon source.

Differential Capacitance Measurements in Solvent-Free Ionic Liquids at Hg and C Interfaces

Abstract: Series-stacked, double-layer carbon capacitors are slated to be used in electric vehicles for power management as well as in consumer electronics for memory backup and burst power. Nonaqueous electrolytes are preferred over aqueous electrolytes, since a wider voltage window can be accessed in the former electrolytes, thereby requiring fewer cells in the series stack. However, it has historically been difficult to assess whether the organic solvent and/or the supporting electrolyte determine the anodic limit. We have eliminated this ambiguity by using solvent-free ionic liquids where the source of anodic oxidation may be ascribed to the anion alone. Even though the new ionic liquids manifested high oxidation limits, we found that when used in practical capacitors comprising high-surface-area carbon cloth electrodes, a much lower capacitance (compared to smooth electrodes) was achieved. To understand whether the observed decrease in capacitance might be due to the microporosity of the carbon cloth electrode or to practical limitation of the device itself. we first measured the differential capacitance (C dl ) at a Hg/1-ethyl-3-methyl imidazolium imide. The integral capacitance at the Hg interface was then calculated and compared with that of a smooth glassy carbon electrode, a carbon yarn, and a cloth electrode. In addition, the effect of (CF 3 SO 2 ) 3 C - , (CF 3 SO 2 ) 2 N - , CF 3 SO 3 - , an BF 4 - on C dl were interpreted based on existing theories of double-layer structure.

Li‐Ion Battery Electrolyte Formulated for Low‐Temperature Applications

Abstract: Low-temperature (<0 C) applications of Li-ion batteries have prompted the search for improved, high-conductivity electrolytes. Because the performance of the carbonaceous anode is highly sensitive to changes in electrolyte composition, the authors focused their efforts on this electrode. Electrolytes containing LiAsF{sub 6}, LiPF{sub 6}, LiN(SO{sub 2}CF{sub 3}){sub 2}[lithium bis(trifluoromethanesulfonyl)imide], or LiIm, and LiC(SO{sub 2}CF{sub 3}){sub 3} [lithium tris(trifluoromethanesulfonyl)methide], or LiMe, in methyl formate (MF)-ethylene carbonate (EC) solvent mixtures were tested in lithium-graphite half-cells. The graphite electrodes could be cycled at ambient temperature with high reversible capacity. The best supporting electrolyte was found to be LiAsF{sub 6}, and the presence of a high concentration of ethylene carbonate and up to 300 ppm H{sub 2}O in the solution considerably increased the reversible capacity upon cycling. The conductivity values of a binary solvent mixture of methyl formate and ethylene carbonate containing LiAsF{sub 6} or LiMe were measured between {minus}40 C and room temperature. Graphite electrodes cycled at {minus}2 C in these electrolytes obtained reasonable reversible capacity, approaching 50%.

Electrochemistry of Conductive Polymers XXII. Electrochemical and Spectroelectrochemical Studies of Polyazulene Growth and Its Characterization

Abstract: Electrochemical growth of the polyazulene film and its electrochemical and spectroscopic properties were studied at platinum electrodes in acetonitrile by ac impedance and in situ spectroelectrochemical techniques. During polymer growth from the acetonitrile solution containing a millimolar concentration of azulene with 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte, absorption bands were observed at 380, 480, and 680 nm. These bands were assigned to arise from neutral oligomers, their first oxidation products, and their second oxidation products, respectively. When the polymer film was oxidized in acetonitrile after the polymer is fully grown, absorption bands were red shifted significantly to 580 and around 800 nm, respectively, suggesting that the absorption bands are dependent on the chain lengths.

Electrolytic removal of metals using a flow-through cell with a reticulated vitreous carbon cathode

Abstract: The aim of this work was to establish an electrolytic method for the removal of lead from wastewater using a three-dimensional, reticulated vitreous carbon cathode. During the development of the experimental setup, particular attention was paid to the electrolyte flow rate and to the cathode porosity. The electrolytic cell employed potential values in such a way that the lead reduction reaction occurred under mass transport control. The potentials were determined by hydrodynamic voltammetry using a borate/nitrate solution as the supporting electrolyte on a viterous carbon rotating disc electrode. The cell proved to be efficient in removing lead and was able to reduce the levels of this metal to 0.1mgdm-3 in 20min of recirculation, using the ratio catholyte volume/cathode volume equal to 0.027. The best configuration for this removal was a cathode porosity of 80ppi and a solution flow rate of 240dm3h-1.

Effect of Counterion Valence and Polymer Charge Density on the Pair Potential of Two Polyions

Abstract: A theory of attractive forces between identical highly charged polyions in a 1:1 supporting electrolyte, generated by the interaction of the layers of condensed counterions, is extended to a Z:Z‘ electrolyte and to rodlike polymer segments of any charge density. If the combined charge density of a polyion pair does not exceed the charge density critical for counterion condensation, then there is no attraction. But an attraction arises if the combined charge density does exceed the condensation threshold, even if the individual polymer charge densities lie below the critical value. The intensity of the attraction at a fixed ionic strength decreases with higher values of counterion valence. The effect of increased ionic strength is to decrease both the range and the intensity of the attractive force.

Amperometric Sensor for Monitoring Ethylene

Abstract: Sensing electrodes were fabricated by chemical deposition of gold onto a Nafion membrane. These electrodes, with suitable choice of real surface area, applied potential, supporting electrolyte, and gas flow rate, showed a detection limit of 40 ppb ethylene for a signal-to-noise ratio of 3. A linear response was observed up to at least 500 ppm. The choice of flow rate was especially influential as it was found to affect the mechanism of the sensor response, which is kinetically controlled even at relatively low flow rates. No cross-sensitivity was found to ethanol and acetaldehyde which might be present in certain applications of the sensor, such as in horticulture.

Carbon paste electrodes plated with a gold film for the voltammetric determination of mercury(II)

Abstract: Carbon pastes of different compositions were tested as supports for gold film electrodes. A paste containing highly-viscous silicone oil as a binder was found the most suitable. Experimental conditions were optimized in order to develop a differential pulse anodic stripping voltammetric method for the determination of mercury in water samples. In a supporting electrolyte containing HNO3 (0.1 mol/L), (KCl 0.02 mol/L) and tetrachloroaurate(III) for in situ plating with a gold film, the response of Hg was found to be linear in a concentration range from 1 × 10−9 up to 2.5 × 10−7 mol/L (0.2–50 μg/L; accumulation potential +0.2V (vs. Ag/AgCl); accumulation time 30 s–5 min) with a detection limit of about 2.5 × 10−10 mol/L (50 ng/L) after accumulating for 30 min. Reliability of the method depended mainly on the concentration of AuIII in the electrolyte which had to be chosen adequately, with respect to the concentration level of HgII in the sample. Interferences from some ions with the determination of mercury were also studied. The method was tested on both model and real samples and the results agreed well with those obtained by reference determinations.

Electrochemically Assisted Sol−Gel Process for the Synthesis of Polysiloxane Films Incorporating Phenothiazine Dyes Analogous to Methylene Blue. Structure and Ion-Transport Properties of the Films via Spectroscopic and Electrochemical Characterization

Abstract: A sol−gel process is directed toward an electrode surface via electrochemical manipulation of the solubility of trimethoxysilyl group-modified methylene blue in an aqueous environment. The process lasts for 2−3 h and results in electrode derivatization with a polysiloxane network incorporating methylene blue. Cross-linking (curing) is completed by drying the films at room temperature for 2 days. The concentration of the phenothiazine moieties in the resulting xero films was calculated at ∼3.9 M, and the film density at ∼2.6 g/cm3. The average distance between phenothiazine moieties is <5 A allowing interactions between their π-systems. The narrow pores between monomer units restrict movement of hydrated charge-compensating ions, so that the redox switching of the films depends upon the chemical identity of both the cation and anion of the supporting electrolyte. The films retain the electrochromic and electrocatalytic properties of the parent dye; for example, gold electrodes derivatized with the film med...

Preparation of conducting fibers via the electrochemical polymerization of pyrrole

Abstract: Cyclic voltammograms of pyrrole monomer and polypyrrole films prepared potentiostatically at 1.0 V versus an Ag/AgCl electrode have been examined. The insulating natural fibers, such as cotton, silk, and wool become electrically conducting when they are subjected to electrical treatment in the polymerizing solution of pyrrole in acetonitrile containing p-toluenesulfonic acid as a supporting electrolyte. The weight gain and the electrical conductivity of the fibers increase with the time of electrolysis and impressed current levels. The conductivities are in the range of 0.2 to 15 s/cm and dependent on the nature of the fibers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1073–1077, 1997

Charge Transport in Ferrocene-Containing Polyacrylamide-Based Redox Gels

Abstract: The charge transport properties of the redox gels formed from the copolymerization of vinylferrocene (VF) with acrylamide and N,N‘-methylenebisacrylamide (PVAB gels) are described. The apparent electron diffusion coefficients (Dap) measured by cyclic voltammetry and chronocoulometry exhibit an upward bowlike curvature vs immobilized VF concentration ([VF]imm) within the range 0.1−1.60 mM in 0.10 M supporting electrolyte (NaH2PO4/Na2HPO4, NaClO4, or NaNO3). The range of measured Dap values [(0.6−6.0) × 10-7 cm2 s-1] can be interpreted in terms of the mean-field model developed by Blauch and Saveant (Blauch, D. N.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 3323). The large Dap values are attributed to the large λ (range of molecular motion permitted to the ferrocene residues) and large kex (bimolecular electron self-exchange rate constant of the ferrocenes in the PVAB gels). It was further observed that Dap decreases on increasing the electrolyte (NaClO4 or NaNO3) concentration from 0.10 to 0.50 M, especi...

Electrolytic Partial Fluorination of Organic Compounds. 22.1 Highly Regioselective Anodic Monofluorination of Oxindole and 3-Oxo-1,2,3,4-tetrahydroisoquinoline Derivatives: Effects of Supporting Fluoride Salts and Anode Materials

Abstract: Highly regioselective monofluorination of 1-aryl-3-(phenylthio)oxindoles and 2-substituted-3-oxo-4-(phenylthio)-1,2,3,4-tetrahydroisoquinolines can be successfully carried out in the presence of Me4NF·4HF or Et4NF·3HF as a supporting electrolyte to provide the corresponding 3-fluorinated oxindole and 4-fluorinated isoquinoline derivatives in good yields. Carbon anodes as well as a platinum anode were found to be effective for the fluorination when Me4NF·4HF was used as the supporting electrolyte.

Enantioselective Cathodic Reduction of 4‐Methylcoumarin: Dependence of Selectivity on Reaction Conditions and Investigation of the Mechanism

Abstract: The cathodic reduction of 4-methylcoumarin (1) in acidic methanol/water in the presence of yohimbine leads to formation of a mixture of the hydrogenation product 4-methyl-3,4-dihydrocoumarin (2), with an enantiomeric excess (ee) of (R)-2 of 0–67%, and the hydrodimer 3. The relative yields of 2 and 3 and the ee of 2 depend on a number of experimental parameters such as pH, supporting electrolyte, working potential, and the concentrations of substrate and yohimbine, as demonstrated by a series of preparative-scale experiments. In addition, a series of voltammetric and kinetic measurements were carried out to investigate the influence of the individual experimental parameters. Three mechanistic possibilities have been examined, and by combination of the analytical data with the results of the preparative experiments, a single model is put forward which is in accord with the available results. The main features of the mechanistic model can be summarized as follows: 1) under acidic conditions (pH 2–3) the electroactive species is a complex between 1 and H3O+, the reduction of which leads to an enolic radical; 2) this radical is not reduced at the working potential but tautomerizes into the more easily reduced keto radical or dimerizes; 3) the keto radical is reduced and further protonated; 4) the function of the yohimbineH+ is to catalyze the tautomerization and enantioselectively protonate the final carbanion. Additionally, we conclude that the concentration of yohimbine in the immediate vicinity of the electrode is considerably higher than its stoichiometric concentration. Quantum chemical calculations demonstrate that si protonation of the intermediate anion by yohimbineH+ to give (R)-2 is energetically favored.