Nonadditivity of Faradaic currents and modification of capacitance currents in the voltammetry of mixtures of ferrocene and the cobaltocenium cation in protic and aprotic ionic liquids

Unexpected nonadditivity of currents encountered in the electrochemistry of mixtures of ferrocene (Fc) and cobaltocenium cation (Cc+) as the PF6 - salt has been investigated by direct current (dc) and Fourier-transformed alternating current (ac) cyclic voltammetry in two aprotic (1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate) and three protic (triethylammonium formate, bis(2-hydroxyethyl)ammonium acetate, and triethylammonium acetate) ionic liquids (ILs). The voltammetry of the individual Fc0/+ and Cc+/0 couples always exhibits near-Nernstian behavior at glassy carbon and gold electrodes. As expected for an ideal process, the reversible formal potentials and diffusion coefficients at 23 ( 1 °C in each IL determined from measurement on individual Fc and Cc+ solutions were found to be independent of electrode material, concentration, and technique used for the measurement. However, when Fc and Cc+ were simultaneously present, the dc and ac peak currents per unit concentration for the Fc0/+ and Cc+/0 processes were found to be significantly enhanced in both aprotic and protic ILs. Thus, the apparent diffusion coefficient values calculated for Fc and Cc+ were respectively found to be about 25 and 35% larger than those determined individually in the aprotic ILs. A similar change in the Fc0/+ mass transport characteristics was observed upon addition of tetrabutylammonium hexafluorophosphate (Bu4NPF6), and the double layer capacitance also varied in distinctly different ways when Fc and Cc+ were present individually or in mixtures. Importantly, the nonadditivity of Faradaic current is not associated with a change in viscosity or from electron exchange as found when some solutes are added to ILs. The observation that the 1H NMR T1 relaxation times for the proton resonance in Cc+ also are modified in mixed systems implies that specific interaction with aggregates of the constituent IL ionic species giving rise to subtle structural changes plays an important role in modifying the mass transport, double layer characteristics, and dynamics when solutes of interest in this study are added to ILs. Analogous voltammetric changes were not observed in studies in organic solvent media containing 0.1 M added supporting electrolyte. Implications of the nonadditivity of Faradaic and capacitance terms in ILs are considered.

Electrochemistry of room temperature protic ionic liquids

Eighteen protic ionic liquids containing different combinations of cations and anions, hydrophobicity, viscosity, and conductivity have been synthesized and their physicochemical properties determined. In one series, the diethanolammonium cations were combined with acetate, formate, hydrogen sulfate, chloride, sulfamate, and mesylate anions. In the second series, acetate and formate anions were combined with amine bases, triethylamine, diethylamine, triethanolamine, di-n-propylamine, and di-n-butylamine. The electrochemical characteristics of the eight protic ionic liquids that are liquid at room temperature (RTPILs) have been determined using cyclic, microelectrode, and rotating disk electrode voltammetries. Potential windows of the RTPILs have been compared at glassy carbon, platinum, gold, and boron-doped diamond electrodes and generally found to be the largest in the case of glassy carbon. The voltammetry of IUPAC recommended potential scale reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, have been evaluated and found to be ideal in the case of the less viscous RTPILs but involve adsorption in the highly viscous ones. Other properties such as diffusion coefficients, ionic conductivity, and double layer capacitance also have been measured. The influence of water on the potential windows, viscosity, and diffusion has been studied systematically by deliberate addition of water to the dried ionic liquids. The survey highlights the problems with voltammetric studies in highly viscous room temperature protic ionic liquids and also suggests the way forward with respect to their possible industrial use.

A novel non-enzymatic glucose sensor based on nickel (II) oxide electrospun nanofibers

A new enzymeless glucose sensor has been fabricated via electrospinning technology and subsequent calcination. The morphology and structure of the as-prepared nanofibers have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The electrocatalytic oxidation of glucose in alkaline medium at nickel oxide modified glassy carbon electrodes has been investigated. The modified electrodes offer excellent electrocatalytic activity toward the glucose oxidation at low positive potential (0.3 V). Glucose has been determined chronoamperometrically at the surface of NiO nanofibers modified electrode in 0.5 mM NaOH. Under the optimized condition, the calibration curve is linear in the concentration range of 2 × 10−3 mM∼1 mM, and 1 mM∼9.5 mM. The detection limit (signal-to-noise 3) and response time are 3.394 × 10−6 M and 2 s, respectively. The NiO electrospun nanofibers is easy to prepare and feasible in economy. The modified electrode is steady and can be used repeatedly, so it is reasonable to expect its broad use in non-enzymatic glucose sensor.

The voltammetric reduction, deprotonation and surface activity of ruthenium photovoltaic sensitizers in acetone

The reductive voltammetry of the photovoltaic sensitizer [(H2-dcbpy)2Ru(NCS)2] (H2-dcbpy=2,2′-bipyridine-4,4′-dicarboxylic acid) and [(H3-tctpy)Ru(NCS)3]− (H3-tctpy=2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylic acid) has been investigated in acetone. Significant surface interactions at both platinum and glassy carbon electrodes occur at 0.6 V prior to the reversible potential expected for ligand-based reduction process of the fully protonated acids. The origin of the surface interactions are attributed to the acid–base behaviour of the compounds, combined with overall deprotonation and reduction to hydrogen, since repetitive cycling of the potential reveals well-defined reversible reduction processes in the negative potential range, resulting from formation of doubly deprotonated [(H-dcbpy−)2Ru(NCS)2]2− and singly deprotonated [(H2-tctpy−)Ru(NCS)3]2−, respectively. The extent of the surface interactions has been estimated by electrochemical quartz crystal microbalance and chronocoulometric measurements. Under certain conditions, a thick conducting polymer consisting of several hundred monolayers is formed.

Cerium acetylacetonates—new aspects, including the lamellar clathrate [Ce(acac)4]·10H2O

Reactions of CeCl₃·7H₂O and Ce(NO₃)₃·6H₂O with Naacac or NH₄acac in aqueous solution at 21 and 45 °C yielded the trihydrate [Ce(acac)₃(H₂O)₂]·H₂O and the dihydrate [Ce(acac)₃(H₂O)₂], respectively, whereas similar treatment of (NH₄)₂[Ce(NO₃)₆] gave the trihydrate at both temperatures. Desiccation of the hydrates over silica gel left the dihydrate unchanged, whereas the trihydrate underwent decomposition rather than dehydration. Aerial oxidation of [Ce(acac)₃(H₂O)₂] in CH₂Cl₂ and toluene yielded α-[Ce(acac)₄] and β-[Ce(acac)₄], respectively, the structure of the former being re-determined with improved precision. Careful treatment of aqueous (NH₄)₄[Ce(SO₄)₄] and Hacac (initially pH 1–2) with aqueous ammonia to pH 5 precipitated hydrated [Ce(acac)₄], from which [Ce(acac)₄]·10H₂O was isolated as unstable, light-sensitive single crystals, and the structure was determined. The complex is a laminar clathrate containing layers of Ce(acac)₄ molecules sandwiched between extensive hydrogen-bonded layers of water molecules which do not interact with the metal. Electrochemical experiments confirmed the unstable nature of hydrated Ce^III(acac)₃, while the reduction of [Ce(acac)₄] yielded well-defined cyclic voltammograms in acetonitrile and acetone, corresponding to a quasi-reversible process. For the [Ce^IV(acac)₄]/[Ce^III(acac)₄]⁻redox couple, a calculated reversible potential of 0.22±0.02 V versus SHE was obtained in acetone or acetonitrile (0.1 M Bu₄NPF₆) at both gold and glassy carbon electrodes. This potential is consistent with the ease of both oxidation and reduction of cerium acetylacetonate complexes as found in the synthetic studies.

Physical and electrochemical properties of thioether-functionalized ionic liquids

The preparation and characterization of a series of ionic liquids based on S-alkyl thiolonium, S-alkyl thiotetrazolium, or S-alkyl thiobenzolium cations coupled with bis(trifluoromethanesulfonyl)amide, trifluoromethanesulfonate, alkyl phosphate, chloride, and hexafluorophosphate anions are reported. All are liquid at room temperature, except the chloride salt, which has a melting point of 92 °C. The electrochemical characteristics of this class of ionic liquid have been determined by cyclic voltammetry. Potential windows of the ionic liquids have been obtained at glassy carbon, platinum, and gold electrodes and found to be the largest at glassy carbon, but are limited by oxidation of the thioether-functionalized cation. The voltammetry of IUPAC reference potential scale systems, ferrocene/ferrocenium, cobaltocenium/cobaltocene, and decamethylferrocene/decamethylferrocenium have been evaluated, with the last being most widely applicable. Nonadditivity of Faradaic current is found in the voltammograms of decamethylferrocene in the presence of ferrocene and cobaltocenium. Diffusion coefficient, viscosity, ionic conductivity, double layer capacitance, and other physical properties have also been measured. The dependence of the diffusion coefficient vs viscosity follows the Stokes−Einstein relationship. The properties of the ionic liquids are compared with the related imidazolium family of ionic liquids.

Zn electrochemistry in 1-Ethyl-3-Methylimidazolium and N-Butyl-N-Methylpyrrolidinium Dicyanamides: promising new rechargeable Zn battery electrolytes

 We have studied both 1-ethyl-3-methylimidazolium (C2mim) and N-butyl-N-methylpyrrolidinium (C4mpyr) dicyanamide (dca) ionic liquids (ILs) containing 3 wt % H2O and 9 mol % Zn(dca)2 salt for their ability to support Zn0/2+ electrochemistry in the context of a rechargeable Zn battery. Despite the similarities of the two IL electrolyte systems [identical H2O and Zn(dca)2 contents], the system based on [C2mim] supported much higher current densities for Zn electrochemistry at greatly reduced overpotentials [−0.23 V vs. Zn pseudo-reference, 32 mA cm−2 (red) and 61 mA cm−2 (ox)] compared to the [C4mpyr]-based electrolyte [−0.84 V vs. Zn pseudo-reference, 8 mA cm−2 (red) and 15 mA cm−2 (ox)]. The overpotential for Zn deposition is reduced by 0.13 V on Zn metal surfaces compared to glassy carbon (GC), regardless of the electrolyte used. The morphologies of the Zn deposits on both GC and Zn surfaces were also studied. The Zn surfaces promote a deposition that displays a smooth morphology, resulting from an instantaneous nucleation mechanism demonstrated by chronoamperometric experiments. Finally, both [C2mim] and [C4mpyr] electrolytes were tested in symmetrical Zn|Zn cells, where it was determined that the [C2mim] system could sustain over 90 cycles at 0.1 mA cm−2, whereas the [C4mpyr] based system could only achieve 15 cycles at the more modest current density of 0.05 mA cm−2.

One-pot facile synthesis of platinum nanoparticle decorated reduced graphene oxide composites and their application in electrochemical detection of rutin

We report on the synthesis of platinum nanoparticle-reduced graphene oxide (PtNP-rGO) composites and their application as a novel architecture in electrochemical detection of rutin. PtNPs anchored over rGO are synthesized through a facile one-pot synthesis method, where the reduction of GO and in situ generation of PtNPs occurred concurrently. The characterization results of transmission electron microscopy (TEM) demonstrate that PtNPs with small particle sizes are dispersed on the rGO matrix. Electrochemical measurements reveal that a PtNP-rGO modified glass carbon electrode (GCE) directly catalyzes rutin oxidation and displays an enhanced current response compared with a bare GCE. Under the optimal experimental conditions, the peak current was linear with rutin concentration in the range of 5 × 10^-8 to 1 × 10^-5 M with the detection limit of 1 × 10^-8 M (S/N = 3) by differential pulse voltammetry. The proposed method was successfully applied to determine rutin in tablet samples with satisfactory results. This journal is

Reduction of oxygen in a trialkoxy ammonium-based ionic liquid and the role of water

The oxygen reduction reaction in a novel trialkoxy ammonium-based ionic liquid, N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy)ethyl)ethan-1-aminium bis(trifluoromethylsulfonyl)imide, [N2(20201)(20201)(20201)] [NTf2] has been studied on glassy carbon and gold electrodes, showing faster electrokinetics on glass carbon because of weaker adsorption of the IL. This has been demonstrated by theoretical calculations and electrochemical studies. In the neat IL, the oxygen is reduced to superoxide (O2 -) through a one electron process; however, better performance is attained in the presence of water (42 mol%), in terms of current density, and onset potential of the reduction process via a reversible 2-electron process. Furthermore, a remarkable increase in cyclic coulombic efficiency is observed for the wet IL (66% in comparison with the neat IL (24%), showing the practicality of a reversible O2/H2O2 system for energy storage.

Electrochemical performance of polyaniline nanofibres and polyaniline/multi-walled carbon nanotube composite as an electrode material for aqueous redox supercapacitors

Polyaniline (PANI) nanofibres are synthesized by interfacial polymerization and their electrochemical performance is evaluated in an aqueous redox supercapacitor constituted as a two-electrode cell. The initial specific capacitance of the cell is 554 F g−1 at a constant current of 1.0 A g−1, but this value rapidly decreases on continuous cycling. In order to improve the cycleability of the supercapacitor, a composite of polyaniline with multi-walled carbon nanotubes (CNTs) is synthesized by in situ chemical polymerization. Its capacitive behaviour is evaluated in a similar cell configuration. A high initial specific capacitance of 606 F g−1 is obtained with good retention on cycling. In both supercapacitors, the effect of charging potential on cycling performances is investigated.