Robert A. Osteryoung

The Room Temperature Ionic Liquid 1‐Ethyl‐3‐methylimidazolium Tetrafluoroborate: Electrochemical Couples and Physical Properties

Room temperature molten salts composed of the 1-ethyl-3-methylimidazolium cation and a chloroaluminate anion have received much attention for use in a variety of commercial applications such as batteries, photovoltaics, metal deposition, and capacitors. The room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF{sub 4}) was demonstrated as a versatile electrolyte by examining three representative electrochemical couples: ferrocene and tetrathiafulvalene oxidations and lithium ion reduction. Square-wave voltammetric data for ferrocene oxidation were fit to a reversible one-electron process using the COOL algorithm to give a half-wave potential of 0.490 V vs. Al/Al(III) and a diffusion coefficient of 5.1 {times} 10{sup {minus}7} cm{sup 2}/s. The two-electron oxidation of tetrathiafulvalene was reversible and proceeded through two consecutive one-electron steps; although data collected at lower square-wave frequencies indicated a slow precipitation of the TTF{sup +} species. Lithium ion was reduced to lithium metal at a Pt electrode following the addition of water to the EMIBF{sub 4} electrolyte, whereas lithium ion reduction at an Al wire produced the {beta}-LiAl alloy. Conductivities and kinematic viscosities of EMIBF{sub 4} were measured from 20 to 100 C and had values of 14 mS/cm and 0.275 cm{sup 2}/s, respectively, at 25 C.

Rechargeable Lithium and Sodium Anodes in Chloroaluminate Molten Salts Containing Thionyl Chloride

Lithium and sodium deposition-stripping studies were performed in room temperature buffered neutral chloroaluminate melts containing low concentrations of thionyl chloride (SOCl{sub 2}). The SOCl{sub 2} solute promotes high cycling efficiencies of the alkali metals in these electrolytes. Staircase cyclic voltammetry and chronopotentiometry show cycling efficiencies of approximately 90% for both lithium and sodium. High cycling efficiencies are maintained following extended exposure of the melt to the dry box atmosphere and after time delays at open circuit. The performance of the SOCl{sub 2}-promoted systems is substantially improved over previous studies in room temperature melts containing hydrogen chloride as the promoting solute.

Electrochemical Studies of Cu(I) and Cu(II) in an Aluminum Chloride‐N‐(n‐Butyl)Pyridinium Chloride Ionic Liquid

Etude des systemes redox Cu(II)-Cu(I)-Cu dans le melange liquide donne, a laide dune electrode a disque tournant (carbone vitreux) et dune electrode a disque et anneau, ainsi que par coulometrie et potentiometrie

Switching Potentials and Conductivity of Polypyrrole Films Prepared in the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate

Polypyrrole (ppy) and poly(N-methylpyrrole) (pmpy) films were prepared galvanostatically at the ring of a rotating ring-disk electrode from the corresponding monomers dissolved in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6 ) Rotating ring-disk voltammetry was used to determine switching potentials and conductivity of the films while immersed in bmitnPF 6 . Switching potentials of 0.63 ± 0.04 and 1.07 ± 0.03 V vs. the cobaltocenium/cobaltocene couple, [CoCp 2 ] +/0 , were determined for the ppy and pmpy films, respectively, from negative potential scan voltammograms. Positive potential scan voltammetry was used to obtain the potential-dependent conductivity of the films as the films switched from their insulating (at potentials more negative than switching potential) to a quasi-metallic state. Two different computational models (nonlinear vs. linear conductance gradients) and two different redox probes ([CoCp 2 ] +/0 and decamethylferrocenium/ decamethylferrocene) were used in the calculation of the film conductivity. The conductivity of the ppy films increased by an order of magnitude for every 72 ± 8 mV change in the applied potential. At potentials less than the switching potential, the conductivity of the pmpy films increased tenfold for every 110 ± 15 mV change in the applied potential. At potentials greater than the of teh pmpy films the pmpy films required a 200 mV change to induce the same degree of change in film conductivity.

Electrochemical Properties of Alkali Metals in 1-Butyl-3-methylimidazolium Hexafluorophosphate

Formal potentials and diffusion coefficients of the alkali metal couples of lithium, sodium, and potassium hexafluorophosphate salts dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate were determined using a hanging mercury drop electrode as the working electrode. Chronoamperometry was used to determine the diffusion coefficients, which ranged from 1.4 X 10 -8 to 4.5 × 10 -8 cm 2 /s for the Na and K cations, respectively. Formal reduction potentials of -2.96 ± 0.01 V and -3.35 ± 0.01 V vs. the ferrocene/ferrocenium couple were obtained for sodium and potassium using normal pulse voltammetry. The presence of adsorption maxima in the lithium normal pulse voltammograms necessitated the use of chronopotentiometry to determine a lithium formal potential of -2.45 ± 0.02 V.

Apparent Anomaly during Rotating Disk Voltammetry in Ionic Liquids

An apparent anomaly is described in which maxima on rotating disk voltammograms for ferrocene oxidation in an ionic liquid were found. The ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, bmimPF 6 , is a neoteric solvent of current interest. The maxima are shown to he due to the very high viscosity of the solvent, 3.26 poise, which yields an unusually high Schmidt number (Sc) of 3.5 × 10 7 , compared to a value of about 10 3 for aqueous solutions. It is estimated that over 100 revolutions of the disk are needed to achieve the Levich limiting current following application of a voltage step to the disk. The current transient resulting from application of a voltage step to the rotating disk is found to be in agreement with previously developed theory, even for these highly viscous systems. Analysis of the transient permits a determination of Sc values without measurement of the viscosity and density of the solvent, or diffusion coefficient of the reacting species.

In Situ Optical Microscopy Investigations of Lithium and Sodium Film Formation in Buffered Room Temperature Molten Salts

Previous work performed in both sodium and lithium buffered chloroaluminate molten salts have shown that the addition of small amounts of SOCl{sub 2} promotes the reversible stripping behavior of lithium and sodium metal with cycling efficiencies between 80 and 90%. The authors have performed a series of optical studies in conjunction with electrochemical experiments at varying SOCl{sub 2} concentrations in both lithium and sodium chloride buffered melts. On investigation, the lithium deposit is dendritic in nature and does not form a uniform film on the tungsten electrode. After discharging at moderate current densities, disconnected lithium metal is observed at the electrode surface. In contrast, the sodium deposits as a uniform, flat film on the tungsten electrode with little or no dendritic growth. The sodium electrodeposits undergo complete stripping from the tungsten electrode without dendritic or disconnected sodium metal left on the electrode surface.

Benzene polymerization in 1-ethyl-3-methylimidazolium chloride-AlCl3 ionic liquid

Abstract Poly(p-phenylene) films were prepared by the electropolymerization of benzene in an ambient-temperature molten salt, or ionic liquid, consisting of 1-ethyl-3-methylimidazolium chloride-aluminum chloride. Polymerization was carried out in melts of varying acidity in neutral buffered melts. The films were then examined in 1-ethyl-3-methylimidazolium chloride-aluminum chloride melts of varying acidity. The polymerization and electrochemical behavior of the films are independent of melt acidity; the films show reasonably facile electrochemical behavior.

A silane-based electroactive film prepared in an imidazolium chloroaluminate molten salt

Electrochemical oxidation of Ph[sub 3]SiCl (Ph = phenyl) in the room-temperature molten salt AlCl[sub 3]:EMICl (EMICl = 1-ethyl-3-methylimidazolium chloride) results in the formation of an adherent electroactive film on the surface of solid electrodes. The film is reversibly oxidized and reduced in the molten salt and is conducting when in the oxidized state. Based on x-ray photoelectron spectroscopy analysis, the film consists of both a silane and an imidazole component, and the charge storage properties of the film are localized on the heterocyclic rings. This electroactive film compares favorably to other conducting polymers currently being investigated as positive electrodes in room-temperature molten salt batteries.

Buffering of 1-Ethyl-3-methylimidazolium Chloride/Aluminum Chloride Ionic Liquids Using Alkali Metal Bromides and Iodides

The buffering of 1--ethyl-3-methylimidazolium chloride-aluminum trichloride (AlCl 3 ) room-temperature ionic liquids (melts) using alkali metal bromides and iodides was studied. The bromide or iodide salts buffer the melts by reaction with the Al 2 Cl - 7 ion, but the bromide or iodide ions do not replace the chloride from the AlCl - 4 in the melts. Unlike melts buffered with alkali metal chlorides, it is easy to deposit the alkali metals, and thus it may be possible to use these buffered melts in power sources. In melts buffered with a mixture of lithium chloride and iodide, it is possible to both deposit and strip lithium metal. As seen with melts buffered with alkali metal chlorides, the buffered melts appear to he more acidic than expected from the low concentration of the acidic Al 2 Cl 7 ion.