Joan Fuller

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.

Ionic liquid–polymer gel electrolytes from hydrophilic and hydrophobic ionic liquids

Abstract Ionic liquid–polymer gels were prepared by incorporating hydrophilic EMIBF 4 and EMI(triflate) (EMI + =1-ethyl-3-methylimidazolium) and hydrophobic BMIPF 6 (BMI + =1-(1-butyl)-3-methylimidazolium) room-temperature ionic liquids into a poly(vinylidene fluoride)–hexafluoropropylene copolymer (PVdF(HFP)) matrix. Gel electrolytes prepared with ionic liquid:PVdF(HFP) mass ratios of 2:1 exhibited ionic conductivities of >10 −3 S cm −1 at room temperature and >10 −2 S cm −1 at 100°C. The BMIPF 6 -PVdF(HFP) gel was incorporated into electrochemical cells, employing graphite intercalation electrodes for both the anode and cathode, to construct single and bipolar cells displaying open-circuit voltages of 3.77 and 7.86 V, respectively. New inexpensive preparative routes for the hydrophilic ionic liquids were developed that utilize the metathesis reaction of EMICl with the appropriate ammonium salt in acetone or acetonitrile to produce high purity products.

Ionic Liquid‐Polymer Gel Electrolytes

New rubbery gel electrolytes have been prepared from room temperature ionic liquids and poly(vinylidene fluoride)-hexafluoropropylene copolymer [PVdF(HFP)]. The ionic liquids employed in these preparations were 1-ethyl-3-methylimidazolium salts of triflate (CF 3 SO 3 - ) and BF 4 - . When properly processed, the ionic liquid-PVdF(HFP) gels are freestanding, flexible films with room temperature conductivities ranging from 1.1 to 5.8 mS cm -1 . Because both the ionic liquids and the PVdF(HFP) are nonvolatile and are thermally stable, the gels can be operated at elevated temperatures without performance degradation. An ionic conductivity of 41 mS cm -1 was measured for a triflate ionic liquid-PVdF(HFP) gel at 205°C.

Dual Intercalating Molten Electrolyte Batteries

The reductive and oxidative intercalation of ions into graphite from room-temperature and low temperature molten salts is demonstrated. For this investigation, the molten salts use 1-ethyl-3-methylimidazolium (EMI[sup +]) or 1,2-dimethyl-3-propylimidazolium (DMPI[sup +]) as the cation and AlCl[sup [minus]][sub 4], BF[sup [minus]][sub 4], PF[sup [minus]][sub 6], CF[sub 3]SO[sup [minus]][sub 3], or C[sub 6]H[sub 5]CO[sup [minus]][sub 2] as the anion. In a two-electrode battery configuration, the molten salt electrolyte provides both the cation and anion which are intercalated into the graphite anode and cathode, respectively. A cell employing a (DMPI)(AlCl[sub 4]) electrolyte and two graphite rod electrodes achieved an open-circuit voltage of 3.5 V and a cycling efficiency of 85%.

Structure of 1-ethyl-3-methylimidazolium hexafluorophosphate: model for room temperature molten salts

The crystal structure of 1-ethyl-3-methylimidazolium (EMI+) hexafluorophosphate consists of interionic interactions dominated by cation–anion coulombic forces with minimal hydrogen bonding and serves as a model for EMI+ room temperature molten salts containing weakly complexing anions.

Microelectrode Evaluation of Transition Metal‐Aluminum Alloy Electrodepositions in Chloroaluminate Ionic Liquids

Chronoamperometric data collected at a 250 μm tungsten microelectrode were analyzed under near-steady-state conditions to determine the composition of MAl x alloys (M = Co, Ni, Fe, Cu, and Ag) electrodeposited from 1.5:1.0 AlCl 3 :1-ethyl-3-methylimidazolium chloride room temperature ionic liquids. The analysis method relied on the fact that these alloys are produced by an underpotential deposition mechanism. Results were consistent with previous studies showing that the CoAl x , FeAl x , and CuAl x systems tended to produce alloys with x 1. Analysis of the NiAl x data was complicated by kinetic phenomena, while data analysis of the AgAl x system was precluded by dendritic growth of the electrodeposit. All the alloy systems showed complex anodic stripping voltammetric behavior, and the nature of the oxidation processes (e.g., metal anodization, alloy anodization, or selective dealloying) are different for electrodeposits produced in specific potential regimes. Nonlinear curve fitting of the chronoamperometric data to the appropriate short-time and long-time equations gave diffusion coefficients from 3.9 x 10 -7 to 8.3 x 10 -7 cm 2 s -1 for the transition metal ions in the ionic liquid electrolyte at ca. 22°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.

Electrochemistry of room-temperature chloroaluminate molten salts at graphitic and nongraphitic electrodes

The electrochemistry of unbuffered and buffered neutral AlCl3-EMIC-MC1 (EMIC =1-ethyl-3-methylimidazolium chloride and MC1= LiCl, NaCl or KCl) room-temperature molten salts was studied at graphitic and nongraphitic electrodes. In the case of the unbuffered 1 : 1 AlCl3 : EMIC molten salt, the organic cation reductive intercalation at about −1.6 V and the AlCl4− anion oxidative intercalation at about +1.8 V were evaluated at porous graphite electrodes. It was determined that the instability of the organic cation in the graphite lattice limits the performance of a dual intercalating molten electrolyte (DIME) cell based on this electrolyte. In buffered neutral 1.1 :1.0:0.1 AIC13: EMIC : MCl (MC1= LiCl, NaCl and KCl) molten salts, the organic cation was intercalated into porous and nonporous graphite electrodes with similar cycling efficiencies as the unbuffered 1 : 1 melt; however, additional nonintercalating processes were also found to occur between 1 and −1.6 V in the LiCl and NaCl systems. A black electrodeposit, formed at −1.4 V in the LiCl buffered neutral melt, was analysed with X-ray photoelectron spectroscopy and X-ray diffraction and was found to be composed of LiCl, metallic phases containing lithium and aluminium, and an alumina phase formed from reaction with the atmosphere. A similar film appears to form in the NaCl buffered neutral melt, but at a much slower rate. These films are believed to form by reduction of the AlCl4− anion, a process promoted by decreasing the ionic radius of the alkali metal cation in the molten salt. The partially insulating films may limit the usefulness of the LiCl and NaCl buffered neutral melts as electrolytes for rechargeable graphite intercalation anodes and may interfere with other electrochemical processes occurring negative of −1 V.

Ionic liquid–polymer gel catalytic membrane

A novel catalytic membrane for heterogeneous hydrogenation is fabricated by incorporating palladium into a gas-permeable ionic liquid–polymer gel composed of 1-n-butyl-3-methylimidazolium hexafluorophosphate and poly(vinylidene fluoride)–hexafluoropropylene copolymer.

Reversible Lithium‐Graphite Anodes in Room‐Temperature Chloroaluminate Melts

Lithium rocking-chair batteries employing a lithium/carbon intercalation node and a transition metal oxide intercalation cathode have captured a significant portion of battery research throughout the world. The intercalation of lithium into a carbon matrix provides a more stable and safer means for lithium utilization than an elemental lithium anode. Reduction of a graphite rod electrode in an AlCl[sub 3]:EMlC;LiCl room-temperature molten salts leads to reversible lithium intercalation into the reduced graphite lattice. The cycling efficiency of the Li-graphite electrode ranges from 80 to 90% for current densities of 0.2 to 1mA cm[sup [minus]2]. In the staircase cyclic voltammogram the oxidation process occurs at a potential approximately 1 V positive of the reduction process.