Christopher M. Lang

Dentrite-Free Electrochemical Deposition of Li–Na Alloys from an Ionic Liquid Electrolyte

The deposition of Li-Na alloys from an ionic liquid medium has been demonstrated and evaluated with respect to dendrite growth, oxidation potential, and stability. The maximum coulombic efficiency for the reoxidation of the Li-Na alloy was found to be 91%. The conductivity of the ionic liquid medium containing the alloy salts was 364-466 μS/cm 2 . Upon addition of sodium to the lithium-ion electrolyte, a Li-Na alloy was deposited (mA/cm 2 current density range) that appears to suppress dendrite formation.

The Role of Additives in the Electroreduction of Sodium Ions in Chloroaluminate-Based Ionic Liquids

Ionic liquids are an ionically conductive medium that can provide a wide potential window for the study of electrochemical processes. We have observed that the degree of ionization of the ions depends on the charge density of the ions with significant ion pairing possible. Previously, it was shown that sodium ions can be reduced only to sodium metal if an acidic additive (e.g., SOCl 2 ) is added to the liquid. It is shown here that the additive increases the degree of dissociation of the Na + from its counterion in the liquid, making it available for electrodeposition. The observed increase in ionic conductivity provided by the SOCl 2 supports this proposed mechanism. It is believed that the additive coordinates with chloride in the liquid, to provide greater freedom for the Na + ion. In addition, conditions were found for the reduction of sodium ions to sodium metal without the use of an additive.

Electrochemical Investigation of Quaternary Ammonium/Aluminum Chloride Ionic Liquids

Quaternary ammonium salts have been studied as ionic liquids for electrochemical applications, including sodium batteries. Mixtures of benzyltrialkylammonium chlorides with chloroaluminate formed ionic liquids near room temperature. The maximum coulombic efficiency for the reduction and re-oxidation of sodium ions with benzyltriethylammonium chloride ionic liquid was over 91%. The self-discharge current for a sodium electrode in this ionic liquid was 32.7 and 18 μA/cm 2 by chronopotentiometry at tungsten electrodes at 6.37 and 2.55 mA/cm 2 , respectively. These are comparable to values in 1-methyl-3-propylimidazolium chloride melt. Issues with the coulombic efficiencies and the self-discharge currents are discussed.

High-energy density, room-temperature carbonate fuel cell

4In this study, the viability of a high-energy density, low-power, room-temperature carbonate fuel cell has been investigated. A solid carbonate conducting electrolyte based an anion exchange membrane was used. The pH sensitivity of the membrane was addressed by converting it to the bicarbonate/carbonate form. The resistivity of the membranes was measured and chemical stability in methanol evaluated. Hydrogen, 1M, and pure methanol have been shown to be viable anode fuels. Carbon dioxide was observed at the anode exhaust when operating on hydrogen. Experimental Calcium hydroxide 99.5%, Fisher Scientific and methanol 99.9%, Fisher Scientific were used as-received or diluted with deionized DI water. 1-butyl-3-methylimidazolium tetrafluoroborate BMIBF4, 97%, Fluka was used as-received. Carbon dioxide, hydrogen, oxygen, and nitrogen gases were obtained from Air Products. Carbonate anion exchange membranes were prepared by soaking chloride containing AFN anion exchange membranes AFN, Somerset, New Jersey in aqueous solutions of sodium bicarbonate 99.9%, Fisher Scientific and sodium carbonate 99.5%, EMD Chemicals. Upon soaking in 1 M sodium carbonate, the membranes darkened from a light brown to near black and were found to be unusable as carbonate exchange membranes due to the high pH. The aqueous solution also changed from clear to yellow. In an attempt to prevent damage to the membranes, sodium bicarbonate was added to lower the pH of the solution resulting in green transparent membranes. The resulting membrane was measured to be 150 m thick. Fuel cells were constructed in two ways. The cells used for the hydrogen tests were formed by sandwiching the carbonate anion exchange membrane between two carbon electrodes coated on one side with platinum 20 wt % Pt/Vulcan XC-72 1m g/cm 2 Pt, ElectroChem, Inc. and hot-pressed together. The cells used in the methanol tests were constructed using epoxy to attach a rubber gasket with a hole of known area punched out to the electrode and membrane. An EG&G Princeton Applied Research model 263A potentiostat was used for the electrochemical measurements.

Properties of Asymmetric Benzyl-Substituted Ammonium Ionic Liquids and Their Electrochemical Properties

Benzyl-substituted quaternary ammonium ions were used to form room-temperature ionic liquids with chloroaluminate ions. Asymmetric ammonium structures significantly lowered the melting point of the ionic liquid. Asymmetric benzyl-substituted ammonium chlorides were mixed with AlCl 3 to form acidic room-temperature ionic liquids. It is shown that the melting point and viscosity are a function of the symmetry of the quaternary ammonium ion and its molecular weight. Asymmetry and low molecular weight favor lower viscosity and melting point, and higher conductivity. These liquids were neutralized with NaCI and tested as electrolytes for sodium batteries. The neutralized ionic liquid of benzyldimethylethylammonium chloride had a low self-discharge current (3.96 μA/cm 2 ) at room temperature on a platinum electrode substrate.

Investigation of Ether-Substituted Quaternary Ammonium Ionic Liquids

The effect of structural rigidity in quaternary ammonium-based ionic liquids (ILs) was investigated. [EtOMe]EtMe 2 NCl(I) and [MeOEt]EtMe 2 NCl(II), both 8 atom (7 carbons, 1 oxygen) ether-substituted quaternary ammonium chlorides (Quats), were synthesized and investigated. In acetonitrile, placement of the oxygen one atom from the nitrogen (Quat I) resulted in a significant decrease in the reductive stability. While the IL form showed similar stability for the two systems, placing the oxygen close to the nitrogen (methyl ethyl ether) appears to localize a greater portion of the positive charge on the nitrogen. This results in an IL with a higher melting point (58-62°C) than the nonether analog, BuEtMe 2 NCl (111, 51.3°C) IL. In contrast, placing two carbon atoms between the nitrogen and ether group, ethyl methyl ether (Quat II), delocalizes the positive charge from the nitrogen and produces a room-temperature IL. Neutralization of the acidic IL with sodium chloride resulted in an increase in the bulk conductivity, due to the polar ether group. Upon addition of CDCl 3 or SOCl 2 , which both form a solid-electrolyte interphase on the substrate surface, the increased free sodium ion concentration allowed sodium reduction and oxidation currents greater than 4 mA/cm 2 at room temperature.

Simulations of Photopumping in Doubly Illuminated Liquid Membranes Containing Photoactive Carriers

A steady-state model used to simulate photofacilitated active transport against a concentration gradient, called photopumping, is described. Central to this model is the idea that the carrier can be in either a strongly binding or a weakly binding form and light can be used to control the interconversion rate between the two forms. Most experimental and theoretical studies have focused on systems in which only one side of the membrane is illuminated at a time to form singly illuminated liquid membranes. This study explores membranes in which both the feed and the sweep side are illuminated simultaneously with light of different wavelengths to form doubly illuminated liquid membranes. Doubly illuminated liquid membranes can sustain transport against concentration gradients in which the solute concentration in the sweep is a factor of 10 or more higher than that in the feed, while transport in singly illuminated liquid membranes falls off at lower concentration gradients. In addition, carrier properties that are important in single illumination such as the interconversion rates between the weakly and the strongly binding forms of the carrier are not important for doubly illuminated membranes, meaning that the range of suitable carriers will be much greater for double illumination than for single illumination.