Graham T. Cheek

Measurement of hydrogen uptake by palladium using a quartz crystal microbalance

In this communication it is shown that these large frequency shifts can be attributed to stresses arising in the metal as the lattice expands due to the hydrogen (deuterium) charging. We also want to alert others using this technique of the possible role stresses can play in these measurements

Redox behavior of the nickel oxide electrode system: quartz crystal microbalance studies

The electrochemical reactions occurring in the nickel oxide electrode system have been studied with the quartz crystal microbalance. Following cathodic deposition from nickel sulfate solutions, α-nickel hydroxide films were cycled in 1.0M alkali metal hydroxide solutions. The oxidation process produced frequency decreases in these solutions, indicating a corresponding increase in mass in the electrode layer. Upon subsequent reduction, a return to the initial frequency value was observed. These shifts ranged from a relatively small change for lithium hydroxide to progressively larger shifts as the cation size increased. It is clear from these results that alkali metal cations are being taken up into the electrode structure during oxidation. A mechanism based upon a net 1.7 electron oxidation has been proposed, involving predominant oxidation of nickel from the +2 to the +4 state as indicated by recent EXAFS studies. The mechanism is consistent with the quartz crystal microbalance data provided that alkali metal cations enter, and water molecules leave, the electrode during oxidation. β-Nickel hydroxide films were prepared by contacting α-Ni(OH)2 films with hot 8 M KOH. Cycling these films in 1.0 M alkali hydroxide solutions gave frequency changes which were positive for the lighter cations and negative for the heavier ones, in contrast to the results for α-Ni(OH)2 films. Cation uptake during oxidation is also occurring in this case; however, a small but significant amount of water is also expelled, producing the observed frequency response. The mechanism in this case also involves some oxidation to the +4 state, as shown by XANES studies.

Measurement of H/D uptake characteristics at palladium using a quartz crystal microbalance

Abstract Frequency measurements at AT- and BT-cut crystals have been carried out for electrolytic loading of palladium films with H and D from 0.10 M LiOH + H 2 O (0.10 M LiOD + D 2 O) solutions. The results at BT-cut crystals have shown that film stressing occurs during the early part of H or D deposition and that this stressing is largely reversible upon removal of hydrogen species. In experiments involving addition of H 2 O to D 2 O solutions, it has been found that deposition of H into the palladium film is favored over that of D and that H uptake occurs exclusively at H 2 O levels above 10%, although these experiments have been carried out at much lower current densities than is typical for these investigations.

The Structure of Nickel Chloride in the Ionic Liquid 1-Ethyl-3-methyl Imidazolium Chloride/Aluminum Chloride: X-ray Absorption Spectroscopy

The structure of anhydrous nickel chloride in the ionic liquid 1-ethyl-3-methyl imidazolium chloride and aluminum chloride has been investigated with extended X-ray absorption fine structure (EXAFS) in both Lewis acid and Lewis base solutions. The EXAFS data of NiCl{sub 2} {center_dot} 6H{sub 2}O crystals were also recorded and analyzed to demonstrate the difference file technique. The difference file technique is used to obtain the structural information for the very closely spaced coordination shells of chloride and oxygen in NiCl{sub 2} {center_dot} 6H{sub 2}O and they are found to agree very closely with the X-ray diffraction data. The difference file technique is then used to analyze the nickel chloride in the ionic liquid solutions. Even though anhydrous NiCl{sub 2} is more soluble in the basic solution than in the acidic solution, the EXAFS data show a single coordination of four chlorides in a tetrahedron around the nickel atom in the basic solution. In a weak acid solution, there are six chlorides in a single octahedral coordination shell around the nickel. However, in a strong acid solution, in addition to the octahedral chloride-coordination shell, there is a second coordination shell of eight aluminum atoms in the form of a simple cube.

An XAFS Study of Nickel Chloride in the Ionic Liquid 1-Ethyl-3-Methyl Imidazolium Chloride/ Aluminum Chloride

The electrodeposition of metals from aqueous solutions has a successful history for many metals. However, some metals cannot be deposited from aqueous solutions because their potentials fall outside of the window of stability for water. Using ionic liquids for the electrodeposition of metals can avoid some of these difficulties because they have a larger region of stability than water. The electrochemical window can be tailored to fit a particular application by choosing appropriate anions and cations to form the melt. There is also the possibility to deposit pure metals without the oxides and hydrides that can form in aqueous solutions. The study of the structure of metal salts in ionic liquids is an important step towards achieving these goals.

XAFS STUDIES OF Ni, Ta AND Nb CHLORIDES IN THE IONIC LIQUID 1-ETHYL-3- METHYL IMIDAZOLIUM CHLORIDE / ALUMINUM CHLORIDE

The structure of anhydrous nickel, niobium and tantalum metal chlorides in ionic liquids IL) is important in the development of new electrochemical alloy deposition techniques. In order to understand the interactions of these metal ions with ionic liquids, we have investigated their structures in situ with X-ray Absorption Spectroscopy (XAS). The ionic liquid used in this study is 1-methyl-3-ethylimidazolium chloride (EMIC)/aluminum chloride (AlCl3) and is formed by the addition of the Lewis acid AlCl3 to the EMIC. The acid-base character of the IL can be changed by varying the ratio of the AlCl3 to EMIC and thus changing the concentration of Cl. This dramatically changes the structure of species formed when the metal chlorides dissolve in the solution. The coordination of NiCl2 changes from tetrahedral in basic solution to octahedral in acidic solution. The NiCl2 is also a stronger Lewis acid in that it can induce the aluminum chloride to share its chlorides in the strong acid solution forming a structure with six near Cl ions and 8 further away Al ions which share all the Cl ions surrounding the Ni 2+ . When Nb2Cl10, a dimer, is added to the acidic or basic solution, the dimer breaks up and forms two species. In the acid solution, two trigonal bipyramids are formed with five equal chloride distances while in the basic solution, a square pyramid with four chlorides forming a square base and one shorter axial chloride bond. Ta2Cl10 is also a dimer and breaks in half in the acidic solution and forms two trigonal bipyramids. However, in the basic solution the dimer divides in half but the specie formed is sufficiently acidic that it attracts two additional chloride ions and forms a seven coordinated tantalum specie.

Electrochemical Studies of Lewis Acid / Ketone Interactions in Ionic Liquids

The voltammetric behavior of 9-fluorenone, benzophenone, and benzil has been investigated in the ionic liquids 1-ethyl-3- methylimidazolium tetrafluoroborate (EMIm BF4) and 1-butyl- 2,3-dimethylimidazolium tetrafluoroborate (BDMIm BF4). At 100 mV/s, protonation of the radical anions of benzophenone and benzil by the EMIm cation is extensive; however, the protonation reaction is slow and is not significant at 1000 mV/s. Use of BDMIm BF4, in which the acidic 2 position is substituted, precludes protonation of these radical anions. It has been found that addition of BF3 etherate to solutions of benzophenone in BDMIm BF4 , and to benzil in EMIm BF4, results in formation of ketone : BF3 complexes.

Voltammetric Study of Titanium Chlorides in the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate

The electrochemical behavior of titanium species is important to an understanding of its electrodeposition. Recent work has involved exploration of alternatives to the Kroll process for titanium production, particularly in molten CaCl2 medium (1). Other efforts have involved titanium deposition in ionic liquids (2). In this work, we present the electrochemical behavior for various oxidation states of titanium chloride.

Electrochemical Study of Aromatic Ketones in 1-Ethyl-3-methylimidazolium tetrafluoroborate

The electrochemical reduction of 9-fluorenone and benzophenone has been studied in the 1-ethyl-3-methylimidazolium tetrafluoroborate ( EMIBF4 ) ionic liquid. A comparison of potential separation for successive 9-fluorenone reduction processes in EMIBF4 at 100 mV/s with those in acetonitrile provides evidence for complexation of the 9-fluorenone dianion by the EMI cation. Complexation of the 9-fluorenone carbonyl oxygen by added BF3 etherate has also been studied, producing a positive potential shift for the reduction process. The reduction of benzophenone in EMIBF4 shows two successive electron transfers only at higher scan rates ( 1 V/s ), suggesting that proton transfer and / or complexation of benzophenone reduction products by the EMI cation may be involved. The peak potential separation for the benzophenone reduction processes at 1 V/s is similar to that for 9-fluorenone at 100 mV/s, indicating that the EMI cation also complexes the benzophenone dianion.

Determination of Water in 1-Ethyl-3-methylimidazolium Tetrafluoroborate

Determination of water in 1-ethyl-3-methylimidazolium tetrafluoroborate has been carried out using both electrochemical and infrared spectroscopic methods. At platinum electrodes, voltammetric responses for water oxidation and reduction have been used to prepare calibration curves. It has been found that the water oxidation current at +2.4V vs Ag/AgCl is suitable for water determination to at least 40 mM. Infrared spectroscopy of water in quartz cells in the OH stretching region revealed an IR feature at 3550 cm -1 for water additions to 20 mM, with another feature at 3650 cm -1 appearing at higher water concentrations. For the lowfrequency feature, a calibration curve is linear to 14 mM. Both electrochemical and IR methods provide a lower limit of detection of approximately 1 mM water in this ionic liquid.