Phase behaviour and conductivity study on multi-component mixtures for electrodeposition in supercritical fluids
- 13 November 2009
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Physical Chemistry Chemical Physics
- Vol. 12 (2), 492-501
- https://doi.org/10.1039/b918981e
Abstract
Electrochemistry in supercritical CO2 (scCO2) is difficult because the very low dielectric constant of the fluid restricts the solubility of ionic species and the conductivity of dissolved electrolytes. To overcome this problem to allow us to carry out electrodeposition at macroelectrodes from scCO2 we have investigated the use of co-solvents and modified electrolyte salts chosen to increase their solubility and dissociation in the supercritical fluid. Here we report results of phase behaviour studies for mixtures of CO2 with [NBun 4][BF4] and either methanol (CH3OH) or acetonitrile (CH3CN) as the co-solvent. These show that the solubility of [NBun 4][BF4] is approximately 5 times larger when CH3CN is the co-solvent rather than CH3OH. Consequently the phase behaviour of the ternary of CO2–[NBun 4][BF4]–CH3CN was studied in greater detail over a range of compositions. To enhance the conductivity of scCO2–CH3CN a range of electrolyte salts was synthesised in which the [NBun 4]+ and/or [BF4]− ion were replaced by different derivatives. Results for the phase behaviour and conductivity of these modified electrolyte salts in scCO2–CH3CN are reported for several different compositions. We find that increasing the degree of fluorination and size of the ions increases the solubility of the electrolyte salt in scCO2–CH3CN. Of the 11 electrolytes investigated [NBun 4][B{3,5-C6H3(CF3)2}4] appears the most suitable for use in scCO2–CH3CN with a molar conductivity of 22–26 S cm2 mol−1 and a maximum measured conductivity of ∼3 mS cm−1 for 0.07 M [NBun 4][B{3,5-C6H3(CF3)2}4] dissolved in scCO2–CH3CN (molar ratio CH3CN : CO2 ≈ 0.12) at 20 MPa and 328.15 K. This is an order of magnitude improvement over similar results for the [NBun 4][BF4] parent. Studies of the conductance as a function of the electrolyte concentration suggest that triple ions make an important contribution to the conductivity of the supercritical fluid.This publication has 30 references indexed in Scilit:
- Electrodeposition of metals from supercritical fluidsProceedings of the National Academy of Sciences of the United States of America, 2009
- Large-aperture variable-volume view cell for the determination of phase-equilibria in high pressure systems and supercritical fluidsReview of Scientific Instruments, 2004
- Conductivity of (C4H9)4N BF4 in Liquid and Supercritical HydrofluorocarbonsThe Journal of Physical Chemistry B, 2000
- Electrochemical investigations in liquid and supercritical 1,1,1,2-tetrafluoroethane (HFC 134a) and difluoromethane (HFC 32)Journal of Electroanalytical Chemistry, 1998
- Electrochemical investigations in supercritical carbon dioxideJournal of the Chemical Society, Faraday Transactions, 1996
- Use of carbon dioxide to shift benzene/acetonitrile and benzene/cyclohexane azeotropesFluid Phase Equilibria, 1994
- The search for larger and more weakly coordinating anionsChemical Reviews, 1993
- [(3,5-(CF3)2C6H3)4B]-[H(OEt2)2]+: a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexesOrganometallics, 1992
- Studies in the tetraarylborates: The preparation and reagent properties of sodium tetrakis(p-trifluoromethylphenyl) borate and sodium tetrakis (m-fluorophenyl) borateAnalytica Chimica Acta, 1969
- Perfluorophenyl derivatives of the elements : I. Tris(pentafluorophenyl)boronJournal of Organometallic Chemistry, 1964