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Phase behaviour and conductivity study on multi-component mixtures for electrodeposition in supercritical fluids

Phase behaviour and conductivity study on multi-component mixtures for electrodeposition in supercritical fluids
Phase behaviour and conductivity study on multi-component mixtures for electrodeposition in supercritical fluids
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 [NBun4][BF4] and either methanol (CH3OH) or acetonitrile (CH3CN) as the co-solvent. These show that the solubility of [NBun4][BF4] is approximately 5 times larger when CH3CN is the co-solvent rather than CH3OH. Consequently the phase behaviour of the ternary of CO2–[NBun4][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 [NBun4]+ 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 [NBun4][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 [NBun4][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 [NBun4][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.
1463-9076
492-501
Bartlett, Philip N.
d99446db-a59d-4f89-96eb-f64b5d8bb075
Cook, David C.
0fca0ffb-eef4-422f-b513-ce9a1f7de7cc
George, Michael W.
09040935-b1da-4a63-ad2e-5ba29e39f80a
Ke, Jie
e4b531d4-7642-4367-9a4e-d06f126c2599
Levason, William
e7f6d7c7-643c-49f5-8b57-0ebbe1bb52cd
Reid, Gillian
37d35b11-40ce-48c5-a68e-f6ce04cd4037
Su, Wenta
78e12e98-09de-461d-8af3-91e5b0e018b7
Zhang, Wenjian
1f80ac5e-d4c2-4720-b19e-be700cd411e7
Bartlett, Philip N.
d99446db-a59d-4f89-96eb-f64b5d8bb075
Cook, David C.
0fca0ffb-eef4-422f-b513-ce9a1f7de7cc
George, Michael W.
09040935-b1da-4a63-ad2e-5ba29e39f80a
Ke, Jie
e4b531d4-7642-4367-9a4e-d06f126c2599
Levason, William
e7f6d7c7-643c-49f5-8b57-0ebbe1bb52cd
Reid, Gillian
37d35b11-40ce-48c5-a68e-f6ce04cd4037
Su, Wenta
78e12e98-09de-461d-8af3-91e5b0e018b7
Zhang, Wenjian
1f80ac5e-d4c2-4720-b19e-be700cd411e7

Bartlett, Philip N., Cook, David C., George, Michael W., Ke, Jie, Levason, William, Reid, Gillian, Su, Wenta and Zhang, Wenjian (2010) Phase behaviour and conductivity study on multi-component mixtures for electrodeposition in supercritical fluids. Physical Chemistry Chemical Physics, 12 (2), 492-501. (doi:10.1039/b918981e).

Record type: Article

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 [NBun4][BF4] and either methanol (CH3OH) or acetonitrile (CH3CN) as the co-solvent. These show that the solubility of [NBun4][BF4] is approximately 5 times larger when CH3CN is the co-solvent rather than CH3OH. Consequently the phase behaviour of the ternary of CO2–[NBun4][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 [NBun4]+ 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 [NBun4][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 [NBun4][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 [NBun4][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.

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Published date: January 2010

Identifiers

Local EPrints ID: 144995
URI: https://eprints.soton.ac.uk/id/eprint/144995
ISSN: 1463-9076
PURE UUID: 233ccf40-f42f-4a18-958d-403e831cf7ea
ORCID for Philip N. Bartlett: ORCID iD orcid.org/0000-0002-7300-6900
ORCID for William Levason: ORCID iD orcid.org/0000-0003-3540-0971
ORCID for Gillian Reid: ORCID iD orcid.org/0000-0001-5349-3468

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Date deposited: 15 Apr 2010 14:08
Last modified: 20 Jul 2019 01:28

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Contributors

Author: David C. Cook
Author: Michael W. George
Author: Jie Ke
Author: William Levason ORCID iD
Author: Gillian Reid ORCID iD
Author: Wenta Su
Author: Wenjian Zhang

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