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

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.