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

Abstract

Electrochemistry in supercritical CO(2) (scCO(2)) 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 scCO(2) 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 CO(2) with [NBu(n)(4)][BF(4)] and either methanol (CH(3)OH) or acetonitrile (CH(3)CN) as the co-solvent. These show that the solubility of [NBu(n)(4)][BF(4)] is approximately 5 times larger when CH(3)CN is the co-solvent rather than CH(3)OH. Consequently the phase behaviour of the ternary of CO(2)-[NBu(n)(4)][BF(4)]-CH(3)CN was studied in greater detail over a range of compositions. To enhance the conductivity of scCO(2)-CH(3)CN a range of electrolyte salts was synthesised in which the [NBu(n)(4)](+) and/or [BF(4)](-) ion were replaced by different derivatives. Results for the phase behaviour and conductivity of these modified electrolyte salts in scCO(2)-CH(3)CN 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 scCO(2)-CH(3)CN. Of the 11 electrolytes investigated [NBu(n)(4)][B{3,5-C(6)H(3)(CF(3))(2)}(4)] appears the most suitable for use in scCO(2)-CH(3)CN with a molar conductivity of 22-26 S cm(2) mol(-1) and a maximum measured conductivity of approximately 3 mS cm(-1) for 0.07 M [NBu(n)(4)][B{3,5-C(6)H(3)(CF(3))(2)}(4)] dissolved in scCO(2)-CH(3)CN (molar ratio CH(3)CN : CO(2) approximately 0.12) at 20 MPa and 328.15 K. This is an order of magnitude improvement over similar results for the [NBu(n)(4)][BF(4)] 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.

Electrochemistry in supercritical fluids: A mini review

“…Organic co-solvents form a single phase with scCO 2 and considerably improve the solubility of ionic species. This mixed solvent phase has been largely documented in the field electrodeposition [3,18,22], with Bartlett et al recently reviving interest in SCFs as electroplating media. Their research efforts have demonstrated that the unique mass transport properties and lack of surface tension in supercritical media permit the formation of well-defined nano-structured electrodeposits [3].…”

“…An extensive study by Bartlett et al evaluated the conductivity of a series of electrolytes and determined that cation fluorination has little effect but for the anion it was crucial [18]. Specifically BARF anions, fluorinated derivatives of [BAr 4 ] − , were the most soluble and conductive in a mixed scCO 2 /acetonitrile fluid [18]. An alternative strategy proposed to enhance the ionic conductivity of the scCO 2 is to add a highly polar and miscible co-solvent.…”

“…Bulky, hydrophobic electrolytes have since been commonplace in scCO 2 electrochemistry, with recent advances focusing on the halogenation of the ions. An extensive study by Bartlett et al evaluated the conductivity of a series of electrolytes and determined that cation fluorination has little effect but for the anion it was crucial [18]. Specifically BARF anions, fluorinated derivatives of [BAr 4 ] − , were the most soluble and conductive in a mixed scCO 2 /acetonitrile fluid [18].…”

“…An alternative strategy proposed to enhance the ionic conductivity of the scCO 2 is to add a highly polar and miscible co-solvent. To-date methanol [18,19], dimethylformamide [20] or acetonitrile [18,21] has been employed. Organic co-solvents form a single phase with scCO 2 and considerably improve the solubility of ionic species.…”

Measurements of the double layer capacitance for electrodes in supercritical CO2/acetonitrile electrolytes

“…Addition of acetonitrile (CH 3 CN) as a polar co-solvent increases the effective dielectric constant for the fluid and hence the solubility of electrolytes and the solution conductivity [20]. Supercritical CO 2 /CH 3 CN has been used in our group for the electrodeposition of both copper [14] and silver [15] and Toghill et al [21] synthesised and purified as described elsewhere [13,20].…”