Phase behaviour and conductivity study of electrolytes in supercritical hydrofluorocarbons

Abstract

The purpose of this work was to characterise supercritical hydrofluorocarbons (HFC) that can be used as solvents for electrodeposition. The phase behaviour of CHF₃, CH₂F₂, and CH₂FCF₃ containing [NBuⁿ₄][BF₄], [NBuⁿ₄][B{3,5-C₆H₃(CF₃)₂}₄] and Na[B{3,5-C₆H₃(CF₃)₂}₄] was studied and the conditions for forming a single supercritical phase established. Although all three HFCs are good solvents for [NBuⁿ₄][BF₄] the results show that the CH₂F₂ system has the lowest p_r for dissolving a given amount of [NBuⁿ₄][BF₄]. The solubility of Na[B{3,5-C₆H₃(CF₃)₂}₄] in CH₂F₂ was found to be unexpectedly high. Studies of the phase behaviour of CH₂F₂ containing [NBuⁿ₄][BF₄] and [Cu(CH₃CN)₄][BF₄] showed that the copper complex was unstable in the absence of CH₃CN. For CHF₃, [Cu(hfac)₂] was more soluble and more stable than [Cu(CH₃CN)₄][BF₄] and only increased the phase-separation pressure by a moderate amount. Studies of the conductivity of [NBuⁿ₄][B(C₆F₅)₄], [NBuⁿ₄][B{3,5-C₆H₃(CF₃)₂}₄], [NR_fBuⁿ₃][B{3,5-C₆H₃(CF₃)₂}₄] (R_f = (CH₂)₃C₇F₁₅), and Na[B{3,5-C₆H₃(CF₃)₂}₄] were carried out in scCH₂F₂. The results show that these salts are more conducting than [NBuⁿ₄][BF₄] under the same conditions although the increase is much less significant than that reported in previous work in supercritical CO₂ + CH₃CN. Consequently, either [NBuⁿ₄][BF₄] or the corresponding BARF salts would be suitable background electrolytes for electrodeposition from scCH₂F₂.