Phase behaviour and conductivity of supporting electrolytes in supercritical difluoromethane and 1,1-difluoroethane

Abstract

We present investigations into a variety of supporting electrolytes and supercritical fluids probing the phase and conductivity behaviour of these systems and show that they not only provide sufficient electrical conductivity for an electrodeposition bath, but match the requirements imposed by the different precursors and process parameters, e.g. increased temperature, for potential deposition experiments. The two supercritical fluids that have been explored in this study are difluoromethane (CH₂F₂) and 1,1-difluoroethane (CHF₂CH₃). For CH₂F₂, the phase behaviour and electrical conductivity of eight ionic compounds have been studied. Each compound consists of a cation and an anion from the selected candidates i.e. tetramethylammonium ([N(CH₃)₄]⁺), tetrabutylammonium ([N(ⁿC₄H₉)₄]⁺), 1-ethyl-3-methylimidazolium ([EMIM]⁺) and 1-butyl-3-methylimidazolium ([BMIM]⁺) for cations, and tetrakis(perfluoro-tert-butoxy)aluminate ([Al(OC(CF₃)₃)₄]⁻), chloride (Cl⁻), trifluoromethyl sulfonimide ([NTf₂]⁻) and tris(pentafluoroethyl)trifluorophosphate ([FAP]⁻) for anions. For CHF₂CH₃, [N(ⁿC₄H₉)₄][BF₄] and [N(ⁿC₄H₉)₄][B{3,5-C₆H₃(CF₃)₂}₄] have been investigated for comparison with the previously measured solubility and conductivity in CH₂F₂. We have found that [N(ⁿC₄H₉)₄][Al(OC(CF₃)₃)₄], [N(ⁿC₄H₉)₄][FAP] and [N(CH₃)₄][FAP] have much higher molar conductivity in scCH₂F₂ at similar conditions than [N(ⁿC₄H₉)₄][BF₄], a widely used commercial electrolyte. Additionally, scCHF₂CH₃ shows potential for use as the solvent for supercritical fluid electrodeposition, especially at high temperatures since high density of this fluid can be achieved at lower operating pressures than similar fluids that can be used to produce electrochemical baths with comparable conductivity.