| Summary: | Ionic liquids (ILs) were studied as solvents for electrochemical reactions with the intent to devise metallurgical processes for Al, Mg and Ti that are less energy intensive and operate at lower temperatures than current industrial practice. Tetra-alkyl phosphonium ILs are on the low end of the IL cost spectrum and are regarded as understudied compared with imidazolium and pyridinium ILs. They are also known to be more thermally stable.
The density, viscosity and conductivity of the phosphonium ILs and metal salt-IL mixtures were measured. The conductivity of the phosphonium ILs tested were found to be roughly an order of magnitude lower than imidazolium ILs; this is attributed to the relatively large cation size and localized charge. Linear density-temperature functions are presented. The viscosity and conductivity temperature relationship was modeled using the Vogel-Tamman-Fulcher (VTF) equation.
The electrochemical window of A10341'14,6,6,610 was studied on a Pt substrate over a wide range of A1C13 concentrations using cyclic voltammetry (CV). It was found that the tetra-alkyl phosphonium cation is on the order of 800 mV more electrochemically stable than the 1-ethyl-3-methyl imidazolium (EMI+).
Cathodic and anodic polarization of Al in A1C13-[P14,6,6,6]C1 (Xmc13 = 0.67) was studied at temperatures ranging from 347 to 423 K. The Butler-Volmer equation was fitted to the plots by varying the kinetic parameters. The cathodic reaction was found to be diffusion limited and the anodic reaction is limited by passivation at lower temperatures. The overpotential required for electrodissolution of Al was found to be higher than for electrodeposition.
Aluminium was electrodeposited using both an electrowinning setup (chlorine evolution anode reaction) and electrorefining setup (Al dissolution anode reaction). The deposits were characterized in terms of morphology, current efficiency and power consumption. A variety of deposit morphologies were observed ranging from smooth, to spherical to dendritic, and in some cases, the IL was occluded in the deposit. The current efficiency and power consumption were negatively impacted by the presence of H2O and HCl present in the as-received ILs and by C12(g) generated by the anode reaction in the case of the electrowinning setup. HC1 was removed by cyclic polarization or corrosion of pure Al, resulting in current efficiencies above 90%. Aluminium was electrodeposited using the electrorefining setup with anode-cathode spacing of 2 mm at power consumption as low as 0.6 kWhr/kg-Al. This is very low compared with industrial Al electrorefining and Al electroplating using the National Bureau of Standards bath, which require 15-18 kWhr/kg-Al and 18 kWhr/kg-Al, respectively. However, due to low solution conductivity the power consumption increases significantly with increased anode-cathode spacing.
Titanium tetrachloride was found to be soluble in [P14,6,6,6]Cl and increases the conductivity of the solution. Attempts to reduce the Ti(IV) included corrosion of titanium metal, corrosion of magnesium metal powder and cathodic polarization. Despite a few attempts, the electro-deposition of Ti was not observed. At this point, titanium electrodeposition from phosphonium based ILs does not appear feasible.
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