Electrochemical Deposition of Aluminum from Binary Phenolate-Based Electrolytes
Mosman, D.; Ambar, A; Silguero, C.; Antonetti, J. M.; Vasylevski, S.; Aubrey, M. L.
Abstract
The five binary aluminum phenoxide electrolytes Al2(THF)2(phenoxide)6, Al2(2,5-dimethylphenoxide)6, Al2(3-trifluoromethyl-phenoxide)6, Al2(4-trifluoromethyl-phenoxide)6, and Al2(perfluorophenoxide)6—henceforth Al2(OC6F5)6—were prepared by reaction of their respective phenols with trimethylaluminum. The room temperature ion conductivities of each electrolyte were determined by A.C. impedance spectroscopy, where the addition of fluorinated functional groups was found to increase both solubility and dissociation of ions in dimethoxyethane (DME) solutions. The most conductive binary aluminum salt, 0.5 M Al2(OC6F5)6, displayed a conductivity of 7.28 mS cm–1. By linear sweep and cyclic voltammetry, the oxidative stability limit of Al2(OC6F5)6 was determined to be approximately 1.5 V on platinum, tungsten, and stainless-steel working electrodes. At reducing potentials, Al2(OC6F5)6 was found to deposit aluminum quasi-reversibly. The solid-state structure of the Al2(OC6F5)6 was found to be a charge-neutral dinuclear complex. Speciation of Al2(OC6F5)6 in DME was analyzed by 19F and 27Al NMR spectroscopy with at least three chemically distinct aluminum species identified and at least four species identified by 19F NMR at lower concentrations. By 19F DOSY measurements, diffusivities of perfluorophenolate-bearing species were determined and used to estimate fractional ionization of the electrolyte in DME by comparison to ionic conductivity measurements.