Electrochemical Deposition of Aluminum from Binary Phenolate-Based Electrolytes

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 AC 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.