Abstract
The anodic dissolution of aluminum metal was investigated in the Lewis acidic chloroaluminate ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride. The investigation was conducted on aluminum rotating disk electrodes as a function of potential, ionic liquid composition, and temperature. Two different dissolution mechanisms were realized. At modest overpotentials, dissolution takes place under mixed kinetic-mass transport control. However, as the overpotential is increased to induce higher dissolution rates and/or the ionic liquid is made more acidic, the dissolution reaction transitions to a potential-independent passivation-like process ascribed to the formation of a porous solid layer of AlCl3(s). At a fixed temperature and composition, the limiting passivation current density displays Levich behavior and also scales linearly with the concentration of AlCl4− in the ionic liquid. The heterogeneous kinetics of the Al dissolution reaction were measured in the active dissolution potential regime. The exchange current densities were independent of the composition of the ionic liquid, and the anodic transfer coefficients were close to zero and seemed to be independent of the Al grain size.
Highlights
Room-temperature chloroaluminate ionic liquids are obtained by combining aluminum chloride with certain anhydrous quaternary ammonium chloride salts
Characterization of the pre-electrolysis Al anode film in AlCl3EtMeImCl.—As noted in the experimental section, the acidic AlCl3EtMeImCl ionic liquid is conveniently purified by electrolysis between Al electrodes
The anode rod sometimes developed a water-insoluble black coating. The formation of this black surface film on Al anodes in chloroaluminate molten salts and ionic liquids has been reported by many workers[8,11,12,15,16] and has been attributed to finely divided Al resulting from the disproportionation of subvalent Al species produced during an initial one-electron oxidation reaction.[15,16]
Summary
Characterization of the pre-electrolysis Al anode film in AlCl3EtMeImCl.—As noted in the experimental section, the acidic AlCl3EtMeImCl ionic liquid is conveniently purified by electrolysis between Al electrodes. The anode rod sometimes developed a water-insoluble black coating The formation of this black surface film on Al anodes in chloroaluminate molten salts and ionic liquids has been reported by many workers[8,11,12,15,16] and has been attributed to finely divided Al resulting from the disproportionation of subvalent Al species produced during an initial one-electron oxidation reaction.[15,16] Gale and Osteryoung[11] investigated this phenomenon in some detail in acidic AlCl3-NaCl, but were unable to confirm the participation of subvalent Al ions. The first is an active dissolution process represented by the overall reaction, Al + 7AlCl4− 4Al2Cl7− + 3e−
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