Graduate Student Chen Wang will Present “Aluminum Anodization in Haloaluminate Molten Salts/Ionic Liquids” to the Department.
The anodization of aluminum was examined in the low-melting inorganic molten salt (mp 86 0C), LiAlBr4-NaAlCl4-KAlCl4 (3:5:2) molten salt1 and the room-temperature ionic liquid, AlCl3-1-Ethyl-3-methylimidazolium Chloride in an undivided electrochemical cell. In both cases, the primary three-electron anodization reaction
Al + 7AlCl4– <–> Al2Cl7– + 3e–
proceeds under mixed kinetic and mass-transport control at low anodic potentials. At more positive potentials, the anodization reaction transitions to a mass-transport-limited process resulting from the steady-state formation and dissolution of a passive layer of AlCl3 on the Al electrode surface.2 In the LiAlBr4-NaAlCl4-KAlCl4 (3:5:2) molten salt, the heterogeneous rate constant at the equilibrium potential, k0, was found to 6.6 x 10-7 cm s-1 at 151 ◦C. However, k0 cannot be measured directly in the room-temperature IL, as a result of the back reaction associated with the reducible Al3+ (Al2Cl7–). However, it is possible to measure the exchange current density, jo, by using the classical Tafel analysis based on the Butler-Volmer kinetic approach.3 The apparent activation energies of the reaction shown above are similar in the two molten salts/ionic liquids examined in this research, indicating that the electrode reactions in these two different ionic solvents may be similar.
1) H. A. Hjuler, S. von Winbush, R. W. Berg, and N. J. Bjerrum, J. Electrochem. Soc., 136, 901 (1989).
2.) G. L. Holleck and J. Giner, J. Electrochem. Soc., 119, 1161 (1972)
3.) A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2000.