Aluminum anodic behavior in aqueous sulfur electrolytes

We report on an unexpected domain of high Coulombic efficiency for electrochemical oxidation of aluminum in aqueous polysulfide solutions at high current density for the process: Al + 3OH(-) --> Al(OH)(3) + 3e(-). This high-efficiency domain, of importance to battery processes, includes aluminum oxidation in a wide range of solutions containing concentrated dissolved zerovalent sulfur. As expected at lower concentrations of dissolved sulfur, aluminum electrochemical oxidation is inefficient, due to various exothermic parasitic reactions, including: Al(c) + yS(x)(2-)(aq) + yH(2)O(1) reversible arrow 1/2Al(2)S(3)(c) + yOH(-)(aq) + yHS(-)(aq), y = 1.5/(x - 1), and Al(c) + 1.5S(2)(2-)(aq) + 3H(2)O(1) reversible arrow Al(OH)(3)(amorph) + 3HS(-)(aq). However, at high polysulfide and sulfur concentrations, the Coulombic efficiency can approach 100%. This domain of high efficiency is correlated to an observed cathodic shift with increasing sulfur concentration, leading to improved chemical passivation at the aluminum surface.