Abstract
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) + ySx2-(aq) + yH2O(l) ⇄ 1/2Al2S3(c) + yOH-(aq) + yHS-(aq), y = 1.5/(x − 1), and Al(c) + 1.5S22-(aq) + 3H2O(1) ⇄ 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.
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