Understanding the role of Br- during the electrooxidation of I- in aqueous media: I2Br-(aq)-formation without the precipitation of an iodine film

Abstract

Zn-polyiodide redox flow battery is considered to be promising energy storage systems. However, the rate of the I-3(-)(aq)/I-(aq) half redox reaction could be limited by a metastable iodine film in aqueous solutions. In this article, we found that the addition of Br- could inhibit the formation of an iodine film (I-2-F) and form soluble I2Br-(aq) during the electrooxidation of I-. I- was electrochemically oxidized to the soluble form I-3(-)(aq). The depletion of I-(aq) and an increase of I-3(-)(aq) would lead to the formation of I-2 and form I-2-F. Then, under a steady state, I-(aq) was electrochemically oxidized to I-3(-)(aq) via I-2-F. However, the reaction pathway for the electrooxidation of I- was significantly altered by the addition of Br-. I- was electrochemically oxidized to I-3(-)(aq) and was further oxidized to I2Br-(aq) without the formation of I-2-F. Moreover, we estimated the stability constant of I2Br-(aq) and consequently the fractional diagrams of I-2(aq), I-3(-)(aq), and I2Br-(aq) existing in a solution with both I- and Br- during the electrooxidation of I-, providing the necessity of a highly concentrated Br- condition in the vicinity of an electrode to form I2Br-(aq) as the main iodine species through the electrooxidation of I-.