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

• Published in: Bulletin of the Korean Chemical Society Vol. 42; no. 12; pp. 1678 - 1685
• Main Authors: Park, Cheolmin, Chang, Jinho
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley‐VCH Verlag GmbH & Co. KGaA 01-12-2021; 대한화학회
• Subjects: I2Br; I3−(aq)/I−(aq) redox reaction; iodine film; rotating ring‐disk electrode; Zn‐polyiodide redox flow battery; 화학
• ISSN: 1229-5949; 0253-2964; 1229-5949
• DOI: 10.1002/bkcs.12406

Zn‐polyiodide redox flow battery is considered to be promising energy storage systems. However, the rate of the I3−(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 (I2‐F) and form soluble I2Br−(aq) during the electrooxidation of I−. I− was electrochemically oxidized to the soluble form I3−(aq). The depletion of I−(aq) and an increase of I3−(aq) would lead to the formation of I2 and form I2‐F. Then, under a steady state, I−(aq) was electrochemically oxidized to I3−(aq) via I2‐F. However, the reaction pathway for the electrooxidation of I− was significantly altered by the addition of Br−. I− was electrochemically oxidized to I3−(aq) and was further oxidized to I2Br−(aq) without the formation of I2‐F. Moreover, we estimated the stability constant of I2Br−(aq) and consequently the fractional diagrams of I2(aq), I3−(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−.
Zn‐polyiodide redox flow battery is considered to be promising energy storage systems. Nevertheless, the overall battery performance could suffer from the rate of the triiodide/iodide half redox reaction due to in situ formation of a metastable iodine film in aqueous solution. In this article, we present that the addition of bromide could inhibit the formation of an iodine film and form soluble iodine bromide anion during the electro‐oxidation of iodide.