Effect of an Iodine Film on Charge-Transfer Resistance during the Electro-Oxidation of Iodide in Redox Flow Batteries

The use of iodide as the positive redox-active species in redox flow batteries has been highly anticipated owing to its attractive features of high solubility, excellent reversibility, and low cost. However, the electro-oxidation reaction of iodide (I–) is very complicated, giving various possible products such as iodine (I2), polyiodides (I2n+1–), and polyiodines (I2n+2) with n ≥ 1. In particular, the electro-oxidation of I–/I3– and I3–/I2 occurs in competition depending on the applied potential. Although the former reaction is adopted as the main reaction in most redox flow batteries because I3– is highly soluble in an aqueous electrolyte, the latter reaction inevitably occurs together and a thick I2-film forms on the electrode, impeding the electro-oxidation of I–. In this study, we investigate the variation of the interface between the electrode and the electrolyte during the development of an I2-film and the corresponding change in the charge-transfer resistance (Rct). Initially, the I2-film builds upon the electrode surface in the form of a porous layer and the aqueous I– ions can easily reach the electrode surface through pores inside the film. I– ions are electro-oxidized to I3– or I2 at the interface between the aqueous I– phase and electrode with a small Rct of less than 16.5 ohm·cm2. Over time, the I2-film is converted into a dense layer and I– ions diffuse through the film in the form of I3–, possibly by a Grotthuss-type hopping mechanism. I3– can then be electro-oxidized to I2 at the new interface between the I2-film and electrode, resulting in a dramatic 9-fold increase of Rct to 147.4 ohm·cm2. This increase of Rct by the dense I2-film is also observed in the actual flow battery. At high current densities above 400 mA·cm–2, the overpotential begins to show an abrupt increase in the amplitude of more than 300 mV after reaching a critical charging capacity at which the dense I2-film appears to have begun to form on the felt electrode. Therefore, the I2-film exerts a serious negative effect on the performance of the flow battery depending on the current density and electrolyte SoC (state-of-charge).