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).