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 (I_2n+1^-), and polyiodines (I_2n+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 (R_ct). 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 R_ct 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 R_ct to 147.4 ohm·cm². This increase of R_ct 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).

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