RESUMEN
Based on the ambipolar characteristics and high solubility of ZnI2, zinc-polyiodide flow batteries (ZIFB) have attracted attention as high-energy density flow batteries. However, due to the various oxidation products of iodide (I-) and the formation of iodine (I2) solid precipitates at the positive electrode, the limiting state-of-charge (SoC) of ZIFB has not been clearly defined. Herein, a clear definition of SoC in ZIFBs is given based on the thermodynamic relationship among I-(aq), I3-(aq), I5-(aq), and I2(aq) in the electrolyte. Conventional ZIFBs are limited by their maximum attainable SoC of 87%, at which the fully charged catholyte includes I-, I3-, and I5- ions at molar ratios of 49.6, 32.2, and 18.1%, respectively. Furthermore, two effective strategies to extend the maximum SoC are suggested: (1) increasing the formation constant (Keq) of I3- can raise the availability of I- for electrooxidation by suppressing I2 precipitation, and (2) promoting the production of higher-order polyiodides such as I5- can increase the oxidation state of the charged electrolyte. The addition of 5 vol % triethylene glycol (tri-EG) to the electrolyte increased Keq from 710 to 1123 L mol-1; this increase was confirmed spectrophotometrically. Tri-EG stabilized I5- ions in the form of the I5-/tri-EG complex, thereby converting the main oxidation product from I3- to I5-. The preferred electrochemical production of I5- in the tri-EG electrolyte was observed by electrochemical and computational analyses. As a result, the maximum attainable SoC was enhanced remarkably to 116%, yielding molar ratios of I-, I3-, and I5- ions of 9.1, 11.2, and 79.7%, respectively. This SoC extension effect was confirmed in the ZIFB flow cell with stable charge-discharge cycling at the SoC 120% limit, demonstrating the highest energy density, 249.9 Wh L-1, among all reported ZIFBs.
RESUMEN
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).