Triiodide-in-Iodine Networks Stabilized by Quaternary Ammonium Cations as Accelerants for Electrode Kinetics of Iodide Oxidation in Aqueous Media

Abstract

The Zn-polyiodide redox flow battery is considered to be a promising aqueous energy storage system. However, in its charging process, the electrode kinetics of I- oxidation often suffer from an intrinsically generated iodine film (I2-F) on the cathode of the battery. Therefore, it is critical to both understand and enhance the observed slow electrode kinetics of I- oxidation by an electrochemically generated I2-F. In this article, we introduced an electrogenerated N-methyl-N-ethyl pyrrolidinium iodide (MEPI)-iodine (I2) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I- oxidation were dramatically enhanced compared to an I2-F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I3-), which exists in the shape of a I3-in-I2 network, [I3-·(I2)n]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I3- to [I3-·(I2)n] via I5- as the anodic overpotential increased. We also confirmed that I- was electrochemically oxidized on a MEPIS-modified Pt electrode with fast electrode kinetics, which is clearly contrary to the nature of an I2-F derived from a NaI solution as a kinetic barrier of I- oxidation. Through stochastic MEPIS-particle impact electrochemistry and electrochemical impedance spectroscopy, we revealed that the enhanced electrode kinetics of I- oxidation in MEPIS can be attributed to the facilitated charge transfer of I3- oxidation in [I3-·(I2)n]. In addition, we found that the degree of freedom of I3- in a quaternary ammonium-based I2-F can also be critical to determine the kinetics of the electro-oxidation of I-, which is that MEPIS showed more enhanced charge-transfer kinetics of I- oxidation compared to tetrabutylammonium I3- due to the higher degree of freedom of I3-.