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

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 (I-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 I-F. In this article, we introduced an electrogenerated -methyl--ethyl pyrrolidinium iodide (MEPI)-iodine (I) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I oxidation were dramatically enhanced compared to an I-F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I), which exists in the shape of a I-in-I network, [I·(I)]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I to [I·(I)] via I 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 I-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 I oxidation in [I·(I)]. In addition, we found that the degree of freedom of I in a quaternary ammonium-based I-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 I due to the higher degree of freedom of I.