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 2 -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 2 -F. In this article, we introduced an electrogenerated N -methyl- N -ethyl pyrrolidinium iodide (MEPI)-iodine (I 2 ) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I - oxidation were dramatically enhanced compared to an I 2 -F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I 3 - ), which exists in the shape of a I 3 - -in-I 2 network, [I 3 - ·(I 2 ) n ]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I 3 - to [I 3 - ·(I 2 ) n ] via I 5 - 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 2 -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 3 - oxidation in [I 3 - ·(I 2 ) n ]. In addition, we found that the degree of freedom of I 3 - in a quaternary ammonium-based I 2 -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 3 - due to the higher degree of freedom of I 3 - .