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 sillier 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- ox.da_.on 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(-)center dot(I-2)(n)]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I-3(-) to [I-3(-)center dot(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,- oxidation in [I-3(-)center dot(I-2)(n)]. In addition, we found that the degree of freedom of I-3(-) 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 I-3(-) due to the higher degree of freedom of I-3(-).

Automated summary

This article introduces an electrochemically generated MEPIS solution and demonstrates that it significantly improves the electrode kinetics of I- oxidation compared to conventional electrolyte conditions. The study finds that the fast electro-oxidation of triiodide and the facilitated charge transfer of I- oxidation in MEPIS contribute to the enhancement of electrode kinetics.