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

Hyeonmin Kim; Kyung Mi Kim; Jungju Ryu; Sehyeok Ki; Daewon Sohn; Junghyun Chae; Jinho Chang

doi:10.1021/acsami.1c21429

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

ACS Appl Mater Interfaces. 2022 Mar 16;14(10):12168-12179. doi: 10.1021/acsami.1c21429. Epub 2022 Mar 7.

Authors

Hyeonmin Kim¹, Kyung Mi Kim², Jungju Ryu¹, Sehyeok Ki¹, Daewon Sohn¹, Junghyun Chae², Jinho Chang^{1

3}

Affiliations

¹ Department of Chemistry and Research Institute for Convergence of Basic Science, Hanyang University, Seoul 04763, Republic of Korea.
² Department of Chemistry, Sungshin Women's University, 55, Dobong-ro 76 ga-gil, Gangbuk-gu, Seoul 142-732, Republic of Korea.
³ Department of HY-KIST Bio-convergence, Hanyang University, Seoul 04763, Republic of Korea.

PMID: 35254047
DOI: 10.1021/acsami.1c21429

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^-.

Keywords: Zn-polyiodide redox flow battery; electro-oxidation of I−; electrode kinetics; quaternary ammonium iodide−iodine solution; triiodide-in-iodine network.