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 (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^-.

Temporally Resolved Electrochemical Interrogation for Stochastic Collision Dynamics of Electrogenerated Single Polybromide Droplets

In this article, we present electrochemical interrogation for collision dynamics of electrogenerated individual polybromide ionic liquid (PBIL) droplets through chronoamperometry combined with fast scan cyclic voltammetry (CA-FSCV). In the CA mode of CA-FSCV, a Pt ultramicroelectrode (UME) acts as the electrochemical generator for PBIL droplets by holding the oxidation potential for Br^- in a given time, while FSCV is repetitively performed at a certain frequency. In the FSCV mode of CA-FSCV, a Pt UME serves as the probe to electrochemically monitor Br₃^- reduction for an adsorbed PBIL droplet during collision with a high temporal resolution. Based on the newly introduced CA-FSCV, we can estimate the dynamic changes in the following parameters for a short collision time: the contact radius of a PBIL droplet on a Pt UME, the concentration of Br^- in the droplet, and the apparent charge transfer rate constant for electro-reduction of Br₃^- to Br^- in the droplet, k^o_app. Moreover, a computational calculation using molecular dynamics is presented that can explain the change in k^o_app as a function of time for Br^- electrolysis in a PBIL droplet. Based on the quantitative estimation of the above parameters, we suggest a more advanced mechanism for the stochastic electrochemical collision process of a PBIL droplet. These findings are important for understanding QBr_2n+1/QBr half redox reactions in aqueous energy storage systems, such as Zn-Br redox flow batteries and Br-related redox enhanced electrochemical capacitors.