EIS Investigation of Redox Reactions (Quinone/Hydroquinone Couple) - Relevant to Organic Redox Flow Batteries

Abstract

The evolution of internal mass-transfer feature and the effect of Nafion on the transport of inorganic redox species (VO2+/VO2 +, V2+/V3+, Ti3+/Ti4+, and Fe2+/Fe3+) across the porous thin-film electrode/electrolyte interface is reported in the literature [1, 2]. Redox reactions of organic redox couple are investigated on Nafion-free and Nafion-containing porous carbon (Vulcan XC-72) modified glassy carbon disk electrodes (GCD) in acidic medium. The model system chosen is the p-benzoquinone (BQ)/hydroquinone (BQH2) couple, since BQ derivatives are common to most of the organic redox couples. The electrochemical impedance spectroscopy (EIS) patterns show an unexpected rise in mass-transport resistance with time, rotation speed (rpm), and Nafion content in the porous electrode. The increase in resistance is remarkable with Nafion-containing GCDs. It is observed that one of the redox species of the organic redox couple strongly interact with cation/anion exchange membrane and may tremendously impact the ion exchange membrane conductivity. Moreover, the internal and external mass-transfer features are better resolved, unlike that with the metal-ion based inorganic counterparts. These findings have enormous implications to the development of ORFBs, organic synthesis, supercapacitors, and sensors. Figure caption :Effect of rotation speed of electrode on the impedance of equimolar solution of 5 mM BQ/BQH2 (1:1) in 1 M H2SO4 on carbon-modified GCD for various Nafion concentration. References P. Leuaa, D. Priyadarshani, A. K. Tripathi, and M. Neergat, J. Phys. Chem. C, 123, 21440–21447 (2019).P. Leuaa, D. Priyadarshani, D. Choudhury, R. Maurya, and M. Neergat, RSC Advances, 10, 30887–30895 (2020). Figure 1