Tracking real-time chemical evolutions in redox flow batteries using modern ultrafast 2D NMR and localized MRI techniques

Abstract: Aqueous organic Redox flow batteries (AORFBs) have the potential to emerge as a cost-effective and sustainable alternative to conventional lithium-ion batteries (LIBs) and vanadium redox-flow batteries (VRFBs). The decoupling of energy and power density, makes them unique, which is critical for energy distribution. The tunability of redox properties due to their structural diversity and designability makes them suitable for desired energy storage systems.

Various redox mediators such as quinone, anthraquinone, viologens, alloxazine and 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) have been studied for gaining insights to their redox chemistry.

Phenazine based derivatives have recently emerged as promising redox mediators due to high solubility. Phenazine based 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS) and isomeric analogues of dihydroxy phenazines (1,4-DHP and 1-6 DHP) as anolytes has showed excellent volumetric capacity and stable cycling, with very low capacity fade. Despite of these benefits, understanding of their redox chemistry is limited and unclear due to poor resolution, sensitivity and sparse sampling based on ex-situ 1D 1H NMR.

The proposed research will use a multi-scale approach to fully understand the reaction chemistry of three phenazine based, DHPS and 1,4-DHP and 1,6-DHP, anolytes. Real-time Ultrafast (UF) 2D NMR in combination with 1D 1H NMR will be performed sequentially by pumping the anolyte from externally placed RFB through NMR probe. UF experiments will be performed ex-situ for gaining sensitivity, and resolution for correlation with online data.

A miniaturized RFB will be designed and used for operando measurements by placing the cell inside NMR probe to perform localized 1D and 2D NMR to study redox mechanisms near the electrode. In summary, this proposed research will provide an exclusive understanding of the redox chemistry for improving and designing economic, environmental friendly and sustainable Phenazine based AORFB systems.