Multifaceted Effects of Ring Fusion in Charged Dialkoxyarene Redoxmers

Abstract

Redox flow batteries (RFBs) hold promises for long-duration energy storage, with various chemistries demonstrated by academia and industry. However, their practical applications remain limited due to high active material cost, low energy density, narrow voltage window, and limited cyclability. Improving the performance of redox active materials (redoxmers) in RFBs is crucial for addressing these challenges. In this study, to better understand the impact of π-conjugation of aromatic redox cores on the redoxmers’ electrochemical behavior, we systematically investigated a series of dialkoxy benzene based redoxmers, including 1,4-dimethoxy benzene (DMB), 1,4-dimethoxynaphthalene (DMN), 2-ethyl-9,10-dimethoxyanthracene (EDMA). As the conjugation increases from DMB to DMN, we observed a lower redox potential, improved kinetic stability, and longer cycling life. However, further expanding the conjugation as in EDMA resulted in rapid O-dealkylation of the radical cation, possibly due to increased strain in methoxy groups. For DMN, the radical cation is observed to be prone to neutralization (likely via disproportionation) and can be recharged, suggesting a longer cycling life than the calendar life. Our study indicates that extending the π-system alters reactivity in multiple ways, and lowering the oxidation potential through π-conjugation to improve redoxmer stability should be cautiously pursued.