Zhiguang Li

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.

Redoxmers or redox-active organic materials, arc one critical component for nonaqueous redox flow batteries (RFBs), which hold high promise in enabling the time domain of the grid. While tuning redox potentials of redoxmers is a very effective way to enhance energy densities of NRFBs, those improvements often accompany accelerated kinetics of the charged species, undermining stability and cycling performance. Herein, a strategy for designing redoxmers with simultaneous improvements in redox potential and stability is proposed. Specifically, the redoxmer 1,4-di-tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene (ANL-C46) is developed by incorporating fluorinated substitutions into the dialkoxybenzene-based platform. Compared to the non-fluorinated analogue, ANL-C46 demonstrates not only an increased (similar to 0.41 V) redox potential but also much enhanced stability (1.6 times) and cyclability (4 times) evidenced by electron paramagnetic resonance kinetic study, H-cell and flow cell cycling. In fact, the cycling performance of ANL-C46 is among the best of high potential (>1.0 V vs Ag/Ag+) redoxmers ever reported. Density functional theory calculations suggest that while the introduced fluorine substitutions elevate the redox potentials, they also help to depress the decomposition reactions of the charged redoxmers, affording excellent stability. The findings represent an interesting strategy for simultaneously improving energy density and stability, which could further prompt the development of high-performance redoxmers.