Enabling Long-Life Aqueous Organic Redox Flow Batteries with a Highly Stable, Low Redox Potential Phenazine Anolyte

Abstract

Aqueous organic redox flow batteries (AORFBs) are considered a promising energy storage technology due to the sustainability and designability of organic active molecules. Despite this, most of AORFBs suffer from limited stability and low voltage because of the chemical instability and high redox potential of organic molecules in anolyte. Herein, we propose a new phenazine derivative, 4,4′-(phenazine-2,3-diylbis(oxy))dibutyric acid (2,3-O-DBAP), as a water-soluble and chemically stable anodic active molecules. By combining calculations and experiments, we demonstrate that 2,3-O-DBAP exhibits a higher solubility, a lower redox potential (−0.699 V vs SHE), and greater chemical stability than other O-DBAP isomers. Then, we demonstrate a long-lasting flow cell with an average discharge voltage of 1.12 V, a low fade rate of 0.0127%, and a lifespan of 62 days at pH 14 using 2,3-O-DBAP paired with ferri/ ferrocyanide. The negligible self-discharge behavior also verifies the high stability of 2,3-O-DBAP. These results highlight the importance of molecular engineering for AORFBs.

Organic Electrode Materials for Energy Storage and Conversion: Mechanism, Characteristics, and Applications

“…b) Synthetic route of O -DBAP isomers, where the branched chains (R′) can be repositioned at multiple positions (i.e., 1–8) of the aromatic. Reproduced with permission . Copyright [2024] [ACS].…”

“…c) Calculated octanol–water partition coefficients of O -DBAP isomers with branched chains (R′) at different positions. Reproduced with permission . Copyright [2024] [ACS].…”

“…d) HOMO–LUMO plots and correlated energy level of 1,6- O- DBAP, 2,3- O- DBAP, and 2,7- O -DBAP. Reproduced with permission . Copyright [2024] [ACS].…”

“…On this basis, we reported a 4,4′-(phenazine-2,3-diylbis­(oxy))-dibutyric acid (2,3- O -DBAP) as the anolyte for AORFBs (Figure a) . We design and synthesize a series of O -DBAP isomers (Figure b), and then through a combination of DFT calculations and experiment, we compare the several O -DBAP isomers and illustrate that 2,3- O -DBAP is the most promising as the active species in the anolyte in consideration of solubility (Figure c) and electrochemical performance (Figure d) in aqueous solution . We further demonstrate a long-lasting flow battery based on 2,3- O -DBAP with a 40-day lifespan and a low-capacity fade rate of as low as 0.0127%/day, which is among the best in this field (Figure e) .…”

“…However, many organic molecules are dissoluble in liquid electrolyte, making OEMs promising as catholytes or anolytes for redox flow batteries (RFBs). During the operation of RFBs, a pair of reversible redox reactions take place in the regions of RFBs separated by a membrane, while the electroactive materials dissolved in the liquids are continuously supplied by the outer tanks via pumps (Figure a) …”