Biomimetic Amino Acid Functionalized Phenazine Flow Batteries with Long Lifetime at Near-Neutral pH

Aqueous organic redox flow batteries (AORFBs) are a promising electrochemical technology for large-scale energy storage. We report a biomimetic, ultra-stable AORFB utilizing an amino acid functionalized phenazine (AFP). A series of AFPs with various commercial amino acids at different substituted positions were synthesized and studied. 1,6-AFPs display much higher stability during cycling when compared to 2,7- and 1,8-AFPs. Mechanism investigations reveal that the reduced 2,7- and 1,8-AFPs tend to tautomerize and lose their reversible redox activities, while 1,6-AFPs possess ultra-high stability both in their oxidized and reduced states. By pairing 3,3'-(phenazine-1,6-diylbis(azanediyl))dipropionic acid (1,6-DPAP) with ferrocyanide at pH 8 with 1.0 M electron concentration, this flow battery exhibits an OCV of 1.15 V and an extremely low capacity fade rate of 0.5 % per year. These results show the importance of molecular engineering of redox-active organics for robust redox-flow batteries.