A π‐Conjugation Extended Viologen as a Two‐Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries

Extending the conjugation of viologen by a planar thiazolo[5,4‐d]thiazole (TTz) framework and functionalizing the pyridinium with hydrophilic ammonium groups yielded a highly water‐soluble π‐conjugation extended viologen, 4,4′‐(thiazolo[5,4‐d]thiazole‐2,5‐diyl)bis(1‐(3‐(trimethylammonio)propyl)pyridin‐1‐ium) tetrachloride, [(NPr)2TTz]Cl4 , as a novel two‐electron storage anolyte for aqueous organic redox flow battery (AORFB) applications. Its physical and electrochemical properties were systematically investigated. Paired with 4‐trimethylammonium‐TEMPO (NMe‐TEMPO) as catholyte, [(NPr)2TTz]Cl4 enables a 1.44 V AORFB with a theoretical energy density of 53.7 Wh L−1. A demonstrated [(NPr)2TTz]Cl4/NMe‐TEMPO AORFB delivered an energy efficiency of 70 % and 99.97 % capacity retention per cycle.