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 thiazolo5,4-dthiazole (TTz) framework and functionalizing the pyridinium with hydrophilic ammonium groups yielded a highly water-soluble π-conjugation extended viologen, 4,4'-(thiazolo5,4-dthiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium) tetrachloride, (NPr)2 TTzCl4 , 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)2 TTzCl4 enables a 1.44 V AORFB with a theoretical energy density of 53.7 Wh L-1 . A demonstrated (NPr)2 TTzCl4 /NMe -TEMPO AORFB delivered an energy efficiency of 70 % and 99.97 % capacity retention per cycle.