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

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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)₂ TTz]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)₂ TTz]Cl₄ enables a 1.44 V AORFB with a theoretical energy density of 53.7 Wh L⁻¹ . A demonstrated [(NPr)₂ TTz]Cl₄ /NMe -TEMPO AORFB delivered an energy efficiency of 70 % and 99.97 % capacity retention per cycle.