Two‐Electron Tetrathiafulvalene Catholytes for Nonaqueous Redox Flow Batteries

Abstract Tetrathiafulvalene (TTF) exhibits two reversible oxidation steps and is used as a novel multi‐electron catholyte for nonaqueous organic redox flow batteries. To increase solubility in polar organic solvents, TTF derivatives with polar side chains are synthesized. 4‐Methoxymethyltetrathiafulvalene emerges as a promising two‐electron catholyte because it is a liquid at room temperature and miscible with acetonitrile. Bulk‐electrolysis experiments and UV‐vis‐NIR absorption spectroscopy reveal excellent cycling stability for the first and second electrochemical oxidations. In the doubly oxidized state, the TTF derivatives show a reversible about 1 % loss of the state of charge per day, due to extrinsic effects. In a symmetric redox flow battery, 4‐methoxymethyltetrathiafulvalene shows a volumetric capacity loss of only 0.2 % per cycle with a Coulombic efficiency (CE) of 99.6 %. In an asymmetric redox flow battery with a pyromellitic diimide as two‐electron anolyte, the capacity loss is 0.8 % per cycle, with CE>99 % in each cycle.