Two-Electron Tetrathiafulvalene Catholytes for Nonaqueous Redox Flow Batteries

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.