Hydrobromide Salt of Tribromodopamine as a Positive Electroactive Species with a Three-Electron Redox Process for Redox Flow Battery Applications

The hydrobromide salt of tribromodopamine (DABr3·HBr) was synthesized for redox flow battery applications. It exhibited two redox processes at ∼0.74 V (due to the DABr3/DQBr3 redox couple) and 1.0 V (due to the Br–/Br2 redox couple) vs Ag/AgCl in H2SO4, thereby leading to a 3e– reaction per molecule. An aqueous organic redox battery was constructed using DABr3·HBr catholyte and anthraquinone-1,5-disulfonic acid disodium salt (1,5-AQDS) anolyte. A capacity decay of only ∼0.028%/cycle was observed over 2500 galvanostatic charge–discharge (GCD) cycles. The high cycle life (2500 GCD cycles) with ∼99% Coulombic efficiency (CE) is due to the effective Br2-capturing ability of DABr3·HBr through ammonium polybromide formation. The energy efficiency (EE) of the DABr3·HBr-1,5-AQDS static battery was found to be 66% at a current density of 20 mA cm–2.