Communication—Tris(bipyridyl)iron Complexes for High-Voltage Aqueous Redox Flow Batteries

While the choices of active materials for redox flow batteries (RFBs) have grown substantially, very few can adequately serve in the positive side of a RFB. We attempt to address the challenge with an inexpensive, high-potential tris(bipyridyl)iron complex. Paired with methyl viologen, the complex enables a 1.4 V neutral RFB with 215 cycles at a current efficiency of 99.8% and capacity retention over 99.9% per cycle. The discharging process displays two peculiar plateaus due to the dimerization of ferric complexes, as confirmed by ex situ cyclic voltammetry and the UV–vis spectroscopy.