Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery

The limited availability of a high-performance catholyte has hindered the development of aqueous organic redox flow batteries (AORFB) for large-scale energy storage. Here we report a symmetry-breaking design of iron complexes with 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide ligands. By introducing two ligands to the metal centre, the complex compounds (M 4 [Fe II (Dcbpy) 2 (CN) 2 ], M = Na, K) exhibited up to a 4.2 times higher solubility (1.22 M) than that of M 4 [Fe II (Dcbpy) 3 ] and a 50% increase in potential compared with that of ferrocyanide. The AORFBs with 0.1 M Na 4 [Fe II (Dcbpy) 2 (CN) 2 ] as the catholyte were demonstrated for 6,000 cycles with a capacity fading rate of 0.00158% per cycle (0.217% per day). Even at a concentration near the solubility limit (1 M Na 4 [Fe II (Dcbpy) 2 (CN) 2 ]), the flow battery exhibited a capacity fading rate of 0.008% per cycle (0.25% per day) in the first 400 cycles. The AORFB cell with a nearly 1:1 catholyte:anolyte electron ratio achieved a cell voltage of 1.2 V and an energy density of 12.5 Wh l –1 .