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Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery
Nature Energy  (IF60.858),  Pub Date : 2021-08-02, DOI: 10.1038/s41560-021-00879-6
Li, Xiang, Gao, Peiyuan, Lai, Yun-Yu, Bazak, J. David, Hollas, Aaron, Lin, Heng-Yi, Murugesan, Vijayakumar, Zhang, Shuyuan, Cheng, Chung-Fu, Tung, Wei-Yao, Lai, Yueh-Ting, Feng, Ruozhu, Wang, Jin, Wang, Chien-Lung, Wang, Wei, Zhu, Yu

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 (M4[FeII(Dcbpy)2(CN)2], M = Na, K) exhibited up to a 4.2 times higher solubility (1.22 M) than that of M4[FeII(Dcbpy)3] and a 50% increase in potential compared with that of ferrocyanide. The AORFBs with 0.1 M Na4[FeII(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 Na4[FeII(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.