Designer Ferrocene Catholyte for Aqueous Organic Flow Batteries

The aqueous organic flow battery (AOFB) holds enormous potential as an energy storage device for fluctuating renewable electricity by exploiting the redox reactions of water-soluble organic molecules. The current development is impeded by lack of organic molecules adequate as catholyte, yet how the catholyte structure impacts the battery lifetime remains unexplored. Here, six ferrocene derivatives with deliberately tuned chemical structure were devised. They underwent reversible redox reactions in water, and the redox potentials were inversely related to the lowest unoccupied molecular orbital (LUMO) energy of their energized forms. The stability of the ferrocene derivatives was evaluated in full flow cells and in symmetric cells. Density function theory calculations, along with experimental results, revealed that the localized LUMO density on Fe led to fast capacity fading. BQH-Fc, which has the lowest LUMO density on Fe, showed the highest stability. No capacity loss was observed for the BQH-Fc/BTMAP-Vi cell at 0.1 m, and a high capacity retention rate of 99.993 % h^-1 was recorded at 1.5 m, which could be attributed to electrolyte crossover. To facilitate explorations of robust and high capacity catholytes, a method was established to predict the water solubility of ferrocene molecules, and calculations were in good accordance with measured values.