A robust anionic sulfonated ferrocene derivative for pH-neutral aqueous flow battery

Despite that aqueous organic redox flow batteries (AORFBs) have recently been extensively investigated for low-cost large-scale energy storage, the option for viable catholyte remains limited. This is further constrained by the incompatibility of those positively charged redox species with the cation-exchange membranes. Here, we report an anionic sulfonated ferrocene derivative (Fc-SO3Na) bearing negatively charged moiety to tackle the above issue. The molecule has a solubility of 2.5 M in water and reveals remarkable electrochemical reversibility and stability in pH-neutral solution. A flow battery with Fc-SO3Na cathodic redox species and zinc metal as anode demonstrated superb performance in conjunction with sulfonated poly (ether ether ketone)/titanium oxide composite membrane (SPEEK/TiO2) as separator. The cell exhibits a capacity retention of 99.9975% per cycle at low concentration and at high concentration it shows an energy density of 27.1 Wh L-1 which is fairly comparable to the VFBs (25-30 Wh L-1). The volumetric capacity of catholyte could be further promoted to 293.5 Ah L-1 via redox-targeting reaction when LiFePO4 is loaded into cathodic tank.