Simple-Synthesized Sulfonated Ferrocene Ammonium for Aqueous Redox Flow Batteries

To achieve sustainable development, the development and utilization of redox-active organic species in aqueous redox flow batteries provide broad prospects for large-scale and sustainable energy storage. Herein, we report a sulfonated ferrocene derivative, ferrocene ammonium sulfonate, that is stable and easy to synthesize. We applied it as a catholyte in zinc hybrid flow batteries. Through the adjustment of the electrolyte, the battery showed high capacity retention and efficiency. When the current density was 20 mA/cm 2 and 0.5 M NaCl + 0.5 M NH 4 Cl solution was used as the supporting electrolyte, the capacity retention reached 92.55% after 100 consecutive charge/discharge cycles. In addition, the coulombic efficiency, energy efficiency, and voltage efficiency stayed at 98.33, 83.19, and 84.60%, respectively. This battery design puts forward an idea for developing low-cost materials and improving capacity retention.