Redox flow batteries (RFBs) have been recognized as a promising option for scalable and dispatchable renewable energy storage (e.g. solar and wind energy). Zinc metal represents a low cost, high capacity anode material to develop high energy density aqueous redox flow batteries. However, the energy storage applications of traditional inorganic Zn halide flow batteries are primarily plagued by the material challenges of traditional halide cathode electrolytes (e.g. bromine) including corrosion, toxicity, and severe crossover. As reported here, we have developed a bipolar Zinc-ferrocene salt compound, Zinc 1,1’-bis(3-sulfonatopropyl)ferrocene, Zn[Fc(SPr)2] (1.80 M solubility or 48.2 Ah/L charge storage capacity) – a robust, energy-dense, bipolar redox-active electrolyte material for high performance Zn organic RFBs. Using a low-cost porous Daramic membrane, the Zn[Fc(SPr)2] aqueous organic redox flow battery (AORFB) has worked in dual-flow and single-flow modes. It has manifested outstanding current, energy, and power performance, specifically, operating at high current densities of up to 200 mA/cm2 and delivering an energy efficiency of up to 81.5% and a power density of up to 270.5 mW/cm2. A Zn[Fc(SPr)2] AORFB demonstrated an energy density of 20.2 Wh/L and displayed 100% capacity retention for 2000 cycles (1284 hr or 53.5 days). The Zn[Fc(SPr)2] ionic bipolar electrolyte not only offers record-setting, highly-stable, energy-dense, and the most powerful Zn-organic AORFBs to date, but it also provides a new paradigm to develop even more advanced redox materials for scalable energy storage.
Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries