(201f) An Investigation of a Polysulfide – Polyiodide Aqueous Redox Flow Battery

Redox flow batteries (RFBs) have attracted considerable academic and industrial interest based on their favorable combination of performance and safety in the widespread integration of renewable, non-dispatchable energy sources and in the improvement of energy efficiency of the electric grid. However, at present, RFBs are broadly considered too expensive for widespread deployment. A cost-effective aqueous RFB based on alkaline metal polysulfide (PS) and polyiodide (PI) is shown here for the application grid-scale energy storage. The proposed RFB applies earth-abundant elements, which shows great potential to meet the stringent cost target for RFBs ($100/kWh). The OCV of the proposed RFB is ~ 0.88 V with a theoretical energy density as high as ~ 46 Wh/L that is ~84% higher than that of all vanadium redox flow batteries. Galvanostatic and potentiostatic studies were carried out on both the negative and positive sides, which sheds light on the electrochemistry of sulfide and iodide species, respectively. The cell performance of the proposed PS-PI chemistry was further studied using a flow battery architecture operated at 5 mL/min and 20 degree Celsius. The resultant RFB displayed stable performance at 15 mA/cm² for over 150 hours (25 cycles) with relatively small capacity decay. A system diagnosis after the cycling experiment revealed that membrane (sodiated Nafion membrane) degradation is the primary reason for the significantly decreased energy efficiency of the cell after 25 cycles.