Optimized and cost-effective elemental-sulfur sodium polysulfide/sodium bromide aqueous electrolytes for redox flow batteries

Redox flow batteries (RFBs) can potentially revolutionize large-scale energy-storage technologies for both conventional (fossil fuel) and modern (renewable) electric power systems. This has stimulated the development of new methods for reducing the costs of such batteries by lowering material costs and increasing the energy density. Driven by the abundance and low costs of sulfur and bromine salts, this study investigates the viability of an aqueous flow battery system, in which sodium bromide (NaBr) is used as a catholyte, and a novel electrolyte called elemental added sulfur sodium polysulfide (EASSP) is utilized as an anolyte. The molar ratio of elemental sulfur to sodium (S/Na) in the sodium polysulfide solution is maintained at 1:4. Various concentrations of the EASSP and NaBr electrolytes are examined, and their optimal values corresponding to the minimum overpotential are determined. A bromine–polysulfide redox flow battery (BPRFB) containing a 1.4 M EASSP anolyte and 8 M NaBr catholyte is charged and discharged for 5 min, which results in a capacity of 17 mAh (3.4 mAh cm⁻²) at a current density of 40 mA cm⁻². The cyclic performance of the BPRFB is characterized by a capacity of 17 mAh at 40 mA cm⁻², which is retained over 150 cycles with a 98 % coulombic efficiency. The assembled BPRFB cell with the optimized electrolyte composition (1.4 M EASSP: 8 M NaBr) exhibits constant current (without leakage) and potential. The findings of this work demonstrate a high application potential of the developed anolyte for BPRFBs, which is highly soluble in water, inexpensive, and can be scaled up for energy storage applications.