Improved Formulation and Optimization of Sodium Polysulfide/Bromine Electrolytes for Redox Flow Battery

Abstract

Sodium polysulfide/bromine RFBs are one the most promising electrolyte couples for scale-up technology. Sodium polysulfide/bromine RFB utilizes aqueous sodium polysulfide and sodium bromide solution as anolyte and catholyte, respectively, separated by a sodium ion exchange membrane.Polysulfide electrolyte of different concentrations ranging from S:Na ratio of 0.5 that is without the addition of elemental sulfur, up to saturation limit having S:Na ratio of 2.4 with the addition of elemental sulfur are prepared.Studies of electrolyte composition have been essential to establishing the optimum conditions for practical use. These involve various parameters such as sulfide concentration, the ratio of dissolved sulfur to sulfide, speciation, pH, and the effect of temperature on battery stability and lifetime. Different experimental methodologies were used to investigate the speciation in these concentrated solutions. A titration method was developed to find the concentration of HS- and OH- species in any sodium sulfide and polysulfide solution. Sulfide and polysulfide chemistry is presented in more detail by comparing the UV-ATR spectra. It was found that polysulfide electrolyte shows very complex system containing species such as HS-, OH-, H+ and Sn 2- where n = 2-5 and sulphur oxoanions.Furthermore, we have established that controlled speciation of polysulfide solution can substantially impact on the electrochemistry in these electrolytes. Controlling the pH, concentrations of HS- and OH- ions in polysulfide electrolytes is extremely important to overcome many challenges of the polysulfide/bromine RFB system.Therefore, we have developed an improved polysulfide electrolytes with high charge densities but more soluble species in the presence of excessive HS- species. Charging and discharging data on our improved and well optimized polysulfide electrolyte will be presented.