Electrolytes for bromine-based flow batteries: Challenges, strategies, and prospects

Bromine-based flow batteries (Br-FBs) have been widely used for stationary energy storage benefiting from their high positive potential, high solubility and low cost. However, they are still confronted with serious challenges including bromine cross-diffusion, sluggish reaction kinetics of Br2/Br− redox couple and sometimes dendrites. To impel the further industrial development of Br-FBs, great efforts have been made in their key materials of the electrodes, membranes and electrolytes. Among them, electrolyte optimization is one of the most promising strategies because of its great economy and feasibility, which includes electrolyte composition optimization and electrolyte flow optimization. Herein, we first summarize the physicochemical properties and composition of electrolytes for Br-FBs. Notably, the spectroscopic characterization methods are also overviewed in this review, in order to provide the comprehensive analysis methods to guide the electrolyte design and modification. Then we analyze the main challenges and summarize corresponding strategies related to electrolytes. The promising opportunities for electrolyte optimization to achieve high-performance and long-lifespan Br-FBs are outlined as well. As a result, this review provides inspiration and guidance for further demonstration applications of Br-FBs.