Electrocatalysts and Membranes for Aqueous Polysulfide Redox Flow Batteries

Redox flow batteries have demonstrated attractive attributes in large-scale stationary energy storage, but practical applications are impeded by high capital cost. Polysulfides are exceedingly cost-effective candidates of redox-active materials for achieving cost reduction, and a recent revival has been witnessed. But the slow conversion kinetics and irreversible crossover loss of polysulfides are daunting challenges that have caused severe technoeconomic stress and even system failure. Solutions to these issues capitalize on the innovations of powerful electrocatalysts and permselective membranes. To inspire viable development strategies and further advance polysulfide redox, this Review presents a critical overview of the state of the art of electrocatalysts and membranes, highlighting their working mechanisms, design protocols, and performance metrics. We briefly describe the complicated processes of the polysulfide reaction and the major spectroscopic methods for polysulfide speciation. Next, we point out the specific characteristics of polysulfide redox and summarize the metallic, metal sulfide, and molecular electrocatalysts to elucidate the fundamental requirements for imparting strong catalytic effects. We then discuss the possible origins of polysulfide crossover and outline the major families of membrane chemistries targeting polysulfide retention. Finally, the remaining challenges and the future perspectives for potential considerations are provided, aiming to realize efficient, durable polysulfide flow batteries.