Synergy of single atoms and sulfur vacancies for advanced polysulfide–iodide redox flow battery

Aqueous redox flow batteries (RFBs) incorporating polysulfide/iodide chemistries have received considerable attention due to their safety, high scalability, and cost-effectiveness. However, the sluggish redox kinetics restricted their output energy efficiency and power density. Here we designed a defective MoS₂ nanosheets supported Co single-atom catalyst that accelerated the transformation of S²⁻/S_x²⁻ and I⁻/I₃⁻ redox couples, hence endow the derived polysulfide–iodide RFB with an initial energy efficiency (EE) of 87.9% and an overpotential of 113 mV with an average EE 80.4% at 20 mA cm⁻² and 50% state-of-charge for 50 cycles, and a maximal power density of 95.7 mW cm⁻² for an extended cycling life exceeding 850 cycles at 10 mA cm⁻² and 10% state-of-charge. In situ experimental and theoretical analyses elucidate that Co single atoms induce the generation of abundant sulfur vacancies in MoS₂ via a phase transition process, which synergistically contributed to the enhanced adsorption of reactants and key reaction intermediates and improved charge transfer, resulting in the enhanced RFB performance.