An active and durable molecular catalyst for aqueous polysulfide-based redox flow batteries

Aqueous redox flow battery (RFB) is one of the most promising technologies for grid-scale energy storage systems. Polysulfides are particularly attractive active materials owing to their low cost and high capacity, but the low energy efficiency and low operating current density limit their practical applications. Here we report an active and durable molecule catalyst, riboflavin sodium phosphate (FMN-Na), to transform sluggish polysulfide reduction reactions to fast redox reactions of FMN-Na via homogeneous catalysis. The FMN-Na catalyst substantially reduces the overpotential of a polysulfide–ferrocyanide RFB (S-Fe RFB) from more than 800 mV to 241 mV at 30 mA cm^‑2. A catalysed S-Fe flow cell was demonstrated for 2,000 cycles at 40 mA cm^‑2 with a low decay rate of 0.00004% per cycle (0.0017% per day). A catalysed polysulfide–iodide RFB operated for 1,300 cycles under 40 mA cm^‑2 without capacity decay. This work addresses the bottleneck of polysulfide-based RFBs for long-duration energy storage applications.