Polypyrrole-Assisted Formation of Co−N x Sites for Accelerating Redox Kinetics of Polysulfide Conversion in Alkaline S−Fe Redox Flow Battery

The alkaline polysulfide/ferricyanide redox flow battery (S−Fe RFB) possesses advantages of safety, design flexibility, and low cost, which is promising for large-scale energy storage. However, the sluggish redox kinetics of polysulfide conversion on the negative electrode severely restricts the energy efficiency. To address this issue, a Co-PPy/C with polypyrrole-assisted formation of Co−Nx sites is designed as the catalyst for the negative electrode of S−Fe RFB. This catalyst integrates the excellent electrical conductivity of polypyrrole with the superior catalytic activity of Co−Nx sites, thereby accelerating the redox kinetics of polysulfide conversion. The Co-PPy/C-modified battery reaches an energy efficiency of 87.5% at 20 mA cm−2, representing a 31.2% improvement over the battery with the pristine electrode (56.3%). Moreover, the S−Fe RFB with Co-PPy/C is able to function for 500 cycles at 80 mA cm−2 with an energy efficiency decay rate of only 0.00442% per cycle, indicating excellent long-term cycling stability. These findings highlight the potential of Co-PPy/C as an efficient catalyst for enhancing the electrochemical performance of redox flow batteries.