Sandwiched Cu7S4@graphite felt electrode for high performance aqueous polysulfide/iodide redox flow batteries: Enhanced cycling stability and electrocatalytic dynamics of polysulfides

Aqueous polysulfide/iodide redox flow battery (RFB) with highly soluble active species are attractive candidates for scalable energy storage, offering noticeable reduced RFB cost. Though, poor reversibility and electrochemical activity of the redox reaction of polysulfide couples on graphite felt electrode restricts its energy efficiency. Herein, a facile method to improve the electrocatalytic activity by fabrication of skeletal Cu 7 S 4 hollow nanocages with copper vacancies is presented. Cu 7 S 4 nanoparticles can selectively boost the electrocatalytic activities of S 2− /S x 2- redox reactions and suppress the hydrogen evolution side reaction. In addition, the hollow opening structure of skeletal Cu 7 S 4 served as the catalytic active site (both the inner side and outer surface), accelerating the electrolyte transport and promoting the charge transfer process. A sandwiched electrode was constructed by coating Cu 7 S 4 nanocages into the interlayer of graphite felt. The polysulfide/iodide flow battery with SKE-Cu 7 S 4 sandwich structure electrode can generate high energy efficiency of 78.5% at 20 mA cm −2 , power density of 30.6 mW cm −2 and a stable energy efficiency retention of 92% after approximately 200 continuous cycles suggesting their potential use for future scalable RFBs. The developing of highly efficient and stable electrocatalyst for the electro-conversion of polysulfides will boost the performance and application of polysulfide-based flow batteries and other energy devices involved with polysulfides such as, quantum dots sensitized solar cells, Li–S batteries.