Single vs Dual Shuttle Cycling of Polyferrocenyl Cathodes for Redox Targeting Flow Batteries

Redox-targeting flow batteries (RTFBs) merge solution and solid-state redox chemistries to create scalable storage systems with the potential for high energy densities. This study systematically evaluates ferrocenyl derivatives as shuttles that charge/discharge an insoluble ferrocenyl polymer. The polymer utilization, voltaic efficiency, and discharge voltage of cells cycled by each shuttle alone (single system) or by combinations of shuttles (dual system) are directly compared. Here, the performance of a dual system exceeded that of any other system tested, and stable cycling was achieved with added polymer that quintupled the system’s capacity. Most importantly, experimental and computational mechanistic studies provide insights into the interplay between the shuttle’s redox potential (thermodynamic parameter) and the shuttle’s rate of redox targeting reaction (kinetic parameter) with the solid. These data reveal that the rate of redox targeting dictates a system’s performance and depth of charge once the potentials of shuttles and solids are comparable.