Rational Design of Electrochemical Iodine-Based Redox Mediators for Water-Proofed Flexible Fiber Supercapacitors

Flexible supercapacitors based on electrostatic double-layer capacitors as next-generation energy storage systems hold great promise for wearable and flexible electronic devices owing to their high flexibility and electrochemical cyclability. However, the challenge of low charge-storing ability still remains, as electronic devices require a high charge storage capability. As one of the advanced strategies for enhancing charge-storing ability in flexible supercapacitors, the introduction of a redox mediator (RM) into a gel electrolyte has recently attracted great interest. Here, we present a fiber-based flexible energy storage system (f-FESS) integrated with iodine-based chemical species as a novel RM, carbon fiber-based electrodes, and a solid-state polymer-gel electrolyte so as to enhance the charge storage capability of flexible supercapacitors. The as-prepared f-FESS-RM exhibits enhanced charge storage capabilities of up to 461.8 F L^-1 and 64.14 mWh L^-1, which are 3.6 times higher than those of f-FESS without RM. The enhanced capacitive properties of f-FESS-RM are attributed to their additional Faradic redox reaction of iodine-based chemical species in the electrolyte as well as electrical double-layer capacitive behavior. Also, the f-FESS-RM shows remarkably superior mechanical robustness under various bending, winding, knotting, and weaving conditions. Furthermore, we have demonstrated that the f-FESS-RM is stably workable under deionized water and base electrolytes. In combination, these outcomes provide a novel advanced strategy to enhance the charge-storing ability with strong mechanical robustness in flexible supercapacitors.