Iodide-Mediated Charge Storage in Deep Eutectic Solvent Electrolytes for Nontoxic Printed Supercapacitors

Deep eutectic solvents (DESs) are promising sustainable electrolytes for printed energy storage devices owing to their low toxicity, low cost, and tunable physicochemical properties. Herein, a redox-active DES electrolyte based on choline chloride-urea was developed for nontoxic printed symmetric supercapacitors (SCs) by incorporating potassium iodide (KI) as a charge-storage enhancer. The influence of KI concentration on electrolyte stability, rheological behavior, and electrochemical performance was systematically investigated. While the addition of 1 wt% water effectively suppressed DES crystallization during storage, KI introduced reversible iodide/triiodide redox reactions that enhanced charge storage beyond conventional electric double-layer capacitance (EDLC). The DES-1H₂O-2.5KI electrolyte provided a balanced performance, with a specific capacitance of 55 F g^-1@ 0.1 A g^-1, an energy density of 24.8 W h kg^-1, and capacitance retention of 98% after 10,000 cycles. Trasatti analysis revealed a balanced charge storage mechanism with contributions from both EDLC (47%) and pseudocapacitance (53%). In contrast, higher KI concentrations increased electrolyte viscosity and diffusion limitations, resulting in reduced electrochemical efficiency. The results demonstrate that controlled incorporation of iodide redox species provides an effective strategy for improving the performance of DES electrolytes while maintaining electrolyte stability and sustainability. These findings highlight the potential of redox-active DES electrolytes for sustainable printed SCs.