Reducing self-discharge of iodide-based redox supercapacitors through highly concentrated salt solution

Iodide-based redox supercapacitors (SCs) can increase the specific capacitance of conventional SCs by introducing redox species in the electrolyte. However, the rapid shuttling effect of I - /I 3 - would exacerbate the self-discharge behavior of the redox SCs. Herein, we developed a novel composite electrolyte solution consisting of 0.1 M potassium iodide (KI) and highly concentrated salt (e.g., 9 M potassium acetate (KAc)), which is used for the assembly of iodide-based redox SCs. The composite electrolyte solution not only extends the operating voltage of the aqueous redox SCs to 2.0 V, but also leads to a low self-discharge rate. Specifically, at 1.6 V, the self-discharge rate of the 9 M KAc-based SCs decreases from 62.58 to 19.58 mV/h, compared to the 1 M KAc-based SCs. Meanwhile, the suppressed shuttling effect of I - /I 3 - by highly concentrated salt solution is well studied by in-situ UV–vis absorption spectrum, viscosity, conductivity, Raman spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. It is concluded that highly concentrated electrolyte can limit the diffusion of active species and thus transform the self-discharge mechanism from an activation-controlled faradaic reaction to a diffusion-controlled faradaic reaction. This work not only deepens understanding the working mechanism of highly concentrated composite electrolyte for alleviating the self-discharge of redox SCs, but also opens a new universal avenue to fabricate good cycling stability and high performances of redox SCs.