Hydrated Eutectic Electrolyte for Self-discharge Suppression and Enhanced Stability in Bromide-Based Redox Electrochemical Capacitors

Abstract

Redox-enhanced electrochemical capacitors (redox ECs) offer a solution to the low energy density issue prevalent in supercapacitors. Their primary challenge, however, is self-discharge, which results from the cross diffusion of redox-active materials. Utilizing a bromine complexing agent like tetrabutylammonium bromide (TBAB) has proven effective in reducing the self-discharge of tribromide, but it necessitates the inclusion of water, typically leading to the use of aqueous electrolytes. These aqueous electrolytes are limited by a narrow potential window and a propensity for evaporation, which diminishes their electrochemical and thermal stability. In this study, we developed a hydrated deep eutectic solvent (HDES) electrolyte incorporating tetrabutylammonium bromide (TBAB), aimed at addressing both the self-discharge concern and the stability issues related to aqueous electrolytes. The substantial water content in the electrolyte facilitated the effective suppression of tribromide self discharge. This strategy also resolved the issues of high viscosity and low ionic conductivity, common with traditional deep eutectic solvent (DES) electrolytes. Consequently, the HDES electrolyte demonstrated superior electrochemical and thermal stability compared to aqueous electrolytes. The full cell utilizing HDES electrolyte with a butyl viologen (BV) as the anolyte showed an 85% energy retention after 10,000 cycles (or 1730 hours)