Harnessing Hybrid Aqueous/Organic Electrolytes for High Energy Density Supercapacitors.

Supercapacitors (SCs) emerged as promising energy storage devices to address the energy storage demands of the modern era. The limited energy density of SCs due to a narrow voltage window hinders their competitiveness. The electrodes of SCs drive the mechanism responsible for charge storage. However, electrolytes play a critical role in shaping key parameters that directly affect the operating voltage window, performance, and cost of SCs. This work highlights current research, addressing critical issues and solutions, focusing on advancing hybrid aqueous/organic electrolytes to widen the operating voltage window of SCs. The solvation chemistry and mechanistic studies responsible for widening the voltage window of SCs, especially with water as a primary solvent and organic additive as a co-solvent, have been explored. The tailored coordination of solvents (water and organic co-solvent) with electrolyte ions in hybrid electrolytes reduces the ion size and the availability of free water molecules for undesired hydrogen/oxygen evolution reaction (HER/OER), thereby widening the voltage window in SCs. The challenges in finding a suitable co-solvent and maintaining all essential properties of hybrid electrolytes have been highlighted and discussed. This study is vital to developing high-quality electrolytes for SCs to meet the high energy density demands for practical applications.