Addressing Intrinsic Limitations of Aqueous Electric Double-Layer Capacitors with Designer Redox-Active Deep Eutectic Solvent Electrolytes

Despite their environmentally friendly and cost-effective operation, aqueous electric double-layer capacitors (EDLCs) are limited by lower energy density compared with conventional batteries. Additionally, the inherent properties of aqueous electrolytes present challenges including fast self-discharge, limited operational voltage, and susceptibility to freezing. To address these issues, we introduce redox-active deep eutectic solvent (DES)-based electrolytes comprising choline chloride, ethylene glycol, and ethyl viologen dibromide. Incorporating this redox-active DES into EDLCs not only increases energy density and reduces self-discharge rates but also ensures stable operation at an elevated voltage of 1.8 V and in subzero temperatures down to −10 °C.