Synthesis and characterization of betaine-based natural deep eutectic solvents for electrochemical application

• The betaine-based deep eutectic solvent is an electrochemically stable electrolyte. • Supercapacitor with investigated electrolyte achieved high capacitance. • Betaine hydrochloride may replace choline chloride as a hydrogen bond acceptor. Lately, deep eutectic solvents (DESs) have been taken into detailed consideration as potential electrolytes for use in various electrochemical devices. This study reports a successful synthesis of three novel natural deep eutectic solvents (NADESs) based on betaine hydrochloride as the hydrogen bond acceptor and glycerol, glycolic acid, and ethylene glycol as hydrogen bond donors. Comprehensive research, including ATR-FTIR, surface tension, rheological properties, and ionic conductivity, revealed that the betaine hydrochloride–ethylene glycol (BETCl − :MEG) system exhibited superior physicochemical characteristics. This NADES was subsequently evaluated as an electrolyte in an electric double-layer capacitor (EDLC), demonstrating excellent electrochemical performance with a specific capacitance of 94 F g −1 (0.5 A g −1 ) and excellent electrochemical stability. Notably, this study represents the first successful application of a betaine hydrochloride-based NADES as a liquid electrolyte in an energy storage device, highlighting its potential as a sustainable alternative to other innovative electrolyte systems, such as deep eutectic solvents based on choline chloride. These findings suggest that betaine-based NADESs hold significant promise for future applications in energy storage technologies.