Tailoring electrolyte solvation for improved Na-based supercapacitor efficiency: an operando characterization approach

Abstract

The development of safe and high-performance sodium-based supercapacitors critically depends on the electrolyte choice, which governs ionic transport, the material’s electrochemical stability, and the device’s long-term durability. In this work, we present a comprehensive comparative study of four distinct classes of sodium-based electrolytes: highly concentrated aqueous solutions (WiSE), organic carbonate solvents, ionic liquids (IL), and deep eutectic solvents (DES). By combining classical electrochemical characterization studies (CV, GCD, and EIS) with operando electrochemical mass spectrometry (OEMS), we provide novel insights into the interplay between electrolyte composition, ion accessibility in porous carbon electrodes, and gas evolution processes. Our results reveal that the NaClO4:[C3mpip][NTf2] ionic liquid system delivers the highest specific capacitance and energy, while the NaOTf:PEG400 deep eutectic solvent demonstrates intermediate behavior with moderate stability improvements. Conversely, the 17 M NaClO4 aqueous system, despite its superior ionic conductivity, is prone to the gas evolution reaction and instabilities at the electrode/electrolyte interface. The OEMS integrated analysis allowed us to verify a direct correlation between the gas evolution rate and electrolyte degradation mechanisms, shedding light on the formation of a solid electrolyte interface (SEI) and solvent stability. The integrated method establishes a framework for rational electrolyte design and highlights the importance of balancing ionic transport with interface stability. The novelty of this work mainly lies in bridging electrochemical performance with real-time gas evolution analysis using different electrolytes, providing a roadmap for the development of the next-generation sodium-based energy storage devices.