Strategic modulation of hydrogen bonding networks in aqueous electrolytes using green deep eutectic solvents: Experimental and computational insights for enhanced capacitive performance in symmetric supercapacitors

This study explores the integration of deep eutectic solvents (DESs) with aqueous electrolytes, paving a pathway to modulate the hydrogen-bonding network, suppress water decomposition, and enhance electrochemical performance of a well-established electroactive reduced graphene oxide (rGO) based symmetrical supercapacitor (SSC). A strategic approach is proposed to control water dissociation in aqueous Na₂SO₄ electrolytes by incorporating a choline chloride (ChCl) based green DES, specifically ChCl and ethylene glycol (EG) (commonly referred to as ethaline, ETH). Systematic investigations reveal that varying water content (0 %, 40 %, 60 %, and 90 %) in the DES-water mixture affects its physicochemical properties, including conductivity, viscosity, and capacitive performance. The conductivity of neat ETH (ETH-1000) increased with up to 60 % water addition (ETH-4060) but dropped sharply to 90 % water content (ETH-1090) due to excessive dilution. The viscosity of ETH-1090 was six times lower than ETH-1000, reflecting hydrogen bond weakening among ETH components due to dilution. Among these, ETH-1090 demonstrated the highest capacitive performance compared to ETH-1000. Furthermore, the addition of ETH-1090 DES to the aqueous Na₂SO₄ electrolyte simultaneously disturbed the pre-existing hydrogen bonding network while fostering new and unique types of hydrogen bonds in ETH-1090-NS based electrolyte. Thus, this strategic modulation suppressed water decomposition and enhanced the electrochemical stability of aqueous Na₂SO₄ electrolyte for application in rGO-based SSC. This leads to a significant increase in the capacitive performance of aqueous Na₂SO₄ electrolyte from 445 to 645 mF cm^-2 with greater conductivity and extended electrochemical potential window up to 1.6 V. To support these findings, classical molecular dynamics (MD) simulations and quantum chemical calculations were conducted. Computational study provides insights into molecular interactions between ETH DES and aqueous electrolyte, revealing the unique role of hydrogen bonds in manipulating the conventional aqueous electrolyte. The excellent long-term cyclic stability (70 % retention of capacitance up to 10,000 cycles at high current density) is profoundly validating the robustness of the developed ETH-1090-NS based electrolyte system. This comprehensive study highlights the potential of unique DES based modifications of conventional aqueous electrolytes at the atomic level to enhance performance and advance energy storage systems.