Thermodynamic Water Activity Explains the Unusual Electrochemical Stability of Aqueous Deep Eutectic Solvents

Journal Article · Tue Mar 25 00:00:00 EDT 2025 · Chemistry - A European Journal

DOI:https://doi.org/10.1002/chem.202500717· OSTI ID:2548170

Prado, Desiree Mae ^[1]; Gonzaga, Aaron Niño ^[1]; Carter, Brady ^[2]; Burda, Clemens ^[1]

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1. Case Western Reserve University, Cleveland, OH (United States)
2. Novasina AG, Morgan, UT (United States)

The presence of water in nonaqueous deep eutectic solvent (DES) electrolytes has been debated in recent years, with efforts ranging from its complete removal to willful addition. It was shown that controlled amounts of water can be beneficial, as it not only enhances the physicochemical properties of these electrolytes but also has no significant detrimental effects on their electrochemical stability. Despite these advantages, there is still limited understanding of how water interacts with DES systems at the molecular level. This study examines the water activity in ethylene glycol and glycerol, as well as their binary mixtures with choline chloride to form the DESs ethaline and glyceline, respectively. In this work, we show that the high electrochemical stability of glyceline is related to its lower water activity compared to ethaline and can be attributed to the robust H-bonding network formed by the three hydroxyl groups of glycerol. Its 3D H-bond network effectively integrates water molecules within its solvent structure, reducing degradation and maintaining stability at higher water contents. The deviations from the ideal Raoult's law behavior are reflected in the water activity and activity coefficients, which highlight the intricate H-bond interactions within DES-water mixtures. Water acts like a lubricant within the more viscous DES mixtures without being detrimental to their electrochemical performance. The presented results emphasize the necessity of customizing DES-water compositions to enhance their performance as electrolytes, especially in flow battery applications where electrochemical stability, ionic conductivity, and fluidity are of utmost importance.

• Research Organization: Case Western Reserve University, Cleveland, OH (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0019409
• OSTI ID: 2548170
• Journal Information: Chemistry - A European Journal, Journal Name: Chemistry - A European Journal Journal Issue: 24 Vol. 31; ISSN 0947-6539
• Publisher: Wiley
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
deep eutectic solvents
electrochemical stability window
hydrogen bonding
water activity
water activity coefficient