Cooperative H2O–CO2 Regulation Reinforces Transport and Interfacial Stability in Deep Eutectic Solvent Electrolytes

Abstract Deep eutectic solvents (DESs), as representative hydrogen-bonded electrolytes, offer unique solvation environments, high CO2 affinity, and broad opportunities in sustainable fields. However, the high viscosity, sluggish diffusion, and the low-temperature solidification tendency of DESs severely impede their electrochemical performance. This work demonstrates an H2O–CO2 cooperative regulation strategy, in which water effectively lowers the viscosity and preserves fluidity of DESs even at −40 °C, while CO2 stabilizes the hydrated DES interfaces and expands the electrochemical window over a wide temperature range. Such interfaceial stabilization is achieved through a CO2-mediated interfacial molecular mechanism rather than a conventional surface passivation process. Combining electrochemical vibrational spectroscopy, atomic force microscopy, and molecular dynamics simulations, it elucidates that the CO2 lowers the population of electrochemically accessible water at the cathodic interface and reorganizes the local hydrogen-bond network in hydrated DESs, thereby suppressing the hydrogen evolution reaction while preserving the fluidity provided by hydration. This stabilized interface further benefits an increased CO2-to-CO reduction rate by approximately 1 order of magnitude. By resolving the persistent transport-stability trade-off in DES electrolytes, this H2O–CO2 cooperative regulation concept offers molecular insights into nanoscale interfacial hydrogen-bond engineering at viscous electrochemical interfaces and provides a general strategy for CO2 conversion, electrosynthesis, etc.