Reversed rectification of ionic liquid/water mixtures in conical nanochannels

Because of their remarkable properties, room-temperature ionic liquids (RTILs) are used widely in electrochemistry, fuel cells, supercapacitors, and even DNA sequencing, and many of these applications involve the transport of RTILs in nanoscale media. Particularly for single-molecule detection, the RTIL must be mixed with a solvent (e.g., water) so that the electrolyte has both high viscosity and conductivity to obtain excellent signals. If a RTIL contains a quantity of water in bulk, this has a significant effect on its properties (e.g., the electrochemical window), thereby limiting some applications. However, the physicochemical properties of RTILs containing water in nanoconfined spaces remain unclear, especially their ionic transport behavior. Therefore, reported here is a study of the ionic transport behavior of mixed RTIL/water solutions at the nanoscale using a single conical nanochannel as a nanofluidic platform. The conductivity of the mixtures in the nanoconfined space was closely related to the nanochannel size, and highly diluted mixed solutions resulted in a nonlinear rectification-reversed current, which was possibly due to the adsorption of cations on the nanochannel wall. The maximum rectification ratio was 114, showing excellent rectification that could be used to realize newly conceptualized nanofluidic diodes. In summary, this work provides an exhaustive understanding of the nonlinear ion transport of RTIL/water mixtures and a theoretical foundation for applying RTILs in energy storage and conversion and bio-sensing. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).