Hua LI1,2, Jianan WANG1, Gregory WARR3, Rob ATKIN1
1School of Molecular Sciences, The University of Western Australia, Perth, Australia
2Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia
3School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, Australia
The concerted motion of concentrated electrolytes at electrodes is critical for diverse applications like metal-ion batteries, capacitors, and lubricants, but remains almost unexplored. Here, we use state-of-the-art video-rate atomic force microscopy (AFM) to record the motion of the liquid nanostructure of ethylammonium nitrate (EAN) and its mixtures with water above mica, and the effect of the surface potential is investigated using highly oriented pyrolytic graphite (HOPG). This is the first time the motion of any liquid has been recorded at the nanometer scale in real time. Diffusion coefficients determined from the AFM videos reveal that the nanostructure diffuses many orders of magnitude more slowly than ions in the bulk for both mica and HOPG systems, attributed to solvophobic interactions between ions in the near-surface nanostructure and Stern layer ions. This means that, rather than free-flowing liquid, near-surface IL ions are better conceptualized as aggregates diffusing slowly over the counter ion-rich Stern layer. This new and surprising insight affects wide-ranging processes involving the interfacial dynamics of concentrated electrolytes.