(Invited) Control of Diffusion Behavior of Metal Ions at Ionic Liquid/Electrode Interface and Its Effect on Electrodeposits
Authors: Imanishi Akihito 
Published: February 2020

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices. However, we still lack enough knowledge on the interface structure and spatial distribution of chemical species during the electrochemical processes. In our previous study, we succeeded in the observation of the spatial distribution of XPS spectra close to the electrode under the applied potential using scanning electrochemical photoelectron spectroscopy (Scanning EC-PES) we originally developed. Interestingly, the shape of the diffusion layer (The spatial distribution of the concentration of the metal ions) was largely different from the concave function usually supposed in the case of aqueous solution. In the case of ionic liquid solution, the spatial distribution of concentration was convex and the depletion layer where the concentration of metal ions is quite small (We named this region as “depletion layer”) was formed close to the electrode in a wide region.
By a numerical simulation, we revealed that the metal ions diffused by the hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of 'holes' (vacancies formed between IL molecules) acting as hopping sites. In addition, we also revealed that the diffusion behavior (diffusion velocity) of metal ions can be controlled by controlling the concentration of the holes by using of the small amount of additive ions. These results indicate that we will be able to propose the new strategy for controlling the diffusion behavior of solutes in ionic liquid solutions close to the electrode. On the other hand, it is known that the diffusion behavior of metal ions affects the shape of the electrodeposits on the electrode. Therefore, the controlling of the diffusion behavior of metal ions close to the electrode enable us to control the shape of the electrodeposits.
In this strudy, we investigated how we can control the diffuosion behavior of the metal ions based on the new diffusion model we previously proposed and we also investigated that how the diffusiton behavior of the metal ions in IL affects the shape of the electrodeposits on the surface.
In the case of the ~200 mM Ag+/BMI-TFSA((1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide) solution, the large depletion layer was formed close to the electrode, whereas the depletion layer was decreased in the case that Li+ (as an electrochemically inactive additive) was added to the solution. This result indicates that the hopping-like diffusion was suppressed by the addition of the Li+ ion (Note that the depletion layer was formed by the fast hopping-like diffusion mechanism). On the other hand, the shape of the electrodeposit formed in the former solution is largely different from that formed in the latter solution. This is mainly due to the total diffusion velocity of Ag+ ion was largely decreased by the addition of Li+ ion. These result indicates that we can effectively control the shape of the electrodeposits by controlling the diffusion behavior of metal ions.