Ionic liquids (ILs) have attracted much attention as promising electrolytes for electrochemical devices due to their wide electrochemical window (stability) and negligible vaporization. For such applications, nanostructured electrodes have advantage on their large surface area leading to accumulation of large density energy at the interface. However, recent reports on IL/electrode interfaces revealed that quite strange structures can be formed and it is not clear how the interface forms and how it changes by forming the characteristic local structure. In this study, we investigated the local structure and physicochemical properties of the interfacial properties of nanostructured materials using XPS and electrochemical impedance spectroscopy (EIS).

Polystyrene beads were self-assembled in a close packed form on a gold substrate by dipping in polystyrene beads solution and rapid drying. Au electrodeposition on thus prepared surface followed by removal of the beads resulted in the fabrication of a nanostructured Au electrode with periodic 100 nm-sized Au dimples. Electrochemical behavior of a ferrocene dissolved ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide (BMI-TFSA)) at the nanostructured Au electrode was investigated by EIS. It is obvious that the capacitance of the nanostructured electrode was smaller than that of the flat electrode in the whole potential range. This result suggests that the thickness of the electric double layer formed in the nano-sized dimple is thicker than that formed on the flat surface. The dimple size dependence on the capacitance will also be discussed.

On the other hand, it is an interesting subject to study how ILs and solutes behave in the nano-micro sized pore. We investigated the properties of IL(BMI-TFSA) and solute ions (Au³⁺, Ag⁺, Mg²⁺ etc.) confined in the porous silica whose diameter was 1.8 nm using XPS, IR and TEM. It was revealed that the molar ratio of all the ions existing in the pore is largely different from that in the bulk. Interestingly, the molar ratio of Au³⁺ to BMI⁺ (or TFSA^-) was very large comparing with that of as prepared BMI-TFSA solution. The result of N-1s XPS spectra revealed that cation and anion of IL were organized in the nanopores. Such characteristic local structure in nanospace may affect the catalytic reaction in the pore.