In recent times, ionic liquids (IL) have been studied as a new solvent because of their unique features, in particular, tunability and designability. The electrochemical window, viscosity, conductivity, and hydrophobicity of ILs can be tuned by altering their molecular structures. One of the applications of ILs is an electrolyte solution for electrochemical devices. In the last two decades, many researchers have investigated the IL/electrode interfaces, where the electrochemical reactions exactly occur. It has been well known that both the electrode surface and the electrical double layer (EDL) structure affect the kinetics of electrochemical reactions. However, the IL/electrode interface, especially, the EDL structure has not been well understood. The EDL formed in ILs is assumed to have different structure from that formed in aqueous electrolytes due to IL's high ionic strength. In this study, the EDL structure formed in the deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM-d₁₅]TFSA) was elucidated using neutron reflectivity (NR) technique. To analyze the EDL structure, it is important that the technique must not the disturb the EDL structure during measurements and the measurement should be carried out under the ideal electrochemical condition. The NR technique satisfies the both conditions. Further, neutron does not decompose organic compounds, including ILs. Therefore, the NR is suitable for the analysis of EDL structure.

[BMIM-d₁₅]TFSA was synthesized at the National Deuteration Facility (NDF), Australian Nuclear Science and Technology Organization (ANSTO). Before use, [BMIM-d₁₅]TFSA was dehydrated at 80°C under vacuum for 24 h. Si(100) electrode was used as the working electrode. The working electrode was cleaned with acetone, ethanol, and Milli-Q. Pt wires were used as the reference and counter electrodes. The electrode potential was controlled by IviumStat. The specially designed electrochemical cell for NR measurements made from Kel-F was used and it was assembled in a glove box filled with Ar gas. NR measurements were carried out at BL-17 SHARAKU, Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC). NR profiles were analyzed using Motofit.

Firstly, an impedance measurement was carried out to elucidate the flat band potential of Si electrode in [BMIM]TFSA. The flat band potential was estimated at –0.88 V vs Pt. In NR measurements, NR profiles were taken at open circuit voltage (OCV), –1.1 V (negative than the flat band potential) and +0.5 V (positive than the flat band potential). The analysis reveal that at any electrode potential, where we measured in this study, the EDL consists of IL molecule layers and a bulk IL layer. At the electrode potential, E = –1.1 V, [BMIM]⁺ molecules adsorb on the electrode surface and a TFSA^– molecules adsorb on the [BMIM]⁺ layer. On the other hand, at E = +0.5 V, the stacking order was opposite, i.e., the first and second layer consist of TFSA^– and [BMIM]⁺ molecules, respectively. The electrode surface is negatively charged when the electrode potential is negative than the flat band potential and vice versa. Therefore, these results show that the EDL structure reflects the charge of electrode surface. The detail measurements and analysis are under way.