(Invited) Behavior of Ionic Liquid and Solute Molecules at Electrode/Ionic Liquid Interface
Authors: Imanishi Akihito 
Published: February 2020

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices such as Li-ion battery, wet-type solar battery, etc. For most of such applications, the interface between ionic liquid and electrodes plays an important role in their performance. However, we still lack enough knowledge on the interface structure and spatial distribution of chemical species during the electrochemical processes.

1. Nanostructured electrode

In an application of electrochemical devices, 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 an electric double layer (EDL). We investigated the interfacial capacitance of nanostructured Au electrodes with periodic 100 nm-sized dimples using electrochemical impedance spectroscopy (EIS). It was revealed that the capacitance of the nanostructured electrode was largely different from 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 different from that formed on the flat surface. The dimple size dependence on the capacitance was also observed.
2. ILs confined in Nanopore

It is known that ILs confined in small spaces exhibit characteristic properties which are different from those of bulk ILs. Therefore, 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. So, we tried to synthesize Au nanoparticles via a reductive reaction in an ionic liquid confined in a nanopore using an X-ray irradiation technique[1,2]. High density Au particles whose size was limited by the pore diameter were observed (Fig. 1 left). This result is very interesting and benefits not only for the formation of the high density carrier but also for the development of the porous electrode used for various kinds of electrochemical devices and/or techniques such as battery, electrochemical deposition, impurity removing devices and so on, because condensation of reactant ions in the nanopore leads to the effective electrochemical (or chemical) reaction.

3. Diffusion behavior of solute ions at IL/electrode interface

It is known that most of ILs show extremely low vapor pressure, which enables us to apply various techniques that need vacuum conditions such as photoelectron spectroscopy. We recently developed the electrochemical photoelectron spectroscopy (EC-PES) to carry out in-situ XPS and UPS measurements under electrochemical condition using 3-electrodes type cell (Fig. 1 right). By observing the spatial distribution of concentration of contained ions close to the electrode (The mole fraction of solute and ionic liquid ions was plotted against the distance from the edge of the working electrode), we firstly succeeded in revealing the extraordinary diffusion behavior of solute ion (Ag+) in ionic liquid (BMI-TFSA). Under the applied potential of 0 V vs. Pt, the concentration of Ag+ ion was uniform. Under the applied potential of -1.2 V vs. Pt, a drastic change was observed: the concentration of Ag+ ion decreased at the edge of electrode, and gradually increased at larger distance from the electrode. This indicates that the diffusion layer was formed by the reductive reaction of Ag+ (Ag+ + e- => Ag) at the electrode. The thickness of the observed diffusion layer was 240 μm, which was much larger than the value usually supposed. By the 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’ (vacancy formed between IL molecules) acting as hopping sites.
[1] A. Imanishi, M. Tamura, S. Kuwabata, Chem. Commun. (2009)1775. [2] A. Imanishi, S. Gonsui, T. Tsuda, S. Kuwabata, K. Fukui, Phys. Chem. Chem. Phys., 13(2011)14823.

Figure 1