Study of the microscopic structure of the electric double layer formedat the ionic liquid/electrode interface using Neutron reflectivity
Japan Atomic Energy Agency

Kazuhisa Tamura

1. Introduction
A salt, which exists as a liquid at room temperature, is called “ionic liquid”. In the last twodecades, ionic liquids (ILs) have been recognized as new type solvents. One of the characteristics ofILs is that ILs have high electric conductivity without adding any electrolyte. Further, ILs have avery low vapor pressure and a wide electrochemical window (high electrochemical stability).
Therefore, many studies for the usage of IL as electrolyte solutions have been conducted. On theother hand, fundamental studies have not been carried out enough. Especially, the interface between
IL and the electrode surface, where electrochemical reactions indeed occur, has not been wellunderstood. Therefore, we have studied the electrochemical reactions in ILs in terms of IL/electrodeinterfacestructure.

We

havefocusedon

the1-ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)amide ([BMIM]TFSA)/Si(100) interface structure and studied it using
electrochemical and quantum beam scattering techniques. Previously, it was found that there arethree phases of the interface structure including one transition state. Further, there observed ahysteresis and irreversibility. In this study, we carried out neutron reflectivity measurements toelucidate the detail of the potential dependence of interface structure.

2. Experiment
The working electrode was a Si(100) (40  40 mm, t =3 mm). Prior to measurements, Si(100)electrode was pretreated with acetone, ethanol, and Milli-Q water. The reference and counterelectrodes were Pt wires. The d-[BMIM]TFSA was synthesized in ANSTO and dehydrated undervacuum for 24 h before measurements. The electrochemical cell was assembled in a glove box filledwith Ar gas. The electrochemical cell was placed at the center of a goniometer under the potential
controlled condition. Reflectivity were measured between Q = 0.005 and 0.23 Å-1 at BL17.

3. Results
Reflectivity profiles were measured every 0.26 V
between E = -1.1 and 0.46 V. Figure 1 shows the potential
dependence of reflected neutron intensity measured at Q =0.1 Å-1. In the potential scan, there observed ahysteresis and irreversibility as observed in ourprevious electrochemical and X-ray reflectivitymeasurements. According to the electrochemicalmeasurements, both the peak potentials observed inFig. 1 almost correspond to the transition potential
between a steady state phase and a transition stateFig. 1 The potential dependence of reflected

phase. Therefore, this result indicates that in theneutron intensity measured at Q = 0.1 Å-1.

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phase transition state, the interface structure drastically changes and there is a hysteresis.
Toelucidate

thedetailoftheinterface structure fitting analysis

wascarried out. In the fitting procedure, layeredand monolayer structure models were used.
Fig. 2 shows the result of the fitting analysis.
It was found that in the steady state phase
described as “Phase 1” in Fig. 2, IL molecules
form a layered structure and the first andsecond layers are cation and anion layer,respectively. In the Phase1 state, both thecationandanion

molecules

Fig. 2 Result of fitting analysis

adsorbperpendicular to the surface. Further, theanion layer becomes rough near the transition potential. In the transition phase described as “Phase2“ in Fig. 2, IL molecules form monolayer structure and the orientation of the cation molecules
depends on the electrode potential.
We also carried out another neutron reflectivity measurement using partially deuterated[BMIM]TFSA to elucidate the inner structure of the adsorbed cation layer. At this point, the analysis
is underway.

4. Conclusion
In this study, the potential dependence of the [BMIM]TFSA/Si(100) electrode interface
structure was elucidated using in-situ neutron reflectivity technique. It was found that the IL
molecules form a stable layered structure in the “Phase 1”; on the other hand, IL molecules form adynamic structure in the “Phase 2”. It was also found that there are hysteresis and irreversibility inpotential dependence.

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