The Electric Double Layer in an Ionic Liquid Incorporated with Water Molecules: Atomic Force Microscopy Force Curve Study

Abstract

Electrochemical techniques and atomic force microscopy based force curve measurements under potential control are combined to investigate the effect of small amounts of water on the structure of the electric double layer of an Au(111)/1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) interface. Three to five layering structures, including two charged layers, are observed at the Au(111)/BMPTFSA interface at potentials more negative than the point of zero charge. With an increase in the water concentration, the stiffness values for both the first and second layers decrease, which demonstrates that more water molecules adsorb on the Au(111) surface or interact with the ionic liquid, and thus weaken the interactions between cations, anions, and the electrode surface and lower the stability of the layering structures. The thicknesses of the charged interior layers (the first and second layers in this system) increase with an increase in the water concentration and the thicknesses of neutral exterior layers (the next one to three layers in this system) remain almost unchanged. The structure of the first layer of the interface varies dramatically with the change in the water concentration.

Atomic Force Spectroscopy on Ionic Liquids

“…Even small amounts of water can modify the interface structure of an ionic liquid under applied electric potential [108]. On the interface between butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA], also referred to as [Py 1,4 ][TFSI]) and Au(111), Zhong et al [109], using AFS, observed that upon a water increase from 30 ppm to 90 ppm, the stiffness of the first interface layers decreased significantly while the thickness of the layers increased. This indicates that at this low concentration, water absorbs on the surface and distorts the defined orientation of ions in the layer.…”

Minimizing the electrosorption of water from humid ionic liquids on electrodes

“…They did not observe evidence of electrosorption of water on ions layered at the ionophilic gold electrodes. However, AFM force curves showed that the incorporated water molecules weaken the stability of layered structures 39 . Despite these modeling and experimental studies, some critical questions, regarding the electrosorption of water on electrodes, remained unanswered, such as (1) effects of the nature of electrode materials, from gold to carbon, on water electrosorption, (2) differences between hydrophobic and hydrophilic RTILs, and (3) how to prevent water adsorption on electrode surfaces to avoid electrochemical reaction.…”

Adding salt to expand voltage window of humid ionic liquids

“…For humid ILs, it has been found that water molecules prefer to accumulate on charged electrode surfaces 13 – 15 . Very unfavorable to EES devices, the surface-adsorbed water can effectively narrow the electrochemical windows of ILs due to its electrolysis, resulting in performance degradation 13 , 16 .…”

“…It has been reported that due to the strong interaction with anions, water molecules are excluded from the negatively charged electrode with hydrophilic ILs 18 . However, it is still an unaddressed issue that, for hydrophobic ILs that have been widely used as electrolytes 19,20 , they become humid when exposed to the atmosphere 12 , and unfortunately, their water much favors adsorption on both negatively and positively charged electrodes 15,16,18 . To avoid compromising their wide electrochemical windows, it is of great importance to minimize surface-adsorbed water and its impact, and thus enhance the voltage window of humid hydrophobic ILs.…”