Researchers Observe Behavior of Ionic Liquids at Ionic Liquid–Electrode Interfaces

Researchers are progressively analyzing the probability of using ionic liquids in supercapacitors, batteries, and transistors. Ionic liquids are salts produced by integrating negatively charged molecules, or anions, with positively charged molecules, or cations.

At comparatively lower temperatures, that is, below ambient temperature, these salts exist in liquid state. The distinctive chemical and physical characteristics of the ionic liquids—such as low flammability and volatility, good ionic conductivity, and high thermal stability—render them highly appropriate for such applications. However, there are thousands of ionic liquids, and there is no clear knowledge on the interaction of these liquids with the electrified surfaces of electrodes. Therefore, selecting the appropriate ionic liquid for a specific application is challenging.

At present, researchers from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have developed a new technique for performing real-time detection of the way in which ions in such liquids move and rearrange when the electrodes are applied with different voltages. The technique has been reported in a paper in the online edition of the journal Advanced Materials, published on 12 May 2017.

When ionic liquid electrolytes come into contact with an electrified electrode, a special structure consisting of alternating layers of cations and anions—called an electric double layer (EDL)—forms at that interface. But tracking the real-time evolution of the EDL, where the electrochemical reactions take place in batteries, is difficult because it is very thin (only a few nanometers thick) and buried by the bulk portion of the ionic liquid.

Wattaka Sitaputra, Scientist, Center for Functional Nanomaterials, Brookhaven

To date, researchers were only able to observe the initial as well as final EDL structures through spectroscopic and microscopic methods. However, it has been difficult to observe the intermediate structure. In order to observe the structural changes of the EDL and the movement of ions during the application of voltage to the electrodes, the Brookhaven researchers employed an imaging method known as photoemission electron microscopy (PEEM). In this method, an energy source is used to excite surface electrons that are accelerated into an electron microscope. The surface electrons go through magnifying lenses and are projected onto a detector. The detector records the electrons reflected from the surface. Contrast images of the surface are formed by using local discrepancies in the photoemission signal intensities. Here, the researchers used ultraviolet light for exciting the electrons on the surface of not only the ionic liquid (called as EMMIM TFSI) deposited by the researchers as thin films but also the two gold electrodes fabricated by them.

Imaging the whole surface, including the electrodes and the space between them, allows us to study not only the evolution of the structure of the ionic liquid–electrode interface but also to probe both electrodes at the same time while changing various conditions of the system.

Jerzy (Jurek) Sadowski, Scientist, CFN

In the preliminary analysis, the researchers altered the voltage applied to the electrodes, the ionic liquid films’ thickness, as well as the temperature of the system. They carried out these changes simultaneously while observing changes in photoemission intensity.

The researchers discovered that the ions—that usually get layered in a checkerboard-like fashion in the case of this ionic liquid—moved and configured on their own based on the sign and magnitude of the voltage applied. Anions moved toward the electrode with the positive bias to counter the charge, and the cations moved toward the negatively charged electrodes.

When there is an increase in potential between the two electrodes, a very dense layer of anions or cations gets accumulated at the biased electrode, thus blocking further ions of the same charge from accumulating there (i.e. overcrowding) and minimizing ion mobility.

The researchers also found that more counter-charged ions accumulate at the biased electrode in thicker films.

For very thin films, the number of ions available for rearrangement is small so the EDL layer may not be able to form. In the thicker films, more ions are available and they have more room to move around. They rush to the interface and then disperse back into the bulk upon overcrowding to form a more stable structure.

Wattaka Sitaputra, Scientist, Center for Functional Nanomaterials, Brookhaven

The team additionally investigated the significance of mobility in the reconfiguration process by cooling the thicker film to a point at which the ions almost ceased from moving.

The researchers stated that the application of PEEM to an operando experiment is a very innovative thought and has not been performed before for ionic liquids.

“We had to overcome several technical challenges in the experimental setup, including designing and fabricating the gold-patterned electrodes and incorporating the sample holder in the electron microscope,” described Sadowski. “Ionic liquids probably have not been investigated through this technique because putting a liquid into an ultrahigh vacuum–based microscope seems counterintuitive.”

The researchers aim to prolong their study by employing the new aberration-corrected low-energy electron microscope (LEEM)/PEEM system. The system was installed by means of collaboration between CFN and the National Synchrotron Light Source II (NSLS-II), another DOE Office of Science User Facility at Brookhaven, at NSLS-II’s Electron Spectro-Microscopy beamline. This system will allow the researchers to analyze the electronic and structural changes as well as the chemical changes in the interface between the ionic liquid and the electrode, in a single experiment. Ascertaining such distinctive characteristics will help researchers to choose the most appropriate ionic liquids for particular energy storage applications.

The research was partially supported by the DOE’s Laboratory Directed Research and Development program and is a partnership between the CFN and Brookhaven’s Chemistry and Sustainable Energy Technologies Divisions.

The scientists acquired this photoemission electron microscopy movie while biasing gold electrodes covered with three monolayers of the studied ionic liquid. The change in contrast corresponds to a change in the photoemission signal intensities from the ionic liquid–electrode interface, signifying the migration of ions in response to voltage biases on the electrodes. A positive voltage bias results in a darker contrast (higher density of anions near electrode) and a negative one in brighter contrast (higher density of cations near electrode).