Ionic liquids—salts made by combining positively charged molecules (cations) and negatively charged molecules (anions) that are liquid at relatively low temperatures, often below room temperature—are increasingly being investigated for uses in batteries, supercapacitors, and transistors. Their unique physical and chemical properties, including good ionic conductivity, low flammability and volatility, and high thermal stability, make them well suited for such applications. But thousands of ionic liquids exist and exactly how they interact with the electrified surfaces of electrodes remains poorly understood, making it difficult to choose one for a particular application.

The team's research showed that the ionic reconfigurations occurring near the gold electrodes (yellow bars) depend on the thickness of the studied ionic liquid films, as illustrated in the above schematic. Anions (green circles) and cations (blue circles) are structured in a checkerboard-like pattern (left) without an applied voltage but rearrange when one of the electrodes is biased (-U). The thicker film (b) has a second layer of cations near the -U electrode.

Brookhaven National Laboratory scientists have demonstrated a new method for observing in real time how the ions of such liquids move and reconfigure as different voltages are applied to the electrodes.

When ionic liquid electrolytes come into contact with an electrode, a special structure consisting of alternating layers of cations and anions — called an electric double layer (EDL) — forms at the 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.

Until now, scientists have only been able to look at the initial and final EDL structures by using microscopy and spectroscopy techniques; the intermediate structure has been harder to probe. To visualize the structural changes of the EDL and the movement of ions as voltage is applied to the electrodes, the Brookhaven team used an imaging technique called photoemission electron microscopy (PEEM). In this technique, surface electrons are excited with an energy source and accelerated into an electron microscope, where they pass through magnifying lenses before being projected onto a detector that records them. Local variations in the photoemission signal intensities are then used to generate contrast images of the surface.

In this initial demonstration, the team changed the voltage applied to the electrodes, the thickness of the ionic liquid films, and the temperature of the system, all while monitoring changes in photoemission intensity.

They found that the ions move and arrange themselves according to the sign and magnitude of the applied voltage. Cations gravitate toward the electrode with the negative bias to counter the charge, and vice versa for anions. As the difference in potential increases between the two electrodes, a highly dense layer of cations or anions can accumulate near the biased electrode, preventing further ions of the same charge from moving there — a phenomenon called overcrowding — and reducing ion mobility.

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. The team further explored the importance of mobility in the rearrangement process by cooling the thicker film until the ions virtually stopped moving.

The team plans to continue their research using the new aberration-corrected low-energy electron microscope (LEEM)/PEEM system. This will enable them to study not only the structural and electronic changes, but also the chemical changes of the ionic liquid-electrode interface, all in a single experiment. By determining these unique properties, scientists will be able to select the optimal ionic liquids for specific energy storage applications.

For more information, contact Ariana Tantillo at 631-344-2347.

Photonics & Imaging Technology Magazine

This article first appeared in the November, 2017 issue of Photonics & Imaging Technology Magazine.

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