Researchers Peel Back Another Layer of Chemistry with 'Tender' X-rays

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Scientists can now directly probe a previously hard-to-see layer of chemistry thanks to a unique X-ray toolkit developed at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). The X-ray tools and techniques could be extended, researchers say, to provide new insight about battery performance and corrosion, a wide range of chemical reactions, and even biological and environmental processes that rely on similar chemistry.

Berkeley Lab’s Ethan Crumlin, working with other researchers, found a new way to study chemical processes at work in batteries and in other chemical reactions using a specialized X-ray toolkit developed at the lab’s Advanced Light Source, an X-ray source. The technique was pioneered at the ALS’s Beam Line 9.3.1. (Credit: Marilyn Chung/Berkeley Lab)

In a first-of-its-kind experiment at Berkeley Lab’s Advanced Light Source, an X-ray source known as a synchrotron, researchers demonstrated this new, direct way to study the inner workings of an activity center in chemistry known as an “electrochemical double layer” that forms where liquids meets solids—where battery fluid (the electrolyte) meets an electrode, for example (batteries have two electrodes: an anode and a cathode).

A key breakthrough enabling the latest experiment was in tailoring “tender” X-rays—which have an energy range tuned in a middle ground between the typical high-energy (or “hard”) and low-energy (or “soft”) X-rays used in research—to focus on chemistry within the double layer of a sample electrochemical system. The related study was published Aug. 31 in Nature Communications.

Drilling down on the double layer

In a battery, this electrochemical double layer describes the layer of charged atoms or molecules in the battery’s fluid that are drawn in and cling to the surface of the electrode because of their opposite electrical charge—an essential step in battery operation—and a second and closely related zone of chemical activity that is affected by the chemistry at the electrode’s surface. The complex molecular-scale dance of charge flow and transfer within a battery’s double layer is central to its function.

This stylized representation shows an electrochemical double layer, the heart of solid/liquid chemical interactions such as those occurring around a battery’s electrode. An experiment at Berkeley Lab used X-rays to study the properties of the double layer that formed as positively or negatively charged particles (ions, shown as plus and minus symbols) were drawn to a gold electrode (left). The experiment featured neutrally charged pyrazine molecules (dark blue) suspended in the water-based electrolyte, composed of potassium hydroxide. Researchers precisely measured changes in the charge properties of molecules caused by changes to the electric charge applied to the electrode and to the charged-particle concentration of the electrolyte in the double-layer region. (Credit: Zosia Rostomian/Berkeley Lab)

The latest work shows changes in the electric “potential” in this double layer. This potential is a location-based measure of the effect of an electric field on an object—an increased potential would be found in an electric charge moving toward a lightbulb, and flows to a lower potential after powering on the lightbulb.

“To be able to directly probe any attribute of the double layer is a significant advancement,” said Ethan Crumlin, a research scientist at Berkeley Lab’s ALS who led the experiment. “Essentially, we now have a direct map, showing how potential within the double layer changes based on adjustments to the electrode charge and electrolyte concentration. Independent of a model, we can directly see this—it’s literally a picture of the system at that time.”

He added, “This will help us with guidance of theoretical models as well as materials design and development of improved electrochemical, environmental, biological, and chemical systems.”

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