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

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Zahid Hussain, division deputy for scientific support at the ALS, who participated in the experiment, added, “The problem of understanding solid/liquid interfaces has been known for 50-plus years—everybody has been using simulations and modeling to try to conceive of what’s at work.” The latest work has narrowed the list of candidate models that explain what’s at work in the double layer.

Hussain more than a decade ago had helped to pioneer X-ray tools and techniques at the ALS, which dozens of other research sites have since adopted, that allow researchers to study another important class of chemical reactions: those that occur between solids and gases.

There was a clear need to create new study tools for solid/liquid reactions, too, he said. “Solid/liquid interfaces are key for all kinds of research, from batteries to fuel cells to artificial photosynthesis,” the latter which seeks to synthesize plants’ conversion of sunlight into energy.

Hubert Gasteiger, a chemistry professor at the Technical University of Munich and the university’s chair of technical electrochemistry who is familiar with the latest experiment, said, “This work is already quite applicable to real problems,” as it provides new insight about the potential distribution within the double layer.

“No one has been able to look into this roughly 10-nanometer-thin region of the electrochemical double layer in this way before,” he said. “This is one of the first papers where you have a probe of the potential distribution here. Using this tool to validate double-layer models I think would give us insight into many electrochemical systems that are of industrial relevance.”

Probing active chemistry in changing conditions

In the experiment, researchers from Berkeley Lab and Shanghai studied the active chemistry of a gold electrode and a water-containing electrolyte that also contained a neutrally charged molecule called pyrazine. They used a technique called ambient pressure X-ray photoelectron spectroscopy (APXPS) to measure the potential distribution for water and pyrazine molecules across the solid/liquid interface in response to changes in the electrode potential and the electrolyte concentration.

A view inside the experimental chamber used in a chemistry experiment at Berkeley Lab’s Advanced Light Source. Researchers used ‘tender’ X-rays to explore a nanometers-thick region known as the electrochemical double layer at ALS Beam Line 9.3.1. (Credit: Marilyn Chung)

The experiment demonstrated a new, direct way to precisely measure a potential drop in the stored electrical energy within the double layer’s electrolyte solution. These measurements also allowed researchers to determine associated charge properties across the interface (known as the “potential of zero charge” or “pzc”).

Upgrade, new beamline will enhance studies

Importantly, the technique is well-suited to active chemistry, and there are plans to add new capabilities to make this technique more robust for studying finer details during the course of chemical reactions, and to bring in other complementary X-ray study techniques to add new details, Hussain said.

An upgrade to the X-ray beamline where the experiment was conducted is now in progress and is expected to conclude early next year. Also, a brand new beamline that will marry this and several other X-ray capabilities for energy-related research, dubbed AMBER (Advanced Materials Beamline for Energy Research) is under construction at the ALS and is scheduled to begin operating in 2018.

“What’s absolutely key to these new experiments is that they will be carried out in actual, operating conditions—in a working electrochemical cell,” Hussain said. “Ultimately, we will be able to understand how a material behaves down to the level of electrons and atoms, and also to understand charge-transfer and corrosion,” a key problem in battery longevity.

Researchers from the Joint Center for Artificial Photosynthesis, the Joint Center for Energy Storage Research, the Gwangju Institute of Science and Technology in the Republic of Korea, the Shanghai Institute of Microsystem and Information Technology in China, and the School of Physical Science and Technology in China participated in this research. The work was supported by the U.S. Department of Energy Office Science, the National Natural Science Foundation of China, and the Chinese Academy of Sciences-Shanghai Science Research Center.

The Advanced Light Source is a DOE Office of Science User Facility.

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