A comprehensive investigation of the EDL structure and associated charge transfer processes constitutes an essential step towards understanding and improvement of a variety of electrochemical processes, such as electrocatalysis, electrochemical energy storage, ion transport through biological membranes and corrosion.

… despite … initial studies on the EDL structure, the information of the electrical potential profile at the solid/liquid-electrified interface, particularly as a function of the applied potential, was still elusive. In this work, we report the direct probing of the potential drop (PD) as well as the potential of zero charge (PZC) by means of ambient pressure X-ray photoelectron spectroscopy (APXPS) performed under polarization conditions.

By analyzing the spectra of the solvent (water) and a spectator neutral molecule with numerical simulations of the electric field, we discern the shape of the EDL profile. In addition, we determine how the EDL changes as a function of both the electrolyte concentration and applied potential.

—Favaro

The researchers used ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to study the ECL. AP-XPS is a technique available from two of the beamlines at the Advanced Light Source at the Lab.

X-ray photoelectron spectroscopy (XPS) is a powerful and versatile surface characterization technique that can provide quantitative information about elemental composition and chemical specificity. However, conventional XPS measurements require ultra-high vacuum (UHV) conditions to avoid electron scattering with gas molecules as well as the surface contaminations.

To apply XPS to liquid and gas phases, researchers developed the ambient pressure XPS (AP-XPS) technique enabling the use of a laboratory-based X-ray source in near ambient pressure conditions.

In 2015, Berkeley Lab researchers used AP-XPS to explore the electrochemical oxidation of the Pt electrode at an oxygen evolution reaction (OER) potential (Axnanda et al.).

A key breakthrough enabling the latest experiment was in tailoring “tender” X-rays (also used in the Axnanda study)—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.

Stylized representation of an electrochemical double layer. 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 a 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 ion concentration of the electrolyte in the double-layer region. (Credit: Zosia Rostomian/Berkeley Lab) Click to enlarge.

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.

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

—Ethan Crumlin, a research scientist at Berkeley Lab’s ALS who led the experiment

Schematization of the combined operando APXPS, electrochemical and numerical simulation-based approach for the direct probing of the potential drop within the EDL. Favaro et al. Click to enlarge.

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. Solid/liquid interfaces are key for all kinds of research, from batteries to fuel cells to artificial photosynthesis.” The latest work has narrowed the list of candidate models that explain what’s at work in 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. 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.

—Hubert Gasteiger, a chemistry professor at the Technical University of Munich

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 AP-XPS 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.

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”).

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. 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.

—Zahid Hussain

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 US 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.

Resources

• Marco Favaro, Beomgyun Jeong, Philip N. Ross, Junko Yano, Zahid Hussain, Zhi Liu & Ethan J. Crumlin (2016) “Unravelling the electrochemical double layer by direct probing of the solid/liquid interface” Nature Communications 7, Article number: 12695 doi: 10.1038/ncomms12695

• Stephanus Axnanda, Ethan J. Crumlin, Baohua Mao, Sana Rani, Rui Chang, Patrik G. Karlsson, Mårten O. M. Edwards, Måns Lundqvist, Robert Moberg, Phil Ross, Zahid Hussain & Zhi Liu (2016) “Using “Tender” X-ray Ambient Pressure X-Ray Photoelectron Spectroscopy as A Direct Probe of Solid-Liquid Interface” Scientific Reports 5, Article number: 9788 doi: 10.1038/srep09788