X-ray spectral microscopy reveals the active edges of a water-splitting material
X-rays penetrate a working electrode to determine the structure and chemistry in play when water enters the electrochemically active layers.
image: Artist’s rendition of a working electrode used to separate water by electrochemical means. Water penetrates the atomic layers and splits into its component elements when an applied voltage from attached probe changes the atomic configuration at the edges. view more
Credit: Image courtesy of SLAC National Accelerator Laboratory
The Science
Hydrolysis is a chemical reaction that involves water molecules breaking down chemical bonds. One key type is electrolysis—using electricity to break water into oxygen and hydrogen gas. In this study, researchers used an array of specialized techniques to determine the structure and chemical activity of an electrical conductor involved in electrolysis. The researchers used a method called resonant X-ray absorption microscopy to describe the chemical changes around the atoms of the electrode . This allowed them to examine inside the electrode and the surrounding liquid while they applied an electrical voltage. They could then create a map of the chemical changes during the electrolysis process.
The Impact
Splitting water into hydrogen and oxygen is a key process for energy storage. The chemical transitions involved in splitting water requires energy. Researchers are designing more efficient new electrodes with energy saving catalytic properties. This work shows the atomic structure and chemical mechanisms under actual working conditions as electrical voltages rearrange the atomic bonds and split the water molecule. The resulting understanding of this process will help scientists engineer better electrodes.
Summary
To understand materials transformations during the actual conditions of working systems for energy conversion, researchers need analytical tools that can penetrate the working environment without disturbing the process. X-ray absorption microscopy penetrates and provides high resolution chemical maps. In this research, the working electrode was coated with a liquid water layer. The researchers focused bright and broad-energy X-rays from a synchrotron source with a specialized X-ray lens. They used a monochromator to select an energy where X-ray absorption depends on the chemical state of the atoms in the electrode. By making and comparing maps taken at specific energies associated with a particular chemical state, the researchers could identify regions of varying chemical activity.
The researchers mapped the electrochemical reaction current, physical structure, and cobalt oxidation state across single crystal nanoplatelets while they were actively splitting water. The researchers determined that the oxygen state around metal atoms in the electrode changes dramatically during electrolysis. The greatest changes and most active sites for oxygen production were at the edges of the electrode nanoplatelets. This information will allow materials engineers to tweak the chemistry at the platelet edge and possibly lower the energy cost for creating hydrogen by splitting water.
Funding
The research was funded by the Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Part of this work was performed at the Stanford Nano Shared Facilities/Stanford Nano-fabrication Facility, supported by the National Science Foundation. Equipment support was provided by the DOE Office of Science, Office of Basic Energy Sciences, Small Business Innovation Research program. X-ray work was performed at the Advanced Light Source, a DOE Office of Science user facility.