The same metal can have many faces, and each tells a different chemical story. At the atomic scale, those different faces can push the same reaction along very different paths.
Within a single crystal of a metal, faces are flat surfaces that expose the atoms of the material in a different geometric pattern. Depending on the direction the crystal is cut, the atoms are stacked differently. One face might reveal atoms that are packed very tightly together while another face might show atoms that are farther apart or arranged in rows. Although the atoms are of the same element, chemical reactions depend on how they are organized at the surface.
"Think of a cut diamond," explained Pietro Papa Lopes, an electrochemist at the U.S. Department of Energy's (DOE) Argonne National Laboratory and a Consortium for Advanced Science and Engineering scientist at the University of Chicago. "Each face can reflect light differently. Here, each crystal face gives you a different chemistry."
Iridium belongs to the same group of metals as platinum and gold, but its behavior is not nearly as well understood. In electrocatalysis, where electricity drives chemical reactions on a metal surface, limited understanding makes it harder to predict how iridium will behave as the interface that facilitates those reactions.
To establish a benchmark, researchers at Argonne compared several well-defined iridium crystal surfaces during reactions important to energy technologies, including hydrogen production and fuel cells.
They found that iridium's electrocatalytic performance depends strongly on which crystal face is exposed and whether the surrounding liquid environment is acidic or alkaline. No single iridium surface performed best across all reactions, highlighting the need to match the right catalyst structure to the job. These results provide a critical reference point for researchers and could help guide the design of better catalysts.
"Iridium is useful but still underexplored in its metallic form," Papa Lopes said. "In the larger context of electrocatalysis, having well-defined data on pristine surfaces - where you know exactly what the surface is - lets you start building a database. That gives the community more data points and from there, you can start developing deeper insights."
A Tale of Two Environments
To begin building that database, the team exposed three iridium crystal faces to different pH values, a measure of how acidic or alkaline a solution is. Comparing acidic and alkaline conditions is important because both are widely used in electrochemical technologies, and each can change how reactions happen at a metal surface.
Acidic liquids generally conduct electricity more efficiently, but many materials are not stable in acid. Alkaline liquids can be less corrosive and may allow the use of less expensive materials, though they come with tradeoffs of their own.
"From a fundamental perspective, we need to establish the reactivity of the surfaces first so people designing technologies know how they behave. Then they can evaluate the tradeoffs: cost, activity, stability and conductivity," Papa Lopes said.
The team then tested how those surfaces behaved in four reactions important to electrochemical technologies: making hydrogen, using hydrogen as fuel, adding electrons to oxygen (called oxygen reduction) and handling hydrogen peroxide - an unwanted side product.
How the Surface Changes the Ending
They found that different iridium surfaces are better at different reactions and in different liquid environments, meaning no single surface is best at everything. One iridium face performed better for hydrogen-related reactions, while a different face was better for oxygen reduction, a key reaction in many fuel cells. The hydrogen-related reactions became much slower in alkaline conditions. Under these conditions, the surface first has to split water molecules to get that hydrogen, which takes more energy. But in acid, the surface can draw hydrogen directly from the plentiful protons in the liquid.
Oxygen reduction behaved differently. Its overall reaction speed was less affected by acidic and alkaline conditions, but the products it formed changed much more. In acidic conditions, the iridium surfaces produced almost no hydrogen peroxide. In alkaline conditions, however, hydrogen peroxide production increased sharply under some conditions. Too much hydrogen peroxide can lower energy efficiency and, like an accidental bleach spill, can damage parts of electrochemical devices.
These results show that catalytic performance depends not only on the metal itself but also on the surrounding environment. By using carefully controlled crystal surfaces, the researchers were able to isolate the behavior of specific iridium structures.
"An important contribution of this research is not just whether iridium is a good or bad material overall," Papa Lopes said. "It's that we are providing the community with benchmark reactivity data for well-defined iridium surfaces."
With this data, scientists can compare iridium with more widely used catalyst materials such as platinum and use those insights to design improved catalysts for energy technologies.
This research was done as part of the DOE Early Career Research Program, a prestigious funding initiative that supports exceptional early-career scientists to stimulate new research directions in mission critical areas supported by the DOE's Office of Science.
Papa Lopes emphasized that the program was instrumental to this work.
"I am really grateful for the support of the Early Career Research Program," he said. "It let us ask fundamental questions and explore them deeply. And from doing that, we are able to learn things that other researchers can then translate into applications."
The results of this research were published in the American Chemical Society Catalysis.
Other contributors to this work include Kavyasree Anjanarambath, who participated in this study as a doctoral student at the University of Illinois Chicago, and Kaline Nascimento da Silva, an Argonne postdoctoral appointee.
This study was funded by the DOE Office of Science Materials Science and Engineering Division, Materials Chemistry Program via the Early Career Research Project Award.
Amber Rose is a science writer and editor for Argonne specializing in coverage of chemical sciences and engineering, materials science, microelectronics and physics. She holds a master's degree in chemistry from the University of California San Diego. Rose joined Argonne in 2024 and has been a science writer for more than 3 years. She previously worked as a science writer for the University of Illinois Urbana-Champaign Grainger College of Engineering.