By unraveling iridium's hidden chemistry at interfaces between a solid and liquid, Argonne scientists are helping strengthen the scientific basis for technologies that produce fuels and valuable chemicals and critical materials.
Iridium is a key component in many electrochemical technologies used for chemical transformations. These include producing hydrogen fuel from water, manufacturing chlorine from seawater for use as a disinfectant and extracting metals from their ores. Yet scientists still know surprisingly little about how this metal behaves at the very spot where those reactions unfold - the thin boundary where the surface of a solid electrode meets a water-based electrolyte.
"What happens at that interface decodes iridium's electrochemical properties, such as its activity and stability for many processes," said Kaline Nascimento da Silva, a postdoctoral appointee at the U.S. Department of Energy's (DOE) Argonne National Laboratory.
A new Argonne-led study offers the most detailed view yet of that hidden world. The research team used several experimental tools and theoretical simulations to track how water and its derivative species - hydrogen, oxygen and a combination of the two (hydroxyl) - form, vanish and rearrange on an iridium surface under different voltages. The findings reveal a surface far more dynamic than previously understood. Its behavior shifts dramatically depending on the metal's crystal structure and the applied voltage.
"When you look at another key metal used in electrochemical technologies, platinum, we've had decades to understand how its surfaces behave," said Argonne scientist Pietro Papa Lopes. "But for iridium, the knowledge is scattered. We needed a clear, unified picture of what the interface actually looks like."
To build that picture, the team immersed a single crystal of iridium in an acidic water-based electrolyte, applied a controlled electric field to it and monitored what happened at the interface. They investigated iridium's three primary surface facets of the crystal, named Ir(111), Ir(100) and Ir(110). Each has its own atomic arrangement and chemical "personality." A molecule that binds readily to one surface may barely attach to another.
The single crystals are a model system. "They're like a lighthouse," Papa Lopes said. "They give us ground truths, something we can understand extremely well, and those ground truths help guide studies of more complex, practical materials."
A technique called cyclic voltammetry allowed the team to quantify how much of the three surface facets were covered by different derivative species from water. Carbon monoxide, introduced as an external probe, helped identify whether those species carried a positive or negative charge at different voltages applied to an electrode.
But the biggest advance came from a specialized form of Raman spectroscopy, a technique based on how matter interacts with light. The team used gold nanoparticles covered with a thin layer of glass and sprinkled these "nano-antennas" on each iridium surface. This allowed them to detect the vibrational fingerprints of hydrogen, oxygen, hydroxyl groups and their interactions at the interface with surrounding water - details that had never been resolved before.
To interpret these signals, the group paired their measurements with theoretical simulations of how each molecule should vibrate. The combined approach uncovered more than 20 distinct vibrational markers. Each marker revealed how the iridium interface changed, moment by moment, as the voltage varied. These data captured any changes not resolved by the early measurements with cyclic voltammetry.
Electrocatalysis is a powerful tool for converting chemicals into energy, as well as in making new molecules. One surprising finding here was that the electrocatalytic properties of Ir(111) outperformed platinum for both hydrogen evolution and hydrogen oxidation reactions. The former is important for hydrogen production and, consequently, many industrial processes. The latter is pivotal to enabling the use of fuel cells that convert fuels into electricity. The team also observed that iridium produces far less hydrogen peroxide during the oxygen reduction reaction. Hydrogen peroxide can degrade fuel cell components.
While the study used aqueous solution as the electrolyte, the authors noted that the principles uncovered here can extend well beyond water-based systems.
"Because this investigation is really about the description of the interface itself, these methods can be applied to entirely different electrolytes and surfaces," Nascimento da Silva said. "If you want to understand how any liquid interacts with your surface, these concepts give you a starting point." The results could help guide improvements in many electrochemical technologies.
The research appeared in The Journal of the American Chemical Society. In addition to Nascimento da Silva and Papa Lopes, coauthors include Caroline Katherine Williams and Peter Zapol (Argonne); Kavyasree Anjanarambath (Argonne and the University of Illinois at Chicago); and Haiying He (Valparaiso University). Funding was provided by the DOE Office of Basic Energy Sciences through the Early Career Research Program.