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Copper has many uses—in electrical wires, plumbing and even coins. With its abundance and relatively low price tag, copper has also long been used as a catalyst to speed up chemical reactions—notably water and carbon dioxide electrolysis, where copper serves as an electrode and catalyst for using electricity to produce fuels.

The trouble is, ordinary copper isn't the most durable catalyst, so researchers have been searching for ways to improve on that. One approach is to oxidize it, a process essentially the same as rusting iron. In the 1970s, chemist Marcel Pourbaix theorized that particularly durable forms of highly oxidized copper should exist. Researchers have been hunting for those forms ever since.

Now, at last, a team led by researchers at the U.S. Department of Energy's (DOE) SLAC National Accelerator Laboratory have found this elusive form of copper through advanced computational methods and state-of-the-art experimental techniques.

The team—including researchers from Lawrence Berkeley National Laboratory (Berkeley Lab), Stanford University, the National Institute for Standards and Technology (NIST), the University of California, Berkeley, and the National Renewable Energy Laboratory—is part of the Liquid Sunlight Alliance (LiSA), a DOE Fuels from Sunlight Energy Innovation Hub.

Published in the Journal of the American Chemical Society, their map out under what conditions this special form of copper is most stable, paving the way to make more durable copper catalysts.

To produce this material—specifically, a kind of copper hydroxide with chemical formula CuOOH—the researchers applied electricity to copper electrodes submerged in an electrolyte bath.

But with the precise electrical voltage, acidity and many other variables to consider, producing and identifying this copper compound wasn't simply a matter of turning the system on. To address that challenge, co-lead author and SLAC and SUNCAT Center for Interface Science and Catalysis postdoctoral fellow Pooja Basera used powerful computational methods to predict conditions where they could produce the kinds of copper compounds they were after.

With the help of a supercomputer at the National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab, they did just that. "It matched very well with Pourbaix's hypothesis," said Basera. "We were excited to be able to pinpoint where we could find this form of copper."

The team next turned to the Stanford Synchrotron Radiation Lightsource's (SSRL's) bright X-rays at SLAC to test those predictions. Because catalytic reactions take place in the first few atomic layers of the catalyst, they needed techniques sensitive to surface reactions under operating conditions to capture the formation of oxidized copper compounds in detail.

One novel technique has that sensitivity. Developed by SSRL and Berkeley Lab researchers, modulation excitation X-ray absorption spectroscopy cycles electrical pulses on and off at rapid rates while probing the sample with X-rays, revealing "structural fingerprints" in the copper electrodes.

"We could see, as predicted by the calculations, a new copper spectral signature we haven't seen before," indicating the presence of copper hydroxide, said Angel T. Garcia-Esparza, an SSRL staff scientist.

The team also wanted to understand another important piece of the puzzle: how this copper compound forms. Yang Zhao, postdoctoral researcher, and Shannon Boettcher, senior scientist, at Berkeley Lab utilized another specialized technique, operando Raman spectroscopy. They shined visible light on the sample to measure how the bonds between atoms were vibrating. These molecular vibrations act like fingerprints that help identify different chemical species.

As the voltage increased to a high level—beyond what is typically used in copper studies—a new signal appeared. This signal matched computational predictions, providing strong evidence that the copper transformed into a CuOOH phase.

These calculations and fingerprints show that, in the right form, copper can withstand higher operating voltages, increasing its durability, said Michal Bajdich, a SLAC staff scientist and lead author of this study.

Increasing the durability of copper catalysts has important implications in electrochemical water splitting, the process of splitting water into oxygen and hydrogen, which could help create the fuels society needs in a more cost-efficient, less energy-intensive way, particularly if energy from the sun is used instead of other sources.

Whereas copper is now only used in the negatively charged water-splitting electrode, the results open the door to using copper for both the negatively and positively charged electrodes, thereby potentially replacing more expensive and scarce materials used now.

The combination of advanced computational capabilities with cutting-edge techniques we are developing at SLAC allows us to uncover elusive catalytic states, said Dimosthenis Sokaras, co-principal investigator and SSRL senior scientist. Such fundamental studies contribute toward establishing new or emerging chemical transformation technologies.

Having solved the mystery of copper, the team said their approach can help find higher oxidation states of other catalytic materials with the overall goal of designing more stable, durable catalysts for not only water splitting, but other industrially relevant chemical reactions as well.