Exploring the mysteries of copper, researchers at the Lawrence Berkeley National Laboratory have discovered yet another use for this "new oil" of the clean energy future – converting carbon dioxide and water into fuels and chemicals. But to witness this discovery, the scientists had to design an entirely new nanoimaging technology.

The idea of water-powered engines has been waiting for its moment to jump from the pages of science fiction into the world of science reality. As it turns out, this intriguing concept just needed a catalyst – copper.

Since the 1970s, scientists have been aware that copper has a special ability to transform CO2 into valuable chemicals and fuels. Yet, for decades, researchers have struggled to understand how this common metal works as an electrocatalyst, a mechanism that uses energy from electrons to chemically transform molecules into different products.

Now, scientists at Berkeley Lab have made a real-time showing of copper nanoparticles as they evolve to convert CO2 and H2O into renewable fuels and chemicals – with these new insights, it could help advance the next generation of solar fuels.

"This is very exciting," said Peidong Yang, senior faculty scientist in Berkeley Lab's Materials Sciences and Chemical Sciences Divisions who led the study. "After decades of work, we're finally able to show – with undeniable proof – how copper electrocatalysts excel in CO2 reduction."

Published in the journal Nature, the scientists were able to capture copper particles engineered at the scale of a billionth of a meter as they converted CO2 and H2O into renewable fuels and chemicals such as ethanol, ethylene, and propanol, among others.

"Knowing how copper is such an excellent electrocatalyst brings us steps closer to turning CO2 into new, renewable solar fuels through artificial photosynthesis," added Peidong Yang.

This work was made possible by combining the power of an electron probe with X-rays, two techniques that typically can't be performed by the same instrument. However, through a new imaging technique called operando 4D electrochemical liquid-cell scanning transmission electron microscopy (STEM) with a soft X-ray probe, the team was able to observe otherwise hidden functions of the building blocks of the universe.

New imaging technology

Scientists who study artificial photosynthesis materials and reactions have been seeking to combine the power of an electron probe with X-rays for several decades, but the two techniques have never been capable of operating on a single device.

Electron microscopes, such as STEM and TEM, use beams of electrons and excel at characterizing the atomic structure in parts of a material. In recent years, 4D STEM instruments, such as those at Berkeley Lab's Molecular Foundry, have pushed the boundaries of electron microscopy even further, enabling scientists to map out atomic or molecular regions in a variety of materials, from hard metallic glass to soft, flexible films.

On the other hand, lower-energy X-rays are useful for identifying and tracking chemical reactions in real time in a real-world environment.

Now, scientists can have the best of both worlds. At the heart of this new technique is an electrochemical "liquid cell" sample holder with remarkable versatility. A thousand times thinner than a human hair, the device is compatible with both STEM and X-ray instruments.

Due to its ultrathin design, reliable imaging of delicate samples is possible as it protects them from electron beam damage.

Through the use of a special electrode custom-designed by co-author Cheng Wang, a staff scientist at Berkeley Lab's Advanced Light Source, this new device enabled the team to conduct X-ray experiments with the electrochemical liquid cell.

Combining the two allowed researchers to comprehensively characterize electrochemical reactions in real-time and at the nanoscale.

With the tools in hand, the question of copper's electrocatalyst capabilities could now be uncovered.

Nanoparticles to grains

During the 4D-STEM experiments, first author of the study Yao Yang and colleagues used this new electrochemical liquid cell to observe copper nanoparticles, ranging in size from seven to 18 nanometers, evolve into active nanograins during CO2 electrolysis – a process that uses electricity to drive a reaction on the surface of an electrocatalyst.

These experiments quickly revealed a surprise – copper nanoparticles combined into larger metallic copper nanograins within seconds of the electrochemical solution.

To understand this, the team turned back to Wang, who pioneered a technique known as "resonant soft X-ray scattering (RSoXS) for soft materials" more than a decade ago. With his help, the team used the same electrochemical liquid cell, but this time during RSoXS experiments.

These experiments would inform the scientists whether copper nanograins facilitate CO2 reduction.

By using RSoXS, the researchers were able to monitor multiple reactions between thousands of nanoparticles in real time and accurately identify chemical reactants and products.

The RSoXS experiments at Advanced Light Source – along with additional evidence gathered at Cornell High Energy Synchrotron Source – proved that metallic copper nanograins serve as active sites for CO2 reduction.

During CO2 electrolysis, the copper nanoparticles changed their structure during a process called "electrochemical scrambling." The copper nanoparticles' surface layer of oxide degrades, creating open sites on the copper surface for CO2 molecules to attach.

And as CO2 "docks" or binds to the copper nanograin surface, electrons are then transferred to CO2, causing a reaction that simultaneously produces ethylene, ethanol, and propanol along with other multicarbon products.

"The copper nanograins essentially turn into little chemical manufacturing factories," Yao Yang said.

Validating earlier postulates by Peidong Yang, these new copper nanograins could potentially boost the energy efficiency and productivity of some of the catalysts designed for artificial photosynthesis, a field that is furthering research into producing solar fuels from sunlight, water, and CO2 – like all plant life on Earth.

"The technique's ability to record real-time movies of a chemical process opens up exciting opportunities to study many other electrochemical energy conversion processes," said Peidong Yang. "It's a huge breakthrough, and it would not have been possible without Yao and his pioneering work."

Witnessing not just the makeup but the process through which molecules operate and bond is perhaps the largest step forward in materials science since first placing eyes upon them. Much like the comparison between photographs to moving pictures, this is the first glimpse into the activities of life's building blocks, seen by the naked eye as it happens.

Although the significance of peering into copper's capabilities cannot be undervalued, it must be noted that there are still 118 known chemical elements that exist on the periodic table – while not all will be able to be filmed like copper, much more work is yet to be done, and much more is yet to be learned.