The evolution of a copper catalyst that can convert CO₂ into valuable chemicals and fuels

Copper is the most promising catalyst for turning carbon dioxide into valuable chemical feedstocks and liquid fuels through reactions that are driven by electricity. But those reactions are not as efficient or selective as they need to be, and the electrochemical reactors where they take place aren’t sturdy enough for deployment on an industrial scale.

Despite decades of work and progress, researchers haven’t been able to fix those flaws, because they haven’t had a way to specifically observe the few copper atoms that actively participate in the catalytic reactions—on the surface of a copper film that’s hundreds of layers thick—while ignoring all the rest.

Now researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a way to do that.

Rather than running the electrochemical reactor continuously, they turned it on and off several times per second while probing the catalyst with X-rays from SLAC’s synchrotron, the Stanford Synchrotron Radiation Lightsource (SSRL). Then they analyzed the X-ray data from the brief intervals when the electric pulses were switched on and the catalyst atoms were active.

Like a rapidly flashing strobe light, this technique clearly illuminated the individual steps of near-surface reactions taking place and allowed scientists to time them down to a few thousandths of a second, all while the reactor was running under realistic operating conditions.

The research team, led by SSRL senior scientist Dimosthenis Sokaras and Berkeley Lab senior scientist Junko Yano, published their work in the journal Energy & Environmental Science.

Their novel approach is suitable for studying a wide range of electrochemical conversion technologies, such as electrolyzers, fuel cells and batteries, Sokaras said, and the team is already using it to push the energy efficiency of catalysts that generate oxygen gas from water.

“It also gives us critical insights into fleeting changes that occur in catalytic reactions powered by intermittent energy sources,” Sokaras said. “Understanding these phenomena will drive advanced research, accelerate the development of robust electrochemical technologies and position national labs to lead innovation in energy and chemical manufacturing.”

Yano, who is a principal investigator for the Liquid Sunlight Alliance (LiSA) project, said, “Seeing how the chemical states change and on what time scale is very important. This new method is like creating thousands of tiny windows into what’s going on, and it gives us information that we could not get before.”

Harnessing energy from the sun

SLAC and Berkeley Lab are two of the major partners in LiSA, led by the California Institute of Technology, which began in 2020 to pursue ways to convert carbon dioxide into chemicals and fuels using energy from the sun. This experiment was the latest of many the team has carried out at SSRL, which produces extremely bright beams of X-ray light to advance strategic areas of research relevant to national goals.

The new method, modulation excitation X-ray absorption spectroscopy (ME-XAS), was developed and refined at SSRL. It allows researchers to generate reaction-triggering electric pulses and modulate, or change, their frequencies, voltages and shapes. They vary the timing of the pulses—for instance, one tenth of a second on, one tenth of a second off—while X-rays bounce off the surface of the copper film and into a detector, recording data the whole time.

Then the data is sorted into little bins corresponding to the times when the pulses were on or off. The team combs through this data to find the tiniest discernible differences that match the timing, or frequency, of the pulse.

“Any little thing—fluctuations in temperature, instability of the catalyst, random noises—can affect those differences,” said SLAC staff scientist Angel T. Garcia-Esparza, lead author of the study. “To make it work, Dean Skoien—an SSRL staff engineer—had to develop complex customized electronics for triggering, recording and saving gigabytes of data while analyzing them on the fly.”

The analysis effort also drew on the expertise of Berkeley Lab project scientist Philipp Simon, who developed customized routines that helped extract meaningful signals from highly dynamic and fluctuating datasets.

This experiment didn’t attempt to run the whole series of surface reactions that goes into a copper-driven catalytic reaction—just a few fundamental first steps. First, hydroxide ions adhere to active copper atoms on the surface; then cuprous oxide forms.

“If the reactions were to proceed further, they would leave a complex coating of copper hydroxide and cupric oxide on the surface of the copper film that can affect how the catalyst performs,” Garcia-Esparza said. Therefore, it is crucial to understand the chain of chemical reactions in great detail for the development of next-generation electrochemical conversion devices.