New Study Sheds Light on How Copper Catalysts Degrade During Artificial Photosynthesis

New research has uncovered the fundamental mechanisms that limit the performance of copper catalysts, essential components in artificial photosynthesis systems that convert carbon dioxide and water into valuable fuels and chemicals.

In a study co-led by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and SLAC National Accelerator Laboratory, researchers used advanced X-ray techniques to directly observe how copper nanoparticles change during the catalytic process.

By applying small-angle X-ray scattering (SAXS), a method traditionally used for studying soft materials like polymers, the team gained rare insights into catalyst degradation that has puzzled scientists for decades.

This research is part of the Liquid Sunlight Alliance (LiSA) DOE Energy Innovation Hub. Led by Caltech in close collaboration with Berkeley Lab, LiSA unites more than 100 researchers from national labs (SLAC, NREL) and universities (UC Irvine, UC San Diego, University of Oregon).

Since its launch in 2020, the team has been developing the scientific groundwork for efficiently and selectively generating liquid fuels from sunlight, water, carbon dioxide, and nitrogen. (Learn more about LiSA’s progress in “Five Ways LiSA is Advancing Solar Fuels.”)

The electrochemical reduction of CO₂ (CO₂RR) has long fascinated scientists as a potential method to produce fuels and valuable chemicals.

A breakthrough in the 1980s identified copper as a standout catalyst for this process, capable of converting CO₂ and water into building blocks for products like ethylene and ethanol.

Later studies revealed that copper contains active sites where electrocatalysis occurs—electrons from the copper surface interact with CO₂ and water, step-by-step, to create desired chemical products. Researchers have since been trying to fine-tune these active sites to selectively produce compounds like ethanol, ethylene, and propanol.

However, copper’s exceptional catalytic abilities degrade over time during CO₂RR, leading to a decline in performance. Despite years of effort, the specific chemical and physical processes behind this degradation have remained elusive.

The new study, published recently in the Journal of the American Chemical Society, provides fresh clarity. 

Using a combination of scattering and imaging techniques, the team identified two competing mechanisms that drive copper nanoparticle degradation: particle migration and coalescence (PMC), where smaller particles merge into larger ones, and Ostwald ripening, where larger particles grow at the expense of smaller ones.

Our approach allowed us to explore how the nanoscale size distribution evolves as a function of operating conditions, and to identify two different mechanisms that we can then use to guide our efforts to stabilize these systems and protect them from degradation.

Walter Drisdell, Study Co-Corresponding Author and Staff Scientist, Chemical Sciences Division, Lawrence Berkeley National Laboratory

For their experiments, the researchers used SAXS at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC to track changes in the size and shape of uniformly sized 7-nanometer copper oxide nanoparticles. They tested various voltages in a custom-designed electrochemical cell with an aqueous electrolyte.

During one-hour reaction tests, they observed that the PMC process dominated the first 12 minutes, after which Ostwald ripening took over. Initially, nanoparticles migrated and coalesced into clusters. As the reaction progressed, smaller nanoparticles dissolved and redeposited onto larger ones, a phenomenon similar to the formation of crunchy water crystals in ice cream.

The study also found that lower voltages, which slow down reactions, favor particle migration and coalescence, while higher voltages accelerate Ostwald ripening by speeding up dissolution and redeposition.

Additional in situ X-ray absorption spectroscopy (XAS) measurements confirmed that the copper oxide nanoparticles reduced to copper metal before restructuring began. Post-experiment imaging, performed with advanced electron microscopy at Berkeley Lab’s Molecular Foundry, verified that the nanoparticles had migrated and formed large agglomerates.

“These results suggest various mitigation strategies to protect catalysts depending on the desired operating conditions, such as improved support materials to limit PMC, or alloying strategies and physical coatings to slow dissolution and reduce Ostwald ripening,” Drisdell added.

Looking ahead, Drisdell and his colleagues plan to test various protective approaches and collaborate with LiSA partners at Caltech to design catalytic coatings with organic molecules. They aim to explore whether these coatings can help steer CO₂RR reactions to selectively produce specific fuels and chemicals.

This research was supported by the DOE Office of Science.
The Molecular Foundry and SSRL are DOE Office of Science national user facilities at Berkeley Lab and SLAC, respectively.

Journal Reference:

Lee, S. H. et al. (2025) Structural Transformation and Degradation of Cu Oxide Nanocatalysts during Electrochemical CO₂ Reduction. Journal of the American Chemical Society. doi.org/10.1021/jacs.4c14720