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Recent findings have shed light on the essential limitations inherent in copper catalysts, which play a vital role in artificial photosynthesis by converting carbon dioxide and water into useful fuels and chemicals.
This investigation, spearheaded by researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) and SLAC National Accelerator Laboratory, utilized advanced X-ray techniques to observe the transformations of copper nanoparticles during the catalytic process.
By employing small-angle X-ray scattering (SAXS)—a method typically applied to study materials like polymers—the team achieved unprecedented insights into catalyst degradation, a phenomenon that has posed challenges for researchers for many years.
This research is part of the Liquid Sunlight Alliance (LiSA), a Department of Energy Energy Innovation Hub. Established in 2020 and led by Caltech in collaboration with Berkeley Lab, LiSA unites over 100 scientists from various national labs and universities, focusing on efficiently and selectively generating liquid fuels from sunlight, water, carbon dioxide, and nitrogen.
The CO2 electrochemical reduction reaction (CO2RR) has long attracted attention for its potential to convert CO2 into valuable fuels and other essential compounds. A pivotal discovery in the 1980s identified copper as an effective catalyst, facilitating the transformation of CO2 and water into foundational ingredients for liquid fuels such as ethylene and ethanol.
Studies have indicated that copper possesses active sites where electrocatalytic reactions occur; electrons from the copper interact with CO2 and water in a series of steps to form products that include ethanol for fuel and ethylene for plastics. Researchers are now probing ways to fine-tune these active sites to selectively produce desired chemicals like ethanol, ethylene, and propanol.
However, the extraordinary catalytic capabilities of copper tend to diminish during CO2RR, leading to reduced efficiency over time. Despite numerous efforts to combat this performance degradation, the underlying chemical and physical processes remained largely unexplored.
In the recent study—published in the Journal of the American Chemical Society—the researchers employed innovative scattering and imaging techniques to uncover and observe dual mechanisms contributing to the degradation of copper nanoparticles in CO2RR: particle migration and coalescence (PMC), where smaller particles merge into larger ones, and Ostwald ripening, where larger particles grow by dissolving smaller ones.
“Our method enabled us to investigate how the nanoscale size distribution evolves with varying operational conditions, allowing us to pinpoint two distinct mechanisms that can assist our efforts in enhancing stability and preventing degradation,” stated Walter Drisdell, a co-author and staff scientist in Berkeley Lab’s Chemical Sciences Division, also affiliated with LiSA.
In this study, the team utilized SAXS at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC to monitor the size and shape of uniformly shaped 7-nanometer copper oxide nanoparticles under various electrical voltages in a specially designed electrochemical cell with an aqueous electrolyte.
During a one-hour CO2RR reaction, the researchers observed that the PMC process dominated the initial 12 minutes, after which Ostwald ripening became prevalent. Under PMC, nanoparticles migrate and cluster together, while in Ostwald ripening, smaller particles dissolve and redeposit onto larger ones—analogous to how crunchy ice crystals form in ice cream.
Further analyses indicated that lower voltages, which yield slower reactions, facilitate the migration and clustering associated with PMC, whereas higher voltages accelerate reactions, thus boosting Ostwald ripening.
In situ X-ray absorption spectroscopy (XAS) measurements conducted at SSRL demonstrated that the copper oxide nanoparticles convert to metallic copper before any restructuring occurs, with post-mortem imaging validating that the nanoparticles migrated and formed larger agglomerates, using advanced electron microscopy techniques at Berkeley Lab’s Molecular Foundry.
“These findings suggest various strategies to protect catalysts based on the desired operational conditions, including enhancements in support materials to limit PMC or employing alloying methods and physical coatings to mitigate dissolution and reduce Ostwald ripening,” Drisdell added.
Looking ahead, Drisdell and the team plan to experiment with different protective strategies and continue their collaboration with LiSA colleagues at Caltech to develop catalytic coatings using organic molecules, examining these coatings’ effectiveness in steering CO2RR towards specific fuels and chemicals.
More information: Soo Hong Lee et al, Structural Transformation and Degradation of Cu Oxide Nanocatalysts during Electrochemical CO2 Reduction, Journal of the American Chemical Society (2025). DOI: 10.1021/jacs.4c14720
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phys.org