Scientists Crack Decades-Old Puzzle in CO2-to-Fuel Conversion 

“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,” said Walter Drisdell, a co-corresponding author on the paper who is also a staff scientist in Berkeley Lab’s Chemical Sciences Division and principal investigator with LiSA.

In this study, the researchers used a technique called small angle X-ray scattering (SAXS) at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC to track the size and shape distributions of uniformly shaped 7-nanometer copper oxide nanoparticles under various electrical voltages in a custom-designed electrochemical cell with an aqueous electrolyte.

When running the CO₂RR reaction for an hour, the researchers found that the PMC process dominates in the first 12 minutes, and then after that, Ostwald ripening takes over. Under the PMC mechanism, the nanoparticles migrate and coalesce into clusters. When the Ostwald ripening process takes over, smaller nanoparticles dissolve and redeposit onto larger nanoparticles, the same process that can create crunchy water crystals in ice cream.

Further analyses in the current study showed that lower voltages, where reactions are slower, trigger the migration and agglomeration of the PMC process – and larger voltages speed reactions up, increasing the dissolution and redeposition process of Ostwald ripening.

Separate in situ X-ray absorption spectroscopy (XAS) measurements at SSRL show that the copper-oxide nanoparticles reduce to copper metal before restructuring begins, and post-mortem imaging confirmed that the nanoparticles had migrated and formed large agglomerates. The imaging was achieved using advanced electron microscopy techniques at Berkeley Lab’s Molecular Foundry.

“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 said.

In future studies, Drisdell and team plan to test different protection schemes, and continue working with their LiSA colleagues at Caltech to design catalytic coatings with organic molecules, and test these coatings’ ability to steer CO₂RR reactions into producing specific fuels and chemicals.

This work was supported by the DOE Office of Science.

The Molecular Foundry is a DOE Office of Science national user facility at Berkeley Lab.

The Stanford Synchrotron Radiation Lightsource (SSRL) is a DOE Office of Science national user facility at SLAC National Accelerator Laboratory.

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