Apr 8, 2025
11:30am - 11:45am
Summit, Level 3, Room 342
Julian Feijoo1,2,Yao Yang1,2,3,Maria Fonseca Guzman1,2,Alfred Vargas1,Chubai Chen1,2,Christopher Pollock4,Peidong Yang1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Miller Institute for Basic Research in Science3,Cornell University4
Julian Feijoo1,2,Yao Yang1,2,3,Maria Fonseca Guzman1,2,Alfred Vargas1,Chubai Chen1,2,Christopher Pollock4,Peidong Yang1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2,Miller Institute for Basic Research in Science3,Cornell University4
Using renewable electricity to upcycle CO
2 into valuable chemical fuels and feedstocks is a promising strategy to close the carbon cycle. Copper, the only element to form deeply reduced products at appreciable rates, suffers from poor selectivity and high overpotentials. A deeper understanding of the active sites and molecular mechanisms is required to advance the field. Efforts to obtain such insights are complicated by the severe morphological changes copper nanocatalysts undergo under CO
2 reduction reaction (CO
2RR) conditions, limiting the effectiveness of ex-situ characterization and instead calling for the use of
operando techniques to directly observe the reaction under realistic conditions. We employ high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) to advance our understanding of the processes at the electrified solid-liquid interface. Specifically, we study how the structural evolution of copper nanocatalysts leads to the formation of active sites and microenvironments that can catalyze the formation of valuable multicarbon products from CO
2.
In this talk, I will focus on a family of high-performing copper nanoparticles catalyst, and share our insights into how they achieve lower overpotentials compared to most other state-of-the-art materials. The pristine particles are mostly oxidized and surrounded by an organic ligand shell. Using
operando HERFD-XAS, we can directly follow dynamic changes of the catalyst as an electrochemical potential is applied: We observe the gradual desorption of the ligand from the nanoparticle surface, as well as the complete reduction of Cu
2O to metallic copper. For further insights into the evolution and active state of this catalyst, we used EC-STEM to image the nanoparticles through a thin layer of electrolyte as they perform the catalytic reaction. We were able to directly observe the aggregation of individual nanoparticles into 50-100 nm clusters as the major restructuring mechanism. Intriguingly, fitting of the extended X-ray absorption fine structure (EXAFS) indicates a greatly reduced Cu-Cu coordination number compared to what would be expected for such a morphology. With additional evidence from operando electron diffraction, we are able to build a cohesive explanation for all our data: Rather than fully fusing, the nanoparticles aggregate into nanogranular structures with a high density of grain boundaries. As a result, the surface has a high concentration of undercoordinated sites, which serve as active sites for the CO
2RR.
By sharing this example, I hope to convey the message that although Cu catalysts have the strong tendency to completely restructure under CO2RR conditions, we can nevertheless control their reactivity by carefully designing their initial structure.