Capping-Ligand Density on Cu Nanoparticles Determining Multi-Carbon Product Selectivity in Electrochemical CO₂ Reduction

Carbon dioxide (CO2) emission from the combustion of fossil fuel causes global warming and climate change. Much attention has been paid for recycling of CO2 gas, namely converting this greenhouse gas to high-value fuels such as multi-carbon products (ethylene, ethanol, propanol, etc.). In particular, electrochemical CO2 reduction reaction (CO2RR) is one of the promising methods operated at ambient pressure and room temperature. By using gold and silver catalysts, carbon monoxide (CO) is successfully yielded with >80% Faradaic efficiency. In contrast, it is still challenging to produce multi-carbon (i.e., C2+) products. Copper (Cu) is known as the sole catalyst to yield C2+ products by affording the suitable binding energy to the intermediate of *CO (the asterisk denotes the surface adsorption). Although various synthetic methods for the preparation of Cu nanoparticles (NPs) have been developed to increase the surface area and design of the preferred facets for the CO2 reduction, the highly sensitive Cu surface in air and from contaminants resulted in variable Faradaic efficiencies for C2+ production. In particular, the capping ligands, which coat the Cu surface to stabilize the designed nanostructures, should significantly influence the catalytic efficiency and stability. However, their effects have been little studied. Here, we show the enhanced catalytic activity and stability of Cu NPs for the C2+ yield by eliminating the capping ligands.[1] The pristine Cu NPs with ~8 nm of diameter were prepared by using a capping ligand of tetradecylphosphonate (TDP). After UV-ozone treatment, the TDP ligands were eliminated in part; The phosphorous signal was attenuated, and the Cu surface was oxidized in X-ray photoelectron spectra. In addition, the carbonyl oxygen (O=C) emerged from the damaged TDP. In contrast, the carbon substrate and the surface roughness of the Cu were insignificantly changed. These UV-ozone-treated Cu NPs showed ~50% Faradaic efficiency of the C2+ conversion (FEC2+) at –0.98 V vs. RHE, which was twice as high as that of the pristine Cu NPs (~25%). In particular, the FEC2+ gradually increased as the remaining capping ligands were removed, indicating the increased catalytic sites for 3 h CO2RR. In sharp contrast, the FEC2+ from the pristine Cu NPs was consistently low despite the partial stripping of the capping ligands, suggesting the negligible formation of the C2+ active sites. For prolonged chronoamperometry tests for 20 h, these FEC2+ values were retained for both catalysts, while the size and shape of Cu catalysts were differently changed. The UV-ozone-treated ones proceeded little agglomeration and contained many grain boundaries. In contrast, the pristine Cu NPs were transformed to the cubic particles with ~100 nm size. The distinct alternations of Cu structures are closely related to the yield of the C2+ active sites, which I will discuss in this presentation. [1] J. Mater. Chem. A, 9, 11210 (2021) Figure 1

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