Fundamental Understandings on Electrochemical CO₍₂₎ Reduction on Cu and Cu-based Catalysts

Anthropogenic activities have been strongly reliant on fossil fuels since the industrial revolution. Combustion of fossil fuels releases a great amount of greenhouse gases, particularly, carbon dioxide (CO₂), resulting in global warming and further catastrophic impacts on the earth and human beings. Inclining the primary energy source to renewable energy such as solar radiation and wind would help mitigate carbon emissions. Yet, power produced from the sun and the wind is intermittent due to its strong weather-condition dependency, and thus needs to be stored in a stable and efficient way. Electrochemical CO₂ reduction reaction (eCO2RR) is such a strategy that stores renewable energy in chemical bonds by electrochemically converting CO₂ into chemicals and fuels, and thus close the carbon cycle.
Copper (Cu) has been known as the only monometallic eCO2RR catalyst that produces a variety of fuels, such as methane, ethylene, acetaldehyde, ethanol, etc. However, most of the products are simultaneously produced during eCO2RR. Improving the selectivity toward specific species requires mechanistic insights into the obscure reaction mechanism and pathways of eCO2RR. To this end, Cu single crystals with well-defined surface structures are employed as a model platform for building up the structure-performance correlation. Electrochemical CO(₂) reduction reactions are carried out on an electrochemistry-mass spectrometry (EC-MS) system which enables real-time gas and volatile liquid product detection and analyzation. Upon optimizing the mass spectrometer parameters, not only gas products (including methane, ethylene, and hydrogen from the side-reaction water splitting), but also the liquid product acetaldehyde are successfully detected and qualitatively tracked (in molar flux). The production of another volatile liquid product ethanol, on the other hand, cannot be tracked with a suitable descriptor, which may be attributed to the strong interference from hydrogen via the molecule-ion reaction mechanism. Considering that *CO is the most important intermediate in reducing CO₂ to multicarbon products, electrochemical CO reduction is carried out on four single crystal Cu electrodes: (100), (110), (111), and (211). The preference of methane and ethylene production on the Cu(110) and (100) facets, respectively, as well as the shared reaction pathway between acetaldehyde and ethylene are spectrometrically testified. Acetaldehyde chemistry in alkaline is also investigated. This work provides mechanistic information on the facet-dependent eCO2RR product distribution on Cu, as well as the acetaldehyde activity under the reaction conditions, and therefore helps steer the product selectivity towards acetaldehyde and ethanol.
In addition to the surface structure of Cu catalysts, the formation of multicarbon products with high energy density requires a high coverage of *CO intermediate. Hence, combing Cu with a CO-producing co-catalyst is expected to improve the local CO concentration and thus *CO coverage, facilitating multicarbon product formation. Silver (Ag) is a promising co-catalyst in this regard. It has been found that introducing Ag atoms into the Cu lattice can modulate product preference. However, the synergistic effects between Cu and Ag, and thus the catalytic performance, are strongly influenced by catalyst morphology, electrolyzer configuration, reaction conditions, etc. Operando measurements can provide explicit information on the catalyst dynamic variation during the reaction, but their operation and analysis are challenging. herein, CuAg multiphase alloy catalysts are prepared by magnetron sputtering, which allowed for investigating the intrinsic interaction between Cu and Ag. Electrochemical CO₂ reduction performance exhibited an improved selectivity towards carbonyls at the expense of hydrogen and hydrocarbons. The partially alloyed Cu and Ag phases were confirmed by operando X-ray diffraction. By means of combining operando X-ray measurements and density functional theory (DFT) calculations, the preferred carbonyl production is attributed to the reduced electron density and compressive strain of Cu due to Ag incorporation, which leads to a deeper d-band center and therefore weakened intermediate adsorption and oxophilicity. This work provides evidence of the intrinsic structural and electronic interaction between Cu and Ag during eCO2RR. The obtained information will facilitate the design of new bi-/multi-phase metallic or alloy electrocatalysts.
Works presented in this thesis provide mechanistic information for a better understanding on the eCO2RR pathways, as well as for the design and fabrication of new alloy catalysts for eCO2RR.