Controlling the Oxidation State of the Cu Electrode and Reaction Intermediates for Electrochemical CO₂ Reduction to Ethylene

Understanding the role of the oxidation state of the Cu surface and surface-adsorbed intermediate species in electrochemical CO₂ reduction is crucial for the development of selective CO₂-to-fuel electrocatalysts. In this study, the electrochemical CO₂ reduction mechanism over the Cu catalysts with various oxidation states was studied by using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), in situ soft X-ray absorption spectroscopy (Cu L-edge), and online gas chromatography measurements. The atop-adsorbed CO (COatop) intermediate is obtained on the electrodeposited Cu surface which primarily has the oxidation state of Cu(I). COatop is further reduced, followed by the formation of C₁ product such as CH₄. The residual bridge-adsorbed CO (CObridge) is formed on the as-prepared Cu surface with Cu(0) which inhibits hydrocarbon formation. In contrast, the CV-treated Cu electrode prepared by oxidizing the as-prepared Cu surface contains different amounts of Cu(I) and Cu(0) states. The major theme of this work is that in situ SEIRAS results show the coexistence of COatop and CObridge as the reaction intermediates during CO₂ reduction and that the selectivity of CO₂-to-ethylene conversion is further enhanced in the CV-treated Cu electrode. The Cu catalysts modulated by the electrochemical method exhibit different oxidation states and reaction intermediates as well as electrocatalytic properties.