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 (CO_atop) intermediate is obtained on the electrodeposited Cu surface which primarily has the oxidation state of Cu­(I). CO_atop is further reduced, followed by the formation of C₁ product such as CH₄. The residual bridge-adsorbed CO (CO_bridge) 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 CO_atop and CO_bridge 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.