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Abstract:
Revealing the active nature of oxide derived copper (OD-Cu) is of key importance to understand its remarkable catalytic performance during the cathodic CO2 reduction reaction (CO2RR) to produce valuable hydrocarbons. Using advanced spectroscopy, electron microscopy and electro active surface area characterization techniques, the electronic structure and the changes in the morphology/roughness of thermally oxidized copper thin films were revealed during CO2RR. For this purpose we developed an in situ cell for X-ray spectroscopy able to be operated accurately in the presence of gases or liquids to unite the role of the initial thermal oxide-phase and its active phase during the electrocatalytic reduction of CO2. It was found that the Cu(I) species formed during the thermal treatment are readily reduced to Cu0 during the CO2RR, whereas Cu(II) species are hardly reduced. In addition, Cu(II) oxide electrode dissolution was found to yield a porous/void structure, where the lack of electrical connection between isolated islands prohibits the CO2RR. Therefore, the active/stable phase for CO2RR is metallic copper, independent of its initial phase, with a significant change in its morphology upon its reduction yielding the formation of a rougher surface with higher number of under coordinated sites. Thus, the initial thermal oxidation of copper in air controls the reaction activity/selectivity due to the changes induced in electrode surface morphology/roughness and the presence of more under coordinated sites during the CO2RR.
