Revisiting Electrochemical Reduction of CO₂ on Cu Electrode: Where Do We Stand about the Intermediates?

Electrochemical CO₂ reduction on Cu electrode has attracted the attention of many researchers in the last decades, because of its potential to generate significant amounts of hydrocarbons at high reaction rates over sustained periods of time. As a result, substantial effort has been devoted to determining the unique catalytic performance of Cu and to elucidate the mechanism through which hydrocarbons are formed. Here we report new insights into CO₂ reduction on Cu by electrochemical impedance spectroscopy (EIS) in terms of adsorption/desorption of the reduction intermediates. The potential dependence of charge transfer kinetics is discussed on the basis of EIS results. We revisit the mechanism of the formation of hydrocarbons, taking into account the pH adjacent to the electrode surface, adsorption of HCO₃ ^- and CO₃ ^2-, and the role of active hydrogen. In addition to the enol-like intermediate, proposed previously, we proposed that∗COOH^• radicals, originating from the active involvement of HCO₃ ^- and/or CO₃ ^2- upon reduction are key intermediates for the formation of a variety of C2 and C3 products. Thus, our results provide an additional crucial guideline for the design of future catalysts that can efficiently and selectively reduce CO₂ into value-added chemicals.