Time-resolved observation of C–C coupling intermediates on Cu electrodes for selective electrochemical CO2 reduction

Abstract

In the electrochemical CO₂ reduction reaction (CO₂RR), Cu has been spotlighted as the only electro-catalyst that can produce multi-carbon molecules, but the mechanism of the selective C₂₊ production reaction remains elusive. Here, we directly monitored CO₂RR intermediates by employing time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), with particular attention to the C₁ and C₂₊ pathways beyond the formation of *CO. Electrodeposited Cu and Cu(OH)₂-derived Cu were synthesized, and subsequently employed as a C₁ and C₂₊ activating catalyst and C₂₊ activating catalyst, respectively. For the first time, a kinetically linked dimer intermediate (*OCCO) was observed and identified as the C₂₊ path triggering intermediate. The ATR-SEIRAS results suggest that C–C coupling occurs exclusively by CO dimerization toward *OCCO, without the participation of *CHO, which is an intermediate for CH₄ production. In the real-time measurements, CO dimerization occurred concurrently with CO adsorption (∼5 s), while proton-coupled reduction toward *CHO has slower kinetics (∼30 s). We demonstrated that the sites showing a high vibrational frequency of *CO on the fragmented Cu surface are the potential active sites for the fast dimerization of CO. This work provides mechanistic insights into the CO₂RR pathways and enables the design of efficient C₂₊-producing catalysts.