In Situ Raman Study of Potential‐Dependent Surface Adsorbed Carbonate, CO, OH, and C Species on Cu Electrodes During Electrochemical Reduction of CO2

Abstract Using in situ surface‐enhanced Raman spectroscopy (SERS), and 13 C/ 12 C and D 2 O/H 2 O isotopic labeling for assignment, we show potential dependent transients in surface composition of Cu‐catalyzed electrochemical reduction of CO 2 in carbonate solution. First, reduction of Cu(I)oxide is accompanied by adsorption of predominantly monodentate carbonate at ∼1067 cm −1 starting in the potential range from [+0.2 V→−0.2 V]. Contrary to recently advocated hypotheses, and based on the significant presence at anodic potential, a band in this potential range at ∼1540 cm −1 can be assigned to bidentate carbonate. As expected, appearance of surface CO was observed in the range of [−0.4 V→−1.0 V], clearly identified by the Cu−CO vibration at 360 cm −1 . Most importantly, at the more negative end of this potential range, we identified the formation of surface OH, and for the first time a surface Cu−C species, showing Raman bands at ∼525 cm −1 (Cu−OH) and ∼500 cm −1 (Cu−C), respectively. In the potential range of [−1.0 V→−1.4 V], surface CO disappears, while the Cu−OH and Cu−C species are persistent. Interestingly positive polarization at >0.1 V removes these species and restores the surface to Cu(I)oxide, rendering the surface processes completely reversible. Implications of this study for mechanistic understanding of electrode deactivation and practical operation are discussed.