Sub-second Time-resolved Surface Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper.

Electrocatalytic reduction of carbon dioxide (CO2) into value-added products (e.g., ethylene) is a promising approach for greenhouse gas mitigation, but many details of electrocatalytic CO2 reduction reactions (CO2RR) remain elusive. Raman spectroscopy is suitable for in situ characterization of CO2RR mechanisms, but the low signal intensity and resulting poor time resolution (often up to minutes) hampers the application of conventional Raman spectroscopy for the study of the dynamic CO2 reduction reaction, which requires sub-second time resolution. By using Time-Resolved Surface Enhanced Raman Spectroscopy (TR-SERS) we were able to successfully monitor CO2RR over Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 V vs. the reversible hydrogen electrode (RHE) and subsequent surface oxide reduction (below -0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hot-spots for TR-SERS, enhanced time resolution (down to ~ 0.7 s) and improved CO2RR efficiency (i.e., four-fold increase in ethylene faradaic efficiency). With TR-SERS, the initial formation of hot-spots for SERS and CO2RR was followed (<7 s), after which a stable copper surface surrounded by increased local alkalinity was formed. Our measurements revealed that a highly dynamic CO intermediate, with a characteristic vibration below 2060 cm-1, is related to C-C coupling and ethylene production (-0.9 V vs. RHE), whereas lower cathodic bias (-0.7 V vs. RHE) resulted in gaseous CO production from isolated and static CO surface species with a distinct vibration at 2092 cm-1. Our results provide valuable time-resolved insights into the dynamic nature of the electrode surface and adsorbed intermediates during CO2 electrochemical reduction on copper and showcase the potential of TR-SERS in copper-based electrocatalysis to follow reaction dynamics.