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

The electrocatalytic carbon dioxide (CO2 ) reduction reaction (CO2 RR) into hydrocarbons is a promising approach for greenhouse gas mitigation, but many details of this dynamic reaction remain elusive. Here, time-resolved surface-enhanced Raman spectroscopy (TR-SERS) is employed to successfully monitor the dynamics of CO2 RR intermediates and Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 V vs. RHE and subsequent surface oxide reduction (below -0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hotspots for TR-SERS, enhanced time resolution (down to ≈0.7 s) and fourfold improved CO2 RR efficiency toward ethylene. With TR-SERS, the initial restructuring of the Cu surface was followed (<7 s), after which a stable 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 .