Publication date: 6th November 2020
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) still remain elusive. Raman spectroscopy is suitable for in-situ characterization of CO2RR mechanisms, but the low signal intensity and resulting poor time resolution (up to minutes) hampers the application of conventional Raman spectroscopy for the study of this dynamic reaction on the second timescale. In this work, we show that Time-Resolved Surface Enhanced Raman Spectroscopy (TR-SERS) can be successfully performed during CO2RR with sub-second time resolution. Anodic treatment (1.55 V vs. RHE) and surface oxide reduction (below −0.4 V vs. RHE) induced roughening of the electrode surface, which resulted in hot-spots for TR-SERS, enhanced time resolution (down to 717 milliseconds) and improved CO2RR efficiency (two-fold increase in ethylene faradaic efficiency). With TR-SERS, the initial formation of hot-spots for SERS and CO2RR was followed, and potential-dependent dynamical coupling of surface-bound carbon monoxide (CO) intermediates was investigated. Our measurements reveal that a highly dynamic CO intermediate, with a characteristic vibration below 2050 cm−1, is a crucial species for C-C coupling at ethylene-dominating potential (−0.9 V vs. RHE), whereas lower cathodic bias (−0.7 V vs. RHE) results in CO production from isolated CO. Our results provide valuable time-resolved insights into the mechanism of C-C coupling during CO2 electrochemical reduction on copper, and showcase the potential of TR-SERS in electrocatalysis to follow reaction dynamics.