Restoring Formate Selectivity: In Situ Raman Study of Deactivated Electrodes for CO ₂ Electroreduction

Careful monitoring and control of material changes during operation are crucial for identifying active sites, understanding reaction or degradation pathways, and guiding the design of improved catalysts. Under electrochemical conditions, catalysts often undergo structural transformations induced by interactions with nearby species. Here, we study Bi and Sn electrodes, two metals that are widely investigated for electrochemical CO2 reduction toward formate. By combining in situ Raman spectroscopy with density functional theory calculations, we show that the formation of an oxo-carbonate (subcarbonate) surface species positively influences catalytic performance toward formate production after surface reoxidation. Our results indicate that the frequently reported high Faradaic efficiency and high current density at low overpotentials for Sn and Bi catalysts may arise from the coexistence of this subcarbonate species on the surface. This work highlights that structural modifications occurring before or during catalysis can strongly influence performance, providing important insights into the rational design of more-active and more-stable electrocatalysts.