New aspects of operando Raman spectroscopy applied to electrochemical CO₂ reduction on Cu foams

The mechanism of electrochemical CO₂ reduction (CO₂RR) on copper surfaces is still insufficiently understood. Operando Raman spectroscopy is ideally suited to elucidate the role of adsorbed reaction intermediates and products. For a Cu foam material which has been previously characterized regarding electrochemical properties and product spectrum, 129 operando spectra are reported, covering the spectral range from 250 to 3300 cm^-1. (1) The dendritic foam structure facilitates surface-enhanced Raman spectroscopy (SERS) and thus electrochemical operando spectroscopy, without any further surface manipulations. (2) Both Raman enhancement and SERS background depend strongly on the electric potential and the "history" of preceding potential sequences. (3) To restore the plausible intensity dependencies of Raman bands, normalization to the SERS background intensity is proposed. (4) Two distinct types of ^*CO adsorption modes are resolved. (5) Hysteresis in the potential-dependent ^*CO desorption supports previous electrochemical analyses; saturating ^*CO adsorption may limit CO formation rates. (6) HCO₃ ^- likely deprotonates upon adsorption so that exclusively adsorbed carbonate is detectable, but with strong dependence on the preceding potential sequences. (7) A variety of species and adsorption modes of reaction products containing C-H bonds were detected and compared to reference solutions of likely reaction products, but further investigations are required for assignment to specific molecular species. (8) The Raman bands of adsorbed reaction products depend weakly or strongly on the preceding potential sequences. In future investigations, suitably designed potential protocols could provide valuable insights into the potential-dependent kinetics of product formation, adsorption, and desorption.