Flow-Cell-Compatible Operando Surface-Enhanced Raman Spectroscopy for Probing Reaction Intermediates during Carbon Dioxide Reduction Reaction.

Elucidating reaction intermediates under practically relevant conditions remains a fundamental challenge in electrocatalysis. For an electrochemical CO2 reduction reaction (CO2RR), reaction mechanisms can differ substantially between conventional H-cells and high-rate flow cells, complicating accurate operando characterization. In this study, we develop a flow-cell-compatible operando surface-enhanced Raman spectroscopy platform by integrating silica-coated Au nanorods (AuNRs@SiO2) optimized for 785 nm excitation. The plasmonic resonance enhances Raman signals while suppressing fluorescence, and the inert SiO2 shell prevents catalytic perturbation, enabling sensitive measurements under practical catalytic conditions. Using a benchmark copper catalyst, we directly detect key transient intermediates, including *CO2- and *HOCCOH, that are inaccessible by conventional Raman spectroscopy. Deconvolution of the adsorbed *CO reveals bridge-bonded, low-frequency, and high-frequency components, establishing a quantitative correlation between *CO speciation and C2 selectivity. Parallel flow-cell and H-cell comparisons highlight the necessity of realistic mass transport. This work establishes a general operando spectroscopic strategy for mechanistic studies under realistic CO2RR conditions.