Operando Spectroscopic Insights into the Influences of Organic Modifiers on the Electrochemical CO2 Reduction Microenvironments

Abstract Developing efficient carbon dioxide (CO2) conversion technology is imperative to address global warming and advance carbon neutrality. The electrochemical CO2 reduction reaction (CO2RR) offers a promising route for converting CO2 into value-added products using renewable electricity; however, its practical implementation is hindered by high overpotential requirements and competition with the hydrogen evolution reaction (HER). Metal electrodes (Au, Ag, Cu, Sn, and others) display distinct product selectivities in CO2RR, ranging from CO and formate to multi-carbon hydrocarbons and alcohols, governed by the metal identity and the surface binding affinities of key intermediates, rendering the reaction pathway highly sensitive to the surface and interface microenvironment. Operando spectro-electrochemical techniques, including infrared (IR) and Raman spectroscopy, enable real-time monitoring of microenvironmental changes that are inaccessible by ex-situ analysis. Surface-enhanced variants, such as surface-enhanced infrared absorption spectroscopy (SEIRAS) and surface-enhanced Raman spectroscopy (SERS), further enhance signal quality and sensitivity, providing mechanistic insights at the molecular level. This mini review organizes operando spectroscopic insights into the role of organic modifiers in enhancing CO2RR activity on metal electrodes. Organic surface modifications employing amines, pyridines, quinones, triazines, and imidazolium moieties reshape the interfacial reaction environment. We classify the effects of these modifiers into five mechanistic roles: (1) activation of CO2 molecules, (2) tuning the binding strength and surface coverage of key reaction intermediates, (3) modulating interfacial water structure, (4) regulating local pH, and (5) preserving the oxidation state of the electrode. Each role has been substantiated by operando spectroscopic evidence, manifested as shifts in vibrational band positions or changes in band intensities of intermediate signals. Importantly, these roles are interrelated: most modifiers act through several, and a given spectral change may have multiple origins. Mechanistic assignment thus requires cross-validation across complementary operando probes. Together, this framework rationalizes CO2RR at organic-modified interfaces and guides catalyst design.