Cation-Regulated Interfacial Proton Transfer for Enhanced C–C Coupling in Electrochemical CO Reduction

Electrochemical CO reduction reaction (CORR) on copper is a prototypical platform for studying C–C coupling toward multicarbon (C2+) products, yet the fundamental factors governing this process remain under debate. Particularly, how electrolyte composition regulates interfacial proton transfer and thereby governs C–C coupling pathways is still unclear. This paper describes the role of alkali metal cations in controlling C–C coupling in CORR by restructuring interfacial water and modulating proton transfer dynamics on Cu electrodes. By varying cation identity (Li+, Na+, K+, Cs+) and concentration, we demonstrate that cations with intrinsic larger ionic radii in the electrolyte shift selectivity toward C2+ products. Combining differential electrochemical mass spectrometry (DEMS) analysis with in situ surface-enhanced Raman spectroscopy (SERS) and attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) investigations, it is revealed that alkali metal cations with large intrinsic radii have weak hydration shells that disrupt the interfacial hydrogen-bond (H-bond) network and increase the population of cation-coordinated water molecules at the interface, which may serve as more effective proton transfer channels toward surface intermediates. This reconstruction of interfacial water can accelerate *CO hydrogenation, thus promoting the C–C coupling for C2+ production. These findings identify interfacial water as an active regulator of interfacial proton transfer, thereby supporting a mechanistic link between electrolyte composition and C–C coupling pathways. This work emphasizes the importance of the interfacial microenvironment and molecular-level understanding of electrochemical reactions.