In Situ Raman Spectroscopy Reveals the Multifunctional Role of Interfacial Water in CO2-to-C2 Electroreduction on Cu(hkl) Surfaces

Abstract

Interfacial water serves as both a proton donor and a competitor for active sites at the copper–catalyst interface in the electrochemical CO₂ reduction reaction (CO₂RR). However, its precise impact on C₂₊ product selectivity remains debatable. Here, through the utilization of in situ Raman spectroscopy and theoretical calculations, we have discovered that the population of K⁺ cation hydrated water (K-H₂O) rises concurrently with the increase of C₂ yield on atomically flat model Cu(hkl) single crystal surfaces. The K⁺ not only stabilizes the *CO + *CO intermediate by direct coordination but also reshapes the configuration of solvated interfacial water to create a hydrogen-bond-absent environment. This prevents surrounding hydrogen from interacting with the *CO species, ingeniously suppressing its hydrogenation along the C₁ pathway while promoting C–C coupling toward C₂ products. Our results further clarify the CO₂RR mechanism and provide definitive evidence that cationic hydrated water is critical to tuning the product selectivity.