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

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.