In Situ Raman Spectroscopy Reveals the Multifunctional Role of Interfacial Water in CO 2 -to-C 2 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 CO 2 reduction reaction (CO 2 RR). However, its precise impact on C 2+ 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-H 2 O) rises concurrently with the increase of C 2 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 C 1 pathway while promoting C–C coupling toward C 2 products. Our results further clarify the CO 2 RR mechanism and provide definitive evidence that cationic hydrated water is critical to tuning the product selectivity.