A Holistic View of Ions and Water Arrangements at the Electrified Cu Surface during Electrochemical CO2 Conversion

Resolving the competitive adsorption of ions and water at the electrified surface is vital for understanding the local microenvironments controlling all surface reactions. Herein, we conducted thorough in situ spectroscopy studies of the microenvironments of electrified Cu during its electrochemical CO2 reduction reaction (CO2RR) utilizing in situ attenuated total reflection infrared absorption spectroscopy. The potential-dependent evolution of various solution species, including cations, anions, and interfacial water, was systematically resolved. The formation of adsorbed CO (COads), the key reaction intermediate for the CO2RR, was found to involve notable displacement of interfacial water and the formation of free H2O isolated from the bulk hydrogen-bonding framework. The general trend of ion and water movement was verified to be universal for all the utilized alkali cations (Li+, Na+, K+, and Cs+), with the cation size additionally modifying the interfacial water density. Unlike the classical view of a stationary electric double layer structure and cation effects at the electrified surface, we propose that “cation–water–COads” may be considered as a better collective elementary building block of these ubiquitous surfaces for further mechanistic discussion of CO2RR and related electrocatalytic systems.