Multifaceted roles of water in the electrochemical reduction of CO2

Electrochemical carbon dioxide reduction reaction (CO 2 RR) offers a sustainable approach to mitigating CO 2 emissions while producing value-added fuels and chemicals. Despite significant advances in electrocatalysts and mechanistic understandings, achieving practical applications remains a considerable challenge. Water (H 2 O), as the key electrolyte component in CO 2 RR, plays multifaceted roles beyond that of a simple solvent. It participates in proton-coupled electron transfer, stabilizes key reaction intermediates through solvation, and modulates the microenvironment at electrode–electrolyte interfaces. Conversely, sluggish water dissociation kinetics, competition from the hydrogen evolution reaction, and dynamic restructures of interfacial water hinder CO 2 RR activity and selectivity. This review provides a comprehensive examination of the multifaceted roles of water in CO 2 RR. The structural and chemical properties of water, which govern electrolyte function, catalyst activation, and product distribution, are discussed, along with pathways by which uncontrolled water dynamics promote undesired reactions. Strategies for regulating water behaviors, including engineering catalysts, adjusting electrolyte compositions, tuning electrode wettability, and optimizing reaction operational conditions, are systematically summarized. Further, advances in characterization techniques that probe interfacial water dynamics are highlighted. Finally, key challenges and future opportunities are outlined for controlling multifaceted roles of water to enable efficient, selective, and industrially viable CO 2 RR. • H 2 O dictates electrolyte behaviors, catalyst activation, and product distribution in electrochemical CO 2 reduction. • Strategies for regulating H 2 O behaviors: catalyst engineering, electrolyte formulation, electrode wettability tuning, and optimization of operational conditions. • New operando characterization techniques can probe interfacial H 2 O dynamics under reaction conditions. • Key challenges and future opportunities for controlling the multifaceted roles of H 2 O to enable efficient, selective, and industrially relevant CO 2 RR.