Multi-scale roles of water in electrocatalytic CO 2 and CO reduction

The electrocatalytic CO₂ reduction reaction (CO₂RR) in aqueous electrolytes is a cornerstone technology for achieving a carbon-neutral society. Beyond the direct CO₂RR, the sequential reduction of CO₂ to CO followed by the CO reduction reaction (CORR) offers an alternative pathway. Water plays a pivotal and complex multi-scale role in CO₂RR/CORR systems. Its influence spans multiple scales, including micro-scale water-mediated reaction kinetics, meso-scale regulation of the local aqueous microenvironment, and macro-scale optimization of operational conditions. Factors at these different scales are interconnected and mutually interactive, ultimately determining the overall reaction performance. Therefore, this review establishes a comprehensive multi-scale framework for water regulation in both the CO₂RR and the CORR. We systematically elucidate the roles of water, survey advanced regulation strategies, and discuss state-of-the-art in situ characterization and simulation techniques. Finally, we identify key challenges and provide a forward-looking perspective, emphasizing that as this technology matures towards large-scale applications, such synergistic, multi-scale regulation becomes increasingly critical. This work aims to offer a clear roadmap for designing high-performance, stable, and large-scale CO₂RR/CORR systems and accelerating their practical implementation.