Selective and stable CO 2 electroreduction at high rates via control of local H 2 O/CO 2 ratio.

Controlling the concentrations of H 2 O and CO 2 at the reaction interface is crucial for achieving efficient electrochemical CO 2 reduction. However, precise control of these variables during catalysis remains challenging, and the underlying mechanisms are not fully understood. Herein, guided by a multi-physics model, we demonstrate that tuning the local H 2 O/CO 2 concentrations is achievable by thin polymer coatings on the catalyst surface. Beyond the often-explored hydrophobicity, polymer properties of gas permeability and water-uptake ability are even more critical for this purpose. With these insights, we achieve CO 2 reduction on copper with Faradaic efficiency exceeding 87% towards multi-carbon products at a high current density of -2 A cm -2 . Encouraging cathodic energy efficiency (>50%) is also observed at this high current density due to the substantially reduced cathodic potential. Additionally, we demonstrate stable CO 2 reduction for over 150 h at practically relevant current densities owning to the robust reaction interface. Moreover, this strategy has been extended to membrane electrode assemblies and other catalysts for CO 2 reduction. Our findings underscore the significance of fine-tuning the local H 2 O/CO 2 balance for future CO 2 reduction applications.