Multifunctional Binding Interface Drives Near‐Unity CO Selectivity in Acidic CO2 Electrolysis

Abstract The electrocatalytic carbon dioxide (CO 2 ) reduction is challenged by the parasitic hydrogen evolution reaction (HER) especially in acidic media. Here, we elaborate that redox‐active isoindigo, acting as a multifunctional co‐catalyst, can pre‐activate CO 2 ‐bound intermediates and suppress HER upon the synergistic effects of Lewis acid‐base adduct formation, intramolecular hydrogen‐bond interaction, and interfacial water structure modulation. Modifying a silver catalyst with isoindigo substantially decreases the energy barrier for CO 2 ‐to‐*COOH conversion, which is regarded as the potential‐limiting step of carbon monoxide production. Accordingly, superior catalytic performances are obtained at pH 2, where Faradaic efficiencies surpass 99% at industrial‐relevant current densities. Moreover, we find that assembling an additional polyamine‐coated layer in front of gas flow channels improves CO 2 transport to the catalyst layer, optimizing the trade‐off of conversion and selectivity at low flow rates.