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

The electrocatalytic carbon dioxide (CO₂) 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 CO₂-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 CO₂-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 CO₂ transport to the catalyst layer, optimizing the trade-off of conversion and selectivity at low flow rates.