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.