Differential Adsorption on Synergistic Cu─Cd Sites Enables Direct Hydrogenation in Acidic CO2 Electroreduction.

The direct involvement of proton in acidic CO2 reduction is key to boosting energy efficiency and preventing carbonate formation. However, the competitive adsorption of proton at low overpotential limits CO production by inhibiting the activation and hydrogenation of CO2 to form the key intermediates, *COOH. To address this challenge, we developed a Cu─Cd alloy catalyst, which optimizes the proton-coupled electron transfer (PCET) pathway through a synergistic regulation between dual Cu and Cd active sites. In situ spectroscopy and theoretical calculations reveal that Cd incorporation enhances *CO2 adsorption and lowers the energy barrier for *COOH formation. By proving direct electrochemical hydrogenation, our catalyst achieved a remarkable CO Faradaic efficiency (FE) of 98.6% at -1.12 V versus RHE, and operated for over 250 h at a total current of 0.4 A under a voltage of 2.55 V in a proton exchange membrane electrode assembly. This research presents a powerful new insight for high-performance acidic CO2 electrolysis in proton-abundant electrolyte environments.