Electrocatalytic CO2 reduction promoted by catalyst-bound proton-abstracting ligands

The electrochemical CO₂ reduction reaction (eCO₂RR), which is well suited for reducing atmospheric CO₂ levels and generating clean energy. Water is the main proton source in the electrochemical reduction process. However, water features a high dissociation energy barrier, and most protons are involved in HER, so that only a small amount of H⁺ is used in the CO₂ reduction reaction. Herein, we reveal that appropriately chosen ligands on the catalyst surface can abstract protons from water, store them, and transfer them to CO₂ to promote the formation of COOH and improve the proton utilization efficiency of the eCO₂RR. Moreover, the activity of the ligand complexed protons is shown to be lower than that of free ones and thus result in HER inhibition our ligands with different pKa values (i.e., proton release capacities) are used to functionalize ZnO, and the plot of pKa vs. the FE_CO exhibits a volcano shape. Overly small pKa leads to the combination of abundant protons with each other and the dominance of the HER, whereas overly large pKa does not promote the utilization of H⁺ by CO₂. Ligands with suitable pKa can properly bind and release protons, which is conducive to the formation of *COOH, thus accelerating eCO₂RR. In particular, mercaptobenzoic acid (MBA) considerably enhances catalytic activity because of its moderate pKa (FE_CO = 90 %, which is 2 times higher than the FE_CO of unmodified ZnO, and j_CO is nearly 5 times higher). In situ infrared spectroscopic analysis proves that the above ligand can abstract protons from H₂O and then release them, promoting the dissociation of H₂O. Theoretical calculations show that pKa affects the ability of ligands to abstract and release protons.