Inhibiting Hydrogen Evolution using a Chloride Adlayer for Efficient Electrochemical CO₂ Reduction on Zn Electrodes

A challenge in electrochemical CO₂ reduction is inhibiting hydrogen evolution reaction (HER). Herein, a bulk Zn electrode was chosen to reveal the effect of specifically adsorbed Cl^- ions on HER during CO₂ reduction. We found that in the Cl^- ion-containing electrolyte, the Zn-Cl adlayer formed because the weakly solvated Cl^- ions can strip the solvation shell and form a direct chemical bond with the metal Zn. As a result, the Zn-Cl bonds on the Zn electrode notably blocked the hydrogen evolution and significantly facilitated the electron transfer process via the Cl to the vacant orbitals of inert CO₂, therefore stabilizing the CO₂^- radical and leading to a four times higher CO Faradaic efficiency for CO₂ reduction in the KCl electrolyte than that in the KHCO₃ electrolyte at a moderate potential of -1.2 V versus a reversible hydrogen electrode.