Chloride-modified defective bismuth nanosheets providing a local proton for enhanced CO₂ reduction

Proton affinity or transfer is crucial in determining the activity and selectivity of the electroreduction of CO₂. However, optimizing proton supply during CO₂ reduction while simultaneously enhancing the activity of catalytic sites and inhibiting hydrogen evolution poses a significant challenge. In this work, we report a rapid strategy for preparing Cl-modified Bi nanosheets by decorating the defective sites of Bi nanosheets with abundant Cl⁻, permitting systematical study on how Cl⁻ impact on the catalytic activity of defective Bi sites and the proton transfer during CO₂ reduction process. It is demonstrated that rich Cl⁻ sites around defective Bi sites could form Cl–H species during catalytic process (as confirmed by operando spectroscopy) and function as unique proton relay stations for the reaction intermediate, significantly accelerating the conversion of CO₂-to-formate. The resulting Cl-modified Bi nanosheets achieve nearly 100% formate Faradaic efficiency (FE) at 400 mA/cm² and high formate FEs (> 90%) across 100 to 700 mA/cm², as well as a long-term stability over 130 h in acidic electrolyte, much superior than those of Bi catalysts without Cl⁻ modification. This work provides new guidance for designing advanced electrocatalysts for CO₂ electroreduction.