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.

CO₂ electroreduction to formic acid/formate would contribute to alleviating the energy and climate crisis. This work reports a Bi-based catalyst derived from the in-situ electroreduction of Bi₂O₂CO₃ modified with iodine and pyrenyl-graphdiyne (PGDY) on the surface for efficient electroreduction of CO₂ in acidic electrolyte, with a high partial current density of 98.71 mA·cm⁻² and high Faradaic efficiency (FE) > 90% over the potential range from −1.2 to −1.5 V vs. reversible hydrogen electrode (RHE), as well as the long-term operational stability over 240 h without degradation in H-type cell. Experimental results and density function theory calculations show that the synergistic effect of surface iodine and PGDY is responsible for this active and extremely stable process of CO₂ electroreduction via lowering the energy barriers for formation of *OCHO intermediate, suppressing the competitive HER by enhancing the concentration of both K⁺ and CO₂ at reaction interface, as well as preventing the dissolution and re-deposition of active Bi atoms on surface during catalytic reaction. This work provides new insight into designing highly active and stable electrocatalysts for CO₂ reduction.

A visible-light-response, efficient and robust photo-catalyst for CO₂ reduction is highly desirable. Herein, we demonstrate that single titanium-oxide species implanted in two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) matrix (2D TiO-CN) can efficiently photo-catalyze the reduction of CO₂ to CO under the irradiation of visible light. The synergistic interaction between single titanium oxide species and g-C₃N₄ in 2D TiO-CN not only enhances the separation of photo-excited charges, but also results in visible light response of single titanium-oxide species, realizing high activity of CO₂ photo-reduction with extremely high CO generation rate of 283.9 μmol·h⁻¹·g⁻¹, 5.7, 6.8 and 292.2 times larger than those of TiO₂/CN hybrid material, CN and commercial TiO₂, respectively. Time-resolved fluorescence and electron spin resonance spectroscopy revealed the catalytic mechanism of the fabricated 2D TiO-CN photocatalysts for CO₂ reduction.