Defect-Mediated Selectivity: Sulfur Vacancy Engineering in Tin Sulfide Boosts CO2 Electroreduction to Formate

Introducing S vacancies into tin disulfide (SnS₂) is crucial for regulating its electrocatalytic CO₂ reduction activity. However, conventional methods for generating vacancies often result in uncontrolled defect concentrations. In this study, SnS₂ with identical nanolayer structures but varying S-vacancy concentrations was synthesized by reacting different tin salts with high-temperature molten potassium thiocyanate. The results show that although SnS₂ derived from SnCl₂ exhibits a high S-vacancy concentration, its Faradaic efficiency (FE) for formic acid reaches only 48.2%. In contrast, SnS₂ derived from SnSO₄ contains fewer vacancies and exhibits an FE of 85.6%. Adjusting the KSCN/Sn salt ratio and applying H₂ posttreatment can further optimize the S-vacancy concentration and enhance the CO₂ reactivity. DFT calculations confirm that introducing an appropriate S-vacancy concentration can enhance the CO₂ adsorption capacity, improve the affinity for the *OCHO intermediate, and accelerate the CO₂RR kinetics. This precise vacancy engineering highlights the significance of balancing the defect concentration and catalytic efficiency.