Stabilizing Mixed-Valence Sn Active Sites With Oxygen Vacancies for Enhanced Acidic CO₂-to-HCOOH Conversion

Electrocatalytic CO₂ reduction reaction (CO₂RR) converts atmospheric CO₂ into valuable chemicals using renewable energy. However, acidic CO₂-to-HCOOH electrolysis with high CO₂ utilization still faces challenges such as the competing hydrogen evolution reactions, acidic corrosion, and low selectivity. In this study, we synthesized a series of SnO₂ catalysts with tunable oxygen vacancy concentrations by high-temperature (300-900°C) calcination. The obtained SnO₂-600 catalyst achieved over 90% Faradaic efficiency (FE) across a wide current density range of -0.30 to -1.0 A cm^-2, with a peak FE of 96.2% at -1.0 A cm^-2, a formic acid production rate of 17.9 mmol h^-1 cm^-2. The catalyst could maintain 80% FE of HCOOH over 80 h. In situ Raman spectroscopy revealed that under CO₂RR conditions, the SnO₂-600 catalyst with moderate oxygen vacancies could convert to stable mixed-valence state SnO_x (Sn₂O₃ and Sn₃O₄) active species, while those without or excessive oxygen vacancies will be over-reduced. This study establishes a correlation between oxygen vacancy and acidic CO₂RR performance in Sn-based catalysts, highlighting mixed-valence SnO_x species as key active sites and providing a foundation for designing high-activity, stable CO₂RR catalysts for industrial applications.