Stabilizing Mixed‐Valence Sn Active Sites With Oxygen Vacancies for Enhanced Acidic CO 2 ‐to‐HCOOH Conversion

Electrocatalytic CO 2 reduction reaction (CO 2 RR) converts atmospheric CO 2 into valuable chemicals using renewable energy. However, acidic CO 2 ‐to‐HCOOH electrolysis with high CO 2 utilization still faces challenges such as the competing hydrogen evolution reactions, acidic corrosion, and low selectivity. In this study, we synthesized a series of SnO 2 catalysts with tunable oxygen vacancy concentrations by high‐temperature (300–900°C) calcination. The obtained SnO 2 −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 CO 2 RR conditions, the SnO 2 −600 catalyst with moderate oxygen vacancies could convert to stable mixed‐valence state SnO x (Sn 2 O 3 and Sn 3 O 4 ) active species, while those without or excessive oxygen vacancies will be over‐reduced. This study establishes a correlation between oxygen vacancy and acidic CO 2 RR performance in Sn‐based catalysts, highlighting mixed‐valence SnO x species as key active sites and providing a foundation for designing high‐activity, stable CO 2 RR catalysts for industrial applications.