In-situ generation of appropriate mixed-valence tin oxides on carbon nanofibers tuning CO2 reduction to formate

Excessive carbon dioxide emissions and the energy crisis trigger ongoing challenges. To address these issues, the electrochemical carbon dioxide reduction reaction (CO 2 RR) has emerged as a sustainable pathway for converting CO 2 into valuable chemicals. Among various products, formic acid has attracted widespread attention due to its broad industrial application prospects. Although tin-based catalysts are cost-effective and accessible, they still face challenges including active site aggregation and insufficient long-term stability. In this study, mixed-valence SnO x was supported on bacterial cellulose (BC) surface via an in-situ impregnation method. Sn/SnO/Sn 2 O 3 /SnO 2 @CNFs, Sn/Sn 2 O 3 /SnO 2 @CNFs, and Sn/SnO 2 @CNFs catalysts with different phase composition and oxygen vacancies (OVs) concentrations were obtained by simply changing the heat treatment temperature (400∼600°C). In situ Fourier transform infrared spectroscopy (FTIR) confirmed that synergistic interaction between mixed-valence SnOx species and the CNFs facilitated adsorption and stabilization of the key intermediate *OCHO to drive CO 2 conversion to formate. Density functional theory (DFT) calculations proved that Sn/Sn 2 O 3 /SnO 2 @CNFs catalyst with moderate OVs concentrations exhibits stronger adsorption capacity for *OCHO. Systematic characterization demonstrated that the CNFs not only provide a high specific surface area (871.71 m 2 ·g −1 ) to promote active site exposure but also stabilize the oxidation states of SnO x . Especially, the Sn/Sn 2 O 3 /SnO 2 @CNFs catalyst prepared at 500°C exhibited the highest Faradaic efficiency for formate (93.53%) while maintaining persistent stability for 60 h. In summary, the synergistic effect between porous CNFs and Sn/Sn 2 O 3 /SnO 2 with moderate OVs concentrations plays a key role in promoting CO 2 conversion to formate. This work provides a promising approach for the rational design of efficient and stable tin-based CO₂RR catalysts toward formate production.