Boosting electrochemical CO₂ reduction via valence state and oxygen vacancy controllable Bi-Sn/CeO₂ nanorod

Electrocatalytic CO₂ reduction reaction (CO₂RR) to produce formate has been recognized as one of the most efficient strategies to convert CO₂ to energy-rich products and store renewable energy compared with other methods such as biological reduction, thermal catalytic reduction, and photocatalytic reduction. Developing an efficient catalyst is crucial to enhance the formate Faradaic efficiency (FE_formate) and retard the competing H₂ evolution reaction. The combination of Sn and Bi has been demonstrated to be effective in inhibiting the evolution of H₂ and the generation of CO, promoting the formation of formate. Herein, we design Bi- and Sn-anchored CeO₂ nanorods catalysts with the valence state and oxygen vacancy (V_o) concentration controllable for CO₂RR by reduction treatment at different environments. The m-Bi₁Sn₂O_x/CeO₂ with moderate H₂ composition reduction and suitable Sn/Bi molar ratio achieves a remarkable FE_formate of 87.7% at -1.18 V vs. RHE compared with other catalysts. Additionally, the selectivity of formate was maintained over 20 h with an outstanding FE_formate of above 80% in 0.5 M KHCO₃ electrolyte. The outstanding CO₂RR performance was attributed to the highest surface Sn²⁺ concentration which improves the formate selectivity. Further, the electron delocalization effect between Bi, Sn, and CeO₂ tunes electronic structure and V_o concentration, promoting the CO₂ adsorption and activation as well as facilitating the formation of key intermediates HCOO* as evidenced by the in-situ Attenuated Total Reflectance-Fourier Transform Infrared measurements and Density Functional Theory calculations. This work provides an interesting measure for the rational design of efficient CO₂RR catalysts via valence state and V_o concentration control.