Construction of Bi-Sn(In)O Integrated Electrode for Efficient CO₂ Electroreduction to Formate

Tin oxide is widely recognized as a promising candidate for the electrochemical synthesis of formate from CO₂. However, challenges such as suboptimal performance and a limited understanding of the reaction mechanisms persist. In this study, we developed an integrated electrode by optimizing a hierarchical metal-metal oxide nanoarray via a combination of hydrothermal synthesis and electrodeposition. This design enables direct growth of the catalyst on the current collector, eliminating the need for polymer binders. The constructed metal-loading structure effectively regulates the electronic structure of Sn and facilitates the formation of Sn^δ+ species, which thereby enhances the adsorption strength of *OCHO intermediate, significantly enhancing formate production. The resulting electrocatalyst, Bi-Sn(In)O, demonstrates a Faradaic efficiency (FE) of 83.66% for the reduction of CO₂ to formate, with a maximum production rate of 768.12 μmol·h^-1·cm^-2 and a current density of 54.45 mA·cm^-2. In situ infrared characterization and theoretical calculations reveal that local charge redistribution and the presence of high-valence Sn^δ+ improve the adsorption and stabilization of the crucial *OCHO intermediate, thereby lowering the energy barrier for HCOOH formation. Additionally, the integrated electrode design enables the exposure of more active sites, substantially increasing the electrochemical surface area and catalytic activity. This study offers valuable insights into the catalyst design for high-valence Sn^δ+ in CO₂ reduction.