Restructuring-Regulated Bismuth Catalyst Promotes Electrochemical CO₂ Reduction to Formic Acid in Acidic Electrolyte

Electrochemical CO₂ reduction (eCO₂R) in acidic electrolytes is appealing due to its high CO₂ utilization efficiency. For this reaction, bismuth (Bi)-based catalysts have drawn considerable attention for their potential in producing formate/formic acid. However, the presynthesized materials for Bi-based catalysts often undergo restructuring during electrocatalysis, resulting in altered electrochemical performance. Furthermore, the mechanisms underlying the restructuring of Bi-based catalysts in acidic environments have not yet been clearly elucidated. Herein, distinct restructuring mechanisms are revealed in structurally different Bi-based compounds, such as Bi₉O_7.5S₆ and Bi₂O₂S. Among them, the Bi₉O_7.5S₆ precatalyst exhibits high selectivity and activity for formic acid production, attributed to its unique structure, featuring stacking of [Bi₂O₂]²⁺ and [BiS₂]^- layers. In contrast, the conventional Bi₂O₂S catalyst, characterized by alternating [Bi₂O₂]²⁺ layers with S^2- ions, delivers inferior eCO₂R performances. Quasi-in situ X-ray diffraction and in situ Raman spectra results reveal that metal elements situated between two [Bi₂O₂]²⁺ layers can resist decomposition and prevent the over-reduction of catalysts, leading to the restructuring in Bi/Bi₂O₂CO₃ composite material with active Bi-Bi₂O₂CO₃ interface for formic acid production. As a result, the Bi₉O_7.5S₆ precatalyst achieves a high Faraday efficiency above 95% at 100 mA cm^-2 and remarkable stability of 117 h in a flow cell.