Anion-regulated reconstruction of bismuth-based electrocatalysts for enhanced electrocatalytic CO2 reduction

Elucidating the active site formation mechanism of bismuth (Bi)-based catalysts in electrochemical CO2 reduction remains challenging for achieving high activity, selectivity, and long-term stability. Here we confirm through experimental results that Bi-based catalysts containing halogen ions (I−, Cl−, Br−) and SO42− maintain the system stability, keeping Faraday efficiency of formic acid above 90% in the current range of 50–800 mA cm−2. In contrast, anions containing S2− and NO3− in the electrolyte can be reduced to produce by-products. These anions and their by-products could poison the active center, leading to increased side reactions and thus significantly reducing the Faraday efficiency of formic acid. The combination of non-in situ and in situ characterization results revealed that the Bi-based catalysts all underwent the transition from the initial state to the Bi/Bi2O2CO3 (BOC) intermediate state in high-concentration KHCO3 solution, and the different anions could selectively modulate the degree of exposure of specific crystalline surfaces of BOC. At the late stage of the reaction, BOC was completely converted to metal Bi and became the real active center. Combined with in situ IR and DFT calculations, it is further verified that *OCHO is the key intermediate on the metallic Bi surface, which is most favorable for formic acid formation. This study reveals the key mechanism by which anions affect the formation of active sites via modulating the catalyst reconstruction process, which provides an important theoretical basis for the design and optimization of test conditions of Bi-based catalysts.