Critical Role of Surface Hydroxyls on Bismuth in Steering CO ₂ Electroreduction to Formate

The electrocatalytic reduction of CO2 to formic acid (HCOOH) is a promising route toward carbon neutrality, offering high selectivity and Faradaic efficiency. However, the essential mechanism governing the reduction of CO2 to HCOOH remains elusive, as the reaction intermediates have not been conclusively identified. Herein, we discovered that the hydroxylated metallic Bi (OH–Bi) achieves a formate FE of up to 99.9%, markedly surpassing the 5% FE of commercial Bi powder. In situ Raman spectroscopy revealed that OH–Bi promotes the formation of carbonate intermediates during the CO2 reduction reaction (CO2RR). Isotopic labeling experiments with 18O in online differential electrochemical mass spectrometry (DEMS) confirmed that the oxygen atoms from surface hydroxyl groups actively participate in formate production. Density functional theory calculations proposed a feasible pathway from CO2 to HCOOH, demonstrating that the hydroxylated Bi surface lowers the free energy change for CO2 hydrogenation from 6.40 to 0.72 eV by facilitating elongation of C═O bonds in CO2 molecules and blocking the hydrogen evolution reaction, which enables the OH–Bi catalyst exhibiting near-unity formate selectivity and enhanced stability. This study clarifies the essential mechanism of HCOOH production by electrocatalytic CO2 reduction, providing theoretical guidance for the rational design of efficient catalysts.