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

The electrocatalytic reduction of CO 2 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 CO 2 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 CO 2 reduction reaction (CO 2 RR). Isotopic labeling experiments with 18 O 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 CO 2 to HCOOH, demonstrating that the hydroxylated Bi surface lowers the free energy change for CO 2 hydrogenation from 6.40 to 0.72 eV by facilitating elongation of C═O bonds in CO 2 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 CO 2 reduction, providing theoretical guidance for the rational design of efficient catalysts.