Halide-guided active site exposure in bismuth electrocatalysts for selective CO 2 conversion into formic acid

It remains a challenge to identify the active sites of bismuth catalysts in the electrochemical CO 2 reduction reaction. Here we show through in situ characterization that the activation of bismuth oxyhalide electrocatalysts to metallic bismuth is guided by the halides. In situ X-ray diffraction results show that bromide promotes the selective exposure of planar bismuth surfaces, whereas chloride and iodide result in more disordered active sites. Furthermore, we find that bromide-activated bismuth catalysts outperform the chloride and iodide counterparts, achieving high current density (>100 mA cm –2 ) and formic acid selectivity (>90%), suggesting that planar bismuth surfaces are more active for the electrochemical CO 2 reduction reaction. In addition, in situ X-ray absorption spectroscopy measurements reveal that the reconstruction proceeds rapidly in chloride-activated bismuth and gradually when bromide is present, facilitating the formation of ordered planar surfaces. These findings show the pivotal role of halogens on selective facet exposure in activated bismuth-based electrocatalysts during the electrochemical CO 2 reduction reaction.