Mechanistic insights into the impact of bromide ion adsorption and surface bromination on Cu2O for enhanced selectivity and activity in electrochemical CO2 reduction

Abstract

The enhanced selectivity for C₂₊ products in the electrochemical CO₂ reduction reaction (ECO₂RR) is critically dependent on the regulation of the elemental existence state on the surface of the electrocatalyst. In this study, Cu₂O nanowires featuring multiple grain boundaries were successfully synthesized. Two distinct model catalysts were prepared: one through surface adsorption of Br⁻ (denoted as Cu₂O_Br) and the other via surface bromination (denoted as Cu₂O@CuBr). These models were employed to systematically investigate the influence of the differences between Br⁻ adsorption on the Cu₂O surface and surface bromination on activity and product selectivity. The integration of in-situ characterization techniques with electrochemical measurements revealed that Br⁻ adsorption induces a stable charge distribution on the catalyst surface, accompanied by a consistent potential drop within the double layer. This signifies stable and efficient processes of CO₂ adsorption, electron transfer, and mass transfer at the electrode/electrolyte interface. Under these conditions, Cu₂O_Br exhibits high stability. In contrast, catalyst surfaces modified via surface bromination are prone to Br⁻ dissolution during electrolysis, leading to structural changes and significant surface reconstruction, which ultimately compromise the catalyst’s selectivity. Notably, the Cu₂O_Br catalyst achieved a maximum Faradaic efficiency (FE) of 98 % for CO production at −0.4 V vs. RHE and 42 % for C₂H₆ production at −0.6 V vs. RHE. Additionally, the Cu₂O_Br catalyst reached an optimal FE_C2+ of 60 % at −0.6 V, which is 1.5 times higher than that of the pure Cu₂O catalyst under the same potential and 2.5 times higher than that of the Cu₂O@CuBr catalyst at −0.9 V. This work provides new insights into enhancing the selectivity and activity of carbon dioxide electroreduction by modulating halide ion adsorption on the catalyst surface and surface halogenation.