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

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 FEC2+ 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.