The effects of halide ions (F-,Cl-,Br-, and I- ) on nanoporous Ag electrode for reduction of CO2 to CO were systematically assessed. The electrochemical measurements indicate that the catalytic performance of nanoporous Ag is closely related to the affinity of halide ions to Ag. The current density and Faradaic efficiency for CO production simultaneously increase with the increase of the radius of halide ions from F- to Cl- and then to Br-, and the disparity between catalytic performance will further widen with the negative shift of potential. While the nanoporous Ag in the presence of I- first exhibits the poorest electrocatalytic performance at low potential (<-0.69 V vs. RHE), then reverses the situation at high potential (>-0.89 V vs. RHE) in comparison to the other halide ions. Tafel plots indicate that the adsorption of halide ions on the surface of nanoporous Ag influences mechanistic pathways for CO2 reduction reaction. 8 h durability test demonstrates that the nanoporous Ag is catalytically stable in a mixed electrolyte containing bromide with only less than 5% deactivation. The density functional theory calculations enlighten that the weakened interaction between Ag atoms on the surface of electrocatalyst in the presence of halides can significantly enhance the binding energy of the intermediate COOH*, and thus effectively lowers the overpotential of CO2 reduction reaction. (C) 2019 Elsevier Ltd. All rights reserved.