Abstract Utilization of the excess capacity from power plants by electrocatalytic methods to reduce the products from ammonia-based carbon capture technology to chemicals such as syngas is valuable and meaningful. Direct electrocatalytic reduction of NH4HCO3 electrolyte to syngas without CO2 bubbling is rarely reported. A porous Br-modified Ag catalyst with trace amounts of Br on the surface was investigated in 1.0, 2.0 M, and saturated NH4HCO3 electrolyte without CO2 bubbling. This catalyst can generate CO and H2 at ratios with ranges from 2:1 to 3:1. The highest CO Faradaic efficiency of 77.8% was observed with the CO current density of 13.8 mA cm−2 at −0.6 V vs RHE in saturated NH4HCO3 electrolyte. By contrast, the Ag catalyst with high selectivity for electrochemical reduction of CO2 to CO cannot catalyze CO production under this condition. On the Br-modified Ag catalyst, the trace amounts of Br formed a chemical bond with Ag in the Helmholtz surface, leading to changes in the electronic state and structure of Ag. The results are beneficial to the adsorption of intermediates. Thus, the remaining Br may serve as active contributors to promote the selectivity and catalytic activity of the electrochemical reduction both on the Br-modified Ag catalyst and in the NH4HCO3 electrolyte.