Electrochemical CO₂ reduction reaction (CO2RR) using renewable energy sources is a promising method for converting CO₂ to chemical fuels such as CO, formate HCOOH, and hydrocarbons. Direct conversion of flue gas CO₂ emitting from industry to valuable chemicals is an important issue for carbon capture and utilization in an economic approach. The flue gas emitted from coal-fired power and steel industry contains metal impurities at hundreds of ppm levels such as Fe₂⁺, Zn₂⁺, and gaseous impurities of SO_x, NO_x and H₂S gases. Such impurities dissolving in the electrolyte could degrade the catalytic activities due to the surface poisoning by the deposition of metal impurities during the CO₂RR. To mitigate this problem, pretreatment of the electrolyte with several ligands such as ethylenediaminetetraacetic acid (EDTA) or solid-supported iminodiacetate (IDA) was employed to suppress the metal deposition on the surface by forming the metal-ligand complex. In contrast, efforts to understand the effect of gaseous impurities on catalytic properties have been limited although billion tons of CO₂ (~8% of global CO₂ emissions) with 0.3 – 0.9% of H₂S impurity have been emitted from the steel industry every year. The sulfur species could react with metal to form catalytic active metal compounds by the formation of metal-sulfur bonds rather than degrade the performance of CO₂ reduction reaction. Until now, most studies have been conducted on electrochemical CO₂ reduction using pure CO₂ gas without any impurities such as H2S gas. This work present a novel method to directly convert H2S-containing CO₂ gas into value-added chemicals through CuS_x nanostructures rapidly grown by immersing 3-nm-thick Ag-coating Cu foil (Ag/Cu) into a H₂S-containing CO₂ electrolyte. Herein, we investigated the effect of H₂S on morphology, composition, and catalytic activity of thin Ag layers deposited on Cu foil (Ag/Cu) for the electrochemical reduction reaction of modeled sulfur-containing CO₂ (CO₂+H₂S). The bimetallic Ag (3 nm)/Cu spontaneously changed to CuS_x nanostructures during the sulfur-containing CO₂ reduction reaction. Ag initially transformed to AgS_x with nanoporous structure, leading to exposure of underlying Cu surface to the electrolyte. Then the Cu dissolved preferentially because the difference in the reduction potential between Cu and Ag drives the Ag-mediated corrosion reaction. The dissolved Cu ions readily react with sulfur species in the electrolyte, thereby resulting in formation of thick CuS_x nanostructures at the surface. Together with the facilitated charge-transfer activity and increased the electrochemical surface area, CuS_x nanostructures on Ag (3 nm)/Cu showed relatively higher value of average FE_HCOOH = 87.37% (+/-1.7%) at -0.6 V_RHE and j_HCOOH = 9.60 (+/-1.1) mA/cm² at -1.2 V_RHE compared to that of CuS_x on Cu foil (average FE_HCOOH = 69.09% (+/-2.0%), j_HCOOH = 4.78 (+/-0.6) mA/cm²). This study provides an evidence on the origin of galvanic corrosion of bimetallic catalysts and their beneficial effect on the catalytic activity for sulfur-containing CO₂ conversion.