Selective conversion of carbon dioxide (CO2) to a reusable form of carbon via electrochemical reduction has attracted intensive interest for the storage of renewable energy. However, the achievement of efficient bulk monolithic electrocatalysts still remains a challenge. Herein, a facile oxidative etching of the Au20Cu80 alloy was developed for the synthesis of a monolithic nanoporous core–shell structured AuCu3@Au electrode, which showed a faradaic efficiency (FE) of 97.27% with a partial current density of 5.3 mA cm−2 at −0.6 V vs. RHE for the production of CO. The FE value is about 1.45 times higher than that over the Au nanocatalyst. Unlike single nanoporous Au, AuCu3@Au maintained an excellent performance in a broad potential window. Furthermore, a 23 cm long nanoporous AuCu3@Au bulk electrode with good ductility was prepared, over which the active current reached up to 37.2 mA with a current density of 10.78 mA cm−2 at −0.7 V vs. RHE, pushing the reduction of CO2 to industrialization. The unsaturated coordination environment with a coordination number of 8.2 over the shell gold and curved interface determined this high electrocatalytic performance. Density functional theory calculations suggested that the double-dentate adsorption structure in the AuCu3@Au catalyst effectively improves the stability of the *COOH intermediate. The density of states indicates that the introduction of Cu causes the d-band-centre of AuCu3@Au to move toward the Fermi level, directly bonding with *COOH. Therefore, the adsorption of *COOH on the surface of the AuCu3@Au catalyst is strengthened, facilitating the formation of CO. This work opens an avenue to achieve self-supported porous electrodes for various useful catalytic conversions.