• • AuAgCuPdPt HEAs achieve a peak Faradaic efficiency (FE) of 96.5% for CO at −0.3 V vs. RHE, with FE >90% across −0.2 to −0.7 V, demonstrating superior selectivity for CO2-to-CO conversion.
• • In situ ATR-FTIR reveals that Pt addition promotes water dissociation to generate protonic hydrogen and facilitates *CO desorption, directly addressing the rate-limiting steps in CO2RR.
• • Kinetic isotope effect (H-D) experiments confirm water dissociation as the key proton donor, providing mechanistic evidence for the reaction pathway.
• • In a Zn-CO2 battery, the AuAgCuPdPt cathode delivers 90.23% FE for CO and a power density of 3.474 mW cm−2, showcasing practical energy storage and CO2 conversion integration.