*CO-Driven Selectivity Switching of C2+ Products over Au-Cu Biphasic Heterostructures in CO2 Electroreduction.

The electrochemical reduction of carbon dioxide (CO2) to multicarbon (C2+) products offers a sustainable strategy for carbon utilization, but the selective formation of target C2+ species remains challenging due to competitive reaction pathways. In this study, we design phase-separated Au-Cu biphasic heterostructures by assembling Au nanoparticles onto Cu nanowires through 4,4'-bipyridine linkers, creating a tunable tandem catalytic interface. Electrochemical tests demonstrate that Au loading regulates C2+ selectivity, where Au1Cu10 predominantly produces ethylene (C2H4) and Au1Cu5 favors ethanol (C2H5OH) formation. Operando spectroelectrochemical measurements and finite-element simulations reveal that modulating the surface density of Au nanoparticles controls the *CO concentration near Cu sites, dictating the reaction pathway. Moderate *CO coverage favors the *OCCHO pathway toward C2H4, whereas high *CO accumulation promotes the *OCCOH pathway leading to C2H5OH. Density functional theory calculations further confirm that *CO coverage modulates the thermodynamic preference for carbon-carbon coupling intermediates. This work establishes a mechanistic link between interfacial *CO availability and C2+ selectivity, offering new design principles for engineered tandem electrocatalysts in CO2 conversion.