Electrocatalytic reduction of carbon dioxide (CO2ER) to reusable carbon resources is a significant step to balance the carbon cycle. This Communication describes a seed-mediated growth method to synthesize ultrathin Pd–Au alloy nanoshells with controllable alloying degree on Pd nanocubes. Specifically, Pd@Pd3Au7 nanocrystals (NCs) show superior CO2ER performance, with a 94% CO faraday efficiency (FE) at −0.5 V vs reversible hydrogen electrode and approaching 100% CO FE from −0.6 to −0.9 V. The enhancement primarily originates from ensemble and ligand effects, i.e., appropriately proportional Pd–Au sites and electronic back-donation from Au to Pd. In situ attenuated total reflection infrared spectra and density functional theory calculations clarify the reaction mechanism. This work may offer a general strategy for the synthesis of bimetallic NCs to explore the structure–activity relationship in catalytic reactions.