CO-selective metals (e.g., Ag) on Cu catalysts improve the selec-tivity of multi-carbon (C2+) products in electrochemical CO2 reduc-tion. However, the origin of the improvement remains unclear due to the convolution of tandem and interface effects. Here, Ag@C@Cu core-shell catalysts were synthesized, in which the thin carbon interlayer inhibited the direct interaction between Ag and Cu while still allowing the reduction of CO2 on Ag, thus isolating the tandem effect from other effects. This catalyst produced higher ratios of ethanol to ethylene relative to the monometallic Cu catalyst, demonstrating that the locally increased CO concentration promoted the ethanol pathway over the ethylene pathway. Further, the selectivity of ethanol was optimized by tuning the thickness of the Cu shell. This work provides a rational approach to design core-shell catalysts for understanding structure-performance relationships and demonstrates the key role of the tandem effect in tuning the selectivity of C2+ products.