Synergistic effects at metal/metal oxide interfaces often give rise to highly active and selective catalytic motifs. So far, such interactions have been rarely explored to enhance the selectivity in the electrochemical CO2 reduction reaction (CO2RR). Herein, Cu/CeO2-x, heterodimers (HDs) are synthesized and presented as one of the prime examples where such effects promote CO2RR, A colloidal seeded-growth synthesis is developed to connect the two highly mismatched domains (Cu and CeO2-x) through an interface. The Cu/CeO2-x HDs exhibit state-of-the-art selectivity toward CO2RR (up to similar to 80%) against the competitive hydrogen evolution reaction (HER) and high faradaic efficiency for methane (up to similar to 54%) at -1.2 V-RHE, which is similar to 5 times higher than that obtained when the Cu and CeO2-x nanocrystals are physically mixed. Operando X-ray absorption spectroscopy along with other ex-situ spectroscopies evidences the partial reduction of Ce4+ to Ce3+ in the HDs during CO2RR A Density Functional Theory (DFT) study of the active site motif in reducing condition reveals synergistic effects in the electronic structure at the interface. The proposed lowest free energy pathway utilizes an O-vacancy site with intermediates binding to both Cu and Ce atoms, a configuration which allows one to break the CHO*/CO* scaling relation. The suppression of HER is attributed to the spontaneous formation of CO* at this interfacial motif and subsequent blockage of the Cu-sites.