CV was performed at by identifying the real impedance contribution from potentio electrochemical impedance spectroscopy. Current density for Cu/CeO2-δ and Cu/FTO has been normalized to geometric surface area (cm 2 geo), as normalization by double layer capacitance would also include an unknown contribution from the CeO2-δ and FTO supports. Electrochemically active surface area (ECSA) of Cu foils was determined by sweeping from 0 to 0. Cdl by the average of specific capacitances measured in alkaline (150 mF/cm 2 ) and acidic (50 mF/cm 2 ) electrolytes. Selectivity Selectivity measurements were performed in a Pine Research low volume separated Hcell, with ca. are detailed in the Supplemental Information.