Increased hydrogen coverage at more cathodic potentials promotes formation of hydrogenated products: ammonium requires 10 H + /NO3 -while nitrite only requires 2 H + /NO3 -. In contrast, material-dependent selectivity may stem from the activity of oxygen within the CeO2-δ lattice. Surface science literature has identified that oxidized nitrogenous species (e.g. nitric oxide and nitrogen dioxide) have high affinity for non-stoichiometric CeO2-δ and adsorb poorly on stoichiometric CeO2. Under sufficiently cathodic potentials, CeO2-δ is known to form 𝑣 𝑂 •• , stabilizing the adsorption of oxidizers such as nitrate and electrosynthetic oxygen redox reactions. NO3RR FE (Figure ) at less-cathodic applied potentials (-0. ) and is in line with speciation measurements in low nitrate concentrations (Figure ). While NO3RR FE for Cu/CeO2-δ-C is lower than Cu/C-Felt at 50 mM NaNO3, a greater proportion of charge goes to nitrite production (Figure ), leading to slightly higher nitrate conversions (Figure ). Nitrite is undesirable from a wastewater treatment perspective, though has a more diverse product portfolio. This suggests pairing metals know to selectively reduce nitrite to desirable products (e.g. dinitrogen, hydroxylamine, or ammonium) with Cu/CeO2-δ may prove to be a useful catalyst design strategy for overcoming the complex NO3RR pathway.