Nitrite is the primary reduction product at low conversion (Figure )) on Cu/CeO2-δ, increasing monotonically with e -/NO3 -(0 to 4 e -/NO3 -, Figure ). As charge increases to > 4 e -/NO3 -, the concentration of nitrite decreases, indicating its conversion to more reduced species exceeds the rate at which it is produced by nitrate reduction. At 10 e -/NO3 -an appreciable decrease in NTot (ca. 16 ± 5% for Cu/CeO2-δ-C, Figure ; 23 ± 8% for Cu/C-Felt, Figure ) is ascribed to evolution of nitrogen-containing species not directly identified or quantified (ΔNTot). We suggest that ΔNTot is primarily caused by reduction of aqueous-phase nitrite, known to produce gas-phase nitrogen/oxides and hydroxylamine in other systems, where aqueous-phase nitrite concentration is only appreciable at higher nitrate conversion (> 4 e -/NO3 -). Ammonium production is nominally linear with increasing e -/NO3 -, suggesting ~zeroth-order reaction kinetics at the nitrate and nitrite concentrations observed here. Concentration-dependent reaction kinetics for nitrate reduction to ammonium have recently been observed on Ni foam in sodium sulfate electrolyte, ranging from first (200 ppm NO3 --N) to zeroth order (2000 ppm NO3 --N). Similar trends in selectivity are observed for Cu/C-Felt (Figure ). From 0.2 to 2 e -/NO3 -passed, total NO3RR FE (Eqn. 1) is higher for Cu/C-Felt than for Cu/CeO2-δ-C (Figure ). We note that the higher phosphate deprotonation current observe on Cu/CeO2-δ at -0.4 VRHE is likely the source of lower NO3RR FE for Cu/CeO2-δ-C relative to Cu/C-Felt (Figure ). This may also lead to the somewhat lower nitrate conversion (Eqn. 3)