Here we investigate NO3RR kinetics and selectivity of supported Cu in buffered sodium phosphate electrolyte (pH 7; NaxH3-xPO4). We observe a competitive adsorption mechanism between nitrate and adsorbed hydrogen via changes in activity as a function of cyclic voltammetry (CV) cathodic limit and selectivity measurements. The NO3RR activity is greater for Cu films electrodeposited over CeO2-δ (expected to form 𝑣 𝑂 •• ) than Cu electrodeposited on FTO (not expected to form 𝑣 𝑂 •• ), corroborating that the NO3RR ratelimiting step involves nitrate adsorption. Nitrate has a greater affinity for CeO2-δ, relative to FTO, which increases the coverage of nitrate and slightly shifts selectivity towards nitrite.