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Electrocatalytic reduction of waste nitrate to ammonium provides a circular process with reduced carbon dioxide emissions compared to current nitrate treatment and ammonia production processes. However, electrocatalysts require a delicate balance between a surfaces’ activity for the competing hydrogen evolution (HER) and nitrate reduction reactions (NO3RR). We measure ammonium Faradaic efficiencies (FEs) of several transition metals (TMs) ranging from 3.6±6.6% (on Ag) to 93.7±0.9% (on Co) in neutral buffered media. A microkinetic model identifies competitive adsorption between nitrate and hydrogen adatoms (H*) as the origin of voltage-dependent nitrate rate order. NO3RR FE is described via competition for electrons with the HER, decreasing sharply for TMs with high work function or hydrogen adsorption energy. Density functional theory calculations indicate Co maximizes ammonium selectivity by: (1) binding intermediate nitrite strongly to enable subsequent reduction; and (2) promoting subsequent nitric oxide dissociation, leading to selective reduction of nitrogen adatoms (N*) to ammonium.