Nitrate accumulation represents a growing threat to global drinking water resources and human health. At the same time, ammonia production (the largest anthropogenic disruption to For graphical abstract only the nitrogen cycle) 2 emits more carbon dioxide than production of any other commodity chemical. Together, this motivates research in technologies that can generate green ammonia, preferably circularly by upgrading waste forms of nitrogen. Electrochemical reduction offers a scalable, distributable, and increasingly economical technology for the upgrading of waste nitrate to value-added products (ammonium and hydroxylamine ) or benign dinitrogen gas. Nitrate reduces to ammonium by a complex 8-electron and 10-proton process. In the ratelimiting step, adsorbed nitrate reduces to nitrite. Nitrite is the predominant side product in neutral and alkaline electrolytes, reducing further to nitric oxide when remaining adsorbed on the surface (and more complex products when present in the aqueous phase ). Nitric oxide serves as a critical intermediate in determining selectivity between nitrogen/oxides and ammonium or hydroxylamine. Mechanistically, adsorbed nitric oxide (NO * ) forms ammonium by one of two pathways: (1) Eley-Rideal like proton-coupled electron transfer (PCET) reducing NO * to hydroxylamine (NH2OH * ) and further to ammonium; or (2) dissociation of NO * into N * and O * adatoms and subsequent Langmuir-Hinshelwood like hydrogenation of N * by H * to ammonium. Despite these complexities, recent electrochemical NO3RR literature has demonstrated appreciable FE and selectivity to ammonia in highly alkaline electrolytes. For example, strained Ru nanoparticles-where Ru is a benchmark thermal Haber Bosch catalyst-have achieved 100% FE at potentials as cathodic as -0.2 V vs the reversible hydrogen electrode (RHE), albeit at high nitrate concentrations (1 M). First row transition metals such as Co and alloys including NiCu have also demonstrated high FE. However, while such examples exist under alkaline pH, a fundamental understanding of the physicochemical factors driving NO3RR selectivity towards ammonium is lacking, particularly in the circumneutral pH range expected of most wastewaters. Here we investigate electrocatalytic NO3RR activity and selectivity at low conversion for a series of polycrystalline 3d (Ti, Fe, Co, Ni, Ni0.68Cu0.32, and Cu) and 4d (Ag) transition metal foils (TMs) in buffered sodium phosphate electrolyte (pH 7; NaxH3-xPO4). Trends between phosphate-mediated HER activity and H chemisorption energy correspond with the familiar Sabatier relationship between HER exchange current and metal-H * bond strength, providing an in-situ probe of H * affinity for different TM surfaces. Microkinetic modeling of the competition between the HER and NO3RR captures experimentally observed potentialdependent nitrate rate order, where peak rate order magnitude and potential are well-described by the difference in H * and nitrate adsorption free energies (Δ𝐺 𝐻 Δ𝐺 𝑁𝑂 3 -). The HER and NO3RR compete for electrons, where HER activity descriptors (TM work function by photoemission and H chemisorption energy ) play key roles in describing NO3RR FE. The d-band model provides a fundamental understanding of the relationship between TM electronic structure and nitric oxide adsorption energies, describing to a first order the increased selectivity towards ammonium as the TM d-band center energy (Ed) approaches the Fermi level (EF). Selectivity can be further explained by the differences in calculated reaction free energies for nitrite reduction (Δ𝐺 𝑁𝑂 Δ𝐺 𝑁𝑂 2 -) and nitric oxide dissociation (Δ𝐺 𝑁 +𝑂 Δ𝐺 𝑁𝑂 ) ,