Considering NO3RR FE and ammonium selectivity graphically provides a simple means to identify promising electrocatalysts (Figure ), which appear increasingly towards the upperright. TMs such as Cu and Ag, while demonstrating appreciable NO3RR FE, provide poor selectivity to ammonium. At increasingly cathodic potentials, selectivity towards ammonium does increase for these TMs, though at the cost of NO3RR FE (Supplementary Figures and). Alternatively, TMs such as Ni and Ti demonstrate the opposite behavior: providing high selectivity towards ammonium with low NO3RR FE. Optimally, Fe and (in particular) Co demonstrate both appreciable NO3RR FE and selectivity towards ammonium. As NO3RR FE represents electron selectivity between HER and nitrate reduction, it is best described via H * affinity towards TM surfaces and the associated electronic descriptor of work function (Figure and Supplementary Figure ). NO3RR FE is appreciable (>70%) for TMs Cu, Co, Fe), but drops for TMs with either high work function (e.g. Ni, Figure ) or high H chemisorption energy (e.g. Ti, Supplementary Figure ). Ni (and Ni0.68Cu0.32) has the greatest work function, and consequently greatest HER activity (Supplementary Figure ), PCET or hydrogenation). The dependence of nitrate rate order on Δ𝐺 𝐻 Δ𝐺 𝑁𝑂 3 - discussed in Section 3.1.1 may provide additional context for understanding the relationship between NO3RR FE and work function as a future direction for the field. Differences in NO3RR