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In light of the enormous energy footprint of the Haber–Bosch process (1–2% of global energy consumption), alternative green routes of generating ammonia (NH 3 ) are needed. The electrochemical reduction of NO 3 − from waste streams is a promising method to produce NH 3 using renewably-sourced electricity. However, catalyst selectivity is a grand challenge that hinders NO 3 − to NH 3 conversion technologies. In this manuscript, we fabricate Nafion-modified metal catalysts for NO 3 − reduction. Although Nafion composites are commonly used to facilitate proton transfer, this work investigates electrodes covered by Nafion overlayers, which possess unique reactivity. We find that Cu versions of these catalysts reduce NO 3 − to NH 3 with a faradaic efficiency of up to (91 ± 2)%, making them among the most selective catalysts reported. Voltammetry studies, surface-enhanced Raman spectroscopy, and density functional theory calculations indicate that the Nafion overlayer activates the N–O bond of a key Cu–NO intermediate, thus facilitating NH 3 production. Lastly, we demonstrate that these catalysts are effective at denitrifying polluted groundwater samples in the field.