Membrane-modified electrocatalysts for nitrate reduction to ammonia with high faradaic efficiency

Abstract

In light of the enormous energy footprint of the Haber–Bosch process (1–2% of global energy consumption), alternative green routes of generating ammonia (NH₃) are needed. The electrochemical reduction of NO₃⁻ from waste streams is a promising method to produce NH₃ using renewably-sourced electricity. However, catalyst selectivity is a grand challenge that hinders NO₃⁻ to NH₃ conversion technologies. In this manuscript, we fabricate Nafion-modified metal catalysts for NO₃⁻ 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 NO₃⁻ to NH₃ 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 NH₃ production. Lastly, we demonstrate that these catalysts are effective at denitrifying polluted groundwater samples in the field.